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CAI XinYu, WANG Wei, TIAN Yang. Neoproterozoic Tectonic Evolution in the Middle Segment of the Jiangnan Orogenic Belt: Implications from detrital zircon U-Pb and Lu-Hf isotopes[J]. Acta Sedimentologica Sinica, 2024, 42(6): 1986-2005. doi: 10.14027/j.issn.1000-0550.2024.036
Citation: CAI XinYu, WANG Wei, TIAN Yang. Neoproterozoic Tectonic Evolution in the Middle Segment of the Jiangnan Orogenic Belt: Implications from detrital zircon U-Pb and Lu-Hf isotopes[J]. Acta Sedimentologica Sinica, 2024, 42(6): 1986-2005. doi: 10.14027/j.issn.1000-0550.2024.036
shu

Neoproterozoic Tectonic Evolution in the Middle Segment of the Jiangnan Orogenic Belt: Implications from detrital zircon U-Pb and Lu-Hf isotopes

doi: 10.14027/j.issn.1000-0550.2024.036
  • 1.

    State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences, China University of Geosciences (Wuhan), Wuhan 430074, China

  • 2.

    Wuhan Center of China Geological Survey (Geosciences Innovation Center of Central South China), Wuhan 430205, China

Funds:

National Natural Science Foundation of China 42272228

National Natural Science Foundation of China 41972242

Fok Ying-Tong Education Foundation, China 171013

  • Received Date: 2023-09-27
  • Accepted Date: 2024-04-09
  • Rev Recd Date: 2024-03-04
    • Available Online: 2024-04-09
  • Publish Date: 2024-12-10
  • Objective Determining the depositional age, sedimentary sources and tectonic background of Neoproterozoic sedimentary strata in Hunan province is one of the keys to understanding tectonic evolution in the middle segment of the Jiangnan orogenic belt. Methods Six clastic rock samples were collected from the Lengjiaxi Group and Banxi Group in the middle segment of the Jiangnan orogenic belt. Provenance and tectonic setting of the sedimentary basins were determined by studying the morphology, trace elements, and U-Pb/Lu-Hf isotopic composition of detrital zircons, and collating published data for detrital zircons and zircons from source areas. Results The results indicated that the Lengjiaxi Group was formed at about 852⁃825 Ma, and the Banxi Group was formed at about 820⁃720 Ma. The most important detrital zircon age peaks in the Lengjiaxi Group and Banxi Group occur at 920⁃790 Ma； age peaks also appear at 1 750⁃1 620 Ma and 2 500⁃2 450 Ma in the Lengjiaxi Group, and at 1 950⁃1 790 Ma and 2 420⁃2 330 Ma in the Banxi Group. The Hf isotopic characteristics show that the Lengjiaxi Group mainly received sediments from the Yangtze Block, and the Banxi Group received detritus from both the Yangtze and Cathaysia Blocks. Conclusions The U-Pb and Lu-Hf isotopic compositions of detrital zircons indicate that deposition in the Lengjiaxi Group and Banxi Group occurred before and after the amalgamation of the Yangtze and Cathaysia Blocks, respectively. The Lengjiaxi Group was deposited in the back-arc basin of a trench-arc-basin system which was closed by 825 Ma accompanied by strong folding in the sedimentary strata and a large number of S-type granite intrusions. The Banxi Group was deposited in an intraplate rift setting related to post-collision extension. Rifting-related magma erupted around 780⁃760 Ma, and gradually weakened along the sedimentation of the Banxi Group.
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Publishing history
  • Received:  2023-09-27
  • Revised:  2024-03-04
  • Accepted:  2024-04-09
  • Published:  2024-12-10

Neoproterozoic Tectonic Evolution in the Middle Segment of the Jiangnan Orogenic Belt: Implications from detrital zircon U-Pb and Lu-Hf isotopes

doi: 10.14027/j.issn.1000-0550.2024.036
  • 1. State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences, China University of Geosciences (Wuhan), Wuhan 430074, China
  • 2. Wuhan Center of China Geological Survey (Geosciences Innovation Center of Central South China), Wuhan 430205, China
Funds:

National Natural Science Foundation of China 42272228

National Natural Science Foundation of China 41972242

Fok Ying-Tong Education Foundation, China 171013

  • Received Date: 2023-09-27
  • Accepted Date: 2024-04-09
  • Rev Recd Date: 2024-03-04
  • Available Online: 2024-04-09
  • Publish Date: 2024-12-10

