Geology and geochemistry of a granite-greenstone association in the southeastern Vredefort dome, South Africa

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1 CRISTIANO LANA, ROGER L. GIBSON, WOLF U. REIMOLD AND RICHARD C.A. MINNITT 291 Geology and geochemistry of a granite-greenstone association in the southeastern Vredefort dome, South Africa Cristiano Lana * and Roger L. Gibson Impact Cratering Research Group, School of Geosciences, University of the Witwatersrand, P.O. Wits 2050, Private Bag 3, Johannesburg, South Africa *Present address: Department of Earth Sciences and Engineering, South Kensington Campus, Imperial Collage London, SW72AZ,UK lanac@science.pg.wits.ac.za, gibsonr@geosciences.wits.ac.za Wolf U. Reimold Impact Cratering Research Group, School of Geosciences, University of the Witwatersrand, P.O. Wits 2050, Private Bag 3, Johannesburg, South AfricaCorresponding author: Fax: ; Tel reimoldw@geosciences.wits.ac.za Richard C.A. Minnitt Department of Mining Engineering, University of the Witwatersrand, Private Bag 3, P.O. Wits 2050, Johannesburg, South Africa minnitt@egoli.min.wits.ac.za 2003 Geological Society of South Africa ABSTRACT A sequence of mafic to ultramafic chlorite-actinolite-hornblende-talc rocks with komatiitic and komatiitic basalt compositions has been mapped in the southeastern Vredefort dome. A volcanic origin for the rocks is indicated by pillow structures, flow banding, variolites and spinifex textures in low-strain domains. They are intercalated with quartz-sericite-biotite schists. The rocks are metamorphosed to mid-greenschist-facies (~400 C) and are heterogeneously deformed, with a dominant subvertical, northwest-trending schistosity (S3) that developed under peak metamorphic conditions. To the northwest, the greenstones are bounded by a kilometre-wide dip-slip shear zone, which includes poorly-exposed high-grade (~700 C, 5 kbar) amphibolites and migmatitic trondhjemitic gneisses, and pegmatitic granite, and which separates the low-grade rocks from the high-grade gneissic core of the dome. The variation in metamorphic grade across the mapped area is attributed primarily to the displacement across the shear zone (southeast side down), but it may also reflect, in part, deeper levels of exhumation towards the centre of the dome. In contrast to the komatiitic greenstones, the amphibolites within the shear zone have a tholeiitic composition; however, they also show features consistent with volcanic precursors and are interleaved wirh banded ironstones and quartz-sericite-biotite schists. The mafic and ultramafic metavolcanics display close similarities to the komatiites and komatiitic basalts in the Johannesburg dome, but are relatively depleted in Fe 2 O 3, TiO 2 and CaO and enriched in Al 2 O 3 relative to the Barberton greenstones. They are compositionally distinct from the dismembered granulite-facies greenstone remnants in the central parts of the dome. Introduction The granite-greenstone associations of the Kaapvaal craton constitute one of the most comprehensive records of Palaeo- to Meso-Archaean crustal evolution on Earth (Figure 1a) (e.g., De Wit et al., 1992; Anhaeusser, 2001). Most of the information concerning the Archaean geological history of southern Africa is derived from studies of the granite-greenstone associations in the eastern parts of the craton, such as the Barberton greenstone belt and the Ancient Gneiss Complex of Swaziland (e.g., De Wit et al., 1992; Lowe, 1994; 1999; Poujol et al., 2003). De Wit et al. (1992) and Lowe (1994, 1999) proposed that the granite-greenstone associations in the eastern parts of the craton were formed during an initial phase of rift-related and/or intraoceanic mafic and ultramafic volcanism, which was followed by magmatic accretion of tonalite-trondhjemite-granodiorite (TTG) suites and extrusion of consanguineous dacites, rhyodacites and rhyolites. This sequence of magmatic events gave rise to several thickened, buoyant crustal fragments, which were assembled into a relatively unstable cratonic shield between ~3.6 and ~3.2 Ga (De Wit et al., 1992). De Wit et al. (1992) and Lowe (1994; 1999) proposed that the cratonic shield attained stability during renewed partial melting of the TTG-greenstone rocks, which had led to the intrusion of voluminous potassic granitoids between ~3.1 and ~3.0 Ga. Recent field, geochemical and isotopic age data for areas such as the Murchison and Kraaipan greenstone belts and the Johannesburg dome (Figure 1a), seem to indicate similar, albeit younger, mafic-ultramafic and, 2003,VOLUME 106, PAGE

