Metamorphism and timing of the nappe movement in the Asaji metamorphic area, eastern Kyushu

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  • Horie Kenji
    Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology
  • Fujii Masahiro
    Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University
  • Hayasaka Yasutaka
    Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University

Bibliographic Information

Other Title
  • 九州東部，朝地変成岩地域の変成作用とナップ運動の時期
  • キュウシュウ トウブ チョウチ ヘンセイガン チイキ ノ ヘンセイ サヨウ ト ナップ ウンドウ ノ ジキ
Published
2008-03-15
Resource Type
journal article
DOI
  • 10.5575/geosoc.114.127
Publisher
The Geological Society of Japan

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Description

The pre-Upper Cretaceous basement rocks distributed in the Notsuharu area, eastern Kyushu, are classified structurally from bottom to top into the Chokai Unit, the Ultramafic Unit and the Hikata Unit. The Chokai Unit known as the Asaji metamorphic rocks consists mainly of pelitic, psammitic and mafic rocks, and chert. Metamorphic zonation and the chemical compositions of metamorphic garnet and biotite indicate that the metamorphism of the Chokai Unit is due to the thermal effects of the syntectonic intrusion of the Nioki granite. Metamorphic conditions are estimated to be approximately 610°C at 2 kbar. SHRIMP Zircon U-Pb dating of the Nioki granite shows a concordia age of 134.7 ± 2.8 Ma (2σ). The uppermost Hikata Unit consists of non-metamorphic sedimentary rocks, and forms a southeastern vergenced nappe structure (Hikata nappe). The Hikata nappe truncates the thermal structure within the Chokai Unit, and is unconformably overlain by the Upper Cretaceous Onogawa Group. Therefore, the movement of the Hikata nappe can be limited to the period between the intrusion of the Nioki granite (ca. 135 Ma) and initial sedimentation of the Onogawa Group (ca. 100 Ma).

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