Zircon U-Pb age and geochemistry of Middle Triassic porphyritic granite in the north of Kudi, Xinjiang
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摘要:
西昆仑山是塔里木地块和羌塘地块在晚中生代碰撞造山的产物，古特提斯洋分支库尔良-阿羌裂谷闭合时限亦存在争议。以库地北斑状二长花岗岩为研究对象，进行岩石学、地球化学和年代学研究。获得LA-ICP-MS锆石U-Pb年龄244.42±0.87Ma，时代为中三叠世。斑状二长花岗岩的SiO2含量为65.57%~67.70%，Mg#值为35~38，A/CNK值为0.90~1.01，属于偏铝质高钾钙碱性岩石。稀土元素总量为193×10-6~339×10-6，δEu=0.72~0.78，具弱负Eu异常。微量元素Rb、K、Th、U、La、Ce、Zr富集，Nb、Ti、P中等-强亏损。斑状二长花岗岩属于I型花岗岩，源岩为安山岩，形成于俯冲的大陆岛弧环境。结合区域资料和前人研究成果表明，斑状二长花岗岩形成于中三叠世大陆岛弧环境，中三叠世库地一带存在古特提斯洋壳俯冲活动，库尔良-阿羌裂谷尚未消减闭合。
Abstract:The West Kunlun Mountains is the product of the Tarim block and the Qiangtang plots in the Late Mesozoic collision orogeny. The closing time of the Kurdish-Aqiang rift valley is also controversial. In this paper, we studied the petrology, geochemistry and chronology of prophyritic monzonitic granite of the Kudi northern. In this paper, the LA-ICP-MS zircon U-Pb age is 244.42±0.87Ma, and the age is Middle Triassic. The SiO2 content of the porphyritic granite is 65.57%~67.70%, Mg#=35~38, A/CNK=.90~1.01, belonging to the aluminum-alkali high-potassium calc-alkaline rock. The total amount of rare earth elements is 193×10-6~339×10-6, δEu=0.72~0.78, with weak negative anomaly。Trace elements Rb, K, Th, U, La, Ce, Zr are enriched, Nb, Ti, P medium-strong loss. The porphyritic granite belongs to the I-type granite, and the source rock is the Andesite, which is formed in the subduction continental island arc environment. Regional data, previous achievements and research in this paper indicate that the porphyritic granite was formed in the continental arc environment of the Middle Triassic, and that the ancient Tethys oceanic crust subduction activity existed in the reservoir area of the Middle Triassic, Kulliag-A Rift Valley has not yet reduced closure.
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致谢: 样品分析得到河北省区域地质矿产调查研究所实验室、天津地质调查中心同位素实验室及耿建珍博士大力支持，审稿专家提出了宝贵的修改意见，在此表示衷心的感谢。
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图 1 库地地区地质简图①
Qhal—全新统冲积物；C2K—上石炭统库尔良群；C1T—下石炭统他龙群；∈2Y—中寒武统依莎克群；Pt3St.—新元古界赛图拉岩群；Jxb—蓟县系博查特塔格组；Pz1Tmlg—早古生代构造混杂岩；Pz1οψ—库地蛇绿岩；T2ηγ—晚三叠世二长花岗岩；O3ηγ—晚奥陶世二长花岗岩
Figure 1. Simplified geological map of Kudi area
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图 2 斑状二长花岗岩野外（a）和显微结构（b）照片
Figure 2. Field (a) and microstructure (b) photographs of prophyritic granite
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图 3 斑状二长花岗岩锆石阴极发光图像、U-Th-Pb同位素测点位置及206Pb/238U年龄