2 292 GRANITE-GREENSTONE ASSOCIATION IN THE SOUTHEASTERN VREDEFORT DOME Figure 1. (a) Schematic map of the Kaapvaal craton (modified after De Wit et al., 1992) showing the main greenstone belts and Archaean gneiss complexes. (b) Simplified geological map of the central parts of the Vredefort dome showing the dominant Meso-Archaean lithologies and fabrics in the ~40km wide Archaean gneiss complex (after Lana et al., 2003a, b). The granulite-facies greenstones of the Steynskraal Formation (Stepto, 1990) and the location (black dot) of the metaperidotite (Hart et al., 1990) discussed in the text are also indicated. felsic magmatic episodes to those exposed in the eastern parts of the craton (Anhaeusser, 1992; 1999; Poujol et al. 1996; 1999; 2001; 2003; Poujol and Anhaeusser, 2000; Hirner, 2003). In contrast to the eastern parts of the craton and, to a lesser extent, the western craton and the Johannesburg dome, comparatively little is known about the greenstone component of the Archaean basement complex in the core of the Vredefort dome. Recent lithological mapping and geochemical and petrographic studies indicate that the TTG suites in the core of the Vredefort dome record evidence of significant partial melting, melt segregation and emplacement of granites and granodiorites during a polyphase tectonometamorphic event shortly before the deposition of the overlying Dominion Group supracrustal rocks (Lana et al., 2003a; b). Most of the greenstones associated with the TTG suite in the core of the dome were dismembered and highly deformed and metamorphosed during this event (Stepto, 1979; 1990). Consequently, they show little, if any, of their primary characteristics. In contrast, a large body of less strongly-metamorphosed and variably deformed greenstone rocks is exposed in the southeastern sector of the dome (Figure 1b). Here we describe these greenstones and their relationship to the TTG rocks that constitute the bulk of the Archaean basement complex in the Vredefort dome. Geological Setting The Vredefort dome, located ~120km southwest of Johannesburg (Figure 1a), is the ~90km-wide eroded remnant of the central uplift of a giant, 250 to 300km diameter, meteorite impact structure that formed in the central parts of the Kaapvaal craton ~2.02 Ga ago (Gibson and Reimold, 2001, and references therein). Uplift was sufficiently large to expose the >~3.1 Ga early Archaean granite-greenstone basement beneath the ~15km-thick Late Archaean to Palaeoproterozoic supracrustal succession that dominates much of the craton (Figure 1b) (e.g., Slawson, 1976; Stepto, 1979; 1990; Hart et al., 1981; 1990a; Henkel and Reimold, 1998; Lana et al., 2003a; b). The bulk of this basement complex comprises polydeformed Archaean migmatitic gneisses and granitoids that contain a few small, scattered, metasedimentary and mafic granulite xenoliths, interpreted by Stepto (1979; 1990) as parts of a dismembered greenstone sequence (Figure 1b). The metamorphic grade in these rocks ranges from upper amphibolite facies, in the outer parts of the core, to

3 CRISTIANO LANA, ROGER L. GIBSON, WOLF U. REIMOLD AND RICHARD C.A. MINNITT 293 Figure 2. Detailed lithological and structural map of the granite-greenstone association in the southeastern sector of the Vredefort dome. Localities of samples selected for geochemistry and mineral chemistry are indicated by labelled black dots. Square indicates locality of samples selected for whole-rock U-Pb and Rb-Sr isotopic analysis, and triangle indicates locality of samples selected for U-Pb single zircon dating. granulite facies in the central parts, consistent with the exposure of progressively deeper crustal levels as a result of dome formation (Figure 1b) (Slawson, 1976; Stepto, 1979; 1990; Hart et al., 1981; 1990a; Henkel and Reimold, 1998; Lana et al., 2003a; b). This mid-crustal regional metamorphism (Stevens et al., 1997), which predated the ~3.07 Ga deposition of the Dominion Group (Armstrong et al., 1991; Hart et al., 1999; Moser et al., 2001; Lana et al., 2003b), is variably overprinted by shock deformation features and a static metamorphism induced by the impact event that reached temperatures of between 550 C and >1000ºC in the core of the dome (Gibson et al., 2002; Gibson, 2002) and that, locally, produced granofelsic textures in the Archaean gneisses in the innermost 5km of the core of the dome (Figure 1b). The southeastern and southern parts of the dome are generally poorly exposed, with outcrop mostly restricted to deeply weathered inliers of rock underlying horizontal Karoo Supergroup shales and dolerite sills (Figure 1b) (Minnitt et al., 1994). In the largest of these inliers, greenstones and subsidiary migmatitic gneisses and granites are exposed over an area of ~8km 2 (Figures 1b; 2).

4 294 GRANITE-GREENSTONE ASSOCIATION IN THE SOUTHEASTERN VREDEFORT DOME Figure 3. (a) Komatiite on farm Oceaan 99, showing millimetre- to centimetre-scale compositional layering. Layers consist dominantly of actinolite + chlorite and talc + serpentine + actinolite. (b) Variolitic textures in undeformed komatiitic basalt (farm Hattingsrust 68). (c) Pillow structures (P) in komatiitic basalts on farm Hattingsrust 68. The tricuspate intersection of three pillows is filled with quartz (white arrow) d) Radiating aggregates of oriented chlorite, talc and actinolite in spinifex-textured komatiite (farm Enkelbosch, 98). Lithologies in the southeastern sector of the Vredefort basement complex The outcrops in the inlier are dominated by actinolitechlorite schists with subsidiary, largely undeformed, mafic metavolcanics, banded iron formation, felsic schists and amphibolites (Figure 2). To the northwest, these rocks are bounded by strongly foliated, coarsegrained, migmatitic trondhjemitic gneiss and granitic pegmatites (Figure 2). The greenstone schists are locally intruded by these pegmatites and by small lensoid bodies of feldspar-quartz porphyry and spodumene-rich pegmatite. Several post-metamorphic gabbroic and doleritic sills and dykes of various ages occur in the area. This granite-greenstone association has been termed the Greenlands Formation by the South African Council for Stratigraphy (Minnitt and Reimold, 2000), but was previously referred to as the Greenlands Greenstone Complex (Minnitt et al., 1994). Mafic and ultramafic metavolcanics and metasediments Poorly-exposed greenstone rocks constitute more than 90% of the outcrop area within the inlier (Figure 2). In the southern part of the area, these rocks comprise massive komatiitic metabasalts, with volcanic features such as fine-scale layering, variolites up to 1 to 3cm wide, and pillows (Figures 3a; b; c). Serpentinised ultramafic rocks are found alternating with talc-chlorite schist and spinifex-textured komatiites (Minnitt et al., 1994) (Figure 3d). Komatiites and ultramafic rocks consist predominantly of serpentine, talc, actinolite and chlorite. Massive komatiitic basalts consist of random crystals of hornblende (90 to 95 volume %), magnetite (5 to 10 volume %), chlorite (5 volume %), and rare oligoclase (1 volume %). In the remainder of the area, however, the dominant lithology is mediumto fine-grained actinolite-chlorite schist, which consists of actinolite (90 volume %), chlorite (5 to 10 volume %), magnetite (<5 volume %), and talc + carbonate (<1 volume %). Quartz-sericite-biotite schist occurs in several metre-long lenses intercalated with medium-grained actinolite-chlorite schist on farm Blaauwboschpoort 13 in the southern part of the area. This schist consists of sericite (80 to 90 volume %), quartz (5 to 10 volume %) and biotite (5 volume %).

