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    柴北缘从大洋俯冲到陆陆碰撞:来自开屏沟造山带M型橄榄岩的证据

    蔡鹏捷 许荣科 郑有业 陈鑫 刘嘉 俞军真

    引用本文:
    Citation:

    柴北缘从大洋俯冲到陆陆碰撞:来自开屏沟造山带M型橄榄岩的证据

      作者简介: 蔡鹏捷(1988-), 男, 博士研究生, 主要从事造山带橄榄岩地球化学研究.
      通讯作者: 许荣科, E⁃mail: xurongke1968@126.com
    • 基金项目:

      教育部长江学者和创新团队发展计划 IRT14R54

      中国地质调查局项目 12120113032800

    • 中图分类号: P597

    From Oceanic Subduction to Continental Collision in North Qaidam: Evidence from Kaipinggou Orogenic M-Type Peridotite

      Corresponding author: Xu Rongke, E⁃mail: xurongke1968@126.com
    • 摘要: 造山带幔源(M型)橄榄岩虽然在高压/超高压变质带分布不多,但由于其来自俯冲板块上覆的岩石圈地幔,因此是研究俯冲隧道内俯冲板片与地幔楔之间相互作用的重要对象,对于还原超高压变质带的演化有重要意义.柴北缘鱼卡榴辉岩-片麻岩区边部附近的开屏沟存在一套橄榄岩,其岩石类型、成因、时代等都缺乏研究.对开屏沟橄榄岩全岩的主量和微量元素及铂族元素、橄榄石主量元素、锆石U-Pb年龄和Hf同位素进行了研究.结果显示,其全岩具有高的Mg#、Mg/Si和Ni值,同时表现出亏损难溶的HFSE和HREE,轻微富集LILE和LREE中与流体活动性相关的元素;橄榄石具有较高的Fo值(90.11~92.77)与NiO含量(0.32%~0.45%)、低的CaO含量(< 0.02%);PGEs的球粒陨石标准化配分模式与交代橄榄岩和残留橄榄岩近似;两组变质锆石年龄为459.5±3.6 Ma和417.5±2.7 Ma,对应的εHft)值为-0.71~9.45和-11.96~-1.20,分别反映了洋壳流体(或早期大陆俯冲板片流体)和陆壳流体交代的性质和时限.开屏沟橄榄岩来源于俯冲带上覆地幔楔,遭受了不同来源流体不同程度的交代作用而获得地壳特征,同时为柴北缘大洋俯冲到陆陆碰撞的构造演化提供了新证据.
    • 图 1  柴北缘造山带及毗邻区域地质图(a)与开屏沟地区地质简图(b)

      Figure 1.  Geological sketch of the North Qaidam orogen and adjacent areas in northern Tibet (a) and geological sketch of the Kaipinggou area (b)

      朱小辉等(2014)张建新等(2015)修改

      图 2  柴北缘造山带开屏沟橄榄岩的野外(a, b)和显微镜下照片(c~f)

      Figure 2.  Photographs showing field occurrence (a, b) and microscope (c-f) for peridotites at Kaipinggou in the Qaidam orogen

      a.野外辉长岩侵入超基性岩; b.蛇纹石化橄榄岩; c.斑状蛇纹石内残留橄榄石(单偏光); d.孤岛状残留的橄榄石(正交偏光); e.蛇纹石化单斜辉石(正交偏光); f.蛇纹石内金红石包体(单偏光). Ol.橄榄岩, Cpx.单斜辉石, Srp.蛇纹石, Rut.金红石, Spl.尖晶石.据Whitney and Evans(2010)

      图 3  柴北缘造山带开屏沟橄榄岩球粒陨石标准化稀土元素配分模式(a)和原始地幔标准化微量元素蛛网图(b)

      Figure 3.  Chondrite normalized REE pattern (a) and PM-normalized trace element spider diagram (b) for peridotite at Kaipinggou in the Qaidam orogen

      原始地幔及球粒陨石标准化值据McDonough and Sun(1995)

      图 4  开屏沟橄榄岩铂族元素球粒陨石标准化配分模式

      Figure 4.  Chondrite-normalized PGE pattern for peridotite at Kaipinggou

      底图据Su et al.(2016);标准化值据McDonough and Sun(1995)

      图 5  柴北缘造山带开屏沟橄榄岩锆石CL图像

      Figure 5.  Zircon CL images for peridotite at Kaipinggou in the Qaidam orogen

      括号外数值表示年龄, 括号内数值表示锆石Hf同位素组成

      图 6  柴北缘造山带开屏沟橄榄岩锆石U-Pb年龄谐和图

      Figure 6.  Zircon U-Pb concordia diagrams for peridotite at Kaipinggou in the Qaidam orogen

      图 7  柴北缘造山带开屏沟橄榄岩锆石中矿物包裹体的代表照片和拉曼光谱

      Figure 7.  Representative photographs and raman spectra of mineral inclusions in zircon for peridotite at Kaipinggou in the Qaidam orogen

      Zrn.锆石;Ol.橄榄石;Cpx.单斜辉石;Cal.方解石.据Whitney and Evans (2010)

      图 8  柴北缘造山带开屏沟橄榄岩全岩Mg/Si-LOI(a)和Mg#-LOI(b)图解

      Figure 8.  Plots of Mg/Si-LOI (a) and Mg#-LOI (b) for peridotite at Kaipinggou in the Qaidam orogen

      图 9  柴北缘造山带开屏沟橄榄岩全岩Mg/Si-Mg#(a)和Ni-Mg#(b)图解

      Figure 9.  Plots of Mg/Si- Mg# (a) and Ni-Mg# (b) for peridotite at Kaipinggou in the Qaidam orogen

      Chen et al.(2015)

      图 10  柴北缘造山带开屏沟橄榄岩内橄榄石NiO-Fo(a)和CaO-Fo(b)图解

      Figure 10.  Plots of NiO-Fo (a) and CaO-Fo (b) of peridot from peridotite at Kaipinggou in the Qaidam orogen

      Su et al. (2016)

      图 11  柴北缘造山带开屏沟橄榄岩锆石Th-U图解(a)和稀土元素球粒陨石标准化配分图(b)

      Figure 11.  Zircon Th-U plot (a) and chondrite-normalized REE pattern (b) for peridotite at Kaipinggou in the Qaidam orogen

      图 12  柴北缘造山带开屏沟橄榄岩锆石176Hf/177Hf-176Lu/177Hf图解(a)和εHf(t)-t图解(b)

      Figure 12.  Plots of 176Hf/177Hf-176Lu/177Hf (a) and εHf(t)-t (b) of zircon from peridotite at Kaipinggou in the Qaidam orogen

      图 13  壳幔相互作用流体与上覆地幔交代的柴北缘造山模式

      Figure 13.  Crust-mantle interaction through metasomatic reaction of the overlying mantle wedge peridotite with fluids in North Qaidam

      Chen et al.(2017)修改

      表 1  开屏沟造山带橄榄岩主量元素(%)和微量元素(10-6)分析结果

      Table 1.  Major elements (%) and trace elements (10-6) results for peridotite at Kaipinggou in the Qaidam orogen

      样品P73Bb0-1P73Bb1-1P73Bb2-1P73Bb3-1P73Bb6-1P73Bb8-1P73Bb9-1
      SiO237.2839.1136.9537.8038.2038.8838.22
      TiO20.040.020.030.020.040.030.04
      Al2O31.041.041.081.114.211.982.01
      Fe2O36.527.227.197.976.134.784.78
      FeO2.952.453.202.753.803.203.20
      MnO0.130.110.110.100.150.100.11
      MgO34.8337.0435.4036.3033.9635.0734.30
      CaO2.810.201.790.883.462.273.10
      Na2O0.100.130.490.100.250.130.10
      K2O0.020.010.030.020.020.030.05
      P2O50.020.010.010.010.010.020.02
      LOI13.3011.7412.7912.079.0912.4913.06
      H2O+10.0611.2010.3111.078.9011.3111.02
      H2O-0.730.580.610.610.780.790.74
      S199.28276.48782.55473.76425.15636.73675.33
      La0.250.100.180.150.140.290.47
      Ce0.580.230.400.280.280.590.96
      Pr0.0680.0260.0450.0280.0380.0680.110
      Nd0.280.0990.160.120.180.310.47
      Sm0.0710.0230.0460.0290.0550.0720.11
      Eu0.0180.007 90.0510.0520.0330.0360.048
      Gd0.0760.0260.0590.0360.0920.1100.120
      Tb0.0140.004 10.009 10.006 30.0170.0200.023
      Dy0.0950.0380.0470.0350.1200.1200.160
      Ho0.0180.0060.0130.0080.0270.0300.036
      Er0.0550.0260.0430.0330.0790.100.10
      Tm0.0100.0060.0070.0050.0130.0200.019
      Yb0.0690.0470.0560.0370.0850.1400.130
      Lu0.0120.0080.0110.0090.0160.0210.021
      ΣREE1.620.641.140.831.171.942.79
      LREE1.270.480.890.670.731.382.18
      HREE0.350.160.240.170.440.560.61
      LREE/HREE3.632.973.653.931.642.443.55
      (La/Yb)N2.491.422.232.731.081.452.40
      (La/Sm)N2.242.702.503.261.552.542.61
      (Gd/Yb)N0.900.450.850.790.870.660.76
      δEu0.720.972.964.911.411.221.23
      Li1.981.900.981.041.071.001.18
      Be0.0240.0090.1500.1500.0150.0380.026
      Sc6.226.716.907.2410.607.207.32
      V34.331.629.229.542.036.737.0
      Cr5 8056 0264 7874 7774 5965 9485 874
      Co124124116126109105103
      Ni1 9611 9382 1472 1081 6422 1102 070
      Cu15.606.405.972.561.2827.7039.40
      Zn53.049.645.444.346.435.439.6
      Ga1.741.911.331.302.402.202.28
      Rb0.790.150.360.270.180.921.47
      Sr60.75.598.866.411.832.255.2
      Y0.530.220.390.240.720.840.94
      Zr0.920.660.480.380.561.493.25
      Nb0.1400.0520.0960.0880.0550.1300.170
      Sn0.1000.1000.1300.0960.0570.0940.110
      Cs0.0940.0220.0540.0370.0440.2400.330
      Ba71.128.523.324.14.113.039.6
      Hf0.0310.0270.0180.0140.0220.0570.10
      Ta0.008 90.003 60.003 60.003 60.005 40.007 90.012 0
      Tl0.0330.0340.0430.0380.0200.0490.059
      Pb0.930.541.731.950.691.392.39
      Th0.0960.0250.0500.0290.0210.0900.150
      U0.190.060.140.120.040.790.95
      下载: 导出CSV

