15:00 〜 15:15
[SMP24-06] Petro-tectonic setting of the Karagwe–Ankole Belt (Rwanda) and implications for the amalgamation of Rodinia
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 and the implications for Rodinia assembly. Four categories of granitic rocks have been recognised, providing constraints on the geodynamic evolution of the KAB.
Anorogenic (A-type) garnet-biotite granite was emplaced at 1368 ± 5 Ma. Its major, trace and rare earth element composition, Nd TDM model age of up to 2.33–1.96 Ga, high initial 87Sr/86Sr (0.7119–0.7216) and negative εNd (-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 within Archean 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 εNd (-9.4 to -10.1) and a Nd TDM model age of 2.82–2.40 Ga, consistent with derivation by partial melting of a pelitic source containing Paleoproterozoic and Archean components, possibly with a minor contribution from magma derived from the lower crust. The magmatism occurred in a convergent setting between the Congo and Tanzania cratons during an early stage of Rodinia amalgamation. Muscovite granite and leucogranite resulting from fluid-fluxed partial melting of metapelites were emplaced at ca. 1011–976 Ma in a late to post-collisional setting of Rodinia. Their high initial 87Sr/86Sr (0.7289–0.7446) and strongly negative εNd (-6.8 to -12.4) indicate a supracrustal source. One younger S-type leucogranite (614 ± 9 Ma) was emplaced in a volcanic-arc setting, possibly during the West Gondwana Orogeny. The combined available evidence points to evolution of the Mesoproterozoic KAB during accretion-collisional tectonism.
Anorogenic (A-type) garnet-biotite granite was emplaced at 1368 ± 5 Ma. Its major, trace and rare earth element composition, Nd TDM model age of up to 2.33–1.96 Ga, high initial 87Sr/86Sr (0.7119–0.7216) and negative εNd (-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 within Archean 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 εNd (-9.4 to -10.1) and a Nd TDM model age of 2.82–2.40 Ga, consistent with derivation by partial melting of a pelitic source containing Paleoproterozoic and Archean components, possibly with a minor contribution from magma derived from the lower crust. The magmatism occurred in a convergent setting between the Congo and Tanzania cratons during an early stage of Rodinia amalgamation. Muscovite granite and leucogranite resulting from fluid-fluxed partial melting of metapelites were emplaced at ca. 1011–976 Ma in a late to post-collisional setting of Rodinia. Their high initial 87Sr/86Sr (0.7289–0.7446) and strongly negative εNd (-6.8 to -12.4) indicate a supracrustal source. One younger S-type leucogranite (614 ± 9 Ma) was emplaced in a volcanic-arc setting, possibly during the West Gondwana Orogeny. The combined available evidence points to evolution of the Mesoproterozoic KAB during accretion-collisional tectonism.