Amphibolite facies metamorphism in lower Witwatersrand Supergroup rocks exposed in the Vredefort Dome – a Ventersdorp LIP connection

Abstract

Metamorphism in the lower Witwatersrand Supergroup exposed in the Vredefort Dome has previously been proposed to be related to elevated heat flow linked to the 2.06 Ga Bushveld magmatic event; however, there are no unambiguous chronological data to confirm this timing. Microtextural and mineral compositional analysis of a garnet-bearing metapelite in the northwestern collar of the Vredefort Dome suggests at least two metamorphic events with distinctly different P-T conditions preceding the Dome-forming meteorite impact at 2.02 Ga. THERMOCALC mineral equilibrium calculations yield P-T conditions of 500°C, 3.1 kbar for the M1 mineral assemblage garnet1-plagioclase-muscovite-biotite-chlorite-ilmenite. Thin, discontinuous, garnet2 overgrowths on the garnet1 porphyroblasts define a subsequent, M2, event with P-T conditions of 530°C, 5 kbar. Garnet Sm-Nd chronology yields an isochron age of 2 796.0 ± 1.5 Ma, indicating an early Ventersdorp (Klipriviersberg) timing of M1 metamorphism. Although the M2 garnet overgrowths are volumetrically too small to date, the calculated M2 pressure is consistent with the predicted overburden thickness above the lower Witwatersrand Supergroup during emplacement of the Bushveld Complex. While elevated, the M2 apparent geotherm (30°C/km) is significantly lower than the M1 apparent geotherm (46°C/km); however, thermal modelling suggests both events benefitted from local perturbations caused by contemporaneous sill emplacement. Our results thus show that the initial garnet-forming, mid-amphibolite facies metamorphism in the collar of the Vredefort Dome is not related to the emplacement of the Bushveld Complex, but rather to the early stages of magmatism associated with the Ventersdorp Large Igneous Province (LIP). Nonetheless, elevated heat flow associated with the Bushveld LIP also reached comparable amphibolite facies conditions.