Margarite-paragonite-muscovite assemblages from the low-grade metapelites of the Tono metamorphic aureole, Kitakami Mountains, Northeast Japan.

Abstract

Margarite occurs in the low-grade zone of the contact aureole of the Tono granitic mass, Kitakami Mountains, Northeast Japan, coexisting with muscovite, paragonite, chlorite and quartz, but without carbonates and feldspars. The margarite is produced by the reaction between pyrophyllite and zoisite in the progressive metamorphism at a temperature near the upper stability limit of the assemblage margarite +quartz. The solubility of muscovite component in paragonite in the three white micas is suppressed as compared to that in the muscovite-paragonite assemblage, whereas the paragonite solubility in muscovite of the three-phase assemblage is similar to that of the two-phase assemblage. Paragonite component in margarite in three-phase assemblage of this area is about 17.3-22.5 mol% ; this value is the smallest for the margarite in three-phase assemblages so far reported. The low paragonite solubility of margarite in this area is probably due to the low pressure condition of the contact metamorphism of the Tono area.