Local equilibrium during the contact metamorphism of siliceous dolomites in Kasuga-mura, Gifu-ken, Japan

Abstract

The contact aureole around the Kaizuki-yama granite, recognized with a width of more than 3 km, ranges from the greenschist to the amphibolite facies. Dolomite and quartz in siliceous dolomites started to react at the lower grade part of the amphibolite facies. The increase of metamorphic grade brought three-, four- and five-mineral assemblages, with calcite, talc, tremolite, diopside and forsterite arriving successively. The four- and five-mineral assemblages can be related to divariant and univariant assemblages respectively in the system CaO-MgO-SiO2-CO2-H2O, and are considered to represent components of reactions that buffered the chemistry of intergranular fluids. The dolomite-calcite solvus geothermometry gives temperatures 465 ° C for the assemblage dolomite-quartz-talc-tremolite-calcite, 555 ° C for the assemblage dolomite-quartz-tremolite-diopside-calcite, and 595 ° C for the assemblage dolomite-tremolite-diopside-forsterite-calcite. Equilibrium fluid pressures, calculated with available experimental data, are 2.6 kb for the first five-mineral assemblage, and 3.5 kb for the latter two. Fluid compositions were controlled by reactions in each local system, but changed toward a water-rich one when the reactions ceased. The fluid pressures as distinct from the possible rock pressures and the controlling of the fluid composition by reactions are interpreted in terms of the ratio of reaction to fluid migration. Of a skarn zone developed around a chert nodule its mineralogy and deduced composition of the fluids enabled to estimate the size of the microdomain in which equilibrium was maintained. It is of millimeter size. Equilibrium systems of larger sizes are concluded to represent overlapped microdomains of a size.