NEW CONTRAINTS ON THE ERUPTION OF CENOZOIC BASALTIC FLOWS IN NW BUKAVU CITY (SOUTH KIVU, DEMOCRATIC REPUBLIC OF THE CONGO)

Bukavu city and surrounding areas are located inside the system of the East African Rift (EARST) region. Volcanic rocks that cropout in the region extend from Miocene to current (Kampunzu and Al. 1986). The region is characterized by a paucity of outcrops due to a thick vegetation cover leading to the observed surficial weathering of the basaltic flows.

Several phases of basaltic flows are observed that can grouped into two: : the upperflows are relatively fresh and overly a 3m thick paleosoil and the lower flows are strongly weathered, making it easy to map the discontinuous volcanic activity in the region.

These basalts are formed of phenocrystals of Olivine, Pyroxene (mainly clinopyroxene) and Plagioclase, in a mesostase made of oxides. Silica content varies from 45 to 54% indicating that the magma was not strongly differentiated. The majority of the basalts are of the tholeiitic series, but some are of calc-alkaline origin, with a possible association with the transitional series in the extensional zone characterized by anorogenic magmatism.