Geochronology of volcanic rocks and evolution of the Cenozoic western branch of the East African Rift system

New K-Ar age data from the Virunga, Bukavu and Mwenga-Kamituga Volcanic Provinces in the Western Branch of the East African Rift System are given. In the Virunga Province, the earliest volcanic episode, hereafter 'initial' Virunga, includes fissural eruptions of continental tholeiites and Na alkaline lavas. These volcanics erupted along normal faults (sub-)parallel to the main north-south rift axis. Tholeiitic flows (first cycle) are directly overlain by Na alkaline lavas (second cycle). The ages of the volcanics from both petrological groups range approximately between 11 and 9 Ma. The data suggest that both the tholeiitic and Na alkaline lavas probably accumulated in a few bursts. The last flows of Na alkaline lavas, dated at around 9 Ma, were outpured in the Bishusha and Tongo areas and mark the initial phase of rift subsidence in the northwest part of the Virunga Basin. Pliocene-Pleistocene volcanism associated with the eight major central volcanoes at the Virunga Range are highly undersaturated, potassic, ultra-alkaline lavas, outpoured after a long period of quiescence (between ca 9 to < 3 Ma). This ultra-alkaline volcanism developed along fracture systems transverse to the main axis of the rift system. In the Bukavu Province, tholeiites are also the earliest flows (first cycle) overlain by Na alkaline lavas (second cycle). The field relationships indicate that there is some overlap between the last and earliest flows of the above two volcanic cycles. New K-Ar datings show that the earliest flows of the second cycle of alkaline affinity are ca 8 Ma old, whereas the youngest olivine-normaltive tholeiites are ca 6 Ma old. The transition from tholeiitic to alkaline volcanism (8-6 Ma) date the initiation of the Bukavu Rift Basin. Southwest of Bukavu, the initial volcanism in the Mwenga-Kamituga Province is poorly dated but should be close to 6 Ma. The Kamituga half-graben developed around 4-3 Ma. The volcanism in the Rungwe Volcanic Province started around 9 Ma. The age of the earliest volcanics along the main axis of the Western Branch of the East African Rift System decreases from Virunga (ca 11 Ma) to Bukavu (ca 10 Ma) and then to Rungwe (ca 9 Ma). In addition, the rift basin is older in the Edward-Albert Lakes zone (16-13 Ma). All together, these dates display a north to south polarity of ages marking an along-axis propagation of lithospheric extension in the Western Branch of the East African Rift System. The Tanganyika Basin is marked by an important subsidence but is typically avolcanic. Available data indicate underplating of magmas below the Tanganyika Rift. In all the examples examined, the changes in composition of the magmas with time appear to mark major phases of rift evolution.