Petrogenesis and Sedimentary Environment of Permian Wujiaping Formation Siliceous rocks in Lichuan, Southwestern Hubei

Key words: 
• siliceous rocks

Abstract: The Upper Permian siliceous rocks of Wujiaping Formation develop in Lichuan, southwestern Hubei. Containing radiolarian and sponge, the siliceous rocks appear with stone coal and volcanic claystone. The radiolarian assemblage is dominated by Spumellaria, followed by Entactinaria and rarely Latentifistularia, which represents the outer shelf environment. The siliceous rocks are rich in terrestrial elements (for example Al, Ti), poor in hydrothermal elements (such as Fe, Mn). Their Al/(Al+Fe+Mn) values are 0.64~0.83, with an average of 0.73. In the Al-Fe-Mn ternary, chert samples locate in or close to Non-Hydrothermal siliceous rocks region. In SiO₂- (K₂O+Na₂O), SiO₂-Al₂O₃, (K^₂O+Na₂O)-Al₂O₃ binary discrimination diagram, samples plot in or near volcano-sedimentary region. Comparing with abundance of trace elements in sedimentary rocks, life elements: Mo and V assemble, hydrothermal elements: Ba, As,  Sc deplete. ∑REE is lower  and average ΣLREE/ΣHREE is 3.02, with Ce (δCe:0.75~0.83, average 0.79) and Eu (δEu: 0.61~0.95, average 0.84) slight negative anomaly. Their NASC-normalized REE distribution patterns are nearly horizontal. These geochemical characteristics prove that siliceous rocks  are formed by volcano sedimentation with biological effect. Redox indexes: U/Th, V/(V+Ni) and Ce_anom, sedimentary environment indicators: MnO/TiO₂, La_N/Yb_N and discrimination diagrams: 100×TFe₂O₃/SiO₂-100 Al₂O₃/SiO₂, TFe₂O₃/(100-SiO₂)-Al₂O₃/(100-SiO₂), TFe₂O₃/TiO₂-Al₂O₃/(Al₂O₃+TFe₂O₃), La_N/Ce_N-Al₂O₃/(Al₂O₃+TFe₂O₃) and ΣLREE/ΣHREE-100×Eu/ΣREE comprehensively make sure that the siliceous rocks deposited in continental shelf under anoxic condition.