Tectonic attribute of the Shiquanhe-Namco ophiolitic belt: Constraint from geochemistry of the island-arc basalts in the Asa mélange zone, central Tibet
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摘要:
明确狮泉河-纳木错蛇绿岩带的构造属性是重建西藏中部中特提斯演化的关键之一。报道了西藏中部阿索地区蛇绿混杂岩带中岛弧玄武岩的岩石学、地球化学特征，并讨论了该岩石的构造意义。野外和镜下观察结果显示，该样品具有枕状构造并具有细碧岩的岩石特征，表示其形成于海底喷发环境。全岩地球化学分析结果显示，该样品属于亚碱性系列中的钙碱性玄武岩。该岩石富集轻稀土元素，具有平坦的重稀土元素分布特征，轻、重稀土元素分异较强。岩石富集Ba、Th、Pb等大离子亲石元素，亏损Nb、Ta等高场强元素。地球化学分析结果显示，岩石起源于被俯冲沉积物熔体交代的亏损地幔的部分熔融，形成于洋内岛弧环境之下。结合前人研究，发现狮泉河-纳木错蛇绿岩带并非仅存在洋壳成因的蛇绿岩，还保存了一些岛弧成因的岩浆岩。这些岛弧成因岩浆岩的存在表明，狮泉河-纳木错蛇绿岩带中的岩浆岩并非单一构造背景下形成的蛇绿岩，而是包含了多种构造背景之下的岩浆产物。研究表明，狮泉河-纳木错蛇绿岩带并非仅保留了弧后盆地的遗迹，其包含了洋内俯冲环境中多种成因的岩浆作用产物。
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关键词:
- 青藏高原 /
- 狮泉河-纳木错蛇绿岩带 /
- 岛弧玄武岩 /
- 地球化学 /
- 中特提斯洋
Abstract:The tectonic setting of the Shiquanhe-Namco ophiolitic belt in central Tibet is a key to understanding the Mesozoic tectonic evolution of the Meso-Tethys Ocean.Based on the analysis of major and trace element compositions of island arc basalts in the Asa mélange zone, central Tibet, its tectonic significance was discussed.Field observation and microscopic study indicate that they were formed in submarine eruption environment.Whole-rock geochemical analyses suggest that these rocks belong to calc-alkaline rocks, rich in light rare earth and Ba, Th, Pb large-ion lithophile elements, and depleted in Nb and Ta, with clear differentiation of light and heavy rare earth elements and flat distribution of heavy rare earth elements.Geochemical analysis shows that the rocks were originated from partial melting of depleted mantle metasomatized by melt of subducting sediments and formed under the oceanic island arc environment.Combined with previous work, it is suggested that there are some island arc basalts widely preserved in the Shiquanhe-Namco ophiolitic belt.The existence of the island-arc magmatic rocks within the Shiquanhe-Namco ophiolitic belt reveals that the Shiquanhe-Namco ophiolitic belt cannot be interpreted as an ophiolite in a single back-arc setting but contains magmatic products under various tectonic settings.The results show that the Shiquanhe-Namco ophiolite belt does not only retain the remains of backarc basin, but also contains the magmatic products of various genesis in the subduction environment.
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致谢: 野外工作得到了吉林大学西藏科研队的队员们和后勤工作人员的帮助，全岩地球化学测试分析由中国地质大学(北京)科学研究院帮助完成，在此一并致以真挚的谢意，并感谢审稿专家的建议和意见。
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图 1 藏北尼玛县阿索乡阿索混杂岩带地质简图(据参考文献[17]修改)
a—青藏高原区域地质简图；b—研究区区域地质图；①—龙木错-双湖-澜沧江板块缝合带；②—班公湖-怒江板块缝合带；③—狮泉河-纳木错蛇绿岩带；④—沙莫勒-米拉山断裂带；⑤—印度-雅鲁藏布江板块缝合带
Figure 1. Geological map of the Asa mélange in Asa Town of Nima County, northern Tibet
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图 2 阿索混杂岩带岛弧玄武岩实测剖面(a)和野外(b~d)、镜下照片(e~g)
