Comendite Melts of the Early Mesozoic Bimodal Sant Association (Central Mongolia) and Mechanisms of Their Formation

Abstract

Based on the investigation of melt inclusions using electron and ion microprobe analysis, we estimated the composition, evolution, and formation conditions of magmas producing the the comendites of the Sant bimodal volcanic association (Central Mongolia). The mechanisms of the formation of melts were determined. The primary melt and coexisting crystalline inclusions in quartz from three samples of comendites collected from different parts of the volcanic section were studied. Among the crystalline inclusions, sanidine, zircon, and the REE diortosilicate–chevkinite were identified. The phenocrysts of the comendites were determined to crystallize at temperatures of 880–960°C. The homogeneous glasses of melt inclusions have both trachydacite and rhyolite compositions. They are characterized by high concentrations of Zr, Nb, Rb, Y, Th and REE. Significant differences were determined in concentrations of Li and volatile component (H₂O and F) in the glasses: some of the melts are enriched in these components, whereas other are depleted in them.

Analysis of the composition of the glasses of the homogenized melt inclusions in quartz of comendites from the Sant bimodal association allowed us to recognize magmatic processes responsible for formation of the comendite melts. The dominant role among them belongs to crystallization differentiation of the magma, accompanied by a process of liquid immiscibility with participation of fluoride melts.