塘沽地区沙河街组下部含云质泥岩主微量元素地球化学特征及地质意义

细粒泥岩记录并反映着源区及沉积区的重要地质信息.塘沽地区沙河街组下部发育一套湖相含云质泥岩, 为了解该地区的构造背景、沉积背景、风化程度以及物源属性, 对25块含云质泥岩的地球化学特征进行了全面的分析.经"去白云石"调整后, 各类元素地球地化参数及图解反映出了一致的地质信息.其中, 高∑REE值(154.0×10-6~219.3×10-6)、中等强度的负铕异常(0.64~0.73)以及包括K₂O/Na₂O-SiO₂, SiO₂/Al₂O₃-K₂O/Na₂O和判别函数几类双变量图解均指示塘沽地区具有主动大陆边缘(安第斯型)的构造背景; SiO₂-(K₂O+Na₂O+Al₂O₃)图解指示岩石沉积期气候干旱, 相当B(200.17~313.21)指示沉积水体为咸水类型, 近似黄铁矿矿化度(approximate degree of pyritization, 简称DOP_apx)(0.02~0.38)、自生铀(0.14~1.22)及U/Th(0.17~0.44)指示水体具备常氧属性; 化学蚀变指数(chemical index of alteration, 简称CIA)(51.26~65.74), 长石蚀变指数(plagioclase index of alteration, 简称PIA)(51.65~78.06), 修正成分变异指数(modified index of composition variation, 简称ICV_m)(0.67~1.24)及A-CN-K图解反映了母岩经受了弱-中等强度的风化程度; 判别函数图解、K-Rb双变量判别图解、A-CN-K图解、Al₂O₃/TiO₂指标(23.37~28.58)、与上地壳(upper continental crust, 简称UCC)相似的稀土配分模式及负铕异常则共同确定长英质岩石为含云质泥岩的母岩来源.在上述背景条件的约束下, 研究表明塘沽地区沙河街组含云质泥岩的物质来源应主要由燕山褶皱带提供. 

Major and Trace Elements Geochemistry and Geological Implications of Dolomite-Bearing Mudstones in Lower Part of Shahejie Formation in Tanggu Area,Eastern China

Fine mudstone can record and reflect geological aspects of source and sedimentary area. A set of lacustrine dolomite-bearing mudstones is developed in Lower Shahejie Formation of Tanggu area. To understand its tectonic setting, depositional setting, weathering and provenance, the geochemical characteristics of 25 rock samples were analyzed thoroughly. With dolomite-free modification, different indices and diagrams reveal identical geological information. Active continental margin setting (Andean-type) is revealed by the combination of high ∑REE(154.0×10-6-219.3×10-6), moderate negative Eu anomaly (0.64-0.73) and tectonic setting discriminant diagram (K₂O/Na₂O-SiO₂, SiO₂/Al₂O₃-K₂O/Na₂O, and discriminant function bivariate diagram). SiO₂-(K₂O+Na₂O+Al₂O₃) bivariate discriminant diagram, equivalent B(200.17-313.21) and approximate degree of pyritization (DOP_apx) (0.02~0.38), authigenic U(0.14-1.22), U/Th(0.17-0.44) indicate arid, saline and oxic depositional condition respectively. Chemical index of alteration (CIA)(51.26-65.74), plagioclase index of alteration (PIA)(51.65-78.06), modified index of composition variation (ICV_m)(0.67-1.24) and A-CN-K diagram reflect mild to moderate weathering of source rock. Discriminant function diagram, K-Rb bivariate diagram, A-CN-K diagram, Al₂O₃/TiO₂ index(23.37-28.58), REE pattern similar to upper continental crust (UCC) and negative Eu anomaly deduce the felsic rocks as the major source for the dolomite-bearing mudstones. Under such geological constrain, it is concluded that the source materials are mostly derived from Yanshan fold and thrust belt.