PHASE-EQUILIBRIA AND THE PRESSURE-TEMPERATURE PATH OF THE HIGHEST-GRADE RYOKE METAMORPHIC ROCKS IN THE YANAI DISTRICT, SW JAPAN

The garnet-cordierite zone, the highest-grade zone of the Ryoke metamo rphic rocks in the Yanai district, SW Japan, is defined by the coexist ence of garnet and cordierite in pelitic rocks. Three assemblages in t his zone are studied in detail, i.e, spinel + cordierite + biotite, ga rnet + cordierite + biotite and garnet + biotite, all of which contain quartz, K-feldspar and plagioclase. The Mg/(Fe + Mg) in the coexistin g minerals decreases in the following order: cordierite, biotite, garn et and spinel. Two facts described below are inconsistent with the par agenetic relation in the K2O-FeO-MpO-Al2O3-SiO2-H2O (KFMASH) system in terms of an isophysical variation. First, garnet and biotite in the l ast assemblage have Mg/(Fe + Mg) higher than those in the second. Seco nd, the first two assemblages are described by the reaction, spinel biotite + quartz = garnet + cordierite + K-feldspar + vapour, while th ey occur in a single outcrop. The addition of MnO, ZnO and TiO2 to the system can resolve the inconsistencies as follows. The assemblage gar net + biotite can consist of garnet and biotite higher in Mg/(Fe + Mg) than those in garnet + cordierite + biotite as long as they are enric hed in spessartine and depleted in Al, respectively. The assemblage ga rnet + cordierite + biotite becomes stable relative to spinel + cordie rite + biotite with increasing spessartine content or decreasing gahni te content and the Ti content of biotite. The constituent minerals of the assemblages, spinel + corderite + biotite and garnet + cordierite + biotitel preserve several reaction microstructures indicative of pro grade reactions, sillimanite + biotite + quartz = cordierite + spinel + K-feldspar + vapour, sillimanite + biotite + quartz = garnet + cordi erite + K-feldspar + vapour and biotite i cordierite i quartz = garnet + K-Feldspar + vapour, together with retrograde reactions, spinel + q uartz = cordierite and garnet + K-feldspar + vapour = biotite + cordie rite + quartz. This suggests that the pressure-temperature path of the rocks includes an isobaric heating and an isobaric or decompressional cooling. The high-grade areas consisting of the K-feldspar-cordierite zone, sillimanite-K-feldspar zone and garnet-cordierite zone have pro grade paths involving isobaric heating and show a southwards increase in pressure with a thermal maximum in the middle. These high-grade zon es are closely associated with the gneissose granitic rocks, suggestin g that the Ryoke metamorphism, one of the typical low-pressure type, i s caused by the heat supply from the syn-tectonic granitic rocks that emplaced at the middle level of the crust.