Phenocrystic pigeonite from Senganmori, Iino-machi, Fukushima Prefecture

Abstract

In Senganmori, southeast of Fukushima City, is found volcanic neck which was intruded into the basement consisting of granites, during the period of sedimentation of the Ryozen formation (middle Miocene). Lithologically these volcanic rocks are defined as compact andesitic basalts having scarce phenocrysts. The phenocrysts are diopside, rhombic pyroxene, pigeonite, bytownite, and olivine replaced by cholorite. The groundmass consists of pigeonite, augite, bytownite to labradorite, and brown glass. The mode of occurrence of pigeonite phenocrysts shows the following four types: (I) occurrng as single crystals, (2) forming rims around diopside phenocrysts and intergrowing with the latter, (3) inverted from rhombic pyroxene and (4) surrounding augite and in the intergrowth relation with the latter. The chemical composition of pyroxenes, obtained through measurements of their optical properties, are plotted in Fig. 5. It is understood from the figure that the crystallization course of augite from Senganmori roughly coincides with that of augite in the Skaergaard intrusion and that the augite crystallized out in the stage generally corresponding to pigeonite.