Sub-lithospheric source for Quaternary alkaline Tepi shield, southwest Ethiopia

Mineral chemistry, major and trace elements and Sr-Nd isotopes are presented for basalts and trachybasalts from the Quaternary alkaline Tepi shield from southwest Ethiopia. The lavas are variably porphyritic with phenocrysts of olivine (Fo67–87), clinopyroxene (Wo45–47En38–40Fs13–16), plagioclase (An57–68) and Ti-magnetite set in a microcrystalline matrix. Although there are two high-MgO lavas (10.6–10.8 wt.% MgO), the majority of specimens are more evolved (MgO = 4.7– 6.2 wt.%). The major element variations can be explained by olivine + clinopyroxene ± plagioclase fractionation, most likely consistent with low P fractionation. The geochemical variations observed in the Tepi lavas are interpreted in terms of variable open system fractional crystallization of a magma sourced from primitive mantle like source.