揚子北緣奧陶紀—志留紀之交泥巖地球化學(xué)特征及其源區(qū)、構(gòu)造背景

劉春來, 熊國慶, 董國明, 崔偉

1) 河北省地質(zhì)礦產(chǎn)勘查開發(fā)局第二地質(zhì)大隊, 河北唐山, 063004; 2) 中國地質(zhì)調(diào)查局成都地質(zhì)調(diào)查中心，成都，610081; 3) 自然資源部沉積盆地與油氣資源重點實驗室，成都，610081

內(nèi)容提要: 近年來，揚子地塊北緣奧陶紀—志留紀之交沉積構(gòu)造背景一直存在不同認識。本文對渝東北城口厚坪五峰組—龍馬溪組砂、泥巖進行系統(tǒng)采樣和主、微量元素測試分析，從地球化學(xué)特征角度，來探討該時期砂、泥巖物源源區(qū)及其構(gòu)造環(huán)境。地球化學(xué)特征顯示，五峰組—龍馬溪組砂、泥巖主量元素SiO2含量為39.57%～87.16%，多數(shù)高于北美頁巖，Al2O3含量為3.41%～14.98%，均低于北美頁巖，MgO、CaO、MnO2和P2O5含量明顯低于北美頁巖，相比于四川盆地川南長寧—雙河五峰組—龍馬溪組泥巖，渝東北城口具有高SiO2和低MgO、CaO的特點，表明城口五峰組—龍馬溪組泥巖碎屑組分主要以粉砂質(zhì)和硅質(zhì)等陸緣碎屑為主；微量元素Sc、Sr、Co、Zr、Hf較北美頁巖相對虧損，V、U、Sb、Ba相對富集，Cr、Ni、Nb、Ta含量相近。稀土元素北美頁巖標準化均表現(xiàn)為略微向右傾斜的配分模式，多數(shù)具有δCe負異常，觀音橋段和龍馬溪組δEu正異常指示了該時期存在海底熱液活動。砂、泥巖主、微量元素的物源和構(gòu)造背景圖解表明沉積物源區(qū)源巖具有多樣的物質(zhì)組分，以長英質(zhì)巖為主，同時混入了花崗質(zhì)和安山質(zhì)巖石組分；源巖形成時的構(gòu)造背景主要為被動陸緣環(huán)境，少數(shù)為活動陸緣和大陸島弧環(huán)境，這一結(jié)果與揚子地塊北緣新元古代—早古生代處于被動大陸邊緣的區(qū)域地質(zhì)背景一致。

碎屑巖是由母巖風(fēng)化產(chǎn)物或火山碎屑經(jīng)搬運、沉積、壓實和膠結(jié)而形成的巖石，一般認為這些碎屑巖組分主要受物源區(qū)母巖性質(zhì)和構(gòu)造背景影響(McLennan et al., 1993; Fralick and Kronberg, 1997)。因此，碎屑組分常作為一種恢復(fù)沉積盆地的物源和構(gòu)造背景的傳統(tǒng)方法(Dickinson, 1983; Morton, 1991)。研究表明，在風(fēng)化、搬運和成巖過程中，碎屑巖中一些相對不易遷移的地球化學(xué)組分(如稀土元素(REE)和Zr、Th、Sc、Y等一些微量元素)，不僅能提供了物源區(qū)成分的信息(Taylor and McLennan, 1985; Girty et al., 1993)，Bhatia(1983)和Roser等(1986)還據(jù)此提出了不同構(gòu)造背景下砂、泥巖的地球化學(xué)判別方法，隨后被廣泛用于碎屑巖的物源及其沉積構(gòu)造背景分析(Bhatia and Crook, 1986; McLennan et al., 1993)。

