Abstract
Scoriae of Z-To5 tephra in Zao volcano, NE Japan were formed by the mixing of three magmas: basalt with high-Mg (Fo〜81) olv and An-rich plg; basaltic andesite with Mg-rich opx (〜78, Mg#) and cpx (〜78, Mg#), lower-Mg olv (Fo〜78), and An〜85 plg; and andesite with Mg-poor opx (61-66, Mg#) and cpx (64-68, Mg#), and An-poor plg. The basaltic magma was formed by fractionation of Fo〜85 olv from a less differentiated magma during fast ascent from depth. The andesitic shallow magma chamber was heated by underplating of the basaltic magma, and then, the basaltic andesite magma was formed by mixing of the basaltic and andesitic magmas in the chamber. Textural and chemical evidence for the rapid growth of phenocrysts in the basaltic andesite magma suggests that the magma residence time was short.
