Petrology and geochronology of the Wami river granulites

Thumbnail Image
Journal Title
Journal ISSN
Volume Title
University of Dar es Salaam
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.
Available in print form, East Africana Collection, Dr. Wilbert Chagula Library, Class mark (THS EAF QE508.W2 M2)
Pertology, Tanzania, Wami river, Geological time
Maboko, M. A. H (1984) Petrology and geochronology of the Wami river granulites, Master dissertation, University of Dar es Salaam