渝東北地區(qū)位于揚子陸塊北緣，奧陶紀—志留紀之交，沉積了一套深水陸棚相五峰組—龍馬溪組黑色頁巖。這一時期，揚子陸塊北緣究竟是處于秦嶺洋南側(cè)的揚子被動大陸邊緣(吉讓壽等，1990；張國偉等，1988，1995，1996，2001)，還是南秦嶺溝—弧—盆體系的活動大陸邊緣(王宗起等，2009)，或是克拉通盆地南側(cè)的揚子陸緣(姜涌泉，1998)，仍存有爭議。前人通過常量元素的地球化學(xué)方法對渝東北地區(qū)五峰組—龍馬溪組泥巖所形成的構(gòu)造環(huán)境進行了初步討論(熊小輝等，2015；劉田等，2019)，但缺乏微量元素，尤其是不活潑的元素的深入系統(tǒng)地分析，其結(jié)論有待進一步驗證。本文從渝東北城口厚坪五峰組—龍馬溪組泥巖的地球化學(xué)特征入手，詳細探討揚子陸塊北緣奧陶紀—志留紀之交沉積物的物源及其沉積構(gòu)造背景，對該地區(qū)這一時期沉積盆地所處的大地構(gòu)造背景及其沉積古地理源區(qū)分析具有重要的地質(zhì)意義。

1 區(qū)域地質(zhì)背景

大巴山地區(qū)處于盆山過渡帶，北與東秦嶺造山帶、南與四川盆地相接，東西夾持于武當山隆起、神農(nóng)架隆起和米倉山隆起之間，呈向南凸出的弧形構(gòu)造帶，城巴斷裂將其分為南、北大巴山兩個沉積—構(gòu)造單元(圖1a，李智武等，2006)。研究區(qū)位于大巴山西段的川渝陜交界，區(qū)內(nèi)自下而上發(fā)育震旦系、寒武系、奧陶系、志留系、二疊系及三疊系，缺失泥盆系和石炭系，北大巴山早古生代地層中輝綠巖脈順層發(fā)育(圖1b)，時代上為奧陶紀—志留紀(向忠金等，2016)。

晚元古代的晉寧運動，華北陸塊與揚子陸塊逐漸拼合對接，最終導(dǎo)致南秦嶺在內(nèi)各個塊體形成統(tǒng)一的克拉通褶皺基底(羅志立，1998)。隨著全球Rodinia超大陸解體，華北陸塊和揚子陸塊發(fā)生裂解，逐漸演化形成“秦嶺洋”。有觀點認為，秦嶺洋發(fā)生在商南—丹鳳一線，也稱“商丹洋”，兩側(cè)為被動大陸邊緣，奧陶—志留紀開始，揚子陸塊向華北陸塊之下的北向俯沖，南秦嶺南緣此時發(fā)生局部拉伸，基性巖漿活動就位(胡健民等，2002)，洋盆于泥盆—石炭紀消亡，與此同時，沿勉縣—略陽一帶打開一個新的勉略洋盆，最終形成了秦嶺造山帶“三塊夾兩縫”的構(gòu)造格局(張國偉等，1995a，b，1996，2001，2003，2004)；也有觀點認為，揚子陸塊和華北陸塊之間的秦嶺造山帶只存在一個統(tǒng)一的原始大洋盆地——“秦嶺洋”，時間為新元古—早中生代(約840～220 Ma)(閆全人等，2007，2009，2014)，晚古生代秦嶺洋向南俯沖增生，南秦嶺南緣構(gòu)成了安康弧、北大巴山弧后混雜巖和寒武紀—志留紀弧后盆地構(gòu)造體系，南大巴山為弧后盆地陸緣，屬于揚子臺地穩(wěn)定沉積(王宗起等，2009；王剛等，2014)。隨著早中生代洋盆消亡，印支—燕山期，秦嶺開始轉(zhuǎn)入陸內(nèi)碰撞造山階段，喜馬拉雅期定型，最終形成了現(xiàn)今的大巴山弧形構(gòu)造帶。

