早寒武世早期热液沉积特征:以塔里木盆地西北缘玉尔吐斯组底部硅质岩系为例

埃迪卡拉纪-寒武纪重要地质历史转折时期全球多陆块广泛沉积硅质岩.塔里木盆地西北缘下寒武统玉尔吐斯组底部所发育的薄层状硅质岩，以富含残余颗粒、磷质结核和重晶石结核而显得非常特殊和罕见，明显有别于国内外报道的常规硅质岩.为了揭示该套硅质岩的成因及沉积模式，通过剖面测量（库勒剖面）、薄片鉴定、SEM观察、能谱探针、主微量元素特征、氧硅同位素分布等岩石学-地球化学分析方法，认为玉尔吐斯组硅质岩垂向上共分5小段，其主要矿物是石英、白云石，自生矿物主要有重晶石、方沸石、黄铁矿等，不同硅质岩段微观结构显示出十分明显的组构变化，包含残余颗粒结构、残余晶粒结构、隐晶质结构和微晶结构.所测剖面硅质岩沉积于靠近大陆/古隆起的高盐度浅海陆架（高Al₂O₃/（Al₂O₃+Fe₂O₃）比值，高MgO/Al₂O₃比值），其陆源碎屑主要来自大陆地壳或基底花岗岩的风化产物，并且受到SiO₂的稀释作用影响（Th/Ti与Al/Ti关系）.氧化还原敏感元素（如Ba、V、Ni、Cu、Zn、U等）不同程度的富集及氧化还原指标（V/Cr介于0.81~8.34）特征表明硅质岩段沉积早期处于氧化环境，水体能量较强，显微镜下见圆度较好的内碎屑颗粒、微生物席碎片；中晚期经历了水体循环不畅的有氧/缺氧分层环境（Ce/Ce*负异常），显微镜下微生物球粒、粪球粒、浮游藻（粘结）团块发育，有机质含量较高（TOC可达2%以上）.稀土元素特征（低∑REE、中等Ce负异常、显著的Y正异常和Eu正异常）以及δ30Si和δ18O值分布范围暗示硅质岩地球化学特征主要继承自热液流体和海水，受陆源碎屑影响较弱.自近台内裂陷中心上升的富含多金属的富硅热液以及陆源碎屑提供大量营养，为微生物大繁盛和硅质的富集成矿奠定物质基础.水体中硅的溶解度降低（温度降低、pH降低和水体盐度升高）促使硅质胶体颗粒化学沉淀，而微生物（特别是嗜热微生物）的新陈代谢活动往往加速了这一过程. 

Depositional Characteristics of Early Cambrian Hydrothermal Fluid: A Case Study of Siliceous Rocks from Yurtus Formation in Aksu Area of Tarim Basin,Northwest China

Siliceous rocks were widely deposited in many continents during an important geological period, Ediacaran-Cambriam transition. The well developed thin-bedded siliceous rocks in the bottom of Yurtus Formation, northwestern margin of Tarim basin, rich in residual grains, phosphatic nodules and barite concretions, are very special and rare, which is obviously different from conventional siliceous rocks reported at home and abroad. In order to reveal the causes and sedimentary model of this set of siliceous rocks, the advantages of petrological and geochemical methods were used, such as detailed profile measurement (Kule profile), SEM observation, energy spectrum probe, characteristics of main and trace elements, oxygen and silicon isotope distribution. The siliceous rock section can be divided into five units in vertical direction. Its main minerals are quartz and dolomite, and authigenic minerals are mainly pyrite, barite and zeolite; the microstructures of different siliceous rock units show obvious fabric variation, including residual grain structure, residual crystal structure, cryptocrystalline structure and micro-crystal structure. Siliceous rocks of measured profile are deposited in shallow continental shelf near the continental or paleo-uplift with high salinity (high Al₂O₃/(Al₂O₃+Fe₂O₃) ratios, high MgO/Al₂O₃ ratios), and their terrigenous clastics, affected by the dilution effect of SiO₂ (Th/Ti and Al/Ti), are mainly from the weathering products of continental crust or basement granite. Redox sensitive elements (such as Ba, V, Ni, Cu, Zn and U) with different degrees of enrichment and redox indicators (V/Cr ratios from 0.81 to 8.34) indicate that siliceous rocks are developed in the oxidizing environment during early depositional period due to the strong water energy, also supported by many internal clastic particles and microbial mat fragments with good roundness seen under the microscope. And they experienced aerobic/anoxic stratified environment (negative Ce/Ce* anomaly) with limited water circulation during middle to late depositional period, also supported by well-development of microbial pellets, faecal pellets and phytoplankton (bonding) mass seen under the microscope with high organic matter content (TOC up to 2%). Characteristics of rare earth elements (low ∑REE, medium Ce negative anomaly, significant positive Y anomaly and positive Eu anomaly) and distribution range of δ30Si and δ18O values suggest that geochemical characteristics of siliceous rocks have mainly inherited from hydrothermal fluid and seawater, and the terrigenous influence is weak. The polymetallic and Si-rich hydrothermal fluid ascending from rift center near platform and terrigenous clastics provide plenty of nutrition which offers the material foundation for microbial prosperity and the mineralization of SiO₂. The decreased solubility of silicon dissolved in water column (due to temperature decreased, the decrease of pH and increase of salinity) promotes the chemical precipitation of silica colloidal particles, and the metabolic activity of microorganisms (especially thermophilic microorganisms) has accelerated this process.