Peridotite xenoliths of Harrat ar-Rahah-Uwayrid alkaline basalts, NW Saudi Arabia: fragments of a multiply metasomatized Neoproterozoic mantle

Numerous Upper Miocene to Quaternary harrats (basalt fields) litter western Saudi Arabia along the Red Sea between Jordan and Yemen and are related to the initial rifting of the Red Sea system at ~30–25 Ma. Many of these harrats contain alkaline scoria bombs and lesser lavas that bear mantle xenoliths, and one of these is Harrat Ar-Rahah-Uwayrid (HARU), which comprises a NW-SE oriented elongate volcanic field of ~7150 km² and which extends for about ~230 km in NW Saudi Arabia. This study presents new whole-rock and mineral chemistry data of HARU mantle xenoliths to provide new insights into their petrogenesis and the nature of the subcontinental lithospheric mantle of Arabia. HARU xenoliths are mostly spinel lherzolite with lesser harzburgite and rare wehrlite. Xenoliths range from cm-scale to fist-size and are found primarily within Pleistocene basanite scoria bombs from cinder cones in the central HARU segment; others were recovered, for the first time, from the Upper Miocene alkali basalt and hawaiite lavas of the southern plateau. No distinctive features are apparent between peridotite xenoliths of the older and younger host rocks. Fo# of olivines from most of the HARU peridotite xenoliths ranges from 83.5 to 93.1, and spinel Cr# ranges widely from 0.09 to 0.59. Using F = 10 ln(spinel Cr#) + 24, estimated degrees of partial melting are low to high, ranging from 2% to 19%. On the basis of coexisting olivine-orthopyroxene-spinel pairs, oxygen fugacity ranges from −1.89 to −0.22 relative to FMQ, suggesting equilibration under reduced conditions. Thermobarometry calculations based on Ca in orthopyroxene suggest that the peridotite xenoliths equilibrated at temperatures of ~860–1000°C, at relatively shallow depths of ca. 35–42 km. Alike other harrats in Saudi Arabia, the host basalts are derived from partial melting of spinel lherzolite and garnet lherzolite. Overall, HARU peridotite xenoliths represent multiply metasomatized fragments of Neoproterozoic mantle.