2 樣品采集及測試

2.1 樣品采集

本文研究的樣品采自南大巴山西段渝東北城口縣厚坪(HP)剖面和蒙蒙溪剖面(MM)，兩者處于同一向斜的南北兩翼，直線距離約10 km(圖1c)。厚坪剖面位于城溪路S301省道至厚坪的X026縣道邊(圖1b)，晚奧陶世臨湘組—早志留世龍馬溪組地層連續(xù)。臨湘組與下伏寶塔組灰色中層狀龜裂紋灰?guī)r整合接觸，主體上為一套淺水陸棚黃綠色泥巖夾灰色泥灰?guī)r，頂部為灰色薄層狀泥質(zhì)白云巖、泥質(zhì)砂巖，表現(xiàn)為向上變淺的沉積層序，頂界面土黃色泥質(zhì)風(fēng)化殼表明曾短暫暴露(熊國慶等，2019)，其上五峰組深水陸棚黑色中層狀硅質(zhì)頁巖、硅質(zhì)碳質(zhì)泥巖，為一套快速海侵沉積，上覆觀音橋段時代上與全球赫蘭特冰期相當，底部以黑色薄板狀泥質(zhì)粉砂巖，向上變?yōu)榛液谏袑訝罴毶皫r、中—粗砂巖及灰色厚層狀含砂礫白云巖，與五峰組碳質(zhì)泥巖漸變過渡，反映了一次逐漸海退的沉積序列(熊國慶等，2019)；隨著赫蘭特冰川消融，全球海平面迅速上升，龍馬溪組一套深水陸棚黑色薄層狀粉砂質(zhì)碳質(zhì)泥巖快速沉積于白云巖之上(圖1c)，未見頂。蒙蒙溪剖面位于明中鄉(xiāng)至彭家灣的鄉(xiāng)道邊，交通方便(圖1b)，露頭出露較為完整，晚奧陶世臨湘組—早志留世龍馬溪組地層連續(xù)。臨湘組淺黃綠色泥巖與五峰組黑色硅質(zhì)碳質(zhì)泥巖直接接觸，表現(xiàn)為一次快速海侵過程，隨后發(fā)育的觀音橋段白云巖呈透鏡狀，區(qū)域上可對比，為一套赫蘭特期冰期沉積，其上龍馬溪組黑色粉砂質(zhì)碳質(zhì)泥巖為一套冰期后全球海平面再次快速上升形成的深水陸棚相沉積(圖1c)。

2.2 樣品處理及測試

采樣時已盡量剝除風(fēng)化表面，保證樣品新鮮和足量。全巖樣品首先粉碎至200目以下，常量、微量和稀土元素測試分析在核工業(yè)北京地質(zhì)研究院分析測試研究中心完成，測試時，室內(nèi)溫度20℃，相對濕度30%。常量元素分析儀器為AB-104L，PW2404 X射線熒光光譜儀，測試方法和依據(jù)為GB/T 14506.14-2010《硅酸鹽巖石化學(xué)分析方法 第14部分：氧化亞鐵量測定》，GB/T 14506.28-2010《硅酸鹽巖石化學(xué)分析方法 第28部分：16個主次成分量測定》，相對誤差為±1%～±4%。微量、稀土元素測試采用ELEMENT XR等離子質(zhì)譜分析儀，測試方法和依據(jù)為GB/T 14506.30-2010《硅酸鹽巖石化學(xué)分析方法 第30部分：44個元素量測定》，相對誤差為±3%～±12%。常量元素及微量、稀土元素測試分析結(jié)果分別列入表1、表2、表3和表4中。

表3 揚子地塊北緣城口厚坪剖面五峰組—龍馬溪組砂、泥巖微量元素分析及計算結(jié)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)

樣品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樣品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

樣品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樣品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

樣品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 揚子地塊北緣城口蒙蒙溪剖面五峰組—龍馬溪組泥巖微量元素分析及計算結(jié)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 結(jié)果

