Joumal of African Earth Sciences. Vol. 31, No. 314. pp. 499-521. Zoo0 o 2001 Elrevier Science Ltd

Pll:S0899-!5362(00) All rights reserved. Printed in Great aiiain

oFz99-5362/01 :- see front matter

The Anna’s Rust Sheet and related gabbroic intrusions in the Vredefort Dome-Kibaran magmatic event on the

Kaapvaal Craton and beyond?

W.U. REIMOLDl,*, G.Q.J. PYBUS,*, F.J. KRUGER3, P.W. LAYER4 and C. KOEBERL5 ‘Impact Cratering Research Group, Department of Geology, University of the Witwatersrand,

Private Bag 3, PO Wits 2050, Johannesburg, South Africa 2Present address: De Beers Geoscience Centre, PO Box 82232,

Southdale 2135, South Africa 3Hugh Allsopp Laboratory, University of the Witwatersrand, Private Bag 3,

PO Wits 2050, Johannesburg, South Africa 4Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks,

Alaska 99775-7320, USA Ynstitute of Geochemistry, University of Vienna, AlthanstraBe 14, A-l 090 Vienna, Austria

ABSTRACT-The Anna’s Rust Sheet (ARS) and a suite of mineralogically and chemically related intrusions in the core and collar of the Vredefort Dome (in particular, the Vredefort Mafic Complex: VMC) represent a newly recognised type of high Ti gabbro in this central part of the Kaapvaal Craton. This lithology, referred to as the Vredefort Type IV mafic intrusion, is distinguished from chemically similar Type V intrusions (the Karoo dolerites) by the presence of glomeroporphyritic plagioclase and higher Th content and from Type Ill intrusions ( - 1600 Ma gabbro) by the lack of cross-cutting pseudotachylitic breccia veinlets. Petrographic features and both major and trace element compositions of all Type IV intrusions are very similar. Based on its Rb-Sr isotope age and character, a gabbroic intrusion from Majuba Colliery (Mpumalanga Province) is also thought to belong to the ARS (Type IV) suite of tholeiitic intrusions. Rb-Sr isotopic analysis resulted in a preferred age of 1052 f 11 Ma (2a) for biotite and plagioclase data for ARS, VMC and Majuba samples. The Rb-Sr age for the ARS is further supported by QAr-3gAr stepheating ages for plagioclase and pyroxene separates from two ARS and VMC samples, which favour formation of this gabbroic intrusion at ca 1000 Ma ago. These results suggest that an - 120 m thick sheet intrusion may be present throughout a major part of the Vredefort Dome. While Kibaran-age (ca l-l .2 Ga) alkaline, both mafic and felsic, magmatism, as well as tectonic and hydrothermal activity at that time, have been known in the central Kaapvaal Craton, a widespread tholeiitic magmatic component has now been added to this record. There is a strong likelihood that this magmatic event occurred throughout the southern African subcontinent and perhaps into Antarctica. o 2001 Elsevier Science Limited. All rights reserved.

RESUME-La sequence de Anna Rust (ARS) et les suites intrusives, mineralogiquement et petrologiquement associees, affleurent au cceur et en bordure du dome de Vredefort (le complexe mafique du Vredefort, VMC, plus precisement). Elles representent, dans cette partie centrale du craton de Kaapvaal, un nouveau type de gabbro riche en Ti. Cette lithologie, conside& comme une intrusion mafique de type IV, se distingue des intrusions chimiquement similaires de type V (les dolerites de Karoo) par une plus grande teneur en Th et des plagioclases glomero-porphyriques. Elle se distingue Bgalement des intrusions de type III (gabbros - 1600 Ma) par I’absence de

*Corresponding author 065wur@cosmos.wits.ac.za

Joumaiof Aftican Earth Sciences 499

W.U. REIMOLD et al.

veines de pseudotachylites brechiques s&antes. Toutes les intrusions de type IV presentent des figures petrologiques et des compositions en elements majeurs et en traces tres similaires. Compte tenu des caracteristiques isotopiques Rb-Sr et des ages obtenus, I’intrusion gabbroique de Majuba Colliery (Province de Mpumalanga) semble egalement faire partie des intrusions thol@itiques (type IV) de I’ARS. Les analyses isotopiques Rb-Sr realisees sur les biotites et les plagioclases des dchantillons de I’ARS, du VMC et de Majuba, ont donnd un Bge preferentiel de I’ordre de 1052 + 11 Ma (2a). Cet age est conforte par les Sages plateaux 40Ar-3gAr obtenus sur plagioclases et pyroxenes s&par&s, de deux echantillons de I’ARS et du VMC, qui donnent un age de - 1000 Ma, interpret6 comme I’Bge de mise en place des gabbros. Les resultats suggerent qu’une intrusion stratiforme d’environ 120 m d’ipaisseur devait Btre presente dans la majeure partie du dtjme de Vredefort. Ce nouvel episode de magmatisme thol&tique est desormais a integrer a I’histoire d’8ge Kibaran (ca l-l .2 Ga) marquee par un magmatisme alcalin, mafique et felsique, et par une activite tectonique et hydrothermale deja connue dans la partie centrale du craton. II y a de forte probabilite que cet evenement magmatique se soit produit a travers I’ensemble du continent sud africain et peut-&re egalement en Antarctique. Q 2001 Elsevier Science Limited. All rights reserved.

(Received 16110199: revised version received 3/6/00: accepted 17/l 1100)

INTRODUCTION

The ca 70 km wide Vredefort Dome is situated - 120 km southwest of Johannesburg, nearly central to the surrounding Witwatersrand Basin. Recent multi- disciplinary research has provided strong support for the impact hypothesis for the origin of the Vredefort Structure (Reimold and Gibson, 1996). Modelling of the regional distribution of shock metamorphic effects (Therriault et al., 1997a) and of the regional gravity and magnetic fields (Henkel and Reimold, 1998) indicates that the original diameter of the Vredefort Impact Structure (hereafter referred to as the Vrede- fort Structure) was of the order of 250-300 km, thus encompassing the whole extent of the Witwatersrand Basin.

The 45-50 km wide core of the dome consists of Archaean crystalline basement rocks. It is surrounded by the ca 20 km wide collar of volcano-sedimentary successions belonging to the Archasan Dominion Group and the Witwatersrand and Ventersdorp Super- groups, and the Proterozoic Transvaal Supergroup. The collar strata are intercalated with abundant mafic intrusions (Hall and Molegraaff, 1925; Nel, 1927a, 1927b; Bisschoff, 1972a, 1972b; Stepto, 1990; Jackson et a/. ,I 992; P/bus, 19951, which comprise an estimated 30% of the collar (Nel, 1927a, 1927b). A large number of mafic bodies also occur in the core (e.g. Stepto, 1990; compare Pig. 1, after R/bus, 1995).

Available chronological data for the Vredefort Dome have recently been reviewed by Reimold and Gibson (1996). Important intervals relevant to this study are:

i) formation of the granitoid basement to the Vredefort Dome around 3.1-3.3 Ga (single zircon U/ Pb chronology by Kamo et al., 1996);

# Dominion Group lavas extruded at 3.074 kO.009 Ga (Armstrong et al., 1991);

500 Journal of African Earth Sciences

iiil deposition of the Witwatersrand Supergroup between ca 2.9 and 2.7 Ga (references in Robb and Meyer, 1995);

iv) Ventersdorp Supergroup extrusives formed at - 2.7 Ga (Armstrong er al., 1991); and

vl deposition of the carbonate-dominated Transvaal Supergroup, which took place between ca 2.60 and 2.25 Ga and emplacement of the Bushveld Complex at 2.05-2.06 Ga (e.g. Walraven et a/., 1990; Walraven and Martini, 1995; Gibson and Stevens, 1998). The age of the impact event was determined by Kamo et al. (1996) through U-Pb dating of single zircons of igneous morphology from pseudotachylitic breccia to be 2023 f 4 Ma (cf. also Spray et al., 1995; Moser, 1997; Gibson et a/. , 1997).