      表 2  开屏沟造山带橄榄岩中橄榄石主量元素含量(%)

      Table 2.  Contents of major elements (%) of olivine for peridotite at Kaipinggou in the Qaidam orogen

      样品Olv-1Olv-2Olv-3Olv-4Olv-5Olv-6Olv-7Olv-8Olv-9Olv-10Olv-11Olv-12Olv-13Olv-14Olv-15Olv-16Olv-17
      Na2O0.030.000.010.000.000.030.010.000.000.000.000.010.000.000.000.030.02
      SiO238.6738.6438.5437.7938.1238.5637.4338.3438.2138.1938.2138.4138.3038.5039.0039.4138.92
      TiO20.010.000.000.000.000.000.000.000.010.020.020.000.020.010.020.000.02
      CaO0.010.010.000.010.010.000.000.000.010.000.010.000.010.020.010.000.00
      ZrO20.020.000.000.000.000.000.000.000.020.000.000.000.000.000.000.000.00
      FeO9.379.829.759.839.469.349.749.679.819.369.389.459.437.357.427.458.14
      CoO0.030.000.000.000.000.020.010.000.000.050.050.000.000.040.040.000.00
      Al2O30.000.010.000.000.000.000.000.000.000.030.040.020.030.000.000.070.03
      MgO50.9950.1951.1050.8951.0250.8951.9051.0051.0051.1451.0951.1451.1952.8952.1851.9351.50
      K2O0.000.000.000.000.000.000.000.000.000.010.000.000.000.000.000.000.00
      Cr2O30.000.030.010.000.000.000.010.020.000.000.000.000.000.000.000.000.00
      MnO0.160.200.170.150.160.150.170.160.150.140.150.160.140.160.160.180.16
      NiO0.320.390.390.430.360.380.400.410.450.380.360.390.400.380.400.350.36
      ZnO0.010.000.000.000.000.000.000.000.000.000.000.000.000.000.000.000.00
      Total99.6199.3099.9899.1199.1599.3999.6799.6199.6699.3299.3299.5999.5399.3799.2499.4399.14
      Si0.960.960.950.940.950.960.930.950.950.950.950.950.950.950.960.970.96
      Ti0.000.000.000.000.000.000.000.000.000.000.000.000.000.000.000.000.00
      Al0.000.000.000.000.000.000.000.000.000.000.000.000.000.000.000.000.00
      Cr0.000.000.000.000.000.000.000.000.000.000.000.000.000.000.000.000.00
      Fe2+0.190.200.200.210.200.190.200.200.200.190.190.200.200.150.150.150.17
      Mn0.000.000.000.000.000.000.000.000.000.000.000.000.000.000.000.000.00
      Mg1.881.861.881.891.891.881.921.891.891.891.891.891.891.941.911.901.90
      Ni0.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.01
      Ca0.000.000.000.000.000.000.000.000.000.000.000.000.000.000.000.000.00
      Total3.043.043.053.063.053.043.073.053.053.053.053.053.053.053.043.033.04
      Fo90.6690.1190.3490.2290.5890.6690.4790.3990.2690.6990.6790.6190.6392.7792.6192.5591.85
      Fa9.339.879.659.769.419.329.519.609.739.309.329.389.357.227.377.448.13
      Tp0.160.20.170.150.160.150.160.160.150.140.150.160.140.160.160.180.16
      下载: 导出CSV

      表 3  开屏沟造山带橄榄岩铂族元素含量(10-6)

      Table 3.  Concentrations of PGE (10-6) for peridotite at Kaipinggou in the Qaidam orogen

      样品OsIrRuRhPtPdPd/IrPt/Pt*
      KP010.660.825.611.657.0412.615.40.26
      KP021.750.605.550.762.332.173.60.31
      KP030.960.892.033.9154.450.656.90.66
      KP040.571.593.702.402.080.940.60.24
      KP050.380.402.830.9210.08.3620.90.62
      注:${\rm{Pt/P}}{{\rm{t}}^*} = \left({{\rm{Pt}}/8.3} \right) \times \sqrt {\left({{\rm{Rh/1}}{\rm{.6}}} \right) \times \left({{\rm{Pd/4}}{\rm{.4}}} \right)} .$
      下载: 导出CSV

      表 4  开屏沟造山带橄榄岩锆石U-Pb同位素测试结果

      Table 4.  Zircon LA-ICPMS U-Pb isotope data for peridotite at Kaipinggou in the Qaidam orogen

      点号元素含量Th/U同位素比值年龄(Ma)
      PbThU207Pb/206Pb1σ207Pb/235U1σ206Pb/238U1σ207Pb/235U1σ206Pb/238U1σ
      1267.51 494.91 797.00.830.055 680.001 200.523 730.011 500.067 740.000 67427.67.7422.64.1
      264.2264.1436.50.610.053 250.001 680.493 260.015 480.067 170.000 92407.110.5419.15.6
      373.9305.4455.90.670.060 850.002 030.570 080.019 050.067 780.000 86458.112.3422.75.2
      4294.71 381.21 849.30.750.053 600.001 250.490 180.011 390.066 010.000 68405.07.8412.14.1
      5127.3286.11 382.40.210.055 330.001 400.505 880.012 890.066 100.000 82415.78.7412.65.0
      6268.11 225.41 495.90.820.061 790.001 390.579 510.012 440.067 760.000 71464.18.0422.74.3
      7226.7596.92 370.60.250.057 730.001 170.528 750.010 660.066 150.000 70431.07.1412.94.2
      8426.11 640.73 073.10.530.055 910.001 170.516 960.010 600.066 670.000 63423.17.1416.13.8
      9267.41 012.02 104.00.480.053 600.001 030.497 150.009 460.067 020.000 74409.86.4418.24.5
      10179.7638.0815.20.780.057 750.003 470.538 530.030 550.067 010.001 08437.520.2418.16.5
      1169.7166.2339.10.490.063 330.009 370.554 700.063 910.067 290.002 77448.141.8419.816.7
      12122.6419.2638.50.660.052 610.004 850.493 860.045 090.067 120.001 31407.530.7418.87.9
      13729.22 494.54 573.50.550.052 860.002 230.494 290.020 060.066 750.000 88407.813.6416.55.3
      14121.1434.6898.10.480.056 380.001 490.574 820.014 470.073 960.000 80461.19.3460.04.8
      15458.42 393.41 753.91.360.057 530.001 430.591 910.014 510.074 320.000 61472.19.3462.13.7
      16203.31 018.4910.41.120.056 170.001 670.559 130.016 560.072 310.000 76451.010.8450.14.6
      17179.8446.71 386.70.320.053 770.002 730.544 930.026 550.073 530.001 15441.717.5457.46.9
      18144.747.32 236.10.020.060 440.003 030.633 260.030 340.075 600.001 05498.118.9469.86.3
      19139.1326.91 325.40.250.060 290.005 310.622 270.049 120.074 720.001 12491.330.8464.56.7
      20168.1481.91 011.90.480.055 510.002 940.567 350.029 640.073 330.001 05456.319.2456.26.3
      21294.5101.34 961.50.020.055 570.002 080.573 110.021 040.073 870.000 88460.013.6459.45.3
      22770.71 177.94 372.40.270.071 700.001 171.188 830.027 380.119 990.002 52795.412.7730.514.5
      23702.9992.2901.11.100.091 140.001 683.256 070.063 500.256 410.002 751470.615.11471.414.1
      24794.9753.31 852.60.410.095 430.001 593.067 780.062 240.230 740.003 011424.715.51338.415.8
      25243.4406.8680.80.600.072 600.001 561.692 730.037 630.168 290.001 981005.814.21002.710.9
      26446.7138.51 309.90.110.098 000.001 553.849 260.068 810.282 530.003 151603.114.41604.115.8
      27179.5348.41 368.60.250.059 150.001 260.696 920.015 350.085 050.000 98536.99.2526.25.8
      28159.1154.7723.60.210.071 650.001 701.696 920.060 480.168 750.004 021007.422.81005.222.2
      29220.11 058.12 274.80.470.063 230.001 220.772 790.014 740.088 470.000 85581.48.4546.55.0
      30279.7721.41 076.70.670.067 760.001 451.099 460.029 630.116 040.001 66753.114.3707.79.6
      31352.1398.9563.00.710.094 950.001 753.612 240.069 600.274 420.002 531552.215.31563.212.8
      32239.5333.31 060.00.310.067 530.001 591.335 340.035 400.142 170.001 80861.215.4856.910.2
      33351.4601.41 256.60.480.072 350.001 331.537 930.032 240.152 800.001 53945.712.9916.68.6
      34229.0305.31 129.50.270.067 770.001 441.295 190.030 260.137 530.001 35843.613.4830.77.7
      3576.794.8363.50.260.073 030.002 101.395 960.040 910.137 950.001 28887.217.3833.07.3
      36240.5413.2795.70.520.069 950.001 421.490 310.032 390.153 650.001 44926.413.2921.48.1
      37462.5766.11 682.10.460.069 240.001 191.434 960.026 670.149 590.001 40903.611.1898.77.8
      38205.0386.2691.30.560.067 280.001 421.349 860.029 070.145 190.001 36867.512.6873.97.6
      39253.7407.4884.20.460.061 180.002 791.196 340.056 490.140 630.001 98798.926.1848.211.2
      40225.9362.0538.80.670.068 540.003 411.345 880.062 150.141 700.002 20865.826.9854.312.4
      41155.8282.0309.90.910.069 730.004 141.388 780.080 950.142 530.002 67884.234.4859.015.1
      42147.4191.2515.00.370.069 930.003 581.527 380.078 720.156 460.002 55941.431.6937.114.2
      43917.2438.46253.10.070.070 750.002 371.419 640.048 460.143 570.002 15897.220.3864.812.1
      下载: 导出CSV

