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

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 similar to2100 and similar to2000 Ma and are interpreted as reflecting an initial metamorphism (M1) accompanying the similar to2060 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 samples from elsewhere in the Witwatersrand Basin. Despite temperatures in excess of 500degreesC during M2, disturbance of the M1 40Ar/39Ar spectra is generally limited, reflecting the short-lived and largely anhydrous nature of the M2 event and the absence of accompanying strain. Single-grain biotite laser-probe data confirm that the extent of within-sample resetting varies, and probably reflects more intense deformation and alteration effects towards ubiquitous pseudotachylite-bearing microfaults that formed during the impact event. The complex spectra in M1 muscovite and amphibole are difficult to interpret but may at least partly reflect contamination by excess argon during M1.