METAMORPHISM OF CALC-SILICATE AND ASSOCIATED ROCKS IN THE PAN-AFRICANKAAIMANS GROUP, SALDANIA BELT, SOUTH-AFRICA

Phase relations and mineral compositions indicate a polymetamorphic hi story of carbonate and siliciclastic sediments constituting the Kaaima ns Group of the Pan-African Saldania Belt along the south coast of Sou th Africa. Regional metamorphism during Pan-African orogeny did not ex ceed greenschist facies conditions but was superimposed locally onto l ow- to medium-grade contact metamorphism related to the intrusion of s yn-orogenic granitoids. Petrogenetic grids, constructed for the system CaO-MgO-Al2O3-SiO2-H2O-CO2 and based on available thermodynamic data, reveal that vesuvianite, grossular-rich garnet and wollastonite beari ng mineral assemblages, found in calc-silicate rocks adjacent to the s yn-orogenic Rooiklip granite gneiss sheet, were formed at temperatures possibly as low as 450 degrees C and pressures between 2 and 3 kbar. These mineral assemblages were in equilibrium with an H2O-CO2 fluid th at was characterized by a CO2/(CO2 + H2O) mole fraction of less than 0 .02. The fluid composition in the calc-silicate rocks was buffered ent irely externally through magmatic/hydrothermal hydrothermal fluids rel ated to the granite intrusion. The fluid flux was enhanced by syn-intr usive bedding-parallel shearing in the sedimentary rocks. Vesuvianite, which is of highly variable but generally Fe-rich composition, contai ns up to 2.1 wt.% F and hence acted as sink for fluorine. A regional i ncrease in Pan-African metamorphic grade from the tectonically lowest to highest formation suggests an overturned position of the whole Kaai mans Group. Later metamorphism during the Cape Orogeny caused only ver y low- to low-grade retrogression.