Coexisting 3T and 2M1 megacrysts of titanium oxybiotite in Shavaryn-Tsaram basalt (Mongolia)

Abstract

The Quaternary Shavaryn-Tsaram volcano (central Mongolia) is one of the richest known repositories of mantle nodules and megacrystals. Magnesial biotite, rhombic pyroxene, pyrope, sanidine and ilmenite have been found in the megacrysts. The first of the mica specimens investigated by X-ray diffractometry contained a rather uncommon type of biotite. A megacryst of 3Y trioctahedral biotite that we found in the Shavaryn-Tsaram basanite is interesting not only as a find of an association of two polytypes of similar composition, but also for our understanding of the origin of basanite and megacrysts and the state of the mantle material in the magma-generation zone. Our data indicate unambiguously that two trioctahedral modifications of high-titanium magnesian oxybiotite, namely 2M, and 3T, occur in the basanitic Shavaryn-Tsaram volcano. The two modifications are similar in composition and in unit cell volume (calculated for one layer).