刘春来, 熊国庆, 董国明, 崔伟

1) 河北省地质矿产勘查开发局第二地质大队, 河北唐山, 063004; 2) 中国地质调查局成都地质调查中心，成都，610081; 3) 自然资源部沉积盆地与油气资源重点实验室，成都，610081

内容提要: 近年来，扬子地块北缘奥陶纪—志留纪之交沉积构造背景一直存在不同认识。本文对渝东北城口厚坪五峰组—龙马溪组砂、泥岩进行系统采样和主、微量元素测试分析，从地球化学特征角度，来探讨该时期砂、泥岩物源源区及其构造环境。地球化学特征显示，五峰组—龙马溪组砂、泥岩主量元素SiO2含量为39.57%～87.16%，多数高于北美页岩，Al2O3含量为3.41%～14.98%，均低于北美页岩，MgO、CaO、MnO2和P2O5含量明显低于北美页岩，相比于四川盆地川南长宁—双河五峰组—龙马溪组泥岩，渝东北城口具有高SiO2和低MgO、CaO的特点，表明城口五峰组—龙马溪组泥岩碎屑组分主要以粉砂质和硅质等陆缘碎屑为主；微量元素Sc、Sr、Co、Zr、Hf较北美页岩相对亏损，V、U、Sb、Ba相对富集，Cr、Ni、Nb、Ta含量相近。稀土元素北美页岩标准化均表现为略微向右倾斜的配分模式，多数具有δCe负异常，观音桥段和龙马溪组δEu正异常指示了该时期存在海底热液活动。砂、泥岩主、微量元素的物源和构造背景图解表明沉积物源区源岩具有多样的物质组分，以长英质岩为主，同时混入了花岗质和安山质岩石组分；源岩形成时的构造背景主要为被动陆缘环境，少数为活动陆缘和大陆岛弧环境，这一结果与扬子地块北缘新元古代—早古生代处于被动大陆边缘的区域地质背景一致。

碎屑岩是由母岩风化产物或火山碎屑经搬运、沉积、压实和胶结而形成的岩石，一般认为这些碎屑岩组分主要受物源区母岩性质和构造背景影响(McLennan et al., 1993; Fralick and Kronberg, 1997)。因此，碎屑组分常作为一种恢复沉积盆地的物源和构造背景的传统方法(Dickinson, 1983; Morton, 1991)。研究表明，在风化、搬运和成岩过程中，碎屑岩中一些相对不易迁移的地球化学组分(如稀土元素(REE)和Zr、Th、Sc、Y等一些微量元素)，不仅能提供了物源区成分的信息(Taylor and McLennan, 1985; Girty et al., 1993)，Bhatia(1983)和Roser等(1986)还据此提出了不同构造背景下砂、泥岩的地球化学判别方法，随后被广泛用于碎屑岩的物源及其沉积构造背景分析(Bhatia and Crook, 1986; McLennan et al., 1993)。

渝东北地区位于扬子陆块北缘，奥陶纪—志留纪之交，沉积了一套深水陆棚相五峰组—龙马溪组黑色页岩。这一时期，扬子陆块北缘究竟是处于秦岭洋南侧的扬子被动大陆边缘(吉让寿等，1990；张国伟等，1988，1995，1996，2001)，还是南秦岭沟—弧—盆体系的活动大陆边缘(王宗起等，2009)，或是克拉通盆地南侧的扬子陆缘(姜涌泉，1998)，仍存有争议。前人通过常量元素的地球化学方法对渝东北地区五峰组—龙马溪组泥岩所形成的构造环境进行了初步讨论(熊小辉等，2015；刘田等，2019)，但缺乏微量元素，尤其是不活泼的元素的深入系统地分析，其结论有待进一步验证。本文从渝东北城口厚坪五峰组—龙马溪组泥岩的地球化学特征入手，详细探讨扬子陆块北缘奥陶纪—志留纪之交沉积物的物源及其沉积构造背景，对该地区这一时期沉积盆地所处的大地构造背景及其沉积古地理源区分析具有重要的地质意义。

1 区域地质背景

大巴山地区处于盆山过渡带，北与东秦岭造山带、南与四川盆地相接，东西夹持于武当山隆起、神农架隆起和米仓山隆起之间，呈向南凸出的弧形构造带，城巴断裂将其分为南、北大巴山两个沉积—构造单元(图1a，李智武等，2006)。研究区位于大巴山西段的川渝陕交界，区内自下而上发育震旦系、寒武系、奥陶系、志留系、二叠系及三叠系，缺失泥盆系和石炭系，北大巴山早古生代地层中辉绿岩脉顺层发育(图1b)，时代上为奥陶纪—志留纪(向忠金等，2016)。

晚元古代的晋宁运动，华北陆块与扬子陆块逐渐拼合对接，最终导致南秦岭在内各个块体形成统一的克拉通褶皱基底(罗志立，1998)。随着全球Rodinia超大陆解体，华北陆块和扬子陆块发生裂解，逐渐演化形成“秦岭洋”。有观点认为，秦岭洋发生在商南—丹凤一线，也称“商丹洋”，两侧为被动大陆边缘，奥陶—志留纪开始，扬子陆块向华北陆块之下的北向俯冲，南秦岭南缘此时发生局部拉伸，基性岩浆活动就位(胡健民等，2002)，洋盆于泥盆—石炭纪消亡，与此同时，沿勉县—略阳一带打开一个新的勉略洋盆，最终形成了秦岭造山带“三块夹两缝”的构造格局(张国伟等，1995a，b，1996，2001，2003，2004)；也有观点认为，扬子陆块和华北陆块之间的秦岭造山带只存在一个统一的原始大洋盆地——“秦岭洋”，时间为新元古—早中生代(约840～220 Ma)(闫全人等，2007，2009，2014)，晚古生代秦岭洋向南俯冲增生，南秦岭南缘构成了安康弧、北大巴山弧后混杂岩和寒武纪—志留纪弧后盆地构造体系，南大巴山为弧后盆地陆缘，属于扬子台地稳定沉积(王宗起等，2009；王刚等，2014)。随着早中生代洋盆消亡，印支—燕山期，秦岭开始转入陆内碰撞造山阶段，喜马拉雅期定型，最终形成了现今的大巴山弧形构造带。

