Neoproterozoic pre- and post-deformational metamorphism in the Western Domain of the Karagwe-Ankole Belt reconstructed by Lu-Hf garnet geochronology in the Kibuye-Gatumba area, Rwanda

Abstract

The timing of metamorphism in the Karagwe-Ankole Belt (KAB, Central Africa) has never been investigated in detail. Although its metamorphic history and geochronology would provide a unique and crucial contribution, these aspects have largely been overlooked in most published geodynamic reconstructions of this belt. In this study, well-constrained Lu-Hf garnet dates obtained on four samples (three metasiltstones, one schist) can be directly linked to pre- and post-deformational greenschist metamorphism and broadly constrain the timing of Neoproterozoic deformation in the Karagwe-Ankole Belt. Depending on the host rock composition, garnet porphyroblasts grew over intervals of <2 Myr up to 9 Myr. Three Neoproterozoic garnet growth episodes were identified in the Western Domain of the Karagwe-Ankole Belt. In the Satinsyi Complex, just west of Gatumba, two prograde, pre-deformational garnet generations of Tonian age were identified. The first generation, mostly preserved as relict cores, grew before 920 Ma. Around 880 Ma, a second garnet generation formed new crystals, and overgrowths on first generation porphyroblasts. In the Kibuye area, garnet growth occurred between 618.9 ± 1.6 and 602.3 ± 1.1 Ma (Ediacaran period) during post-deformational greenschist metamorphism. In the western part of the Karagwe-Ankole Belt, the youngest deformation, probably of Pan-African origin, occurred after 880 Ma (2nd garnet growth episode), and had ceased by 620 Ma (3rd garnet growth episode) at the latest. The latter age is at least 70 Myr older than the Pan-African culmination and deformation observed in the northeastern part of the KAB (NW Tanzania), and the southern Ubendian Belt but corresponds to the timing of the West Gondwana Orogeny in the Congo Craton west of the study area. The Lu-Hf dates additionally indicate a limited timespan between Rodinia break-up (705–748 Ma) and Gondwana amalgamation (>620 Ma).