Geochemistry across an exposed section of Archaean crust at Vredefort, South Africa: with implications for mid-crustal discontinuities

Abstract

The central region of the Vredefort structure consists of a semi-circular multi-layered sequence of crystalline rocks which are nearly vertical in attitude, and which increase in metamorphic grade towards the core of the structure. Together with the overlying Precambrian strata, this sequence provides a cross-section through almost the entire crustal section of the Kaapvaal craton (36 km). The upper part of the Vredefort crystalline crust consists of a 3.0-Ga sequence of differentiated felsic rocks in amphibolite facies. The lower crust consists of a complex and heterogeneous (both chemically and isotopically) high-grade metamorphic terrain of charnockites, granulites (mafic and felsic) and supracrustal rocks. The upper crust is separated from the lower crust by the Vredefort discontinuity, a brittle-ductile shear zone characterised by a high concentration of pseudotachylite and brecciated rock. Petrographic, chemical and isotopic evidence suggest that the upper and lower crust have undergone very different styles of evolution. This is indicative of different geological environments prior to their present juxtaposition. We speculate that the upper and the lower parts of the Vredefort crystalline crust were juxtaposed during intracratonic thrusting ∼2.8 Ga ago, and that the entire 36-km section of crust, including the Vredefort discontinuity, were rotated and uplifted into their present vertical position during the 2.0-Ga Vredefort catastrophe.