The Cryogenian Nanhuan System (South China) during the interglacial-glacial transition: geochemistry, sedimentary provenance, and tectonic setting

Abstract

ABSTRACT Samples of the Nanhuan System during the interglacial-glacial transition in the Shennongjia area, northern Yangtze block, were analysed via major and trace elements, clay mineralogy, and sulphur isotopes to infer provenance, paleoweathering intensity of the source rocks, tectonic setting, and marine chemistry. The magmatogenic sedimentary rocks are typical of acidic igneous sources, whereas the remaining samples show a sedimentary trend. The tectonic discrimination of a passive continental margin was determined via major element diagram. The samples showing subduction-affinity geochemistry may represent provenance virtually entirely comprising old recycled material. Low chemical index of alteration values and high illite contents of the sedimentary deposits reveal that the basin is characterized by physical erosion owing to the combined effects of tectonic movement and long-term cool and dry climatic conditions. Extreme δ34S enrichment in the Datangpo Formation indicates that this Cryogenian formation was a restricted rift basin with limited exchange with the open ocean. The δ34Ssulfide and illite values increase rapidly at the end of the Sturtian glaciation followed by a sharp decline at the end of inter-glacial silt deposition. Low oceanic sulphate concentrations and intense bacterial reduction of sulphate may account for the 34S enrichment in this restricted deep-water basin.