ISOTOPE (SR, ND, PB, O) AND TRACE-ELEMENT GEOCHEMISTRY OF VOLCANICS FROM THE ERTAALE RANGE (ETHIOPIA)

The Erta'Ale range is the most important axial volcanic chain of the A far region. The petrographic diversity of lavas erupted in this area i s very important, ranging from magnesian transitional basalts to rhyol ites. The variation in isotopic compositions and incompatible element ratios in the basalts (e.g., epsilon(Nd),, = +4.5 to +7.5, (La/Yb)(n), = 2.5 to 11) demonstrates the heterogeneous character for their mantl e sources. Such heterogeneity can be interpreted by the participation of two mantle reservoirs: a depleted MORE and a HIMU GIB-type sources. These reservoirs are indistinguishable from those recently identified in the southern part of the Red Sea region. The isotopic data indicat e that the contribution of continental sialic components is not signif icant in the petrogenesis of the volcanic rocks. Our data further sugg est that the crust beneath the Erta'Ale volcanic range is not a thinne d (Pan-african?) sialic crust, but could be gabbroic in nature. The ac id volcanics have originated mostly through fractional crystallisation of basaltic magmas as revealed from major-and trace-element data and from the relatively homogeneous Sr and Nd isotopic ratios. However, th e delta(18)O variation in the acid lavas suggests an important contrib ution from a low delta(18)O component in the petrogenesis of some low delta(18)O acid lavas. (C) 1998 Elsevier Science B.V.