DATING THE SHOCK-WAVE AND THERMAL IMPRINT OF THE GIANT VREDEFORT IMPACT, SOUTH-AFRICA

U-Pb geochronology of single grains of zircon and monazite has been us ed to date an episode of intense postimpact metamorphism in the core o f the deeply eroded Vredefort impact structure of South Africa. Result s from two basement units exposed in the uplifted central region indic ate that the impact and a later pyroxene hornfels metamorphic event we re penecontemporaneous at 2020 +/- 3 Ma. Discovery of a synimpact to p ostimpact dike of norite that intruded at 2019 +/- 2 Ma is the first r ecognition of mafic igneous activity related to impact. The dike is ei ther derived from a Sudbury-type impact melt layer (since eroded) or i s the product of decompression melting of Kaapvaal mantle in response to the ablation of >15 km of crust at the center of the crater, The co mbined heating effects of the shock wave and impact-triggered magmas a re thought to have created the 300 km(2) thermal imprint of the astero id collision with Kaapvaal craton, and account for the nearly coeval t iming relationship between core metamorphism and shock revealed by thi s study.