3.1 主量元素

厚坪五峰組—龍馬溪組砂、泥巖SiO2含量為53.4%～79.43%，平均72.98%(表1)，高于北美頁巖64.8%，也高于同一盆地內(nèi)川南長寧—雙河五峰組—龍馬溪組SiO2含量55.67%的平均值。五峰組頂部2件泥巖SiO2含量為53.4%和56.21%，較其他樣品明顯偏低，這與樣品的高燒失量有關(guān)；觀音橋段2件砂、泥巖SiO2含量為69.03%和69.71%，龍馬溪組1件樣品70.91%，低于其他樣品，高于北美頁巖；其余10件樣品SiO2含量平均77.54%，五峰組與龍馬溪組基本相同，遠高于北美頁巖(表1)。蒙蒙溪五峰組—龍馬溪組泥巖SiO2含量為39.57%～87.16%，平均73.31%(表2)，均高于北美頁巖和川南長寧—雙河。龍馬溪組一個異常低的SiO2值與其高鐵含量和高燒失量有關(guān)，其余絕大多數(shù)均大于70%(表2)。厚坪五峰組—龍馬溪組砂、泥巖Al2O3含量為3.41%～11.47%，平均7.73%，均低于北美頁巖16.9%，也低于川南長寧—雙河五峰組—龍馬溪組Al2O3含量10.13%的平均值。五峰組頂部2件泥巖Al2O3含量為11.09%和11.49%，明顯高于其他樣品，與川南長寧—雙河接近，觀音橋段2件樣品Al2O3含量最低，僅為3.41%～5.62%，其余11件樣品Al2O3含量平均7.67%，五峰組與龍馬溪組基本接近(表1)。蒙蒙溪泥巖Al2O3含量為4.26%～14.98%，平均8.98%，均低于北美頁巖16.9%，也低于川南長寧—雙河，但略高于厚坪(表2)。五峰組Al2O3含量為4.26%～11.48%，平均7.95%，龍馬溪組Al2O3含量為4.82%～14.98%，平均9.42%，龍馬溪組高于五峰組。厚坪五峰組和龍馬溪組MgO和CaO含量分別介于0.393%～1.01%，平均0.642%，0.064%～0.803%，平均0.227%，觀音橋段含量遠高于五峰組、龍馬溪組，分別為2.31%～4.31%和6.99%～7.39%；蒙蒙溪兩者含量分別為0.285%～0.997%，平均0.59%，0.112%～0.644%，平均0.156%，與厚坪含量相當，但均遠低于川南長寧—雙河MgO和CaO含量3.0%和9.51%的平均值(李艷芳等，2015)。川南長寧—雙河富有機質(zhì)層段SiO2含量高，Al2O3含量低，貧有機質(zhì)層段SiO2含量低，Al2O3含量高，這種富有機質(zhì)段高SiO2含量與生物活動強烈導(dǎo)致的生物硅增加有關(guān)，貧有機質(zhì)段Al2O3含量增加與陸源碎屑物質(zhì)輸入有關(guān)(李艷芳等，2015)。相比川南長寧—雙河，渝東北城口厚坪具有高SiO2和低MgO、CaO的特點，表明城口五峰組—龍馬溪組泥巖碎屑組分主要以粉砂質(zhì)和硅質(zhì)等陸緣碎屑為主，長寧—雙河地區(qū)還含有大量的鈣質(zhì)組分。

表1 揚子地塊北緣城口厚坪剖面五峰組—龍馬溪組砂、泥巖主量元素分析結(jié)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 揚子地塊北緣城口蒙蒙溪剖面五峰組—龍馬溪組泥巖主量元素分析結(jié)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(%)

樣品地層及巖性深度(m)SiO2Al2O3Fe2O3MgOCaONa2OK2OMnO2TiO2P2O5FeO燒失量總含量MM18MM19MM20MM21MM22MM23MM24MM25MM26MM27MM28MM29MM30MM31MM32MM33MM34MM35MM36MM37MM38MM39MM40MM41MM42MM43MM44MM45MM46MM47MM48MM49MM50MM51MM52MM53MM54MM55MM56MM57龍馬溪組泥巖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

在主量元素北美頁巖標準化圖中，厚坪五峰組—龍馬溪組泥巖的Fe2O3+FeO(即全鐵TFeO)、MgO、CaO、MnO2和P2O5含量明顯低于北美頁巖，Na2O、K2O和TiO2含量與北美頁巖相近，而觀音橋段2件砂、泥巖MgO、CaO、MnO2和P2O5含量接近或高于北美頁巖(圖2)，這與該時期沉積水體變淺和上升洋流的特殊沉積環(huán)境有關(guān)(熊國慶，2020)；蒙蒙溪五峰組—龍馬溪組泥巖主量元素含量也顯示相似的特征(圖2)。主量元素北美頁巖標準化圖也顯示了渝東北城口厚坪和蒙蒙溪比川南長寧—雙河SiO2含量更高、MgO、CaO和MnO2含量更低的特征。蒙蒙溪Fe2O3+FeO(即全鐵TFeO)含量相對較高，尤其是龍馬溪組中、下部接近或高于北美頁巖，這一特征也表現(xiàn)在川南長寧—雙河龍馬溪組中、下部，表明這一時期可能處于最大海泛面，盆內(nèi)沉積水體極度缺氧，有利于鐵富集，而厚坪未表現(xiàn)出類似的特征，可能是樣品采自龍馬溪組底部，層位較低。

圖2 揚子地塊北緣城口五峰組—龍馬溪組砂、泥巖主量元素北美頁巖標準化圖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

