ABSTRACT

In order to elucidate carbon and oxygen isotopic systematic in metamorphosed limestone and isotopic behavior during metamorphism, microscale carbon and oxygen isotopic compositions of graphite and calcite in crystalline limestone were determined for the contact metamorphic aureole of the polymetamorphosed Hida metamorphic belt, central Japan. In this area granulite facies metamorphic rocks were overprinted by contact metamorphism related to a granitic intrusion. Carbon isotopic fractionation between individual crystalline graphites and the cores of coexisting calcite grains gives a constant value independent of the sampling localities. Graphites armored by silicates have the same δ¹³C as other graphites in calcite. These indicate that carbon isotopic equilibrium between graphite and calcite was completely attained during granulite facies metamorphism. Values of δ¹⁸O in the cores of individual calcite grains vary over about 2‰ near the granite contact, and the range of variation tends to become smaller with increasing distances from the contact. In calcite grains near the granite contact, δ¹⁸O values sharply decrease by up to 7‰ within 200∼300 μm wide zones toward grain boundaries, whereas δ¹³C remain almost constant. This δ¹⁸O trough at the grain boundaries is less pronounced with increasing distance from the granite contact and is almost no longer seen at distance about 1.3 km. Oxygen isotopic zonation of calcite at grain boundaries strongly suggests partial isotopic exchange with water-rich metamorphic fluids that migrated along grain boundaries. This fluid-rock interaction occurred preferentially in the grain boundaries with very limited scale at distances up to 1.3 km from the granite contact, although the contact aureole defined by the mineralogical paragenesis is limited to within several tens of meters of the contact. Skeletal graphites growing over the substrata of large hexagonal graphite grains have lower δ¹³C values than those of the host graphite. It is considered that the fine-grained graphite crystals were formed in isotopic equilibrium with low δ¹³C carbon in the metamorphic fluids.