As part of a comprehensive mineralogical, chemical and chronological study of mafic intrusives from the central part of the Witwatersrand Basin (Jackson et a/. , 1992; Reimold et al,, 1995a, 1995b; Pybus et al., 1994, 19951, Pybus (1995) carried out a detailed petrographic and chemical investigation of numerous intrusions from the region of the Vredefort Dome. In this paper, part of the results of this study are reported, focusing on a particular type of gabbroic intrusion, which is typified by the Anna’s Rust Sheet north and northeast of the town of Parys.

CLASSIFICATION OF VREDEFOKl- MAFIC INTRUSWES

Hall and Molengraaff (1925) and Nel(l927a) believed that the gabbroic/dioritic and alkali granitic intrusions of the region (Reimold and Gibson, 1996) were all related, both in age and in origin. In addition, Nel

The Anna’s Rust Sheet and relatedgabbroic intrusions in the Vredefort Dome-K&wan magmatic event

- 26” 50’S /

m- -\ 2P71

.

i k USA2 I7

‘ANNA’S RUST

t BOREHOLES

%H’:Z~ w WS2I228

iid

KAROO SEDIMENTS, ALLUVIUM

KAROO DOLERITE

-L, -r”L

. , I

1 Okm I I

MAFlClULTRAMAFlC INTRUSIONS

GREENSTONE TERRANE

1”----1 ARCHRAN GRANlTElKAROO OR ALLUVIUM CONTACT

m ARCHAAN GRANITE/DOMINION OR WITS SG CONTACT

@ure 1. Distribution of mafic intrusions in the region of the Vredefort Dome, based on the geological map of Nel (1927bl. All Type IV intrusions identified by q/bus (19951 are indicated. Localities are marked by their abbreviated names and/or denoted by the numbers of samples taken for petrographic and chemical analysis. WW: Winddam wehrlite; VMC: Vredefort Mafic Complex. Other denominations, such as UPxx, GPx, etc. are sample numbers corresponding to individual analyses listed in the tables or marked on the diagrams in this paper.

(I 927a) related a ‘slaty looking igneous rock’ (SLIR) subdivision of the Vredefort mafic intrusive rocks was intruding Kimberley-Elsburg quartzite of the collar to possible and that intrusions of different ages are the Ventersdorp Supergroup and placed the ca 180 present. Bisschoff (I 969, 1972a, 1972b) indicated Ma (Allsopp et al. 1984; Eales et al. 1984) Karoo that intrusives that could be related to the Bushveld dolerites into a separate category. Bisschoff (I 969, Complex were prominent in the area of the Vredefort 1972a, 1972b) and Stepto (I 990) showed that further Dome. Recently, A.A. Bisschoff @ers. comm. to WUR,

Journal of African Earth Sciences 50 1

W.U. REIMOLD et al.

2OOD) indicated that Bushveld-related intrusives were particularly abundant in the outer collar strata. Based on work of the authors on the core and inner collar (e.g. Pybus, 1995), it is concluded that mafic intrusions, which could be positively considered correlatives of the Bushveld Complex, are rare in these parts of the Vredefort Dome.

Most recently, Jackson eta/. (1992), R/bus (1995) and Pybus et al. (1995) have classified the intrusions in and around the Vredefort Structure using geo- chemical criteria in addition to field observations and petrographic and chronological data. Their investi- gations did not include the occurrences of Dominion Group metamorphosed lavas (amphibolites) occurring in a narrow zone along the core-collar boundary (CL Jackson, 1992, 1994). Besides Dominion Group metalavas, five types of mafic intrusions have been delimited in this study:

17 Type I: the ‘primitive mafic-ultramafic’ intrusions of Stepto (1990) and Jackson et a/. (1992);

17) Type II: intrusive bodies related to Ventersdorp magmatism, which generally represent altered and metamorphosed, low TiO, gabbro;

3) Type Ill: high TiO, gabbro of - 1600 Ma age and characterised by the presence of pseudotachylitic breccia veinlets (Reimold et al., 1988);

iv) Type IV: high TiO, gabbro of Kibaran age (the subject of this work); and

WI Type V: ca 180 Ma Karoo doleriie intrusions with intermediate TiO, contents (e.g. Reimold et al., 1995b).

Pybus (1995) determined that the Anna’s Rust Sheet (ARS), previously termed ‘Anna’s Rust Sill’ or ‘Anna’s Rust Gabbro’ (Fig. 1) (Nel, 1927a; Bisschoff, 1969) represents a characteristic member of the Type IV group. Other Type IV intrusions that occur in the core and collar of the Vredefort Dome are the Vredefort Mafic Complex (VMC), the Hester and Oceaan intrusions, intersections in drillcore from the Inlandsee pan in the centre of the Vredefort Dome, boreholes in the southwestern and western (USA- 65) sector of the collar, and a small number of other surface exposures scattered throughout the northern and western parts of the core of the Vredefort Dome. These localities are indicated in Fig. 1. Type IV material was also sampled in two Anglo-American Corporation boreholes (GH-1 and NG-1) to the south of the Vredefort Dome.

THE ARS AND RELATED INTRUSIONS

The ARS represents an undulose, sub-horizontal sheet intrusion, the thickness of which can be gauged from elevation contrasts on the surface as being in excess of - 60 m. Its main outcrops are located to the east

502 Journal of African E&h Sciences

of the Vaal River in the Vredefort Structure, extending from the core-collar transition in the north to the area southeast of the town of Parys (Fig. 2). The ARS intrudes both the Outer Granite Gneiss and the metasedimentary rocks of the West Rand Group (Lower Witwatersrand Supergroup). Contact relation- ships with adjacent lithologies can only rarely be studied. The body crosscuts several sills of Venters- dorp (2.7 Ga) affinity, as well as a dyke of Vredefort Granophyre (e.g. Reimold et al., 1990a; Therriault et al., 1996, 1997b) (compare Fig. 2). Re-0s isotope studies (Koeberl et a/., 1996) showed that this granophyre contains a small meteoritic component, which convincingly proved that this rock type represents impact melt rock. Due to this crosscutting relationship, the ARS must be younger than the Vredefort impact event. Furthermore, no occurrences of pseudotachylitic breccia, which is generally related to the Vredefort impact event, have been found in the ARS or in any of the other Type IV intrusions (e.g. Reimold and Colliston, 1994; Reimold, 1995, 1998) in contrast to the otherwise similar (cf. below) Type Ill intrusions.

The morphology of the ARS is characterised by a topographically prominent outcrop, at least in the nor- thern part of the exposed body (Fig. 2), whereas typical exposures of the VMC to the east of the town of Vrede- fort (Fig. 1) are C 30 cm high, discontinuous outcrops, boulder-strewn fields and a few flat erosion plateaus. Samples from the VMC complex exhibit a pattern of glomeroporphyritic plagioclase development and veinlets of pyroxene segregations similar to those in the ARS. While it is difficult to see in the field whether the VMC might have a (sub)horizontal geometry, it is perhaps significant that Nel(1927b; his cross-section marked A-E) already assigned such an attitude to this intrusion and indicated a possible subsurface connection between the ARS and the VMC.

