40Ar/39Ar constraints on the age of metamorphism in the Witwatersrand Supergroup, Vredefort dome (South Africa)

Abstract

Muscovite, biotite and amphibole bulk step-heating and laser-probe 40Ar/39Ar results from metasediments and meta-lavas of the Witwatersrand Supergroup and Dominion Group exposed in the Vredefort dome provide constraints on the age of greenschist to amphibolite facies metamorphism of these rocks. The majority of apparent ages from biotite and sericite grains cluster between ∼2100 and ∼2000 Ma and are interpreted as reflecting an initial metamorphism (M1) accompanying the ∼2060 Ma Bushveld magmatic event and a later lower-grade overprint (M2) accompanying the 2023 ± 4 Ma Vredefort impact event. The spectra also show variable degrees of post-2000 Ma resetting that has also been observed in samp..