2 样品采集及测试

2.1 样品采集

本文研究的样品采自南大巴山西段渝东北城口县厚坪(HP)剖面和蒙蒙溪剖面(MM)，两者处于同一向斜的南北两翼，直线距离约10 km(图1c)。厚坪剖面位于城溪路S301省道至厚坪的X026县道边(图1b)，晚奥陶世临湘组—早志留世龙马溪组地层连续。临湘组与下伏宝塔组灰色中层状龟裂纹灰岩整合接触，主体上为一套浅水陆棚黄绿色泥岩夹灰色泥灰岩，顶部为灰色薄层状泥质白云岩、泥质砂岩，表现为向上变浅的沉积层序，顶界面土黄色泥质风化壳表明曾短暂暴露(熊国庆等，2019)，其上五峰组深水陆棚黑色中层状硅质页岩、硅质碳质泥岩，为一套快速海侵沉积，上覆观音桥段时代上与全球赫兰特冰期相当，底部以黑色薄板状泥质粉砂岩，向上变为灰黑色中层状细砂岩、中—粗砂岩及灰色厚层状含砂砾白云岩，与五峰组碳质泥岩渐变过渡，反映了一次逐渐海退的沉积序列(熊国庆等，2019)；随着赫兰特冰川消融，全球海平面迅速上升，龙马溪组一套深水陆棚黑色薄层状粉砂质碳质泥岩快速沉积于白云岩之上(图1c)，未见顶。蒙蒙溪剖面位于明中乡至彭家湾的乡道边，交通方便(图1b)，露头出露较为完整，晚奥陶世临湘组—早志留世龙马溪组地层连续。临湘组浅黄绿色泥岩与五峰组黑色硅质碳质泥岩直接接触，表现为一次快速海侵过程，随后发育的观音桥段白云岩呈透镜状，区域上可对比，为一套赫兰特期冰期沉积，其上龙马溪组黑色粉砂质碳质泥岩为一套冰期后全球海平面再次快速上升形成的深水陆棚相沉积(图1c)。

2.2 样品处理及测试

采样时已尽量剥除风化表面，保证样品新鲜和足量。全岩样品首先粉碎至200目以下，常量、微量和稀土元素测试分析在核工业北京地质研究院分析测试研究中心完成，测试时，室内温度20℃，相对湿度30%。常量元素分析仪器为AB-104L，PW2404 X射线荧光光谱仪，测试方法和依据为GB/T 14506.14-2010《硅酸盐岩石化学分析方法 第14部分：氧化亚铁量测定》，GB/T 14506.28-2010《硅酸盐岩石化学分析方法 第28部分：16个主次成分量测定》，相对误差为±1%～±4%。微量、稀土元素测试采用ELEMENT XR等离子质谱分析仪，测试方法和依据为GB/T 14506.30-2010《硅酸盐岩石化学分析方法 第30部分：44个元素量测定》，相对误差为±3%～±12%。常量元素及微量、稀土元素测试分析结果分别列入表1、表2、表3和表4中。

表3 扬子地块北缘城口厚坪剖面五峰组—龙马溪组砂、泥岩微量元素分析及计算结果(×10-6)Table 3 Analysical and calculated results of trace elements in sandstone and mudstones of the Wufeng Formation and the Longmaxi Formation from Houping profile in Chengkou County, northern margin of the Yangtz block(×10-6)

