Sedimentary record of environmental evolution since ca 2000 cal yr B P ago in Qehan Lake, Inner Mongolia
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摘要:
以内蒙古东部查干淖尔湖西湖深97cm的浅井剖面为对象, 根据取得的7个AMS14C测年数据建立其2070cal a BP以来的年代序列, 结合孢粉组合特征与粒度组成的综合分析, 重建该地区2070cal a BP以来气候环境变化过程。研究结果表明, 查干淖尔湖2070cal a BP以来气候环境变化具体可以分为3个阶段:2070~1150cal a BP, 孢粉总浓度较高, 以蒿属、藜科为主, 沉积物各粒级组分变幅较小且以粉砂为主, 水动力条件较弱, 湖水位较高, 气候温凉偏湿; 1150~825cal a BP, 孢粉总浓度显著降低, 耐旱的麻黄属达到最高值, 沉积物粗颗粒含量显著增加, 水动力条件增强, 湖水位降低, 气候冷干, 其中940~870cal a BP期间气候极端干旱; 825cal a BP以来, 总体温暖偏湿, 有冷干事件发生。太阳活动可能是导致查干淖尔湖过去2000a气候变化的主要驱动力。
Abstract:A 97cm long lacustrine section from Qehan Lake, situated in eastern Inner Mongolia, was used to reestablish the environ-ment and climate changes since the last 2070cal a BP. The chronological framework was built on seven AMS14C ages. On the basis of pollen-spore characteristics and grain size distribution, three environmental stages can be recognized. From 2070~1150cal a BP, this interval was featured by high total pollen concentration, with the dominance of Artemisia and Chenopodiaceae; lacustrine sediments were mainly composed of silt and each fraction presented quite low amplitudes of increases, indicating the weak hydrological condi-tions and high lake level; this period was characterized by a warm, cool and slightly humid climate. During the period of 1150~825cal a BP, the total pollen concentration significantly decreased and the pollen percentages of xerophyte, especially Ephedra, reached the maximum; the coarse particle content showed obviously increase, sugge sting strong hydrodynamic conditions and low lake level; so in this period, the climate was cold and dry, especially from 940cal a BP to 870cal a BP. Since 825cal a BP, the climate mainly tend-ed to become warm and slightly humid with the occurrence of cold and dry oscillations. Solar activities might have been the domi-nant force that drove the climate changes in Qehan Lake area since 2000 years ago.
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Keywords:
- 2000a /
- pollen /
- grain size /
- climate change /
- Qehan Lake /
- Inner Mongolia
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全球古地理重建方面的研究表明,罗迪尼亚大陆在新元古代(约825Ma)进入裂谷阶段,并在750~ 600Ma发生全面裂解[1]。在这个过程中,古南美、澳大利亚、印度、阿拉伯、南极洲大陆等从罗迪尼亚大陆分离出来。在新元古代末―寒武纪初,泛非造山带将这些来自罗迪尼亚大陆的陆块拼接组成冈瓦纳超大陆[2-11]。位于东冈瓦纳大陆的泛非造山带主要有东非造山带、达马拉造山带、平贾拉造山带等[11]。在喜马拉雅地体中,寒武纪—奥陶纪花岗岩、不整合和变质作用被广泛报道[8, 12-18]。有学者认为其是泛非造山运动的产物[19-21],而更多学者认为喜马拉雅这些早古生代地质记录与冈瓦纳大陆周缘增生造山过程相关[11, 18, 22-23]。Zhu等[22]指出,拉萨地体中发育的寒武系—奥陶系角度不整合[24]及早古生代岩浆活动[25, 22]也是冈瓦纳大陆周缘增生造山的产物。然而,前人对青藏高原早古生代造山事件的研究主要集中在喜马拉雅、拉萨等青藏高原南部地区,在青藏高原东南缘的研究并不多。
