COPYRIGHT AND CITATION CONSIDERATIONS FOR THIS THESIS/ DISSERTATION

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1 COPYRIGHT AND CITATION CONSIDERATIONS FOR THIS THESIS/ DISSERTATION This copy has been supplied on the understanding that it is copyrighted and that no quotation from the thesis may be published without proper acknowledgement. Please include the following information in your citation: Name of author Year of publication, in brackets Title of thesis, in italics Type of degree (e.g. D. Phil.; Ph.D.; M.Sc.; M.A. or M.Ed. etc.) Name of the University Website Date, accessed Example Surname, Initial(s). (2012) Title of the thesis or dissertation. PhD. (Chemistry), M.Sc. (Physics), M.A. (Philosophy), M.Com. (Finance) etc. [Unpublished]: University of Johannesburg. Retrieved from: (Accessed: Date).

2 Towards a magmatic barcode for the south-easternmost terrane of the Kaapvaal Craton, South Africa by ASHLEY PAUL GUMSLEY DISSERTATION Submitted in fulfilment of the requirements for the degree of MAGISTER SCIENTAE in GEOLOGY at the FACULTY OF SCIENCE of the UNIVERSITY OF JOHANNESBURG, SOUTH AFRICA SUPERVISOR: M.W. KNOPER CO-SUPERVISOR: M.O. DE KOCK May 2013

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4 DECLARATION I hereby declare that this dissertation submitted for the Magister Scientae degree to the Faculty of Science at the University of Johannesburg, apart from the help recognised, is my own original work and has not been formally submitted in the past, or is being submitted, for a degree or examination at any other university. A.P. Gumsley i

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6 ACKNOWLEDGEMENTS Any system is the sum of its moving parts, and in this work, there is no difference, no matter how small the part may be; as each bigger part is ultimately composed of a number of equally critical smaller parts First, I wish to thank my two supervisors, Michael Knoper and Michiel de Kock. My supervisors not only provided me with guidance and patience in my study, but also allowed me the freedom to pursue my thoughts and feelings with regard to my work and what we wished to accomplish. I would also like to thank them for their friendship, thoughts and passion for geology, as well as their constructive criticisms and belief in me while working on my thesis. I am indebted to the Department of Geology at the University of Johannesburg, and more specifically the Palaeoproterozoic Mineralisation Group which provided me with a scholarship. In addition I wish to thank Richard Ernst, Wouter Bleeker and Ulf Söderlund who not only provided finances for doing my U-Pb baddeleyite age dating through the Supercontinent Project ( but also helped with my baddeleyite separation and TIMS age dating, particulary Ulf. The Jim and Gladys Taylor Trust must also be thanked for providing me with living expenses while travelling and staying in Sweden during my analytical work. I would also like to acknowledge Rajesh Harirajan, Johan Olsson, Herman van Niekerk, Bertus Smith, Lauren Blignaut, Nic Beukes, George Belyanin, Jan Kramers, Barbara Cavalazzi, Andrea Agangi, Bryony Richards, Craig McClung, Christian Reinke, Fanie Kruger, Lisborn Mangwane, Baldwin Tshivhiahuvhi, Diana Khoza, Eve Kroukamp, Herwe Wabo and Hennie Jonker who all played a role in assisting me throughout my studies, whether it was through friendship, advice, criticisms or help during my study, I cannot state this enough. Last but not least I wish to thank my parents, without whose love and support I have received over these many years, none of this would have been possible. iii

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8 ABSTRACT The south-easternmost Kaapvaal Craton is composed of scattered inliers of Archaean basement granitoid-greenstone terrane exposed through Phanerozoic cover successions. In addition, erosional remnants of the supracrustal Mesoarchaean Pongola Supergroup unconformably overlay this granitoid-greenstone terrane in the same inliers. Into this crust a variety of Precambrian intrusions occur. These are comprised of SE-, ENE- and NE-trending dolerite dykes. Also, the Hlagothi Complex intrudes into Pongola strata in the Nkandla region, particularly the quartzites of the basal Mantonga Formation. The whole area, including Phanerozoic strata, has in turn been intruded by Jurassic sills and dykes related to the Karoo Large Igneous Province. All the rocks of the Archaean inliers, with the exception of the Jurassic sills and dykes have been subjected to greenschist facies metamorphism and deformation, with petrographic, Ar-Ar geochronologic and palaeomagnetic studies attesting to this. This metamorphism and deformation is associated with the Mesoproterozoic orogeny from the nearby Namaqua-Natal Mobile Belt located to the south. This orogeny has a decreasing influence with distance from the cratonic margin, and is highly variable from locality to locality. However, it is generally upper greenschist facies up to a metamorphic isograd 50 km from the craton margin. Overprints directions seen within the palaeomagnetic data confirm directions associated with the post-pongola granitoids across the region and the Namaqua-Natal Mobile Belt. The dolerite dykes consist of several trends and generations. Up to five different generations within the three Precambrian trends have potentially been recognised. SEtrending dykes represent the oldest dyke swarm in the area, being cross-cut by all the other dyke trends. These dykes consist of two possible generations with similar basaltic to basaltic andesite geochemistry. They provide evidence of a geochemically enriched or contaminated magma having been emplaced into the craton. This is similar to SE-trending dolerite dyke swarms across the Barberton-Badplaas region to the north from literature. In northern KwaZulu-Natal the SE-trending dolerite dyke swarms have been geochronologically, geochemically and paleomagnetically linked to either ca or ca Ga magmatic events across the Kaapvaal Craton. v

9 The 2866 ± 2 Ma Hlagothi Complex is composed of a series of layered sills intruding into Nkandla sub-basin quartzites of the Pongola Supergroup. The sills consist of meta-peridotite, pyroxenite and gabbro. At least two distinct pulses of magmatism have been recognised in the sills from their geochemistry. The distinct high-mgo units are compositionally different from the older Dominion Group and Nsuze Group volcanic rocks, as well as younger Ventersdorp volcanic rocks. This resurgence of high-mgo magmatism is similar to komatiitic lithologies seen in the Barberton Greenstone Belt. It is indicative of a more primitive magma source, such as one derived from a mantle plume. A mantle plume would also account for the Hlagothi Complex and the widespread distribution of magmatic events of possible temporal and spatial similarity across the craton. Examples include the layered Thole Complex, gabbroic phases of the ca to 2870 Ma Usushwana Complex, and the 2874 ± 2 Ma SE-trending dykes of northern KwaZulu-Natal already described above and dated herein. A generation of NE-trending dolerite dykes in northern KwaZulu-Natal can also be palaeomagnetically linked to this event with either a primary or overprint direction. Flood basalts seen within the upper Witwatersrand and Pongola Supergroups (i.e., Crown, Bird, Tobolsk and Gabela lavas) may also be related. This large, voluminous extent of magmatism allows us to provide evidence for a new Large Igneous Province on the Kaapvaal Craton during the Mesoarchaean. This new Large Igneous Province would encompass all of the above mentioned geological units. It is possible that it could be generated by a shortlived transient mantle plume(s), in several distinct pulses. This plume would also explain the development of unconformities within the Mozaan Group. This is reasoned through thermal uplift from the plume leading to erosion of the underlying strata, culminating in the eruption of flood basalts coeval to the Hlagothi Complex. Marine incursion and sediment deposition would occur during thermal subsidence from the plume into the Witwatersrand-Mozaan basin. This magmatic event also assists in resolving the apparent polar wander path for the Kaapvaal Craton during the Meso- to Neoarchaean. Between existing poles established for the older ca Ga Nsuze event, to poles established for the younger ca Ga Ventersdorp event, a new magnetic component for this ca Ga magmatic event can be shown. This new component has a virtual geographic pole of 23.4 N, 53.4 E and a dp and dm of 8.2 and 11.8 for the Hlagothi Complex, with a similar magnetic direction seen in one generation of NE-trending dolerite dykes in the region. This new ca Ma addition to the magmatic barcode of the Kaapvaal Craton allows for comparisons to be made to other vi

10 coeval magmatic units on cratons from around the world. Specific examples include the Millindinna Complex and the Zebra Hills dykes on the Pilbara Craton. Precise age dating and palaeomagnetism on these magmatic units is needed to confirm a temporal and spatial link between all the events. If substantiated, this link would assist in further validating the existence of the Vaalbara supercraton during the Mesoarchaean. After the Hlagothi Complex event, different pulses of magma can be seen associated with the Neoarchaean Ventersdorp event. A generation of NE-trending dolerite dykes in the region was dated herein at 2652 ± 11 Ma. In addition, a primary Ventersdorp virtual geographic pole established in Lubnina et al. (2010) from ENE-trending dolerite dykes was confirmed in this study. This ENE-trending dolerite dyke has a virtual geographic pole of 31.7 S, 13.6 E and a dp and dm of 7.0 and 7.2. This date and virtual geographic poles from NE- and ENE-trending dolerite dyke swarms in northern KwaZulu-Natal match up with NE- and E-trending palaeostress fields seen in the Neoarchaean Ventersdorp and proto- Transvaal volcanics by Olsson et al. (2010). Both generations of dolerite dykes also demonstrate variable geochemistry. The NE-trending dolerite dyke swarm is tholeiitic, and the ENE dolerite dyke swarm is calc-alkaline. In addition, some of the tholeiitic NE-trending dolerite dykes have a similar magnetic component to NE-trending dolerite dykes much further to the north in the Black Hills area according to Lubnina et al. (2010). This magnetic component is also similar to the Mazowe dolerite dyke swarm on the Zimbabwe Craton. The NE-trending dolerite dykes in the Black Hills area differ geochemically from those in northern KwaZulu-Natal though, but are also of ca Ga age. The Mazowe dolerite dyke swarm was linked to the dyke swarm of the Black Hills dyke swarm through palaeomagnetic studies. The Mazowe dolerite dyke swarm however is geochemically similar to the NE-trending dolerite dykes of northern KwaZulu-Natal, creating greater complexity in the relationship between the three dyke swarms. It is clear from the complex array of dolerite dyke swarms and other intrusions into these Archaean inliers of northern KwaZulu-Natal, that much more work on the dykes within the south-easternmost Kaapvaal Craton needs to be done. This will resolve these complex patterns and outstanding issues with regard to their palaeo-tectonic framework. vii

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12 Table of Contents DECLARATION ACKNOWLEDGEMENTS ABSTRACT i iii v 1. CHAPTER: 01 INTRODUCTION Statement Of The Problem LIPs and reconstructions using barcoding and palaeomagnetism Locality Methodology CHAPER: 02 GEOLOGICAL SETTING Regional Geology The amalgamation of the Kaapvaal Craton Meso- to Neoarchaean supracrustal successions and intrusions Palaeoproterozoic supracrustal successions and intrusions The Mesoproterozoic to the Mesozoic Local Geology The Archaean Basement The Pongola Supergroup The Hlagothi Complex Dyke and sill swarms and provinces CHAPTER: 03 GEOLOGY Introduction The Hlagothi Complex Dolerite Dykes SE-trending dolerite dykes ENE-trending dolerite dykes NE-trending dolerite dykes Dolerite dykes of other ages CHAPTER: 04 PETROGRAPHY Introduction The Hlagothi Complex Dolerite Dykes SE-trending dolerite dykes ENE-trending dolerite dykes NE-trending dolerite dykes CHAPER: 05 GEOCHEMISTRY Methodology The Hlagothi Complex Rock Alteration/Classification Magmatic Variation/Affinity Further Characterisation/Tectonic Setting Dolerite Dykes Rock Alteration/Classification 82 ix

13 Magmatic Variation/Affinity Further Characterisation/Tectonic Setting CHAPTER: 06 GEOCHRONOLOGY Introduction Ar-Ar Methodology Ar-Ar Result(s) U-Pb Methodology U-Pb Result(s) The Hlagothi Complex Hlagothi Dyke Swarm Rykoppies Dyke Swarm CHAPTER:0 7 PALAEOMAGNETISM Introduction Methodology The Hlagothi Complex SE-trending dolerite dykes ENE-trending dolerite dykes NE-trending dolerite dykes CHAPTER: 08 DISCUSSION Intrusion and metamorphism Geochemistry and petrogenesis Correlation to strata-bound igneous units Correlation with the ca Ga Nsuze Group dykes and lavas Correlation with the ca Usushwana and Thole layered complexes, as well as Mozaan and Witwatersrand lavas Correlation with the ca Ga Ventersdorp dykes and lavas Correlation with the ca Ga Soutpansberg dykes and lavas Dyke swarms of potentially other ages Tectonic model and a new large igneous province The Nsuze igneous event The Hlagothi igneous event The Ventersdorp igneous event The Soutpansberg-Mashonaland igneous event Palaeomagnetism Correlations with the Pilbara Craton CHAPTER: 09 CONCLUSION CHAPTER: 10 REFERENCE(S) 171 APPENDIX: A SAMPLE LOCALITIES 201 APPENDIX: B PETROGRAPHY 203 APPENDIX: C WHOLE-ROCK GEOCHEMISTRY 213 x

14 Chapter: 1 Introduction Chapter: 1 Introduction 1.1. Statement Of The Problem Toward the goal of obtaining a magmatic barcode and an apparent polar wander path (APWP) for the Kaapvaal Craton during the Mesoarchaean to Palaeoproterozoic, a geological, geochemical, geochronological and palaeomagnetic study was initiated on the mafic intrusive units of the south-easternmost region of the Kaapvaal Craton. This particular region has received only limited or no such study. These mafic intrusive units include: SE-, NE- and ENE-trending dolerite dyke swarms, as well as the layered intrusion known as the Hlagothi Complex. This portion of the Kaapvaal Craton is south of the Swaziland and Barberton-Badplaas regions. The Archaean Kaapvaal Craton in southern Africa is found within the eastern half of South Africa and encompasses most of Lesotho and Swaziland, with its western extension into Botswana. It is bounded on all sides by younger orogens (see Fig. 1). It is one of the few cratons in the world that has retained an almost complete Mesoarchaean to Palaeoproterozoic stratigraphy. This geologic record is also relatively unmetamorphosed and undeformed, with several of these stratigraphic successions having been dated, such as the Nsuze and Ventersdorp Supergroups (e.g., Eglington and Armstrong, 2004; Poujol et al., 2003). The ca to 3.10 Ga Archaean granitoid-greenstone basement exposed in the eastern part of the Kaapvaal Craton is also intruded by numerous mafic dyke swarms with different trends, as well as a variety of sill provinces and layered complexes. Until the reconnaissance study of Hunter and Reid (1987), there had been a complete lack of any geological studies on which to base interpretations between mafic dyke and sill emplacement and evolution of the crust on the Kaapvaal Craton. This work has since been expanded upon by Havenga (1995), Hunter and Halls (1992), McCarthy et al. (1990), Meier et al. (2009) and Uken and Watkeys (1997)

15 Chapter: 1 Introduction Figure: 1 The Kaapvaal Craton within the context of southern Africa, including all the surrounding belts. These are in decreasing age: the Limpopo, Kheis / Magondi, Namaqua-Natal and Damara / Sinclair / Ghanzi- Chobe / Gariep / Cape / Mozambique orogens; along with the Kasai, Rehoboth, Richtersveld and Zimbabwe cratons and crustal blocks (modified after de Kock, 2007). Numbers 1 to 5 correspond to the Swaziland, Witwatersrand, Pietersburg, Kimberley and Okwa terranes of the Kaapvaal Craton respectively. The focus area of this study in northern KwaZulu-Natal, South Africa is denoted in red Using geochemistry for example, it has been suggested that NE-trending dykes in the Johannesburg Dome area fed the juxtaposed Klipriviersberg lavas (McCarthy et al., 1990). SE-trending dykes in the Barberton-Badplaas area fed Nsuze lavas (Hunter and Halls, 1992). Recently, these studies have been further expanded upon by the work of Klausen et al. (2010), Lubnina et al. (2010), Olsson (2012) and Olsson et al. (2010; 2011), who focused on dykes emplaced into the eastern, north-eastern and south-eastern basement of the Kaapvaal Craton using geochronological, geochemical and palaeomagnetic studies. These authors stated that there are at least three dominant regional-scale Precambrian dyke swarms across these areas of the Kaapvaal Craton s Archaean basement (see Fig. 4). These - 2 -

16 Chapter: 1 Introduction dyke swarms appear to collectively radiate from a centre within the eastern lobe of the ca Ga Bushveld Complex (Olsson et al., 2010; 2011; Uken and Watkeys, 1997). These dolerite dyke swarms include: A southerly ca Ga SE-trending dolerite dyke swarm that cuts through the ca. 3.6 to 3.1 Ga granitoid-greenstone terrane (Brandl et al., 2006). A widespread ca Ga dolerite dyke swarm in the eastern Archaean basement. Dykes of this swarm are NE-, E- and SE-trending. A dense and extensive ca Ga N- to NE-trending dolerite swarm that can be observed to cut across the ca to 2.06 Ga Transvaal Supergroup and Bushveld Complex (Cawthorn et al., 2006; Eriksson et al., 2006;). These authors have further stated that SE-trending dykes in the Barberton-Badplaas area fed ca Ga Nsuze lavas, as was first proposed by Hunter and Halls (1992). In addition, the radiating NE-, E- and SE-trending dykes in the Black Hills, Rykoppies and Barberton- Badplaas areas are coeval with ca Ga Allanridge lavas within the Ventersdorp Supergroup based on geochronological, geochemical and palaeomagnetic constraints. Olsson et al. (2010) however, also speculated that lava successions within the proto-basinal fills to the Transvaal Supergroup, such as the Wolkberg and Godwan Groups may be coeval to these dolerite dykes. Klausen et al. (2010) studied the geochemistry of the different dolerite dyke swarms and assigned them different geochemical attributes. This study was elaborated on in Maré and Fourie (2012) within the Barberton-Badplaas area and was critical on geochemical correlations made by Klausen et al. (2010). Maré and Fourie (2012) illustrated a much greater geochemical variation with the different trending dyke swarms, with different trends overlapping considerably geochemically. Klausen et al. (2010), Lubnina et al. (2010), Olsson (2012) and Söderlund et al. (2010) have all argued for some NEtrending dykes within the Black Hills area acting as feeders for ca Ga Soutpansberg lavas. Thus most of these dykes can now be geochronologically, compositionally and palaeomagnetically matched as potential feeder dykes to volcanic successions which are known Large Igneous Provinces (LIPs) based on the original definition by Coffin and Eldholm (1994). These studies now provide better data in order to re-evaluate the tectonic association between mafic dyke emplacement in the Kaapvaal Craton and the various Mesoarchaean to Palaeoproterozoic volcanic packages within the stratigraphy of the - 3 -

17 Chapter: 1 Introduction Kaapvaal Craton. This can also be further used to separate out the various dolerite dyke swarms from each other. Extensive work was also done on the northern Kaapvaal Craton by Jourdan et al. (2004; 2006). Jourdan (2004; 2006) specifically addressed Proterozoic ages seen amongst the presumed Jurassic ESE-trending Okavango dolerite dyke swarm and NNE-trending Olifants River dyke swarm. These studies showed that the apparent triple junction formed by radiating dyke swarms of Karoo age is not a Jurassic structure. It rather reflects weakened lithospheric pathways that can no longer be considered a mantle plume marker as previously proposed (Jourdan et al., 2004; 2006). It does not preclude the possible existence of a mantle plume during the Precambrian along the same structure however. Hanson et al. (2004a) targeted dolerite sills within the Waterberg and Soutpansberg strata on the Kaapvaal Craton, and related them to the same ca Ga Soutpansberg Large Igneous Province (LIP) already described above. Olsson (2012) has added more ages to this same LIP on the north-eastern Kaapvaal Craton from NE-trending dolerite dykes. Hanson et al. (2011) and Söderlund et al. (2010) have further described the Mashonaland sills on the Zimbabwe Craton as being related to the greater ca Ga LIP. This LIP appears to be extensive across both the Zimbabwe and Kaapvaal cratons with ages mostly varying from ca to 1.87 Ga. The dolerite dyke swarms of the eastern and northern Kaapvaal Craton, and many greenstone belts terminate at high angles to the current craton margins. This indicates that the Kaapvaal Craton has been truncated by plate tectonic processes and/or mantle plumes along rifted margins (Bleeker, 2003; Bleeker and Ernst, 2006). These dolerite dykes provide potential piercing points with which to match spatially removed cratonic blocks. The truncation of dyke swarms and greenstone belts can indicate that the Kaapvaal Craton may have belonged to a larger supercraton. Bleeker (2003) defined a supercraton as a large ancestral landmass of Archaean age with a stabilised core that on break-up spawned several independently drifting cratons. The supercraton which the Kaapvaal was part of must have been fragmented at different stages during the Mesoarchaean to Palaeoproterozoic. This was prior to the amalgamation of the Limpopo, Kheis, Namaqua-Natal and Mozambique mobile belts (e.g., Jacobs et al., 2008). Various possible Precambrian continental arrangements have been proposed for the Kaapvaal Craton, particulary in the Neoarchaean - 4 -

18 Chapter: 1 Introduction (e.g., Aspler and Chiarenzelli, 1998; Rogers, 1996). The only consensus regarding nearest neighbours for the Kaapvaal Craton during this time is the linkage with the Pilbara Craton, called Vaalbara (e.g., Cheney, 1996; de Kock et al., 2009; Eriksson et al., 2009; Nelson et al., 1999; Wingate, 1998). Other possible nearest neighbour links with the Kaapvaal Craton remain elusive during the Archaean-Palaeoproterozoic. The exception is the Grunehogna crustal fragment of western Dronning Maud Land in eastern Antarctica (e.g., Basson et al., 2004; Groenewald et al., 1991). The identification of other blocks formerly adjacent to other sides of the Kaapvaal in the Mesoarchaean to Palaeoproterozoic is relatively unknown. The Kaapvaal-Zimbabwe craton connection is known from at least the Palaeoproterozoic (e.g., Söderlund et al., 2010), although Hanson et al. (2011) proposed a greater than 2000 km displacement. The significance of such correlations and docking histories lies in accurate palaeogeographic reconstructions. This is essential to understanding the full tectonic framework for a particular craton or crustal fragment (e.g., Bleeker, 2003; Bleeker and Ernst, 2006; Ernst et al., 2013). This tectonic framework can be used in conjunction with all the other remaining cratons and crustal fragments. This can help to further validate plate tectonic models, processes and reconstructions back into the Archaean (e.g., Ernst et al., 2013). In addition, there is the potential economic significance by tracing metallogenic belts between crustal fragments. Examples include the Witwatersrand Basin and Bushveld Complex. The ca Ma Bushveld LIP, is known for its economic platinum group element (PGE), chromium, and nickel deposits. It follows that satellite intrusions of the Bushveld LIP should be present on the former nearest neighbours to the Kaapvaal Craton within the Palaeoproterozoic geologic record. The robust identification of former nearest neighbours to the Kaapvaal Craton in the Neoarchaean to Palaeoproterozoic is of significance. It will allow the correlation of geological units, prominent structures and sedimentary basins between the Kaapvaal Craton and its former neighbours, such as the Pilbara Craton in western Australia and the Grunehogna Craton of eastern Antarctica. Cratonic magmatic barcoding in combination with palaeomagnetic studies and geochemistry on mafic intrusions such as mafic dykes and sills offers a robust approach for producing wellconstrained reconstructions of the past, as discussed in the next section (Ernst et al., 2013)

19 Chapter: 1 Introduction 1.2. LIPs and reconstructions using barcoding and palaeomagnetism LIPs were first defined in detail by Coffin and Eldholm (1994) as anomalously large volume emplacements of predominantly mafic extrusive rock as well as related intrusives into the crust. Many LIPs contain felsic components too, and sometimes carbonatites, kimberlites and lamprophyres, which are often neglected (Bryan et al., 2002; Ernst et al., 2013). These emplacements of magmatic material manifest in supracrustal volcanic successions, forming flood basalts. The plumbing system of such volcanic rocks is typically dyke swarms, sill provinces and layered complexes, which represent the feeders to the volcanic succession (e.g., Bryan and Ernst, 2008; Ernst and Buchan, 2001). LIPs have further been defined as having a spatial extent of at least 0.1 MKm 2 (Coffin and Eldholm, 2001). However, most LIPs are greater than 1.0 MKm 2, and are usually up to 10 km thick prior to erosion of the volcanic pile (Courtillot and Renne, 2003; Eldholm and Coffin, 2000; Ernst et al., 2005). Where little or no LIP supracrustal volcanic succession remains, minimum areal extent is usually determined using the coverage of the feeder intrusive plumbing system related to that particular event, despite the possible uncertainty (e.g., Yale and Carpenter, 1998). This is particulary true in the Palaeozoic and Precambrian rock record. The bulk of LIPs are generally transient, being deposited in less than 10 Ma, with most of the volcanism occurring in less than 1 Ma (e.g., Coffin and Eldholm, 1994; 2005). In some cases, persistent LIPs may last tens of millions of years and produce hotspot or seamount chains. A lot depends on melt production rate and nature of the mantle processes involved (e.g., Ernst et al., 2005 and references therein). LIPs can be found globally throughout Earth s history, mostly as continental and oceanic flood basalts. The LIPs in the Mesozoic and Cenozoic being the best preserved. Many Mesozoic and Cenozoic LIPs are associated with continental rifting and break-up, and are commonly seen on volcanic-rifted margins (e.g., Coffin and Eldholm, 1994; 2001; Cox, 1980; Menzies et al., 2002; Storey, 1995; White and McKenzie, 1989). Volcanic-rifted margins are truncated, thickened crust with LIPs, whereas nonvolcanic-rifted margins have a LIP-free transition from continental to oceanic crust of normal thickness (Wilson et al., 2001). Palaeozoic and Proterozoic LIPs are typically deeply eroded and fragmented, - 6 -

20 Chapter: 1 Introduction metamorphosed and deformed, with just their plumbing system usually remaining. LIP fragments may also be found in the Archaean to a much more limited extent in greenstone belts. In greenstone belts they occur as highly deformed and thick metamorphosed basalt packages, with minor komatiite flows (Arndt, 1999; 2003; Bleeker, 2002; Campbell et al., 1989; Kerr et al., 2000; Nelson, 1998; Tomlinson and Condie, 2001). Coffin and Eldholm (1994) described LIPs as forming through processes unlike normal seafloor spreading and subduction, being the manifestation of mantle-driven processes, such as mantle plumes. Variations in depth into the Earth and fertility within the mantle will lead to different amounts of melt, and may be responsible for the variety in size and duration of LIPs (e.g., Ernst et al., 2005 and references therein). Apart from mantle plumes, alternative models proposed for LIP development include lithospheric delamination (e.g., Şengör, 2001), back-arc processes (e.g., Taylor, 1995), a continent over-riding a spreading centre (Gower and Krogh, 2002), enhanced mantle convection at the edge of a craton (shallow mantle edge convection, e.g., King and Anderson, 1998), lithospheric fracturing (e.g., Sheth, 1999), melting of fertile mantle (e.g. Anderson, 2005) and bolide impact (e.g., Boslough et al., 1996). The study of LIPs and their relationship (or lack thereof) with mantle plumes has intensified since the early 1990s as new information and techniques have developed. These techniques include integrated mapping and remote sensing, seismic tomography, ICP-MS trace element geochemistry, petrogenetic and geodynamic modelling, as well as more precise palaeomagnetic measurements. The significant development in age dating techniques has also assisted these new methodologies, particularly with regard to Ar- Ar and U-Pb isotopic measurements. LIPs can be associated with the breakup of continents (e.g., Bleeker and Ernst, 2006; Ernst et al., 2013). When breakup does occur, the result is LIP remnants on conjugate margins of continental crust (e.g. Courtillot et al., 1999; Storey, 1995). Studies of LIPs and their associated mafic dyke swarms and sill provinces have also allowed further understanding of the Earth s palaeogeography through time, as well as the supercontinent cycle (Bleeker and Ernst, 2006; Ernst et al., 2013). This can assist in the tracing of metallogenic belts between continents, and also understanding of the evolving Earth system as a whole (e.g., Bleeker and Ernst, 2006). Little reliable palaeogeographic information is available for the Earth prior to 250 million years ago, which represents the time of final - 7 -

21 Chapter: 1 Introduction consolidation of the supercontinent Pangaea (Jacoby, 1981). The absence of preserved oceanic crust that formed before the Pangaean supercontinent, the lack of an abundant a fossil record before the Cambrian, and palaeomagnetic overprinting complicate plate reconstructions in the Precambrian according to Bleeker and Ernst (2006) and Ernst et al. (2013). Continental crust usually consists of an Archaean cratonic core. Many cratonic cores are composite, composed of a variety of terranes, which are in turn surrounded by progressively younger terranes and orogenic belts (e.g., Bleeker, 2003; Ernst et al., 2013). This crust was embedded in various supercontinental frameworks through time (Bleeker, 2003). However, these supercontinental frameworks become uncertain as determined from continental geology using ages of granitic intrusions, variable orogenic belts, metamorphism and deformation of sedimentary basins and piercing points such as structural trends and dyke swarms (Bleeker and Ernst, 2006; Ernst and Bleeker, 2010; Ernst et al., 2013), especially against non-distinct margins. This is the case when viewing the uncertainty surrounding the ca to 700 Ma supercontinent of Rodinia (Li et al., 2008). Reconstructions of Columbia even further back into the Proterozoic (ca to 1300 Ma), and Kenorland from the Neoarchaean to Palaeoproterozoic respectively are, at best, speculative and require further highly precise geochronology and palaeomagnetism (e.g., Ernst et al., 2013; Meert, 2012). Bleeker (2003) stated that several supercratons may have existed during the Archaean, instead of one single supercontinent. Smaller scale (i.e., nonglobal) reconstructions are possible with selected better preserved crustal fragments by looking at the geology and palaeomagnetic record from one craton to the next. A good example is the Kaapvaal and Pilbara (or Vaalbara) cratonic connection (Byerly et al., 2002; Cheney, 1996; Cheney et al. 1988; de Kock et al., 2009; Nelson et al., 1999; Strik et al., 2001; Trendall et al., 1990; Wingate, 1998; Zegers et al., 1998). Also, the growth of cratons through looking at the docking histories of individual terranes within a single composite craton can be established qualitatively, such as with the Superior Craton (Bleeker and Ernst, 2006) and the Kaapvaal Craton (Schmitz et al., 2004). With modern analytical techniques, reconstructions as far back as ca. 2.7 Ga and beyond may be possible, which is the age of stabilisation of a significant portion of the Archaean cratons (e.g., Bleeker, 2003; Bleeker and Ernst, 2006; Ernst et al., 2013). Mafic - 8 -

22 Chapter: 1 Introduction dyke swarms are integral parts of LIPs, being emplaced in a small time interval (e.g. Ernst et al. 2013). These dyke swarms are usually spatially extensive, extending far into the cratonic hinterland away from its margins. This allows for more distal portions to be less susceptible to deformation and metamorphism, providing better geochronologic and palaeomagnetic constraints for key poles, (e.g., Buchan, 2000; Buchan and Halls, 1990; Ernst et al., 2013; Halls, 1982). Dykes also make good piercing points, as they are geophysically and geochemically distinguishable, as well as being sub-vertical, which makes their preservation insensitive to uplift and erosion, and avoids structural complication. Data gathered from them thus allow continental fragments to be placed according to Bleeker and Ernst (2006): At a specific latitude. At a specific time. With a known orientation such that piercing points provided by dyke swarms are satisfied and in a position that optimises general geological continuity prior to breakup and dispersal, provided they are not heavily altered along old sutures in orogenic belts. Baddeleyite (ZrO 2 ) is an excellent geochronometer for mafic rocks, providing both accurate and precise emplacement ages (Heaman and LeCheminant, 1993). Baddeleyite is also more susceptible to being pseudomorphed by zircon during metamorphism instead of developing zones, assuring that the ages reflect emplacement rather than metamorphic overprints (Heaman and LeCheminant, 1993). The recent improvements in the separation and recovering of baddeleyite grains from mafic dykes and sills, as well as layered intrusions have significantly enhanced the success rate in dating both fine-grained and coarse-grained silica-undersaturated rocks (Söderlund and Johansson, 2002). U-Pb age dating of baddeleyite is important in providing emplacement ages, and allows for LIP remnants, such as complex dyke swarm and sill provinces to be interpreted

23 Chapter: 1 Introduction Figure: 2 - Piercing points and craton reconstruction. (a) A hypothetical (super)craton with various geological elements just prior to break-up. A LIP, with flood basalts and associated intrusions, is emplaced along the incipient rift. (b) Break-up of the supercraton has spawned two cratons (A and B). As long as both cratons are not too modified (e.g. South American and African conjugate margins), they are easily fitted together again us: P-R, the fitting of promontories and re-entries along the rifted margins; PM, general correlation and fitting of the conjugate passive margins; P1, piercing points and reconstruction of the LIP; P2, piercing points provided by older sedimentary basins; P3, piercing points provided by an ancient orogenic front or fold-thrust belt; and P4, the non-precise piercing points provide by orogenic internides. (c) The more general case where further break-up up has occurred (craton C) and craton margins have been abraded, modified, and differentially uplifted. Craton B was strongly uplifted and its sedimentary cover has been eroded. Piercing point P3, if still recognizable as such, has strongly shifted, and an exhumed granitoid belt is unmatched in Craton A. Craton C was also uplifted, virtually erasing piercing point P2. Dykes related to the LIP, however, remain on all three cratons and precise age dating (x Ma) yields a critical clue that they might be part of a single event. Primary palaeomagnetic data may yield additional geometrical clues (North arrows), if not palaeo-latitudes. (d) Reconstruction of the original supercraton, based only on the precise piercing points and other information derived from the dyke swarms (after Bleeker and Ernst, 2006)

