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川东北地区晚埃迪卡拉纪灯影期海水氧化还原环境重建

赵坤 满玲 贺然 李松倬 祝圣贤 郎咸国

赵坤, 满玲, 贺然, 李松倬, 祝圣贤, 郎咸国. 川东北地区晚埃迪卡拉纪灯影期海水氧化还原环境重建[J]. 沉积学报, 2023, 41(1): 183-195. doi: 10.14027/j.issn.1000-0550.2021.083
引用本文: 赵坤, 满玲, 贺然, 李松倬, 祝圣贤, 郎咸国. 川东北地区晚埃迪卡拉纪灯影期海水氧化还原环境重建[J]. 沉积学报, 2023, 41(1): 183-195. doi: 10.14027/j.issn.1000-0550.2021.083
ZHAO Kun, MAN Ling, HE Ran, LI SongZhuo, ZHU ShengXian, LANG XianGuo. Redox Conditions of the Late Ediacaran Dengying Period in Northeastern Sichuan, China[J]. Acta Sedimentologica Sinica, 2023, 41(1): 183-195. doi: 10.14027/j.issn.1000-0550.2021.083
Citation: ZHAO Kun, MAN Ling, HE Ran, LI SongZhuo, ZHU ShengXian, LANG XianGuo. Redox Conditions of the Late Ediacaran Dengying Period in Northeastern Sichuan, China[J]. Acta Sedimentologica Sinica, 2023, 41(1): 183-195. doi: 10.14027/j.issn.1000-0550.2021.083

川东北地区晚埃迪卡拉纪灯影期海水氧化还原环境重建

doi: 10.14027/j.issn.1000-0550.2021.083
基金项目: 

成都理工大学珠峰科学研究计划 ZF11406

详细信息

Redox Conditions of the Late Ediacaran Dengying Period in Northeastern Sichuan, China

Funds: 

Everest Scientific Research Orogram of Chengdu University of Technology ZF11406

  • 摘要: 晚埃迪卡拉纪全球海洋发生了大面积的缺氧,海洋化学结构呈现明显的非均质性,直接影响了埃迪卡拉型生物的演化与分布。四川盆地发育完整的晚埃迪卡拉系地层,以灯影组巨厚层碳酸盐岩沉积为代表。但是对于该套巨厚层碳酸盐岩沉积时的古海水氧化还原性质备受争议。为了解决这一问题,对川东北地区鹿池剖面的灯影组地层开展了系统的沉积学和稀土元素地球化学分析。该地区灯影组的岩石类型主要为泥微晶白云岩、黏连白云岩、叠层/层纹白云岩,以及溶蚀白云岩,沉积环境为开阔碳酸盐岩台地相。地球化学数据结果显示灯影组碳酸盐岩普遍具有较低的稀土总量(∑REE+Y值为0.4~3.3 μg/g)、较低的Mn/Sr值(0.2~2.8)和较高的Fe含量(55.9~1 772.6 μg/g)。灯影组的REE+Y配分曲线(经页岩标准化)可划分为四个阶段,且Ce异常指示该地区经历了弱氧化到弱还原再到缺氧状态,表明埃迪卡拉纪晚期海洋浅部水体也发生了缺氧现象。
  • 图  1  鹿池剖面地理位置(修改自文献[24])

    Figure  1.  Geographical background of the Luchi profile (modified from reference [24])

    Fig.1

    图  2  鹿池剖面灯影组野外宏观沉积特征

    (a)溶蚀角砾白云岩;(b)微生物白云岩;(c)似葡萄状花边状白云岩;(d)纹层状白云岩

    Figure  2.  Field sedimentological characteristics of the Dengying Formation in the Luchi profile

    Fig.2

    图  3  鹿池剖面灯影组泥微晶白云岩的岩石学特征

    (a)泥微晶白云岩,含较多的黄铁矿,单偏光,17号样;(b)泥微晶白云岩,单偏光,58号样;(c)泥微晶白云岩,单偏光,59号样;(d)泥微晶白云岩,单偏光,75号样

    Figure  3.  Petrological characteristics of argillaceous microcrystalline dolomite from the Dengying Formation

    Fig.3

    图  4  鹿池剖面灯影组黏连云岩的岩石学特征

    (a)黏连云岩,藻团块黏连,单偏光,43号样;(b)黏连云岩,砂屑黏连,正交偏光,57号样;(c)黏连云岩,线纹层状黏连,单偏光,51号样;(d)黏连云岩,粒屑黏连,正交偏光,54号样

    Figure  4.  Petrological characteristics of boundstone from the Dengying Formation

    Fig.4

    图  5  鹿池剖面灯影组叠层/层纹云岩的岩石学特征

    (a)叠层云岩,空腔发育,单偏光,13号样;(b)层纹云岩,空腔不发育,单偏光,30号样;(c)叠层云岩,空腔发育,正交偏光,33号样;(d)层纹云岩,空腔不发育,单偏光,76号样

    Figure  5.  Petrological characteristics of alga dolomite from the Dengying Formation

    Fig.5

    图  6  鹿池剖面灯影组微生物白云岩的岩石学特征

    (a)细粉晶白云岩,重结晶纹层,单偏光,85号样;(b)泥晶白云岩,残余藻结构,单偏光,90号样

    Figure  6.  Petrological characteristics of microbial dolomite from the Dengying Formation

