Geochemistry of vein and wallrock carbonates from the Ediacaran system in South China: Insights into the origins of depositional and post-depositional fluids

Abstract

A combined study of carbon–oxygen isotopes and major-trace elements was carried out for vein and wallrock carbonates from the Lantian Formation of the Ediacaran system in South China. The results provide geochemical constraints on the origins of depositional and post-depositional fluids with respect to recognition of the post-depositional alteration. The Lantian Formation was subdivided into the Upper and Lower carbonate units. For the Upper Unit (UU), petrographic observations indicate that wallrock carbonates only experienced limited post-depositional alteration. The wallrock exhibits δ18O values of −19.2 to −13.1‰ and δ13C values of −9.6 to −8. 9‰ (both relative to VPDB). The vein carbonates are divided into Groups A and B based on their δ18O values and REE+Y patterns. Group A veins show nearly similar δ18O and δ13C ranges to, but different REE+Y patterns from, the wallrock. These data indicate that the origin of fluids for Group A veins may be similar to that of wallrock, which was probably the mixture of continental and marine glacial meltwaters. Group B veins show both different δ18O values and REE+Y patterns from, but similar δ13C values to, the wallrock. This indicates that post-depositional fluids are different from the depositional fluids and thus of external origin. Low δ18O values −23.7 to −18.1‰ for Group B veins indicate that the post-depositional fluids were derived from continental glacial meltwater. There is no correlation between δ18O and δ13C values for these vein and wallrock carbonates, suggesting that the wallrock was not extensively altered by the post-depositional fluids.For the lower unit (LU), veins exhibit different C–O isotope compositions and REE+Y patterns from wallrock carbonate, indicating that post-depositional fluids have different geochemical compositions from depositional fluids and thus are of external origin. The simultaneous changes in δ18O and δ13C values with different laminations in the wallrock carbonates suggest the preservation of the primary geochemical compositions. The δ18O values and REE+Y patterns for the LU veins indicate that the post-depositional fluids were probably the mixtures of continental and marine glacial meltwaters. Therefore, the Lantian carbonates record the incorporation of continental glacial meltwater into the marginal sea of Ediacaran age. Nevertheless, the geochemical screening can provide constraints on the preservation of primary signatures and thus on the origins of depositional and post-depositional fluids in the Neoproterozoic marine carbonates.