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Article Navigation >  Acta Sedimentologica Sinica  > 2024  >  42(6): 2135-2143

WEI HengYe, ZHANG Gan, ZHANG Xuan, HU Die, GONG JiaXin. Termination Age of the Chert Deposits in the Late Permian Dalong Formation in Middle and Upper Yangtze Area, China[J]. Acta Sedimentologica Sinica, 2024, 42(6): 2135-2143. doi: 10.14027/j.issn.1000-0550.2024.065
Citation: WEI HengYe, ZHANG Gan, ZHANG Xuan, HU Die, GONG JiaXin. Termination Age of the Chert Deposits in the Late Permian Dalong Formation in Middle and Upper Yangtze Area, China[J]. Acta Sedimentologica Sinica, 2024, 42(6): 2135-2143. doi: 10.14027/j.issn.1000-0550.2024.065
shu

Termination Age of the Chert Deposits in the Late Permian Dalong Formation in Middle and Upper Yangtze Area, China

doi: 10.14027/j.issn.1000-0550.2024.065
  • 1.

    School of Geoscience and Technology, Southwest Petroleum University, Chengdu 610500, China

  • 2.

    Qiangtang Institute of Sedimentary Basin, Southwest Petroleum University, Chengdu 610500, China

  • 3.

    State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, China

Funds:

National Natural Science Foundation of China 42272118

Natural Science Foundation of Sichuan Province 2023NSFSC0279

  • Received Date: 2024-02-26
  • Accepted Date: 2024-06-08
  • Rev Recd Date: 2024-05-09
    • Available Online: 2024-06-08
  • Publish Date: 2024-12-10
  • Objective Several intraplatform deep-marine trough basins in the northern Yangtze Platform developed bedded chert in the Dalong Formation during the Late Permian. The organic-rich black rocks in the trough basins are shale gas exploration targets in the Sichuan Basin. The black chert deposition in the Dalong Formation started at 258.77 Ma, related to waning of the Emeishan mantle plume. However, its termination date and the cause of the disappearance of the intraplatform trough basins were still unclear. The results of this study help to better understand the evolution of these deep-marine troughs, offering a theoretical basis for potential assessment and exploration of Permian shale gas. Methods Zircon U-Pb LA-ICP-MS age dating and trace element measurement in the bentonite interbeds were carried out in the upper part of the Dalong Formation chert beds. Results The zircon grains are euhedral and show clear zoning, with Th/U ratios higher than 0.46. The zircon grains are enriched in heavy rare earth elements （HREE） with positive Ce anomalies and negative Eu anomalies, showing a rising trend from light REE to HREE in the chondrite-normalized REE patterns. The U-Pb LA-ICP-MS age of the zircon was 253.0±1.3 Ma. Diagrams of Th/Nb vs. Hf/Th and Th/U vs. Nb/Hf in the zircon show arc magma origin. Conclusions Chert deposition in the Dalong Formation in South China ceased in the mid-Changhsingian Stage of the Late Permian at 253.0±1.3 Ma. The arc volcanism eruption was not related to intraplate Emeishan mantle plume activity, but was caused by arc activity around the Yangtze Block, and was probably related to the disappearance of the deep-marine troughs in the northern Yangtze area.
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  • Received:  2024-02-26
  • Revised:  2024-05-09
  • Accepted:  2024-06-08
  • Published:  2024-12-10

Termination Age of the Chert Deposits in the Late Permian Dalong Formation in Middle and Upper Yangtze Area, China

doi: 10.14027/j.issn.1000-0550.2024.065
  • 1. School of Geoscience and Technology, Southwest Petroleum University, Chengdu 610500, China
  • 2. Qiangtang Institute of Sedimentary Basin, Southwest Petroleum University, Chengdu 610500, China
  • 3. State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, China
Funds:

National Natural Science Foundation of China 42272118

Natural Science Foundation of Sichuan Province 2023NSFSC0279

  • Received Date: 2024-02-26
  • Accepted Date: 2024-06-08
  • Rev Recd Date: 2024-05-09
  • Available Online: 2024-06-08
  • Publish Date: 2024-12-10