    Fig.6

    图  7  鹿池剖面灯影组碳酸盐岩的稀土元素地球化学散点图

    Figure  7.  Cross⁃plots of rare earth element (REE) geochemistry of dolomite in the Dengying Formation, Luchi profile

    Fig.7

    图  8  鹿池剖面灯影组碳酸盐岩的地球化学柱状图

    Figure  8.  Stratigraphic distribution of Ce/Ce*, Mn/Sr, Mn, and Fe contents from the Dengying Formation in the Luchi profile

    Fig.8

    图  9  鹿池剖面灯影组碳酸盐岩的REE+Y配分模式图

    Figure  9.  Post⁃Archean Average Shale (PAAS)⁃normalized REE+Y distribution patterns of the Dengying Formation carbonate in the Luchi profile

    Fig.9

    表  附表1  鹿池剖面灯影组碳酸盐岩ICP⁃MS激光原位常量和微量元素数据

    样品Stage厚度/m岩性Al2O3/%MgO/%CaO/%Mn/(μg/g)Fe/(μg/g)Sr/(μg/g)Y/(μg/g)La/(μg/g)Ce/(μg/g)Pr/(μg/g)Nd/(μg/g)Sm/(μg/g)Eu/(μg/g)Gd/(μg/g)Tb/(μg/g)Dy/(μg/g)Ho/(μg/g)Er/(μg/g)Tm/(μg/g)Yb/(μg/g)Lu/(μg/g)
    54MMD0.0918.429.538.1407.551.20.2520.1840.3680.0440.1790.0320.0100.0340.0060.0350.0070.0210.0030.0240.003
    87BD0.0218.929.469.3289.345.40.1860.1020.1440.0160.0800.0130.0040.0200.0030.0230.0040.0130.0020.0110.001
    1010B0.0621.533.947.3441.941.60.2220.0870.1770.0210.1210.0320.0070.0300.0040.0280.0060.0130.0020.0100.002
    1220BD0.0316.425.022.2200.131.60.2500.1000.1720.0270.1400.0340.0070.0310.0050.0330.0070.0130.0020.0110.001
    1430MMD0.0417.825.760.51 284.428.70.3700.0820.2670.0440.2300.0550.0180.0560.0080.0480.0100.0210.0020.0130.003
    1535MMD0.0416.224.637.4580.738.80.8080.2580.8620.1190.6630.1690.0500.1690.0200.1020.0210.0480.0040.0200.004
    2359MMD0.0518.427.074.0395.231.10.3370.1620.2410.0310.1660.0400.0100.0510.0070.0520.0090.0250.0020.0160.002
    2763MMD0.0511.918.017.8105.522.10.2510.2880.1890.0510.2250.0460.0080.0380.0070.0380.0060.0230.0030.0140.003
    3171MMD0.0716.525.622.3212.036.70.1110.0890.2150.0200.1040.0190.0040.0170.0030.0130.0040.0070.0010.0100.001
    3478MMD0.0316.826.510.4131.132.40.1370.1570.3580.0350.1170.0250.0050.0260.0030.0220.0040.0120.0010.0080.002
    3785MMD0.1825.037.529.9259.471.70.3600.2740.5500.0560.2310.0480.0130.0500.0080.0540.0110.0340.0050.0310.006
    4093MMD0.0418.229.670.6237.038.40.2200.1530.3430.0310.1260.0320.0070.0270.0040.0330.0080.0200.0020.0180.003
    4196MMD0.0218.530.194.5258.033.70.1290.0820.1800.0190.0960.0180.0050.0180.0030.0240.0030.0090.0020.0120.003
    47100MMD0.0718.529.953.0221.257.50.3980.3130.7010.0810.3450.0760.0140.0720.0120.0730.0150.0350.0040.0310.004
    54116AD0.0119.433.423.7121.436.50.1860.0370.1780.0140.0610.0150.0050.0230.0030.0180.0050.0120.0010.0090.001
    58118.9BD0.0320.034.45.977.331.70.0800.0220.1040.0100.0700.0180.0040.0260.0030.0170.0020.0060.0010.0050.001
    59119.1MD0.0113.924.036.9411.666.20.3170.1120.5320.0410.1940.0460.0120.0480.0080.0500.0090.0240.0040.0200.003
    67130MD0.0115.731.123.1109.437.60.1880.0540.2770.0220.1240.0270.0050.0330.0060.0280.0050.0140.0020.0100.002
    69139MD0.0314.928.717.4127.842.80.6380.1050.6970.0520.2830.0970.0230.0930.0170.1030.0210.0600.0080.0420.008
    75152MD0.1115.529.529.5261.635.90.5410.1850.9480.0990.4720.1010.0200.0960.0130.0940.0170.0370.0040.0280.005
    78161MD0.0316.732.748.2131.854.50.2550.0640.3610.0300.1560.0360.0090.0360.0080.0420.0080.0240.0030.0190.003
    82170MD0.016.217.913.055.951.20.1750.0300.2210.0140.0870.0210.0050.0270.0030.0260.0050.0130.0020.0110.002
    84175MD0.027.622.226.31 772.641.60.1970.0350.1630.0220.1360.0370.0080.0340.0050.0340.0070.0190.0020.0200.003
    注:MMD.泥微晶白云岩;BD.黏连白云岩;B.溶蚀角砾白云岩;AD.藻白云岩;MD.泥晶白云岩。
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  • 收稿日期:  2021-02-18
  • 修回日期:  2021-06-21
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目录

