Geochemical and chronologic characteristics of ultramafic rocks in Gaxian ophiolitic mélange of the Da Hinggan Mountains and their geological significance
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摘要:
嘎仙蛇绿混杂岩位于新林-嘎仙-吉峰蛇绿混杂岩带中段。岩石主要由镁铁-超镁铁质岩石组成,呈构造岩块的形式与变质细碎屑岩基质混杂产出。对超镁铁质岩系统的岩石学、地球化学和年代学研究结果表明,超镁铁质岩石SiO2含量平均值为44.87%,Mg#平均值为87.06,锆石U-Pb同位素年龄为649.6±4.2 Ma,时代为新元古代。稀土元素配分曲线总体呈平坦型,具有富集型洋脊玄武岩(E-MORB)向洋岛玄武岩(OIB)过渡的趋势。样品亏损Nb、Ta、Ti元素,相对富集Th、U、Zr、Hf元素,具有岛弧岩浆岩的特征。在Nb/Yb-Th/Yb构造环境判别图解中,样品点落入E-MORB和OIB交汇部位。通过与区域构造带岩石组合和地球化学特征对比,认为嘎仙超镁铁岩岩浆源区可能为受俯冲流体交代的富集地幔,属俯冲型(SSZ)蛇绿岩,其就位机制与持续洋内俯冲有关。
Abstract:The Gaxian ophiolitic melange belt is located in the middle of Xinlin-Gaxian-Jifeng ophiolitic melange belt.This ophiolite mainly consists of ultramafic and mafic rocks, mixed with metamorphic fine clastic rock matrix in the tectonic block form.This paper focuses on the systematic petrology, geochemistry and chronology of ultramafic rocks.The result shows that, in the ultramafic rock, the average value of SiO2 is 44.87%, the average value of Mg# is 87.06, and the zircon U-Pb age is 649.6±4.2 Ma, suggesting Neo-proterozoic.The curve of REE is of flat type, which shows MORB's transition to OIB.Geochemical data of the sample show depletion of Nb, Ta, Ti and enrichment of Th, U, Zr, Hf, exhibiting characteristics of the arc magma.In the Nb/Yb-Th/Yb tectonic environment discrimination diagram, it is located at the intersection of E-MORB and OIB.A comparison with the rock association of the regional tectonic and geochemical characteristics shows that the magma source area of the Gaxian ultramafic magma may be an enriched mantle replaced by subductive fluid, belonging to the SSZ type ophiolite, and its emplacement mechanism might have been related to the continuous intra-ocean subduction.
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Key words:
- Gaxian /
- ophiolitic mélange /
- SSZ type /
- Da Hinggan Mountains
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图 1 嘎仙蛇绿混杂岩分布区地质简图(图b据参考文献[2]修改)
Figure 1.
图 4 嘎仙超镁铁质岩TAS图解(a)和Nb/Y-Zr/TiO2岩石分类图解(b)(a、b底图据参考文献[11])
Figure 4.
图 8 嘎仙超镁铁质岩Nb/Yb-Th/Yb构造环境判别图解[31]
Figure 8.