1—复理石砂岩；2—白云母石英片岩；3—大理岩；4—层状玄武岩；5—枕状玄武岩；6—片麻岩；7—石英岩；8—断层；9—采样点；Cpx—单斜辉石；Pl—斜长石；Chl—绿泥石；Pum—绿纤石
Figure 2. Geological section(a), field photos(b~d)and micrographs(e~g)of the island arc basalt from Asa mélange
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图 4 阿索岛弧玄武岩球粒陨石标准化稀土元素配分模式图(a)及N-MORB标准化微量元素蛛网图(b) (原始地幔和N-MORB, E-MORB, OIB的标准值据参考文献[27])
N-MORB—正常型大洋中脊玄武岩；E-MORB—富集型大洋中脊玄武岩；OIB—洋岛玄武岩
Figure 4. Chondrite-normalized rare earth element patterns(a)and N-MORB-normalized trace element diagram(b)of the Asa island arc basalt
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图 6 阿索岛弧玄武岩Hf/3-Th-Nb(a)、Y /15-La/10-Nb/8 (b)和Ce-Yb(c)构造环境判别图解
图a:A-洋岛/板内玄武岩;B-富集型洋中脊玄武岩; C-亏损型洋中脊玄武岩;D-岛弧玄武岩 图b:1A-钙碱性玄武岩;1B-过渡区域;1C- 火山弧 拉斑玄武岩;2A-大陆玄武岩;2B-弧后盆地玄武岩;3A-碱性玄武岩;3B、3C-富集型洋中脊玄武岩; 3D-亏损型洋中脊玄武岩
Figure 6. Hf/3-Th-Nb(a)，Y/15—La/10-Nb/8(b) and Ce-Yb(c) tectonic setting discrimination diagrams of Asa island arc basalt
表 1 阿索岛弧玄武岩主量、微量和稀土元素测试结果
Table 1 Whole-rock major, trace elements and REE data of the Asa island arc basalt
样品号 NT38H1 NT38H2 NT38H3 NT38H4 NT38H5 样品号 NT38H1 NT38H2 NT38H3 NT38H4 NT38H5 SiO2 54.18 52.37 49.95 54.29 54.85 Nb 3.81 4.30 5.98 3.89 4.02 TiO2 0.92 1.00 0.96 0.92 0.97 Ta 0.24 0.27 0.32 0.23 0.23 Al2O3 16.35 17.41 14.76 17.27 16.67 La 12.02 12.89 30.33 13.02 13.69 TFe2O3 6.99 7.36 6.98 6.68 6.79 Ce 22.66 25.29 54.94 24.59 24.95 MnO 0.12 0.13 0.13 0.13 0.13 Pb 5.74 4.22 5.67 6.20 3.58 MgO 5.85 5.15 5.50 4.92 5.14 Pr 2.72 3.04 6.10 2.92 3.03 CaO 5.69 7.02 12.19 5.45 5.66 Sr 257.2 438.2 455.7 447.2 339.4 Na2O 6.73 6.25 4.77 6.84 7.12 Nd 10.35 11.33 20.35 10.98 11.33 K2O 0.08 0.07 0.44 0.07 0.07 Zr 88.72 98.57 84.86 92.34 93.33 P2O5 0.13 0.14 0.21 0.13 0.14 Hf 2.22 2.46 1.98 2.26 2.25 烧失量 2.68 2.95 4.09 3.02 2.16 Sm 2.51 2.82 3.77 2.67 2.76 总计 99.71 99.84 99.98 99.72 99.69 Eu 0.78 0.92 1.06 0.85 0.82 Mg# 66.11 61.98 64.75 63.18 63.86 Gd 2.71 3.06 3.47 2.88 2.95 Sc 29.24 31.54 32.87 29.32 30.40 Tb 0.48 0.52 0.53 0.49 0.50 V 211.0 225.4 244.67 209.8 220.2 Dy 3.24 3.50 3.39 3.25 3.32 Cr 90.66 74.12 229.59 73.40 92.70 Y 21.58 23.04 22.90 22.08 23.54 Co 26.58 27.56 28.65 27.28 25.64 Ho 0.69 0.74 0.71 0.69 0.71 Ni 50.22 48.12 81.40 53.16 49.96 Er 2.03 2.12 2.04 2.00 2.07 Mn 880.6 1027.4 1004.6 968.4 990.8 Tm 0.29 0.31 0.29 0.28 0.29 Cu 53.36 50.58 34.46 54.98 64.64 Yb 1.82 1.94 1.84 1.83 1.87 Zn 59.60 67.24 68.32 68.68 66.68 Lu 0.28 0.29 0.27 0.27 0.28 Ga 19.87 22.04 18.35 19.13 16.31 ∑REE 62.59 68.76 129.09 66.72 68.57 Cs 1.77 1.39 0.77 2.47 0.79 Eu/Eu* 0.91 0.96 0.90 0.94 0.88 Rb 2.37 2.26 10.18 1.99 2.27 (La/Yb)N 4.73 4.77 11.85 5.10 5.25 Ba 63.62 82.44 178.41 80.64 63.92 (La/Sm)N 3.09 2.96 5.19 3.15 3.20 Th 4.83 5.24 6.97 4.72 4.73 (Dy/Yb)N 1.19 1.21 1.24 1.19 1.19 U 0.74 0.68 1.18 0.65 0.76 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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