    川东北地区晚埃迪卡拉纪灯影期海水氧化还原环境重建

    doi: 10.14027/j.issn.1000-0550.2021.083
      基金项目:

      成都理工大学珠峰科学研究计划 ZF11406

      作者简介:

      赵坤,男,1996年出生,博士研究生,地质学,E-mail: geozhaokun@163.com

      通讯作者: 满玲,女,工程师,沉积学,E-mail: manling9999@petrochina.com.cn
    • 中图分类号: P512.2

    摘要: 晚埃迪卡拉纪全球海洋发生了大面积的缺氧,海洋化学结构呈现明显的非均质性,直接影响了埃迪卡拉型生物的演化与分布。四川盆地发育完整的晚埃迪卡拉系地层,以灯影组巨厚层碳酸盐岩沉积为代表。但是对于该套巨厚层碳酸盐岩沉积时的古海水氧化还原性质备受争议。为了解决这一问题,对川东北地区鹿池剖面的灯影组地层开展了系统的沉积学和稀土元素地球化学分析。该地区灯影组的岩石类型主要为泥微晶白云岩、黏连白云岩、叠层/层纹白云岩,以及溶蚀白云岩,沉积环境为开阔碳酸盐岩台地相。地球化学数据结果显示灯影组碳酸盐岩普遍具有较低的稀土总量(∑REE+Y值为0.4~3.3 μg/g)、较低的Mn/Sr值(0.2~2.8)和较高的Fe含量(55.9~1 772.6 μg/g)。灯影组的REE+Y配分曲线(经页岩标准化)可划分为四个阶段,且Ce异常指示该地区经历了弱氧化到弱还原再到缺氧状态,表明埃迪卡拉纪晚期海洋浅部水体也发生了缺氧现象。

    English Abstract

    赵坤, 满玲, 贺然, 李松倬, 祝圣贤, 郎咸国. 川东北地区晚埃迪卡拉纪灯影期海水氧化还原环境重建[J]. 沉积学报, 2023, 41(1): 183-195. doi: 10.14027/j.issn.1000-0550.2021.083
    引用本文: 赵坤, 满玲, 贺然, 李松倬, 祝圣贤, 郎咸国. 川东北地区晚埃迪卡拉纪灯影期海水氧化还原环境重建[J]. 沉积学报, 2023, 41(1): 183-195. doi: 10.14027/j.issn.1000-0550.2021.083
    ZHAO Kun, MAN Ling, HE Ran, LI SongZhuo, ZHU ShengXian, LANG XianGuo. Redox Conditions of the Late Ediacaran Dengying Period in Northeastern Sichuan, China[J]. Acta Sedimentologica Sinica, 2023, 41(1): 183-195. doi: 10.14027/j.issn.1000-0550.2021.083
    Citation: ZHAO Kun, MAN Ling, HE Ran, LI SongZhuo, ZHU ShengXian, LANG XianGuo. Redox Conditions of the Late Ediacaran Dengying Period in Northeastern Sichuan, China[J]. Acta Sedimentologica Sinica, 2023, 41(1): 183-195. doi: 10.14027/j.issn.1000-0550.2021.083
      • 埃迪卡拉纪(635~541 Ma)是宜居地球演化中的关键转折期,全球生物、大气和海洋等圈层均发生了显著改变。雪球地球结束后,多细胞藻类、大型带刺疑源类以及可能的动物胚胎迅速繁盛和出现[1]。在华南地区,以蓝田生物群、瓮安生物群、庙河生物群、石板滩生物群和高家山生物群为代表的埃迪卡拉生物群的相继出现[27],表明埃迪卡拉纪的生物圈开始由简单向复杂,由低等到高等,由单细胞向多细胞发生逐步转变。与此同时,埃迪卡拉纪大气圈组分也发生了变化,大气氧气含量迅速升高,以真核生物为主导的初级生产者开始取代原核为主的初级生产者,导致有机碳具有了更高的埋藏效率,全球碳循环发生明显的波动,出现了地质历史时期最大的碳同位素负偏事件[8]

        埃迪卡拉纪,大气氧气含量的迅速升高,诱发深部海洋发生间歇性氧化。根据估计,埃迪卡拉纪时期的大气氧含量由10%逐步升高接近现代大气氧水平[910],部分地球化学证据揭示在551 Ma后大洋完全氧化[1012]。然而,区域性的缺氧仍是埃迪卡拉纪的主要特点,特别是在深水盆地区域,存在以缺氧富铁为主导,夹间歇性的硫化环境[1316]。全球海洋结构呈显著的化学分层,以“三明治”模型为特征的硫化楔是埃迪卡拉纪海洋化学结构的最主要特点。除此之外,埃迪卡拉纪晚期还发生了生物矿化现象,被认为是海洋化学与生命演化紧密耦合相关的结果。因此认识该时期的海洋环境变化,对于理解E-C转折期的地球系统和早期生命演化具有重要意义。

