Ordovician island arc volcanic rocks in Halahu area of Qilian Mountain and their tectonic significance
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摘要:
通过1:5万区域地质调查,对祁连山哈拉湖地区火山岩进行了野外地质、岩石学、锆石U-Pb同位素年龄、地球化学等研究。LA-ICP-MS锆石U-Pb同位素定年结果显示,该火山岩年龄为466.3±2.4Ma(n=9,MSWD=1.4),形成于中奥陶世。岩石地球化学研究表明,哈拉湖地区火山岩为低钾拉斑玄武系列,绝大多数样品的亲石元素Rb、Th、U、Ce、Zr、Nd和稀土元素La、Sm富集明显;高场强元素(Sr、P、Ti)强烈亏损,Zr、Hf微弱富集,Ba明显亏损。总的特征显示,少数火山岩具有过渡型洋脊玄武岩的特征,大多数火山岩样品显示岛弧火山岩的特征。此外,哈拉湖地区的岛弧火山岩与晚奥陶世的岛弧花岗岩伴生在一起。这些特征表明,与俯冲有关的奥陶纪岛弧岩石可能与拉脊山地区古大洋的闭合有关。
Abstract:Based on 1:50000 regional geological survey, the authors studied volcanic rocks of Hulahu area in Qilian Mountain in the aspects of field geology, petrology, geochronology and geochemistry. Zircon LA-ICP-MS U-Pb dating results show that the Halahu volcanic rocks were formed in Middle Ordovician (466.3 ±2.4Ma, MSWD=1.4). Geochemical data show that the Halahu volcanic rocks belong to low-K tholeiite series, and are characterized by enrichment of LILEs (Rb, Th, U, Ce, Zr and Nd) and LREEs and depletion of Sr, P, Ti and Ba. On the whole, a few samples have features similar to those of T-MORB, but most samples show affinities with island arc related to subduction. In addition, the Halahu volcanic rocks were associated with Late Ordovician arc granitoids. These features indicate that the island arc rocks were associated with Ordovician paleo-oceanic closure in Lajishan area.
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Key words:
- volcanic rocks /
- island arc /
- Ordovician /
- Halahu area, Qilian Mountain
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图 5 哈拉湖火山岩稀土元素配分图(a)和原始地幔标准化微量元素蛛网图(b)(底图据参考文献[15])
Figure 5.
表 1 哈拉湖地区火山岩地球化学分析结果
Table 1. Geochemical compositions of volcanic rocks in Halahu area
样品号 ⅠC1014-2 ⅠC1014-3 IC1014 ⅠPm103/2-1 ⅠPm103/16-1 ⅠPm103/32-1 ⅠPm103/32-2 ⅠPm103/32-5 岩性 玄武岩 玄武岩 玄武岩 玄武安山岩 玄武安山岩 安山岩 安山岩 安山岩 SiO2 46.24 45.71 46.61 50.44 50.45 47.70 48.00 48.89 TiO2 0.38 0.40 0.58 0.83 0.90 0.66 0.78 0.53 Al2O3 7.57 7.97 12.39 12.19 12.27 10.51 11.70 10.51 Fe2O3 1.54 1.70 1.15 2.23 2.14 0.80 1.08 1.16 FeO 6.87 6.83 8.23 7.65 7.67 7.43 7.13 7.37 MnO 0.17 0.18 0.21 0.17 0.18 0.30 0.18 0.15 MgO 20.45 20.59 10.44 9.07 8.99 9.07 10.14 13.66 CaO 9.64 9.22 7.34 10.08 10.11 9.99 9.89 6.28 Na2O 0.49 0.49 0.17 2.10 2.34 0.89 2.07 0.19 K2O 0.19 0.18 0.42 1.59 1.43 0.04 0.33 0.03 P2O5 0.14 0.14 0.17 0.17 0.23 0.20 0.16 0.18 烧失量 5.11 5.37 11.11 2.40 2.27 11.42 7.65 10.25 总量 99.66 99.67 99.68 99.76 99.74 99.77 99.74 99.75 Mg# 82.95 83.00 68.16 65.41 65.25 67.59 70.49 75.65 A/NK 7.48 7.96 16.88 2.36 2.27 6.97 3.11 30.46 A/CNK 0.73 0.81 1.56 0.89 0.88 0.96 0.95 1.62 La 7.46 5.60 10.57 16.1 18.1 14.80 11.03 10.44 Ce 14.69 13.24 21.44 32.1 36.5 29.16 22.79 21.15 Pr 2.10 2.06 2.75 4.13 5.14 3.77 3.07 2.76 Nd 9.44 9.15 10.91 16.8 21.6 15.74 13.25 11.74 Sm 2.11 2.19 2.49 3.62 4.71 3.39 3.07 2.61 Eu 0.59 0.61 0.49 0.963 1.21 0.89 0.96 0.78 Gd 2.09 2.19 2.15 3.62 4.42 3.16 3.35 2.69 Tb 0.33 0.34 0.40 0.586 0.707 0.52 0.57 0.43 Dy 1.96 2.05 2.44 3.27 3.73 3.12 3.49 2.68 Ho 0.39 0.40 0.52 0.680 0.762 0.59 0.73 0.53 Er 1.14 1.13 1.46 1.85 2.09 1.70 2.10 1.50 Tm 0.19 0.18 0.24 0.297 0.315 0.26 0.32 0.24 Yb 1.18 1.13 1.51 2.00 2.04 1.47 2.02 1.43 Lu 0.18 0.17 0.23 0.319 0.300 0.21 0.27 0.20 Sr 59.17 57.33 145.4 428 449 241.2 272.6 156.3 Rb 3.73 1.48 