Miscibility of Tremolite and Hornblende in Progressive Skagit Metamorphic Suite, North Cascades, Washington

Abstract

The Sanbagawa metamorphic terrain in the Sazare area, central Sikoku, is divided into three zones of progressive metamorphism, A, B, and C, on the basis of mineral assemblages of pelitic schists. The characteristic mineral assemblage of zone A is phengite+chlorite and that of zone C is phengite+chlorite+garnet with biotite possibly at a higher-grade part. Zone B is transitional between A and C. Variation of the chemistry of garnet rim and chlorite with increasing grade is conspicuous: the MnO and FeO contents of chlorite decrease and the FeO content of garnet rims increase with grade. The Fe-Mg partition coefficient for the chlorite—garnet rim pair also changes systematically with the grade defined by the assemblage suggesting that the progressive metamorphism is primarily due to temperature increase. The difference in assemblages between the Barrovian biotite zone and zone C of the present area is due either to the common occurrence of clinozoisite in the latter or to the different paragenetic relations induced by higher-pressure of equilibrium in the Sanbagawa belt as compared with the Scottish one.