        四川盆地灯影组地层发育巨厚的碳酸盐岩,是研究埃迪卡拉纪晚期海洋环境的重要素材。该套地层以微生物白云岩为主,且是四川盆地重要的油气储层之一。来自灯影组碳酸盐岩组分的Fe元素含量研究认为,藻纹层发育的碳酸盐岩具有较低的Fe含量,反映的是水体富氧环境,海底大量微生物藻席的发育,大大降低了孔隙水Fe扩散对碳酸盐岩沉积的影响,进而为后生动物的出现提供了很好的富氧条件[17]。然而,灯影组碳酸盐岩的U同位素证据显示,在埃迪卡拉纪晚期,全球海洋缺氧面积发生显著扩张,并且提出缺氧可能是导致埃迪卡拉型生物发生灭绝最主要的控制因素[18]。由此可见,灯影组古海洋环境的研究仍然存在争议。基于此,在前人对灯影组岩相古地理研究充分的条件下[1922],本文选取灯影组发育完整的四川盆地东北部地区的鹿池剖面,结合沉积学和稀土元素地球化学对该时期碳酸盐岩沉积和古海洋环境进行系统研究。

      • 华南板块包括扬子地块和华夏地块,始于~820 Ma新元古代罗迪尼亚超大陆的裂解与聚合[23],经历多期次的陆内裂解与凹陷。华南埃迪卡拉系地层在整个扬子地块均有沉积,在早埃迪卡拉纪沉积陡山沱组地层,台地相沉积厚度为40~180 m,而盆地相沉积厚度通常小于100 m[24]。陡山沱组地层以湖北峡东地区最为典型,自下而上可以划分为四段[2530]。第一段普遍沉积盖帽碳酸盐岩(3~5 m),具板状裂缝、帐篷构造和重晶石;第二段为黑色富有机质页岩(~80 m),夹薄层含燧石结核白云岩;第三段由块状、薄层含燧石的白云岩与含灰质/白云质泥岩组成(~60 m);第四段为黑色富有机质页岩、泥岩(~10 m)。埃迪卡拉纪晚期,在扬子东南缘的深水地区(斜坡、盆地)沉积一套以硅质岩为主的地层[3132],被称为留茶坡组或老堡组,其厚度可超过100 m[13]。在浅水地区,即扬子台地,广泛沉积以碳酸盐岩为主的灯影组地层,碳酸盐岩从几十米到几百米不等[27,3334],发育叠层石、鲕粒白云岩,以及葡萄花边状的藻白云岩[3536]。目前在国内普遍将四川盆地及周缘的灯影组碳酸盐岩地层进行四段划分[3739]:灯一段为贫藻的泥微晶白云岩;灯二段为富藻白云岩,葡萄状花边发育;灯三段以碎屑岩为主,贫藻;灯四段为含硅质条带的泥微晶白云岩,偶见藻纹层发育。且普遍认为以蓝细菌为主导的微生物的发育是富藻白云岩形成的重要因素[4042]

        鹿池剖面位于四川盆地东北缘(图1),地处陕西镇巴县、陕西紫阳县和四川万源市三地交界的地方,GPS坐标为32°17′11″ N,108°9′60″ E。该地区埃迪卡拉纪地层发育齐全,自下而上依次出露陡山沱组(~37 m)、灯影组地层(~330 m)。其中,灯影组与陡山沱组地层为整合接触,与上覆寒武系石牌组地层平行不整合接触。鹿池剖面陡山沱组表现为碳酸盐岩—碎屑岩混积体系,以粉砂质泥岩、泥质粉砂岩和泥—粉晶白云岩为主。灯影组地层(~180 m)以白云岩为主,偶见溶蚀角砾白云岩(图2a),微生物白云岩发育(图2b)。在灯影组中部可见似葡萄状花边的藻白云岩(图2c),以及纹层状白云岩(图2d)。

        图  1  鹿池剖面地理位置(修改自文献[24])

        Figure 1.  Geographical background of the Luchi profile (modified from reference [24])

        图  2  鹿池剖面灯影组野外宏观沉积特征

        Figure 2.  Field sedimentological characteristics of the Dengying Formation in the Luchi profile

      • 所有的碳酸盐岩样品做薄片和激光厚片处理,薄片与激光厚片一一对应,使用微区激光剥蚀系统(LA-ICP-MS)在激光厚片上进行测试分析。碳酸盐的原位微量元素测试分析在西安矿谱地质勘查技术有限公司完成,所用仪器为Agilent 7500 ICP-MS及与之配套的GeolasPro 193 nm准分子激光剥蚀系统。激光剥蚀所用斑束直径为120 μm,频率为5 Hz,以高纯度氦气为载气。测试前先用NIST 610进行调试仪器,使之达到最优状态。LA-ICP-MS激光剥蚀采样采用单点剥蚀的方式,测试过程中首先进行空白背景采集20 s,然后进行样品连续剥蚀采集45 s,停止剥蚀后继续吹扫20 s清洗进样系统,单点测试分析时间为85 s。每隔10个剥蚀点插入一组NIST 610、NIST 612、MACS-3,以对元素含量进行定量计算。对分析数据的离线处理(包括对样品和空白信号的选择、仪器灵敏度漂移校正、元素含量计算)采用软件ICPMSDataCal完成,常量元素的测试精度优于1%,微量元素的测试精度优于0.1%。

