金峰山花崗岩体の地質と貫入定置メカニズム

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タイトル別名
  • Geology and Emplacement Mechanism of the Kinpusan Granitic Pluton, Central Japan
  • キンプサン カコウガンタイ ノ チシツ ト カンニュウテイチ メカニズム

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説明

The Kinpusan granitic pluton is a member of the middle Miocene Kofu granitic complex in the South Fossa Magna region, central Japan. The Kinpusan granitic pluton consists of three plutons: the Kinpusan north, Kinpusan south and Shosenkyo, which comprise the ilmenite-series monzo-granite. The granitic rocks of the Kinpusan north pluton are por-phyritic and those of the Kinpusan south pluton are equigranular. The grain size of groundmass quartz of porphyritic granite shows altitudinal variation and decrease from 0.6 to 1.1mm in the lower horizon below 1500m in altitude to 0.1 to 0.5mm in the upper horizon above1600m in altitude. The graphic intergrowth of quartz and K-feldspar is predominant in the area above1500m in altitude, and microcline K-feldspar is characteristic in the horizon below 1500m in altitude. The granitic rocks of the Kinpusan pluton comprise quartz, plagioclase, K-feldspar and biotite. There is no altitudinal variation in the modal composition and whole-rock chemistry of the granitic rocks, the SiO2 content of which is about 75 to 78wt.% and corresponds to high silica rhyolite in composition. The aplite consists of sheets and dikes in addition to small amount of massive one, whose width is less than 30cm in general. The aplite sheets are concentrated to the horizon between 1600 and 2200m in altitude. The miarolitic cavities are conspicuous above1600m in altitude. Abundant felsic magmatic inclu-sions and small amount of mafic magmatic inclusions are included in the granitic pluton. The phenocryst contents of fel-sic magmatic inclusion are 9 to 39 vol.%, which comprise quartz, plagioclase, K-feldspar and biotite. Their whole-rock SiO2 contents are 74 to 77 wt.%, and they are more abundant in TiO2, Al2O3, FeO*, MnO, MgO, CaO and P2O5 than host granitic rocks; the magma of felsic magmatic inclusions are less differentiated than that of the host granitic rocks. The magma of felsic magmatic inclusion may be higher in temperature and less hydrous than that of the host granite; they are chilled and solidified when both magmas were encountered. The felsic magmatic inclusions are concentrated to form a layer with a thickness of 100 to 400m; the number of such layers including those without inclusions is six in the horizon above 1500m in altitude. The intermittent incremental emplacement by the subsidence of large blocks or underground cauldron explains the construction of the layering.

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  • 研究紀要

    研究紀要 56 39-68, 2021-03-05

    日本大学文理学部自然科学研究所

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