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贵州遵义牛蹄塘组黑色岩系的硒同位素变化及其环境指示初探

A tentative discussion on the variation of selenium isotopes in black shale of Niutitang Formation,Zunyi,Guizhou Province,and its implications for depositional environment

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【作者】 朱建明Johnson Thomas M罗泰义朱祥坤

【Author】 ZHU Jian-ming1,2,JOHNSON Thomas M2,LUO Tai-yi3 and ZHU Xiang-kun4(1.State Key Lab of Environmental Geochemistry,Institute of Geochemistry,Chinese Academy of Sciences,Guiyang 550002,China;2.Department of Geology,245 NHB,University of Illinois at Urbana-Champaign,Urbana,IL 61801,USA;3.State Key Lab of Ore Deposit Geochemistry,Institute of Geochemistry,Chinese Academy of Sciences,Guiyang 550002,China;4.Key Lab of Isotopic Geology,Ministry of Land and Resources,Institute of Geology,Chinese Academy of Geological Sciences,Beijing 100037,China)

【机构】 中国科学院地球化学研究所环境地球化学国家重点实验室伊利诺伊大学厄巴纳-香槟分校地质系中国科学院地球化学研究所矿床地球化学国家重点实验室中国地质科学院地质研究所国土资源部同位素地质重点实验室 贵州贵阳550002伊利诺伊大学厄巴纳-香槟分校地质系厄巴纳61801美国贵州贵阳550002北京100037

【摘要】 西南早寒武世牛蹄塘组是分布在中国南方扬子地台的一套黑色岩系,区域上发育有典型富集多金属元素的硫化物矿化层。选取遵义松林小竹牛蹄塘组下部富硒、钼、镍等元素的黑色岩系剖面,利用Se同位素初步探讨了该剖面岩石的沉积环境和硒的可能来源。结果表明,小竹牛蹄塘组下部剖面黑色岩系的δ82/76SeSRM3149比值变化较大,下部底层含碳斑脱岩与磷块岩的变化范围在-4.35‰~+4.11‰之间;中间镍钼层及碳质页岩、碳质碳酸盐岩的δ82/76SeSRM3149变化范围窄,平均值为0.91‰±0.23‰(n=4);上层碳质页岩为-1.24‰。结合已发表的钼同位素数据,认为中间岩石沉积于缺氧/无氧环境,但存在盆地海水与热液或充氧水团的混合;下层的岩石曾一度位于充氧与贫氧环境的边界面,局部岩石曾暴露地表经历了较强的风化和蚀变作用,海水中硒有可能来自底部富硒斑脱岩的氧化淋滤或海底热液。据此推测遵义松林小竹牛蹄塘组下部岩石的沉积环境极可能处在局限盆地靠近陆地的边缘部分,经历了充氧→贫氧→缺氧/无氧→贫氧的演化阶段。

【Abstract】 Black shale of the Lower Cambrian Niutitang Formation is widely distributed in the Yangtze Platform of South China,and there is regionally developed a typical conformable polymetallic sulfide horizon.A lower profile of the black shale enriched in Se,Mo and other metal elements in Xiaozhu of Zunyi area was selected for Se isotope determination so as to investigate the probable source of Se and its depositional environment.The results show that remarkable variation of δ82/76SeSRM 3149 exists in the studied black shale sequence.The middle carbonaceous shale,Ni-Mo layer and carbonaceous carbonate have a narrow range of δ82/76SeSRM 3149,from +0.65‰ to +1.15‰ with a mean of 0.91‰±0.23 ‰(n=4);the upper carbonaceous shale has a value of-1.24‰;the bottom K-bentonite and phosphorite have a wide range of δ82/76SeSRM 3149 between-4.35‰ and +4.11‰.Combined with the published δ98/95MoJMC data of Ni-Mo layer in the Huangjiawan section,it is held that the middle black shale was deposited in an anoxic/euxinic environment,but an uncertain mixing process between basin seawater and oxic ocean water or hydrothermal fluid must have happened at that time.The bottom K-bentonite experienced a strong weathering and alteration,and the oxidization-reducing process probably occurred near the boundary between oxidizing water and anoxic water.Se in seawater might have been leached from the Se-rich bottom K-bentonite or tuff or from submarine hydrothermal fluid.Se isotopes indicate that the depositional environment of black shale in the Niutitang Formation of Zunyi area was possibly situated on the continental margin of a restricted basin and experienced different evolution stages from oxidizing-suboxic-anoxic/euxinic to suboxic.

【基金】 国家自然科学基金项目(40573050);国家863计划资助项目(2007AA06Z125);教育部留学回国人员科研启动基金
  • 【文献出处】 岩石矿物学杂志 ,Acta Petrologica Et Mineralogica , 编辑部邮箱 ,2008年04期
  • 【分类号】P597.2
  • 【被引频次】37
  • 【下载频次】771
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