城口和蒙蒙溪五峰組—龍馬溪組砂、泥巖微量元素北美頁巖標準化蛛網(wǎng)圖(圖3)顯示，五峰組—龍馬溪組泥巖的微量元素變化趨勢基本一致。五峰組—龍馬溪組泥巖大離子親石元素Sc、Sr較北美頁巖相對虧損，元素Sb、Ba相對富集，而觀音橋段砂、泥巖Sc、Rb、Cs相對虧損，Sr、Sb、Ba元素相對富集。五峰組—龍馬溪組泥巖親鐵元素V相對北美頁巖富集，Co元素相對虧損，Cr、Ni元素含量接近，而觀音橋段砂、泥巖V、Cr、Co元素相對虧損，這與五峰組—龍馬溪組富有機質(zhì)泥巖具有較強的吸附能力有關(guān)。五峰組—龍馬溪組泥巖高場強元素U相對北美頁巖富集，Zr、Hf元素相對虧損，Nb、Ta元素含量相近，而厚坪觀音橋段砂、泥巖高場強元素也具有相似的變化趨勢(圖3)。

圖3 揚子北緣城口五峰組—龍馬溪組砂、泥巖微量元素北美頁巖標準化蛛網(wǎng)圖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

厚坪和蒙蒙溪五峰組—龍馬溪組砂、泥巖稀土元素測試結(jié)果見表3和表4，稀土元素北美頁巖標準化圖如圖4所示。文中δCe和δEu異常分別按δCe=CeN/(La×Pr)N1/2，δEu=EuN/(Sm×Gd)N1/2公式計算而得(Taylor and McClennan, 1985)。

圖4 揚子北緣城口五峰組—龍馬溪組砂泥巖稀土元素北美頁巖標準化圖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

厚坪五峰組—龍馬溪組砂、泥巖稀土元素含量∑REE為66.83～247.07 μg/g，平均149.77 μg/g(表3)。其中五峰組上部∑REE為120.42～247.07 μg/g，平均175.26 μg/g，觀音橋段2件樣品∑REE為66.83 μg/g和208.37 μg/g變化較大，龍馬溪組∑REE為100.4～155.77 μg/g，平均為124.09 μg/g，總體上五峰組∑REE略高于龍馬溪組(表3)；LREE/HREE(5.86～11.08)和(La/Yb)N(6.33～11.95)表明輕稀土富集、重稀土虧損(表3)；蒙蒙溪五峰組—龍馬溪組泥巖稀土元素含量∑REE為68.67～308.69 μg/g，平均151.18 μg/g(表4)。其中五峰組∑REE為68.67～226.69 μg/g，平均143.65 μg/g，向上略有增加的趨勢，龍馬溪組∑REE為97.17～308.69 μg/g，平均158.28 μg/g，具有下高、中低、上高的變化趨勢(表4)；LREE/HREE(6.53～13.66)和(La/Yb)N(4.68～15.74)也反映了輕稀土富集、重稀土虧損(表4)。

厚坪和蒙蒙溪五峰組—龍馬溪組砂、泥巖稀土元素北美頁巖標準化均表現(xiàn)為略微向右傾斜的配分模式(圖4)，多數(shù)具有δCe負異常，觀音橋段和龍馬溪組還顯示出δEu正異常，正Eu異常通常指示了海底熱水活動的存在(Klinkhammer et al., 1983；嚴德天等，2009)，同期地層中大量的重晶石結(jié)核充分說明了這次熱液活動的存在(昝博文等，2017)，可 能與該時期與揚子北緣南秦嶺陸緣裂谷發(fā)育有關(guān)，裂谷作用過程中地幔柱上涌致使巖漿活動強烈，不僅堿性玄武巖噴發(fā)，還形成了同期侵入的超基性、基性巖墻(向忠金等，2016)。

4 討論

4.1 物源分析

細粒沉積物中主量元素最能反映沉積物物源區(qū)的物質(zhì)組成特征(Rollinson, 1993)，Roser和Korsch(1988)最早提出了砂、泥巖物源區(qū)判別函數(shù)(表5)。主量元素判別函數(shù)圖(F11-F21)顯示(圖5a)，渝東北城口五峰組—龍馬溪組砂、泥巖均落在成熟大陸石英質(zhì)物源區(qū)(P4)。

表5 砂、泥巖源區(qū)判別函數(shù)的變量及其系數(shù)(Roser and Korsch, 1988)Table 5 Variables and their coefficients of discriminant function for provenance of sandstone and mudstone

