Petrogenesis of Middle Triassic bimodal volcanic rocks in the Weixi area, Yunnan Province and geological implication for the formation and evolution of the Jinshajiang arc-basin
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摘要:
金沙江缝合带及其西侧发育的岩浆活动记录了大洋俯冲-碰撞过程，是反演大洋演化的关键。江达-维西陆缘弧维西地区的中三叠世火山岩（崔依比组）在时空上构成独特的双峰式火山岩组合，以基性火山岩与大量酸性火山岩交互出现为主要特征，流纹岩LA-ICP-MS锆石U-Pb年龄为244±1.3 Ma，表明崔依比组火山岩形成于中三叠世晚期。地球化学分析结果显示，玄武岩亏损Nb、Ta、Ti等高场强元素，Zr含量、Zr/Y值和（Th/Nb）N值低，兼具板内及弧火山岩的双重特性；流纹岩相对富硅，贫TiO2和MgO，具有较低的Al2O3，富集Rb、Ba等大离子亲石元素，亏损Nb等高场强元素，Sr、Ti均具有弱的负异常特征，也兼具有板内及弧火山岩的双重特性。综合认为，维西地区中三叠世崔依比组双峰式火山岩形成于与板块俯冲过程有关的板内伸展环境，是金沙江洋壳西向俯冲过程中板片断离导致区域伸展背景下岩浆活动的产物。
Abstract:The magmatic activities developed in the Jinshajiang suture zone and its west side record the process of ocean subduction and collision, which is the key to the inversion of ocean evolution.The Middle Triassic volcanic rocks (Cuiyibi Formation) constitute a unique bimodal volcanic assemblage in space and time in the southern Jomda-Weixi continental arc, which is mainly characterized by the interaction of basic and acidic volcanic rocks.The SHRIMP zircon U-Pb age of 244±1.3 Ma of rhyolite indicates that the Cuiyibi Formation volcanic rocks were formed in the Late Middle Triassic.The basalts are highly depleted in high field strength elements such as Nb, Ta and Ti, with low Zr content, Zr/Y and (Th/Nb)N values, which is similar to the geochemical characteristics of arc magma.The rhyolite are relatively rich in silicon, poor in TiO2 and MgO, and characterized by low Al2O3.The rhyolite are highly depleted in high field strength elements such as Nb.Sr and Ti show weak negative anomaly.They all have characteristics of intra-plate and arc volcanic rocks.Combined with previous studies, it is suggested that the bimodal volcanic rocks of the Cuiyibi Formation is the product of magmatic activity derived from plate fragmentation under the background of regional extension caused by the westward subduction of the Jinshajiang oceanic crust.
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Key words:
- Jomda-Weixi continental arc /
- Cuiyibi Formation /
- bimodal volcanic rocks /
- slab break-off /
- geotectonics
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表 1 维西崔依比组流纹岩LA-ICP-MS锆石U-Th-Pb同位素分析结果
Table 1. LA-ICP-MS zircon U-Th-Pb isotope analytical results of the rhyolite in the Cuiyibi Formation
