ABSTRACT

¹³C/¹²C and ¹⁸O/¹⁶O ratios were determined for carbonate minerals and graphites in. marbles from the contact metamorphic aureole in the Kasuga area, Gifu Prefecture, central Japan. ¹³C/¹²C ratios were determined also for carbonaceous materials in pelitic rocks. ¹³C/¹²C ratios of graphites in marbles approach the carbon isotopic range of coexisting carbonates, although they are considered according to their occurrence to be of biogenic origin. In a typical traverse of the carbonate aureole, carbon isotopic fractionation between calcite and coexisting graphite becomes regularly smaller toward the igneous contact. This strongly suggests that calcite and coexisting graphite approached isotopic equilibrium during contact metamorphism. When we assume an isotopic equilibrium between calcite and graphite, we can estimate carbon isotopic temperatures, which reasonably agree with temperatures estimated from the solubility of magnesium in calcite coexisting with dolomite and from a heat flow model. Carbon isotopic temperatures for the maximum contact temperature are estimated to be from 600 to 650°C. The thermal gradient estimated from carbon isotopic ratios is gentle and agrees with that estimated from a heat flow model. Carbon isotopic temperature is considered to represent a temperature, which is closest to the maximum temperature during metamorphism. Dolomite and coexisting calcite are considered to be in disequilibrium as to oxygen isotopes, although equilibrium between calcite and graphite is attained. This is probably due to the difference in resistivity between calcite and dolomite to oxygen isotopic exchange with hydrothermal water.