四国西部, 幡多半島東岸の第三系四万十累層群に含まれる火砕岩の産状・岩石記載とその意義  [in Japanese] Occurrence and petrography of volcaniclastic rocks in the Tertiary Shimanto Supergroup along the east coast of the Hata Peninsula, southwestern Shikoku, Southwest Japan, and its significance  [in Japanese]

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Abstract

四国西部,幡多半島東岸地域の後期漸新世~前期中新世の清水層・在岬層の混在相は,火山礫岩や火山性砂岩,含火山岩礫泥岩などの火砕岩を含む.火砕岩は,中期始新世の石灰岩を伴うことがある.火砕岩の多くは,土石流や乱泥流などの堆積物重力流に由来する.火砕岩中の火山岩は,アルカリ岩系の火山岩であり,アルカリ玄武岩,粗面安山岩,粗面岩およびコメンダイトからなる.アルカリ玄武岩や粗面安山岩には緑泥石や方解石で充填された発泡痕を有する.アルカリ玄武岩にはカンラン石仮像が含まれる.火山岩類はかなりの程度の変質作用を被っており,ほとんどの苦鉄質鉱物は,緑泥石などの粘土鉱物やFe-Ti酸化鉱物で置換されている.変質過程での移動が少ないと考えられる元素を使ったいくつかの判別図から,火山岩は海洋島起源のプレート内アルカリ岩と推定される.後期漸新世~前期中新世のある時期に四国西部の収束域で海山の衝突があったと考えられる.<br>

Volcaniclastic rocks such as volcanic breccia, volcanic conglomerate, volcanic sandstone, and mudstone with volcanic rock gravels are found in small bodies scattered sporadically in the Shimanto Supergroup in the east coast area of the Hata Peninsula, western Shikoku, SW Japan. The volcaniclastic rocks are contained in chaotic facies made up mainly of mudstone and sandstone of the Late Oligocene-Early Miocene (Burdigalian) Shimizu and Arimisaki Formations, and are occasionally accompanied by Middle Eocene limestone blocks. Most of the volcaniclastic rocks were originally deposited from sediment gravity flows such as debris flow and turbidity flow, although, the mechanism of their emplacement in the chaotic facies still remains unsolved. Volcanic rocks are composed of alkali basalt, trachyandesite, trachyte and comendite, which are all classified into alkaline volcanic rock series. Amygdules filled with chlorite and calcite are abundant in alkali basalt and trachyandesite, indicating that they were formed at a shallow depth. Most volcanic rocks have been subjected to some degree of alteration. Pseudomorphs of olivine are commonly found in the alkali basalts and all mafic minerals broke down to mixture of chlorite and Fe-Ti oxide minerals. The alkali basalts have geochemical affinity to within-plate basalts of seamount origin judging from some discrimination diagrams using alteration-resistant elements. All observations suggest that collision of a seamount would have occurred during the Late Oligocene-Early Miocene at the convergent margin of western Shikoku. And the fragments of it incorporated into the accretionary prism.<br>

Journal

  • The Journal of the Geological Society of Japan

    The Journal of the Geological Society of Japan 112(2), 107-121, 2006-02-15

    The Geological Society of Japan

References:  31

Codes

  • NII Article ID (NAID)
    110004078731
  • NII NACSIS-CAT ID (NCID)
    AN00141768
  • Text Lang
    JPN
  • Article Type
    ART
  • ISSN
    00167630
  • NDL Article ID
    7852652
  • 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  IR  J-STAGE 
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