Magma Composition and Formation Conditions of Alkaline-Salic Rocks of the Early Mesozoic Adaatsag Bimodal Association of the Kharkhorin Rift Zone, Central Asia: Data from the Study of Melt Inclusions in Minerals

Abstract

The composition and formation conditions of magmas of comendites of the Early Mesozoic Adaatsag volcanic association are determined using electron and ion microprobe analysis of melt inclusions, and mechanisms favorable for the accumulation of trace and rare earth elements are identified. The melt and fluid inclusions are found in quartz of comendites sampled in various parts of the section of a volcanic sequence. The melt inclusions consist of glass, a gas bubble, and daughter minerals (fluorite, polylithionite, and K-feldspar). The composition of a gas phase in melt inclusions was studied using Raman spectroscopy. The dominant gas phase components include water and hydrogen. The fluid inclusions are aqueous fluids probably corresponding to KF. The thermometric studies of melt inclusions and the analysis of glasses of homogenized melt inclusions in quartz phenocrysts of comendites showed that magmas crystallized from water-saturated rare metal melts with high Li, Zr, F, Rb, Nb, Y, and Th contents at a temperature of 880–930°C and a pressure of 1000 bar at a depth of ~3.5 km accompanied by degassing. The comparison of our data of the study of melt inclusions in phenocrysts of alkaline salic rocks of the coeval Adaatsag, Dzarta-Khuduk, and Sant volcanic associations, which occur within the Kharkhorin Rift Zone, revealed common principles of magma evolution. This allowed us to propose a similar mechanism of their formation, involving the accumulation of many rare and rare-earth elements, as well as volatile components (F, H2O) in the process of crystallization differentiation. Subsequently, a salt melt rich in Li, F and water could be separated from such comendite magmas. The detection of fluoride aqueous inclusions in quartz allows us to suppose the further evolution of the salt melt leading to the appearance of a concentrated aqueous fluid and the possible participation of the latter in metasomatic processes.