M. Poujol a,b, *, C.R. Anhaeusser b. Abstract

Transcription

1 Precambrian Research 108 (2001) The Johannesburg Dome, South Africa: new single zircon U Pb isotopic evidence for early Archaean granite greenstone development within the central Kaapvaal Craton M. Poujol a,b, *, C.R. Anhaeusser b a Hugh Allsopp Laboratory, Uni ersity of the Witwatersrand, Pri ate Bag 3, Johannesburg WITS 2050, South Africa b Economic Geology Research Institute, Uni ersity of the Witwatersrand, Pri ate Bag 3, Johannesburg WITS 2050, South Africa Accepted 30 November 2000 Abstract The Johannesburg Dome, located in the central part of the Kaapvaal Craton, constitutes one of the key areas to better understand the Archaean crustal evolution of this part of the craton. The dome comprises a variety of Archaean granitic rocks intruded into mafic ultramafic greenstone remnants. This study presents new precise U Pb single zircon dating for seven different granitoid samples and an amphibolite dyke collected from the Johannesburg Dome. A trondhjemitic gneiss sampled on the northwestern part of the dome yielded an age of Maand represents the oldest granitoid phase recognized so far. This result has important implications with regard to the age of the mafic and ultramafic greenstone remnants scattered throughout the dome as it implies that the greenstone remnants are older than c.3.34 Ga. This initial magmatic episode, involving early greenstone development and the intrusion of trondhjemitic and tonalitic granitoids on the northern half of the dome, was followed by the emplacement of a Ma hornblende biotite tonalite gneiss in the south. Following the trondhjemite tonalite gneiss emplacement a further period of magmatism took place on the dome, which resulted in the intrusion of mafic dykes that are manifest as hornblende amphibolites. The age of these dykes has yet to be determined quantitatively, but they fall within the time constraints imposed by the age of the trondhjemitic gneisses ( Ma) and later, crosscutting, potassic granitoids. These rocks, consisting dominantly of granodiorites constitute the third magmatic event and occupy an area of batholithic dimensions extending across most of the southern portion of the dome. The southern and southeastern parts of the batholith consist mainly of medium-grained, homogeneous, grey granodiorites dated at Ma. Their porphyritic granodiorite equivalents in the southwestern part of the dome yielded an age of Ma. An age of Ma, from zircons extracted from one of the mafic dykes possessing granitic microveins, provided confirmation of the timing of this third magmatic event. Lastly, pegmatites that crosscut all these earlier granitoid events are younger than 3114 Ma and might be at least 3.0 billion-years old. These new data provide confirmation of the conclusion that the Witwatersrand Basin was deposited after c.3074 Ma on an Archaean * Corresponding author. Fax: address: 065marc@cosmos.wits.ac.za (M. Poujol) /01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S (00)

2 140 M. Poujol, C.R. Anhaeusser / Precambrian Research 108 (2001) basement as young as c.3120 Ma. The data, combined with that from other parts of the Kaapvaal Craton, further supports the view that the evolution of the Craton was long-lived and episodic, and that it grew by accretionary processes, becoming generally younger to the north and west of the c.3.5 Ga Barberton-Swaziland granite greenstone terrane situated in the southeastern part of the Craton Elsevier Science B.V. All rights reserved. Keywords: U Pb; Zircon; Archaean; Greenstone belt; Kaapvaal Craton 1. Introduction The Johannesburg Dome, which is unconformably overlain by the sedimentary successions of the Witwatersrand Supergroup, is one of the few mid-archaean granite greenstone inliers exposed in the central part of the Kaapvaal Craton. With an areal extent of approximately 700 km 2,it provides a unique window through which the Archaean basement rocks in this part of the craton can be examined. Anhaeusser (1973) provided the first comprehensive geological map of the dome. This work described the Johannesburg Dome as a mosaic of different granitic rocks that had intruded an older Archaean mafic ultramafic greenstone crust. The granitic rocks display distinctive field characteristics and variable geochemical, mineralogical and textural properties. The oldest granitic rocks comprise a suite of tonalitic and trondhjemitic gneisses and migmatites that occupy most of the northern half of the dome (Fig. 1). The south-central portion consists mainly of a variety of homogeneous, medium-grained granodioritic rocks which, in the west, are somewhat coarser grained and are commonly porphyritic in texture. Pegmatitic dykes and veins are also common. Despite the fact that the Johannesburg Dome provides an opportunity to better understand the Archaean history of this part of the Kaapvaal Craton, very few geochronological data are presently available (see later). The purpose of this work is to present new U Pb single zircon ages for the Johannesburg Dome. Seven samples representative of the different granitic rock types were collected. These include three trondhjemitic gneisses from the northern part of the dome, one tonalitic gneiss from the south and three homogeneous granodioritic rocks from other more central localities shown in Fig. 1. In addition, a mafic dyke cropping out on the northern part of the dome was also sampled (Fig. 2). 2. Geological and chronological settings 2.1. General geology A variety of mafic and ultramafic rocks, many displaying affinities with komatiites, high-magnesian basalts and tholeiites, were described as the earliest recognized greenstone rocks exposed on the Johannesburg Dome (Anhaeusser, 1977, 1978, 1992). Field mapping undertaken by Anhaeusser (1973) did not reveal the presence of any ancient gneissic crust predating these greenstones, the remnants of which have also been equated with similar rocks elsewhere on the Kaapvaal Craton (e.g. the c.3500 Ma rocks of the Barberton greenstone belt), that have been intruded, metamorphosed and migmatized by successive granitoid events as described by Anhaeusser and Robb (1981). Regional mapping, coupled with selected detailed studies of key outcrops, such as the Nooitgedacht migmatite platform (Fig. 2) seen in a river exposure in the northwestern sector of the dome, led to the establishment of a field-based relative chronology of granitic emplacement events (Anhaeusser, 1973, 1999). The earliest granitoid rocks include a suite of trondhjemitic and tonalitic gneisses (TTG s), most of which occupy the northern half of the Johannesburg Dome (e.g. samples JHBD 98-8, 98-9, 98-10, Fig. 1). Exposures of similar rocks also occur on the southern edge of the dome (represented by hornblende tonalite gneiss sample JHBD 98-1, Fig. 1) and unconformably underlie the Witwatersrand Supergroup sediments that dip to the south. Field relations (Anhaeusser, 1973) suggested that the