5 CRISTIANO LANA, ROGER L. GIBSON, WOLF U. REIMOLD AND RICHARD C.A. MINNITT 295 Figure 4. (a) Banded amphibolite (farm Benshoop 74). Compositional banding comprises quartz + plagioclase alternating with hornblende + plagioclase + quartz. (b) Detail of elongated felsic vesicles in amphibolite (farm Benshoop 74). Vesicles display varied aspect ratios due to strain partitioning during deformation. (c) Stromatic leucosome (L) and mesosome (M) defining the S3 fabric in the migmatite in the northwestern part of the inlier. (d) Detail of the migmatitic layering showing biotite + hornblende schlieren (S) in the leucosome (farm Broodkop 304). (e) Detail of the muscovite clots (black arrow) in the nebulitic migmatite (farm Goedgunst 315). The muscovite clots define the subvertical L4 lineation. (f) Pegmatite in the homogeneous to nebulitic migmatite at the northeastern flank of the inlier (farm Benshoop 74). The pegmatite shows up to 10cm long anhedral to subhedral K-feldspar crystals (arrow). Metasediments such as banded iron formation and banded ferruginous chert occur in scattered outcrops on farm Benshoop 74 (Figure 2). The banded iron formation consists of 2 to 5mm wide alternating layers of recrystallised chert, and bands of grunerite (30 to 40 volume %) + ferro-hornblende (10 to 15 volume %) + quartz (20 to 30 volume %) + magnetite (10 to 20 volume %) + ilmenite (<5 volume %). The amphibole- rich bands are commonly thicker (approximately 2:1) than the chert bands. Intermediate to felsic rocks Isolated outcrops of fine-grained, strongly foliated amphibolites occur on farms Benshoop 74 and Avondale A112 (Figure 2). These amphibolites consist of quartz (30 to 40 volume %), plagioclase (20 to 30 volume %),

6 296 GRANITE-GREENSTONE ASSOCIATION IN THE SOUTHEASTERN VREDEFORT DOME biotite (20 to 30 volume %), hornblende (10 to 15 volume %), epidote (5 volume %), muscovite + sphene (< 1 volume %). Locally, the amphibolites show compositional banding comprising 1 to 3mm wide plagioclase-biotite-hornblende bands that alternate with 2 to 5mm wide bands of quartz and plagioclase (Figure 4a) or discontinuous, foliation-parallel, quartz layers or lenses, up to a few centimetres wide. In addition, they may also show 1 to 3cm long quartzplagioclase vesicles (Figure 4b), or up to 5mm long biotite + amphibole clots (see Figure 8f) in a quartzofeldspathic matrix. Contact relationships between the amphibolites and the chlorite-actinolite-tremolite schists are not exposed, but the facts that the amphibolites (a) show some relict volcanic features (vesicles), and (b) are interleaved with banded iron formation suggest that they are part of the greenstone association. Poorly exposed feldspar-quartz porphyry is also observed among patchy outcrops of the amphibolites in the northern part of the complex (Figure 2). This rock type has been described by Bisschoff and Bisschoff (1988) as a dacite porphyry or granodiorite. It is found in close proximity to several spodumene-bearing pegmatite bodies in this area. The feldspar-quartz porphyry consists dominantly of quartz (30 to 40 volume %), hornblende (30 to 40 volume %), plagioclase (20 to 30 volume %), K-feldspar (10 volume %), biotite (<5 volume %), and epidote + sphene + ilmenite (<1 volume %). Bisschoff and Bisschoff (1988) noted fibrous holmquistite (lithium amphibole) in association with biotite, which they attributed to metasomatic effects during emplacement of the spodumene-bearing pegmatite bodies. The lower degree of strain in the porphyry relative to the surrounding amphibolites suggests a late-tectonic timing for intrusion (late-d4; see below). Migmatite and granitoids The metavolcanics and metasediments are flanked to the north and northwest by trondhjemitic to granitic migmatite, which is exposed in an 8km long and 1 to 2km wide zone of patchy outcrops (Figure 2). The migmatite consists dominantly of stromatic granitic leucosome bands alternating with thin trondhjemitic melanosomes (Figures 4c; d). The melanosomes are characterised by intensely recrystallized plagioclase (50 to 60 volume %) and quartz (20 to 30 volume %), and oriented biotite (10 to 20 volume %) and hornblende (>5 volume %). Granitic leucosomes consist of coarse-grained (0.9 to 2mm) plagioclase (35 to 45 volume %), quartz (25 to 35 volume %) and K-feldspar (20 to 30 volume %), and medium- to finegrained biotite (5 to 10 volume %) and garnet (<1 volume %). These leucosomes contain biotitehornblende schlieren, which are oriented parallel to the migmatitic banding (Figure 4d). Locally, elongated xenoliths of tonalitic gneisses (biotite + plagioclase + quartz ± hornblende) and amphibolites (hornblende + biotite + plagioclase + quartz), up to a metre long, occur within 2 to 5m wide, coarse-grained granitic leucosomes (plagioclase + K-feldspar + quartz). These features are similar to those observed in the migmatite that dominates the remainder of the crystalline basement outcrops in the Vredefort dome (Lana et al., 2003b). Several of the outcrops of migmatite on farm Broodkop 304 were described in detail by Colliston and Reimold (1990), who noted the presence of a subvertical, northeast-trending, protomylonitic to mylonitic fabric overprinting and rotating the migmatitic structures. On farms Goedgunst 315 and Benshoop 74 (Figure 2), the migmatite is coarse-grained to pegmatitic, nebulitic to schlieric, and has been intruded by 2 to 5m wide bodies of quartz-plagioclase-muscovite ± garnet pegmatite, in which grain sizes may reach 20 to 30cm (Figures 4e; f). The pegmatites are characterised by 5 to 10cm long oriented muscovite clots (Figure 4e). Other granitoid phases (mainly quartz-feldspar pegmatites) also occur as small bodies or metre-wide veins in the actinolite-chlorite schist mainly close to the contact with the migmatite on farm Broodkop 304 (Figure 2). Similarly to the leucosomes in the migmatite, these veins consist of coarse-grained plagioclase, quartz, K-feldspar, muscovite and garnet. Spodumene pegmatite In the northern part of the inlier (Figure 2), the foliated amphibolites are intruded by several bodies of spodumene pegmatite that are characterised by composite internal mineral layering. The Li content of these pegmatites has previously been investigated by Bisschoff and Bisschoff (1988), who showed that the spodumene crystals contain up to 85 mole % LiAlSi 2 O 6. Bisschoff and Bisschoff (1988) also observed that lithium-bearing fluids emanating from the spodumene pegmatite caused replacement of hornblende by a lithium amphibole (holmquistite) in a metre-wide metasomatic aureole within the host amphibolites and porphyry. The pegmatites contain layering defined by variation in both grain-size and modal mineralogy, but are not foliated. Zones of 1 to 2m width consist of 30 to 40 volume % spodumene crystals in a coarse-grained (2 to 10cm) matrix comprising plagioclase, quartz and muscovite. Spodumene crystals are normally 2 to 10cm long and 1 to 5cm wide, but locally may attain lengths of 30cm and widths of 5cm. The lack of internal deformation in the pegmatites suggests a late- to post- D4 timing (see below) for their emplacement. A preimpact origin is confirmed by the presence of many veinlets and pods of pseudotachylitic breccia cutting across the pegmatite bodies. Post-metamorphic mafic intrusions The granite-greenstone association is intruded by several sheet-like mafic bodies of tholeiitic affinity (Pybus 1995; Reimold et al., 2000) that do not show any evidence of the Archaean deformation or metamorphism. Pre-impact mafic intrusions comprise tholeiitic dykes that are

