Granitic rocks from Rwanda: Vital clues to the tectonic evolution of the Karagwe–Ankole Belt

Abstract

Petrological, geochemical and geochronological studies of granitic rocks from the Karagwe–Ankole Belt (KAB) have helped to establish the tectonic setting of the orogenic belt. Four categories of granitic rocks have been recognized, providing constraints on the geodynamics and evolution of the KAB. Anorogenic (A-type) garnet-biotite granite was emplaced at 1372 ± 5 Ma along deep, weak crustal structures. Its major, trace and rare earth element composition, TDM model age of 2.33–1.96 Ga, high initial 87Sr/86Sr (0.71193–0.72159) and negative ƐNd (−1.9 to −4.0), indicates derivation from a much older mafic source, probably originated from lower-crustal granulitic residue or lower continental lithospheric mantle contaminated by crustal material. Ascent of the small-batch A-type magma was aided by a localized crustal-scale zone of weakness within Archaean crust underlying both the Eastern and Western Domains of the KAB. The rest of the granites have S-type orogenic characteristics. Large batholiths of two-mica granite emplaced at 1369 ± 5 Ma have major, trace and rare earth element compositions, high initial 87Sr/86Sr (0.70808–0.71795), strongly negative ƐNd (−9.4 to −10.1) and a TDM model age of 2.82–2.40 Ga, indicative of derivation by partial melting of a pelitic source containing Palaeoproterozoic and Archaean components, possibly with a minor contribution from magma of lower crustal derivation. This magmatism occurred during the amalgamation of the Congo and Tanzania cratons at an early stage of Rodinia formation. Muscovite granite and leucogranite, derived from fluid-fluxed partial melting of metapelites, were emplaced at 1011 ± 18 to 976 ± 11 Ma in a late to post-collisional setting during final Rodinia formation. Their high initial 87Sr/86Sr (0.72893–0.74456) and strongly negative ƐNd (−6.8 to −12.4) indicate a supracrustal source. One younger S-type leucogranite (614 ± 9 Ma) was emplaced in a volcanic-arc setting during the assembly of West Gondwana. The combined available evidence points to tectonic evolution of the Mesoproterozoic KAB during accretion-collisional tectonism.