Pyroxenite Xenoliths from Al Ashaer Volcano, Syria: Constraints on the Thermal State of the Subcontinental Arabian Lithosphere

Abstract

Pyroxenite xenoliths from the Pliocene Al Ashaer volcano, Syria, have been investigated to expand the database on pyroxenite xenoliths in the western part of the Arabian plate, and to complement the existing data for associated spinel peridotite xenoliths. Such data are essential for establishing the chemical composition and thermal state of the Arabian plate lithosphere, and for evaluating possible geographic and secular variations in the mantle. Sixteen samples of Al Ashaer pyroxenite xenoliths were analyzed by X-ray fluorescence for major and minor elements, and mineral compositions in six samples were determined by electron microprobe. Al Ashaer pyroxenites belong to the Al-augite group of mantle xenoliths, include spinel- and garnet-bearing varieties, and are characterized by a mosaic texture. Rock and mineral compositions indicate that the pyroxenites formed by high-pressure crystal accumulation from alkali basaltic melts in the upper mantle. Equilibration conditions for Al Ashaer pyroxenites are 970 to 1140°C and 11.5 to 13.8 kbar; these values are comparable to those for other pyroxenite xenoliths in the Arabian plate, and are consistent with T-P conditions for accompanying spinel peridotites. T-P conditions for xenolith suites from different locations in the western Arabian plate are unrelated to geographic position and tectonic setting. Pyroxenite and peridotite equilibration conditions are well above those predicted by the present-day surface heat flow of ∼45 mW/m2, and reflect a geologically young thermal pulse whose effects have yet to reach the surface. This thermal perturbation is caused most likely by advective heating of the subcontinental lithosphere by extensive and voluminous Neogene magmas, although high mantle temperatures immediately east of the Red Sea may be caused by convective thinning of the lithosphere associated with rifting.