Prograde and Retrograde P-T Paths of the Late Paleozoic Glaucophane Eclogite from the Renge Metamorphic Belt, Hida Mountains, Southwestern Japan

Abstract

Late Paleozoic glaucophane eclogite and garnet glaucophane schist are intercalated with pelitic schist from the Omi eclogitic unit in the Renge metamorphic belt, Hida Mountains, southwestern Japan. Eclogite and garnet glaucophane schist have MORB-like bulk compositions (major and trace), but garnet glaucophane schist is free from omphacite, reflecting a higher Mg/(Mg+Fe) ratio bulk-rock composition. In the eclogite, three different metamorphic stages (stages I to III) are defined on the basis of microstructures and mineral zoning. In particular, prograde-zoned porphyroblastic garnet preserves the transition from blueschist to eclogite facies. The inclusion trails in the rims of garnet are parallel to a penetrative foliation S1 in the matrix, and consist of eclogite-facies minerals (omphacite, glaucophane, epidote, rutile, and quartz). Its core contains the preceding epidote blueschist-facies minerals (glaucophane, epidote, titanite, albite, and quartz) instead of eclogite-facies minerals, and has an internal fabric S0 at a high angle to the surrounding S1 foliation. In the foliated matrix, the eclogite-facies mineral assemblage garnet + omphacite + glaucophane + epidote + rutile + quartz + phengite is partly replaced by secondary minerals such as chlorite, titanite, albite, and calcite, which coexist with recrystallized glaucophane. These petrographic features show a "hair-pin"-like P-T path that passes from the epidote blueschist facies (stage I) through the eclogite facies (stage II), and then is retraced on a nearly coincident path to epidote blueschist facies (stage III). The systematic decrease from core to rim in K D value between garnet and omphacite, and a significant increase in jadeite component of omphacite suggest a rise in both temperature and pressure during prograde eclogitization. The host pelitic schist contains the primary mineral assemblage of quartz + paragonite + phengite + garnet + ferroglaucophane + clinozoisite + rutile, and is strongly replaced by secondary albite, chlorite, and titanite. Pseudomorphs after omphacite and possible kyanite are also found in host pelitic schist. Using geothermobarometry for eclogite and a petrogenetic grid for the host pelitic schist, conditions of peak eclogite-facies metamorphism are estimated to be around 550-600°C and at least 1.8 GPa, indicating an apparent paleo-geothermal gradient of ~10°C/km. The eclogite-bearing Renge metamorphic belt in southwestern Japan may be a new candidate for the eastern extension of a suture zone in east-central China. Late Paleozoic blueschist and eclogite metamorphism may be related to subduction of oceanic crust between the Sino-Korean and Yangtze blocks, prior to their collision.