Petrological and geochemical constraints on the composition of the lithospheric mantle beneath the Syrian rift, northern part of the Arabian plate

A suite of mantle xenoliths from the Neogene-Quaternary volcanic province of Jabel El Arab (Syria) is dominated by spinel ± amphibole harzburgites, with rare lherzolites and wehrlites that were equilibrated at temperatures of 900-1100 °C. The major elements of pristine minerals and trace element compositions of clinopyroxene and amphibole indicate that the lithospheric mantle experienced various degrees of melt extraction (olivine Mg-number = 89.4-91.8, spinel Cr-number = 10.4-46.4), followed by a multistage metasomatic history. The primary clinopyroxene has variable and high Mg-number (89.1-93.6) and highly variable major element concentrations (Al₂O₃ 2.7-7.6 wt%, Na₂O 0.5-2.5 wt% and Cr₂O₃ 0.4-2.5 wt%). Three groups of harzburgites were identified oil the basis of petrographical, mineralogical and geochemical data. Group I harzburgites have compositions indicating a residual origin after polybaric partial melting with F <20%, which started in the garnet stability field and continued in the spinel stability field. Group II harzburgites are interpreted as a result of a percolation mechanism involving the infiltration of large volumes of undifferentiated basaltic melts through the residual lithosphere. Finally, the mineral major element compositions and the selectively enriched trace element contents of clinopyroxenes in group III harzburgites (high (La/SM)_N and Th, U, Sr and low high field strength element contents) are attributed to a percolation mechanism involving small volume melt fractions. Such small melt fractions correspond to CO₂-bearing alkaline silicate magmas that have evolved to CO₂-rich melts during repeated percolation-reaction within the Syrian lithospheric mantle. Shortly before eruption, some xenoliths were infiltrated by small silicate melt fractions, which produced discrete reaction zones composed of cpx ± ol ± sp ± glass surrounding reacting primary spinels. The glass in the melt pockets has a trachytic to trachy-andesitic composition and its composition suggests that glass is derived from melting of pre-existing amphibole in the lithospheric mantle, triggered by infiltration of a Na-rich metasomatic agent.