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

Abstract

Granulite- and amphibolite-facies rocks in the Uluguru Mountains of the polyorogenic Mozambique Belt 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 ∼800 to ∼900°C and pressures from ∼9 kbar to ∼12 kbar. Rim compositions of analysed minerals consistently give temperatures and pressures of ∼800°C and ∼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 ∼950°C and pressures from ∼13 to ∼15 kbar. The highest temperatures of ∼1100°C and pressures of ∼15 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.