K-Ar Ages for the Middle Miocene Muro Volcanic Rock, Southwest Japan

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  • UTO Kozo
    Geological Survey of Japan
  • ANNO Ken
    Graduate School of Human and Environmental Studies, Kyoto University
  • SUDO Masafumi
    Division of Earth and Space Science, Graduate School of Science, Kyoto University:(Present) School of Earth Sciences, Faculty of Integrated Human Studies, Kyoto University
  • UCHIUMI Shigeru
    Geological Survey of Japan

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Other Title
  • 西南日本,中期中新世室生火山岩のK-Ar年代
  • セイナン ニホン チュウキ チュウシンセイ ムロウ カザンガン ノ KAr

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Abstract

The Muro Volcanic Rock is a large scale welded pyroclastic flow deposit in the central part of Kii Peninsula, Southwest Japan. It belongs to the Middle Miocene Setouchi Volcanic Rocks and has been considered to have erupted before Southwest Japan made a clockwise rotation. K-Ar dating was performed using two diffrent procedures for biotite separates from two vitrophyres, which are about a few tens meters apart from each other. The conventional isotopic dilution method gives the age of 14.7±0.5 Ma (weighted mean of four ages), while the unspiked method gives the age of 14.44±0.16 Ma (weighted mean of two). These ages are slightly younger than previous dates, and indicate that the clockwise rotation of Southwest Japan occrred after 14.5 Ma.

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