Genesis of the pliocene to recent bimodal mafic-felsic volcanism in the Debre Zeyt area, central Ethiopia: volcanological and geochemical constraints

The Debre Zeyt volcanic district is located about 40 km southeast of Addis Ababa, on the western shoulder of the Ethiopian Rift. Two main phases of volcanic activity are shown to have occurred in the area, both younger than the main episodes of rifting. Eruption of acid lavas and pyroclastics, between 4 and 1 Ma, brought to the formation of the central volcanoes of Yerer, Bede Gebabe and Zikwala. Younger activity generated basaltic cinder cones, maars and lava flows, all aligned along the main rift direction. A few intermediate and acid volcanics are associated with the younger basalts.

Petrological and geochemical investigations have shown that the rocks from central volcanoes consist of peralkaline rhyolites and trachytes with a few trachyandesitic lava flows. All these rocks have high concentrations of Rb, Zr, LREE and other incompatible elements, and variable Ba and Sr contents. Measured Nd isotopic ratios are close to the bulk earth value, whereas Sr isotopic ratios are very variable, due to the combined effects of the high to extreme Rb/Sr values, variable ages and, possibly, different initial isotopic signatures. Basalts range from transitional to weakly alkaline in composition and display relatively homogeneous incompatible element contents and Nd and Sr isotopic ratios. Incompatible element ratios such as Ba/Rb are very variable in the basalts. The younger acid rocks have a large range of incompatible elements concentration with some rhyolites displaying very low values of some hygromagmaphyle trace elements (HYGE), such as Zr, Nb and LREE. The younger intermediate rocks have comparable HYGE contents as the basalts and define linear trends between basalts and low-HYGE rhyolites on several interelement variation diagrams.

Geochemical modelling indicates that the major and trace element composition of the acid rocks from central volcanoes can be satisfactorily explained by a derivation from basaltic parents by fractional crystallization. Nd-isotopic ratios which, for the largest part, fall within the range of younger basalts suggest that, except for the Yerer rhyolites, the assimilation of the upper continental crust did not play a major role during magma evolution.

The relatively constant HYGE contents of basaltic and intermediate younger volcanics exclude an evolution by fractional crystallization for this suite. The linear trends on inter-element diagrams suggest that mixing processes between basaltic magmas and an acid rock or liquid may be responsible for the generation of the intermediate rocks. This hypothesis is supported by Ba/Rb vs. Rb relationships revealing hyperbolic mixing trends between acid and basaltic end-members. This process also generated important geochemical variations within the basalts.