Petrology and geochronology of the Wami river granulites

The Wami river granulites represent a volcano-sedimentary sequence of rocks that have been metamorphosed under conditions of intermediate-to high-pressure granulites facies to form two-pyroxene and garnet-clinopyroxene mineral assemblages. Contemporaneous with the granulite-facies metamorphism, the rocks were deformed by a process of progressive flattening of isoclinals recumbent folds culminating in a complete elimination of fold hinges and the development of a new transposition foliation. A quartz rodding lineation plunging gently to the south-east formed during the deformation. Peak metamorphic conditions during the granulite facies event are estimated, using mineral geothermometry and geobarometry, at 700-820˚c and 8-9 Kb. The granulite-facies event is dated at about 700 Ma by the U-Pb systematic of zircons. Following the granulite-facies metamorphism there was an episode of partial retrogression into the amphibolites facies, which resulted in the development of biotite and hornblende rims on granulite-facies garnets and pyroxenes, the exsolution of K-feldspar from plagioclase, and the formation of sphere. The retrogression was accompanied by an episode of slip folding which resulted in the formation of open, nearly vertical folds. An east-south-east slip direction is inferred from the effect of the slip folding on the lineation formed during the first deformation. Garnet-biotite geothermometry indicates peak metamorphic temperatures of about 600˚c corresponding to a pressure of about 5 Kb during the retrogression. The termination of the retrogressive event is dated at about 500 Ma by Rb-Sr determinations on biotites and U-Pb lower-intercept ages of two suites of microns. The granulite complex, as well as the other granulite occurrences in the Mocambique belt of eastern Africa, are interpreted in terms of a plate-tectonic model involving a continent collision, either between East and West Condwane or between smaller (nicor) plates during early Pan-African time (about 700 Ma ago). This collision resulted in the thrusting and welding of the granulites on the foreland basement accompanied by amphibolites- facies metamorphism in the latter and partiat retrogression in the granulites. All tactono-thermal activity was terminated at the end of the amphibolote-facies metamorphism about 500 Ma ago.