Isotope (Sr, Nd, Pb, O) and trace-element geochemistry of volcanics from the Erta'Ale range (Ethiopia)

The Erta'Ale range is the most important axial volcanic chain of the Afar region. The petrographic diversity of lavas erupted in this area is very important, ranging from magnesian transitional basalts to rhyolites. 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 mantle 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 indicate that the contribution of continental sialic components is not significant in the petrogenesis of the volcanic rocks. Our data further suggest that the crust beneath the Erta'Ale volcanic range is not a thinned (Pan-african?) sialic crust, but could be gabbroic in nature. The acid 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, the delta(18)O variation in the acid lavas suggests an important contribution from a low delta(18)O component in the petrogenesis of some low delta(18)O acid lavas. (C) 1998 Elsevier Science B.V.