华南下寒武统Ni-Mo多金属层的厘定及其古海洋环境意义

Identification of the Lower Cambrian polymetallic Ni-Mo layer in South Chinaand its paleo-ocean environmental significance

华南下寒武统牛蹄塘组底部及其相当地层发育一层Ni-Mo多金属层,被当作华南下寒武统区域对比的标志层。然而,目前华南部分重要剖面的Ni-Mo多金属层位置依然没有明确,妨碍了基于精确地层对比的寒武纪早期海洋环境信息的揭示。本研究中,我们对云南梅树村剖面和贵州平引剖面地层进行采样,采用电感耦合等离子质谱仪(ICP-MS)对样品微量元素含量进行分析,以确定这两条剖面的Ni-Mo多金属层位置。在此基础上,进一步对平引剖面牛蹄塘组底部样品进行Fe组分分析,结合前人数据,初步揭示华南下寒武统Ni-Mo多金属层沉积时期(~521 Ma)南华盆地海水的氧化还原信息。微量元素含量结果显示,梅树村剖面玉案山组底界之上3 m的地层(Ni:135×10^(-6),Mo:583×10^(-6))和平引剖面牛蹄塘组底界之上2.5 m的地层(Ni:2480×10^(-6),Mo:16341×10^(-6))记录了各自剖面的Ni、Mo、As、Tl等元素含量的峰值。因此,我们认为梅树村剖面Ni-Mo多金属层应位于该剖面玉案山组底界之上3 m层位,平引剖面Ni-Mo多金属层应位于该剖面牛蹄塘组底界之上2.5 m层位。平引剖面牛蹄塘组底部样品Fe组分数据表明,该剖面Ni-Mo多金属层及其附近地层记录了海水硫化信息(FeHR/FeT>0.38,FePy/FeHR>0.80)。结合已有的华南其他代表性剖面Ni-Mo多金属层及其附近层位(或相当层位)的Fe组分数据,我们对南华盆地(包括台地带、斜坡带和盆地带)Ni-Mo多金属层沉积时期的海水氧化还原状态进行重建,结果表明该时期南华盆地发生了一次深层海水硫化扩张事件。这一事件可能对寒武纪生命幕式辐射产生了重要影响。

A polymetallic Ni-Mo layer at the base of the Niutitang Formation(and its equivalents)in the lower Cambrian of South China,serves as a marker bed for regional correlation of lower Cambrian strata.However,up to now,the precise stratigraphic position of polymetallic Ni-Mo layer is uncertain for some sections in South China,which hinders the precise correlation of stratigraphic and makes our interpretation of the paleo-ocean changes unsolid.In this study,we collected rock samples from the Meishucun and the Pingyin section and conducted trace element analysis using inductively coupled plasma mass spectrometer(ICP-MS)with identifying of the precise position of polymetallic Ni-Mo layer in two sections.Furthermore,the samples from the base of the Niutitang Formation in the Pingyin section were analyzed for Fe speciation.Combined with previous data,our data reveal the redox condition of the seawater in Nanhua basin during the deposition of polymetallic Ni-Mo layer(~521 Ma).Trace-element content results show:Ni,Mo peaks(Ni:135×10^(-6),Mo:583×10^(-6))with the basal Yu􀆳anshan Formation(3 m above the base boundary)at the Meishucun section and maximum Ni,Mo concentrations(Ni:2480×10^(-6),Mo:16341×10^(-6))at the base of the Niutitang Formation(2.5 m above the base boundary)at the Pingyin section.Therefore,we suggest that the polymetallic Ni-Mo layer should lie within the basal of the Yu􀆳anshan Formation(3 m above the lower boundary)at the Meishucun section,and the polymetallic Ni-Mo layer should lie within the base of the Niutitang Formation(2.5 m above the lower boundary)at the Pingyin section.Fe speciation of samples from the Pingyin section indicates that the seawater euxinia information(FeHR/FeT>0.38,FePy/FeHR>0.80)was recorded in the polymetallic Ni-Mo layer(or near strata)at the Pingyin.Combination with previously published Fe speciation data from the polymetallic Ni-Mo layer(and near strata,or its equivalents)of representative sections in South China,reconstructing the redox condition of seawater during the deposition of polymetallic Ni-Mo layer in Nanhua basin(including the platform,slope and basin zone).Indicating that an extensification of euxinia deep seawater occurred in Nanhua basin during this period.This event may have a profound impact on Cambrian life episodic radiation.