滇黔北昭通地区晚奥陶世—早志留世黑色页岩地球化学特征及其地质意义

Element Geochemical Characteristics and Their Geological Significance of Late Ordovician-Early Silurian Black Shale in Zhaotong Area of Northern Yunnan and Guizhou

【目的】滇黔北昭通示范区地处四川盆地边缘,在晚奥陶世—早志留世时期,相较于盆内的长宁—威远地区,其沉积水体相对更浅、更靠近物源区,该地区五峰组—龙马溪组页岩的沉积可能与盆内存在差异。因此,厘清滇黔北昭通示范区晚奥陶世—早志留世黑色页岩的硅质成因、物质来源以及源区背景,具有重要意义。【方法】以昭通示范区太阳区块为研究区,利用区内四口井样品的主量元素、微量元素和稀土元素测试数据,探讨研究区五峰组—龙马溪组页岩的源区背景、源岩属性及硅质来源。【结果】研究区五峰组—龙马溪组黑色页岩中的硅质主要来自硅质生物和陆源碎屑物质,生物成因硅自下而上先增加后减小,表现出与陆源成因硅相反的变化规律;晚奥陶世存在短暂的冰期,直到早志留世冰川发生融化,海侵事件发生,随后海平面逐渐下降,水体不断变浅,陆源输入总体表现为先减小再增加的特征,沉积速率也呈现相同的规律。部分样品稀土元素球粒陨石标准化配分模式曲线小幅度波动,反映研究区五峰组—龙马溪组页岩沉积可能存在混合物源。【结论】研究区五峰组—龙马溪组页岩原始物质可能主要来自康滇古陆和黔中隆起的花岗岩和沉积岩,沉积时受一定海底热液的影响,但原始热液组分极少;相关指标综合反映五峰组—龙马溪组页岩物源区的构造背景主要为被动大陆边缘。

[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 area 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 region,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 elements,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 intrusions,and then the gradual falling of sea level,leading to the continuous shallow water bodies.The overall manifestation 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 during sedimentation,but the original hydrothermal components are extremely small.The relevant indicators comprehensively reflect that the tectonic background of the shale source area of the Wufeng Formation-Longmaxi Formation is a passive continental margin.