Petrology and geochronology of Asuo ophiolite in central Tibetan Plateau
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摘要:
阿索蛇绿岩位于尼玛县阿索乡西南，大地构造上归属于狮泉河-永珠-嘉黎蛇绿岩带中段。蛇绿岩以岩片形式混杂在晚侏罗世-早白垩世复理石中，岩石组合较齐全，由下至上为蛇纹岩、辉石岩、堆晶辉长岩、席状岩墙及火山熔岩，同时存在蛇绿岩上覆沉积岩系。辉长岩获得LA-ICP-MS锆石U-Pb谐和年龄为117.5±0.5Ma，时代为早白垩世。狮泉河-永珠-嘉黎蛇绿岩带中的蛇绿岩形成于晚三叠世-早白垩世，主要分布在219~178Ma、165~149Ma和117~114Ma三个年龄段，代表了大洋演化的扩张、俯冲、弧后拉张3个阶段。
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关键词:
- 青藏高原 /
- 狮泉河-永珠-嘉黎蛇绿岩带 /
- 阿索蛇绿岩 /
- 岩石学 /
- 锆石U-Pb定年
Abstract:Asuo ophiolite belongs to Shiquanhe-Yongzhu-Jiali ophiolite zone, located in southwestern Asuo County. The rock combination is complete, and consists from the bottom to the top of serentine, pyroxenite, cumulate gabbro, mafic dyke swarms and volcanic lava, with the existence of sedimentary rocks overlying ophiolite. In this paper, LA-ICP-MS U-Pb dating was carried out for the zircons of the gabbro dyke of Asuo ophiolite. LA-ICP-MS zircon U-Pb dating of gabbro dyke yielded a concordant age of 117.54±0.58Ma, indicating that it was formed in the Early Cretaceous.The ophiolite of Shiquanhe-Yongzhu-Jiali ophiolite zone was formed in Late Triassic to Early Cretaceous and mainly distributed in three age groups of 219~178Ma, 165~149Ma and 117~114Ma, representing the ocean expansion, subduction and back-arc extension respectively.
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致谢: 野外工作得到吉林大学西藏科研队的队员们和后勤工作人员的帮助，锆石同位素测试分析由中国地质大学（北京）科学研究院老师帮助完成，在此一并致以真挚的谢意。
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图 1 藏北尼玛县阿索乡阿索蛇绿岩地质简图
1—第四系冲洪积物；2—下白垩统郎山组；3—晚侏罗世-早白垩世复理石岩片；4—中下侏罗统接奴组；
5—上二叠统昂杰组；6—下二叠统拉嘎组；7—闪长岩；8—石英钠长斑岩；9—花岗岩；10—玄武岩岩片；
11—辉绿岩岩片；12—辉长岩岩片；13—堆晶岩岩片；14—超基性岩岩片；15—变质岩岩片；
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-永珠-嘉黎蛇绿岩带；
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 Asuo ophiolite in Asuo Town of Nima County, northern Tibet
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图 2 尼玛县阿索乡加布内热蛇绿岩剖面
Figure 2. Profile of Jiabuneire ophiolite in Asuo Town, Nima County
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图版Ⅰ
a.辉石岩野外露头；b.粗粒堆晶辉长岩野外露头；c.侵入堆晶岩中的辉长岩；d.斜长花岗岩野外照片；
e.辉长质岩墙；f.辉绿玢岩岩墙；g.玄武岩枕状构造；h.放射虫硅质岩图版Ⅰ.
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图版Ⅱ
a.辉石岩镜下照片；b.细粒堆晶辉长岩镜下照片；c.斜长花岗岩镜下照片；d.辉长岩辉长辉绿结构；
e.辉长质岩墙；f.辉绿玢岩岩墙；g.细碧岩中空骸晶结构；h.放射虫硅质岩镜下照片。Chl—绿泥石；
Cpx—单斜辉石；Pl—斜长石；Am—角闪石；Qtz—石英；Ep—绿帘石；Ura—纤闪石图版Ⅱ.