Abstract: Objective Several intraplatform deep-marine trough basins in the northern Yangtze Platform developed bedded chert in the Dalong Formation during the Late Permian. The organic-rich black rocks in the trough basins are shale gas exploration targets in the Sichuan Basin. The black chert deposition in the Dalong Formation started at 258.77 Ma, related to waning of the Emeishan mantle plume. However, its termination date and the cause of the disappearance of the intraplatform trough basins were still unclear. The results of this study help to better understand the evolution of these deep-marine troughs, offering a theoretical basis for potential assessment and exploration of Permian shale gas. Methods Zircon U-Pb LA-ICP-MS age dating and trace element measurement in the bentonite interbeds were carried out in the upper part of the Dalong Formation chert beds. Results The zircon grains are euhedral and show clear zoning, with Th/U ratios higher than 0.46. The zircon grains are enriched in heavy rare earth elements （HREE） with positive Ce anomalies and negative Eu anomalies, showing a rising trend from light REE to HREE in the chondrite-normalized REE patterns. The U-Pb LA-ICP-MS age of the zircon was 253.0±1.3 Ma. Diagrams of Th/Nb vs. Hf/Th and Th/U vs. Nb/Hf in the zircon show arc magma origin. Conclusions Chert deposition in the Dalong Formation in South China ceased in the mid-Changhsingian Stage of the Late Permian at 253.0±1.3 Ma. The arc volcanism eruption was not related to intraplate Emeishan mantle plume activity, but was caused by arc activity around the Yangtze Block, and was probably related to the disappearance of the deep-marine troughs in the northern Yangtze area.

WEI HengYe, ZHANG Gan, ZHANG Xuan, HU Die, GONG JiaXin. Termination Age of the Chert Deposits in the Late Permian Dalong Formation in Middle and Upper Yangtze Area, China[J]. Acta Sedimentologica Sinica, 2024, 42(6): 2135-2143. doi: 10.14027/j.issn.1000-0550.2024.065
Citation: WEI HengYe, ZHANG Gan, ZHANG Xuan, HU Die, GONG JiaXin. Termination Age of the Chert Deposits in the Late Permian Dalong Formation in Middle and Upper Yangtze Area, China[J]. Acta Sedimentologica Sinica, 2024, 42(6): 2135-2143. doi: 10.14027/j.issn.1000-0550.2024.065
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0.   引言

  • 我国华南地区上二叠统上部地层普遍发育一套黑色层状硅质岩（也称为硅岩）沉积,统称为大隆组。在中上扬子区,这套硅质岩形成于台内盆地环境,例如广旺海槽和鄂西海槽等[1⁃2]。在广旺海槽沉积盆地,由东部的燕儿洞至西部的剑阁上寺,大隆组硅质岩沉积厚度逐渐减薄,在斜坡环境相变为硅质灰岩（图1）,盆地环境的沉积厚度约10 m；在鄂西盆地如恩施地区,大隆组硅质岩也具有类似的沉积厚度（图1）。大隆组硅质岩的下部地层为吴家坪组碳酸盐岩,地层过渡段由硅质灰岩突变或渐变为硅质岩,发育密集的火山灰层（图1）。因此,虽然大隆组硅质岩从岩相证据上指示为放射虫、硅质海绵等生物成因[5⁃8],但地球化学信号反映硅具有生物、热液以及火山喷发物质的混合来源[7,9⁃10]。这些热液和火山作用与峨眉山地幔柱（亦称峨眉地裂）活动、特提斯洋裂离以以及秦岭勉略洋的裂陷有关[5,11],说明吴家坪浅水台地沉积之后形成的大隆组台内盆地硅质岩沉积是在伸展构造背景下形成的断陷盆地[12],是东吴运动在中上扬子区的伸展裂陷作用产物[11]。大隆组硅质岩沉积的结束时间指示中上扬子区东吴运动裂陷活动的结束时代,尽管大隆组硅质岩开始沉积的年龄已经明确（258.77±0.67 Ma[4]）,但目前仍缺乏相应的绝对年龄的约束。因此,选择广旺海槽的旺苍燕儿洞剖面大隆组硅质岩的火山灰夹层斑脱岩为研究对象,开展锆石LA-ICP-MS U-Pb年龄分析,研究大隆组硅质岩沉积结束的年龄,约束大隆组富有机质黑色岩系的沉积年龄,为华南二叠系页岩气勘探提供依据。

