PREDOMINANTLY MESOPROTEROZOIC U-PB DETRITAL ZIRCON AGES OF THE SEOCHANGNI FORMATION FROM THE NORTHEASTERN OKCHEON METAMORPHIC BELT, SOUTH KOREA

The age-unknown Seochangni Formation is distributed over the northeastern part of the Okcheon Metamorphic Belt in central South Korea. It overlies the Paleoproterozoic Busan gneiss complex exposed as a small dome. The Seochangni Formation is in contact with the Cambrian-Ordovician Joseon Supergroup of the Taebaeksan Basin to the east. However, their relationship has not been clarified. We separated detrital zircons from the quartzite units of the Seochangni Formation and determined their SHRIMP U-Pb ages. We discovered a marked presence of Mesoproterozoic ages with two age peaks at ca. 1200 Ma and ca. 1600 Ma. It is worthwhile to notice that quite similar Mesoproterozic ages were also reported from the Neoproterozoic Sangwon System in southern North Korea and Tumon Group in the Shandong Peninsula located at the southern margin of the North China Craton. In North Korea, the Cambrian-Ordovician sedimentary successions of the Pyeongnam Basin unconformably overlie the Mesoproterozoic to Neoproterozoic Sangwon and Guhyeon Systems. Considering apparent similarities in ages between the lower Paleozoic successions of the Taebaeksan and Pyeongnam basins and also between the Mesoproterozoic detrital zircons of the Seochangni Formation and the Sangwon System, we suggest the possibility that the boundary between the Seochangni Formation and the Joseon Supergroup is an unconformity. Following North Korea and southern North China Craton, the discovery of the significant occurrences of the Mesoproterozoic ages in South Korea indicates that that wide areas of East Asia shared common Mesoproterozoic provenances including significant contributions from Grenvillian magmatic rocks. Considering that these areas have very similar crustal evolution patterns, having Paleoproterozoic high-grade metamorphic rocks, Mesoproterozoic-Neoproterozoic low-grade metasedimentary rocks, and Cambrian-Ordovician sedimentary rocks, we suggest that they have evolved together as a part of the Sino-Korean Block at least since the Paleoproterozoic.