圖5 揚子地塊北緣城口五峰組—龍馬溪組砂、泥巖物源分析(a)及構(gòu)造背景判別(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—基性火山巖物源區(qū)；P2—中性火山巖物源區(qū)；P3—酸性火山巖物源區(qū)；P4—成熟大陸石英質(zhì)物源區(qū)；PM—被動陸緣；OIA—大洋島?。籆IA—大陸島??；ACM—活動陸緣；(a) F11-F21判別函數(shù)圖(Roser and Korsch, 1988)；(b) F2-F1判別函數(shù)圖(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 砂巖構(gòu)造環(huán)境判別函數(shù)的變量及其系數(shù)(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及高場強元素Th、Zr、Hf、Ta、Nb、Sc和Co等不活潑微量元素在沉積和變質(zhì)作用過程中具有較強的抗遷移性，被轉(zhuǎn)移到沉積物中仍能反映母巖的地球化學(xué)性質(zhì)，因此，這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)，被廣泛應(yīng)用于沉積源區(qū)的分析(Taylor and McLennan, 1985)。微量元素La/Th—Hf圖解(圖6a)中，渝東北城口五峰組—龍馬溪組大多數(shù)砂、泥巖落在長英質(zhì)巖與基性巖混合源區(qū)到安山巖島弧物源區(qū)。在Co/Th—La/Sc圖解(圖6b)上，大部分泥巖具有低而相對穩(wěn)定的Co/Th值，介于0.1～1.0之間，而La/Sc的值較高，多數(shù)大于4，樣品分布區(qū)域在長英質(zhì)火山巖和花崗巖之間，反映源巖以長英質(zhì)巖為主，并伴有花崗質(zhì)和安山質(zhì)巖石的混入。

圖6 揚子北緣城口五峰組—龍馬溪組砂、泥巖物源分析: (a) La/Th—Hf 圖解(Floyd and Leveridge, 1987)；(b) Co/Th—La/Sc 圖解 (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 構(gòu)造背景

Bhatia(1983)通過砂巖主量元素分析，最早提出了其形成的大地構(gòu)造環(huán)境判別函數(shù)(表5)及一些碎屑巖的構(gòu)造環(huán)境判別圖解，Roser和Korsch(1986)認為與砂巖相伴生的粉砂巖和泥、頁巖等細碎屑巖地球化學(xué)特征相似，具有相同的物源和沉積構(gòu)造背景。因此，細粒沉積物中的微量元素和稀土元素也被廣泛應(yīng)用于沉積源區(qū)的確定、構(gòu)造背景及大陸生長的分析(Bhatia and Taylor, 1981; Taylor and McLennan, 1985; Bhatia, 1985; Bhatia and Crook, 1986)。

在碎屑巖主量元素函數(shù)構(gòu)造環(huán)境函數(shù)判別圖(F2-F1)(圖5b)中，渝東北城口五峰組—龍馬溪組砂、泥巖均落在被動大陸區(qū)域內(nèi)；K2O/Na2O—SiO2圖解上，幾乎所有樣品都落在被動陸緣區(qū)域，極少島弧和活動陸緣區(qū)(圖7a)；在TiO2—TFeO/MgO和Al2O3/SiO2—TFeO/MgO圖解(TFeO代表全鐵含量，即Fe2O3+FeO)(圖7b、7c)中，渝東北城口五峰組—龍馬溪組砂、泥巖絕大多數(shù)落在被動陸緣區(qū)內(nèi)，少數(shù)在活動陸緣區(qū)。微量元素La/Y—Sc/Cr構(gòu)造環(huán)境判別圖解(圖7d)中，渝東北城口五峰組—龍馬溪組砂、泥巖大部分落在被動陸緣區(qū)內(nèi)，少數(shù)在大陸島弧區(qū)；La—Th—Sc圖解(圖7e)中，研究區(qū)多數(shù)樣品落在被動陸緣和活動陸緣區(qū)域內(nèi)，少數(shù)在大陸島弧區(qū)，而在Th—Sc—Zr/10圖解(圖7f)上，樣品多數(shù)落在活動陸緣區(qū)，部分位于大陸島弧區(qū)。上述結(jié)果表明，渝東北城口五峰組—龍馬溪組砂、泥巖物源區(qū)源巖形成時的構(gòu)造背景主要為被動陸緣環(huán)境，少數(shù)為活動陸緣和大陸島弧環(huán)境。