样品MM1MM2MM3MM4MM5MM6MM7MM8MM9MM10MM11MM12MM13MM14Nb19.98.6610.46.3612.24.1514.611.78.7613.19.2211.317.312.7Ta1.020.5630.7520.4510.311.420.8450.6030.9090.6560.7071.040.806Zr13555.971.539.677.129.310486.759.382.762.378.811082.8Hf3.21.471.851.162.160.8962.611.881.742.41.812.083.172.29La4926.332.520.734.613.945.533.929.538.728.235.442.226.1Ce79.44460.536.359.425.679.357.948.869.651.462.673.647.5Pr8.925.048.044.87.433.3910.77.256.029.046.417.968.666.09Nd30.917.830.518.426.314.239.923.821.334.123.331.731.823.5Sm5.042.624.352.743.782.776.32.822.925.013.235.034.144.48Eu1.030.6021.060.7891.030.8960.9580.3690.6951.210.9851.681.211.12Gd4.712.834.12.573.562.244.752.682.924.773.194.514.063.85Tb0.6880.4230.6750.3740.530.3780.7740.4130.3920.7010.4630.6810.5880.64Dy4.022.293.421.942.91.983.652.1423.612.433.493.123.55Ho0.9190.5210.6450.4160.6310.4220.7050.4470.3990.720.480.6640.6630.7Er3.141.572.011.292.051.242.081.371.312.261.541.982.212.09Tm0.6160.2580.3030.2190.3530.1970.3650.2510.2350.3470.2530.3130.3810.374Yb4.181.621.941.422.341.262.61.811.472.231.632.082.472.34Lu0.6870.2560.2810.2270.3660.1990.3860.2550.2530.3440.2720.3260.4110.357ΣREE193.25106.13150.3292.19145.2768.67197.97135.41118.21172.64123.78158.41175.51122.69LREEHREE9.199.8710.249.9010.417.6811.9313.4612.1710.5211.0710.2811.627.83La/Yb11.7216.2316.7514.5814.7911.0317.5018.7320.0717.3517.3017.0217.0911.15(La/Yb)N8.4111.6512.0210.4610.617.9112.5513.4314.3912.4512.4112.2112.268.00δEu0.650.680.770.910.861.100.540.410.730.760.941.080.900.82δCe0.930.940.920.890.910.910.880.910.900.910.940.910.940.92样品MM15MM16MM17MM18MM19MM20MM21MM22MM23MM24MM25MM26MM27MM28Sc7.027.674.534.837.336.817.348.725.68.49.316.475.25.94V339283163288316320263401260352301272263263Cr50.95928.441.75155.848.569.147.655.960.448.238.943.6Co5.215.185.484.977.747.266.121.7238.7910.49.078.788.36Ni80.272.458.273.991.794.663.758.662.710510410298.7103Cu48.743.230.444.862.353.347.419.526.668.867.255.15350Zn30.945.314330.263.766.253.433.347.511713914859.5114Rb10012457.378.21041061111488311010979.158.871.8Sr61.794.664.558.256.476.587.21238881.368.459.442.653.5Y19.824.421.922.525.227.829.241.420.928.632.928.618.526.1Sb6.915.940.5684.345.22.472.331.7645.364.823.64.114.07Cs6.067.722.914.046.226.086.798.574.516.575.844.482.714.08Ba31793291362972783258318433193170505346994625529934513579Pb18.921.815.727.48.2534.82916.625.518.420.828.220.116.2Th12.29.785.38.884.05118.377.6212.48.7810.611.47.378.75U10.823.710.314.38.9516.7118.0416.310.213.813.313.118.2Nb19.710.46.3612.24.1514.611.78.7613.19.2211.317.37.688.88Ta1.670.7520.4510.311.420.8450.6030.9090.6560.7071.040.5890.637Zr11071.539.677.129.310486.759.382.762.378.81105765.2Hf3.471.851.162.160.8962.611.881.742.41.812.083.171.321.88La40.653.718.219.834.93845.15329.638.337.729.122.426.9Ce68.496.333.536.359.568.984.297.153.269.768.651.53848.3Pr8.2311.34.444.757.048.499.5312.86.38.898.477.124.996.31Nd29.641.517.719.12632.23545.323.535.832.928.518.123.6Sm3.825.283.624.234.885.615.026.643.836.476.766.243.214.95Eu1.131.391.4211.521.571.470.9990.8381.61.561.510.361.52Gd3.925.293.243.614.355.024.75.523.545.695.565.312.844.23Tb0.550.7030.5340.6450.70.8060.7180.9190.5490.93610.9220.5080.725

样品MM15MM16MM17MM18MM19MM20MM21MM22MM23MM24MM25MM26MM27MM28Dy3.143.421.942.91.983.652.1423.612.433.493.122.613.81Ho0.6350.6450.4160.6310.4220.7050.4470.3990.720.480.6640.6630.5150.789Er1.982.011.292.051.242.081.371.312.261.541.982.211.452.26Tm0.3560.3030.2190.3530.1970.3650.2510.2350.3470.2530.3130.3810.2510.384Yb2.241.941.422.341.262.61.811.472.231.632.082.471.692.25Lu0.3620.2810.2270.3660.1990.3860.2550.2530.3440.2720.3260.4110.2460.36ΣREE164.96150.3292.19145.2768.67197.97135.41118.21172.64123.78158.41175.5197.17126.39LREEHREE11.5110.249.9010.417.6811.9313.4612.1710.5211.0710.2811.628.617.54La/Yb18.1316.7514.5814.7911.0317.5018.7320.0717.3517.3017.0217.0913.2511.96(La/Yb)N13.0012.0210.4610.617.9112.5513.4314.3912.4512.4112.2112.269.518.58δEu0.890.770.910.861.100.540.410.730.760.941.080.900.361.02δCe0.920.920.890.910.910.880.910.900.910.940.910.940.880.91样品MM29MM30MM31MM32MM33MM34MM35MM36MM37MM38MM39MM40MM41MM42Sc6.366.996.085.384.66.447.198.779.55.799.255.998.837.79V227286220232162199194281240195254179180146Cr45.149.943.241.431.542.547.661.168.248.667.949.449.440.8Co8.058.38.387.586.4910.61.0111.45.032.681.755.38.216.03Ni87.592.793102738643.91165947.350.254.173.654.3Cu43.559.543.241.731.250.32176.838.432.426.458.385.767.4Zn83.242.611767.452.31012616447.724.535.829.137.625.4Rb76.685.973.97054.976.410810714183.81309697.878.5Sr55.967.749.344.743.454.281.31341021221456971.463.3Y25.824.426.524.117.52216.423.131.81625.220.526.519Sb0.6864.082.962.811.92.5633.733.350.3082.071.810.9792.45Cs4.324.574.073.752.754.175.736.248.494.436.835.765.514.31Ba34843391339727332874381637723203519296933502347154633560Pb1420.314.513.710.715.819.920.626.214.829.7212117.9Th9.5710.58.467.166.610.411.912.915.89.5914.912.312.611U17.516.618.513.211.22013.912516.88.618.412.814.312.1Nb9.1310.48.867.726.8310.513.321.335.911.81813.813.310.7Ta0.6620.7460.6130.5250.4380.681.061.453.690.751.120.8810.8520.724Zr73.174.865.657.348.864.510811922777.612192.610673.1Hf2.022.21.941.311.031.813.223.667.542.213.542.632.762.09La28.73125.622.920.429.948.744.974.833.350.936.63831.2Ce49.853.247.639.936.75287.882.61335688.666.166.654.7Pr6.437.246.225.735.046.4710.410.714.36.911.27.98.866.74Nd24.527.323.822.220.125.337.342.252.724.243.927.429.724Sm5.25.374.964.434.145.034.857.388.063.456.993.664.493.6Eu1.581.631.590.9170.7541.531.111.552.010.9011.81.250.9621.38Gd4.364.524.44.043.44.084.425.576.943.375.693.824.263.63Tb0.7690.7560.7770.7370.5550.6950.6280.8781.110.4860.8520.5940.7070.58Dy4.043.94.23.692.923.623.334.386.012.554.543.234.043.12Ho0.8390.8080.9040.7380.5450.7560.6090.8071.170.5170.8860.6930.8490.617Er2.422.352.421.921.42.152.012.643.591.682.872.142.851.91Tm0.4050.3830.3940.3140.2250.3280.3430.4130.6130.2820.4830.3630.4710.315Yb2.452.422.382.031.532.132.222.673.821.883.12.232.982.08Lu0.40.3860.3720.2780.210.3330.3610.4240.5680.3340.4740.3650.4770.319ΣREE131.89141.26125.62109.8297.92134.32204.08207.11308.69135.85222.29156.35165.25134.19LREEHREE7.418.106.936.998.088.5313.6610.6511.9611.2410.7610.648.939.67La/Yb11.7112.8110.7611.2813.3314.0421.9416.8219.5817.7116.4216.4112.7515.00(La/Yb)N8.409.197.728.099.5610.0715.7412.0614.0512.7111.7811.779.1510.76δEu1.011.011.040.660.611.030.730.740.820.810.871.020.671.17δCe0.900.870.920.850.890.920.960.921.000.910.910.950.890.92