在青藏高原东南缘,地质方面的讨论主要集中在古特提斯及新生代印度亚洲碰撞之后的相关问题,而对早古生代构造事件的研究则较薄弱,尤其是对岩浆岩方面。一些学者发现,腾冲地体和保山地体中出露许多早古生代花岗岩,并对其岩石学、地球化学特征、原岩类型和构造环境进行了探讨,认为这些花岗岩为高钾钙碱性过铝质花岗岩,主要源于古老地壳物质的重熔,并不同程度地混入了幔源物质[26-32],为理解青藏高原东南缘早古生代构造演化提供了重要资料。然而,人们对早古生代基性岩浆岩的研究较欠缺,对此地区早古生代岩浆构造事件的理解不够全面和深刻。本文在详细的野外观察基础上,结合岩石学、地球化学、锆石U-Pb年代学及Sm-Nd同位素研究,对保山地体寒武纪基性岩浆岩的时限及岩浆起源进行阐述,为探讨青藏高原早古生代演化提供了科学证据。
1. 地质背景
青藏高原东南缘滇西地区处于印度板块、扬子地块及印支地块的拼接地带,以发育强烈的走滑构造、岩浆活动及丰富多样的矿产资源为特征。3条主要缝合带从西向东依次为高黎贡缝合带、昌宁-孟连缝合带和哀牢山-松马缝合带[33-37],将滇西地区依次划分为腾冲地体、保山地体及思茅地体[38-39]。研究区位于滇西邦迈地区,大地构造上属于保山地体(图 1-a)。多数学者认为,保山地体是滇缅马地体的向北延伸[39-41],但也有学者认为保山地体和滇缅马地体为2个不同单元[40, 42]。保山地体的早古生代地层主要由粉砂岩、砂岩、泥岩、板岩、千枚岩夹少量变质火山岩和硅质岩组成,称为蒲满哨群(保山地体北部)或公养河群(保山地体南部)。蒲满哨群中变火山岩类型主要为斜长角闪岩、绿帘斜长角闪岩、黑云斜长角闪岩等①。镇安-平达早古生代花岗岩体[28-29]侵入新元古代—早古生代蒲满哨群或公养河群中,上覆寒武系—中生界碎屑岩、碳酸盐岩、玄武岩等[43]。本文对保山—潞西地区开展了野外工作,采集了保山镇安县邦迈地区蒲满哨群中一系列早古生代玄武岩样品进行岩石学、地球化学和UPb同位素年代学及Sm-Nd同位素示踪工作。
2. 样品描述与测试方法
12个样品YC2411~YC2416和YC3001~YC3006为变质基性火山岩,采自保山地区镇安县邦迈村附近(图 1-b)。样品较新鲜,颜色呈深灰色、灰绿色,块状构造,变余斑状结构,具有弱面理(图 2-a、b)。第一组变质基性岩(6个样品YC2411~YC2416)以YC2411为代表,主要由斜长石(25%~35%)和角闪石(50%~60%)组成,含少量铁氧化物(5%~8%)、磷灰石等(2%)(图 2-c),岩石名称为斜长角闪岩。第二组(6个样品YC3001~YC3006)以YC3001为代表,矿物组成主要为斜长石(30%~40%)、角闪石(40%~ 50%)及黑云母(10%~15%),含少量铁氧化物(5%)、绿帘石(2%)、榍石(1%)等(图 2-d),岩石名称为黑云斜长角闪岩。
选择样品YC2411和YC3001进行LA-ICPMS锆石U-Pb测年实验。将样品粉碎,采用重力和磁选方法分选出锆石,然后在显微镜下挑选出较完整、典型的锆石颗粒,将其粘置于环氧树脂中制靶、并打磨抛光,使锆石内部充分暴露。然后对锆石样品进行透射光、反射光及阴极发光照相,此过程在中国地质科学院地质研究所大陆动力学实验室完成。LA-ICP-MS锆石U-Pb定年测试分析在中国地质科学院矿产资源研究所LA-ICP-MS实验室完成。锆石U-Pb测年分析所用仪器为Finnigan Neptune型MC-ICP-MS及与之配套的Newwave UP 213激光剥蚀系统。激光剥蚀斑束直径为25μm,频率为10Hz,能量密度约为2.5J/cm2,以氦气为载气。信号较小的207Pb、206Pb、204Pb(+204Hg)和202Hg用离子计数器(multi-ion-counters)接收,208Pb、232Th和238U信号用法拉第杯接收,实现所有目标同位素信号的同时接收且不同质量数的峰基本上都是平坦的,进而获得高精度的数据。详细测试过程参考侯可军等[44]。数据处理采用ICPMSDataCal程序[45]。锆石206Pb/238U年龄加权平均值误差为2σ,测试结果如表 1所示。
表 1 保山邦迈变质基性岩LA-ICP-MS锆石U-Th-Pb测年分析结果Table 1. LA-ICP-MS zircon U-Th-Pb dating results of the metabasites of Bangmai area, Baoshan样品 U/10-6 Th/10-6 Th/U 207Pb/235U 206Pb/238U 207Pb/235U 206Pb/238U 比值 误差 比值 误差 年龄/Ma 误差/Ma 年龄/Ma 误差/Ma 样品YC241 测点01 293 324 1.10 0.7153 0.0129 0.08696 0.00112 547.9 7.7 537.6 6.6 测点02 1561 659 0.42 0.6931 0.0067 0.08706 0.00072 534.7 4.0 