长江三峡地区成冰纪-埃迪卡拉纪转换时期微量元素和稀土元素地球化学特征

摘要

通过微量元素和稀土元素地球化学分析，对长江三峡地区陡山沱组层型剖面---田家园子剖面成冰系南沱组顶部和埃迪卡拉系陡山沱组下部102个岩石样品进行了地球化学研究。重点分析了氧化还原敏感元素（Zn，Co，U，Mo，Ni，V）的富集特征，并探讨其可能成因机制以及三峡地区成冰纪-埃迪卡拉纪转换时期的水体特征。结果表明：在南沱组顶部仅 Zn和 Co 富集；在盖帽白云岩下部，氧化还原敏感元素均富集，而在盖帽白云岩上部，除 Zn和 V 外，其他氧化还原敏感元素均亏损；在陡山沱组Ⅱ段下部，氧化还原敏感元素由最初的亏损，逐渐变为较稳定的富集。在陡山沱组下部，出现2次明显的富集峰值，分别出现在剖面的0.4m 处（盖帽白云岩中间）和6.5m处（陡山沱组Ⅱ段下部）。整个剖面大部分样品具有 Eu 的轻微正异常（Eu/Eu*＜1.6），而在剖面0.4m和6.5m处，Eu具有明显的正异常，结合稀土配分类型、Y/Ho值、La 异常、Ce 异常等指标，推测这2次异常均可能受到深海热液流的影响，而缺氧海水的上涌造成水体缺氧，导致这些元素出现富集峰值。U/Th、V/（V＋Ni）以及稀土元素指标综合指示，三峡地区南沱组顶部冰碛岩应为氧化环境下的沉积物；随着冰川消融，冰融淡水注入古海洋，陡山沱组盖帽碳酸盐的沉积受冰融淡水的影响，深部缺氧海水的上涌使沉积水体经历氧化-缺氧-氧化的转变，海水的分层性较强；而陡山沱组Ⅱ段下部沉积环境以分层性较弱的弱氧化环境为主。%Geochemical characteristics of trace elements and rare earth elements were analyzed on1 02 rock samples including tillites from the top of Nantuo Formation in Marinoan glaciation and carbonate from the lower part of Doushantuo Formation (cap carbonate of Member Ⅰ and lower part of Member Ⅱ) of Ediacaran in Tianjiayuanzi section,the stratotype section of Doushantuo Formation,in Yangtze Gorges area.Enrichment characteristics of redox-sensitive elements (RSEs)(Zn,Co,U,Mo,Ni and V) and its probable genetic mechanism and depositional water environment during the Cryogenian-Ediacaran tran-sition in Yangtze Gorges area were systemically researched.The results indicated that only Zn and Co were enriched in the Nantuo Formation,in the lower part of cap carbonate all RSEs changed to be enriched, while in the top of cap carbonate all RSEs were depleted except Zn and V.In the lower of Member Ⅱ of Doushantuo Formation,RSEs changed from depleted in the lower part to relatively stable in the upper.In the lower Doushantuo Formation,there were twice significant enrichments of RSEs.One happened in 0.4 m position of cap carbonate,and the other happened in 6.5 m position of the section (the lower part of Member Ⅱ of Doushantuo Formation ).Most of the rock samples had slightly positive Eu anomalies (Eu/Eu*<1.6),but two significant positive Eu anomalies happened at the same places with the two sig-nificant enrichments of RSEs.Combined with palaeoenvironmental proxies (REE distribution patterns, Y/Ho,Ce anomalies,and La anomalies)in Doushantuo Formation,it was suggested that both of the a-nomalies were derived primarily from hydrothermal fluids,and upwelling of anoxic water caused those ele-ments reach to peak.U/Th,V/(V+Ni)values and palaeoenvironmental proxies of REE suggested that the sedimentary environment during the top of Nantuo Formation was oxic on the whole,and with glacial ablation,deposition of cap carbonate was affected by melting ice water and sedimentary environment had endured an oxic-anoxic-oxic change with an increasing of the water column stratification.In the lower part of MemberⅡ of Doushantuo Formation,the depositional environment was mainly suboxic and with a weak water column stratification.