样品MM43MM44MM45MM46MM47MM48MM49MM50MM51MM52MM53MM54MM55MM56MM57Sc7.37.19610.29.7610.68.117.737.789.856.877.257.129.038.46V18518016415098.2155117120132129116108135148148Cr52.853.248.557.154.86047.151.352.658.147.247.451.254.545.2Co10.66.423.856.999.3613.68.6611.112.115.85.915.095.576.632.33Ni97.855.242.953.651.768.84338.964.567.623.721.539.846.622.8Cu89.750.56857.47845.542.642.842.260.543.966.662.166.345.3Zn16.430.532.140.929.712048.454.411212033.632.537.537.835.2Rb10198.685.3116119129105115110122109117113124117Sr67.566.368.211985.480.383.179.798.489.778.474.575.370.893.7Y19.119.31821.920.6272014.119.716.415.919.118.719.222.8Sb2.852.812.730.8891.620.640.8231.11.311.340.7651.31.291.80.644Cs4.965.444.836.645.757.145.36.185.477.356.386.886.487.196.28Ba265234443496290827513392390730856231302635593023362429806738Pb21.822.519.91818.218.217.82023.619.323.415.82420.324.6Th12.610.99.5913.21213.911.612.413.312.11112.21310.912.3U9.6510.88.510.712.112.79.68.2720.615.77.3512.911.610.311.7Nb10.912.37.8313.312.114.112.414.112.714.313.31516.314.915.9Ta0.8650.8940.3430.940.8340.9630.8630.8510.8990.8510.8531.020.9440.7710.933Zr81.981.570.710410410489.610511310796.112511391.2121Hf1.892.41.912.952.593.042.382.853.182.582.773.092.912.012.75La32.634.43437.836.739.434.132.633.935.23435.837.436.738Ce6061.657.469.67172.562.160.26269.562.470.169.26767.9Pr7.877.626.938.929.438.857.636.737.917.227.296.988.547.048.8Nd28.32825.732.534.233.829.223.529.428.327.125.92925.531.9Sm4.423.973.855.76.066.734.873.345.194.73.514.014.43.835.29Eu0.7691.271.091.30.9821.641.560.7971.220.5661.190.5931.310.5831.07Gd3.583.743.684.664.765.454.183.024.283.693.313.173.823.264.24Tb0.5860.610.5760.7650.7790.9360.690.4570.6920.5640.4720.5110.5830.50.714Dy3.043.273.043.814.125.013.542.443.562.982.52.73.252.913.95Ho0.5970.6680.6120.7390.7820.970.720.4380.7090.5140.4840.5390.6250.5480.816Er1.692.041.922.362.172.942.161.522.051.851.681.932.0222.61Tm0.2850.3440.3370.3770.3590.4730.3670.2390.3510.2530.290.2860.3430.2790.456Yb1.942.262.162.382.43.062.381.482.221.661.841.792.121.762.89Lu0.2720.3480.3530.3770.3330.4710.40.2370.3510.2570.2910.2660.340.2740.477ΣREE145.95150.14141.65171.29174.08182.23153.90137.00153.83157.25146.36154.58162.95152.18169.11LREEHREE11.1710.3110.1710.0710.098.449.6612.949.8212.3612.4712.8111.4412.209.47La/Yb16.8015.2215.7415.8815.2912.8814.3322.0315.2721.2018.4820.0017.6420.8513.15(La/Yb)N12.0510.9211.2911.3910.979.2410.2815.8010.9515.2113.2514.3512.6514.969.43δEu0.591.010.890.770.560.831.060.770.790.421.070.510.980.500.69δCe0.920.930.920.930.940.950.941.000.931.070.971.090.951.020.91

表4 扬子地块北缘城口蒙蒙溪剖面五峰组—龙马溪组泥岩微量元素分析及计算结果(×10-6)Table 4 Analysical and calculated results of trace elements in mudstone of the Wufeng Formation and the Longmaxi Formation from Mengmengxi profile in Chengkou County, northern margin of the Yangtz Block(×10-6)

