湖南慈利溪口剖面埃迪卡拉系陡山沱组碳酸盐岩微量元素特征及其古环境意义

以湖南慈利溪口埃迪卡拉系陡山沱组剖面为研究对象,分析了64件碳酸盐岩样品的微量元素以及稀土元素特征。结果表明,利用U/Th、Ce异常和Ca/Mg等多种指标识别出的陡山沱期古氧化还原环境与层序地层学的研究结果基本一致,说明碳酸盐岩的这3个指标在重建埃迪卡拉纪古海洋环境的研究中具有一定的适用性。稀土元素配分模式在陡山沱组底部和中部均发生了明显的由典型淡水模式向典型古海水模式的转变,很可能指示了在Marinoan冰期和Gaskier冰期结束之后,大量的间冰期冰融淡水注入海洋并逐步被古海水取代。较高的Eu异常值以及前人在溪口剖面发现的多期古地震证据可能暗示了海底热液活动几乎贯穿于整个埃迪卡拉纪陡山沱期,且具有早期剧烈、晚期逐渐趋于平缓的特征。多期次的海底热液活动为古海洋提供了重要的磷质来源,对埃迪卡拉纪生物的出现与繁盛具有重要的意义。

Trace element characteristics of carbonate rocks from the Ediacaran Doushantuo Formation of Xikou section,Cili County,Hunan Province and its palaeoenvironmental significance

Trace element and rare earth element(REE)of 64 carbonate rock samples from the Ediacaran Doushantuo Formation of Xikou section,Cili County,Hunan Province are analyzed. Palaeoredox indicated by the combination of U/Th,Ce anomalies and Ca/Mg is in accordance with that by sequence stratigraphic studies,suggesting that the three proxies in carbonates are applicable in reconstructing palaeoredox environment of the Ediacaran. In the bottom and middle of Doushantuo Formation,the REE distributional pattern changes from typical fresh water to typical sea water,probably indicating the injecting of deglacial meltwater and thus being replaced by normal sea water during the post-Marinoan and post-Gaskier period. The strong Eu anomalies,relatively higher Eu value as well as the seismites discovered in Xikou section by previous research may suggest that the hydrothermal activity probably occurs during the whole Doushantuo Age,being strong in the early and then weakened in the late stage. The hydrothermal activity may be of great significance to the appearance and the bloom of Ediacaran biota because of the important phosphorus supply.