7 CRISTIANO LANA, ROGER L. GIBSON, WOLF U. REIMOLD AND RICHARD C.A. MINNITT 297 Figure 5. (Fe total +Ti)-Al-Mg and FeO total -(Na 2 O+K 2 O)-MgO ternary diagrams (after Jensen, 1996; and Irvine and Baragar, 1971, respectively) for the mafic and ultramafic volcanics from the Vredefort and Johannesburg domes and the Barberton Greenstone Belt. These also include the mafic granulites from the Steynskraal Formation (Stepto, 1990) and metaperidotites from the central part of the Vredefort dome (Hart et al., 1990b). crosscut by thin veinlets of pseudotachylitic breccia. These dykes display varied textures and mineral compositions. The largest pre-impact mafic intrusion is a 200m wide, 2km long gabbroic body exposed on the farm Goedgunst 315 (Figure 2). This rock exhibits a brown, distinctively pitted, surface as a result of preferential weathering of pyroxene grains and consists dominantly of euhedral to subhedral plagioclase in an augite-magnetite-rich matrix. Smaller dykes of gabbro on farms Benshoop 74, Enkelbosch 98 and Blaauwboschpoort 13 consist mainly of plagioclase, amphibole, pyroxene and minor magnetite. Although having similar compositions, they exhibit different degrees of alteration. The dykes on Enkelbosch 98 and Blaauwboschpoort 13 are intensely altered and consist of partially saussuritized plagioclase, amphibole, and rare remnants of orthopyroxene. In contrast, the dyke on Benshoop 74 is relatively unaltered. It consists of subophitic plagioclase (60 volume %), orthopyroxene (30 volume %), amphibole (10 volume %) and minor magnetite. Alteration is restricted to uralitization of pyroxene. Post-impact mafic magmatism is represented by the largest mafic body in the central area of the inlier on farms Broodkop 304, Ocean 99, and Van der Merwes Dam 37 (Figure 2). This gabbroic intrusion consists of 50 to 60 volume % plagioclase, 30 volume % clino- and orthopyroxene (in apparently equal proportions), up to 15 volume % amphibole, and minor magnetite. Gabbros with similar igneous textures and mineralogy are found at other localities (e.g., Goud Laagte 236, Verdeel 278, Geluk 237) in 5 to 10m wide dykes. Previous results of geochemical and Rb-Sr isotopic analysis (Pybus, 1995; Reimold et al., 2000) related this rock type to the ~1.1 Ga (Reimold et al., 2000) Annas Rust Sheet in the northern sector of the Vredefort dome. Geochemistry Samples of mafic and ultramafic lavas, amphibolites, and feldspar-quartz porphyry were collected for bulk rock major and trace element analysis (Table 1; sample localities in Figure 2). Samples were analysed by X-ray fluorescence spectrometry (XRF) at the School of Geosciences, University of the Witwatersrand, Johannesburg. Relevant accuracy and precision values for XRF chemical analyses were reported by Reimold et al. (1994). Every attempt was made during sampling to obtain rocks with the lowest degrees of alteration, principally from the southern sector of the inlier where deformation and chemical alteration appear to have been lowest. Despite this effort, two of the metavolcanic samples listed in Table 1 (GGB 1, GGB2) show unusually high SiO 2 and low MgO values and have, thus, been excluded from discussion. In the ternary classification diagram of Jensen (1976) (Figure 5a), the metavolcanics lie within the komatiitic basalt and komatiite fields. The komatiitic samples can be readily distinguished from the komatiitic basalt samples based on MgO, CaO and Al 2 O 3 contents (Table 1) - the komatiites are characterised by relatively narrow ranges of MgO (31.92 to weight %), CaO (2.05 to 2.72 weight %), and Al 2 O 3 (4.72 to 5.78 weight %), whereas the MgO, CaO and Al 2 O 3 contents in the komatiitic basalts lie between 9.5 and 18.6 weight %, 4.5 and weight %, and 7.16 and weight %, respectively. Although the komatiitic basalts are characterised by a wide range of CaO, Na 2 O, K 2 O and MnO contents (Figure 6), their compositions are very

8 298 GRANITE-GREENSTONE ASSOCIATION IN THE SOUTHEASTERN VREDEFORT DOME Figure 6. Major oxide versus MgO variation diagrams for mafic and ultramafic metavolcanics from the Vredefort and Johannesburg domes and the Barberton Greenstone Belt. These also include the mafic granulites from the Steynskraal Formation (Stepto, 1990) and metaperidotites from the central part of the Vredefort dome (Hart et al., 1990b). Legend as in Figure 5.