测点 含量/10-6 Th/U 同位素比值(已扣除普通铅) 年龄/Ma 谐和度 Pb Th U 207Pb/206Pb 1σ 207Pb/235U 1σ 206Pb/238U 1σ 207Pb/235U 1σ 206Pb/238U 1σ 1 37 230 366 0.63 0.045 0.003 0.246 0.019 0.039 0.001 252 10.5 241 2.8 94% 2 64 376 612 0.61 0.048 0.003 0.257 0.016 0.039 0.001 247 10.1 241 2.7 91% 3 74 475 783 0.61 0.052 0.003 0.275 0.013 0.039 0.001 272 12.0 250 2.7 77% 4 38 252 361 0.70 0.043 0.004 0.222 0.019 0.039 0.001 257 13.0 248 3.5 93% 5 54 353 499 0.71 0.051 0.003 0.273 0.016 0.039 0.001 453 20.6 263 4.0 74% 6 101 671 925 0.73 0.050 0.003 0.264 0.013 0.039 0.001 253 12.3 241 3.3 98% 7 40 237 316 0.75 0.054 0.005 0.291 0.028 0.039 0.001 234 9.3 245 3.0 91% 8 81 496 762 0.65 0.053 0.003 0.285 0.015 0.039 0.001 262 9.6 248 2.9 99% 9 109 699 981 0.71 0.053 0.003 0.285 0.014 0.040 0.001 233 9.2 242 2.7 95% 10 108 749 1005 0.75 0.049 0.002 0.263 0.013 0.039 0.001 243 9.5 243 3.0 87% 11 61 402 631 0.64 0.049 0.003 0.267 0.017 0.040 0.001 249 12.2 245 3.4 88% 12 63 428 494 0.87 0.045 0.003 0.237 0.017 0.039 0.001 245 10.3 246 3.1 99% 13 125 783 1298 0.60 0.052 0.002 0.280 0.011 0.039 0.000 246 9.7 243 2.9 93% 14 81 532 801 0.66 0.054 0.003 0.287 0.013 0.039 0.001 251 15.9 248 3.6 99% 15 87 546 830 0.66 0.052 0.003 0.280 0.015 0.039 0.001 249 12.0 245 3.3 97% 16 69 471 688 0.69 0.049 0.003 0.262 0.016 0.039 0.001 253 11.5 240 3.4 99% 17 91 549 988 0.56 0.049 0.003 0.261 0.013 0.039 0.001 255 8.7 245 2.7 91% 18 63 393 615 0.64 0.043 0.003 0.231 0.015 0.039 0.001 243 14.1 243 3.4 94% 19 176 970 2297 0.42 0.051 0.002 0.274 0.009 0.038 0.000 236 8.4 244 2.7 95% 20 105 725 1025 0.71 0.050 0.003 0.268 0.014 0.039 0.001 218 14.7 252 3.5 73% 21 122 608 1097 0.55 0.054 0.002 0.303 0.011 0.041 0.000 253 12.0 248 3.2 84% 22 71 325 753 0.43 0.046 0.002 0.251 0.012 0.039 0.001 251 12.3 248 2.9 91% 23 75 408 656 0.62 0.049 0.003 0.263 0.014 0.039 0.001 257 9.1 250 2.6 87% 24 89 433 916 0.47 0.050 0.002 0.265 0.012 0.039 0.000 265 10.6 249 3.1 93% 
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表 2 维西崔依比组双峰式火山岩主量、微量和稀土元素分析结果
Table 2. Major, trace and rare earth elements analyses of the bimodal volcanic rocks from the Cuiyibi Formation
样品编号 PM3-55 PM3-59 PM3-59 PM3-61 PM3-63 PM3-67 PM4-21 PM4-23 PM2-03 岩性 流纹岩 流纹岩 流纹岩 流纹岩 流纹岩 流纹岩 玄武岩 玄武安山岩 玄武安山岩 SiO2 78.2 76.2 76.4 77.7 78.0 76.6 47.9 51.6 50.2 Al2O3 11.9 11.1 10.9 10.9 10.8 11.3 14.9 14.2 14.6 Fe2O3 1.42 1.88 2.18 1.19 0.82 2.88 8.24 8.45 12.4 CaO 0.08 0.77 0.59 0.05 0.03 0.08 7.22 6.89 10.9 MgO 0.78 0.81 0.95 0.18 0.26 0.77 7.50 4.90 7.07 K2O 3.84 6.28 6.07 7.38 8.22 5.38 1.12 0.67 0.63 Na2O 1.41 1.23 1.07 1.01 0.08 1.26 