The latest Early Cretaceous detrital zircons from clastic rocks in the Koshibu-gawa area of the central Akaishi Mountains

  • Shimura Yusuke
    Department of Earth and Planetary Sciences, Graduate School of Environmental Studies, Nagoya University
  • Nakamura Yoshihiro
    Research Institute of Geology and Geoinformation, Geological Survey of Japan, AIST
  • Tokiwa Tetsuya
    Department of Geology, Faculty of Science, Shinshu University
  • Sugimoto Taishi
    Department of Geology, Graduate School of Science and Technology, Shinshu University Present Address: CTI Engineering Co., Ltd.
  • Mito Soya
    Department of Geology, Graduate School of Science and Technology, Shinshu University Present Address: INPEX Co.

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Other Title
  • 赤石山地中央部の小渋川地域に分布する陸源砕屑岩から得られた前期白亜紀最末期の砕屑性ジルコン
  • アカイシ サンチ チュウオウブ ノ ショウシブカワ チイキ ニ ブンプ スル リク ゲンサイクズガン カラ エラレタ ゼンキ ハクアキ サイマッキ ノ サイクズセイ ジルコン

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<p>We conducted U-Pb dating of detrital zircons from three clastic rock samples, which have been classified into the Chichibu Belt in the Koshibu-gawa area, central Akaishi Mountains. U-Pb ages of the samples fall into two main age groups at 280-100 Ma and 2600-1700 Ma. The youngest single grain ages are 105.5±1.3 Ma, 109.6±1.2 Ma, and 111.1±1.9 Ma, and the youngest cluster ages are 115.9±1.0 Ma, 115.1±0.4 Ma, and 113.1±1.3 Ma. These Aptian to Albian (latest Early Cretaceous) ages are considerably younger than previously reported youngest ages from the Jurassic accretionary complex of the Chichibu Belt in the southern part of the Koshibu-gawa area (Okuchausu-yama area). Our results indicate that the geological bodies, including clastic rocks studied in the Koshibu-gawa area, should be assigned to the Cretaceous accretionary complex of the Shimanto Belt, and not to the Jurassic accretionary complex of the Chichibu Belt. Reexamination of the structural evolution and geological architecture in the Koshibu-gawa area is thus warranted.</p>

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