538.1 4.3 测点03 812 227 0.28 0.6901 0.0089 0.08751 0.00089 532.9 5.3 540.8 5.2 测点04 1537 648 0.42 0.6948 0.0061 0.08746 0.00067 535.7 3.6 540.5 4.0 测点05 804 217 0.27 0.7001 0.0068 0.08684 0.00071 538.8 4.1 536.8 4.2 测点06 834 226 0.27 0.7153 0.0063 0.08707 0.00062 547.9 3.7 538.2 3.7 测点07 819 222 0.27 0.7096 0.0066 0.08734 0.00067 544.5 3.9 539.8 4.0 测点08 1768 829 0.47 0.6998 0.0069 0.08637 0.00086 538.7 4.1 534.0 5.1 测点09 1176 489 0.42 0.6893 0.0093 0.08686 0.00131 532.4 5.6 536.9 7.8 测点10 1465 618 0.42 0.6865 0.0067 0.08639 0.00072 530.7 4.0 534.1 4.3 测点11 751 202 0.27 0.7174 0.0085 0.08708 0.00072 549.1 5.0 538.2 4.3 测点12 509 260 0.51 0.6974 0.0077 0.08658 0.00067 537.2 4.6 535.3 4.0 测点13 628 558 0.89 0.6855 0.0096 0.08647 0.00095 530.1 5.8 534.6 5.7 测点14 605 479 0.79 0.6744 0.0080 0.08624 0.00087 523.4 4.9 533.3 5.2 测点15 645 508 0.79 0.6811 0.0075 0.08654 0.00076 527.4 4.5 535.1 4.5 测点16 477 246 0.52 0.7030 0.0089 0.0863 0.0007 540.6 5.3 533.6 4.2 测点17 675 600 0.89 0.6927 0.0103 0.08627 0.00101 534.4 6.2 533.4 6.0 测点18 851 381 0.45 1.7693 0.0122 0.1704 0.0009 1034.3 4.5 1014.1 4.9 测点19 336 28 0.08 0.9493 0.0068 0.1108 0.0006 677.7 3.5 677.6 3.3 测点20 352 60 0.17 1.5980 0.0128 0.1621 0.001 969.4 5.0 968.3 5.6 样品YC3001 测点01 1106 926 0.84 0.6808 0.0129 0.08541 0.00099 527.0 8.0 528.0 6.0 测点02 1012 879 0.87 0.6864 0.0125 0.0854 0.00048 531.0 7.0 528.0 3.0 测点03 429 133 0.31 0.6914 0.0132 0.08608 0.00173 533.6 7.9 532.3 10.2 测点04 1774 834 0.47 0.6894 0.0064 0.08617 0.00065 532.4 3.8 532.9 3.8 测点05 1701 806 0.47 0.6894 0.0086 0.08606 0.00089 532.4 5.2 532.2 5.3 测点06 158 166 1.05 0.6873 0.0135 0.08556 0.00092 531.2 8.1 529.2 5.5 测点07 1465 618 0.42 0.6865 0.0067 0.08639 0.00072 530.7 4.0 534.1 4.3 测点08 1768 829 0.47 0.6998 0.0069 0.08637 0.00086 538.7 4.1 534.0 5.1 测点09 1861 1453 0.78 0.6893 0.0124 0.08648 0.00101 532.0 7.0 535.0 6.0 测点10 915 782 0.85 0.6807 0.0139 0.08649 0.00103 527.0 8.0 535.0 6.0 测点11 1861 877 0.47 0.6967 0.0090 0.08667 0.00086 536.8 5.4 535.8 5.1 测点12 1176 489 0.42 0.6893 0.0093 0.08686 0.00131 532.4 5.6 536.9 7.8 测点13 151 72 0.48 1.3325 0.0108 0.1407 0.0008 860.0 4.7 848.8 4.3 测点14 217 95 0.44 1.4870 0.0122 0.1520 0.0007 925.1 5.0 912.1 4.0 测点15 908 94 0.10 1.5191 0.0086 0.1563 0.0006 938.1 3.5 936.0 3.4 测点16 90 70 0.78 1.5196 0.0152 0.1564 0.0009 938.3 6.1 936.8 4.9 测点17 847 397 0.47 1.8812 0.0123 0.1775 0.0009 1074.5 4.3 1053.3 4.9 YC2411~YC2416和YC3001~YC3006共12件样品(采样位置见图 1-b),样品新鲜且具有代表性。将这些样品破碎研磨至200目,进行主量和微量元素分析。主量元素采用X射线荧光光谱仪方法(XRF)分析,分析误差小于0.5%。微量元素采用等离子质谱仪(ICP-MS)(PE300D)分析,相对标准偏差小于5%。测试过程在国家地质实验测试中心完成。测试结果见表 2。
