Abstract

Alkali basalts from the Hanish-Zukur and Zubair island groups in the southern Red Sea have been analysed for major, trace elements and Sr, Nd and Pb isotopes. They have close to primary compositions with high Mg# (60–70) and Ni and Cr concentrations up to 298 ppm and 597 ppm respectively. Many have only experienced olivine fractionation and this can be readily corrected. Compositional variations in the corrected analyses are interpreted to result from variations in the degree of melting, and a simple inversion of the corrected incompatible element abundances reveals a source composition intermediate between that of N-MORB and primitive mantle. This is suggested to result from the interaction of depleted asthenospheric mantle with less depleted material derived from the nearby Afar plume.

The overall geochemical characteristics of the basalts are comparable with ocean island basalts erupted through thin (young) lithosphere. SiO₂ varies from 44.0–47.9%, Nb/Y from 0.8–1.4 and Ce/Y averages 1.64, close to values from Ascension Island. ¹⁴³Nd/¹⁴⁴Nd ratios show minimal variation between 0.51301–0.51309 whereas ⁸⁷Sr/⁸⁶Sr ranges between 0.70323–0.70396, and the analyses define a horizontal trend extending to the right of the mantle array. Pb isotopes also show marked variation, ²⁰⁶Pb/²⁰⁴Pb values varying from 18.675–19.077, but the analyses remain within the ranges defined by MORB and are generally comparable with samples from the Red Sea axial trough and the Gulf of Aden.

Comparison with basalts from Afar and the Main Ethiopian Rift (MER), together with those from other parts of the Red Sea and Gulf of Aden reveal systematic shifts in isotope ratios with degree of extension. Along the constructive plate margins and including the island basalts, isotope results are dominantly asthenospheric, showing variations from N-MORB towards a possible plume end-member. In Afar, where β factors are lower (<2.5), isotope ratios show a greater deviation from oceanic basalts and this is most marked in the MER basalts where β factors are lowest (< 1.2).

These data are tentatively related to a model of passive extension over a mantle plume, driven by plate motions, with the possibility of active, plume-driven extension being restricted to the early stages of Ethiopian flood basalt volcanism and the subsequent development of the MER during the Miocene.