表 1 嘎仙超镁铁质岩(WJE032)锆石U-Th-Pb同位素分析结果
Table 1. The zircon U-Th-Pb isotope analyses of Gaxian ultramafic rocks(WJE032)
测点号 含量/10-6 Th/U 同位素比值 年龄/Ma Pb U Th 206Pb/238U ±1σ 207Pb/235U ±1σ 207Pb/206Pb ±1σ 206Pb/238U ±1σ 207Pb/235U ±1σ 1 17 148 83 0.5564 0.1068 0.0011 0.9187 0.013 0.0624 0.0008 654 7 662 9 2 22 170 173 1.0193 0.1071 0.0011 0.9159 0.0223 0.062 0.0016 656 7 660 16 3 19 156 114 0.7287 0.1054 0.0011 0.8882 0.0144 0.0611 0.0009 646 7 645 10 4 62 481 507 1.0539 0.1054 0.0011 0.8967 0.012 0.0617 0.0007 646 7 650 9 5 33 285 152 0.5341 0.1067 0.0011 0.8974 0.0119 0.061 0.0007 654 7 650 9 6 4 38 18 0.4618 0.105 0.0011 0.9108 0.0198 0.0629 0.0013 644 7 657 14 7 10 88 58 0.6582 0.105 0.0011 0.9297 0.0166 0.0642 0.0011 643 7 667 12 8 2 21 10 0.461 0.1078 0.0012 0.9395 0.0437 0.0632 0.0029 660 8 673 31 9 8 61 67 1.102 0.1052 0.0011 0.915 0.0471 0.0631 0.0032 645 7 660 34 11 14 120 77 0.6444 0.1062 0.0011 0.9249 0.0137 0.0632 0.0008 651 7 665 10 12 13 106 81 0.7668 0.1066 0.0011 0.9246 0.0141 0.0629 0.0008 653 7 665 10 13 10 212 171 0.4889 0.1054 0.0011 0.91 0.0208 0.0626 0.0014 646 6 657 15 14 4 39 19 0.4356 0.1056 0.0011 0.8981 0.0118 0.0617 0.0007 647 6 651 9 15 4 40 18 0.5169 0.1052 0.0011 0.9147 0.0266 0.063 0.0018 645 7 660 19 16 25 225 98 0.6328 0.1042 0.0011 0.9139 0.031 0.0636 0.0021 639 7 659 22 17 3 26 13 0.6353 0.1033 0.0011 0.8867 0.0493 0.0623 0.0034 634 7 645 36 18 3 24 15 0.5307 0.1051 0.0011 0.8927 0.0137 0.0616 0.0008 644 7 648 10 19 2 19 12 0.5976 0.1012 0.001 0.8907 0.0172 0.0638 0.0012 621 6 647 12 20 17 154 82 0.3979 0.1067 0.0011 0.9143 0.0146 0.0622 0.0009 653 7 659 11 21 14 128 76 0.5901 0.1004 0.0011 0.8836 0.0295 0.0639 0.002 617 7 643 21 22 11 96 38 0.7213 0.1078 0.0011 0.9212 0.0133 0.062 0.0008 660 7 663 10 23 6 57 31 0.528 0.1073 0.0011 0.9069 0.0126 0.0613 0.0008 657 7 655 9 24 8 77 45 0.7996 0.1064 0.0011 0.908 0.0229 0.0619 0.0015 652 7 656 17 25 17 145 105 0.3972 0.1068 0.0011 0.9249 0.0363 0.0628 0.0024 654 7 665 26 26 26 232 123 0.6243 0.1065 0.0011 0.9028 0.0469 0.0615 0.0032 652 7 653 34 27 10 87 73 0.5056 0.1084 0.0011 0.909 0.0133 0.0608 0.0008 664 7 657 10 28 14 120 77 1.125 0.1077 0.0011 0.9173 0.0345 0.0618 0.0023 659 7 661 25 29 13 106 81 1.4355 0.1241 0.0015 1.1738 0.0231 0.0686 0.0012 754 9 788 16 30 10 212 171 0.6178 0.109 0.0011 0.9382 0.0129 0.0624 0.0008 667 7 672 9 表 2 嘎仙超镁铁质岩岩石地球化学特征