The Oceaan intrusion in the southeastern sector of the dome (Fig. 1; cf. Minnitt ef a/., 1994) also has a sub-horizontal attitude. Glomeroqsts of plagioclase have not been observed in samples from this body, but only a few samples have been studied petro- graphically to date. It must be noted that Nel(1927b) had marked an occurrence of pseudotachylitic breccia on this body, but the authors’ fieldwork did not reveal any in situ pseudotachylitic breccia in the Oceaan intrusion (though outcrop on this body is extremely sparse). Float of pseudotachylitic breccia observed in the area could be derived from the abundant breccia occurrences in migmatitic gneisses exposed in the environs of the Oceaan body. On the balance of the evidence, the Oceaan intrusion is assigned to Type IV, although it cannot be entirely ruled out that it is of Type Ill.

The Anna 5 Rust Sheet and related gabbroic intrusions in the Vredefort Dome-Kibaran magmatic event

2718' 2T31'

2650’

26’53

2718' 0 2km

2731'

I,',,,

a GABBRO SILL - ROADS

I-‘. OUTER GRANITE GNEISS /f FAULTS

m EPIDIORITES c

_I-- STRIKE & DIP

0 COLLAR SEDIMENTARY ROCKS _” INFERRED CONTACT m GRANOPHYRE OBSERVED CONTACT

2653'

Figure 2. Map of the Anna’s Rust Sheet northeast of Parys, South Africa.

Journal of African Earth Sciences 503

W. lJ. REIMOL D et al.

Another exposure on the farm, Hester, east of the Inlandsee (Fig. 11, forms an extensive, but shallow, ridge in the otherwise flat morphdogy of this part of the dome. Exposures on this ridge are limited to isolated shallow exposures and float. However, in the course of the extension of the Nl freeway towards the town of Kroonstad in the late 1980s material from the Hester body was quarried for filling of the road base, and a large quarry existed for several years (although it has since been filled in). Samples were collected covering the whole extent of this quarry. Glomerocrystic plagioclase aggregates do occur in some of these samples, and pyroxene veinlets were also observed.

In a drillcore obtained within the Inlandsee pan near the centre of the dome (IS in Fig. 11, an apparently horizontal mafic intrusion was intersected at - 182 m depth. The lower limit of this body was not reached when drilling was terminated at 256 m depth. Similar mafic material was intersected in another borehole only a few kilometres to the northeast of the Inlandsee drilling site, with upper and lower contacts at 140.4 and 240.5 m depths. Pybus (I 995) established that this sub- horizontal intrusion must be classified as Type IV on the basis of combined petrographic and chemical evidence. Similar rocks also occur in several exploration boreholes sunk by Gold Fields of South Africa into the south- western sector of the collar (Fig. I), as described in detail by Pybus (1995).

A shallow exploration borehole sunk by a dimension stone company near the western core-collar boundary (UP-65; Fig. 1) also intersected such material. Other surface exposures of Type IV material were identified during random sampling of exposed mafic intrusives. Whereas the ARS, VMC and Hester bodies have the glomerocrystic feldspar aggregates in common, all the other Type IV intrusions were classified following chemical studies in combination with geological information (such as a lack of pseudotachylitic breccia).

Petrography The ARS gabbro contains - 60 ~01% plagioclase, 35 vol. % clinopyroxene, < 2 ~01% orthopyroxene, 10 vol. % titanomagnetite and ilmenite, and a few volume percent of biotite fin some samples). Its ophitic to subophitic texture is dominated by euhedral to subhedral feldspar. Grain size is variable from area to area at the thin section scale, but mostly limited to l-2 mm. The (sub)ophitic texture is formed by plagioclase laths and prisms that are partly, or wholly, enclosed by clinopyroxene. Plagioclase often exhibits normal zonation. The majority of feldspar grains are reasonably fresh, but abundant fractures in plagioclase give a considerable number of grains a cloudy appearance. Some fractures are filled with secondary sericite. Minor sericitisation and saussuritisation were

504 Journal of African Earth Sciences

noted along some plagioclase grain boundaries. Electron microprobe analysis of two ARS samples (GPl and USA-21 7) gave plagioclase compositions

of An55_75. The fracturing occasionally observed can be tentatively linked to deformation resulting from the late Kibaran tectonic overprint experienced by this part of the Kaapvaal Craton, as, for example, dis- cussed by Friese et al. (19951, or to an even later deformation event, such as that associated with the emplacement of the abundant Karoo dolerite dykes.

Pyroxene, commonly occurring in anhedral forms, is generally fresh and typically yielded compositions of En,_,,Fs,_,,Wo,_,,. A single low Ca clino- pyroxene (pigeonite) of En,Fs,Wo, composition was analysed in an exsolution lamella in host augite. Some grains of pyroxene display rims of a green-brown secondary biotite. Primary biotite crystals occur as late stage interstitial growth, which is often associated with opaque grains.

Plagioclase glomerocrysts found in the ARS and VMC are large and characterised by a coarsening of the plagioclase grain size, near complete lack of pyroxene crystals in these patches and a change from lath- to plate-like habits of the feldspar crystals. The formation of these porphyritic aggregates is described and discussed by Bate (19951, Bate et a/. (1995) and Coetzee et a/. (1995) in terms of flow differentiation, based on the internal variation of modal contents along a borehole through the ARS.

Accessory primary minerals, besides biotite, include an opaque component formed by intergrowths of titaniferous magnetite and ilmenite. Only one of the ARS samples contained a few grains of olivine, and another two contained serpentine pseudomorphs after olivine. No quartz was identified in any of the samples.

Besides the mineral fracturing mentioned, no significant deformation effects are noted in any of the samples studied from Type IV intrusions. Together with the complete lack of cross-cutting pseudotachy- litic breccia, this observation requires emplacement of these Type IV intrusions at times later than the 2023 Ma Vredefort impact event.

All petrographic detail reported for the ARS sample suite, as typically representative of the Type IV group, can be applied to samples from the other related intrusions. Results of electron microprobe analysis of plagioclase, pyroxene and opaque minerals are shown in Fig. 3a-c. All analysed samples, despite being from different bodies, are very similar with regard to their mineral chemical compositions.

Geochemistry Analytical procedures Major, and some trace element, geochemical data for these sample suites were obtained by standard

The Anna’s Rust Sheet and related gabbroic intrusions in the Vredefort Dome-Kibaran magmatic event

Anorthite

Hd

” ” ” Fs

TiOp

cl

olJSA 217

v v v v v v v v MAGNETITE Fe24

Figure 3. Electron microprobe analyses of minerals in Type IV intrusions. la) Plagioclase compositions; Ibl pyroxene data; and fcl Fe-Ti oxide compositions. Sample numbers correspond to localities marked in Fig. 1.

Jwmsl of Afdwn Eatth Sciences 505

W. U. REIMOLD et al.

X-ray fluorescence (XRF) spectrometry at the Department of Geology, University of the Witwaters- rand. Selected samples were also analysed for a range of trace elements, including the rare earth elements (REE), by instrumental neutron activation analysis (INCA) at the University of Vienna, according to standard procedures and to accuracy levels previously reported by Koeberl er al. (I 987) and Koeberl (1993). REE data were normalised against Cl chondrite data from Taylor and McLennan (1985).

