Trace Elements As Redox Paleoenvironments Proxies In Chuanlinggou Formation Of The Mesoproterozoic In North China And Their Geological Significances

The geochemical characteristics of trace elements in the sediments can be used to infer the evolutionary history of sedimentary basins,depositional environments,and the source of sediments.With the use of geochemical methods,by studying the characteristics trace elements of the dark shale from Chuanlinggou Formation in North China Platform,the ancient sedimentary envronments can be rebuilt.Redox conditions in marine settings controlled the concentrations of redox sensitive trace elements in sediments and sedimentary rocks.Thus,the concentrations of them could be used to reconstruct the paleoredox.Redox sensitive trace elements Mo,Re,V,U and Cd,have fewer origins and mostly do not move after deposition and burial.Consequently,the sediments and sedimentary rocks retained the primary concentrations of these elements and could reflect the characteristics of the depositional conditions.Mo,U,V,Re and Cd are the best recorders of redox conditions as a good value proxies for paleoredox.In oxic to suboxic conditions,Mo,Re,V,U and Cd do not enrich;however,Re,U and V enrich in anoxic and euxinic conditions but Mo just enriches in euxinic settings.Therefore,the combined use of U,V and Mo concentrations may allow us to distinguish the redox conditions.In this study,we collected 42 sediment samples of dark shale from Chuanlinggou Formation,and analyzed their redox sensitive trace elements.The results show that the total redox sensitive trace elements content are be lacked.So,we should use some traditional proxies for paleoredox,such as V/(V+Ni),Uaut,V/Cr,Ceanom andU/Th.Combined with these methods,we consider the Chuanlinggou Formation was formed in the anoxic environment.The abundances of rare earth elements(REE)of the dark shale from Chuanlinggou Formation were measured by using inductively coupled plasma mass spectrometry(ICP-MS).The results show that the total REE content is 217.52×10-6-412.05×10-6,the average one is 302.78×l0-6;the ratio of ?LREE to?HREE is 4.64-5.37,which shows that LREE are relatively more intensive than HREE,have negative Eu anomaly(?Eu=0.49-0.75).?Ce=0.899-0.978.They have a distinctively negative Ce-anomaly which reflects that a rock deposits under the condition of relatively hypoxic sea.The ratio of rock(La/Sm)N is between4.53and 5.09,which represents that it has added with the materials from deep resource.The chondrite normalized patterns of dark shale incline to the right.Combined with the discrimination diagrams of La/Ce and La/Yb-REE,the black shale of Chuanlinggou Formation formed in a hypoxia sea,with the participation of some hydrothermal fluids in the formation process.