Figure 3. Zircon CL images, U-Th-Pb isotope location and 206Pb/238U age of prophyritic granite
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图 4 斑状二长花岗岩LA-ICP-MS锆石U-Pb年龄谐和图
Figure 4. LA-ICP-MS zircon U-Pb concordia diagrams of prophyritic granite
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表 1 斑状二长花岗岩LA-ICP-MS锆石U-Th-Pb同位素数据
Table 1 LA-ICP-MS zircon U-Th-Pb data of prophyritic granite
测点号 元素含量/10-6 Th/U 同位素比值 年龄/Ma Pb U 208Pb/232Th 206Pb/238U 1σ err% 207Pb/235U 1σ err% 207Pb/206Pb 1σ 206Pb/238U 1σ 207Pb/235U 1σ 207Pb/206Pb 1σ 1 66 1688 0.3029 0.0142 0.0388 0.0003 0.85 0.2837 0.0025 0.89 0.0530 0.0004 245 2 254 2 329 19 2 46 1190 0.2932 0.0140 0.0388 0.0003 0.77 0.2771 0.0028 1.00 0.0519 0.0005 245 2 248 2 279 23 3 66 1670 0.6018 0.0081 0.0384 0.0003 0.81 0.2891 0.0027 0.92 0.0546 0.0005 243 2 258 2 395 19 4 51 1298 0.3020 0.0142 0.0391 0.0003 0.81 0.2924 0.0028 0.97 0.0542 0.0005 247 2 260 3 381 20 5 57 1458 0.2467 0.0156 0.0395 0.0004 0.97 0.2831 0.0028 1.00 0.0520 0.0005 250 2 277 3 285 22 6 46 1220 0.2555 0.0135 0.0384 0.0003 0.88 0.2708 0.0028 1.03 0.0512 0.0005 243 2 243 3 249 23 7 53 1278 0.6471 0.0111 0.0386 0.0003 0.85 0.2709 0.0026 0.96 0.0510 0.0005 244 2 243 2 239 21 8 68 1810 0.3453 0.0101 0.0380 0.0003 0.89 0.2697 0.0024 0.88 0.0515 0.0004 240 2 242 2 265 19 9 47 1222 0.3392 0.0100 0.0390 0.0005 1.17 0.2921 0.0028 0.96 0.0543 0.0005 247 3 260 3 385 22 10 48 1276 0.2780 0.0107 0.0386 0.0003 0.83 0.2784 0.0026 0.94 0.0524 0.0005 244 2 249 2 301 21 11 47 1164 0.6412 0.0090 0.0387 0.0003 0.87 0.2921 0.0030 1.04 0.0547 0.0005 245 2 260 3 400 22 12 50 1290 0.3586 0.0111 0.0384 0.0003 0.78 0.2730 0.0027 1.00 0.0516 0.0005 243 2 245 2 267 22 13 49 1273 0.3017 0.0116 0.0389 0.0003 0.78 0.2783 0.0026 0.93 0.0519 0.0005 246 2 249 2 281 21 14 40 1057 0.3885 0.0103 0.0381 0.0003 0.73 0.2702 0.0030 1.11 0.0514 0.0006 241 2 243 3 260 25 15 42 1089 0.2870 0.0108 0.0389 0.0005 1.37 0.2759 0.0029 1.04 0.0514 0.0005 246 3 247 3 260 22 16 46 1209 0.3063 0.0112 0.0387 0.0003 0.88 0.2794 0.0026 0.93 0.0523 0.0005 245 2 250 2 298 20 17 48 1239 0.3059 0.0118 0.0386 0.0004 0.92 0.2915 0.0036 1.25 0.0548 0.0007 244 2 260 3 404 27 18 49 1289 0.3949 0.0108 0.0374 0.0003 0.93 0.2692 0.0033 1.21 0.0521 0.0006 237 2 242 3 291 27 19 47 1218 0.3601 0.0109 0.0384 0.0004 0.93 0.2654 0.0025 0.95 0.0501 0.0005 243 2 239 2 199 22 20 43 1112 0.3088 0.0112 0.0387 0.0003 