Abstract: Objective Determining the depositional age, sedimentary sources and tectonic background of Neoproterozoic sedimentary strata in Hunan province is one of the keys to understanding tectonic evolution in the middle segment of the Jiangnan orogenic belt. Methods Six clastic rock samples were collected from the Lengjiaxi Group and Banxi Group in the middle segment of the Jiangnan orogenic belt. Provenance and tectonic setting of the sedimentary basins were determined by studying the morphology, trace elements, and U-Pb/Lu-Hf isotopic composition of detrital zircons, and collating published data for detrital zircons and zircons from source areas. Results The results indicated that the Lengjiaxi Group was formed at about 852⁃825 Ma, and the Banxi Group was formed at about 820⁃720 Ma. The most important detrital zircon age peaks in the Lengjiaxi Group and Banxi Group occur at 920⁃790 Ma； age peaks also appear at 1 750⁃1 620 Ma and 2 500⁃2 450 Ma in the Lengjiaxi Group, and at 1 950⁃1 790 Ma and 2 420⁃2 330 Ma in the Banxi Group. The Hf isotopic characteristics show that the Lengjiaxi Group mainly received sediments from the Yangtze Block, and the Banxi Group received detritus from both the Yangtze and Cathaysia Blocks. Conclusions The U-Pb and Lu-Hf isotopic compositions of detrital zircons indicate that deposition in the Lengjiaxi Group and Banxi Group occurred before and after the amalgamation of the Yangtze and Cathaysia Blocks, respectively. The Lengjiaxi Group was deposited in the back-arc basin of a trench-arc-basin system which was closed by 825 Ma accompanied by strong folding in the sedimentary strata and a large number of S-type granite intrusions. The Banxi Group was deposited in an intraplate rift setting related to post-collision extension. Rifting-related magma erupted around 780⁃760 Ma, and gradually weakened along the sedimentation of the Banxi Group.

CAI XinYu, WANG Wei, TIAN Yang. Neoproterozoic Tectonic Evolution in the Middle Segment of the Jiangnan Orogenic Belt: Implications from detrital zircon U-Pb and Lu-Hf isotopes[J]. Acta Sedimentologica Sinica, 2024, 42(6): 1986-2005. doi: 10.14027/j.issn.1000-0550.2024.036
Citation: CAI XinYu, WANG Wei, TIAN Yang. Neoproterozoic Tectonic Evolution in the Middle Segment of the Jiangnan Orogenic Belt: Implications from detrital zircon U-Pb and Lu-Hf isotopes[J]. Acta Sedimentologica Sinica, 2024, 42(6): 1986-2005. doi: 10.14027/j.issn.1000-0550.2024.036
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0.   引言

  • 新元古代是地球历史演化的重要时期,这一时期最重要的地质过程是Rodinia超大陆的聚合和裂解及伴随的诸多全球性关键地质事件[1]。作为Rodinia超大陆重建重要组成之一的华南板块,记录了新元古代的岩浆—构造—沉积—成矿事件、古气候变化、埃迪卡拉纪多细胞生命演化等,同样也引起广泛的关注[2⁃5]。江南造山带是连接扬子地块和华夏地块的缝合带,记录了华南板块的形成过程,对于理解华南在该时期的地质过程,以及随后的古气候、古环境、生命演化等都非常重要。前人针对与江南造山带形成相关的岩浆时间进行了大量的研究,取得了诸多成果,主要是关于扬子地块和华夏地块拼合的界限、时间以及动力学机制等问题[1,5⁃15]。其中核心争论是扬子地块和华夏地块沿江南造山带（又称四堡或晋宁造山带）在何时拼合[16],华南板块在新元古代时期的构造属性如何。一些研究认为,扬子地块和华夏地块之间的碰撞发生在约1 000~880 Ma[1,17⁃21]；另外一些研究则发现该拼合发生在约860~800 Ma[4,22⁃28]。关于两板块的拼合方式包括向南俯冲[29]、向北俯冲[30⁃33]、发散式双重俯冲[11,26,34]和剪切闭合[32,35]等。相应地,主要有地幔柱假说（plume-rift）、板片—岛弧模型（slab-arc model）和板片—裂谷模型（platerift model）三种不同的构造演化模型[17,22,30,36]。

    前人对于华南板块新元古代这一时期的研究多依托于岩浆事件的信息,沉积岩方面的研究则相对匮乏[37⁃39]。当原生的岩浆—变质—构造事件受到后期改造很难完整保留下来时,沉积盆地的物源和构造背景分析则可以为盆—山过程提供关键的信息,尤其是对于前寒武纪的造山事件。江南造山带中段湖南地区发育完整的新元古代地层,主要包括下伏的冷家溪群和上覆的板溪群,二者被与造山事件相关的角度不整合所分割。因此,探究它们的沉积时代、物源和构造背景,对于理解江南造山带中部地壳的新元古代构造演化过程具有重要意义。

