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Peraluminous / metaluminous and ilmenite-series / magnetite-series zoning within a granitic body; Case study of the Toki granitic body, Central Japan

Yuguchi, Takashi; Tsuruta, Tadahiko  ; Nishiyama, Tadao*

The Toki granite, a single pluton, is located in the Cretaceous Sanyo Belt in the Inner Zone of Southwest Japan. 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 (mol Al$$_{2}$$O$$_{3}$$ / (CaO + Na$$_{2}$$O + K$$_{2}$$O)), Al$$_{2}$$O$$_{3}$$, K$$_{2}$$O and Na$$_{2}$$O change systematically from MBG through HBG to BG, corresponding to a systematic variation from peraluminous granite in the margin to metaluminous in the interior. The Fe$$^{3+}$$ / Fe$$^{2+}$$ 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. 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.



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