山口県東部,柳井地域に産する領家帯蒲野花崗閃緑岩のマグマ過程  [in Japanese] Magma process of Gamano granodiorite in Ryoke belt, Yanai region, Yamaguchi, Southwest Japan  [in Japanese]

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Abstract

<p>山口県東部,屋代島西部と室津半島には,領家帯の高変成度岩と変成岩の構造と調和的な蒲野花崗閃緑岩が分布する.蒲野花崗閃緑岩は鉱物組み合わせから,黒雲母花崗閃緑岩,ザクロ石花崗閃緑岩,角閃石花崗閃緑岩そして黒雲母花崗岩に区分される.黒雲母花崗閃緑岩は広域に分布するのに対し,ザクロ石花崗閃緑岩と角閃石花崗閃緑岩は泥質片麻岩と同時期に貫入した閃緑岩の周囲に分布する.鉱物・全岩化学組成の特徴は,蒲野花崗閃緑岩の岩相変化が黒雲母花崗閃緑岩マグマと泥質片麻岩との同化作用や閃緑岩マグマとの混合・混交作用によって生じたことを示す.一方,黒雲母花崗岩は,黒雲母花崗閃緑岩から分化したマグマに由来した可能性が高い.</p>

<p>Magma processes associated with plutonic rocks are known to involve fractional crystallization, assimilation with crustal materials, and magma mixing and/or mingling with coeval intrusive rocks. We studied such processes in the Yanai region, in the western part of Yashiro-jima Island and the Murotsu Peninsula (Yamaguchi Prefecture). In this area, the Gamano granodiorite and high-grade rocks of the Ryoke metamorphic belt are exposed. Field relations and geochronological data indicate that the emplacement of the Gamano granodiorite occurred simultaneously with peak metamorphism. We identified four lithologies in the Gamano granodiorite (biotite granodiorite, garnet granodiorite, hornblende granodiorite, and biotite granite). The main facies is the biotite granodiorite, whereas garnet granodiorite and hornblende granodiorite occur around the pelitic gneiss and coeval diorite, respectively. The garnet granodiorite contains zoned garnet crystals, characterized by an almandine-rich core, possibly derived from pelitic gneiss xenocrysts, and a spessartine-rich rim, which was crystallized from the granitic magma. Mixing and/or mingling microscopic textures are recognized in the hornblende granodiorite. The initial values of Sr-Nd isotopic ratios support the idea that the garnet granodiorite and hornblende granodiorite interacted with the pelitic gneiss and the diorite, respectively. The biotite granite, which is the fourth lithology in the Gamano granodiorite, is recognized as small stocks intruding into the biotite granodiorite. Geochemical variation diagrams show overlapping trends of the biotite granite and biotite granodiorite. In addition, these two lithologies show similar isotopic compositions. Based on petrological considerations, it is suggested that the garnet granodiorite was produced by assimilation of the biotite granodiorite magma with the host pelitic gneiss. In contrast, the hornblende granodiorite was likely produced by magma mixing and/or mingling between the biotite granodiorite and the diorite magmas. The biotite granite likely represents an evolved magma derived from the biotite granodiorite.</p>

Journal

  • The Journal of the Geological Society of Japan

    The Journal of the Geological Society of Japan 125(2), 167-182, 2019

    The Geological Society of Japan

Codes

  • NII Article ID (NAID)
    130007660915
  • NII NACSIS-CAT ID (NCID)
    AN00141768
  • Text Lang
    JPN
  • ISSN
    0016-7630
  • NDL Article ID
    029547400
  • NDL Call No.
    Z15-174
  • Data Source
    NDL  J-STAGE 
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