      • 碳酸盐岩的微区激光剥蚀测试分析结果表明(见附表1),碳酸盐岩的稀土总量很低,24个样品的∑REE值介于0.3~2.5 μg/g(平均值为0.9 μg/g,n=24),∑REE+Y值介于0.4~3.3 μg/g(平均值为1.2 μg/g,n=24)。其中轻稀土含量(LREE)介于0.3~2.3 μg/g(平均值为0.8 μg/g,n=24),重稀土含量(HREE)介于0~0.3 μg/g(平均值为0.1 μg/g,n=24),轻重稀土比值LREE/HREE为4.9~14.1(平均值为7.9,n=24),指示相对重稀土,轻稀土富集。常量元素中,碳酸盐岩样品的CaO、MgO含量较高,分别为17.9%~42.1%(平均值为28.7%,n=24)、6.2%~25.0%(平均值为17.0%,n=24);Al2O3含量极低,平均值不到0.1%(为0.02%,n=24)。除此之外,一些微量元素,如Mn、Sr含量比较低,分别为5.9~94.5 μg/g(平均值为37.4 μg/g,n=24)、22.1~71.7 μg/g(平均值为41.7 μg/g,n=24),而Fe含量较高,为55.9~1 772.6 μg/g(平均值为350.9 μg/g,n=24)。Ce和Eu的异常值计算方法分别是Ce/Ce*=CeN/(PrN2/NdN),Eu/Eu*=EuN/[(SmN2×TbN)13][43]

        表 附表1  鹿池剖面灯影组碳酸盐岩ICP⁃MS激光原位常量和微量元素数据

        样品Stage厚度/m岩性Al2O3/%MgO/%CaO/%Mn/(μg/g)Fe/(μg/g)Sr/(μg/g)Y/(μg/g)La/(μg/g)Ce/(μg/g)Pr/(μg/g)Nd/(μg/g)Sm/(μg/g)Eu/(μg/g)Gd/(μg/g)Tb/(μg/g)Dy/(μg/g)Ho/(μg/g)Er/(μg/g)Tm/(μg/g)Yb/(μg/g)Lu/(μg/g)
        54MMD0.0918.429.538.1407.551.20.2520.1840.3680.0440.1790.0320.0100.0340.0060.0350.0070.0210.0030.0240.003
        87BD0.0218.929.469.3289.345.40.1860.1020.1440.0160.0800.0130.0040.0200.0030.0230.0040.0130.0020.0110.001
        1010B0.0621.533.947.3441.941.60.2220.0870.1770.0210.1210.0320.0070.0300.0040.0280.0060.0130.0020.0100.002
        1220BD0.0316.425.022.2200.131.60.2500.1000.1720.0270.1400.0340.0070.0310.0050.0330.0070.0130.0020.0110.001
        1430MMD0.0417.825.760.51 284.428.70.3700.0820.2670.0440.2300.0550.0180.0560.0080.0480.0100.0210.0020.0130.003
        1535MMD0.0416.224.637.4580.738.80.8080.2580.8620.1190.6630.1690.0500.1690.0200.1020.0210.0480.0040.0200.004
        2359MMD0.0518.427.074.0395.231.10.3370.1620.2410.0310.1660.0400.0100.0510.0070.0520.0090.0250.0020.0160.002
        2763MMD0.0511.918.017.8105.522.10.2510.2880.1890.0510.2250.0460.0080.0380.0070.0380.0060.0230.0030.0140.003
        3171MMD0.0716.525.622.3212.036.70.1110.0890.2150.0200.1040.0190.0040.0170.0030.0130.0040.0070.0010.0100.001
        3478MMD0.0316.826.510.4131.132.40.1370.1570.3580.0350.1170.0250.0050.0260.0030.0220.0040.0120.0010.0080.002
        3785MMD0.1825.037.529.9259.471.70.3600.2740.5500.0560.2310.0480.0130.0500.0080.0540.0110.0340.0050.0310.006
        4093MMD0.0418.229.670.6237.038.40.2200.1530.3430.0310.1260.0320.0070.0270.0040.0330.0080.0200.0020.0180.003
        4196MMD0.0218.530.194.5258.033.70.1290.0820.1800.0190.0960.0180.0050.0180.0030.0240.0030.0090.0020.0120.003
        47100MMD0.0718.529.953.0221.257.50.3980.3130.7010.0810.3450.0760.0140.0720.0120.0730.0150.0350.0040.0310.004
        54116AD0.0119.433.423.7121.436.50.1860.0370.1780.0140.0610.0150.0050.0230.0030.0180.0050.0120.0010.0090.001
        58118.9BD0.0320.034.45.977.331.70.0800.0220.1040.0100.0700.0180.0040.0260.0030.0170.0020.0060.0010.0050.001
        59119.1MD0.0113.924.036.9411.666.20.3170.1120.5320.0410.1940.0460.0120.0480.0080.0500.0090.0240.0040.0200.003
        67130MD0.0115.731.123.1109.437.60.1880.0540.2770.0220.1240.0270.0050.0330.0060.0280.0050.0140.0020.0100.002
        69139MD0.0314.928.717.4127.842.80.6380.1050.6970.0520.2830.0970.0230.0930.0170.1030.0210.0600.0080.0420.008
        75152MD0.1115.529.529.5261.635.90.5410.1850.9480.0990.4720.1010.0200.0960.0130.0940.0170.0370.0040.0280.005
        78161MD0.0316.732.748.2131.854.50.2550.0640.3610.0300.1560.0360.0090.0360.0080.0420.0080.0240.0030.0190.003
        82170MD0.016.217.913.055.951.20.1750.0300.2210.0140.0870.0210.0050.0270.0030.0260.0050.0130.0020.0110.002
        84175MD0.027.622.226.31 772.641.60.1970.0350.1630.0220.1360.0370.0080.0340.0050.0340.0070.0190.0020.0200.003
        注:MMD.泥微晶白云岩;BD.黏连白云岩;B.溶蚀角砾白云岩;AD.藻白云岩;MD.泥晶白云岩。
      • 针对扬子西北缘的岩相古地理分析指出,研究区的灯影组地层在埃迪卡拉纪晚期整体处于开阔碳酸盐台地相[19]。沉积岩石学分析表明,鹿池剖面发育的碳酸盐岩类型多样,包括泥微晶白云岩、黏连云岩(图2b),以及叠层/层纹云岩(图2c,d),局部发育溶蚀角砾白云岩(图2a)。在部分泥微晶白云岩中,可见大量的自型晶黄铁矿(图3a)。整个剖面白云岩化程度较高,重结晶作用明显,显微结构丰富。

