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Low-grade Progressive Metamorphism of Pelitic Schists of the Sazare area, Sanbagawa Metamorphic Terrain in central Sikoku, Japan |
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| Authors: | KURATA HISAYUKI; BANNO SHOHEI |
| Institution: | Department of Earth Sciences, Kanazawa University Kanazawa, 920, Japan 1 Present address: Tokushima Plant, Nippon Denko Co.,Ltd., Tokushima, Japan. 2 On leave at the Department of Geology, University of Manchester Manchester, M13 9PL, England |
| Abstract: | The Sanbagawa metamorphic terrain in the Sazare area, centralSikoku, is divided into three zones of progressive metamorphism,A, B, and C, on the basis of mineral assemblages of peliticschists. The characteristic mineral assemblage of zone A isphengite+chlorite and that of zone C is phengite+chlorite+garnetwith biotite possibly at a higher-grade part. Zone B is transitionalbetween A and C. Variation of the chemistry of garnet rim andchlorite with increasing grade is conspicuous: the MnO and FeOcontents of chlorite decrease and the FeO content of garnetrims increase with grade. The Fe-Mg partition coefficient forthe chlorite—garnet rim pair also changes systematicallywith the grade defined by the assemblage suggesting that theprogressive metamorphism is primarily due to temperature increase.The difference in assemblages between the Barrovian biotitezone and zone C of the present area is due either to the commonoccurrence of clinozoisite in the latter or to the differentparagenetic relations induced by higher-pressure of equilibriumin the Sanbagawa belt as compared with the Scottish one. The thermal structure of the present area revealed by zonalmapping requires the presence of a large-scale overturned structure,which postdated the major mineral formation. |
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