Concentrations for those elements analysed by both XRF analysis and INAA are generally very similar. Where differences in Cr contents are noted, the INAA data are preferred due to long-term moni- toring of interlaboratory effects and greater con- fidence in reproducibility of Cr standard values in the Vienna laboratory.

Results The geochemical signature of the mafic intrusions of Type IV was described by P/bus (1995) as being very similar to that of the Type V intrusions, which were correlated with Karoo magmatism; but samples of these respective groups could be distinguished on the basis of textural appearances (typically large poikilitic ot-thopyroxene crystals in the Type V Karoo intrusives), Sm-Nd isotopic records (Reimold et a/. , 199513) and some 40Ar-3sAr stepheating dating (Pybus 1995). In particular, the Sm-Nd results for three Type IV samples, in comparison with data for other types of Vredefort mafic intrusives, indi- cated that Type IV material can be well distinguished from other types, with the exception of the Karoo dolerites (Type V), of very similar Sm-Nd isotopic signature. According to the data and discussion of Pybus (1995), the Type III and Type IV intrusions are chemically indistinguishable.

In terms of their major element compositions (e.g. in the MgO-A&O,-CaO diagram; Fig. 4a, after Jensen, 1976), the Type IV intrusions are chemi- cally similar to high Fe tholeiitic basalts according to the classification of Jensen (1976). In the classification scheme of Viljoen et al. (1982), the Type IV samples occupy a very limited field in the tholeiitic basalt area (Fig. 4a). The TiO, content of the intrusions is the most discriminatory parameter when comparing Types Ill and IV with Types II and V, respectively (compare Fig. 4b). However, the TiO, contents of samples from a specific group are not tightly constrained. For most other combinations of elements (an example is given with the Hf-Th-Ta variation diagram of Fig. 4c) these groups are not well separated. The means and standard deviations for analyses from Type IV and Type III samples are given in Table 1.

506 Journal of African Earth Sciences

The geochemical similarities between the main intrusions of Type IV are also illustrated in Table 2, in which the mean compositions of samples from the ARS, Oceaan, and VMC bodies, and from the bore- holes in the core and southwestern collar of the Vredefort Structure, are compared. Within the internal variation limits for a given body, the major element abundances for all these Type IV bodies compare very well and are indistinguishable from Type Ill intrusions. The standard deviations given in Table 2 signify that each body is internally rather homogeneous. Notice- able variations, for example in some of the V, Cr and Ni contents, are thought to be possibly related to different amounts of minor or trace minerals; for example, the association of V with Fe-Ti oxide min- erals is typical.

Bate ef a/. (1995) and Coetzee et al, (1995) reported chemical and petrographic data for a 120 m long borehole through the ARS. The observed vertical variation was explained as the result of flow differentiation processes believed to have resulted in a concentration of plagioclase phenocrysts and glomerocrysts in two distinct layers. In the current

Table 1. Mean compositions (and standard deviations) for samples belonging to Types Ill and IV (individual analyses in Pybus, 1995)

MEAN o MEAN o

SiO, 50.57

TiO, 1.58

Al203 14.75

Fez03 13.72 Fe0 0.06 MnO 0.19

MgD 6.31 CaO 9.74 Na,O 2.25

KzO 0.69

w5 0.17 LOI 0.23 Ba 221 Rb 36 Sr 173 Y 30 Zr 131 Nb 8 V 294 Cr 151 co 44 Ni 93 cu 115 Zn 113

0.77 50.06

0.26 1.57

0.81 14.87

1.29 13.77 0.00 0.01 0.02 0.20 0.67 6.59 0.53 9.86 0.62 2.17

0.19 0.69

0.04 0.2 0.42 0.43

74 205 11 35 29 162

6 26 32 132

2 7 58 244

107 203 5 42

17 103 45 93 47 99

0.821 0.21

0.81

0.68 0.05 0.01 0.58 0.42 0.56

0.26

0.06 0.61

80 20 84 12 56

4 110 155

13 37 42 44

The Anna’s Rust Sheet and related gabbroic intrusions in the Vredefott Dome-Kibaran magmatic event

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I I 1 I ! 50 100 200 250

Figure 4. IaJ Chemical compositions of Type IV tholektic intrusions in the MgO-CaO-AI,O, classification diagram of ViJoen et al. (1982J. lbJ TiO, versus Zr abundances in Type II, ill, IV and V mafic intrusions from the Vredefott area (data from Pybus, 1995J in comparison with the variation of compositions for Umkondo dolerites and lavas (after Munyanyiwa, 1999J. (cl Hf- Th-Ta systematics for samples of Types II, III, IV and V (data from Fybus, 19951.

study Pybus, 19951, extensive surface sampling was small, but distinct, differences in chemical compo- carried out, which allowed the investigation of sition: MgO concentrations generally are somewhat chemical variation over some 60 m of elevation higher in the central part of the body (accompanied contrast. In addition, the internal chemical com- by sympathetic decreases of Zr and Y concentra- position of the ARS was investigated along several tions), and outer parts of the intrusion are relatively horizontal sampling traverses. This demonstrated enriched in more incompatible elements. In samples

Journal of African Earth Sciences 507

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The Anna’s Rust Sheet and related gabbroic intrusions in the Vredefofl Dome-K&ran magmatic event

from topographically higher positions, MgO is with total REE abundances; in contrast, no obvious relatively depleted in comparison with samples from correlation between U concentrations and REE the central part of the body, and CaO values follow abundances can be reported for Type IV samples). the same trend. The general trends observed in this Type IV intrusives have some similarities to Karoo study are consistent with the model by Bate et al. magmatic compositions (e.g. similar Fe-Ti-rich com- (1995) favouring concentration of large plagioclase positions). In detail, the Type IV intrusions from the crystals and aggregates. However, areas rich in Vredefort Dome are characterised by the following glomerocrysts were excluded from sampling for this parameters: study, as bulk representation was aimed at for com- D mineralogical composition: augitic clinopyroxene, parative studies of different intrusions (some of plagioclase, ilmenite/titanomagnetite; which are not characterised by the presence of i) classification: tholeiitic basalt; glomerocrysts). 3 major element contents: TiO,> 1.15 wt%; SiO,

REE patterns for Type IV intrusions (Fig. 5) are 48-53 wt%; Fe,O, 1 l-l 7 wt%; P,O, 0.12-0.21 enriched relative to Cl chondrite and display only a wt%; and slight LREE enrichment relative to the HREE. Some iv) trace element abundances: (in ppm) Y 20-38; slight differences between samples are noted with Nb 5-10; V 220-320; Cu 70-220; Zn 80-l 30; Zr respect to absolute REE abundances. Some are due IOO-150;Th 2-4; Eu 11-18. to mineralogical differences (e.g. a crude negative In contrast, the chemically very similar Karoo correlation between MgO abundances and total REE dolerites are distinguished by slightly lower TiO, ( i 1 abundances can be related to the plagioclase/pyroxene wt%) and marginally lower trace element contents ratio). Other differences are due to small variations (generally at the lower end of the ranges given for in the trace mineral contents of individual samples Type IV material). The Th contents of Karoo dolerites (e.g. Hf, an element presumed to reside mainly in from this part of the craton are generally lower (1 .O- accessory minerals, forms a crude positive correlation 1.5 ppm) than those of Type IV intrusions (2-4 ppm).

