ABSTRACT

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 (Fo_67-87), clinopyroxene (Wo_45-47En_38-40Fs_13-16), plagioclase (An_57-68) 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.