ASTHENOSPHERE VERSUS LITHOSPHERE AS POSSIBLE SOURCES FOR BASALTIC MAGMAS ERUPTED DURING FORMATION OF THE RED-SEA - CONSTRAINTS FROM SR, PB AND ND ISOTOPES

Representative basalts from the axial trough of the Red Sea and from volcanic fields of the Arabian Peninsula ranging in composition from N-type MORB to basanite and in age from Early Miocene to Recent show a limited variation in their isotopic compositions: 87Sr 86Sr = 0.70240-0.70361, 206Pb 204Pb = 18.040-19.634, 207Pb 204Pb = 15.496-15.666, 208Pb 204Pb = 37.808-39.710, 143Nd 144Nd = 0.513194-0.512670. There is a poorly constrained correlation between chemical composition and isotope ratios: with increasing alkalinity, Sr and Pb isotope ratios increase and the Nd isotope ratio tends to decrease. In Pb isotope variation diagrams most of the basalts plot significantly above the NHRLs, irrespective of tectonic setting, i.e. thickness of underlying crust and/or lithosphere. MORBs from the axial trough of the Red Sea have higher Pb isotope ratios for a given 87Sr 86Sr than MORBs from the Indian Ocean ridges, including the Carlsberg Ridge. It is therefore suggested that both spreading ridges tap different convective systems in the asthenosphere. The tectonic setting of the basalts is reflected in their NdSr isotope characteristics. Basalts from areas where the continental lithosphere is drastically thinned or absent (i.e. Red Sea axial trough and coastal plain, Afar) plot along a reference line defined by N-type MORB and Tristan da Cunha. Basalts erupted in areas with Pan-African crust of normal thickness and moderately thinned lithospheric mantle (i.e. rift shoulder) are characterized by relative low 143Nd 144Nd ratios and plot below the reference line towards an EM I component which is also found in the subcontinental lithospheric mantle. These differences in the NdSr isotopic compositions of the basalts are independent of bulk-rock chemistry and are therefore controlled by tectonic setting alone. It is suggested that the low- 143Nd 144Nd trend of basalts from the Arabian rift shoulder is caused by a significant contribution of the pre-rift lithospheric mantle underneath Arabia. Basalts from the Red Sea coastal plain are mainly derived from an asthenospheric source but locally a lithospheric component seems to be present as well. A pure asthenospheric source is assumed for the basalts from the axial trough of the Red Sea. © 1990.