Metamorphism and geochronology of granulites and migmatitic granulites from the Magondi Mobile Belt, Zimbabwe

The Magondi Mobile Belt, which crops out to the west of the Zimbabwe Archaean craton, is an early- to mid-Proterozoic feature within which sediments of the Magondi Supergroup were deformed and metamorphosed. Metamorphic grade increases northwards along the strike of the belt from greenschist facies in the south to granulite facies in the north. The granulite-facies rocks were partially melted during metamorphism at pressures of 5-7 kbar and temperatures of about 700-750°C, with fluids buffered to low values of P_fl. Late stages of partial melting were driven by vapour-poor dehydration melting reactions. An age for the metamorphism is indicated by Rb-Sr ages of 1890 ± 260 Ma from enderbitic granulites at the Rukomeshe River causeway, east of Makuti, and of 1780 ± 280 Ma from garnet-bearing and granitic granulites from the Nyaodza region just east of lake Kariba. These ages confirm an early Proterozoic age for the Magondi orogeny, which can be correlated with deformation in the Kheis Belt occupying a similar position on the western margin of the Kaapvaal craton. Later brittle deformation and cataclasis, distributed throughout the granulites but locally concentrated in N-S-striking shear zones, are the result of deformation in the footwall of the south-verging Pan-African thrust sheets.