The Nubian-Arabian shield mantle and Cenozoic magmatism of western Arabia

Voluminous Cenozoic lava fields are widespread in the western Arabian Peninsula, covering an area of 180000 km2 from Yemen, through Saudi Arabia, Jordan, Syria and Israel. Although most studies agree on a genetic link between the western Arabia alkaline magmatism with the plume head impingement beneath the Afar depression and the subsequent lithospheric thinning related to the Red Sea rifting, the nature of their mantle source is still a matter of debate. Here, we report on new whole rock radiogenic isotope and olivine geochemistry of basalts from five Arabian lava fields. Similar to olivine phenocrysts from ocean island basalts (OIB), olivine grains from our samples have high Ni, high Ni/Co and low 100*Fe/Mn, which suggest melting of a modal pyroxene-enriched source. However, the olivine compositions of Arabian basalts with HIMU-type signatures (initial 206Pb/204Pb up to 20.3) differ from most OIBs in their lower Al, Sc, Cr and higher Ni contents. Although it cannot be excluded that their enriched signatures could have originated from a HIMU-component in the underlying mantle affected by the Afar plume, their high MREE/HREE ratios and K/K* suggest that the HIMU-type signature in the western Arabia alkali basalts is due to pyroxene/amphibole-rich metasomes located in the lowermost portion of the Arabian shield lithosphere. Indeed, the chemical fingerprint of these metasomes is best revealed at low degrees of melting, being gradually diluted by mixing with melts generated at higher degrees of melting by a depleted asthenosphere.