表 2 保山邦迈地区变质基性岩的主量、微量和稀土元素测试结果Table 2. Major, trace and rare earth element concentrations of the metabasites in Bangmai area, Baoshan样品号 YC2411 YC2412 YC2413 YC2414 YC2415 YC2416 YC3001 YC3002 YC3003 YC3004 YC3005 YC3006 SiO2 47.96 48.18 47.90 48.40 48.80 49.23 47.94 48.25 49.06 49.14 48.04 49.03 TiO2 1.71 1.88 1.94 1.86 1.87 1.83 3.03 2.73 2.66 2.57 3.12 2.81 Al2O3 13.81 14.03 13.85 13.82 13.80 13.73 12.94 14.23 13.04 13.08 12.49 12.51 Fe2O3 2.22 2.15 1.86 2.56 2.21 2.79 2.98 3.90 4.24 4.13 2.68 2.21 FeO 13.65 12.84 13.44 12.56 12.74 12.20 11.97 11.93 11.99 11.67 12.78 12.33 MnO 0.45 0.34 0.35 0.35 0.35 0.34 0.55 0.47 0.51 0.54 0.42 0.57 MgO 6.81 6.82 6.82 6.83 6.79 6.62 5.97 4.28 4.55 4.48 5.78 4.51 CaO 11.48 11.50 11.04 11.35 11.67 11.14 9.93 8.95 8.96 9.12 11.05 10.92 Na2O 0.54 0.56 0.54 0.59 0.54 0.50 2.13 2.54 2.24 2.29 1.07 2.24 K2O 0.19 0.32 0.38 0.34 0.21 0.18 0.94 0.51 0.74 0.59 0.87 0.84 P2O5 0.15 0.18 0.18 0.17 0.17 0.16 0.52 0.51 0.58 0.63 0.58 0.42 烧失量 1.08 1.27 1.61 1.26 0.92 1.39 1.22 1.07 1.06 1.58 1.03 1.37 总量 99.90 99.91 99.76 99.90 99.90 99.90 100.12 99.37 99.63 99.82 99.91 99.76 Mg# 45.56 47.01 46.57 46.76 46.95 46.16 43.63 34.27 35.05 35.31 42.08 37.67 La 10.10 10.60 13.30 11.50 10.20 10.80 30.82 40.01 18.72 24.15 42.7 40.71 Ce 22.30 23.00 28.60 24.40 22.20 21.50 62.68 80.78 44.32 49.51 94.86 75.15 Pr 3.36 3.46 4.27 3.62 3.27 3.16 7.72 11.15 6.11 6.89 11.49 10.18 Nd 16.80 17.10 21.10 17.80 16.20 15.90 27.21 46.42 20.85 24.68 42.87 34.09 Sm 4.89 4.80 5.95 4.95 4.51 4.46 8.01 10.57 6.14 7.04 9.37 8.85 Eu 1.80 1.88 1.84 1.80 1.71 1.65 2.29 3.61 1.91 2.06 3.26 3.47 Gd 4.68 4.63 5.57 4.64 4.35 4.28 9.17 11.14 6.45 8.16 8.55 10.11 Tb 0.98 0.95 1.14 0.95 0.89 0.93 1.29 1.87 1.08 1.21 1.42 1.85 Dy 6.88 6.56 7.68 6.54 6.24 5.86 8.73 12.36 5.96 7.56 7.89 10.46 Ho 1.30 1.25 1.46 1.24 1.18 1.04 1.45 2.06 1.07 1.26 1.34 1.73 Er 3.77 3.65 4.20 3.61 3.47 3.05 3.44 4.56 2.55 2.74 2.85 3.64 Tm 0.58 0.56 0.64 0.56 0.53 0.47 0.45 0.58 0.3 0.37 0.35 0.47 Yb 3.70 3.56 4.01 3.53 3.39 3.01 2.58 3.32 1.71 1.67 1.89 2.92 Lu 0.50 0.49 0.54 0.47 0.45 0.41 0.43 0.42 0.23 0.25 0.27 0.35 Sr 121.00 137.00 133.00 132.00 125.00 98.60 