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Reactions Leading to the Disappearance of Pumpellyite in Low-grade Metamorphic Rocks of the Sanbagawa Metamorphic Belt in Central Shikoku, Japan 总被引：2，自引：0，他引：2
The major mineral assemblages of the metabasites of the Omoiji-Nagasawaarea in central Shikoku are hematite+epidote+chlorite+actinolite,riebeckitic actinolite+epidote+chlorite, epidote+chlorite+actinolite,and pumpellyite+epidote+chlorite+actinolite. The constituentminerals are often heterogeneous and assemblages in the fieldof a thin section sometimes do not obey the phase rule, butif grains apparently in non-equilibrium with others are excludedand domains of chemical equilibrium are appropriately chosenthe assemblages approximately obey the phase rule. The stability of hematite, pumpellyite, and epidote associatedwith chlorite and actinolite can be dealt with in terms of aternary system with appropriate excess phases. By fixing theFe2+/(Fe2+ +Mg) ratio of chlorite, it is dealt with in termsof stability relations in the system Ca2Al3Si3O12(OH)–Ca2AlFe2Si3O12(OH)with excess chlorite, actinolite, quartz, and controlled PH2O.The maximum and minimum Fe3+ contents of epidote in this modelsystem are determined by hematite+epidote+chlorite+actinoliteand pumpellyite+epidote+chlorite+actinolite assemblages. Themaximum Fe3+ of the three phase assemblage epidote+chlorite+actinoliteis insensitive to temperature, but the minimum Fe3+ contentof epidote is sensitive to temperature and can be used to definethe metamorphic grade by a continuous quantity related to temperature.The phase relations expected for the model system are in goodagreement with the parageneses of the Sanbagawa terrain in centralShikoku and offer an explanation to the rule of Miyashiro &Seki (1958a) that the compositional range of epidote enlargeswith increasing temperature. The model also makes it possibleto estimate semi-quantitatively the temperature range in whichthe assemblage pumpellyite+epidote+chlorite+actinolite is stable.The possible maximum range is about 120 ?C, but the assemblageis stable in metabasite only for about 90 ?C. The higher temperaturelimit of the pumpellyite-actinolite facies defined by the disappearanceof pumpellyite in metabasite corresponds to the temperatureat which epidote with Fe3+/(Fe3+ +Al) = 0.10 0.15 coexistswith pumpellyite, actinolite, and chlorite. The compositions of epidotes in the metabasites of the Omoiji-Nagasawaarea cluster around Fe3+/(Fe3+ +Al) = 0.33. The grade of thisarea is close to the lower temperature stability limit of thepumpellyite+epidote+chlorite+actinolite assemblage. 相似文献
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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. 相似文献
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