圖7 揚子北緣城口五峰組—龍馬溪組砂、泥巖沉積構(gòu)造背景判別圖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圖解(Roser and Korsch, 1986)；(b) TiO2—TFeO/MgO圖解和(c) Al2O3/SiO2—Hf圖解(Bhatia, 1983)；(d) La/Y—Sc/Cr圖解；(e) La—Th—Sc圖解和(f) Th—Sc—Zr/10圖解(Bhatia, 1983; Bhatia and Crook, 1986); A—大洋島弧；B—大陸島??；C—活動陸緣；D—被動陸緣(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

研究認為揚子板塊北緣在晚元古—早古生代期間是一個被古東秦嶺洋(商丹洋)分隔的大西洋型被動大陸邊緣(高長林等，1990；吉讓壽等，1990)，從早奧陶世中期(～470 Ma)開始向北單邊俯沖，俯沖作用使得華北地塊南緣由被動大陸邊緣轉(zhuǎn)化為活動大陸邊緣，揚子地塊北緣被動大陸邊緣發(fā)生前陸隆起(熊國慶等，2017)，該隆起處于城口以北、紫陽芭蕉口以南，東至鎮(zhèn)坪一帶，被稱為巴山隆起或巴山古陸(葉儉等，1992)。隨著巴山古陸受構(gòu)造抬升，致使芭蕉口以南地區(qū)坡度變陡，伴隨強烈的火山活動，導(dǎo)致濁流發(fā)生，為北側(cè)芭蕉口地區(qū)的濁流沉積提供了充足的物源(趙健，1987)。米倉山地區(qū)漢南古陸周緣地區(qū)在早古生代(中寒武世至早奧陶世大巴山西段)發(fā)生了南鄭上升，導(dǎo)致大巴山西段整體上升成陸并與漢南古陸連成一體，甚至可能與龍門山古陸相連(陳旭等，1990)。因此，奧陶紀—志留紀之交，揚子地塊北緣的漢南古陸和巴山隆起為盆內(nèi)源源不斷地提供了物質(zhì)來源，少量細粒沉積物也可能來自華北南緣的活動陸緣和大陸島弧物質(zhì)，此外，南秦嶺南緣紫陽—嵐皋—竹山一帶局部拉伸所產(chǎn)生的火山巖漿活動也為盆內(nèi)供給了一定的物源。

5 結(jié)論

基于對揚子地塊北緣奧陶紀—志留紀之交砂、泥巖的沉積地球化學(xué)特征分析，得出了以下結(jié)論：

(1)渝東北城口五峰組—龍馬溪組砂、泥巖主量元素Al2O3、MgO、CaO、MnO2和P2O5含量明顯低于北美頁巖，SiO2、Na2O、K2O和TiO2含量與北美頁巖相近，相比于川南長寧—雙河五峰組—龍馬溪組泥巖，渝東北城口具有高SiO2、低MgO和CaO的特點，表明五峰組—龍馬溪組泥巖碎屑組分主要以粉砂質(zhì)和硅質(zhì)等陸緣碎屑為主。

(2)微量元素Sc、Sr、Co、Zr、Hf較北美頁巖相對虧損，V、U、Sb、Ba相對富集，Cr、Ni、Nb、Ta含量相近。LREE/HREE和(La/Yb)N均反映了輕稀土富集、重稀土虧損。

(3)稀土元素北美頁巖標準化均表現(xiàn)為略微向右傾斜的配分模式，多數(shù)具有δCe負異常，觀音橋段和龍馬溪組顯示為δEu正異常，指示了該時期存在海底熱液活動。

(4)主、微量元素的物源和構(gòu)造背景圖解表明，沉積物源區(qū)源巖具有多樣的物質(zhì)組分，以長英質(zhì)巖為主，同時混入了花崗質(zhì)和安山質(zhì)巖石組分；源巖形成時的構(gòu)造背景主要為被動陸緣環(huán)境，部分為活動陸緣和大陸島弧環(huán)境，這與揚子地塊北緣新元古代—早古生代處于被動大陸邊緣的區(qū)域地質(zhì)背景一致。

致謝：野外期間得到江新勝研究員和閆劍飛博士的大力支持和幫助，審稿專家對本文提出了諸多有益的建議和意見，在此深表謝意。