四国中央部,三波川帯と四万十帯のテクトニクス境界 Tectonic boundary between the Sanbagawa belt and the Shimanto belt in central Shikoku, Japan

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Abstract

四国中央部大歩危地域に分布する三波川結晶片岩類は，原岩層序に基づき構造的上位から下位に向かい三縄層，小歩危層，川口層に分類されてきた．今回，それぞれの層から火成ジルコンを分離し，LA-ICP-MSを用いてU-Pb年代測定を行った結果，三縄層中のジルコンの多くが1900－1800 Maの火成年代を示した．一方，小歩危層・川口層中のジルコンから得られた火成年代は三縄層よりも若く，その中で最も若い年代は，それぞれ 92±4 Ma，82±11 Ma であった．両層の堆積年代は更に若く，その年代は，四万十帯北帯の付加年代と一致する．このことから，従来，三波川帯と見なされてきた小歩危層，川口層は，四万十帯北帯であることが明らかになった．したがって，三縄層と小歩危層の層境界は，三波川帯／四万十帯境界にあたる．また，大歩危地域における構造的累重関係は，三波川変成帯が四万十帯の構造的上位に位置することを示している．<br>

In order to make it clear the mode of occurrence of the Sanbagawa belt, we carried out in situ U-Pb isotope analyses of igneous zircon grains from the Oboke area that was a type area of the Sanbagawa belt in central Shikoku, Japan. Analyzed igneous zircons were separated from psammitic schist in the Minawa and Kawaguchi Formations and from igneous cobbles in the Koboke Formation. Spot analyses were performed on the laser ablation-inductively coupled plasma mass spectrometer (LA-ICP-MS). The youngest U-Pb ages of zircon grains from the Koboke Formation and the Kawaguchi Formation showed 92±4 Ma and 82±11 Ma, respectively. On the other hand, zircons from the Minawa Formation yielded remarkably older ages clustered around 1900-1800 Ma. There is a large chronological gap between protolith sedimentary clasts of the Minawa and those of the other two formations. The protolith sedimentary ages of the Sanbagawa belt have been well constrained as older than 130 Ma based on fossil and U-Pb isotopic ages. The peak metamorphism occurred in 120-110 Ma. Therefore, both Koboke and Kawaguchi Formations must not belong to the Sanbagawa belt, because the timing of formation of accretionary complex must be later than 92±4 Ma for the Koboke Formation and 82±11 Ma for the Kawaguchi Formation. Both the Koboke and Kawaguchi Formations correspond to the late Cretaceous accretionary complex, and they are equivalent to the Northern Shimanto belt. The tectonic boundary between the Sanbagawa and the Northern Shimanto belts is reverse fault and the Northern Shimanto belt appears as a tectonic window in the Sanbagawa belt, central Shikoku. The whole package of the Sanbagawa and underlying Shimanto belts are deformed by the secondary fault movement and doming after the tectonic juxtaposition at the mid-crustal levels.<br>

Journal

  • The Journal of the Geological Society of Japan

    The Journal of the Geological Society of Japan 113(5), 171-183, 2007-05-15

    The Geological Society of Japan

References:  48

Cited by:  10

Codes

  • NII Article ID (NAID)
    110006279282
  • NII NACSIS-CAT ID (NCID)
    AN00141768
  • Text Lang
    ENG
  • Article Type
    Journal Article
  • ISSN
    00167630
  • NDL Article ID
    8841057
  • NDL Source Classification
    ZM49(科学技術--地球科学--地質)
  • NDL Call No.
    Z15-174
  • DOI
    10.5575/geosoc.113.171
  • Data Source
    CJP  CJPref  NDL  NII-ELS  J-STAGE 
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