U-Pb Geochronology and Trace Element Compositions of Zircon in Biotite Granite from the Bagaladong Pb-Zn Deposit, Tibet and Their Geological Significance
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摘要: 巴嘎拉东铅锌矿床位于冈底斯弧背断隆带东段，研究程度较低，尚未开展成岩成矿年代学研究。本文选取该矿床黑云母花岗岩进行了LA-ICP-MS锆石U-Pb年代学和微量元素组成测试，并利用锆石Ti温度计方法获得锆石结晶温度。结果表明，黑云母花岗岩锆石均为典型岩浆成因锆石，14个测点得到的锆石206Pb/238U加权平均年龄为129.1±2.3 Ma（MSWD=1.5），岩体侵位于早白垩世中期，与前人获得的该期岩浆侵位年龄一致。锆石ΣLREEs=13.21~530.28 μg/g，平均值61.90 μg/g，ΣHREEs=849.16~3981.54 μg/g，平均值1826.91 μg/g，具有轻稀土亏损、重稀土富集的左倾配分模式；δCe=1.20~701.77，δEu=0.01~0.12，表现出明显的铈正异常和铕负异常特征。锆石Ti含量分布在0.60~7.40之间，结晶温度范围为593.9~795.3℃，平均温度724.3℃，一定程度反映了成岩温度。可以推断，巴嘎拉东黑云母花岗岩可能形成于班公湖-怒江洋闭合后的碰撞造山挤压阶段，黑云母花岗岩形成时代的厘定代表了成矿时代的上限，为区域上同时代铅锌矿找矿勘查提供了重要依据。Abstract: The Bagaladong Pb-Zn deposit is located in the eastern segment of the Gangdese back-arc uplift belt and lacks enough study with no available ages for ores and rocks. Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) zircon U-Pb dating and in situ trace elements were carried out for mineralization-related biotite granite in the Bagaladong Pb-Zn deposit and are reported in this paper. The crystallization temperatures of zircons in the biotite granite are obtained by using the zircon Ti thermometer. Trace element composition of zircons in the biotite granite indicates a magmatic origin. Fourteen zircons yield a weighted average 206Pb/238U ages of 129.1±2.3 Ma (MSWD=1.5), suggesting that the biotite granite was established in the middle of the Early Cretaceous, which is consistent with the age of magmatic intrusions obtained by previous studies. Zircon has ΣLREEs values of 13.21-530.28 μg/g with an average of 61.90 μg/g and ΣHREEs values of 849.16-3981.54 μg/g with an average of 1826.91 μg/g. Zircon has left-inclined REEs patterns with relative LREEs depletion and HREEs enrichment. These zircons have δCe and δEu values of 1.20-701.77 and 0.01-0.12, respectively, indicating obviously positive Ce anomalies and negative Eu anomalies. Zircons have Ti contents varying from 0.60 to 7.40 and crystallization temperatures of 593.9-795.3℃ with an average of 724.3℃, which partially reflect the diagenetic temperature. It can be concluded that the Bagaladong biotite granite was probably formed during the extrusion stage of collision after the closure of the Bangong Co-Nujiang ocean basin. The age of the biotite granite in the Bagaladong deposit represents the upper limit of the metallogenic epoch and provides important evidence for prospecting the Early Cretaceous Pb-Zn mineralization in the east section of the Gangdese back-arc fault uplift area.
