Petrogenesis of basaltic and associated rhyolitic rocks of the Alaje Formation from the Aiba area in Tigray, Northern Ethiopia

摘要

An integrated field, petrological, as well as major and trace element geochemical study of a suite of basaltic and associated rhyolitic rocks of the Alaje Formation from the continental flood basalts of the Aiba area of northern Ethiopia has been conducted. The study constrained the geochemical and petrogenetic association of the basaltic and rhyolitic rocks. The results show that the Alaje Formation is constituted by three layers of transitional to tholeiitic basaltic suites intercalated with three layers of alkaline, pantelleritic trachytes/trachy dacites and comenditic rhyolites. The dominant phenocrysts in the porphyritic basalts are plagioclase, with some clinopyroxene, Fe-Ti oxide and olivine. The basalts: (i) have high-Ti concentrations, (ii) show uniform LREE enriched and slightly HREE depleted pattern, and (iii) have low Y/Nb and Zr/Nb ratios, all implying their co-genesis and evolution from homogeneous source parental magma, and their derivation by shallow level fractional crystallization of magma sourced from an enriched component of the mantle. The associated rhyolitic rocks: (i) contain some plagioclase phenocrysts, (ii) plot along the differentiation lines of the associated basalts, (iii) show strong negative anomalies in P, Ti, and Sr, (iv) show general LREE enrichment and HREE depletion, and (v) have very low La/Nb, Rb/Nb, Ta/Nb and Th/Nb ratios. All these suggest their derivation by low-pressure fractionation of mantle derived basaltic magma, with little or no crustal contamination. The rhyolitic rocks of the Alaje Formation from the Aiba area have nearly the same geochemical signatures as other well-constrained rhyolitic rocks of similar stratigraphic context and age in other parts of the Northwestern Ethiopian plateau.