The geochemical characteristics and tectonic significance of Devonian bimodal volcanic rocks in Yemaquan area of Beishan, Inner Mongolia
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摘要:
北山造山带南缘野马泉一带的泥盆纪火山岩(三个井组)在时空上构成独特的双峰式火山岩组合，以大量基性火山岩与酸性火山岩交互出现为主要特征。测得三个井组玄武岩、流纹岩LA-ICP-MS锆石U-Pb年龄分别为404.8±3.9 Ma、402.3±1.8 Ma，为早泥盆世。其中，玄武岩具有富铝、高钠、低钾及钙碱性系列的特征。火山岩地球化学特征表明，玄武岩母岩浆来自于地幔的部分熔融，且发生了一定程度的地壳混染，反映其形成于板内伸展环境。流纹岩相对富硅和全碱，贫TiO2和MgO，以及较低的Al2O3值，具有较典型A型花岗岩类的特征，为碰撞后伸展环境下底侵基性岩浆结晶分异的产物。早泥盆世三个井组双峰式火山岩的厘定进一步证实北山造山带形成于碰撞造山后的伸展环境，代表其形成的地球动力学背景为碰撞后转换为拉伸构造演化阶段，表明从早泥盆世开始，不断隆升的山系前陆出现局部引张下陷形成山前裂陷带，对研究北山地区早古生代构造格局及板块构造体制提供了新的地质依据。
Abstract:Devonian volcanic rocks from Sangejing Formation in Yemaquan area in the south margin of Beishan orogenic belt form a unique bimodal volcanic rock assemblage in time and space, which is characterized by the interaction of basic volcanic rocks and acid volcanic rock.The U-Pb ages of zircons from basalts and rhyolites in the Sangejing Formation are 404.8±3.9 Ma and 402.3±1.8 Ma, respectively, which belong to the Early Devonian.The basalt is rich in aluminum and sodium, and is depleted in potassium.And it belongs to calc alkaline series.The geochemical characteristics of the volcanic rocks show that the basaltic mother magma in the study area came from the partial melting of the mantle, and a certain degree of crustal contamination occurs.The geochemical characteristics of basalts reflect that they formed in the intraplate extensional environment.The rhyolites in the study area are relatively rich in silicon and AKL, depleted in TiO2 and MgO, and low in Al2O3.They are typical A-type granitoids.The rocks are the products of crystallization and differentiation of underplating basic magma in the post collision extensional environment.The determination of the bimodal volcanic rocks of the Sangejing Formation in the Early Devonian further confirmed that the Beishan orogenic belt was formed in the extension transformation period of collisional orogeny.The geodynamic background of its formation is the stage of transformation from collision to extensional tectonic environment, which indicated that from the Early Devonian the uplifted foreland of the mountain appeared local extension and subsidence to form the piedmont rift belt.It provides a new theoretical basis of the study of the Early Paleozoic structural pattern of Beishan area.
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表 1 研究区构造单元划分
Table 1. Division of tectonic units in the study area
一级构造单元 二级构造单元 三级构造单元 塔里木板块(Ⅱ) 敦煌微陆块(Ⅱ1) 马鬃山南-白石山-古硐井南晚古生代断陷盆地(Ⅱ12) 红柳大泉-红山井晚古生代裂陷带(Ⅱ13) 石板山-大奇山-神螺山晚古生代陆内裂谷带(Ⅱ14) 敦煌地块(DH) 
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表 2 北山造山带野马泉一带早泥盆世三个井组火山岩主量、微量和稀土元素地球化学分析结果
Table 2. Composition of major, trace and rare earth elements for Sangejing Formation in Yemaquan area, Beishan Orogenic Belt
