Archean age for the granulite facies metamorphism near the center of the Vredefort structure, South Africa

Granulite facies metamorphic assemblages in rocks exposed near the center o f the 2.02 Ga Vredefort impact structure previously have been interpreted e ither as Early Proterozoic, genetically related to the 2060 Ma Bushveld Com plex, or as Archean, and representative of lower crust that rebounded to up per crustal levels following an impact event. Zircon and monazite recovered from the granulite facies rocks record high-grade metamorphism at 3107 +/- 9 Ma and a primary age of greater than or equal to 3425 Ma for detrital zi rcon, A shock-deformed, but otherwise pristine, dolerite dike that intrudes the granulite terrane yields a U-Pb zircon age of greater than or equal to 2560 Ma, providing a minimum age for the metamorphism. These isotopic age data are difficult to reconcile with a regional high-grade metamorphic even t in the crust beneath Vredefort at 2060 Ma. Instead, the preimpact, high-t emperature metamorphic history observed in the Vredefort lower crustal rock s indicates an enigmatic high-temperature event during the stabilization of diamondiferous Archean tectosphere.