Geochemistry of the Lunayyir and Khaybar Volcanic Fields (Saudi Arabia): Insights into the Origin of Cenozoic Arabian Volcanism

This paper reports on a detailed geochemical study of rocks from Harrats Lunayyir and Khaybar, two large lava fields located in the central portion of the western Arabian Peninsula. Lavas from young flows north of Al Birk were also considered. Sample composition ranges from basanite to basalts with transitional to alkaline affinity. Their incompatible trace element signatures are consistent with alkaline magmas produced by an enriched mantle source, akin to that producing continental flood magmatism in other locations of the Arabian-Nubian plate. Large variations in major (Al2O3, CaO, NaO, TiO2) and trace (e.g., Ni, Cr, Nb, Sr, Zr, Ti, Y and REE) element compositions at a given Mg/(Mg+Fe) indicate that magmatic evolution occurred in magma chambers located at or close to the crust-mantle boundary, constrained by fractionation of olivine, clinopyroxene, plagioclase and Ti-Fe oxides. Their Ba/Nb and K/La ratios (7-10 and 200-300, respectively) are similar to those of ocean island basalts (OIBs) and with no evidence of crustal assimilation. Fractionations between incompatible trace elements are used to investigate differences in mantle composition and melting conditions in the studied localities and in other lava fields in Arabia, Yemen and Syria. Variable La/Yb and Dy/Yb fractionations of the lavas can be reproduced by mixing different proportions of partial melts produced within the garnet and the spinel stability fields. Lavas from Harrats Lunayyir, Khaybar and Al-Birk show relative depletion in Rb, Th and U compared to Ba and Nb, and negative K anomalies. These compositions are consistent with the local occurrence of an amphibole-bearing source, located most likely in the subcontinental lithospheric mantle. This model agrees with the idea that the sub-continental mantle in this region may have formed in a supra-subduction environment with residual amphiboles that preferentially withheld Ba and Nb during fluid-fluxed melting. Our results, when analyzed together with existing data from the region, suggest that Cenzoic alkaline volcanism in western Arabia formed mainly by decompression melting of ancient fusible components in the sub-Arabian lithospheric mantle, that were remobilized by lithospheric thinning due to Red Sea rifting. Additionally, our data are consistent with progressive thinning of the lithosphere toward the Red Sea and lengthening of the melting column over time.