24 Chapter: 1 Introduction Figure: 3 Magmatic barcode record of the Kaapvaal and Zimbabwe cratons. The width of individual bars corresponds to the 2σ error in radiometric ages. Arrow depicts possible age match of mafic extrusions and intrusions (dyke swarms, sill provinces and volcanic successions) between cratons. The record reveals three post 2.0 Ga magmatic events common to both the Zimbabwe and Kaapvaal cratons, while no matches occur in pre 2.0 Ga times, in favour of formation of Kalahari after ca. 2.0 Ga (modified after Söderlund et al., 2010) An appropriate example would be the information than can be gained on deeply eroded cratons dominated by Archaean rocks with indistinct and/or inter-mixed dyke swarms of radiating or linear trends (e.g., the Superior Craton: Bleeker and Ernst, 2006; or the Zimbabwe Craton: Söderlund et al., 2010; see Fig. 2 and 3). Combined with good geological, geochemical and palaeomagnetic studies, age determinations of mafic intrusions yield invaluable clues to ancient tectonic settings. They also potentially provide correlations to supracrustal units such as volcanic successions in the stratigraphic record both within, and between cratons and crustal blocks. This is critical for obtaining reliable global plate reconstructions, especially for supercontinents and cratons of the past (e.g., Bleeker and Ernst, 2006; Buchan et al., 1996; Ernst et al., 2013; Halls, 1982;). Ernst et al. (2013, and

25 Chapter: 1 Introduction references therein) state that using precise and accurate age matches among short-lived mafic magmatic events is a first and highly efficient filter to identify whether two cratons, which may now be distant, may have possibly been adjacent pieces of crust (see Fig. 3). Such possible LIP events are usually associated with continental break-up, thus creating conjugate margins (e.g., Bleeker and Ernst, 2006). These LIP events lead to the creation of a unique mafic magmatic barcode for each individual craton or terrane, which can be compared and correlated to that of other cratons or terranes (Bleeker and Ernst, 2006). Several precisely dated magmatic events provide a fingerprint or barcode for a craton stated Bleeker (2003), Ernst and Bleeker (2010) and Ernst et al. (2013). Originally continuous crustal fragments will share essential parts of their barcodes, and matches beyond a single event will indicate a sharing of events (e.g., Bleeker and Ernst, 2006). Obtaining magmatic barcodes for all of the Archaean cratons worldwide would then greatly assist in continental reconstructions, in addition to other key attributes such as a geological setting, distribution, palaeomagnetism and geochemistry (Bleeker, 2003; Bleeker and Ernst, 2006; Ernst et al., 2013). In addition, there have been several previous palaeomagnetic studies (as listed in the global palaeomagnetic database of Pisarevsky, 2005), particulary relevant for Neoarchaean and Palaeoproterozoic units on the Kaapvaal Craton (e.g., de Kock et al. 2006; 2009; Evans et al., 2002; Strik et al., 2007). However, the general problem with obtaining precise Precambrian palaeomagnetic poles, not only in the Kaapvaal Craton but globally, has been the absence of precise dates on the units studied palaeomagnetically. This problem was noted by Buchan et al. (2000) who surveyed the available key palaeomagnetic poles for Laurentia and Baltica. These are palaeopoles for which there was a robust palaeomagnetic direction that: Averaged secular variation through comprehensive sampling of a single site, in addition to providing data from multiple sites within an acceptable statistical error. Demonstrated that the direction was primary on the basis of a field test, such as a baked contact or fold test. Had a precise age date of better than ± 20 Ma obtained through U-Pb or Ar-Ar geochronology

26 Chapter: 1 Introduction Of the hundreds of poles published for Precambrian units, only a handful are considered key poles, and of these, the majority are on the Canadian shield. A reliable Mesoarchaean to Palaeoproterozoic Apparent Polar Wander Path (APWP) for the Kaapvaal Craton is poorly constrained. It must be said that the late Palaeoproterozoic APWP segment of the Kaapvaal Craton is known to some extent (e.g., de Kock et al, 2006; Evans et al., 2002; Hanson et al., 2004a; 2011) 1.3. Locality The inliers of Archaean crust of northern KwaZulu-Natal in South Africa form part of the south-easternmost terrane of the Kaapvaal Craton, and are the focus of this study (see Fig. 4). The largest of these inliers is the White Mfolozi inlier. These inliers consist of granitoidgreenstone basement including the Nondweni and Ilangwe greenstone belts and fragments, as well as the White Mfolozi and Nkandla portions of the supracrustal Pongola Supergroup. Mafic- to ultramafic intrusions into the south-easternmost inliers of the Archaean crust includes the Hlagothi Complex, which is the most prominent and well-studied. The Hlagothi Complex is composed of a series of layered sills which intrude the Mesoarchaean Nsuze Group of the Pongola Supergroup (du Toit, 1931; Groenewald, 1984, 1988, 2006). These sills correlate well in terms of age and composition with the Thole Complex (Groenewald, 2006), as well as gabbroic portions of the Usushwana Complex located further to the north. These units potentially represent feeders to volcanic units within the upper Witwatersrand and Pongola Supergroups. In addition, three large-scale dolerite dyke swarms of SE-, ENE- and NE-trends may be found. However, sub-parallel dyke swarms within these dolerite dyke trends may reveal the presence of additional dyke swarms in the area. These dyke swarms may also be exploiting former lines of weakness in precursor dykes such as was noted in Jourdan et al. (2006) for Jurassic aged dykes in the Okavango dolerite dyke swarm. The dolerite dykes of northern KwaZulu-Natal cross-cut the granitic basement and associated greenstone belts and fragments, as well as the Pongola Supergroup in some cases. Limited work on the dykes was carried out by Klausen et al. (2010) and Lubnina et al. (2010), who correlated them with dykes seen further to the north as was noted above. These authors

27 Chapter: 1 Introduction carried out limited geochemical and palaeomagnetic studies within this region of the craton on selected dolerite dykes. Figure: 4 - Simplified geological map of the Kaapvaal Craton illustrating the exposed Archaean basement and supracrustal successions relevant to this study, modified after Robb et al. (2006). The bottom right box is the outline of the south-easternmost window of the craton, and forms the focus area for this study. Intrusions into the eastern Kaapvaal Craton are demarcated in red. From north to south: Black Hills 2.65 and 1.90 Ga NEtrending dykes, Rykoppies 2.65 E-trending dykes, Barberton-Badplaas 2.95 and 2.65 Ga SE-trending dykes and 1.90 NE-trending dykes, the 2.99 Ga Usushwana Complex and the 2.87 Thole Complex. Lastly the Hlagothi Complex, SE-, NE- and ENE-trending dykes in the bottom right box in northern KwaZulu-Natal

28 Chapter: 1 Introduction Lubnina et al. (2010) assigned ca Ga, 2.65 Ga and 1.90 Ga ages to the SE-, ENE- and NEtrends respectively using palaeomagnetic studies. Klausen et al. (2010) however, assigned ages of ca Ga and 1.90 Ga to the ENE- and NE-trending mafic dykes based on geochemistry. The inliers of Archaean crust are overlain by the Natal Group and Karoo Supergroup sedimentary rocks in which the above mentioned intrusions are not present. The whole area, however, is in turn intruded by Jurassic SSE-trending dolerite dykes and sills related to the breakup of Gondwana Methodology This study focuses on the dyke and sill swarms and provinces on the south-easternmost portion of the Kaapvaal Craton in northern KwaZulu-Natal, South Africa south of the Swaziland and Barberton-Badplaas region. This south-easternmost window into the Kaapvaal Craton has received little attention due to a variety of reasons including remote, rugged terrane, complex geology associated with metamorphism and deformation due to the proximity of the cratonic margin to the south with the Namaqua-Natal Mobile Belt. The aim of this thesis is to obtain new palaeomagnetic and geochemical results on the intrusions precisely dated during this study, following the same methodologies employed above by Klausen et al. (2010), Lubnina et al. (2010) and Olsson et al. (2010). Targets for dyke, sill and layered complex samples were identified using geological and geophysical maps, as well as remote sensing imagery from Google Earth. In addition, this data was digitised on maps. This was used to calculate cumulative segment lengths and strikes of dykes, which were plotted on histograms at 5 strike intervals in order to resolve the dominant strike trends, following Klausen et al. (2010). Dyke and sill samples were gathered from across the south-easternmost Kaapvaal Craton in northern KwaZulu-Natal south of Vryheid and north of Eshowe. Geological field relationships were studied, as was petrography, geochemistry and geochronology within each intrusive unit. In general, several samples were gathered from each dyke and sill outcrop, usually large or composite samples up to 1 kg. The coarser-grained central parts of

29 Chapter: 1 Introduction dykes and sills were preferentially sampled, particulary for geochronology and geochemistry. Usually between 6 and 8 samples from these sampling sites were also taken for palaeomagnetic studies, with the dyke centre to contact sampled. The contact zone with the country rock, as well as the host rock itself was also sampled to produce a baked contact test. Samples were taken from localities where the effects of alteration and weathering were minimal, or were able to be mostly removed in the field. All sample processing and preparation was done at the University of Johannesburg s Department of Geology. The exception was U-Pb baddeleyite separation, which was done at Lund University in Sweden

30 Chapter: 2 Geological Setting Chapter: 2 Geological Setting 2.1. Regional Geology The ca to 3.10 Ga Kaapvaal Craton is one of the world s oldest and best preserved granite-greenstone cratonic terranes. Most of the craton s formation took place during the Palaeo- to Mesoarchaean (see Fig. 5). It is located within South Africa, and occupies most of Lesotho and Swaziland, with its north-western extension into Botswana. It is bounded on all sides by younger orogens. It has a surface area of approximately 1.2 x 10 6 km 2, with a relatively cool, reduced lithosphere that extends down to a depth between 250 and 300 km (de Wit et al., 1992). The Kaapvaal Craton has a near complete geologic record of sedimentation and volcanism preserved within its Mesoarchaean to Palaeoproterozoic stratigraphy (Hunter et al., 2006). Supracrustal successions cover more than 85% of the basement. However, along the eastern and southeastern margins, large exposures of the Archaean granitoid-greenstone basement can be seen. This basement hosts numerous mafic to ultramafic intrusions, and in particular dyke swarms which are useful for magmatic barcoding and palaeomagnetic studies The amalgamation of the Kaapvaal Craton The Kaapvaal Craton formed during a series of Palaeo- to Mesoarchaean orogenic stages according to de Wit et al. (1992), Eglington and Armstrong (2004) and Poujol et al. (2003). Several granitoid sub-domains amalgamated, along with a number of greenstone belts around a > 3600 Ma nucleus, which is the Ngwane gneiss of the Ancient Gneiss Complex in north-western Swaziland. The process is akin to modern day plate tectonics (e.g., de Wit et al., 1992; Eglington and Armstrong, 2004; Poujol et al., 2003)

31 Chapter: 2 Geological Setting Figure: 5 - Simplified geological map of the Kaapvaal Craton illustrating the exposed Archaean basement and supracrustal successions relevant to this study, modified after Robb et al. (2006). The bottom right rectangle is the outline of the south-easternmost window of the craton, and forms the focus area for this study Mantle xenoliths in kimberlites and their associated diamonds suggest the lithospheric keel of the craton developed at approximately 3.5 Ga (Irvine et al., 2012). Its evolution can be linked to four tectonic and geochronologically distinct domains or blocks. These include: the ca. 3.6 to 3.1 Ga Swaziland block (of which the Ngwane gneiss is a part), the ca. 3.3 to 3.0 Ga Witwatersrand block, the ca. 3.2 to 2.7 Pietersburg block and the ca. 3.0 to 2.7 Ga Kimberley block (Eglington and Armstrong, 2004; Poujol et al., 2003). The most reliable age constraints

32 Chapter: 2 Geological Setting for the Ngwane gneiss of the Ancient Gneiss Complex are zircon ages of 3644 ± 4 Ma on tonalitic gneiss (Kröner and Compston, 1988), and 3663 ± 1 Ma on a nearby outcrop (Schoene et al., 2008). Zircon xenocrysts within the Ancient Gneiss Complex have yielded older ages of 3702 ± 1 Ma and 3683 ± 10 Ma (Kröner and Tegtmeyer, 1994; Kröner et al., 1996), and may indicate an older crustal component. A variety of greenstone belts and a series of granitoid intrusions amalgamated upon the Ancient Gneiss Complex up until ca Ga when the craton began to stabilise, forming a small proto-continental block (de Wit et al., 1992). Among the numerous greenstone belts of the Kaapvaal Craton, the ca Ga Barberton Greenstone Belt is the largest, oldest and most rigorously investigated (e.g., Armstrong et al., 1990; Kamo and Davis, 1994; Kröner and Compston, 1988; Kröner et al., 1996). Various tectonic regimes have been proposed for the Barberton Greenstone Belt, with many proponents favouring the idea that the belt formed through subduction processes similar to modern plate tectonic regimes (e.g., de Wit et al., 1992; Diener et al., 2005; Dziggel et al., 2002; Lowe, 1994). However, other studies argue vertical tectonics played a role, stating it is a structural complex of many discrete terranes, which were fused together in tectonic stacking (e.g., Kröner et al., 1996), or formed through foundering (e.g., van Thienen et al., 2004), plumes and gravitational collapse (e.g., Anhaeusser, 2001) and extensional orogenic collapse resulting in fault-bounded basins (e.g., Diener et al., 2005; Kisters and Anhaeusser, 1995;). The Barberton Greenstone Belt, along with the numerous other greenstone belts, and early granitoids were then stitched together by more potassic granitoid batholiths of varying ages, predominantly at ca and ca Ma. The craton is sub-divided into four major terranes as stated above (see Fig. 4), where the presumed Archaean micro-continents amalgamated on to the oldest south-eastern Swaziland block from the west and north (Eglington and Armstrong, 2004). The terrane collision and suturing between the Swaziland block and the Witwatersrand block (forming a greater Witwatersrand block) played an important role in the development of the Mesoarchaean supracrustal basins (de Wit et al., 1992; Schmitz et al., 2004). In addition, the collision between this greater Witwatersrand block and Kimberley block along the Colesberg lineament to the west assisted. As the Dominion lavas erupted on the central half of the craton and the Nsuze lavas on the eastern side, the craton continued to grow along the northern and western margins. These juvenile arcs incorporating the Amalia, Kraaipan and

33 Chapter: 2 Geological Setting Madibe greenstone belts accreted during the protracted terrane collision between the Witwatersrand and Kimberley blocks along the north-south Colesberg Lineament in the west between ca and 2.88 Ga (Schmitz et al., 2004). The Kaapvaal Craton was already joined with the Pietersberg terrane along the east-west Thabazimbi-Murchison lineament in the north during this time according to de Wit et al. (1992) Meso- to Neoarchaean supracrustal successions and intrusions The Meso- to Neoarchaean stratigraphy and geochronology for the Kaapvaal Craton is summarised in Table 1 (see Fig. 6). Fragments of a relatively complete Mesoarchaean to Palaeoproterozoic stratigraphic sequence are remarkably well-preserved on the Kaapvaal Craton. The stabilisation of the craton allowed a fundamental transition in style of crustal evolution as supracrustal successions began to form. The oldest preserved of which is the rift dominated 3074 ± 6 Ma volcanic sedimentary Dominion Group on the centre of the craton (Armstrong et al., 1991; Marsh, 2006). This succession represents the oldest cratonic sedimentary basin on Earth, and although its present outline represents only a structural remnant of the original depository, it may have occupied an area in excess of km 2 (Lowe and Tice, 2007). The lower Nsuze Group of the Pongola Supergroup on the eastern side of the craton can possibly be correlated with the Dominion Group (Cole, 1994). Both are rift-fill successions of volcanics and sediments. Both also bear testimony to the uplift and erosion of the Kaapvaal Craton prior to the onset of sedimentation. The Kaapvaal Craton then underwent further uplift and erosion, followed by thermal subsidence due to post-rift/plume cooling. This led to the erosion of the majority of the Dominion Group before the deposition of the proximal and shallow marine West Rand Group of the Witwatersrand Supergroup, and the distal deep marine lower Mozaan Group (McCarthy, 2006). Both supergroups can be almost completely correlated bed for bed (Beukes and Cairncross, 1991). Specific to this study, the 2914 ± 8 Ma Crown lava (Armstrong et al., 1991), as well as the Bird lavas of the Witwatersrand Supergroup and the Tobolsk and Gabela lavas from the Mozaan Group represent the best evidence for a Witwatersrand- Mozaan correlation (see Fig. 7). Both sub-basins reflect upward-coarsening sequences

34 Chapter: 2 Geological Setting Table: 1 - Summary of reported ages for the Archaean supracrustal volcanic successions and related intrusions Extrusives Intrusives Age Error System Mineral/Whole Rock Reference Dominion lavas U-Pb zircon Armstrong et al. (1991) Agatha lavas U-Pb zircon Burger and Coertze (1973) Rb-Sr whole rock Burger and Coertze (1973) U-Pb zircon Hegner et al. (1994) U-Pb zircon Hegner et al. (1994) Rb-Sr whole rock Hegner et al. (1984) U-Pb zircon Mukasa et al. (2013) U-Pb zircon Nhleko (2003) U-Pb zircon Mukasa et al. (2013) U-Pb zircon Hegner et al. (1984) Sm-Nd whole rock Hegner et al. (1984) Barberton-Badplaas Dyke Swarm U-Pb baddelyeite Olsson (2012) U-Pb baddelyeite Olsson et al. (2010) U-Pb baddelyeite Olsson et al. (2010) Usushwana Complex U-Pb baddelyeite Olsson (2012) U-Pb baddelyeite Olsson (2012) b-sr, Sm-N whole rock Layer et al. (1988) Sm-Nd whole rock Hegner et al. (1984) Rb-Sr whole rock Davies et al. (1970) Ar-Ar pyroxene Layer et al. (1988) Ar-Ar pyroxene Layer et al. (1988) Ar-Ar amphibole Layer et al. (1988) Crown lavas U-Pb zircon Armstrong et al. (1991) Tobolsk lavas U-Pb zircon Mukasa et al. (2013) Klipriviersberg/Derdepoort/Khanye lavas U-Pb zircon Wingate (1998) U-Pb zircon Moore et al. (1993) U-Pb zircon Moore et al. (1993) U-Pb zircon Grobler and Walraven (1993) U-Pb zircon Walraven et al. (1996) U-Pb zircon Walraven et al. (1994) U-Pb zircon Armstrong et al. (1991) Goedgenoeg/Makwassie lavas U-Pb zircon de Kock et al. (2012) U-Pb zircon Armstrong et al. (1991) /-59 Rb-Sr whole rock Walraven et al. (1987) Rb-Sr whole rock van Niekerk and Burger (1978) Rietgat lavas U-Pb zircon de Kock et al. (2012) Rykoppies Dyke Swarm U-Pb baddelyeite Olsson et al. (2011) U-Pb baddelyeite Olsson et al. (2011) U-Pb baddelyeite Olsson et al. (2011) U-Pb baddelyeite Olsson et al. (2010) U-Pb baddelyeite Olsson et al. (2010) U-Pb baddelyeite Olsson et al. (2011) U-Pb baddelyeite Olsson et al. (2010) U-Pb baddelyeite Olsson et al. (2010) U-Pb baddelyeite Olsson (2012) U-Pb baddelyeite Olsson et al. (2011) U-Pb baddelyeite Olsson et al. (2010)

35 Chapter: 2 Geological Setting Figure: 6 Cumulative Archaean stratigraphy for the Kaapvaal Craton, compiled by combining maximum thickness estimates from various sources after Klausen et al. (2010) and references therein The Witwatersrand sub-basin shows shallow marine deposits progressively overlain by more fluvial braided river deposits (McCarthy, 2006). The lower stratigraphy of the Witwatersrand Supergroup comprises the shale, banded iron formation and sandstone-dominant West Rand Group. The upper stratigraphy consists of the sandstone and conglomerate-dominant Central Rand Group. Two flood basalt successions occur in the Witwatersrand sub-basin, with the 2914 ± 8 Ma Crown lava at the top of the West Rand Group, and the Bird lava in the middle of the Central Rand Group (Armstrong et al., 1991). The origins of the lavas are somewhat enigmatic, with changes in tectonic regime or plume magmatism having been suggested according to Frimmel et al. (2005) and Nhleko (2003). Numerous tectonic settings have been proposed for the Witwatersrand basin, with a foreland basin the preferred. The Central Rand Group was deposited in a much smaller and restricted basin dominated by uplift to the west and north (e.g., Myers et al., 1990). The age of the Pongola Supergroup is poorly constrained, with an upper age given by the / 2 Ma granitoid basement beneath the Pongola Supergroup (Kamo and Davis, 1994). A lower age limit is given by a cross-cutting granitoid intrusion and quartz-feldspar porphyry, dated at 2863 ± 8 Ma and 2837 ± 5 Ma respectively (Gutzmer et al., 1999; Reimold et al., 1993). There are also a variety of ages between 2985 ± 1 Ma and 2934 ± 114 Ma for the Nsuze (Agatha) lavas (Hegner et al., 1984, 1994; Mukasa et al., 2013 and references therein; Nhleko, 2003)

36 Chapter: 2 Geological Setting Figure: 7 - Stratigraphy of the Dominion, West Rand and Central Rand, Nsuze and Mozaan Groups (modified after Duncan and Marsh, 2006; Gold, 2006; McCarthy, 2006). Correlation between Witwatersrand and Pongola strata based on the work by Beukes and Cairncross (1991) and Cole (1994)

37 Chapter: 2 Geological Setting There is also the cross-cutting ca Ma to 2860 Ma Usushwana Complex (Hunter and Reid, 1987; Olsson, 2012). The Mozaan Group consists of several formations that lie unconformably on top of the volcanic and rift-fill sedimentary rocks of the Nsuze Group. It is composed of alternating sandstones and mudrocks, with iron formations nearer to the stratigraphic base. The Klipwal diamictite marks the end of deeper marine deposition and a transition into a shallower marine or continental setting (Gold, 2006). The Tobolsk, Gabela and Ntanyana lavas occur stratigraphically within the upper Mozaan basin, and are thought to be flood basalts produced from fissure eruptions in a continental setting (Hammerbeck, 1982; Nhleko, 2003). The Tobolsk lavas unconformably overlie the Delfkom (Nconga) Formation with an erosive base (Nhleko, 2003). The Tobolsk lavas vary in thickness between 50 and 574 m and have been correlated with the Crown Formation lavas. The Gabela lavas are approximately 150 m thick and have been stratigraphically matched with the Bird Formation lavas (Beukes and Cairncross, 1991; Gold, 2006). The Ntanyana lavas occur at the top of the Mozaan Group in the Nongoma graben, and have only be seen in drill core (Nhleko, 2003). They are approximately 64 m thick. Both the Gabela and Ntanyana lavas consist of various flows intercalated with sediments and tuffs. These lavas may also be associated with the Bird lavas of the eastern part of the Witwatersrand basin (Beukes and Cairncross, 1991). The Pongola Supergroup was in turn intruded by the Usushwana Complex and the Thole Complex between 2990 Ma and ca Ma (Hammerbeck, 1982; Hunter and Reid, 1987; Olsson, 2012). The SE-trending Barberton Badplaas dyke swarm has also recently been dated between 2966 ± 1 Ma and 2967 ± 1 Ma in the south-eastern region of the craton. This dyke swarm could possibly represent feeders to the Nsuze volcanic pile (Olsson et al., 2010). The Witwatersrand Basin is unconformably overlain by the volcanic-sedimentary ca Ga Ventersdorp Supergroup, in a setting once again dominated by rifting (Eriksson et al., 2002; Marsh et al., 1992). The lower, middle and upper Klipriversberg, Platberg and Pniel volcanic sequences respectively appear to have erupted relatively rapidly within approximately 35 million years. This is shown in zircon ages of 2714 ± 8 Ma and 2709 ± 5 Ma for the basal basaltic lavas of the Klipriviersberg Group and middle felsic lavas of the

38 Chapter: 2 Geological Setting Makwassie Group respectively (Armstrong et al., 1991). Author s such as de Kock (2007), de Kock et al. (2012) and Wingate (1998) argue that the significantly older 2782 ± 5 Ma Derdepoort basalts represent the most basal rocks of the Ventersdorp Supergroup, and that the 2714 ± 8 Ma age should be regarded as a minimum age. In addition, ages of 2733 ± 4 and 2724 ± 6 Ma on Platberg Group volcanic equivalents (Hartswater Group) by de Kock et al. (2012). This further casts the 2714 ± 8 Ma and 2709 ± 5 Ma ages into doubt. This study implies a volcanic history of less than 100 million years instead, from less than 2782 Ma to greater than 2662 Ma (Cheney, 1998; Eriksson et al., 2002; Wingate, 1998). Possible 2701 ± 11 Ma to 2659 ± 3 Ma feeder dykes were also emplaced along radiating NE-, E- to SEtrending directions known as the Rykoppies dyke swarm (Olsson, 2012; Olsson et al., 2010). These were once inferred to be emplaced by the Bushveld Complex (Uken and Watkeys, 1997). A period of sedimentation then occurred after the Allanridge lavas terminated the infilling of the Ventersdorp basin (van der Westhuizen et al., 1991). It is postulated that the Ventersdorp basin was composed of NE-trending Basin-and-Range type rift structures of horst and graben blocks. The structures were found across the central and western parts of the Kaapvaal Craton (Stanistreet and McCarthy, 1991; van der Westhuizen et al., 2006). It is debated whether the Ventersdorp flood basalts (mainly the Klipriviersberg Group) are related to the collision between the Kaapvaal and Zimbabwe cratons (Burke et al., 1985; Light, 1982; Stanistreet and McCarthy, 1991) or to a mantle plume event (Eriksson et al., 2002; Hatton, 1995). The whole southeastern region of the Kaapvaal Craton was also intruded by numerous potassic post-pongola granites such as the 2863 ± 8 Ma Godlwayo and the 2671 ± 3 Ma Kwetta granites (Reimold et al., 1993) Palaeoproterozoic supracrustal successions and intrusions Several groups of mainly clastic sedimentary rocks unconformably overlie the Ventersdorp Supergroup, with some volcanic rocks near the base (Eriksson and Reczko, 1995). These groups are exposed as definite sedimentary basin-fills along the northern and eastern exposed base of the Transvaal Supergroup. They are tentatively interpreted to have been deposited in a rift environment (Eriksson et al., 2001). Lava in the Buffelsfontein Group of

39 Chapter: 2 Geological Setting one of these basin-fills has yielded Rb-Sr ages of ca Ma (SACS, 1993) and U-Pb ages of 2664 ± 1 Ma (Barton et al., 1995), providing a depositional age for these successions. The Kaapvaal Craton is disconformably overlain by two well-preserved supergroups: The ca to 2.05 Ga Transvaal Supergroup towards the north, of which the distribution is wholly confined to the craton. The ca to 0.18 Ga Karoo Supergroup across the southern extent of the Kaapvaal Craton, as well as overlying the nearby Namaqua-Natal Mobile Belt. Both supergroups are comprised of relatively thick and varied sedimentary sequences capped in each case by rapidly emplaced and extraordinarily voluminous igneous deposits, i.e., the Bushveld LIP (Cawthorn et al., 2006) and the Karoo LIP (Duncan and Marsh, 2006). The predominantly clastic and carbonate Transvaal Supergroup overlies the protobasinal successions within its E-trending Transvaal Basin. It also unconformably overlies the Ventersdorp Supergroup within the NE-trending Griqualand West basin and lesser Kanye basin in Botswana (Eriksson et al., 2001). Minor lavas occur in the Abel Erasmus, Ongeluk- Hekpoort and Machadodorp sequences (Crow and Condie, 1990). The 2222 ± 13 Ma Ongeluk-Hekpoort lavas are the youngest dated unit in the Transvaal strata (Cornell et al., 1996), but these lavas are conformably overlain by another few thousand meters of sedimentary rock. The best age estimate for the base of the Transvaal Supergroup is derived from detrital zircons in the 2642 ± 2 Ma Vryburg Formation from the Griqualand West subbasin. This formation is considered to be correlative with the Black Reef Formation in the Transvaal sub-basin (SACS, 1980; Walraven and Martini, 1995). The Transvaal Supergroup is capped and discordantly overlain and intruded by the massive Bushveld Complex (Cawthorn et al., 2006). This ca Ga old complex is made up of: The 2061 ± 2 Ma bimodal Rooiberg Group lavas of basalt and rhyolite (Walraven, 1997). The world s largest layered mafic intrusion the 2058 ± 2 to 2054 ± 1 Ma Rustenburg layered suite at ca km 2, which also hosts the world s largest deposits of PGE s,

40 Chapter: 2 Geological Setting Cr and V (Lee, 1996; Olsson et al., 2010; Scoates and Friedman, 2008), as well as Cu and Ni in its satellite intrusions. The late 2053 ± 12 Ma Rashoop granophyre and 2054 ± 2 Ma Lebowa granite suite intrusions (Coertze et al., 1978; Walraven and Hattingh, 1993). The large sill/lopolith that gave rise to the Rustenburg layered suite was likely emplaced through centralised conduits (e.g., Clarke et al., 2009), as suggested by an absence of recognised dyke-shaped satellite intrusions. Its emplacement may have occurred roughly at the time that subduction was occurring along the western margin of the Kaapvaal Craton prior to a collision with the Congo Craton along the Kheis-Magondi Belt, or even the Zimbabwe Craton along the Limpopo Belt (e.g. Jacobs et al., 2008). It may also be a plumeinduced LIP, produced through delamination of the underlying lithosphere (e.g., Olsson et al., 2011). Connected to the Bushveld Complex are smaller sill-, dyke- or plug-like satellite intrusions in the central parts of the craton, such as the Moshaneng, Uitkomst and Marble Hall complexes (Anhaeusser, 2006; Cawthorn et al., 2006). The Bushveld event was superseded by predominantly clastic sedimentation within an assumed foreland basin. This formed the red-bed sequences of the Waterberg Group. Subsequent ca to 1.87 Ga igneous activity is recorded on the Kaapvaal Craton as intrusive dykes and sills into the Waterberg and Soutpansberg groups (Hanson et al., 2004a; 2011). Also the Sibasa and Ngwanedzi lavas within the Soutspansberg Group were produced in the same event (Barker et al., 2006; Barton, 1979; Crow and Condie, 1990). These intrusives and lavas may be coeval with the 1928 ± 4 Ma Hartley lavas from the Olifantshoek Supergroup on the western margin of the craton, which were caught up in the Kheis orogeny (Cornell et al., 1998; Moen, 2006; van Niekerk, 2006). This ca Ga igneous event is otherwise recognised as mafic dykes and sills across the northern and eastern Kaapvaal Craton, as well as covering large parts of the by then attached Zimbabwe Craton (e.g., Klausen et al., 2010; Olsson, 2012; Söderlund et al., 2010; Stubbs et al., 1999). The dolerite dyke swarm associated with this event has been termed the ca Ga NEtrending Black Hills dyke swarm by Olsson et al. (2010) and the Olifants River dyke swarm by Uken and Watkeys (1997). The timing of collision of the Zimbabwe and Kaapvaal Cratons along the Limpopo Belt is constrained by this magmatic record (Söderlund et al., 2010). This is shown by the apparent absence of the ca Ga Bushveld magmatism on the Zimbabwe

41 Chapter: 2 Geological Setting Craton, and the corresponding absence of magmatism of the same age as the ca Ga Great Dyke of Zimbabwe on the Kaapvaal Craton. This suggests that the Zimbabwe and Kaapvaal Cratons were still separate at these times (Bleeker, 2003). Furthermore, since ca Ga magmatism is present in both Zimbabwe and Kaapvaal Cratons, then collision and welding of these two cratons must have occurred between 1.90 Ga and 2.06 Ga (Söderlund et al., 2010). However, palaeomagnetic evidence suggests an almost 2000 km displacement between the Kaapvaal and Zimbabwe cratons during this time, adding to the complexity of this LIP (Hanson et al., 2011) The Mesoproterozoic to the Mesozoic After the deposition of the Waterberg, Soutpansberg and Olifantshoek successions, the deposition of a magnificent record of Mesoarchaean to Palaeoproterozoic volcanic/clastic strata across the Kaapvaal Craton terminated. Successive orogenies occurred around the Kaapvaal Craton as it was surrounded by active continental margins that led to the expansion of the craton through crustal accretion. This formed a larger combined Kaapvaal- Zimbabwe Craton, called the Kalahari Craton. Crustal growth occurred along the south and eastern margins during the Mesoproterozoic Namaqua-Natal orogeny (Cornell et al., 2006; Jacobs et al., 2008). The only record from the interior of the craton during this time comes from intra-continental alkaline volcanism (Verwoerd, 2006). Towards the end of the Namaquan Epoch, the interior of the Kalahari Craton recorded the emplacement of the ca Ga Umkondo LIP across parts of what had now grown into the Rodinian supercontinent (Hanson et al., 2004b; 2006). Rodinia then began to break apart between ca and 0.75 Ga. The growth of the Gondwana and Pangaea supercontinents was then recorded between ca to 0.50 Ga. This is evident in the Mozambique, Damaran, Cape and Gariep orogenic belts around the Kalahari Craton, as well as in the Antarctic, then occurred (Jacobs et al., 2008). Finally, the Kaapvaal Craton was covered by Phanerozoic Karoo Supergroup sedimentary rocks, deposited up to the time of the Jurassic emplacement of the Karoo LIP (Duncan and Marsh, 2006). The ca Ga Karoo LIP is related to Gondwana break-up. This was when a vast area that stretches across South America, southern Africa and Antarctica was affected by voluminous mafic magmatism

