Metamorphic evolution of the northwestern Ogcheon metamorphic belt, South Korea

Abstract

The Ogcheon metamorphic belt (OMB) comprises Late Proterozoic to Paleozoic metasedimentary and metavolcanic sequences which are intruded by Mesozoic granitoid plutons. To delineate the metamorphic evolution of the OMB, we have investigated mineral parageneses, metamorphic reactions and P–T conditions of pelitic and mafic schists in the Chungju area, northwestern part of the OMB. The regional metamorphic grade increases north-westward from biotite to garnet zones, although low-pressure assemblages have developed in contact aureoles around Jurassic granites. Garnet crystals show chemical zoning, typical for prograde metamorphism, with decreasing Mn, and increasing Fe and Mg from core to rim. Compositions of amphiboles constituting the common assemblage in the mafic schist are consistent with those of biotite- and garnet-zones documented in the medium-pressure metabasites. P–T conditions of the garnet zone, estimated from garnet–biotite, garnet–chlorite and amphibole–plagioclase geothermometers together with garnet–plagioclase–biotite–muscovite/quartz geobarometers, are in the range of 5–8 kbar and 520–590°C. Retrograde P–T path based on fluid inclusion studies suggests that the exhumation of the OMB has passed through the P–T range of 1–3 kbar and 350–500°C, following the isochore curves of the CO 2 inclusions. In conjunction with structural and geochronologic data, we conclude that the OMB has experienced a polycyclic P–T evolution characterized by (1) crustal thickening during the Middle Paleozoic time and (2) regional retrograde metamorphism in the Triassic. Our result further suggests that the Triassic collision belt in east-central China does not pass through the OMB.