Metamorphic and metasomatic evolution of the Western Domain of the Karagwe-Ankole Belt (Central Africa)

Abstract

The tectonometamorphic evolution of the Western Domain of the Karagwe-Ankole Belt, containing widespread granite-related W–Nb–Ta–Sn mineralization in pegmatites and hydrothermal quartz veins of Early Neoproterozoic age, is largely unknown. This study aims to characterize the Meso- and Neoproterozoic metamorphism and metasomatism in the Karagwe-Ankole Belt, to reconstruct the temperature evolution and to investigate its temporal relation to deformation and the widespread granite magmatism and mineralization. A quantitative geothermometric study was conducted on metasiltstones and amphibolites, and applies thin section petrography, garnet–biotite and chlorite geothermometry on samples collected in the representative Kibuye-Gitarama-Gatumba area in West Rwanda. The presence of garnet, staurolite and kyanite in metasiltstones, and hornblende and andesine-labradorite feldspar in amphibolites indicates prograde Barrovian metamorphism up to syn-to post-deformational (D1 or D2) lower amphibolite facies (up to 630 °C). This peak metamorphism was followed by post-D2 greenschist facies metamorphism (c. 525 °C–440 °C; garnet, biotite, chlorite, muscovite in metasiltstone), probably related to the East African Orogeny as part of the Gondwana assembly. A geothermometric evolution with high temperature conditions (>500 °C) at least from the flare-up of Early Neoproterozoic tin granites and their metasomatic haloes onwards for most of the Neoproterozoic is proposed, in close correspondence with the geodynamic evolution of the neighboring terranes. Additionally, this high temperature regime is an important factor to be taken into account when interpreting thermal diffusion-sensitive geochronological data.