Pyroxenite xenoliths from Al Ashaer volcano, Syria: Constraints on the thermal state of the subcontinental Arabian lithosphere

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 associat ed spinel peridotite xenoliths. Such data are essential for establishing th e 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 belon g to the Al-augite group of mantle xenoliths, include spinel- and garnet-be aring varieties, and are characterized by a mosaic texture. Rock and minera l compositions indicate that the pyroxenites formed by high-pressure crysta l accumulation from alkali basaltic melts in the upper mantle. Equilibratio n conditions for Al Ashaer pyroxenites are 970 to 1140 degrees C and 11.5 t o 13.8 kbar; these values are comparable to those for other pyroxenite xeno liths in the Arabian plate, and are consistent with T-P conditions for acco mpanying spinel peridotites. T-P conditions for xenolith suites from differ ent locations in the western Arabian plate are unrelated to geographic posi tion and tectonic setting. Pyroxenite and peridotite equilibration conditio ns are well above those predicted by the present-day surface heat flow of s imilar to 45 mW/m(2), 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 exten sive and voluminous Neogene magmas, although high mantle temperatures immed iately east of the Red Sea may be caused by convective thinning of the lith osphere associated with rifting.