42 Chapter: 2 Geological Setting Thus, the eastern Kaapvaal Craton potentially hosts Jurassic (Karoo) dykes that may be difficult to distinguish from Precambrian dykes. While Karoo dykes and sills have been distinguished on geological maps as less altered, this may be an unreliable criterion. For instance, Jourdan et al. (2006) conducted a reconnaissance 40 Ar/ 39 Ar study on the NNEtrending Olifants River dyke swarm in the north-eastern part of the Kaapvaal Craton. That study revealed, contrary to map classifications, only mafic dykes of Precambrian age. In general, however, it is suspected that the following intrusions are Jurassic: N-trending dykes, because these parallel the Lebombo Group monocline (e.g., Klausen, 2009). ESE-trending dykes across the north-eastern part of the Kaapvaal Craton, because these parallel the Okavango dyke swarm (Jourdan et al., 2006). All sills and potential feeders to these, near or within the Karoo Supergroup cover across the southern parts of the eastern Kaapvaal Craton. This tremendous record of sedimentary and volcanic strata and associated intrusions allows for a formulation of a magmatic barcode for the Kaapvaal Craton. This enables comparison against other cratons (see Fig. 8) Local Geology The south-easternmost terrane of the Kaapvaal Craton in northern KwaZulu-Natal hosts numerous inliers of Precambrian basement (see Fig. 9). This terrane is truncated to the south and east by the Natal Thrust Front. This marks the boundary between the Kaapvaal Craton and the Mesoproterozoic Namaqua-Natal Mobile Belt to the south, and the Neoproterozoic to early Palaeozoic Mozambique Belt to the east, which is covered by Quaternary sands. The margin of the craton in the area thus is deformed and metamorphosed. The intensity of this metamorphism and deformation decreases with distance away from the cratonic boundary (Elworthy et al., 2000). The Precambrian inliers are overlain by Phanerozoic rocks of the Natal and Karoo sedimentary successions, as well as intruded by dykes and sills related to the Jurassic Karoo LIP. The inliers preserve the ca

43 Chapter: 2 Geological Setting 3.29 Ga to 2.61 Ga granitoid greenstone basement, which is unconformably overlain by the Pongola Supergroup (Elworthy et al., 2000; Matthews et al., 1989). Figure: 8 Magmatic barcode for the eastern and western sides of the Kaapvaal Craton with ages from a variety of sources discussed in the text

44 Chapter: 2 Geological Setting

45 Chapter: 2 Geological Setting The Archaean Basement The basement of the Kaapvaal Craton in the area of study is composed predominantly of the granitic Anhalt Suite in the White Mfolozi inlier, occupying the whole inlier east of the Nondweni Greenstone Belt. Locally, the Anhalt Suite is referred to as the Mvunyana granodiorite (du Toit, 1931; Hunter, 1990; 1991; Hunter and Wilson, 1988; Hunter et al., 1992). It is intrusive into the ca Ma Nondweni Greenstone Belt, having an age of ca Ma (Matthews et al., 1989). Grey tonalitic gneiss has also been documented in the area which predates the Mvunyana granodiorite. Further south, the granitoids between Babanango and Nkandla remain undifferentiated. They are however, similar to the Mvunyana granodiorite (Robb et al., 2006). In addition, a wide variety of granitoids occur adjacent to the Ilangwe Greenstone Belt near the margin with the Natal Thrust Front. These consist of early to late post- Nondweni granitoids, gneisses and migmatites according to Mathe (1997). They become increasingly metamorphosed and deformed closer to the Natal Thrust Front. Numerous greenstone belt fragments have also been identified in the Buffalo River Gorge, the Empagneni, as well as near the Nzimane areas and in the Mfule Gorge. Post-Pongola granitoids were identified in northern KwaZulu-Natal, with the isolated Nzimane granite having been dated at 2733 ± 3 Ma (Thomas et al., 1997). Further inliers of undifferentiated granitoids that have as yet not been dated occur in the Buffalo River Gorge inlier, as well as associated with the Empangeni greenstone belt fragments. All units are unconformably overlain by the Pongola Supergroup The Pongola Supergroup The Pongola Supergroup is exposed as inliers that are subdivided into two broad sub-basins in the region: the bigger Pongola sub-basin and the smaller Nkandla sub-basin. The Nkandla sub-basin is separated from the main Pongola sub-basin by a palaeo-high according to Cole (1994). It preserves rocks of the Nsuze Group only. It is structurally more complex, with a stratigraphy that is different from the main Pongola sub-basin according to Groenewald (1984). Cole (1994) thought that the stratigraphy was essentially the same, although several

46 Chapter: 2 Geological Setting successions were structurally thickened or removed from the strata in the sub-basin. The White Mfolozi inlier in northern KwaZulu-Natal falls into the main Pongola sub-basin area. The smaller Archaean inliers in the Nkandla area preserve the Pongola successions falling into the Nkandla sub-basin (see Fig. 10). The area being studied consists of pre-pongola granitoid basement rocks unconformably overlain by north-east dipping Pongola Supergroup rocks in the main Pongola sub-basin. In the region of the Nkandla sub-basin, the granitic basement is overlain by Pongola Supergroup rocks of the Nsuze Group only, dipping gently to the south. Deformation and metamorphism is associated with the nearby Namaqua-Natal Mobile Belt (Groenewald, 1984). Figure: 10 Geological map of the Nkandla sub-basin of the Pongola Supergroup showing the intrusions of the Hlagothi Complex modified after Groenewald (2006). Sample sites for this study are indicated The Nsuze Group comprises six formations in the White Mfolozi and Nkandla inliers, with an average thickness of ~ 3.5 km: the Mantonga, Nhlebela/Pypklipberg, White Mfolozi, Agatha, Langfontein, Mkuzane and Ekombe Formations from oldest to youngest according to Cole (1994). The Mantonga Formation represents initial sedimentation of sandstones, and minor shales and tuffs on top of the basement granite, and is related to basin formation and subsidence through rifting. This was followed by continental rift volcanism of the basaltic and andesitic Nhlebela/Pypklipberg lavas. Volcanism then ceased due to the

47 Chapter: 2 Geological Setting formation of a trailing plate margin, with subsidence and transgression depositing shallow marine sandstones, minor shales and the Chobeni carbonates of the White Mfolozi Formation (Cole, 1994). The dolomites have been documented as containing stromatolites and algal mats (von Brunn and Mason, 1977). Uplift and erosion then occurred, after which the volcanism of the basaltic to rhyolitic Agatha lavas were extruded. A small transgression led to the deposition of Ntambo shales near the end of volcanism. The end of volcanism is marked by Roodewal agglomerates and pyroclastics, which are part of the Langfontein Formation. The Langfontein Formation then continues with sandstones deposited during a marine transgression from subsidence. The basin then underwent further deepening, depositing the Mkuzane Formation shales in a deeper marine setting. A further unconformity seen only in the Nkandla sub-basin marks localised volcanism of the Ekombe lavas ending Nsuze Group sedimentation and volcanism (Cole, 1994). The Mozaan Group in the White Mfolozi inlier comprises a sedimentary sequence consisting essentially of a lower arenaceous and an upper argillaceous sequence. According to Linström (1987), it oversteps approximately m of Nsuze Group strata in a southeastwards direction. The characteristics of the Mozaan Group succession in the White Mfolozi area indicate that it was deposited as fluvial strata. This was followed by subsidence and deposition of deep marine sedimentation during an overall period of sea-level transgression (Linström, 1987). The upper sedimentary and volcanic packages of the Mozaan Group in this region are absent The Hlagothi Complex The Hlagothi Complex intrudes into this region of the craton (see Fig. 4 and 10). It is composed of a series of layered sills of meta-peridotite, pyroxenite and gabbro. It intruded into the basal quartzites of the Nsuze Group in the Nkandla sub-basin of the Pongola Supergroup near the craton margin. These sills were named the Hlagothi Complex by du Toit (1931), who first mapped and described them, as the differentiating products of a single reservoir. Groenewald (1984) carried out extensive mapping and petrography coupled with some geochemistry. Groenewald (1984) noted at least five sills with E-trends. The sills dip concordantly with the Nsuze beds gently to the south. The complex is scattered

48 Chapter: 2 Geological Setting through small inliers of Archaean crust over 18 km in length and 8 km in width between the towns of Babanango and Nkandla. All the rocks of these inliers have been deformed through folding and faulting associated with the Natal Thrust Front. The lithologies appear massive, except at the contacts with the quartzites where they become schistose. They are predominantly overlain by undeformed Permian-Carboniferous Dwyka Group diamictites. Prior to this study, the Hlagothi Complex was believed to be near contemporaneous with the ca Ga Nsuze Group, with a poorly constrained whole rock Pb-Pb age of between 2980 and 3050 Ma (Hegner et al., 1981) Dyke and sill swarms and provinces The eastern areas of the Kaapvaal Craton host the largest exposures of the Archaean basement of the craton. It hosts numerous intrusions of mafic dyke swarms (see Fig. 4) which have received the most study until the present. At least three major dyke swarms were identified from the work of Klausen et al. (2010), Lubnina et al. (2010), Olsson (2012) and Olsson et al. (2010): A ca Ga SE-trending swarm in the Barberton-Badplaas area. A ca Ga NE-, E- and SE-trending swarm which is intermixed with the older 2.95 Ga dykes in the Barberton-Badplaas area, forming a radial pattern. A ca Ga NE-trending swarm in the Black Hills area, also known as the Olifants River swarm of Uken and Watkeys (1997). The Archean basement of the south-easternmost Kaapvaal Craton is intruded by a number of prominent mafic dyke swarms with several ages and trends. These can be compared to dyke swarms located further north on the eastern side of the craton (e.g., Hunter and Reid, 1987; Olsson et al., 2010; Uken and Watkeys, 1997). There were no accurate and precise, absolute ages on the eastern Kaapvaal Craton dykes prior to the U-Pb dating by Olsson et al. (2010; 2011), Olsson (2012) and Söderlund et al. (2010). In the southeasternmost region, this study concentrates on the dyke swarms limited to the Archaean inliers of similar trends on which age dating, geochemistry and palaeomagnetism was done further to the north on the eastern Kaapvaal Craton. This includes:

49 Chapter: 2 Geological Setting NW- to SE-trending dolerite dykes, which may be compared to the same trending Barberton-Badplaas dyke swarm of Olsson et al. (2010). SW- to NE-trending dykes, which may be compared to the same trending Black Ridge dyke swarm of Olsson (2012) and Söderlund et al. (2010) on the eastern Kaapvaal Craton. This comparison was made by both Klausen et al. (2010) and Lubnina et al. (2010), despite the almost 1000 km separation. WSW- to ENE-trending dyke swarm which is comparable to the E-W trending Rykoppies dyke swarm, for which new ages are available (Olsson, 2012; Olsson et al., 2010). This comparison was made by Lubnina et al. (2010), but was contradicted by Klausen et al. (2010), who inferred a relationship to the Barberton-Badplaas dyke swarm of Olsson et al. (2010). Prior to the present study, no age dating has been carried out on these dykes directly, but in one locality a SE-trending dyke is seen to be cross-cut by an ENE-trending dyke, which is in turn cross-cut by a SSE-trending dyke. The SSE-trending dyke also intrudes into Phanerozoic Karoo strata, while all the other dyke trends are absent from it. Lubnina et al. (2010) assigned ages of ca. 2.95, 2.65, 1.90 and 0.18 Ga for the SE-, ENE-, NE- and SSEtrending dykes of the region respectively based on the cross-cutting relationships and palaeomagnetic studies. Klausen et al. (2010) contradicted this with a possible age of ca Ga for the ENE-trending dykes using petrography and geochemistry. No other prior study has been presented for the dykes in the south-easternmost area of the Kaapvaal Craton. Therefore the following literature review below focuses on the dykes located further north with which they may be compared (see Fig. 4). The Barberton-Badplaas area of the Kaapvaal Craton has been more intensively investigated because it includes the world-famous Barberton Greenstone Belt. The remarkable high-standing SE-trending dolerite dyke ridges extend across many contrasting pale granitoids, and these are believed to have been feeding Nsuze lavas within the Pongola Supergroup (e.g., Hunter and Halls, 1992; Klausen et al., 2010; Lubnina et al., 2010; Olsson et al., 2010). The assignment of the ca Ga age to the oldest SE-trending dyke swarm is in agreement with the work of Hunter and Halls (1992) and Uken and Watkeys (1997). These authors tentatively correlated these dykes with volcanism within the Nsuze Group of the Pongola Supergroup. Neoarchaean post-pongola Supergroup granitoids have been observed

50 Chapter: 2 Geological Setting to cut many of the dykes that are orientated sub-parallel to the postulated Pongola Rift of Weilers (1990). The ca Ga age for the E-trending dykes in the Rykoppies area is in disagreement with the studies of Anhaeusser (2006) and Uken and Watkeys (1997), who stated that these dykes represented feeder dykes to the E-W elongated Bushveld Complex. However, more than one age of dyke events is present in the area for the following reasons: A SE- to ESE-trending dyke in the Barberton-Badplaas area is of comparable age to the E-trending Rykoppies dyke swarm located further north. This defines a fanning swarm of this age in conjunction with coeval NE-trending dolerite dykes observed also in the Black Hills area (Olsson et al., 2010; 2011) SE-trending dykes may be subdivided into two geochemically different groups which may be of different ages (e.g., Hunter and Halls, 1992; Klausen et al., 2010) In detail, SE-trending dykes have two distinct trends between 120 and 150 which probably represent different swarms. In addition, there are sporadic N-trending dykes of presumed Jurassic age, and also more subdued SW- to NE-trending dykes of likely ca Ga (Söderlund et al., 2010). The Barberton-Badplaas dyke swarm is ~ 80 km wide, and can be followed from Swaziland in the south-east for at least 100 km before disappearing under the cover sequences of the Transvaal Supergroup in the north-west (Hunter and Halls, 1992; Hunter and Reid, 1987; Uken and Watkeys, 1997). It has intruded into the Ancient Gneiss Complex, but also into younger Archean granitoid bodies such as the 3227 ± 4 Ma Kaap Valley pluton (Kamo and Davis, 1994), the 3212 ± 2 Ma Nelshoogte pluton (York et al., 1989) and the 3105 ± 3 Ma Mpuluzi batholith (Kamo and Davis, 1994). Since the dolerite dykes of this swarm are absent in the 2691 ± 2 Ma Mbabane pluton (Layer et al., 1989), Hunter and Halls (1992) suggested that the majority of these dykes may have been emplaced between ca Ga and 2.70 Ga. This is because they are generally cut by post-pongola (Neoarchaean) granitoids, which was confirmed by Olsson et al. (2010). This dyke swarm also parallels the mafic ca to 2.87 Ga Ushushwana Complex (Hunter and Reid, 1987; Olsson, 2012). Burke et al. (1985) stated that rocks of the Pongola Supergroup have been deposited in a sub-parallel, NW-trending rift system, with dykes more commonly bifurcating towards the north-west. This implies a predominant magma movement from the south-east (Hunter and

51 Chapter: 2 Geological Setting Halls, 1992). Also, the dykes have roughly similar compositions to Nsuze lavas (Klausen et al., 2010). The dolerite dykes can be divided into high-ti and low-ti types (Hegner et al., 1993). The high-ti dykes have geochemical characteristics that resemble the 2984 ± 3 Ma lavas from the Nsuze Group. It was argued that the low-ti dykes have a geochemical signature that is typical of early Proterozoic dykes (Hunter and Halls, 1992). Geochemical variations (including that of Ti) can also be explained by variable degrees of crustal contamination and low-pressure pyroxene and plagioclase fractionation of a common primary basaltic melt (Hunter and Halls, 1992; Olsson et al., 2010). The latter explanation is more consistent with the inability of Klausen et al. (2010) to make a similar low-sr/v and high-sr/v discrimination between the Hunter and Halls (1992) high-ti and low-ti dykes, respectively. The geochemical study of Maré and Fourie (2012) further contradicted the inferences made by Klausen et al. (2010), showing even greater geochemical variability. E-trending dykes appear to concentrate in the Rykoppies area of the eastern Kaapvaal Craton, in addition to those in the south-easternmost Kaapvaal Craton in northern KwaZulu-Natal (see Fig. 4). The E- to W-trending Rykoppies Dyke Swarm is more than 50 km wide and 100 km long. This dyke swarm follows the long axis of the E-trending elongated Transvaal Supergroup (including the Rustenburg Layered Suite). Relatively few NE-, S- and SE-trending dykes appear to cross-cut this well-constrained succession. The age of the sericitised and silicified shear zones in the Rykoppies area is unconstrained (Walraven and Hartzer, 1986). However, they are oriented conspicuously sub-parallel to the ca Ga SEtrending Barberton-Badplaas dyke swarm (Klausen et al., 2010; Lubnina et al., 2010; Olsson et al., 2010). Such a tentative correlation may at first be discounted according to Lubnina et al. (2010) by the prevalent right-lateral offsets of all ca Ga E-trending dykes along this shear zone. The question is whether these systematic offsets were caused by a postintrusive tectonic shear, or by propagating dykes consistently following a pre-existing shear zone in right-lateral fashion according to Lubnina et al. (2010). The following observations convinced Lubnina et al. (2010) that the right-lateral offsets along these ca Ga dolerite dykes are of a primary intrusive origin: Three cases of conspicuous dykelets located consistently to the left of a corresponding major right-lateral offset which resemble typical bayonets extending from magmatic dyke offsets

52 Chapter: 2 Geological Setting Dykes do not appear nearly as sheared as the host rock. The NL-23 sample of Lubnina et al. (2010) is offset by a greater amount than NL-22 across the same fault zone. Thus, while being aware of the possibility of any secondary deformation, Lubnina et al. (2010) were confident that samples from the sites within the Rykoppies area are unaffected by this shear zone. The most prominent dolerite dyke within this swarm is the Rykoppies dyke itself, which marks the central axis of the dyke swarm and predates the cross-cutting Timbavati Gabbro, of ca Ga Umkondo age (Burger and Walraven, 1979, 1980; Hargraves et al., 1994; Hanson et al., 2004b). The dyke swarm appears to be exposed only in the Archaean basement and is discordantly overlain by the ca to 2.06 Ga Transvaal Supergroup (Walraven and Martini, 1995). Thus Hunter and Reid (1987) proposed an Archaean age for the Rykoppies Dyke Swarm. Uken and Watkeys (1997) suggested that the swarm could be a feeder system to the ca Ga Bushveld Complex. This was not only because of its E-trend parallel to the long axis of the Bushveld Complex, but also because of a possible link with dykes and sills within the Transvaal Supergroup (e.g., Sharpe, 1981). This view was also upheld in a recent compilation of South Africa s mafic- to ultramafic intrusions (Anhaeusser, 2006). However, consistent ca Ga ages negate previous inferences suggesting that E-trending Rykoppies dykes were feeding the neighbouring E-W elongated Bushveld Complex (Anhaeusser, 2006; Uken and Watkeys, 1997). Instead, this swarm appears to be radiating (Olsson et al., 2010; 2011), and probably acted as feeders to the upper (Allanridge) lava formation within the Ventersdorp Supergroup (Klausen et al., 2010; Olsson et al., 2010). Satellite imagery and aeromagnetic compilations clearly reveal the predominance of NE-trending dykes in the Black Hills area, which appear to become denser towards the Jurassic Lebombo-Mwenetzi-Okavango triple-junction (e.g., Jourdan et al., 2006; Klausen, 2009; Reeves, 2000). However, only Precambrian and no Phanerozoic dykes were recorded among these in the Jourdan et al. (2006) 40 Ar- 39 Ar reconnaissance study. An apparent kink from a more E-trending swarm across the Kaapvaal Craton basement to a more northerly trend across the Transvaal Supergroup is possibly an artefact of the juxtaposition of two different swarms (Uken and Watkeys, 1997). There being a more E-trending ca Ga dyke swarm that does not cut the Transvaal Supergroup and a more N-trending and

53 Chapter: 2 Geological Setting presumed ca Ga swarm that does (Klausen et al., 2010). This conjugate pattern between approximately NE-trending dykes is evident throughout the north-eastern part of the Kaapvaal Craton. The older, more E-trending set of dykes is more metamorphosed and therefore, less suited for geochronology (Klausen et al., 2010). A ca Ga age for the NEtrending Black Hills dyke swarm argues for a widespread magmatic event. This magmatic event is correlatable to sills in the Waterberg Group (Hanson et al., 2004a; 2011), lavas within the Soutpansberg Group (Barker et al., 2006), and even dykes and sills in Mashonaland within the juxtaposed Zimbabwe Craton (Hanson, 2004a; 2011; Klausen et al., 2010; Söderlund et al., 2010)

54 Chapter: 3 Geology Chapter: 3 Geology 3.1. Introduction Northern KwaZulu-Natal south of the town of Vryheid in South Africa consists mainly of Permian to Jurassic sedimentary rocks of the Natal and Karoo successions. Inliers of Precambrian crust, have been exposed through the weathering and erosion of the Phanerozoic strata as a result of uplift of the southern Africa region since the Cretaceous (Partridge and Maud, 1987). The area is cross-cut by deeply incised braided and meandering rivers, which have exposed these Precambrian rocks. To the east, along the coastal plain the Phanerozoic sedimentary rocks have been in turn been covered by Quaternary sands. The Precambrian rocks in these inliers consist of the Mesoarchaean to Neoarchaean granite-greenstone terrane north of the town of Eshowe, with remnants also of the Pongola Supergroup, which have been truncated to the south by the Natal Thrust Front running eastwest between Eshowe and Melmoth. This front is easily distinguished by an escarpment in the area forming the boundary between the Archaean Kaapvaal Craton and the Mesoproterozoic Namaqua-Natal Mobile Belt south of it. The Archaean inliers consist of granitoids and the Nondweni and Ilangwe greenstone belts and fragments. Overlying this are scattered remnants of the Pongola Supergroup, with the Nkandla sub-basin in the vicinity of Nkandla separated from the Pongola sub-basin near Ulundi by a palaeo-high (Cole, 1994; Groenewald, 1984). The Nkandla sub-basin preserves rocks only of the Nsuze Group, whereas in the Pongola sub-basin, the complete Pongola sequence can be seen (Cole, 1994). It is this Archaean crust that is the focus of this dissertation. It is poorly studied, probably due to limited infrastructure and rugged terrane. The climate also renders it difficult to work in this area over summer, with its heavy rainfall and high temperatures. This has also led to a deep weathering profile and a scarcity of outcrop, making mapping

55 Chapter: 3 Geology and sampling difficult, except along river section pavements, which are often flooded in summer however. Into these Archaean inliers numerous mafic- to ultramafic intrusions can be found in the form of dyke swarms and layered complexes. These intrusions are useful for magmatic barcoding and palaeomagnetic studies. These intrusions in this area have received little study, and are also not mapped in great detail or differentiated. Intrusions into the crust closer to the Natal Thrust Front in the proximity of the Ilangwe Greenstone Belt have not been mapped or studied at all, nor have dykes present in the more scattered inliers of Archaean crust in the Nkandla area. Sampling GPS co-ordinates from this study can be found in Appendix A The Hlagothi Complex The isolated and scattered Archaean inliers of the Nkandla area can be reached by turning off the R68 between Babanango and Melmoth toward the town of Nkandla. The terrain consists of rolling hills, with incised meandering rivers. These rivers expose the Archaean basement beneath the mainly Phanerozoic Karoo strata consisting mostly of the Dwyka and Ecca Groups of diamictite and shale. The inliers are dominated by the structurally complex Nsuze Group remnants of the Nkandla sub-basin (Pongola Supergroup). Metamorphism and deformation is also higher in this region, being approximately 50 km away from the Natal thrust front. Into these inliers of Nsuze Group, the Hlagothi Complex intrudes (see Fig. 11). This is the only intrusion which has been mapped or sampled in any detail in the area by du Toit (1931) and Groenewald (1984; 1988; 2006). Outcrops of the granite-greenstone terrane itself occur in the eastern regions adjacent to the Ilangwe inlier. This inlier is composed of the Ilangwe Greenstone Belt and associated granitoids. An intrusive syenite has also been identified and mapped in the vicinity of the Mhlatuze River. No dykes have been mapped or reported, apart from feeder dykes to the Hlagothi Complex by Goenewald (1984)

56 Chapter: 3 Geology Figure: 11 The Nsongeni and Hlagothi sheets of the Hlagothi Complex within the Archaean inliers of the Nkandla area, with sample localities shown (modified after Groenewald, 1984). Sample sites for this study are indicated, as well as section lines depicted in Figure 12 The Hlagothi Complex consists of alternating layered sills of peridotite, pyroxenite and gabbro. It has been extensively altered through deformation and metamorphism associated with the Namaqua-Natal Mobile Belt located approximately 50 km to the south. The complex is scattered through small inliers of Archaean crust over 18 km in length and 8 km in width between the towns of Babanango and Nkandla. The work of Groenewald (1984) noted at least five sills with E-trends that vary in thickness from 50 to 250 m, with a combined thickness of over 500 m measured along the Nsongeni and Nsuze rivers. The thickest single sheet is about 200 m thick. The sills dip concordantly with the Nsuze sedimentary strata gently to the south, varying from 10 in the northern Nsongeni sheet

57 Chapter: 3 Geology exposures to 30 in the south within the outcrops of the Hlagothi sheets. However, the sills do transgress from this accepted dip locally, and across observed bedding. The northern sills are termed the Nsongeni sheets, and the southern sills the Hlagothi sheets, both of which are located west of Nkandla. Scattered outcrops also occur east of Nkandla, and have been termed the Wonderdraai sheets. Further to the east, poorer outcrop exposures are found, most of which is predominantly along rivers from which the areal extent of the Hlagothi Complex has been extrapolated. These sills are termed the Mhlatuze sheets. The whole complex is predominantly overlain by undeformed Permian Carboniferous Dwyka Group diamictites (see Fig. 12). Figure: 12 Simplified E-W and N-S cross-sections (not to scale) through the various sheets of the Hlagothi Complex. E-W section represents an 18 km long section from Wonderdraai to the headwaters of the Nsuze River. The N-S section shows the 8 km long outcrop area along the upper Nsuze River valley (after Groenewald, 1984). HC-04 shows a representative hand specimen of the peridotites, whereas HC-08 shows a representative hand specimen of the gabbros

58 Chapter: 3 Geology Samples HC-01, HC-02, HC-03, HC-04, HC-05, HC-06, HC-07 and AG-I_core were taken from the ultramafic sills within the Nsongeni sheets (see Fig. 9, 10 and 11). These sills are dark grey-black, massive and fine- to medium-grained (see Fig. 13a). Along the contact between the complex and the quartzite of the Nsuze Group, a zone of green, foliated and fine-grained rock was observed. It was described in the field as talc-chlorite schist. It has possibly developed along the contact between the complex and the quartzites due to stress partitioning along a zone of competency contrast. One sample was taken from here, AG- I_contact (see Fig. 13b and c). The pyroxenite sills are usually less than 10 m thick, and consist of grey-green, massive and medium- to coarse-grained rocks. They are only observed directly at the base of the Hlagothi sheets. No pyroxenite sill portions were judged adequate for sampling due to weathering and/or alteration. The gabbros make up the bulk of the complex and range from 10 to 70 m in thickness. The contact between the gabbros and the underlying lithologies was not directly seen, but it was inferred within approximately 10 m. The gabbros are coarse-grained and massive, being also grey in colour, with samples HC-08 and HC-09 taken from them in the Hlagothi sheets located to the south of the Nsongeni sheet. In addition, one sample was taken from the Wonderdraai inlier. The Wonderdraai inliers form part of a series of small, isolated inliers of Nsuze Group, and the Hlagothi Complex rocks which intrude into them east of the main inliers containing the Hlagothi Complex. Sample HC-10 was taken here, where a black coarse-grained and massive rocktype, possibly of the Hlagothi Complex (although its exact relationship is unknown), was seen to intrude into the Nsuze Group quartzite. It must be noted the contact itself wasn t directly visible. Skeletal plagioclase feldspar was clearly visible in hand specimen, as was scattered grains of sulphides, such as pyrite. This rock has been termed a diorite (see Fig. 13d). Finally, samples AG-D, AG-E, AG-F, AG-G and AG-H were also collected from the Mhlatuze sheets. These inliers host ultramafic to mafic intrusions which have been correlated with the Hlagothi Complex located further west. They are also inter-sliced with greenstone belt fragments and possibly volcanic samples of the Nsuze Group (Groenewald, 1984). Due to poor outcrop exposure which is limited to along river sections, as well as structural and metamorphic complexities, the exact relationship of the Mhlatuze sheets with the Hlagothi Complex are unknown. The river section sampled is composed of alternating fine- to medium-grained gabbros and pyroxenites which range in colour from dark grey to green, and are massive. In addition, the greater Hlagothi Complex is intruded by

59 Chapter: 3 Geology porphyritic dykes which pre-date the Karoo Supergroup sedimentation in the region, and which may be related to the Namaqua-Natal Mobile Belt to the south. Figure: 13 Selected geological features observed in the various Archaean inliers hosting intrusions of the Hlagothi Complex in the Nkandla area. (a) The peridotite intrusions of the Nsongeni sheets of the complex seen dipping gently toward the south. (b) The contact between the peridotites of the Nsongeni sheets (right) and the basal Nsuze Group quartzites (left). (c) The schistose contact of the Hlagothi Complex with the quartzites of the Nsuze Group. (d) The diorite seen in the Wonderdraai inlier Dolerite Dykes The White Mfolozi inliers of northern KwaZulu-Natal are the largest inliers of Archaean crust of the south-easternmost Kaapvaal Craton. The nearest towns in the region are Nondweni,

60 Chapter: 3 Geology Melmoth and Vryheid. Nondweni is reached by turning off the R68 from Nqutu, whereas the R34 from Melmoth to Vryheid bisects the more eastern side of the area. The area is drained by the White Mfolozi River which has exposed a large area of rolling hills composed of the Mvunyana granodiorite, as well as the Nondweni Greenstone Belt in the western areas of the inliers as well. The Pongola Supergroup outcrops to the east and northeast, which contain both Nsuze and Mozaan rocks. Good exposures of the Pongola Supergroup can be seen along river sections, and are remarkably well preserved, illustrating a near complete Pongola stratigraphy according to Cole (1994), with the exception of the uppermost Mozaan Group. Metamorphism and deformation is far less than in the inliers located to the south in the Ilangwe Greenstone Belt area. This is due to the White Mfolozi inliers being at a greater distance from the Natal Thrust Front. The granodiorite to the south becomes increasingly gneissic, and is undifferentiated. There is also a large amount of young granitoids present. Pongola Supergroup rocks also occur west of the Ilangwe inliers, as well as in scattered remnants from inliers south of the Nondweni Greenstone Belt. These windows into the south-easternmost Kaapvaal Craton in northern KwaZulu- Natal are more heavily cross-cut by a complex array of dolerite dykes and sills than seen in areas immediately to the north. Some of these intrusions extend into the overlying Karoo Supergroup and therefore are almost certainly Jurassic in age, and related to the Karoo LIP. Dykes in the region have been mapped by Linström (1987), and predominantly show NE- and ENE-trends. Lesser ~ 135 and 165 SE-trending dolerite dykes have been noted. The 135 dolerite dyke trend is seen in the north-western areas, and 165 trend seen in the south-east (herein referred to as SE135 and SE165 dolerite dykes respectively). The ENEtrending dolerite dykes were only noted in the south-western portions of the inliers, and consist of a dominant ~ 075 trend only. The NE-trending dolerite dykes were noted to be widespread across the whole area, but are predominantly in the north-western portions of the inliers, and consist of a dominant ~ 030 dolerite dyke trend (NE030). A minor trend across the whole region can be found at between 045 and 055 (herein referred to as NE050). Samples of the dolerite dykes hosted in the oldest (Palaeo- to Neoarchaean), Swaziland terrane basement granitoids in the area were collected. In general, tonalitetrondhjemite-granodiorite (TTG) gneisses predominate within the Palaeo- to Mesoarchaean. A granodiorite-monzonite-syenite (GMS) suite of intrusions makes up most of the late

