Spinel Peridotite Xenoliths from the Al Ashaer Volcano, Syria: A Contribution to the Elemental Composition and Thermal State of Subcontinental Arabian Lithosphere

Abstract

Spinel peridotite xenoliths from the Pliocene Al Ashaer volcano, Syria, have been investigated to expand the database on mantle-derived xenoliths in the western part of the Arabian plate and to evaluate possible geographic and secular differences in the major-element composition and thermal state of the Arabian plate lithospheric mantle. Eighteen Al 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 fertile 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 peridotite xenoliths worldwide that have been erupted through Phanerozoic terranes, in contrast to xenoliths that have been erupted through Proterozoic 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 composition 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° to 1100°C and f(O2) within −.7 to +0.1 log units of FMQ (calculated at Ptotal = 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 analyzed xenoliths were extracted from shallow depths of 40 to 60 km in the uppermost mantle. Temperatures for the Al Ashaer and other Arabian plate peridotite xenoliths are below the solidus for dry lherzolite, but above a steady-state conductive geotherm predicted by the regional low surface heat flow (∼45 mW/m2). The mantle in the western part of the Arabian plate has been heated by an event young enough such that surface heat flow has not yet equilibrated with lithospheric temperatures. There is no apparent correlation between peridotite xenolith temperature and geographic position with respect to the Red Sea rift and the Dead Sea transform, but temperatures for the Pliocene Al Ashaer xenoliths are slightly higher than those for Quaternary xenoliths in the Arabian plate. We suggest that heating of the Arabian plate lithosphere was caused by Neogene magmatism, as recorded in Pliocene xenoliths, and that a small amount of thermal relaxation and cooling occurred by the time that Quaternary xenoliths were erupted.