Table 2. Major, trace and rare earth element characteristics of Gaxian ultramafic rocks
样品编号 WJE032 WJE059 WJE065 WJE070 WJE071 名称 蛇纹石化橄榄岩 蛇纹石化橄榄辉石岩 蛇纹石化橄榄岩 蛇纹石化橄榄辉石岩 蛇纹石化橄榄辉石岩 SiO2 43.80 45.47 45.44 44.64 45.01 Al2O3 8.65 3.54 5.93 8.90 2.65 TiO2 0.81 0.27 0.61 1.12 0.10 Fe2O3 1.18 0.98 1.21 1.59 2.00 FeO 8.90 8.11 6.41 7.66 5.80 CaO 6.59 5.85 7.12 6.45 4.51 MgO 23.21 30.47 26.97 26.37 34.19 K2O 0.08 0.01 0.02 0.03 0.01 Na2O 0.13 0.04 0.13 0.22 0.05 MnO 0.18 0.13 0.19 0.18 0.10 P2O5 0.12 0.05 0.10 0.16 0.16 TFeO 9.96 9.00 7.50 9.10 7.61 Mg# 83.0 87.7 88.3 85.9 90.4 灼失量 6.00 4.55 5.48 2.22 4.85 总和 99.67 99.47 99.61 99.53 99.45 K2O/Na2O 0.62 0.16 0.12 0.12 0.23 K2O+Na2O 0.22 0.05 0.15 0.24 0.06 Y 19.0 9.30 8.44 19.6 5.72 La 11.0 5.21 7.90 20.7 5.36 Ce 23.3 13.0 14.9 42.2 9.31 Pr 3.40 1.82 1.80 5.29 1.15 Nd 15.4 7.77 6.83 21.0 4.66 Sm 3.89 1.90 1.52 4.18 0.98 Eu 0.90 0.31 0.51 0.95 0.32 Gd 2.96 1.62 1.37 3.84 1.06 Tb 0.57 0.30 0.25 0.66 0.19 Dy 3.44 1.74 1.55 4.12 1.18 Ho 0.65 0.32 0.29 0.78 0.22 Er 1.78 0.91 0.84 2.29 0.58 Tm 0.30 0.15 0.14 0.41 0.09 Yb 1.75 0.90 0.84 2.67 0.56 Lu 0.25 0.14 0.13 0.35 0.08 Li 14.8 8.97 7.27 7.16 9.31 Be 0.63 0.44 0.64 1.04 0.16 Sc 26.6 17.6 22.4 23.8 14.0 V 117 80.7 95.5 171 56.0 Cr 1224 1542 1908 2152 2463 Co 93.6 166 75.4 68.4 82.9 Ni 1171 2897 1187 965 1330 Cu 55.3 33.0 15.1 12.6 16.9 Zn 73.8 58.6 58.0 76.9 61.8 Ga 8.87 4.40 7.20 13.1 2.95 Rb 8.42 1.60 6.80 6.39 2.4 Sr 98.8 190 366 57.7 243 Zr 76.6 27.0 55.8 105 8.6 Nb 15.4 4.97 8.10 23.8 1.04 Mo 1.27 0.44 0.10 1.16 0.23 Cs 2.44 0.37 0.53 1.02 0.21 Ba 24.2 6.52 41.7 39.9 61.6 Hf 2.05 0.76 1.63 2.96 0.34 Ta 0.78 0.28 0.38 1.77 0.10 W 2.29 0.61 0.86 0.57 1.38 Tl 0.19 0.03 0.05 0.03 0.04 Pb 12.4 0.11 1.99 1.11 4.93 Bi 0.13 0.15 0.13 0.14 0.08 Th 2.74 0.95 0.97 2.39 0.39 U 0.87 0.26 0.47 0.99 0.15 Nb/Ta 19.7 18.0 21.4 13.4 10.8 (La/Yb)N 4.53 4.16 6.71 5.58 6.85 (La/Sm)N 1.84 1.77 3.36 3.21 3.54 (Gd/ Yb)N' 1.40 1.50 1.34 1.19 1.56 δEu 0.27 0.17 0.35 0.24 0.31 δCe 3.81 4.22 3.95 4.03 3.75 ΣREE 88.6 45.4 47.3 129.0 31.5 注:σ=(Na2O+ K2O)2/(SiO2-43)(wt%);TFeO=FeO+0.8998Fe2O3;Mg#={100× n(Mg2+)/[(n(Mg2+)+ n(Fe2+)]};主量元素含量单位为%,微量和微土元素含量单位为10-6 -
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