559.54 307.09 500.58 246.31 532.92 221.18 K 1586 2665 3146 2814 1718 1478 7803 4234 6143 4898 7222 6973 Rb 6.65 10.60 10.10 12.10 8.57 7.88 25.64 27.08 34.15 35.12 29.46 34.54 Ba 77.80 65.10 86.00 82.30 79.80 86.30 140.42 365.01 214.03 108.42 292.25 105.94 Th 0.91 1.04 1.11 1.08 0.96 0.79 4.64 5.02 5.78 4.65 4.25 5.46 Ta 0.53 0.56 0.61 0.52 0.52 0.45 2.55 1.98 2.43 2.27 2.63 2.45 Nb 8.30 8.57 9.99 8.23 8.24 7.85 38.74 33.03 36.19 34.13 37.76 36.18 Ce 22.30 23.00 28.60 24.40 22.20 21.50 62.68 80.78 44.32 49.51 94.86 75.15 P 655 764 781 746 729 698 2270 2226 2531 2750 2531 1833 Zr 122.50 116.30 118.40 117.00 96.60 117.80 210.64 225.34 205.32 192.07 186.85 237.32 Hf 3.13 3.13 3.22 3.12 2.60 2.94 4.36 6.17 3.91 3.75 5.07 4.95 Sm 4.89 4.80 5.95 4.95 4.51 4.46 8.01 10.57 6.14 7.04 9.37 8.85 Ti 10251 11271 11630 11151 11211 10971 18143 16378 15923 15389 18686 16846 Y 33.80 32.20 38.00 31.60 30.20 30.40 22.09 29.21 21.13 20.13 22.58 22.12 Yb 3.70 3.56 4.01 3.53 3.39 3.01 2.58 3.32 1.71 1.67 1.89 2.92 Sc 50.90 48.00 56.10 48.50 46.10 42.80 20.68 35.42 14.31 24.01 17.37 22.23 Cr 153.00 73.40 69.20 75.70 65.70 75.30 122.96 132.75 98.52 141.19 125.48 125.12 Nb/Yb 2.24 2.41 2.49 2.33 2.43 2.61 15.02 9.95 21.16 20.44 19.98 12.39 Th/Yb 0.25 0.29 0.28 0.31 0.28 0.26 1.80 1.51 3.38 2.78 2.25 1.87 Nb/Y 0.25 0.27 0.26 0.26 0.27 0.26 1.75 1.13 1.71 1.70 1.67 1.64 Zr/Ti2O 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 Nb/La 0.82 0.81 0.75 0.72 0.81 0.73 1.26 0.83 1.93 1.41 0.88 0.89 (Th/Ta)PM 0.83 0.90 0.87 0.99 0.90 0.85 0.88 1.22 1.15 0.99 0.78 1.07 (Th/Nb)PM 0.92 1.02 0.93 1.10 0.98 0.84 1.01 1.27 1.34 1.14 0.94 1.27 注:主量元素含量单位为%,微量和稀土元素含量为10-6 选择YC2411~YC2415和YC3001~YC3005共10件样品进行Nd同位素分析。分析过程在南京内生金属矿床成矿机制研究国家重点实验室完成。采用HIBA作为淋洗剂在0.6mL的阳离子交换树脂中分离Sm和Nd。同位素分析采用Finnigan MAT公司的TRITON TI表面热电离质谱仪,元素分离条件实验时采用Finnigan MAT公司的Element2(ICP-MS)。Nd同位素以143Nd/144Nd=0.7219标准化,测得143Nd/144Nd=0.511676±8。具体实验细节参考濮巍等[46]。测试结果如表 3所示。
表 3 保山邦迈变质基性岩的Sm-Nd同位素分析结果Table 3. Sm-Nd isotope compositions of zircon in the metabasites of Bangmai area, Baoshan样品 年龄/Ma Sm Nd 147Sm/144Nd 143Nd/144Nd 2σ Sm/Nd INd εNd(0) εNd(t) 2σ TDM/Ma TDMC/Ma YC2411 536.7 4.89 16.8 0.175867 0.513003 9 0.291 0.5123846 7.12 8.56 0.18 599 561 YC2412 536.7 4.8 17.1 0.169601 0.512951 5 0.281 0.5123547 6.11 7.98 0.10 693 609 YC2413 536.7 5.95 21.1 0.170380 0.512968 7 0.282 0.5123689 6.44 8.25 0.14 