SHRIMP U-PB ZIRCON AGES OF METAMORPHIC ROCKS FROM THE SAMGOT UNIT, YEONCHEON COMPLEX IN THE IMJINGANG BELT, KOREA: IMPLICATIONS FOR THE PHANEROZOIC TECTONICS OF EAST ASIA

The Imjingang belt in the middle part of the Korean peninsula is considered to be a possible eastward extension of the Qinling-Dabie-Sulu belt, the Triassic suture zone between the North and South China continents. However, precise geochronological data are still lacking for the metamorphic rocks of the Imjingang belt. We obtained SHRIMP U-Pb ages for zircons from an amphibolite and a garnet-biotite paragneiss of the Samgot unit, Yeoncheon Complex in the Imjingang belt. The amphibolite zircons show magmatic zoning, and gave a crystallization age of 861.2±7.3 Ma (95% confidence level). This age of the Samgot amphibolite, along with its geochemical affinity to intra-continental rift-related alkalic basalt, suggests that its protolith may have formed during the Neoproterozoic break-up of the Rodinia supercontinent, as witnessed in the South China continent. The paragneiss zircons exhibit prismatic or isometric metamorphic overgrowth on detrital cores. The zircon cores yielded two distinct Paleozoic age groups of 447.0±4.4 Ma (Late Ordovician) and 396.7±6.1 Ma (Early Devonian), and some older ages up to 2,728±5 Ma. The overgrowth zircons gave a peak metamorphic age of 252.9±1.9 Ma. The Paleozoic ages from the detrital zircon cores indicate presence of Caledonian igneous activities, unidentified so far, in the Korean Peninsula. Also, the youngest age of detrital zircons and the metamorphic age constrain that the protolith of the paragneiss was deposited between the Early Devonian and the Late Permian, indicating that at least a part of the Samgot unit is much younger than previously thought (Meso- to Neoproterozoic). Our U-Pb zircon ages suggest possible eastward continuation of the Imjingang belt to the Hida belt of Japan which reportedly has similar geochronological aspects as in this study, such as Precambrian and Paleozoic terranes metamorphosed at about 250 Ma. Our data also suggest that the timing of peak metamorphism recorded in the Chinese Qinling-Dabie-Sulu collisional belt is about 20 Ma younger than those in the Imjingang and Hida belts, apparently supporting the indentation collision model between the North and South China continents proposed by Yin and Nie (1993).