3.96 3.56 1.97 TiO2 0.27 0.29 0.27 0.26 0.28 0.32 0.72 0.77 1.16 P2O5 0.03 0.04 0.04 0.03 0.03 0.04 0.13 0.13 0.14 MnO 0.01 0.03 0.02 0.01 0.01 0.01 0.13 0.12 0.20 烧失量 2.00 1.57 1.52 1.30 1.04 1.36 8.87 8.61 1.38 总计 99.9 99.4 99.4 99.9 99.6 99.8 93.6 93.1 99.2 Sc 29.3 9.49 9.24 6.94 3.65 4.00 31.7 31.7 31.4 Ti 10780 1632 1606 1446 1601 1462 3794 3698 3720 V 227 7.09 5.16 4.37 4.89 6.83 208 187 188 Cr 9.45 197 6.68 2.17 2.61 2.71 135 196 186 Mn 699 32.9 229 6.05 8.56 71.2 1126 196 186 Co 20.8 0.94 0.79 0.48 0.67 1.13 31.3 37.5 31.2 Ni 4.35 41.3 1.96 0.76 1.25 1.83 34.1 50.9 49.3 Cu 8.69 10.7 21.7 3.16 4.71 1.58 42.7 28.0 24.6 Zn 114 29.2 34.2 15.3 18.3 27.3 66.9 135 92.5 Ga 22.4 20.2 19.8 15.9 16.7 18.3 17.6 19.3 17.9 Ge 9.00 4.45 5.38 4.92 4.03 4.62 7.02 6.99 7.12 Rb 23.5 206 180 190 219 179 53.4 8.45 23.9 Sr 122 16.1 17.6 11.8 16.7 19.5 192 186 322 Y 36.5 68.7 61.1 47.4 34.9 29.2 41.2 42.2 42.0 Zr 342 401 356 594 591 274 66.6 220 214 Nb 12.5 25.9 24.0 22.5 21.3 21.0 2.80 12.9 12.7 Cs 3.12 7.85 2.58 4.27 2.30 5.21 9.67 0.60 1.44 Ba 158 336 1309 845 898 1132 176 191 383 La 22.5 72.9 58.6 58.1 50.0 44.2 6.79 7.89 9.21 Ce 50.1 142 112 113 91.1 78.1 13.9 51.3 50.4 Pr 6.03 17.1 13.6 13.5 11.2 9.62 1.78 6.43 6.28 Nd 23.9 65.1 51.4 50.9 42.3 36.1 7.41 26.7 26.4 Sm 5.29 13.6 10.8 10.4 8.50 6.90 1.82 6.19 6.20 Eu 1.53 1.73 1.55 1.56 1.33 0.91 0.66 1.39 1.54 Gd 5.29 11.7 9.52 8.98 7.13 5.78 1.78 6.84 6.84 Tb 0.99 1.99 1.73 1.54 1.19 0.97 0.34 1.18 1.15 Dy 6.34 11.7 10.5 9.03 6.87 5.93 2.16 7.40 7.14 Ho 1.37 2.36 2.20 1.84 1.38 1.20 0.47 1.57 1.52 Er 4.15 7.16 6.79 5.30 3.92 3.57 1.36 4.44 4.28 Tm 0.65 1.08 0.97 0.80 0.57 0.52 0.21 0.67 0.65 Yb 4.29 7.04 6.26 5.12 3.58 3.24 1.37 4.22 4.02 Lu 0.68 1.08 0.96 0.77 0.54 0.47 0.21 0.63 0.61 Hf 5.01 7.42 6.61 9.80 10.0 5.03 1.11 5.32 5.18 Ta 0.90 1.85 1.73 1.64 1.68 0.94 0.19 0.89 0.85 Pb 3.82 5.08 3.05 9.13 11.4 3.81 4.35 84.8 10.7 Th 8.65 28.2 24.9 21.3 16.0 16.7 1.59 5.75 5.42 U 2.34 6.16 5.87 4.32 4.90 4.94 0.54 1.54 1.44 ∑REE 154 405 329 327 273 229 48.0 241 164 δEu 0.29 0.14 0.15 0.16 0.17 0.14 0.37 0.21 0.24 (Th/Ta)N 9.61 15.2 14.4 13.0 9.52 17.8 8.37 6.46 6.38 (La/Yb)N 5.24 10.3 9.35 11.3 14.0 13.6 4.96 5.57 5.72 (La/Sm)N 4.25 5.35 5.41 5.56 5.88 6.41 3.73 3.80 3.71 注: 主量元素含量单位为%，微量、稀土元素含量单位为10-6；δEu=2EuN/(SmN+GdN)，样品∑REE为样品La系元素含量与Y含量总和
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① 云南省地质矿产局.1∶25万临沧,滚龙幅(国内部分)区域地质调查报告.2003.
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