LA-ICP-MS zircon U-Pb dating and geochemical characteristics of gabbro in Qingshuiquan, east section of East Kunlun, and its tectonic significance
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摘要:
清水泉辉长岩体位于东昆仑中缝合带清水泉蛇绿岩北侧,主要岩性为角闪辉长岩。该岩体的LA-ICP-MS锆石U-Pb测年结果为452.1±5Ma,形成时代为晚奥陶世。辉长岩体样品SiO2含量为43.89%~45.99%,Na2O含量为1.30%~2.32%,K2O含量为0.68%~1.39%,P2O5含量为0.26%~0.40%,TiO2含量较低,为0.74%~0.95%,MgO含量为5.72%~6.54%,Mg#值较低,为53~54。岩石地球化学特征显示,该岩体属于拉斑玄武岩系列,稀土元素表现出轻稀土元素富集、重稀土元素亏损的特点,微量元素以富集大离子亲石元素(Cs、Rb、Ba等,尤其以Ba的正异常最为突出)、亏损高场强元素(Nb、Ta、Ti、Zr、Hf)为特征。微量元素构造判别显示,其形成于活动陆缘岛弧构造环境。结合区域地质资料研究表明,清水泉辉长岩体可能形成于以清水泉蛇绿岩为代表的东昆仑中弧后有限洋盆向北俯冲的初始阶段。
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关键词:
- 清水泉 /
- 辉长岩体 /
- 岛弧 /
- LA-ICP-MS锆石U-Pb测年 /
- 晚奥陶世
Abstract:Located in central Eastern Kunlun suture zone on the northern side of Qingshuiquan ophiolite, the Qingshuiquan gabbro consists mainly of hornblende gabbros. The LA-ICP-MS zircon U-Pb age is 452.1±5Ma, suggesting the formation of gabbro in Late Ordovician. The gabbro samples have 43.89%~45.99% SiO2, 1.30%~2.32% Na2O, 0.68%~1.39% K2O, and 5.72%~6.54% MgO, with low P2O5 (0.26%~0.40%), TiO2(0.74%~0.95%) and Mg#(53~54). The geochemical characteristics indicate that the gabbro belongs to tholeiitic rocks; REE patterns show that the rocks are enriched in LREE and depleted in HREE; the trace elements are characterized by enrichment of LILE (Cs,Rb,Ba,especially positive anomaly of Ba) and obvious depletion of HFSE (Nb,Ta,Ti,Zr and Hf). Tectonic discrimination diagrams indicate that the gabbro formed in an island arc setting of active continental margin. Combined with regional studies, the authors hold that Qingshuiquan gabbro rocks might have formed at the initial stage of the northward subduction of Central Eastern Kunlun back-arc limited ocean, represented by the Qingshuiquan ophiolite.
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Keywords:
- Qingshuiquan /
- gabbro /
- island arc /
- LA-ICP-MS zircon U-Pb dating /
- Late Ordovician
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致谢: LA-ICP-MS 锆石U-Pb 同位素测定得到天津地质矿产研究所同位素实验室李怀坤研究员和耿建珍博士的大力支持,主量和微量元素分析测试得到长安大学西部矿产资源与地质工程教育部重点实验室王柱命老师的帮助,野外和室内工作得到长安大学地球科学与资源学院徐通博士和中国冶金地质总局西北局魏博工程师的大力帮助,在此一并表示衷心的感谢。
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图 1 东昆仑东段地质简图[16]
1—第四系;2—侏罗系羊曲组;3—下三叠统洪水川组;4—上石炭统浩特洛哇组;5—下石炭统哈拉郭勒组;6—泥盆系牦牛山组;7—下古生界纳赤台岩群;8—新元古界万宝沟岩群;9—中元古界苦海岩群;10—中元古界小庙岩组;11—古元古界白沙河岩组;12—三叠纪花岗岩体;13—早三叠世香加南山花岗岩体;14—泥盆纪念堂花岗岩体;15—二叠纪花岗岩体;16—晚奥陶世辉长岩体;17—蛇绿岩;18—断层;19—角度不整合;20—剖面及剖面位置
Figure 1. Geological sketch map of the eastern section of East Kunlun region
图 2 清水泉辉长岩体实测剖面(剖面位置见图 1)
Figure 2. Measured geological section of Qingshuiquan gabbro
表 1 清水泉辉长岩体(XRD098/2)LA-ICP-MS 锆石U-Th-Pb 分析结果
Table 1 LA-ICP-MS zircon U-Th-Pb data for Qingshuiquan gabbro
