Geochemical Characteristics and Genesis of Cherts of Dengying Formation on the North Rim of Yangzi Block

Field survey indicates that a large scale of cherts occur in Dengying formation of Sinian Systemin the tectonic belt of Dabashan and Micangshan, together with carbonatite. The results of trace and rare earth elements analysis indicate that the cherts have the characteristics of hydrothermal origin. Among the trace elements, Ba is enriched and most of the Ba/Sr ratio is greater than 1. Th/U ratio is less than 1 and the values from all samples fall into the hydrothermal sediment area in the Th-U diagram. Zr is also enriched and its content in all samples are neighboring the hydrothermal sediment trend line in the Zr-Cr diagram. The ∑REE of cherts is very low. Ce is negative anomaly in general while Eu is obviously positive. HREE is enriched in the radiolarian cherts, but not distinctly enriched in the siliceous dolostone. The Ce/Ce*, La_N/Ce_N, V/Y ratios indicate that the cherts are formed in the transition zone from oceanic basin to continental margin. The Th/Sc, Th/U, V/Cr, V/(V+Ni) ratios indicate that the cherts are mostly formed in the reducing environment, with a small number formed in the oxidation environment. Considering the trace and rare earth elements characteristics and regional evolution of tectonics during Sinian period, it is inferred that the cherts are formed in shallow-sea shelf oxygen-enriched environment and anoxic intracontinental taphrogenic trough environmentin the complex continental margin.