3 结果

3.1 主量元素

厚坪五峰组—龙马溪组砂、泥岩SiO2含量为53.4%～79.43%，平均72.98%(表1)，高于北美页岩64.8%，也高于同一盆地内川南长宁—双河五峰组—龙马溪组SiO2含量55.67%的平均值。五峰组顶部2件泥岩SiO2含量为53.4%和56.21%，较其他样品明显偏低，这与样品的高烧失量有关；观音桥段2件砂、泥岩SiO2含量为69.03%和69.71%，龙马溪组1件样品70.91%，低于其他样品，高于北美页岩；其余10件样品SiO2含量平均77.54%，五峰组与龙马溪组基本相同，远高于北美页岩(表1)。蒙蒙溪五峰组—龙马溪组泥岩SiO2含量为39.57%～87.16%，平均73.31%(表2)，均高于北美页岩和川南长宁—双河。龙马溪组一个异常低的SiO2值与其高铁含量和高烧失量有关，其余绝大多数均大于70%(表2)。厚坪五峰组—龙马溪组砂、泥岩Al2O3含量为3.41%～11.47%，平均7.73%，均低于北美页岩16.9%，也低于川南长宁—双河五峰组—龙马溪组Al2O3含量10.13%的平均值。五峰组顶部2件泥岩Al2O3含量为11.09%和11.49%，明显高于其他样品，与川南长宁—双河接近，观音桥段2件样品Al2O3含量最低，仅为3.41%～5.62%，其余11件样品Al2O3含量平均7.67%，五峰组与龙马溪组基本接近(表1)。蒙蒙溪泥岩Al2O3含量为4.26%～14.98%，平均8.98%，均低于北美页岩16.9%，也低于川南长宁—双河，但略高于厚坪(表2)。五峰组Al2O3含量为4.26%～11.48%，平均7.95%，龙马溪组Al2O3含量为4.82%～14.98%，平均9.42%，龙马溪组高于五峰组。厚坪五峰组和龙马溪组MgO和CaO含量分别介于0.393%～1.01%，平均0.642%，0.064%～0.803%，平均0.227%，观音桥段含量远高于五峰组、龙马溪组，分别为2.31%～4.31%和6.99%～7.39%；蒙蒙溪两者含量分别为0.285%～0.997%，平均0.59%，0.112%～0.644%，平均0.156%，与厚坪含量相当，但均远低于川南长宁—双河MgO和CaO含量3.0%和9.51%的平均值(李艳芳等，2015)。川南长宁—双河富有机质层段SiO2含量高，Al2O3含量低，贫有机质层段SiO2含量低，Al2O3含量高，这种富有机质段高SiO2含量与生物活动强烈导致的生物硅增加有关，贫有机质段Al2O3含量增加与陆源碎屑物质输入有关(李艳芳等，2015)。相比川南长宁—双河，渝东北城口厚坪具有高SiO2和低MgO、CaO的特点，表明城口五峰组—龙马溪组泥岩碎屑组分主要以粉砂质和硅质等陆缘碎屑为主，长宁—双河地区还含有大量的钙质组分。

表1 扬子地块北缘城口厚坪剖面五峰组—龙马溪组砂、泥岩主量元素分析结果(%)Table 1 Analysical results (%) of major elements in sandstone and mudstone of the Wufeng Formation and the Longmaxi Formation from Houping profile in Chengkou County, northern margin of the Yangtz Block

表2 扬子地块北缘城口蒙蒙溪剖面五峰组—龙马溪组泥岩主量元素分析结果(%)Table 2 Analysical results of major elements in mudstones of the Wufeng Formation and the Longmaxi Formation from Mengmengxi profile in Chengkou County, northern margin of the Yangtz block(%)

样品地层及岩性深度(m)SiO2Al2O3Fe2O3MgOCaONa2OK2OMnO2TiO2P2O5FeO烧失量总含量MM18MM19MM20MM21MM22MM23MM24MM25MM26MM27MM28MM29MM30MM31MM32MM33MM34MM35MM36MM37MM38MM39MM40MM41MM42MM43MM44MM45MM46MM47MM48MM49MM50MM51MM52MM53MM54MM55MM56MM57龙马溪组泥岩10.8679.227.172.680.4750.1580.5792<0.0040.380.0720.886.86100.4711.1572.269.624.430.6960.250.7642.670.0050.5290.0730.818.28100.3911.4474.799.073.230.5920.1480.7152.48<0.0040.4970.0770.987.98100.5612.0271.7710.382.670.7330.1360.5812.860.0090.5620.0631.059.87100.6812.3170.9311.732.070.8240.2130.7673.220.0040.6150.0871.829.02101.3012.6074.68.172.440.5140.2690.9022.190.0040.4590.0891.0110.12100.7713.0470.389.634.440.6070.1610.9422.530.0070.4850.0741.0410.26100.5613.5572.1710.042.90.6490.1930.8892.670.0080.5660.1010.999.44100.6214.4272.438.034.930.510.1280.8422.070.0070.4220.1040.9610.2100.6315.2939.574.8231.790.3370.1230.4231.230.0050.3020.071.2721.33101.2716.4575.047.273.70.4670.160.7561.920.0080.3890.1091.089.88100.7816.8976.787.562.630.4820.1310.79520.0050.4160.1110.738.87100.5117.2570.167.497.40.4850.1340.7842.030.0070.4310.1171.4110.66101.1119.0777.297.082.830.4570.1430.7271.850.0060.3840.1021.328.85101.0420.5977.817.012.970.4370.1290.7571.830.0070.3540.0911.48.24101.0422.1277.776.612.930.4020.1190.7491.70.0060.330.0821.058.97100.7222.7072.968.563.790.5040.1241.082.110.0070.4270.0881.6310.18101.4623.8672.3510.31.460.6940.1251.152.820.0040.5810.051.2310.08100.8425.5368.3210.854.160.6610.1411.122.60.0090.5630.1082.9210.96102.4126.6262.5214.983.270.9970.1571.673.80.0080.8680.0941.8611.33101.5529.0275.038.162.30.5170.1280.9872.150.0050.4770.0391.499.71100.9930.4769.3911.752.270.7820.1350.9183.120.0050.5980.0851.7910.7101.5430.5474.538.452.710.5130.1311.042.30.0060.4930.0391.439.49101.1331.0474.149.013.080.5660.1181.112.330.010.4780.0431.148.78100.8131.6076.967.762.820.4560.121.041.990.0080.4350.040.988.05100.6632.5472.159.753.370.6230.1121.042.530.0090.5050.0461.089.44100.6632.9874.928.642.780.5760.1210.9812.290.0060.4750.0471.398.75100.9833.6177.277.842.060.480.1151.131.940.0050.410.0461.158.34100.7934.2468.7513.063.410.7940.1521.393.240.0090.6480.0862.568.29102.3934.8673.9210.732.930.6690.141.122.760.0090.5330.0560.946.86100.6735.4972.511.363.180.7640.1321.132.940.0090.570.060.977.03100.6536.1275.239.593.190.6020.1371.082.490.0080.4870.0751.126.85100.8637.3772.4111.183.060.6830.1491.492.780.0080.5460.0690.957.3100.6338.9473.119.544.930.6390.1920.9492.540.0080.5120.0712.497.29102.2740.0770.6111.4840.7750.1580.9932.970.010.5820.0910.988.03100.6840.5073.6910.263.320.6550.1861.042.780.0040.5520.0442.247.12101.8940.6372.0211.420.7170.1361.252.870.0060.5740.04118.65100.6641.1374.6810.112.390.660.1190.9052.720.0050.5440.0451.177.48100.8342.2271.5210.473.110.6980.12612.790.0060.5860.0541.19.29100.7543.7575.339.712.520.6910.1140.6962.580.0040.520.0491.437.31100.95

