鄂西宜昌地区寒武系页岩过剩钡成因及其对有机质富集的指示  

Origin of excess barium in the Cambrian shale of Yichang area,western Hubei,and its implication for organic matter accumulation

作　　者：罗欢 邵德勇 孟康 张瑜[2] 宋辉 闫建萍 张同伟 LUO Huan;SHAO Deyong;MENG Kang;ZHANG Yu;SONG Hui;YAN Jianping;ZHANG Tongwei(Gansu Key Laboratory of Mineral Resources in Western China,School of Earth Sciences,Lanzhou University,Lanzhou 730000,China;State Key Laboratory of Continental Dynamics,Department of Geology,Northwest University,Xi’an 710069,China;Bureau of Economic Geology,The University of Texas at Austin,Austin TX 78713,USA)

机构地区：[1]兰州大学地质科学与矿产资源学院,甘肃省西部矿产资源重点实验室,甘肃兰州730000 [2]西北大学地质学系大陆动力学国家重点实验室,陕西西安710069 [3]Bureau of Economic Geology,The University of Texas at Austin,Austin TX 78713,USA

出　　处：《地学前缘》2023年第3期66-82,共17页Earth Science Frontiers

基　　金：国家自然科学基金重点项目“中国南方寒武系页岩有机质、流体和孔隙演化耦合机制研究(41730421)”。

摘　　要：通过对鄂西宜昌地区鄂阳页1井(EYY1)寒武系底部岩家河组和水井沱组下部的总有机碳(TOC)、总无机碳(TIC)、微量元素(Ba、U、Th)和稀土元素分析,并结合前人对中上扬子地区7个位于不同沉积环境区寒武系剖面的研究结果和已有数据报道,讨论了寒武系水井沱组富有机质页岩中过剩钡(Ba过剩)的成因、来源及其对海洋古生产力水平的指示。结果表明,中上扬子地区寒武系富有机质页岩普遍富集Ba过剩,而且从浅水内陆棚到深水外陆棚——大陆斜坡,再到热水沉积区,寒武系页岩的Ba过剩富集程度趋于增加,反映了与古陆物源区的距离远近以及古地理背景对Ba过剩富集的控制。寒武系富有机质页岩中所富集的Ba过剩有热液成因和生物成因两种来源,Ba过剩-Eu异常图版可用于推测其主要来源:黔北—湘西北地区所处的热水沉积区内Ba过剩含量极高(>10000μg/g),具有显著的正Eu异常,且Ba过剩与Eu异常值具有正相关性,指示Ba过剩主要为热液成因,其含量变化反映了热液活动的强度;鄂西地区和黔南地区Ba过剩含量相对较高(5000~10000μg/g),四川盆地西部Ba过剩含量相对较低(<1000μg/g),而且都不具备正Eu异常,指示Ba过剩为生物成因,其含量变化与古生产力水平有关。与中上扬子其他地区寒武系页岩的TOC含量、U/Th值及Ba过剩富集程度的综合对比表明,宜昌地区较高的古生产力水平和强还原条件为有机质的来源和保存提供了物质基础和有利条件,共同决定了寒武系水井沱组页岩有机质的富集。We analyzed the total organic and inorganic carbon,trace elements,and rare earth elements in shale samples from the Lower Cambrian Yanjiahe and lower Shuijingtuo Formations of well EYY1,Yichang area,western Hubei Province.Combining with already published studies and data on other 7 Cambrian sections in the middle-upper Yangtze region,we discussed the origin and source of excess barium(Ba ex)in the organic-rich Cambrian Shuijingtuo shale and their implications for paleo-productivity.According to our analysis,there is a general barium enrichment in the organic-rich Cambrian shale of the middle-upper Yangtze region.The Ba ex level increases from the shallow-water inner shelf to the deepwater outer shelf and slope then to the hydrothermal depositional area,which implies that the Ba ex level increase is controlled by the distance from terrigenous source and by the paleo-geographic background.The excess Ba is of hydrothermal or biogenic origins,and the Ba ex-Eu anomaly plot can be utilized to infer its main source.The Ba ex levels are extremely high(>10000μg/g)in the hydrothermal depositional regions across northern Guizhou to northwest Hunan,with obvious positive Eu anomalies in these regions.The anomaly values are positively correlated with Ba ex levels,indicating that Ba ex is mainly of hydrothermal origin and change in Ba ex reflects the intensity of hydrothermal activity.In contrast,the Ba ex levels are not as high(500010000μg/g)in western Hubei and southern Guizhou and relatively low(<1000μg/g)in the western Sichuan Basin,where no positive Eu anomalies are present in these regions,indicating Ba ex is of biogenic origin and change in Ba ex is related to paleo-productivity level.By comparing TOC contents,U/Th ratios,and Ba ex levels in Cambrian shales from different areas of middle-upper Yangtze,we found the Yichang area has a relatively high paleo-productivity level and strong reducing depositional environment,which provides rich organic source and favorable preservation conditions for organic matter accumulati

关 键 词：中上扬子地区 寒武系 水井沱组 鄂阳页1井 富有机质页岩 过剩钡 稀土元素 

分 类 号：P595[天文地球—地球化学]