BIOTITE FROM MEGACRYST ASSEMBLAGES IN ALKALI BASALTOIDS OF THE VITIM PLATEAU

Abstract

Ti-biotite megacrysts, glimmerites, and intergrowths of biotite, clinopyroxene, and ilmenite megacrysts were found in Quaternary basanites and Miocene picrobasaltic tuff of the Vitim Plateau. The TiO2 content in the micas reaches 11.1 wt %. Different trends of the biotite composition in megacryst assemblages indicate specific formation conditions of the Quaternary basanite and Miocene picrobasalt. The biotite trend of Quaternary basanite is characterized by enrichment in Fe, Na, Si and depletion in Ti. Data point clusters on plots for biotite and clinopyroxene compositions presumably reflect the sinking of clinopyroxene, biotite, and ilmenite megacrysts in a magmatic chamber; according to P-T calculations, it was located at a depth of 36-40 km. The megacryst formation was related to the supply of evolved magma from a deep-seated source and occurred during the same episode of volcanic activity when the host basanite was produced. The biotite from the picrobasalt crystallized at the final stage of melt fractionation. The biotite enrichment in both Fe and Ti was controlled by clinopyroxene fractionation.