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

Basalts from the Tadjoura Gulf (Afar area, northeastern Africa) have b een analyzed for chemical and isotopic (Sr, Nd, O) compositions in ord er to understand processes of magma genesis in relation to the active rifting in this region. We use these data to characterize the mantle s ource of the basalts and to examine the potential role of lithospheric mantle in their genesis. The isotopic compositions of Sr, Nd, and O s how 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 an d trace-element data, these isotopic results suggest that the basalts were essentially produced by partial melting of the mantle sources com parable to those of southern Red Sea basalts; namely, both depleted ma ntle and somewhat enriched plume sources are responsible for the gener ation of the magmas. The continental lithospheric mantle seems to have played only a very minor role in the petrogenesis. Prior to the openi ng of the Tadjoura Gulf and the Asal Rift, some lavas penetrated throu gh continental crust, resulting in variable degrees of crustal contami nation. At present, the crust of the Tadjoura Gulf and the Asal Rift i s dominantly basaltic. Some lavas of the ''recent series'' show very u nusual chemical and isotopic characteristics: high Ta/Th ratios, posit ive Eu anomalies, and low deltaO-18 values. These features can be best explained by a contamination process involving assimilation of hydrot hermally altered gabbros by some basaltic magmas.