646 586 YC2414 536.7 4.95 17.8 0.168023 0.512956 8 0.278 0.5123652 6.20 8.18 0.16 653 592 YC2415 536.7 4.51 16.2 0.168208 0.512957 6 0.278 0.5123656 6.22 8.19 0.12 652 592 YC3001 532.0 8.01 27.2 0.177864 0.512697 17 0.29 0.512077 1.15 2.43 0.33 1930 1056 YC3002 532.0 10.57 46.4 0.137580 0.512625 12 0.23 0.512145 -0.25 3.77 0.23 1054 948 YC3003 532.0 6.14 20.9 0.177929 0.512692 11 0.29 0.512072 1.05 2.33 0.21 1955 1064 YC3004 532.0 7.04 24.7 0.172350 0.512686 22 0.29 0.512085 0.94 2.60 0.43 1714 1043 YC3005 532.0 9.37 42.9 0.132060 0.512601 10 0.22 0.512141 -0.72 3.68 0.20 1028 955 注:TDM为一阶段年龄;TDMC为二阶段年龄 3. 测试结果
3.1 LA-ICP-MS锆石U-Pb测年结果
保山邦迈村斜长角闪岩样品YC2411中的锆石呈短柱状或板状,个别为长柱状,半透明、深黄色-深褐色。粒度为80~200μm。阴极发光图像显示,锆石晶形较完整,有的呈略微磨圆状,大多数具有结晶环带,显示典型的岩浆成因(图 3-a)。选择结晶环带清晰的锆石进行测年,测试数据列于表 1。结果显示,17个测点的U含量为293×10-6~1768×10-6;Th含量为202×10-6~892×10-6,Th/U值为0.27~1.1,进一步说明其为岩浆成因。17个锆石分析点位于谐和线附近,分布较集中,获得206Pb/238U年龄加权平均值为536.7±2.2Ma(MSWD=0.3)(图 3-b)。
样品YC3001中的锆石晶形较好,呈短柱状或板状,颜色为浅褐色-深褐色,半透明。长度为100~ 200μm,长宽比为1.5:1~2:1。阴极发光图像显示,大多数锆石具有弱韵律环带,表现出岩浆锆石的特征(图 3-a)。选择锆石环带部位进行U-Pb测试,结果见表 1。锆石U含量为158×10-6~1865×10-6,Th含量为133×10-6~1453×10-6。所有锆石测点Th/U值大于0.1。12个锆石分析点位于谐和线附近,206Pb/238U年龄加权平均值为532.0±2.8Ma(MSWD= 0.4)(图 3-c)。YC3001年龄值与YC2411相近,说明二者的形成时代相近,可能是同一期构造事件的产物。2个样品中部分锆石的206Pb/238U年龄较大且位于谐和线附近,代表岩浆形成过程中捕获锆石的年龄(表 1)。
3.2 地球化学特征
对采自云南保山邦迈地区的12件样品进行主量和微量元素分析,结果列于表 2。12个样品烧失量较小(0.92~1.61),说明其较新鲜。斜长角闪岩样品(YC2411~YC2416)的SiO2含量为47.90% ~ 49.23%,黑云斜长角闪岩样品YC3001~YC3006的SiO2含量为47.94%~49.14%,说明这些样品皆属于基性岩系列。斜长角闪岩样品YC2411~YC2416的MgO含量为6.62%~6.83%(平均值为6.78%),与EMORB(富集型大洋中脊玄武岩)或OIB(洋岛玄武岩)型玄武岩类似,明显低于N-MORB的9.67%[47]。Mg#值为46~47。TiO2含量为1.71%~1.94%(平均值为1.85),接近洋中脊玄武岩TiO2含量(1.5%),明显高于岛弧和活动大陆边缘玄武岩(小于1.25%)[48]。Al2O3含量为12.49%~14.23%(平均值为13.05%),与大西洋、太平洋及印度洋MORB(大洋中脊玄武岩)的Al2O3含量相近(分别为15.6%, 14.86%和15.15%)[49]。黑云斜长角闪岩样品YC3001~YC3006的MgO含量为4.28%~5.97%(平均值为4.93%),与OIB型玄武岩类似,明显低于N-MORB的9.67%[47]。Mg#值(34~44)较低,说明岩浆来源于地幔,且经历了明显的橄榄石和辉石分离结晶过程。TiO2含量为2.57% ~3.12%(平均值2.82%),与典型OIB相近(2.87%),K2O含量为1.07%~ 2.54%(平均值为2.09%),略高于典型OIB,P2O5含量为0.42%~0.63%(平均值为0.54%),Al2O3含量平均值为13.05,Al2O3和SiO2之间没有明显的负相关性,说明岩浆没有经历斜长石分离结晶过程。
保山邦迈斜长角闪岩(YC2411~YC2416)和黑云斜长角闪岩(YC3001~YC3006)的稀土及微量元素模式皆有明显差异,说明其具有不同的岩浆起源。YC2411~YC2416的轻、重稀土元素分异不明显,稀土元素配分曲线总体较平坦,(La/Yb)N为40.26~287.59。没有明显的Eu异常,δEu=0.52~ 0.81,说明岩浆斜长石分离结晶作用不明显。参考典型N-MORB、E-MORB及OIB稀土元素含量特征[50],样品YC2411~YC2416与E-MORB稀土元素配分模式较相似(图 4-a),原始地幔标准化微量元素配分曲线较平坦,也较接近E-MORB微量元素配分模式,而有别于N-MORB(图 4-b)。不同于斜长角闪岩(YC2411~YC2416),黑云斜长角闪岩样品YC3001~YC3006的稀土元素配分曲线显示明显富集LREE(轻稀土元素),整体呈向右倾模式(图 4-a),无明显Eu异常,δEu=0.52~0.81。原始地幔标准化微量元素配分曲线整体呈隆起特征,富集不相容元素Rb、Ba、Th、Nb、Ta等。这种稀土和微量元素配分模式与OIB相似(图 4-b)。黑云斜长角闪岩样品YC3001~YC3006的部分样品相对于OIB微量元素配分曲线较亏损Sr、K及Ba,可能与样品变质、海水蚀变等因素有关。