1.   地质背景

  • 江南造山带为NEE走向且呈NW向弧形拱出的地质构造单元,东西向长约1 500 km,其位于华南板块中部靠东的位置[40⁃42]（图1）。造山带西南起桂北地体往南,东北至怀玉地体,整个造山带跨越了浙江、安徽、江西、湖南、贵州、广西等区域[43⁃46],是一套由弱变质、强变形的巨厚火山—沉积建造及同时期的侵入岩体所组成的地质单元。

    Figure 1.  Geological sketch of the Precambrian in the northern Hunan area of Jiangnan orogenic belt of the South China Block (basemap from references [40⁃42])

    江南造山带中段湘中—湘东（北）地区的新元古代沉积地层自下而上（从老到新）依次可以分为冷家溪群、板溪群、南华系/成冰系和震旦/埃迪卡拉系[6,47]（表1）。其中扬子地块南部的新元古代沉积基底中包含两套低级变质的地层,并被区域的角度不整合所隔开,其中不整合面之下为冷家溪群,与之相当的同时期地层单元主要包括广西的四堡群、贵州的梵净山群和江西的双桥山群等。冷家溪群作为组成江南造山带的褶皱基底之一,早期学者通过研究其地层中的侵入岩而限定其为中元古代地层[48⁃50]。随着地质定年精度的大幅提高,研究得到基底内侵入岩的年龄主要为840~790 Ma[51⁃54]。此外,碎屑锆石和火山凝灰岩锆石U-Pb年龄对基底地层的时代进一步限制在860~820 Ma[7,55⁃59]。新的年龄数据基本可以确定江南造山带褶皱基底形成于新元古代而非中元古代。冷家溪群具有低级变质（绿片岩相）、强变形的特征,主要为深绿色低级变质的砂岩、粉砂岩、泥岩组成的类复理石沉积,局部夹基性—酸性火山岩（如具有柱状节理或枕状构造的熔岩和火山碎屑岩）,地层显示高角度紧闭褶皱及等斜褶皱[19,43,60]。孙海清等[61]依据区域岩石组合特征指出冷家溪群下部为海相深水盆地沉积细碎屑岩系,上部为盆地斜坡相浊流（扇）沉积粗碎屑岩系。冷家溪群自下而上可以分为六个岩组,依次为易家桥组、潘家冲组、雷神庙组、黄浒洞组、小木坪组、大药姑组[62],研究区仅出露下部五个组。

    Table 1.  Comparison of the comprehensive division of stratigraphy in the study area (modified from references [6,47])

    上覆于角度不整合的为板溪群,与之相当的同时期地层以丹州群、高涧群等为代表（表1）。现有的地质年代学研究表明,板溪群形成于815~715 Ma[5,33,63⁃67]。板溪群整体变形变质较弱,仅在局部发育较为宽缓的褶皱构造,主要由砂岩、板岩、砾岩、泥灰岩、碳酸盐岩、页岩和火山碎屑岩组成,并被新元古代基性、超基性岩石所侵入[16,60,68]。研究区板溪群自下而上分为宝林冲组、横路冲组、马底驿组、通塔湾组、五强溪组、多益塘组、百合垅组、牛牯坪组[62]。

2.   野外产状和样品采集

  • 研究区位于扬子地块东南部江南造山带中段湘中—湘东北地区（图2a沧水铺地区、图2b板溪村地区、图2c长沙地区、图2d衡阳地区）。区内最老地层为新元古代冷家溪群,向上为板溪群、南华—震旦纪、显生宙地层,且有第四系覆盖物堆积。研究区内发生多期次岩浆、构造活动,以燕山期、喜马拉雅期次为主,发育多组断层裂隙[43],沿NNW、NNE展布居多。

    Figure 2.  Geological map of the study area and sampling point (based on 1∶200 000 geological map width, namely Anhua amplitude, Changsha amplitude, Liuyang amplitude and Hengyang amplitude, stratigraphic histogram modified from references [41,62,67])

    研究区出露冷家溪群变砂岩、板岩,层内广泛发育紧闭褶皱,局部可见槽模构造、水平层理等现象,整体指示了较强的变形作用,局部地层发生倒转（图3）。野外出露板溪群砾岩、砂岩、板岩,层内变质变形较弱,发育多种层理构造,冲洗交错层理、槽状交错层理、透镜状层理等,指示其沉积于水动力较强,水流方向不断变化的环境之下（图3）。对野外露头实地考察并采集代表性岩石样品共六件,根据所属地层从老到新样品号依次为ww259、ww253、ww295、ww380、ww375、ww343,包括来自冷家溪群雷神庙组的长石砂岩、黄浒洞组的砂质板岩以及小木坪组的黄绿色泥质粉砂岩共三件样品,以及来自板溪群底部的紫红色砂质砾岩、紫红色砂岩以及板溪群上部五强溪组上段的长石砂岩共三件样品（图3）。样品的获取和制备过程中尽量避免了脉体、蚀变和风化产物等的干扰。

