9:45 AM - 10:00 AM
[SMP26-04] Identification of granulite-facies metamorphism in the southeastern Kivu region (eastern DR Congo, Central Africa)
Keywords:Ruzizian basement, Granulite-facies metamorphism, Ubendian belt, Kivu region
Mobile belts play a crucial role in understanding the geodynamics of the Earth’s crust by providing key information on different aspects of crustal evolution. In Africa, they represent essential components of the basement where they join together or border the three major Archean cratons, namely the West African, Central Africa (Congo-Tanzania), and Southern Africa (Zimbabwe-Kaapvaal) cratons as well as some minor cratonic fragments. In Central Africa, the Kibaran and Ubendian belts are among the mobile orogens developed between the Congo craton, Tanzania craton, and Bangweulu block.
The Kibaran belt is a Mesoproterozoic orogen (1.4-1.0 Ga) split into two major domains by the NW-striking Paleoproterozoic Ubendian belt thought to extend northwards into the poorly investigated Ruzizian belt, situated northwest of Lake Tanganyika. Although it is one of the key pieces of the Rodinia puzzle, the geodynamic evolution of the Kibaran belt remains poorly constrained, resulting in conflicting interpretations. In addition, very little is known about its basement. In such a context, understanding the geological features of this key area would provide valuable clues to the overall geodynamic evolution of the region.
The preliminary field and petrography data indicate that the study area is predominantly composed of gneisses, along with granitoids, metabasites, schists, quartzites, and carbonates. Biotite (Bt) orthogneisses are widely distributed in the area and display migmatitic textures in some locations. A garnet (Grt)-Bt-sillimanite (Sil) metapelitic gneiss is also present, although not extensive. Other rock types include metabasic layers (amphibolites along with meta-dolerite), metasediments (Grt-Bt and Bt- schists, quartzites, carbonates), and the granitoids consisting of Grt-Bt granite, Bt-granite, Bt-muscovite (Ms) granite, and Bt-Ms pegmatite. On a structural note, a nearly consistent NW to NNW structural trend is observed across the area, expressed locally by a strong foliation in gneisses and in amphibolites, as well as strong schistosity in related rocks. The gneisses are also characterized by quartzo-feldspathic layer parallel intrusions.
Mineral chemistry of the metapelitic gneiss yielded an almandine-rich composition for garnet (Xalm= 0.81~0.85), biotite has large XMg variations (0.37~0.62), and plagioclase grains are oligoclase (XAb= 0.75~0.99). Consequently, preliminary estimates of peak P-T conditions led to a burial depth of around 17 km (P ≈ 4.8 kbar, T ≈ 760°C), most likely under granulite-facies conditions. Thus, it can be suggested that this part of the Kivu region has experienced medium- to high-grade metamorphic conditions, ranging from amphibolite or upper-amphibolite facies to granulite facies. Since there are no reports of any granulite-facies metamorphic event from the adjacent Ruzizian window in southwestern Rwanda, it may be most closely linked to the final Ubendian event, which peaked at ca.1.84 Ga (Boniface and Tsujimori, 2021) and interpreted as a result of collision between the Tanzania craton and Bangweulu with a possible participation of the Congo craton. Interestingly, this protracted stage which might have started at ca. 2.05 Ga, witnessed in between, accretionary thermal events synchronous with deformation and which were recorded in both Rwanda and Tanzania (Tack et al., 2010; Kazimoto et al., 2015; Nambaje et al., 2020). Therefore, ongoing dating may help constrain the postulated northward prolongation of the Ubendian belt as well as provide the first evidence of a basement for the overlying Kibaran segment in the Kivu region.
The Kibaran belt is a Mesoproterozoic orogen (1.4-1.0 Ga) split into two major domains by the NW-striking Paleoproterozoic Ubendian belt thought to extend northwards into the poorly investigated Ruzizian belt, situated northwest of Lake Tanganyika. Although it is one of the key pieces of the Rodinia puzzle, the geodynamic evolution of the Kibaran belt remains poorly constrained, resulting in conflicting interpretations. In addition, very little is known about its basement. In such a context, understanding the geological features of this key area would provide valuable clues to the overall geodynamic evolution of the region.
The preliminary field and petrography data indicate that the study area is predominantly composed of gneisses, along with granitoids, metabasites, schists, quartzites, and carbonates. Biotite (Bt) orthogneisses are widely distributed in the area and display migmatitic textures in some locations. A garnet (Grt)-Bt-sillimanite (Sil) metapelitic gneiss is also present, although not extensive. Other rock types include metabasic layers (amphibolites along with meta-dolerite), metasediments (Grt-Bt and Bt- schists, quartzites, carbonates), and the granitoids consisting of Grt-Bt granite, Bt-granite, Bt-muscovite (Ms) granite, and Bt-Ms pegmatite. On a structural note, a nearly consistent NW to NNW structural trend is observed across the area, expressed locally by a strong foliation in gneisses and in amphibolites, as well as strong schistosity in related rocks. The gneisses are also characterized by quartzo-feldspathic layer parallel intrusions.
Mineral chemistry of the metapelitic gneiss yielded an almandine-rich composition for garnet (Xalm= 0.81~0.85), biotite has large XMg variations (0.37~0.62), and plagioclase grains are oligoclase (XAb= 0.75~0.99). Consequently, preliminary estimates of peak P-T conditions led to a burial depth of around 17 km (P ≈ 4.8 kbar, T ≈ 760°C), most likely under granulite-facies conditions. Thus, it can be suggested that this part of the Kivu region has experienced medium- to high-grade metamorphic conditions, ranging from amphibolite or upper-amphibolite facies to granulite facies. Since there are no reports of any granulite-facies metamorphic event from the adjacent Ruzizian window in southwestern Rwanda, it may be most closely linked to the final Ubendian event, which peaked at ca.1.84 Ga (Boniface and Tsujimori, 2021) and interpreted as a result of collision between the Tanzania craton and Bangweulu with a possible participation of the Congo craton. Interestingly, this protracted stage which might have started at ca. 2.05 Ga, witnessed in between, accretionary thermal events synchronous with deformation and which were recorded in both Rwanda and Tanzania (Tack et al., 2010; Kazimoto et al., 2015; Nambaje et al., 2020). Therefore, ongoing dating may help constrain the postulated northward prolongation of the Ubendian belt as well as provide the first evidence of a basement for the overlying Kibaran segment in the Kivu region.