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Abstract:
The Late Pleistocene-Holocene Dilo-Dukana and Mega volcanic fields (Ririba rift, South Ethiopia) formed through monogenetic eruptions of limited volumes of alkaline-basalts, rich in mantle xenoliths. This activity postdated the emplacement of voluminous Pliocene subalkaline basaltic lavas related to the main rifting phase. Both volcanic fields display vent alignments that abruptly cut the rift-related features, suggestive of an emplacement after rift abandonment.We provide new petrological, geochemical and isotopic data on these still poorly studied products aimed at investigating the nature and temporal variation of the mantle source compared to the previous Pliocene activity, and the main differentiation processes. All data discriminate the younger alkaline lavas from the Pliocene products, indicating a variation of the mantle source during the two activities possibly related to the local rifting evolution. The Dilo-Dukana and Mega products overlap both in major, trace elements and radiogenic (Sr-Nd-Pb) isotopes. Well-defined trends, as displayed by incompatible trace elements and by major elements variations, indicate the prominent role of fractional crystallization driving magma differentiation, without evidence of shallow storage levels. No clear geochemical correlation is evident with respect to location, xenoliths content or eruptive style. Light-REEs roughly discriminate between the products erupted at Dilo-Dukana and Mega, pointing to possible variations in the degree of melting and/or crystallization conditions of different magma pulses in the two areas. These data indicate that the volcanic fields are fed by two different systems of deep structures, unrelated to rifting but associated to old inherited fabrics, directly transferring mantle melts to the surface.
