LA-ICP-MS zircon U-Pb age, geochemical characteristics and tectonic implications of the Bangdong granite intrusions in Longling area, western Yunnan Province
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摘要:
保山地块是滇西特提斯多岛洋的重要组成部分，属滇-缅-马-泰微大陆的北端。出露于保山地块西部的蚌东花岗岩体的LA-ICP-MS锆石U-Pb年龄为228.3±1.7Ma，记录了特提斯洋关闭后的弧-陆碰撞过程。该岩体以正长花岗岩为主，岩石具有高硅、富碱、低镁、低钙、富铬、富镍的特点；在B-A图解上显示了堇青石过铝花岗岩的演化特征；总体上表现为富集大离子亲石元素、亏损高强场元素的特点，可划归后碰撞-后造山花岗岩。但是，该岩体远离昌宁-孟连主特提斯弧-陆碰撞带，可能是主造山过程晚期，滇-缅-马-泰微大陆内部各微小陆块相互作用导致地幔岩石圈发生拆沉作用的产物，或是昌宁-孟连洋盆闭合过程中俯冲板块的拖拽在后缘的岩石圈伸展作用的产物。
Abstract:Located at the northern end of the Yunnan-Myanmar-Malaysia-Thailand microcontinent, the Baoshan block is an important part of multi-island ocean in Paleozoic Tethys of western Yunnan. Bangdong granite intrusion, which is located in western Baoshan block and has LA-ICP-MS zircon U-Pb weighted average age of 228.3±1.7Ma, recorded the process of arc-land collision after the closure of Tethys. The intrusion consists of main orthogranite, with rich silicon and alkali, poor magnesium and calcium as well as rich chromium and nickel, showing evolution tendency of cordierite per-aluminous granite in B-A diagram. On the whole it is enriched in LILE (large ionic lithophile elements) and depleted in HFSE (high field strong elements), thus can be classified as postcollision or post-orogenic granite. Nevertheless, Bangdong granite intrusions, which stay away from Changning-Menglian Tethys main arc-land collision belts, resulted from the mantle lithosphere delamination by micro-block interaction in Yunnan-MyanmarMalaysia-Thailand microcontinent. And it probably resulted from the lithosphere extension at the trailing edge by subduction plate dragging during the closure of Changning-Menglian Tethys.
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表 1 蚌东花岗岩LA-ICP-MS锆石U-Th-Pb分析结果
Table 1. LA-ICP-MS U-Th-Pb analyzed data of the zircons from Bangdong granites
序号 样品编号 元素含量/10-6 Th/U 同位素比值 年龄/Ma Pb Th U 207Pb/206Pb 1σ 207Pb/235U 1σ 206Pb/238U 1σ 208Pb/232Th 1σ 207Pb/206Pb 1σ 207Pb/235U 1σ 206Pb/238U 1σ 206Pb/238Th 1σ 1 D2681-1-01 30.5 133 696 0.19 0.0507 0.0010 0.2511 0.0059 0.0360 0.0006 0.0106 0.0002 228 42 227 5 228 4 213 5 2 D2681-1-02 37.7 158 742 0.21 0.0556 0.0012 0.2768 0.0071 0.0360 0.0005 0.0152 0.0006 435 48 248 6 228 3 306 11 3 D2681-1-03 39.7 168 896 0.19 0.0508 0.0008 0.2528 0.0050 0.0361 0.0005 0.0111 0.0002 232 69 229 4 228 3 223 4 4 D2681-1-04 10.4 67.3 158 0.42 0.0545 0.0019 0.2709 0.0098 0.0361 0.0005 0.0107 0.0003 391 84 243 8 228 3 215 5 5 D2681-1-05 36.2 142 836 0.17 0.0523 0.0010 0.2597 0.0058 0.0360 0.0006 0.0114 0.0002 298 44 234 5 228 4 229 4 7 D2681-1-07 32.9 144 744 0.19 0.0510 0.0010 0.2538 0.0059 0.0361 0.0006 0.0106 0.0002 239 44 230 5 228 4 214 4 8 D2681-1-08 48 154 993 0.15 0.0516 0.0010 0.2571 0.0053 0.0361 0.0005 0.0165 0.0052 333 38 232 4 229 3 332 103 9 D2681-1-09 18.5 113 313 0.36 0.0528 0.0018 0.2612 0.0084 0.0361 0.0006 0.0109 0.0003 320 78 236 7 228 4 220 5 10 D2681-1-10 34.5 132 734 0.18 0.0595 0.0015 0.2966 0.0083 0.0362 0.0008 0.0130 0.0003 587 56 264 7 229 5 261 7 11 D2681-1-11 29.8 124 736 0.17 0.0517 0.0011 0.2579 0.0069 0.0361 0.0006 0.0109 0.0002 333 50 233 6 228 3 220 5 13 D2681-1-13 28.8 125 571 0.22 0.0539 0.0017 0.2702 0.0117 0.0361 0.0008 0.0133 0.0005 365 68 243 9 229 5 268 9 14 D2681-1-14 18.1 120 295 0.41 0.0513 0.0015 