在主量元素北美页岩标准化图中，厚坪五峰组—龙马溪组泥岩的Fe2O3+FeO(即全铁TFeO)、MgO、CaO、MnO2和P2O5含量明显低于北美页岩，Na2O、K2O和TiO2含量与北美页岩相近，而观音桥段2件砂、泥岩MgO、CaO、MnO2和P2O5含量接近或高于北美页岩(图2)，这与该时期沉积水体变浅和上升洋流的特殊沉积环境有关(熊国庆，2020)；蒙蒙溪五峰组—龙马溪组泥岩主量元素含量也显示相似的特征(图2)。主量元素北美页岩标准化图也显示了渝东北城口厚坪和蒙蒙溪比川南长宁—双河SiO2含量更高、MgO、CaO和MnO2含量更低的特征。蒙蒙溪Fe2O3+FeO(即全铁TFeO)含量相对较高，尤其是龙马溪组中、下部接近或高于北美页岩，这一特征也表现在川南长宁—双河龙马溪组中、下部，表明这一时期可能处于最大海泛面，盆内沉积水体极度缺氧，有利于铁富集，而厚坪未表现出类似的特征，可能是样品采自龙马溪组底部，层位较低。

图2 扬子地块北缘城口五峰组—龙马溪组砂、泥岩主量元素北美页岩标准化图Fig. 2 NASC-normalized patterns of major elements of the Wufeng Formation and the Longmaxi Formation sandstones and mudstones in Chengkou County, northern margin of Yangtz Block

3.2 微量元素

城口和蒙蒙溪五峰组—龙马溪组砂、泥岩微量元素北美页岩标准化蛛网图(图3)显示，五峰组—龙马溪组泥岩的微量元素变化趋势基本一致。五峰组—龙马溪组泥岩大离子亲石元素Sc、Sr较北美页岩相对亏损，元素Sb、Ba相对富集，而观音桥段砂、泥岩Sc、Rb、Cs相对亏损，Sr、Sb、Ba元素相对富集。五峰组—龙马溪组泥岩亲铁元素V相对北美页岩富集，Co元素相对亏损，Cr、Ni元素含量接近，而观音桥段砂、泥岩V、Cr、Co元素相对亏损，这与五峰组—龙马溪组富有机质泥岩具有较强的吸附能力有关。五峰组—龙马溪组泥岩高场强元素U相对北美页岩富集，Zr、Hf元素相对亏损，Nb、Ta元素含量相近，而厚坪观音桥段砂、泥岩高场强元素也具有相似的变化趋势(图3)。

图3 扬子北缘城口五峰组—龙马溪组砂、泥岩微量元素北美页岩标准化蛛网图Fig. 3 NASC-nornalized spider diagram of trace elements of the Wufeng Formation and the Longmaxi Formation sandstones and mudstones in Chengkou County, northern margin of Yangtz Block

3.3 稀土元素

厚坪和蒙蒙溪五峰组—龙马溪组砂、泥岩稀土元素测试结果见表3和表4，稀土元素北美页岩标准化图如图4所示。文中δCe和δEu异常分别按δCe=CeN/(La×Pr)N1/2，δEu=EuN/(Sm×Gd)N1/2公式计算而得(Taylor and McClennan, 1985)。

图4 扬子北缘城口五峰组—龙马溪组砂泥岩稀土元素北美页岩标准化图Fig. 4 NASC-normalized patterns of major elements of Wufeng Formation and Longmaxi Formation sandstones and mudstones in Chengkou County, northern margin of Yangtz block

