Abstract

Granulite facies metamorphic assemblages in rocks exposed near the center of the 2.02 Ga Vredefort impact structure previously have been interpreted either as Early Proterozoic, genetically related to the 2060 Ma Bushveld Complex, or as Archean, and representative of lower crust that rebounded to upper 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 ≥ 3425 Ma for detrital zircon. A shock-deformed, but otherwise pristine, dolerite dike that intrudes the granulite terrane yields a U-Pb zircon age of ≥ 2560 Ma, providing a minimum age for the metamorphism. These isotopic age data are difficult to reconcile with a regional high-grade metamorphic event in the crust beneath Vredefort at 2060 Ma. Instead, the preimpact, high-temperature metamorphic history observed in the Vredefort lower crustal rocks indicates an enigmatic high-temperature event during the stabilization of diamondiferous Archean tectosphere.

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