We report on the rare earth and Nd-Sr-Pb isotopic composition of basalts dredged along the Sheba Ridge axis in the Gulf of Aden and its extension into the Gulf of Tadjoura and subaerial basalts from the Ardoukoba Rift in east Afar. The sampling profile provides a means to study the evolutionary nature of the mantle sources involved in the melting process associated with the interaction of the head of a starting mantle plume with continental lithosphere and an ocean basin at a nascent stage of formation. An 800-km-long Nd-Sr-Pb isotopic and La/Sm gradient, sinusoidally modulated, is apparent from the Afar eastward. The first enrichment peak occurs in the Gulf of Tadjoura, where diffuse extension of the Danakil-Aisha continental lithospheric block and westward rift propagation is currently progressing. The second enrichment peak at 46 ¿E is associated with a mantle buoyancy anomaly and related constructional volcanism. East of 48 ¿E, the MORBs are typically light rare earth element depleted, whereas 206Pb/204Pb and 87Sr/86Sr slightly increase, suggesting recent decoupling. In Nd-Sr-Pb isotope ratio space, three distinct vector trends are observed within a plane. The mixing vectors point toward three mantle source end-members which can be interpreted as Pan-African continental lithosphere along the Gulf of Tadjoura (a hybrid EM-1--EM-2), a mantle plume (relatively young HIMU-like) which dominates the 46 ¿E anomaly, and the depleted asthenosphere east of 48 ¿E (DUPAL-like). Combined data from the Gulf of Aden--Red Sea--Afar--Ethiopian rifted zones suggest radial pattern of geochemical and isotopic variation about the Afar.

A working dynamical-thermal model is presented for the past 30--40 m.y. history of the Horn of Africa. It invokes both passible rifting/seafloor spreading in the Red Sea/Gulf of Aden and the flattening and interaction of the starting head of a toruslike thermal mantle plume with the Pan-African continental lithosphere which is slowly moving northeastward with the plume head attached at its base. The plume flattened into a pancakelike form, twice the diameter of the original head which is estimated to be of the order of 700 km in diameter. The thinning of the lithosphere by stretching and thermal erosion by the mantle plume has not yet been completed. A working ternary mixing model constrained by the isotope data indicates that within the 800--1000 km radius of influence of the Afar mantle plume, melting of the lithosphere mantle and the depleted asthenosphere apparently entrained by the ascending mantle plume dominates initially. Only along the three rifting zones intersecting the flattened plume ring, 450¿150 km in radius, composed of original HIMU-like plume material does the original plume component play a more dominant role. Judging from the spatial isotopic composition variation of the basalts, the plume torus may be apparent along (1) the 46 ¿E Gulf of Aden anomaly where seafloor spreading is now well established; (2) the 13¿--16 ¿N southern Red Sea segment, which represents a rift zones at a transient stage of either development or abandonment (overlapping with the Afar NW neovolcanic zone), where ocean island alkali volcanism dominates and diffuse lithosphere extension may still operate; (3) the high alkaline field of the Aden Volcanic Series; and (4) the Ethiopian Rift around 8 ¿N in a purely continental setting.

The NW Afar neovolcanic zone, which is essentially at a nascent stage of seafloor spreading and is overlapping the ring and the center of the pancakelike flattened mantle plume, is dominated by tholeiites derived from depleted asthenospheric material entrained by the plume during its original ascent. Plate reconstructions further suggest that the original center of the flattened mantle plume head has moved with the lithosphere some 900 km northeastward. The stem feeder of the plume has now been drawn or tilted toward the Afar as a result of the migration of the Gulf of Aden/Red Sea spreading centers which act as sinks of asthenospheric material and the likelihood that the feeder of the mantle plume is encountering with time an African lithosphere increasing in age, thickness, and rigidity. ¿ American Geophysical Union 1992