Petrogeochemical and Pb-Nd-Sr isotope characteristics of ultrabasic rocks in the Yizhuxingla area, Eastern Tibet
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摘要:
对出露于班公湖-怒江缝合带东段衣珠兴拉地区超基性岩的野外地质调查，以及岩石学、岩石地球化学、Pb-Nd-Sr同位素等研究表明：岩石类型为强烈蛇纹石化的方辉橄榄岩，具有富MgO（45.33%~47.70%）、贫CaO（0.15%~0.37%）和Al2O3（0.08%~0.39%）、低TiO2（0.011%~0.014%）的特征；稀土元素含量低于原始地幔，稀土元素配分模式呈平缓右倾型，轻、重稀土元素分馏明显，具有明显的正Eu异常和弱负Ce异常；过渡金属元素表现为不对称的配分型式，在Ti和Cu处形成明显的负异常；岩石具有高（87Sr/86Sr）i、低εNd（t）的特征。上述特征表明，衣珠兴拉超基性岩相当于SSZ型地幔橄榄岩，由原始地幔经15%~25%部分熔融形成亏损地幔岩，形成于洋内岛弧环境，在俯冲消减作用过程中，混染了20%~40%的地壳物质。
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关键词:
- 岩石地球化学 /
- Pb-Nd-Sr同位素 /
- 衣珠兴拉 /
- 亏损地幔
Abstract:The field geological survey and petrology study of the ultrabasic rocks in the Yizhuxingla area of the eastern Bangonghu-Nujiang suture zone show that the rocks in this area are strongly serpentinized peridotite.Geochemical studies show that these rocks are rich in MgO (45.33%~47.70%), poor in CaO (0.15%~0.37%) and Al2O3 (0.08%~0.39%), and deficient in TiO2 (0.011%~0.014%).Its total content of REE is lower than that in the primitive mantle, and the REE distribution pattern is gentle to the right, the fractionation of light and heavy REE is obvious, and there are obvious positive Eu anomaly and weak negative Ce anomaly.Transition metal elements show an asymmetric W-type distribution pattern, forming obvious negative Ti and Cu anomalies, and the rocks have high (87Sr/86Sr)i and low εNd(t) characteristics.The above characteristics indicate that the ultrabasic rocks in the Yizhuxingla area is similar to SSZ type mantle peridotite.The depleted mantle rocks resulted form the partial meltingof original mantle(15%~25%).These ultrabasic rocks were formed in the oceanic island arc environment and had mixing of 20%~40% crustal material during the subduction process.
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Key words:
- petrogeochemistry /
- Pb-Nd-Sr isotope /
- Yizhuxingla ultrabasic rocks /
- depleted mantle
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表 1 衣珠兴拉超基性岩主量、微量和稀土元素分析结果
Table 1. Major, trace elements and REE data of Yizhuxingla ultrabasic rocks
样号 YZ-B6 YZ-B7 YZ-B8 YZ-B9 YZ-B10 YZ-B11 YZ-B12 YZ-B13 YZ-B14 YZ-B15 YZ-B16 SiO2 37.54 38.38 38.52 38.88 38.84 39.60 38.94 39.54 36.42 35.41 36.38 TiO2 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 Al2O3 0.21 0.32 0.21 0.26 0.22 0.28 0.14 0.21 0.07 0.06 0.21 Fe2O3 4.26 4.94 4.33 4.44 3.62 4.86 4.61 4.42 4.64 5.20 5.24 FeO 2.26 1.29 2.15 2.53 2.74 1.67 1.01 2.44 2.26 1.78 1.70 MnO 0.08 0.08 0.07 0.08 0.08 0.08 0.07 0.08 0.08 0.08 0.08 MgO 38.27 36.44 37.75 38.85 39.06 38.31 37.61 39.88 39.55 38.99 37.29 CaO 0.31 0.25 0.17 0.32 0.26 0.25 0.16 0.20 0.18 0.13 0.12 Na2O 0.039 0.037 0.033 0.036 0.031 0.041 0.042 0.034 0.032 0.047 0.038 K2O 0.027 0.018 0.022 0.022 0.017 0.022 0.029 0.022 0.017 0.024 0.022 P2O5 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 烧失量 13.58 15.24 13.10 12.02 11.92 12.94 14.92 10.84 13.22 15.00 15.16 总计 96.59 97.00 96.37 97.45 96.79 98.07 97.55 97.68 96.48 96.73 96.25 TFeO 7.34 7.01 7.26 7.63 7.06 7.09 6.24 7.38 7.71 7.89 7.90 Mg 0.86 0.86 0.86 0.86 0.87 0.86 0.88 0.86 0.86 0.86 0.85 m/f 4.84 4.90 4.81 4.59 5.03 4.89 5.62 4.79 4.74 4.65 4.48 Di 0.89 0.06 0.08 0.61 0.23 0.44 0.44 0.56 0.82 0.59 0.24 Hy 26.21 37.21 33.16 28.88 29.65 34.12 36.26 28.28 18.08 15.92 25.63 Ol 69.67 59.15 63.54 67.04 66.90 62.08 60.59 68.10 78.43 80.69 70.86 La 0.30 0.29 0.28 0.28 0.28 0.23 0.19 0.26 