61 Chapter: 3 Geology Meso- to Neoarchaean (Robb et al., 2006). Neoarchaean granitoids are not directly relevant to the study, except for the fact that for example, most SE-trending mafic dykes do not cut the post-pongola granitoids (GMS suites) seen further to the north in the Barberton and Swaziland region. This is in agreement with this swarm s syn-pongola age of ca Ga (Olsson et al., 2010). The south-easternmost Kaapvaal Craton includes the Nondweni and Ilangwe greenstone belts. The lava sequences within these are generally too dark to provide any good contrast to the same cross-cutting mafic dyke swarms that are more readily mapped in equally old granitoids. The same is true of the mafic lavas from the Pongola. The width and length of the dykes in the inliers are estimated from outcrop, as well as from Google Earth imagery and 1: geological maps from the Council of Geoscience for the region (1: Dundee and Vryheid maps). These estimates should be regarded as approximate (see Fig. 14). Cross-cutting relationships are described below (see Fig. 15). Figure: 14 - Cumulative dyke lengths at 5 intervals for the White Mfolozi Archaean inliers on the southeasternmost Kaapvaal Craton. Histogram (A) was generated for the north-western areas, and (B) for southeastern areas

62 Chapter: 3 Geology Figure: 15 Field relationships seen within the Archaean inliers of northern KwaZulu-Natal, with the yellow Karoo succession being weathered and eroded to expose the pink Archaean inliers of granitoids and lesser greenstone belts. The supracrustal Pongola Supergroup is also shown in blue. The green SE-trending dolerite dykes are cut by green ENE-trending dolerites which in turn are cross-cut by green NE-trending dolerite dykes. These dykes are metamorphosed at greenschist facies and limited only to the Archaean inliers. The whole area is then in turn cross-cut by unaltered grey S-trending dolerite dykes SE-trending dolerite dykes SE-trending dolerite dykes can be found throughout the White Mfolozi inliers, and are absent from the overlying Karoo strata, indicating that they are Precambrian age. These dykes are less visible in satellite imagery than the NE- and ENE-trending dykes within the area. They generally show no or negative relief, with two possible generations of dolerite dykes present in the inliers, i.e., two SE-trending sets of dolerite dykes offset by up to 30 from one another in the northwest and southeast (SE135 and SE165). In one locality, already noted by Lubnina et al. (2010) and Klausen et al. (2010) within the White Mfolozi River itself, these dykes have been seen to be cross-cut by both SSE-trending dolerite dykes and ENEtrending dykes. This makes them potentially the oldest in the inliers (see Fig. 15, and Fig. 17). They have also been noted in some instances to cross-cut the Pongola Supergroup lithologies (Lubnina et al., 2010). The two different trends of SE-trending dolerite dykes were studied. The first generation (SE165), represented by samples DY-01, AG-Bc and AG-K

63 Chapter: 3 Geology (see Fig. 9), was only noted in the south-eastern portions of the White Mfolozi inliers. Dykes of this generation are medium- to coarse-grained, green-grey and orientated at 165 with no obvious fabric. A possible slight plunge of 10 to the vertical to the south-west was noted in these dykes, and these dolerite dykes were observed to be approximately 10 m wide (see Fig. 16a). In addition, another dyke generation (SE135) which was braided in nature, with fine and coarse-grained phases was observed, along with devitrified glassy veinlets of dyke material seen splaying off from the main dyke. Sample AG-J was taken from one of them. Generally, they are aphanitic, however, and green, with no observable fabric. These dykes are orientated at roughly 135 with variable widths of between 10 and 35 m. Xenoliths have been observed within these dykes, accounting for their braided nature. Xenoliths are more numerous along contacts between dykes and the country rock (see Fig. 16b). These SEtrending dykes were preliminarily linked to the ca Ga dyke swarm that is dated near Badplaas by Olsson et al. (2010). Figure: 16 Sample localities for the various SE-trending dolerite dykes. (a) a SE-trending dolerite dyke from which sample AG-Bc was gathered (b) another SE-trending dyke from which sample AG-K was gathered. Sample localities are also shown from various SE-trending dolerite dykes with hand sample pictures

64 Chapter: 3 Geology Figure: 17 The intersection of three dolerite dykes in the White Mfolozi inlier within the White Mfolozi River. A SE-trending dolerite dyke was observed to be cross-cut by an ENE-trending dolerite dyke, which is in turn cross-cut and off-set by a N-trending Jurassic aged dolerite dyke

65 Chapter: 3 Geology ENE-trending dolerite dykes The ENE-trending dolerite dykes are restricted to the south-eastern areas of the White Mfolozi inliers (see Fig. 9). Like SE-trending dykes in the region, they are absent from the overlying Karoo strata. In addition, these dykes are observable in satellite imagery from the area, as vegetation is attracted to their outcrops; particulary aloes (see Fig. 18). They do not vary as much in their width and direction as the SE-trending dolerite dykes. As was already established by Klausen et al. (2010) and Lubnina et al. (2010) within the White Mfolozi River, these dykes have been cross-cut by SSE-trending dolerite dykes, whereas they have in turn cross cut the SE-trending braided dolerite dykes (SE135). This makes them possibly the second oldest observed dykes in the inliers (see Fig. 15, 17, 18 and 19a). Klausen et al. (2010) and Lubnina et al. (2010) have also observed them to be cross-cut by NE-trending dolerite dykes. These dykes have been noted by Klausen et al. (2010) to be heavily altered during metamorphism. They are largely indistinguishable from those within the E-trending Rykoppies dyke swarm, and may likewise contain variable amounts of basement xenoliths. These dykes were noted to be grey, massive and fine-grained (see Fig. 19b), with one locality containing a large number of xenoliths, which are usually less than a centimetre to greater than a meter long (Klausen et al., 2010). Figure: 18 Google Earth image with the strikes of various ENE-trending dolerite dykes indicated along with sample localities. Sample sites for this study are indicated

66 Chapter: 3 Geology They have also been noted to be preferentially orientated in certain localities, according to Klausen et al. (2010), at 30 to the vertical. In addition, the dykes were noted to be small, massive and approximately 5 m in width, as well as orientated at approximately 075. Sharp contacts with the Mvunyana granodiorite are common. Sampling was done on these dykes, with samples AG-Ba, AG-Bb and AG-Cb representing three dykes from the White Mfolozi inlier (see Fig. 19). Figure: 19 Sample localities for the various ENE-trending dolerite dykes. (a) an ENE-trending dolerite dyke near where a SE- trending dolerite dyke sample AG-J was gathered, (b) another ENE-trending dyke from which samples AG-Ca and b were gathered. Sample localities with various ENE-trending dolerite dykes also shown with hand sample pictures NE-trending dolerite dykes NE-trending dykes are found mostly in the north-western portions of the White Mfolozi inlier, and intrude into the Precambrian basement, as well as the overlying Pongola Supergroup strata. They are absent from the Phanerozoic cover of the Dwyka and Ecca

67 Chapter: 3 Geology groups of the Karoo Supergroup. The NE-trending dolerite dykes with 030 (NE030) trends are remarkably high-standing and wide, whereas the dykes with more subdued 045 to 055 (NE050) trends are much thinner. These dykes were observed to cross-cut the SE-trending dykes, making at least some of them younger than this event. ENE- trending dykes have also been observed in turn to be cross-cut by some of these NE-trending dykes, in particular the NE030 dolerite dyke trend (Klausen et al., 2010; Lubnina et al., 2010). No other cross-cutting relationship of these dykes has been observed (see Fig. 15). The NE030 dykes are very distinct on Google Earth imagery, being greater than 30 m wide, and also being marked by linear features of outcrop and indigenous trees and shrubs in an otherwise undulating landscape of grassland (see Fig. 20). Figure: 20 - Google Earth map with strikes of NE-trending dolerite dykes indicated along with sample localities Pink to white phenocrysts were observed in the NE030 dolerite dykes. In addition, these dykes have been described as porphyritic by Klausen et al. (2010), with zoned plagioclase feldspar phenocrysts ranging from 1 to 8 cm in length, with anorthite cores and albite rims. The phenocrysts coarsen toward the centre of the dykes. These phenocrysts are more sub-angular and anhedral near the margins of the dykes and more sub-rounded and euhedral near the centres of the dykes. In addition, the dykes are not massive, showing evidence of flow banding with the plagioclase feldspar phenocrysts forming bands varying between 6 and 60 cm in width. The dykes are coarse-grained and dark grey-green in colour

68 Chapter: 3 Geology Sampling was done on these dykes in the north-western area, where sample AG-A was taken (see Fig. 21b). These dykes have been interpreted to stretch as far as Swaziland and Barberton by Klausen et al. (2010). However, a second generation of NE-trending dolerite dykes was observed at 050 orientation (NE050). These dykes are less than 10 m in width and are massive, dark-grey and fine-grained, with numerous dykelets splaying off them. No phenocrysts were observed within these dykes, in contrast to the NE030 dolerite dykes described above. These dykes were found to be widespread across the whole inlier(s), with samples DY-02_m and DY-02_s and AG-Ca collected from them (see Fig. 21a). Figure: 21 Sample localities for the various NE-trending dolerite dykes. (a) a NE-trending dolerite dyke which is < 5 m wide and from which sample DY-02 was gathered, (b) another NE-trending dyke from which sample AG-A was gathered, note the phenocrysts. Both localities are in the north-western portions of the White Mfolozi inliers. Sample localities with various NE-trending dolerite dykes are also shown with hand sample pictures

69 Chapter: 3 Geology Dolerite dykes of other ages More pristine S- to SSE-trending dykes can be assumed to be Jurassic in age because they run parallel to the Rooi Rand dyke swarm along the southern Lebombo monocline (Klausen, 2009). They may also be erratic step-feeders to juxtaposed sills, as was noted by Klausen et al. (2010). These dykes can be easily distinguished from the altered NE-, ENE- and SEtrending dolerite dykes, as they have experienced little metamorphism and deformation, unlike the Precambrian examples. Samples AG-L and AG-M were taken from them near the Natal Thrust Front in order to compare against the older generations of dolerite dykes, however no further work was done on them during this study

70 Chapter: 4 Petrography Chapter: 4 Petrography 4.1. Introduction Petrographical and mineralogical studies were conducted on the 31 samples taken from a variety of dolerite dykes and sills across the south-easternmost terrane of the Kaapvaal Craton, including the Hlagothi Complex. Samples were selected from the two possible generations of SE- and NE-trending dykes, as well as ENE-trending dykes. The various phases of the Hlagothi Complex were also studied, including the meta-peridotites and gabbros taken from the Nsongeni and Hlagothi sheets. The study was done principally by optical microscopy in both transmitted and reflected light, as well as X-Ray diffractrometry (XRD). In addition, selected features were studied by scanning electron microscopy (SEM), using traditional imagery as well as electron dispersive spectrometry (EDS); see Appendix B for further details. Based on macroscopic observations in hand sample as well as microscopic observations from thin and polished sections, the original mineralogy in the dolerite dykes as well as the Hlagothi Complex was noted to be poorly preserved. In most cases this poor preservation was due to alteration through metamorphism (or in some cases, weathering), with a few exceptions. In addition, many of the dolerite dyke samples were too fine-grained for good optical petrography study. Therefore, XRD was particulary useful in conjunction with CIPW norms calculated following Cox et al. (1979), from whole-rock geochemistry presented in the next chapter. Samples from the Mhlatuze and Wonderdraai sheets of the Hlagothi Complex were omitted due to either being highly mineralised (HC-10), highly altered (AG-E, AG-F and AG-H) or extremely fine-grained (AG-D and AG-G). The relationship between these samples and the Hlagothi Complex was cast into doubt due to uncertain field relationships. This was in addition to anomalous geochemical and palaeomagnetic data

71 Chapter: 4 Petrography discussed in later chapters. The fresh Karoo dolerite dykes are also not discussed (AG-L and AG-M) The Hlagothi Complex The various phases of the Hlagothi Complex were studied, including the meta-peridotites and gabbros taken from the Nsongeni and Hlagothi sheets. Groenewald (1984; 1988; 2006) proposed a four-fold subdivision of the complex based on petrographic studies, updating the work of du Toit (1931). Groenewald (1984; 1988; 2006) noted that the Hlagothi Complex generally consists of three types of lithologies, as well as a chilled margin phase against upper contacts with the Nsuze Group in the gabbroic phases of the complex. Prior to alteration and metamorphism, the lower portion of each sill consisted of cumulate peridotites (feldspathic wehrlite, olivine websterite or lherzolite) and pyroxenites with clinoand orthopyroxene (websterites). The upper portions consist of gabbros and gabbro-norites according to Groenewald (1984). Primary minerals were probably olivine, ortho- and clinopyroxene as well as plagioclase feldspar. Other minerals include magnetite, chromite and biotite. Groenewald (1984; 1988; 2006) also noted chilled marginal phases along the upper contacts of the gabbroic phase; but it was not observed in this study. Metamorphism varies from lower to upper greenschist facies. The peridotite mineralogy consists of fine- to medium-grained amphibole, chlorite, talc, magnetite and serpentine; with talc or serpentine and magnetite replacing olivine; chlorite and amphibole replacing orthopyroxene, and more minor clinopyroxene (see Fig. 22). There are instances where the original subhedral to anhedral clino- and orthopyroxene are still preserved. Usually only relict textures of the cleavage directions remain, with the pyroxenes almost always enclosing the olivine grains. Mostly the pyroxene has been pseudomorphed by amphibole and chlorite. Grain size of the pyroxenes and olivine vary between 0.2 and 7.0 mm. The amphibole and chlorite are usually intergrown, with the amphibole forming small acicular and needle-like crystals. This amphibole-chlorite phase can constitute modal % of the peridotite

72 Chapter: 4 Petrography Figure: 22 Petrography of the Hlagothi Complex with (a) and (c) showing the meta-peridotites in planepolarised light and cross-polarised light, (b) and (d) showing the meta-gabbros in plane-polarised light and cross-polarised light. Abbreviations are: ta talc, cl chlorite, cpx clinopyroxene, amp amphibole, mg magnetite and plg plagioclase feldspar The layers vary from olivine-rich to olivine-poor (10 to 40 modal % of the rock-type), with 1 to 5 mm subhedral to euhedral grains of olivine having broken down completely to talc or serpentine, and with magnetite rimming the original cumulus olivine on its boundaries and in cracks. Talc and serpentine can also be intergrown with each other. The original plagioclase feldspar is mostly preserved, making up less than 20 modal % of the sample, although it is highly sericitised and/or saussiritised. It is anhedral and occurs interstitially within the groundmass. Magnetite and chromite form the minor phases. Two different generations of magnetite and chromite are recognised: a minor generation of fine-grained magnetite in the core of former olivine cumulus crystals, and larger blebby grains of magnetite and chromite rimming the cumulus crystals. Magnetite can make up to 2 modal %

73 Chapter: 4 Petrography of the lithology, with chromite making up to 1 modal %. Orthoclase feldspar can sometimes be seen occurring in the groundmass and makes up less than 1 modal % of the lithology. XRD analysis recognised the presence of actinolite, clinochlore and lizardite/antigorite as the dominant amphibole, chlorite and serpentine respectively. SEM also revealed the presence of baddeleyite, ilmenite, pentlandite and biotite as accessory minerals in equal amounts. All samples have been chloritised and serpentinised. At the contact with the Nsuze Group quartzites, these peridotite bodies become completely altered into schists. These schists are composed of talc, chlorite, amphibole and serpentinite, in which all the relict textures have been completely obliterated. The finer talc, amphibole and chlorite are typically intergrown, and may reflect the original groundmass of ortho- and clinopyroxene, with irregular amounts of magnetite and chromite also present. The olivine is typically replaced by talc, serpentine and chlorite. In weathered outcrop, clay minerals are common. Between the peridotite and gabbro, small layers and lenses of rather homogeneous pyroxenite occur. Although most samples were highly altered through weathering, relict textures and minerals remain in fresher samples. These pyroxenites consist of ortho- and minor clinopyroxene (which make up almost 70 modal % of the lithology) with plagioclase feldspar consisting of up to 10% of the samples. The pyroxenites appear considerably less altered than the peridotite through metamorphism however, with well-preserved crystals and relict cleavage planes, particulary where amphibole has replaced pyroxene. However, larger euhedral orthopyroxenes have been completely replaced by amphibole. Finer-grained lath-like clinopyroxenes as well as equant and subhedral orthopyroxene are chloritised or uralitised. These amphiboles are commonly intergrown in the groundmass between the larger amphibole crystals. The orthopyroxene is in far greater abundance (60 modal %), and typically encloses the clinopyroxene (10 modal %), creating a poikilitic texture. Plagioclase feldspar grains show evidence of alteration, specifically sericitisation. Magnetite and ilmenite occur in small amounts, making up less than 2 modal % of the total lithology. XRD shows that the common amphiboles are actinolite and tremolite, with the presence of some primary pigeonite, hornblende and augite. Clinochlore is the common chlorite. SEM revealed chromite, orthoclase feldspar, apatite and baddeleyite. The gabbros makeup the bulk of the Hlagothi Complex and have experienced the most extensive alteration of the original pyroxenes and plagioclase feldspar. These minerals

74 Chapter: 4 Petrography have become, with rare exceptions, a quartz, plagioclase feldspar, amphibole, chlorite and epidote mineral assemblage (see Fig. 22). The dominant plagioclase feldspar is albite. Equant to elongate subhedral grains of amphibole are the primary alteration product of the original orthopyroxenes, making up between 40 and 50 modal % of the gabbroic phase and ranging between 1 and 15 mm in length. The amphibole is intergrown with chlorite, which can also occur as an alteration product of plagioclase feldspar as well as pyroxene. Anhedral plagioclase feldspar appears heavily sericitised, making up around 50 modal % of the samples, and usually occurring in the groundmass; zoned, mm, equant epidote has also replaced it, along with about 10 modal % quartz. Intergrowths of plagioclase feldspar also occur in the quartz. Biotite was also identified in association with the quartz. XRD showed the main amphibole to be actinolite and hornblende. The hornblende appears brown and euhedral, with grain sizes up to 2 mm. The plagioclase feldspar, when present, is labrodorite. Approximately 4 modal % microcline feldspar is present too. Opaque minerals are less common, and only about 1 modal % magnetite and ilmenite was noted. SEM did reveal that what little magnetite and chromite is present is titanium-rich. Chromite, apatite and baddeleyite occur in trace amounts. Groenewald (1984, 2006) noted very fine-grained rocks within the uppermost gabbro sheets, generally within 4 m of the upper contact. This fine-grained rock type is a chilled margin with the Nsuze Group quartzites; however, these were not observed in this study Dolerite Dykes All the dolerite dyke samples restricted to the Precambrian basement granitoid-greenstone terrane of the south-easternmost inliers of the Kaapvaal Craton generally show high concentrations of amphibole, chlorite and plagioclase feldspar in optical microscopy, which was confirmed by XRD analyses. This type of alteration is indicative of low to intermediate grades of metamorphism. It could also be possible syn-magmatic alteration through the interaction of dyke material along the contact with meteoric or magmatic fluids. Almost no talc was present in the dykes, which could suggest the presence of metamorphic fluids with less than 10% carbon dioxide. However, this type of alteration is very typical of olivine,

75 Chapter: 4 Petrography which is not present within any of the dykes. Most samples show sericitised and epidotised plagioclase feldspar intergrown with orthopyroxene that is replaced by amphibole (usually actinolite). Fine-grained marginal rocks are typically highly weathered, whereas the dyke centres are coarser, and much fresher. However, coarse-grained phenocrysts of plagioclase feldspar can sometimes be seen along the margins of the dykes. Xenoliths and xenocrysts from the country rock also tend to occur along margins of the dykes. Both phenocrysts and xenocrysts can provide anomalous geochemical results. Granophyric textures along these margins can also be common, with high amounts of interstitial quartz derived through partial melting of the granodiorite host rock SE-trending dolerite dykes Both possible generations of the SE-trending dolerite dykes are medium- to coarse-grained and variably metamorphosed, with one dolerite dyke, AG-Bc, relatively fresh and unaltered to a large extent (see Fig. 23). Both SE-trending dyke generations generally have lower greenschist facies mineral assemblages, making identification of the primary mineralogy more difficult. Most grains are greater than 0.5 mm in size. These dykes consist of plagioclase feldspar (40 modal %), amphibole (20-40 modal %) and chlorite (10-20 modal %). Other common minerals include a small amount of microcline feldspar and variety of opaques, such as magnetite, which constitutes up to 5 modal % of the samples. Anhedral plagioclase feldspar is variably sericitised and cloudy, with grain sizes ranging from mm. Amphibole can be fibrous, and is a pseudomorph after orthopyroxene, which has been uralitised and has grain sizes between 0.5 and 1.0 cm. The amphibole can be euhedral to subhedral in some cases. Amphibole is often enclosed by large poikilitic plagioclase feldspar, in which case primary orthopyroxene can be preserved. Chlorite is also common, and can form as a pseudomorph after clinopyroxene and plagioclase feldspar, forming small granular aggregates of flakes in the groundmass, which are rarely greater than 0.2 mm. It is usually intergrown with actinolite or plagioclase feldspar. Various amounts of interstitial quartz can occur, usually only up to 10 modal %, which may be granophyric, or may occur as intergrowths with plagioclase feldspar. Under reflected light and SEM, magnetite and ilmenite are the common opaque minerals

76 Chapter: 4 Petrography Figure: 23 Petrography of the SE-trending dolerite dykes with (a) and (c) showing dyke AG-Bc in planepolarised light and cross-polarised light, (b) and (d) showing dyke AG-J in plane-polarised light and crosspolarised light. Abbreviations are: cpx clinopyroxene, amp amphibole, mg magnetite, and plg plagioclase feldspar Opaques account for less than 10 modal % of the samples, and are usually less than 0.2 mm. They appear embayed with chlorite, with only skeletal crystals left. Small amounts of biotite, apatite, baddeleyite, serpentine and epidote have been noted in two of the dykes within the south-western areas. Indistinguishable material ranges from 10 to 30 modal % depending on how fine-grained the rocks are. XRD analysis confirmed actinolite and tremolite as the amphiboles present, clinochlore as the chlorite, and albite as the primary plagioclase feldspar

77 Chapter: 4 Petrography ENE-trending dolerite dykes All the dolerite dyke samples restricted to Precambrian basement show high concentrations of amphibole and chlorite in optical microscopy after pyroxene. This type of alteration is indicative of low grades of metamorphism. Almost all of the samples show sericitised plagioclase feldspar intergrown with pyroxene replaced by amphibole (usually actinolite). Transmitted and reflected light microscopy showed that the ENE-trending dykes are finegrained, with the mineralogy difficult to assess in thin section, with most grains usually being less than 0.5 mm in size. The minerals that were present showed a high degree of alteration to greenschist-facies mineral assemblages (see Fig. 24). Figure: 24 Petrography of the ENE-trending dolerite dykes with (a) and (c) showing dyke AG-Ba in plane- polarised light and cross-polarised light, (b) and (d) showing dyke AG-Cb in plane-polarised light and crosspolarised light. Abbreviation are: qz quartz, and ch chlorite

78 Chapter: 4 Petrography These dolerite dykes consist primarily of plagioclase feldspar (10-40 modal %), amphibole (20-40 modal %) and chlorite (10-20 modal %), as determined with the assistance of CIPW norms. Other minerals include quartz, microcline, calcite, apatite and various opaques. The plagioclase feldspar mostly has elongate- to lath-like crystal shapes with a cloudy appearance indicative of sericitisation, where visible. SEM imagery and EDS analysis showed that the plagioclase feldspar grains range in size from mm, and can be partially enclosed by amphibole. The amphibole present is mostly fibrous or acicular, and is almost certainly a pseudomorph after clinopyroxene, with grain sizes of less than 0.2 mm. Chlorite can also form as a pseudomorph after clinopyroxene and plagioclase feldspar. Chlorite forms small flakes (very seldom greater than 0.1 mm) within the plagioclase feldspar, or it forms large aggregates. Quartz occurs in quantities ranging from 10 to 30 modal % of quartz, and grain sizes from mm. From 5-10 modal % opaques also occur, which can be up to 0.2 mm. Indistinguishable material ranges from 10 to 50 modal %. Rarely there is calcite, sphene and apatite present. XRD analysis confirmed the presence of actinolite as the amphibole, with a smaller amount of hornblende, which could be a relict of the primary product, as well as clinochlore as the chlorite and albite as the plagioclase feldspar. SEM analysis found that the primary opaques include hematite or magnetite, ilmenite and rutile NE-trending dolerite dykes The first described generation (NE030 in chapter 3) of dolerite dykes in chapter 3 consists of the coarser-grained 030 NE-trending dykes (see Fig. 25a). Like the dolerite dykes documented above, these dykes consist primarily of plagioclase feldspar, amphibole and chlorite under optical microscopy, which have been altered during greenschist facies metamorphism. Grain sizes are coarse, with most grains being greater than 0.5 mm. Plagioclase feldspar phenocrysts, however, sometimes are greater than 1.0 cm. Plagioclase feldspar is often sericitised, and comprises between modal % of the dykes, and up to 60 modal % of the sample if phenocrysts are included outside of the groundmass. The phenocrysts are zoned, and show no obvious signs of preferred orientation. Amphibole is present in abundance. It is fibrous, comprising between modal % of the samples, with grain sizes usually less than 0.8 mm. It is usually a pseudomorph after clinopyroxene, which

79 Chapter: 4 Petrography was uralitised. Small amounts of chlorite are also evident, usually up to 10 modal % of the samples, with grain sizes typically at around 0.2 mm. It is a pseudomorph after clinopyroxene too. Quartz is rare; usually less than 5% of the sample, with grains up to 0.7 mm. Opaques can be up to modal % of the dykes, and have grain sizes of 0.4 mm. Some sulphide mineralisation has been noted, usually pyrite. Indistinguishable material is usually only about 5-10 modal %. XRD analysis confirmed the presence of actinolite as the amphibole present, as well as clinochlore as the chlorite and albite as the plagioclase feldspar. SEM analysis found that the primary opaques are ilmenite and titanium-rich magnetite and chromite. Figure: 25 Petrography of the NE-trending dolerite dykes with (a) and (c) showing dyke AG-A (NE030) in plane-polarised light and cross-polarised light, (b) and (d) showing dyke DY-02 (NE050) in plane-polarised light and cross -polarised light. Abbreviations are: plg plagioclase feldspar, amp amphibole and mg magnetite

80 Chapter: 4 Petrography The second generation of NE-trending dolerite dykes (NE050) is fine-grained and highly altered at greenschist facies, with most grains usually less than 0.2 mm in size. This makes it difficult to distinguish the original mineralogy (see Fig. 25b). Assisted by CIPW norms from whole rock geochemistry, XRD and SEM analytical work, these dykes consist primarily of plagioclase feldspar (10-40 modal %), amphibole (30-40 modal %), chlorite (5-20 modal %) and quartz (20-30 modal %). Other minerals include various opaques. The plagioclase feldspar mostly has a cloudy appearance due to sericitisation. It grain size ranges from 0.1 to 0.4 mm. Identifying the outline of the plagioclase from the surrounding groundmass proved difficult. One amphibole present is brown hornblende and appears to be primary, although it may be recrystallised. Other amphiboles are also present, but they tend to be green-brown and are probably pseudomorphs after pyroxenes. Grain sizes for amphibole are typically in the range of 0.2 mm. Chlorite forms as a pseudomorph after pyroxene and plagioclase feldspar. It forms small aggregates in the groundmass and is extremely fine-grained, being less than 0.1 mm in size. Large amounts of quartz have been distinguished in quantities ranging from 20 to 30 modal % of the sample. It is usually xenocrystic and intergrown with plagioclase feldspar. The quartz also usually shows undulatory extinction and appears to be recrystallised. Grain sizes vary from mm in size. The opaques are hard to distinguish, but are probably less than 10 modal % of the sample. Opaque grains are typically less than 0.1 mm. Indistinguishable material ranges from 20 to 50 modal %. XRD analysis confirmed the presence of actinolite and hornblende as the amphiboles present, pigeonite as one of the pyroxenes, clinochlore as the chlorite, and albite as the plagioclase feldspar. SEM analysis found that the opaques include magnetite, ilmenite and rutile

81 Chapter: 4 Petrography

82 Chapter: 5 Geochemistry Chapter: 5 Geochemistry 5.1. Methodology A total of 17 samples from across the Hlagothi Complex in the inliers from the Nkandla subbasin of the Pongola Supergroup, were selected to provide a representative spectrum across the various rock types for geochemistry. However, 6 of these samples provided anomalous results. In addition, 11 samples were taken from selected SE-, NE- and ENE-trending dolerite dykes. Sampling was done in such a way as to achieve homogeneity, and to avoid alteration. For the analyses, samples were cleaned and any weathered/altered areas not removed in the field were cut off using a diamond rock saw, paying particular attention to using the saw as sparingly as possible. The cut pieces were then cleaned thoroughly in an ultrasonic bath and oven-dried overnight. The samples were then crushed by hand, using a large mortar and pestle, which was cleaned and dried with both water and acetone respectively to limit possible contamination between each sample. The material was then sieved to collect only rock fragments 1 cm 3 in size, with no obvious evidence of being cut by the rock saw. The remainder of the material was discarded because it was either too small or bearing cut grooves or smears from the rock saw. This was done to limit contamination. The rock fragments were then cleaned and dried in the ultrasonic bath and oven overnight once again. This crushed material was then split and powdered for approximately 1 minute within a chrome-steel ring mill. Between samples, all equipment was thoroughly brushed, vacuumed, washed, cleaned and dried with water and then acetone. Between all samples, the ring mill processed pure quartz sand. Following this, an approximate amount of 10 g of sample was dispatched to ACME Laboratories in Vancouver, Canada (acmelab.com) for major and trace element analysis. Major and trace element contents for all representative whole-rock samples were determined using X-Ray Fluorescence Spectrometry (XRF) and Inductively Coupled Plasma Mass Spectrometry (ICPMS), respectively. XRF was done for major and minor elements on glass beads prepared from powdered whole-rock samples with a sample-to-flux ratio of 1:10. Volatiles were determined by loss on ignition. Precision

83 Chapter: 5 Geochemistry for the different elements was better than ± 1 % of the reported values. Trace and rare earth element (REE) compositions were determined using ICPMS. The samples were dissolved in a 1.5 g lithium meta/tetraborate flux fusion, and the resultant molten bead was then rapidly digested in a weak dilute nitric acid solution. Precision and accuracy based on replicate analysis of international rock standards were between 2 and 5% (2σ) for most elements and ± 10 % for U, Sr, Nd and Ni. The whole rock geochemical analyses for the SE-, NE-, and ENE-trending dolerite dykes and the Hlagothi Complex are presented in Table 2 and Table2 3; and additional geochemical data can be found in Appendix C Hlagothi Complex A total of eleven samples from across the Hlagothi Complex are presented here to characterise and provide a broad, representative spectrum of the various rock types of the complex for geochemistry. Similar fractionation histories for the different layered sills are likely, and therefore the combined data are considered to provide broad geochemical characteristics for the whole complex. The sampling is not considered adequate for petrogenetic modelling, and no sampling of the pyroxenites was done. Samples from the Wonderdraai and Mhlatuze sheets were omitted due to their uncertain relationships with the complex and/or anomalous behaviour through alteration from weathering and/or metamorphism. The whole rock geochemical analyses for the Hlagothi Complex are presented in Table 2. Available geochemical data presented by Groenewald (1984) and Hammerbeck (1982) for the Hlagothi Complex and Thole Complex is also shown in the diagrams Rock Alteration/Classification The alteration box plot of Large et al. (2001) demonstrates geochemical changes through alteration for the major igneous rock types see in the previous chapter (see Fig. 26). The

84 Chapter: 5 Geochemistry gabbros of the Hlagothi Complex fall within the least altered field of basalt and andesite. The pyroxenite and peridotite groups, however, show significant chloritisation. However, it must be noted that this alteration diagram of Large et al. (2001) was developed specifically for rhyolite, dacite, andesite and basalt rock types, and not ultramafic lithologies. This shows the limitation of this diagram for these lithologies. In addition, while some alkalis may have been leached from the Hlagothi Complex, all samples have maintained their original rocktype classifications in Winchester and Floyd (1977) classification diagrams using SiO 2, Nb/Y and Zr/TiO 2 (see Fig. 27). SiO 2 and Nb/Y appear to be the best discriminators between the rock types of the complex, with Zr/TiO 2 being the least useful. SiO 2 is the best for dividing the peridotitic from the pyroxenitic and gabbroic phases, with Nb/Y good for distinguishing between the pyroxenitic and gabbroic phases. This suggests that these rocks have not suffered secondary large ionic lithophile (LIL) element mobilisation due to a clear correct classification. Table: 2 Whole rock geochemical analyses on 11 samples from the Hlagothi Complex Major elements (wt. %) Trace elements (ppm) Sample HC-08 HC-09 AG-I_core AG-I_contact HC-01 HC-02 HC-03 HC-04 HC-05 HC-06 HC-07 SiO Al2O MnO CaO Na2O K2O Fe2O3T MgO TiO P2O Cr2O LOI Total Cs Rb Ba Th U Nb Ta La Ce Pb Pr Sr Nd Zr Hf Sm Eu Gd Tb Dy Ho Er Tm Yb Y Lu V

