Geochemical and isotopic constraints on the recent magmatic activity of the Dilo-Dukana and Mega volcanic fields (Ririba rift, South Ethiopia): implications for rift evolution

The Late Pleistocene-Holocene Dilo-Dukana and Mega volcanic fields (Ririba rift, South Ethiopia) formed through monogenetic eruptions of variable 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 and lava flows that abruptly cut inactive rift-related features, suggestive of an emplacement after rift abandonment. We provide new petrological, geochemical and isotopic data on these still poorly studied basaltic products aimed at investigating the nature and temporal variation of the mantle source compared to the previous Pliocene activity, and the main differentiation processes involved. Petrological and geochemical data clearly discriminate the younger alkaline lavas from the Pliocene products, indicating a variation within the mantle source during the two activities possibly related to the local rifting evolution. In detail, the Dilo-Dukana and Mega products show porphyritic textures, with mainly olivine and clinopyroxene as phenocrysts, minor oxides and rare plagioclase, which dominate in the microcrystalline groundmass. Crustal and mantle micro-xenoliths are commonly observed dispersed within the lava samples. From the compositional point of view, the analyzed samples overlap both in major, trace elements and radiogenic (Sr-Nd-Pb) isotopes. However, well-defined trends are displayed by incompatible trace elements and by major elements, indicating the prominent role of fractional crystallization (dominated by olivine and clinopyroxene) driving magma differentiation. The negligible amount of plagioclase among the fractionating assemblage and the presence of large xenoliths, suggest that these magmas rapidly ascended without stopping at 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. Overall, 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. The isotopic signature suggests a decreasing though time of the Sub Continental Lithospheric Mantle contribution, consistent with a general cooling of the lithosphere following the abandonment after the rift-related Pliocene effusive phase.