GEOCHRONOLOGIC EVIDENCE FOR LATE PALEOZOIC OROGENY IN THE OGCHEON METAMORPHIC BELT, SOUTH KOREA

The Ogcheon metamorphic belt (OMB) is a northeast-trending orogenic belt, which is often correlated with the Triassic ultrahigh-pressure belt in China. The OMB mainly consists of a series of metasedimentary and metavolcanic rocks of Late Proterozoic to Late Paleozoic times. This belt is interpreted to represent a stack of syn-metamorphic nappes produced by intracontinental extension and subsequent closure processes of an aborted rift. As a consequence, metamorphic evolution of the OMB is characterized by a clockwise P-T evolution. Peak metamorphic conditions estimated from various geothermobaromers are in the range of 4.2-9.4 kbar and 490-630 deg. C, but the timing of peak metamorphism is poorly constrained. In this study, we report both U-Pb step-leaching garnet ages and Sm-Nd whole rock-garnet age estimated from several samples of garnet and staurolite-garnet schists.

Three samples taken from the upper structural unit yielded the consistent U-Pb garnet ages of ca. 285-300 Ma. This result is corroborated by a Sm-Nd whole rock-garnet age of 295+/- 42 Ma. On the other hand, slightly younger ages were obtained from the lower structural unit: Pb-Pb whole rock age of ca. 260-270 Ma estimated from black slates and an electron-microprobe chemical age of uraninite dated as ca. 280 Ma. The apparent difference in metamorphic ages between two structural units could be attributed to the prolonged tectonometamorphism or to the progression of syn-metamorphic nappes towards the southeast across strike of the OMB. In summary, our geochronologic data suggest that the peak metamorphism has occurred during Late Carboniferous to Early Permian time, in contrast to earlier considerations favoring either Silurian or Triassic orogeny in the OMB. Consequently, tectonic correlation of the OMB with the Triassic ultrahigh-pressure belt in China is not likely.