Enigmatic provenance of carbonate clasts in Cryogenian glacial diamictite of the Nantuo Formation in South China

Abstract

Glacial diamictites of the Cryogenian Nantuo Formation of the Yangtze Craton of South China record major environmental transitions during the Marinoan glaciation. Although the provenance of the siliciclastic materials in the Nantuo Formation have been constrained via detrital zircon analysis, the source areas for the abundant carbonate clasts within the formation have yet to be determined. Given that the Mesoproterozoic Shennongjia Group is the only major extant pre-Cryogenian sedimentary carbonate succession in South China, it has been proposed as a source of the Nantuo dolostone clasts in the northern Yangtze Craton and may have been a major supplier of Cryogenian carbonate clasts across the entire Yangtze Craton, but this speculative provenance has not been tested or formally investigated. To test this hypothesis, δ13C and δ18O values of 18 clasts collected near Yichang, Hubei Province were analyzed along with petrographic examination to determine lithology, diagenetic alteration, and relationship to the Shennongjia Group carbonates. Our data show that the δ18O and δ13C values of these clasts are distinct from and thus unlikely sourced from the Shennongjia Group. Therefore, the Shennongjia Group cannot be the sole source of Nantuo carbonate clasts. Instead, we propose the Nantuo carbonate clasts in the study area were derived from either (1) pre-Marinoan carbonate successions in South China that were lost due to erosion or (2) another craton (e.g., the Indian Craton) adjacent to the Yangtze Craton during the Cryogenian. Given that paleogeographic and detrital zircon provenance analysis suggest separation between the Yangtze Craton and the Indian Craton in the Cryogenian Period, we favor the former origin, and that the Shennongjia Group is not the exclusive source of carbonate clasts from the Nantuo Formation.