The Features of the Compositional Evolution of Felsic Rocks in the Low-Potassium Calc-Alkaline Series of the Zavaritskii Volcano, Kurile Arc, Simushir Island

摘要

This article is focused on dacitic pumices, which are the felsic members of the basalt-andesite- dacite series. The phenoscrysts of all of the rocks from this series are the same: plagioclase, olivine, clino- and orthopyroxenes, and titanomagnetite. The groundmass of dacitic pumices that contain microlites of the same minerals and felsic glass has been studied in detail. Quartz and K-Na feldspar are absent. The study of that microlite zoning that formed in the upper parts of the channels or at the surface under the most nonequilibrium conditions was one of the most important tasks; it revealed several interesting features. As an example, anorthite plagioclases were found as microlites. The resorption zones are absent in both plagioclase phenocrysts and microlites, which implies the major role of fractionation rather than magma mixing.