0.81 0.2716 0.0026 0.94 0.0509 0.0005 245 2 244 2 237 21 21 44 1142 0.2382 0.0148 0.0391 0.0005 1.17 0.2833 0.0034 1.19 0.0525 0.0006 247 3 253 3 308 26 22 83 2118 0.3890 0.0133 0.0383 0.0004 0.96 0.2695 0.0022 0.81 0.0510 0.0004 242 2 242 2 241 18 23 47 1256 0.2496 0.0122 0.0386 0.0004 0.91 0.2723 0.0026 0.96 0.0512 0.0005 244 2 245 2 250 22 24 55 1409 0.3397 0.0123 0.0391 0.0004 0.98 0.2734 0.0026 0.94 0.0507 0.0005 247 2 245 2 226 21 25 89 2323 0.2546 0.0123 0.0389 0.0004 1.08 0.2821 0.0023 0.81 0.0526 0.0004 246 3 252 2 312 18 
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表 2 斑状二长花岗岩元素分析结果及相关参数
Table 2 Analytical results and some parameters of prophyritic granite
样号 P4YQ1 P4YQ2 P4YQ3 P22YQ1 P22YQ2 P22YQ3 SiO2 65.56 67.7 67.58 65.23 65.99 66.07 TiO2 0.43 0.33 0.45 0.5 0.43 0.44 Al2O3 16.16 15 14.99 15.76 15.61 16.08 Fe2O3 1.76 1.56 2.03 2.14 1.82 1.74 FeO 2.01 1.6 1.94 2.35 2.08 2.01 MnO 0.12 0.11 0.12 0.13 0.12 0.12 MgO 1.21 0.97 1.22 1.47 1.27 1.19 CaO 3.9 2.19 3.92 4.36 2.53 4.24 Na2O 3.92 3.13 3.53 3.64 3.55 3.96 K2O 3.91 5.83 3.27 3.1 4.62 2.92 P2O5 0.18 0.15 0.19 0.22 0.19 0.18 烧失量 0.66 1.26 0.65 0.96 1.61 0.9 总计 99.83 99.81 99.87 99.85 99.82 99.86 Rb 144.9 191.2 154.8 133.8 243.1 145.8 Sr 348 314.8 345.9 400.1 303.2 374.7 Ba 721.7 1062.1 538.4 622.3 974 550.3 Ga 18.5 14.9 18.2 19.2 18.9 20.4 Nb 25.4 19.6 24.9 24 23.4 24.2 Ta 2.8 2.1 2.5 2.3 2.3 2.3 Zr 185.2 135.4 194.1 205.3 192.7 194.6 Hf 7.4 6.5 7.4 9.2 8.6 9 Th 14.8 23 23.4 18.1 20.3 24 V 60.4 41.7 65.3 72.5 67.2 64 Cr 5.6 4.2 5.8 5.6 6.3 5.2 Co 6.6 5.2 5.9 7.2 6.4 6.1 Sc 2.9 2.9 3.2 3.9 3.6 3.9 U 71.3 44.2 87.4 77.8 74.3 63.3 La 128.8 85.1 153.2 140.2 133.7 115.3 Ce 13.7 9.5 15.5 14.4 13.6 12.3 Pr 46.2 32.1 51.1 48.8 45.4 41.9 Nd 7.6 5.5 8 7.8 7.3 6.9 Sm 1.8 1.2 1.8 1.8 1.8 1.6 Eu 6.3 4.6 6.9 6.7 6.3 5.9 Gd 0.9 0.7 1 1 0.9 0.9 Tb 4.9 3.6 5.1 5 4.6 4.7 Dy 1 0.8 1 1 0.9 0.9 Ho 2.9 2.3 3.1 3 2.7 2.7 Er 0.5 0.4 0.6 0.5 0.5 0.5 Tm 3.4 2.7 3.5 3.3 3.1 3 Yb 0.4 0.3 0.4 0.4 0.4 0.4 Lu 28.4 21.3 29.5 28.2 26.4 26.5 Y 65.6 67.7 67.6 65.2 66 66.1 σ 2.72 3.25 1.88 2.04 2.9 2.05 A/CNK 0.91 0.97 0.91 0.91 1.01 0.92 ∑REE 290 193 339 312 295 260 δEu 0.78 0.74 0.72 0.74 0.78 0.76 (La/Yb)N 14.11 10.9 16.71 15.77 16.24 14.03 (La/Sm)N 5.88 5.09 6.87 6.24 6.43 5.77 (Gd/Yb)N 1.49 1.35 1.59 1.63 1.65 1.56 Nb/U 8.92 6.85 7.7 6.1 6.53 6.18 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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