    Figure 1.  Stratigraphic framework in the Dalong Formation in Upper Permian, South China

1.   地质背景

  • 晚二叠世早期,扬子区发生东吴运动,在我国西南地区喷发了大量的玄武岩,构成了峨眉山大火成岩省。峨眉山溢流玄武岩的分布西至理塘,东至桂西,南至越北,北至川东北达州（图2a）,分布面积为0.25×106 km2,体积为0.5×106 km3[14⁃15,17]。近年来,四川盆地油气勘探发现在开江、梁平以及忠县一带的DY-1、FT1等钻井在中—上二叠统界线钻遇玄武岩[16,18],同时在广元西北乡野外剖面的茅口组一段发育辉绿岩岩墙侵入体。峨眉山溢流玄武岩已被证实与地幔柱活动有关,称为峨眉山地幔柱[15,19⁃21]。地幔柱隆升活动过程包括地幔岩浆上涌、到达并撞击岩石圈底部、地壳抬升、火山喷发、地壳沉降等五个过程[22]。地幔柱头部到达岩石圈之后,发生侧向扩张延展,由原来的400 km直径拓展2倍至800~1 000 km,分别对应着内带和中带的范围[23]。在峨眉山大火成岩省主喷发之前和之后分别发育了中二叠统孤峰组和上二叠统大隆组硅质岩沉积。

    Figure 2.  Maps of geological setting in the study area

    由于东吴运动,华南晚二叠世时期古地理发生了根本性的变化,在原来中二叠世巨型碳酸盐岩台地基础上形成了隆凹格局的古地理面貌（图2b）。在西昌—攀枝花—昆明一带形成南北向的康滇古陆,古陆周围向外分布带状的海陆过渡相、浅海碳酸盐岩台地相和深水盆地相,总体展示穹状隆起的古地理特征[23⁃24]。位于外带的深水盆地相沉积了大隆组黑色硅质岩,例如位于川北的广元至梁平的广旺—开江梁平海槽以及湖北西部的鄂西海槽发育大量的富有机质硅质岩夹页岩。该套页岩和硅质岩目前是四川盆地及其周缘二叠系页岩气勘探目的层,近年来更是取得了突破性的进展,获得了可观的工业气流,是非常规油气勘探的新层位和新领域[25⁃26]。

2.   样品与实验方法

  • 在燕儿洞剖面采集大隆组上部硅质岩夹层中斑脱岩样品约1 kg（图3）。斑脱岩夹层为黄白色,夹于黑色页岩和黑色硅质岩之间,遇水黏手。该类斑脱岩在大隆组黑色硅质岩和页岩中发育多层,本次选择大隆组硅质岩段顶部的斑脱岩夹层作为研究对象（图1）。样品烘干后,采用传统锆石分选方法,经过磁选和重液分选后在双目镜下挑选晶形规则、颗粒较大的锆石。将挑选好的锆石切割并抛光以便露出锆石内部结构,用环氧树脂包埋固定,在扫描电子显微镜以及光学显微镜下进行观察并拍照阴极发光和透反射相片。锆石原位U-Pb年龄的测定采用激光剥蚀电感耦合等离子体质谱（Laser Ablation Inductively Coupled Plasma mass Spectrometry,LA-ICP-MS）技术,在武汉上谱分析公司实验室完成。激光剥蚀系统为COMPexPro 102 ArF 193 nm准分子激光剥蚀系统（GeoLas HD）,ICP-MS型号为Agilent 7900,激光束斑和频率分别为32 μm和5 Hz,锆石U-Pb同位素定年和微量元素含量处理采用锆石标准91500和玻璃标准物质NIST610作外标进行同位素和微量元素分馏校正。

