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On the oxidized and reduced granites found in quarries from Okayama city, Southwest Japan (Ishihara et al.) Bulletin of the Geological Survey of Japan, vol.56 (1/2), p. 1-8 , 2005 On the oxidized and reduced granites found in quarries of Okayama city, Southwest Japan Shunso Ishihara, Shin-ichi Yoshikura, Shigeo Horikawa, Masatsugu Ogasawara, Itaru Nishio and Shigeru Terashima (2005) On the oxidized and reduced granites found in quarries from Okayama city, Southwest Japan. Bull. Geol. Surv. Japan, vol. 56 (1/2), 1-8, 6 figs, 2 tables.Abstract: Two different granites in their oxygen fugacity during the formation, are seen in the drill cores from the northwestern part of Okayama City. One is coarse-grained granite having no or little rock-forming magnetite, while the other is fine-grained aplitic granite occurring in sheet-like form and containing coarse-grained magnetite in miarolitic aggregates of K-feldspar and quartz. The major and trace elements chemistry indicates that the aplitic granite is crystallized from fractionated melts of the coarse-grained granitic magma. It is suggested that the fractionated magma became rich in H2O which dissociated into O2 and H2 and the hydrogen diffused out to the roof rocks; then the oxygen fugacity was increased to form magnetite. Löllingite was discovered along cracks of the coarse-grained granite at one quarry, reflecting a low sulfur fugacity of the post-magmatic hydrothermal activity of these granites.

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