Pan-African high pressure isobaric cooling: Evidence from the mineralogy and thermobarometry of the granulite-facies rocks from the Uluguru Mountains, eastern Tanzania

Granulite- and amphibolite-facies rocks in the Uluguru Mountains of the polyorogenic Mozambique Bell are composed of basic granulites, meta-anorthosites, paragneisses and marbles as major lithotypes. Both granulite-facies metamorphism and exhumation took place during the Pan-African Orogeny, as indicated by the published U-Pb and Rb-Sr data. In this work minerals of the garnet + clinopyroxene + ilmenite + plagioclase + hornblende +/- orthopyroxene +/- quartz bearing ultramafic granulites, meta-anorthosites, meta-anorthositic granulites, garnet pyroxene granulites and amphibolites from the belt were analysed for thermobarometry. Pressures and temperatures obtained by several calibrations, using suitable mineral parageneses, give temperatures ranging from similar to 800 to similar to 900 degrees C and pressures from similar to 9 kbar to similar to 12 kbar. Rim compositions of analysed minerals consistently give temperatures and pressures of similar to 800 degrees C and similar to 9-10 kbar, which are comparable to those obtained from the Furua Granulite Complex 200 km south of the study area. Mineral cores give higher PT values. Thus, core-rim relationships record a nearly isobaric cooling path. Meta-anorthosites give rim temperatures and pressures comparable to garnetiferous granulites, but their cores give temperatures of up to similar to 950 degrees C and pressures from similar to 13 to similar to 15 kbar. The highest temperatures of similar to 1100 degrees C and pressures of similar to 15 to similar to 17 kbar were obtained from mineral cores of the ultramafic granulites. The obtained PT data imply a minimum depth of about 35 km for the formation of these granulite-facies rocks. The preservation of the zoning and exsolution lamellae in some minerals considered to result from a relatively rapid exhumation of the granulite-facies rocks in the Uluguru Mountains soon after isobaric cooling in the lower crust. Copyright (C) 1997 Elsevier Science Ltd.