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广东三水盆地玄武岩源区特征与南海早期演化

张煜 方念乔

张煜, 方念乔. 广东三水盆地玄武岩源区特征与南海早期演化[J]. 海洋地质与第四纪地质, 2021, 41(3): 95-113. doi: 10.16562/j.cnki.0256-1492.2020092902
引用本文: 张煜, 方念乔. 广东三水盆地玄武岩源区特征与南海早期演化[J]. 海洋地质与第四纪地质, 2021, 41(3): 95-113. doi: 10.16562/j.cnki.0256-1492.2020092902
ZHANG Yu, FANG Nianqiao. Source characteristics of basalts in Sanshui Basin and the early tectonic evolution stage of the South China Sea[J]. Marine Geology & Quaternary Geology, 2021, 41(3): 95-113. doi: 10.16562/j.cnki.0256-1492.2020092902
Citation: ZHANG Yu, FANG Nianqiao. Source characteristics of basalts in Sanshui Basin and the early tectonic evolution stage of the South China Sea[J]. Marine Geology & Quaternary Geology, 2021, 41(3): 95-113. doi: 10.16562/j.cnki.0256-1492.2020092902

广东三水盆地玄武岩源区特征与南海早期演化


doi: 10.16562/j.cnki.0256-1492.2020092902
详细信息
    作者简介:

    张煜(1992—),男,博士研究生,研究方向为岩石大地构造,E-mail:zhangyu_cugb@foxmail.com

  • 基金项目:  国家自然科学基金面上项目“三水盆地及周边地区古近纪火山-沉积组合与南海早期演化”(41572207)
  • 中图分类号: P736.1

Source characteristics of basalts in Sanshui Basin and the early tectonic evolution stage of the South China Sea

More Information
  • 摘要: 南海在扩张前是否经历了陆内裂谷阶段是南海成因研究中一个重要的问题。三水盆地位于南海北部陆缘,其新生代以来喷发的双峰式火山岩具备大陆裂谷的岩石组合特征。通过对其中玄武岩主微量元素分析认为三水盆地玄武岩可以分为亚碱性和碱性玄武岩系列,两者均显示出明显的Nb、Ta正异常,相对于大陆地壳具有较低的Th/Sc、La/Nb和U/Al×1000,陆壳混染程度低;首次对盆地内玄武岩进行40Ar-39Ar测年,结合前人年代学结果表明玄武质岩浆强烈喷发的时段为61~54 Ma,其中亚碱性玄武岩喷发时间(60 Ma)早于碱性玄武岩(56 Ma);通过熔融柱模型反演得到亚碱性岩浆源区起止熔融温压分别为1 517 ℃(3.03 GPa)和1 471 ℃(2.25 GPa),深度为101~76 km,碱性岩浆源区起止熔融温压分别为1 555 ℃(3.33 GPa)和1 506 ℃(2.48 GPa),深度为110~84 km,整体为石榴石-尖晶石橄榄岩过渡区且呈逐渐变深的趋势。综合岩浆源区特征以及岩石组合特征认为三水盆地在古新世具备大陆裂谷特征。通过对比三水盆地与南海扩张期岩浆活动的分布时段及源区特征,发现三水盆地与南海扩张期岩浆活动时间分布存在较长间隔,深部过程差异较大,三水盆地岩浆活动与南海扩张并无直接因果联系。
  • 图  1  三水盆地位置及地质简图

    a.南海北部陆域火山岩盆地分布(据文献[13]进行修改),b.三水盆地地质简图。

    Figure  1.  Location and geological map of Sanshui Basin

    a. the distribution of volcanic basins to the northern part of South China Sea (modified from reference [13]), b. the geological map of Sanshui Basin.

    图  2  三水盆地玄武岩TAS图解

    橙色线为碱性-亚碱性界线,据文献[28]。

    Figure  2.  TAS diagram of basalt in Sanshui Basin

    The orange line is the alkaline-subalkaline boundary, according to reference[28].

    图  3  三水盆地玄武岩稀土及微量元素分布

    a.三水盆地玄武岩球粒陨石标准化稀土元素配分图,b.原始地幔标准化微量元素蛛网图;原始地幔以及球粒陨石数据来自文献[28],LCC(下地壳)和UCC(上地壳)数据来自文献[29],U1500(约33 Ma)为南海扩张初期洋壳样品,数据来自文献[30];a、b图例相同,ZD、WJG、DQC及STC含义同表1

    Figure  3.  Distribution of rare earth and trace elements in basalts in Sanshui Basin

    a.trace element pattern, b.REE pattern of the basalt; data are normalized to primitive mantle and chondrite of [28], LCC (lower continental crust) and UCC (upper continental crust) is from [29], U1500 (about 33 Ma) are the ocean crust of SCS from [30]; the legends of a and b are the same, and the meanings of ZD, WJG, DQC and STC are same as those in Table 1.

    图  4  三水盆地玄武岩40Ar/39Ar测年结果

    Figure  4.  40Ar/39Ar dating results of basalt in Sanshui Basin

    图  5  三水盆地玄武岩Haker图解

    Figure  5.  Haker diagram for basalts in Sanshui Basin

    图  6  三水盆地亚碱性-碱性玄武岩FeOT、Na2O含量回归校正

    a.FeOT回归校正,b.Na2O回归校正;ZD-紫洞,WJG-王借岗。

    Figure  6.  FeOT and Na2O regression correction of sub-alk (ZD) and alk (WJG) basalts

    a.regression correction of FeOT, b. regression correction of Na2O; ZD-ZiDong, WJG-WangJieGang.

    图  7  亚碱性(ZD)-碱性(WJG)玄武岩熔融柱模型FeO-Na2O关系图

    P0表示部分熔融作用发生时的压力,Pf表示部分熔融作用结束时的压力;ZD-紫洞,WJG-王借岗。

    Figure  7.  FeO-Na2O diagram of sub-alk (ZD) and alk (WJG) mantle melting column

    P0 represents the pressure when partial melting occurs, and Pf represents the pressure at the end of partial melting; ZD-ZiDong, WJG-WangJieGang.

    图  8  三水盆地源区性质(La/Yb)N-(Sm/Yb)N图解

    Figure  8.  Mantle source (La/Yb)N-(Sm/Yb)N diagram of Sanshui Basin

    图  9  三水盆地火山岩年龄分布

    年龄数据来自[16],[17]及[21]。

    Figure  9.  Age distribution of volcanic rocks in Sanshui Basin

    Age datas are from [16], [17] and [21].

    10  三水盆地演化模式及其与南海演化的关系

    10.  The evolution model of the Sanshui Basin and its relationship with the evolution of the South China Sea

    11  三水盆地玄武岩与OIB、MORB、玳瑁海山以及U1500样品源区对比

    彩色箭头为不同区域样品熔融柱模型计算结果,箭尾和箭头分别为部分熔融作用开始和停止时的的源区特征;彩色区块为前人对不同区域样品源区范围的计算结果,其中夏威夷OIB、东太平洋海岭及中大西洋洋脊MORB源区据文献[52];灰色实线(近垂直)为固体地幔在不发生熔融条件下绝热上升线,据文献[52];灰色虚线为熔融分数等值线,二辉橄榄岩固相线及熔融分数等值线据文献[53];F-部分熔融分数;U1500 MORB源区计算结果据文献[30]。

    11.  Comparison of basalt source in Sanshui Basin with OIB, MORB, Daimao seamount and U1500 samples.

    The colored arrows are the calculation results of the melting column model for samples in different regions, the arrow tails and arrows represents the source area characteristics when partial melting starts and stops respectively; the colored blocks are the calculation results of the source area of the samples in different areas, while Hawaii OIB, the Eastern Pacific Rise and the Mid-Atlantic Ridge MORB source areas are based on [52]; the solid gray line (near vertical) represents the solid mantle adiabats, according to [52]; the gray dashed line is the melt fraction isopleths, according to [53]; F-partial melting fraction; the source of U1500 MORB according to [30].

    表  1  三水盆地玄武岩野外及镜下特征

    Table  1.   Field and microscopic characteristics of basalts in Sanshui Basin

    采样地区野外特征镜下结构主要矿物及含量矿物特征
    紫洞(ZD)灰黑色、块状构造,露头呈现明显的柱状节理间粒-间隐结构橄榄石(1%~3%)、单斜辉石(10%~12%)、斜长石(15%~19%)橄榄石无色(0.2~0.3 mm),他形粒状包裹于辉石颗粒中;辉石(2 mm)具筛状熔蚀特征,自形短柱状;斜长石(2~6 mm),发育聚片双晶,可见环带结构。
    王借岗(WJG)黑色、块状构造间隐结构橄榄石(3%~5%)、辉石(10%~15%)、斜长石(20%~36%)橄榄石呈椭圆形粒状(1.5~3 mm)。部分细粒橄榄石被具有环带的辉石包裹在核部。单斜辉石裂隙发育,多见呈八边形的横切面。
    邓群村(DQC)黑色、块状构造间隐结构辉石(5%~10%)、斜长石(约20%)辉石(1 mm)分为两类,一类为未发生变质的新鲜辉石,呈他形,另一类为表面发生滑石化的辉石。
    石头村(STC)黑色、块状构造间粒-间隐结构辉石(约15%)、斜长石(约15%)单斜辉石多见呈八边形的横切面,亦可见发育裂纹,发育较弱的环带。斜长石自形板状,发育清晰、完整的环带结构。部分长石具有熔蚀结构。基质具间隐结构,主要为斜长石微晶。
    下载: 导出CSV