3 M. Poujol, C.R. Anhaeusser / Precambrian Research 108 (2001) Fig. 1.

4 142 M. Poujol, C.R. Anhaeusser / Precambrian Research 108 (2001) Fig. 2. Geological map of the Nooitgedacht migmatite platform (after Anhaeusser, 1999).

5 M. Poujol, C.R. Anhaeusser / Precambrian Research 108 (2001) TTG granitoid suite, which includes dioritic, tonalitic and trondhjemitic gneisses and migmatites, may have been emplaced at different stages; hence it became important to establish the isotopic ages of the various granitic phases distinguishable on the basis of their mineralogical, geochemical and textural differences. Following the emplacement of the TTG suite, an early mafic dyke event (sample JHBD 98-11) can be recognized on the dome (e.g. on the Nooitgedacht platform, Fig. 2). These mafic dykes, now represented by hornblende amphibolites, preceded the intrusion of the potassic granite suite that occupies most of the southern half of the dome (Fig. 1). The potassic granitoids consist of a variety of homogeneous granodiorites that differ texturally across the dome. Homogeneous, grey, mediumgrained granodiorites occur in the south-central and southeastern sectors (sample JHBD 98-3, Fig. 1) whereas coarser-grained, homogeneous, porphyritic granodiorites occupy the southwestern sector (sample JHBD 98-5, Fig. 1). A further textural variation of the homogeneous granodiorite suite is developed along the southern contact of the main potassic massif or batholith, adjacent to the hornblende tonalite gneisses. These medium-to-coarse-grained, pinkish granodiorites are represented in this study by sample JHBD 98-2 (Fig. 1). Fine-grained, homogeneous granodioritic dykes, considered to be genetically related to the potassic granitoids described above, transgress the trondhjemite gneiss migmatite terrane on the northern half of the dome. Also transgressing these gneiss migmatite exposures are coarse-textured pinkish pegmatite dykes that probably represent the final stages of granitoid emplacement on the dome. The pegmatites, which are also encountered in the homogeneous granodiorites, were sampled for isotopic dating, but the few zircons found in these rocks proved to be unsuitable for this purpose. The Johannesburg Dome has also participated in various episodes of tectonism and epeirogenic uplift beginning in the early Archaean and extending to post-transvaal Supergroup times ( 2250 Ma), or even to post-bushveld Complex times ( 2000 Ma). Shear zones, like the prominent north northeast-trending structure shown in Fig. 1, and many others like it recorded by Anhaeusser (1973), were reactivated by successive periods of uplift and tectonic disturbance on the Kaapvaal Craton (Anhaeusser, 1973; Hilliard, 1994). No evidence could be found in support of a claim by Roering et al. (1990) that a series of northwardverging thrust faults were responsible for widespread ramping of granite sheets on the dome itself. Hence, in this paper, all the granitic relationships discussed are regarded as in situ magmatic and not structural in origin. What thrusting exists in the vicinity of the dome was probably of a thin-skinned variety involving the supracrustal cover rocks, but not the granitic basement. The dome was also intruded in Ventersdorp and Transvaal times ( Ma, Walraven et al., 1990) by numerous dykes. These include post- Transvaal mafic dykes, which preceded and accompanied the emplacement of the 2060 Ma Bushveld Complex. Later intrusive events included the subalkaline and mafic dykes associated with the c.1300 Ma Pilanesberg Alkaline Complex, and mafic dykes linked to the early Mesozoic Karoo igneous activity that occurred between Ma ago Pre ious geochronological studies Allsopp (1961) carried out the first geochronological investigations using the Rb Sr system on the granitic rocks. He examined both whole-rock and separated mineral fractions from samples collected exclusively from the granodioritic phases developed in the central portion of the dome. A whole-rock Rb Sr age (recalculated with = per year) was found to be Ma (Allsopp, 1961) with an initial 87 Sr/ 86 Sr ratio of (Allsopp, 1964). Widely differing apparent ages were obtained for the separated mineral fractions and Allsopp concluded that the discordance of the mineral ages was the result of the diffusion of radiogenic strontium from mineral to mineral. Contrasting with this whole-rock Rb Sr age is a 207 Pb/ 206 Pb zircon age of Ma (Burger and Walraven, 1979) obtained from one of the granodiorite samples analyzed by

