EARLIEST MAGMATISM IN ETHIOPIA - EVIDENCE FOR 2 MANTLE PLUMES IN ONE FLOOD-BASALT PROVINCE

Tertiary magmatism in Ethiopia has been linked to the thermal influenc e of the Afar mantle plume. However, new laser Ar-40/Ar-39 ages for th e volcanic succession in southern Ethiopia confirm the presence of two distinct magmatic phases at 45-35 Ma and 19-12 Ma. The earliest phase predates both extension and magmatism in northern Ethiopia by 15 m.y, and cannot be related to any simple model of melting in response to e xtension over a single mantle plume. We propose a model in which the E thiopian province was initially related to the thermal influence of th e Kenyan, and subsequently, the Afar mantle plume during northward mov ement of the African plate in the Tertiary. Support for this model com es from paleogeographic evidence that places southern Ethiopia similar to 1000 km farther south than its current position during the early m elting event at 45 Ma. Moreover, the rate of migration of the onset of magmatism from southern Ethiopia to Tanzania is similar to the rate o f migration of the African plate over the same period. Comparable erup tion rates in southern Ethiopia and Kenya further strengthen this link , In the light of this evidence, eruption rates ascribed to melting of the Afar mantle plume may be overestimated, which calls into question the potential for the Afar mantle plume to have had a significant eff ect on the biosphere.