日本列島の誕生場 : 古太平洋の沈み込み開始を示す飛騨外縁帯の520Maの熱水活動 Juvenile Japan : Hydrothermal Activity of the Hida-Gaien Belt Indicating Initiation of Subduction of Proto-Pacific Plate in ca. 520Ma

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 This paper examines the early stage of the geotectonic history of the Japanese Islands on the basis of finding hydrothermal jadeitite including zircons of <i>ca.</i> 520 Ma in serpentinite mélange of the Itoigawa-Omi area of the Hida-Gaien belt, Central Japan. Hydrothermal jadeitite contains euhedral jadeite in natrolite veins and patches, and consists of jadeite-albite and jadeite-natrolite without quartz. These minerals were crystallized from an aqueous fluid phase at the low-pressure and high-temperature side of the reaction boundary of albite = jadeite + quartz in the system NaAlSiO<sub>4</sub>-SiO<sub>2</sub>-H<sub>2</sub>O. The occurrence of rounded relict hornblende mantled by omphacite rimmed by fine-grained aggregates of jadeite in the matrix of jadeite and albite suggests a pervasive hydrothermal fluid flow, through which metabasite was extensively replaced by jadeitite.<br> This rather high-temperature hydrothermal activity of <i>ca.</i> 520 Ma did not occur in an ordinary subduction zone but in a newly-formed mantle wedge suffering severe hydration from a subducting slab. Recently accumulated U-Pb ages of zircon of <i>ca.</i> 450-500 Ma from paleozoic sediments and granitic rocks of the Hida-Gaien belt were due to initiation of subduction followed by subduction zone magmatism.<br> Protolith of serpentinite in the Hida-Gaien belt includes highly depleted harzburgite, thus requiring tectonic setting of a high-temperature-rift zone rather than a low-temperature-slow spreading ridge. Subduction was initiated at <i>ca.</i> 520 Ma along the boundary between low-density harzburgitic rift zone peridotite and lherzolitic spreading ridge peridotite with a slightly higher density, resulting in the common occurrence of harzburgitic serpentinite in the oldest part of the accretionary complex of Southwest Japan.<br> An area including the Japanese Islands was born around the Yangtze block by the breaking up of the Rodinia supercontinent, because the oldest K-Ar age of biotite actinolit rock of 672 Ma (Matsumoto <i>et al.</i>, 1981) and the subduction initiation of <i>ca.</i> 520 Ma are in accord with the paleogeographic history of the Yangtze block, and because <i>ca.</i> 300 Ma Renge schists of the Hida-Gaien belt did not suffer the <i>ca.</i> 280-200 Ma collision-type metamorphism of the Hida metamorphic belt that is an eastern extension of the suture between the Sino-Korea and Yangtze blocks.

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  • 地學雜誌

    地學雜誌 119(2), 279-293, 2010-04-25

    Tokyo Geographical Society

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各種コード

  • NII論文ID(NAID)
    10030368664
  • NII書誌ID(NCID)
    AN00322536
  • 本文言語コード
    JPN
  • 資料種別
    ART
  • ISSN
    0022135X
  • NDL 記事登録ID
    10704752
  • NDL 雑誌分類
    ZM41(科学技術--地球科学)
  • NDL 請求記号
    Z15-169
  • データ提供元
    CJP書誌  CJP引用  NDL  J-STAGE 
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