Melt Pockets and Spongy Clinopyroxenes in Mantle Xenoliths from the Plio-Quaternary Al Ghab Volcanic Field, NW Syria: Implications for the Metasomatic Evolution of the Lithosphere

Abstract

Spongy minerals, especially clinopyroxenes, and fine-grained, often glass-bearing mineral assemblages , commonly referred to as melt pockets, occur in many mantle xenolith suites worldwide, but their origins remain far from being clearly understood. We describe a suite of spongy clinopyroxene- and melt pockets-bearing peridotite xenoliths from the Plio-Quaternary volcanic field in the Al Ghab Depression, a pull-apart basin of the Dead Sea Fault System in northwestern Syria. The melt pockets comprise fine-grained olivines, clinopyroxenes, spinels and feldspars ± amphiboles ± glasses. Petrography and major and trace element mineral chemistry suggest that the xenoliths have experienced at least two stages of metasomatism with the formation of the melt pockets being associated with the latest event. The first metasomatic episode featuring LREE, Na, Th, U enrichment and relative Ti and Zr depletion in the cores of primary clinopyroxenes involved metasomatism by a low-silica, CO2-rich agent. The second metasomatic episode was associated with the development of melt pockets which evolved from decompressional and perhaps metasomatism-induced incongruent melting of clinopyroxene ± spinel. The spongy clinopyroxenes that occur as coronas around clear, primary clinopyroxenes represent a transitional stage of the partially melted crystals. The incongruent melting produced a liquid that evolved within the melt pockets and eventually migrated out to form amphiboles and micas elsewhere in the lithosphere. Albeit with some uncertainty, geothermobarometric estimations reveal significant, systematic differences in the equilibration pressures between the primary minerals (0.8–1.4 GPa), and spongy and melt-pocket minerals (0.7–0.9 GPa), lending good support for the decompressional origins of the spongy clinopyroxenes and melt pockets. It is interpreted that decompression resulted from local transtension associated with the development of the Al Ghab pull-apart basin, a step-over zone of the Dead Sea Fault System, in Plio-Quaternary time.