Neogene volcanic rocks of the eastern part of the Koma Mountain, Yamanashi Prefecture

Abstract

The Kushigatayama Subgroup, Kurosawa basalt-mudstone and Tsukimibashi andesite of the Nishiyatsushiro Group of middle Miocene are distributed in the eastern part of the Koma mountain and are composed of basaltic and andesitic lavas and pyroclastic rocks and small amounts of mudstones and acid tuffs. Most of the volcanic rocks were altered to low-grademetamorphic rocks ranging from laumontite to prehnite-pumpellyite facies.
Volcanic rocks of the Kushigatayama Subgroup are composed of olivine basalt, diopsideolivine basalt, diopside-augite basalt, augite andesite, hypersthene-augite andesite and augitehornblende andesite. Almost of olivine phenocrysts are altered to saponite or chlorite/saponite mixed layered mineral. Phenocryst clinopyroxenes are diopside, endiopside, salite and augite and groundmass clinopyroxenes range augite through subcalcic augite to pigeonite. Mafic phenocryst content is 38.8 percent in maximum. Abnormal phenocryst content results from the presence ofmegacryst or cumulate.
Most of volcanic rocks may belong to tholeiite series. Basalts having FeO^*/MgO ratios(0.91-1.00) are very close to the primitive basalt in composition, although some basalts include cumulates. Differentiation from olivine basalt to andesite is mainly interpreted by fractionation of olivine, clinopyroxene and plagioclase.
From whole rock chemical compositions, P₂O₅/MnO/TiO₂ ratios in Mullen's diagram and chondrite-normalized REE concentration, basalts of this area as well as those of the Misaka and Tanzawa areas are obvious to be island arc tholeiite.