9 CRISTIANO LANA, ROGER L. GIBSON, WOLF U. REIMOLD AND RICHARD C.A. MINNITT 299 Figure 7. Trace elements versus MgO variation diagrams comparing data for mafic and ultramafic rocks from the Vredefort and Johannesburg domes, the Barberton Greenstone Belt, and the mafic granulites from the Steynskraal Formation (Stepto et al., 1990) and metaperidotites from the central part of the Vredefort dome (Hart et al., 1990b). Legend as in Figure 5.

10 300 GRANITE-GREENSTONE ASSOCIATION IN THE SOUTHEASTERN VREDEFORT DOME similar to those in the Johannesburg dome and Barberton Greenstone Belt (Anhaeusser, 2002; Byerly, 1999; Vennemann and Smith, 1999). Figures 6 and 7 show the variations of major and trace elements between komatiites and komatiitic basalts in the Vredefort and Johannesburg domes and in the Barberton Greenstone Belt (BGB). Most samples from the southeastern sector of the Vredefort dome fall along single trends defined by the samples from the Johannesburg dome (Anhaeusser, 1978; 1979; 1992; 1999) and Barberton Greenstone Belt (e.g., Anhaeusser, 2002; Byerly, 1999; Vennemann and Smith, 1999) (Figure 6). Significantly, the rocks in the Vredefort inlier show a degree of hydration similar to that of the rocks in the Johannesburg dome and Barberton Greenstone Belt (compare loss of ignition values LOI versus MgO; Figure 6). In contrast to the major elements, Co, Sr, Ba, Zn, Y and Zr trace element abundances in the komatiitic basalts are largely similar to those in the komatiites (Figure 7). This is particularly true for komatiitic basalts with MgO > 14 weight %. The komatiitic basalts are, however, depleted in Cr and Ni relative to komatiites, which might indicate processes related to crystallisation of these rocks, such as fractionation of olivine and pyroxene (e.g., Pyke et al., 1973; Nesbitt and Sun, 1976; Nesbitt et al., 1982; Arndt et al., 1995). Rocks with MgO < 14 weight % show wide ranges of trace element abundances, in particular for the relatively mobile elements Ba, Sr, Rb and Zn. The komatiitic basalts from the Johannesburg dome, however, show even higher contents of Ba, Sr and Rb, which appears to be consistent with their higher contents of Na 2 O and K 2 O relative to the Vredefort komatiitic basalts. The Johannesburg dome rocks are in contact with voluminous tonalite and trondhjemite bodies and are intruded by several generations of granite dykes (Anhaeusser, 1999), which suggests that their compositions (or at least their mobile element contents) might have been altered during later magmatic events. The komatiitic basalts in the Vredefort dome have similar Na 2 O, K 2 O, Rb, Sr and Ba contents to the mafic and ultramafic rocks in the Barberton area. It is worth noting, however, that most of the samples of komatiites and komatiitic basalts in the Vredefort and Johannesburg domes are slightly depleted in CaO, TiO 2, and Fe 2 O 3 and enriched in Al 2 O 3, relative to the Barberton samples (Figure 6). The distinction can be clearly seen in the Al-Fe+Ti-Mg ternary diagram of Jensen et al. (1976) (Figure 5a) and in the variation diagrams in Figure 6, where the Barberton samples plot as a distinct group relative to the samples from the Vredefort and Johannesburg domes. The amphibolites in the northern part of the Vredefort inlier have higher SiO 2 and lower MgO, Ni and Cr contents relative to the komatiites and komatiitic basalts (Table 1, Figures. 6; 7). They strongly resemble some tholeiitic basalts in the Kromberg Formation of the Barberton Greenstone Belt (Vennemann and Smith, 1999) (Figures 5 to 7). In Figure 5, they plot into the field defined by tholeiitic basalts and lie along a differentiation trend defined by the komatiites and komatiitic basalts. It is not clear, however, if these amphibolites represent a highly evolved part of the komatiitic sequence or if they represent a different generation of magma. The feldspar-quartz porphyry has calc-alkaline affinity (Figure 6) and a dacitic to rhyolitic composition. Structural geology The metavolcanics are dominated by a subvertical, northwest-trending, actinolite-chlorite schistosity that decreases in intensity towards the south of the inlier (Figure 2). This fabric is relatively sinuous and locally swings to more easterly and northerly trends in the central and northeastern parts of the inlier (Figures. 2; 8a). Elsewhere in the core of the dome, Lana et al. (2003a; c) have noted a similar subvertical, northwesttrending, albeit higher-grade, peak metamorphic fabric that they identified as S3 (Figure 1b), as it transposes a subhorizontal S2 foliation, which itself contains relics of an older S1 fabric. The truncation of S3 by the corecollar unconformity in the northwestern sector (Figure 1b; Lana et al. 2003c) indicates that D3 predated the deposition of the ~3.07 Ga (Armstrong et al., 1991) Dominion Group lavas. The S1 fabric has not been observed in the southeastern inlier, and S2 is only visible in low-strain lenses within the S3 foliation, where it defines shallowlyplunging, upright, metre-scale, open folds with axial planes parallel to S3 (Figure 2), or millimetre-scale tight folds with hinges plunging 40 to 60 to the northwest (Figure 2). The metre-scale folds are normally associated with parasitic kink folds with hinges plunging 40 to 50 to the northwest. The S2 fabric is locally accentuated by foliation-parallel quartz veins, which are strongly recrystallised. The S3 schistosity is also locally associated with a shallowly southeast-plunging mineral lineation (L3; Figures. 2; 8a) defined by individual crystals or aggregates of chlorite and amphibole. In the southern part of the area, the pillows and variolites show moderate (2:1 maximum ratio) elongation parallel to S3 and L3 (Figure 8b), and a weak, spaced S3 cleavage. The pillows and variolites are also symmetric with respect to S3 and indicate a strong component of horizontal northeast-southwest directed coaxial flattening strain during D3. Lana et al. (2003a) reached a similar conclusion for D3 in the remainder of the dome based on the geometries of conjugate D3 extensional shear bands and boudins in the high-grade gneisses. The northwestern and northern flanks of the inlier are dominated by a kilometre-wide vertical to subvertical, NE-trending, D4 mylonitic shear zone (Figure 2), which has variably transposed and rotated the S3 fabric (Figures 2; 8c). S3 is folded into doubleplunging folds with axial planes subparallel to S4 (Figures 8d; e). Vertically to subvertically-plunging sheath folds are locally observed in the shear zone (Lana

