MIDCRUSTAL GRANULITE-FACIES METAMORPHISM IN THE CENTRAL KAAPVAAL CRATON - THE BUSHVELD COMPLEX CONNECTION

The numerous greenstone remnants which occur as inclusions in the Arch aean gneissic granitoid basement exposed in the core of the Vredefort Dome display evidence of very high grades of granulite facies metamorp hism and an unusually complex three-stage metamorphic history: (1) in metapelites, migmatites and refractory restites were produced in a mid -crustal (+/- 0.5 GPa) M-1 anatectic event, at temperatures in excess of 900 degrees C (in the restites); (2) in the migmatites, the peak me tamorphic assemblages are variably overprinted by a high-grade retrogr ade reaction which involved crystallizing anatectic melts (M-2); (3) b oth the peak metamorphic assemblages and the retrogression textures ar e cross-cut by pseudotachylitic breccia veins which mark a shock metam orphic event associated with the formation of the Vredefort Dome; and (4) the peak metamorphic assemblages and the M, retrogression textures are overprinted by new generations of remarkably fine-grained, post-s hock, high-grade phases which crystallised at a pressure some 0.25 GPa lower than that of both the peak metamorphic conditions and the retro gression (M,). These data indicate an anticlockwise P-T evolution wher e mid-crustal anatexis most probably resulted from the intraplating of Bushveld Complex-related ultramafic magmas at 2.06 Ga. A period of ap proximately isobaric cooling and retrogression was followed by the Vre defort Catastrophe, which resulted from the impact of a large meteorit e into the terrane at 2.02 Ga. This shock event halted the natural mid -crustal metamorphic progression prior to the terrane cooling below 65 0 degrees C, and resulted in the rapid exhumation of the terrane by so me 9 km. In the process, it provided a window into the deep levels of the central Kaapvaal craton not seen elsewhere in the region. (C) 1997 Elsevier Science B.V.