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The Adwa – Yeha felsic plugs and domes are chain of towering inverted cone-shaped isolated plugs and circular domes with wide basal diameters and steep-sided, narrow tops. They form contrasting prominent topography from the Trap basalt ridges of the Adwa – Axum - Tekeze volcanic ridge, located just west of the Northern Afar Depression. The felsic plugs and domes are concentrated in the Adwa – Yeha area, in a zone of about 20 km wide and 30 km long and generally aligned along NNE-SSW direction, parallel to the regional structural grain of the Precambrian basement and the Miocene - Pliocene rift margin faults. In this study, we investigated these felsic plugs and domes using petrological and major and trace element geochemical methods in order to constrain their petrogenetic evolution. Their possible tectonic link to the Western Afar Margin has also been investigated combining our geochemical data, with assessment of regional tectonic setting and geochronological data from previous studies. The results ascertain that the felsic plugs and domes are peralkaline trachytes and peralkaline to sub-alkaline rhyolites, formed by Assimilation Fractional Crystallization (AFC) of basaltic magma which involved removal of olivine, pyroxene, plagioclase, and Fe–Ti oxides, and moderate amount of crustal input. The eruption of the felsic plugs and domes occurred along reactivated Precambrian weakness zones during the formation of the western Afar boundary faults. • The Adwa – Yeha felsic plugs are concentrated along a NNE-SSW aligned zone west of the Western Afar Margin. • The felsic plugs and domes erupted during the Miocene along reactivated Precambrian weakness zones along terrane boundaries. • The eruption of the felsic plugs and domes possibly occurred following the formation of the western Afar boundary faults. • The plugs and domes are peralkaline rhyolites and trachytes. • The felsic plugs and domes are formed by Assimilation Fractional Crystallization of basaltic magma. [ABSTRACT FROM AUTHOR]

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. • The Alaje Formation is a cyclic intercalation of tholeiitic to transitional basalts and pantelleritic trachytes and rhyolites. • All the basalt layers show uniform geochemical signature implying their co-genesis from homogeneous source parental magma. • The basalts are derived by shallow level fractional crystallization of magma sourced from an enriched mantle component. • The rhyolitic rocks are derived by low-pressure fractionation of mantle derived basaltic magma. [ABSTRACT FROM AUTHOR]