Melt Inclusions in Minerals of Granulitic Xenoliths from Diatremes of Eastern Pamir, Tajikistan: Evidence of Incongruent Melting at the Crustal–Mantle Boundary

Melt inclusions (MIs) in migmatites and granulites provide crucial information about the anatexis of crustal material in various geological conditions. This paper presents the first data on the composition (including trace elements and H2O) and age of the MIs in peritectic garnets, kyanite, quartz, and accessory minerals (zircon, monazite, and apatite) from xenoliths of various granulites (garnet–kyanite, garnet–clinopyroxene, and garnet–orthopyroxene) of the Eklogitovaya alkali basaltic pipe (Eastern Pamir). The composition of the MIs corresponds to K-rich felsic melts from rhyodacites to rhyolites. Their Н2О content varies from 1 to 4 wt %, and the CO2 content is estimated at ~1 wt %. The felsic composition and the low Н2О content of the MIs indicate their formation as a result of dehydrated (incongruent) melting of mica (muscovite and/or biotite). Garnet and the melt are the products of these reactions; thus, the low content of heavy rare earth elements of the MIs most likely indicate the melt–garnet equilibrium. The findings of MIs in zircon and monazite are evidence of equilibrium between the melt and accessory minerals. The presence of MIs (with a high CO2 content) and syngenetic CO2 inclusions with a high density indicates that the CO2 fluid played an important role in crustal melting and petrogenesis of these melts. According to microthermometric data on the MIs, the mineralogical thermobarometry, and SHRIMP dating of zircon with a MI, the melts of inclusions and their minerals formed at a temperature of 950‒1000°C and a pressure of >1.5 GPa (14.5 million years) shortly before the removal of these xenoliths by alkali-basic melts, the age of which is 11 Ma.