100

. GP5 + IS-225 l SHl-475 A UP71 n WS2-228 v UP16 x USA-59 * IIPGS

I I I I I I I I I I I I I I I J

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

F&we 5. Chondrite-normalised (after Taylor and McLennan, 1985) REE patterns for selected samples from Type IV Vredefort intrusions. All samples analysed are characterised by very similar patterns and limited variations in REE abundances (see Table 41. Sample numbers as corresponding to localities marked in Fig. 1.

Journal of Afriwn Earth Sciences 509

W.U. REIMOLD et al.

Another distinguishing factor between Type IV and Karoo intrusions from the Vredefort Dome is the widespread occurrence of pyroxene poikiloblasts enclosing plagioclase crystals in Karoo dolerite, as well as the general absence of plagioclase glomero- trysts in Karoo intrusives. As shown by Pybus (19951, REE patterns or absolute concentrations can- not be used to distinguish between major groups of mafic intrusives from the central part of the Wit- watersrand Basin, including the Ventersdorp, Type IV, and Karoo suites.

Munyanyiwa (1999) reported a detailed geo- chemical study of the tholeiitic Umkondo dolerites and lavas. This suite of rocks has a primary mineral- ogical composition which is very similar to that of the ARS and associated intrusions. It is dominated by calcic plagioclase laths, clinopyroxene and Fe-Ti oxides but, in contrast to the Type IV intrusions, has been metamorphosed to lower-most greenschist-facies grade. In terms of major elements, the average Umkondo composition, calculated from Munyanyiwa’s data, is quite different from the Type III and IV com- positions of this study. This finding relates to both the mean values and the standard deviations, which are generally larger in the case of the Umkondo composition than for the Vredefort sample suites. At least partially, this can be attributed to effects of metamorphism and/or alteration (e.g. higher loss on ignition and K contents in some Umkondo samples could be linked to relatively enhanced sericitisation of feldspar).

In the TiO, versus Zr diagram of Fig. 4b, Umkondo data are compared to the variations in the Vredefort Type II-V sample suites (after Pybus, 1995). This illustrates that the bulk of the Umkondo samples plot into the Type V (the Karoo dolerite) field, but that a significant number of data scatter more widely. Comparison of the REE patterns for Um- kondo and Type IV samples show a very similar spread of abundance data and similar pattern shapes, but slightly more pronounced positive Eu anomalies in the case of the Umkondo suite (Mun- yanyiwa, 1999).

In conclusion, Umkondo and Type IV Vredefort rocks are, to an extent, geochemically similar but not identical. Both groups of rocks have characteristics that are typical for basaltic rocks emplaced in a continental environment (e.g. CeNb ratios of about 10; compare Table 3).

Geochronology Once the ARS group had been recognised as a distinct group of intrusions that post-dates the Vredefort impact event and is mineralogically and texturally different from the Karoo dolerite suite

5 10 Journal of African Earth Sciences

(Type V; P/bus, 19951, it became necessary to con- strain the age of this group. Selected, represen- tative and fresh samples from the ARS and VMC bodies were analysed by the Rb-Sr and 40Ar-3sAr stepheating dating techniques.

Methodokgy The analytical methods for the Rb-Sr isotopic study, which was carried out at the Hugh Allsopp Laboratory of the University of the Witwatersrand, are essentially those described by Smith et al. (1985) and Brown et al. (19891, except that leaching techniques were not applied. Accuracies attained in the laboratory throughout the period of this study can be gauged from the average 87Sr/BBSr ratio determined for the Eimer and Amend standard at 0.70800 *4 (1 o standard deviation for 81 analyses) and for the SRM987 standard at 0.71022&3 (la for 59 analyses). Accuracy of Rb and Sr concentration determinations was periodically monitored by analysis of the K-feldspar SRM607 standard. Total chemistry blank values determined during this study were of the order of 2 ng for Rb and 2.5 ng for Sr, which are significant factors in the context of this study. Thus, all reported data (Table 4) are blank-corrected. Regression of the isotopic data was carried out with the GEODATE version 2.2 software (Eglington and Harmer, 19911, applying the decay constant for 87Rb of 1.42 x IO-“y-‘.

Plagioclase, pyroxene and biotite were separated from an ARS (#GP18) and a VMC (#UP1 61 sample. A total of ten sub-samples were prepared for analysis: a whole rock sample, as well as pyroxene and plagioclase separates from each of the two samples, and four biotite separates from the ARS sample. Sample preparation involved standard separation techniques and final handpicking of least- altered material. Only the ARS sample contained sufficient biotite for easy separation by handpicking. The ARS biotite separate was split into four fractions, 2 coarse-grained ones and 2 finer-grained ones, with one specimen of each sub-sample pair containing either relatively fresh or altered grains, respectively. The separation of biotite and analysis of these multiple fractions was deemed necessary to provide a sufficient spread in *6Rb/67Sr values.

In the course of the ARS study, several samples from a sill-like intrusion at Majuba Colliery near the town of Amersfoort in Mpumalanga Province (compare Fig. 8) became available, which were mineralogically and chemically similar to the ARS samples. The Rb and Sr concentrations and isotopic compositions for four mineral separates from two samples of this intrusion were determined and are also listed in Table 4.

The Anna’s Rust Sheet and relatedgabbroic intrusions in the Vredefott Dome-Kibaran magmatic event

Table 3. Results of instrumental neutron activation analysis

Sample Na (96) SC Cr Fe (%I co Ni Zn Ga AS

Se Br Rb Sr Zr

Ag Sb cs Ba La Ce Nd Sm Eu Gd Tb

BY Tm Yb Lu Hf Ta W

Ir (ppb) Au (ppb)

Hg Th U

YS2-228 UP-l 6 GP-5 IS-225 SHl-475 UP-71 USA59 UP-65 UP-68 1.96 1.69 1.67 1.88 1.79 2.15 1.76 1.65 38.5 34.9 36.3 37.5 37.8 31.9 37.7 32.2 113 200 147 207 119 29.9 185 174 9.38 8.75 8.79 9.24 9.72 11.9 9.92 8.65 51.7 50.7 53.2 48.7 50.9 55.9 49.1 49.9 95 120 130 85 108 71 116 85 162 166 157 158 157 173 163 145 20 20 17 33 24 22 45 11

1 .BB 0.62 1.04 0.46 1.27 0.51 1.11 0.63 0.16 2.21 0.09 0.12 0.08 0.25 1.05 0.6 0.96 1.01 1.05 0.51 0.68 0.41 0.32 0.81 20.5 36.4 35.2 26.4 32.8 12.3 27.1 23.7 252 167 81 96 135 72 110 220 165 230 120 60 175 255 55 103 0.22 <0.2 0.45 0.27 0.33 0.16 0.11 <0.2 0.36 0.17 0.19 0.1 0.14 0.07 0.21 0.05 2.31 0.81 1.53 1.68 2.44 0.45 1.41 0.61 96 172 92 80 88 81 160 149

15.5 11.1 10.3 12.7 13.2 16.5 14.5 11.8 33.7 20.4 25.4 30.1 31.6 35.3 36.1 22.9 19.8 13.4 14.6 17.7 17.9 19.7 21.6 15.1 4.71 3.35 3.35 4.03 4.27 4.93 4.35 3.58 1.61 1.31 1.23 1.41 1.45 1.71 1.42 1.35 5.1 3.9 4.4 4.6 5.1 5.8 5.2 4.7

