The age and thermal evolution of the Vredefort impact structure: A single-grain UPb zircon study

Abstract

Single-grain UPb SHRIMP analyses of a heterogeneous zircon population from a small granite body in the Archean basement core of the Vredefort Dome indicate an age of 2017 ± 5 Ma for the granite, which is identical, within errors, to the previously reported age determinations for the Vredefort impact event. Petrographic evidence from the granite indicates that it crystallized after the impact shock event, but some doubt remains whether it is a true impact melt, a decompression melt generated during rapid uplift of the crater basement following the impact event, or a melt generated during an earlier, 2.05–2.06 Ga, granulite facies anatectic event that was still at least partially molten at the time of the Vredefort impact event. Rare zircon xenocrysts displaying planar microdeformation features and giving discordant Archean ages are overgrown by polycrystalline zircon rims with UTh compositions similar to that of the main ∼2.02 Ga zircon population, but with discordant 207Pb/ 206Pb ages, indicating more recent Pb-loss.