Composition and Mechanisms of Formation of Comendite Melts of the Early Mesozoic Bimodal Association of Sant, Central Mongolia

Abstract

Based on the investigation of melt inclusions in minerals using electronic and ion microprobe analysis, we estimated the composition and evolution of the melts involved in the formation of comendites of the bimodal association of Sant’s volcanites (Central Mongolia), the mechanisms of their formation were revealed. Primary melt inclusions and crystalline inclusions coexisting with them were studied in quartz in three samples of comendites selected in different parts of the volcanic section. Sanidin, zircon, and chevkinite were determined among crystalline inclusions. It was found that the crystallization of phenocrysts of comendites was carried out in the temperature range of 880–960 °C. It was determined that the glasses of homogeneous melt inclusions correspond to the compositions of trachydacites and rhyolites. They are also characterized by high concentrations of Zr, Nb, Rb, Y, Th and rare earth elements. At the same time, significant differences in the composition of melt inclusion glasses in the concentrations of Li and volatile components (H2O and F) were revealed. Some of the melts are enriched in these components, whereas other are depleted in them. Analysis of composition of glasses of homogenized melt inclusions in phenocrysts of alkaline salic rocks of bimodal association of Sant allowed to reveal magmatic processes responsible for the formation of comendite melts and leading to the accumulation of rare and rare earth elements in the latter. The dominant role among them is played by the process of magma crystallization differentiation accompanied by the process of liquid immiscibility with participation of fluoride melts.