    Figure 3.  Typical outcrops in the sedimentary⁃tectonic field of the Lengjiaxi Group and Banxi Group in the study area

3.   分析方法

  • 将3~5 kg的岩石样品粉碎至60~100目,之后进行重力淘洗并使用电磁分选以分离锆石重矿物。得到的锆石颗粒在双目显微镜下进行手工挑选,嵌入环氧树脂中,然后抛光至大概2/3,并进行透、反射光和阴极发光照片的拍摄。

    • 3.1.   锆石U⁃Pb同位素分析

  • 锆石的U-Pb同位素的分析在中国科学院地球化学研究所矿床地球化学国家重点实验室完成。将型号为GeoLas Pro型193nm ArF准分子激光剥蚀系统和Agilent 7700x型四极杆电感耦合等离子质谱仪（ICP-MS）联合进行实验测试。193nm ArF准分子激光,激光能量密度为10 J/cm2,剥蚀束斑直径为32 μm,频率为5 Hz,持续时间为45 s（相当于225个脉冲）。使用氦气作为载气,将剥蚀的气溶胶有效地输送到ICP-MS。测试之前在样品表面进行5~10 s预剥蚀处理,排除或减少普通铅的污染。锆石91500被用作校正元素分馏的主要标样,同时将锆石GJ-1和Plešovice[69]作为未知样品进行了质量控制分析。锆石的微量元素组成以NIST SRM 610作为外标,91Zr作为二级标准。使用软件ICPMSDataCal进行了背景和锆石信号的离线选择和整合,以及微量元素分析和U-Pb测年的时间漂移校正和定量校准[70]。普通铅的校正采用MS Excel程序ComPbCorr#3.17进行校正[71],数据使用Isoplot程序进行处理[72]。测试结果显示检测标样GJ-1（601±17 Ma,2σ,MSWD=1.3）和PL（334.2±7.5 Ma,2σ,MSWD = 2.9）的一致年龄与推荐值（GJ-1：599.8±1.7 Ma,2σ；PL：337.13±0.37 Ma,2σ）一致[69,73]。

    • 3.2.   锆石Lu⁃Hf同位素分析

  • 锆石Lu-Hf同位素分析与U-Pb同位素定年在同一颗粒相同位置上进行。使用香港大学Nu Plasma等离子HR多接收器电感耦合等离子体质谱仪（MC-ICPMS）和M-50 193nm准分子激光烧蚀系统对锆石进行Hf同位素分析。实验采用的测试束斑直径为55±2 μm,频率为10 Hz,能量密度为15 J/cm2,在静态收集模式下同时测量原子质量为172~179的同位素。176Yb与176Hf的同量异位干扰已针对176Yb/172Yb比值矫正为0.588 6[74]。Yb同位素比值的仪器质量偏差校正使用指数定律归一化为1.352 74的172Yb/173Yb[74],使用指数定律,Hf同位素比率归一化为0.732 5的179Hf/177Hf[75]。获得Yb和Hf之间的关系βYb=0.872βHf,并将其用于质量偏置偏移[76]。通过测量无干扰的175Lu同位素的强度并使用推荐的176Lu/175Lu比值0.026 55来校正176Lu与176Hf的干扰[77]。由于176Lu的信号强度对176Hf的贡献不大（通常<1%）,可以假设Lu同位素的质量偏差（βLu）与βHf相同[78]。在分析过程中,通过测量未知样品的锆石标准91500和GJ-1进行质量控制,以评估分析数据的可靠性。

4.   结果

    4.1.   锆石阴极发光图像和Th/U比值

  • 冷家溪群和板溪群沉积岩样品中的锆石颗粒大多数为浅黄色至无色,透明至半透明,锆石颗粒大小集中在90~200 μm。阴极发光图像显示,几乎所有的新元古代以来的晶粒保留了较好的自形—半自形棱柱状,呈现振荡环带,指示其为岩浆成因。古元古代晚期—中元古代的锆石多不规则,但仍保留较为明显的振荡环带,部分锆石存在较薄的亮边。较为古老的古元古代早期的碎屑锆石多浑圆状,存在核—边、核—幔—边结构,核部显示继承锆石的特征（图4）。

    Figure 4.  Typical detrital zircon cathodoluminescence images of Lengjiaxi Group and Banxi Group

