Cation Exchange Between Piemontite and Garnet in Piemontite-Quartz Schists from Asemi-Gawa Area of Central Shikoku, Sanbagawa Metamorphic Belt, Japan

In the Asemi-gawa area of intermediate high-pressure and low-temperature Sanbagawa metamorphic belt in central Shikoku, Japan, piemontite-quartz schists are common in which cation exchange between piemontite and garnet has been studied. Piemontite in contact with garnet usually contains two zones in which core is enriched in Mn3+ and surrounded by a rim rich in Fe3+. Garnet is Ca-Fe-bearing spessartine and is slightly heterogeneous in which core rich in Mn is surrounded by a narrow rim poor in Mn and rich in Ca. The chemical composition of piemontite changes in contact with garnet in matrix or whenever it is included in the garnet porphyroblast. In both types of contacts, piemontite tends to be richer in Mn3+ and poorer in Fe3+ while garnet tends to be richer in Ca and Fe3+ and poorer in Mn2+. P-T curves were calculated using piemontite-garnet equilibria and show that the chemical variation from core to rim in piemontite-garnet pair is related to increasing temperature whilst cation exchange in boundary of piemontite-garnet can be caused by decreasing temperature. Log f (O2)-T calculations show that cores of piemontite-garnet pair have been formed under more oxidizing condition than their rims. Calculations also show cation exchange between piemontite and garnet has been occurred under more oxidizing condition than the piemontite-garnet rim.