0.88 0.68 0.87 0.76 0.81 1.04 0.93 0.78 5.7 4.4 5.3 4.9 5.1 6.8 5.3 4.4

0.52 0.39 0.46 0.45 0.46 0.62 0.49 0.37 3.41 2.61 2.61 3.13 3.14 4.07 3.09 2.57 0.49 0.37 0.37 0.43 0.46 0.58 0.45 0.39 2.48 2.54 3.01 3.32 3.08 4.06 3.15 2.61 0.48 0.28 0.31 0.61 0.41 0.48 0.31 0.55 0.63 2.37 0.62 0.55 1.24 0.62 0.98 0.31 <l <0.7 Cl <l <l <0.3 <l CO.6 1.2 2.5 2.1 1.7 1.5 2.1 3.2 1.1

0.16 co.1 0.18 0.28 0.21 0.05 0.03 0.62 3.78 2.64 2.72 3.08 3.44 0.53 3.42 2.64 0.79 0.28 1.03 0.61 0.69 0.54 0.49 0.64

1.89 35.2 207 9.32 48.4 132 145 21

0.61 1.22 1.16 27.2 97 115

co.3 0.08

174 12.9 29.7 17.1 3.94 1.35 4.4

0.82 5.1

0.44 2.74 0.39 2.99 0.26 0.69 <0.6 0.8

<O.l 2.81

All data are in ppm, unless stated otherwise.

For the Ar thermochronological experiments, the freshest parts of each sample were crushed and then sieved to grain sizes of 250-350pm. These sieved fractions were subjected to magnetic separation, and the resulting enriched feldspar and magnetic mafic mineral fractions were further purified by handpicking. Of one ARS and one VMC sample each, a feldspar and a pyroxene separate were produced.

The final separates were wrapped in Al foil and placed, together with three samples of the Mmhbl-1 hornblende monitor of 513.9 Ma age (Samson and Alexander, 1987; Lanphere et a/., 19901, into a 2.5

cm wide and 4.5 cm long Al can. The samples were irradiated for 70 MWh in position 5c in the enriched U research reactor at McMaster University (Hamilton, Ontario). After return from the reactor, samples and monitors were heated in a low-blank, double-wall Ta furnace (Modifications, Inc.) connected to an ultra- high vacuum extraction line. The monitors were fused at 16OOOC. The samples were heated and degassed stepwise in IO- 12 steps between 500 and 16ooOC. Argon purification was achieved using a liquid N cold trap, as well as a SAES Zr-Al getter operated at 4XPC. Purified Ar was then analysed for its isotopic

Journal of African Earth Sciences 5 11

W. U. REIMOL D et al.

Table 4. Results of Rb-Sr isotpic analysis of whole rock and mineral separate specimens from the Vredefort Mafic Complex (GPl8), Anna’s Rust Sheet UP1 61, and from a dyke at Majuba Colliery (samples 451 A/B and 460A/B)

Sample

UP16 PY UP16 PL UP16 WR

GP18 WR GP18 PL GP18 PY GP18 Bl GP18 B2 GP18 B3 GP18 B4

i451A Mica i451 B PY 460A Mica 14608 PY

Weight Rb Sr 87Rb/86Sr “Sr/%r 1 (T

(ma) (ppm) (ppml Error 29.02 1.206 12.16 0.2871 0.711480 11

65.010 25.400 167.700 5.430 1.744 297.200

38.280 1.520 11.970 5.110 377.100 7.093 9.430 100.400 4.868 2.910 405.800 6.624 2.460 143.400 5.504

0.4384 0.712800 12 0.0170 0.705580 12 0.3676 0.712600 13 199.30 3.726890 8 65.57 1.718010 4

239.50 4.299760 14 84.51 1.946160 33

352.4000 10.4700 113.6000 2.408800 37 0.2495 1.6890 0.4276 0.711550 24

357.1000 12.3700 95.5200 2.174800 24

3.16 1.574 315.60 0.0144 0.705371 15 64.38 20.250 174.70 0.3355 0.711420 13

0.3539 1.5570 0.6581 0.714750 24

PY: pyroxene; PL: plagioclase; WR: whole rock; B: biotite; errors refer to last two digits and are lo standard errors: All data are blank-corrected.

composition in a Nuclide 6-60-SGA mass spectro- meter at the Geophysical Institute in Fairbanks, Alaska. isotopic compositions were corrected for system blank and mass discrimination, as well as for Ca, K and Cl interferences, following procedures out- lined in McDougall and Harrison (1988). The weighted mean of the results obtained on the monitor samples of the batch was used to calibrate the age calcu- lations. Results are summarised in Table 5, with all ages quoted at the 1 o confidence level and calculated with the standard values given by Steiger and JIger (I 977). Apparent age spectra are presented in Figs 7a-d, with Ca/K ratio plots accompanying each age spectrum.

It must be noted that the authors are well aware of the recent controversy about the true age of the Mmhbl-1 standard (Renne er a/. , 1998). However, while this matter is still being debated and not fully resolved, the authors will continue to adhere to the previously recommended and accepted age value.

Results Rb-Sr model ages were calculated for the four biotite separates from Vredefort sample GP18 using an initial ratio of 0.7052 (this value was derived from the regression of the other isotopic data, cf. below). The resulting ages are: GP18 Bl : 1060 Ma; GPI 8 B2: 1079 Ma; GPI 8 B3: 1049 Ma; and GPI 8 84: 1027 Ma. These model ages are within 50 Ma of each

512 Journal of Aft&an Earth Sciences

other and probably give a fairly good estimate of the age of the ARS, in the absence of any petro- graphic evidence for significant post-crystallisation processes that might have affected this intrusion.

Various combinations of the 14 isotopic results were regressed, with all results yielding ages between 1020 and 1100 Ma, except for a single regression, which was carried out without any bio- tite data. In attempting to use the maximum amount of data to obtain a balance of representation re- lative to clear deviations from a representative line, all the mica data were accepted, as well as the py- roxene data from the Majuba samples and the plagioclase data from the ARS and the VMC sample. The pyroxene and whole rock data from ARS and VMC clearly show disturbances that suggest Rb loss or radiogenic Sr gain, which is not clearly understood. This requires further work for a fuller understanding, but does not, at this stage, detract from the rather precise age indicated by the rest of the data. The age calculated for these selected data is 1052 f 11 Ma (2a), with an initial ratio of 0.70517 f 12 (20).

This regression (compare Fig. 6) is regarded as the best representation of the age of these bodies, because of the low MSWD value and correspondence to the Ar chronological results (cf. below); it includes the six biotite and two plagioclase data that provide a good constraint on the initial ratio.

The Anna’s Rust Sheet and related gabbroic intrusions in the Vt-edefort Dome-Kibaran magmatic event

1052fll Ma Initial Ratio: 0.70517&l 2

3.86 on 10 samples cut-off: 2.10

All errors 2 sigma and augmented by

Sqrt (MS WD/Cut-oQ

I I I I I I 0 50 100 150

“Rb18%r 200 250

Figure 6. Results of Rb-Sr isotopic analysis of whole rock and mineral specimens from the ARS, VMC and Mejuba intrusions. inset emphasises low RbBr data (squere data points refer to date excluded from the statistical treatment).

Clearly, the nature of the biotite is extremely important for the interpretation of this age. The petrographic study showed that the vast majority of the biotite in all ARS samples represents late stage growth of primary (igneous) biotite. The age of 1052 f 11 Ma is, therefore, interpreted as a satis- factory approximation of the emplacement age of the ARS and the Majuba intrusion.

