Low pressure high temperature metamorphism in the Vredefort Dome, South Africa: Anticlockwise pressure-temperature path followed by rapid decompression

Pelitic metasediments of the Witwatersrand Supergroup in the Vredefort Dome contain textural evidence for a two-stage metamorphic history. The pressure-temperature (P-T) path derived for the rocks indicates that they were subjected to a high geothermal gradient (approximate to 40 degrees C km(-1)), mid-amphibolite facies, Mla metamorphic event concomitant with thickening of the overlying upper crust. Peak temperatures of 570-600 degrees C were reached during Mla at depths of 14-16 km. After initial isobaric cooling following Mla, the rocks experienced rapid exhumation during a high strain rate deformation event associated with the formation of the dome. This event led to the development of abundant pseudotachylite and a brittle cleavage in these rocks. These features are overgrown, together with the Mla assemblages, by a low P Mlb paragenesis comprising microporphyroblastic cordierite+biotite.-Estimated P-T conditions during Mlb were <3.5 kbar, approximate to 500-530 degrees C. The two-stage P-T path is incompatible with existing contact metamorphic and diapiric models that have attempted to link the mid-amphibolite facies metamorphism with the formation of the Vredefort Dome. Instead, it indicates that the doming event occurred some time after the peak of a regional low P-high T metamorphic event, during cooling of the terrain. Given an age of 2.02 Ga for the doming event, the Mla event is attributed to a widespread magmatic-thermal event on the Kaapvaal Craton that accompanied the formation of the Bushveld Complex at 2.05-2.06 Ga. The Mlb event developed in response to the localized exhumation of deep parts of this terrain while the crustal geotherm was still elevated (approximate to 30 degrees C km(-1)). The association, elsewhere in the dome, of pseudotachylite with shock metamorphic features generated at pressures in excess of 20-120 kbar indicates that exhumation was accomplished by the impact of a large meteorite into the terrain.