A 2.023 GA AGE FOR THE VREDEFORT IMPACT EVENT AND A FIRST REPORT OF SHOCK METAMORPHOSED ZIRCONS IN PSEUDOTACHYLITIC BRECCIAS AND GRANOPHYRE

U-Pb isotope systematics of shock metamorphosed zircon grains from pse udotachylitic breccias and Granophyre from the controversial Vredefort Structure, South Africa, provide new and compelling evidence for an i mpact origin for this structure, Zircon grains from these rocks exhibi t planar microstructures and polycrystalline textures similar to those from the Chicxulub crater breccia, K/T boundary ejecta, and rocks fro m the Sudbury Structure. A concordant 2023 +/- 4 Ma (2 sigma) age for newly crystallized, unshocked zircon grains from recrystallized pseudo tachylitic breccia from the central part of the Vredefort Structure pr ovides a good approximation of the time of impact. This age indicates that the impact post-dates Bushveld magmatism by at least 30 m.y. U-Pb isotopic results for individual, pre-impact zircon grains indicate cr ystallization ages from about 3060 to 3300 Ma and Pb loss at ca. 2000 Ma. Data for high U grains plot below a discordia line from 3060 to 20 23 Ma and indicate both impact- and post-impact related Pb loss, The d ata and granular morphology of a zircon grain from the Granophyre indi cates probable ca. 2.0 Ga and ca. 1.0 Ga Pb loss. Although planar micr ostructures in zircon are ubiquitous, there are also some unshocked, l ow-U grains, and these record a ca. 3.1 Ga primary age, The older ca, 3.1-3.3 Ga ages for shocked zircons reflect formation and modification of granitoid crust in the region of the Vredefort Structure prior to and during a metamorphic event at about 3080 Ma. The resilience of zir con shock features to post-impact alteration and annealing, in combina tion with precise U-Pb dating of individual shocked grains provide a v aluable method for identifying ancient, metamorphosed and tectonically modified impact structures.