SPINEL PERIDOTITE XENOLITHS FROM THE AL-ASHAER VOLCANO, SYRIA - A CONTRIBUTION TO THE ELEMENTAL COMPOSITION AND THERMAL STATE OF SUBCONTINENTAL ARABIAN LITHOSPHERE

Spinel peridotite xenoliths from the Pliocene Al Ashaer volcano, Syria , have been investigated to expand the database on mantle-derived xeno liths in the western part of the Arabian plate and to evaluate possibl e geographic and secular differences in the major-element composition and thermal state of the Arabian plate lithospheric mantle. Eighteen A l Ashaer peridotite xenoliths vary widely in major-element composition (MgO = 37.8-45.0 wt%; CaO = 0.61-3.00 wt%; Mg# = 88.1-90.1; Cr#; 2.7- 30.2), resulting from up to 40% extraction of partial melt from a fert ile protolith. This wide range in composition spans almost the entire range for all published analyses of other peridotite xenoliths in the Arabian plate. With respect to olivine composition and modal content, peridotite: xenoliths from the Arabian plate have affinities with peri dotite xenoliths worldwide that have been erupted through Phanerozoic terranes, in contrast to xenoliths that have been erupted through Prot erozoic or Archean terranes. Al Ashaer spinel peridotite xenoliths are well equilibrated, and individual samples yield similar temperatures from geothermometers based on the two-pyroxene miscibility gap, the co mposition of orthopyroxene in equilibrium with spinel and olivine, and Mg-Fe2+ exchange between olivine and spinel. Minerals in four samples were analyzed by electron microprobe, and all have temperatures from 1000 degrees to 1100 degrees C and f(O-2) within -.7 to +0.1 log units of FMQ (calculated at P-total = 15 kbar). The maximum pressure, 18.8 kbar, for the most aluminous and highest-temperature xenolith and the thickness of crust beneath the Al Ashaer volcano require that the anal yzed xenoliths were extracted from shallow depths of 40 to 60 km in th e uppermost mantle. Temperatures for the Al Ashaer and other Arabian p late peridotite xenoliths are below the solidus for dry Iherzolite, bu t above a steady-state conductive geotherm predicted by the regional l ow surface heat flow (similar to 45 mW/m(2)). The mantle in the wester n part of the Arabian plate has been heated by an event young enough s uch that surface heat flow has not yet equilibrated with lithospheric temperatures. There is no apparent correlation between peridotite xeno lith temperature and geographic position with respect to the Red Sea r ift and the Dead Sea transform, but temperatures for the Pliocene Al A shaer xenoliths are slightly higher than those for Quaternary xenolith s in the Arabian plate. We suggest that heating of the Arabian plate l ithosphere was caused by Neogene magmatism, as recorded in Pliocene xe noliths, and that a small amount of thermal relaxation and cooling occ urred by the time that Quaternary xenoliths were erupted.