Zircon U-Pb age and geochemical characteristics of Ordovician pluton in Geleaobao, Inner Mongolia
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摘要:
内蒙古格勒敖包位于兴蒙造山带内，对该区奥陶纪岩浆岩的研究，有助于进一步认识该造山带早古生代的构造格局及其演化。正长花岗岩和斑状二长花岗岩样品LA-ICP-MS锆石年龄为441.6±3.7~454.9±4.6Ma，形成时代为奥陶纪。经地球化学分析，岩体为高钾钙碱性系列；铝饱和指数显示其为过铝质花岗岩。岩石富集大离子亲石元素（LILEs）Rb、Th、U、K，亏损Ba、Sr；富集高场强元素（HFSEs）La、Ce、Nd、Zr，亏损Nb、P、Ti；稀土元素总量为38.7×10-6~120.23×10-6，轻稀土元素较重稀土元素分异明显，分馏程度较高，表现出较弱的负Eu异常。综合区域地质资料，认为格勒敖包奥陶纪岩浆源区为上地壳物质的部分熔融，其形成环境为弧后盆地，在古亚洲洋向北俯冲之后经历了岛弧岩浆作用旋回，为弧后伸张阶段的产物。
Abstract:The Geleaobao area of Inner Mongolia is located in the Xingmeng orogenic belt. By studying the Ordovician magmatic rocks in this area, researchers can further understand the structure and evolution of the Early Paleozoic strata in this orogenic belt. The ages of the LA-ICP-MS zircons of syenite and mottled monzogranite samples studied in this work are 441.6±3.7Ma to 454.9±4.6Ma. The age of formation is Ordovician. Geochemical analysis shows that the pluton has high-K calc-alkaline characteristics. Aluminum saturation index shows that it is peraluminous granite. The characteristics of trace elements in rocks indicate that they are enriched in large ionic lithophilic elements (LILEs) Rb, Th, U, K, depleted in Ba, Sr; enriched in high field strength elements (HFSEs) La, Ce, Nd, Zr, and depleted in Nb, P, Ti. The total amount of rare earths in the rocks is 38.7×10 -6 to 120.23×10 -6, and the light rare earths exhibit obvious differences in heavier rare earths, with higher fractionation and weaker negative Eu anomalies. Based on regional geological data, it is believed that the source region of the Ordovician magma in the Geleaobao area is partially melted upper crustal materials, and the formation environment was a back-arc basin. After the subduction of the ancient Asian Ocean to the north, it experienced an island-arc magmatic process cycle, being product of extension stage.
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Keywords:
- Geleaobao /
- Ordovician /
- intrusive rock /
- geochemical characteristics /
- back-arc basin
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致谢: 成文过程中得到内蒙古自治区地质调查院正高级工程师武利文、王忠的指导，实验数据处理过程中得到内蒙古自治区岩浆活动成矿与找矿重点实验室肖建伟博士、呼日乐女士的帮助，特此表示衷心的感谢。
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图 2 格勒敖包岩体野外露头（a、c）及显微照片（b、d）
a、b—正长花岗岩；c、d—斑状黑云母二长花岗岩；Q—石英；Pl—斜长石；Kf—钾长石；Bi—黑云母；Ch—绿泥石
Figure 2. Field outcrop(a, c) and microphotographs(b, d) of the Geleaobao pluton
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图 5 格勒敖包岩体主量元素关系图
a—岩浆系统全碱-硅(TAS)分类[15]；b—A.R-SiO2(碱度率)图解[16]；c—SiO2-K2O图解[17]；d-A/CNK-A/NK图解[18]。
1—橄榄辉长岩；2a—碱性辉长岩；2b—亚碱性辉长岩；3—辉长闪长岩；4—闪长岩；5—花岗闪长岩； 6—花岗岩；7—硅英岩；8—二长辉长岩；9—二长闪长岩；10—二长岩；11—石英二长岩；12—正长岩； 13—副长石辉长岩；14—副长石二长闪长岩；15—副长石二长正长岩；16—副长正长岩；17—副长深成岩； 18—霓方钠岩/磷霞岩/粗白榴岩；Ir—Irvine分界线，上方为碱性，下方为亚碱性Figure 5. Major element variation diagrams of Geleaobao pluton
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图 6 微量元素原始地幔标准化蛛网图（a）和稀土元素球粒陨石标准化配分模式图（b）
（原始地幔、球粒陨石标准化值据参考文献[19]）
