Petrogenesis and Sedimentary Environment of Siliceous Rocks of Yangjiabao Formation in Zhushan Area, Northwestern Hubei

Key words: 
• Yangjiabao Formation /
• siliceous rocks /
• depositional environment /
• geochemistry

Abstract: Deepwater siliceous rocks developed during the Early Cambrian in Zhushan area which located on the north margin of the Yangtze block. These rocks mainly comprise pure siliceous rocks, with little dolomite and shale. The siliceous rocks are composed of microcrystalline quartz(content of more than 85%) and some carbon, flint and sericite. They show high SiO₂ content(93.4%, on average) by geochemical analysis. Contents of Al₂O₃ and TiO₂, are as low as 0.19%~1.45% and 0.01%~0.09%, respectively. And TFe₂O₃ accounts for 0.053%~0.342%. The ratio of Al/(Al+Fe+Mn) is from 0.58 to 0.83(0.73, on average), with Eu/Eu^* being in 0.74~3.61 range(1.65, on average), which indicates the silicon comes from biological activities and hydrothermal injection. Meanwhile, the ratio of TFe₂O₃/TiO₂ and Al₂O₃/(Al₂O₃+Fe₂O₃) are evidently low to 2.16~12.56 and 0.65~0.87 Y is positive anomaly(Y/Ho value is 34.81~60.19) and Ce is negative anomaly(Ce/Ce^* value is 0.28~0.92), implying that these rocks were deposited in an ocean basin with no input of terrigenous materials. Negative anomaly of Ce are less evident to upside, which records the expansion of the passive continental margin rift basin on the northern Yangtze block.