鄂西南利川二叠纪吴家坪组硅质岩成因及沉积环境

鄂西南利川吴家坪组硅质岩与海相石煤及多层火山粘土岩共生，产海绵骨针及放射虫化石，放射虫组合以泡沫虫类为主，内射球虫类次之，隐管虫类很少，代表外陆棚沉积环境。硅质岩的陆源元素Al、Ti含量较高，热水来源元素Fe、Mn含量较低，Al/(Al+Fe+Mn)值在0.64~0.83之间，平均值0.73。在Al-Fe-Mn三角图中，样品位于或紧邻非热水沉积区，在 SiO₂- (K₂O+Na₂O)、SiO₂-Al₂O₃， (K₂O+Na₂O)-Al₂O₃ 图解中大多数样品落在火山成因区，少数位于火山成因与生物成因区之间。相对于沉积岩的微量元素丰度，生命元素Mo、V富集，指示热水沉积的特征元素Ba、As、Sc亏损。∑REE较低，∑LREE/∑HREE平均值为3.02，Ce、Eu均轻度负异常（δCe:0.75~0.83，平均值0.79；δEu:0.61~0.95, 平均值0.84），经北美页岩标准化后稀土元素配分曲线近水平。以上地球化学特征表明研究区硅质岩属火山成因，并伴有生物作用。氧化还原判别指标U/Th、V/(V+Ni)及Ce_anom，沉积环境判别指标MnO/TiO₂，La_N/Yb_N 与判别图解100×TFe₂O₃/SiO₂-100 Al₂O₃/SiO₂、TFe₂O₃/(100-SiO₂)-Al₂O₃/(100-SiO₂)、TFe₂O₃/TiO₂-Al₂O₃/(Al₂O₃+TFe₂O₃)、La_N/Ce_N-Al₂O₃/(Al₂O₃+TFe₂O₃)与∑LREE/∑HREE-100×Eu/∑REE一致表明硅质岩形成于大陆边缘缺氧环境。

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

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, (K₂O+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.