Figure 6. Primitive mantle-normalized trace element spidergrams（a）and chondrite-normalized REE patterns （b）for the Geleaobao pluton
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图 8 格勒敖包岩体构造判别图解
a—花岗岩 SiO2-Al2O3构造环境判别图解[18]；b—花岗岩构造环境（Y+Nb）-Rb判别图解[24]；c—Rb/10-Hf-3Ta判别图解[26]； d—Rb/30-Hf-3Ta判别图解[26]；syn-COLG—同碰撞花岗岩；WPG—板内花岗岩；VAG—火山弧花岗岩；ORG—洋脊花岗岩； IAG—岛弧花岗岩；CAG—大陆弧花岗岩；GCG—大陆碰撞花岗岩；POG—造山后花岗岩；RRG—与裂谷有关的花岗岩； CEUG—与大陆的造陆抬升有关的花岗岩
Figure 8. Tectonic discrimination diagrams of the GeLeAoBao pluton
表 1 格勒敖包地区岩体LA-ICP-MS锆石U-Th-Pb测年结果
Table 1 LA-ICP-MS zircon U-Th-Pb dating results of the Geleaobao granite pluton
测点编号 含量/10-6 Th/U 同位素比值 同位素年龄/Ma U Pb 206Pb/238U 1σ 207Pb/235U 1σ 207Pb/206Pb 1σ 206Pb/238U 1σ 207Pb/235U 1σ 207Pb/206Pb 1σ 粗中粒正长花岗岩(样品TW4072) 1 318 25 0.64 0.0737 0.0009 0.5665 0.0114 0.0557 0.0010 459 6 456 9 442 38 2 1022 70 0.18 0.0699 0.0009 0.5849 0.0100 0.0608 0.0008 435 5 468 8 631 29 3 518 36 0.16 0.0725 0.0009 0.5814 0.0105 0.0580 0.0008 451 5 465 8 531 32 4 725 51 0.16 0.0719 0.0009 0.5925 0.0111 0.0596 0.0009 448 5 472 9 591 33 5 567 39 0.22 0.0698 0.0008 0.5607 0.0100 0.0583 0.0008 435 5 452 8 540 32 6 276 20 0.30 0.0737 0.0009 0.5918 0.0120 0.0582 0.0010 458 6 472 10 539 38 7 440 32 0.18 0.0743 0.0009 0.5922 0.0114 0.0578 0.0009 462 6 472 9 524 35 8 363 27 0.58 0.0700 0.0008 0.5360 0.0100 0.0556 0.0009 436 5 436 8 436 36 9 399 30 0.44 0.0738 0.0009 0.5820 0.0112 0.0572 0.0009 459 6 466 9 499 34 10 218 17 0.59 0.0721 0.0009 0.5633 0.0136 0.0567 0.0012 449 5 454 11 478 48 11 804 54 0.13 0.0705 0.0008 0.5460 0.0092 0.0562 0.0008 439 5 442 7 460 30 12 505 37 0.31 0.0739 0.0010 0.5441 0.0101 0.0534 0.0008 460 6 441 8 348 33 13 301 22 0.39 0.0724 0.0009 0.5592 0.0117 0.0560 0.0010 451 6 451 9 453 40 14 151 11 0.46 0.0709 0.0008 0.5624 0.0154 0.0575 0.0014 442 5 453 12 511 55 15 409 31 0.53 0.0714 0.0009 0.5428 0.0102 0.0552 0.0009 444 5 440 8 421 35 16 516 41 0.68 0.0734 0.0009 0.5694 0.0103 0.0563 0.0008 457 6 458 8 463 32 18 702 50 0.25 0.0725 0.0009 0.5467 0.0096 0.0547 0.0008 451 6 443 8 400 31 19 516 38 0.54 0.0708 0.0008 0.5264 0.0095 0.0539 0.0008 441 5 429 8 367 34 20 353 26 0.53 0.0698 0.0008 0.5433 0.0103 0.0565 0.0009 435 5 441 8 472 37 21 415 30 0.30 0.0738 0.0009 0.5594 0.0105 0.0550 0.0008 459 6 451 8 410 34 23 207 16 0.50 0.0725 0.0009 0.5522 0.0142 0.0552 0.0013 451 6 446 11 421 51 24 246 18 0.47 0.0712 0.0009 0.5351 0.0122 0.0546 0.0011 443 5 435 10 397 45 25 476 33 0.15 0.0714 0.0009 0.5556 0.0101 0.0564 0.0008 445 5 449 8 468 33 26 893 61 0.12 0.0716 0.0009 0.5557 0.0095 0.0563 0.0008 446 5 449 8 465 30 27 458 34 0.28 0.0738 0.0010 0.5779 0.0115 0.0568 0.0009 459 6 463 9 482 34 斑状中粒黑云母二长花岗岩(样品TW1132) 3 659 47 0.22 0.0736 0.0009 0.5647 0.0098 0.0557 0.0008 458 6 455 8 440 31 4 660 50 0.42 0.0739 0.0009 0.5671 0.0099 0.0557 0.0008 460 6 456 8 439 31 5 885 60 0.16 0.0706 0.0008 0.5374 0.0090 0.0552 0.0007 440 5 437 7 421 30 6 819 60 0.34 0.0733 0.0009 0.5657 0.0095 0.0560 0.0008 456 5 455 8 453 30 7 673 47 0.24 0.0730 0.0009 0.5691 0.0100 0.0566 0.0008 454 6 457 8 