    超过98%的碎屑锆石Th/U值均大于0.1（图5）,结合碎屑锆石的阴极发光图像,认为所测试的锆石点位均为岩浆成因。

    Figure 5.  Th/U ratio vs. U⁃Pb age binary plot for detrital zircons

    • 4.2.   锆石U⁃Pb地质年代学

  • 总计对六件样品中428颗碎屑锆石进行了U-Pb地质年代学分析,部分分析结果展示在图6中。其中206Pb/238U年龄数据用于小于1 000 Ma的颗粒、207Pb/206Pb年龄数据用于大于1 000 Ma的颗粒,此外共收集冷家溪群、板溪群碎屑锆石U-Pb年龄1 576个数据点[16,30,35,40,56,61,66,79⁃81]（图7）。

    Figure 6.  U⁃Pb age concordance diagrams for detrital zircons

    Figure 7.  U⁃Pb age KDE atlas of detrital zircons

    • 4.2.1.   冷家溪群（样品ww259、ww253和ww295）

  • 样品ww259是从冷家溪群下部雷神庙组采集的长石砂岩。对所含的62颗锆石进行分析,其中55个分析点的U-Pb年龄具有较高的协和度（91%~110%),年龄介于2 586~848 Ma,并给出了三个年龄峰,分别在2 406 Ma、1 740 Ma和918 Ma左右（图7f）。ww259样品最年轻的两个锆石206Pb/238U年龄显示852.3±4.23 Ma的加权平均年龄,限定衡阳地区的冷家溪群下部雷神庙组开始沉积于852 Ma或之后。样品ww253是从冷家溪群中部黄浒洞组采集的砂质板岩。对所含的77颗锆石进行分析,除2个点外其余数据显示较高的协和度（92%~99%）。75颗高协和度锆石的U-Pb年龄在2 985~822 Ma,并显示三个年龄峰值,分别约为2 335 Ma、1 750 Ma和872 Ma（图7d）。该样品最年轻的两个锆石206Pb/238U年龄显示821±3.19 Ma的加权平均年龄,指示衡阳地区的冷家溪群黄浒洞组开始沉积晚于821 Ma。样品ww295是从冷家溪群上部小木坪组采集的黄绿色泥质粉砂岩。对所含的47颗锆石进行了分析,除7个点外其余数据显示较高的协和度（90%~99%),其余40颗协和的锆石给出2 444~835 Ma的年龄范围,并具有三个年龄峰值,分别为2 422 Ma、1 620 Ma和857 Ma（图7b）。该样品最年轻的8个锆石206Pb/238U年龄显示840±3.54 Ma的加权平均年龄,且在最年轻的锆石835±5 Ma在误差范围之内,表明长沙地区冷家溪群小木坪组最大沉积年龄晚于835 Ma。

    • 4.2.2.   板溪群（样品ww380、ww375和ww343）

  • 样品ww380是从板溪群宝林冲组所采集的一块紫红色砂质砾岩,位于不整合面之上约10 m。该样品共分析了80颗碎屑锆石,其中77个分析点的U-Pb年龄具有较高的协和度（97%~104%),年龄范围在2 606~806 Ma,并给出了2 479 Ma、1 790 Ma和851 Ma处的三个主要年龄峰值（图7e）,其中较年轻的一组锆石的加权平均年龄815.6±3.6 Ma,解释为沧水铺地区板溪群的底界年龄。样品ww375是在样品ww380附近所采集的一块紫红色砂岩,前者层位较后者稍微向上20 m,层位为板溪群横路冲组。共从该样品中分析了80颗碎屑锆石,其中77颗协和的锆石（91%~106%）定义了2 656~769 Ma的年龄范围,并给出了三个年龄峰,分别为2 480 Ma、1 880 Ma和850 Ma左右（图7c）。样品最年轻的3颗锆石206Pb/238U年龄显示773±2.68 Ma的加权平均年龄,说明沧水铺地区板溪群下部的横路冲组的最大沉积年龄在773 Ma之后。样品ww343为采集于板溪群上部百合垅组上部的一块长石砂岩。共从该样品中分析了80颗碎屑锆石,除14个点外其余数据显示较高的协和度（90%~104%）,66个协和的年龄数据显示介于2 495~746 Ma,且在2 455 Ma、1 950 Ma和794 Ma处显示出三个主要年龄峰值（图7a）。样品最年轻的2个锆石206Pb/238U年龄显示747±4.18 Ma的加权平均年龄,指示在板溪村地区板溪群上部的百合垅组的最大沉积年龄在747 Ma之后。