        图  3  鹿池剖面灯影组泥微晶白云岩的岩石学特征

        Figure 3.  Petrological characteristics of argillaceous microcrystalline dolomite from the Dengying Formation

      • 泥微晶白云岩在鹿池剖面灯影组中普遍发育,在剖面的中下部和上部均有分布,总体以上部分布较多,即灯影组后期。显微镜下白云石分布均一,泥—微晶结构,且灯影组下部的样品白云石粒径相对较大(图3a~c),灯影组上部白云石粒径较小(图3d)。此外,在灯影组下部的泥微晶白云岩中,可见较多的自型晶黄铁矿呈分散状随机分布(图3a),个别的黄铁矿晶粒可达100 μm。

      • 四川盆地灯影组中普遍发育黏连组构,与埃迪卡拉纪晚期四川盆地独特的古地理和水体沉积环境相关。黏连云岩的普遍发育是鹿池剖面灯影组的特点。形态特征有团块状(图4a,b)、絮状和不规则条带状(图4c,d)等,甚至部分样品有残余凝块结构。胶结物主要为亮晶白云石,晶粒较小。凝块的形成与蓝细菌等菌落生长作用有一定联系[44],且不同的水体环境会造成凝块形态的差异,可以在一定程度上反映不同的水体环境[19]:分散状的凝块往往指示水动力条件较弱,顺层状的反映中等的水动力条件;而以团块、斑状链接呈格架的表明较强的水动力条件。在鹿池剖面中,大多数的黏连云岩在显微镜下多为絮状或不规则条带状(图4c,d),且残余凝块结构较发育,表明该地区灯影组沉积期水动力条件中等。

        图  4  鹿池剖面灯影组黏连云岩的岩石学特征

        Figure 4.  Petrological characteristics of boundstone from the Dengying Formation

      • 鹿池剖面灯影组叠层/层纹结构明显,明暗相间,纹层形态多样,有不规则波状(图5a)、近平行状(图5b)、缓坡状(图5c)、水平状(图5d),横向上连续展布,且单层厚度在50~500 μm之间,反映不同的微生物藻丝体对藻纹层的贡献。纹层状白云岩中的亮层主要为白云石,以他形—半自形为主(图5a,c),白云石的纤维柱状结构特征不明显,亮层的白云石晶粒最大可以达到400 μm(图5c),以粉—细晶白云石为主(图5a,c)。部分叠层/层纹白云岩的亮层,以微晶白云石为主(图5b,d)。鹿池剖面的叠层/层纹白云岩纹层结构特征表明主要发育在静水的沉积环境,其成因可能与微生物席的黏结增长和诱导矿化作用密切相关[4445]

        图  5  鹿池剖面灯影组叠层/层纹云岩的岩石学特征

        Figure 5.  Petrological characteristics of alga dolomite from the Dengying Formation