6 144 M. Poujol, C.R. Anhaeusser / Precambrian Research 108 (2001) Allsopp. More recently, Barton et al. (1999) conducted a Rb Sr, Pb Pb and Sm Nd study on granitoid rocks from the Johannesburg Dome. The whole-rock Rb Sr data on the granodiorites define ages at and Ma, respectively. The Pb-isotope data for the same units define ages at and Ma, while zircon evaporation data define an age of Ma. These authors concluded that the granodiorites were emplaced 3090 Ma ago and were derived from a source between 3300 and 3500 Ma old. Very few data are available for the tonalitic trondhjemitic gneisses. An U Pb age of Ma (Anhaeusser and Burger, 1982) was determined from multiple zircons obtained from a tonalite cropping out on the southern edge of the Johannesburg Dome. The least discordant isotopic data were found to closely conform to a 3200 Ma Wasserburg-type diffusion curve. More recently, Barton et al. (1999) obtained a wholerock Pb age of / 146 Ma for tonalite from the same sample locality. In addition, this tonalite yielded a whole-rock Rb Sr age of Ma and a biotite Rb Sr age of Ma. 3. Sampling Seven samples of the different granitic phases and one sample of the early mafic dykes were collected from various localities on the Johannesburg Dome (Fig. 1). The tonalitic trondhjemitic gneisses (TTG) were sampled at three different locations; JHBD 98-1 is a hornblende biotite tonalitic gneiss cropping out along the southern margin of the dome, whereas JHBD 98-9 represents a sample of leuco-biotite trondhjemitic gneiss from the northwestern sector of the dome (Fig. 1). Samples JHBD 98-8 and consist of leuco-biotite trondhjemitic gneisses from the Nooitgedacht migmatite platform (Fig. 2) described recently by Anhaeusser (1999). The sample JHBD corresponds to a mafic dyke cropping out on the same platform. This exposure also occurs on the northwestern side of the dome and is situated approximately 2 km east of locality JHBD The homogeneous potassium-rich granitoid suite was sampled at four separate localities. Sample JHBD 98-2 is a relatively coarse-grained, homogeneous granodiorite from the southern half of the dome; sample JHBD 98-3 is a medium-finegrained granodiorite from the south-central part of the dome, and sample JHBD 98-5 is a coarsegrained porphyritic granodiorite from the westcentral part (Fig. 1). 4. Methodology All the samples were prepared and analyzed at the Hugh Allsopp Laboratory, University of the Witwatersrand, Johannesburg. Rock samples were pulverized using a heavy-duty hydraulic rock splitter, jaw crusher and swing mill. Mineral separation involved the use of a Wilfley Table, heavy liquids (bromoform and methylene iodide) and a Frantz Isodynamic Separator. Zircons were examined with a binocular microscope in order to assess grain quality, degree of fracturing and the possible existence of inherited cores. Handpicked zircons were abraded using the techniques of Krogh (1982) and washed in ultra-pure acetone, diluted nitric acid and hydrochloric acid. Single grains or small populations of zircons were then placed into 0.35 ml Teflon vials together with 30- l HF and a mixed 205 Pb 235 U spike. Eight of these Teflon vials were then placed in a Parr Container for 2 days at 220 C. The samples were chemically processed without separating U and Pb (Lancelot et al., 1976) and loaded on a rhenium filament together with a 0.25 N phosphoric acid silica-gel mixture. The analyses were performed on an automated VG54E mass spectrometer using a Daly collector and corrected by ( 0.05%) for mass fractionation. Total Pb blanks over the period of the analyses range from 15 to 30 pg and a value of 30 pg was assigned as the laboratory blank ( 206 Pb/ 204 Pb= , 207 Pb/ 204 Pb= and 208 Pb/ 204 Pb= ). The calculation of common Pb was made by subtracting blanks and then assuming that the remaining common Pb has been incorporated into the crystal and has a composition determined from the model of Stacey and Kramer

