Mineral chemistry of anorthite megacryst and its inclusions from Ogi peninsula, Sado Island, Niigata Prefecture

Abstract

We investigated anorthite magecrysts (grain size > 10 mm) occurring in mid-Miocene basaits from Ogi Peninsula, Sado Island, Niigata Prefecture, Japan, using elctron probe microanalyzer (EMPA) and scanning electron microscope (SEM). Some mineral and melt inclusions in the anorthite megacrysts were also analyzed in this study. EMP analyses showed that the anorthite megacrysts are chemically homogeneous (An 92 - 94 %) from core to rim and can be distinguished from the other plagioclase phenocrysts (grain size: 0.1 - 0.2 mm, An 79 - 84 %) by chemical composition. In addition, Fe-Cu-Ni sulfide globules and S, Cl-bearing mafic glass inclusions occur within the anorthite megacrysts. Many previous stuides on anorthite megacrysts occurring along the volcanic front (e.g., Kimata et al. 1995) reported that these anorthite megacrysts contains corroded olivine as mineral inclusion. These differences in inclusions suggests that the formation conditions of anorthite megacrysts between the volcanic front and back-arc are different each other.