      表 5  开屏沟造山带橄榄岩锆石微量元素(10-6)分析结果

      Table 5.  Trace elements (10-6) results of zircon for peridotite at Kaipinggou in the Qaidam orogen

      点号TiNbLaCePrNdSmEuGdTmYbLuYHfTa
      137.215.56.671856.2843.638.717.91131141 0741853 49727 8076.64
      211.78.020.00025.20.0701.152.830.41413.940.241376.797630 0474.46
      33.319.550.04730.30.0451.183.240.59518.044.746182.61 13928 7145.03
      415.320.80.80488.30.5406.9110.83.2259.31141 1762142 89927 9907.78
      59.259.100.0216.610.1032.8710.50.16491.73753 6135549 02834 1886.12
      614.511.50.8901160.7677.279.864.0045.51081 1572262 52230 3314.86
      721.811.90.06516.80.1813.0511.70.271043323 0815068 56736 8778.88
      837.912.526.11149.9266.254.16.852051661 5152445 57128 8376.88
      915.212.87.0411141.7910.18.173.2939.891.39941872 16429 6576.96
      1017.96.600.00085.50.0403.766.911.4336.276.28201622 17838 2603.59
      1138.45.400.0006.490.0731.737.790.00058.61291 2832283 98837 9642.00
      1228.18.480.00029.20.0001.8911.60.24158.31141 0881913 43838 7654.58
      1314.515.44.501544.1530.525.43.501011151 2112203 85539 79010.4
      1438.37.500.21722.210.2534.2211.50.50977.51871 7282784 80531 4965.64
      1550.610.42.631102.4630.934.08.981381831 8213635 80722 0364.52
      1628.54.910.10097.00.80013.121.76.1293.91091 0692063 60026 0192.34
      1720.86.482.2920.31.7011.818.00.7221022272 2143866 73339 9414.95
      188.172.126.4415.51.499.954.550.15718.346.949796.11 44442 9465.65
      1927.35.000.83614.40.7972.6212.50.67179.52462 3154116 82042 6914.20
      2019.55.370.00015.70.0642.169.920.32490.01901 8263155 88435 7733.57
      2132.529.60.0484.980.0360.323.273.7340.71061 2002553 11659 27813.2
      229.613.490.04743.90.0251.023.500.80016.868.08121661 52633 2262.72
      2330.66.242.1435.94.6233.026.77.1678.764.05981052 49135 2865.79
      2457926.93.6440.32.7525.932.82.911631831 6692846 18526 1866.10
      2528.74.140.00040.40.2635.1612.01.8378.71371 3342454 23629 6002.66
      2613.54.811.1518.70.8689.5717.41.461021111 0772063 62629 9622.47
      2733.316.797.734046.620753.21.8593.51171 1061943 67034 1198.72
      2813.23.322.2112.12.5324.749.31.332682422 0603449 66734 1691.36
      2923.87.890.16617.80.2202.684.200.31423.640.142890.51 11328 1323.51
      3020.815.50.41332.10.3495.2012.70.91373.41111 0191773 56831 4766.56
      3115.824.83.2558.01.389.4111.20.48770.51331 2482184 07535 14211.7
      3233.412.70.00029.30.2114.029.570.78768.51069881723 39230 3655.26
      3316.73.140.00041.40.0952.174.150.84424.546.95071041 24735 6292.64
      349.103.590.04420.00.85310.720.94.501041331 3662564 24121 3631.34
      3547.13.700.00056.30.2858.9710.54.2653.396.79991982 80024 4561.49
      3622.62.590.1465.310.2174.9211.30.20563.51231 1542093 76140 0871.10
      3715.912.84.1132.03.8924.022.81.3994.03183 1335268 64749 21913.2
      3859216.61915051477864651226983072 65742011 36329 8678.34
      3945.410.20.1711240.3499.8018.33.8979.91371 2932173 73125 6105.17
      4019.016.90.62157.00.8109.0016.95.7367.01191 1802073 02332 13211.4
      4126.013.41.4337.70.7616.099.870.69152.093.58431382 54027 7436.33
      4214.87.500.05213.20.1612.313.991.0026.655.25961211 28531 2142.77
      4360.25.080.04940.90.4556.4412.71.8962.474.06781072 16332 8663.10
      下载: 导出CSV

      表 6  开屏沟造山带橄榄岩锆石Lu-Hf同位素

      Table 6.  Zircon Lu-Hf isotope data for peridotite at Kaipinggou in the Qaidam orogen

      点号176Yb/
      177Hf
      2σ176Lu/
      177Hf
      2σ176Hf/
      177Hf
      2σt
      (Ma)
      (176Hf/
      177Hf)i
      εHf(0)2σεHf (t)2σtDM1tDM2f
      10.032 2920.000 3150.001 2050.000 0070.282 3310.000 0244220.282 322-16.060.84-7.130.841 3071 833-0.96
      20.022 7850.000 1390.000 9010.000 0050.282 3180.000 0284160.282 311-16.520.98-7.510.981 3151 857-0.97
      30.009 3070.000 1010.000 4410.000 0070.282 2230.000 0164220.282 219-19.890.56-10.760.561 4302 062-0.99
      40.008 0410.000 0450.000 3460.000 0020.282 1880.000 0174190.282 185-21.110.60-11.960.611 4742 137-0.99
      50.015 0440.000 0760.000 6950.000 0030.282 4950.000 0204180.282 489-10.270.72-1.200.721 0621 458-0.98
      60.019 9120.000 1460.000 8730.000 0060.282 3330.000 0204120.282 326-16.000.69-6.980.691 2941 824-0.97
      70.045 1590.000 1320.001 7560.000 0050.282 3110.000 0144120.282 297-16.760.48-7.990.481 3551 887-0.95
      80.044 9660.000 6870.001 7640.000 0280.282 2470.000 0284120.282 233-19.031.00-10.261.011 4472 030-0.95
      90.013 7260.000 1990.000 6420.000 0080.282 3800.000 0184220.282 375-14.330.64-5.240.641 2201 714-0.98
      100.033 3190.000 1560.001 2650.000 0060.282 3440.000 0224180.282 334-15.590.79-6.640.791 2911 804-0.96
      110.023 1670.000 1350.000 9910.000 0050.282 2380.000 0214190.282 230-19.360.75-10.310.751 4302 036-0.97
      120.003 4630.000 1710.000 1980.000 0070.282 2430.000 0144180.282 241-19.180.51-9.940.511 3942 012-0.99
      130.008 0540.000 0330.000 4420.000 0020.282 2610.000 0174160.282 258-18.530.58-9.400.581 3781 976-0.99
      140.008 4880.000 0780.000 3880.000 0040.282 7250.000 0224570.282 721-2.140.777.930.77735909-0.99
      150.061 9120.000 2020.002 8270.000 0110.282 7150.000 0224640.282 690-2.490.786.980.78800975-0.92
      160.022 5280.000 2520.000 9020.000 0060.282 4850.000 0154560.282 478-10.590.53-0.710.531 0811 458-0.97
      170.016 8850.000 2630.000 8130.000 0110.282 5580.000 0264620.282 551-8.020.922.020.929771 289-0.98
      180.021 0690.000 1850.001 2280.000 0080.282 7730.000 0364590.282 763-0.411.279.451.27683814-0.96
      190.008 2780.000 1770.000 3860.000 0080.282 7060.000 0234590.282 703-2.800.827.320.82761950-0.99
      200.041 8910.000 6490.002 5710.000 0440.282 7680.000 0454580.282 745-0.621.618.811.61716854-0.92
      210.005 8370.000 0340.000 2830.000 0020.282 5890.000 0104620.282 587-6.920.373.290.379201 209-0.99
      下载: 导出CSV
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    出版历程
    • 收稿日期:  2018-04-27
    • 刊出日期:  2018-08-01

    柴北缘从大洋俯冲到陆陆碰撞:来自开屏沟造山带M型橄榄岩的证据

      通讯作者: 许荣科, xurongke1968@126.com
      作者简介: 蔡鹏捷(1988-), 男, 博士研究生, 主要从事造山带橄榄岩地球化学研究
    • 1. 中国地质大学地质调查研究院, 湖北武汉 430074
    • 2. 中国地质大学资源学院, 湖北武汉 430074
    • 3. 湖北省地质调查研究院, 湖北武汉 430074
    基金项目:  教育部长江学者和创新团队发展计划 IRT14R54中国地质调查局项目 12120113032800

    摘要: 造山带幔源(M型)橄榄岩虽然在高压/超高压变质带分布不多,但由于其来自俯冲板块上覆的岩石圈地幔,因此是研究俯冲隧道内俯冲板片与地幔楔之间相互作用的重要对象,对于还原超高压变质带的演化有重要意义.柴北缘鱼卡榴辉岩-片麻岩区边部附近的开屏沟存在一套橄榄岩,其岩石类型、成因、时代等都缺乏研究.对开屏沟橄榄岩全岩的主量和微量元素及铂族元素、橄榄石主量元素、锆石U-Pb年龄和Hf同位素进行了研究.结果显示,其全岩具有高的Mg#、Mg/Si和Ni值,同时表现出亏损难溶的HFSE和HREE,轻微富集LILE和LREE中与流体活动性相关的元素;橄榄石具有较高的Fo值(90.11~92.77)与NiO含量(0.32%~0.45%)、低的CaO含量(< 0.02%);PGEs的球粒陨石标准化配分模式与交代橄榄岩和残留橄榄岩近似;两组变质锆石年龄为459.5±3.6 Ma和417.5±2.7 Ma,对应的εHft)值为-0.71~9.45和-11.96~-1.20,分别反映了洋壳流体(或早期大陆俯冲板片流体)和陆壳流体交代的性质和时限.开屏沟橄榄岩来源于俯冲带上覆地幔楔,遭受了不同来源流体不同程度的交代作用而获得地壳特征,同时为柴北缘大洋俯冲到陆陆碰撞的构造演化提供了新证据.