85 Chapter: 5 Geochemistry The three geochemical groups of the Hlagothi Complex can be identified, in agreement with the major element classification. Figure: 26 Alteration box plot of Large et al. (2001) for the Hlagothi Complex Complying with the recommendations of the IUGS sub commission of Le Bas et al. (1986), the majority of samples from the Hlagothi Complex broadly fall within two subdivisions of either basalts to picro-basalts and basaltic andesites on the total alkali-silica (TAS) classification diagram (see Fig. 28a), with the basaltic andesites further separated into two different groupings. Peridotites of the complex fall in the basalt to picro-basalt grouping. The basalt to picro-basaltic compositions from the more ultramafic phases of the complex contain <2 wt% Na 2 O+K 2 O and wt% SiO 2. The basaltic andesite group is represented by the more mafic phases of the complex containing approximately 1 wt% Na 2 O+K 2 O with wt% SiO 2 for the pyroxenites, 2-4 wt% Na 2 O+K 2 O and SiO 2 wt% for the gabbros. Geochemically, the peridotites and pyroxenites have well defined ranges whereas the gabbros are more variable. However, the limited number of samples from the literature for the pyroxenites must be noted

86 Chapter: 5 Geochemistry Figure: 27 Trace element classification diagrams of Winchester and Floyd (1977) for the Hlagothi Complex

87 Chapter: 5 Geochemistry Figure: 28 - Total alkali-silica classification diagram of Le Bas et al. (1986) for the Hlagothi and Thole complexes (a), and classification diagram showing the variation in K 2 O content with respect to SiO 2 (b). Total alkali-silica classification diagram for high-mg rocks (c) of Le Bas et al. (2000)

88 Chapter: 5 Geochemistry The gabbroic portions of the Hlagothi Complex have a medium-potassium content of wt% K 2 O (see Fig. 28b). The peridotitic and pyroxenitic portions show low-potassium content of <0.5 wt% K 2 O. Another TAS classification diagram of Le Bas (2000) is useful for high-mg rocks, and this diagram classifies the peridotites as komatiites due to the fact that the peridotites have more than 18 wt% MgO and less than 1 wt% TiO 2 (see Fig. 28c). The three geochemical groupings of the Hlagothi Complex are sub-alkaline and broadly tholeiitic (see Fig. 29a), with the gabbroic phase straddling the tholeiitic to calcalkaline boundary. The pyroxenitic and peridotitic phases, however, are more tholeiitic on the classification diagram of Irvine and Baragar (1971). The same trend is seen on the Jensen (1976) classification diagram, with the pyroxenitic and peridotitic phases both defined as komatiitic and the gabbroic phase as tholeiitic (see Fig. 29b). Figure: 29 AFM (a) and Jensen (b) classifications diagram of Irvine and Baragar (1971) and Jensen (1976) for the Hlagothi Complex

89 Chapter: 5 Geochemistry Magmatic Variation/Affinity Major and trace variation diagrams illustrate two distinct populations using Mg number (Mg#), with no obvious trend line existing between the two groupings for the Hlagothi Complex. The mafic- to ultramafic lithologies (peridotites and pyroxenites) can be defined as SiO 2 -poor and MgO-rich (high Mg#), containing wt% MgO, with a Mg# between The mafic to intermediate lithologies (gabbros) are more SiO 2 -rich and MgO-poor (low Mg#). This population contains between 5-10 wt% MgO (low Mg#), with the Mg# from SiO 2 content is wt% for the high Mg# group and between for the low Mg# group. Total Fe as Fe 2 O 3 is constant throughout the complex at between 5-10 wt%, as is MnO at between wt%. TiO 2 content is <0.5 wt% and P 2 O 5 <0.1 wt%. The low Mg# end members of the Hlagothi Complex are enriched in Al 2 O 3, CaO, Na 2 O and K 2 O at 10-15, 8-12, and wt respectively. The high Mg# end members are depleted at 5-10, 2-8, 0-1 and wt% respectively for these oxides (see Fig. 30). Ni and Cr are more enriched in the high Mg# portions of the Hlagothi Complex, with between ppm for Ni and ppm for Cr, as they are both compatible elements. It must be noted however that Cr contamination is likely from the steel ring mill during sample preparation. The low Mg# portions display between ppm for Ni and ppm for Cr. The low Mg# phases typically contain ppm V, ppm Y, ppm Zr, 3-6 ppm Nb, ppm Rb and ppm Sr. In contrast, the high Mg# phases have ppm V, Y, ppm Zr, 1-3 ppm Nb, 0-30 ppm Rb and 0-60 ppm for Sr. In addition, the low Mg# phases display high levels of variability, which the high Mg# phases do not (see Fig. 31). SiO 2, CaO and Ni can be used to distinguish between the peridotites and pyroxenites Further Characterisation/Tectonic Setting Using the multi-element trace geochemistry for the samples, spider diagrams were constructed and normalised using data from McDonough and Sun (1995) to either primitive mantle for the trace elements or C1 chondrite for the rare earth elements (see Fig. 32 and 33)

90 Chapter: 5 Geochemistry Figure: 30 Major element variation diagrams for the Hlagothi Complex

91 Chapter: 5 Geochemistry Figure: 31 Trace element variation diagrams for the Hlagothi Complex

92 Chapter: 5 Geochemistry All samples for the Hlagothi Complex exhibit an overall enriched multi-element pattern when normalised to primitive mantle, with more or less consistently negative Nb-Ta anomalies, a common feature of most mafic rocks on the Kaapvaal Craton (e.g., Duncan, 1987). For the Hlagothi Complex, all samples display enrichment in some of the large ion lithophile (LIL) elements relative to primitive mantle, and are depleted in the high field strength (HFS) elements relative to LIL elements. For the complex, two distinct populations can be seen for high Mg# and low Mg# samples. The low Mg# population shows more enrichment with respect to primitive mantle and C1 chondrite compared to the high Mg# samples, which are less enriched. Both populations show negative Nb-Ta anomalies. The low Mg# samples also display slight negative Pb, Sr, P, Eu and Ti anomalies, and the high Mg# samples have deep negative Ba and Sr anomalies and slightly negative Eu and Ti anomalies. They also have a slight positive Pb anomaly. Using the prior classification and discrimination of the Hlagothi Complex, two distinct geochemical groupings of the Hlagothi Complex can thus be further validated. Figure: 32 Trace element primitive mantle normalised spider diagram for the Hlagothi Complex (McDonough and Sun, 1995) The rare earth element (REE) patterns reflect the trends already seen above (see Fig. 33). For all the lithologies of the Hlagothi Complex there is a very slight enrichment of light REE relative to heavy REE. There is also a small negative Eu anomaly in both the low and

93 Chapter: 5 Geochemistry high Mg# samples. The REE pattern also shows a general enrichment of the REEs in the low Mg# samples relative to the high Mg# samples, with a relatively flat trend. This is illustrated with (La/Yb) N, with both high Mg# and low Mg# phases having values of between 2.8 and 4.3. Figure: 33 Rare earth element C1 chondrite normalised diagram for the Hlagothi Complex (McDonough and Sun, 1995) Tectonic setting discrimination diagrams using trace elements were also plotted for the Hlagothi Complex (see Fig. 34a, b and c). Both phases of the complex display an arc, plate margin or continental flood basalt-type signature using a variety of tectonic discrimination diagrams, suggestive of an enriched mantle signature beneath the Kaapvaal Craton or crustal contamination Dolerite Dykes In total, 4 samples of the SE-trending dolerite dykes were taken from the north-western as well as south-eastern portion of the White Mfolozi Archaean basement inliers. Another 3 samples of the ENE- and 4 from NE-trending dolerite dykes were also taken across the White Mfolozi Archaean basement inliers. Data from Klausen et al. (2010) and Maré and

94 Chapter: 5 Geochemistry Fourie (2012) was used for comparison from various SE-, NE- and ENE-trending dolerite dykes in the region. Figure: 34 Tectonic discrimination diagrams for the Hlagothi Complex. (a) Discrimination diagram of Zr/Y versus Ti/Y after Pearce and Gale (1977). (b) Mantle source discrimination diagram (Condie, 1997); DM depleted mantle; PM primitive mantle; HIMU high U/Pb mantle source; LC lower continental crust; UC upper continental crust. (c) Ternary discrimination diagram of Ti, Zr and Y after Pearce and Cann (1973). 1 within-plate basalts, 2 low-potassium tholeiite; 3 ocean floor basalt and 4 calc-alkaline basalt

95 Chapter: 5 Geochemistry The whole rock geochemical analyses of these dolerite dykes are presented in Table 3. Table: 3 Whole rock geochemical analyses on various trends of dolerite dykes in northern KwaZulu-Natal Major elements (wt. %) Trace elements (ppm) Trend SE-trend ENE-trend NE-trend Sample AG-J AG-Bc AG-K DY-01 AG-Ba AG-Bb AG-Cb AG-A DY-02m DY-02s AG-Ca SiO Al2O MnO CaO Na2O K2O Fe2O3T MgO TiO P2O Cr2O LOI Total Cs Rb Ba Th U Nb Ta La Ce Pb Pr Sr Nd Zr Hf Sm Eu Gd Tb Dy Ho Er Yb Y Lu Tm V Rock Alteration/Classification The alteration box plot of Large et al. (2010) enables distinction of various types of alteration, and is useful for rhyolites, dacites, andesites and basalts, as well as their intrusive equivalents, such as dolerite dykes. The dolerite dykes generally fall along the boundary between least altered and altered, with minimal alteration into the epidote and chlorite end members (see Fig. 35). Alkalis may have leached from the dolerite dykes, but the samples have maintained their original rock-type classifications in Winchester and Floyd (1977)

96 Chapter: 5 Geochemistry classification diagrams using SiO 2, Nb/Y and Zr/TiO 2. This suggests that the effects of weathering and metamorphism on the geochemistry of these dykes are minimal. Large variability in SiO 2 contents is in agreement with the major element classification for SE-, ENE- and NE-trending dolerite dykes from basalt to dacite. Nb/Y distinguishes the dykes as alkali or sub-alkali basalts. Zr/TiO 2 distinguishes the dykes as andesitic basalt to dacite, as well as alkali, demonstrating the weakness in using Zr/TiO 2 in classification of the dolerite dykes (see Fig. 36). Figure: 35 Alteration box plot of Large et al. (2001) for the various dolerite dyke trends From the IUGS recommendations (Le Bas et al., 1986), the NE-, SE- and ENE-trending dolerite dykes show increasing levels of variability with respect to SiO 2 content. NE-trending dolerite dykes show low variability, and are generally composed of basalts with between wt% SiO 2, and 2-4 wt% Na 2 O+K 2 O. ENE-trending dolerite dykes are highly variable, with basalt to andesite compositions of wt% SiO 2 and 2-6 wt% Na 2 O+K 2 O. The SE-trending dolerites are intermediate in variability between NE- and ENE-trending dolerite dykes

97 Chapter: 5 Geochemistry Figure: 36 Trace element classification diagrams of Winchester and Floyd (1977) for the dolerite dykes of northern KwaZulu-Natal compared to published data

98 Chapter: 5 Geochemistry Figure: 37 - Total alkali-silica classification diagram of Le Bas et al. (1986) for the dolerite dykes (a), and classification diagram showing the variation in K 2 O content with respect to SiO 2 after Le Bas et al. (2000), (b)

99 Chapter: 5 Geochemistry However, they are generally composed of basalts to basaltic andesites, and contain between 2-4 wt% Na 2 O+K 2 O, as well as wt% SiO 2 respectively on the TAS classification plot (see Fig. 37a). In addition, the SE-trending dolerite dykes can be shown to have medium-potassium content (see Fig. 37b), having between wt% K 2 O. The NE- and ENE-trending dolerite dykes also have medium-potassium content between as well as between wt% K 2 O for the NE- and ENE-trending dolerite dykes respectively. However, two generations can be seen in NE-trending dolerite dykes in northern KwaZulu- Natal: one of lower and one of higher potassium content, corresponding to the NE030 and NE050 dolerite dykes respectively. The generation of low-k conforms to the variably feldspar-phyric dykes of Klausen et al. (2010). In addition, although most dolerite dykes are not high-mg (greater than 18 wt% MgO), there is a proportion of SE- and ENE-trending dolerite dykes across the south-eastern and south-easternmost areas that are boninitic (greater than 8 wt% MgO and less than 0.5 wt% TiO 2 ). Klausen et al. (2010) illustrated by way of an AFM diagram that most of the andesitic dykes on the eastern basement of the Kaapvaal Craton were correspondingly calc-alkaline, whereas basaltic dykes are tholeiitic. Results here are similar (see Fig. 38a). All dykes are also sub-alkaline in northern KwaZulu-Natal. There appears to be a large grouping of SEtrending dykes with a trend that straddles the tholeiitic to calc-alkaline borderline, although the dykes in this study in northern KwaZulu-Natal are all tholeiitic. The ENE-trending dykes are clearly borderline tholeiitic to calc-alkaline, and can be compared with the Rykoppies dyke swarm and the Barberton-Badplaas dyke swarm seen further to the north, both of which are Archaean. NE-trending dolerite dykes can further be seen in two categories, a clear tholeiitic trend like the other NE-trending dykes on the eastern Kaapvaal Craton, and a more tholeiitic to calc-alkaline trend only seen in northern KwaZulu-Natal, comparable to SE- and ENE-trending dolerite dykes. The first group can be compared to the ca Ga NEtrending Palaeoproterozoic dykes observed further to the north in the Black Hills dyke swarm according to Klausen et al. (2010). Using the Jensen plot, these trends are reaffirmed. The SE-trending dykes showing tholeiitic affinity, different from tholeiitic to calcalkaline dykes seen further to the north, with some komatiitic affinity too in both regions. ENE-trending dykes as above fall between the tholeiitic and calc-alkaline field, with the NEtrending dykes showing tholeiitic affinity only. This illustrates that there is clearly a greater

100 Chapter: 5 Geochemistry complexity to the mafic dyke swarms seen in the northern KwaZulu-Natal window of the south-easternmost Kaapvaal Craton (see Fig. 38b). Figure: 38 AFM (a) and Jensen (b) classifications diagram of Irvine and Baragar (1971) and Jensen (1976) for the dolerite dykes of northern KwaZulu-Natal compared to dolerite dykes on the eastern Kaapvaal Craton Using only the major and trace element classification diagrams, the SE- and ENEtrending dolerite dykes do not deviate from the consistent separation from the ca Ga SE-trending and radiating ca Ga NE-, E- to SE-trending dolerite dykes of Olsson et al. (2010) and Klausen et al. (2010) seen further to the north. This is true also within northern KwaZulu-Natal. Herein however, two groups of the NE-trending dykes can be identified conclusively, in agreement with the major element classification, and geology seen within chapter 3. One of these geochemical groups is not represented further to the north in comparison with the data set of Klausen et al. (2010) and Maré and Fourie (2012), however Magmatic Variation/Affinity

101 Chapter: 5 Geochemistry Major and trace variation diagrams were constructed for the different trending dolerite dykes using Mg# and compared to data obtained by Klausen et al. (2010) and Maré and Fourie (2012) for the same trending dolerite dyke swarms (see Fig. 39 and 40). Almost all the dykes vary between a Mg# of The dolerite dykes seen within northern KwaZulu- Natal in this study and within Klausen et al. (2010) fall between 30-50, making them far less variable than further to the north. Mg# is lowest in the NE- and ENE-trending dolerite dykes, with values from and in these dykes respectively. Mg# in SE-trending dolerite dykes varies from Fe is more enriched in the NE-trending dolerite dykes and depleted in the SE- and ENE-trending dolerite dykes, with 15, 12 and 9 wt% FeO+Fe 2 O 3 respectively. The opposite is true of SE- and ENE-trending dolerite dykes, and particulary for SE-trending dolerite dykes compared to NE-trending dolerite dykes, with values of MgO wt% of 10, 9 and 7 respectively. SiO 2 content is wt% for all the dolerite dykes in the region, with ENE-trending dolerite dykes having up to 56 wt%, and NE-trending dolerite dykes the lowest, at wt%. MnO varies between wt%. One NE-trending dolerite dyke generation is clearly separated from the other in MnO, however. TiO 2 content is in the range from wt%, with NE-trends being more enriched, and ENE-trends more depleted in TiO 2. P 2 O 5 contents of the dykes are usually <0.2 wt%, except the one generation (NE030) with P 2 O 5 contents up to Al 2 O 3 varies between wt%. Alkali wt% varies between 2-3 for Na 2 O, and for K 2 O. Na 2 O wt% in ENE-trending dykes can be as high as 4 wt%. CaO varies between 6-12 wt% (see Fig. 39). Ni and Cr contents are <400 and <1000 ppm respectively for all trends within northern KwaZulu-Natal. V varies from ppm, Y from ppm and Zr contents vary from between ppm. Nb, Rb and Sr vary widely, with two generations of NEtrending dolerite dykes being evident. Nb concentrations are generally 3 ppm for all dykes, with some NE- and SE-trends varying up to 9-20 ppm. The same geochemical grouping is seen in Sr, with most SE-trends and NE-trending dolerite dykes at 200 ppm and ENE-trends at 380 ppm. The one geochemical grouping of NE dolerite dykes (NE050) can be up to between ppm Sr. Rb varies widely from ppm (see Fig. 40). ENE-trending dolerite dykes can be distinguished from the other dykes by their Na 2 O content, and the one generation of NE-trending dykes (NE050) by their P 2 O 5 and Sr contents. SE-trending dolerite dykes appear to vary the most, and NE-trending dykes the least. However most dykes in

102 Chapter: 5 Geochemistry northern KwaZulu-Natal do not show the same level of variability as seen in dolerite dykes across the wider eastern basement of the Kaapvaal Craton

103 Chapter: 5 Geochemistry Figure: 39 Major element variation diagrams for the dolerite dykes

104 Chapter: 5 Geochemistry Figure: 40 Trace element variation diagrams for the Hlagothi Complex

105 Chapter: 5 Geochemistry Further Characterisation/Tectonic Setting Using the multi-element trace geochemistry for the dolerite dyke samples, variation diagrams were constructed and normalised to either primitive mantle for the trace elements or C1 chondrite for the rare earth elements (McDonough and Sun (1995). All samples from the SE- and ENE-trending dolerite dykes exhibit overall enriched multielemental patterns with more or less consistently negative Nb-Ta anomalies (see Fig. 41), which as noted before is a common feature of most mafic rocks on the Kaapvaal Craton according to Duncan (1987). Using the prior classification and discrimination of the SE- and ENE-trending dykes, the SE- and ENE-trending dykes are more LIL element enriched to primitive mantle, with HFS elements less so, enabling comparison to the low Mg# samples of the Hlagothi Complex. They also have distinct negative P and Ti anomalies, as well as having generally higher positive Pb anomalies, and some slight positive K anomalies. This makes the ENE-dolerite dykes essentially the same geochemically as SE-trending dolerite dykes within the south-easternmost inliers of northern KwaZulu-Natal, as well as with SE- and E-trending dykes seen within the Barberton-Badplaas and Rykoppies areas by Klausen et al. (2010). However, ENE-trending dolerite dykes do have a slight negative Ba anomaly and a more pronounced positive Pb anomaly. U and Th contents are also slightly higher. Not all NE-trending dolerite dykes have consistently negative Nb-Ta anomalies however. The two generations of NE-trending dykes stated already in prior chapters using the classification and affinity diagrams can be further validated. The two generations of NE-trending dykes exhibit unusual patterns, with two very distinct chemical groupings. One grouping (NE050) is much more LIL and HFS element enriched than the other, and its pattern is comparable with both SE- and ENE-trending dykes. It, however, has a pronounced positive Ba anomaly and negative Pb and Ti anomalies. The second grouping (NE030) is characterised by very low Th and U concentrations and no apparent negative Nb-Ta anomalies. These dykes are also marked by positive Pb and K anomalies that can correlate with their feldspar contents. Both these trends are different from observed NE-trending dolerite dykes to the north in the Black Hills area by Klausen et al. (2010). The REE patterns reflect the trends already seen above (see Fig. 42). For the SE- and ENE-trending dolerite dykes there is a slight enrichment of light REE (LREE) relative to heavy REE (HREE)

106 Chapter: 5 Geochemistry Figure: 41 Trace element primitive mantle normalised spider diagrams for the dolerite dykes of northern KwaZulu-Natal (McDonough and Sun, 1995). Grey areas denote data from Klausen et al. (2010) for the area. REE patterns are also relatively smooth, with no obvious anomalies. For the NE-trending dolerite dykes, both groupings are enriched relative to C1 chondrite, however, one population is much more enriched than the other. The one generation of NE-trending dykes (NE030) is also distinct with a flat trend, and no enrichment of LREE compared to HREE as is

107 Chapter: 5 Geochemistry observed in the other trend (NE050), which is highly enriched in LREE compared to HREE. This gives dolerite dykes with the NE030 trend a very primitive signature. Figure: 42 Rare earth element C1 chondrite normalised diagrams (McDonough and Sun, 1995) for the various dolerite dykes. Grey areas denote data from Klausen et al. (2010) for northern KwaZulu-Natal It may also be possible to relate some of this depletion to the common abundance of large and highly segregated feldspar phenocrysts, according to Klausen et al. (2010). Thus, feldspar-rich samples exhibit the lowest element concentrations around some of the most positive anomalies for elements that preferentially partition into feldspars (i.e., Rb, K, Pb and Sr)

108 Chapter: 5 Geochemistry Figure: 43 Tectonic discrimination diagrams for the dolerite dyke swarms. (a) Discrimination diagram of Zr/Y versus Ti/Y after Pearce and Gale (1977). (b) Mantle source discrimination diagram (Condie, 1997); DM depleted mantle; PM primitive mantle; HIMU high U/Pb mantle source; LC lower continental crust; UC upper continental crust. (c) Ternary discrimination diagram of Ti, Zr and Y after Pearce and Cann (1973). 1 within-plate basalts, 2 low-potassium tholeiite; 3 ocean floor basalt and 4 calc-alkaline basalt However, Klausen et al. (2010) noted on the other hand, that one feldspar-barren sample exhibited the highest element concentrations around a negative Sr anomaly. These element

109 Chapter: 5 Geochemistry patterns correlate roughly with moderate phenocryst contents are consistent with flowsegregated bands of feldspars in the NE030 generation of NE-trending dykes. The (La/Yb) N ratio shows that SE-trending dolerite dykes are extremely variable, with values of between 3.8 and 10.0, whereas ENE-trending dykes vary between 3.0 and 7.7. The NE-trending dolerite dykes show the greatest variability, with the one generation (NE050) between and 35.0, whereas the NE030 generation is at 1.4. Tectonic discrimination diagrams using trace elements were plotted for the SE-, ENE- and NE-trending dyke swarms (see Fig. 43). The SE- and ENE-trending dykes display arc-, plate margin or continental flood basalt-type signature, suggestive of an enriched mantle signature beneath the Kaapvaal Craton or crustal contamination. The flatter REE-patterns of the one generation of NE-trending dykes (NE030) may have been inherited from a more primitive mantle source and show some affinities to oceanic plateau basalt. The other generation of NE-trending dykes (NE050), however, shows an arc signature, as was seen to be common in the other dykes too

110 Chapter: 6 Geochronology Chapter: 6 Geochronology 6.1. Introduction In this chapter, preliminary Ar-Ar amphibole ages for NE- and SE-trending dykes are presented. In addition, precise U-Pb baddeleyite emplacement ages for the Hlagothi Complex as well as NE- and SE-trending dolerite dykes were obtained respectively. The SEtrending swarm is herein referred to as the Hlagothi Dyke Swarm. The NE-trending swarm is tentatively compared to the Neoarchaean Rykoppies Dyke Swarm seen further to the north by Olsson et al. (2010). These results require a re-evaluation of previously proposed linkages to volcanic and tectonic events within the relatively well-constrained Mesoarchaean to Palaeoproterozoic South African stratigraphy (e.g., Johnson et al., 2006) Ar-Ar Methodology A total of three samples were selected for 40 Ar/ 39 Ar geochronology. They were crushed and milled to fine sand, with finer sediment remaining in solution removed through water-based separation. Hand-picked fresh amphibole separates, usually composite grains (5 10 grains for step-heating experiments, defined below), were carefully selected under a binocular microscope. The samples were irradiated for 20 hours in the NTP s Safari1 nuclear reactor at NECSA s Pelindaba facility in South Africa. It was run at 20 MW, and this procedure typically produced a J-factor of It was run in position B2W along with standards hornblende Hb3gr as well as McCure Mountains hornblende (MMHb). The total neutron flux density during irradiation was neutrons/cm 2. The estimated error bar on the corresponding 40 Ar/ 39 ArK ratio is ± 0.2 % (1σ) in the volume where the samples were included. After 3 weeks of "cooling", samples were placed in pits in an aluminium disc and evacuated in an UHV sample port with a quartz window. Small clusters of amphibole (for step-heating

111 Chapter: 6 Geochronology experiments) using a defocused beam from a continuous Nd-YAG laser beam (1064 nm), and isotopic measurements of 40 Ar, 39 Ar, 38 Ar, 37 Ar and 36 Ar were measured in sequence, usually in 7 cycles, on a single collector MAP mass spectrometer with electron multiplier used in analogue mode. Step-heating experiments on amphibole bulk samples were performed with a double-vacuum high-frequency furnace. The mass spectrometer is composed of a 120 M.A.S.S.E. tube, a Baur Signer GS 98 source and a Balzers electron multiplier. The three samples were heated only with a few steps, with the aim of discriminating between Jurassic and Precambrian dykes. The usual criteria according to Jourdan et al. (2004) used to define a plateau age are: At least 70 % of the 39 Ar released. A minimum of three successive steps in the plateau. The integrated age of the plateau should agree with each apparent age of the plateau within a two sigma confidence interval (2σ). Plateau and integrated ages are given at the 2σ level, but individual apparent ages are given at 1σ level. The uncertainties on the 40 Ar/ 39 Ar ratios of the monitors are included in the calculation of the integrated and plateau age uncertainties, but the error on the age of the monitor is not included in the calculation. Data acquisition and reduction were done using in-house software. Blank correction was routinely done and signals were extrapolated to the time of gas admission from the mass spectrometer. Errors were propagated by a Monte Carlo procedure Ar-Ar Result(s) Analytical Ar-Ar data are shown in Table 4, and the results are presented graphically (see Fig. 44), with ages reflecting the uncertainties in Ar decay constants shown

112 Chapter: 6 Geochronology Table: 4 Ar-Ar amphibole data for SE- and NE-trending dolerite dykes DY02 Step Name % 39Ar Age Ma ± 2σ % err Ca/K ± 2σ % err Pseudo-Plateau age DY Pseudo-Plateau age AG-A Pseudo-Plateau age In the step-heating experiments, a first step was performed to degas atmospheric and alteration phase argon. The second and following steps (including fusion) represent 80% to 90% of the radiogenic 40 Ar released. The three samples yielded poorly defined second- and third-step ages of between Ma for DY-02 and Ma for AG-A, respectively. However, the second- and third-step ages of DY-01 produced ages of 1440 Ma and 1538 Ma, corresponding to the rather flat Gaussian curve in an age probability density distribution diagram with a peak age at 1623 Ma. The biggest plateau age step for DY-02 and AG-A was 907 Ma and 1319 Ma, which released 44 and 45% 39 Ar gas respectively, which corresponds to the fourth step-heating age. The 1623 Ma age plateau for sample DY-01 produced 47% of 39 Ar gas released in the sample. The plateau ages were: 1596 ± 16 Ma for DY-01, 831 ± 30 for DY-02 and 1388 ± 18 Ma for AG-A

113 Chapter: 6 Geochronology Figure: Ar/ 39 Ar ratio spectra from dolerite dyke amphibole separates

114 Chapter: 6 Geochronology The Ca/K ratio spectra associated with the plateau ages display relatively irregular and disturbed patterns, with values mainly ranging from 8-12 for DY-01, 6-26 for DY-02 and for AG-A, highlighting the role of alteration phases in these ages. The results obtained on the samples by step-heating experiments display Proterozoic ages. Results are difficult to interpret because it appears that the very variable apparent ages may be the result of alteration (clearly visible on the analysed samples that were amphibole grains probably derived from pyroxenes during greenschist facies metamorphism, accounting for the variable Ca/K ratios and probably some excess argon). It is therefore not possible to attribute precise emplacement ages to these Proterozoic samples, with the exception that the ages may reflect overprinting U-Pb Methodology Baddeleyite was extracted from rock samples weighing between 1-2 kg. The extraction was performed at Lund University in Sweden, using the water-based technique of Söderlund and Johansson (2002). Approximately baddeleyite grains were extracted from each sample and between 1-6 grains were combined in each fraction analysed. U-Pb chemistry and mass spectrometry were performed at the Laboratory of Isotope Geology at the Swedish Museum of Natural History in Stockholm. Upon separation, the baddeleyite grains were transferred to Teflon capsules and washed in 2-3 M HNO 3 on a hot plate and repeatedly rinsed with distilled and deionised H 2 O. A mixture of HF and HNO 3 was then added to the capsules. The baddeleyite grains were then completely dissolved after 24 hours under high pressure and temperature ( ~ 210 C). After dissolution, samples were dried. A small amount of a 205 Pb U spike was added together with a 3.1 M HCl solution and re-dissolved before being loaded on 50 μl columns filled with a pre-washed anion exchange resin (Bio-Rad Mesh chloride). The Zr-Hf-REE cut was washed out with 3.1 M HCl solution and disposed of. U and Pb were washed out with H 2 O and collected in the same Teflon capsules as used for the dissolution. A small amount of H 3 PO 4 was added to each capsule and the samples were put on a hot plate to evaporate overnight. U and Pb were

115 Chapter: 6 Geochronology loaded on the same single Re filament together with a small amount of silica gel produced from the recipe of Gerstenberger and Haase (1997). Baddeleyite fractions for samples AG-A, AG-B and AG-I were analysed by mass spectrometry on a Thermo Finnigan Triton thermal ionisation multi-collector mass spectrometer. The intensities of 208 Pb, 207 Pb, 206 Pb and 205 Pb were measured using Faraday collectors whereas the intensity of 204 Pb was measured simultaneously in an ETP SEM equipped with an RPQ filter. SEM to Faraday gain was controlled by measuring a ~ 5-10 mv signal between runs. The Pb isotopic measurements were performed at filament temperatures in the C range. Isotopes of U were measured subsequently in dynamic mode on the SEM at filament temperatures exceeding 1350 C. Data reduction was performed using an in-house program written in Microsoft Excel (Per-Olof Persson, Museum of Natural History, Stockholm) with calculations from Ludwig (1991). Analytical results and regressions have been calculated and plotted using the Excel Macro Isoplot (Ludwig, 2003); decay constants for 238 U and 235 U follow those of Jaffey et al. (1971). All errors in age and isotopic ratios are quoted at the 95% confidence level. Procedural blanks were 2 pg Pb and 0.2 pg U, and the initial Pb isotope compositions were corrected using the model compositions of Stacey and Kramers (1975) at 2960 Ma, 2700Ma and 2060 Ma U-Pb Result(s) Analytical U-Pb data are shown in Table 5, and the results are presented graphically (see Fig. 45), with ages reflecting the uncertainties in U decay constants shown Hlagothi Complex Sample AG-I was collected from an approximately 200m thick layered sill along the Nsongeni River. The sill has been identified as one of the type examples of the Hlagothi Complex, and is a massive, medium-grained, green-grey meta-peridotite (Groenewald, 2006)

116 Chapter: 6 Geochronology Figure: 45 U-Pb Concordia diagrams showing results from the peridotites of the Hlagothi Complex (AG-I) and dolerite dykes of the Hlagothi Dyke Swarm (AG-B) and Rykoppies Dyke Swarm (AG-A)