11 CRISTIANO LANA, ROGER L. GIBSON, WOLF U. REIMOLD AND RICHARD C.A. MINNITT 301 Table1. Major and trace element data for metavolcanics, amphibolites and feldspar-quartz (FQ) porphyry. Altered Mafic metavolcanics Ultramafic metavolcanics Amphibolites FQ Porphyry Sample GGB1 GGB2 GGB3 GGB5 UP-77 UP-78 UP-79 UP-80 UP-84 GR2-1 GR2-2 GR2-4 GR2-9 GR3-9 UP-81 UP-82 Gr4-1 Gr4-2 Gr4-3 Gr4-4 Ton01-01 Ton01-02 Major elements (wt. %) SiO TiO Al2O Fe2O3t MnO * * MgO CaO * * Na2O * * K2O P2O LOI Total Trace elements (ppm) V Cr Co Ni Cu * Zn Rb * * * Sr Y Zr Nb Ba Total (t) Fe as Fe2O3; *below detection limited

12 302 GRANITE-GREENSTONE ASSOCIATION IN THE SOUTHEASTERN VREDEFORT DOME Figure 8. (a) Lower hemisphere equal area projection of poles to the S3 cleavage and L3 mineral lineation. The spread in S3 orientations is a consequence of rotations related to the D4 shearing event. (b) Flattened variolites in komatiitic basalts on farm Hattingsrust 68. Maximum extension is parallel to L3. (c) Lower hemisphere equal area projection showing poles to S4 and L4 lineation. The regional S3 trend can be distinguished from local variations of the S3 orientation caused by the D4 high-strain shear zones. (d) Folded S3 migmatitic layering along the S4 mylonitic fabric in the D4 shear zone on farm Broodkop 304 (boxed area shown in e). (e) Mylonitic fabric transposing stromatic migmatite layering in the D4 shear zone (farm Broodkop 304). (f) Subvertical L4 lineation defined by elongated hornblende + biotite clots in amphibolite (farm Benshoop 74).

13 CRISTIANO LANA, ROGER L. GIBSON, WOLF U. REIMOLD AND RICHARD C.A. MINNITT 303 Table 2a. Averages of electron microprobe analyses of amphiboles in feldspar-quartz porphyry (BO), amphibolites (GDL), amphibolite xenoliths in the migmatites (Gr15), undeformed pillow lava (GR80), actinolite-clorite schist (Kink; Gr4), and banded iron formation (Gr6; GR2) Mineral Horn.-pargasite Horn.-pargasite Horn.-pargasite Mg-honrblende Actinolite Actinolite Act.-hornblende Act.-hornblende Grunerite Ferro-hornblende Samples Gr15 BO GDL RG08 kink1 Gr4 Gr4 kink GR6 GR2 No. N=6 N=8 N=6 N=5 N=3 N=6 N=3 N=1 N=4 N=4 analyses average st. dev. average st. dev. average st. dev. average st. dev. average st. dev. average st. dev. average st. dev. average st. dev. average st. dev. average st. dev. Major elements (wt%) MnO Na2O K2O SiO Al2O MgO FeO CaO TiO Total oxygen Tetrahedral sites TSi TAl Sum-T C sites CAl CTi CMg CFe Sum-C B sites BFe BMn BCa BNa Sum-B A sites ACa ANa AK Sum-A Sum-cat Sum-oxy

