Zoning of rock facies and chemical composition in the Toki granitic body, Central Japan

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  • YUGUCHI Takashi
    Mizunami Underground Research Laboratory, Geological Isolation Research and Development Directorate, Japan Atomic Energy Agency
  • TSURUTA Tadahiko
    Mizunami Underground Research Laboratory, Geological Isolation Research and Development Directorate, Japan Atomic Energy Agency Department of Earth and Environmental Sciences, School of Science, Graduate School of Science and Technology, Kumamoto University
  • NISHIYAMA Tadao
    Department of Earth and Environmental Sciences, School of Science, Graduate School of Science and Technology, Kumamoto University

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Other Title
  • 中部日本土岐花崗岩体の岩相と化学組成の累帯変化
  • チュウブ ニホン トキ カコウガンタイ ノ ガンソウ ト カガク ソセイ ノ ルイタイ ヘンカ

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  The Toki granite, a single pluton, is located in the Cretaceous Sanyo Belt in the Inner Zone of Southwest Japan. 483 samples collected from 19 borehole sites in the Toki granite display spatial variations in mineral assemblage, mode, bulk chemical composition and mineral composition, indicative of a zoned pluton. Based on the mode and the mineral assemblages with or without hornblende, the Toki granite can be divided into three rock facies from the margin to the interior: muscovite-biotite granite (MBG), hornblende-biotite granite (HBG) and biotite granite (BG). The ASI values [molar Al2O3/(CaO+Na2O+K2O)], Al2O3, K2O and Na2O change systematically from MBG through HBG to BG, corresponding to a systematic variation from peraluminous in the margin to metaluminous granite in the interior. Fe3+/Fe2+ ratios gradually increase from MBG through HBG to BG, corresponding to a systematic change from ilmenite-series in the margin to magnetite-series in the interior. The sedimentary rocks of the Mino terrane (Kamiasou unit) were intruded and metamorphosed to hornfels by the Toki granitic magma. Na2O/K2O and Fe3+/Fe2+ ratios become closer to those of the hornfels towards the margin of the body, suggesting the assimilation of the crustal host rock by the Toki granitic magma. However, the same ranges in SiO2 among three rock facies leave the possibility that each rock facies derived from a different magma. Both petrography and bulk chemistry imply that 1) the chemical variation of the Toki granite is not solely due to fractional crystallization, and 2) assimilation of crustal host rocks or simultaneous intrusion of different magmas has played a significant role during the emplacement process.<br>

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