样品号 YMQ-1 YMQ-2 YMQ-3 YMQ-4 YMQ-5 YMQ-6 YMQ-7 YMQ-8 岩性 玄武岩 流纹岩 SiO2 49.73 48.31 50.26 49.19 48.38 77.06 74.6 76.68 TiO2 2.92 1.67 2.78 3.27 1.85 0.19 0.18 0.16 Al2O3 13.74 16.57 13.38 13.72 16.78 11.76 12.16 11.74 Fe2O3 6.84 1.92 3.94 2.27 2.1 0.29 0.56 2.1 FeO 7.11 6.73 8.78 10.02 8.24 0.65 0.42 0.12 MnO 0.275 0.17 0.21 0.24 0.18 0.02 0.024 0.017 MgO 4.62 6.8 4.73 4.88 6.56 0.36 0.47 0.06 CaO 7.01 9.65 7.65 8.31 9.06 0.72 1.87 0.82 Na2O 4.24 3.39 3.78 3.86 3.29 3.5 3.27 4.4 K2O 1.06 0.64 0.84 0.67 0.88 4.29 4.27 2.62 P2O5 0.327 0.21 0.35 0.52 0.32 0.033 0.033 0.03 烧失量 2.1 3.82 3.16 2.9 2.23 1 2.05 1.14 总计 99.98 99.88 99.86 99.84 99.86 99.86 99.92 99.89 Rb 42.32 7.4 41.38 20.14 31.9 97.7 200 69.3 Ba 164.3 161.73 255.22 430.26 180 850 503 407 Th 1.94 0.56 2.32 2.46 1.96 20.1 24.3 18.7 U 0.99 0.28 0.96 0.64 1.17 3.82 4.88 4.96 Ta 0.39 0.46 0.38 0.65 0.72 1.19 1.18 1.26 Nb 5.04 4.16 4.67 9.74 11.5 16 14.3 16.5 Sr 335 305 282 253 329 58.9 60.3 45.2 Zr 232 143 179 269 182 362 287 357 Hf 7.71 3.91 5.35 7.66 5.93 10.7 8.95 9.85 Y 47.1 29.3 46.8 51.7 38.1 49.6 55.7 63.2 Yb 9.43 2.08 4.35 2.34 4.33 3.46 7.79 22.4 Lu 0.84 0.53 0.63 0.82 0.8 1.2 1.07 1.51 La 10.6 7.65 13.1 21.8 20.5 51.2 30.4 34.1 Ce 25.9 18.1 32 53 40.3 96.8 65.5 58.3 Pr 4.29 3.29 4.43 7.13 5.52 11.7 9.23 9.5 Nd 22 16.8 21.5 33.2 24.2 44.1 39.7 40.8 Sm 6.1 4.65 5.88 8.55 5.45 8.87 9.38 9.52 Eu 1.94 1.6 2.03 2.78 1.56 0.85 1.4 1.65 Gd 4.87 4.42 5.31 7.44 4.48 8.88 8.49 8.77 Tb 1.25 0.98 1.19 1.6 0.98 1.64 1.86 1.88 Dy 8.7 6.28 8.04 10.3 6.51 9.98 11.4 11.9 Ho 1.76 1.26 1.55 2.03 1.31 2 2.32 2.52 Er 4.28 3.25 4.28 5.57 3.48 5.86 6.01 6.86 Tm 0.84 0.58 0.76 0.95 0.69 0.99 1.15 1.32 ∑REE 98.37 72.92 105.15 161.05 120.15 251.15 194.99 196.57 δEu 1.05 1.07 1.09 1.04 0.94 0.29 0.47 0.54 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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表 3 玄武岩(YMQ-TW1)LA-ICP-MS锆石U-Pb测试结果
Table 3. Zircon U-Pb isotopic data analyzed by LA-ICP-MS of basalt(YMQ-TW1)
样品号 含量/10-6 同位素比值 年龄/Ma Pb U 207Pb/206Pb 1σ 207Pb/235U 1σ 206Pb/238U 1σ 207 Pb/ 206 Pb 1σ 207Pb/ 235Pb 1σ 206Pb/ 238U 1σ RZ1 114 400 0.0549 0.0027 0.4939 0.0240 0.0653 0.0012 409 114 408 16.3 408 7 RZ2 21.6 116 0.0543 0.0046 0.4798 0.0404 0.0634 0.0017 383 191 398 27.7 396 10 RZ3 29.2 127 0.0545 0.0041 0.4833 0.0335 0.0647 0.0017 391 169 400 22.9 404 11 RZ4 72 261 0.0556 0.0035 0.4840 0.0294 0.0628 0.0013 435 134 401 20.1 393 8 RZ5 19.6 108 0.0545 0.0053 0.4467 0.0446 0.0593 0.0019 391 214 375 31.3 371 12 RZ6 63 206 0.0547 0.0035 0.4887 0.0302 0.0646 0.0013 398 175 404 20.6 403 8 RZ7 33.5 143 0.0545 0.0040 0.4727 0.0342 0.0624 0.0015 391 167 393 23.6 390 9 RZ8 30.5 133 0.0574 0.0056 0.5167 0.0435 0.0667 0.0018 506 214 423 29.1 416 11 RZ9 15.8 63.9 0.0624 0.0068 0.6297 0.0599 0.0790 0.0032 687 235 496 37.3 490 19 RZ10 72 254 0.0554 0.0035 0.4834 0.0266 0.0635 0.0012 428 143 400 18.2 397 7 RZ11 105 390 0.0540 0.0028 0.4733 0.0236 0.0627 0.0012 369 117 393 16.2 392 7 RZ12 62 206 0.0596 0.0040 0.5270 0.0339 0.0635 0.0014 587 146 430 22.5 397 8 RZ13 20.6 94.6 0.0585 0.0087 0.5095 0.0651 0.0660 0.0030 546 328 418 43.8 412 18 RZ14 18.7 95.9 0.0569 0.0057 0.4662 0.0429 0.0603 0.0019 487 219 389 29.7 378 12 RZ15 