117 Chapter: 6 Geochronology Table: 5 U-Pb baddeleyite data for the Hlagothi Complex, as well as SE- and NE-trending dolerite dyke Analysis no. U/Th Pbc/ 206 Pb/ 207 Pb/ ± 2σ 206 Pb/ ± 2σ 207 Pb/ 206 Pb/ 207 Pb/ ± 2σ Concordance (number of grains) Pbtot Pb 235 U % err 238 U % err 235 U 238 U 206 Pb % err raw 2 [corrected] 3 [age, Ma] Sample AG-A, S, E Bd-a (3 grains) Bd-b (3 grains) Sample AG-B, E, S Bd-a (3 grains) Bd-b (1 grain) Bd-c (2 grains) Bd-d (5 grains) Bd-e (6 grains) Bd-f (2 grains) Bd-g (1 grain) Sample AG-I, E, S Bd-a (3 grains) Bd-b (5 grains) Bd-c (3 grains) ) Pbc = common Pb; Pbtot = total Pb (radiogenic + blank + initial). 2) measured ratio, corrected for fractionation and spike. 3) isotopic ratios corrected for fractionation (0.1% per amu for Pb), spike contribution, blank (0.5 pg Pb and 0.05 pg U), and initial common Pb. Initial common Pb corrected with isotopic compositions from the model of Stacey and Kramers (1975) at 2875 Ma Sample AG-I yielded only 15 yellow-brown and tabular baddeleyite grains with lengths of approximately 30 to 60 μm. This population of baddeleyite had definite traces of secondary alteration, seen as frosty surfaces. The most transparent grains were preferentially selected for the analyses. Regression comprising 3 fractions yielded an age of 2866 ± 2 Ma (MSWD = 0.040) which is interpreted as the crystallisation age. The lower intercept is 122 ± 69 Ma Hlagothi Dyke Swarm AG-B was sampled in a river bed of exposed Mvunyana granodiorite 100m to the west of the R34 road between Melmoth and Vryheid. It was collected from a massive, dark grey, coarsegrained and sub-vertical 20m wide SE-trending dolerite dyke This SE-trending dyke has a well-exposed strike length of ~ 1 km along the river, and is up to 20m wide. The country rocks are the basement granitoids of the White Mfolozi Inlier. Mineral separation yielded 50 brown and tabular-shaped baddeleyite crystals, some with frosty surfaces (presumably due to partial replacement of baddeleyite with zircon). Typical lengths of the crystals varied from μm. Regression through seven analyses of AG-B yields an upper intercept age of 2874 ± 2 Ma (MSWD = 3.2), which is interpreted as the crystallization age of this sample, and a lower intercept of 0 ± 100 Ma, which was a forced regression. Two single-grain

118 Chapter: 6 Geochronology fractions were analysed without U-Pb column chemistry, in order to obtain more concordant data and to minimise the blank contamination Rykoppies Dyke Swarm Sample AG-A was collected from the NE-trending dyke, which has a well-exposed strike length of approximately 11 km and a width of m. Baddeleyite extraction resulted in approximately 100 dark-brown and wafer-shaped grains with an overall length of μm. Regression yields an upper intercept age of 2652 ± 11 Ma and a lower intercept of 104 ± 43 Ma. The sample had very low counts of U and Pb, and therefore only two fractions were analysed. It was decided at the time not to continue with further analysis, and the relatively high analytical uncertainty. This cannot be regarded at this stage as a rigorous emplacement age, and is only used here as a preliminary result

119 Chapter: 6 Geochronology

120 Chapter: 7 Palaeomagnetism Chapter: 7 Palaeomagnetism 7.1. Introduction A palaeomagnetic study has been conducted on the various inliers of the Hlagothi Complex, as well as on a variety of SE-, ENE- and NE-trending dolerite dyke swarms across the southeastern Kaapvaal Craton, thought to represent swarms with ages of ca. 2.95, 2.65 and 1.90 Ga (Lubnina et al., 2010). However, from the previous chapter, it is seen that there are ca Ga SE-trending dykes in the area, and possibly ca NE-trending dolerite dykes as well. In the Nkandla inliers, between six and eight orientated samples were collected from the Hlagothi Complex, in addition to one to three orientated samples from the basal ca Ga Nsuze Group quartzite host rock (Pongola Supergroup). Intrusive contacts were sampled for a baked contact test. Between six and fourteen oriented samples were collected from each dolerite sill or dyke site. In addition to this, four to eight samples were collected from a host rock intruded by these dykes or sills, as well as the contact region. Most often the host rock was an undifferentiated granitoid of either Palaeoarchaean or Mesoarchaean age. Tectonic disruptions during Gondwana break-up are not thought to have significantly rotated any of the sample sites because rift-related deformation was concentrated along the African continent s passive margins according to Klausen (2009). Also, Karoo Supergroup sedimentary rocks display minimal dips, and dip very gently away from these margins and across most of the southern part of the eastern Kaapvaal Craton

121 Chapter: 7 Palaeomagnetism 7.2. Methodology The samples were collected using a water-cooled, portable, petrol drill. Most samples were oriented with both a magnetic and sun compass. There were no significant differences between sun and magnetic compass orientation measurements. The oriented drill core samples were cut into standard cylinders of ~ 2 cm 3 in size. Measurements were completed at the palaeomagnetic laboratory of the University of Johannesburg, South Africa. Remanence measurements were performed using a 2G Enterprises 755-4K Superconducting Cryogenic Rock Magnetometer. Samples were progressively demagnetised using alternating field (AF) and thermal demagnetisation methods. Demagnetisation generally consisted of between 15 to 30 steps. Four AF steps were done in 2.5 mt steps from mt, followed by 22 systematically decreasing incremental temperature steps from C. For selected dolerite dyke specimens, 20 AF steps were done between 2-85 mt. AF demagnetisation was done using a tumbling demagnetiser, while thermal demagnetisation was achieved with stepwise heating within an ASC model TD48 shielded oven. NRM components were visually identified by using stereographic and orthogonal projections (Zijderveld, 1967). The directions of components were quantified via principal components through least-squares analyses (Kirschvink, 1980), which is based in all cases on at least three or more vector endpoints. Only components with a maximum angular deviation (MAD) less than 10 were accepted for further interpretation. Mean directions were calculated according to Fisher (1953). These calculations and a graphic representation of the results were carried out using software by Jones (2002) and Cogné (2003) The Hlagothi Complex A palaeomagnetic study has been applied to various inliers and lithologies of the Hlagothi Complex. A total of 90 oriented samples were collected from 12 sampling sites from the Hlagothi Complex as well as from possible eastward extensions of it along the Mhlatuze

122 Chapter: 7 Palaeomagnetism River. Samples AG-I, HC-01, HC-03, HC-05 and HC-07 were gathered from the Nsongeni sheets, and sample HC-08 from the Hlagothi sheets. A sample from the Wonderdraai sheets was also collected, HC-10. In addition, samples (AG-D, AG-E, AG-F, AG-G and AG-H) were taken from the possible eastward extension of the complex along the Mhlatuze River (Mhlatuze sheets). Samples from the Mhlatuze sheets of the complex were omitted due to erratic behaviour during demagnetisation, possibly through alteration or metamorphism present in the inlier, as well uncertain structural relationships. A regional tilt correction was applied to samples from the Nsongeni sheets, where a strike and dip was apparent, particulary at the contact with the Nsuze Group quartzites. This was measured from outcrop as 256 /13. No tilt correction was applied to the Hlagothi or Wonderdraai sheets. Table: 6 Palaeomagnetic data for the Hlagothi Complex Component Site Lithology Site location Present coordinates Tilt-corrected coordinates Long Decl. Incl. α95 Decl. Incl n/n L/P k k (in E) (in ) (in ) (in ) (in ) (in ) Lat (in N) PLF AG-I peridotite, sheared peridotite, /14 14/ baked quartzite HC-01 peridotite /8 8/ HC-03 pyroxenite /6 6/ HC-05 peridotite /5 5/ A HC-07 peridotite /11 3/ HC-08 gabbro /6 3/ HC-10 diorite /6 5/ Component A mean = B HC-03 pyroxenite /6 1/ HC-01 peridotite /8 8/ HC-05 peridotite /6 6/ HC-07 sheared peridotite, baked /11 6/ C quartzite HC-08 gabbro /6 0/ AG-I sheared peridotite, baked quartzite /14 5/ Component C mean = D AG-I peridotite /11 8/ HC-07 peridotite /11 2/ n/n = number of samples included/number analysed, L/P = line verses plane least squares analyses, Decl. = mean declination in degrees, Incl. = mean inclination in in degrees, k = Fisher s precision parameter, except when in modified form for where both line and plane data were combined and α95 = radius of 95% confidence cone about the mean PLF is a present local field component. Components 'A' and 'B' are overprint directions associated with post-pongola granitic intrusions between 2850 and 2650 Ma. Component 'C' is related to an overprint from the Meso- to Neoproterozoic Namaqua-Natal orogen, whereas component 'D' is taken as the primary 2866 Ma direction. Underlined values represent data used to calculate component means, which are given in bold letters. α95 (in )

123 Chapter: 7 Palaeomagnetism At each site, between six and eight oriented samples were collected from the Hlagothi Complex. In addition, one to three orientated samples from the basal ca Ga Nsuze Group quartzite host rock (Pongola Supergroup) into which the Hlagothi Complex intrudes was sampled for a baked contact test. The schistose contact, where present, was also sampled. The palaeomagnetic results from the Hlagothi Complex are summarised in Table 6. Examples of the demagnetisation behaviour and summaries of the remanent magnetisation are also shown (Fig. 46, 47 and 48). The meta-peridotites returned good intra-site reproducibility, whereas samples from the meta-gabbro sites behaved more erratically during demagnetisation. Apart from spurious low coercivity magnetic components, five stable and geologically significant magnetic components (i.e., PLF, A, B, C and D) were identified. Each sample recorded a maximum of two of these components. During AF demagnetisation and thermal demagnetisation steps up to 530 C, peridotite samples from site AG-I revealed steep northerly magnetic components with negative inclination, which is parallel to the present local geomagnetic field at the sampling site. This component was labelled PLF (i.e., present local field). The most widespread component (i.e., identified within peridotite, gabbro and diorite samples) is a steep easterly magnetic component with negative inclination. This component, labelled A, was variably demagnetised during AF and thermal demagnetisation steps between 200 C and 515 C at six sampling sites. Following the removal of either PLF or A components, three characteristic remanent magnetisations (i.e., B, C or D) were identified in the various lithologies via thermal demagnetisation up to 580 C, but the signal of most samples dropped below the noise level of the sample holder (1 x 10-9 A.m 2 ) before this temperature step was reached, and the samples started to show erratic behaviour. Component B was only identified in meta-peridotite samples from site HC-03. It generally demagnetised as great-circle arc trajectories away from component A towards an antipodal, westerly and downward direction. One sample reached a stable end-point of demagnetisation at 515 C, which allowed for definition of component B when combined with great-circle arcs of the five other samples (McFadden and McElhinny, 1988). By far the most widespread of the characteristic remanent magnetisations is component C. It was identified within meta-peridotite, the sheared contacts, baked host quartzite of the Nsuze Group, and meta-gabbro

124 Chapter: 7 Palaeomagnetism Figure: 46 Representative sample demagnetisation behaviour using Zjiderveld projections for samples taken from the Hlagothi Complex. Various components shown with coloured straight lines

125 Chapter: 7 Palaeomagnetism Figure: 47 Representative sample demagnetisation behaviour using equal area projections for samples taken from the Hlagothi Complex

126 Chapter: 7 Palaeomagnetism Figure: 48 Equal-area plots of component means for components sft, PLF, C, B, A and D fitted by the least-squares line analysis method of Kirschvink (1980)

127 Chapter: 7 Palaeomagnetism These units represent some of the most extensively altered lithologies sampled during this study. After removal of A and PLF components, demagnetisation trajectories generally followed linear tracts towards the origin. Least-squares analyses reveal moderately negatively inclined eastward directed characteristic components. In contrast to this, the demagnetisation of samples from the least altered lithologies (i.e., meta-peridotite), which were generally collected far away from intrusive contacts, revealed a north-north-westerly characteristic remanent component with moderate positive inclination. Figure: 49 Palaeomagnetic sample sites AG-I and HC-07 showing sample positions at the sites in order to produce a baked contact test. Also shown is the baked contact test using for site AG-I

128 Chapter: 7 Palaeomagnetism We named this component D. This component was only identified from two sampling sites (AG-I and HC-07), from eight and two samples, respectively (see Fig. 46, 47 and 48). The baked contact test shows the existence of the same component C that was seen in the Hlagothi Complex, as well as in the sheared contact between the complex and the quartzites. The same component was seen in the quartzites of the Nsuze Group up to 30m away from the contact, providing evidence toward a failed contact test. However, the component D in two sites from the least altered sills of the Hlagothi Complex was revealed only after the removal of component C and A within these sills. Figure: 50 VGPs plotted for each component observed within each site of the Hlagothi Complex (components A, B, C and D), with the Kaapvaal Craton in its present orientation and position shown in orange

129 Chapter: 7 Palaeomagnetism This indicates an origin before the overprinting by component C in all lithologies. Although this is indicative of an inconclusive baked contact test, the complex, sheared contact and host rock has been overprinted by component C, some of the least altered lithologies in the Hlagothi Complex have retained an earlier high magnetic component, D (see Fig. 49). The virtual geomagnetic poles (VGPs) are plotted below for each site for each component, labelled poles A, B, C and D in relationship to the Kaapvaal Craton in its present position (see Fig. 50), with A and B plotting at similar low latitudes, intermediate latitudes offset by up to 50. and C and D in 7.4. SE-trending dolerite dykes Palaeomagnetic studies were conducted on two of the SE-trending dolerite dykes, from which samples AG-K and AG-J were collected. In total, 31 oriented samples were collected from the two sampling sites. Sample behaviour from the 10m wide SE-trending dolerite dyke AG-J was erratic and chaotic, possibly due to alteration or metamorphism, or due to the dyke s anastomosing to braided nature, as well as the influence posed by the ENE- and SSE-trending dolerite dykes 10m and 50m away respectively. For site AG-J, 10 samples were taken of both the coarse-grained and fine-grained phases of the dyke, in addition to one sample from a small glassy-textured dyke splaying off from the main dyke. Further, three samples were obtained from within 0.3m of the contact with the surrounding Mvunyana granodiorite. The granodiorite itself was sampled 10m away from the dyke in order to produce a baked contact test. At the site AG-K, six oriented samples were collected from the second 10m wide SE-trending dolerite dyke. In addition, five orientated samples were taken from the contact zone and a xenolith of the country rock within the margin of the dyke. This sampling zone was less than 1m on either side of the contact, with the Mvunyana granodiorite. Three orientated samples 20m away in the unbaked granodiorite were also collected. This was done in order to conduct a baked contact test. The palaeomagnetic results from the SE-trending dolerite dykes are summarised in Table 7. Examples of the

130 Chapter: 7 Palaeomagnetism demagnetisation behaviour and summaries of the remanent magnetisation are shown (see Fig. 51). Table: 7 Palaeomagnetic data for SE-trending dolerite dykes Site location Present coordinates Tilt-corrected coordinates Lat Long Decl. Incl. α95 Decl. Incl α95 Component Site Lithology n/n L/P k k (in N) (in E) (in ) (in ) (in ) (in ) (in ) (in ) AG-J dolerite /17 17/ C AG-K dolerite /14 13/ n/n = number of samples included/number analysed, L/P = line verses plane least squares analyses, Decl. = mean declination in degrees, Incl. = mean inclination in in degrees, k = Fisher s precision parameter, except when in modified form for where both line and plane data were combined and α95 = radius of 95% confidence cone about the mean Component 'C' is related to an overprint thought to be associated with the Meso- to Neoproterozoic Namaqua-Natal orogen The SE-trending dolerite dykes returned good site reproducibility at AG-K, whereas samples from the site AG-J were more erratic during demagnetisation, probably due to alteration and/or metamorphism. Also, several phases are present. The effect of the two later dyke events in the vicinity may also have affected the result. Two spurious low coercivity magnetic components were noted in AG-K, whereas no low coercivity magnetic components were identified in AG-J. One stable and geologically significant magnetic component, C was identified from both sites however. During AF demagnetisation and thermal demagnetisation steps up to 500 C, followed by alternating field demagnetisation up to 8.5 mt, the dolerite dyke samples from site AG-K revealed a reproducible and characteristic magnetic component. This component is in a moderate easterly direction, with a positive inclination, which is similar to the Meso- to Neoproterozoic overprint component, C, already identified for the Hlagothi Complex (see Fig. 46, 47 and 48). This component was labelled C also, as both dolerite dykes are in the same geologic vicinity as the Hlagothi Complex. This component is observed within the dolerite dykes, as well as the contact zones and unbaked host rock of the Mvunyana granodiorite, suggesting this component may represent a younger overprint. The means for these components are plotted on stereographic projections (see Fig. 52), and are also plotted geographically as VGPs (see Fig. 53), in a moderate northern latitude already seen in the component C from the Hlagothi Complex (see Fig. 50)

131 Chapter: 7 Palaeomagnetism Figure: 51 Representative sample demagnetisation behaviour using Zjiderveld and equal area projections for all samples taken from the SE-trending dolerite dykes. Various components are highlighted with coloured straight lines

132 Chapter: 7 Palaeomagnetism Figure: 52 Equal-area plots of component means for components sft and C, fitted by the least-squares line analysis method of Kirschvink (1980) Figure: 53 VGPs plotted for each component observed within each site of the SE-trending dolerite dykes (component C) with the Kaapvaal Craton in its present orientation and position 7.5. ENE-trending dolerite dykes Palaeomagnetic studies were done on two of the ENE-trending dolerite dykes. Sample sites included AG-B and AG-C. In total, 23 oriented samples were collected from these sites. Samples from dolerite dyke AG-B showed chaotic behaviour, whereas sample behaviour from the ENE-trending dolerite dyke AG-C was reproducible and consistent. For AG-C, seven

133 Chapter: 7 Palaeomagnetism orientated samples were drilled into the dyke, and five samples were also drilled into the contact zone with the surround Mvunyana granodiorite and the granodiorite itself in order to produce a baked contact test. The palaeomagnetic results from this ENE-trending dolerite dyke are summarised in Table 8. Examples of the demagnetisation behaviour and summaries of the remnant magnetisation are also shown (Fig. 54 and 55). Table: 8 Palaeomagnetic data for the ENE-trending dolerite dyke Site location Present coordinates Tilt-corrected coordinates Component Site Lithology Lat Long Decl. Incl. α95 Decl. Incl α95 n/n L/P k k (in N) (in E) (in ) (in ) (in ) (in ) (in ) (in ) E AG-C dolerite /13 9/ F AG-C dolerite /13 7/ n/n = number of samples included/number analysed, L/P = line verses plane least squares analyses, Decl. = mean declination in degrees, Incl. = mean inclination in in degrees, k = Fisher s precision parameter, except when in modified form for where both line and plane data were combined and α95 = radius of 95% confidence cone about the mean Components 'E' is an overprint direction associated with either the 180 Ma Karoo LIP or present local field Component 'F' is taken as a primary 2650 Ma direction. The ENE-trending dolerite dyke at site AG-C returned very good site reproducibility, probably due to its homogeneity and fine grain size. Figure: 54 Representative sample demagnetisation behaviour using Zjiderveld and equal area projections for all samples taken from the ENE-trending dolerite dyke AG-C. The various components are illustrated with straight coloured lines

134 Chapter: 7 Palaeomagnetism Figure: 55 Equal area plots of component means for components sft, E and F fitted by least squares line analysis method of Kirschvink (1980) The dyke produced two stable components at intermediate and high temperature steps during demagnetisation. After a spurious low coercivity magnetic component was removed by initial AF demagnetisation steps up to 0.5 mt within the dolerite dyke, component E was then demagnetised from 0.5 mt up to temperature steps of 350 C. This component has an intermediate negative north-west component, and is unique to the intrusion. The high temperature component was isolated during thermal demagnetisation above 350 C up to approximately 530 C, after which sample behaviour became erratic. This component has a west-south-west orientation with a steep positive inclination, which due to a positive baked contact test described below, is taken to be the primary direction for this ENE-trending dolerite dyke. Figure: 56 VGPs plotted for each component observed within the ENE-trending dolerite dykes (component E and F) compared with the Kaapvaal Craton in its present orientation and position

135 Chapter: 7 Palaeomagnetism The means for these components are plotted on equal area projections (see Fig. 55), and plotted geographically as VGPs (see Fig. 56), with component E in moderate northern latitudes and F in moderate southern latitudes. A baked contact test was attempted at locality AG-C. Samples were collected within the baked contact zone up to 1m either side of the contact, as well as within the host rock granitoids, which were collected up to about 20 m away from the 5m wide ENE-trending dolerite dyke. The samples of baked granitoids in the contact zone exhibit two components during demagnetisation. Figure: 57 Palaeomagnetic site AG-C showing sample positions at the site in order to produce a baked contact test. Also shown is the baked contact test using a equal-area projection

136 Chapter: 7 Palaeomagnetism The low-coercivity or soft component demonstrates a present-day geomagnetic field component. An intermediate positive south-easterly component then unblocks up to temperatures of about 500 C. A different component, however, was seen in the host rock granodiorite collected away from the chilled margin and dyke, with two components seen unblocking up to a 500 C. A component was seen toward the south-south-east with a moderate negative inclination up to 300 C, as well as a component toward the north-west with a moderate negative inclination from C, showing a positive baked contact test. (see Fig. 57) NE-trending dolerite dykes Palaeomagnetic studies were applied on both generations of the NE-trending dolerite dykes, from which samples AG-A (NE030) and DY-02 (NE050) were collected. In total, 29 oriented samples were collected from 2 sampling sites. Sample behaviour from AG-A was chaotic, with only samples from DY-02 returning reproducible results. From this 3m wide dyke, six samples were drilled into the dolerite dyke, and three into a small dolerite dyke splaying off from the main dyke. In addition, three samples were drilled into the surrounding Mvunyana granodiorite in order to produce a baked contact test. From the baked contact, four were taken at the contact between the dyke and the host rock. The palaeomagnetic results from the NE-trending dolerite dyke DY-02 are summarised in Table 9. Examples of the demagnetisation behaviour and summaries of the remanent magnetisation are shown (Fig. 58 and 59). Table: 9 Palaeomagnetic data from a NE-trending dolerite dyke Site location Present coordinates Tilt-corrected coordinates Lat Long Decl. Incl. α95 Decl. Incl α95 Component Site Lithology n/n L/P k k (in N) (in E) (in ) (in ) (in ) (in ) (in ) (in ) D DY-02 dolerite /9 7/ n/n = number of samples included/number analysed, L/P = line verses plane least squares analyses, Decl. = mean declination in degrees, Incl. = mean inclination in in degrees, k = Fisher s precision parameter, except when in modified form for where both line and plane data were combined and α95 = radius of 95% confidence cone about the mean Component 'D' is taken as a 2866 Ma direction associated with the Hlagothi Complex

137 Chapter: 7 Palaeomagnetism Figure: 58 Representative sample demagnetisation behaviour using Zjiderveld and equal area projections for all samples taken from the NE-trending dolerite dyke DY-02. The one component is shown with a straight coloured line Figure: 59 Equal area plots of component means for component D fitted by the least-squares line analysis method of Kirschvink (1980) The NE-trending dolerite dyke returned very good site reproducibility at DY-02, probably due to its homogeneity and fine grain size. The dyke produced one stable component at intermediate and high temperature steps during demagnetisation. After a variety of spurious low coercivity magnetic components were removed in initial AF demagnetisation steps up to 10 mt within the dolerite dyke, component D was then demagnetised from

138 Chapter: 7 Palaeomagnetism mt up to temperature steps of 540 C, and alternating field steps up to 85 mt, upon which sample behaviour became erratic. This component has a positive and shallow north-northeast direction, and is unique to the intrusion. The mean for this component is illustrated below (see Fig. 59), with the VGP plotted in intermediate to high northern latitudes (see Fig. 60). Figure: 60 VGP plotted for the component observed within the NE-trending dolerite dyke DY-02 (component D) with the Kaapvaal Craton in its present orientation and position A baked-contact test was attempted at locality at DY-02. Sampling was carried out within the baked-contact zone, as well as the host rock granitoids up to about 20 m away from the NE-trending (NE050) dolerite dyke contact. The samples of baked granitoids produced erratic and chaotic directions during demagnetisation. The baked-contact test is therefore inconclusive, but suggesting that the dolerite dyke is different from the host rock in magnetisation

139 Chapter: 7 Palaeomagnetism

140 Chapter: 8 Discussion Chapter: 8 Discussion 8.1. Intrusion and metamorphism The inliers of the Palaeo- to Neoarchaean granitoid-greenstone terrane of the southeasternmost Kaapvaal Craton are intruded by numerous dolerite dykes of SE-, ENE- and NEtrends. These dykes are absent from the overlying Phanerozoic cover of the Karoo Supergroup, indicating that they are Precambrian in age. The Mesoarchaean Pongola Supergroup overlies this Archaean basement terrane. The Hlagothi Complex is intrusive into the basal quartzite strata of the Mantonga Formation of the Nsuze Group, the Nkandla subbasin. It was once understood to be coeval with the ca Ga Nsuze Group (Hegner et al., 1981). The Hlagothi Complex now has a newly determined U-Pb baddeleyite emplacement age of 2866 ± 2 Ma, dated herein. It consists of a layered series of sills of meta-peridotite, pyroxenite and gabbro. Several other intrusives into the Pongola strata have been noted and mapped in the region by Groenewald (1984) and Gold (1993), with very little further study. These deformed and metamorphosed inliers of Archaean crust are in turn overlain by relatively fresh strata of the Karoo Supergroup diamictites and shales of the Dwyka and Ecca groups. This strata is also intruded across the region by Jurassic SSE-trending dolerite dykes and sills. In addition, the Kaapvaal Craton on this south-easternmost side is in close proximity to the craton margin, and the Namaqua-Natal Mobile Belt to the south and the Mozambique Mobile Belt east of it. Units of the Namaqua-Natal orogen in places have been thrust up and on to the craton. Elworthy et al. (2000), and Jacobs and Thomas (2001) noted that the Kaapvaal Craton more than 50 km away from the Natal Thrust Front was experienced lesser burial and exhumation from the orogeny, whereas localities further to the south experienced greater metamorphism and deformation. Rb-Sr whole rock ages on the granitoids to the south of this 50 km line returned a mean age of 967 ± 24 Ma, whereas

141 Chapter: 8 Discussion granitoids to the north of it produced ages of 2614 ± 74 Ma (Elworthy et al. 2000). The Hlagothi Complex straddles this zone, being emplaced approximately 50 km from the cratonic margin. The various Precambrian dolerite dyke generations of the White Mfolozi inlier were emplaced between 50 and 80 km from the cratonic margin. Other dolerite dykes are indeed present to the south of this 50 km line, but have not been mapped or described in any detail. One generation each of SE- and NE-trending dykes have been dated in this study by the U-Pb baddeleyite method at 2874 ± 2 Ma and 2652 ± 11 Ma. The first result is thought to represent an emplacement age. The age of ca Ma on the NE-trending dolerite must, however, be regarded as preliminary. Ar-Ar amphibole dates were obtained from SE- and NE-trending dolerite dykes in the White Mfolozi inlier as well. These dates are 1596 ± 16 for a SE-trending dolerite dyke and 831 ± 30 Ma and 1388 ± 18 Ma from NEtrending dykes, respectively. These pseudo plateau ages, as well as their Ca/K ratios however, were disturbed, and may reflect alteration and metamorphism related to the cooling and exhumation of the Namaqua-Natal orogeny to the south, as no known plutonic or volcanic event can be associated with these ages. The rock types in this south-easternmost terrane of the Kaapvaal Craton have seen strong alteration of the original lithologies at greenschist facies. The presence of uralitic amphiboles, chlorite, talc, serpentine, epidote, and sericite attest to this in both the Hlagothi Complex and variously trending dolerite dykes, as these secondary minerals are the products of greenschist facies metamorphism and alteration of the original ortho- and clinopyroxenes, olivine and plagioclase feldspar. Relict textures and relict primary pyroxene in the dolerite dykes attest to the heterogeneous nature of the metamorphic alteration, as do the 40 Ar/ 39 Ar ages obtained. This means that although the 50 km metamorphic isograd of Elworthy et al. (2000) is correct, the area has sustained a much more protracted and heterogeneous set of metamorphic conditions. This is also reflected in the palaeomagnetic data (discussion to follow). From the data presented, and from the literature, it would appear that there were metamorphic episodes or overprinting at ca. 2870, 2800 to 2600 Ma, 1600 to 800 Ma and 180 Ma. These events can be linked with the Hlagothi Complex LIP event in the Mesoarchaean, post-pongola intrusions and Ventersdorp volcanism in the Neoarchaean, the protracted Namaqua-Natal orogeny in the Meso- to Neoproterozoic for the region, and lastly the Jurassic Karoo LIP

142 Chapter: 8 Discussion For the Hlagothi Complex, Groenewald (1984) argued that shallow intrusion of the complex into water-rich rocks and subsequent syn-metasomatism placed the timing of intrusion shortly after the deposition of the Nsuze Group, thus accounting for their alteration. He did note post-intrusion metamorphic and deformation events, which could in part be associated with the Meso- to Neoproterozoic Namaqua-Natal orogeny. The palaeomagnetic and 40 Ar/ 39 Ar geochronological data provide additional information about the possible timing of this post-intrusive alteration. A remanence component A identified in this study was seen across all lithologies in the Hlagothi Complex, while component B was revealed only within gabbroic samples from one site (see Fig. 61). These two magnetic directions were isolated in the more altered lithologies, i.e. the peridotites, the sheared contacts with the quartzites of the Nsuze Group, and the gabbros. Relict textures are common, and in some cases there is unaltered minerals too, particulary pyroxenes. Component B is antipodal to component A, and they are interpreted to represent a dual polarity overprint. The VGPs of these two components are in a similar position as poles obtained for the Agatha basalts of the Nsuze Group by Strik et al. (2007), and poles for the 2782 ± 5 Ma Derdepoort basalts by Wingate (1998) for example. It is interesting to note that between 2850 and 2650 Ma, the whole southeastern Kaapvaal Craton experienced the intrusion of voluminous potassic granitic plutons, collectively known as the post-pongola granites. The ages of these granites appear to be coeval with the Rb-Sr mean ages of 2614 ± 74 Ma obtained by Elworthy et al. (2000). We suggest that the Agatha basalt pole of Strik et al. (2007) may very well also represent a magnetic overprint caused by the intrusion of these post-pongola granites into the rocks of the Pongola Supergroup in the area studied. In addition, this pole shows a significant discrepancy to the similar age Nsuze dolerite dykes and basalt pole of Lubnina et al. (2010), which is further enhanced by baked contact tests. A primary component, F in the ENE-trending dolerite dykes across the region is also seen in this study, and by Lubnina et al. (2010), which are in a similar region to ca Ga poles. It is also similar to the Agatha basalt pole of Strik et al. (2007), further arguing that the Agatha basalt pole is not primary. Another component seen in most lithologies and interpreted as a younger magnetic overprint is component C. It was seen in the peridotites and gabbros, as well as the quartzites into which the Hlagothi Complex was intruded. In addition, some SE-trending

143 Chapter: 8 Discussion dykes appear to show the same component. Baked contact tests on the quartzites and granitoids into which the Hlagothi Complex and SE-trending dolerite dykes intrude also display the same component, and further attest to both the dykes and granitoid host rocks being affected by the same regional metamorphic event. It cannot reflect a primary direction. When compared to other known palaeopoles from the Kaapvaal Craton, the VGPs for this component resembles the pole for the ca Ma Ntimbankhulu pluton in the Margate terrane of the Natal sector within the Namaqua-Natal Mobile Belt obtained by Maré and Thomas (1998). The age of the Ntimbankhulu granite-charnockite pluton carries significant uncertainty, but it is known to have been produced during the Meso- to Neoproterozoic. It is believed component C represents an overprint related to this event, given the SE-trending dolerite dykes and the Hlagothi Complex's proximity to the Natal Thrust Front (see Fig. 9). Other components include the possibility of a magnetic component D for the Hlagothi Complex and component F seen within NE-trending dolerite dykes respectively. Components D and F will be presented further in this chapter, as they may be primary. Figure: 61 Palaeopoles and VGPs for various units within the Mesoarchaean to Mesoproterozoic stratigraphy of the Kaapvaal Craton (component codes with references, see Table: 10), as well as overprints associated with the Namaqua-Natal Mobile Belt, with component D1 and D2 associated with the Hlagothi Complex and a NEtrending dolerite dyke (NE050); A and B associated with overprinting from the post-pongola granitoids in the Hlagothi Complex; F with the Ventersdorp volcanics in ENE-trending dolerite dykes, and C1, C2 and C3 with overprinting from the Namaqua-Natal orogenic belt in the Hlagothi Complex and the two SE-trending dolerite dykes respectively

144 Chapter: 8 Discussion 8.2. Geochemistry and petrogenesis There is a geochemical separation between predominantly NE-trending depleted tholeiitic basalt dykes (NE030), and more enriched, borderline tholeiitic to calc-alkaline dykes of other trends. This includes ca Ga ENE-trending dykes, ca and 2.87 Ga SE-trending dykes and NE-trending dolerite dykes (NE050) which possibly have an age of ca Ga. This is consistent with more aeromagnetically distinct NE-trending dykes having higher modal irontitanium oxide contents, compared to more quartz-bearing ENE-, NE- and SE-trending dolerite dykes (Klausen et al., 2010). However, these dykes are more resistant to weathering and likely to form geomorphological ridges; providing a means to roughly discriminate between Archean and Proterozoic dykes in the field according to Klausen et al. (2010). Multi-element statistical discrimination and spider diagrams elaborate on the calc-alkaline affinity of Archean ENE-, NE- and SE-trending dykes, which typically have higher LIL element concentrations and steeper REE patterns, and where Sr/V and La/Yb ratios most clearly distinguish these from the Proterozoic NE-trending tholeiites (Klausen et al., 2010). Finally, a geochemical comparison with published data on coeval volcanic rocks indicates according to Klausen et al. (2010) that: The ca Ga and ca Ga SE-trending (and NE050) dolerite dykes, are feeders to Nsuze and Mozaan Group lavas. The ca Ga ENE-trending dykes resemble Allanridge Formation lavas more than other Ventersdorp Supergroup lavas, and some NE-trending dykes may also be related to Ventersdorp age events (NE030), although they are much more primitive in composition. Geochemically, these NE030 dykes resemble the Mazowe dolerites on the Zimbabwe Craton, which have been associated with the ca Ga event in the past based on palaeomagnetic studies. Some ca Ga NE-trending dykes might belong to ca Ga Soutpansberg- Mashonaland LIP that extends across both the Kaapvaal and Zimbabwe cratons and includes coeval lava remnants within both the Soutpansberg and Olifantshoek Group based on geochemistry and palaeomagnetism

