The Mesoarchaean Basement Complex of the Vredefort Dome—A Mid-Crustal Section Through the Central Kaapvaal Craton Exposed by Impact

Abstract

Mesoarchaean Basement Complex rocks exposed in the core of the Vredefort Dome represent a composite terrane in which polydeformed high-grade TTG gneisses and granites with subsidiary greenstone remnants were tec- tonically juxtaposed against low-grade mafic–ultramafic metavolcanic greenschists along a major NE-trending shear zone prior to 3.07 Ga. The high-grade portion exposes a ?10 km section through the mid-crust as a result of extreme rotation and differential uplift related to the 2.02 Ga Vredefort impact event. Syntectonic, 3.13– 3.10 Ga, TTG gneisses and late-tectonic, 3.10–3.08 Ga, granites intruded and partially assimilated 3.3–3.5 Ga old supracrustal gneiss protoliths. Prior to impact rotational effects, the gneisses contained subhorizontal S2 and subvertical, NW-trending, S3 foliations. D3 straddled the metamorphic peak, which ranged from *680 °C, 0.5 GPa to *885 °C, 0.71–0.77 GPa. Together with the lithological and structural evidence, the tight clock- wise P-T path derived from migmatitic-restitic metapelitic gneisses is interpreted to suggest the formation and docking of a volcanic arc with the southwestern edge of the Kaapvaal protocraton at 3.13–3.08 Ga. The D4 shear zone that juxtaposes the gneisses against greenschist facies (400 °C) komatiite to komatiitic metabasalts to the southeast is interpreted as an originally shallow NW-dipping, retrograde, crustal-scale extensional feature that aided rapid exhumation of the high-grade terrane prior to deposition of the 3.07 Ga Dominion Group. In addition to differential uplift of the Basement Complex, the impact caused significant post-shock heating that generated metamorphic reaction coronas, melting and annealing of Archaean features to produce granofelses under ultrahigh-T/low-P (>1050 °C, 0.2–0.3 GPa) condi- tions in the centre of the dome at 2.02 Ga.