20.3 69.7 0.0586 0.0087 0.5405 0.0709 0.0705 0.0038 550 332 439 46.8 439 23 RZ16 26.6 98.3 0.0589 0.0053 0.5595 0.0402 0.0726 0.0019 565 198 451 26.2 452 11 RZ17 29.6 142 0.0543 0.0047 0.4921 0.0437 0.0653 0.0017 383 198 406 29.8 408 10 RZ18 264 583 0.0571 0.0031 0.5502 0.0288 0.0693 0.0014 494 120.4 445 18.9 432 8 RZ19 24.2 89.4 0.0594 0.0087 0.5225 0.0659 0.0668 0.0026 589 319 427 44.0 417 16 RZ20 19.7 103 0.0562 0.0064 0.4700 0.0484 0.0626 0.0021 461 254 391 33.4 392 13 RZ21 34.3 166 0.0523 0.0033 0.4678 0.0321 0.0628 0.0016 302 144 390 22.2 393 9 RZ22 15.4 82.6 0.0545 0.0053 0.4683 0.0400 0.0624 0.0016 391 216 390 27.7 390 10 RZ23 21.6 76.4 0.0582 0.0062 0.5812 0.0546 0.0751 0.0026 539 233 465 35.1 467 16 RZ24 30.9 101 0.0599 0.0062 0.5171 0.0469 0.0686 0.0021 598 221 423 31.4 427 13 RZ25 14.4 84.0 0.0552 0.0049 0.4752 0.0355 0.0634 0.0020 420 200 395 24.4 396 12 RZ26 40.3 120 0.0568 0.0058 0.4946 0.0418 0.0634 0.0024 483 229 408 28.4 396 15 RZ27 19.1 90.8 0.0568 0.0054 0.4922 0.0445 0.0646 0.0020 483 211 406 30.3 404 12 RZ28 81 306 0.0520 0.0030 0.4936 0.0292 0.0678 0.0013 287 133 407 19.8 423 8 RZ29 33.3 163 0.0566 0.0036 0.5350 0.0344 0.0678 0.0013 476 143 435 22.8 423 8 RZ30 15.6 59.7 0.0651 0.0078 0.4928 0.0414 0.0636 0.0025 776 254 407 28.2 398 15 RZ31 22.4 83.4 0.0553 0.0052 0.4880 0.0405 0.0647 0.0021 433 211 404 27.7 404 13 RZ32 75 233 0.0553 0.0031 0.5042 0.0286 0.0659 0.0014 433 123 415 19.3 411 8 RZ33 85 413 0.0537 0.0035 0.4458 0.0279 0.0595 0.0014 367 145 374 19.6 372 9 RZ34 152 429 0.0545 0.0027 0.5069 0.0266 0.0656 0.0011 391 115 416 17.9 410 6 RZ35 30.2 137 0.0606 0.0063 0.5797 0.0534 0.0731 0.0023 633 226 464 34.3 455 14
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表 4 流纹岩(YMQ-TW2)LA-ICP-MS锆石U-Pb测试结果
Table 4. Zircon U-Pb isotopic data analyzed by LA-ICP-MS of rhyolite(YMQ-TW2)
样品号 含量/10-6 同位素比值 年龄/Ma Pb U 207Pb/206Pb 1σ 207Pb/235U 1σ 206Pb/238U 1σ 207 Pb/206 Pb 1σ 207Pb/ 235Pb 1σ 206Pb/238U 1σ RZ1 44.25 416.80 0.06101 0.00156 0.54892 0.01393 0.06519 0.00100 639 56 444 9 407 6 RZ2 42.70 406.31 0.05890 0.00188 0.53071 0.01572 0.06514 0.00104 565 66 432 10 407 6 RZ3 25.57 315.19 0.05640 0.00175 0.50368 0.01609 0.06453 0.00065 478 101 414 11 403 4 RZ4 45.45 478.19 0.05937 0.00172 0.53085 0.01634 0.06465 0.00077 589 63 432 11 404 5 RZ5 31.97 313.71 0.05577 0.00144 0.49938 0.01284 0.06492 0.00070 443 57 411 9 406 4 RZ6 67.50 737.76 0.05406 0.00123 0.48456 0.01085 0.06506 0.00076 372 56 401 7 406 5 RZ7 29.52 328.53 0.06069 0.00197 0.54005 0.01604 0.06498 0.00087 628 66 438 11 406 5 RZ8 27.82 335.62 0.05602 0.00125 0.49981 0.01119 0.06482 0.00057 454 18 412 8 405 3 RZ9 79.11 798.53 0.05922 0.00157 0.52682 0.01448 0.06450 0.00069 576 57 430 10 403 4 RZ10 28.25 363.14 0.05264 0.00162 0.46779 0.01397 0.06470 0.00068 322 73 390 10 404 4 RZ11 14.71 141.31 0.05882 0.00221 0.52656 0.01966 0.06525 0.00086 561 81 430 13 408 5 RZ12 34.84 433.07 0.05506 0.00142 0.49172 0.01286 0.06484 0.00099 413 53 406 9 405 6
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