7 M. Poujol, C.R. Anhaeusser / Precambrian Research 108 (2001) (1975). Data were reduced using PbDat (Ludwig, 1993a). Analytical uncertainties are listed at 2 and age determinations were processed using Isoplot (Ludwig, 1993b). 5. Results 5.1. Tonalitic and trondhjemitic gneisses The hornblende biotite tonalitic gneiss (JHBD 98-1; equivalent to sample RP7 of Anhaeusser, 1971, 1973) consists mainly of quartz, sodic-plagioclase, hornblende and biotite. Accessory minerals include sphene, apatite, magnetite, zircon and microcline. The plagioclase (albiteoligoclase) is generally saussuritized (to epidote) or sericitized, whereas the hornblende is partly or totally altered to chlorite. The tonalitic gneisses have a distinctive chemical composition characterized by high Na 2 O (4.23 wt.%) and low K 2 O (2.24 wt.%) contents. As mentioned earlier this tonalitic gneiss was found to be approximately 3170 Ma by Anhaeusser and Burger (1982) using multiple zircon populations. In the present study, all the zircons extracted from sample JHBD 98-1 were found to be translucent and pink in colour. Six individual grains were analyzed (Table 1) and the results were plotted in a concordia diagram (Fig. 3). Four sub-concordant points define an upper intercept age of Ma (MSWD=3.5) with a weighted mean 207 Pb/ 206 Pb age of Ma. These new data, together with that from Anhaeusser and Burger (Fig. 3 inset) define a similar age of Ma. Consequently, we consider that the emplacement age of this tonalite Fig. 3. Concordia diagram for hornblende biotite tonalite sample JHBD Inset diagram shows these data together with that from Anhaeusser and Burger (1982).

8 146 M. Poujol, C.R. Anhaeusser / Precambrian Research 108 (2001) Table 1 U Pb isotopic data for the samples from the Johannesburg Dome a Grain 206 Weight ( g) U (ppm) Pb (ppm) Pb/ 204 Pb Radiogenic Ratios Apparent Ages (Ma) 206 Pb/ 238 U (%) 207 Pb/ 235 U (%) 207 Pb/ 206 Pb (%) 206 Pb/ 238 U 207 Pb/ 235 U 207 Pb/ 206 Pb Cor. Coef JHBD 98-1 Zr 1-1, p, t Zr 1-2, p, t Zr 1-3, p, t Zr 1-4, p, t Zr 1-5, p, t Zr 1-6, p, t JHBD 98-2 Zr 2-1, p, t Zr 2-2, y, t Zr 2-3, p, d Zr 2-4, p, d JHBD 98-3 Zr 3-1, p Zr 3-2, p Zr 3-3, p, t Zr 3-4, p, d Zr 3-5, p, d Zr 3-6, p, d Zr 3-7, p, d Zr 3-8, p, d JHBD 98-5 Zr 5-1, p, t Zr 5-2, p, d Zr 5-3, p, t Zr 5-4, p, d Zr 5-5, p, d JHBD 98-8 Zr 8-1, p, t Zr 8-2, p, t Zr 8-3, p, t Zr 8-4, p, t Zr 8-5, p, t Zr 8-6, p, d Zr 8-7, p, t Zr 8-8, p, d Zr 8-9 (4, t) Zr 8-10, p, d Zr 8-11, p, d

9 M. Poujol, C.R. Anhaeusser / Precambrian Research 108 (2001) Table 1 (Continued) Grain 206 Weight ( g) U (ppm) Pb (ppm) Pb/ 204 Pb Radiogenic Ratios Apparent Ages (Ma) 206 Pb/ 238 U (%) 207 Pb/ 235 U (%) 207 Pb/ 206 Pb (%) 206 Pb/ 238 U 207 Pb/ 235 U 207 Pb/ 206 Pb Cor. Coef JHBD 98-9 Zr 9-1, p, t Zr 9-2, p, t Zr 9-3, p, d Zr 9-4, p, t Zr 9-5, p, d Zr 9-6, p, d JHBD Zr 10-1, p, d Zr 10-2, p, t JHBD Zr 11-1, p, t Zr 11-2, p, t Zr 11-3, p, t Zr 11-4, p, t Zr 11-5, (2, y) Zr 11-6, p, t Zr 11-7, t Zr 11-8, p, t a Errors are listed at 2. p, pink; y, yellow; t, translucent; d, dark. Numbers between brackets indicate the number of grains analyzed.

10 148 M. Poujol, C.R. Anhaeusser / Precambrian Research 108 (2001) Fig. 4. Concordia diagram for the trondhjemitic gneiss sample JHBD 98-9 from the northwestern part of the Johannesburg Dome (Fig. 1). is 3200 Ma. Zircons 1 and 5 (Table 1), although identical in shape, are slightly discordant and plot well to the left of the discordia defined by the other four grains. It is suggested that the position of these grains may be the consequence of a multiple discordancy with some Pb loss at an early stage in addition to some more recent Pb loss. The presence of such grains within the zircon population of the tonalite gneiss may also explain the slightly younger age ( 3170 Ma) recorded by Anhaeusser and Burger (1982). Sample JHBD 98-9 is a trondhjemitic gneiss (equivalent to sample SK7 of Anhaeusser, 1971, with 6.25 wt.% Na 2 O and 0.97 wt.% K 2 O) from the northwestern part of the dome (Fig. 1). Zircons extracted from this sample were typically pink in colour and most often translucent. However, some of the zircons were darker. Six grains in total were analyzed from this rock (Table 1). Plotted in a concordia diagram (Fig. 4) they are slightly to highly discordant. All the translucent grains define a discordia pointing to a well-defined upper intercept age of Ma (MSWD=1.7) with a lower intercept age of 5 13 Ma. This 3340 Ma age is considered to be the best estimate for the emplacement of the trondhjemite. Two of the darker zircons are discordant (Fig. 4) and, relative to the others, are characterized by younger 207 Pb/ 206 Pb ages as well as very low 208 Pb/ 206 Pb ratios (Table 1). These grains can, therefore, be interpreted as a reflection of a post-crystallization (migmatization or gneissforming?) event leading to a complex lead loss and/or partial recrystallization.

