Multiple stages of piemontite formation in piemontite-quartz schists of the Sanbagawa metamorphic belt in central Shikoku, Japan

Piemontite with variable chemical composition occurs in piemontite-quartz schists in the Asemi-gawa (garnet zone) and Besshi (albite-biotite zone) areas of high P - low T Sanbagawa metamorphic belt in central Shikoku, Japan. Even though piemontite is usually a matrix phase, it also occurs as inclusion in the core and rim of albite. In matrix piemontite from Asemi-gawa, three zones can be distinguished in which core is enriched in Mn³⁺ and surrounded by a mantle rich in Al and Fe³⁺. The mantle is also surrounded by a rim which again is enriched in Mn³⁺. In matrix piemontite from Besshi area two types of zoning can be identified; in one type core is richer in Mn³⁺ and poorer in Fe³⁺, whilst in another type core contains more Fe³⁺ than the rim. Piemontite from Besshi area contains more X_Fe3+ and X_Al (X_Fe3+ = Fe³⁺/Al + Mn³⁺ + Al and X_Al = Al/Fe³⁺ +Mn³⁺ +Al) in comparison with the Asemi-gawa piemontite. Increasing X_Fe3+ and X_Al in piemontite from garnet zone towards albite-biotite zone could be related to increasing metamorphic grade. Log fO₂-T curves were calculated using piemontite-garnet oxybarometer for Asemi-gawa and Besshi samples between 400 and 800 ºC temperature and show that Asemi-gawa samples yield conditions more oxidizing than Besshi samples. Calculations also show a relation between X_Mn3+ and X_Fe3+ in piemontite and fO₂, in which by increasing fO₂, the X_Mn3+ of piemontite also increases.

Keywords: Piemontite-quartz schists, Piemontite-garnet oxybarometer, Sanbagawa metamorphic belt, Japan