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

Magmatism in extensional tectonic settings, such as the East African Rift System, is mainly modulated by the interaction of plume upwelling and plate dynamics, resulting in complex tectonic processes associated to rifting (e.g., rift migration, focusing) and generating a large compositional variability of the erupted products, even over small distances and short time periods. The Ririba rift, formed from the southward propagation of the Main Ethiopian Rift, well exemplifies these complexities providing a unique opportunity to investigate in detail both the spatial and temporal evolution of mantle sources involvement and magma ascent dynamics related to tectonics. The activity of the Ririba rift is associated to the emplacement of subalkaline basalts producing a widespread basaltic lava basement during Pliocene. Later, the Quaternary Dilo-Dukana and Mega volcanic fields (VFs) formed through monogenetic eruptions of limited volumes of alkaline-basalts, rich in mantle and crustal xenoliths. Both volcanic fields form lineaments that abruptly cut the rift-related features, suggestive of an emplacement after rift abandonment.

We provide new petrological, geochemical and isotopic data on both Pliocene and Quaternary products aimed at investigating the nature of mantle component(s) contributing to magma genesis during the two periods of activity and detail the processes driving magma evolution during the more recent magmatic phase.

All data discriminate the younger alkaline lavas from the Pliocene products, indicating they originated from different mantle sources. The Quaternary Dilo-Dukana and Mega products, on the contrary, overlap in major, trace elements and radiogenic (Sr-Nd-Pb) isotopes. No clear geochemical correlation is observed with respect to vent location (rift floor, scarp or plateau), xenoliths content or eruptive style (lava flows or tuff cones). However, well-defined trends, displayed by major and incompatible trace elements, coupled with the absence of a correlation between the evolutionary degree and isotopes, indicate the prominent role of fractional crystallization driving magma differentiation. The small variation in silica, decorrelated from MgO variation, indicates fractionation of mainly femic phases, probably in deep, transient, storage levels. Only light-REEs, together with some HFSE ratios and LILE contents, roughly discriminate between the products erupted at Dilo-Dukana and Mega VFs pointing to variations in the degree of partial melting and/or different phase proportion in the crystallizing assemblage of the ascending magmas between the two VFs.

We infer that the Dilo-Dukana and Mega VFs were fed by two different systems of deep structures, unrelated to rifting but associated to old inherited fabrics, directly transferring mantle melts to the surface without important differentiation in shallow storage levels before eruption. Moreover, from rift-related Pliocene sub-alkaline basalts to the Quaternary basanites, the nature of the involved mantle source changed towards a more plume-dominated signature with minor contribution from the sub-continental lithosphere mantle (SCLM). This can be correlated to a variation in the thermal and rheological state of the lithosphere following the transitions from the main rift phase (Pliocene) to the later rift deactivation (Quaternary) that limited the SCLM melting and promote a direct magma uprise in the more recent activity.