K–Ar and 40Ar/39Ar phengite ages of Sanbagawa schist clasts from the Kuma Group, central Shikoku, southwest Japan |
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Authors: | Nguyen D Nuong Tetsumaru Itaya Hironobu Hyodo and Kazumi Yokoyama |
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Institution: | Research Institute of Natural Sciences, Okayama University of Science, Okayama 700-0005, Japan (email: ), and;Department of Geology, National Science Museum, Hyakunin-cho 3-23-1, Tokyo 169-0073, Japan |
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Abstract: | Conglomerates of the Kuma Group, central Shikoku, southwest Japan contain Sanbagawa schist clasts with a variety of metamorphic grades and lithologies. K–Ar and 40Ar/39Ar dating of phengite show all the pelitic schist clasts from low- to high-grade zones have similar phengite ages (82–84 Ma) that are significantly older than those from the in situ Sanbagawa sequence of central Shikoku. This is because the Kuma–Sanbagawa sequence was exhumed earlier than the in situ Asemi sequence with an exhumation process intermediate between those for the Kanto Mountains and the in situ Asemi sequences. 40A/39Ar plateau ages (103 and 117 Ma) of phengite in amphibolites indicate the timing of the early stage of the exhumation of the metamorphic pile, probably close to the peak metamorphic age. |
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Keywords: | 40Ar/39Ar age argon closure system exhumation tectonics K–Ar age Kuma Group phengite Sanbagawa schist clasts |
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