        在鹿池剖面的灯影组白云岩中可见藻纹层(图6a),以及在泥晶白云岩中有残余的藻丝体(图6b)。从显微镜下看,残余藻丝体特征明显,有三种不同形态的藻丝体特征可以清晰的辨识,也可能是同一类藻丝体的不同聚集体形式。第一种是藻纹层平行状分布在白云岩中,藻纹层之间充填他形—半自形的白云石,且白云石有一定程度的泥晶化(图6a),此外藻纹层也受重结晶作用改造,仅仅有外观形态保存下来;第二种是似水滴状的残余藻丝体碎片(图6b,黄线区域),藻丝体互相交错、环绕,局部可见残余藻丝体的管状横切面(图6b,黄色箭头所指),藻丝体受白云岩化程度影响,仅保留藻丝体的形态学特征;第三种是片状的残余藻丝体结构,藻体上可见长条管状的藻丝体平行排列(图6b,红色箭头指示区域),残余藻丝体碎片可能是原地堆积,也可能是近源搬运过来沉积的,能够代表该地区灯影组沉积时期的藻类活动。此外,片状藻丝体的泥晶化程度较高,且残余藻丝体碎片之间充填亮晶白云石,白云石以微晶为主。不同的残余藻丝体结构的存在表明灯影组在该时期以高能的水体环境为主,部分残余藻丝体碎片杂乱分选(图6b),同时也反映藻白云岩的形成是藻微生物共同作用的结果[4647]

        图  6  鹿池剖面灯影组微生物白云岩的岩石学特征

        Figure 6.  Petrological characteristics of microbial dolomite from the Dengying Formation

      • 元古代在全球不同地区的台地相发育巨厚的碳酸盐岩沉积[4849],是探索早期古海洋环境变化、生物地球化学循环的重要素材[5052]。目前针对古代或者现代的碳酸盐岩的沉积环境,稀土元素中的Ce/Ce*异常分析在其中扮演越来越重要的角色[5355],然而,碳酸盐岩在沉积过程中或沉积之后不可避免地受到成岩作用的影响,导致碳酸盐岩的微量元素和稀土地球化学意义失效。因此,需要筛选出没有受到明显的成岩作用或较小的成岩作用影响的微量元素和稀土元素数据,才能真实地反映碳酸盐岩沉积期的海水地球化学信号。通常,在碳酸盐岩的成岩作用过程中,元素表现为Sr、Na、Mg的丢失和Zn、Fe、Mn的获取[56]。因此,Mn/Sr值被广泛用来判定碳酸盐岩所经历的成岩作用强度,未受成岩作用影响的Mn/Sr值为小于2[5758]

        在鹿池剖面,所有地球化学分析数据均是在白云石矿物上采用激光原位剥蚀,相较于全岩测试已尽可能地规避陆源碎屑和样品处理过程中可能的污染。所有的碳酸盐岩样品具有较低的Mn(平均值为37.4 μg/g)和Mn/Sr值(平均值为1.0),相对较高的Fe含量(平均值为350.9 μg/g),指示较弱的成岩作用影响[59]。稀土中的轻稀土含量(LREE)和稀土总量(∑REE)与陆源相关的元素Al显示较弱的线性关系(图7a),表明碳酸盐岩的稀土主要来源于海水,而非陆源。同时,Ce/Ce*异常作为碳酸盐岩评估古海洋环境的重要参数[43,55,6061],其与LREE和∑REE也没有明显的线性关系(图7b)。除此之外,碳酸盐岩的Y/Ho值、Mn的氧化物等也会影响Ce/Ce*异常[54],造成Ce/Ce*异常信号的变化,不能准确反映海水特征。而鹿池剖面碳酸盐岩中Ce/Ce*与Mn、Y/Ho,甚至是Fe,均没有相关性(图7c,d),反映Ce/Ce*的信号没有遭受后期成岩作用影响。在岩石学镜下观察,白云石多为早期白云岩化作用结果,并未有明显的后期成岩流体活动迹象(图3~6),同时稀土元素地球化学数据也采用激光原位剥蚀,已最大程度地规避成岩作用的影响,样品的数据可靠,能够用于沉积环境的分析。

        图  7  鹿池剖面灯影组碳酸盐岩的稀土元素地球化学散点图

        Figure 7.  Cross⁃plots of rare earth element (REE) geochemistry of dolomite in the Dengying Formation, Luchi profile

      • 应用稀土元素中的Ce在示踪碳酸盐岩沉积环境的时候,主要通过判断Ce的异常情况来推测沉积水体的环境[43]。本文采用Tostevin et al.[62]的标准来识别鹿池剖面灯影组碳酸盐岩的Ce异常,即Ce/Ce*值小于0.9,代表负异常;Ce/Ce*值为0.9~1.3,代表无明显异常;Ce/Ce*值大于1.3,代表正异常。基于这个判别方法,灯影组下部大部分的样品是无明显异常或弱的Ce/Ce*负异常(图8),而灯影组上部样品为显著的Ce正异常(图8)。特别的,由于文石或者方解石的早期白云岩化作用并不会改变海水的Ce异常信号,且在对鹿池剖面灯影组碳酸盐岩的岩石学薄片镜下观察中,也并未有明显的后期成岩作用(图3~6)。此外,海水的REE信号不易受成岩作用的影响,特别是对于古代海相非骨骼碳酸盐岩更是如此[55]

        图  8  鹿池剖面灯影组碳酸盐岩的地球化学柱状图

        Figure 8.  Stratigraphic distribution of Ce/Ce*, Mn/Sr, Mn, and Fe contents from the Dengying Formation in the Luchi profile

