焦作煤田石炭—二叠纪泥质岩地球化学特征及古环境意义

Geochemical Characteristics of Mudstones in the Permo-Carboniferous Strata of the Jiaozuo Coalfield and Their Paleoenvironmental Significance

泥质岩的地球化学特征蕴含了大量地质信息,在物源和沉积环境分析中的应用十分广泛。利用X射线荧光光谱仪和电感耦合等离子体质谱仪(ICP-MS)对华北板块南缘焦作煤田石炭—二叠纪泥质岩样品进行常量元素、微量元素(包括稀土元素)进行分析,并据此讨论泥质岩的物源性质、物源区岩石风化作用强度及当时古环境特征。研究区泥质岩的高Al_2O_3/TiO_2值及低Cr/Zr和K2O/Al_2O_3值特征,以及泥质岩Zr-TiO_2和Th/Co-La/Sc源岩判别图、Ni-Cr图和(Gd/Yb)N-Eu/Eu*图等的判别分析结果,都揭示了研究区泥质岩物源主要为后太古宙长英质岩石;Al_2O_3-CaO*+Na_2O-K_2O(A-CN-K)图和较高的黏土矿物成分反映了泥质岩源区受到钾交代作用的影响,致使CIA(化学蚀变指数)值被低估;高CIW(化学风化指数)值和Al_2O_3-CaO*-Na_2O(A-CN)判别图分析结果表明泥质岩物源区母岩在石炭—二叠纪经历了强烈的风化作用;Sr/Ba和B/Ga比值变化指示本溪组沉积时期到太原组沉积早期为海相为主的沉积环境,太原组沉积中期到山西组沉积早期为海陆过渡相为主的沉积环境,山西组沉积中期到上石盒子组沉积时期为陆相沉积环境。

Geochemical characteristics of mudstones contain a large amount of geological information,which are widely used in the analysis of provenance and sedimentary environments.The Permo-Carboniferous mudstones from the Jiaozuo Coalfield of Henan Province,northern China,were analyzed by means of X-ray fluorescence spectrometer and ICP-MS,and the major elements,trace elements,rare earth elements(REE)and minerals were determined.The provenance,source area weathering and paleoenvironmental characteristics are discussed.Argillaceous rocks were predominantly derived from post-Archean felsic igneous rocks based on the high values of Al 2 O 3/TiO 2,low values of Cr/Zr,K 2 O/Al 2 O 3 and plots of Zr-TiO 2,Th/Co-La/Sc,Ni-Cr and(Gd/Yb)N-Eu/Eu*.The values of CIA were underestimated because of potassium metasomatism of the argillaceous source area reflected by Al 2 O 3-CaO*+Na 2 O-K 2 O diagrams and high clay mineral composition.Palaeo-weathering indices(CIW ratios)and Al 2 O 3-CaO*-Na 2 O diagrams suggest that the source rocks were subjected to an extreme chemical weathering.Trace element composition characteristics(Sr/Ba,B/Ga)indicate that from the Benxi Formation to the early Taiyuan Formation,the sedimentary environment is marine facies,from the mid Taiyuan Formation to the early Shanxi Formation,the sedimentary environment is marine-continental transitional facies,from the mid Shanxi Formation to the Shangshihezi Formation,and the sedimentary environment is continental facies.