14.50 35.9 28.9 3.18 9.63 1.58 Ba 37.09 26.84 111.4 380 440 23.70 104.2 36.40 Th 3.85 3.18 4.08 4.96 5.68 4.10 3.79 3.76 Ta 0.35 0.23 0.23 1.28 0.82 0.41 0.44 0.33 Nb 3.33 2.10 3.58 5.53 6.12 4.58 5.18 3.59 Zr 33.03 32.48 67.9 67.7 75.0 68.91 71.67 51.98 Hf 0.98 1.13 2.9 2.04 2.36 3.03 2.61 2.15 Cr 1177 1281 331.8 275 279 711.8 831.6 708.2 V 133.6 135.7 170.6 273 252 170.3 197.3 187.0 U 0.83 0.78 1.07 1.03 1.24 1.09 1.11 0.81 Y 10.27 10.37 13.33 17.40 19.09 14.92 17.93 13.55 Sc 24.5 24.89 27.91 38.2 41.2 26.25 34.28 39.33 注:主量元素含量单位为%,微量、稀土元素含量单位为10-6 表 2 哈拉湖地区火山岩(IU-PbC1014)锆石U-Th-Pb同位素测定结果
Table 2. Zircon U-Th-Pb isotope analyses for volcanic rocks (IU-PbC1014) in Halahu area
测试号 Pb U 206Pb/238U 1σ 207Pb/235U 1σ 207Pb/206U 1σ 208Pb/232U 1σ 232Pb/238U 1σ 年龄/Ma 10-6 206Pb/238U 1σ 207Pb/235U 1σ 1 12 162 0.07545 0.00041 0.58197 0.00920 0.05594 0.00087 0.03014 0.00023 0.25267 0.00072 469 3 466 7 2 50 242 0.18414 0.00094 1.92580 0.01367 0.07585 0.00052 0.06177 0.00014 0.61816 0.00325 1090 6 1090 8 3 22 257 0.07385 0.00049 0.57869 0.01587 0.05683 0.00130 0.03671 0.00126 0.59360 0.00911 459 3 464 13 4 27 304 0.02979 0.00021 0.41911 0.00597 0.10205 0.00134 0.01808 0.00014 0.37823 0.00066 498 3 652 9 5 12 150 0.08039 0.00042 0.90074 0.01293 0.08127 0.00111 0.03683 0.00040 0.37754 0.00088 480 3 541 9 6 30 348 0.07976 0.00042 0.90553 0.01919 0.08234 0.00165 0.03635 0.00067 0.36581 0.00102 495 3 655 14 7 29 338 0.07888 0.00041 0.80625 0.01234 0.07413 0.00108 0.03137 0.00036 0.40672 0.00074 489 3 600 9 8 50 651 0.07551 0.00042 0.58558 0.00430 0.05625 0.00044 0.03269 0.00025 0.27742 0.00099 469 3 468 3 9 28 333 0.07534 0.00040 0.58593 0.01031 0.05640 0.00095 0.02570 0.00011 0.65253 0.00045 468 2 468 8 10 51 583 0.07905 0.00041 0.84468 0.00868 0.07750 0.00080 0.03047 0.00018 0.51798 0.00059 490 3 622 6 11 39 99 0.37093 0.00208 6.41381 0.03874 0.12541 0.00073 0.11708 0.00039 0.28248 0.00033 2034 11 2034 12 12 15 158 0.07948 0.00048 0.94601 0.01960 0.08633 0.00188 0.03708 0.00025 0.57647 0.00213 493 3 676 14 13 10 70 0.14135 0.00078 1.62255 0.05211 0.08325 0.00264 0.07423 0.00183 0.20732 0.00080 852 5 979 31 14 75 733 0.07966 0.00041 1.49718 0.00952 0.13632 0.00082 0.06244 0.00012 0.43148 0.00037 494 3 929 6 15 18 218 0.07444 0.00039 0.58308 0.00954 0.05681 0.00087 0.03029 0.00042 0.57697 0.00333 463 2 466 8 16 54 620 0.07514 0.00041 0.58008 0.01090 0.05599 0.00101 0.02416 0.00028 0.85623 0.00264 467 3 465 9 17 90 418 0.19838 0.00144 2.83062 0.02120 0.10349 0.00083 0.08891 0.00107 0.34191 0.00144 1167 8 1364 10 18 20 252 0.07482 0.00040 0.58176 0.00953 0.05639 0.00092 0.02340 0.00011 0.50771 0.00041 465 2 466 8 19 95 477 0.18679 0.00099 2.65184 0.02096 0.10296 0.00089 0.07092 0.00022 0.32939 0.00432 1104 6 1315 10 20 34 148 0.20761 0.00107 2.34458 0.01698 0.08191 0.00057 0.05787 0.00022 0.62972 0.00335 1216 6 1226 9 21 66 840 0.07541 0.00040 0.58401 0.00386 0.05617 0.00039 0.03284 0.00013 0.34566 0.00080 469 2 467 3 22 9 261 0.02893 0.00016 0.50801 0.00872 0.12737 0.00214 0.02838 0.00039 0.21948 0.00081 184 1 417 7 23 43 556 0.07486 0.00041 0.58123 0.00449 0.05631 0.00041 0.02048 0.00004 0.48987 0.00153 465 3 465 4 24 70 455 0.06704 0.00044 5.14905 0.04494 0.55703 0.00333 0.27698 0.00185 0.25872 0.00082 418 3 1844 16 -
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