    • 4.3.   锆石Lu⁃Hf同位素

  • 选择冷家溪群上部小木坪组和板溪群下部宝林冲组、横路冲组的三个样品ww295、ww380以及ww375,对来自其中显示出协和U-Pb年龄的碎屑锆石共64颗进行原位Hf同位素分析,结果显示不整合面上下地层样品的锆石εHf(t)与U-Pb年龄分布图如图8a所示。整体上,大部分锆石具有低于球粒陨石均一库的εHf(t)值（-0.7~-37.7）,说明这些锆石的母岩浆以古老大陆地壳重熔为主。在中、新元古代时期,少量锆石的εHf(t)值大于0,尤其是新元古代锆石的εHf(t)值接近亏损地幔值（+10.5）,表明有幔源物质的贡献。冷家溪群和板溪群样品中锆石的Hf同位素显示不同的模式年龄分布。冷家溪群上部岩石中锆石的二阶段Hf模式年龄基本介于3.5~1.9 Ga,在3.1~2.7 Ga有一年龄峰值（图8b）。对于板溪群样品,其锆石二阶段Hf模式年龄介于3.6~1.0 Ga,在3.3~3.0 Ga、2.7~2.2 Ga和1.8~1.5 Ga处有三个年龄峰值（图8c,d）。

    Figure 8.  (a) Zircon εHf(t) vs. U⁃Pb age binary plot; (b⁃d) histograms of Hf isotope two⁃stage mode age statistics of the sample

5.   讨论

    5.1.   冷家溪群、板溪群的沉积时代

  • 根据获得的测年结果,来自冷家溪群雷神庙组、黄浒洞组、小木坪组的三个样品显示冷家溪群的底界年龄小于852 Ma,上部小木坪组沉积上限年龄应在835 Ma之后,这与从其他区域冷家溪群中的碎屑锆石测试结果具有一致性[30,39⁃41,61,66,81⁃88],并且也与冷家溪群相当的其他新元古代沉积地层序列得到的锆石U-Pb测年结果基本相近,包括溪口群、梵净山群、四堡群等[38,80⁃81,85,88⁃94]。此外,在冷家溪群以及同时代地层中报道了825~790 Ma的侵入岩年龄[31,54,84,95⁃97]。结合前人得到的数据,冷家溪群的年龄上限（最大沉积年龄）理论上须小于从碎屑锆石中获得的最年轻锆石约825 Ma[33,61,66,82,87,98],同时应该比所报道的侵入岩更老,因此认为江南造山带中部的冷家溪群的沉积年龄上限约为825 Ma。

    关于不整合面之上板溪群的沉积年龄,本研究中底砾岩样品ww380中最年轻的一组锆石U-Pb加权年龄为815.6±3.6 Ma,同样在湖南长沙望城区剖面、湘北石门县剖面以及湘东北剖面均获得基本一致的锆石U-Pb年龄[66,81,85⁃86]。此外,有研究指出板溪群底部的沧水铺组中的火山岩、火山集块岩年龄为814±12 Ma、821±13 Ma[99⁃100],且马底驿组中凝灰岩年龄为820±3 Ma[82],综上数据认为板溪群沉积始于约820 Ma。本文所采集板溪群上部样品ww343中最年轻锆石约746 Ma,前人报道了板溪群顶部最年轻的锆石年龄约725 Ma[89⁃90,101⁃102],结合上覆南华纪地层底界的沉积年龄小于720 Ma[66,103],以及全球性冰川事件的限制[38,88,91,104⁃106],认为板溪群沉积始于约820 Ma,约725 Ma沉积终止。

    早在20世纪50年代,湖南省地质局研究人员就桃源县茶庵铺剖面发现了夹在冷家溪群和板溪群之间的新元古代角度不整合面,并取名为“武陵运动”的产物,大致与广义的晋宁运动相当。贵州、湖南地区以角度不整合为主,桂北地区丹州群与下部四堡群从角度不整合过渡到平行不整合（表1）。研究通过上下地层的沉积界限来限制该不整合形成的大致时代。桂北地区武陵不整合的时间限制为835~795 Ma[107]或832~803 Ma[108]。贵州梵净山地区结合梵净山群的沉积上限以及不整合上方地层底部的凝灰岩,将武陵不整合的时间限制在816~814 Ma[56,109]。湘西北、湘东北、湘北地区的研究指出武陵不整合时间为830~809 Ma[81]、822~802 Ma[98]、827~798 Ma[66,110]。结合冷家溪群、板溪群的沉积时代,认为该角度不整合形成于825~820 Ma,与前人的报道年龄范围一致。

