抄録
Igneous-limestone contact metamorphic environment is most suited for studies on metasomatism, fluid flow and transport of material. Despite the voluminous studies on contact metamorphic environments, there is still ambiguity in skarn formation mechanism and related processes in high temperature accompanied with fluid flow. The contact aureole at Fuka, Okayama, Japan is renowned for its peculiar occurrence of extensive high-temperature skarn resulting from the intrusion of Mesozoic quartz monzonite into Paleozoic marine carbonate rocks. Here we present the results on the petrologic and systematic stable isotope studies of the high-temperature spurrite-bearing skarn formation at Fuka, and consider the transport of material, formation conditions and origin of fluids. The limestone mine at Fuka exposes several quarry faces having continuous limestone-skarn-igneous rock outcrops. In general, the skarn occurs as three distinct mineralogical zones, the wollastonite zone, the spurrite zone and the gehlenite zone. Gehlenite-zone associate closely with igneous intrusion and are extensive (decimeter to meter thick). Retrogression of gehlenite to hydrogrossular is common. Accessory phases include schrolomite, vesuvianite and perovskite. Predominantly monomineralic spurrite-zone forms in the outer zone of the gehlenite-zone and also as independent veins. Spurrite-zone may extend up to tens of meters. At places tilleyite or rankinite coexists with the spurrite, however larnite is absent. Vesuvianite is the most common accessory phase observed. Retrograde hydration of spurrite to foshagite, scawtite and hillebrandite is commonly observed. Grossular-wollastonite skarn forms very narrow (few centimeter width) zone usually separating the spurrite-skarn and the igneous intrusion. It is also developed along the younger basic intrusive dykes in the region. The spurrite skarn at Fuka formed at high-temperature contact metamorphism with considerable amounts of material transport from the intrusive quartz monzonite. Average aqueous silica concentration of the fluid was about 2.5x10-3 mol/liter. Temperature condition during the spurrite formation is between 980℃ and 1080℃ and the XCO2 was between 0.25 and 0.42. Large-scale carbon and oxygen isotope shifts in the spurrite zone is caused by the combined effect of decarbonation and massive fluid flow. Earlier geochemical studies also suggested extensive mass transport during the skarn formation. The carbon and oxygen isotope profile within marble from spurrite-marble contact indicates that carbon has moved by lattice diffusion, whereas oxygen moved by both lattice and grain boundary diffusion. Also, it is found that during high temperature (1000℃) skarn formation the diffusion constants of carbon and oxygen species resembles, in contrast to the low temperature hydrothermal skarn forming fluids, where oxygen diffuses several times faster than carbon.