    English Abstract

    • 柴北缘超高压变质带位于青藏高原东北部(图 1a),是南祁连地体和柴达木地块的结合部位,东起青海省都兰县的野马滩,西至阿尔金断裂带附近的小赛什腾山,北西向绵延超过400 km,其南北边界分别为柴北缘深断裂及拉脊山-中祁连南缘断裂,东西则以哇洪山-温泉断裂和阿尔金走滑断裂为界(Yang et al., 2001; Shi et al., 2006; 张建新等, 2015; 张延军等, 2016).带内泥质片麻岩锆石含柯石英(Yang et al., 2001)、石榴橄榄岩中的锆石含金刚石包体(Song et al., 2005)和榴辉岩含柯石英(Zhang et al., 2009)等,证实了超高压变质事件存在,发生于420~448 Ma之间(Song et al., 2009; Zhang et al., 2009; 张建新等, 2015).该超高压变质带记录了新元古代裂解、古生代祁连洋从早期的大洋俯冲到大陆碰撞、最终复合造山的过程(Song et al., 2009; Yu et al., 2015; 张建新等, 2015; Chen et al., 2017; Zhang et al., 2017).但由于榴辉岩原岩性质与年龄的复杂性,尤其是大洋俯冲榴辉岩与大陆俯冲形成榴辉岩的空间配位关系不清,对该俯冲带模型还存在进一步刻画的空间.

      图  1  柴北缘造山带及毗邻区域地质图(a)与开屏沟地区地质简图(b)

      Figure 1.  Geological sketch of the North Qaidam orogen and adjacent areas in northern Tibet (a) and geological sketch of the Kaipinggou area (b)

      俯冲带作为地幔和地壳之间能量与物质转换的重要场所,一直以来是研究地幔不均一和地球化学特征的重要对象(Zheng, 2012; Li et al., 2016).造山带橄榄岩在高压(HP)和超高压(UHP)造山带内出露很少,它们不仅能提供俯冲带之上地幔楔组成和演化的重要信息,且由于其大多受到了地壳交代作用(Li et al., 2016),通常保存了汇聚边界内与地壳交代作用相关的丰富的地球动力学信息,是了解大陆俯冲带内地幔楔组成和演化、壳幔相互作用、熔体-岩石、熔体-流体、流体-岩石等反应的重要窗口(Li et al., 2010Zheng, 2012).造山带中的橄榄岩如果受到熔/流体的交代作用不明显,一般能够保留原始的地球化学特征(Li et al., 2010).但是在复杂的造山过程中,岩石经历了俯冲-折返作用,往往受到多期的熔/流体的交代作用,因而造山带橄榄岩的稀土微量元素可以指示交代作用.原始地幔矿物经历交代反应后具有交代组分印记,被地壳来源流体改造的矿物会具有不同的微量元素特征(Scambelluri et al., 2014).通过研究造山带橄榄岩中原始矿物(橄榄石、尖晶石等)能够有效地对橄榄岩的来源和交代进行制约.此外,原始地幔橄榄岩由于低的Zr含量和Si活动而通常无法结晶锆石(Zheng, 2012; Li et al., 2016),一旦出现,多被认为是岩浆侵位过程中发生壳幔相互作用,或在深熔作用前存在源区的壳幔混合(Zheng, 2012).所以出现在橄榄岩中的锆石,不仅能作为受到地壳交代的矿物学证据(Zheng, 2012),而且可以反映交代的时限.因此,根据橄榄岩主量、微量元素和原始矿物原位化学分析及锆石同位素分析,能够揭示大陆碰撞过程中造山带橄榄岩的成因,确定地壳交代作用发生的时间和机制,解析地壳的组成、性质和来源.

      根据造山带橄榄岩侵位前的构造位置,一般将其分为地幔型(M-mantle型)和壳型(C-crust型)(Li et al., 2016).其中M型橄榄岩原本位于俯冲带上覆岩石圈地幔楔底部,后在大陆深俯冲/折返过程中被刮削而最终出露到地壳,是直接记录了俯冲带上覆地幔楔特征的岩石(Zheng, 2012). C型橄榄岩则是原本属于俯冲陆壳中的堆晶橄榄岩,在大陆碰撞过程中随大陆俯冲到地幔深处经受超高压变质后又折返回地壳.目前对于柴北缘造山带橄榄岩的发现与研究主要集中在都兰(Song et al., 2009)、绿梁山(Song et al., 2005; Xiong et al., 2012; Chen et al., 2017)等地区,且前人一致认为其受到来自地壳深俯冲的熔/流体交代.柴北缘鱼卡榴辉岩-片麻岩区边部附近的开屏沟存在一套橄榄岩,其岩石类型、成因、时代等都缺乏研究.因此,本文对开屏沟橄榄岩开展全岩主量和微量元素、铂族元素、主矿物原位化学分析和锆石U-Pb年代学及Hf同位素研究,以揭示其成因、重建大陆碰撞过程中造山带橄榄岩遭受熔/流体交代演化历史,进而探讨大陆俯冲隧道内的壳幔相互作用过程,为柴北缘俯冲碰撞模型提供约束.

      • 整个柴北缘超高压变质带从东向西依次出露4个超高压地体(图 1a),即都兰榴辉岩-片麻岩、锡铁山榴辉岩-片麻岩、绿梁山石榴石橄榄岩-片麻岩和鱼卡榴辉岩-片麻岩地体(Chen et al., 2017Zhang et al., 2017).研究区位于柴达木北缘超高压变质带西段鱼卡榴辉岩-片麻岩单元南部(图 1a).研究区内出露地层主要为元古代达肯大坂岩群斜长角闪岩和片岩,中元古代鱼卡河岩群石英片岩、云母片岩、变粒岩、浅粒岩及榴辉岩,古生代滩间山群浅变质中基性火山岩及碎屑岩、赛什腾组砂砾岩及火山碎屑岩、城墙沟组生物碎屑灰岩(Song et al., 2005; Xiong et al., 2012; 朱小辉等, 2014; Yu et al., 2015; Chen et al., 2017; Zhang et al., 2017).研究区构造以南北向断裂为主,侵入岩包括辉长岩、花岗岩和花岗闪长岩等(图 1b).

        本文研究对象是开屏沟超基性岩(图 1b),累计长约2 km,宽为300~400 m,可见后期辉长岩体大面积侵入(图 2a).岩体西部可见鱼卡河岩群和滩间山群,但接触界面被第四系覆盖,岩体与地层接触关系不明,岩体北东部也被第四系覆盖.超基性岩主要为蛇纹石化纯橄岩(图 2b),其主要由蛇纹石(70%)、残留橄榄石(10%~25%)、单斜辉石(5%~10%)组成,并伴有锆石、尖晶石和磷灰石等副矿物.蛇纹石颗粒通常为深绿色,主要为纤蛇纹石,呈放射状(图 2c)或纤维状(图 2e).橄榄石大部分发生蛇纹石化,少数呈孤岛状残留在蛇纹石中(图 2c),最高干涉色可达Ⅲ级(图 2d).单斜辉石也发生蛇纹石化,可见两组完全解理(2e),同时在蛇纹石内可见金红石包体(图 2f).

        图  2  柴北缘造山带开屏沟橄榄岩的野外(a, b)和显微镜下照片(c~f)

        Figure 2.  Photographs showing field occurrence (a, b) and microscope (c-f) for peridotites at Kaipinggou in the Qaidam orogen

        本文选取了开屏沟超基性岩不同位置的7件橄榄岩样品开展全岩主量和微量及稀土元素分析,5件橄榄岩样品进行全岩铂族元素分析;选取了1个粒度较大且蛇纹石化较弱的橄榄岩样品(KP001,坐标95°58′49″ E、37°53′27″ N)进行锆石U-Pb年龄和Hf同位素分析.

      • 主量元素测试由湖北省地质实验测试中心承担,除H2O采用重量法、CO2采用非水滴定法分析外,其余氧化物都由X-荧光光谱α系数法测定,分析精度(相对误差)除H2O外其他都为1%.微量、稀土元素含量在中国地质大学(武汉)地质过程与矿产资源国家重点实验室采用ICP-MS法测定,经国际标准(AGV-安山岩标样和G-2花岗岩标样)和国家标准(GSR-1、GSR-2、GSR-3)监控,误差小于5%.铂族元素测试由国家地质实验测试中心完成.橄榄石主量元素分析使用武汉理工大学材料研究与分析中心的JEOL jxa-8230电子探针(EPMA)完成.加速电压和束流分别被固定在15 kV和10 nA.光束半径一般设置为2 μm.

        锆石样品选自开屏沟橄榄岩,重量约为20 kg.笔者在河北省廊坊市区域地质调查研究院将样品洗净,使用颚式破碎机将其粉碎成细粉.锆石晶体由传统的重液和磁选技术分离,然后在双目显微镜下挑选.挑选的锆石委托武汉上谱分析科技有限责任公司制靶,并使用该公司JEOL扫描电子显微镜加装阴极发光探头获得阴极发光图像(CL),锆石U-Pb同位素定年和微量元素含量也在该公司利用LA-ICP-MS分析完成.详细的仪器参数和分析流程见Zong et al.(2008),与单矿物原位微区微量元素测试相同.本次分析的激光束斑直径为24 μm, 激光剥蚀样品深度为20~40 μm.U-Pb同位素定年和微量元素含量处理中采用锆石标准91500和玻璃标准物质NIST610作外标分别进行同位素和微量元素分馏校正.每个时间分辨分析数据包括20~30 s空白信号和50 s样品信号.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、元素含量及U-Pb同位素比值和年龄计算)采用ICPMSDataCal软件(Liu et al., 2008)完成.锆石样品的U-Pb年龄谐和图绘制和年龄加权平均计算采用Isoplot软件(Ludwig, 2003)完成.

        锆石Lu-Hf同位素分析在中国地质大学(武汉)地质过程与矿产资源国家重点实验室完成,选择较大锆石测年点或相邻位置测定,激光束斑直径为40 μm,方法详见Hu et al.(2015).εHf(t)的计算采用176Lu的衰变常数1.867 × 10-11 a-1(Söderlund et al., 2004).初始176Hf/177Hf值计算采用(176Hf/177Hf)CHUR=0.282 785和(176Lu/177Hf)CHUR=0.033 6(Bouvier et al., 2008).Hf模式年龄(tDM1tDM2)采用(176Hf/177Hf)DM =0.283 25、(176Lu/177Hf)DM=0.038 4和(176Lu/177Hf)CC=0.015(Griffin et al., 2002).