厚坪五峰组—龙马溪组砂、泥岩稀土元素含量∑REE为66.83～247.07 μg/g，平均149.77 μg/g(表3)。其中五峰组上部∑REE为120.42～247.07 μg/g，平均175.26 μg/g，观音桥段2件样品∑REE为66.83 μg/g和208.37 μg/g变化较大，龙马溪组∑REE为100.4～155.77 μg/g，平均为124.09 μg/g，总体上五峰组∑REE略高于龙马溪组(表3)；LREE/HREE(5.86～11.08)和(La/Yb)N(6.33～11.95)表明轻稀土富集、重稀土亏损(表3)；蒙蒙溪五峰组—龙马溪组泥岩稀土元素含量∑REE为68.67～308.69 μg/g，平均151.18 μg/g(表4)。其中五峰组∑REE为68.67～226.69 μg/g，平均143.65 μg/g，向上略有增加的趋势，龙马溪组∑REE为97.17～308.69 μg/g，平均158.28 μg/g，具有下高、中低、上高的变化趋势(表4)；LREE/HREE(6.53～13.66)和(La/Yb)N(4.68～15.74)也反映了轻稀土富集、重稀土亏损(表4)。

厚坪和蒙蒙溪五峰组—龙马溪组砂、泥岩稀土元素北美页岩标准化均表现为略微向右倾斜的配分模式(图4)，多数具有δCe负异常，观音桥段和龙马溪组还显示出δEu正异常，正Eu异常通常指示了海底热水活动的存在(Klinkhammer et al., 1983；严德天等，2009)，同期地层中大量的重晶石结核充分说明了这次热液活动的存在(昝博文等，2017)，可 能与该时期与扬子北缘南秦岭陆缘裂谷发育有关，裂谷作用过程中地幔柱上涌致使岩浆活动强烈，不仅碱性玄武岩喷发，还形成了同期侵入的超基性、基性岩墙(向忠金等，2016)。

4 讨论

4.1 物源分析

细粒沉积物中主量元素最能反映沉积物物源区的物质组成特征(Rollinson, 1993)，Roser和Korsch(1988)最早提出了砂、泥岩物源区判别函数(表5)。主量元素判别函数图(F11-F21)显示(图5a)，渝东北城口五峰组—龙马溪组砂、泥岩均落在成熟大陆石英质物源区(P4)。

表5 砂、泥岩源区判别函数的变量及其系数(Roser and Korsch, 1988)Table 5 Variables and their coefficients of discriminant function for provenance of sandstone and mudstone

图5 扬子地块北缘城口五峰组—龙马溪组砂、泥岩物源分析(a)及构造背景判别(b)Fig. 5 Discrimination diagram of major elements for provenance(a) and tectonic setting(b) of Wufeng Formation and Longmaxi Formation sandstones and mudstones in Chengkou County, northern margin of Yangtz Block P1—基性火山岩物源区；P2—中性火山岩物源区；P3—酸性火山岩物源区；P4—成熟大陆石英质物源区；PM—被动陆缘；OIA—大洋岛弧；CIA—大陆岛弧；ACM—活动陆缘；(a) F11-F21判别函数图(Roser and Korsch, 1988)；(b) F2-F1判别函数图(Bhatia, 1983) P1—basic volcanic provenance；P2—intermediate volcanic provenance；P3—acid volcanic provenance；P4—matured-continental quartziferous provenance；PM—passive margin；OIA—oceanic island arc；CIA—continental island arc；ACM—active continental margin；(a) F11-F21discrimination function (Roser and Korsch, 1988)；(b) F2-F1discrimination function (Bhatia, 1983)

表6 砂岩构造环境判别函数的变量及其系数(Bhatia, 1983)Table 6 Variables and their coefficients of discriminant function for tectonic setting of sandstone(Bhatia, 1983)

由于稀土元素REE及高场强元素Th、Zr、Hf、Ta、Nb、Sc和Co等不活泼微量元素在沉积和变质作用过程中具有较强的抗迁移性，被转移到沉积物中仍能反映母岩的地球化学性质，因此，这些元素及其比值能很好指示其物源信息(Taylor and McLennan, 1985; Bhatia and Crook, 1986; Prudencio et al., 1989; Crichton and Condie, 1993)，被广泛应用于沉积源区的分析(Taylor and McLennan, 1985)。微量元素La/Th—Hf图解(图6a)中，渝东北城口五峰组—龙马溪组大多数砂、泥岩落在长英质岩与基性岩混合源区到安山岩岛弧物源区。在Co/Th—La/Sc图解(图6b)上，大部分泥岩具有低而相对稳定的Co/Th值，介于0.1～1.0之间，而La/Sc的值较高，多数大于4，样品分布区域在长英质火山岩和花岗岩之间，反映源岩以长英质岩为主，并伴有花岗质和安山质岩石的混入。

图6 扬子北缘城口五峰组—龙马溪组砂、泥岩物源分析: (a) La/Th—Hf 图解(Floyd and Leveridge, 1987)；(b) Co/Th—La/Sc 图解 (Gu et al., 2002)Fig. 6 Discrimination diagram of trace elements for provenance of Wufeng Formation and Longmaxi Formation sandstones and mudstones in Chengkou County, northern margin of Yangtz block: (a) La/Th—Hf diagram (Floyd and Leveridge, 1987)；(b) Co/Th—La/Sc diagram (Gu et al., 2002)

4.2 构造背景

Bhatia(1983)通过砂岩主量元素分析，最早提出了其形成的大地构造环境判别函数(表5)及一些碎屑岩的构造环境判别图解，Roser和Korsch(1986)认为与砂岩相伴生的粉砂岩和泥、页岩等细碎屑岩地球化学特征相似，具有相同的物源和沉积构造背景。因此，细粒沉积物中的微量元素和稀土元素也被广泛应用于沉积源区的确定、构造背景及大陆生长的分析(Bhatia and Taylor, 1981; Taylor and McLennan, 1985; Bhatia, 1985; Bhatia and Crook, 1986)。

