Minerl chemistry and sr isotope composition of anorthite megacrysts from Kyushu-Ryukyu arcs

Abstract

Anorthite megacrysts occur in many of volcanic products in Japanese volcanic arcs. They are recognized as disequilibrium minerals in host basalt and andesite in each volcano. This study focused on the mineral chemistry and Sr isotope composition of anorthite megacrysts in volcanic rocks in the Kyushu-Ryukyu volcanic arc where tholeiitic and calc-alkaline rocks coexist. The lattice constant of anorthite megacrysts are identified to be C=7A type, and their An mol. % are around 92 %. Measured Sr isotope ratios show that the ratios are slightly different between megacryst and host basalt and andesite, but that the ratios of megacrysts and host rocks parallelly increase from north to south (Aso volcano to Akusekijima volcano). We will try to compare the ratios in the Aso and Aira volcanoes where Sr isotope data of various volcanic rocks with long time span are already reported.