贵州五河南沱组形成的氧化还原环境:地球化学与矿物学证据  

Oceanic redox condition of the Nantuo Formation at the Wuhe profile,Guizhou Province:Evidences from its geochemistry and mineralogy

作　　者：冯永 顾尚义[1,2] 吴忠银 沈洪娟 赵思凡 FENG Yong;GU Shang-yi;WU Zhong-yin;SHEN Hong-juan;ZHAO Si-fan(College of Resources and Environmental Engineering,Guizhou University,Guiyang 550025,China;Key Laboratory of Karst Geological Resources and Environment,Ministry of Education,Guiyang 550025,China;Guizhou Geological Environment Monitoring Institute,Guiyang 550000,China)

机构地区：[1]贵州大学资源与环境工程学院,贵州贵阳550025 [2]喀斯特地质资源与环境教育部重点实验室,贵州贵阳550025 [3]贵州省地质环境监测院,贵州贵阳550000

出　　处：《矿物学报》2021年第1期33-44,共12页Acta Mineralogica Sinica

基　　金：国家自然科学基金项目(编号:41663005)。

摘　　要：新元古代海洋氧化还原条件是影响这一时期生物演化的重要原因,海相沉积物记录的各种地球化学信息是研究古海洋氧化还原条件的主要方式。南沱冰期由于缺乏含铁建造等化学沉积岩及黑色页岩,极大地制约了对这一时期氧化还原环境的研究。本文通过对贵州剑河五河剖面南沱组红色富铁杂砾岩开展沉积学、矿物学和地球化学研究,认为杂砾岩中Si、Al、Fe含量与大陆上地壳组成类似,但Ca、Na、Mg显示显著的亏损特征,反映冰期大陆的氧化风化作用。杂砾岩中的氧化还原敏感元素Mo和U反映出不同程度的富集,其协变趋势指示了铁、锰氧化物在氧化还原分层的海水中的穿梭作用。杂砾岩的红色色调是其中赤铁矿的富集所致,红色富铁杂砾岩在南沱组中的不规则分布表明赤铁矿是同冰期海水中亚铁离子氧化后,经后期脱水形成。该剖面南沱组中流水波痕构造、氧化还原敏感元素Mo和U的富集特征和红色杂砾岩中赤铁矿的形成,反映华南地区南沱冰期极端缺氧的还原环境。The redox condition of the late Neoproterozoic ocean exerted fundamental impact on the biological evolution at the time.Various kinds of geochemical information preserved in marine sediments are mainly used to study oceanic redox conditions over geological time.However,during the Nantuo glacial period,the obvious lack of chemical sedimentary rocks such as banded iron formation and black shales resulted in the extremely limitation on the study of the redox environment during the period.In this paper,sedimentological,mineralogical,and geochemical studies on the red ferruginous diamictites of the Nantuo Formation at the Wuhe profile in the Jianhe area,Guizhou Province,have been conducted.The results indicate that contents of Si,Al,and Fe in the diamictites are similar to those of the Upper Continental Crust(UCC),but the contents of Ca,Mg,and Na in the diamictites are significantly lower than those of the UCC.These show that there could be a dominant oxidized weathering during the Nantuo glacial period.The redox sensitive elements of Mo and U are enriched at different degrees in the diamictites.Their co-variation trend in the diamictites suggests that FeMn(oxyhydr)oxides were transported through various leveled seawaters with different redox values in the ocean during the glacial period.The red color of diamictites was mainly caused by the enrichment of hematite in the rock.The irregular distribution of red ferruginous diamictites in the Nantuo Formation indicates that hematite could be formed by the dehydration of iron hydrate which was derived from the oxidized ferrous ion in the seawater at the time.The combined features of sedimentary structure of the wave ripple,the enrichment of redox sensitive elements of Mo and U,and the formation of hematite in the red ferruginous diamictites of the Natuo Formation at Wuhe profile suggest that there was an extremely anoxic reduced environment in the paleo-ocean in the South China during the Nantuo glacial period.

关 键 词：南沱冰期 红色杂砾岩 氧化还原环境 赤铁矿 形成机制 

分 类 号：P579[天文地球—矿物学] P591[天文地球—地质学]