    • 5.2.   冷家溪群、板溪群的沉积物源

  • 碎屑锆石的年龄图谱统计与源区对比已被广泛运用于示踪沉积物源。根据所采集的六件样品（绿色直方图）,以及收集前人研究中的1 576颗碎屑锆石年龄（黄色背景）得到的KDE图谱（图7）。由KDE年龄图谱显示上下地层最为显著的峰值均分布于新元古代920~790 Ma内（图7）。之前的研究报道了江南造山带地区存在大量的新元古代岩浆岩,譬如年龄在800~760 Ma伸展相关的火山岩[80,89⁃90,111],830~800 Ma同碰撞或碰撞后花岗岩[24,31],以及990~840 Ma在江南造山带地区及华夏地块发现的蛇绿混杂岩以及弧相关岩浆岩[24,34,38,88,91,112⁃113]。因此,广泛分布于江南造山带的新元古代岩浆岩可能是该地区冷家溪群至板溪群沉积终止过程中的重要物源之一。之前的研究根据锆石晶型认为冷家溪群、板溪群的物源较近[66],并且本研究中锆石的Th/U比值（图5）以及锆石的Hf同位素与江南造山带新元古代岩浆岩对比结果（图9）也支持这一观点。

    Figure 9.  Binary statistical chart of detrital zircons εHf(t) vs. U⁃Pb age in the Lengjiaxi Group and Banxi Group

    下伏冷家溪群中元古代峰值介于1 750~1 620 Ma,而上覆板溪群年龄峰值介于1 950~1 790 Ma,尤其是冷家溪群上部1 620 Ma与板溪群下部1 790 Ma峰值的明显差异（图7）,且绝大多数冷家溪群碎屑锆石负的锆石εHf(t)也表明其物源以古老大陆地壳物质的重熔为主,而板溪群存在一定比例的正εHf(t)值的锆石（图9）,说明沉积物的源岩具有幔源物质的贡献,上下两套地层古—中元古代的沉积物来源有所差异。江南造山带地区虽然存在这一时期的成岩,但其对两套地层尤其是冷家溪群贡献较小。依据上文所述,两套地层物源来源较近,因此其最可能的源区位于华夏地块或者扬子地块内部,通过对比碎屑锆石及扬子地块和华夏地块岩浆岩的Hf同位素特征发现（图10）,冷家溪群自下而上古元古代晚期1 750~1 620 Ma的峰值与扬子地块西南缘如鱼洞子、后河杂岩的Hf同位素特征接近,而华夏地块这一时期的岩浆事件报道较少或者尚未发现,或者尚未剥露沉积于冷家溪群之中。而在扬子地块和华夏地块均报道了1 950~1 750 Ma的岩浆事件,但与碎屑锆石对比发现,冷家溪群、板溪群锆石的Hf同位素特征与扬子地块内部崆岭杂岩、董岭杂岩以及越南北部的片麻岩更为相似,其中冷家溪群与华夏地块岩浆岩Hf同位素特征范围基本无交集,而板溪群部分锆石点与华夏地块的岩浆岩Hf同位素特征范围有所重叠（图10）。

    Figure 10.  Comparison of Hf isotopic characteristics of detrital zircons in Lengjiaxi Group, Banxi Group and potential magmatic source area

    同样,KDE年龄图谱也显示冷家溪群最古老的锆石年龄峰值介于2 500~2 450 Ma,上覆板溪群则相对年龄介于2 420~2 330 Ma,同样支持两套地层物源有所差异。而新太古代晚期至古元古代早期的碎屑锆石显示2 650~2 450 Ma的沉积物来源主要是扬子地块中的崆岭杂岩、钟祥杂岩,以及扬子地块西缘—西南缘的岩浆岩,并且冷家溪群碎屑锆石Hf同位素特征与扬子地块这些源区更为接近（图10）。2 450~2 300 Ma的物源除了扬子地块西南缘的鱼洞子、后河以及越南北部的来源外,华夏地块也显示其内部存在这一时期的岩石基底,并且板溪群碎屑锆石Hf同位素特征除了能和扬子地块进行对比外,与华夏地块这一时期岩浆岩的锆石也显示出相似性。所收集的岩浆岩也表明扬子地块和华夏地块可能存在广泛的太古宙基底[34,113,120⁃124]。

    综合碎屑锆石年龄KDE图谱以及与潜在源区Hf同位素特征对比,本文认为冷家溪群和板溪群物源有所不同,冷家溪群物源主要来自扬子地块内部,而板溪群物源除了扬子地块来源外,可能还存在华夏地块的物质供给。造成这一结果的主要原因可能是冷家溪群沉积终止之前,扬子地块和华夏地块尚未拼合,而在拼合之后开始沉积板溪群。