Two mineral separates were each analysed for their Ar isotopic compositions from ARS sample GP18 and VMC specimen UP16. Plagioclase of GP18 provided a saddle-shaped apparent age spectrum with an integrated age of 948 + 5 Ma (like all ages in this section, quoted at the 20 error level). Three features characterise this age spectrum:

il an excess Ar errorchron for the first 50% of gas release (age 869 + 54 Ma, 40Ar/36Ari = 1300, MSWD = 17);

ii) a saddle with a three-fraction mean age of 865 f 15 Ma; and

r7 a high-temperature ‘pseudoplateau’ (age 995 f 25 Ma, MSWD =6.9) characterised by several high temperature step results yielding ages between 974 and 1023 Ma (compare Table 5). The Ca/K ratio plot indicates clearly that a relatively more retentive phase (corresponding to the high temperature ages around 1000 Ma) with relatively elevated Ca/K ratios is present in this sample. At low temperatures an intermediate Ca/K ratio phase was degassed, which could be related to the small amounts of sericite occur- ring in the feldspar of this sample. These fractions may also be characterised by admixture of some excess Ar.

GPI 8 pyroxene provided a much smoother spec- trum (Fig. 7b), which is, however, marred by relatively large analytical uncertainties on each apparent age (Table 5). This is most obviously related to low K

Journal of African Earth Sciences 5 13

Tab

le 5

. A

rgon

is

otop

e da

ta,

J-fa

ctor

s an

d ap

pare

nt

ages

fo

r m

iner

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sepa

rate

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(UP

1 6)

and

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mpl

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UP-

16

PX

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0.03

31

g W

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ted

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age

of

J fr

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dard

s =0

.015

649

*0.0

0003

7

Tem

pera

ture

C

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Ar/3

gAr

“Ar1

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r 36

Ar/3

gAr

Volu

me

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3gA

rK

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/3gA

rK

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(Ma)

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70.9

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3 82

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18

2.56

1 15

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0.

320

0.05

1 51

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01

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22

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87.9

26

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875

0.18

96

110.

077

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57

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6 10

25

0.47

33

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047

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6 19

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535

56.1

95

11.5

61

1138

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173.

7 11

00

0.58

57

59.7

30

11.5

83

0.05

3 0.

110

24.9

11

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0.

607

45.1

64

9.96

2 96

4.5

164.

8 12

00

0.67

35

59.9

04

79.2

40

0.08

6 0.

086

32.5

83

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4.

515

42.6

45

12.7

42

922.

3 21

5.8

1600

1

.ooo

o 10

2.74

9 38

6.67

8 0.

181

0.32

0 23

.9

516.

760

7.50

1 10

4.50

1 3.

699

1748

.2

39.6

In

tegr

ated

10

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The Anna’s Rust Sheet and related gabbroic intrusions in the Vredefort Dome-Kibaran magmatic event

concentrations (compare the Ca/K ratio plot of Fig. 7b) determined for the phases in this sample. Weakly constrained apparent ages range from - 1000 to 1450 Ma, and the integrated K-Ar age for this specimen is 1311&88 Ma. This age also corresponds to the plateau age of the sample (plateau is 100% of 3sAr release, MSWD =0.56). However, given the low K content of this sample and the corresponding low precision, this age is not given too much weight in the analysis.

Plagioclase of VMC sample UP16 yielded a ‘pseudoplateau’ of 1068 it 20 Ma (57% 3sAr release, MSWD = 7.1) for four high temperature (I lOO- 1600°C) degassing steps (Fig. 7~). As was seen in GP18 plagioclase, the Ca/K ratio plot indicates relatively high and low Ca phases, which could be attributed to the presence of a small biotite com- ponent, as well as of minor secondary alteration products in plagioclase grains. The integrated age for this specimen is 1034 + 8 Ma. The low temperature portion of the age spectrum shows evidence of excess Ar with an isochron age of 968 f 27 Ma (for seven degassing fractions, 40Ar/36Ari = 382, MSWD = 1.78), similar to that seen in GPI 8 plagio- clase.

The pyroxene spectrum (Fig. 7d) for this sample displays large analytical errors in all degassing fractions, which could be interpreted as being the result of the very low K concentrations in the phases constituting this specimen. The sample has an integrated age of 1378 + 100 Ma and an eight-fraction plateau age of 1092 i 160 Ma (44% 3sAr release, MSWD =0.57). As with GPI 8 pyroxene, the low K content of this mineral prevents obtaining a more precise age for this sample. However, it may be significant, when compared with the other chrono- logical results obtained, that the minimum age for this sample corresponds approximately to 1000 Ma.

DISCUSSION

It is evident from the chemical and mineralogical features presented here that the intrusions in the Vredefor-t Dome area classified as Type IV are very similar. This, combined with a mutual lack of deformation, in the form of cross-cutting pseudo- tachylitic breccia, for example, suggests that these bodies could be coeval. In turn, this implies that these mafic rocks are collectively younger than 1600 Ma, the age of the youngest mafic intrusion in the Vredefort region known to be cut by pseudotachylitic breccia (Reimold et a/. , 1988; Spray et a/. , 1995).

Based on the isotopic data presented here, it is proposed that the Type IV magmatism occurred ca 1 OOO- 1050 Ma ago.

The regional distribution of these intrusions and their occurrence in the form of apparently sub-horizontal bodies in various parts of the core of the Vredefort Dome invites speculation about a possible subsurface connection, as alluded to by Nel(1927a, 1927b). This possibility is supported, but has not yet been proven, by the apparent age similarity between the ARS and the VMC. Even without such a connection, the number and distribution of the samples of similar composition to the ARS implies a larger magmatic event than that exposed in the ARS itself. This significant volume of intrusive material has strong implications for mag- matic activity in the central part of the Kaapvaal Craton during continental collision in the Namaqua- Natal Belt along the southern and southwestern margins of the craton. In the absence of any other coeval activity known from this region at that time, the Namaqua-Natal Orogeny (e.g. Jacobs eta/, 1993) must be considered the probable cause of this ca 1 Ga magmatism.

In recent years, much thermochronological evidence for both tectonic and hydrothermal activity in the region of the Witwatersrand Basin between about 1 .O and 1.2 Ga ago has been obtained, mainly from 40Ar-3sAr stepheating and laser-Ar dating of pseudo- tachylitic and other fault breccias from the Vredefort Dome and environs (Reimold et a/. , 1990b, 1992, 1995c; Trieloff et a/, 1994; Spray et a/, 1995; Friese et a/., 1995). Rb-Sr evidence for a ca 1 Ga event in the history of this region has been pre-sented by Reimold et a/. (1988). Brandt et a/. (1996) reviewed chronological evidence for alkaline (e.g. carbonatite) and mafic (trachyte, phonolite, lamprophyre) intru- sions in the region of the Pienaars River Alkaline Complex, which also coincides with the period be- tween 1.2 and 1 .O Ga. All these results taken together make a strong case for major Kibaran-age tectono- magmatic activity in the central part of the Kaapvaal Craton.

It is interesting to note that a few isolated ca 1 Ga ages were obtained in the earliest dating of single zircons from the Witwatersrand Basin (Barton et a/, 1989; Armstrong et a/, 1991). When these relatively young (for Witwatersrand-Ventersdorp zircon suites) ages were reported, they were considered anomalous and, as such, discarded. However, it must be ques- tioned now whether these results are an indication for growth of possibly authigenic zircon [from hydrothermal solutions, as suggested by the 850-

Figure 7. Apparent age spectra and corresponding Ca/K ratio plots for GPl8 plagioclase /a/ and pyroxene Ibl, and for UP16 plagioclase (cl and pyroxene (dl separates.

