Petrogenesis of basalt and associated silicic rocks from upper Omo valley volcanic section, southwestern Ethiopia

Abstract

Field, petrography, major and trace element analyses of mafic and silicic rocks from upper Omo valley volcanic section, Southwestern Ethiopia are presented to determine petrogenetic evolution. The analyzed rocks belong to the basaltic and rhyolitic suites within the sub-alkaline group and these suites are linked via fractional crystallization as evidenced by occurrence of intermediate rocks (basaltic andesite and andesite rocks). Plagioclase, pyroxene, and Fe–Ti oxide minerals are the prominent phenocryst in basaltic rocks, while alkali feldspar phenocrysts are abundant in rhyolitic rocks. Major element correlations indicate fractional crystallization played a significant role in the evolution of the upper Omo volcanic rocks. The rhyolitic rocks show general enrichment of light rare earth element (LREE) and depletion of heavy rare earth element (HREE), strong negative anomalies in Sr, P and Ti, and low Ba/Nb, La/Nb,and Th/Nb ratios indicate that they were formed through low-pressure fractionation of basaltic lava derived from the depleted mantle with unremarkable crustal input. Correspondingly, REE abundances and incompatible trace element (e.g., Zr/Nb, Nb/Y ratios as well as plots of Nb/Yb vs. Th/Yb) suggest that rhyolite rocks were derived from basalt magma source which resemble ocean island basalt (OIB)-like magma. The source area features and the similarity of melting conditions suggest that the ascent and development of mantle diapirs is the key to elucidate the volcanic activity in the upper Omo valley volcanic section. The geochemical features of the volcanic rocks found in the upper Omo valley volcanic section are similar to those found in the northwest Ethiopian plateaus (such as LT and HT1-type basalt, Lima Limo and Wegel Tena rhyolitic ignimbrite). However, the crustal materials involvement in the genesis of LT-type basalt and Lima Limo rhyolite, Northwestern Ethiopian plateau were determined (Pik et al., 1998, 1999; Ayalew and Yirgu, 2003).