METAMORPHIC EVOLUTION OF THE OGCHEON METAMORPHIC BELT, KOREA

The Ogcheon metamorphic belt (OMB) is a northeast-trending fold-thrust belt, which is often correlated with the Dabie-Sulu ultrahigh-pressure (UHP) belt in China. The OMB comprises Neoproterozoic to Paleozoic meta-sedimentary and -volcanic sequences which are interpreted to represent a stack of syn-metamorphic nappes. Regional metamorphic grade increases north-westward to produce the assemblage, biotite + garnet +/- staurolite +/- kyanite + plagioclase + quartz. Garnet crystals show chemical zoning, typical for prograde metamorphism, with decreasing Mn, and increasing Fe and Mg from core to rim. P-T conditions were estimated as 4.2-9.4 kbar and 490-630^oC, corresponding to the medium-pressure type. In addition, the GIBBS calculation suggests a clockwise P-T-t path corroborating the crustal-thickening event associated with regional metamorphism. Based upon U-Pb and Sm-Nd isotopic ages of garnet containing ilmenite inclusions as well as chemical ages of uraninite, the peak metamorphism has been dated at ca. 300-280 Ma. This result is supported by our SHRIMP U-Pb zircon ages obtained from a granitic gneiss pebble in meta-diamictite, defining a discordia with lower intercept age of ca. 300 Ma. It is thus likely that a significant Pb loss has occurred during the Ogcheon orogeny. Subsequent to this orogeny, the OMB has been affected to a lesser extent by greenschist to amphibolite facies metamorphism in the Triassic time. In summary, several independent sets of geochronologic data suggest that syn-tectonic regional metamorphism has occurred during Late Carboniferous to Early Permian time, in contrast to earlier considerations favoring either Silurian or Triassic orogeny in the OMB. Thus, the OMB is not exactly correlative with the Triassic UHP belt in China. It is noted, however, that the ca. 300 Ma event may correspond to an early stage of collisional episode recently reported from the Dabie belt in east-central China.