Carbon Isotopic Composition of Carbonates of the Okcheon Metamorphic Belt in South Korea from Neoproterozoic to Early Cambrian Potential: Geological and Tectonic Significance

There has been considerable debate as to whether the Korean peninsula has evolved as part of the Sino-Korean Craton since Neoarchean or whether it is a product of the amalgamation of several continental fragments in the early Mesozoic Era. The relationship between the Neoproterozoic Okcheon Metamorphic Belt (OMB) and the Early Paleozoic Tabaeksan Basin (TB) in the central region of the Korean Peninsula has the potential to provide an answer to this question. Various carbonate rocks appear in OMB, showing unique carbon isotope values ​​according to their geologic age.

The Hyangsanni Dolomite is distributed around the Gyemyeongsan Formation with metavolcanics of about 860 Ma. The Hyangsanni Dolomite has δ¹³C values between +2.9 ‰ and +6.2 ‰, markedly higher than the Cambro-Ordovician values, and are consistent with the Neoproterozoic values. Considering the low values ​​of 86Sr/87Sr, the deposition period of the Hyangsanni Dolomite is judged to be Tonian before the Sturtian Glaciation.

The Geumgang Limestone has a maximum thickness of several tens of meters adjacent to the diamictite layer proposed as a glacial deposit but shows a very extensible distribution. The δ¹³C values ​​of the Geumgang Limestone range from -12.25 to -7.88 ‰, suggesting that they may be cap carbonates. However, whether their deposition was related to the Sturtian Glaciation or the Marinoan Glaciation is not yet known.

Between the Cryogenian Seochangri Formation of OMB and the Cambrian Jangsan Formation of TB are carbonate rocks previously considered Ordovician. However, carbon and oxygen isotope values analyzed in this study require different interpretations. Zones with δ¹³C values ​​ranging from -3.4 to +1.3 ‰ agree with Ordovician seawater values. However, over a larger area, δ¹³C values ​​show mostly positive values ​​from +1.9 to +7.8 ‰. Also, a significant negative excursion of δ¹³C values ​​down to -6.9 ‰ occurs near the highest values ​​measured. These values correlate with Ediacaran or Early Cambrian carbonates better than Ordovician seawater. It is the first to discover the possible carbonate rocks of Ediacaran or Early Cambrian in South Korea, supporting that the Neoproterozoic OMB and Early Paleozoic TB have a tectonic evolutionary history of continuous deposition rather than an assembly of different continental fragments.