14 304 GRANITE-GREENSTONE ASSOCIATION IN THE SOUTHEASTERN VREDEFORT DOME et al., 2003d). The mylonitic fabric in the migmatite is characterised by recrystallized biotite, quartz and feldspars. In the amphibolites, S4 is defined by strongly recrystallised biotite, hornblende, quartz and plagioclase, and retrograde epidote, chlorite and muscovite. Oriented crystals of hornblende and biotite, and elongate plagioclase and quartz aggregates, define a subvertical L4 stretching lineation (Figures 8c; f). The L4 mineral lineation and the sheath folds in the migmatite indicate dip-slip movements during D4. These features are consistent with previous field observations by Colliston and Reimold (1990), who interpreted the shear zone as a northwesterly dipping reverse dip-slip fault, with the migmatitic gneisses to the northwest of the shear zone having moved up relative to the greenstone sequence. However, recent numerical models for central uplift formation in large impact events (e.g., Ivanov and Deutsch, 1998; Melosh and Ivanov, 1999) and structural evidence that Archaean fabrics elsewhere in the outer core of the dome have been significantly rotated during the impact event (Lana et al., 2003c) suggest an original subhorizontal to shallow northwesterly dip for S4 (Lana et al., 2003d). The D3 and D4 structures are cut by pseudotachylitic breccia veins and dikes that are found throughout the wider environs of the dome and that are related to the ~2.02 Ga Vredefort impact event (Gibson and Reimold, 2001). Mineral chemistry Mineral compositions in of 12 of the 53 samples selected for petrographic analysis were analysed at the Analytical Facility, Rand Afrikaans University, Johannesburg, using a Cameca 255 electron microprobe operating at 15 kv with a wavelength-dispersive spectrometry system. Results of the probe analyses and calculated mineral compositions are presented in Table 2a-e. Results of the petrographic and electron probe analysis indicate the presence of at least 6 types of amphibole in the lithologies studied. Fine-grained (10 to 90 m) amphiboles defining the S3 fabric in the actinolite-chlorite schists vary from actinolite to actinolite-hornblende, whereas the coarser (0.1 to 0.9mm) amphibole that overgrows the fine-grained assemblage is actinolite (Figures 9a; b; Table 2a). Amphiboles in the banded iron formation vary from colourless grunerite to green ferro-hornblende and are associated with magnetite, ilmenite and quartz (Figure 9c). These amphiboles are coarser-grained (0.2 to 0.9mm) than the actinolite-hornblende in the actinolitechlorite schist and occur in bands of grunerite and ferrohornblende, which alternate with bands of grunerite, magnetite and ilmenite (Table1; Figure 8b). Both grunerite and ferro-hornblende are depleted in MgO (< 4.9 weight %) and SiO 2 (< 52.0 weight %) relative to the actinolite (Table 2a), whereas grunerite is enriched in FeO and depleted in SiO 2 and CaO relative to ferrohornblende. The main amphibole in the amphibolites and feldspar-quartz porphyry is pargasite-hornblende (Figure 9d), which is characterised by high Al 2 O 3 (> 9.0 weight %) and FeO (> 26.0 weight %) and low MgO (< 3.9 weight %) relative to the actinolite (Table 2a). In most samples, medium-grained (0.5 to 1.5mm) hornblende is in textural equilibrium with plagioclase + sphene + quartz ± biotite and defines the S3 fabric in the amphibolite. It has, however, been intensely recrystallised along the S4 mylonitic fabric. Amphibole in the komatiitic basalts is a highly magnesian hornblende, which occurs as fine-grained (10 to 90 m), randomly oriented crystals. It is characterised by significantly higher MgO (>15 weight %), and lower FeO (<15 weight %) and Al 2 O 3 (<5.9 weight %) than the pargasite-hornblende in the amphibolites (FeO > 26 weight %; MgO < 3.5 weight %). Locally, hornblende is replaced by retrograde chlorite and epidote. The chlorite in the actinolite-chlorite schists is clinochlore and occurs in association with peak metamorphic actinolite-hornblende and rare quartz (Figures 9a; b). Retrograde chlorite is observed in other rock types such as amphibolites and quartz porphyry, but it is too fine-grained (<10 m) to provide consistent analytical results by electron microprobe. The clinochlore is characterised by high MgO (>23 weight %) and low FeO (<11 weight %) abundances, which is consistent with the highly magnesian bulk-rock composition of the actinolite-chlorite schists (Table 2b). Significantly, the Al iv in the clinochlore is higher than in chlorites in the aluminous metasediments of the Witwatersrand Supergroup (Stevens and Preston, 1999), which is consistent with higher metamorphic conditions in the greenstones relative to the rocks in the overlying Late Archaean supracrustal sequence (see next section). Biotite and plagioclase only occur together in the layered amphibolites and quartz-feldspar porphyry (Figures 9d to e). These minerals are normally found in bands with or without pargasite-hornblende in the amphibolites. Plagioclase (Ab ) is found as coarsegrained (0.5 to 1mm) porphyroclasts or as intensely recrystallised fine-grained (5 to 90 m) crystals (Figure 9e). The porphyroclasts display undulose extinction and subgrain boundaries (Figure 9e) and may also occur mantled by dynamically recrystallized plagioclase aggregates. Biotite occurs as medium- to fine-grained (10 m to 0.5mm), lath-like crystals with very constant TiO 2 (2.84 to 3.18 weight %), MgO (2.58 to 3.01 weight %) and FeO (31.2 to 33 weight %) (Table 2b). Retrograde epidote and sericite occur as aggregates of fine-grained (1 to 10 m) crystals that replace plagioclase and K-feldspar in the feldspar-quartz porphyry, amphibolites and migmatitic bands. Epidote is characterised by narrow ranges of CaO (22.80 to weight %), SiO 2 (40.51 to weight %), Al 2 O 3 (27.02 to weight %) and FeO (7.21 to 6.15 weight %), and very low (<1 weight %) contents of TiO 2, K 2 O, Na 2 O, MgO and MnO (Table 2c). Medium-grained (0.1 to 0.5mm) magnetite is associated with ferro-hornblende

15 CRISTIANO LANA, ROGER L. GIBSON, WOLF U. REIMOLD AND RICHARD C.A. MINNITT Figure 9. Photomicrographs of representative mineral assemblages in the southeastern sector of the Vredefort dome. (a) Coarse-grained actinolite-chlorite schist (cross-polarised light) with randomly oriented coarse-grained actinolite (Act) overgrowing fine-grained chlorite and actinolite-hornblende (farm Blaauwboschpoort 304). Fine-grained actinolite-hornblende and chlorite (Chl) are aligned parallel to S3. (b) D3 kink fold in finegrained actinolite-chlorite schist with fine-grained oriented crystals of chlorite and actinolite-hornblende (cross-polarised light; farm Blaauwboschpoort 304). (c) Detail of amphibole-rich compositional banding in banded iron formation (plane-polarised light; farm Benshoop 74). Opaque mineral is magnetite with small ilmenite inclusions. d) Biotite + plagioclase + hornblende + quartz assemblage defining the peak metamorphic S3 fabric in amphibolite xenoliths (plane-polarised light; farm Benshoop 74). (e) Detail of S4 mylonitic fabric in the granitic leucosome, with intensely recrystallised plagioclase and quartz (cross-polarised light; Farm Broodkop 304). Plagioclase (Pl) is partially replaced by retrograde epidote (Ep). Note recrystallized quartz ribbons (bottom). 305 and grunerite in the banded iron formation and with Mg-hornblende in the undeformed komatiitic basalts. Fine-grained magnetite also occurs with fine-grained ilmenite, hornblende-pargasite and biotite in the amphibolites. The medium-grained magnetite consists of Fe2O3 (>98 weight %) with traces (<0.1 weight %) of SiO2, Al2O3, MgO, Nb2O5 and V2O5 (Table 2c). Metamorphic conditions The peak metamorphic assemblage of the actinolitechlorite schists involves actinolite + hornblende + Mgchlorite + magnetite + ilmenite ± quartz. The coexistence of actinolite and chlorite is common in greenschist- to amphibolite-facies metabasites (Robinson et al., 1982; Spear, 1993). Actinolite and chlorite form in hydrated metabasites via complex reactions involving zoisite,

