Melt inclusions in xenolith granulite minerals from the diatremes of the eastern Pamir (Tajikistan): evidence of incongruent melting at the crust-mantle boundary

Melt inclusions in migmatites and granulites provide crucial information about the anatexis of the Earth's crust in various geological settings. This study provides the first data on the composition (including trace elements and H₂O) and age of inclusions in peritectic garnets, kyanite, quartz and accessory minerals (zircon, monazite, apatite) from xenoliths of various granulites (garnet-kyanite, garnet-clinopyroxene and garnet-orthopyroxene) from the alkali-basalts of the "Eclogite" pipe (Eastern Pamir). The composition of these inclusions corresponds to potassium-rich acid melts from rhyodacites to rhyolites. The concentration of H₂O in the melts varies from 1 to 4 wt. %, and the concentration of CO₂ is estimated as high as ~ 1 wt. %. The acidic composition and low water content in these inclusions indicate their formation as a result of dehydration (incongruent) melting of mica (muscovite and/or biotite). Garnet and melt are products of these reactions, therefore, low HREE contents in melt inclusions most likely indicate an equilibrium between the melt and garnet. The findings of melt inclusions in zircon and monazite indicate the equilibrium of the melt and accessory minerals. The presence of melt inclusions (with a high CO₂ content) and syngenetic CO₂ inclusions with a high density indicates that the carbon dioxide fluid played an important role in the melting of crustal material and the petrogenesis of these melts. According to the data obtained by us on microthermometry of melt inclusions, mineralogical thermobarometry and SHRIMP dating of zircon with melt inclusions, the melts of inclusions and host minerals were formed at a temperature of 950–1000°C and a pressure of >1.5 GPa (14.5 million years) shortly before the exhumation of these xenoliths by alkaline basalt melts, whose age is 11 million years.