        锆石中矿物包裹体的识别在武汉地质调查中心同位素地球化学实验室RENISHAW激光拉曼光谱仪上完成,激光波长为514.5 nm,束斑直径为1 μm.

      • 开屏沟橄榄岩主量和微量元素测试结果见表 1,主量元素含量均对LOI进行了校正.SiO2=42.34%~44.95%(平均43.75%), TiO2=0.02%~0.05%(平均0.04%),Al2O3=1.19%~4.67%(平均2.03%),Fe2O3=5.53%~9.15%(平均7.32%),FeO=2.81%~4.21%(平均3.54%),MgO=37.64%~42.41%(平均40.55 %),CaO=0.23%~3.83%(平均2.38%).

        样品P73Bb0-1P73Bb1-1P73Bb2-1P73Bb3-1P73Bb6-1P73Bb8-1P73Bb9-1
        SiO237.2839.1136.9537.8038.2038.8838.22
        TiO20.040.020.030.020.040.030.04
        Al2O31.041.041.081.114.211.982.01
        Fe2O36.527.227.197.976.134.784.78
        FeO2.952.453.202.753.803.203.20
        MnO0.130.110.110.100.150.100.11
        MgO34.8337.0435.4036.3033.9635.0734.30
        CaO2.810.201.790.883.462.273.10
        Na2O0.100.130.490.100.250.130.10
        K2O0.020.010.030.020.020.030.05
        P2O50.020.010.010.010.010.020.02
        LOI13.3011.7412.7912.079.0912.4913.06
        H2O+10.0611.2010.3111.078.9011.3111.02
        H2O-0.730.580.610.610.780.790.74
        S199.28276.48782.55473.76425.15636.73675.33
        La0.250.100.180.150.140.290.47
        Ce0.580.230.400.280.280.590.96
        Pr0.0680.0260.0450.0280.0380.0680.110
        Nd0.280.0990.160.120.180.310.47
        Sm0.0710.0230.0460.0290.0550.0720.11
        Eu0.0180.007 90.0510.0520.0330.0360.048
        Gd0.0760.0260.0590.0360.0920.1100.120
        Tb0.0140.004 10.009 10.006 30.0170.0200.023
        Dy0.0950.0380.0470.0350.1200.1200.160
        Ho0.0180.0060.0130.0080.0270.0300.036
        Er0.0550.0260.0430.0330.0790.100.10
        Tm0.0100.0060.0070.0050.0130.0200.019
        Yb0.0690.0470.0560.0370.0850.1400.130
        Lu0.0120.0080.0110.0090.0160.0210.021
        ΣREE1.620.641.140.831.171.942.79
        LREE1.270.480.890.670.731.382.18
        HREE0.350.160.240.170.440.560.61
        LREE/HREE3.632.973.653.931.642.443.55
        (La/Yb)N2.491.422.232.731.081.452.40
        (La/Sm)N2.242.702.503.261.552.542.61
        (Gd/Yb)N0.900.450.850.790.870.660.76
        δEu0.720.972.964.911.411.221.23
        Li1.981.900.981.041.071.001.18
        Be0.0240.0090.1500.1500.0150.0380.026
        Sc6.226.716.907.2410.607.207.32
        V34.331.629.229.542.036.737.0
        Cr5 8056 0264 7874 7774 5965 9485 874
        Co124124116126109105103
        Ni1 9611 9382 1472 1081 6422 1102 070
        Cu15.606.405.972.561.2827.7039.40
        Zn53.049.645.444.346.435.439.6
        Ga1.741.911.331.302.402.202.28
        Rb0.790.150.360.270.180.921.47
        Sr60.75.598.866.411.832.255.2
        Y0.530.220.390.240.720.840.94
        Zr0.920.660.480.380.561.493.25
        Nb0.1400.0520.0960.0880.0550.1300.170
        Sn0.1000.1000.1300.0960.0570.0940.110
        Cs0.0940.0220.0540.0370.0440.2400.330
        Ba71.128.523.324.14.113.039.6
        Hf0.0310.0270.0180.0140.0220.0570.10
        Ta0.008 90.003 60.003 60.003 60.005 40.007 90.012 0
        Tl0.0330.0340.0430.0380.0200.0490.059
        Pb0.930.541.731.950.691.392.39
        Th0.0960.0250.0500.0290.0210.0900.150
        U0.190.060.140.120.040.790.95

        表 1  开屏沟造山带橄榄岩主量元素(%)和微量元素(10-6)分析结果

        Table 1.  Major elements (%) and trace elements (10-6) results for peridotite at Kaipinggou in the Qaidam orogen

        表 1所示,橄榄岩表现出低的∑REE含量(0.64×10-6~9.96×10-6),(La/Yb)N=1.08~2.49, (La/Sm)N=1.55~3.26, (Gd/Yb)N=0.45~0.90,总体上具有轻微的右倾配分特征,LREE相对于HREE轻微富集(图 3a).其微量元素含量总体也较低,具有明显的Ba、U、K、Pb、Sr正异常和Nb、Ta、Ti负异常(图 3b).其中一些样品表现出Lu相对于Ho、Er、Tm和Tb富集.这种配分模式与受过壳源流体交代有关,虽然壳源流体的交代作用比较弱,但由于全岩∑REE含量很低,所以即使微弱的交代也能有所反映.此外,2个样品具有明显的δEu正异常,反映其后期受到流体/熔体交代.同时,Pb正异常指示了有地壳物质的加入(受过地壳流体的交代作用).

        图  3  柴北缘造山带开屏沟橄榄岩球粒陨石标准化稀土元素配分模式(a)和原始地幔标准化微量元素蛛网图(b)

        Figure 3.  Chondrite normalized REE pattern (a) and PM-normalized trace element spider diagram (b) for peridotite at Kaipinggou in the Qaidam orogen

      • 开屏沟橄榄岩中残留的橄榄石具有较为均一的组成(表 2),为镁橄榄石(Fo=90.11~92.77),MgO=50.19%~52.89%,FeO=7.35%~9.83%,SiO2=37.43%~39.00%,NiO=0.32%~0.45%,Cao含量<0.02%,TiO2含量<0.02%,Al2O3含量<0.07%.

        样品Olv-1Olv-2Olv-3Olv-4Olv-5Olv-6Olv-7Olv-8Olv-9Olv-10Olv-11Olv-12Olv-13Olv-14Olv-15Olv-16Olv-17
        Na2O0.030.000.010.000.000.030.010.000.000.000.000.010.000.000.000.030.02
        SiO238.6738.6438.5437.7938.1238.5637.4338.3438.2138.1938.2138.4138.3038.5039.0039.4138.92
        TiO20.010.000.000.000.000.000.000.000.010.020.020.000.020.010.020.000.02
        CaO0.010.010.000.010.010.000.000.000.010.000.010.000.010.020.010.000.00
        ZrO20.020.000.000.000.000.000.000.000.020.000.000.000.000.000.000.000.00
        FeO9.379.829.759.839.469.349.749.679.819.369.389.459.437.357.427.458.14
        CoO0.030.000.000.000.000.020.010.000.000.050.050.000.000.040.040.000.00
        Al2O30.000.010.000.000.000.000.000.000.000.030.040.020.030.000.000.070.03
        MgO50.9950.1951.1050.8951.0250.8951.9051.0051.0051.1451.0951.1451.1952.8952.1851.9351.50
        K2O0.000.000.000.000.000.000.000.000.000.010.000.000.000.000.000.000.00
        Cr2O30.000.030.010.000.000.000.010.020.000.000.000.000.000.000.000.000.00
        MnO0.160.200.170.150.160.150.170.160.150.140.150.160.140.160.160.180.16
        NiO0.320.390.390.430.360.380.400.410.450.380.360.390.400.380.400.350.36
        ZnO0.010.000.000.000.000.000.000.000.000.000.000.000.000.000.000.000.00
        Total99.6199.3099.9899.1199.1599.3999.6799.6199.6699.3299.3299.5999.5399.3799.2499.4399.14
        Si0.960.960.950.940.950.960.930.950.950.950.950.950.950.950.960.970.96
        Ti0.000.000.000.000.000.000.000.000.000.000.000.000.000.000.000.000.00
        Al0.000.000.000.000.000.000.000.000.000.000.000.000.000.000.000.000.00
        Cr0.000.000.000.000.000.000.000.000.000.000.000.000.000.000.000.000.00
        Fe2+0.190.200.200.210.200.190.200.200.200.190.190.200.200.150.150.150.17
        Mn0.000.000.000.000.000.000.000.000.000.000.000.000.000.000.000.000.00
        Mg1.881.861.881.891.891.881.921.891.891.891.891.891.891.941.911.901.90
        Ni0.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.01
        Ca0.000.000.000.000.000.000.000.000.000.000.000.000.000.000.000.000.00
        Total3.043.043.053.063.053.043.073.053.053.053.053.053.053.053.043.033.04
        Fo90.6690.1190.3490.2290.5890.6690.4790.3990.2690.6990.6790.6190.6392.7792.6192.5591.85
        Fa9.339.879.659.769.419.329.519.609.739.309.329.389.357.227.377.448.13
        Tp0.160.20.170.150.160.150.160.160.150.140.150.160.140.160.160.180.16

        表 2  开屏沟造山带橄榄岩中橄榄石主量元素含量(%)

        Table 2.  Contents of major elements (%) of olivine for peridotite at Kaipinggou in the Qaidam orogen

      • 开屏沟橄榄岩的铂族元素分析结果见表 3,橄榄岩的铂族元素球粒陨石标准化配分模式如图 4,与交代橄榄岩和残留橄榄岩相似(Su et al., 2016).

