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

Impact Cratering Research Group, School of Geosciences, University of the Witwatersrand, P.O. Wits 2050, Private Bag 3, Johannesburg, South Africa

Wolf U. Reimold

Impact Cratering Research Group, School of Geosciences, University of the Witwatersrand, P.O. Wits 2050, Private Bag 3, Johannesburg, South Africa

Richard C.A. Minnitt

Department of Mining Engineering, University of the Witwatersrand, Private Bag 3, P.O. Wits 2050, Johannesburg, South Africa, e-mail: minnitt{at}egoli.min.wits.ac.za

Correspondence: Corresponding author: Fax: +27 11 3391697; Tel. 7176565, reimoldw{at}geosciences.wits.ac.za

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 Fe₂O₃, TiO₂ and CaO and enriched in Al₂O₃ relative to the Barberton greenstones. They are compositionally distinct from the dismembered granulite-facies greenstone remnants in the central parts of the dome.

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