Element Geochemical Characteristics and Their Geological Significance of Late Ordovician-Early Silurian Black Shale in Zhaotong Area of Northern Yunnan and Guizhou
[Objective]Located at the southern margin 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 ar-ea in the basin during the Late Ordovician-Early Silurian period.Therefore,the deposition of the Wufeng Formation-Longmaxi Formation shale in this area may be different from that in the basin,it is of great significance to clarify the siliceous genesis,provenance,and tectonic setting of the Late Ordovician-Early Silurian black shale in the Zhaotong area of northern Yunnan and Guizhou.[Methods]Using the Taiyang Block in the Zhaotong area as the research re-gion,this study explores the source area background,source rock properties,and siliceous sources of the shale in the Wufeng Formation-Longmaxi Formation in the region by utilizing the testing data of major elements,trace ele-ments,and rare earth elements from four wells within the area.[Results]The results show that the silica of the black shale mainly came from siliceous organisms and terrestrial clastic materials,and the biogenic silicon first increases and then decreases from bottom to top,showing a trend opposite that 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 intru-sions,and then the gradual falling of sea level,leading to the continuous shallow water bodies.The overall manifesta-tion of terrestrial input is characterized by first decreasing and then increasing,and the sedimentation rate also shows the same trend.The standardized distribution mode curve of chondrite meteorites of rare earth elements in certain samples fluctuated slightly,indicating that there may be a mixture source during the deposition of shale in the Wufeng Formation-Longmaxi Formation in the study area.[Conclusions]Therefore,the parent materials of the Wufeng Formation-Longmaxi Formation shale in the study area may come from granite and sedimentary rocks of the Kangdian paleocontinent and the central Guizhou uplift,which is affected by certain seafloor hydrothermal fluids dur-ing sedimentation,but the original hydrothermal components are extremely small.The relevant indicators compre-hensively reflect that the tectonic background of the shale source area of the Wufeng Formation-Longmaxi Formation is a passive continental margin.
black shalegeochemistryprovenance analysistectonic backgroundnorthern Yunnan and Guizhou
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