        样品OsIrRuRhPtPdPd/IrPt/Pt*
        KP010.660.825.611.657.0412.615.40.26
        KP021.750.605.550.762.332.173.60.31
        KP030.960.892.033.9154.450.656.90.66
        KP040.571.593.702.402.080.940.60.24
        KP050.380.402.830.9210.08.3620.90.62
        注:${\rm{Pt/P}}{{\rm{t}}^*} = \left({{\rm{Pt}}/8.3} \right) \times \sqrt {\left({{\rm{Rh/1}}{\rm{.6}}} \right) \times \left({{\rm{Pd/4}}{\rm{.4}}} \right)} .$

        表 3  开屏沟造山带橄榄岩铂族元素含量(10-6)

        Table 3.  Concentrations of PGE (10-6) for peridotite at Kaipinggou in the Qaidam orogen

        图  4  开屏沟橄榄岩铂族元素球粒陨石标准化配分模式

        Figure 4.  Chondrite-normalized PGE pattern for peridotite at Kaipinggou

      • 开屏沟橄榄岩样品中选定的锆石晶体(KP001)如图 5所示,锆石U-Pb同位素和微量元素分析结果分别见表 4表 5.锆石U、Pb含量分别为310×10-6~6 253×10-6、64×10-6~917×10-6,Th/U比值为0.02~1.36(表 4),锆石基本都具有高陡的REE配分模式(图 6a).锆石206Pb/238U年龄分布在412~1 604 Ma,主要分为3个时段:412~422 Ma、450~469Ma和830~873Ma,加权平均年龄分别为417.5±2.7 Ma、459.5±3.6 Ma和844.0±13.0 Ma(图 6a).

        图  5  柴北缘造山带开屏沟橄榄岩锆石CL图像

        Figure 5.  Zircon CL images for peridotite at Kaipinggou in the Qaidam orogen

        图  6  柴北缘造山带开屏沟橄榄岩锆石U-Pb年龄谐和图

        Figure 6.  Zircon U-Pb concordia diagrams for peridotite at Kaipinggou in the Qaidam orogen

        点号元素含量Th/U同位素比值年龄(Ma)
        PbThU207Pb/206Pb1σ207Pb/235U1σ206Pb/238U1σ207Pb/235U1σ206Pb/238U1σ
        1267.51 494.91 797.00.830.055 680.001 200.523 730.011 500.067 740.000 67427.67.7422.64.1
        264.2264.1436.50.610.053 250.001 680.493 260.015 480.067 170.000 92407.110.5419.15.6
        373.9305.4455.90.670.060 850.002 030.570 080.019 050.067 780.000 86458.112.3422.75.2
        4294.71 381.21 849.30.750.053 600.001 250.490 180.011 390.066 010.000 68405.07.8412.14.1
        5127.3286.11 382.40.210.055 330.001 400.505 880.012 890.066 100.000 82415.78.7412.65.0
        6268.11 225.41 495.90.820.061 790.001 390.579 510.012 440.067 760.000 71464.18.0422.74.3
        7226.7596.92 370.60.250.057 730.001 170.528 750.010 660.066 150.000 70431.07.1412.94.2
        8426.11 640.73 073.10.530.055 910.001 170.516 960.010 600.066 670.000 63423.17.1416.13.8
        9267.41 012.02 104.00.480.053 600.001 030.497 150.009 460.067 020.000 74409.86.4418.24.5
        10179.7638.0815.20.780.057 750.003 470.538 530.030 550.067 010.001 08437.520.2418.16.5
        1169.7166.2339.10.490.063 330.009 370.554 700.063 910.067 290.002 77448.141.8419.816.7
        12122.6419.2638.50.660.052 610.004 850.493 860.045 090.067 120.001 31407.530.7418.87.9
        13729.22 494.54 573.50.550.052 860.002 230.494 290.020 060.066 750.000 88407.813.6416.55.3
        14121.1434.6898.10.480.056 380.001 490.574 820.014 470.073 960.000 80461.19.3460.04.8
        15458.42 393.41 753.91.360.057 530.001 430.591 910.014 510.074 320.000 61472.19.3462.13.7
        16203.31 018.4910.41.120.056 170.001 670.559 130.016 560.072 310.000 76451.010.8450.14.6
        17179.8446.71 386.70.320.053 770.002 730.544 930.026 550.073 530.001 15441.717.5457.46.9
        18144.747.32 236.10.020.060 440.003 030.633 260.030 340.075 600.001 05498.118.9469.86.3
        19139.1326.91 325.40.250.060 290.005 310.622 270.049 120.074 720.001 12491.330.8464.56.7
        20168.1481.91 011.90.480.055 510.002 940.567 350.029 640.073 330.001 05456.319.2456.26.3
        21294.5101.34 961.50.020.055 570.002 080.573 110.021 040.073 870.000 88460.013.6459.45.3
        22770.71 177.94 372.40.270.071 700.001 171.188 830.027 380.119 990.002 52795.412.7730.514.5
        23702.9992.2901.11.100.091 140.001 683.256 070.063 500.256 410.002 751470.615.11471.414.1
        24794.9753.31 852.60.410.095 430.001 593.067 780.062 240.230 740.003 011424.715.51338.415.8
        25243.4406.8680.80.600.072 600.001 561.692 730.037 630.168 290.001 981005.814.21002.710.9
        26446.7138.51 309.90.110.098 000.001 553.849 260.068 810.282 530.003 151603.114.41604.115.8
        27179.5348.41 368.60.250.059 150.001 260.696 920.015 350.085 050.000 98536.99.2526.25.8
        28159.1154.7723.60.210.071 650.001 701.696 920.060 480.168 750.004 021007.422.81005.222.2
        29220.11 058.12 274.80.470.063 230.001 220.772 790.014 740.088 470.000 85581.48.4546.55.0
        30279.7721.41 076.70.670.067 760.001 451.099 460.029 630.116 040.001 66753.114.3707.79.6
        31352.1398.9563.00.710.094 950.001 753.612 240.069 600.274 420.002 531552.215.31563.212.8
        32239.5333.31 060.00.310.067 530.001 591.335 340.035 400.142 170.001 80861.215.4856.910.2
        33351.4601.41 256.60.480.072 350.001 331.537 930.032 240.152 800.001 53945.712.9916.68.6
        34229.0305.31 129.50.270.067 770.001 441.295 190.030 260.137 530.001 35843.613.4830.77.7
        3576.794.8363.50.260.073 030.002 101.395 960.040 910.137 950.001 28887.217.3833.07.3
        36240.5413.2795.70.520.069 950.001 421.490 310.032 390.153 650.001 44926.413.2921.48.1
        37462.5766.11 682.10.460.069 240.001 191.434 960.026 670.149 590.001 40903.611.1898.77.8
        38205.0386.2691.30.560.067 280.001 421.349 860.029 070.145 190.001 36867.512.6873.97.6
        39253.7407.4884.20.460.061 180.002 791.196 340.056 490.140 630.001 98798.926.1848.211.2
        40225.9362.0538.80.670.068 540.003 411.345 880.062 150.141 700.002 20865.826.9854.312.4
        41155.8282.0309.90.910.069 730.004 141.388 780.080 950.142 530.002 67884.234.4859.015.1
        42147.4191.2515.00.370.069 930.003 581.527 380.078 720.156 460.002 55941.431.6937.114.2
        43917.2438.46253.10.070.070 750.002 371.419 640.048 460.143 570.002 15897.220.3864.812.1

        表 4  开屏沟造山带橄榄岩锆石U-Pb同位素测试结果

        Table 4.  Zircon LA-ICPMS U-Pb isotope data for peridotite at Kaipinggou in the Qaidam orogen

        点号TiNbLaCePrNdSmEuGdTmYbLuYHfTa
        137.215.56.671856.2843.638.717.91131141 0741853 49727 8076.64
        211.78.020.00025.20.0701.152.830.41413.940.241376.797630 0474.46
        33.319.550.04730.30.0451.183.240.59518.044.746182.61 13928 7145.03
        415.320.80.80488.30.5406.9110.83.2259.31141 1762142 89927 9907.78
        59.259.100.0216.610.1032.8710.50.16491.73753 6135549 02834 1886.12
        614.511.50.8901160.7677.279.864.0045.51081 1572262 52230 3314.86
        721.811.90.06516.80.1813.0511.70.271043323 0815068 56736 8778.88
        837.912.526.11149.9266.254.16.852051661 5152445 57128 8376.88
        915.212.87.0411141.7910.18.173.2939.891.39941872 16429 6576.96
        1017.96.600.00085.50.0403.766.911.4336.276.28201622 17838 2603.59
        1138.45.400.0006.490.0731.737.790.00058.61291 2832283 98837 9642.00
        1228.18.480.00029.20.0001.8911.60.24158.31141 0881913 43838 7654.58
        1314.515.44.501544.1530.525.43.501011151 2112203 85539 79010.4
        1438.37.500.21722.210.2534.2211.50.50977.51871 7282784 80531 4965.64
        1550.610.42.631102.4630.934.08.981381831 8213635 80722 0364.52
        1628.54.910.10097.00.80013.121.76.1293.91091 0692063 60026 0192.34
        1720.86.482.2920.31.7011.818.00.7221022272 2143866 73339 9414.95
        188.172.126.4415.51.499.954.550.15718.346.949796.11 44442 9465.65
        1927.35.000.83614.40.7972.6212.50.67179.52462 3154116 82042 6914.20
        2019.55.370.00015.70.0642.169.920.32490.01901 8263155 88435 7733.57
        2132.529.60.0484.980.0360.323.273.7340.71061 2002553 11659 27813.2
        229.613.490.04743.90.0251.023.500.80016.868.08121661 52633 2262.72
        2330.66.242.1435.94.6233.026.77.1678.764.05981052 49135 2865.79
        2457926.93.6440.32.7525.932.82.911631831 6692846 18526 1866.10
        2528.74.140.00040.40.2635.1612.01.8378.71371 3342454 23629 6002.66
        2613.54.811.1518.70.8689.5717.41.461021111 0772063 62629 9622.47
        2733.316.797.734046.620753.21.8593.51171 1061943 67034 1198.72
        2813.23.322.2112.12.5324.749.31.332682422 0603449 66734 1691.36
        2923.87.890.16617.80.2202.684.200.31423.640.142890.51 11328 1323.51
        3020.815.50.41332.10.3495.2012.70.91373.41111 0191773 56831 4766.56
        3115.824.83.2558.01.389.4111.20.48770.51331 2482184 07535 14211.7
        3233.412.70.00029.30.2114.029.570.78768.51069881723 39230 3655.26
        3316.73.140.00041.40.0952.174.150.84424.546.95071041 24735 6292.64
        349.103.590.04420.00.85310.720.94.501041331 3662564 24121 3631.34
        3547.13.700.00056.30.2858.9710.54.2653.396.79991982 80024 4561.49
        3622.62.590.1465.310.2174.9211.30.20563.51231 1542093 76140 0871.10
        3715.912.84.1132.03.8924.022.81.3994.03183 1335268 64749 21913.2
        3859216.61915051477864651226983072 65742011 36329 8678.34
        3945.410.20.1711240.3499.8018.33.8979.91371 2932173 73125 6105.17
        4019.016.90.62157.00.8109.0016.95.7367.01191 1802073 02332 13211.4
        4126.013.41.4337.70.7616.099.870.69152.093.58431382 54027 7436.33
        4214.87.500.05213.20.1612.313.991.0026.655.25961211 28531 2142.77
        4360.25.080.04940.90.4556.4412.71.8962.474.06781072 16332 8663.10

