Tectono-metamorphic evolution of the Kibuye-Gitarama-Gatumba area (Rwanda): implications for the geodynamic framework of Nb-Ta-Sn-W granite-related mineralization in the Karagwe-Ankole Belt

This PhD research investigates the tectono-metamorphic history of the Karagwe-Ankole Belt in Central Africa. Earlier studies in this Mesoproterozoic belt have focused on the magmatic events and granite-related Sn-Nb-Ta-W-Au metallogenetic province, and only to a lesser extent on the sedimentological and tectono-metamorphic aspects. Additionally, not all results of these studies could be incorporated into one consistent geological model, leading to conflicting geodynamic interpretations.Therefore, the Kibuye-Gitarama-Gatumba study area, located in West Rwanda and part of the Western Domain of the Karagwe-Ankole Belt, is studied using (thin section) petrography, structural data and multiple analytical geothermometry and geochronology methods. These results are integrated in one consistent tectono-metamorphic model and an updated geodynamic interpretation for the Karagwe-Ankole Belt, especially during the Neoproterozoic and the emplacement of mineralized pegmatites and quartz veins, is proposed.During the Mesoproterozoic, prolonged extension in a failed continental rift setting took place in the Western Domain of the Karagwe-Ankole Belt. This was followed by a full-scale intra-continental orogeny and Barrovian-type metamorphism up to mid-amphibolite facies at ~1.0 Ga, during the Rodinia assembly, after which the granite-related mineralization was emplaced under the post-compressional relaxation regime. An anorogenic thermal event at 880 Ma suggests the onset of a second episode of continental rifting, during Rodinia break-up. The youngest tectono-metamorphic event occurred at the end of the Neoproterozoic (620 - 570 Ma), with upper greenschist to lower amphibolite facies metamorphism and minor compression, in view of the Gondwana assembly. This tectono-metamorphic framework and geodynamic model for the Western Domain differ considerably from those of the Eastern Domain of the Karagwe-Ankole Belt.

Publication year:2020