DETRITAL ZIRCON U-PB GEOCHRONOLOGY OF THE LATE PALEOZOIC PYEONGAN SUPERGROUP IN SOUTHERN KOREA AND ITS TECTONIC AND PALEOGEOGRAPHICAL IMPLICATIONS

The Korean Peninsula was southeastern part of the Sino-Korean (North China) block during the Paleozoic. The Pyeongan Supergroup, late Paleozoic (Middle Carboniferous~Early Triassic) sedimentary succession, is scatteredly distributed in the Okcheon Belt, a NE-SW trending fold-thrust belt in southern Korean Peninsula. Previous studies suggested that the Pyeongan Supergroup was deposited in a foreland-basin setting, and sediments were derived from the east and southeast, based on the paleocurrent data. In the largest and well-studied Samcheok Coalfield, detrital zircon U-Pb ages of the Pyeongan Supergroup were analyzed, and they were compared with those from other areas in the Okcheon Belt. Based on fossil records, lithology, and detrital zircon age, the sedimentary successions of different areas can be age-correlated four time intervals: Bashikirian~early Moscovian, late Moscovian~Cisruralian, Guadalupian~Lopingian, and Lower Triassic. 1.7~2.0 Ga zircons are prominent in all areas, with subordinate amount of syn-depositional zircons. In addition, zircons of 360~420 Ma, 0.8~1.1 Ga, and 2.3~2.7 Ga are notable. The latter age groups are almost absent in the Samcheok Coalfield, but become significant towards the southwest and the northwest. Within the extent of the Pyeongan Supergroup distribution area, the temporal change of zircon age pattern was not evident. The prevalence of 1.7~2.0 Ga zircons suggests that the Pyeongan Supergroup in southern Korea all share the same recycled orogen. The Paleoproterozoic zircons are characteristic of the Precambrian Yeongnam massif located to the southeast of the Okcheon Belt, suggestive of their possible derivation from the Yeongnam massif. In the paleogeographical point of view, the results of this study support the previous studies that the paleo-orogen existed parallel to the Okcheon Belt to the southeast, which formed long before the Early to Late Triassic collision between the North and South China Craton.