U-PB AGES OF DETRITAL ZIRCONS FROM THE SEOCHANGNI FORMATION OF THE NORTHEASTERN OKCHEON METAMORPHIC BELT, SOUTH KOREA: MESOPROTEROZOIC PROVENANCE AND ITS IMPLICATIONS

The Seochangni Formation is distributed over the northeastern part of the Okcheon Metamorphic Belt, South Korea, overlying the Paleoproterozoic Busan gneiss complex exposed as a small dome. The relationship between the Seochangni Formation and the Cambrian-Ordovician Joseon Supergroup of the Taebaek Bain to the east has been controversial; a fault contact between blocks of North and South China affinities or an unconformity. We determined SHRIMP U-Pb ages of detrital zircons separated from the quartzite units of the Seochangni Formation and discovered a marked presence of ages in the Mesoproterozoic Era with two age peaks at ca. 1200 Ma and ca. 1600 Ma. The similar Mesoproterozic ages were previously reported from the Sangwon System in southern North Korea and Tumon Group in the Shandong Peninsula located at the southern margin of the North China Craton, suggesting common Mesoproterozoic provenance of the sediments in the regions. In North Korea, the Cambrian-Ordovician sedimentary successions of the Pyeongan Basin unconformably overlie the Mesoproterozoic to Neoproterozoic Sangwon and Guhyeon Systems. Considering apparent similarities in ages between the lower Paleozoic successions of the Taebaek and Pyeongan basins and also between the Mesoproterozoic detrital zircons of the Seochangni Formation and the Sangwon Syetem, we suggest the possibility that the boundary between the Seochangni Formation and the Joseon Supergroup is an unconformity and they have evolved together as a part of the Sino-Korean Block since the Paleoproterozoic.