[S-MP22] Supercontinents and Crustal Evolution

[SMP22-05] Petrology, geochemistry and geochronology of the granitic rocks from Rwanda: Implications for tectonic settings of the Karagwe–Ankole Belt

*Claude Nambaje^1,2, Madhusoodhan Satish-Kumar¹, Ian S. Williams³, Toshiro Takahashi¹, Krishnan Sajeev⁴ (1.Niigata University, 2.University of Rwanda, 3.Australian National University, 4.Indian Institute of Science)

Keywords:Tectonic settings, A- and S-type granites, Petrogenesis, Rodinia amalgamation

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 (Nambaje et al., 2021). Four categories of granitic rocks with different petrogenesis have been recognised, providing constraints on the geodynamic and evolutionary history of the KAB. Garnet-biotite granite, with anorogenic (A-type) characteristics, was emplaced at 1368 ± 5 Ma. Its major, trace and rare earth element composition, T_DM model age of up to 2.33–1.96 Ga, high initial ⁸⁷Sr/⁸⁶Sr (0.7119–0.7216) and negative Nd epsilon (-1.9 to -4.0), indicates derivation from a much older mafic source, probably asthenospheric or lower continental lithospheric mantle contaminated by crustal material. Ascent of the magma was aided by a localised crustal-scale zone of weakness or transcrustal fault zone 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 (0.7081–0.7329), strongly negative Nd epsilon (-9.4 to -10.1) and a T_DM model age of 2.82–2.40 Ga, indicative of derivation by partial melting of a pelitic source containing Paleoproterozoic and Archean components, possibly with a minor contribution from lower crustal derived magma. Their magmatism occurred at an active continental margin in a convergent setting between the Congo and Tanzania cratons during an early stage of Rodinia amalgamation. Muscovite granite and leucogranite derived from fluid-fluxed partial melting of metapelites, both associated with widespread Sn mineralisation, were emplaced at ca. 1011–976 Ma in a late to post-collisional setting linked to the amalgamation of Rodinia. Their high initial ⁸⁷Sr/⁸⁶Sr (0.7289–0.7446) and strongly negative Nd epsilon (-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 West Gondwana Orogeny at the onset of the East African Orogeny and final amalgamation of Gondwana. The combined available evidence points to evolution of the Mesoproterozoic KAB during collisional tectonism.

Reference: Nambaje, C., Satish-Kumar, M., Williams, I.S., Takahashi, T., Sajeev, K., 2021. Granitic rocks from Rwanda: Vital clues to the tectonic evolution of the Karagwe–Ankole Belt. Lithos 404-405, 106490. DOI: https://doi.org/10.1016/j.lithos.2021.106490 .