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表 1 巴嘎拉东铅锌矿床黑云母花岗岩锆石U-Pb年代学测试结果
Table 1. Isotopic data of U-Pb age determinations on zircon of biotite granite form the Bagaladong Pb-Zn deposit
样品号(LQLZK6-828) 元素含量(μg/g) Th/U 同位素比值 年龄(Ma) Th U 206Pb/238U 1σ 207Pb/235U 1σ 207Pb/206Pb 1σ 206Pb/238U 1σ 207Pb/235U 1σ 207Pb/206Pb 1σ 01 287.80 384.40 0.70 0.02017 0.00038 0.13056 0.00932 0.04653 0.00344 129 2 125 8 33 161 02 226.10 424.20 0.50 0.01972 0.00035 0.14227 0.01447 0.05257 0.00572 126 2 135 13 309 248 03 686.50 1961.80 0.30 0.01965 0.00027 0.14187 0.00571 0.05198 0.00216 125 2 135 5 283 94 04 346.50 721.10 0.50 0.01920 0.0003 0.11689 0.00632 0.04373 0.00237 123 2 112 6 - - 05 325.90 806.00 0.40 0.02093 0.00032 0.14655 0.00741 0.05041 0.00261 134 2 139 7 213 116 06 605.30 640.90 0.90 0.02096 0.00030 0.13141 0.00713 0.04531 0.00261 134 2 125 6 - - 07 274.80 564.40 0.50 0.02036 0.00028 0.13611 0.00649 0.04823 0.00242 130 2 130 6 109 124 08 689.20 731.20 0.90 0.01987 0.00031 0.12872 0.00710 0.04670 0.00258 127 2. 123 6 35 126 09 1430.00 3073.80 0.50 0.02003 0.00023 0.14262 0.00489 0.05093 0.00172 128 2 135 4 239 78 10 384.10 779.20 0.50 0.02062 0.00032 0.15072 0.00798 0.05275 0.00288 132 2 143 7 317 126 11 427.60 1452.70 0.30 0.01968 0.00029 0.14429 0.00638 0.05263 0.00231 126 2 137 6 322 100 12 722.80 1301.20 0.60 0.01996 0.00024 0.13887 0.00646 0.04997 0.00231 127 2 132 6 195 107 13 835.20 3603.90 0.20 0.01945 0.00021 0.14361 0.00523 0.05315 0.00200 124 1 136 5 345 81 14 471.50 1534.00 0.30 0.01961 0.00023 0.13675 0.00535 0.05033 0.00201 125 2 130 5 209 88 注：“-”表示误差数据予以剔除。 
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表 2 巴嘎拉东铅锌矿床黑云母花岗岩锆石微量元素组成及锆石Ti温度计算结果
Table 2. Trace elements compositions and results of crystallization temperture of zircons from the biotite granite in Bagaladong Pb-Zn deposit
测定点号 含量(×10-6) 温度(℃) La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y Ti 01 <0.05 16.29 <0.05 1.45 3.47 0.19 22.70 8.60 110.87 43.40 190.88 40.65 367.17 64.80 1234.25 3.67 728.6 02 0.29 7.20 0.17 1.50 5.10 0.10 29.30 11.40 143.37 54.90 244.49 52.50 489.30 84.65 1583.56 3.45 723.1 03 0.07 20.40 0.10 2.19 6.96 0.19 52.39 24.10 330.88 133.74 618.50 135.65 1225.10 214.50 3889.87 0.60 593.9 04 <0.05 9.48 0.07 1.40 4.48 0.10 32.90 13.44 182.40 72.69 323.20 71.09 634.70 111.20 2077.46 - - 05 <0.05 7.69 0.07 1.38 4.20 0.06 29.76 13.50 179.69 72.79 331.60 71.50 639.10 115.80 2101.87 5.10 759.2 06 1.40 35.00 0.96 6.05 7.45 0.40 31.69 10.85 127.00 48.79 213.10 44.88 403.80 74.18 1419.66 5.90 773.3 07 7.00 25.50 2.88 15.40 7.37 0.60 33.20 10.97 132.18 51.85 226.35 46.98 429.90 78.30 1447.80 7.20 792.8 08 <0.05 28.50 0.10 2.95 5.49 0.20 36.90 13.50 168.40 63.80 281.10 58.40 512.96 91.47 1830.68 3.15 715.3 09 61.68 210.76 29.99 161.20 64.56 2.08 119.40 30.20 303.10 107.95 460.80 95.87 865.70 155.58 3099.90 107.89 1160.1 10 <0.05 7.85 0.10 2.45 5.90 0.07 37.59 15.57 200.47 79.10 355.55 76.20 687.59 124.96 2289.77 7.40 795.3 11 <0.05 7.10 0.05 1.47 6.35 0.09 49.30 21.67 296.10 120.78 560.29 121.30 1098.76 197.25 3508.10 0.90 619.1 12 <0.05 13.50 0.19 3.89 10.16 0.19 71.96 28.28 346.06 137.10 586.60 120.86 1049.20 189.35 3841.36 6.10 776.4 13 0.46 7.29 0.35 3.05 10.10 0.10 76.46 35.85 498.28 198.59 915.97 195.47 1748.37 312.50 5720.00 6.07 775.6 14 <0.05 6.57 0.09 1.00 5.48 <0.05 40.30 18.70 256.05 104.10 480.00 103.79 928.78 172.18 3029.50 1.20 639.1 注：球粒陨石标准化数据值据Sun等[17]，“-”表示低于检出限数据。
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