Isotopic evidence for evolution of sub-continental mantle during Red Sea rifting

Abstract

Tertiary igneous rocks from near Al Lith, Saudi Arabia are alkaline to subalkaline and bimodal in composition, and document early and late rift volcanism (≥30 Ma to ~20 Ma, 11 Ma and 3 Ma) in the central part of the Red Sea rift. Isotopic and trace-element data from twenty-five samples are presented and used to characterize basalt sources in the early rift. Inferred primary isotopic compositions of alkalic basalts and some tholeiites indicate a common mantle source. Many of the felsic volcanics are isotopically indistinguishable from these mafic rocks; they likely represent products of fractional crystallization, or anatectic melts of mafic Al Lith precursors. l43 Nd/ 144 Nd and 87 Sr/ 86 Sr ratios for most Al Lith rocks show limited variation ( 87 Sr/ 86 Sr i = 0.7030 to 0.7033, ε -Nd=+6 to +8). They plot near enriched MORB and between the isotopic compositions of recent magmas from the Red Sea spreading axis and less depleted to enriched basalts from the East African rift. Pb isotopic compositions are MORB-like and moderately radiogenic in 206 Pb/ 204 Pb. Normalized incompatible trace-element abundances and oceanic Pb/Ce indicate sources similar to those for enriched MORB and OIB and distinct from subduction-dominated source regions. As in many continental basalt fields, high l43 Nd/ 144 Nd and relatively low Sr and Pb isotopic ratios, despite incompatible-element enrichment, indicate that source enrichment was not an ancient event; it probably took place as a precursor to rift magmatism. These data are consistent with a multi-stage model of sub-rift mantle evolution. Depleted lithospheric mantle in the spinel-lherzolite fades was veined by incompatible-element rich fluids, then partially melted. Melt equilibration and fractionation took place over a large depth range, producing transitional alkaline-subalkaline basalts during early rift magmatism. In the axis of the rift, depleted asthenospheric mantle upwelled and displaced the veined lithosphere, eventually producing MORB at the Red Sea axis. However, basalts from the veined lithospheric mantle continue to be erupted from the Arabian margin of the rift. Anomalous, old (> 2 Ga) continental-affinity Pb, radiogenic Sr and low ε-Nd values found in some of the subalkaline Al Lith rocks are explained by crustal contamination, not by Dupal mantle sources.