Identification of Ordovician oceanic island basalt in the Changning-Menglian suture zone and its tectonic implications:Evidence from geochemical and geochronological data
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摘要:
西南三江昌宁-孟连构造带传统被认为是晚古生代古特提斯洋的缝合带。在该构造带中段铜厂街蛇绿混杂岩东侧勐勇-芒红一带，新识别出一套近南北向展布的灰绿色气孔-杏仁状玄武岩、安山玄武岩和硅质岩、浅变质泥质粉砂岩组成的火山-沉积岩系。对杏仁状玄武岩样品进行LA-ICP-MS锆石U-Pb定年，获得其206Pb/238U年龄加权平均值为449.3±8.4Ma(MSWD=3.5)，表明该套火山岩喷发于奥陶纪。元素地球化学特征表现为富铝(12.11%~15.04%)、富钠(K2O/N2O=0.03~0.44)、高Mg#值(45.9~61.7)，同时，该套火山岩中MgO=6.78%~12.34%，平均为9.43%，Cr=58.6×10-6~636×10-6，平均为310.3×10-6，Ni=57.4×10-6~410×10-6，平均为202.1×10-6，Nb=18.9×10-6~32.8×10-6，平均为25.06×10-6，具有轻稀土元素强烈富集的"直线状"稀土元素配分模式，(La/Yb)N=5.25，(Ce/Yb)N=4.42，(Ce/Sm)N=1.68，稀土元素总量随全碱含量增大有升高的趋势，表明其为亚速尔型洋岛。总体上，该套火山岩具碱性OIB(洋岛玄武岩)的特征，可能是大洋板内热点前部熔融的产物。亚速尔型洋岛玄武岩的出现代表了特提斯洋盆内洋岛发育早期阶段的物质记录，为重新认识滇西南昌宁-孟连地区特提斯主洋盆开阔多岛洋的格局提供了丰富的岩石学依据。
Abstract:The Changning-Menglian tectonic belt has been traditionally known as one of the Paleo-Tethys suture zones in the Sanjiang orogenic domain, Southwest China. This study focused on the newly found volcano-sedimentary rocks consisting of vesicular and amygdaloidal basalts, andesitic basalt, siliceous rock, and meta-mud siltstone in the Mengyong-Manghong area located to the east of the Tongchangjie ophiolitic rocks in the central Changning-Menglian suture zone. LA-ICP-MS zircon U-Pb dating reveals that the 206Pb/238U weighted mean age of the amygdaloidal basalt is 449.3±8.4Ma(MSWD=3.5), suggesting that this suite of volcanic rocks was emplaced in Ordovician. The basaltic rocks are rich in Al2O3 (12.11%~15.04%), show high K2O/N2O ratios of 0.03~0.44 and high MgO (6.78%~12.34%, 9.43% on average) with high Mg# of 45.9~61.7, and Cr, Ni, Nb values of 58.6×10-6~636×10-6, 57.4×10-6~410×10-6 and 18.9×10-6~32.8×10-6 (310.3×10-6, 202.1×10-6 and 25.06×10-6 on average) respectively. They have flat chondrite-normalized REE patterns with low (La/Yb)N (averagely 5.25), (Ce/Yb)N (4.42 on average) and (Ce/Sm)N (1.68 on average). The ΣREE value of the basaltic rocks increases with the increasing of the total alkaline, which suggests that the rocks are similar to the Azores-type oceanic island basalts (OIB). Generally, this suite of volcanic rocks is alkaline with OIB affinity, which might have resulted from the melting of the head of the hotspot in the Intra-oceanic plate. The new discovery of the Azores-type OIB in the Changing-Menglian records the product of OIB activity in the Early Paleozoic evolution of Tethys Ocean, which provides important basic information for further understanding of the composite arc-basin system in the Tethyan domain of Southwest China.