    表  2  三水盆地玄武岩主量元素测试及标准矿物计算结果

    Table  2.   Major element and CIPW results of basalts in Sanshui Basin

    主量元素含量/(wt%) 标准矿物计算/%
    样品编号SiO2Al2O3TiO2Fe2O3Fe2O3TFeOCaOMgOK2ONa2OMnOP2O5FeOTQOrAbAnNeDiHyOlMtIlHmAp
    紫洞ZD-201-A51.6615.881.963.437.017.866.631.613.310.160.5010.100.309.4928.0023.740.009.6718.950.004.973.720.001.16
    ZD-20151.7716.201.953.396.767.966.331.643.350.160.509.820.499.6728.3124.350.009.5717.830.004.923.700.001.16
    14SSZD-N50.9815.842.0311.437.965.851.713.570.150.4910.280.0010.1730.4022.310.0011.4114.410.006.273.890.001.15
    14SSZD-S251.3515.631.9110.977.576.282.043.650.150.459.860.0012.1231.1120.380.0011.5811.192.746.183.650.001.05
    ZD-10152.4615.981.892.976.857.516.172.073.500.140.479.520.0012.2429.6221.780.0010.0616.231.114.313.580.001.08
    14SSZD-S152.4416.001.9110.797.465.222.003.560.150.459.712.1511.8730.3521.930.009.9512.940.006.113.660.001.05
    ZD-20251.4416.162.016.314.348.016.041.653.350.160.5110.032.6610.0329.2024.870.009.9312.940.005.243.920.001.22
    ZD-20351.7016.052.005.534.967.685.811.983.630.150.529.940.7311.9231.3722.090.0010.6213.000.005.183.880.001.22
    18SS082-151.3116.252.020.0011.206.878.245.311.663.330.140.5310.081.309.8628.4224.670.0010.5414.880.005.213.870.001.25
    18SS082-251.8416.122.030.0010.827.948.175.151.803.380.150.539.740.0310.7428.8723.690.0011.2517.370.002.933.880.001.24
    王借岗WJG-20147.2616.782.855.354.838.547.171.764.510.150.819.64 0.0010.3926.6620.336.2513.050.008.277.765.420.001.87
    WJG-20347.8017.362.794.705.208.476.581.684.450.140.829.060.009.9228.9222.464.7211.130.008.836.825.300.001.91
    WJG-20447.0517.272.764.815.538.786.781.694.370.150.819.360.009.9825.7422.556.0712.410.009.176.975.250.001.87
    14SS012c46.5216.922.8811.438.607.002.203.580.150.7210.280.0013.1322.3823.744.4311.490.0011.506.155.510.001.69
    14SS012b46.8616.652.8211.048.977.092.203.510.150.729.930.0013.0822.1623.324.2213.320.0010.915.935.390.001.68
    14SS01246.8916.882.7610.998.677.232.243.460.150.739.890.0013.3122.8024.093.6211.440.0011.855.915.280.001.71
    18SS083-347.7317.022.7910.578.438.536.531.903.880.150.889.510.0011.3425.0923.624.3810.790.0015.601.765.360.002.07
    石头村14SS004-247.7417.402.7912.119.705.081.712.830.150.4910.89 0.0010.1724.1329.980.0012.337.233.546.145.340.001.15
    18SS079-247.7417.482.8111.798.049.825.411.372.910.150.5310.610.008.1424.8030.910.0012.046.556.764.185.390.001.23
    17SS060-147.7217.342.8511.956.8310.005.291.342.850.170.5010.750.008.0024.3030.780.0012.759.751.446.375.460.001.17
    17SS060-247.9817.712.8111.698.079.955.131.302.720.170.5310.520.007.7523.2332.570.0011.2311.623.003.985.390.001.23
    18SS079-147.9017.282.8211.758.4110.015.231.362.960.150.5410.570.008.0925.2630.160.0013.445.107.773.515.420.001.26
    18SS081-847.9017.492.8411.867.609.905.131.472.710.160.5410.670.008.7523.1431.490.0011.7411.082.124.985.450.001.26
    18SS081-748.1317.362.8111.838.109.945.281.382.560.160.5510.640.008.2421.8432.100.0011.4814.481.074.135.390.001.28
    邓群村17SS063-248.9416.262.7313.066.048.334.021.803.460.231.1611.75 2.7810.7329.4823.670.008.147.990.009.275.220.002.72
    19SS023-149.2515.832.8313.676.527.734.121.613.510.261.1912.313.959.5629.9322.850.006.319.830.009.405.410.002.78
    17SS063-149.1116.332.7012.907.358.553.951.643.370.201.2511.612.149.7928.7424.790.007.9611.580.006.925.180.002.91
    下载: 导出CSV

    表  3  三水盆地玄武岩微量元素测试结果

    Table  3.   Trace elements of basalts in Sanshui Basin

    μg/g
    样品编号YLaCePrNdSmEuGdTbDyHoErTmYbLuLiBeScVCrCo
    紫洞ZD-201-A28.7330.1753.386.8828.276.662.176.241.116.051.102.670.462.880.6222.481.8823.08165.40217.6041.94
    ZD-20127.2629.0751.276.5527.066.502.125.951.065.651.062.540.442.770.6021.491.6521.50155.00192.0037.32
    14SSZD-N29.4028.1053.406.6127.406.232.096.231.005.591.062.810.412.420.361.6121.20159.00139.0042.80
    14SSZD-S228.2028.7054.006.5726.706.011.985.910.955.411.022.730.402.450.371.6720.10148.00184.0038.90
    ZD-10127.4629.5651.976.6127.056.342.095.981.065.571.052.580.432.670.6311.141.7522.47164.90199.3040.25
    14SSZD-S128.5029.3054.706.6927.206.112.016.090.975.441.042.780.412.440.371.5620.70151.00196.0040.80
    ZD-20228.8631.2656.356.7827.536.122.155.810.975.821.072.930.442.830.4719.191.6621.52153.10155.7041.29
    ZD-20329.1032.9958.527.0528.696.302.226.091.015.991.123.060.472.930.4810.121.6421.26152.70147.2039.92
    18SS082-130.0027.8051.606.6427.206.442.255.000.995.171.072.400.462.930.3810.501.3522.10177.00165.0062.30
    18SS082-230.6028.1053.606.6927.306.172.175.280.994.901.072.420.472.840.377.571.7121.90185.00173.0060.50
    王借岗WJG-20128.2641.8976.199.0735.987.312.536.721.045.911.072.890.432.660.4228.131.5823.85211.20154.4041.93
    WJG-20329.7042.8677.979.2136.707.432.556.771.045.941.092.980.432.740.4435.391.6822.69206.40116.2043.74
    WJG-20426.4639.9671.278.5233.696.762.336.290.965.470.992.720.402.520.4132.641.5820.69191.00109.7040.45
    14SS012c27.1037.6070.708.6034.707.062.396.691.005.311.002.610.372.070.321.5621.00211.00116.0045.60
    14SS012b28.5039.2074.209.0136.407.512.507.001.065.651.052.800.392.280.341.5123.40217.00153.0043.50
    14SS01226.1037.2070.008.4834.207.022.386.540.995.230.972.530.352.010.301.5020.70197.00119.0038.20
    18SS083-329.5037.1070.209.1035.707.272.425.761.074.991.092.290.382.520.3725.001.5320.60215.00113.0063.10
    石头村14SS004-224.2024.0049.706.4827.406.192.165.960.925.010.912.400.341.980.291.2524.30240.0067.3042.00
    18SS079-225.1023.1046.306.2025.906.092.204.630.914.460.952.090.352.290.3020.001.2024.90256.0066.7055.30
    17SS060-129.2026.2051.806.8429.606.472.176.081.146.001.122.900.462.720.3818.501.3825.40284.0069.3040.40
    17SS060-226.4023.9050.006.2628.806.362.445.030.974.921.012.120.362.220.3021.701.2624.40272.0068.5081.10
    18SS079-125.1022.8048.406.0927.306.342.134.650.934.711.002.120.342.350.3116.501.2024.60263.0072.9058.10
    18SS081-824.5023.0047.606.4427.506.222.254.730.894.440.982.160.372.290.3132.601.1825.40255.0069.8060.30
    18SS081-725.8023.8050.506.1728.006.442.324.850.994.540.992.140.372.360.3016.301.2023.90248.0066.1058.20
    邓群村17SS063-251.4038.0086.3010.5047.8010.903.8610.501.9410.301.935.090.814.790.6714.001.4929.50176.0040.2038.90
    19SS023-150.4032.5070.209.4342.509.853.599.421.789.211.774.650.764.430.6426.101.7730.20169.0056.0066.90
    17SS063-145.9035.1076.2010.2045.0011.004.077.811.588.481.743.750.663.890.5814.301.5030.10168.0043.0064.00
    样品编号NiCuZnGaRbSrNbMoCdInCsBaTaWTlPbBiThUZrHf
    紫洞ZD-201-A113.1066.44113.9021.9220.80418.2452.551.860.110.080.49394.903.280.640.134.600.033.721.20251.806.03
    ZD-20195.2861.32106.3020.7919.03412.2648.291.660.110.080.39374.303.040.600.223.560.033.751.14250.706.02
    14SSZD-N108.0051.10107.0022.4024.30447.0071.800.48402.004.032.413.821.02256.005.82
    14SSZD-S2126.0042.80102.0021.6039.80414.0074.200.36358.004.152.604.271.15276.006.18
    ZD-101108.2067.06113.1022.3740.96412.9655.071.600.130.080.37363.003.410.510.163.890.033.631.29250.805.79
    14SSZD-S1132.0042.90106.0022.0020.50388.0073.200.18370.004.122.914.271.14280.006.22
    ZD-20292.6844.73102.3821.6520.92444.7054.922.640.080.42398.293.170.530.113.120.0216.471.18263.405.72
    ZD-20386.1040.65105.7021.9134.93410.0054.402.810.090.31367.213.130.600.123.290.0110.291.23239.905.35
    18SS082-1101.0064.80156.0022.3020.90451.0048.802.780.250.070.29391.002.84138.000.092.690.023.310.97129.005.40
    18SS082-2108.0062.60161.0022.7024.50437.0050.003.100.220.070.14367.002.91116.000.072.090.013.140.89137.005.68
    下载: 导出CSV
    续表 3
    样品编号NiCuZnGaRbSrNbMoCdInCsBaTaWTlPbBiThUZrHf


    WJG-20179.5738.12101.9119.0734.53774.8069.841.770.080.97693.932.920.440.108.750.037.991.10181.804.13
    WJG-20377.0638.4788.0419.8633.45825.2074.202.900.081.01739.073.540.470.103.190.0212.401.23202.704.37
    WJG-20472.9434.1687.3718.7831.68829.5070.361.730.070.90742.933.700.450.093.000.0217.891.13192.304.32
    14SS012c93.7036.8083.4019.3046.10895.0090.200.55572.005.072.513.781.09155.003.94
    14SS012b98.3040.3082.3019.7046.90901.0098.000.56605.005.312.713.891.67168.004.34
    14SS01290.7037.2078.4019.0045.80874.0089.700.50572.004.922.443.811.36150.003.94
    18SS083-382.0052.30162.0019.1045.90876.0064.602.830.230.080.71579.003.58116.000.061.780.003.690.83101.004.69


    14SS004-240.7045.60100.0022.6032.40768.0053.100.32318.003.132.572.550.73187.004.56
    18SS079-241.8059.20167.0021.1028.80871.0038.101.990.210.071.90291.002.3096.700.131.380.012.270.6294.504.30
    17SS060-134.7049.00116.0021.4022.50914.0041.101.770.090.071.23699.002.570.350.062.030.012.710.75258.005.55
    17SS060-243.0063.60177.0023.2023.60756.0038.102.070.170.071.01519.002.44201.000.061.600.012.530.6798.804.60
    18SS079-140.0062.20171.0022.0027.20693.0039.902.030.170.071.61329.002.24109.000.104.080.002.410.6398.804.17
    18SS081-841.8066.10172.0022.9025.70610.0038.801.980.210.080.37919.002.24128.000.071.540.002.290.6097.504.48
    18SS081-742.8063.80167.0021.5026.80603.0037.701.900.210.070.58407.002.33107.000.071.480.012.440.6793.304.51


    17SS063-226.6039.20138.0023.1036.20507.0045.702.100.140.093.07498.002.690.470.323.870.012.830.92322.006.75
    19SS023-131.1035.70143.0022.7023.00447.0041.102.160.270.1211.50411.002.99177.000.366.700.034.170.85266.007.16
    17SS063-135.5050.50200.0025.0030.20490.0044.902.510.220.092.50464.002.56129.000.493.390.012.710.85128.005.56
    下载: 导出CSV

