STRONTIUM, CARBON AND OXYGEN ISOTOPIC COMPOSITIONS OF THE HYANGSANNI DOLOMITE FROM THE OKCHEON METAMORPHIC BELT, SOUTH KOREA: POSSIBILITY OF ASSOCIATION WITH NEOPROTEROZOIC GLACIAL EVENT

For decades, stratigraphic interpretation of the Okcheon Metamorphic Belt (OMB) in central South Korea has been controversial. The deposition of some layers of the OMB has been known to occur from the Neoproterozoic to the Paleozoic. However, because of poorly fossiliferous and multiple deformation and metamorphism, it is often difficult to interpret the stratigraphic and sedimentation times of the layers. One of the interesting things is that the sediments of the OMB was deposited through the Neoproterozoic Snowball Earth event as in the South China Craton. In this respect, Hwanggangni Formation and Geumgang Limestone were proposed as glacial sediments and capped limestone, respectively (Choi et al., 2012). However, there was no evidence to support this decisively. We analyzed carbon, oxygen and strontium isotopic compositions from Hyangsanni Dolomite to test the Neoproterozoic glaciation hypothesis.

The Hyangsanni Dolomite in the northeastern part of the Okcheon Metamorphic Belt is distributed along the periphery of Gyemyeongsan Formation, Neoproterozoic metavolcanicss and Devonian Daehyangsan Quartzite is distributed further outward. Analyzed Hyangsanni Dolomite samples have low ΣREE contents and their shale-normalized REE+Y patterns show mostly normal seawater patterns, but there are no obvious positive lanthanum or negative cerium anomalies. Samples yield comparatively consistent δ¹³C(PDB) values (+5~+6‰) and δ¹⁸O(SMOW) values (+21~+24‰). They yield initial ⁸⁷Sr/⁸⁶Sr ratios between 0.7073 and 0.7115. Although Hyangsanni Dolomite was suggested as a Cambrian rock, most of the ⁸⁷Sr/⁸⁶Sr ratios analyzed are distinctly lower than the Early Paleozoic values, indicating that they belong to the Neoproterozoic. Assuming that the lowest ⁸⁷Sr/⁸⁶Sr ratio (0.7073) represents the best estimation of seawater, we suggest that the deposition of Hyangsanni Dolomite was deposited between Sturtian glacial age (ca. 720-740 Ma) and Vendian glacial age (ca. 575-590 Ma) based on Neoproterozoic seawater ⁸⁷Sr/⁸⁶Sr evolution curves in combination with carbon isotope chemostratigraphic curves. These results support the possibility that several sedimentary deposits of the OMB including the Hwanggang Formation were deposited in association with the Snowball Earth event of the Neoproterozoic.