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

Impact Cratering Research Group, Department of Geology, University of the Witwatersrand, P/Bag 3, WITS, Johannesburg 2050, South Africa. E-mail 065rlg{at}cosmos.wits.ac.za;065wur{at}cosmos.wits.ac.za

D. Phillips

Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia

P.W. Layer

Geophysical Institute, University of Alaska, Fairbanks, Alaska 99775-7320, USA

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 samples from elsewhere in the Witwatersrand Basin. Despite temperatures in excess of 500 °C 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.

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