    表  4  三水盆地玄武岩稀土元素特征值

    Table  4.   The characteristic values for REE of basalts in Sanshui Basin

    样品编号 ΣREE ΣLREE ΣHREE LREE/HREE δEu δCe (La/Sm)N (La/Yb)N (Sm/Nd)N (Gd/Yb)N
    ZD-201-A148.66127.5321.136.0354951.0137540.8595642.8495357.0626460.7248781.748391
    ZD-201142.64122.5720.076.1071251.0245220.8605972.8132267.0753810.7390981.73334
    14SSZD-N143.71123.8319.886.2288731.0154990.9126722.8372087.8284460.6996072.077396
    14SSZD-S2143.2123.9619.246.4428271.0044460.9127953.0038657.8976960.6925961.946561
    ZD-101143.59123.6219.976.1902851.0228250.8602572.9328387.4641050.7211721.807326
    14SSZD-S1145.55126.0119.546.4488230.9972110.9065553.0164728.0958490.6911762.014067
    ZD-202150.53130.1920.346.4006881.0870790.8917463.2129987.4470990.6840091.656671
    ZD-203156.92135.7721.156.4193851.0821720.8821313.2939327.5910050.6756581.677244
    18SS082-1140.33121.9318.46.626631.1699540.8863262.7153886.3967850.7285071.377047
    18SS082-2142.37124.0318.346.7628141.1349390.9119612.8647986.6707180.6954071.500245
    WJG-201194.11172.9721.148.1821191.0852930.9003013.60467310.617280.6251342.03861
    WJG-203198.15176.7221.438.2463841.0801070.9029013.62857610.545960.622931.993814
    WJG-204182.29162.5319.768.2252021.0754670.887653.71836210.690780.6173942.014172
    14SS012c180.42161.0519.378.3144041.048340.9124943.35008712.246220.6260252.607968
    14SS012b189.39168.8220.578.2070981.0379520.9169233.28336411.59140.6348272.477477
    14SS012178.2159.2818.928.4186051.0573210.91433.33333312.477610.6315792.625598
    18SS083-3180.26161.7918.478.759611.106330.8944493.2100559.9256270.6265891.844457
    14SS004-2133.74115.9317.816.5092641.0734240.9424342.4388978.1720430.6951152.429
    18SS079-2125.77109.7915.986.8704631.2190460.9143752.3859846.8008170.7234931.631519
    17SS060-1143.88123.0820.85.9173081.0422430.9120522.547246.494070.6725571.80377
    17SS060-2134.69117.7616.936.95571.2759290.963822.3638167.258210.6794871.828359
    18SS079-1129.47113.0616.416.8897011.1484690.970322.2621356.5411120.7145671.59673
    18SS081-8129.18113.0116.176.9888681.2205840.9278742.3260046.7713760.6959441.666757
    18SS081-7133.77117.2316.547.0876661.2198530.9815632.3246846.7990710.7076921.65835
    17SS063-2233.39197.3636.035.4776581.0891351.0238092.1929565.3485080.7016411.768888
    19SS023-1200.73168.0732.665.146051.1243460.9540372.0754874.9461150.7131221.715907
    17SS063-1210.06181.5728.496.3731131.279370.9582462.0071856.083340.7521371.620123
    下载: 导出CSV

    表  5  三水盆地岩浆熔融柱计算结果

    Table  5.   Calculation results of mantle melting column in Sanshui Basin

    FeO8.0/%Na2O8.0/%FeOprim/%Na2Oprim/%P0 /GPaPf /GPaT0 /℃Tf /℃F/ %Z0 /kmZf /km
    ZD10.033.4210.092.923.032.251 5171 4719.22101.287376.6719
    WJG10.723.2510.682.723.322.481 5551 50610.06110.392584.01884
      注:FeO8.0、Na2O8.0分别表示样品中的FeO、Na2O在MgO=8%时的含量;FeOprim、Na2Oprim分别表示样品所代表的母岩浆在未发生橄榄石分离结晶时FeO、Na2O的含量;P0T0Z0分别表示部分熔融作用发生时的初始压力、温度及深度;PfTfZf分别表示部分熔融作用结束时的终止压力、温度及深度;F表示压力从P0下降至Pf的过程中的总部分熔融程度。
    下载: 导出CSV

    表  6  地幔源区矿物组成及La、Yb、Sm总分配系数

    Table  6.   Mineral composition in mantle source and total partition coefficients of La, Yb and Sm

    岩石类型矿物相/% 总分配系数
    橄榄石单斜辉石斜方辉石石榴石尖晶石LaYbSm
    石榴石橄榄岩0.540.090.170.200.005112780.868120.129968
    尖晶石橄榄岩0.460.180.2800.080.009839220.116340.05835
    三水盆地源区0.510.140.240.060.050.007717570.3361550.075927
    下载: 导出CSV
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出版历程
  • 收稿日期:  2020-09-29
  • 修回日期:  2020-12-23
  • 网络出版日期:  2021-02-22
  • 刊出日期:  2021-06-28

广东三水盆地玄武岩源区特征与南海早期演化

doi: 10.16562/j.cnki.0256-1492.2020092902
    作者简介:

    张煜(1992—),男,博士研究生,研究方向为岩石大地构造,E-mail:zhangyu_cugb@foxmail.com

基金项目:  国家自然科学基金面上项目“三水盆地及周边地区古近纪火山-沉积组合与南海早期演化”(41572207)
  • 中图分类号: P736.1

摘要: 南海在扩张前是否经历了陆内裂谷阶段是南海成因研究中一个重要的问题。三水盆地位于南海北部陆缘,其新生代以来喷发的双峰式火山岩具备大陆裂谷的岩石组合特征。通过对其中玄武岩主微量元素分析认为三水盆地玄武岩可以分为亚碱性和碱性玄武岩系列,两者均显示出明显的Nb、Ta正异常,相对于大陆地壳具有较低的Th/Sc、La/Nb和U/Al×1000,陆壳混染程度低;首次对盆地内玄武岩进行40Ar-39Ar测年,结合前人年代学结果表明玄武质岩浆强烈喷发的时段为61~54 Ma,其中亚碱性玄武岩喷发时间(60 Ma)早于碱性玄武岩(56 Ma);通过熔融柱模型反演得到亚碱性岩浆源区起止熔融温压分别为1 517 ℃(3.03 GPa)和1 471 ℃(2.25 GPa),深度为101~76 km,碱性岩浆源区起止熔融温压分别为1 555 ℃(3.33 GPa)和1 506 ℃(2.48 GPa),深度为110~84 km,整体为石榴石-尖晶石橄榄岩过渡区且呈逐渐变深的趋势。综合岩浆源区特征以及岩石组合特征认为三水盆地在古新世具备大陆裂谷特征。通过对比三水盆地与南海扩张期岩浆活动的分布时段及源区特征,发现三水盆地与南海扩张期岩浆活动时间分布存在较长间隔,深部过程差异较大,三水盆地岩浆活动与南海扩张并无直接因果联系。

English Abstract

  • 华南大陆边缘的破裂及南海的打开过程是长期以来备受研究者关注的问题,关于南海形成的原因前人提出了众多观点,如青藏高原碰撞挤出[1]、弧后拉张[2-3]、古南海拖曳[4]、古南海俯冲[5]、海南地幔柱驱动[6]、地幔上升流[7]等。IODP367、IODP368、IODP368X航次钻探均未钻遇蛇纹石化地幔,而在U1500和U1502站位分别钻遇了厚度约为120 m的新鲜洋中脊玄武岩和厚度约为180 m的蚀变玄武岩,推测陆洋转换带很窄,陆洋转换过程较迅速[8],地震剖面显示南海下地壳存在高速体和较多岩浆侵入[9]。以上成果为主动裂谷向火山型被动陆缘演化从而进一步导致南海扩张的观点提供了新的证据。经典的主动裂谷模式中,一个比正常地幔热的地幔(例如热幔柱)会通过热作用来减薄岩石圈的底部,同时使得地表地形与下伏软流圈产生相似的隆起直至裂谷产生(如东非埃塞俄比亚裂谷南部[10]),这种具有热异常的地幔上涌很好地解释了太平洋夏威夷-皇帝海山链火山年龄递增的现象(深源地幔柱假说)。此外,有研究者认为并非所有的地幔柱都是“柱”,有些只是局部熔融异常,这些“似地幔柱”不一定源于核-幔边界。很多远离板块边界零散分布在洋底的海山为地幔异常熔融,可能的解释为岩石圈形成“裂缝”,其下部软流圈物质易熔组分局部熔融[11]。尽管关于地幔柱的形态以及起源深度仍存在较多争论,但所有研究者都认为其源区应当具有比正常地幔更高的热量[12],如夏威夷岛及萨摩亚岛的地幔潜能温度可达1 722 ℃,冰岛稍低一些,但也可达到1 616 ℃。因此,认识南海扩张前火山活动源区性质,尤其是基性岩火山活动的源区性质可以为识别南海在扩张前陆内裂谷阶段的演化及其构造背景提供很大的帮助。前人研究表明南海及其邻区新生代的火山岩以基性岩为主且分布很广,从华南大陆到南海海区、从台湾到中南半岛都有[13],而南海出现洋壳(约33 Ma)之前陆上仅有广东三水、河源以及连平盆地有火山岩的报道[14],其中三水盆地火山岩由于喷发量巨大,时间更接近南海扩张而地位更加突出。盆地内岩石组合呈现为双峰式,暗示三水盆地是南海扩张前经历主动大陆裂谷阶段演化的痕迹[14-19]。对于盆地内基性岩尤其是碱性玄武岩成因的研究相对缺乏且存在大相径庭的意见:张维等[19]认为三水盆地玄武岩来源较深,根据其La/Nb值判断其来源于软流圈地幔或更深,在经历了短暂的橄榄石及单斜辉石分异后到达地表;杨蜀颖[20]通过MELT软件模拟计算认为部分熔融发生在石榴子石和尖晶石的过渡地幔区域,属于岩石圈地幔;朱炳泉等[21]和Chung等[14]研究认为岩浆活动的源区具有亏损地幔(DMM)与富集端员(EM2)混合的特征。总体而言,相对于南海扩张后的岩浆活动,扩张前岩浆活动受关注程度较低,且关于岩浆的源区研究多局限于定性讨论。

    三水盆地作为南海北部陆缘新生代以来岩浆活动记录保存最为完整的盆地,64~38 Ma均有玄武质岩浆活动的记录,并且岩浆的成分出现了较大的变化(Mg#=41.6~60.9),因此,可以将其作为研究南海扩张前华南大陆边缘性质及岩浆活动源区的研究区。本研究对盆地内玄武岩首次进行40Ar-39Ar测年,结合前人的K-Ar测年结果限定了盆地内玄武质岩浆的集中活动时段,利用熔融柱模型对岩浆源区的温压条件进行定量化模拟。通过将得到的结果与南海扩张前、扩张期间岩浆源区进行综合对比,提出三水盆地与南海在岩浆深部演化上不存在直接联系。

    • 三水盆地是南海北部陆缘中最贴近南海的内陆盆地(图1)。盆地主要断裂带为高要-惠来断裂带、西江断裂带、恩平-新丰断裂带以及官窑-沙湾断裂带,新生代地层自下而上有莘庄村组、㘵心组、宝月组和华涌组。根据岩性、与地层接触关系以及同位素测年结果,前人将火山岩分为13期,其中绝大部分发生在古新世―中始新世(63.7~38 Ma)[22],火山岩种类主要为玄武岩、粗面岩和流纹岩。玄武岩地表露头地点主要为紫洞、王借岗、石头村、邓群村、水42(钻井)、小塘,露头上玄武岩不与盆地内流纹岩及粗面岩共生。前人研究表明,虽然在喷发量上玄武岩不占绝对优势(约占火山岩露头面积的20%),但是盆地内普遍发育的粗面岩是由玄武质岩浆进一步结晶分异而形成的[19]。本次研究的玄武岩样品采于紫洞、邓群村、石头村以及王借岗4个地区的露头。

      图  1  三水盆地位置及地质简图

      Figure 1.  Location and geological map of Sanshui Basin

    • 紫洞玄武岩:露头柱状节理发育,手标本观察新鲜面为灰黑色,块状构造,斑状结构。镜下观察斑晶成分有橄榄石、辉石和斜长石,含量约30%,其中橄榄石含量1%~3%,单斜辉石含量10%~12%,斜长石含量15%~19%。橄榄石:无色,他形粒状包裹于辉石颗粒中,大小0.2~0.3 mm;部分辉石具筛状熔蚀特征,自形短柱状,可见呈八边形的横切面,辉石粒径多为约2 mm,个别较大辉石沿(001)面可达7 mm,具有明显环带特征,斑晶较发育裂隙,并且裂隙穿过环带;斜长石斑晶粒径约2~6 mm,发育聚片双晶,可见环带结构,且部分具有循环晶特征,无环带斜长石也多发育反应边。有晚期方解石脉体呈裂隙式贯穿基质和斑晶,基质呈间粒-间隐结构,基质中多为斜长石微晶及少量辉石。

