The relations between felsic and mafic volcanic rocks in continental flood basalts of Ethiopia: implication for the thermal weakening of the crust

Abstract

AbstractSr and Nd isotopic compositions are presented for a Miocene bimodal basalt–rhyolite suite from north Shewa, central Ethiopian plateau. Whole-rock Rb–Sr isochron of the rhyolites yields an age of 20.7±2.4 Ma, marking the onset of volcanism in central Ethiopia c. 20 Ma, 10 Ma after initial magmatism in the northern Ethiopian plateau. Initial 87Sr/86Sr ratios slightly vary in the basalts as well as in the rhyolites, ranging from 0.70440 to 0.70641 and from 0.70563 to 0.70658, respectively. Initial 143Nd/144Nd ratios show significant variations in the basalts (0.51248–0.51274), but remain nearly constant in the rhyolites (0.51273–0.51278). The Sr and Nd isotopic ratios of the basalts are interpreted to reflect their derivation from Afar plume contaminated by crustal materials (up to 15% contamination). The rhyolites evolved dominantly by fractional crystallization of mantle-derived basaltic magmas similar in composition to the exposed flood basalts.