Sapphirine-bearing pyroxenite xenoliths in Cenozoic alkali basalt from Jabel El Arab (Syria): Insights into the nature and composition of the lithosphere beneath the southern Syrian Rift, northern part of the Arabian Plate

摘要

Pyroxenites and composite xenoliths occur in the Cenozoic alkali basalts of Jebel El Arab (the southern extremity of the Syrian Rift). The anhydrous xenolith suite consists mainly of spinel garnet-sapphirine-bearing websterites, spinel-olivine-bearing websterite, as well as rare spinel plagioclase-garnet-bearing websterites and composite garnet-bearing websterite/anorthosite xenoliths. We report for the first time sapphirine coronas around spinel in spinel-garnet-bearing websterite xenoliths from two outcrops in this region. Their compositional and microstructural features are consistent with a reaction from spinel + orthopyroxene + anorthite to clinopyroxene + garnet + sapphirine. The full pyroxenite suite is divided into two groups based on mineralogical characteristics and geochemical composition: (A) tholeiitic to transitional mantle segregates having spinel +/- garnet +/- sapphirine characterized by a high Al2O3 and CaO content, low REE content, LREE depletion, and a highly variable Sr-Nd isotopic ratio; (B) alkaline segregates that include spinel-olivine-bearing websterite and composite xenoliths characterized by a high TiO2 and alkali content, relatively high REE, and low Ni and Cr. Slight LREE enrichments in dinopyroxenes from meta-igneous rocks of tholeiitic to alkaline affinity indicate that they were later metasomatized by small volumes of percolating silicate melts in the upper lithospheric mantle. The xenoliths have been re-equilibrated within the garnet-pyroxenite stability field. Combining xenolith-derived P-T estimates with mineral and chemical composition and seismic data, we show that the websterite xenoliths formed in the shallow lithospheric mantle near the Moho discontinuity at 28-44 km in depth. Nd-model ages and the geodynamic history of the Arabian Plate favor the scenario of tholeiitic-transitional and alkaline magmas being underplated below or intruded at the base of a thinned Syrian crust. This occurred around 200-100 Ma in response to rifting and magmatism beneath the Arabian Plate during the early assembly of the eastern Mediterranean region.