Petrogenesis of latest miocene-quaternary continental intraplate volcanism along the northern Dead Sea Fault System (Al Ghab-Homs volcanic field), western Syria: Evidence for lithosphere-asthenosphere interaction

Abstract

Late Miocene to Quaternary intraplate basaltic volcanism in the Al Ghab and Homs regions, northwestern Syria (the Al Ghab volcanic group and the Homs volcanic group), occurred roughly synchronously with the propagation of the Dead Sea Fault System in these regions. Petrographic evidence as well as major and trace element variations suggest that the basalts have undergone varying amounts of crystal fractionation of olivine ± clinopyroxene ± spinel, with feldspar fractionation only in the most evolved samples. The major and trace element chemistry and Nd-Sr-Pb isotopic variations of the basalts reflect both mantle source heterogeneity and relatively minor crustal contamination. Semi-quantitative assimilation-fractional crystallization modelling suggests that some samples may reflect assimilation of no more than 6% of upper continental crust, probably with Late Proterozoic Arabian Shield characteristics. Amongst the least crustally contaminated and relatively primitive samples, basanites are characterized by marked depletion of K, Rb and Zr relative to other neighbouring incompatible elements on primitive mantle normalized trace element diagrams. This, together with their low SiO2 and high TiO2 and Dy/Yb, is consistent with magma genesis involving a large proportion of garnet-bearing hornblendite or similar amphibole-rich metasomatic veins. Associated alkali and tholeiitic basalts with the higher SiO2, lower TiO2, less negative K, Rb and Zr anomalies, and moderately high Dy/Yb are consistent with melt extraction from a largely peridotitic mantle source. It is suggested that the compositional spectrum from basanite via alkali basalt to tholeiitic basalt can be explained by increasing degrees of metasomatic vein-wall-rock interaction, plus asthenospheric melt assimilation. In agreement with this is the identification of three distinct isotopic and chemical characteristics within the spectrum of mafic lavas, each of which can be referred to a unique mantle source (metasomatic vein, lithospheric wall-rock peridotite mantle and asthenospheric peridotite mantle). A decrease in eruption volume and increase in Si-undersaturation of the lavas from south (Homs) to north (Al Ghab) along the northern Dead Sea Fault System from latest Miocene to Quaternary times suggest a diminishing thermal perturbation and increasing importance of the amphibole-rich veins in magma genesis over time. It is proposed that the genesis of the oldest lavas reflects the arrival of asthenospheric melts beneath the Homs region, which with assimilation of lithospheric metasomatic veins and their wall-rocks produced the parental magmas of the Homs volcanic group. Subsequently, upwelling asthenospheric material could have been channelled northwards at the base of the lithosphere, presumably related to the northward propagation of the Dead Sea Fault System in the Pliocene. Cooling of this channelled asthenospheric material, which did not penetrate the lithosphere during this later period of magma genesis, provided the minimal thermal perturbation necessary for melting of amphibole-rich metasomatic veins and wall-rock peridotite within the lithosphere. © The Author 2011. Published by Oxford University Press.