Carbon, oxygen and strontium isotope geochemistry of the late Neoproterozoic carbonate platform deposit Hyangsanni Dolomite of the Okcheon metamorphic belt, Korea

Abstract

The age-unknown Hyangsanni Dolomite is distributed in the northeastern part of the Okcheon metamorphic belt in central South Korea. It has been claimed as Early to Middle Cambrian because of the misidentified Archaeocyatha fossil in the past, but the time of sedimentation has not yet been confirmed. We performed elemental and isotopic analyses for the Hyangsanni Dolomite. The REE + Y patterns of the samples of the Hyangsanni Dolomite are generally flat and Ce negative anomaly is generally negligible to the extent that it appears only weakly in some samples. Y/Ho values are in the range of 29–43, which is fairly lower than typical Y/Ho ratio of seawater, indicating the influence of terrigenous materials. REE and trace element geochemical discrimination diagrams indicate that the Hyangsanni Dolomite was deposited in a continental margin environment rather than an open ocean. We suggest that the Hyangsanni Dolomite was deposited in a rift basin connected to the ocean rather than in the passive continental margin, considering occurrence of magmatism related to continental rifting within the Okcheon metamorphic belt. Carbon and oxygen isotope analysis was performed on dolomite powders scraped off from polished surfaces unstained with Alizarin-Red-S in order to avoid the recrystallized calcite. The Hyangsanni Dolomite has δ13C(V-PDB) values between +2.9‰ and + 6.2‰, and δ18O(V-SMOW) values between +11.3‰ and + 24.5‰. These δ13C values are markedly higher than the Cambro-Ordovician values, and are consistent with the Neoproterozoic values. The minimum value of 87Sr/86Sr ratio obtained from the Hyangsanni Dolomite also refers to Neoproterozoic, not Phanerozoic. We suggest that the Hyangsanni Dolomite belongs to the sedimentary sequence of the Neoproterozoic, along with other sediments of the northeastern Okcheon metamorphic belt.