0.24 0.25 0.19 Ce 0.41 0.41 0.41 0.41 0.31 0.35 0.30 0.38 0.38 0.33 0.29 Pr 0.06 0.05 0.05 0.05 0.05 0.04 0.04 0.05 0.04 0.04 0.03 Nd 0.21 0.20 0.18 0.18 0.18 0.14 0.13 0.17 0.13 0.14 0.12 Sm 0.07 0.06 0.06 0.04 0.05 0.04 0.05 0.04 0.04 0.03 0.04 Eu 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 Gd 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.00 Tb 0.04 0.02 0.04 0.03 0.03 0.02 0.03 0.03 0.02 0.03 0.02 Dy 0.05 0.04 0.05 0.03 0.04 0.03 0.03 0.04 0.04 0.05 0.03 Ho 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 Er 0.03 0.02 0.03 0.02 0.02 0.02 0.02 0.02 0.03 0.02 0.02 Tm 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Yb 0.04 0.04 0.03 0.03 0.03 0.02 0.02 0.03 0.03 0.03 0.02 Lu 0.01 0.01 0.00 0.01 0.01 0.01 0.00 0.01 0.01 0.01 0.01 Y 0.28 0.28 0.28 0.27 0.32 0.20 0.20 0.23 0.28 0.29 0.21 ΣREE 1.23 1.18 1.15 1.10 1.02 0.94 0.84 1.05 0.98 0.96 0.79 LREE 1.06 1.03 0.98 0.97 0.87 0.82 0.72 0.91 0.85 0.81 0.69 HREE 0.18 0.16 0.17 0.13 0.14 0.12 0.11 0.14 0.13 0.15 0.11 LREE/HREE 5.93 6.62 5.84 7.30 6.11 6.79 6.32 6.70 6.37 5.37 6.49 LaN/YbN 5.63 5.41 6.21 8.12 6.53 7.24 6.96 7.32 5.76 5.81 5.89 δEu 2.10 2.07 1.65 1.80 2.15 2.07 2.05 2.00 2.14 2.69 2.56 δCe 0.76 0.80 0.84 0.83 0.64 0.91 0.86 0.83 0.94 0.80 0.92 Ba 17.30 17.30 15.60 16.00 18.00 15.80 14.90 20.20 20.90 19.70 21.20 Hf 3.86 4.27 4.43 4.11 3.70 3.70 3.78 3.94 4.35 4.11 4.11 Th 6.92 5.34 7.71 9.03 7.10 6.66 3.05 7.98 8.06 6.66 6.39 Zr 7.15 7.24 6.33 7.06 6.79 8.24 5.88 9.33 6.70 7.33 8.42 Co 67.70 69.70 66.80 68.00 66.80 70.60 66.10 71.10 75.30 71.80 67.00 Ni 2054 2082 2118 2056 2109 2122 1999 2175 2405 2275 2064 V 7.18 14.00 7.55 11.80 10.50 12.60 4.85 9.94 5.41 4.95 9.31 Cr 483 810 579 432 711 676 415 542 88 104 538 Cu 4.59 4.69 3.04 2.41 3.61 3.14 2.66 3.55 3.83 2.72 3.11 Zn 36.60 42.50 39.10 26.90 40.50 34.20 37.60 36.60 33.30 35.30 35.40 Li 4.96 2.31 4.62 3.60 2.70 2.46 2.25 2.03 1.40 1.05 2.31 Be 0.03 0.02 0.03 1.00 0.03 0.03 0.02 0.02 0.01 0.02 0.03 Sc 6.17 6.35 5.29 7.47 6.20 6.88 4.51 6.03 4.20 3.82 5.33 Ga 1.51 1.20 1.33 1.33 1.47 1.01 1.11 1.34 1.24 1.26 1.28 Rb 1.33 0.99 1.37 1.15 1.69 0.90 0.75 0.77 0.61 0.57 1.27 Sr 2.26 3.02 3.29 2.21 2.29 2.48 1.97 1.77 2.39 2.07 1.92 Mo 0.39 0.16 0.28 0.15 0.13 0.14 0.10 0.17 0.23 0.18 0.13 Pb 2.84 2.81 1.57 2.12 2.77 2.78 2.21 2.13 2.65 2.60 2.35 Bi 0.02 0.02 0.01 0.03 0.01 0.01 0.02 0.02 0.04 0.02 0.01 U 0.03 0.04 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.03 0.01 Nb 19.00 18.50 19.20 19.80 19.50 18.30 17.60 18.70 19.30 19.80 19.70 Ta 0.12 0.10 0.03 0.10 0.04 0.58 0.13 0.13 0.85 0.95 0.10 注：主量元素含量单位为%，微量和稀土元素含量单位为10-6 
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表 2 衣珠兴拉超基性岩铅同位素分析结果
Table 2. Pb isotope data of Yizhuxingla ultrabasic rocks
样品号 Pb/10-6 Th/10-6 U/10-6 206Pb/204Pb 207Pb/204Pb 208Pb/204Pb (206Pb/204Pb)i (207Pb/204Pb)i (208Pb/204Pb)i YZ-B10 2.77 7.1 0.021 18.98 15.658 38.556 18.963 15.657 36.723 YZ-B15 2.6 6.66 0.031 18.79 15.652 38.556 18.764 15.651 36.729
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表 3 衣珠兴拉超基性岩Nd-Sr同位素分析结果
Table 3. Nd-Sr isotope data of Yizhuxingla ultrabasic rocks
样品号 Sm/10-6 Nd/10-6 147Sm/144Nd 143Nd/144Nd (143Nd/144Nd)i εNd(t) YZ-B10 0.049 0.18 0.1664 0.512055 0.511637 -14.09 YZ-B15 0.031 0.14 0.1320 0.511873 0.511868 -9.58 样品号 Rb/10-6 Sr/10-6 87Rb/86Sr 87Sr/86Sr (87Sr/86Sr)i εSr(t) YZ-B10 1.69 2.29 2.1350 0.715591 0.709 67.5 YZ-B15 0.57 2.07 0.7937 0.734722 0.73227 398
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