Petro-stratigraphy and Geochemistry of volcanic rocks from the eastern escarpment of Main Ethiopian Rift

Abstract

The Dera-Sire section is located in the eastern escarpment of the Central Main Ethiopian Rift. The section consists of, from bottom to top, lower flood basalt (Oligocene), lower ignimbrite, tuff and ash, upper basalt (Pliocene), and upper ignimbrite. The lower flood basalts are characterized by phyric to aphyric textures. The lower and upper ignimbrite units contain crystals of quartz, plagioclase, and lithic fragments embedded within a glassy groundmass. The upper basalts are aphyric to plagioclase phyric in texture. The lower flood basalts and upper basalts are alkaline and sub-alkaline (tholeiitic) in composition, respectively. The geochemical variations suggest at least two dominant mantle components that require the production of both upper tholeiitic and lower alkaline basalts. The components are an OIB-like component, which might be similar to the Afar plume composition, and an E-MORB-like component in the asthenosphere, indicating that there were no temporal mantle source variations from Oligocene to Pliocene magma generations.  However, the alkaline affinity of the lower flood and tholeiitic nature of the upper basalts suggest that the depth of melting of the lower flood basalt is relatively deeper than that of the upper basalts. The fractionated mineral assemblages of the lower flood basalts (plagioclase-olivine-minor clinopyroxene) and upper basalts (dominant plagioclase with minor clinopyroxene) suggest that the mantle-derived magma rose, accumulated, and fractionated in the lower crustal and upper crustal chambers, respectively. Subsequently, the fractionated magma from the lower and upper crustal chambers rose to the surface and produced lower flood basalts and upper basalts, respectively.