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. T he pressure-temperature (P-T) path derived for the rocks indicates tha t they were subjected to a high geothermal gradient (approximate to 40 degrees C km(-1)), mid-amphibolite facies, Mla metamorphic event conc omitant with thickening of the overlying upper crust. Peak temperature s of 570-600 degrees C were reached during Mla at depths of 14-16 km. After initial isobaric cooling following Mla, the rocks experienced ra pid exhumation during a high strain rate deformation event associated with the formation of the dome. This event led to the development of a bundant pseudotachylite and a brittle cleavage in these rocks. These f eatures are overgrown, together with the Mla assemblages, by a low P M lb paragenesis comprising microporphyroblastic cordierite+biotite.-Est imated P-T conditions during Mlb were <3.5 kbar, approximate to 500-53 0 degrees C. The two-stage P-T path is incompatible with existing cont act metamorphic and diapiric models that have attempted to link the mi d-amphibolite facies metamorphism with the formation of the Vredefort Dome. Instead, it indicates that the doming event occurred some time a fter the peak of a regional low P-high T metamorphic event, during coo ling 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 Comple x at 2.05-2.06 Ga. The Mlb event developed in response to the localize d exhumation of deep parts of this terrain while the crustal geotherm was still elevated (approximate to 30 degrees C km(-1)). The associati on, elsewhere in the dome, of pseudotachylite with shock metamorphic f eatures generated at pressures in excess of 20-120 kbar indicates that exhumation was accomplished by the impact of a large meteorite into t he terrain.