      王借岗玄武岩:手标本观察新鲜面为黑色,块状构造。镜下观察斑晶成分有橄榄石、辉石和斜长石,斑晶含量约40%,橄榄石含量3%~5%,辉石含量1%~15%,斜长石20%~36%。橄榄石呈椭圆形粒状,粒径约1.5~3 mm,正高突起。部分细粒橄榄石(0.2~0.3 mm)被具有环带的辉石包裹在核部,发育裂隙及伊丁石化;单斜辉石裂隙发育,辉石多见呈八边形的横切面,部分辉石被具有熔蚀结构的斜长石斑晶包裹于核部,部分辉石环带不清晰,有弱的蚀变现象。斜长石多发育熔蚀边;部分斜长石中可见有少量不透明矿物。基质呈间隐结构,基质中多为斜长石微晶,可见部分斜长石微晶呈弱的定向排列。

      石头村玄武岩:手标本观察新鲜面为黑色,块状构造。镜下呈斑状结构,斑晶成分为斜长石(约15%)和辉石(约15%)。单斜辉石多见呈八边形的横切面,亦可见发育裂纹,发育较弱的环带,且裂纹穿过环带;斜长石自形板状,发育清晰、完整的环带结构。部分长石具有熔蚀结构。基质具间隐结构,主要为斜长石微晶及隐晶质矿物。

      邓群村玄武岩:露头可见球状风化,手标本观察新鲜面呈灰黑色,块状构造。镜下为斑状结构,斑晶主要为斜长石(约20%)和辉石(<10%),斑晶粒径约为1 mm。辉石可以分为两类,一类为未发生变质的新鲜辉石,另一类为表面发生滑石化的自形辉石,基质为间粒-间隐结构。各区域样品镜下简明特征见表1

      表 1  三水盆地玄武岩野外及镜下特征

      Table 1.  Field and microscopic characteristics of basalts in Sanshui Basin

      采样地区野外特征镜下结构主要矿物及含量矿物特征
      紫洞(ZD)灰黑色、块状构造,露头呈现明显的柱状节理间粒-间隐结构橄榄石(1%~3%)、单斜辉石(10%~12%)、斜长石(15%~19%)橄榄石无色(0.2~0.3 mm),他形粒状包裹于辉石颗粒中;辉石(2 mm)具筛状熔蚀特征,自形短柱状;斜长石(2~6 mm),发育聚片双晶,可见环带结构。
      王借岗(WJG)黑色、块状构造间隐结构橄榄石(3%~5%)、辉石(10%~15%)、斜长石(20%~36%)橄榄石呈椭圆形粒状(1.5~3 mm)。部分细粒橄榄石被具有环带的辉石包裹在核部。单斜辉石裂隙发育,多见呈八边形的横切面。
      邓群村(DQC)黑色、块状构造间隐结构辉石(5%~10%)、斜长石(约20%)辉石(1 mm)分为两类,一类为未发生变质的新鲜辉石,呈他形,另一类为表面发生滑石化的辉石。
      石头村(STC)黑色、块状构造间粒-间隐结构辉石(约15%)、斜长石(约15%)单斜辉石多见呈八边形的横切面,亦可见发育裂纹,发育较弱的环带。斜长石自形板状,发育清晰、完整的环带结构。部分长石具有熔蚀结构。基质具间隐结构,主要为斜长石微晶。
    • 本次研究选取盆地内4个地点的玄武岩露头的样品进行主微量元素测试。首先挑选新鲜的岩石样品17件,经超声波洗净并利用玛瑙球磨机粉碎至200目,洗-碎样工作在河北省地调院第一实验室完成。主量元素采用X射线荧光光谱仪(XRF)玻璃熔片法进行测试,测试设备为AB104L,Axios-Max,分析精度小于0.9%;LOI采用高温加热-重量法测定;微量元素采用电感耦合等离子质谱仪进行测试,测试设备为ELEMENT XR,主微量元素测试工作在北京核工业地质研究院实验室进行。

    • 由于部分样品受到氧化,为了后续标准矿物计算以及岩浆源区分析,对ω(Fe2O3)>ω(TiO2)+1.5%的样品进行Fe3+校正,调整后主微量元素数据及标准矿物计算结果(CIPW法)见表2

      表 2  三水盆地玄武岩主量元素测试及标准矿物计算结果

      Table 2.  Major element and CIPW results of basalts in Sanshui Basin

      主量元素含量/(wt%) 标准矿物计算/%
      样品编号SiO2Al2O3TiO2Fe2O3Fe2O3TFeOCaOMgOK2ONa2OMnOP2O5FeOTQOrAbAnNeDiHyOlMtIlHmAp
      紫洞ZD-201-A51.6615.881.963.437.017.866.631.613.310.160.5010.100.309.4928.0023.740.009.6718.950.004.973.720.001.16
      ZD-20151.7716.201.953.396.767.966.331.643.350.160.509.820.499.6728.3124.350.009.5717.830.004.923.700.001.16
      14SSZD-N50.9815.842.0311.437.965.851.713.570.150.4910.280.0010.1730.4022.310.0011.4114.410.006.273.890.001.15
      14SSZD-S251.3515.631.9110.977.576.282.043.650.150.459.860.0012.1231.1120.380.0011.5811.192.746.183.650.001.05
      ZD-10152.4615.981.892.976.857.516.172.073.500.140.479.520.0012.2429.6221.780.0010.0616.231.114.313.580.001.08
      14SSZD-S152.4416.001.9110.797.465.222.003.560.150.459.712.1511.8730.3521.930.009.9512.940.006.113.660.001.05
      ZD-20251.4416.162.016.314.348.016.041.653.350.160.5110.032.6610.0329.2024.870.009.9312.940.005.243.920.001.22
      ZD-20351.7016.052.005.534.967.685.811.983.630.150.529.940.7311.9231.3722.090.0010.6213.000.005.183.880.001.22
      18SS082-151.3116.252.020.0011.206.878.245.311.663.330.140.5310.081.309.8628.4224.670.0010.5414.880.005.213.870.001.25
      18SS082-251.8416.122.030.0010.827.948.175.151.803.380.150.539.740.0310.7428.8723.690.0011.2517.370.002.933.880.001.24
      王借岗WJG-20147.2616.782.855.354.838.547.171.764.510.150.819.64 0.0010.3926.6620.336.2513.050.008.277.765.420.001.87
      WJG-20347.8017.362.794.705.208.476.581.684.450.140.829.060.009.9228.9222.464.7211.130.008.836.825.300.001.91
      WJG-20447.0517.272.764.815.538.786.781.694.370.150.819.360.009.9825.7422.556.0712.410.009.176.975.250.001.87
      14SS012c46.5216.922.8811.438.607.002.203.580.150.7210.280.0013.1322.3823.744.4311.490.0011.506.155.510.001.69
      14SS012b46.8616.652.8211.048.977.092.203.510.150.729.930.0013.0822.1623.324.2213.320.0010.915.935.390.001.68
      14SS01246.8916.882.7610.998.677.232.243.460.150.739.890.0013.3122.8024.093.6211.440.0011.855.915.280.001.71
      18SS083-347.7317.022.7910.578.438.536.531.903.880.150.889.510.0011.3425.0923.624.3810.790.0015.601.765.360.002.07
      石头村14SS004-247.7417.402.7912.119.705.081.712.830.150.4910.89 0.0010.1724.1329.980.0012.337.233.546.145.340.001.15
      18SS079-247.7417.482.8111.798.049.825.411.372.910.150.5310.610.008.1424.8030.910.0012.046.556.764.185.390.001.23
      17SS060-147.7217.342.8511.956.8310.005.291.342.850.170.5010.750.008.0024.3030.780.0012.759.751.446.375.460.001.17
      17SS060-247.9817.712.8111.698.079.955.131.302.720.170.5310.520.007.7523.2332.570.0011.2311.623.003.985.390.001.23
      18SS079-147.9017.282.8211.758.4110.015.231.362.960.150.5410.570.008.0925.2630.160.0013.445.107.773.515.420.001.26
      18SS081-847.9017.492.8411.867.609.905.131.472.710.160.5410.670.008.7523.1431.490.0011.7411.082.124.985.450.001.26
      18SS081-748.1317.362.8111.838.109.945.281.382.560.160.5510.640.008.2421.8432.100.0011.4814.481.074.135.390.001.28
      邓群村17SS063-248.9416.262.7313.066.048.334.021.803.460.231.1611.75 2.7810.7329.4823.670.008.147.990.009.275.220.002.72
      19SS023-149.2515.832.8313.676.527.734.121.613.510.261.1912.313.959.5629.9322.850.006.319.830.009.405.410.002.78
      17SS063-149.1116.332.7012.907.358.553.951.643.370.201.2511.612.149.7928.7424.790.007.9611.580.006.925.180.002.91

      根据测试结果,盆地内基性岩类型分别为粗面质玄武岩、玄武质粗面安山岩以及玄武岩(图2)。SiO2含量为47.26%~52.44%,K2O+Na2O(Alk)含量为3.94%~6.27%,其中AlkWJG=5.71%~6.27%,AlkDQC=5.01%~5.26%,AlkZD=4.92%~5.69%,AlkSTC=3.94%~4.54%,里特曼指数分别为$\bar \sigma $WJG=8.50,$\bar \sigma $DQC=4.31,$\bar \sigma $STC=3.64,$\bar \sigma $ZD=3.15。TiO2含量除紫洞为1.96%外,其余样品含量均大于2%(TiO2=2.70%~2.88%),通常地壳TiO2含量低(0.72%),来自软流圈的岩浆TiO2含量约为1.27%,而与深部地幔物质活动有关的岩浆,TiO2含量通常大于2%[23],三水盆地玄武岩TiO2含量1.96%~2.88%,平均值2.45%,表明来源较深。MgO含量差异较大,王借岗玄武岩MgO含量最高(6.53%~7.23%),邓群村MgO含量最低(3.95%~4.12%),其余样品含量5.08%~6.28%(介于两者之间),正常镁铁质岩浆分异过程是以橄榄石为首要晶出相,随后根据岩浆含水量的不同会先后晶出单斜辉石或斜长石等矿物,熔体中MgO含量随着橄榄石、辉石等矿物的晶出而降低。因此,三水盆地玄武岩MgO含量的差异表明其经历了不同程度的分离结晶过程。

      图  2  三水盆地玄武岩TAS图解

      Figure 2.  TAS diagram of basalt in Sanshui Basin

      前人研究中,对于三水盆地是否发育拉斑玄武岩存在争议[17, 19],袁晓博[17]认为三水盆地玄武岩均为碱性玄武岩,而张维[19]认为王借岗属于碱性玄武岩,紫洞属于大陆拉斑玄武岩,该分歧是由区分碱性和亚碱性的标准差异所导致的。在基本的定义中,拉斑系列是一套硅饱和的岩石,碱性橄榄玄武岩是硅不饱和的岩石,在SiO2含量相同的情况下,碱性橄榄玄武岩比拉斑玄武岩的碱含量更高。此外,根据不同学者的观点,下述条件也可作为判定玄武岩为碱性玄武岩的依据:①标准矿物中不含紫苏辉石[24];②总碱含量超过3%[24];③样品成分投影点落在碱性-亚碱性分界线之上(根据对夏威夷玄武岩的研究划定,图2);④矿物成分中存在似长石和/或者碱性辉石及碱性角闪石[25];⑤标准矿物中含有霞石[26],否则为亚碱性玄武岩。根据②、③本文样品均为碱性玄武岩,根据④判断本文样品则均属于亚碱性玄武岩,而①、⑤又可以将王借岗划为碱性玄武岩,其他3个区域为亚碱性玄武岩。因此,区分碱性-亚碱性的标准较为复杂,焦守涛[27]指出即使IUGS火山岩分类委员会也在该问题上存在矛盾。本文旨在认识三水盆地玄武岩源区,不在该问题上作深入讨论,为了后续叙述的简洁,主要根据标准矿物的计算结果将王借岗玄武岩划分为碱性系列,将紫洞、邓群村和石头村玄武岩划分为亚碱性系列。