    Figure 3.  Bentonite (volcanic ash) bed intercalated by black chert beds

3.   结果与讨论

    3.1.   锆石年龄

  • 斑脱岩样品中含丰富的锆石颗粒,用于铀铅LA-ICP-MS测年的锆石颗粒为自形棱角状,发育清楚的震荡生长环带,是典型的岩浆锆石（图4）。样品测定了17个数据点,测定的锆石年龄与同位素比值见表1。锆石中Th和U的含量一般较高,分析样品的U介于（236~479）×10-6,平均值为337×10-6,Th含量介于（109~305）×10-6,平均值为184×10-6,Th/U比值介于0.46~0.64,平均值为0.54,均大于0.46,同样指示岩浆成因性质[27],这是因为岩浆锆石的Th/U比一般大于或等于0.5[28]。锆石分析点的206Pb/238U年龄介于249~258 Ma,加权平均年龄为253.0±1.3 Ma（95%置信度,MSWD=1.12）（图5）,代表了火山喷发时棱角状锆石结晶年龄。由于该样品火山灰夹层正好位于大隆组硅质岩段的顶部,因此,大隆组硅质岩沉积的结束年龄约为253.0±1.3 Ma,属于长兴期中期年龄,晚于广元上寺剖面硅质灰岩上部年龄253.7 Ma[3],代表大隆组硅质岩终结年龄,也意味着二叠纪—三叠纪过渡期“硅质岩缺口”的开始。

    Figure 4.  Cathodoluminescence images of zircons from bentonite layers in the Dalong Formation in the Yan'erdong section, Wangcang, Sichuan province

    样品号Pb/×10-6Th/×10-6U/×10-6Th/UPb/×10-6207Pb/235U1σ206Pb/238U1σ（207Pb/206Pb）/Ma1σ（207Pb/235U） /Ma1σ（206Pb/238U） /Ma1σ
    YD41-2-0115.051743050.5700.296 60.0110.039 90.000 53618126482523
    YD41-2-0211.201092370.4600.276 90.0130.039 70.000 5220101248102513
    YD41-2-1011.721242360.530.130.284 00.0110.040 60.000 62548825492564
    YD41-2-1117.381803530.510.460.266 30.0100.040 30.000 41068924082553
    YD41-2-1313.041372700.510.060.277 90.0110.039 80.000 42179424992523
    YD41-2-1715.111463080.4700.282 20.0110.040 30.000 52209125292553
    YD41-2-1919.762234040.5500.298 80.0100.040 40.000 53457826582553
    YD41-2-2022.943054790.6400.269 00.0090.039 10.000 51767624272473
    YD41-2-2117.452043560.570.570.285 20.0100.039 80.000 42768825582512
    YD41-2-2315.891773260.5400.286 80.0090.040 50.000 42618425672563
    YD41-2-2416.201863300.570.400.281 90.0120.040 10.000 5217100252102543
    YD41-2-2516.421863300.5600.288 50.0120.040 80.000 52509425792583
    YD41-2-2613.501352790.480.210.297 10.0120.040 00.000 43699426492533
    YD41-2-2721.222674300.620.280.280 30.0100.039 80.000 42207425182523
    YD41-2-2813.411322770.4800.278 80.0110.040 90.000 517613125092583
    YD41-2-3222.752674730.560.010.281 70.0100.039 40.000 42808525282492
    YD41-2-3316.751803430.520.330.279 60.0090.039 90.000 42287825072522

    Table 1.  Isotopic ratios and age dating data of zircon in the bentonite in the Dalong Formation in the Yan'erdong section, Wangcang, Sichuan province

    Figure 5.  (a) LA⁃ICP⁃MS U⁃Pb dating concordia diagram, and (b) individual ages of zircon analyses from bentonite layers in the Dalong Formation in the Yan'erdong section, Wangcang, Sichuan province