图 4 变质基性岩的稀土元素配分曲线(a)和微量元素蛛网图(b)(原始地幔和球粒陨石标准值据参考文献[50])E-MORB—富集型大洋中脊玄武岩;N-MORB—正常大洋中脊玄武岩;OIB—洋岛玄武岩Figure 4. REE abundance patterns(a) and trace element spider diagram (b) of the metabasites3.3 Sm-Nd同位素组成特征
保山邦迈10个变质基性岩样品的Sm-Nd同位素测试结果列于表 3。斜长角闪岩(YC2411~YC2415)的143Nd/144Nd值为0.512651~0.512925,INd值为0.512951~0.513003,εNd(0) 值为0.25~5.60,εNd(t)值为6.11~7.12(t=536Ma),略有变化,亏损程度不明显,表明这些变玄武岩岩浆来源于地幔,可能受到较小程度的壳源物质的混染。黑云斜长角闪岩(YC3001~YC3006)的143Nd/144Nd值为0.512601~ 0.512697,εNd(0) 值为0.25~5.60,εNd(t)值为6.11~7.12(t=536Ma),INd为0.512192~0.512307,εNd(t)值为2.33~3.77(t=532Ma),岩石的Nd同位素特征说明其来自地幔,但受到地壳物质的混染。
4. 讨论
4.1 原岩恢复及构造环境
研究表明,在温度、压力或流体环境改变的情况下,低场强元素(LFSE)和大离子亲石元素(LI⁃ LE)较容易发生迁移,而高场强元素(HFSE)和重稀土元素(HREE)则较稳定,不容易发生迁移[51]。对于发生热液蚀变和变质作用的基性岩,高场强元素及重稀土元素,如Nb、Ti、Th、Ta、Zr、Yb等是判断原岩构造环境的有效因子[50, 52-53]。因此,在判断保山邦迈地区变质基性火山岩原岩类别时,应用Nb/Y-Zr/TiO2图解[53]。6个斜长角闪岩样品(YC2411~YC2416)落在安山岩/玄武岩区域内及与亚碱性玄武岩区域边界处,而黑云斜长角闪岩样品YC3001~YC3006落于碱性玄武岩区域内(图 5)。即斜长角闪岩样品原岩为玄武安山岩,而黑云斜长角闪岩样品原岩为碱性玄武岩。
图 5 变质基性岩的Nb/Y-Zr/TiO2图解[53]Figure 5. Nb/Y-Zr/TiO2 diagram for the metabasites另外,采用微量元素判别图解恢复变基性岩原岩特征。在Nb-Zr-Y、Ti-Zr-Y、Hf-Th-Ta及Zr-Ti图解(图 6)中,斜长角闪岩的6个样品(YC2411~YC2416)皆落在富集型洋中脊玄武岩区域内,而黑云斜长角闪岩样品YC3001~YC3006则大多落于板内玄武岩、板内碱性玄武岩或板内拉斑玄武岩区域内。因此认为,保山邦迈地区的斜长角闪岩原岩为富集型洋中脊玄武岩,黑云斜长角闪岩原岩为板内玄武岩。在Nb/Yb-Th/Yb图解中,斜长角闪岩(YC2411~YC2416)落在E-MORB附近,黑云斜长角闪岩样品YC3001~YC3006落于OIB附近(图 6-e)。采用Agrawal等[58]的ln(La/Th)、ln(Sm/Th)、ln(Yb/Th)及ln(Nb/Th)参数组合进行构造环境投图,可见大多数斜长角闪岩(YC2411~YC2416)落在E-MORB区域,YC3001~YC3006全部落入OIB区域(图 6-f、g)。这与稀土和微量元素配分模式指示的2组样品分别为E-MORB和OIB特征玄武岩一致(图 4)。
图 6 变质基性岩构造环境判别图解a—Nb-Zr-Y图解[54];b—Ti-Zr-Y图解[52];c—Hf-Th-Ta图解[55];d—Zr-Ti图解[56];e—Nb/Yb-Th/Yb图解[57];f、g—不活动元素对数图解[58]。CAB—陆缘弧玄武岩;IAB—岛弧玄武岩;IAT—岛弧拉斑玄武岩;MORB—洋中脊玄武岩;VAB—火山岛弧玄武岩;WPA—板内碱性玄武岩;WPB—板内玄武岩;WPT—板内拉斑玄武岩。f图中的DF1=0.3518ln(La/Th)+0.6013ln(Sm/Th)-1.3450ln(Yb/Th)+2.1056ln(Nb/Th)-5.4763,DF2=-0.3050ln(La/Th)-1.1801ln(Sm/Th)+1.6189ln(Yb/Th)+1.2260ln(Nb/Th)-0.9944;g图中的DF1=-0.5558ln(La/Th)-1.4260ln(Sm/Th)+2.2935ln(Yb/Th)-0.6890ln(Nb/Th)+4.1422,DF2=-0.9207ln(La/Th)+3.6520ln(Sm/Th)-1.9866ln(Yb/Th)+1.0574ln(Nb/Th)-4.4283Figure 6. Tectonic discrimination diagrams of the metabasites4.2 岩石成因与岩浆起源