    • 三水盆地玄武岩的稀土元素含量见3,稀土元素特征值见表4。玄武岩稀土元素总量ΣREE=(142.64~233.39)×10−6,其中邓群村ΣREE最高,平均为214.73×10−6,ΣLREE平均为182.3×10−6,王借岗ΣREE平均为186.12×10−6,ΣLREE平均为166.17×10−6,紫洞与石头村ΣREE和ΣLREE相似,ΣREE平均值分别为145.75×10−6和132.93×10−6,ΣLREE平均值分别为125.94×10−6和115.70×10−6。玄武岩样品整体稀土元素球粒陨石标准化之比(La/Yb)N=4.94~12.24,其中王借岗(La/Yb)N最高,平均值为11.16,同时轻稀土富集程度最高,(La/Sm)N=3.21~3.71,邓群村的(La/Yb)N平均为5.46,(La/Sm)N=2.01~2.19,为所有样品最低,所有样品的重稀土元素分馏均不明显,(Gd/Yb)N=1.38~2.63。在稀土配分曲线图中(图3a),所有样品均显示出轻稀土元素明显分馏的右倾分布型式,LREE/HREE=5.14~8.75,其中王借岗玄武岩LREE/HREE(8.18~8.75)整体高于其他地点玄武岩(最高值6.87),Eu显示轻微正异常(δEu=0.99~1.27, δEu=$ \dfrac{{\mathrm{E}\mathrm{u}}_{\mathrm{N}}}{\sqrt{{\mathrm{S}\mathrm{m}}_{\mathrm{N}}\times {\mathrm{G}\mathrm{d}}_{\mathrm{N}}}} $)。微量元素含量见表3,从表中可以看出,4个不同地区的样品在相容性元素Li、Be、Sc、Co、Cu的含量上相差较小,而在Cr、Ni元素的含量上存在较大差异,其中Cr、Ni在王借岗和紫洞样品中含量较高,$ \overline {{\rm{Cr}}} $WJG=128.05×10−6$\overline {{\rm{Ni}}} $WJG=85.38×10−6$\overline {{\rm{Cr}}} $ZD=176.88×10−6$\overline {{\rm{Ni}}} $ZD=107.04×10−6,邓群村和石头村样品中Cr、Ni含量偏低,$\overline {{\rm{Cr}}} $DQC=46.40×10−6$\overline {{\rm{Ni}}} $DQC=31.07×10−6$\overline {{\rm{Cr}}} $STC=68.98×10−6$\overline {{\rm{Ni}}} $STC=40.50×10−6,Ni、Cr倾向于进入橄榄石,这与镜下观察到紫洞和王借岗样品中较多的橄榄石以及较高的Mg#(Mg#WJG=50.8,Mg#ZD=55.4)相吻合。在微量元素蛛网图中(图3b),部分紫洞、王借岗样品富集Th,含量接近上部陆壳值,石头村样品相对富集Sr同时相对亏损Zr。整体来看,玄武岩微量元素具有较为一致的分配型式,富集高场强元素Nb、Ta及大离子亲石元素Ba,Ti呈现轻微亏损,Th、Pb、Sr、Zr的含量呈现出数倍的差距,整体呈右倾。

      表 3  三水盆地玄武岩微量元素测试结果

      Table 3.  Trace elements of basalts in Sanshui Basin

      μg/g
      样品编号YLaCePrNdSmEuGdTbDyHoErTmYbLuLiBeScVCrCo
      紫洞ZD-201-A28.7330.1753.386.8828.276.662.176.241.116.051.102.670.462.880.6222.481.8823.08165.40217.6041.94
      ZD-20127.2629.0751.276.5527.066.502.125.951.065.651.062.540.442.770.6021.491.6521.50155.00192.0037.32
      14SSZD-N29.4028.1053.406.6127.406.232.096.231.005.591.062.810.412.420.361.6121.20159.00139.0042.80
      14SSZD-S228.2028.7054.006.5726.706.011.985.910.955.411.022.730.402.450.371.6720.10148.00184.0038.90
      ZD-10127.4629.5651.976.6127.056.342.095.981.065.571.052.580.432.670.6311.141.7522.47164.90199.3040.25
      14SSZD-S128.5029.3054.706.6927.206.112.016.090.975.441.042.780.412.440.371.5620.70151.00196.0040.80
      ZD-20228.8631.2656.356.7827.536.122.155.810.975.821.072.930.442.830.4719.191.6621.52153.10155.7041.29
      ZD-20329.1032.9958.527.0528.696.302.226.091.015.991.123.060.472.930.4810.121.6421.26152.70147.2039.92
      18SS082-130.0027.8051.606.6427.206.442.255.000.995.171.072.400.462.930.3810.501.3522.10177.00165.0062.30
      18SS082-230.6028.1053.606.6927.306.172.175.280.994.901.072.420.472.840.377.571.7121.90185.00173.0060.50
      王借岗WJG-20128.2641.8976.199.0735.987.312.536.721.045.911.072.890.432.660.4228.131.5823.85211.20154.4041.93
      WJG-20329.7042.8677.979.2136.707.432.556.771.045.941.092.980.432.740.4435.391.6822.69206.40116.2043.74
      WJG-20426.4639.9671.278.5233.696.762.336.290.965.470.992.720.402.520.4132.641.5820.69191.00109.7040.45
      14SS012c27.1037.6070.708.6034.707.062.396.691.005.311.002.610.372.070.321.5621.00211.00116.0045.60
      14SS012b28.5039.2074.209.0136.407.512.507.001.065.651.052.800.392.280.341.5123.40217.00153.0043.50
      14SS01226.1037.2070.008.4834.207.022.386.540.995.230.972.530.352.010.301.5020.70197.00119.0038.20
      18SS083-329.5037.1070.209.1035.707.272.425.761.074.991.092.290.382.520.3725.001.5320.60215.00113.0063.10
      石头村14SS004-224.2024.0049.706.4827.406.192.165.960.925.010.912.400.341.980.291.2524.30240.0067.3042.00
      18SS079-225.1023.1046.306.2025.906.092.204.630.914.460.952.090.352.290.3020.001.2024.90256.0066.7055.30
      17SS060-129.2026.2051.806.8429.606.472.176.081.146.001.122.900.462.720.3818.501.3825.40284.0069.3040.40
      17SS060-226.4023.9050.006.2628.806.362.445.030.974.921.012.120.362.220.3021.701.2624.40272.0068.5081.10
      18SS079-125.1022.8048.406.0927.306.342.134.650.934.711.002.120.342.350.3116.501.2024.60263.0072.9058.10
      18SS081-824.5023.0047.606.4427.506.222.254.730.894.440.982.160.372.290.3132.601.1825.40255.0069.8060.30
      18SS081-725.8023.8050.506.1728.006.442.324.850.994.540.992.140.372.360.3016.301.2023.90248.0066.1058.20
      邓群村17SS063-251.4038.0086.3010.5047.8010.903.8610.501.9410.301.935.090.814.790.6714.001.4929.50176.0040.2038.90
      19SS023-150.4032.5070.209.4342.509.853.599.421.789.211.774.650.764.430.6426.101.7730.20169.0056.0066.90
      17SS063-145.9035.1076.2010.2045.0011.004.077.811.588.481.743.750.663.890.5814.301.5030.10168.0043.0064.00
      样品编号NiCuZnGaRbSrNbMoCdInCsBaTaWTlPbBiThUZrHf
      紫洞ZD-201-A113.1066.44113.9021.9220.80418.2452.551.860.110.080.49394.903.280.640.134.600.033.721.20251.806.03
      ZD-20195.2861.32106.3020.7919.03412.2648.291.660.110.080.39374.303.040.600.223.560.033.751.14250.706.02
      14SSZD-N108.0051.10107.0022.4024.30447.0071.800.48402.004.032.413.821.02256.005.82
      14SSZD-S2126.0042.80102.0021.6039.80414.0074.200.36358.004.152.604.271.15276.006.18
      ZD-101108.2067.06113.1022.3740.96412.9655.071.600.130.080.37363.003.410.510.163.890.033.631.29250.805.79
      14SSZD-S1132.0042.90106.0022.0020.50388.0073.200.18370.004.122.914.271.14280.006.22
      ZD-20292.6844.73102.3821.6520.92444.7054.922.640.080.42398.293.170.530.113.120.0216.471.18263.405.72
      ZD-20386.1040.65105.7021.9134.93410.0054.402.810.090.31367.213.130.600.123.290.0110.291.23239.905.35
      18SS082-1101.0064.80156.0022.3020.90451.0048.802.780.250.070.29391.002.84138.000.092.690.023.310.97129.005.40
      18SS082-2108.0062.60161.0022.7024.50437.0050.003.100.220.070.14367.002.91116.000.072.090.013.140.89137.005.68
      续表 3
      样品编号NiCuZnGaRbSrNbMoCdInCsBaTaWTlPbBiThUZrHf


      WJG-20179.5738.12101.9119.0734.53774.8069.841.770.080.97693.932.920.440.108.750.037.991.10181.804.13
      WJG-20377.0638.4788.0419.8633.45825.2074.202.900.081.01739.073.540.470.103.190.0212.401.23202.704.37
      WJG-20472.9434.1687.3718.7831.68829.5070.361.730.070.90742.933.700.450.093.000.0217.891.13192.304.32
      14SS012c93.7036.8083.4019.3046.10895.0090.200.55572.005.072.513.781.09155.003.94
      14SS012b98.3040.3082.3019.7046.90901.0098.000.56605.005.312.713.891.67168.004.34
      14SS01290.7037.2078.4019.0045.80874.0089.700.50572.004.922.443.811.36150.003.94
      18SS083-382.0052.30162.0019.1045.90876.0064.602.830.230.080.71579.003.58116.000.061.780.003.690.83101.004.69


      14SS004-240.7045.60100.0022.6032.40768.0053.100.32318.003.132.572.550.73187.004.56
      18SS079-241.8059.20167.0021.1028.80871.0038.101.990.210.071.90291.002.3096.700.131.380.012.270.6294.504.30
      17SS060-134.7049.00116.0021.4022.50914.0041.101.770.090.071.23699.002.570.350.062.030.012.710.75258.005.55
      17SS060-243.0063.60177.0023.2023.60756.0038.102.070.170.071.01519.002.44201.000.061.600.012.530.6798.804.60
      18SS079-140.0062.20171.0022.0027.20693.0039.902.030.170.071.61329.002.24109.000.104.080.002.410.6398.804.17
      18SS081-841.8066.10172.0022.9025.70610.0038.801.980.210.080.37919.002.24128.000.071.540.002.290.6097.504.48
      18SS081-742.8063.80167.0021.5026.80603.0037.701.900.210.070.58407.002.33107.000.071.480.012.440.6793.304.51