11 M. Poujol, C.R. Anhaeusser / Precambrian Research 108 (2001) Samples JHBD 98-8 and are additional trondhjemitic gneisses (equivalent to samples N14 and N2 of Anhaeusser, 1999, and which average 6.08 wt.% Na 2 O and 0.95 wt.% K 2 O) that crop out on the Nooitgedacht migmatite platform (Fig. 2). Zircons from these samples are euhedral, generally pink in colour and vary from translucent to dark. Eleven grains from sample JHBD 98-8 and two grains from sample JHBD were analyzed (Table 1) and plotted in a concordia diagram (Fig. 5). They are concordant to discordant and occur scattered in the diagram. The youngest point (JHBD 98-8, Zr 8, Fig. 5) is concordant with a 207 Pb/ 206 Pb age of Ma, whereas the oldest (JHBD 98-10, Zr 2, Fig. 5) is 4.6% discordant with a 207 Pb/ 206 Pb age of Ma. As shown in Fig. 2, the Nooitgedacht platform is a very complex exposure showing most of the igneous phases recognized on the dome. The scatter of the data could, therefore, be a consequence of the complex history of this platform, which has been influenced by a succession of different fluid injections. This could have led to partial melting of the zircons followed by recrystallization. As these trondhjemitic gneisses are intruded by a 3.12 Ga (see later) granodioritic phase (Fig. 2) they cannot be 3 billion-years old. Therefore, the positions of the data can best be explained in terms of a crisis polygon (Fig. 5), defined by three apices at 3340, 3000 Ma and zero, respectively. The first apex at c.3340 Ma (defined by sample JHBD 98-9) represents the age of the trondhjemite emplacement; the second at c.3000 Ma (defined by the youngest concordant point) could represent the youngest significant event to have influenced the rocks exposed on the platform (crystallization of new zircons associated with the emplacement of the pegmatitic dykes?) and the third, at zero, representing recent lead loss. Fig. 5. Concordia diagram for trondhjemite gneiss samples JHBD 98-8 and from the Nooitgedacht river platform shown in Fig. 2.

12 150 M. Poujol, C.R. Anhaeusser / Precambrian Research 108 (2001) Fig. 6. Concordia diagram for the medium-to-coarse-grained pinkish granodiorite sample JHBD 98-2 located approximately 5 km north of central Johannesburg (Fig. 1) Potassic granitoids Sample JHBD 98-2 (equivalent to sample VP2 of Anhaeusser, 1973, with 4.12 wt.% Na 2 Oand 3.97 wt.% K 2 O) is a coarse-grained, pinkish, homogeneous granodiorite (Fig. 1). The zircons from this rock are generally pink in colour (some are yellowish) and translucent to dark. Four zircons were analyzed (Table 1) and, when plotted in a concordia diagram (Fig. 6), display high degrees of discordance. They point to a relatively poorly defined upper intercept age of Ma (MSWD=22) and a lower intercept age of Ma. This age of 2950 Ma is, therefore, considered to be a minimum age for the emplacement of this potassic granitoid. Sample JHBD 98-3 (similar to sample FD2 of Anhaeusser, 1973, with 3.98 wt.% Na 2 O and 4.30 wt.% K 2 O) is a medium-grained, grey granodiorite cropping out in the central part of the dome (Fig. 1). All the zircons extracted from this sample were translucent to dark-pink in colour. Eight zircons were analyzed (Table 1) and have been plotted in a concordia diagram (Fig. 7). They are sub-concordant to very discordant. The five most concordant zircons analyzed define a relatively well-constrained upper intercept age of Ma (MSWD=0.8) with a lower intercept age of

13 M. Poujol, C.R. Anhaeusser / Precambrian Research 108 (2001) Ma that does not correspond to any relevant geological event. The most concordant grain, zircon 2, defines the absolute minimum age of this sample at Ma. This age is regarded as the best estimate for the emplacement of this granodiorite in the south-central part of the dome. The eighth zircon (Zr 1, Table 1) is slightly discordant and is defined by a 207 Pb/ 206 Pb age of 3227 Ma. This zircon is interpreted as a xenocryst, probably extracted from the earlier TTG granitoid suite. Sample JHBD 98-5 (similar to sample HD30 of Anhaeusser, 1971, with 4.14 wt.% Na 2 O and 4.45 wt.% K 2 O) is representative of the porphyritic granodiorites that crop out in the southwestern part of the dome. Most of the zircons from this sample are pink and translucent and occur together with some darker-pink grains. Five zircons were analyzed (Table 1), two of them being concordant and the remaining three presenting different degrees of discordance (Fig. 8). They define a well-constrained upper intercept age of Ma (MSWD=0.47) with a lower intercept age of Ma, the latter without any apparent geological meaning. The age of Mais once again considered to be the age of emplacement of the porphyritic granodiorite in this part of the dome. Fig. 7. Concordia diagram for the medium-grained granodiorite sample JHBD 98-3 from the central part of the Johannesburg Dome (Fig. 1).