476 31 9 464 33 0.19 0.0731 0.0009 0.5426 0.0096 0.0538 0.0008 455 6 440 8 365 32 10 183 15 0.52 0.0764 0.0010 0.5918 0.0148 0.0561 0.0012 474 6 472 12 457 48 11 2702 198 0.22 0.0755 0.0010 0.5679 0.0101 0.0546 0.0007 469 6 457 8 394 30 12 273 21 0.43 0.0770 0.0010 0.5694 0.0117 0.0538 0.0009 478 6 458 9 361 39 13 494 35 0.26 0.0726 0.0009 0.5572 0.0095 0.0557 0.0008 452 5 450 8 441 31 14 765 54 0.30 0.0729 0.0009 0.5751 0.0101 0.0571 0.0008 454 6 461 8 497 30 15 221 16 0.34 0.0718 0.0008 0.5698 0.0118 0.0576 0.0010 447 5 458 9 515 40 16 1128 89 0.80 0.0714 0.0009 0.5644 0.0094 0.0574 0.0008 444 5 454 8 507 29 17 140 11 0.53 0.0760 0.0009 0.5736 0.0144 0.0549 0.0012 472 6 460 12 407 51 18 832 58 0.28 0.0704 0.0008 0.5475 0.0095 0.0564 0.0008 439 5 443 8 469 32 19 653 47 0.37 0.0704 0.0009 0.5545 0.0111 0.0570 0.0009 438 5 448 9 493 35 21 505 37 0.20 0.0747 0.0009 0.5773 0.0102 0.0561 0.0008 464 6 463 8 457 31 22 746 55 0.42 0.0713 0.0008 0.5482 0.0093 0.0558 0.0008 444 5 444 7 445 30 23 363 27 0.28 0.0741 0.0009 0.5850 0.0111 0.0572 0.0009 461 6 468 9 501 34 25 735 51 0.15 0.0733 0.0009 0.5582 0.0095 0.0552 0.0007 456 6 450 8 422 30 26 448 37 0.86 0.0716 0.0009 0.5377 0.0095 0.0545 0.0008 446 5 437 8 392 33 27 822 60 0.26 0.0747 0.0009 0.5738 0.0097 0.0558 0.0008 464 6 460 8 442 30 29 683 51 0.30 0.0743 0.0010 0.5760 0.0102 0.0562 0.0008 462 6 462 8 462 31 30 595 42 0.22 0.0735 0.0009 0.5554 0.0097 0.0549 0.0008 457 6 449 8 406 31 
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表 2 格勒敖包岩体主量、微量和稀土元素特征
Table 2 Major, trace and rare earth element concentrations of the Geleaobao pluton
样品号 TW4072 TW1132 TW0260 TW0145 岩石名称 正长花岗岩 斑状黑云母 二长花岗岩 二长花岗岩 SiO2 78.74 73.42 72.96 73.73 TiO2 0.17 0.37 0.34 0.16 Al2O3 11.47 13.46 13.64 14.03 Fe2O3 0.71 0.21 0.72 0.83 FeO 0.12 2.45 1.56 0.52 MnO 0.014 0.057 0.06 0.039 MgO 0.16 0.64 0.75 0.38 CaO 0.22 1.77 1.24 0.6 Na2O 3.1 3.72 3.2 3.09 K2O 4.6 3.45 4.2 5.27 P2O5 0.046 0.12 0.12 0.21 烧失量 0.66 0.34 1.04 1.1 DI 96.6 84.62 87.03 92.73 SI 1.85 6.11 7.19 3.77 σ 1.66 1.69 1.82 2.27 A.R 4.86 2.78 2.98 3.67 A/CNK 1.095 1.03 1.131 1.181 Cs 1.8 13.2 10.4 13.2 Rb 60.4 273 199 273 Sr 550 99.6 109 99.6 Ba 102 180 206 180 Ga 23.4 14.4 14.6 14.4 Nb 14.9 6.88 7.42 6.88 Ta 1.15 1.58 1.15 1.58 Zr 441 64 118 64 Hf 12.4 2.46 3.84 2.46 Th 17.9 6.18 9.1 6.18 V 56.8 9.01 25.7 9.01 Cr 22.7 9.25 11.4 9.25 Co 12.3 1.06 3.58 1.06 Ni 15.7 3.99 7.56 3.99 Li 26 20 35.8 20 Sc 11.2 3.89 7.16 3.89 U 2.08 1.7 1.82 1.7 La 6.63 23.9 15.6 9.08 Ce 13 47.6 31.7 18.2 Pr 1.98 5.84 4.29 2.54 Nd 7.5 21.3 16.3 9.74 Sm 1.78 4.27 3.6 2.41 Eu 0.32 0.83 0.53 0.35 Gd 1.57 4.59 3.4 2.16 Tb 0.28 0.68 0.64 0.41 Dy 1.88 3.95 4.46 2.75 Ho 0.41 0.82 0.92 0.51 Er 1.24 2.49 2.66 1.4 Tm 0.26 0.41 0.44 0.23 Yb 1.6 3.08 3 1.52 Lu 0.25 0.47 0.46 0.24 Y 9.96 22.2 25.1 13.7 ∑REE 38.7 120.23 88 51.54 HREE 7.49 16.49 15.98 9.22 LREE 31.21 103.74 72.02 42.32 LR/HR 4.17 6.29 4.51 4.59 (La/Yb)N 2.97 5.57 3.73 4.28 注：主量元素含量单位为%，微量和稀土元素为10-6
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