      17SS063-226.6039.20138.0023.1036.20507.0045.702.100.140.093.07498.002.690.470.323.870.012.830.92322.006.75
      19SS023-131.1035.70143.0022.7023.00447.0041.102.160.270.1211.50411.002.99177.000.366.700.034.170.85266.007.16
      17SS063-135.5050.50200.0025.0030.20490.0044.902.510.220.092.50464.002.56129.000.493.390.012.710.85128.005.56

      表 4  三水盆地玄武岩稀土元素特征值

      Table 4.  The characteristic values for REE of basalts in Sanshui Basin

      样品编号 ΣREE ΣLREE ΣHREE LREE/HREE δEu δCe (La/Sm)N (La/Yb)N (Sm/Nd)N (Gd/Yb)N
      ZD-201-A148.66127.5321.136.0354951.0137540.8595642.8495357.0626460.7248781.748391
      ZD-201142.64122.5720.076.1071251.0245220.8605972.8132267.0753810.7390981.73334
      14SSZD-N143.71123.8319.886.2288731.0154990.9126722.8372087.8284460.6996072.077396
      14SSZD-S2143.2123.9619.246.4428271.0044460.9127953.0038657.8976960.6925961.946561
      ZD-101143.59123.6219.976.1902851.0228250.8602572.9328387.4641050.7211721.807326
      14SSZD-S1145.55126.0119.546.4488230.9972110.9065553.0164728.0958490.6911762.014067
      ZD-202150.53130.1920.346.4006881.0870790.8917463.2129987.4470990.6840091.656671
      ZD-203156.92135.7721.156.4193851.0821720.8821313.2939327.5910050.6756581.677244
      18SS082-1140.33121.9318.46.626631.1699540.8863262.7153886.3967850.7285071.377047
      18SS082-2142.37124.0318.346.7628141.1349390.9119612.8647986.6707180.6954071.500245
      WJG-201194.11172.9721.148.1821191.0852930.9003013.60467310.617280.6251342.03861
      WJG-203198.15176.7221.438.2463841.0801070.9029013.62857610.545960.622931.993814
      WJG-204182.29162.5319.768.2252021.0754670.887653.71836210.690780.6173942.014172
      14SS012c180.42161.0519.378.3144041.048340.9124943.35008712.246220.6260252.607968
      14SS012b189.39168.8220.578.2070981.0379520.9169233.28336411.59140.6348272.477477
      14SS012178.2159.2818.928.4186051.0573210.91433.33333312.477610.6315792.625598
      18SS083-3180.26161.7918.478.759611.106330.8944493.2100559.9256270.6265891.844457
      14SS004-2133.74115.9317.816.5092641.0734240.9424342.4388978.1720430.6951152.429
      18SS079-2125.77109.7915.986.8704631.2190460.9143752.3859846.8008170.7234931.631519
      17SS060-1143.88123.0820.85.9173081.0422430.9120522.547246.494070.6725571.80377
      17SS060-2134.69117.7616.936.95571.2759290.963822.3638167.258210.6794871.828359
      18SS079-1129.47113.0616.416.8897011.1484690.970322.2621356.5411120.7145671.59673
      18SS081-8129.18113.0116.176.9888681.2205840.9278742.3260046.7713760.6959441.666757
      18SS081-7133.77117.2316.547.0876661.2198530.9815632.3246846.7990710.7076921.65835
      17SS063-2233.39197.3636.035.4776581.0891351.0238092.1929565.3485080.7016411.768888
      19SS023-1200.73168.0732.665.146051.1243460.9540372.0754874.9461150.7131221.715907
      17SS063-1210.06181.5728.496.3731131.279370.9582462.0071856.083340.7521371.620123

      图  3  三水盆地玄武岩稀土及微量元素分布

      Figure 3.  Distribution of rare earth and trace elements in basalts in Sanshui Basin

    • 本次研究挑选4个地区烧失量低于2.5%且镜下观察无气孔构造的全岩样品进行测试。首先将样品用石英钵碎至40目的颗粒,称取适量样品进行清洗,用铝箔将每个样品单独包装,将多个样品用石英管融封,外面包裹厚1 mm的镉皮,在中国原子能科学研究院快中子反应堆照射24 h。照射后的样品在超高真空析氩系统双真空炉中进行阶段升温融样,用含有锆铝泵的NG PREP SYSTEM型纯化系统纯化各阶段释放的气体。室温下40Ar本底小于1.0×10−15 mol,1 300 ℃时,本底小于1.0×10−14 mol。用Helix SFT型惰性气体质谱仪静态测定氩的同位素比值。用阶段升温各温度段获得的年龄与累计39Ar百分比含量及ArArCALC ver2.4软件,绘制年龄谱图,并用加权法计算出坪年龄,用直线拟合方法计算出40Ar/39Ar的初始比值及等时线年龄。工作标准为GBW04 418角闪石,其K=(0.729±0.005)%,Ar=109.06×10−6 CCSTP/g,年龄值为(2 060±8)Ma。ZBH-25黑云母,其K=7.599%,Ar=1.816×10−9 mol/g,年龄值为(2 060±8)Ma,测年工作在北京核工业地质研究院完成。测试结果见图4

      图  4  三水盆地玄武岩40Ar/39Ar测年结果

      Figure 4.  40Ar/39Ar dating results of basalt in Sanshui Basin

      本次研究测试的玄武岩其40Ar/36Ar初始值(282.0±45.1~330.9±26.4)接近或略大于尼尔值(理想大气值295.5±5),说明样品中不含过剩氩。紫洞玄武岩样品在低温段视年龄(167~158 Ma)误差极大,且39Ar释放量较低(4.7%),在高温段获得了较为稳定的年龄坪,故采用坪年龄(59.76±0.72)Ma作为紫洞玄武岩的年龄。邓群村玄武岩在所有温阶中获得的视年龄值都较为稳定,其等时线年龄((58.14±0.37)Ma)在误差范围内与全温段加权年龄完全一致,故其年龄取值为(58.70±0.30)Ma。石头村玄武岩除了在低温段出现释放量为0.9%的一个视年龄63 Ma之外,其他温阶的年龄都较为均一,因此,同样采取加权年龄(57.60±0.2)Ma作为年龄值。王借岗玄武岩在高温段获得了较为稳定的年龄坪,与等时线年龄(56.46±0.89)Ma在误差范围内一致,因此,采用坪年龄(57.03±0.62)Ma作为其年龄;王借岗玄武岩在低温段获得了27.90和44.15 Ma两个视年龄,39Ar释放量分别为14.77%和12.41%,可能是由于后期热事件扰动造成的,其中27.90 Ma的年龄坪与袁晓博等[18]测得渐新世中期(29.27 Ma)结果相近,该时间点处于南海扩张期间,可能说明在南海扩张期间北部陆缘仍存在岩浆活动,但该结果为一孤证,其可靠性尚待验证。

    • 陆壳混染作用往往会对样品中微量元素的含量产生影响,以下两点依据则表明陆壳混染对原始岩浆的影响较小:①大陆地壳强烈亏损Nb、Ta和Ti元素,相对富集Zr、Hf元素[18],在原始地幔标准化微量元素蛛网图上(图3b),样品显示出明显的Nb、Ta正异常,Zr、Hf元素并未显示正异常;②与大陆地壳相比,玄武岩样品具有较低的Th/Sc(0.09~0.21,UCCavg≈1.4)、La/Nb(0.39~0.83,UCCavg≈3)和U/Al×1000(0.20~0.55,UCCavg≈1)[29],暗示陆壳混染的程度比较低。因此,三水盆地玄武岩母岩浆在上升过程中并未遭受明显的地壳混染作用。

    • 三水盆地玄武岩的Mg#(41.6~60.9)低于原始岩浆参考值(Mg#=70),其相容元素含量Ni((35.5~132)×10−6)和Cr((43~217.6)×10-6)也显著低于原始岩浆值(Ni≈250×10−6,Cr≈300×10−6[31],结合镜下橄榄石斑晶自形程度低且数量较少,以及全岩MgO含量小于8%,表明其岩浆大多为原始岩浆经分异作用形成的进化岩浆。在基性岩浆演化的过程中,橄榄石的分离结晶会引起熔体中SiO2、Al2O3和强不相容元素含量的升高以及MgO、Ni含量的降低,单斜辉石的分离结晶会使得熔体中强不相容元素(如Ba、Nb等)的升高以及MgO、CaO含量的降低,橄榄石与斜长石同时发生分离结晶则会引起强不相容元素的升高以及Al2O3含量的降低[30]。在哈克图解中,MgO与SiO2和Ni(图5acg)呈较好的正相关,结合样品中强不相容元素Ba、Nb、Ta等的富集,指示熔体经历了橄榄石的分离结晶,MgO与MnO呈负相关关系,可能是因为①橄榄石中Mn含量较低,其分离结晶对熔体中Mn含量影响较小,②玄武岩自身Mn含量较低,容易受到浅部过程影响。MgO与CaO及CaO/Al2O3的相关性不明显(图5def),指示没有发生明显的单斜辉石分离结晶作用,MgO与Sc的负相关及与Cr的正相关(图5hi)可能是尖晶石的分离结晶引起的;样品的稀土元素显示轻微的Eu正异常,结合MgO与Al2O3没有明显的相关性,显示熔体在演化中没有斜长石分离结晶的作用,与前人的研究结果[19-20]一致。

      图  5  三水盆地玄武岩Haker图解

      Figure 5.  Haker diagram for basalts in Sanshui Basin

    • 玄武岩岩浆并不是在上地幔某一固定深度处橄榄岩部分熔融的产物(“点岩浆”或者“面岩浆”),它应当是由发生部分熔融的初始深度与部分熔融作用终止深度所限定的深度范围内部分熔融的产物(“柱岩浆”)[32-33]。本文利用地幔熔融柱模型[32]对本盆地玄武岩源区的部分熔融条件做出限定,该模型在Wang等[34]研究美国盆岭地区岩浆源区的工作中受到了很好的验证。在熔融柱模型中,源区的特征通过开始发生部分熔融作用的压力(P0)以及部分熔融作用停止时的压力(Pf)进行刻画。由于Na2O在地幔橄榄岩中含量非常低,在分配行为中可以视作不相容元素,在熔体最终产生的过程中,其在熔体相中的浓度随着部分熔融的进行而不断被稀释,因此,原始岩浆中Na2O浓度(Na2Oprim)是部分熔融程度(F)的函数。由于FeO(本文中FeO表示全亚铁含量,下同)在橄榄石-熔体间的分配系数主要受压力升高的影响,因此原始岩浆中的FeO浓度(FeOprim)是源区开始发生部分熔融时压力(P0)的函数。运用地幔熔融柱模型计算获得的FeO-Na2O质量百分含量图解中,每一个数据点都代表了与其FeO-Na2O含量相对应的发生部分熔融的温度、压力和部分熔融程度。将能代表原始岩浆成分的玄武岩的FeOprim和Na2Oprim成分投入到FeO-Na2O含量图解中,通过改变初始熔融压力值使得模拟FeO-Na2O成分曲线通过或逼近玄武岩的成分投点,获得形成玄武岩样品的玄武质岩浆发生部分熔融的初始压力(P0)和部分熔融作用结束的终止压力(Pf)。