14 152 M. Poujol, C.R. Anhaeusser / Precambrian Research 108 (2001) Fig. 8. Concordia diagram for the porphyritic granodiorite sample JHBD 98-5 from the west-central part of the Johannesburg Dome (Fig. 1) Mafic dyke Sample JHBD (similar to sample N12 of Anhaeusser, 1999) is representative of the mafic dykes cropping out in the Nooitgedacht migmatite platform. These mafic dykes intruded the trondhjemitic gneisses and the amphibolitic greenstones prior to the late intrusive granodioritic event (Fig. 2) and, in their turn, are intruded by granitic veins (Anhaeusser, 1999). Extreme care was taken to separate the apparently vein-free mafic dyke from its vein-rich equivalent. Zircons were found in both vein-free and vein-rich samples, but were more abundant within the vein-rich sample material. All the zircons were very small in size ( m) and pink in colour. Three grains (Zr 11-1 to Zr 11-3, Table 1) from the vein-free sample and five grains (Zr 11-4 to Zr 11-8, Table 1) from the vein-rich sample were analyzed. Plotted in a concordia diagram (Fig. 9), they are discordant to very discordant, but define a very well-constrained upper intercept age of Ma (MSWD=1.7) with a lower intercept age of Ma. The upper intercept age is undistinguishable from the ages found for the granodiorite and, therefore, it is assumed that all the zircons found in this sample were probably linked to the emplacement of the 3120 Ma granitic veins within the mafic dykes. It was assumed, furthermore, that the granitic veins, which

15 M. Poujol, C.R. Anhaeusser / Precambrian Research 108 (2001) intruded the mafic dykes, were linked to the emplacement of the granodioritic phase within the dome. 6. Discussion The first part of this study focused on the trondhjemitic and tonalitic gneisses occurring mainly on the northern half of the Johannesburg Dome, but which are also represented on the southern margin of the dome. In both localities these gneisses intrude mafic and ultramafic igneous and volcanic rocks. The most interesting result was derived from sample JHBD 98-9, which gave an age of Ma. The two trondhjemitic gneiss samples studied from the nearby Nooitgedacht migmatite platform yielded very scattered data (Fig. 5) that did not provide any direct geochronological constraints. This was interpreted as reflecting the complex multi-stage history that this granitoid platform had undergone, including a late-stage event, possibly linked to the emplacement of pegmatites at approximately 3000 Ma. Fig. 9. Concordia diagram for the zircons extracted from the mafic dyke sample JHBD from the Nooitgedacht migmatite platform.

16 154 M. Poujol, C.R. Anhaeusser / Precambrian Research 108 (2001) The Johannesburg Dome granite greenstone terrane, which was previously considered to be approximately 3170 Ma old (Anhaeusser and Burger, 1982) has now been shown to contain an older granitoid phase. The 3340 Ma age determined for the trondhjemitic gneisses represents the oldest magmatic phase described from the granitoid rocks of the dome. This result may also have important implications with regard to the age of the mafic and ultramafic greenstone remnants scattered throughout the dome and which have not yet been dated because of the absence of material suitable for this purpose. Consequently, it implied that the greenstone remnants, which were considered by Anhaeusser (1999) to have formed in an Archaean oceanic or volcanic arclike geotectonic setting, were older than 3.34 billion years. This initial magmatic episode, involving early greenstone and TTG granitoid development on the northern half of the dome, was followed by the emplacement of the hornblende biotite tonalite in the south at 3200 Ma, as has been demonstrated by the data from sample JHBD 98-1 (Fig. 3). Following the trondhjemite tonalite event there appears to have been a further period of mafic plutonism manifest in the form of the amphibolite dykes displayed on the Nooitgedacht migmatite platform and shown in Fig. 2. Geochemical evidence, in the form of distinctly differing REE abundances, led Anhaeusser (1999) to suggest that more than one dyke event may have occurred. It was argued that if only a single stage of dyke emplacement had been involved then the two magma types would probably have formed from different mantle sources. The age of these dykes has yet to be determined quantitatively, but they fall within the time constraints imposed by the age of the trondhjemitic gneisses ( Ma) and the crosscutting homogeneous granodiorites discussed below ( Ma). The final stages of Archaean crustal evolution evident on the Johannesburg Dome coincided with the emplacement of an extensive homogeneous granodiorite porphyritic granodiorite batholith or massif, the latter seen occupying most of the southern half of the dome (Fig. 1). Manifestations of this event are also seen on the northern half of the dome in the form of granodiorite and pegmatite dykes that intrude the earlierformed greenstones, gneisses and migmatites (Fig. 2). Two samples representative of the mediumgrained and porphyritic potassic granitoids have been dated in this study at (minimum age of Ma) and Ma, respectively. A third sample of coarse-grained granodiorite yielded a poorly constrained minimum age of Ma and consequently did not conflict with the previous ages. Zircons extracted from a mafic dyke, but associated with granitic veins within the dyke, defined an upper intercept age of Ma. Consequently, we consider that the potassic granitoid suite within the Johannesburg Dome was emplaced Ma ago. This age is in good agreement with the Ma age determined by Allsopp (1961), but contradicts the zircon evaporation age of 3090 Ma published recently by Barton et al. (1999). One of the problems of the zircon evaporation technique lies with the difficulty in ascertaining the concordance of the zircons. The data presented in this study show that very few zircons are concordant, some of them giving apparent 207 Pb/ 206 Pb ages at around 3090 Ma (sample JHBD 98-3: Zr Ma, Zr Ma; sample JHBD 98-5: Zr Ma; Zr Ma). It is, therefore, possible to conclude that Barton et al. (1999) were dealing with subconcordant zircons, which yielded younger 207 Pb/ 206 Pb ages. Another explanation might suggest that the age of 3.09 Ga reflects a younger, discreet pulse of magmatism in this area, but this needs to be confirmed. 7. Implications for the evolution of the Kaapvaal Craton Over the past decade much new geochronological data has been published relating to the Kaapvaal Craton (Barton et al., 1999; Kröner et al., 1999, 2000; Nelson et al., 1999; Poujol and Robb, 1999; Kreissig et al., 2000; McCourt et al., 2000). Similarities exist between the Johannesburg Dome and the Barberton terrane where Kamo

