Geochemical Characteristics And Geological Significance Of The Lower Cambrian Black Rock Series In Northwestern Hunan

The black rock series is stable in northwestern Hunan,which is one of the typical areas of the Lower Cambrian black rock series in South China.As a product of environmental mutations in the geological history,the black rock series is a hot spot for scholars to study the paleo-sedimentary environment.This paper takes the black rock series of the Lower Cambrian in the Laodaowan section of the northwest Hunan Province as the research object.Scanning Electron Microscopy and geochemical tests were used to study the distribution characteristics of frambiod pyrite and the main-trace elements of the whole rock in the black rock series of the Late Sinian-Early Cambrian.The enrichment regularity,sedimentary environment and material source of black rock series has been analysed and the following findings have been achieved:?1?The black rock series is obviously enriched in Mo,U,V,Cd,Tl,relatively enriched in Cu,Ni,Zn,and depleted in elements such as Co,Mn,and Ge in study area.The total content of REE is low,and the overall performance shows that the light REE are relatively enriched,the heavy REE are relatively depleted,and the?Ce is obviously negative.The main and trace elements show that the black rock series is characterized by marine hydrothermal sediments:black shale and dolomite are mainly deposited in normal marine facies with the participation of hydrothermal process.Silicate and phosphorus nodules are strongly affected by seawater hydrothermal.?2?Morphological characteristics of pyrite were observed under SEM,black shales and siliceous rocks are mainly frambiod pyrite with redox indicating significance,and dolomite samples are mainly automorphic,no frambiod pyrite is found.The grain distribution range of frambiod pyrite was measured,between 3?m and 7?m;The average grain size of the six samples increased at 44.60?m?5.01?m?4.99?m?5.79?m?5.47?m?6.11?m.It reflects that the black shale is formed in a euinxic?anoxic?suboxic environment from bottom to top in the Niutitang group.?3?Through the change regulation of redox characteristic element ratios U/Th,V/Cr,Ni/Co,V/?V+Ni?,V/Sc,Mo_EF-U_EF covariation relationship and?Ce anomaly values on the section,the sedimentary environment undergoes a weak oxic?oxic?reduction?euinxic??anoxic?suboxic change from bottom to top in the Laodaowan section.The transition of environment from oxic to anoxic occurs at the boundary between the Dengying group and the Niutitang group.The dolomite at the top of the Dengying group is generally formed in an oxic environment,but the siliceous rocks and black shale at the bottom of the Niutitang group are generally formed in an environment of anoxic reduction.?4?The index of CaO/?Fe+CaO???MgO/Al₂O₃?×100 shows that the salinity of seawater decreases gradually,and the hydrodynamic indicator Zr/Rb shows that the hydrodynamic force is gradually weakening.It reflects the general rise of sea level at that time,which coincides with the environmental changes from oxic to anoxic reduction.?5?The black shale of the Niutitang group of the Lower Cambrian in the study area was formed in the shallow sea tectonic setting of the continental margin.The material source was mainly continental crust clastic,and the source rock was feldspathic rock with less alkaline feldspar.