扬子克拉通峡东地区新元古代-寒武纪黑色岩系Os同位素地球化学特征及其地质意义

Os Isotopic Geochemistry of Neoproterozoic-Cambrian Black Shales in Eastern Three Gorges of Yangtze Craton and Its Geological Significance

报道了扬子克拉通峡东地区新元古代至寒武纪含碳黑色泥质岩的Re-Os同位素和微量元素地球化学组成特征,并对地层沉积环境的演化性质及其地质意义进行了讨论.研究表明:(1)峡东地区新元古代晚期-寒武纪早期细粒碎屑沉积岩Os同位素初始比值呈规律变化,且具高Os同位素初始比值的层位与前人研究中发现的C同位素负漂移地层相对应;(2)南沱组冰碛岩具高放射成因Os同位素组成特征,而其上覆盖帽碳酸盐岩为低放射成因Os同位素比值,向上地层Os同位素初始比值表现为局部波动、总体增高的演化趋势;(3)微量元素U/Th比值与δU值指示莲沱组砂岩与南沱组冰碛岩具氧化环境的特征,而陡山沱组、灯影组以及水井沱组底部黑色泥质岩则形成于相对还原环境;(4)Y/Ho比值和δCe值的负相关性特征指示部分地层沉积过程中有海底热液物质参与,应为导致剖面上部分层位出现低放射性成因Os同位素组成的重要原因之一.高Os同位素比值和深海的脉冲式演化以及古生物群的产出之间很好的对应关系为探讨大陆风化与大气和海洋中氧气含量增加的联系提供了新的地球化学证据.南沱组冰碛岩Os同位素以及微量元素特征暗示其并非形成于完全冰封的地球表面环境,而是存在较强的大陆岩石风化作用.

This paper presents an integrated Re-Os isotope and trace element investigation of the Neoproterozoic to Cambrian organic-bearing black mudstones in the eastern Three Gorges, the Yangtze Craton. Their sedimentary environmental evolution and geological significance are then discussed. It shows that （1） initial Os isotopic ratios of fine-grained sedimentary rocks from the strata display regular variations along the stratigraphic column. Of the strata, layers with negative δ13C excursions are coupled with high initial Os isotopic ratios; （2） the Nantuo tillite displays higher initial Os isotopic ratio than that of the overlying cap carbonate, while the overlying successive strata show an increase trend with several fluctuations; （3） geochemical features of U/Th ratio and δU value for the samples suggest that the I.iantuo sandstone and Nantuo tillite were formed within oxidation environments, whereas black mudstones of the Doushantuo, Dengying and Shuijingtuo Formations were formed within reductive environments; （3） negative correlation between Y/Ho ratio and δCe value for the black mudstones is likely indicative of variable involvement of seafloor hydrothermal activity during some sediments deposited, which likely accounts for their low Os isotopic ratios. The correlation between bigh initial Os isotopic ratios and pulsed oxidation of deep ocean as well as appearance of metazoan in I.ate Neoproterozoic Early Cambrian afford a new clue for the linking of continental weathering extent with oxy gcn content in atmosphere and ocean. These Os isotopic and trace element features infer that the Nantuo tillites were formed in an environment where the earth＇s surface had not been fully covered by ice layer, and continent derived inputs by intensive weathering played an important role.