17 M. Poujol, C.R. Anhaeusser / Precambrian Research 108 (2001) and Davis (1994) reported a Ma age from zircons and badeleyites from gabbros intruded into the Komati Formation. In addition, a tuffaceous layer in the uppermost Kromberg Formation gave a 207 Pb/ 206 Pb evaporation age of Ma (Byerly et al., 1996). These two ages are identical, within error margin, to the Ma age found for the emplacement of sample JHBD 98-9 in the northern part of the Johannesburg Dome. In the far northeastern part of the Kaapvaal Craton recent data from Kröner et al. (2000) suggests that the Giyani (Sutherland) greenstone belt was deposited at around 3.2 Ga on a basement as old as c.3.28 Ga. In addition, the oldest rocks in the vicinity of the Murchison greenstone belt, to the south of Giyani, were dated at Ma (Poujol et al., 1996). Similar ages have been reported in the Barberton terrane for the Ma Nelshoogte pluton (De Ronde and Kamo, 2000), the Ma Kaap Valley pluton (Kröner et al., 1991), and the Ma Dalmien pluton (Kamo and Davis, 1994). This, together with the new data from the Johannesburg Dome, suggests that an important period of magmatism occurred between 3.34 and 3.2 billion years ago on the Kaapvaal Craton. At this time it is possible that the Ancient Gneiss terrane of Swaziland and that of the southern and northern Barberton terranes were welded together to form a stable nucleus in the manner described by De Wit et al. (1992). Late potassic batholiths were then emplaced in both the Barberton and central Kaapvaal (Johannesburg Dome) terranes. Kamo and Davis (1994) dated several granitoid batholiths and plutons from the Barberton terrane, including the Stentor ( Ma), Mpuluzi ( Ma), Nelspruit ( Ma) and Salisbury Kop ( Ma) bodies. These all have ages, within error, close to the c Ma emplacement ages of the potassic granitoid suite found on the Johannesburg Dome. In contrast the Pietersburg and Murchison granite greenstone terranes, which are located adjacent to the northern and northeastern margins of the Kaapvaal Craton, are generally younger ( Ga) than the terranes found farther south and in the central part of the Craton (Brandl et al., 1996; Poujol and Robb, 1999; Poujol et al., 1996; Kröner et al., 2000). Furthermore, recent data (Poujol et al., 2000) have demonstrated an episodic granitoid emplacement history in the western part of the Craton (Kraaipan Amalia terrane), which ranges between 3.01 and 2.79 Ga. Thus, the western part of the Craton appears to be significantly younger than the central and eastern portions and can, on the basis of the age relationships, be more suitably correlated with the rocks of the northern portion of the Kaapvaal Craton. These data demonstrate further that the amalgamation of the Kaapvaal Craton was long-lived and episodic, and presumably involved the formation of juvenile magmatic arcs that coalesced with existing continental blocks in the period Ga. The resulting scenario envisages that the Kaapvaal Craton grew by accretionary processes, becoming generally younger towards the north and west with the central portion, represented by the Johannesburg Dome, providing a region with ages overlapping those of the surrounding terranes. 8. Conclusion The Johannesburg Dome consists of a complex mosaic of granitoid rocks manifest by differences in areal extent, composition, texture and age. This geochronological study has demonstrated some of the difficulties that can be encountered in dating Archaean granite gneiss migmatite terranes. Three main magmatic events have been defined. The first involved the emplacement of trondhjemitic rocks at c.3340 Ma in the northern part of the dome, followed by a tonalitic phase at c.3200 Ma in the south. The greenstone remnants occurring widespread on the dome predate the earliest trondhjemitic gneisses and are, therefore, at least 3.34 billion-years old. The mafic dykes that intruded the trondhjemite tonalite gneiss suite were emplaced between c.3340 and c.3120 Ma. The third event, namely the emplacement of the potassic granitoid suite, is shown to have taken place at c Ma, followed by a

