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Geochemical Journal
Geochemical Journal An open access journal for geochemistry
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Sub-lithospheric source for Quaternary alkaline Tepi shield, southwest Ethiopia

DEREJE AYALEW, B. MARTY, P. BARBEY, GEZAHEGN YIRGU, ENDALE KETEFO
Geochemical Journal, Vol. 40, No. 1, P. 47-56, 2006

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 (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.

KEYWORDS

alkaline lava, southwest Ethiopia, primitive mantle, Tepi shield

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