Alteration type and thermoluminescence ages of altered rocks in the Mutsu-Hiuchidake volcanic field, Shimokita peninsula, NE Japan

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  • TOMIYAMA Shingo
    Mitsubishi Materials Natural Resources Development Corporation
  • UMEDA Koji
    Geological Isolation Research and Development Directorate, Japan Atomic Energy Agency
  • HANAMURO Takahiro
    Geological Isolation Research and Development Directorate, Japan Atomic Energy Agency
  • TAKASHIMA Isao
    Faculty of Engineering and resource Science, Akita University
  • HAYASHI Shintaro
    Faculty of Education and Human Studies, Akita University
  • NEGISHI Yoshimitu
    Mitsubishi Materials Natural Resources Development Corporation
  • MASUDOME Yukiko
    Mitsubishi Materials Natural Resources Development Corporation

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Other Title
  • 下北半島，むつ燧岳火山地域の変質帯と変質岩の熱ルミネッセンス年代
  • シモキタ ハントウ ムツ ヒウチダケ カザン チイキ ノ ヘンシツタイ ト ヘンシツガン ノ ネツ ルミネッセンス ネンダイ

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Mutsu-Hiuchidake Volcano has an erosion caldera which shows a horseshoe-like geomorphological feature toward east. Tertiary strata as a basement are distributed in the caldera. Tertialy strata and a part of pyroclastic deposits of the Mutsu-Hiuchidake Volcano have altered strongly to moderately by hydrothermal activities related to the volcanism. This study is to clarify a progress history of the alteration by using a geological mapping, thermoluminescence (TL) dating, x-ray diffraction analysis, an infrared reflection absorption analysis and a fluid inclusion study.<br>      Highly altered zone is recognized in the area of midstream to upstream along the Ohakagawa and the Koakagawa within erosion caldera. The argillic alteration zone surround a silicification zone in the highly altered area shows a circular distribution. The strongly altered areas are along NNW-SSE to NNE-SSW fractures. The alteration areas were divided into the smectite, kaolinite, alunite and pyrophyllite zones.<br>      The kaolinite and alunite zones give the TL ages of quartz 67 ± 13 ka (KG-5), 88 ± 18 ka (OG-4) and 91 ± 23 ka (OG-1). The smectite zone within the argillic alteration zone of outside of collapse caldera, yield the ages 752 ± 215 ka (SO-2) and 615 ± 197 ka (KG-1). These TL ages suggest the hydrothermal activity end at 70 to 90 ka.<br>      The existence of pyrophyllite suggests that hydrothermal temperatures were 200 to 250 °C in these area. This is supported from the homogenization temperatures of fluid inclusions in calcite, 242 °C in average.

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