Metamorphic evolution of the Ogeheon belt, Korea: A review and new age constraints

The Ogcheon metamorphic belt (OMB), which is often correlated with the Dabie-Sulu ultrahigh-pressure (UHP) belt in China, comprises Neoproterozoic to Paleozoic metasedimentary and -volcanic sequences representing a stack of synmetamorphic nappes. Regional metamorphic grade increases northwestward to produce the assemblage, biolite + garnet +/- staurolite +/- kyanite + plagioclase + quartz. Garnet porphyroblasts show chemical zoning typical for prograde metamorphism, with decreasing Mn and increasing Fe and Mg from core to rim. P-T conditions were estimated to be 4.2-9.4 kbar and 490-630degreesC, corresponding to the medium-pressure type. In addition, the GIBBS calculation suggests a clockwise P-T-t path, corroborating a crust-thickening event associated with regional peak metamorphism. Using various isotopic techniques, the timings for intracontinental rift volcanism and syntectonic metamorphism were determined. New SHRIMP U-Pb zircon ages from a felsic tuff reflect the Neoproterozoic rifting at similar to750 Ma. Regional metamorphism has been recently dated at similar to285 Ma, based upon several independent sets of radiometric ages: U-Pb ages of monazite inclusions in garnet, Pb-Pb whole-rock ages of black slates, chemical ages of uraninite, and SHRIMP U-Pb zircon ages from a granitic gneiss pebble in metadiamictite. Thus, the Ogcheon orogeny is newly defined as an earliest Permian event, preceding a high-pressure metamorphic event in the Imjingang belt, central Korea, by similar to30 Ma. During the subsequent Songrim orogeny, at similar to250-220 Ma, the OMB experienced a second regional-thermal metamorphism under greenschistto amphibolite-facies conditions. This Triassic event is interpreted to correspond to a major collisional orogeny in the Dabie-Sulu belt. These multiple metamorphic events in the Ogcheon belt are readily correlated with those reported from not only the UHP bell in east-central China, but also the Hida and Renge belts in Japan. Thus, eastward extension of the Dabie-Sulu belt through the Korean Peninsula to the Hida belt (Ernst and Liou, 1995) remains a valid hypothesis.