    • 5.3.   江南造山带中部沉积—构造演化

  • 冷家溪群上限大致在825 Ma左右,晚于全球性的格林威尔造山运动[125⁃128]。基于冷家溪群中砂岩的地球化学特征,以在江南造山带多个地区报道的冷家溪群中所夹的大陆边缘岛弧环境下形成的拉斑玄武岩和钙碱性玄武岩系列岩石,一些研究者认为冷家溪群是基于沟—弧—盆体系下沉积在弧后盆地的产物[39,42,61,129⁃133]。研究报道了与弧后盆地具有地球化学亲缘性如伏川蛇绿岩的锆石U-Pb年龄为824±3 Ma[134],以及在江南造山带中—西段地区的860~835 Ma的镁铁质岩石[135⁃136],这些研究表明冷家溪群所在的弧后盆地闭合可能始于860 Ma,在825 Ma完全闭合[5,56]。此后,开启扬子地块和华夏地块的拼合。以冷家溪群和板溪群为界的高镁火山岩序列的地球化学特征和地质年代学数据认为扬子地块和华夏地块最终拼合于822~815 Ma[137],碎屑锆石U-Pb年也表明碰撞发生于825~815 Ma[11],根据825 Ma的过铝质花岗质岩浆作用以及805 Ma的双峰式岩浆作用,有学者提出碰撞发生于825~805 Ma,且在805 Ma之后变为板内机制[26,138]。而冷家溪群地层内发育大量的褶皱则可能由洋内弧与扬子地块的拼合作用有关,在碰撞的过程中,大量S型花岗岩侵入了扬子地块的褶皱沉积序列[26,54,139]。此外,对碎屑锆石进行CAD（年龄累积曲线）投图（图11）,冷家溪群的沉积环境显示出汇聚盆地的特征,同样支持了弧后盆地这一观点。

    Figure 11.  CAD discriminant diagram of structural properties of sedimentary basins in Lengjiaxi Group and Banxi Group

    结合已有的岩石学证据以及本文沉积学方面的工作,认为江南造山带中段偏倾向于已提出的板片—裂谷模型。在805 Ma之后,进入碰撞后伸展作用并发育陆内裂谷。碎屑锆石的CAD图谱显示了板溪群沉积于伸展环境中的沉积盆地（图11）。扬子地块和华夏地块拼合后,进入造山后伸展期,地幔流上升,并引发陆内裂谷作用,沿前江南造山带形成南华盆地。有研究得到板溪群凝灰质岩石的年龄为770~766 Ma,微量元素表明其源于大陆地壳,并有后期造山流体的叠加,板溪群凝灰质岩石显示出火山弧的亲缘性,但其在与裂谷事件有关的伸展环境中积累,锆石Hf同位素数据表明新生（即新元古代）弧源物质和古元古代非均质地壳的混合物是这些岩石的岩浆源区物质[115]。此外,研究报道了年龄为780~750 Ma的一系列双峰式碱性岩石,被认为是造山后伸展的产物[16,139,141⁃142],这些岩石同样是软流圈地幔大规模上升流和从造山到造山后构造环境转变的重要标志[142]。在此期间还形成了许多断层、非造山花岗岩侵入体和双峰火山岩[24,80,89,111],以及江南造山带东部约800~760 Ma裂谷相关火山岩的地质年代和地球化学特征,且造山带后伸展可能沿整个造山带具有跨时性[80,89⁃90]。结合拼合后的华南板块的碰撞后伸展和裂谷作用的诸多证据,同裂谷岩浆作用集中发生于780~760 Ma[143⁃144]。板溪群凝灰岩和侵入的S型花岗岩的主、微量元素特征揭示了俯冲作用的影响,地幔上涌引发的快速裂谷作用可能导致地幔和地壳之间的强烈相互作用,正如锆石中变化的Hf同位素所记录的[115],而裂谷作用一直持续到板溪群沉积上限720 Ma以后逐渐减缓。

6.   结论

  • （1） 冷家溪群沉积始于852 Ma之前,沉积上限在825 Ma左右；板溪群沉积下限在820 Ma左右,沉积上限应在720 Ma之后。

    （2） 碎屑锆石Th/U值指示冷家溪群、板溪群碎屑锆石为火成岩来源,江南造山带的新元古代岩浆岩可能是该地区新元古代沉积地体重要物源之一。冷家溪群物源来自扬子地块内部,在冷家溪群沉积终止前扬子地块和华夏地块尚未拼合,拼合之后开始沉积板溪群,板溪群沉积物或有华夏地块的物源贡献。

    （3） 冷家溪群于沟弧盆体系下沉积在弧后盆地,盆地闭合终于冷家溪群沉积上限约825 Ma,此时,沉积地层内发育大量的褶皱以及大量S型花岗岩侵入褶皱基底中。板溪群沉积于碰撞后伸展作用背景下陆内裂谷的盆地,同裂谷岩浆作用集中发生在780~760 Ma,裂谷作用于板溪群沉积结束逐渐减弱。

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