矿床地球化学国家重点实验室知识库

The Lower Cambrian on the Yangtze Block in South China is valuable for understanding the early evolution of life, the global biogeochemical cycles, and the major changes of the ocean. However, both the placement of the Precambrian– Cambrian boundary in South China and the correlation of the Lower Cambrian across the Yangtze Block are still in controversy, which hinders the global correlation of the Lower Cambrian and understanding of environmental changes of the paleo-ocean. Discovery of K-bentonites in the Lower Cambrian in South China will facilitate clarifying the above-mentioned problems. In this study, systematic field investigations and mineralogical and geochemical studies were conducted on Lower Cambrian K-bentonites in South China. The field investigations have shown that the widespread K-bentonites occur in two important stratigraphic levels: the middle Zhujiaqing Formation and the basal Shiyantou Formation and their lateral equivalents. Biostratigraphically, the older K-bentonite bed is preserved in the Anabarites trisulcatus–Protohertzina anabarica Assemblage Zone and the younger K-bentonite in the poorly fossiliferous interzone. Geochemical studies reveal that the Lower Cambrian K-bentonite samples are plotted in the fields of trachyte, trachyandesite, rhyodacite (dacite), and rhyolite in a plot of Nb/Y against Zr/TiO2, suggesting that the K-bentonites are most probably derived from felsic magmas with sub-alkaline to alkaline nature. Compared with the K-bentonite in the basal Shiyantou Formation and its equivalent sequence, the K-bentonite in the middle Zhujiaqing Formation and its correlative succession is characterized by lower Zr and Nb concentrations. Volcanic eruptions recorded by the two K-bentonite beds may have taken place during an interval of tectonic transformation, and the source volcanoes were probably located in the east margin of the Ganze–Songpan Block. Correlation results of the two important Lower Cambrian K-bentonite beds indicate that a previous viewpoint considering the polymetallic Ni-Mo layer in the lowermost Niutitang Formation as the Precambrian–Cambrian boundary in South China is inappropriate. Combined with available geochronological data, the boundary should be placed in the sequence beneath the K-bentonite in the middle Zhujiaqing Formation and its correlative stratigraphic level.