New PGE and Re/Os-isotope data from lower crustal sections of the Vredefort Dome and a reinterpretation of its "crust on edge" profile

Abstract

Granulites exposed in the core of the Vredefort Dome are generally thought to represent components of the Archean middle crust of the central Kaapvaal Craton. These rocks were exhumed from depths of at least 20 km following an ~2.02 Ga meteorite impact event, exposing a “crust on edge” section believed to represent a continuous magmatically-differentiated sequence from upper crust possibly down to Moho. Platinum group element concentrations suggest that mafic and ultramafic enclaves in the lower most sections are fragments of greenstone belt material metamorphosed at granulite grade. Re/Os isotopic data are consistent with previously published U-Pb zircon data, which suggest that the mafic and ultramafic rocks have a primary age of ~3.5 Ga and were metamorphosed to granulite grade at ~3.1 Ga. However, near the core of the Vredefort Dome across a major shear zone, other ~3.5 Ga greenstone belt remnants are at distinctly lower metamorphic grade. The juxtaposition of mafic-ultramafic rocks with a different metamorphic history in the core of the Vredefort structure requires a modified “crust on edge” model that incorporates tectonic stacking of different crustal segments at ~3.1 Ga.