145 Chapter: 8 Discussion These constraints and inferences on the complex array of dykes across the southeasternmost Kaapvaal Craton both complement and partly contradict existing tectonic models for the major igneous events during the Mesoarchaean to Palaeoproterozoic. Compositional variability among SE-, NE- and ENE-trending dolerite dykes as well as the Hlagothi Complex is indicative of increasing concentration of incompatible elements (e.g., Sr, V) with decreasing MgO content, which is consistent with earlier ilmenite fractionation amongst the more borderline tholeiitic and calc-alkaline suites as was noted by Klausen et al. (2010). However, there can be considerable geochemical range within a single dyke, as was noted by Hunter and Halls (1992), with geochemical samples collected from the same dolerite dyke outcrop, as well as by Maré and Fourie (2012) who noted significant contamination between SE- and NE-trending dolerite dykes in the Badplaas-Barberton region. In particular, a highly xenolithic or phenocrystic dyke can clearly demonstrate the dyke s geochemical heterogeneity, without any apparent differentiation trend, probably due to variable contamination from xenoliths from the local host rock, or by granophyric quartz along the dyke-host rock contact zone. Uncritical sampling by Hunter and Halls (1992) could be explained through the sampling of serpentine pseudomorphs after olivine and orthopyroxene crystals from ultramafic xenocrysts from the nearby Barberton Greenstone Belt further to the north for example, as was noted by Klausen et al. (2010). In addition, apart from obvious geochemical separation amongst the Hlagothi Complex and different dyke trends, La/Yb was noted to be a slightly better discriminator between basaltic andesite ENE-trending dykes and the basalt to basaltic andesite SEtrending dykes in the KwaZulu-Natal area for example, and those located further north. In addition, NE-trending dolerite dykes (NE030) from northern KwaZulu-Natal have slightly lower La/Yb than NE-trending dykes from the Black Hills area. However, many dykes and volcanic sequences on the craton show large amounts of possible crustal contamination, or having a very enriched geochemical reservoir beneath the Kaapvaal Craton. The ca Ga NE030 generation of NE-trending dolerite dykes in northern KwaZulu-Natal do not, however. The separation between compatible and incompatible elements among the Hlagothi Complex and dolerite dykes is consistent with magma differentiation due to the

146 Chapter: 8 Discussion fractionation of chromium spinel, olivine and possibly pyroxene. The tholeiitic peridotitic phases of the Hlagothi Complex show higher concentrations of compatible elements, whereas borderline tholeiitic to calc-alkaline SE-, ENE- and NE-trending dolerite dykes, and the gabbroic phases of the Hlagothi Complex have higher concentrations of LIL elements. This discrimination is consistent with inferences based on rock type characteristics and petrography, where more calc-alkaline suites typically are LIL enriched (e.g., McCulloch and Gamble, 1991) and experienced earlier iron-titanium oxide fractionation (e.g., Kuno, 1968) than tholeiitic suites. In addition, geochemically, the Hlagothi Complex shows two or possibly three types of basaltic and basaltic andesitic compositions, which corresponds to the meta-peridotites and meta-pyroxenites/gabbros respectively, with very little fractionation trend seen. Using immobile element discrimination plots, the same trend (or lack thereof) is seen, which also provides evidence for little loss of alkalis during alteration, and which is further affirmed using the alteration box plot of Large et al. (2001). This alteration box is only applicable to mafic lithologies however. Petrographically, there is good evidence for high levels of serpentinisation in the peridotites and chloritisation in the peridotites and pyroxenites. However, such alteration could be associated with synmagmatic alteration or at most upper greenschist facies metamorphism, with no new crystal growth of primary pyroxenes, amphiboles or garnets indicative of amphibolite facies metamorphism. The composition types for the Hlagothi Complex bear no trend or similarity to each other, although du Toit (1931) postulated that the Hlagothi Complex consisted of the differentiating products of a single reservoir. It is quite possible that two separate pulses of magma following the same magmatic pathways at slightly different times led to the emplacement of the complex, which may be supported by the age difference of 4 to 8 Ma accounting for 95% confidence interval errors between the Hlagothi Complex and SEtrending dolerite dykes in the greater region. However, dating of the younger pyroxenites, gabbros and feeder dolerite dykes would be needed to confirm this. The high MgO (Mg#) pulse of the complex is geochemically similar to that of komatiites, as was noted by Groenewald (1984), who did some basic petrogenetic modelling of the complex. This provides evidence for a more primitive, tholeiitic and komatiitic mantle signature, which a mantle plume could provide for example (Ernst and Buchan, 2003). It is worth noting that such a geochemically primitive magma is absent from the Nsuze and Dominion groups. This would imply a different source of magma during these events. Cole (1994), Groenewald

147 Chapter: 8 Discussion (1984) and Gold (1994), however, did note the presence of ultramafic dykes and sills within the Pongola Supergroup stratigraphy in the whole area, as well as within the basement granitoid-greenstone terrane. These intrusions have not been studied in any great detail in order to affect a comparison, or to compare with other such intrusions further to the north, such as the Thole Complex in south-eastern Mpumalanga and Swaziland, which in itself has not been studied in any significant detail. The low MgO (Mg#) end members of the Hlagothi Complex do resemble the Pongola and Ventersdorp dykes, however, as well as portions of the Usushwana Complex. In addition, there are high MgO lava flows at the base of the Allanridge, as well as within the Klipriviersberg lavas, which are classed as komatiitic Correlation to strata-bound igneous units Field evidence, petrographical descriptions and geochemical discriminations in tandem with U-Pb baddeleyite ages are for the most part consistent with the presence of four or five major Precambrian mafic dyke swarms on the south-easternmost Kaapvaal Craton in addition to the Hlagothi Complex. Some of these are coeval with lava formations within equally significant volcanic successions in the Mesoarchaean to Palaeoproterozoic South African stratigraphy. In addition, as a result of palaeomagnetic investigations on the same dyke swarms in the Kaapvaal Craton, at least four or five different components have also been recognised apart from present local field directions, in addition or complimentary to components already established by Lubnina et al. (2010). Their mean directions are summarised below, according to Klausen et al. (2010), Lubnina et al. (2010), Olsson et al. (2010) and Söderlund et al. (2010): The ca Ga dykes within a SE-trending swarm in the Barberton-Badplaas area are potential feeders to lavas within the Nzuse Group. The ca Ga dykes within a radiating NE-, E to SE-trending swarm in the Black Hills, Rykoppies and Barberton-Badplaas areas are potential feeders to lavas within the Ventersdorp Supergroup or proto-basinal fills. The ca Ga dykes within a NE-trending swarm are potential feeders to lavas within the Soutpansberg Group in the Black Hills area

148 Chapter: 8 Discussion Klausen et al. (2010) noted that the Archean dykes appear to have been emplaced towards the end of each, roughly coeval, volcanic event, whereas poorer age constraints on Soutpansberg Group lavas preclude more accurate correlations with ca Ga feeders. A decision needs to be made on whether to include any of northern KwaZulu-Natal s SE-, NE- and ENE-trending dykes in any of the above listed major igneous events, as well as which dykes belong to other igneous events Correlation with the ca Ga Nsuze Group dykes and lavas Uken and Watkeys (1997) considered the SE-trending dyke swarm, later dubbed the Barberton-Badplaas dyke swarm by Olsson et al. (2010), to be associated with rifting at ca Ga. They linked these dykes to the volcanic units of the Nsuze Group, as well as possibly the Dominion Group further to the west. However, their interpretation was based on the observation that the Barberton-Badplaas dyke swarm structurally predates the protobasinal Godwan sequence at the base of the Transvaal Supergroup. Furthermore, Hunter and Halls (1992) noted that many SE-trending dykes cut ca Ga old tabular granitoids such as the Mpuluzi and Heerenveen batholiths, but are not abundant in the ca. 2.7 Ga Mbabane Granite, for example, thus providing a maximum and minimum age for a majority of SE-trending dykes. This, however, is not true for all of the SE-trending dolerite dykes in the region. The ages of the two Barberton-Badplaas dykes determined by Olsson et al. (2010) of 2966 ± 1 Ma and 2967 ± 1 Ma, lie within these estimated age-constraints, and are more precisely synchronous with a 2968 ± 6 Ma porphyritic rhyolite, near the top of the Agatha lavas (Mukasa et al., 2013). In the Pongola Supergroup, the ca. 4.6 km thick Nsuze Group is made up of six formations, dominated by volcanic rocks and subordinate sedimentary strata, specifically the Nhlebela/Pypklipberg, Agatha and Ekombe volcanic formations. These rocks were deposited/extruded within a period of approximately 40 million years, bracketed between the U-Pb age of the 2985 ± 1 Ma and 2984 ± 3 Ma Agatha lava unit (Hegner et al., 1993; 1994) and a 2934 ± 114 Ma and 2940 ± 22 Ma ages for the same unit (Hegner et al., 1984). No ages have been obtained on the Nhlebela/Pypklipberg and Ekombe units, and it could significantly enhance or invalidate stratigraphic correlations

149 Chapter: 8 Discussion both within the Nsuze basin, as well as with the Dominion basin to the west if better age constraints could be obtained. The dykes were characterised geochemically by Hunter and Halls (1992) as belonging to a group of low-ti dykes, which contrast to high-ti dykes, which were inferred to be Palaeoproterozoic in age. Olsson et al. (2010) dated both generations, and the coeval age of ca Ma on both suggests that both low-ti and high-ti dolerite dykes are age equivalents to the Nsuze lavas that is, both groups can be linked to the same Pongola rifting event. This interpretation could only tentatively be extended to encompass all the SEtrending dykes in this part of the craton, which furthermore appear to converge onto the present exposure of Nsuze lavas in the south-easternmost part of the craton, an interpretation also supported by geochemical and palaeomagnetic correlations of Klausen et al. (2010) and Lubnina et al. (2010). Klausen et al. (2010) assigned ages of ca Ga to ENE-trending dolerite dykes in the south-easternmost region on the basis of geochemistry. This argument is invalidated by the palaeomagnetic directions obtained on the ENE-trending dykes in this region by both Lubnina et al. (2010) and the component F obtained in this study. In addition, field, petrographic and geochemical arguments appear to favour the possibility that the area is composed of two slightly different SE-trending dolerite dyke orientations. At least one generation is cross-cut by ENE-trending dykes, which are tentatively thought to be either ca by Klausen et al. (2010) or ca Ga by Lubnina et al. (2010). The age of 2874 ± 2 on one SE-trending dolerite dyke in this study on this southeasternmost region provides a link to the Hlagothi Complex however. This complicates the tentative age assignment done by means of palaeomagnetic directions and geochemistry correlation. Lava samples from the Nzuse Group (Armstrong et al., 1986; Wilson and Grant, 2006) define a continuous sub-alkaline series that ranges from basaltic andesites to rhyolites. Despite their resemblance to compositionally continuous volcanic suites within present-day subduction-zone settings (McCulloch and Gamble, 1991), the Nsuze lavas are not as depleted in iron and thereby straddle the calc-alkaline to tholeiitic boundary. Basaltic andesite samples bear the closest resemblance to ca Ga SE-trending dykes in the Barberton-Badplaas area, as well the SE-trending dykes within northern KwaZulu-Natal. Some of the basalt to basaltic andesite SE-trending dolerite dykes in northern KwaZulu-Natal

150 Chapter: 8 Discussion also define a more MgO-rich group that lines up with a more tholeiitic lava trend, whereas more andesitic SE-trending dykes in the Barberton-Badplaas and northern KwaZulu-Natal can possibly overlap the more calc-alkaline trend defined by the Allanridge Formation in the Ventersdorp Supergroup. Figure: 62 SE-trending dolerite dykes in northern KwaZulu-Natal compared to ca Ga SE-trending dolerite dykes and volcanics in the Barberton-Badplaas region with data from Klausen et al. (2010), Armstrong et al. (1986) and Wilson and Grant (2006) A correspondence analysis by Klausen et al. (2010) from the Nsuze Group lavas and the ca Ga SE-trending dykes identified lava samples that were compositionally most similar to the dykes. From these, a few samples with the highest La/Yb display multielement patterns that partially overlap the ca Ga dykes in both northern KwaZulu- Natal and in the Barberton-Badplaas area. The ca Ga dykes in the Barberton-Badplaas area and in northern KwaZulu-Natal generally have lower heavy rare earth element compositions than the Nsuze Group lavas (see Fig. 62). Instead, the same Nsuze Group lavas are slightly better matched with ESE-trending dykes in the Barberton-Badplaas area according to Klausen et al. (2010), which also have the most similar La/Yb ratios; this opens up the possibility that this swarm is ca Ga in age according to Klausen et al. (2010), contradicting arguments by Lubnina et al. (2010), and the ca Ga obtained herein

151 Chapter: 8 Discussion Figure: 63 Palaeomagnetic results for the Pongola Supergroup volcanics and related intrusives Palaeomagnetic results of Lubnina et al. (2010) identified a component named P (with corresponding pole position named BAD) in some of the SE-trending dolerite dykes already dated by Olsson et al. (2010) in the Barberton-Badplaas area (see Fig. 63). These SE-trending dolerite dykes were studied palaeomagnetically in the south-eastern and south-easternmost areas of the Kaapvaal Craton. Therefore Lubnina et al. (2010) concluded that the ca Ga age of these dykes applies also to the age of their magnetisation. A palaeomagnetic pole, recalculated from this component (BAD), is located in high southern latitudes. This direction is similar to that obtained for the ca Ga Nsuze basalts (NB), also obtained by Lubnina et al. (2010) in the south-easternmost Kaapvaal Craton. These ca Ga poles are about 65 to the west from the ca Ga pole obtained on the Allanridge Formation by Strik et al. (2007) and de Kock et al. (2009), as well significantly different from the pole obtained from the Agatha basalts by Strik et al. (2007). No Nsuze aged magnetic directions were obtained in this study, despite drilling the same SE-trending dolerite dyke (NL-13) of Lubnina et al. (2010). However, the dyke in question in this study (AG-J) was drilled closer to three intersecting dykes, thus possibly explaining the differences in results. The drill holes from the palaeomagnetic sampling of Lubnina et al. (2010) were not observed, and must have been done further afield, which could also explain the better results away from the possible baking and re-magnetisation of the other dolerite dykes seen in the site drilled in this study. It is clear that there are probably two generations of SE-trending dolerite dykes in northern

152 Chapter: 8 Discussion KwaZulu-Natal, conforming to both ca and 2.87 Ga ages. This hypothesis, however, needs further study Correlation with the ca Usushwana and Thole layered complexes, as well as Mozaan and Witwatersrand lavas The Thole Complex, located further north in the Swaziland region of the main Pongola Basin, consists of sills emplaced at several stratigraphic levels into the Pongola Supergroup, Usushwana Complex and basement granitoids (Hammerbeck, 1982). These sills are also layered, consisting of harzburgites at the base grading up into pyroxenites, gabbros and norites. The complex bears the same two distinct compositional groupings as the 2866 ± 2 Ma Hlagothi Complex (Chapter 5), and has been recognised to be a potential correlative of the Hlagothi Complex (Groenewald, 2006). These sills have also seen extensive alteration, despite being far removed from the cratonic margin and its related metamorphism and deformation. This would suggest alteration similar to that experienced by the potentially synchronous Hlagothi Complex as postulated by Groenewald (1984), provided that the alteration was not caused by known post-pongola 2850 to 2650 Ma granites in the region. Flood basalts of the Mozaan and Witwatersrand basins represent another prominent magmatic event (or events) of a possibly similar age. Hammerbeck (1982) and Nhleko (2003) noted that flood basalts present in the Mozaan Group of the Pongola basin (i.e., the Tobolsk, Gabela and Ntanyana lavas) were produced from fissure eruptions in a continental setting. Lavas of similar stratigraphic age are represented in the adjoining upper West Rand Group and middle Central Rand Group of the Witwatersrand basin by the Crown and Bird lavas. A 2914 ± 14 Ma age was reported by Armstrong et al. (1991) for the Crown lava in the Central Rand Group, but this age should be treated as a maximum age. These lavas may have seen crustal contamination, as they are geochemically distinct from the Mozaan lavas, and are of dacitic composition. In addition, zircon grains were few, suffered lead loss and were noted to consist of a variety of morphologies suggesting they may be xenocrysts. In addition, detrital zircons obtained in stratigraphically lower successions in the greater basin produced zircons of younger ages, further validating this argument that the zircons are xenocrystic (see Fig. 64). Beukes and Cairncross (1991) argued that the Mozaan and

153 Chapter: 8 Discussion Witwatersrand groups represent distal and proximal correlatives. A likely setting for these continental flood basalts may be a short-lived transient mantle plume(s). These lavas are located in the upper Mozaan Group of the Pongola Supergroup and the eastern half of the upper Witwatersrand Supergroup closest to the Pongola Supergroup, especially in the case of the less laterally extensive Bird lava. The Crown lava is much more widespread, however, thinning toward the west and south. This suggests a plume centre in the Pongola basin region in the east to south-east. One observable effect of this plume would be regional uplift associated with doming above the plume (e.g., Sengör, 2001). Figure: 64 Detrital zircon ages ranges with peaks shown in black in the Witwatersrand and Mozaan basins after Nhleko (2003). Data for the Crown and Tobolsk lavas are not detrital, however, and reflect possible emplacement ages. The sedimentary strata (yellow) are seen to be interrupted by two volcanic formations (grey). Red crosses are indicative of no data being available for the lava flows or sedimentary strata The development of small local unconformities in the region s sedimentary successions was followed by the eruption of flood basalts, providing possible proof of uplift from a mantle plume before the volcanism

154 Chapter: 8 Discussion The 2866 ± 2 Ma Hlagothi Complex gabbros and the 2874 ± 2 Ma generation of SEtrending dolerite dykes in the south-easternmost Kaapvaal Craton can be shown to be geochemically similar to the Mozaan lavas. In terms of major elements they show similar amounts of SiO 2, but the lavas are richer in Na 2 O and K 2 O, which possibly relates to alteration, fractionation or contamination. Using trace element geochemistry, the Mozaan lavas show an almost identical signature to the gabbroic phases of the Hlagothi Complex, although they are more enriched generally, which is common with greater fractionation. In addition, both magmatic events exhibit negative Nb-Ta anomalies, as well as a negative Sr and anomalies, with a minor negative Ti anomaly. One exception is Th and U. Th is depleted in the Mozaan lavas compared to Hlagothi Complex gabbros. U is enriched in the Mozaan lavas compared to the Hlagothi Complex gabbros (see Fig. 65). These signatures, however, are similar to many such volcanic successions and their plutonic equivalents across the Kaapvaal Craton from the Mesoarchaean to the Jurassic, and show a possible contaminated crustal source or derivation from an enriched sub-arc-like mantle wedge. Figure: 65 Geochemistry of the Hlagothi Complex in northern KwaZulu-Natal compared to Mozaan Group volcanics with data from Nhleko (2003) This signature is found even in rocks of known rift-related tectonic regimes such as the Karoo LIP (Duncan, 1987; Klausen et al., 2010). Tectonic setting diagrams also demonstrate emplacement within an arc-like regime

155 Chapter: 8 Discussion In addition, a magnetic component D was determined for the Hlagothi Complex once the overprint component C was removed during demagnetisation (see Fig. 66). This component shows no similarity to similar-aged magmatic events from the Archaean across the Kaapvaal Craton. An exception is the ca Bushveld Complex, which is of different age. In addition, the same magnetic component D was also obtained from a NE-trending dolerite dyke located further to the north of the Hlagothi Complex. However, in the absence of a conclusive palaeomagnetic field test, whether this component represents a primary direction and thus a correlative to the Hlagothi Complex or whether it is an overprint direction related to the intrusion cannot be determined. However, the influence of the Hlagothi Complex across the south-eastern region of the craton can therefore not be understated, with the existence of numerous possible similarly aged intrusions, volcanic successions and thermal overprints in potentially older strata or intrusions. Figure: 66 Palaeomagnetic results for the Hlagothi Complex (D1) and NE-trending dyke (D2) The complexity of the intrusions of northern KwaZulu-Natal is exemplified by the NEtrending dykes (NE050 and NE030), whose ages indicate connections to ca. 2.87, 2.65 and 1.90 Ga magmatic events across the craton. This adds greater complexity into this area in particular

156 Chapter: 8 Discussion Correlation with the ca Ga Ventersdorp dykes and lavas The scarcity of geochronological data from the upper Ventersdorp and lower Transvaal Supergroups makes the correlations of dolerite dyke ages of Olsson et al. (2010) difficult. This is especially true in the absence of ages on the Allanridge lavas as the stratigraphically youngest unit within the Ventersdorp Supergroup, as well as poor ages existing for lavas within the Wolkberg and Godwan proto-basinal fills, for example. Indeed, speculation exists on whether ages of the basal 2714 ± 8 Ma Klipriversberg lavas and Platberg lavas at 2708 ± 5 Ma of Armstrong et al. (1991) are correct, because they are at odds with the potentially correlatable Derdepoort lavas at 2782 ± 5 Ma (Wingate, 1998). The Hartswater volcanics can also be correlated with the Platberg volcanics, with ages of 2733 ± 3 Ma and 2724 ± 5 Ma (de Kock et al., 2012), further casting doubt on the ages of 2714 ± 8 Ma and 2708 ± 5 Ma of Armstrong et al. (1991). The correlation of the ca Ma Rykoppies dykes of Olsson et al. (2010) and Olsson (2012) to the basaltic andesite lavas of the Allanridge Formation may be possible, since this uppermost part of the Ventersdorp Supergroup is younger than its 2709 ± 5 Ma Makwassie Formation from the middle part (Armstrong et al., 1991). This inference agrees with a similarity in geochemical characteristics between the Rykoppies dykes and lavas from the Allanridge Formation according to Klausen et al. (2010), although the use of geochemistry in correlation for the volcanic sequences has to be cast in doubt, especially given the geochemical similarities between the varieties of magmatic events, particulary in the Archaean. Palaeomagnetic studies are in agreement however. The age of 2662 ± 3 Ma dates the emplacement of a younger sub-swarm within the Rykoppies dyke swarm. All these dykes are interpreted at present to be of the same generation, despite the significant scatter (Olsson, 2012). The later intrusive stage of the Rykoppies swarm coincides with Rb-Sr whole rock ages known for the protobasinal sequences of 2657 to 2659 Ma and 2664 ± 1 Ma for felsic lavas in the Buffelsfontein Group (SACS, 1993; Barton et al., 1995), at the base of the Transvaal Supergroup. The results of Olsson et al. (2010) and Olsson (2012) suggest that at 2.66 to 2.68 Ga, the eastern Kaapvaal Craton was subjected to extension in an approximate north to south direction, which allowed mantle-derived magmas to be emplaced as E- to W-trending dykes. The timing of the Rykoppies swarm appears to coincide with a shift from mainly volcanic rocks of the Ventersdorp Supergroup to predominately sedimentary rocks of the Transvaal

157 Chapter: 8 Discussion Supergroup. According to Burke et al. (1985) and van der Westhuizen et al. (1991), the Allanridge Formation lavas were formed during renewed rifting of the Kaapvaal Craton. The rift structures of the Ventersdorp Supergroup have been assigned a NE-trend (Stanistreet and McCarthy, 1991), but seismic investigations also reveal the presence of E- to W-trending faults within rock units of the Ventersdorp Supergroup (Tinker et al., 2002). Eriksson et al. (2001) proposed that the protobasinal successions at the base of the Transvaal Supergroup were deposited within a rift-type extensional tectonic setting and inferred a tectonic model where the protobasinal successions accumulated in ENE- to WSW-trending rift systems. The age correlations of the younger sub-swarm of dykes with the protobasinal sequences and the proposed correlation between the older sub-swarm and the Allanridge Formation support the idea that the 2.70 to 2.65 Ga Rykoppies Dyke Swarm marks the onset of rifting of the Transvaal Basin, at a time when the rift system also changed direction from NE- to E- trending. This interpretation is supported in the south-easternmost region of the Kaapvaal Craton, with one generation of NE-trending dolerite dykes having been dated by U-Pb in baddeleyite at 2652 ± 11 Ma. In addition, the magnetic component F bears a similar ca Ma direction among the ENE-trending dolerite dykes in the region, being supported by a baked contact test not only in this study, but also across similar ENE-trending dolerite dykes on the south-easternmost Kaapvaal Craton, as was determined by Lubnina et al. (2010); see discussion below. Although it is difficult to verify ages, it is certain that both E-W and SW-NE directed tectonic stresses were applied during the termination of Ventersdorp volcanism and the onset of filling of the Transvaal basin, and that these events were seen across the larger Kaapvaal Craton, as well as being present in this south-easternmost area too. Hatton (1995) and Eriksson et al. (2002) regard the Ventersdorp Supergroup as the product of a mantle plume, possibly related to a global mantle plume event at ca Ga (Condie, 1998; 2001). Hatton (1995) stated that the Ventersdorp magmas were extracted from the mantle at shallow depths (ca. 18 to 40 km), and subsequently underwent crustal contamination, accounting for the calc-alkaline geochemistry. A fast lava extrusion rate up through the lower half of the Ventersdorp Supergroup from 2714 to 2709 Ma is consistent with typical LIPs containing flood basalts (Armstrong et al., 1991). However, these ages are almost certainly incorrect (Wingate, 1998; de Kock et al., 2012). Therefore the proposition

158 Chapter: 8 Discussion that the Rykoppies dyke swarm might be a product of prolonged persistent plume activity may not be correct. A mantle plume origin would agree with the radiating pattern of the Rykoppies swarm from Olsson et al. (2010; 2011). However, the presence of a primitive mantle signature of the 2652 ± 11 Ma NE-trending dolerite dyke swarm in northern KwaZulu-Natal would argue against a radiating pattern, even if it is more indicative of a plume based on geochemistry. In contrast to the mantle plume model, Burke et al. (1985) and Stanistreet and McCarthy (1991) proposed a rifting model related to the collision between the Kaapvaal and Zimbabwe cratons at ca Ga, where the Rykoppies dyke swarm was emplaced within a hinterland horst and graben setting. The 2671 ± 2 Ma U-Pb age of the Matok Granite in the Southern Marginal Zone of the Limpopo Belt (Barton and van Reenen, 1992) can be temporally linked to the Rykoppies swarm, inferring coeval emplacement of magmas in the eastern and northern Kaapvaal Craton and granite intrusions along the northern margin of the craton. Also, peak metamorphism of the Southern Marginal Zone has been constrained to 2691 ± 7 Ma according Kreissig et al. (2001). However, the timing of the Kaapvaal-Zimbabwe collision is debatable, because studies show that metamorphism and deformation within the Central Zone of the Limpopo Belt occurred much later, at approximately 2.00 Ga (e.g., Kamber et al., 1995; Buick et al., 2006; van Reenen et al., 2008; Kroner et al., 1998; Jaeckel et al., 1997). There are, furthermore, no major dyke swarms and LIPs older than 2.00 Ga that mutually exist in both the Kaapvaal and the Zimbabwe cratons, supporting an amalgamation collision forming the Kalahari Craton after ca Ga (e.g., Söderlund et al., 2010). The proposed late formation of Kalahari Craton does not rule out a continental back-arc setting as a plausible tectonic model for the Rykoppies Dyke Swarm at ca to 2.65 Ga according to Olsson et al. (2010). Olsson et al. (2010) state that the Rykoppies swarm could have developed in a backarc rift system at the same time as the northern margin of the Kaapvaal Craton underwent compressional tectonics during south-directed subduction of oceanic lithosphere. Further, Olsson et al. (2010) stated that support for this model lies in the interpretation that the protobasinal sequences at the base of the Transvaal Supergroup represent intra-cratonic depositories, whose provenance can be linked to erosion of Limpopo Belt sedimentary source rocks from work by Eriksson et al. (2002). This would suggest a greater complexity to the tectonic setting of the craton during the closure of the Ventersdorp basin and the opening of the Transvaal basin

159 Chapter: 8 Discussion Ventersdorp Supergroup lava analyses show that the oldest ca Ma Derderpoort lavas are correlatable with the Klipriviersberg lavas (Wingate, 1998). Geochemically distinct lava formations within the overall more basaltic Klipriviersberg Group match NE-trending dykes near the Johannesburg dome according to McCarthy et al. (1990). More andesitic Allanridge Formation lavas are, on the other hand, closer in age to the ca Ga radiating NE-, E- to SE-trending dolerite dyke swarm on the eastern Kaapvaal Craton. Furthermore, the Allanridge lavas show the best compositional overlap with both Rykoppies E-trending dykes and more evolved SE-trending dykes from the Badplaas- Barberton area (see Fig. 67), by Klausen et al. (2010). Figure: 67 ENE-trending dolerite dykes in northern KwaZulu-Natal (black) compared to ca Ga E-trending dolerite dykes in the Rykoppies region with data from Klausen et al. (2010). Both generations of north-east trending dolerite dykes in northern KwaZulu-Natal are also illustrated (red and blue), and compared to NEtrending dolerite dyke data by Klausen et al. (2010) in northern KwaZulu-Natal. Geochemistry of the Allanridge lavas of the Ventersdorp Supergroup is also shown with data from Crow and Condie (1988), Marsh et al. (1992), Nelson et al. (1992) and Keyser (1998) A similarity exists between more calc-alkaline Allanridge Formation lavas, most ca to 2.65 Ga E-trending dykes in the Rykoppies area, and MgO-poor SE-trending dykes from the Badplaas-Barberton area (Klausen et al., 2010). The latter observation is consistent with more evolved (andesitic) dolerites dykes being part of a radiating ca Ga pattern. However, NE-trending ca Ga dolerite dykes have been dated on the south-easternmost

160 Chapter: 8 Discussion region of the Kaapvaal Craton in this study, casting this proposed radiating pattern for ca Ga aged dykes and a mantle plume origin into doubt. According to Klausen et al. (2010), the Allanridge Formation lavas plot relatively close to variably contaminated samples from Rykoppies dyke swarm, as well as ca Ga SE-trending dykes from the Badplaas-Barberton area. Apart from slightly more elevated Nb and Sr, the basaltic andesites from the Allanridge Formation overlap the Rykoppies ridge samples well. The ca Ga SE-trending dykes from the Badplaas-Barberton area have overall lower but sufficiently similar trace element patterns compared to these lava samples, suggesting that the entire radiating ca Ga swarm acted as a feeder system to the Allanridge Formation according to Klausen et al. (2010). Klausen et al. (2010) made no match based on geochemistry to dolerite dykes on the south-easternmost Kaapvaal Craton, although the one generation of NE-trending dolerite dykes (NE030), as well as ENE-trending dykes have a similar age and palaeomagnetism respectively. The ENE-trending dolerite dykes in northern KwaZulu-Natal display a similar, albeit variable pattern with data from the ENE-trending dolerite dykes in the Rykoppies area. However, a much more primitive ca Ga set of NE-trending dolerite exists (NE030), with the only dykes exhibiting a similar geochemical trend coming from the Mazowe dykes on the Zimbabwe Craton of speculated ca Ga age, creating a great deal of controversy between the two sets of dykes trends, ages and geochemistry. Work will need to be done in order to resolve this issue. The dual polarity H component of Lubnina et al. (2010) is characteristic of E-trending dolerite dykes of the Rykoppies swarm of the southeastern and eastern areas of the Kaapvaal Craton. The primary origin of this component is supported by a positive contact test. The age of these dykes is ca Ga, with this also bring the age of the magnetisation (see Fig. 68). The RYK palaeopole, calculated from the H component, is close to the ca. 2.7 Ga pole for the Allanridge Formation (Strik et al., 2007; de Kock et al., 2009). However, the Rykoppies direction is removed by about 20 from the Mbabane pluton direction dated at 2690 Ma (Layer et al., 1988; 1989). This pole, however, is not supported by field stability tests, and can be left out from further discussion. The 2782 ± 5 Ma Derdepoort volcanics have a similar direction, however (Wingate, 1998). The ENE-trending dolerite dykes on the south-easternmost Kaapvaal Craton also show this same palaeopole direction, as shown in this study (Component F), and in work done by Lubnina et al. (2010), indicative that

