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中部日本土岐花崗岩体の岩相と化学組成の累帯変化

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

湯口 貴史; 鶴田 忠彦; 西山 忠男*

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

The Toki granite (TKG), a single pluton, is located at the Cretaceous Sanyo belt in the Inner Zone of Southwest Japan. Samples collected from 19 borehole sites in TKG reveal the spatial variation of mineral assemblage, mode, bulk composition and mineral composition, representing a zoned pluton. Based on the petrography, TKG is divided into three rock facies: the muscovite-biotite granite (MBG), the hornblende-biotite granite (HBG), the biotite granite (BG) from the margin to the interior. ASI (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 S-type in the margin to I-type in the interior. Fe$$^{3+}$$/Fe$$^{2+}$$ become gradually higher from MBG through HBG to BG, corresponding to a systematic change from the ilmenite series in the margin to magnetite series in the interior. The sedimentary rocks of the Mino terrane is intruded and metamorphosed to hornfels by the Toki granitic magma. ASI, Na$$_{2}$$O/K$$_{2}$$O and Fe$$^{3+}$$/Fe$$^{2+}$$ of TKG become closer to those of the hornfels towards the margin of the body, indicating the assimilation of the crustal host rock by the Toki granitic magma. Both petrography and bulk chemistry suggest that (1) the chemical variation is not solely due to a fractional crystallization, and (2) assimilation has played a significant role in its margin.

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