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

被引：7
作者：
Yang Z. [1 ,2 ]
Luo P. [1 ,2 ]
Liu B. [1 ]
Zhou C. [2 ]
Ma J. [3 ]
机构：
[1] School of Earth and Space Sciences, Peking University, Beijing
[2] Tarim Branch, Research Institute of Petroleum Exploration & Development, Beijing
[3] Geophysical Team, Sichuan Provincial Bureau of Geology, Chengdu
来源：
Diqiu Kexue - Zhongguo Dizhi Daxue Xuebao/Earth Science - Journal of China University of Geosciences | 2019年 / 44卷 / 11期
关键词：
Forming cause; Geochemistry; Sedimentary model; Siliceous rock; Yurtus Formation;
D O I：
10.3799/dqkx.2017.502
中图分类号：
学科分类号：
摘要：
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. © 2019, Editorial Department of Earth Science. All right reserved.
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页码：3845 / 3870
页数：25
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