0.2532 0.0069 0.0361 0.0005 0.0107 0.0002 254 69 229 6 228 3 215 4 15 D2681-1-15 48.2 382 507 0.75 0.0531 0.0012 0.2646 0.0064 0.0361 0.0004 0.0115 0.0002 345 50 238 5 229 3 231 4 16 D2681-1-16 30.1 128 701 0.18 0.0504 0.0010 0.2509 0.0059 0.0360 0.0005 0.0112 0.0002 213 46 227 5 228 3 224 5 17 D2681-1-17 22.1 103 473 0.22 0.0495 0.0010 0.2465 0.0052 0.0361 0.0004 0.0111 0.0002 172 46 224 4 229 2 223 5 18 D2681-1-18 22.5 98.1 479 0.20 0.0500 0.0015 0.2480 0.0074 0.0361 0.0006 0.0119 0.0003 198 69 225 6 229 4 240 7 19 D2681-1-19 83 600 944 0.64 0.0496 0.0034 0.2459 0.0171 0.0359 0.0006 0.0117 0.0003 189 161 223 14 227 4 235 6 20 D2681-1-20 23 154 270 0.57 0.0584 0.0027 0.2942 0.0156 0.0360 0.0006 0.0126 0.0004 546 100 262 12 228 4 253 9 21 D2681-1-21 30.7 151 599 0.25 0.0511 0.0011 0.2532 0.0060 0.0361 0.0005 0.0113 0.0002 243 55 229 5 228 3 227 4 25 D2681-1-25 20.4 130 256 0.51 0.0563 0.0029 0.2791 0.0167 0.0360 0.0005 0.0123 0.0006 465 115 250 13 228 3 247 12 注：由湖北地矿测试实验中心完成 
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表 2 蚌东花岗岩岩石化学成分、CIPW标准矿物成分及主要参数
Table 2. Chemical compositions and C I P W normative mineral and main geochemical parameters of Bangdong granites
样品编号 氧化物含量/°% SiO2 TiO2 AI2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 H2O+ H2O- CO2 总和 中细粒正长花岗岩 D2681-1-1 71.17 0.48 14.74 1.75 2.12 0.03 0.76 0.43 2.79 5.43 0.18 0.58 0.10 0.74 99.88 D1712-1-1 74.74 0.23 12.72 1.41 1.74 0.04 0.34 0.83 2.41 5.40 0.13 0.26 0.06 0.47 99.99 D1196-1-1 74.53 0.49 15.43 0.92 1.42 0.01 0.55 0.32 0.76 5.28 0.21 1.90 0.56 1.21 99.92 D1712-1-2 75.38 0.24 12.71 1.16 1.40 0.04 0.34 0.70 2.36 5.53 0.12 0.26 0.06 0.47 99.98 样品编号 CIPW标准矿物含量/% 主要参数 aP ilm mt c q or ab An en fs 总计 σ CI DI AR A/NK A/NCK K2O/Na2O D2681-1-1 0.43 0.91 2.54 3.93 31.87 32.13 23.64 0.96 1.90 1.71 100.01 2.40 87.64 3.37 0.71 1.31 1.95 2.40 D1712-1-1 0.31 0.44 2.04 1.71 37.36 31.91 20.40 3.27 0.85 1.73 100.01 1.92 89.66 3.72 0.77 1.12 2.24 1.92 D1196-1-1 0.50 0.93 1.33 8.39 48.55 31.23 6.44 0.22 1.37 1.06 100.01 1.16 86.21 2.24 0.45 2.04 6.95 1.16 D1712-1-2 0.28 0.46 1.68 1.86 38.24 32.68 19.98 2.69 0.85 1.29 100.01 1.92 90.90 3.86 0.78 1.14 2.34 1.92
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表 3 蚌东花岗岩微量和稀土元素分析结果
Table 3. Trace elements and REE data of Bangdong granites
10-6 样品编号 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y ∑REE ∑REE δCe ∑Ce/∑Y (La/Yb)N D2681-1-1 121.0 104.0 82.8 65.8 39.9 15.5 29.2 25.5 21.0 17.4 16.9 13.9 12.8 12.1 17.1 594.9 594.9 1.02 3.19 9.47 D1712-1-1 84.8 66.8 52.2 39.3 26.2 7.2 20.7 21.3 20.2 17.4 17.6 15.7 14.9 13.4 17.7 435.5 435.5 0.98 2.09 5.68 D1196-1-1 142.6 118.9 91.8 75.3 43.6 18.1 30.7 25.3 19.9 15.7 14.5 12.3 11 9.6 14.9 644.4 644.4 1.01 4.04 13.01 D1712-1-2 87.7 69.3 54.4 41.2 28.5 6.5 21.6 22.2 21.1 17.8 17.3 15.4 14.7 13.0 18.9 449.8 449.8 0.97 2.07 5.97 样品编号 (La/Sm)N (Gd/Yb)N Zr Zn V Th Sc Sr Rb Ni Nb Cu Cr Co Ba Hf Hf U Pb D2681-1-1 3.03 2.29 227 34.2 43.7 27 4.65 113 189 12.9 15.8 13.8 31.6 6.3 2300 10.2 10.2 4.05 27 D1712-1-1 3.24 1.38 118 34 12.8 20.7 2.4 37.5 297 6.35 13.6 11.6 14.3 4.1 419 7.34 7.34 6.48 30.4 D1196-1-1 3.27 2.80 219 25.3 43 26.1 4.7 135 162 8.7 14.2 10 31.3 3.9 2300 6.33 6.33 5.31 40.3 D1712-1-2 3.08 1.47 115 24.8 15.6 22.1 3.4 34.3 307 5 13.1 8.1 21.7 4 444 2.3 2.3 2.9 28.7
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