Element Geochemical characteristics and Their Geological Significance of Late Ordovician-Early Silurian black shale in Zhaotong area of northern Yunnan and Guizhou

Key words: 
• Keywords: black shale /
• Geochemistry /
• Source analysis /
• construct backgrounds /
• Zhaotong Demonstration Area

Abstract: Abstract: Located at the edge of the Sichuan Basin, the Zhaotong Demonstration Area of Northern Yunnan and Guizhou was relatively shallower and closer to the source area than the Changning-Weiyuan area in the basin during the Late Ordovician-Early Silurian period, so the sedimentation of shale of the Wufeng Formation-Longmaxi Formation in this area may be different from that in the basin. In order to clarify the siliceous genesis, material source and source background of the late Ordovician-Early Silurian black shale in the Zhaotong Demonstration Area of Northern Yunnan Province, relevant research was carried out based on the test data of principal elements, trace elements and rare earth elements of core samples in the study area. The results show that the siliceous in the black shale in the study area mainly comes from siliceous organisms and terrestrial clastic materials, and the biogenic silicon first increases and then decreases from bottom to top, showing the opposite change law of terrestrial source silicon. There was a brief glacial period in the Late Ordovician, until the melting of the Early Silurian glaciers, the occurrence of marine intrusions, and then the gradual decline of sea level, the continuous shallow of water bodies, the overall manifestation of terrestrial input is characterized by first decreasing and then increasing, and the sedimentation rate also shows the same law. The standardized distribution mode of rare earth elements reflects that there may be a mixture source during the deposition of shale in the Wufeng-Longmaxi Formation in the study area. The chart of ΣREE-La/Yb and La/Sc-Co/Th indicate that their original materials may mainly come from acid granite and sedimentary rocks, and are affected by certain seafloor hydrothermal fluids during sedimentation. Sc/Cr-La/Y diagrams, SiO2/Al2O3-K2O/Na2O bivariate discriminant diagrams, and some trace and rare earth element ratios comprehensively reflect that the tectonic background of the shale source area of the Wufeng Formation-Longmaxi Formation is mainly passive continental margin.