目前对E-MORB和OIB玄武岩成因的探讨主要依据现代洋盆的研究。在现代洋盆中,相对N-MORB而言,E-MORB较少见,而且主要出现在部分大洋中脊或洋脊附近海山地区。另外,现代弧后盆地也有少量具有E-MORB和OIB特征的玄武岩[59-60]。对产于洋中脊附近的E-MORB的成因,通常认为是由亏损的软流圈地幔受富集组分(如地幔柱、富集地幔包体、洋壳俯冲进入上地幔的富集组分等)影响而形成[47, 61-63]。洋岛玄武岩型变基性岩的源区常为富集地幔柱——布丁型对流地幔[57]。保山邦迈OIB型变质基性岩样的(Th/Ta)PM值(平均值为1.02)、(La/Nb)PM(平均值为1.16)及εNd(t)(2.33~ 3.77)特征说明,其原岩玄武质熔浆受到一定程度的地壳物质混染。这种混染过程可能是通过古老洋壳、深海沉积物、古老大陆地壳等物质的加入而实现的[50]。而现代大洋弧后盆地出现的E-MORB和OIB则被认为是弧后盆地发展到后期阶段,周围相对富集的地幔上涌发生部分熔融而形成的[64]。云南保山邦迈地区的E-MORB和OIB所在的蒲满哨群由深灰色-灰色薄层细砂岩、千枚岩、粉砂质板岩,泥质板岩夹灰岩、泥质灰岩等组成,为半深海-斜坡环境,厚度大于1900m①,岩石变质程度不等,大多为浅变质,岩石种类复杂多样。杨学俊等[65]获得保山邦迈地区变质基性岩499Ma的U-Pb年龄,采样点位于本次所测样品(532~536Ma)的西侧,说明蒲满哨群形成时间跨度大。毛晓长等[66]认为,蒲满哨群中记录了同斜倒转冲断作用的叠瓦构造。以上特征表明,蒲满哨群很可能是洋壳俯冲而形成的增生楔。因此认为,保山邦迈地区具有富集型洋中脊玄武岩和洋岛玄武岩特征的变质基性岩,初始形成于大洋中脊或洋脊附近海山地区,并在俯冲过程中被带到增生楔中。另外,研究表明,蒲满哨群中普遍侵入472~502Ma的花岗岩[28-29, 31],与蒲满哨群沉积时代相近。如此大规模花岗岩的出现,在现代大洋弧后盆地中还没有对应的实例,因此也排除了保山邦迈地区具有E-MORB和OIB型变质基性岩形成于弧后盆地的可能性。
4.3 构造意义
研究表明,当洋中脊部分或全部位于上覆大陆板块之下,形成板片窗,下部不均一地幔经过板片窗上涌,与先前残余地幔发生地幔交代作用,使亏损型地幔转变为富集型地幔,从而形成E-MORB和OIB型岩浆[67-69]。笔者认为,保山邦迈地区的EMORB型和OIB型变质基性岩的成因最有可能与大陆边缘俯冲造山过程相关。大约530Ma,保山邦迈地区的E-MORB和OIB型岩浆在大洋中脊向大陆俯冲过程中喷发到弧前地带,形成弧前海山,并成为增生楔杂岩的重要组成部分。由于洋脊俯冲既提供了热源等条件,使俯冲带上盘地壳物质发生部分熔融而形成保山地区大面积花岗质岩石[59-60, 70-71]。另外,沉积学方面的证据显示,保山地体芒市东弯腰树剖面下奥陶统与蒲满哨群之间的角度不整合[32]、保山南部地区普遍缺失早奥陶世地层[72],也说明保山地体早古生代造山事件的存在。类似于保山地体,在青藏高原其他地区,如羌塘地体、拉萨地体、喜马拉雅地体等也普遍存在早古生代早期岩浆、变质及地层不整合记录[8, 13-15, 17-18, 22-25, 73-82]。青藏高原这些沉积、变质及岩浆记录被越来越多的地质工作者认为是在冈瓦纳大陆周缘增生造山作用的结果[11, 18, 22-23, 32]。新元古代晚期—寒武纪,随着东西冈瓦纳拼合的结束[3-9, 11, 83-84],冈瓦纳大陆北缘原始特提斯洋洋壳开始向南俯冲。位于冈瓦纳大陆北缘的保山、拉萨、羌塘等地体中形成火山弧和弧前增生楔,由于俯冲引起地壳熔融,生成大量早古生代花岗岩。然而,由于后期多期构造事件,尤其古近纪—新近纪印度-亚洲碰撞,使保山地区遭遇大规模构造变动、隆升和剥蚀过程,使得较深层次的早古生代花岗岩出露到地表,而大量较浅层次的岛弧火山岩已被剥蚀殆尽。
5. 结论
(1)保山邦迈地区蒲满哨群中的变质基性岩包括斜长角闪岩和黑云斜长角闪岩,岩浆形成时代为536.7~532.0Ma。
(2)斜长角闪岩与黑云斜长角闪岩的原岩分别为玄武安山岩和碱性玄武岩,主量及微量元素含量分别与E-MORB和OIB相似。
(3)在新元古代末—早古生代时期,保山地体处于冈瓦纳大陆北缘,随着东西冈瓦纳拼合的结束,冈瓦纳大陆北缘原始特提斯洋脊向南俯冲,形成保山地体的早古生代E-MORB、OIB型岩浆,并喷发到弧前增生楔中。
致谢: 孢粉鉴定由中国地质科学院水文地质环境地质研究所童国榜老师完成,在此表示衷心的感谢。 -
表 1 查干淖尔沉积物14C测年结果
Table 1 14C dating results of Qehan Lake sediments
样号 测试材料 深度
/cml4C年龄
/a BP去除碳库后
年龄/a BP日历年龄
(2σ中值cal a
BP)Cg-b09-0-1 全有机质 0~2 760±30 Cg-b09-27 全有机质 27 1630±30 876±30 782 Cg-b09-51 全有机质 51 1950±35 1196±35 1123 Cg-b09-75 全有机质 75 2115士25 1361±25 1291 Cg-b09-86 全有机质 86 2360士30 1606±30 1482 Cg-b09-93 全有机质 93 2665士25 1911±25 1858 Cg-b09-97 全有机质 97 2850士35 2096±35 2068 -
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