大崩山花崗閃緑岩体の鹿川地域における花崗岩とその中に共生する黒雲母と角閃石を化学分析し考察をおこなった。
この地域の花崗閃緑岩の化学組成はSiO₂量において10%に及ぶ変化を示し,この変化はマグマの結晶分化作用によって生じたものである。
花崗閃緑岩中の黒雲母と角閃石ともAlが少なく, Mg/Fe⁺²比が小さい。MgとFe⁺²の平衡定数KD^Bi-Hor=0.75である。黒雲母,角閃石を領家,北上の花崗岩質岩類中のものと比較すると,その化学組成は,火山-深成岩複合火成作用によってできた花崗岩質岩類中のものに近い。

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New chemical analyses of 36 rocks and 24 biotites from Iwaizumi and Otanabe granitic bodies, Cretaceous intrusives in Kitakami mountains, were presented.
Otanbe and Iwaizumi granitic rocks are of different characters in respect of field observations, petrochemistry and mineral chemistry, that is, as for the former rocks, grain size is medium and generally finer than the latter rocks; rock facies changes distinctly; pinkish K-feldspar is contained; place of intrusion was shallow; volcanics are closely associated; variation trend in SiO₂-oxides diagram is different from those of the latter rocks; K₂O and Na₂O in rocks are higher and lower than those of Japanese average granitic rocks, respectively; TiO₂ in biotite is high and Al₂O₃ in biotite and hornblende is low. These evidences suggest that Otanabe rocks are not co-magmatic with Iwaizumi ones. Genetically, therefore, Otanabe body must be treated as separate one from Iwaizumi body, while both have been considered as the same origin by some investigators.
Extraordinarily low K₂O content of Otomo type rocks, composing southern part of Iwaizumi body, is probably originated from elimination of biotite and other mafic minerals from magma due to accumulation, besides assimilation.

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The area surveyed was divided into six zones, Zone I to Zone VI, based on mineral assemblages of pelitic rocks. Zone boundaries were determined by the first appearance of biotite, andesine, cordierite or andalusite, sillimanite and almandine, respectively. Chemical compositions of minerals have been analyzed by means of electron probe microanalyzer.
TiO₂ and Al₂O₃ contents of biotite increase with increasing grade. Al₂O₃ content of chlorite is lower in Zone II than in Zone III. Detrital white mica rich in celadonite component is common in Zones I and II. It is absent above Zone III. Muscovite from most of Zone III and Zone IV are similar in chemical composition. Muscovite in Zone V is slightly poorer in FeO and MgO contents as compared with lower-grade muscovite. Albite coexists with andesine (An_30-34) or oligoclase in Zone III. K₂O content in plagioclase increases with increasing metamorphic temperature. Ab % of K-feldspar remarkably increases from Zone IV to Zone VI. MnO content in ilmenite increases from Zone II to Zone VI.
Partition coefficients of Fe and Mg between biotite, chlorite, and muscovite do not change with grade. Partition coefficient of Ab component between K-feldspar and plagioclase change with grade. According to Stormer's two-feldspar geothermometer, it is 500°C and 550_??_600°C at the sillimanite isograd and slightly above the almandine isograd, respectively, if pressure is about 4000 bars.
Most of biotite are produced from the reaction of white mica, chlorite and muscovite near the andesine isograd. However, formation of biotite occurs above biotite isograd and below medium temperature part of Zone III. Cordierite was formed by the reaction with biotite, sillimanite and quartz in Zones V and VI. The reaction of muscovite plus quartz mainly took place slightly above the sillimanite isograd. K-feldspar and sillimanite coexist in almost all pelites in Zone V. Association of almandine and K-feldspar occurs only in Zone VI.
Zoned plagioclases are common in Zones V and VI. The rims are poor in K₂O content as compared with cores of plagioclases. Zoned plagioclases were produced during retrogressive metamorphism. Many muscovite in Zones V and VI have tiny inclusions of sillimanite and graphite, suggesting that these muscovite were formed during retrogressive metamorphism.
The petrological results were compared with available experimental investigations in order to estimate geothermal gradient of the Ryoke metamorphism. The geothermal gradient estimated is about 6.5 bar/deg under the condition of P_H2O_??_1/4 P_Total.

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