18 156 M. Poujol, C.R. Anhaeusser / Precambrian Research 108 (2001) pegmatite dyke episode possibly as young as 3.0 Ga. A similar age of Ma was obtained from the granitoid basement that pre-dates the Ma upper lava sequence of the Dominion Group, which underlies the Witwatersrand Supergroup successions southwest of Klerksdorp in the North West Province (Armstrong et al., 1991). Consequently, the new data from the Johannesburg Dome, the latter situated approximately 150 km to the northeast of Klerksdorp, provide confirmation that rocks of the Witwatersrand Basin were deposited unconformably on an Archaean basement as young as c.3120 Ma. In conclusion, the Johannesburg Dome appears to represent an intermediary, mid-archaean terrane linking the eastern portion of the Kaapvaal Craton with the northern and western parts of the Craton, with each of these terranes showing progressively younger ages both to the north and to the west. Acknowledgements We would like to acknowledge Sandra Kamo, Jan Kramers and Jay Barton who provided very helpful and insightful reviews of the original manuscript. References Allsopp, H.L., Rb Sr age measurements of total rock and separated mineral fractions from the Old Granite of the Central Transvaal. J. Geophys. Res. 66, Allsopp, H.L., Rubidium/strontium ages from the western Transvaal. Nature 204, Anhaeusser, C.R., The geology and geochemistry of the Archaean granites and gneisses of the Johannesburg-Pretoria Dome. Information Circular 62, Economic Geology Research Unit, Johannesburg, p. 41. Anhaeusser, C.R., The geology and geochemistry of the Archaean granites and gneisses of the Johannesburg-Pretoria Dome. Geol. Soc. S. Afr. Spec. Publ. 3, Anhaeusser, C.R., Geological and geochemical investigations of the Roodekrans Ultramafic Complex and surrounding Archaean volcanic rocks, Krugersdorp District. Trans. Geol. Soc. S. Afr. 80, Anhaeusser, C.R., The geology and geochemistry of the Muldersdrif Complex and surrounding area, Krugersdorp District. Trans. Geol. Soc. S. Afr. 81, Anhaeusser, C.R., Archaean granite-greenstone relationships on the farm Zandspruit 191-IQ, North Riding area, Johannesburg Dome. S. Afr. J. Geol. 95, Anhaeusser, C.R., Archaean crustal evolution of the central Kaapvaal Craton, South Africa: evidence from the Johannesburg Dome. S. Afr. J. Geol. 102, Anhaeusser, C.R., Burger, A., An investigation of the U Pb zircon ages for Archaean tonalitic gneisses from the Johannesburg-Pretoria Dome. Trans. Geol. Soc. S. Afr. 85, Anhaeusser, C.R., Robb, L.J., Magmatic cycles and the evolution of the Archaean granitic crust in the Eastern Transvaal and Swaziland. Spec. Publ. Geol. Soc. Aust. 7, Armstrong, R.A., Compston, W., Retief, E.A., Williams, I.S., Welke, H.J., Zircon ion microprobe studies bearing on the age and evolution of the Witwatersrand triad. Precam. Res. 53, Barton, J.M., Jr, Barton, E.S., Kröner, A., Age and isotopic evidence for the origin of the Archaean granitoid intrusives of the Johannesburg Dome, South Africa. J. Afr. Earth Sci. 28, Brandl, G., Jaeckel, P., Kröner, A., Single zircon age for the felsic Rubbervale Formation, Murchison Greenstone Belt, South Africa. S. Afr. J. Geol. 99, Burger, A.J., Walraven, F., Summary of age determinations carried out during the period April 1977 to March Ann. Geol. Surv. S. Afr. 12, Byerly, G.R., Kröner, A., Lowe, D.R., Todt, W., Walsh, M.M., Prolonged magmatism and time constraints for sediments deposition in the early Archaean Barberton greenstone belt: evidence from the Upper Onverwacht and Fig Tree Groups. Precam. Res. 78, De Ronde, C.E.J., Kamo, S.L., An Archaean arc arc collisional event: a short-lived (ca. 3 Myr) episode, Weltevreden area, Barberton greenstone belt, South Africa. J. Afr. Earth Sci. 30, De Wit, M.J., Roering, C., Hart, R.J., Armstrong, R.A., De Ronde, C.E.J., Green, R.W.E., Tredoux, M., Peberdy, E., Hart, R.A., Formation of an Archaean continent. Nature 357, Hilliard, P., The structural evolution of the Johannesburg Dome, Kaapvaal Craton, South Africa. M.Sc. thesis (unpublished), University of Pretoria, p. 99. Kamo, S.L., Davis, D.W., Reassement of Archaean crustal development in the Barberton Moutain Land, South Africa, based on U Pb dating. Tectonics 13, Kreissig, K., Nagler, T.F., Kramers, J.D., Van Reenen, D.D., Smit, C.A., An isotopic and geochemical study of the northern Kaapvaal Craton and the Southern Marginal Zone of the Limpopo Belt: are they juxtaposed terranes? Lithos 50, Krogh, T.E., Improved accuracy of U Pb ages by the creation of more concordant systems using an air abrasion technique. Geochim. Cosmochim. Acta 46,

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