161 Chapter: 8 Discussion Ventersdorp volcanic activity was present in the vicinity, with NE- and ENE-trending dolerite dykes both showing evidence of it, even in the absence of Ventersdorp strata. The ca Ga NE-trending dolerite dykes, which show a primitive, uncontaminated mantle source indicative of a plume, and perhaps a close proximity to one of the piercing points from which the NE-trending swarm can be traced to the edge of the south-easternmost Kaapvaal Craton. They have a very different geochemistry from the ca Ga ENE-trending dolerite dykes shown above. Figure: 68 Palaeomagnetic results for the Ventersdorp volcanics and related intrusives, including the ENEtrending dolerite dyke (F) and re-magnetised Hlagothi Complex (A and B) Correlation with the ca Ga Soutpansberg dykes and lavas The ca Ga dykes of Klausen et al. (2010) and Olsson (2012) are compared with lava samples from the Soutpansberg Group (Crow and Condie, 1990; Bumby et al., 2001), as well as sill samples from the underlying Waterberg Group (Hanson et al., 2004a), in addition to Mashonaland sill and dyke samples, and Mazowe dyke samples from the Zimbabwe craton (Stubbs et al., 1999; Stubbs, 2000). In contrast to the large compositional range among Archean lavas (i.e., from basaltic andesites to rhyolites), almost all of the ca Ga lavas, sills and dykes previously studied are exclusively basaltic tholeiites, which match most NE

162 Chapter: 8 Discussion trending dykes from the Black Hills area, but not the dated ca Ga NE-trending dykes in northern KwaZulu-Natal. Figure: 69 ENE-trending dolerite dykes in northern KwaZulu-Natal (black) compared to ca Ga Waterberg and Mashonaland sills and dykes, including the ca Ga NE-trending dolerite dykes of the Black Hills area. Interestingly NE-trending dykes in northern KwaZulu-Natal do not compare well to any dykes and sills located further to the north, with the exception of the Mazowe dykes on the Zimbabwe Craton Despite what is stated in Klausen et al. (2010), a poor geochemical comparison invalidates the proposed link between all of these different units, and the NE-trending dykes from northern KwaZulu-Natal. The exception is the Mazowe dolerite dykes, which have essentially flat REE patterns unlike the slightly LREE enriched patterns seen in the other dykes and sills of supposedly coeval intrusions (see Fig. 69). There is REE data available for the Soutpansberg Group lavas, Waterberg sills (Hanson et al., 2004), Mashonaland sills (Stubbs et al., 1999), and Mazowe dykes, however (Stubbs, 2000). Klausen et al. (2010) used only intrusions in multi-elemental comparisons, because especially poor LIL element matches suggest that lava samples may have been altered. Klausen et al. (2010) furthermore stated that: The Waterberg sills have slightly higher Sr/V. Some Mashonaland sills tend to have lower Sr/V

163 Chapter: 8 Discussion Mazowe dykes have (La/Yb) N values that are as low as those of NE-trending dykes in northern KwaZulu-Natal. Thus ca Ga NE-trending dykes of the Black Hills area define a restricted subgroup of roughly parallel patterns which overlaps a tight cluster of the Mashonaland sills, as stated by Klausen et al. (2010), and resemble the ca to 1.87 Ga post-waterberg dolerites of Hanson et al. (2004a). The Mazowe dykes from the northern Zimbabwe Craton have much more depleted patterns, which resemble the variably feldspar-phyric NEtrending dykes from northern KwaZulu-Natal, such as NE030, in the south-easternmost part of the Kaapvaal Craton. Age dating was done on the NE-trending dykes of northern KwaZulu-Natal at ca Ga in this study, but these dykes may be coeval with the Mazowe dykes based on geochemistry too, casting further doubt on the suggestion that are part of a ca Ga age LIP (Wilson et al., 1987; Klausen et al., 2010). Figure: 70 Palaeomagnetic results for the Bushveld Complex and post-bushveld volcanics and related intrusives, including also the Vredefort impact and Black Hills dykes (BH), in comparison to the pole obtained by Lubnina et al. (2010) in northern KwaZulu-Natal from the NE-trending dolerite dykes there. See Table 10 for sources of other poles The component (M) of Lubnina et al. (2010) was found in NE-trending Black Hills dolerite dykes from the north-eastern, south-eastern and south-easternmost areas (see Fig. 70). A palaeomagnetic pole, recalculated from this component (BHD), is similar to the

164 Chapter: 8 Discussion Ma Post-Waterberg dolerites (Hanson et al., 2004a), and also the 2990 to 2875 Ma Ushuswana Complex (Layer et al., 1988; Olsson, 2012). The age of the dykes according to Olsson (2012) and Söderlund et al. (2010) is ca Ga in the Black Hills area of the northeastern Kaapvaal Craton. However, over 500 km away, a NE-trending dolerite swarm on the south-easternmost Kaapvaal Craton displayed a similar magnetic component (M), and the primary origin of magnetisation in these dykes was supported by positive contact, conglomerate and reversal tests. Lubnina et al. (2010) concluded that the age of magnetisation is ca Ga in this region as well, in stark contrast to the ca Ga age, complicating the already complex array of dolerite dyke swarms across this region of the craton. NE-trending dolerite dykes studied for palaeomagnetism in this work, however, showed chaotic distribution and the data thus could not be used, but both the geochronology (done herein) and palaeomagnetism by Lubnina et al. (2010) have to be accepted as correct. In the absence of further more detailed work, NE-trending dolerite dykes may be composed of up to two, and possibly three dolerite dyke events at possibly ca. 2.87, 2.65 and 1.90 Ga. Further study is needed in order to resolve this issue too Dyke swarms of potentially other ages The variably feldspar-phyric NE-trending dykes from northern KwaZulu-Natal have been shown to be as equally depleted as the Mazowe dykes (Stubbs, 2000). However, uncertainties regarding the presumed ca Ga age of the Mazowe dykes (Wilson et al., 1987), the ca Ga age presented for the NE-trending dykes in northern KwaZulu-Natal, and a greater than 1000 km separation between the Mazowe and NE-trending dykes renders such a correlation difficult, although the palaeomagnetism and geochemistry appears similar. Thus, unless the ca Ga Mazowe dyke age is incorrect and/or the ca Ga LIP was much more extensive, and was fringed by depleted mantle source areas, more data is needed to resolve the correlation and tectonic setting of the Mazowe and NEtrending dykes in northern KwaZulu-Natal, which may belong to a newly recognised separate swarm. In addition to the complexities observed above, Lubnina et al. (2010) identified another component across the eastern and south-eastern Kaapvaal Craton. A palaeopole

165 Chapter: 8 Discussion (NSA) was calculated from dykes of the south-eastern area of the craton, that is close to the pole of Ongeluk Formation lavas with an age of ca Ga (Evans et al., 1997) and the Gamagara Formation lavas with an age of ca Ga (Evans et al., 2002). Both these formations occur within the greater Transvaal Supergroup, suggesting that magmatism of this age may be extended across the larger craton. The SE-trending dolerite dykes in northern KwaZulu-Natal are connected with two events: one at ca Ga, and the other at ca Ga. It is clear that the relationship, if any, between these dyke swarms needs to be resolved from the conflicting geochronology and palaeomagnetism. This is also the case for the ages and palaeopoles amongst the ca Ga and ca Ga NE- and ENE-trending dolerite dykes discussed above and below. More data is needed in order to resolve the age of ENE-trending dykes in the northern KwaZulu-Natal and Rykoppies areas. Also, the dykes in the Black Hills area too, because of their more pristine petrography, may have an age younger than the area s ca Ga NE-trending dykes (Lubnina et al., 2010). One possibility is that these more pristine dykes are coeval with the ca Ga Umkondo large igneous province (Hanson et al., 2006), which is found extensively across the whole larger Kalahari Craton, and which may be present in northern KwaZulu-Natal, too. Another component is characteristic of SSE-trending dykes in the south-eastern Kaapvaal Craton, as well as dykes and sills in the south-easternmost area, and which was observed as a low- to medium temperature overprint within ENE-trending dykes within this study. A palaeomagnetic pole (E), was recalculated from this component, and is close to the present local field, as well as the ca Ga pole for a Karoo Dolerite (Hargraves et al., 1997), with dykes and sills related to this event extensive across the south-easternmost Kaapvaal Craton Tectonic model and a new large igneous province

166 Chapter: 8 Discussion In summary, the petrological and palaeomagnetic discriminations between borderline tholeiitic to calc-alkaline SE- and ENE-trending dolerite dykes, as well as the two generations of tholeiitic and calc-alkaline NE-trending tholeiitic dykes are consistent with precise U-Pb baddeleyite ages and palaeomagnetic results obtained herein, and by Olsson et al. (2010) and Lubnina et al. (2010). This indicates the presence of a ca and ca SE-trending dolerite dyke swarms on the south-easternmost Kaapvaal Craton, of which the ca Ga dolerite dyke swarm is also seen further to the north in the Barberton-Badplaas area. In addition, two pulses of ca NE- to ENE-trending dolerite dykes are seen across the south-easternmost part of the craton which have different geochemistry related to NE- and ENE-trending stresses related to the termination of the Ventersdorp and opening of Transvaal basins across the greater craton The Nsuze igneous event SE-trending dolerite dykes primarily outcrop across the south-eastern Kaapvaal Craton in the Barberton-Badplaas area, as well as the south-easternmost area in the vicinity of Vryheid-Melmoth, with this ~250 km wide swarm most likely coinciding with the reconstructed extent of the Pongola Supergroup cover (Weilers, 1990). Sub-parallel trending dykes and rift basin structures (Hunter and Halls, 1992; Uken and Watkeys, 1997), similar dyke and lava ages (Hegner et al., 1994; Olsson et al., 2010), and roughly comparable compositions and palaeomagnetic poles (Klausen et al., 2010; Lubnina et al., 2010) strengthen previous suggestions of a co-genetic lava-feeder system within one of the world s oldest volcanic intra-continental rifts (Burke et al., 1985). An apparent predominance of borderline tholeiitic to calc-alkaline basaltic and basaltic andesite compositions in SE-trending dykes furthermore suggests that more evolved Nsuze Group lavas generally differentiated in more elevated shallow crustal magma chambers, like the elongated and sub-parallel SE-trending ca Usushwana Complex that appears to have been emplaced at the base of the Nsuze Group lava pile. This would also account for the apparent lack of more primitive magma compositions within the volcanic succession and its plutonic equivalents (Armstrong et al., 1986)

167 Chapter: 8 Discussion Coeval granitoid ages across most of the Kaapvaal Craton s north-eastern segment (Olsson et al., 2010; Eglington and Armstrong, 2004; Zeh et al., 2009), are consistent with active subduction along the craton s northern margin, in conjunction with crustal amalgamation onto the craton s western margin along the Colesberg lineament (Poujol et al., 2008; Schmitz et al., 2004). Mountain building along these margins could also provide gold-rich sediments for south- to south-eastward directed fluvial systems into the combined Witwatersrand-Mozaan basin (e.g., McCarthy, 2006). Such a tectonic setting has been used to explain the calc-alkaline and andesitic character of Dominion Group lavas, and could likewise explain the ca Ga Nsuze Group s sub-alkaline series of basaltic andesites to rhyolites, without any significant silica gap (e.g., Klausen et al., 2010; McCulloch and Gamble, 1991). These roughly coeval lavas and associated SE-trending feeder dolerite dykes could have been emplaced along the landward side of an active continental margin or within a more inland continental back-arc setting (e.g., Olsson et al., 2010; Burke et al., 1985; Winter and de La, 1987; Crow and Condie, 1988). Other geochemically based models do not require such a subduction zone setting, favouring either crustally contaminated komatiitic primary mantle melts according to Crow and Condie (1990), or basaltic primary melts from a metasomatised lithospheric mantle (e.g., Klausen et al., 2010; Duncan, 1987; Marsh et al., 1992). Both models could account for generation of LIL element enriched, borderline tholeiitic to calc-alkaline and basaltic to andesitic magmas (with negative Nb, Ta anomalies for example) within Archaean continental rifts across the Kaapvaal Craton. This includes the Dominion, Nsuze and Ventersdorp lavas and their plutonic equivalents, which can also be seen on the south-easternmost Kaapvaal Craton. Structural evidence, including the SE-trending feeder dolerite dyke swarm, favours a volcanic rift setting for both the Dominion (e.g., Lana et al., 2006) and Nsuze Group lavas (Bickle and Eriksson, 1982). The north-west to south-east orientated Pongola volcanic rift is also located very far from, and trends at a high angle to any subduction-zone along either the northern or western margin of the Kaapvaal Craton (Burke et al., 1985). Furthermore, the SE-trending swarm with north-westerly bifurcating dykes indicates emplacement from the south-east (Hunter and Halls, 1992), and may have originated from an igneous plume centre, near a hypothetical passive margin along the Kaapvaal Craton from which northern

168 Chapter: 8 Discussion KwaZulu-Natal s geochemically and palaeomagnetically similar SE-trending dykes also might have been injected as part of a greater swarm, as suggested by Lubnina et al. (2010) The Hlagothi igneous event The 2866 ± 2 Ma sills and 2874 ± 2 Ma SE-trending dykes of the Hlagothi Complex, as well as the Thole Complex, acted as the plumbing system that fed the Mozaan Group flood basalts. The Usushwana Complex can also be shown to be composed of at least two magmatic pulses between 2990 Ma and 2860 Ma (Olsson, 2012; Hammerbeck, 1982). The variation of ages in the Usushwana Complex could be explained if some portions of the complex in fact represent Hlagothi-Thole Complex age intrusions, and not the older 2990 Ma date obtained by Olsson (2012). Shortly after the intrusion of the Hlagothi Complex, the whole region was intruded by potassium-rich post-pongola granites. These granites may have been the result of partial melting induced by the culmination of the same thermal source as that which generated the Hlagothi Complex. The dated SE-trending dykes in the White Mfolozi inlier and the sills of the Hlagothi Complex in this study, coupled with the Thole Complex and also possibly some gabbroic phases within the Usushwana Complex, form a part of a newly recognised large igneous province in the south-easternmost part of the craton (see Fig. 71). The extrusion of the Mozaan lavas and potential correlatives in the central and eastern portions of the West Rand Group and Central Rand Group of the Witwatersrand Supergroup may also belong to this large igneous province. Geochemistry of the Hlagothi Complex and the SE-trending dolerite dykes suggests two different pulses of magmatism, both with evidence of crustal contamination or melting of a sub-arc-like mantle wedge. The geochemical evidence for an arc-like tectonic setting and subduction is strong, but it is common for most volcanic rocks of the Kaapvaal Craton, even those of known rifting settings, such as in the Nsuze and Dominion groups (Klausen et al., 2010). There are also younger LIPs with arc-like signatures acquired from enriched lithospheric mantle, such as the Karoo LIP, which was emplaced in a known extensional tectonic regime (Duncan, 1987). Such a signature is most likely derived from crustal contamination or contamination from metasomatised sub-continental lithosphere, rather than reflecting an in-situ produced arc-like signature, and may also

169 Chapter: 8 Discussion explain the retention of LIL and depletion of HFS elements. It is also important to note that the depositional environment of both the Nsuze and the Mozaan groups reflects an intracratonic basin, and that erosional unconformities prior to volcanic deposition in the Mozaan are suggestive of domal uplift before volcanism (see Fig. 72). Figure: 71 New magmatic barcodes for the eastern and western sides of the Kaapvaal Craton with ages from a variety of sources discussed in the text, highlighting a new possible large igneous province

170 Chapter: 8 Discussion Figure: 72 Tectonic setting for the proximal to distal Witwatersrand and Mozaan basin respectively, with a large igneous province induced by a mantle plume leading to the upward coarsening sedimentary sequences in the basin, culminating in flood volcanism being fed by a series of SE-trending dolerite dykes and layered complexes

171 Chapter: 8 Discussion The basalt sequences in the upper Witwatersrand-Mozaan strata are thick, monotonous sequences of lava, with little interflow of sediment suggestive of flood basalts (with the exception of the Ntanyana lava unit), and were produced through fissure eruptions (Hammerbeck, 1982; Nhleko, 2003). All these associated volcanic sequences, dykes, sills and layered complexes occur over the eastern Witwatersrand block of the Kaapvaal Craton an area of approximately km 2. This evidence is supportive of a LIP, following the definition of Bryan and Ernst (2008) The Ventersdorp igneous event Roughly NE-, E- and SE-trending ca Ga dykes appear to be restricted within a nearly 150 km wide radiating dolerite dyke swarm that converges beneath a younger Transvaal Supergroup basin (Olsson et al., 2011). More northerly-located ENE-trending dykes are probably younger (less than ca Ga and likely ca Ga) in the north-eastern Kaapvaal Craton, whereas tholeiitic ca Ga NE-trending dolerite dykes and calc-alkaline ENEtrending dykes in the northern KwaZulu-Natal window may have fed the same roughly coeval event(s), which is documented by geochronology and palaeomagnetism. The Ventersdorp Supergroup lavas could have erupted in a foreland basin behind a collision zone between the Zimbabwe and Kaapvaal cratons (McCarthy, 2006; Zeh et al., 2009). Such a setting is supported by: Roughly coeval metamorphism along the Limpopo Belt s northern and southern zones (Kramers et al., 2006) A conjugate set of major transfer faults across the Kaapvaal craton, which is compatible with approximately north-south directed compression (Stanistreet and McCarthy, 1991). The borderline tholeiitic to calc-alkaline character of Ventersdorp Supergroup lavas and feeder dykes (Klausen et al., 2010). However, this model is also questioned by:

172 Chapter: 8 Discussion The ca. 2.0 Ga ages along the Limpopo Belt s Central Zone (Kamber et al., 1995; Mourie et al., 2008). General uncertainties regarding age-determinations on fault displacements. The above mentioned possibility that the calc-alkaline affinity of most intracontinental basaltic rocks is inherited either from crustal contamination (Crow and Condie, 1990) or the partial melting of a metasomatised continental lithosphere (Duncan, 1987; Marsh et al., 1992). Despite the plate tectonic setting, the rapid emplacement of the more basaltic Klipriviersberg Group, including its komatiitic base flows, has also been attributed to the presence of a mantle plume (Hatton, 1995; Eriksson et al., 2002). However, the older ages on the Derderpoort basalts, as well as Hartswater rhyolites and tuffs cast doubt on this (Wingate, 1998; de Kock et al., 2012) A sequence of sub-swarms with differently trending feeder dykes and geochemistry provides additional constraints on the tectonic evolution during the emplacement of the Ventersdorp Supergroup. Firstly, NE-trending dykes that are geochemically matched to individual lava formations within the Klipriviersberg Group around the Johannesburg dome (McCarthy et al., 1990) reflect a south-west to north-east directed maximum compressive palaeo-stress field that probably generated sub-parallel rift-structures (e.g., domino-block faulting; van der Westhuizen et al., 2006) during their emplacement at 2782 Ma according to Wingate (1998). The NE-trending dolerite dykes at ca Ga on the south-easternmost Kaapvaal Craton could be coeval with these dykes, however they have a much more primitive geochemistry and could instead be linked to the komatiitic flows at the base of the Ventersdorp (Klipriviersberg lavas), or to the Mazowe dykes on the Zimbabwe Craton. McCarthy et al. (1990) matched NE-trending dolerite dykes in the Johannesburg Dome to the uppermost Lorain Formation of the Klipriviersberg Group, indicating a shift towards a more east-west directed maximum compressive palaeo-stress field near the end of Klipriviersberg Group lava eruptions, and not during Transvaal Supergroup times. Further geochemical matching of ca to 2.65 Ga radiating NE-, E and SE-trending dolerite dykes across eastern Kaapvaal with slightly more andesitic lavas from the younger Allanridge Formation from less than 2708 Ma to greater than 2687 Ma according to Cheney (1996),

173 Chapter: 8 Discussion conforms to McCarthy et al. s (1990) latter palaeo-stress field however. Thereby, this possibly identifies a regional shift towards an east-west orientated maximum compressive stress field. This was maintained during the emplacement of the Platberg Group, Allanridge Formation, Transvaal Supergroup (Eriksson et al., 2006), and the 2.05 Ga Bushveld Complex (Cawthorn et al., 2006); i.e., for another ca. 600 million years. This would account for the ca Ga Ventersdorp palaeomagnetic component identified by Lubnina et al. (2010), on the south-easternmost Kaapvaal Craton. This was seen in the ENE-trending dolerite dykes, different from the ca Ga dolerite dykes in the same region. However, a supposed ca Ga radiating north-east, east to south-east pattern also converges toward an igneous centre below the current eastern lobe of the Rustenburg layered suite, which may coincide with the proposed mantle plume source for the Klipriviersberg Group lavas (Hatton, 1995; Eriksson et al., 2002). This is not, however, supported by the ca Ga age of the NEtrending dolerite dykes of south-easternmost Kaapvaal Craton The Soutpansberg-Mashonaland igneous event Similar tholeiitic basalt compositions are consistent with a unified ca Ga swarm, which may include some NE-trending dolerite dykes in the south-easternmost Kaapvaal Craton, that have been matched palaeomagnetically by Lubnina et al. (2010) to ca Ga dykes further north on the craton. The enriched NE-trending dykes in northern KwaZulu-Natal may be part of a greater than 200 km wide swarm that extends across most of the eastern Kaapvaal Craton already identified, partly hidden by coeval Soutpansberg Group deposits in the north and truncated by the ca to 0.50 Ga East African (Mozambique)-Antarctic Orogenic belt that bounds the eastern margin of the Kalahari Craton (e.g., Jacobs et al., 2008). The locally kinked pattern of more NNE-trending dykes across the eastern Transvaal Supergroup and the underlying Kaapvaal segment (Uken and Watkeys, 1997) is more likely to be primary (e.g., reflecting a regional stress that was distorted beneath the load of the Transvaal basin, including the Bushveld Complex) rather than caused by any post-intrusive left-lateral shear (Klausen et al., 2010). No coeval NE-trending dykes appear to intersect the Zimbabwe Craton, because Plumtree dykes are older (Söderlund et al., 2010) and 40 Ar/ 39 Ar ages (Jourdan et al., 2006)

174 Chapter: 8 Discussion indicate that the ENE-trending Save-Limpopo dyke swarm is Mesoproterozoic, and therefore most likely part of the Umkondo LIP (Hanson et al., 2006). Thus, only the Mashonaland sills (Stubbs et al., 1999), and possibly some E-trending Mazowe dykes (Wilson et al., 1987; Stubbs, 2000), are coeval with some of the NE-trending dolerite dykes on the southeasternmost Kaapvaal Craton. It is possible that these intrusions concentrate near an igneous palaeo-centre at the north-eastern edge of the Zimbabwe Craton, from which a giant NE-trending swarm may have propagated laterally towards the south and west. However, even if Mazowe dykes also are tholeiitic basalts, they are too depleted to have been derived from an enriched mantle source, and are only geochemically similar to northern KwaZulu-Natal s NE-trending dykes in the opposite, south-easternmost part of the Kaapvaal Craton, separated by more than 1000 km. In addition, Hanson et al. (2011) noted paleomagnetic reconstructions consistent with a greater than 2000 km lateral displacement being accommodated in the Limpopo orogenic belt that separates the Kaapvaal and Zimbabwe cratons, further complicating the issue. The more tholeiitic basalt character of most ca Ga and 2.65 Ga NE-trending dykes (including lower LIL element and shallower, flat REE patterns) is distinctly different from ca and ca Ga ENE- and SE-trending dykes, and is possibly related to a typical continental rift setting, or perhaps even radiating from the suggested igneous centre near the north-eastern edge of the Zimbabwe Craton. On the other hand, this extensive igneous event also occurred during a complex tectonic period, including emplacement of the Bushveld Complex and a ca. 2.0 Ga collision of the Zimbabwe and Kaapvaal cratons Palaeomagnetism Our plot (see Fig. 73) of the Kaapvaal Craton palaeopoles in Table 10 illustrates the considerable latitudinal drift undergone by the Kaapvaal Craton through the Mesoarchaean and Neoarchaean. The palaeolatitude of the craton is shown to vary significantly from low latitudes at ca Ga, to much higher latitudes at ca Ga and 2.65 Ga (see Fig. 73)

175 Chapter: 8 Discussion Table: 10 - Summary of new and published palaeomagnetic data for the Kaapvaal Craton and adjacent orogens used in this study Age uncertainty Pole Abbreviation Age (Ma) Age reference (Ma) Longitute in N Latitude in E dp in dm in or (A95 in ) Paleomag reference Nsuze basalts NB 2984 ± 3 Hegner et al., Lubnina et al., 2010 Badplaas dyke swarm BAD ± 1 Olsson et al., Lubnina et al., 2010 Hlagothi comp. D D 2866 ± 2 this study this study NE-trending dykes D ca n/a n/a this study this study Agatha basalts AB 2977 ± 5 Nhleko, (8.9) Strik, 2007 Usushwana Complex UC n/a n/a Olsson et al., 2011; Hunter and Reid, (7.6) Layer et al., 1988; 1989 Hlagothi comp. B B n/a n/a this study this study Hlagothi comp. A A n/a n/a this study (17.1) this study Modipe Gabbro MG 2784 ± 3 Feinberg et al., (10.5) Evans and McElhinny, 1966 Gabarone Granite GG 2783 ± 2 Moore et al., Evans, 1967 Derdepoort basalts DB 2782 ± 5 Wingate, (17.5) Wingate, 1998 ENE-trending dykes F 2650 n/a n/a this study this study Allanridge Formation AFL n/a n/a de Kock et al., (6.1) de Kock et al., 2009 lavas Rykoppies dyke swarm RYD n/a n/a Olsson et al., Lubnina et al., 2010 Mbabane Pluton MP 2687 ± 6 Layer et al., (9.7) Layer et al., 1989 Ongeluk lavas ONG 2222 ± 13 Evans et al., (5.3) Evans et al., 1997 SE trending dykes, south NSA ca n/a n/a Lubnina et al., Lubnina et al., 2010 of Badplaas Gamagara/ Mapedi BGM 2130 ± 92 Evans et al., Evans et al., 2002 Formation Palaborwa Complex PB ± 1 Heaman and LeCheminant, Morgan and Briden, 1981 Bushveld Complex BVC 2058 ± 2 Olsson et al., (5.0) Letts et al., 2009 Waterberg Unconformity bounded WUBS ± 4 Dorland et al., (10.9) de Kock, 2007 sequence 1 Vredefort impact VRED 2023 ± 4 Kamo et al., Salminen et al., 2009 structure Waterberg Unconformity bounded WUBS2 ca n/a n/a de Kock, (9.8) de Kock, 2007 sequence 2 Hartley lavas HAR 1928 ± 4 Evans et al., (16.0) Evans et al., 2002 Post- Waterberg PWD n/a n/a Hanson et al., 2004a (17.3) Hanson et al., 2004a dyke swarm Black Hills dyke swarm BH ca n/a n/a Lubnina et al., Lubnina et al., 2010 Ntimbankulu Granite NG ca n/a n/a Maré and Thomas, (18.0) Maré and Thomas, 1998 Namaqua- Natal NN ca n/a n/a Onstott et al., Hlagothi comp. C C ca n/a n/a this study (19.6) this study SE-trending dolerite dykes C ca n/a n/a this study this study

176 Chapter: 8 Discussion The significant change in the position of palaeopoles between 2.95, 2.87, 2.78 and 2.65 Ga would argue against a single relatively stationary persistent mantle plume being responsible for all the lavas, dykes and sills between the Nsuze, Hlagothi and Ventersdorp events. This rather points toward short-lived transient plumes being operative in the Archaean. The postulated ca Ma Hlagothi plume would have strongly influenced the development of the Pongola and Witwatersrand basins. It would have assisted in causing uplift and subsidence forming several unconformities in the strata. These unconformities would have been a result of uplift and erosion causing marine transgressions and regressions, with cratonic flooding seen during maximum thermal subsidence of a plume. Figure: 73 Pole comparison between VGPs of components A, B, C (1, 2 and 3), D (1 and 2) and F is this study, and VGPs and poles for the Archaean in (a), and the Proterozoic in (b) and (c). The various abbreviations in the diagrams are shown in Table 10 The palaeolatitude of the Kaapvaal Craton changed significantly, moving from northern high latitudes near the poles at ca Ga, to approximately 60 south at ca Ga and again to about 40 south at ca Ga, accounting for the newly established VGPs using component D, A, B and finally F for a position during the time of the Ventersdorp igneous event(s). Between ca Ga and ca Ga the Kaapvaal Craton changed modestly in palaeolatitude (moving slightly northward) but rotated dramatically by about 140 clockwise, which may account for the lack of magmatic events seen during this time. Between ca and ca Ga the position and orientation remained constant giving us

177 Chapter: 8 Discussion more confidence that the data for both ca and 2.15 Ga events are reliable. By the time of the ca Ga Bushveld Complex, the latitude of the craton remained constant and there is a slight further clockwise rotation of the Kaapvaal Craton by about 40. The remaining key pole occurs at ca Ga by which time the palaeolatitude increased slightly to the south, reaching about 45 south and there was still a further 80 clockwise rotation (de Kock et al., 2006; Hanson et al., 2011). Of course there is a polarity ambiguity for each value and therefore, it cannot be excluded that during some of these intervals the Kaapvaal Craton was in the antipodal position (i.e., in the northern hemisphere and rotated by 180 ). As the APWP is developed for other cratonic blocks, this pattern for the Kaapvaal Craton can be usefully compared in order to constrain Neoarchaean to Palaeoproterozoic reconstructions involving the Kaapvaal Craton. In addition, the ability to illustrate the presence of a magnetic remanence component interpreted to be primary (i.e., component D) in the least altered lithologies of the Hlagothi Complex is significant. This component is interpreted as primary, despite it resembling Palaeoproterozoic palaeopoles from the Bushveld Complex, Waterberg Group and Vredefort impact structure. The Hlagothi Complex is of Mesoarchaean age, and in a location far removed from the thermal effects of the Bushveld Complex, the Vredefort meteorite impact and LIPs within the sedimentary basins associated with the Waterberg and Soutpansberg groups. Our VGP for this component is located in a position between the Nsuze Group basalts and dykes obtained by Lubnina et al. (2010) and the post-pongola overprint palaeopoles, as well as the 2782 Ma Derdepoort basalt pole of Wingate (1998) with which the ENE-trending dolerite dykes in the same area bear a similarity. The pole for the Usushwana Complex is now considered discrepant, being cast into considerable doubt given that it is unclear to which age group (i.e., 2990 Ma versus 2860 Ma) the pole belongs. Also, the Usushwana Complex pole does bear remarkable similarity with the ca Ga intrusions of the Palaeoproterozoic that are present in parts of the craton. Work will be needed to be done to resolve this issue, which will assist in confirming the primary nature of either the Nsuze or Hlagothi palaeopoles. The Hlagothi Complex VGP is further significantly different from the poles for the ca NE-trending Rykoppies dyke swarm and other SEtrending dykes (Lubnina et al., 2010), in addition to lavas of the Allanridge Formation (de Kock et al., 2009)

178 Chapter: 8 Discussion 8.6. Correlations with the Pilbara Craton The addition of the new ca Ma LIP provides another intraplate event for the magmatic barcode of the Kaapvaal Craton during the Archaean. This improved magmatic event barcode can be compared to other similar magmatic event barcodes around the world (see Fig. 74). Figure: 74 Correlations between the Kaapvaal and Pilbara cratons during the Meso- to Neoarchaean reconstruction of Vaalbara It has long been postulated that the Kaapvaal and Pilbara cratons were joined in the late Archaean into a supercraton named Vaalbara (e.g., Bleeker, 2003; Cheney, 1996; Cheney et al., 1988; de Kock et al., 2009; Nelson et al., 1992; Wingate, 1998; Zegers et al., 1998). The Vaalbara reconstruction is based on the correlation between magmatic pulses of the Fortescue (Pilbara Craton) and Ventersdorp (Kaapvaal Craton) flood basalts at between 2.8 and 2.7 Ga; paleomagnetism also provides constraints (e.g., de Kock et al., 2009). If this nearest neighbour relationship is correct, the Vaalbara reconstruction may also apply at ca Ma (see Fig. 74). A potential correlative on the Pilbara Craton, the Millindinna Complex yielded a poorly constrained Sm-Nd age of ca Ma (Korsch and Gulson, 1986; Schmidt and Embelton, 1985). However, more recent U-Pb age dating has obtained ages of 2925 and 3015 Ma for different Millindinna intrusions (e.g., van Kranendonk et al., 2007), suggesting that the intrusions represent at least two distinct magmatic events of different ages (and possibly a third if the ca Ma Sm-Nd age was confirmed by U-Pb dating). Alternatively,

179 Chapter: 8 Discussion we might consider the Zebra Hill dykes as possibly ca Ma. The E-W trending Zebra Hill dykes cut one of the Millindinna suites of intrusions, the 2925 Ma Munni Munni intrusion, and are unconformably overlain by the Hardey Sandstone at 2765 Ma (Barnes and Hoatson, 1994; Hoatson and Sun, 2002). If the ca Ma Hlagothi event of the Kaapvaal Craton were also confirmed to be present on the Pilbara Craton, this would further validate the existence of Vaalbara back into the Mesoarchaean. Figure: 75 Palaeo-positions of the Pilbara Craton in relationship to the Kaapvaal Craton during the time of the supercraton of Vaalbara, with two different configurations proposed based on palaeomagnetism and geology: (A) the reconstruction of de Kock et al. (2009) and (B) the reconstruction of Zegers et al. (1998)