Journal of African Earth Sciences 517

W. U. REIMOLD et al.

1350 Ma U-Xe ages obtained by Reimold et a/. (1995c) on Witwatersrand uraninitesl during an event that comprehensively overprinted the whole basin and its environs.

Allsopp et a/ (1989) compared palaeomagnetic data for a number of intrusions from South Africa, Namibia, Botswana and Zimbabwe, including the Anna’s Rust gabbro. One group of such occurrences is the Umkondo dolerites of Zimbabwe, for which these authors presented an approximate age of 1 IOO- 1200 Ma. Recently, a U-Pb single zircon age of 1105 f 2 Ma was reported by Hanson et a/. (1998) for a granophyric zone from the Umkondo Mafic Complex (Zimbabwe). Allsopp et a/. (1989) and Hanson et a/. (1998) concluded that this suggests widespread igneous activity in the subcontinent at that time.

Hargraves et a/. (1994) presented palasomagnetic evidence that could be interpreted to indicate that the Timbavati Gabbro occurring in the Kruger National Park of northeastern South Africa could be related to the Umkondo dykes. Available Ar chronological data for the Timbavati Gabbro lend further support for this suggestion (I 072 + 4 Ma and 1123 f 5 Ma; Burger and Walraven, 1979, 1980). An - 1 .I Ga age has also been given for a thick dolerite sill cutting across

the Premier Mine Kimberlite (Allsopp et a/., 1967). The locations of all these, possibly related, occur- rences of mafic intrusions are shown in Fig. 8. In addition, several types of felsic and mafic igneous rocks of Kibaran age have been described in recent years from Botswana and Zimbabwe (e.g. Schwartz et a/, 1996; Hall et a/. , 1998; Kampunzu et a/, 1998; Ramokate and Mapeo, 1998). This evidence suggests strongly that Kibaran-age igneous activity took place throughout the subcontinent and perhaps extended into Antarctica (Moyes et a/. , 1995; Hanson et a/. , 1998).

Based on the palaeomagnetic and chronological data available, however, it is not impossible that all occurrences of ca 1. l-l .O Ga old mafic intrusives discussed in this paper could belong to a single province of southern African significance, and perhaps even occurring as far afield as in Antarctica (e.g. Moyes et a/. , 1995) - as also argued by Hanson et a/ (1998).

CONCLUSIONS

A group of tholeiitic intrusions of uniform petrographic and geochemical character has been identified in the

ANGOLA I

\ ZAMBIA ,‘. f’ .‘-..-..\

BOTSWANA I_..

KILOMETRES

SOUTH AFRICA

(Allsopp et al., 1969)

@ Hargraves et al. (1994)

Am Arnersfoort VD vmdefortDome PM Premier Mine

Figure 8. Occurrences of possible Type IV intrusions in the southern African subcontinent (after Allsopp et al., 1989; Hergraves et al., 1994; Munyanyiwa, 1999).

518 Joumd of Aftican Earth Sciences

The Anna’s Rust Sheet and relatedgabbroic intrusions in the Vredefort Dome-K&ran magmatic event

region of the Vredefort Dome and has been assigned the name Type IV intrusions. Two of these Type IV intrusions, the ARS and the VMC, yielded ages of ca 1.05 Ga, which is probably also the age of a gabbroic intrusion from Majuba Colliery near Amsterdam.

Regarding the local geological context, it appears possible that these intrusions belong to a common, sub-horizontal sheet intrusion, which may form a coherent layer through a major part of the Vredefort Dome.

Based on the chronological evidence of tholeiitic magmatic activity in the subcontinent and further afield, such as in Antarctica, it can be concluded that mafic magmatism was widespread in this region at - 1.1-l .O Ga, in addition to the alkaline mafic magmatism and tectonic overprint, which have been related to the Namaqua-Natal Belt Orogeny ( - 1250- IlOOMa).

ACKNOWLEDGEMENTS

The Geology Division of Gdd Fields of South Africa kindly allowed sampling of their drillcores from the south- western part of the Vredefort collar. L. Whiffield, D. du Toit and H. Czekanowska provided invaluable technical support. Thorough and helpful reviews by S. Goodman, H. Kampunzu and J.-P. Li6geois are much appreciated. Analyses in Vina were supported by the Austrian FWF, project Y5BGEO (to CK). WUR’s research is funded by the National Research Foundation of South Africa and the Research Council of the University of the Witwatersrand. This paper is University of the Witwatersrand Impact Cratering Research Group Contribution No. 15. Edtiortbl handling - P. 80 wden

Allsopp, H.L., Burger, A.J., van Zyl, C., 1967. A minimum age for the Premier kimberlite pipe yielded by biotite Rb- Sr measurements, with related galena isotopic data. Earth Planetary Science Letters 3, 161-l 66.

Allsopp, H.L., Kramers, J.D., Jones, D.L., Erlank, A.J., 1989. The age of the Umkondo Group, eastern Zimbabwe, and implications for palraomagnetic correlations. South African Journal Geology 92, 11-19.

Allsopp, H.L., Manton, W.I., Bristow, J.W., Erlank, A.J., 1984. Fib-Sr geochronology of Karoo felsic volcanics. In: Erlank, A.J. (Ed.), Petrogenesis of the Volcanic Rocks of the Karoo Province. Geological Society South Africa Special Publication 13. pp. 273-280.

Armstrong, R.A., Compston, W., Retief, E.A., Williams, I.S., Welke, H., 1991. Zircon ion microprobe studies bearing on the age and evolution of the Witwatersrand Triad. Precambrian Research 53, 243-266.

Barton, E.S., Compston, W., Williams, I.S., Bristow, J.W., Hallbauer, D.K., Smith, C.B., 1989. Provenance ages for the Witwatersrand Supergroup and the Ventersdorp Contact Reef: constraints from ion-microprobe U-PI, ages of detrital zircons. Economic Geology 84, 2012-2019.

Bate, M.D., 1995. Petrology of the Anna’s Rust dolerite sill, Vredefort Structure. M.Sc. thesis, Potchefstroomse Universiteit vir C.H.O., Potchefstroom. South Africa, 120~.

Bate, M.D., Coetzee, M.S., Elsanbroek, J.H., 1995. The origin and distribution of glomeroporphyritic plagioclase in the Anna’s Rust dolerite sill - a product of flow differentiation. In: Extended Abstracts, Geocongress ‘95, Centenary Congress Geological Society South Africa, Johannesburg, pp. 549-551.

Bisschoff, A.A., 1969. The petrology of the igneous and metamorphic rocks in the Vredefort Dome and the adjoining parts of the Potchefstroom syncline. D.Sc. thesis, University of Pretoria, Pretoria, South Africa, 243~.

Bisschoff, A.A., 1972a. Tholeiitic intrusions in the Vredefort Dome. Transactions Geological Society South Africa 75, 23-30.

Bisschoff, A.A. 1972b. The dioritic rocks of the Vredefort Dome. Transactions Geological Society South Africa 75, 31-45.

Brandt, D., Reimold, W.U., Smith, C.B., 1996. New Rb-Sr ages for igneous rocks from the area of the Pretoria Saltpan impact crater. South African Journal Geology 99, 293-297.

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