SR ISOTOPE COMPOSITION OF MARBLES AND DETRITAL ZIRCON U-PB AGES FROM THE GUBONGSAN GROUP IN NORTHEASTERN GYEONGGI MASSIF, SOUTH KOREA: IMPLICATIONS ON NEOPROTEROZOIC CRUSTAL EVOLUTION OF THE SINO-KOREAN CRATON

The Gyeonggi massif, located in the central region of the Korean peninsula, has long been known to consist of high-grade gneiss complexes of Paleoproterozoic. Recently, however, small scale metasedimentary rocks distributed throughout the Gyeonggi massif have been found to be deposited at much younger times. We performed U-Pb age determination of detrital zircons separated from the metasedimentary rocks of the Gubongsan Group in the small basin of the Gyeonggi Massif. The zircon U-Pb ages are distributed from Late Paleoproterozoic to Early Neoproterozoic, indicating that it is a sedimentary group younger than Early Neoproterozoic. Having a prominent Mesoproterozoic peak is similar to that of the Neoproterozoic sediments along the southern margins of both North Korea and North China Craton. Marbles of this group yield ⁸⁷Sr/⁸⁶Sr ratios between 0.70562 and 0.71922. Assuming that the lowest Sr isotope value is the seawater value at that time, the sedimentation time is estimated to be about 850-900 Ma. This is slightly older than 750-850 Ma old carbonate rocks in the Okcheon Metamorphic Belt located in the south of the Gyeonggi massif. The occurrences of Neoproterozoic sediments with similar detrital zircon age characteristics suggest that the Gyeonggi Massif has evolved as a single mass at least since Neopoterozoic with North China Craton.