16 306 GRANITE-GREENSTONE ASSOCIATION IN THE SOUTHEASTERN VREDEFORT DOME Table 2b: Averages of electron microprobe analyses of chlorite, plagioclase and biotite in feldspar-quartz porphyry (BO), amphibolites (GDL), amphibolite xenoliths in the migmatites (Gr15), undeformed pillow lava (GR80), and actinolite-clorite schist (Kink; Gr4) Mineral Clinochlore Clinochlore Biotite Biotite Oligoclase Oligoclase Oligoclase Oligoclase Samples Gr4 Kink1 Gr15 BO GDL BO Gr15 GR80 No. N=4 N=5 N=6 N=6 N=5 N=5 N=5 N=5 analyses average st. dev. average st. dev. average st. dev. average st. dev. average st. dev. average st. dev. average st. dev. average st. dev. Major elements (wt%) Major elements (wt%) Major elements (wt%) SiO MnO MnO Al2O Na2O Na2O MgO K2O K2O FeO SiO SiO MnO Al2O Al2O TiO MgO MgO K2O FeO FeO Cr2O CaO CaO CaO TiO TiO H2O Cr2O Total Total Total oxygen and 4 OH 24 oxygen and 4 OH 32 oxygen Si Si Si Al IV Al IV Al Al VI Al VI Ti Ti Ti Fe Fe Fe Mn Cr Cr Ca Mn Mn Na Mg Mg K Ca Na Sum-cat Na K Ab K Sum-cat An XFe OH Or XMg XFe XAb

17 CRISTIANO LANA, ROGER L. GIBSON, WOLF U. REIMOLD AND RICHARD C.A. MINNITT 307 Table 2c. Averages of electron microprobe analyses of epidote and magnetite in banded iron formation (Gr6), undeformed pillow lava (Gr80), feldspar-quartz porphyry (BO), and amphibolite xenoliths in the migmatite (Gr15) Mineral Magnetite Magnetite Epidote Epidote Samples Gr6 Gr80 Gr15 BO No. analyses N=3 N=3 N=11 N=7 average st. dev. average st. dev. average st. dev. average st. dev. Major elements (weight %) Major elements (weight %) SiO FeO Al 2 O MnO MgO TiO MnO CaO TiO Na 2 O Cr 2 O SiO CaO Al 2 O Nb 2 O MgO V 2 O K 2 O Fe 2 O Total Total oxygen 12.5 oxygen Si Fe Al Mn Mg Ti Mn Ca Ti Na Cr Si Ca Al Nb Mg V K Fe Sum-cat Sum-cat carbonates and oxide phases, and their stability fields are largely dependent on bulk-rock composition in addition to temperature and pressure (Spear, 1993). Detailed observations on natural occurrences of actinolite and Mg-chlorite and on Fe-Mg substitution in laboratory experiments (Moody et al., 1983; Apted and Liou, 1983) have indicated that these minerals are likely to remain stable to much higher temperatures in high- Mg rocks than chlorite and actinolite in high-fe rocks. The high Mg content in the chlorites, in addition to the presence of hornblende in equilibrium with actinolite in some of the samples of the actinolite-chlorite schists, indicates peak temperature conditions not lower than 300 C (Moody et al., 1983). This is supported by chlorite thermometry applied to the chlorite compositions. The chlorites show no significant contamination (Ca+Na+K < 0.2 atoms per formula unit; Table 2b) and are, thus, suitable for such temperature estimations (Jiang et al., 1994). The results obtained using calibrations by Kranidiotis and MacLean (1987), Cathelineau (1988), and Zang and Fyfe (1995) indicate consistent temperature results ranging from 393 to 407 C (Table 3). Results from the Cathelineau (1988) calibration indicate average temperatures ~15 C lower than those obtained with the thermometers calibrated by Kranidiotis and MacLean (1987) and Zang and Fyfe (1995). This variation is, however, within the ± 25 C uncertainties typically assigned to the chlorite thermometer (e.g., Cathelineau, 1988; Zang and Fyfe, 1995). A peak metamorphic temperature of 400 C is, thus, inferred for the greenstones southeast of the D4 shear zone. The peak metamorphic assemblage in the migmatitic gneisses and amphibolites along the northwestern flank of the inlier is characterised by quartz + oligoclase + K- feldspar + biotite + ilmenite + sphene ± hornblende. The presence of high-al-hornblende + oligoclase and the absence of epidote and chlorite in the peak metamorphic assemblage suggests temperatures between 650 and 700 C at low to intermediate pressures (Spear, 1993), which is well above the greenschist-facies conditions indicated by the metamorphic assemblage in the meta-lavas to the southeast. Quantification of P-T conditions using the THERMOCALC software (Powell and Holland, 1988) was hampered by the high variance of the assemblages and produced unreliable estimates ranging between 400 and 700 C and 3 and 7 kbar. Consistent pressure estimates were, however, obtained with the Al-in-hornblende barometer calibrated by Hammarstrom and Zen (1986) and Hollister et al. (1987). Results for nineteen hornblende analyses from the