四国西部の秩父累帯北部における構造層序  [in Japanese] Tectonostratigraphy of the northern part of the Chichibu Composite Belt, western Shikoku, SW Japan  [in Japanese]

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Abstract

四国西部の秩父累帯については,黒瀬川帯が認められないため,地体構造区分や構造層序に関して以前から種々の議論があった.秩父累帯北部に位置する大洲市南西部地域の構造層序と構成ユニットの帰属を検討し,新たな知見を得た.調査地域は,御荷鉾緑色岩類,古藪ユニットI・II,大洲ユニットI・IIに区分される.古藪ユニットIはチャートや緑色岩,石灰岩などから,古藪ユニットIIはチャートや砂岩,泥岩などから構成される.大洲ユニットは全体的に変成を受けており,泥質岩や砂質岩からおもに構成される.古藪ユニットは,ほぼ水平な断層を介して大洲ユニットに重なると推定される.岩相的特徴,緑色岩や砂岩の岩石学的・地球化学的特徴,泥質岩の変成白雲母K-Ar年代などを考慮すると,古藪ユニットIは南部秩父帯の三宝山ユニットに,古藪ユニットIIは同帯の斗賀野ユニットに対比され,大洲ユニットは白亜紀四万十帯の深部相に相当すると考えられる.

Tectonostratigraphic divisions of the Chichibu Composite Belt of Ozu-Akehama area, western Shikoku, have been controversial because the Kurosegawa Belt is not exposed in the area. We performed detailed geological survey, chemical analyses of greenstone and sandstone, and K-Ar dating of recrystallized white mica to make clear the tectonostratigraphy in the southwestern area of Ozu City.<br>The study area can be divided into the Mikabu Greenstone, Ozu Unit I/II and Furuyabu Unit I/II on the basis of lithofacies. The Ozu Unit I is composed of sandstone and mudstone, and the Unit II is mainly of mudstone with minor chert and greenstone, and the both have been metamorphosd considerably. The Furuyabu Unit I consists of greenstone, chert and limestone, and the Unit I is made up mainly of chert and siliceous mudstone with minor sandstone. The Mikabu Greenstone is demarcated from the Ozu Unit by a high angle fault. Dips of the strata around the boundary between the Ozu and Furuyabu Units are very gentle. Field evidence suggests that the Ozu Unit is overlain structurally by the Furuyabu Unit.<br>Whole-rock geochemistry of greenstones from the Furuyabu Unit I suggests an oceanic island for their origin. Sandstones from the Furuyabu Unit II have chemical affinities with those from the Togano Unit in Shikoku. The Furuyabu Unit I and II can be correlative to the Sambosan Unit and Togano Unit in the Southern Chichibu Belt, respectively. Sandstone geochemistry and K-Ar muscovite ages of metapelite in the Ozu Unit suggest that the unit is equivalent to the Cretaceous Shimanto accretionary complex. From these, it is concluded that metamorphosed Cretaceous Shimanto accretionary complex is exposed in the northern part of the Chichibu Composite Belt and has been overlain structurally by the Sambosan Unit and Togano Unit in ascending order.

Journal

  • The Journal of the Geological Society of Japan

    The Journal of the Geological Society of Japan 114(1), 31-42, 2008-01-15

    The Geological Society of Japan

References:  35

Codes

  • NII Article ID (NAID)
    110006571425
  • NII NACSIS-CAT ID (NCID)
    AN00141768
  • Text Lang
    JPN
  • Article Type
    ART
  • ISSN
    00167630
  • NDL Article ID
    9368031
  • NDL Source Classification
    ZM49(科学技術--地球科学--地質)
  • NDL Call No.
    Z15-174
  • Data Source
    CJP  NDL  NII-ELS  J-STAGE 
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