A petrological model for the upper mantle and lower crust under the northern part of the Arabian Plate (Syria) has been derived on the basis of petrology of upper mantle and lower crustal xenoliths occurring in the Neogene to Quaternary alkali basalts of the Shamah volcanic fields. The xenolith suite has been classified by texture, mineralogy and chemistry into the following groups: (1) Type I metasomatised and dry Cr diopside xenoliths with protogranular to porphyroclastic textures; (2) Type II Al augite spinel and garnet pyroxenite and websterite which have igneous and/or porphyroclastic textures and abundant phlogopite and/or amphibole; (3) Cr-poor megacrysts; and (4) mafic lower crustal xenoliths. Estimates of Type I xenolith temperatures are 990-1070°C with pressure between 13 and 19 kbar. Type II xenoliths yield temperatures of 930-1150°C and pressures in the range 12-13 kbar. The lower crustal xenolith mineral assemblages and geothermometry based on coexisting minerals suggest equilibration conditions between 6 and 8 kbar and 820-905°C. Mantle plumes, which may be the source of the volatile flux, have implications for melt generation in the Arabian basalt provinces. It is estimated that the lithosphere beneath the Arabian Plate is less than 80 km thick. Xenolith data and geophysical studies indicate that the Moho is located at a depth of 40-37 km and that the crust-mantle transition zone has a thickness of 8-5 km and occurs at a depth of 27-30 km. The boundary between an upper granitic crust and a lower mafic crust occurs at a depth of 19 km. Type I dry xenoliths show a low overall concentration of REE (La/Yb = 1-2 and Sm = 0.7-1.1 times chondrite), whereas Type I hydrous xenoliths are LREE enriched (La/Yb = 6-9 and Sm = 1.1-1.3 times chondrite). Type II xenoliths show high overall LREE enrichment. Petrological and geochemical data for the lower crustal xenoliths indicate that these xenoliths represent basaltic cumulates crystallised at lower crustal pressures. (C) 2000 Elsevier Science Limited. All rights reserved.
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