        表 5  开屏沟造山带橄榄岩锆石微量元素(10-6)分析结果

        Table 5.  Trace elements (10-6) results of zircon for peridotite at Kaipinggou in the Qaidam orogen

        锆石Hf同位素分析主要针对年龄为412~422 Ma和450~469 Ma的锆石,结果列于表 6.其中412~422 Ma锆石的176Hf/177Hf为0.282 188~0.282 495,计算的εHf(t1)为-11.96~-1.20,模式年龄tDM1为1 062~1 474 Ma.而450~469 Ma锆石的176Hf/177Hf为0.282 458~0.282 656,计算的εHf(t2)为-0.71~9.45,模式年龄tDM1为682~1 081 Ma.

        点号176Yb/
        177Hf
        2σ176Lu/
        177Hf
        2σ176Hf/
        177Hf
        2σt
        (Ma)
        (176Hf/
        177Hf)i
        εHf(0)2σεHf (t)2σtDM1tDM2f
        10.032 2920.000 3150.001 2050.000 0070.282 3310.000 0244220.282 322-16.060.84-7.130.841 3071 833-0.96
        20.022 7850.000 1390.000 9010.000 0050.282 3180.000 0284160.282 311-16.520.98-7.510.981 3151 857-0.97
        30.009 3070.000 1010.000 4410.000 0070.282 2230.000 0164220.282 219-19.890.56-10.760.561 4302 062-0.99
        40.008 0410.000 0450.000 3460.000 0020.282 1880.000 0174190.282 185-21.110.60-11.960.611 4742 137-0.99
        50.015 0440.000 0760.000 6950.000 0030.282 4950.000 0204180.282 489-10.270.72-1.200.721 0621 458-0.98
        60.019 9120.000 1460.000 8730.000 0060.282 3330.000 0204120.282 326-16.000.69-6.980.691 2941 824-0.97
        70.045 1590.000 1320.001 7560.000 0050.282 3110.000 0144120.282 297-16.760.48-7.990.481 3551 887-0.95
        80.044 9660.000 6870.001 7640.000 0280.282 2470.000 0284120.282 233-19.031.00-10.261.011 4472 030-0.95
        90.013 7260.000 1990.000 6420.000 0080.282 3800.000 0184220.282 375-14.330.64-5.240.641 2201 714-0.98
        100.033 3190.000 1560.001 2650.000 0060.282 3440.000 0224180.282 334-15.590.79-6.640.791 2911 804-0.96
        110.023 1670.000 1350.000 9910.000 0050.282 2380.000 0214190.282 230-19.360.75-10.310.751 4302 036-0.97
        120.003 4630.000 1710.000 1980.000 0070.282 2430.000 0144180.282 241-19.180.51-9.940.511 3942 012-0.99
        130.008 0540.000 0330.000 4420.000 0020.282 2610.000 0174160.282 258-18.530.58-9.400.581 3781 976-0.99
        140.008 4880.000 0780.000 3880.000 0040.282 7250.000 0224570.282 721-2.140.777.930.77735909-0.99
        150.061 9120.000 2020.002 8270.000 0110.282 7150.000 0224640.282 690-2.490.786.980.78800975-0.92
        160.022 5280.000 2520.000 9020.000 0060.282 4850.000 0154560.282 478-10.590.53-0.710.531 0811 458-0.97
        170.016 8850.000 2630.000 8130.000 0110.282 5580.000 0264620.282 551-8.020.922.020.929771 289-0.98
        180.021 0690.000 1850.001 2280.000 0080.282 7730.000 0364590.282 763-0.411.279.451.27683814-0.96
        190.008 2780.000 1770.000 3860.000 0080.282 7060.000 0234590.282 703-2.800.827.320.82761950-0.99
        200.041 8910.000 6490.002 5710.000 0440.282 7680.000 0454580.282 745-0.621.618.811.61716854-0.92
        210.005 8370.000 0340.000 2830.000 0020.282 5890.000 0104620.282 587-6.920.373.290.379201 209-0.99

        表 6  开屏沟造山带橄榄岩锆石Lu-Hf同位素

        Table 6.  Zircon Lu-Hf isotope data for peridotite at Kaipinggou in the Qaidam orogen

      • 锆石内包裹体保存了其形成时重要的地质信息.拉曼分析结果显示,410~420 Ma锆石的包裹体为橄榄石、单斜辉石(图 7a, 7b),而450~469 Ma锆石的包裹体除了橄榄石(图 7c),还有方解石(图 7d)等不属于橄榄岩的成岩矿物.

        图  7  柴北缘造山带开屏沟橄榄岩锆石中矿物包裹体的代表照片和拉曼光谱

        Figure 7.  Representative photographs and raman spectra of mineral inclusions in zircon for peridotite at Kaipinggou in the Qaidam orogen

      • 高压-超高压变质带中的橄榄岩主要有3种原岩:俯冲带的地幔橄榄岩、俯冲带的基性-超基性岩堆晶杂岩体和来自洋壳地幔的俯冲变质橄榄岩(Li et al., 2010).前两者即M型橄榄岩与C型橄榄岩,而后者橄榄岩先期经历洋底变质作用、随后在洋壳俯冲过程中经历高压变质、最后在折返过程中又经历了退变质作用与热液变质作用(Li et al., 2010).这些橄榄岩位于不同构造位置,经历了不同变质作用从而记录了不同的动力学演化过程.

        开屏沟橄榄岩蛇纹石化较强,Deschamps et al.(2013)认为蛇纹石化会导致全岩主量和微量元素发生迁移.开屏沟橄榄岩的烧失量(LOI)与Mg#、Mg/Si值均没有明显的相关性(图 8),说明蛇纹岩化过程对Mg#和Mg/Si比值影响较小,可以用来示踪橄榄岩原岩和交代过程.开屏沟橄榄岩具有高的Mg#、Mg/Si和Ni值,显示出M型地幔橄榄岩的特征(图 9).开屏沟橄榄岩内橄榄石具有较高的Fo值(90.11~92.77)与NiO含量(0.32%~0.45%)、低CaO含量(<0.02%),表明橄榄石具有地幔残留和交代的特征(图 10).

        图  8  柴北缘造山带开屏沟橄榄岩全岩Mg/Si-LOI(a)和Mg#-LOI(b)图解

        Figure 8.  Plots of Mg/Si-LOI (a) and Mg#-LOI (b) for peridotite at Kaipinggou in the Qaidam orogen

        图  9  柴北缘造山带开屏沟橄榄岩全岩Mg/Si-Mg#(a)和Ni-Mg#(b)图解

        Figure 9.  Plots of Mg/Si- Mg# (a) and Ni-Mg# (b) for peridotite at Kaipinggou in the Qaidam orogen

        图  10  柴北缘造山带开屏沟橄榄岩内橄榄石NiO-Fo(a)和CaO-Fo(b)图解

        Figure 10.  Plots of NiO-Fo (a) and CaO-Fo (b) of peridot from peridotite at Kaipinggou in the Qaidam orogen

        铂族元素(PGE)对母岩浆有明显的继承性,能够很好地反映初始岩浆的性质,因此对于判别幔源和壳源造山带橄榄岩具有重要意义.PGE根据不同溶解度可以分为相容元素组IPGE(Os、Ir、Ru)和不相容组PPGE(Rh、Pt、Pd).前者主要保存在残留橄榄岩内,后者则在金属硫化物内.随着地幔发生部分熔融,PPGE将逐渐进入到熔体中,从而导致残留橄榄岩相对亏损PPGE(Su et al., 2016).前文结果显示PGE的球粒陨石标准化配分模式与交代橄榄岩和残留橄榄岩相似,也说明开屏沟橄榄岩原岩为地幔楔橄榄岩.

        Manning(2004)研究认为,地幔楔底部具有变化的地温梯度,靠近俯冲板块的地幔楔温度一般低于地幔固相线的温度,因此如果板片来源熔/流体没有对其交代,就不会发生富化和富集长英质,也就不能成为岛弧岩浆的地幔源区.高温地幔楔位于低温地幔楔之上,俯冲板片析出的流体在到达高温地幔楔之前必须穿过低温地幔楔,并与之发生强烈的交代作用(陈意等, 2013).开屏沟橄榄岩全岩表现出亏损难溶的HFSE和HREE,轻微富集LILE中与流体活动性相关的元素和LREE中与熔体活动性相关的元素(图 3),这与岛弧岩浆岩的微量元素特征十分相似.当俯冲大洋板块析出的熔/流体进入并交代上覆地幔楔,使受到交代的地幔楔发生部分熔融,产生岛弧岩浆岩(Churikova et al., 2001).结合开屏沟橄榄岩主量元素、微量元素、橄榄石地球化学及铂族元素特征,综合分析其应该是俯冲带的上覆地幔楔橄榄岩.同时如果是富水流体交代地幔楔橄榄岩,则形成蛇纹岩和蛇纹石化橄榄岩; 而含水熔体交代地幔楔橄榄岩,则形成辉石岩和角闪石岩(Zheng, 2012).因此笔者推测早期开屏沟橄榄岩可能形成于浅部的低温地幔楔(位于弧前之下)且受到了俯冲大洋板块析出的富水熔/流体强烈的交代/蚀变作用,后期陆陆碰撞后在深俯冲板片折返过程中地幔楔被刮削而带到地壳层位中.