      Cheng等[35]等根据该模型编制了VBA软件Calmantle1.0。在进行计算时需要原始岩浆的Na2Oprim和FeOprim。首先采用MgO=8%校正法[32]还原得到Na2O8.0和FeO8.0。该方法要求同一组样品数量不少于6个且同一组样品中MgO含量不得远低于或远高于8%,否则无法获得有意义的回归曲线。邓群村以及石头村样品(MgOSTC=5.08%~5.41%,n=7;MgODQC=3.95%~4.12%,n=3)不符合计算要求。因此,我们在计算时使用紫洞玄武岩代表盆地内亚碱性玄武岩,王借岗玄武岩代表碱性玄武岩,将两组样品的Na2O和FeO含量进行回归(图6)。接下来使用的添加橄榄石法[34],以1%的增量向体系中分次加入Fo=89橄榄石,直到得到结果玄武岩岩浆与加入的橄榄石处于平衡状态,使用KD=0.3, KD为橄榄石中与共存岩浆中FeO和MgO的摩尔比,Na2Oprim根据橄榄石的添加量稀释得到,达到平衡时岩浆中的FeO和Na2O即为FeOprim以及Na2Oprim。将得到的数值输入Calmantle1.0,通过调整P0及压力步长使得部分熔融曲线通过投影点(图7),每条曲线代表各原始岩浆发生部分熔融的成分演化路径,曲线起点即为部分熔融作用开始时的FeO和Na2O含量。

      图  6  三水盆地亚碱性-碱性玄武岩FeOT、Na2O含量回归校正

      Figure 6.  FeOT and Na2O regression correction of sub-alk (ZD) and alk (WJG) basalts

      图  7  亚碱性(ZD)-碱性(WJG)玄武岩熔融柱模型FeO-Na2O关系图

      Figure 7.  FeO-Na2O diagram of sub-alk (ZD) and alk (WJG) mantle melting column

      将源区的压力视作其上部的地幔以及地壳产生的静岩压力,因此,可以通过P0以及Pf对深度(h)进行计算;

      $h = \frac{{P + \left( {{\rho _{\rm{m}}} - {\rho _{\rm{q}}}} \right) \times {\rm{g}} \times {h_q}}}{{{\rho _{\rm{m}}} \times {\rm{g}}}}$

      (1)

      式中,h为深度(km);P为源区压力;ρm为地幔平均密度(g/cm3),ρq为地壳平均密度(g/cm3),地幔和地壳的密度分别为3.25和2.85 g/cm3[36];g为重力加速度(m/s2),取值为9.8 m/s2hq为地壳厚度,取值为30 km[37]

      从反演的结果来看(表5),岩浆源区整体属于软流圈地幔,部分熔融停止的深度与温度条件与杨蜀颖[20]的计算结果一致,但此时的岩浆成分应来自于地幔更深部位部分熔融后的产物与源区不断发生混合的结果[33]。此外,碱性玄武岩比亚碱性玄武岩部分熔融程度高,且其源区更深、温度更高。整体来看,三水盆地岩浆活动的源区较深,位于软流圈上部。结合前文的年代学分析,三水盆地岩浆活动的源区在初期(约60 Ma)较浅,约57 Ma盆地内出现了另一种更深源区的岩浆活动。

      表 5  三水盆地岩浆熔融柱计算结果

      Table 5.  Calculation results of mantle melting column in Sanshui Basin

      FeO8.0/%Na2O8.0/%FeOprim/%Na2Oprim/%P0 /GPaPf /GPaT0 /℃Tf /℃F/ %Z0 /kmZf /km
      ZD10.033.4210.092.923.032.251 5171 4719.22101.287376.6719
      WJG10.723.2510.682.723.322.481 5551 50610.06110.392584.01884
        注:FeO8.0、Na2O8.0分别表示样品中的FeO、Na2O在MgO=8%时的含量;FeOprim、Na2Oprim分别表示样品所代表的母岩浆在未发生橄榄石分离结晶时FeO、Na2O的含量;P0T0Z0分别表示部分熔融作用发生时的初始压力、温度及深度;PfTfZf分别表示部分熔融作用结束时的终止压力、温度及深度;F表示压力从P0下降至Pf的过程中的总部分熔融程度。
    • 地幔源区在发生部分熔融的过程中,不相容元素的分配行为遵循能斯特定律,因此,可以利用某些微量元素的比值来确定玄武岩的源区。元素X在部分熔融过程中在熔体中的含量(CL)以及残余固相中的含量(C0)之间的比值受到部分熔融程度(F)以及残余固相中的分配系数DRS的约束:

      $C_X^L/C_X^0 = 1/\left[ {F + D_X^{Rs} \times \left( {1 - F} \right)} \right]$

      (2)

      不同的地幔岩具有不同的矿物组成,因此,相同元素的分配系数也会有所不同。尽管不能确定本次研究的玄武质岩浆所对应的源区稀土元素浓度,但可以假定其稀土元素浓度与原始地幔(PM,primitive mantle)一致,因此对于低程度的部分熔融过程而言,C0=CPM$ {C}_{X}^{L} $/$ {C}_{X}^{0} $可以改写为$ {C}_{X}^{L} $/$ {\mathrm{C}}_{\mathrm{X}}^{\mathrm{P}\mathrm{M}} $,即元素X的原始地幔标准化值XN。本文利用La、Sm、Yb 3种元素对源区的岩性进行判别,上述方程可以转变为:

      ${\left( {{\rm{Sm}}/{\rm{Yb}}} \right)_{\rm{N}}} = \left[ {F + D_{\rm Yb}^{\rm Rs}\left( {1 - F} \right)} \right]/\left[ {F + D_{\rm Sm}^{\rm Rs}\left( {1 - F} \right)} \right]$

      (3)

      ${\left( {{\rm{La}}/{\rm{Yb}}} \right)_{\rm{N}}} = \left[ {F + D_{\rm Yb}^{\rm Rs}\left( {1 - F} \right)} \right]/\left[ {F + D_{\rm La}^{\rm Rs}\left( {1 - F} \right)} \right]$

      (4)

      式中,${D}_{\rm La}^{\rm Rs}$${D}_{\rm Sm}^{\rm Rs}$${D}_{\rm Yb}^{\rm Rs}$分别表示残余固相中La,Sm和Yb的总配分系数。

      杨蜀颖[20]认为三水盆地玄武质岩浆源区在石榴石地幔与尖晶石地幔的过渡区,本文通过上述原理对前人的研究进行验证。根据石榴石地幔以及尖晶石地幔不同的矿物组成以及元素在不同矿物与熔体间的分配系数,可以加权求和分别得到La、Sm、Yb在尖晶石地幔与石榴石地幔中低程度部分熔融下的熔体相/残余固相总分配系数$ {D}_{\rm La}^{\rm Rs} $$ {D}_{\rm Sm}^{\rm Rs} $$ {D}_{\rm Yb}^{\rm Rs} $表6),地幔岩的矿物组成以及矿物与熔体的分配系数见Kelemen等[38],通过改变部分熔融程度F,可以得到尖晶石地幔与石榴石地幔在低程度部分熔融下的(La/Yb)N-(Sm/Yb)N曲线(图8),三水玄武岩样品的(La/Yb)N-(Sm/Yb)N位于理想状态下石榴石/尖晶石地幔的部分熔融曲线之间,指示三水盆地玄武岩源区来自石榴石地幔与尖晶石地幔的过渡区,验证了前人对于源区性质的判断。通过调整源区岩石中的矿物比例,可以得到与盆地内样品拟合程度较好的曲线,此时源区的矿物组成为Ol-51%,Cpx-14%,Opx-24%,Gt-6%,Sp-5%。

      表 6  地幔源区矿物组成及La、Yb、Sm总分配系数

      Table 6.  Mineral composition in mantle source and total partition coefficients of La, Yb and Sm

      岩石类型矿物相/% 总分配系数
      橄榄石单斜辉石斜方辉石石榴石尖晶石LaYbSm
      石榴石橄榄岩0.540.090.170.200.005112780.868120.129968
      尖晶石橄榄岩0.460.180.2800.080.009839220.116340.05835
      三水盆地源区0.510.140.240.060.050.007717570.3361550.075927

      图  8  三水盆地源区性质(La/Yb)N-(Sm/Yb)N图解

      Figure 8.  Mantle source (La/Yb)N-(Sm/Yb)N diagram of Sanshui Basin

    • 从本次研究的测年结果来看,盆地内玄武岩喷发时间为古新世且亚碱性玄武岩(紫洞、邓群村、石头村)的喷发时间早于碱性玄武岩(王借岗)。为了更加直观地认识三水盆地岩浆活动时代,了解玄武岩在盆地火山岩喷发中的时间关系,本文收集了三水盆地所有主要类型的火山岩(玄武岩、粗面岩、流纹岩)年龄数据并绘制了年龄频数分布图(图9)。

      图  9  三水盆地火山岩年龄分布

      Figure 9.  Age distribution of volcanic rocks in Sanshui Basin

      从目前收集到的资料来看,仅有王借岗一个地点喷发碱性玄武岩,其他地点均喷发亚碱性玄武岩,玄武质岩浆活动时间从早古新世(63.7 Ma)到晚始新世(38 Ma)不连续分布。61~54 Ma为玄武岩年龄频数分布最密集的时段,指示该时段岩浆活动最为强烈。亚碱性玄武岩最早喷发于紫洞、小塘、水42(约60 Ma),58 Ma以后邓群村、石头村出现玄武质岩浆活动;碱性玄武岩(王借岗)喷发时间较晚(约56 Ma)并持续活动至始新世晚期。粗面岩年龄频数密集分布于54~44 Ma,分别在52和46 Ma形成了两次活动高峰;流纹岩年龄数据较少,无法拟合出很好的喷发时段。整体来看,三水盆地的岩浆活动在大约66 Ma开始出现,其主体活动停止于约42 Ma,该时间点距离南海开始扩张仍有大约10 Ma的时间间隔,暗示三水盆地的岩浆活动与南海的扩张并无直接的关联。

    • 三水盆地在华南陆缘盆地中距离南海扩张轴最近,自新生代起开始发育以玄武岩-粗面岩和玄武岩-流纹岩组合为特征的双峰式火山岩[14-19, 21],其中粗面岩的性质与红海地区Afar坳陷可以对比[19]。众多前人的研究认为三水盆地具备主动大陆裂谷特征。盆地内从约64 Ma开始出现玄武质岩浆的活动,并在约58 Ma出现玄武质岩浆活动的高峰,与之对应的,盆地在古新世中期㘵心组沉积时期的沉积相主要显示为深湖相,此时盆地湖盆面积最广,地层层序识别为裂陷层序[39]。两种碱性不同的玄武岩源区具有较高的初始部分熔融温度(1 517~1 555 ℃),Putirka[12]通过橄榄石-熔体平衡法计算得到的TpTex来检验岩浆活动源区是否具有热驱动的地幔上涌(地幔柱)特征,其中Tp为地幔潜能温度,即地幔包体沿绝热线在不发生熔融的情况下上升至地表的温度;Tex为过剩地幔温度,指发生熔融的地幔核心温度与周围地幔环境温度的差异,公式为Tex=Tp$ {{T}}_{\mathrm{p}}^{\mathrm{A}\mathrm{M}\mathrm{G}} $(AMG,周围地幔地热ambient mantle geotherm),因为被动的地幔上涌形成的洋中脊玄武岩不存在过剩地幔温度,因此,$ {{T}}_{\mathrm{p}}^{\mathrm{M}\mathrm{O}\mathrm{R}} $=${{T}}_{\rm{p}}^{{\rm{AMG}}} $$ \left( {{{T}}_{{\rm{ex}}}^{{\rm{MOR}}} = T_p^{\rm MOR} - {{T}}_{\rm{p}}^{{\rm{AMG}}} = 0{\text{℃}}} \right)$,故Tex=Tp$ {{T}}_{\mathrm{p}}^{\mathrm{A}\mathrm{M}\mathrm{G}} $可改写为Tex=Tp$ {{T}}_{\mathrm{p}}^{\mathrm{M}\mathrm{O}\mathrm{R}} $。洋中脊平均地幔潜能温度$ {{T}}_{\mathrm{p}}^{\mathrm{M}\mathrm{O}\mathrm{R}} $=1 454 ℃,在夏威夷及冰岛等地的Tex一般超过100 ℃,最高可达268 ℃[12],峨眉山大火成岩省的Tex=96 ℃(Tp=1 550 ℃)[39],本盆地的玄武岩样品源区部分熔融温度最高为1 555 ℃,Tp约为1 540 ℃,即Tex=86 ℃,虽不及夏威夷、冰岛等地高,但盆地源区整体落于夏威夷OIB玄武岩源区,指示盆地在古新世中期受到上涌地幔的影响,三水盆地从古新世中期开始具备主动大陆裂谷盆地特征。