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表 1 老南掌玄武岩(D0100-1-2)LA-ICP-MS锆石U-Th-Pb同位素数据
Table 1. LA-ICP-MS zircon U-Th-Pb dating results of the Laonanzhang basalt
点号 Pb Th U Th/U 同位素比值 同位素年龄/Ma 10-6 207Pb/206Pb 1σ 207Pb/235U 1σ 206Pb/238U 1σ 207Pb/206Pb 1σ 207Pb/235U 1σ 206Pb/238U 1σ 01 210 125 111 1.12 0.0973 0.0023 3.9182 0.0905 0.2900 0.0038 1573 44 1617 19 1642 19 02 45.6 90.4 140 0.64 0.0581 0.0028 0.5906 0.0270 0.0740 0.0012 532 101 471 17 460 7 03 163 160 298 0.54 0.0571 0.0022 0.5539 0.0220 0.0699 0.0009 494 85 448 14 435 6 04 129 176 409 0.43 0.0525 0.0021 0.2810 0.0105 0.0389 0.0005 306 91 251 8 246 3 05 201 196 448 0.44 0.0511 0.0015 0.2636 0.0082 0.0372 0.0005 256 73 238 7 236 3 06 144 219 352 0.62 0.0507 0.0019 0.2568 0.0095 0.0367 0.0004 228 85 232 8 232 3 07 71 352 1424 0.25 0.0526 0.0014 0.3056 0.0086 0.0419 0.0004 309 57 271 7 265 3 08 118 295 464 0.64 0.0569 0.0014 0.5876 0.0147 0.0748 0.0008 487 56 469 9 465 5 09 154 214 212 1.01 0.0683 0.0015 1.1975 0.0276 0.1265 0.0014 880 46 799 13 768 8 10 73 154 217 0.71 0.0662 0.0017 1.2394 0.0334 0.1351 0.0018 813 49 819 15 817 10 11 83 299 354 0.85 0.0564 0.0016 0.6059 0.0174 0.0777 0.0011 478 61 481 11 482 6 12 131 154 326 0.47 0.0508 0.0026 0.2602 0.0128 0.0370 0.0006 232 110 235 10 234 4 13 71 231 1194 0.19 0.0877 0.0016 3.0356 0.0630 0.2493 0.0032 1376 37 1417 16 1435 17 14 162 102 176 0.58 0.0570 0.0024 0.5593 0.0251 0.0709 0.0011 500 94 451 16 442 7 15 447 89.8 157 0.57 0.0558 0.0026 0.5433 0.0251 0.0702 0.0010 443 99 441 17 438 6 16 209 253 227 1.11 0.0552 0.0019 0.5409 0.0185 0.0711 0.0009 420 80 439 12 442 6 17 70 225 527 0.43 0.0576 0.0018 0.4640 0.0146 0.0582 0.0008 522 67 387 10 365 5 18 66 280 786 0.36 0.0550 0.0016 0.2207 0.0066 0.0289 0.0003 413 67 202 6 184 2 19 32.2 69.9 97.2 0.72 0.0582 0.0028 0.5757 0.0265 0.0728 0.0011 539 107 462 17 453 6 20 139 290 1110 0.26 0.0594 0.0017 0.5361 0.0156 0.0651 0.0006 583 63 436 10 406 4 21 50.4 111 179 0.62 0.0588 0.0023 0.5811 0.0225 0.0718 0.0010 561 86 465 14 447 6 22 127 105 308 0.34 0.0747 0.0019 1.8017 0.0422 0.1748 0.0018 1061 57 1046 15 1038 10 23 131 280 373 0.75 0.0562 0.0022 0.5927 0.0220 0.0764 0.0009 461 85 473 14 475 5 24 57.7 133 210 0.63 0.0551 0.0020 0.5553 0.0184 0.0735 0.0009 417 75 448 12 457 6 
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表 2 老南掌玄武岩的全岩主量、微量和稀土元素测试数据
Table 2. Whole-rock major, trace and rare earth elements analyses of the Laonanzhang basalt
