GeoActa Special Publication 1
2008
Acquisto online |
GeoActa
an international Journal of Earth Sciences
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Multiple stages of piemontite formation in piemontite-quartz schists of the Sanbagawa metamorphic belt in central Shikoku, Japan
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PDF (1.2 Mb)
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Abstract
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 XFe3+ and XAl (XFe3+ = Fe3+/Al + Mn3+ + Al and XAl = Al/Fe3+ +Mn3+ +Al) in comparison with the Asemi-gawa piemontite. Increasing XFe3+ and XAl 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 XMn3+ and XFe3+ in piemontite and fO2, in which by increasing fO2, the XMn3+ of piemontite also increases.
Keywords: Piemontite-quartz schists, Piemontite-garnet oxybarometer, Sanbagawa metamorphic belt, Japan
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