      为了探讨三水盆地与南海扩张之间的联系,在空间上需要将三水盆地与南海洋盆之间陆坡-陆架的岩浆活动一并进行讨论,建立三水盆地-珠/琼盆地-南海洋盆体系时空格架,探讨其岩浆活动源区演化规律。前文述及,三水盆地从新生代开始出现火山活动,时间从早古新世一直持续至始新世中期。在古新世―早始新世,火山活动以基性岩浆喷发为主并在约58 Ma达到高峰;整个始新世期间火山活动以中酸性岩浆的喷发为主,分别在约50和46 Ma出现了粗面质岩浆喷发的高峰期,伴随着中酸性岩浆的喷发,基性岩也有少量出露,但规模相对古新世已减小很多;随着约42 Ma一期粗面岩的喷发,整个盆地主体的岩浆活动宣告完结(图9图10)。晚中生代三水盆地处于挤压环境中,此时盆地为一网格状的山间盆地,晚白垩世开始盆地沉积范围逐渐扩大,沉积中心逐渐连通,在经历了古新世期间裂陷-热沉降阶段后,盆地伸展作用结束进入萎缩充填阶段[40]。珠江口盆地有24口井、琼东南盆地有1口井钻遇了新生代火山岩,喷发时间51.7~17.6 Ma均有分布[41]。从岩性及喷发时间来看,古新世期间喷发的岩性以中性的安山岩、安山质凝灰岩、粗面岩为主,伴有少量的基性岩喷发;在始新世期间酸性、中性、基性火山岩均有发育,岩性见流纹质凝灰(熔)岩、流纹质/英安质凝灰岩、英安斑岩、安山质凝灰熔岩和玄武岩;渐新世起珠江口盆地内岩浆活动逐渐以基性岩浆(玄武岩、玄武安山岩)的喷发占据优势;中新世成分单一的大规模玄武质岩浆开始喷发[42]。神狐运动(约54 Ma或更早)是珠江口盆地第一次大规模裂陷构造作用[43],伴随着神狐运动珠江口盆地进入裂陷阶段。晚渐新世珠江口盆地发生南海运动,盆地进入到裂后热沉降阶段,南海洋盆逐渐开始扩张并出现基性岩浆活动,此时三水盆地火山作用已停止了约10 Ma,盆地则停止发育了更长的时间。从空间分布来看,在依次经历了三水盆地张裂、珠/琼盆地的张裂后南海开始扩张,沉陷中心向北向南逐渐迁延(图10)。在经典的大西洋-红海扩张模式中,陆地上均有从扩张轴产生前的点状到其后大体平行扩张轴的线状火山分布体系,但在南海北缘,三水盆地与珠江口盆地南海扩张前的火山喷发仍属少数,没有形成规模。三水盆地岩浆喷发过程中,喷发带始终沿南北向展布,与南海扩张轴高角度相交。

      图  10  三水盆地演化模式及其与南海演化的关系

      Figure 10.  The evolution model of the Sanshui Basin and its relationship with the evolution of the South China Sea

      地幔热柱成因玄武岩高度富集Ta,其Ta/Hf比值一般大于0.3[44],洋岛玄武岩地幔源区ΔNb>0,N-MORB的ΔNb<0(ΔNb=1.74+log(Nb/Y)−1.92log(Zr/Y))[45]。此外分配性质相似或相反的元素之间的比值也可用于对岩浆源区的示踪,Zr/NbMORB=30,Zr/YMORB=2.5[46],而Zr/NbOIB=5.8,Zr/YOIB=9.7[28]。计算得到三水盆地玄武岩Ta/Hf为0.39~1.28,ΔNb=0.07~1.05,Zr/Nb=1.56~7.05,Zr/Y=2.79~9.82,结合玄武岩源区特征说明三水盆地岩浆活动起源于上涌地幔。邹和平报道了珠江口盆地渐新世年龄为45 Ma的石英拉斑玄武岩,其ΔNb=−0.06,Zr/Nb=10.85,Zr/Y=6.85,该样品Th/La=0.39,Ba/Nb=15.59(I型富集地幔Th/La=0.09~0.15,Ba/Nb=9.1~23.4)[47-48],珠江口盆地恩平组玄武安山岩(35.5~33.6 Ma)Ta/Hf为0.29~0.30,ΔNb=−0.16~−0.13,Zr/Nb值为9.05~10.1,Zr/Y值为9.4~9.9,上述两个样品源区同时有OIB及MORB源区组分的参与[49],其中OIB源区组分可能来源于引起三水盆地张裂的地幔上涌,但此时三水盆地已经停止发育,该地幔上涌带来的影响相较于三水盆地发育期已经开始变弱。IODP367航次U1500站位(位置见图1)钻遇的南海扩张初期MORB(约33 Ma)[30],其Ta/Hf=0.12~0.20,ΔNb为−0.05~0.11,Zr/Nb值为13.57~20.88,Zr/Y为2.89~3.52,表明此时岩浆活动的源区主要为亏损地幔[32]。结合前文讨论,三水盆地岩浆活动成因与OIB更为相近,而在南海扩张前珠江口盆地的岩浆活动的源区已成为OIB与MORB组分的混合源区[49],南海扩张伊始洋盆中的岩浆活动中没有明显OIB源区参与的特征,其源区与MORB源区更相近,因此,引起三水盆地张裂的地幔上涌没有持续在区域上产生影响。

      在对南海海盆的样品研究中,U1500 MORB样品的地幔潜能温度(Tp=1 380~1 450 ℃)明显低于地幔柱作用下的夏威夷玄武岩源区[30]图11)。台湾南部Kenting Mélange的样品(约25 Ma)具有亏损地幔的同位素组成,表明海南地幔柱在25 Ma之前对南海洋壳的形成还没有产生贡献,富集地幔软流圈上升流有可能在25 Ma之后到达海底[8]。在南海扩张期间珠江口盆地仍有岩浆活动,珠江组玄武岩 (23.7~21 Ma)Ta/Hf=0.49~0.57,ΔNb=0~0.08,Zr/Nb值为5.6~6.1,Zr/Y为9.6~10.8[49],与恩平组玄武岩不同,此时岩浆源区中又出现了OIB组分,暗示地幔柱活动(可能为海南地幔柱)在此时对珠江口盆地的源区产生了影响。IODP349航次的U1431站位(15°22.5371′N、117°00.0022′E)钻遇了南海扩张末期的MORB(16 Ma)样品,其结晶温度高于N-MORB,U1431站位玄武岩的地球化学和同位素特征表明其地幔源为含丰富EM2型组分的辉石岩。因此,在16 Ma时地幔柱活动对南海产生了强烈的影响[50]。南海扩张停止后(15 Ma至现在),南海海盆和海南地区继续形成大洋岛玄武岩,其地幔源表现为热异常[51]。以上资料表明,海南地幔柱至少在大约25 Ma后才进入上地幔。

      图  11  三水盆地玄武岩与OIB、MORB、玳瑁海山以及U1500样品源区对比

      Figure 11.  Comparison of basalt source in Sanshui Basin with OIB, MORB, Daimao seamount and U1500 samples.

      对岩浆作用源区的温压计算结果也同样指示了引起三水盆地的地幔上涌没有持续产生影响。如图11所示,Yu等[30]分别利用基于SiO2的地质温压计、PREMELT3.0以及橄榄石-熔体平衡温压计对U1500 MORB样品进行计算,结果显示南海扩张初期岩浆源区温压区间约为1 380~1 450 ℃,1.2~1.8 GPa,估算其最深深度约为65~70 km;玳瑁海山为南海停止扩张时(约15 Ma)海盆内玄武质岩浆的活动产物,杨蜀颖等利用MELT对其源区进行了模拟[20],结果显示南海停止扩张时岩浆活动的源区温压区间约为1 350~1 400 ℃、1~1.5 GPa,深度约为50 km。本文利用熔融柱模型再次对上述样品进行源区反演,图11中不同颜色箭头表示不同样品的源区熔融区间,可以看出U1500 MORB源区与前人的计算结果较为一致,其部分熔融作用起始温度为1 370 ℃,压力为1.8 GPa,深度为63 km,部分熔融作用停止温度为1 316 ℃,压力为0.98 GPa,深度为34 km。利用熔融柱模型计算得出的玳瑁海山玄武岩源区部分熔融起始温度为1 303 ℃,压力为1.38 GPa,深度为47 km,部分熔融作用停止温度为1 237℃,压力为0.28 GPa,深度为11 km,与前人计算结果相比深度较浅、温度较低。整体而言,不论利用何种方式进行计算,在南海扩张的过程中岩浆活动源区整体较浅且呈现逐渐变浅的趋势。三水盆地玄武质岩浆的源区温度为1 555~1 471℃,压力为3.32~2.25 GPa,源区为石榴子石地幔-尖晶石地幔过渡区。在源区P-T关系图中,三水盆地源区部分熔融的路径落在了夏威夷OIB源区的中部,明显深于东太平洋和中大西洋MORB源区(图11)。

      结合前文对三水盆地-珠江口盆地-南海海盆岩浆活动的时空分布以及源区特征分析,认为三水盆地在华南陆缘盆地中距离南海扩张轴最近,在南海扩张前呈现Afar型大规模火山喷发,展现出主动裂谷特征,但该裂谷活动与其后发生的南海扩张两者之间并无直接关联,即尽管三水代表陆内裂谷(威尔逊旋回第1阶段),南海扩张形成新的海盆(威尔逊旋回第2、3阶段),两者之间在时空上非常接近,但是在生成演化机制上并无直接的因果联系。

    • (1) 三水盆地玄武岩可以分为亚碱性和碱性玄武岩系列,两者均显示出明显的Nb、Ta正异常,相对于大陆地壳具有较低的Th/Sc、La/Nb和U/Al×1000,陆壳混染的程度低,岩浆在演化过程中主要经历了橄榄石的分离结晶作用。

      (2) 三水盆地亚碱性岩浆源区起止熔融温压分别为1 517 ℃(3.03 GPa)和1 471 ℃(2.25 GPa),深度为101~76 km,碱性岩浆源区起止熔融温压分别为1 555 ℃(3.33 GPa)和1 506 ℃(2.48 GPa),深度为110~84 km,源区整体为石榴石-尖晶石橄榄岩。

      (3) 三水盆地玄武质岩浆从早古新世起开始活动,主要活跃时段为61~54 Ma,盆地亚碱性玄武岩开始活动的时间早于碱性玄武岩,两者间歇持续喷发至中始新世。

      (4) 南海开裂初期(约33 Ma)岩浆源区温压区间约为1 380~1 450 ℃、1.2~1.8 GPa,南海停止开裂时(约15 Ma)岩浆活动的源区温压区间约为1 350~1 400 ℃、1~1.5 GPa,与三水盆地相比,两者岩浆活动的时空分布不匹配,岩浆源区演化趋势不衔接。

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