ABSTRACT

Carbon and oxygen isotopic compositions of graphite and coexisting carbonate were determined for the regional metamorphic areas of Kurobegawa, Toyama Pref. and Kamioka, Gifu Pref. as well as the contact metamorphic aureoles of Kasuga, Gifu Pref., Ikeno, Aichi Pref., Ikenojizo, Kyoto Pref., and Ugakei, Mie Pref. in central Japan. In graphite bearing crystalline limestones, carbonate-graphite pairs are considered to have been in carbon isotopic equilibrium during various metamorphisms. In the contact metamorphic aureoles of the Kasuga area, the carbon isotopic geothermometry gives a temperature similar to that estimated from the mineral assemblages. There is a local variation of carbon isotopic temperatures from 470 to 740°C within one hundred meters from the igneous contact in the Kasuga area. This would be attributed to the heterogeneity of the metamorphic temperatures near the igneous contact. On the other hand, in the regional metamorphic terrane of the Kurobegawa area, the carbon isotopic fractionation between graphite and coexisting carbonate is almost constant for gray limestone. This would show that the metamorphic temperature is constant during metamorphism within an area of at least a few square kilometers. In the polymetamorphic terrane of a part of the Kurobegawa area, the carbon isotopic temperature represents the maximum temperature of the late stage of a progressive metamorphism. In the Kamioka area, the isotopic geothermometry gives a temperature higher than 810°C, which is far above the metamorphic temperature expected from mineral assemblages. This high grade metamorphism would have occurred at an early stage of the Hida polymetamorphism.