    • 3.2.   锆石微量元素指示的构造背景

  • 锆石稀土元素（Rare Earth Element,REE）总量∑REE介于（626~985）×10-6,Hf含量介于（8 032~9 170）×10-6（表2）。锆石稀土元素球粒陨石标准化的配分模式反映重稀土富集、轻稀土亏损、Ce的正异常以及Eu的负异常的特征,轻稀土一般小于100倍的球粒陨石值,重稀土一般大于10 000倍的球粒陨石值,总体表现为从轻稀土向重稀土逐渐爬升的左倾曲线趋势,（图6）,这是未发生蚀变的岩浆锆石的特征[27]。锆石Ce正异常说明锆石是在氧化条件下结晶的,这与样品为喷发火山灰性质一致,而Eu的负异常可能是锆石结晶过程中或结晶之前长石从岩浆中分馏导致岩浆Eu的亏损所致[29]。

    样品号LaCePrNdSmEuGdTbDyHoErTmYbLuTiNbHfTa
    YD41-2-010.003 19.570.0260.832.210.4614.55.1372.230.915635.033475.03.352.278 2220.95
    YD41-2-020.053 08.100.0310.601.400.2011.74.2259.225.613029.528966.32.611.979 1701.09
    YD41-2-100.003 06.880.0531.073.010.6218.06.8690.237.618240.238685.13.451.318 0320.63
    YD41-2-110.009 09.430.0601.162.130.4515.55.8382.235.418241.241093.54.102.568 5241.17
    YD41-2-130.013 08.250.0630.912.820.6018.67.3297.040.720244.543295.94.712.068 0750.90
    YD41-2-170.003 18.850.0410.911.960.6115.15.7583.835.518141.341494.54.102.678 0641.15
    YD41-2-190.003 311.000.0310.791.860.4915.05.7277.834.817539.639089.12.962.648 8891.31
    YD41-2-200.027 012.600.0861.692.970.4218.36.9194.440.420145.6446100.03.103.428 4761.52
    YD41-2-210.001 010.300.0581.352.690.6619.47.2697.942.020846.145099.22.942.598 2030.98
    YD41-2-230.005 78.670.0270.812.240.4413.24.9569.129.014331.530467.23.521.888 3790.91
    YD41-2-2420.300 054.305.57027.106.860.8618.15.8275.331.215233.232370.84.101.958 3050.96
    YD41-2-2509.840.0420.661.720.3913.95.3471.131.014932.931770.04.411.998 6010.91
    YD41-2-260.003 19.300.0661.093.030.5817.96.8894.140.820446.3456102.05.442.408 0830.95
    YD41-2-270.003 211.100.0600.972.270.4416.16.2881.834.316937.535577.73.612.098 5371.12
    YD41-2-280.006 38.120.0400.801.780.4214.15.2671.131.615936.136081.34.132.258 3720.99
    YD41-2-320.009 011.700.0520.641.910.4015.85.9579.535.817841.240792.22.163.019 1351.42
    YD41-2-330.006 19.270.0831.242.950.5219.17.4096.940.820145.444298.72.542.048 9551.14

    Table 2.  Zircon trace element data in the bentonite in the Dalong Formation in the Yan'erdong section, Wangcang, Sichuan province (×10-6)

    Figure 6.  Chondrite⁃normalized REE patterns of zircon from bentonite layers in the Dalong Formation in the Yan'erdong section, Wangcang, Sichuan province

    锆石的微量元素能够反映岩浆母源组成并指示岩浆宿主的构造背景特征[30⁃31]。Hf离子半径与Zr相似,因而锆石中含丰富的Hf,样品锆石Hf含量介于（8 032~9 170）×10-6,平均值达8 472×10-6（表2）。样品锆石中Nb含量很低,介于（1.31~3.42）×10-6,平均为2.30×10-6,与大陆岛弧和洋中脊来源的锆石Nb含量相似,而不同于板内背景下岩浆锆石较高的Nb含量[32⁃33]。Nb一般与岩浆熔融过程中的不相容元素Th为正相关关系,岛弧锆石的Th/Nb比值一般大于10,而板内和洋中脊锆石Th/Nb比值一般小于20[31],同时岛弧岩浆比板内岩浆锆石具有更低的Nb/Hf和更高的Th/U比值[34⁃35]。由于富Th矿物优先结晶,岛弧岩浆锆石具有低Th/Nb和高Hf/Th比值的特征[35⁃37],因此可以利用这些参数来识别锆石岩浆母源的构造背景。Th/Nb与Hf/Th交会图以及Th/U与Nb/Hf交会图中,燕儿洞剖面大隆组顶部和上部火山灰锆石样品落在岛弧岩浆范围内,而不是落在板内造山范围（图7）。其他地区上二叠统吴家坪组以及大隆组的火山灰锆石构造背景的分析也表明[4,38]：湖北西部以及四川剑阁上寺吴家坪阶的火山灰锆石性质为板内造山背景,与峨眉山大火成岩省关系密切[4],而长兴阶的火山灰锆石性质却为岛弧性质。这表明广元旺苍县燕儿洞大隆组沉积晚期的火山喷发可能是周围岛弧性质火山喷发所致,属于汇聚型板块边界的岩浆岛弧性质[38],而不是板内性质峨眉山地幔柱活动的结果。

