贵州渣拉沟剖面下寒武统黑色硅质岩微量元素富集机制

The mechanisms for the enrichment of trace elements in the Lower Cambrian black chert successions from Zhalagou section,Guizhou Province

华南早寒武世发育了一套富有机质黑色硅质岩,其成因尚有较大争议。选择贵州省三都县渣拉沟剖面早寒武世牛蹄塘组底部硅质岩段进行研究,发现该硅质岩具有微量元素富集的特征。其可能的原因包括,静海环境、上升流和热液活动。Ce/Ce*比值显示硅质岩沉积时水体为次氧化条件,不支持静海环境;而Ba、Zn、Cu、Ni、Cd、P与有机碳含量不存在相关关系,也不支持上升流为微量元素富集的主要原因。Al-Fe-Mn三角图、Al2O3/(Fe2O3+Al2O3)、REY配分模式等指标则表明存在热液活动。上述认识与现代上升流、静海环境的微量元素富集系数对比结果相吻合。与华南地区早寒武世初期不同沉积环境硅质岩对比,发现:深海环境并不富集Ba、Zn、Cu、Ni等氧化还原敏感元素,而沿着斜坡相带同沉积断层分布的热液活动,更有可能造成这些微量元素的富集。

Black chert of Early Cambrian age（Niutitang Formation or equivalents）,in South China,show high TOC.The origin of the chert was controversial and poorly understood.The black chert of the Lower Cambrian Niutitang Formation from the Zhalagou section in Sandu County,Guizhou Province,contains high concentrations of trace metals.Three possibilities may have caused enrichment in the trace elements,including euxinic seawater,upwelling and hydrothermal fluids.The moderate to large negative Ce anomalies suggest a semi-oxic environment and thus argue against the euxinic seawater as the source of the trace elements.Upwelling is also less possible due to the lack of positive relationships of Ba,Zn,Cu,Ni,Cd and P to TOC.In contrast,the most likely source for the trace elements are hydrothermal fluids as is evidenced by ternary diagram of Al-Fe-Mn,the ratio of Al2O3/（Fe2O3＋Al2O3） and REY distribution patterns.Compared with modern organic matter-rich sediments from euxinic basin and Peruvian upwelling,Early Cambrian chert shows significant difference in trace element enrichment factors,suggesting that it is unlikely for a unique seawater source to have the metal anomalies.In contrast,the trace elements must have added from a non-seawater source.By further comparison with time-equivalent neighbouring deep basin and slope facies chert successions,it can be found that the deep basin seawater components during the Early Cambrian had nothing in common with the slope environment in terms of enrichment of trace elements,such as Ba,Zn,Cu,Ni and Cd.By contrast,hydrothermal fluids vented from syndepositional fault along the margin-to-slope of Yangtze platform may have played a key role in trace elements enrichment in South China.