摘要
【目的】寒武纪早期是地史时期重要的地质和生物突变时期,不仅在于多门类的带骨骼的后生动物同时出现,而且此时期形成了重要的金属矿产。华南广泛分布的牛蹄塘组就是该时期重要的地层。研究该层位的元素地球化学特征,不仅能反映该时期的沉积成矿环境,且对揭示早期生命演化与环境之间的协同关系具有重要科学意义。【方法】通过对贵州瓮安山屯剖面的岩石学、矿物学和地球化学特征的分析,探讨牛蹄塘组的沉积环境。【结果】贵州瓮安磷矿区的牛蹄塘组可明显划分为上磷矿段和黑色页岩段,多金属层位于二者岩性过渡部位,而牛蹄塘组生物群则主要赋存于多金属层上部及上覆黑色页岩段的底部。上磷矿段底部常见形态不规则的白云岩砾屑,上覆的磷块岩及多金属矿石发育砂砾屑结构,海绿石多呈不规则形态,表明该时期古海洋局部水体具有较强的水动力。多金属层及紧覆其上黑色页岩中发育重晶石透镜体和黄铁矿纹层,矿石中常见黄铁矿、黄铜矿和硫镍矿等矿物共生组合,与现代洋底热液黑烟囱的矿物特征基本一致,代表了海底热液沉积的特征。牛蹄塘组元素地球化学指标(V/Cr、Ni/Co、U/Th和Sr/Ba)显示其沉积环境整体处于缺氧条件,与岩相特征一致。但上磷矿段上部、多金属层及紧覆其上的黑色页岩段底部出现了一系列异常:Sr/Ba比值(Sr/Ba<1)降至最低,而V/Cr、Ni/Co、U/Th比值(U/Th>1)以及P与多种微量元素的含量均达峰值。该层位部分样品呈现明显Ce负异常及轻稀土微富集特征,黑色页岩段中部还出现Eu正异常,反映了热液流体与正常海水混合的沉积环境。多金属层代表了一次古海洋热液上涌事件,该事件可分为三个阶段,其中第二阶段热液贡献最为显著,与古生物记录高度吻合。【结论】元素地球化学特征进一步证明该时期存在海底热液活动。基于此,综合沉积地质背景重建了瓮安磷矿区牛蹄塘组生物群出现前后的古环境演变模式。
[Objective]The early Cambrian period represents a significant epoch of geological and biological transformation in Earth′s history.This period witnessed not only the emergence of numerous skeletal-bearing metazoans but also the formation of significant metal mineral deposits.The Niutitang Formation,widely distributed across South China,represents an important stratigraphic unit from this period.Researching the characteristics of elemental geochemistry in Niutitang Formation reflects not only the environment of sedimentation and deposition of this period,but also it is significant to the exploration of early life evolution and environmental synergy[.Methods]By means of petrological,mineralogical,and geochemical analysis of the Shantun section in Weng′an,Guizhou,the sedimentary environment of the Niutitang Formation is discussed.[Results]The Niutitang Formation in the Weng′an phosphate mining area of Guizhou is clearly divided into the upper phosphate rock and black shale members,with the polymetallic layer located at the transition between these two lithological sections.The Niutitang Formation biota is located in the upper part of the polymetallic layer and at the bottom of the black shale that covers it,suggesting a close relationship between the appearance and prosperity of the Niutitang Formation biota and the special geological event represented by the polymetallic layer.Irregular dolomite debris is commonly found at the bottom of the upper phosphate rock member,with overlying phosphate rock and polymetallic ore developing gravel debris structures,and the morphology of glauconite is commonly irregular,which reflects the stronger hydrodynamic force of the local environment of the ancient ocean at that time.Barite lenses and pyrite laminations are developed in the polymetallic layer and the black shale overlying it.Pyrite,chalcopyrite,and sulfide nickel minerals are commonly found coexisting in the ore,which are consistent with the characteristics of modern ocean floor hydrothermal black chimneys.The geochemical V/Cr and Ni/Co characteristics of the Niutitang Formation indicate that the ancient marine environment during its sedimentary period was anoxic or hypoxic,which is consistent with the sedimentary characteristics of the Niutitang Formation.However,a series of abnormal values were observed in the upper part of the upper phosphate rock member,the polymetallic layer,and the black shale bottom layer closely covering it:V/Cr,Ni/Co,U/Th(U/Th>1),P,and various trace element contents reached their peak,and Sr/Ba(Sr/Ba<1)reached its lowest value.In this layer,samples exhibit clear negative Ce anomalies and slight enrichment of light rare earth elements.Additionally,a sample in the middle of the black shale member showed a positive Eu anomaly.These rare earth element characteristics fall between hydrothermal fluids and typical seawater.In summary,the geological event represented by the polymetallic layer is a hydrothermal upwelling activity event in ancient oceans.Hydrothermal activity can be divided into three distinct stages,with the second stage having the highest hydrothermal composition and being highly consistent with paleontological records.[Conclusions]The geochemical characteristics of the elements further prove the existence of submarine hydrothermal activity during this period.Based on this,a comprehensive reconstruction of the paleoenvironmental evolution model of the Niutitang Formation biota before and after its appearance in the Weng′an phosphate mining area was conducted,taking into account the sedimentary geological background.
作者
庞艳春
林丽
PANG YanChun;LIN Li(Institute of Sedimentary Geology,Chengdu University of Technology,Chengdu 610059,China)
出处
《沉积学报》
北大核心
2025年第5期1796-1813,共18页
Acta Sedimentologica Sinica
基金
国家自然科学基金项目(41173058,41872007)。
关键词
寒武系底部
牛蹄塘组生物群
多金属层
热液活动
充氧事件
bottom of the Cambrian,Niutitang Formation biota
polymetallic layer
hydrothermal activity
oxygenation event
