Abstract: High siliceous rocks are extensively developed in the neriticbathyal environment of South China during the MiddleUpper Ordovician. These rocks are mainly distributed throughout the Yanxi and Duiershi Formations in Hunan and Jiangxi provinces, respectively, generally between the Yangtze and Cathaysian Plates in the EW strip. The stratum thickness gradually decreases from SE to NW, and the layers are mostly thin. Radiolarian Protoentactinia sp. is found in the rocks, sometimes more than 50% in sample content. Analysis results showed that biological and terrigenous Si is the main silicon source—a clear indication of rocks biochemical origin. It was found that the Al/(Al+Fe+Mn) ratio is greater than 0.45, and K2O/Na2O ratio ranges between 1.55 and 42.11. The Al2O3Ni, V, B, Cr diagram showed both discrete and linear relationships and a U/Th ratio of 0.72. Lastly, our study revealed that REE contents in central Hunan and Jiangxi provinces are relatively high, while element Ce in chert in the southeast area showed slightly negative anomaly. In the research area, ∑LREE≈∑HREE, this suggests that the areas from Hengyang to Ganzhou are affected by volcanic and hydrothermal activities.

Key words: Hunan and Jiangxi provinces, Ordovician, Radiolarian, siliceous rock, geochemistry