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图 3 阿索蛇绿岩中辉长岩中典型锆石阴极发光(CL)图像及其206Pb/238U年龄
Figure 3. Zircon CL images and 206Pb/238U ages of typical zircons of gabbro from Asuo ophiolite
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图 4 阿索蛇绿岩中辉长岩锆石稀土元素蛛网图（a）及Y-U/Yb成因判别图解（b）（标准化数据据参考文献[23]）
Figure 4. Chondrite-normalized REE patterns(a)and Y-U/Yb plot (b)for zircons in the gabbro of Asuo ophiolite
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图 5 阿索蛇绿岩中辉长岩LA-ICP-MS锆石U-Pb年龄谐和图
Figure 5. LA-ICP-MS zircon U-Pb concordia diagrams of gabbro from the Asuo ophiolite
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表 1 阿索蛇绿岩中辉长岩(N17T49)LA-ICP-MS锆石U-Th-Pb同位素测定结果
Table 1 LA-ICP-MS U-Th-Pb analyses of zircons from samples of the gabbro (N17T49)
测点 Th U Th/U 同位素比值 年龄/Ma /10-6 207Pb/206Pb 1σ 207Pb/235U 1σ 206Pb/238U 1σ 207Pb/206Pb 1σ 207Pb/235U 1σ 206Pb/238U 1σ 01 539 418 1.29 0.0484 0.0019 0.1225 0.0049 0.0184 0.0002 116 69 117 4 117 1 02 853 480 1.78 0.0489 0.0020 0.1239 0.0051 0.0184 0.0002 141 74 119 5 118 1 03 1198 501 2.39 0.0485 0.0020 0.1236 0.0051 0.0185 0.0002 126 75 118 5 118 1 04 2338 1030 2.27 0.0484 0.0011 0.1238 0.0029 0.0186 0.0002 117 34 119 3 119 1 05 1059 566 1.87 0.0483 0.0018 0.1222 0.0044 0.0184 0.0002 113 63 117 4 117 1 06 1094 610 1.79 0.0483 0.0017 0.1226 0.0044 0.0184 0.0002 115 62 117 4 117 1 07 694 476 1.46 0.0486 0.0020 0.1229 0.0049 0.0184 0.0002 127 71 118 4 117 1 08 1254 663 1.89 0.0484 0.0015 0.1223 0.0039 0.0183 0.0002 118 54 117 3 117 1 09 1833 1012 1.81 0.0519 0.0014 0.1314 0.0034 0.0184 0.0002 280 39 125 3 117 1 10 459 339 1.36 0.0484 0.0028 0.1227 0.0071 0.0184 0.0002 120 105 118 6 117 2 11 193 175 1.11 0.0484 0.0053 0.1222 0.0134 0.0183 0.0003 116 216 117 12 117 2 12 2688 1338 2.01 0.0493 0.0012 0.1251 0.0029 0.0184 0.0002 163 34 120 3 118 1 13 587 369 1.59 0.0484 0.0028 0.1229 0.0070 0.0184 0.0002 117 105 118 6 118 1 14 909 660 1.38 0.0481 0.0018 0.1218 0.0044 0.0184 0.0002 105 63 117 4 117 1 15 1580 898 1.76 0.0481 0.0015 0.1223 0.0037 0.0185 0.0002 103 50 117 3 118 1 16 5112 2216 2.31 0.0485 0.0010 0.1231 0.0025 0.0184 0.0002 124 28 118 2 117 1 17 547 425 1.29 0.0481 0.0021 0.1220 0.0052 0.0184 0.0002 106 74 117 5 117 1 18 725 505 1.43 0.0480 0.0019 0.1219 0.0046 0.0184 0.0002 101 65 117 4 118 1 19 268 215 1.25 0.0484 0.0039 0.1223 0.0099 0.0183 0.0003 118 155 117 9 117 2 20 554 350 1.59 0.0484 0.0028 0.1234 0.0072 0.0185 0.0002 120 109 118 6 118 1 
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表 2 阿索蛇绿岩中辉长岩锆石LA-ICP-MS原位微量、稀土元素分析结果