        与Ce异常的两种情况不同,该剖面的灯影组样品中,经PAAS(Post-Archean Average Shale)标准化后的REE+Y显示出四种不同的配分模式(图9)。在Ⅰ阶段,REE+Y配分模式表现为典型的MREE富集、LREE亏损,异常不明显或Ce正异常,Y正异常和Eu正异常(图9a)。显著的MREE富集特征同样也在高家山剖面灯影组下部的高家山段被观察到[63],两者不同的是,高家山剖面中配分模式具有明显的Ce负异常特征,而鹿池剖面灯影组Ⅰ阶段为异常不明显或Ce正异常(图9a)。通常,碳酸盐岩中具有较高的∑REE含量和MREE特征是因为磷酸盐矿物的风化和铁氧化物的还原[6465],且在高家山剖面的高家山段碳酸盐岩中,Fe的含量占比较高,因此高家山段碳酸盐岩中的MREE富集特征被认为是铁氧化物的还原造成的[63]。而在鹿池剖面灯影组Ⅰ阶段,碳酸盐岩具有极低的∑REE含量(0.4~2.5 μg/g,平均值为1.0 μg/g)和较低的Mn/Sr值(0.7~2.1,平均值为1.2),较高的Y/Ho值(36~46,平均值为39)和Fe含量(200.1~1 284.4 μg/g,平均值为534 μg/g),造成其MREE富集特征的因素可能是有河流的淡水输入,引起孔隙水中的铁氧化物相对升高。

        图  9  鹿池剖面灯影组碳酸盐岩的REE+Y配分模式图

        Figure 9.  Post⁃Archean Average Shale (PAAS)⁃normalized REE+Y distribution patterns of the Dengying Formation carbonate in the Luchi profile

        在Ⅱ阶段,REE+Y配分模式表现为LREE亏损,HREE富集,Y正异常,及Ce负异常和La正异常(图9b),与现代氧化大洋的REE+Y配分模式类似[62,66],而在Ⅱ阶段的碳酸盐岩具有较低的Mn/Sr值(0.8~2.4,平均值为1.6)、较高的Y/Ho值(38~42,平均值为40),没有受到明显的成岩作用(图7),Ce负异常指示Ⅱ阶段碳酸盐岩为弱氧化的沉积环境。在Ⅲ阶段,REE+Y配分模式表现为整体相对平坦,异常不明显或Ce正异常,较弱的Y正异常(图9c),结合其岩石学特征主要为黏连白云岩(图4a)和层纹白云岩(图5d),认为该时期沉积环境主要为含氧量较低的弱还原环境。而Ⅳ阶段,REE+Y配分模式表现为LREE亏损,HREE轻度富集,具有明显的La负异常,以及较弱的Y正异常(图9d),显著的Ce正异常指示其沉积环境以缺氧为主。同时,碳酸盐岩中纹层发育(图5d、图6a),也表明该时期处于相对静水的沉积环境。总体来看,鹿池剖面灯影组早期以弱氧化环境为主,晚期随着水体逐渐加深,沉积环境以缺氧还原为主。

        对四川盆地及周缘的埃迪卡拉纪晚期地层(如灯影组、老堡组、留茶坡组)氧化还原研究指出,浅水地区偏氧化[60],深水地区以缺氧富铁为主[13,16,67]的分层氧化还原环境是这一时期主要的特征。且在鹿池剖面,从Ⅰ阶段到Ⅳ阶段,灯影组沉积水体环境从弱氧化到弱还原再到缺氧(图8,9),反映伴随着埃迪卡拉纪晚期—早寒武世海平面的脉动式变化[68],灯影期的沉积环境也由弱氧化向缺氧转变。并且已有证据表明埃迪卡拉纪晚期海水是一种低氧特征[61],浅水地区并未充分氧化,与U同位素(238U)结果揭示的埃迪卡拉纪晚期深水地区表现为缺氧环境是一致的[14,18]。而海水的加深势必会引起深部缺氧水体的扩张,可能也是早寒武世在不同地区广泛发育黑色页岩的一个重要原因[6972]

      • (1) 川东北鹿池剖面灯影组沉积了巨厚的碳酸盐岩地层,沉积岩石学分析表明碳酸盐岩主要为黏连白云岩、叠层/层纹白云岩和泥微晶白云岩,偶见溶蚀白云岩。纵向上的沉积学特征表明灯影组经历了早期浅水高能向晚期深水低能环境的转变。

        (2) 经成岩作用判别,鹿池剖面灯影组碳酸盐岩受成岩作用影响较小,碳酸盐岩普遍具有较低的Mn/Sr值。经PAAS标准化后的REE+Y配分模式呈现四个典型特征:Ⅰ阶段表现为典型的MREE富集,LREE亏损,具有正的Y异常和Eu异常,较弱的Ce异常;Ⅱ阶段表现为典型的现代海水稀土特征,LREE亏损,Ce负异常和正的Y异常;Ⅲ阶段表现为平坦的配分模式,较弱的偏正的Ce异常和Y异常;Ⅳ阶段表现为典型的LREE亏损,且Ce显著负异常。REE+Y配分模式纵向上的变化表明鹿池剖面灯影组沉积环境经历了弱氧化到弱还原再到缺氧还原,揭示扬子台地埃迪卡拉纪晚期海水环境以缺氧为主。

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