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 A ddis Ababa, on the western shoulder of the Ethiopian Rift. Two main ph ases 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 cen tral volcanoes of Yerer, Bede Gebabe and Zikwala. Younger activity gen erated basaltic cinder cones, maars and lava flows, all aligned along the main rift direction. A few intermediate and acid volcanics are ass ociated with the younger basalts. Petrological and geochemical investi gations have shown that the rocks from central volcanoes consist of pe ralkaline rhyolites and trachytes with a few trachyandesitic lava flow s. All these rocks have high concentrations of Rb, Zr, LREE and other incompatible elements, and variable B a and Sr contents. Measured Nd i sotopic ratios are close to the bulk earth value, whereas Sr isotopic ratios are very variable, due to the combined effects of the high to e xtreme Rb/Sr values, variable ages and, possibly, different initial is otopic 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 s uch as Ba/Rb are very variable in the basalts. The younger acid rocks have a large range of incompatible elements concentration with some rh yolites displaying very low values of some hygromagmaphyle trace eleme nts (HYGE), such as Zr, Nb and LREE. The younger intermediate rocks ha ve comparable HYGE contents as the basalts and define linear trends be tween basalts and low-HYGE rhyolites on several interelement variation diagrams. Geochemical modelling indicates that the major and trace el ement composition of the acid rocks from central volcanoes can be sati sfactorily explained by a derivation from basaltic parents by fraction al crystallization. Nd-isotopic ratios which for the largest part, fal l within the range of younger basalts suggest that, except for the Yer er 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 ev olution by fractional crystallization for this suite. The linear trend s on inter-element diagrams suggest that mixing processes between basa ltic magmas and an acid rock or liquid may be responsible for the gene ration of the intermediate rocks. This hypothesis is supported by Ba/R b vs. Rb relationships revealing hyperbolic mixing trends between acid and basaltic end-members. This process also generated important geoch emical variations within the basalts.