样品号 D0100-1-1 D0100-1-2 D0100-1-3 D0100-1-4 D0100-1-5 D0100-1-7 D0100-1-8 D0100-1-9 D1582-1-1 SiO2 49.72 48.26 45.87 45.64 40.77 50.00 45.43 48.64 49.69 TiO2 2.04 3.00 2.27 1.95 1.79 2.12 2.13 2.34 2.12 Al2O3 11.59 13.60 12.50 10.68 10.62 12.82 12.22 14.38 12.41 Fe2O3 1.63 1.42 1.69 1.85 1.61 1.57 2.06 1.66 3.59 FeO 7.85 9.48 9.97 8.95 7.65 7.61 8.42 7.33 5.98 MnO 0.13 0.18 0.17 0.15 0.16 0.14 0.15 0.14 0.14 MgO 9.36 6.78 10.27 12.28 10.47 7.94 12.34 7.64 7.78 CaO 7.42 7.75 8.21 10.68 11.38 8.02 7.77 8.00 9.22 Na2O 4.07 3.75 2.98 2.09 2.12 4.30 3.04 3.62 3.53 K2O 0.14 0.85 0.20 0.32 0.89 0.77 0.45 1.59 0.88 P2O3 0.22 0.34 0.24 0.21 0.22 0.27 0.31 0.29 0.24 烧失量 5.35 3.99 5.02 4.42 11.66 3.79 5.05 3.66 3.23 合计 99.52 99.40 99.39 99.22 99.34 99.35 99.37 99.29 98.81 Mg# 68.17 64.92 71.13 71.08 65.21 72.47 65.18 56.23 68.30 A/NK 1.69 1.92 2.44 2.82 2.39 1.62 2.23 1.87 1.92 A/CNK 0.85 0.96 0.99 0.79 0.72 0.84 0.97 0.96 0.82 Cu 92.3 97.5 84.4 77.4 73.9 95.5 89.2 86.2 89.1 Zn 94.8 134 112 100 93.7 117 104 101 97.7 Cr 186 58.6 448 636 530 213 299 202 222 Ni 107 57.4 284 410 404 108 239 103 107 Co 38.3 37.7 50.0 51.8 50.6 35.7 46.7 32.7 37.2 Rb 6.50 22.8 3.50 6.10 11.9 20.0 8.70 25.8 19.6 Sr 179 155 101 73.0 468 180 135 149 244 Ba 44.6 430 81.7 49.2 66.3 320 145 307 344 V 239 311 266 248 252 243 268 254 203 Sc 21.5 23.5 26.3 23.9 22.3 24.0 25.3 22.4 25.3 Nb 22.0 32.8 25.7 22.0 18.9 24.0 27.0 30.3 22.8 Zr 147 221 166 140 139 168 174 189 161 Hf 5.17 5.85 5.05 4.44 4.04 4.84 5.04 5.12 4.19 U 0.44 0.71 0.45 0.35 1.49 0.65 0.65 0.68 0.46 Th 1.27 2.31 1.50 1.39 1.40 1.64 1.70 2.10 1.51 La 17.2 30.0 20.0 17.1 16.2 21.2 24.1 23.3 20.0 Ce 38.2 64.8 44.5 38.0 35.4 47.2 51.1 51.9 43.6 Pr 5.23 8.49 5.98 5.18 4.79 6.34 6.86 6.84 5.71 Nd 22.3 35.5 25.2 22.3 20.9 27.2 28.5 28.5 24.3 Sm 5.17 7.60 5.89 5.12 4.59 5.89 6.02 6.24 5.86 Eu 1.60 2.69 1.81 1.60 1.49 1.83 1.95 1.95 2.04 Gd 4.95 7.24 5.60 5.00 4.51 5.73 5.70 5.85 5.45 Tb 0.93 1.34 1.13 0.92 0.91 1.04 1.01 1.08 1.05 Dy 6.16 8.39 6.79 6.00 5.73 6.50 6.51 6.95 6.59 Ho 1.07 1.57 1.18 1.09 1.01 1.17 1.17 1.24 1.26 Er 2.72 3.98 3.13 2.86 2.60 3.07 2.90 3.45 3.28 Tm 0.38 0.58 0.46 0.40 0.37 0.44 0.41 0.53 0.47 Yb 2.32 3.47 2.81 2.47 2.29 2.65 2.47 2.96 2.75 Lu 0.31 0.49 0.38 0.35 0.34 0.38 0.38 0.47 0.44 Y 28.1 42.5 26.9 26.1 25.8 19.5 28.8 18.7 32.5 ΣREE 136.54 218.58 151.77 134.41 126.82 150.07 167.87 159.89 155.25 δEu 0.95 1.09 0.95 0.96 0.99 0.95 1.01 0.97 1.09 (La/Yb)N 5.00 5.84 4.81 4.66 4.78 5.39 6.56 5.29 4.90 (Gd/Yb)N 1.72 1.69 1.61 1.64 1.59 1.74 1.86 1.59 1.60 (Ce/Yb)N 4.26 4.10 3.99 4.00 4.61 5.35 4.53 4.83 4.10 (Ce/Sm)N 1.62 1.62 1.62 1.67 1.72 1.82 1.75 1.79 1.56 注：主量元素含量单位为%，微量和稀土元素含量为10-6；A/NK=摩尔Al2O3/(Na2O+K2O)，A/CNK=摩尔Al2O3/(CaO+Na2O+K2O)；δEu=2EuN/(SmN+ GdN)，其中N为球粒陨石标准化值
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