THE AGE AND THERMAL EVOLUTION OF THE VREDEFORT IMPACT STRUCTURE - A SINGLE-GRAIN U-PB ZIRCON STUDY

Single-grain U-Pb SHRIMP analyses of a heterogeneous zircon population from a small granite body in the Archean basement core of the Vredefo rt Dome indicate an age of 2017 +/- 5 Ma for the granite, which is ide ntical, within errors, to the previously reported age determinations f or 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 g enerated during rapid uplift of the crater basement following the impa ct event, or a melt generated during an earlier, 2.05-2.06 Ga, granuli te facies anatectic event that was still at least partially molten at the time of the Vredefort impact event. Rare zircon xenocrysts display ing planar microdeformation features and giving discordant Archean age s are overgrown by polycrystalline zircon rims with U-Th compositions similar to that of the main similar to 2.02 Ga zircon population, but with discordant Pb-207/Pb-206 ages, indicating more recent Pb-loss. Co pyright (C) 1997 Elsevier Science Ltd.