阿武隈高原の複変成作用 : とくに十字石と藍晶石の発見について

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MIYASHIRO (1958, etc.) has considered that the Abukuma plateau is a late-Mesozoic metamorphic belt of the standard andalusite-sillimanite type. To our opinion, however, the metamorphic history of this belt is never so simple but more complicated. Following after SUGI (1935, 1939), our cooperative study has been made from the stand point of polymetamorphism. Evidences of poly metamorphism disclosed in these three years are: 1) the presence of staurolite in mica schist from Abuki, wherestaurolite is doubly armoured with newly formed andalusite-hercynite simplektite inside and shimmer-aggregate of muscovite and magnetite outside (P1. I-1, 2), and 2) a tiny but convincing kyanite crystal enclosed in oligoclase porphyroblast in two-mica gneiss from Chokoji (Pl. I-3). These are nothing but unstable relics of an older metamorphism of Barrovian type which is essentially different from later superimposing metamorphism of andalusite-sillimanite type. The host rocks are never abnormal but normal pelitic composition. Further andalusite-kyanite-sillimanite (AKS) gneiss was found from the Nishidohira gneiss region. Under the microscope, any two or even three polymorphs of Al_2SiO_5 are seen in side-by-side contact suggesting stable or metastable para-genesis. In conclusion, the metamorphic process leading to the formation of the AKS gneiss might have been in such away as shown by the line I in Fig. 9 which, starting from the field of metastable andalusite, passes through or near the triple point and hardly enters into the sillimanite field. This is the same which, in Abuki as well as Chokoji, produced the aforesaid staurolite and kyanite. Of course the AKS gneiss alone never gives proof of polymetamorphism in the Abukuma belt. The metamorphism II which prevailed throughout the central Abukuma plateau did not give notable effect on the AKS gneiss. Lastly our success in finding the above-mentioned facts could not be achieved without the guidance by URUNO and his pupils in the Miyagi First Girls High School who undertook patient work in heavy mineral investigation of 707 river-sand collections. According to their research, it seems a good deal probable that staurolite and kyanite may distribute more extensively than we have expected from the above data. Then, if it be the case, the kyanite isograd of metamorphism I will be established as k-k line in Figs. 2 and 3.
日本地質学会の論文
1969-01-31

阿武隈高原の複変成作用 : とくに十字石と藍晶石の発見について

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