MAGMA GENESIS IN AN ONGOING RIFTING ZONE - THE TADJOURA GULF (AFAR AREA)

Basalts from the Tadjoura Gulf (Afar area, northeastern Africa) have been analyzed for chemical and isotopic (Sr, Nd, O) compositions in order to understand processes of magma genesis in relation to the active rifting in this region. We use these data to characterize the mantle source of the basalts and to examine the potential role of lithospheric mantle in their genesis. The isotopic compositions of Sr, Nd, and O show considerable variation, with Sr-87/Sr-86 = 0.703-0.707, epsilon(Nd) = +8.1 to -0.5, and deltaO-18 = +4.6 to +8.4. Combined with major and trace-element data, these isotopic results suggest that the basalts were essentially produced by partial melting of the mantle sources comparable to those of southern Red Sea basalts; namely, both depleted mantle and somewhat enriched plume sources are responsible for the generation of the magmas. The continental lithospheric mantle seems to have played only a very minor role in the petrogenesis. Prior to the opening of the Tadjoura Gulf and the Asal Rift, some lavas penetrated through continental crust, resulting in variable degrees of crustal contamination. At present, the crust of the Tadjoura Gulf and the Asal Rift is dominantly basaltic. Some lavas of the ''recent series'' show very unusual chemical and isotopic characteristics: high Ta/Th ratios, positive Eu anomalies, and low deltaO-18 values. These features can be best explained by a contamination process involving assimilation of hydrothermally altered gabbros by some basaltic magmas.