VISOKE AND SABYINYO – UNDERSTUDIED POTASSIC VOLCANOES WITHIN THE VIRUNGA VOLCANIC PROVINCE

Potassic alkaline magmatism is uncommon, occurring mainly within intracontinental extensional tectonic environments. The high concentration of alkalis in these lavas is thought to be related to either the low degree of melting in the upper mantle, or melts derived from metasomatized mantle. While these magmas are considered to be important conduits for cycling of elements from the mantle to crust, uncertainty persists as to the mantle generation processes. Ambiguity arises from magma differentiation within the continental lithosphere. There is thus a compelling need to examine the mechanisms by which potassic magmas evolve within the continental lithosphere. The western branch of the East African Rift provides among the best examples of a potassic magmatic province. Within the western branch of the East African Rift the Virunga Volcanic province is comprised of eight volcanoes. Previous studies have examined the volcanoes throughout the region but have focused predominantly on the more primitive magmatic end-members. Visoke and Sabyinyo, two of the more central and evolved volcanoes in the region, remain understudied. Here we examine the magmatic evolution of these volcanoes using petrographic and geochemical techniques. New field work has recovered 92 samples from these two volcanoes, which will be combined with additional sampling from Karisimbi, the largest volcano in the province. These data represent a significant increase on the existing body of knowledge within the Virunga Volcanic Province. Karisimbi hawaiites exhibit dominantly fractional crystallization processes involving plagioclase, clinopyroxene and olivine, consistent with geochemical trends. However, evidence of disequilibrium is also present in the form of rimmed biotite crystals, suggesting more complex magmatic evolution in Karisimbi. We compare this evidence of disequilibrium with latites from Sabyinyo, which have previously been hypothesized to originate from mixing between mantle-derived basanites and crust-derived silicic magma. Results of this study provide critical constraints on crustal processes which modify the parental melts allowing for better resolution of mantle processes controlling potassic alkaline magmatism.