点号 含量/10-6 同位素比值 年龄/Ma Pb U 206Pb/238U 1σ 207Pb/235U 1σ 207Pb/206Pb 1σ 208Pb/232Th 1σ 206Pb/238U 1σ 07Pb/235U 1σ 207Pb/206Pb 1σ 1 114 1476 0.0736 0.0005 0.5742 0.0056 0.0566 0.0005 0.0206 0.0001 458 3 461 5 477 21 2 6 72 0.0789 0.0006 1.2361 0.0322 0.1136 0.0029 0.0600 0.0009 490 4 817 21 1858 47 3 31 409 0.0735 0.0003 0.5688 0.0074 0.0561 0.0007 0.0246 0.0001 457 2 457 6 457 28 4 54 731 0.0725 0.0004 0.5868 0.0064 0.0587 0.0006 0.0229 0.0001 451 3 469 5 555 24 5 58 798 0.0724 0.0005 0.5692 0.0064 0.0570 0.0006 0.0237 0.0001 450 3 458 5 493 24 6 12 166 0.0727 0.0004 0.5752 0.0125 0.0574 0.0012 0.0340 0.0004 452 2 461 10 508 47 7 11 146 0.0738 0.0004 0.8185 0.0188 0.0804 0.0018 0.0438 0.0010 459 3 607 14 1206 44 8 18 234 0.0729 0.0004 0.5735 0.0114 0.0570 0.0011 0.0313 0.0004 454 2 460 9 493 43 9 19 255 0.0716 0.0003 0.5640 0.0108 0.0571 0.0011 0.0252 0.0002 446 2 454 9 495 41 10 29 373 0.0731 0.0003 0.5690 0.0090 0.0565 0.0009 0.0267 0.0002 455 2 457 7 472 34 11 21 312 0.0709 0.0004 0.5513 0.0105 0.0564 0.0011 0.0281 0.0005 441 2 446 8 468 42 12 29 379 0.0725 0.0003 0.5918 0.0104 0.0592 0.0010 0.0300 0.0003 451 2 472 8 575 36 13 28 353 0.0751 0.0003 0.6055 0.0086 0.0585 0.0008 0.0322 0.0002 467 2 481 7 548 30 14 18 249 0.0722 0.0003 0.5701 0.0108 0.0572 0.0011 0.0245 0.0003 450 2 458 9 501 41 15 18 230 0.0748 0.0004 0.8007 0.0136 0.0776 0.0013 0.0343 0.0003 465 2 597 10 1137 33 16 46 629 0.0721 0.0004 0.5716 0.0067 0.0575 0.0007 0.0256 0.0001 449 2 459 5 512 26 17 61 725 0.0729 0.0004 0.5741 0.0062 0.0571 0.0006 0.0275 0.0000 453 3 461 5 497 24 18 15 232 0.0671 0.0004 0.5312 0.0122 0.0574 0.0013 0.0637 0.0026 419 3 433 10 506 51 19 20 268 0.0720 0.0003 0.5703 0.0126 0.0574 0.0012 0.0317 0.0005 448 2 458 10 508 47 20 32 414 0.0725 0.0003 0.5658 0.0075 0.0566 0.0007 0.0328 0.0001 451 2 455 6 475 29 21 14 183 0.0724 0.0004 0.7672 0.0153 0.0768 0.0015 0.0469 0.0009 451 2 578 12 1115 39 表 2 清水泉辉长岩体的主量、微量和稀土元素组成
Table 2 Major, trace and rare earth elements abundances of Qingshuiquan gabbro
编号 XRD098/2 XRD098/3 XRD098/4 XRD098/5 SiO2 43.89 43.97 44.16 45.99 TiO2 0.84 0.93 0.95 0.74 Al2O3 19.86 19.74 19.88 20.43 TFe2O3 11.04 11.14 10.87 10.01 MnO 0.17 0.16 0.17 0.17 MgO 6.54 6.29 6.15 5.72 CaO 11.62 11.77 11.61 11.04 Na2O 1.30 1.64 1.72 2.32 K2O 1.39 0.80 0.88 0.68 P2O5 0.32 0.27 0.40 0.26 LOI 2.13 1.53 2.74 1.74 Total 99.10 98.24 99.53 99.10 Na2O+K2O 2.69 2.44 2.60 3.00 Mg# 54.00 52.80 52.85 53.10 Be 1.08 1.10 1.53 2.05 Sc 39.89 39.85 36.32 32.0 V 281.14 300.04 284.59 219.67 Cr 49.63 34.55 48.24 51.21 Ni 307.25 301.01 294.14 267.27 Co 33.62 20.88 19.84 16.34 Cu 25.14 14.11 13.04 6.24 Zn 94.98 104.30 106.97 97.43 Ga 27.39 29.87 29.73 27.97 Rb 54.42 23.95 30.17 23.12 Sr 867.64 943.00 1007.80 1060.36 Y 29.71 41.06 38.71 22.14 Zr 61.40 55.58 105.87 104.26 Nb 5.49 7.80 7.78 6.01 Cs 1.54 1.73 0.84 0.70 Ba 698.03 372.31 447.41 286.13 La 26.93 29.70 33.40 28.42 Ce 68.81 78.85 83.18 67.51 Pr 8.03 9.34 10.04 7.95 Nd 43.52 51.20 50.67 40.76 Sm 8.55 10.47 10.15 8.12 Eu 1.22 1.34 1.25 1.12 Gd 7.63 9.29 9.01 7.26 Tb 0.67 0.86 0.82 0.65 Dy 3.70 4.87 4.55 3.57 Ho 0.74 0.98 0.92 0.70 Er 2.08 2.81 2.64 1.97 Tm 0.29 0.39 0.39 0.29 Yb 1.87 2.49 2.24 1.657 Lu 0.29 0.38 0.35 0.26 Hf 1.20 1.18 1.77 1.59 Ta 0.16 0.25 0.25 0.17 Th 0.40 0.41 0.61 0.29 U 0.24 0.17 0.26 0.21 Ti 5036 5575 5695 4436 K 5769 3320 3652 2822 P 698 589 872 567 ∑REE 174.33 202.96 209.62 170.22 LREE/HREE 9.09 8.20 9.02 9.41 (La/Sm)N 1.98 1.78 2.07 2.20 (La/Yb)N 9.69 8.04 10.05 11.61 δEu 0.45 0.41 0.39 0.44 -
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