Table 2 Trace element and REE compositions of zircons in gabbro of Asuo ophiolite as measured by LA-ICP-MS
10-6 测点 1 2 3 4 5 6 7 8 9 10 Y 9824.58 15398.54 9284.82 13990.57 2505.87 4887.58 10160.05 2489.12 11917.05 10087.65 Nb 11.93 9.84 16.44 22.98 2.18 3.48 25.66 3.03 5.51 4.55 La 0.07 0.22 0.19 0.22 0.08 0.09 0.08 0.09 0.14 0.11 Ce 102.37 89.69 188.13 217.35 13.49 31.39 157.27 24.85 52.64 65.38 Pr 1.15 1.57 2.29 2.86 0.1 0.47 0.86 1.2 1.06 0.95 Nd 23.62 27.26 43.59 54.08 2.13 7.99 17.99 21.48 20.26 14.92 Sm 52.98 62.87 89.86 105.72 7.57 18.48 42.88 42.01 46.96 31.86 Eu 7.82 8.53 4.1 9.05 1.93 3.37 3.24 5.78 7.28 2.44 Gd 244.9 319.24 349.58 455.08 44.08 92.62 216.77 141.55 252.4 165.98 Tb 91.16 126.21 117.68 155.89 18 36.38 82.77 41.4 97.74 69.5 Dy 1061.86 1552.21 1155.24 1653.81 231.12 436.56 969.5 359.45 1193.66 893.98 Ho 371.91 553.57 330.66 508.31 88.24 162.35 333.05 91.17 428 338.55 Er 1514.42 2346.45 1127.33 1942.02 380.98 701.92 1342.41 290.28 1761.05 1499.71 Tm 342.79 544.65 222.75 383.68 94.57 174.76 297.02 55.27 408.11 361.82 Yb 3622.39 5721.35 2042.4 3565.22 1072.46 1966.76 2936.81 539.69 4350.33 3865.13 δEu 0.06 0.12 0.1 0.1 0.1 0.1 0.1 0.12 0.06 0.11 δCe 56.64 25.29 16.94 40.12 43.22 26.39 29.79 28.67 73.89 23.76 ΣREE 4285.5 7383.11 9952.55 5299.71 4586.49 7703.84 4571.58 8155.72 7815.5 3906.64 测点 11 12 13 14 15 16 17 18 19 20 Y 4789.46 3430.63 8233.49 2376.99 6835.81 6508.85 7409.12 16039.71 4906.82 6664.45 Nb 4.9 3.57 4.26 1.06 2.41 9.1 2.22 38.5 3.46 5 La 0.07 0.05 0.08 0.13 0.11 0.08 0.07 0.07 0.06 0.08 Ce 40.07 17.11 30.68 17.38 20.43 36.02 20.88 198.47 23.85 16.81 Pr 0.22 0.13 0.61 0.19 0.55 0.21 0.29 1.12 0.2 0.27 Nd 4.91 2.63 10 2.64 10.53 5.13 6 24.05 4.38 6.28 Sm 14.29 7.9 23.7 6.44 26.45 18.01 18.05 69.65 13.71 19.7 Eu 2.52 1.48 3.68 2.02 6.59 3.1 4.97 8.57 3.38 4.72 Gd 88.92 49.8 132.45 34.24 135.1 99.67 120.52 372.88 83.45 122.18 Tb 35.07 21.38 56.45 14.9 51.89 44.08 49.37 151.28 34.96 48.79 Dy 431.21 291.45 726.01 193.87 654.44 580.47 668.43 1774.79 439.24 633.91 Ho 159.24 114.13 274.97 78.27 244.29 224.5 261.27 588.24 162.89 229.03 Er 711.7 518.44 1223.12 376.76 1064.44 980.44 1131.95 2081.1 683.41 910.85 Tm 171.53 131.61 303.98 99.19 265.48 237.61 264.44 410.32 155.33 205.34 Yb 1849.8 1514.91 3381.98 1202.84 2953.94 2537.51 2662.4 3740.38 1591.12 2041.04 ΔEu 0.11 0.1 0.12 0.07 0.08 0.06 0.06 0.07 0.1 0.14 ΔCe 25.99 119.94 19.44 70.28 95.39 230.11 56.84 39.58 26.32 18.37 ΕREE 2547.44 6461.63 6408.46 3093.19 6894.38 11864.59 4553.28 4967.05 2846.32 5474.27
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