      • 事实上,在造山过程中变质具有多阶段性,因此微量元素的表现特征只能指示其发生了交代作用,但无法判定其是早期熔/流体的反应还是晚期地壳物质的交代.这就需要能够在时间尺度有所响应的证据,锆石就是一个很好的研究对象.造山带M型橄榄岩中含有新生锆石和残余的锆石(Zheng, 2012).对于新生锆石的成因仍然存在争议,一种观点是其来自地幔来源的交代流体的结晶(Zheng et al., 2006),另一种观点是其从深俯冲地壳脱水产生的交代流体中结晶(Chen et al., 2017).但是无论哪种观点都表明,原始地幔是显著亏损Zr的,其无法直接结晶锆石,只有通过交代作用才能在超基性岩中形成锆石.而造山带橄榄岩中的残余锆石则可能是在交代过程中从俯冲陆壳中机械搬运形成的(Zheng, 2012).

        开屏沟橄榄岩中锆石的206Pb/238U年龄分布在412~1 604 Ma,主要集中在412~422 Ma、450~469 Ma和830~873 Ma,加权平均年龄分别为417.5±2.7 Ma、459.7±3.3 Ma和844±13 Ma.前两组锆石均具有模糊的振荡环带或者无环带(图 5),同时它们具有变化的Th/U比值(图 11a),稀土元素配分模式中重稀土曲线陡峭,正的Ce异常和亏损的Eu异常(图 11b),εHf(t1)为-11.96~-1.20、εHf(t2)为-0.71~9.45(表 6, 图 12),这些特征指示它们是地壳来源流体在交代橄榄岩过程中新形成的锆石.460~400 Ma对应的是柴北缘俯冲造山的时间(查显锋等, 2016).

        图  11  柴北缘造山带开屏沟橄榄岩锆石Th-U图解(a)和稀土元素球粒陨石标准化配分图(b)

        Figure 11.  Zircon Th-U plot (a) and chondrite-normalized REE pattern (b) for peridotite at Kaipinggou in the Qaidam orogen

        图  12  柴北缘造山带开屏沟橄榄岩锆石176Hf/177Hf-176Lu/177Hf图解(a)和εHf(t)-t图解(b)

        Figure 12.  Plots of 176Hf/177Hf-176Lu/177Hf (a) and εHf(t)-t (b) of zircon from peridotite at Kaipinggou in the Qaidam orogen

        对于时代更老的锆石,环带较为明显,时间上也远远早于超高压变质的时代,因此认为其是在俯冲隧道中板块与地幔界面局部发生变质脱水作用或进一步发生熔融作用的过程中、橄榄岩中卷入的深度俯冲地壳内的锆石;与苏鲁大别一带造山带橄榄岩内的残留锆石一样(Li et al., 2016).其他锆石年代分布在526~1604 Ma,其中830~873 Ma的锆石较为集中,与区域榴辉岩原岩年龄接近(750~800 Ma),很可能来自当时基底地壳的残留.

        实验岩石学已证明,难溶的HFSE其实能够随超高压流体迁移,主要通过硅、卤素和碱金属等络合反应进行(Louvel et al., 2013).超高压变质岩内形成金红石脉和锆石的事实,就说明了在俯冲带内地幔深度HFSE可以发生迁移(Chen et al., 2012).在俯冲过程中,上覆地幔楔与流体发生作用,会沉淀富含HFSE的副矿物——锆石和金红石等.因此,这些锆石的U-Pb年龄和Lu-Hf同位素与橄榄岩形成时代无关,反映的是壳幔物质作用的变质交代年龄,同时提供了交代流体中地壳组分的地球化学信息(Zheng, 2012; Li et al., 2016).郑永飞等(2007)认为在壳幔混合作用时,锆石具有低的εHf(t)和相对老的tDM,指示了岩浆源区主要受到壳源物质的影响,反之锆石具有高的εHf(t)和相对年轻的tDM则表明岩浆源区受到幔源物质的贡献较大.对于大洋地壳的生长来说,虽然其既有来自亏损地幔的部分熔融,也有来自不亏损地幔的部分熔融,但其锆石的εHf(t)总体上仍为正值(郑永飞等, 2007).因此对于大洋地壳流体交代作用形成的锆石,应该也继承此Hf同位素特征.

        年龄为412~422 Ma的锆石,εHf(t)为-11.96~-1.20(平均-8.10),tDM1为1 062~1 474 Ma(平均1 338 Ma),锆石包裹体为橄榄石、单斜辉石,说明其应该是具有地壳特征的熔/流体在交代地幔楔橄榄岩过程中结晶形成的.而450~469 Ma锆石的εHf(t)为-0.71~9.45(平均6.19),tDM1为682~1 081 Ma(平均811 Ma),锆石包裹体除了橄榄石、还有方解石等不属于橄榄岩的成岩矿物,指示这些锆石不是在原始橄榄岩形成阶段结晶,而是在交代过程中形成.以上表明这2组变质锆石受到不同变质流体作用的影响.这与洋壳榴辉岩和陆壳榴辉岩的Hf同位素组成一致(Chen et al., 2017).不可否认的是柴北缘造山带大陆板片也存在部分具有正εHf值的超高压变质岩(Xiong et al., 2012),450~460 Ma也可能对应于大陆板片的俯冲进变质年龄,指示的是早期大陆板片俯冲流体作用.

        Chen et al.(2017)总结了整个柴北缘地区榴辉岩原岩类型及时代,认为在新元古代中期产生的大陆玄武岩是Rodinia大陆裂解形成的,在发生大陆俯冲之前有一个早于460 Ma的洋盆将柴达木地块与祁连地块分开,柴达木地块被大洋板片拖曳俯冲到祁连块体之下.当洋壳俯冲消耗后,柴达木与祁连地块开始碰撞.在大陆板块和大洋板块之间的过渡地带,附着的小的洋壳残片(包括大洋型的榴辉岩)被夹带到大陆地壳中一同经历了超高压变质.因此在过渡带形成的洋壳榴辉岩与陆壳榴辉岩具有相似的变质演化历史.所以,从构造背景上看,开屏沟橄榄岩中412~422 Ma的锆石与450~469 Ma的锆石分别反映其受到了大陆地壳和大洋地壳或早期大陆板片俯冲流体交代,这也论证了前文关于橄榄岩成因的推论.

      • 关于柴北缘超高压变质带构造演化的研究已长达20多年,但对其俯冲带模型一直存在争议(Yang et al., 2001; Song et al., 2009; Yu et al., 2015; 张建新等, 2015; Chen et al., 2017; Zhang et al., 2017).解释出不同模型的原因就在于榴辉岩原岩性质与年龄的复杂性.前文已经提到,随着不同类型榴辉岩的发现与分析,现在大多数学者认为柴北缘超高压变质带是一个由洋壳俯冲到陆壳碰撞的构造演化产物(Chen et al., 2017).

        其实,在柴北缘地区已有少量早古生代洋壳残片及其上覆岩系的报道,如锡铁山地区的洋岛玄武岩(~521 Ma;朱小辉等, 2014).沙柳河地区蛇绿岩剖面也表明在柴达木和祁连地体之间存在过早古生代的大洋——南祁连洋,该剖面内具有岩浆震荡生长环带特征的变质辉长岩锆石年龄范围为480~544 Ma,加权平均年龄为516±8 Ma,为此洋壳的持续时间提供了时间限定(Zhang et al., 2009).而此蛇绿岩组合的变质年龄约为450 Ma,表明450 Ma之前存在过持续的洋壳俯冲.都兰东北部的旺尕岛弧型辉长岩时代为468~522 Ma,也为早古生代洋盆的存在提供了限定(朱小辉等, 2014).绿粱山地区弧后盆地型蛇绿岩中变辉长岩的形成时代为535 Ma,侵入其内的斜长花岗岩的形成时代为493 Ma,表明该蛇绿岩发育的时代至少在493~535 Ma之间(朱小辉等, 2014);说明晋宁运动后柴北缘地区已进入大洋地壳的发育及演化阶段,沿柴达木盆地北缘连续分布的岩石记录表明该洋盆可能在早古生代已具有一定的规模.与此同时,在柴北缘地区广泛分布的滩间山群火山岩的主体为形成在俯冲带环境下的岛弧火山岩,其时代介于450~514 Ma(Shi et al., 2006).柴北缘超高压变质作用发生的时代介于420~460 Ma(Zhang et al., 2009; Yu et al., 2013Song et al., 2015).同时关于柴北缘地区古生代花岗岩的研究也表明本地区存在多期次的岩浆活动: 475~460 Ma、435~410 Ma、410~400 Ma和375~370 Ma(Wang et al., 2014Song et al., 2015).

        结合本次开屏沟橄榄岩变质锆石记录的流体性质及全岩地球化学特征,不难推测出柴北缘确实经历了一个从大洋到大陆的深俯冲过程(Chen et al., 2017; 图 13).开屏沟橄榄岩来源于俯冲带上覆的岩石圈地幔,先经历了深俯冲的大洋板片或早期大陆板片析出的熔/流体的强烈交代/蚀变作用而获得岛弧岩浆特征(450~469 Ma).随着进一步俯冲,大陆开始碰撞,橄榄岩在俯冲隧道经历另一阶段的大陆地壳流体交代作用(412~422 Ma),最后在俯冲板片折返过程中被刮削而返回地壳层位(400~412 Ma).

        图  13  壳幔相互作用流体与上覆地幔交代的柴北缘造山模式

        Figure 13.  Crust-mantle interaction through metasomatic reaction of the overlying mantle wedge peridotite with fluids in North Qaidam

      • (1) 开屏沟橄榄岩全岩主量元素、微量元素、PGE和橄榄石地球化学特征显示其具有M型地幔橄榄岩的特征,受到多期壳源流体交代作用.

        (2) 两组变质锆石(412~422 Ma与450~469 Ma)的Hf同位素显示开屏沟橄榄岩先后受到了洋壳(或早期陆壳)和陆壳俯冲流体交代作用.

        (3) 该造山带橄榄岩记录了俯冲隧道中的壳幔相互作用,同时佐证了柴北缘经历了从大洋到大陆的深俯冲过程.

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