在碎屑岩主量元素函数构造环境函数判别图(F2-F1)(图5b)中，渝东北城口五峰组—龙马溪组砂、泥岩均落在被动大陆区域内；K2O/Na2O—SiO2图解上，几乎所有样品都落在被动陆缘区域，极少岛弧和活动陆缘区(图7a)；在TiO2—TFeO/MgO和Al2O3/SiO2—TFeO/MgO图解(TFeO代表全铁含量，即Fe2O3+FeO)(图7b、7c)中，渝东北城口五峰组—龙马溪组砂、泥岩绝大多数落在被动陆缘区内，少数在活动陆缘区。微量元素La/Y—Sc/Cr构造环境判别图解(图7d)中，渝东北城口五峰组—龙马溪组砂、泥岩大部分落在被动陆缘区内，少数在大陆岛弧区；La—Th—Sc图解(图7e)中，研究区多数样品落在被动陆缘和活动陆缘区域内，少数在大陆岛弧区，而在Th—Sc—Zr/10图解(图7f)上，样品多数落在活动陆缘区，部分位于大陆岛弧区。上述结果表明，渝东北城口五峰组—龙马溪组砂、泥岩物源区源岩形成时的构造背景主要为被动陆缘环境，少数为活动陆缘和大陆岛弧环境。

图7 扬子北缘城口五峰组—龙马溪组砂、泥岩沉积构造背景判别图Fig. 7 Discrimination diagram of major and trace elements for tectonic setting of Wufeng Formation and Longmaxi Formation sandstones and mudstones in Chengkou County, northern margin of Yangtz block(a) K2O/Na2O—TiO2图解(Roser and Korsch, 1986)；(b) TiO2—TFeO/MgO图解和(c) Al2O3/SiO2—Hf图解(Bhatia, 1983)；(d) La/Y—Sc/Cr图解；(e) La—Th—Sc图解和(f) Th—Sc—Zr/10图解(Bhatia, 1983; Bhatia and Crook, 1986); A—大洋岛弧；B—大陆岛弧；C—活动陆缘；D—被动陆缘(a) K2O/Na2O—TiO2diagram (Roser and Korsch, 1986)；(b) TiO2—TFeO/MgO diagram；(c) Al2O3/SiO2—Hf diagram (Bhatia, 1983)；(d) La/Y—Sc/Cr diagram；(e) La—Th—Sc diagram；(f) Th—Sc—Zr/10 diagram (Bhatia, 1983; Bhatia and Crook, 1986); A—oceanic island arc；B—continental island arc；C—active continental margin；D—passive continental margin

研究认为扬子板块北缘在晚元古—早古生代期间是一个被古东秦岭洋(商丹洋)分隔的大西洋型被动大陆边缘(高长林等，1990；吉让寿等，1990)，从早奥陶世中期(～470 Ma)开始向北单边俯冲，俯冲作用使得华北地块南缘由被动大陆边缘转化为活动大陆边缘，扬子地块北缘被动大陆边缘发生前陆隆起(熊国庆等，2017)，该隆起处于城口以北、紫阳芭蕉口以南，东至镇坪一带，被称为巴山隆起或巴山古陆(叶俭等，1992)。随着巴山古陆受构造抬升，致使芭蕉口以南地区坡度变陡，伴随强烈的火山活动，导致浊流发生，为北侧芭蕉口地区的浊流沉积提供了充足的物源(赵健，1987)。米仓山地区汉南古陆周缘地区在早古生代(中寒武世至早奥陶世大巴山西段)发生了南郑上升，导致大巴山西段整体上升成陆并与汉南古陆连成一体，甚至可能与龙门山古陆相连(陈旭等，1990)。因此，奥陶纪—志留纪之交，扬子地块北缘的汉南古陆和巴山隆起为盆内源源不断地提供了物质来源，少量细粒沉积物也可能来自华北南缘的活动陆缘和大陆岛弧物质，此外，南秦岭南缘紫阳—岚皋—竹山一带局部拉伸所产生的火山岩浆活动也为盆内供给了一定的物源。

5 结论

基于对扬子地块北缘奥陶纪—志留纪之交砂、泥岩的沉积地球化学特征分析，得出了以下结论：

(1)渝东北城口五峰组—龙马溪组砂、泥岩主量元素Al2O3、MgO、CaO、MnO2和P2O5含量明显低于北美页岩，SiO2、Na2O、K2O和TiO2含量与北美页岩相近，相比于川南长宁—双河五峰组—龙马溪组泥岩，渝东北城口具有高SiO2、低MgO和CaO的特点，表明五峰组—龙马溪组泥岩碎屑组分主要以粉砂质和硅质等陆缘碎屑为主。

(2)微量元素Sc、Sr、Co、Zr、Hf较北美页岩相对亏损，V、U、Sb、Ba相对富集，Cr、Ni、Nb、Ta含量相近。LREE/HREE和(La/Yb)N均反映了轻稀土富集、重稀土亏损。

(3)稀土元素北美页岩标准化均表现为略微向右倾斜的配分模式，多数具有δCe负异常，观音桥段和龙马溪组显示为δEu正异常，指示了该时期存在海底热液活动。

(4)主、微量元素的物源和构造背景图解表明，沉积物源区源岩具有多样的物质组分，以长英质岩为主，同时混入了花岗质和安山质岩石组分；源岩形成时的构造背景主要为被动陆缘环境，部分为活动陆缘和大陆岛弧环境，这与扬子地块北缘新元古代—早古生代处于被动大陆边缘的区域地质背景一致。

致谢：野外期间得到江新胜研究员和闫剑飞博士的大力支持和帮助，审稿专家对本文提出了诸多有益的建议和意见，在此深表谢意。