UC San Diego

Volcanism in Toro Ankole and Virunga provinces at the northernmost tip of the Western Branch of East African Rift system (EARS) is associated with the earliest stages of rifting. Although the provinces are spatially close, Toro Ankole lavas are relatively more silica-undersaturated, potassic, carbonatitic, and geochemically enriched than Virunga lavas. New petrographic, major-trace element and Sr-Pb isotopic data indicate such compositional differences are due to the different stages of tectonic rifting and concomitant magmatism in the region. Its underlying continental lithospheric mantle (CLM) is being metasomatized by volatile-rich small degree partial melts from the African Superplume. With decreasing pressure, carbonatitic-rich volatiles exsolve ahead of the upwelling partial melts and accumulate in the uppermost CLM layers beneath the cold, strong crust prior to rifting. Initial rifting in Toro Ankole generates explosive volcanism drawn from the uppermost carbonatitic metasomatized CLM layers. As rifting progresses in Virunga, less explosive volcanism emanates from the deeper silicate metasomatized CLM. More advanced rifting in the nearby South Kivu province additionally involves the asthenospheric mantle. The Ethiopian rifts in the Eastern Branch of EARS are most likely advanced stages of the same dynamic tectono-magmatic evolutionary process. Additionally, however, the Western Branch is underlain by the relatively more stable Paleoproterozoic to Archean basement whereas the Eastern Branch in Ethiopia is underlain by the Neoproterozoic Mozambique Belt. Thus, rifting and magmatism in the entire EARS is due to the dynamic interaction between the African Superplume and local CLM.