    Figure 7.  Tectonic setting crossplots for zircons from bentonite in the Dalong Formation in the Yan'erdong section, Wangcang, Sichuan province

    • 3.3.   大隆组硅质岩年龄的意义

  • 四川盆地广旺海槽及其周缘鄂西盆地大隆组硅质岩含多层火山灰夹层,大隆组下部吴家坪阶的硅质岩内火山灰夹层锆石指示板内造山性质[4],说明该期火山喷发与板内造山性质的峨眉山大火成岩省（或地幔柱）活动有关。地幔柱形成过程中,地壳抬升之后发生裂谷或地堑地垒式台内盆地[22,39],诱发海底活跃的热液活动[40]。峨眉山大火成岩省（地幔柱）中心位于攀枝花地区,地幔柱头部在到达岩石圈之前约400 km,碰撞岩石圈底部之后侧向扩展为800~1 000 km,也即峨眉山大火成岩省中带与外带的分界面[17,41]。而广旺海槽以及鄂西海槽位于外带边缘（图1）,远离峨眉山地幔柱头部岩浆的影响范围。但是,近年来四川盆地油气勘探钻井发现,在广旺海槽钻遇大量的峨眉山玄武岩[16,18]以及部分野外辉绿岩岩墙,地幔柱岩浆在侧向上可以通过岩墙和岩石圈底部软流圈进行远距离扩张[42⁃43]。峨眉山地幔柱岩浆的影响范围可能包括广旺海槽、开江—梁平海槽以及鄂西海槽的外带。峨眉山大火成岩省（地幔柱）在中—晚二叠世过渡期大规模喷发之后,可能形成多个裂谷性质（地堑地垒）断陷盆地或海槽,海底频繁的热液活动以及陆地喷发的玄武岩风化带来的硅质在海洋较深水盆地中富集形成大隆组硅质岩。

    然而,大隆组上部和顶部长兴阶硅质岩内火山灰夹层锆石与二叠纪末期火山灰锆石一样,属于岛弧造山构造背景[4,38],说明大隆组沉积晚期火山喷发与峨眉山大火成岩省活动无关,而与扬子板块周围的岛弧火山的活动有关[43]。推测长兴期扬子板块北部出现明显的岩浆岛弧活动,该活动可能与大隆组硅质岩盆地环境萎缩灭亡密切相关。

4.   结论

  • 四川盆地广旺海槽上二叠统大隆组硅质岩上部火山灰夹层锆石的铀铅LA-ICP-MS测年表明,大隆组硅质岩沉积结束于253.0±1.3 Ma,属于长兴期中期。锆石稀土元素展示重稀土富集、轻稀土亏损、Ce正异常和Eu负异常的左倾配分曲线特征,痕量元素Th/U比绝大部分大于0.5,指示岩浆锆石的特征。锆石Th/Nb与Hf/Th以及Th/U与Nb/Hf交会图反映锆石岩浆母源为岛弧造山构造背景,与二叠纪末期扬子板块周围岩浆岛弧活动有关,而与板内造山成因的峨眉山大火成岩省无关。晚二叠世长兴中期的岛弧火山活动可能与扬子北缘多个海槽在二叠纪末期萎缩消失密切相关。

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