Geochemistry and zircon U-Pb age of the Tongchanggou porphyry Cu-Mo deposit on the southern margin of Geza arc, northwest Yunnan Province, and its geological significance
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摘要:
云南格咱岛弧构造-岩浆成矿带是义敦岛弧的重要组成部分，属于西南三江地区铜多金属矿床富集区之一。对格咱岛弧南缘铜厂沟成矿斑岩的岩石学、地球化学特征及锆石U-Pb年龄进行了研究，系统分析了区内花岗岩的岩石成因、形成的构造环境及成岩成矿的地球动力学背景。铜厂沟成矿斑岩主要为花岗闪长斑岩，用LA-ICP-MS测得其中锆石的206Pb/238U年龄为87.62±0.59Ma。岩石地球化学特征表明，铜厂沟花岗闪长斑岩具有高硅、富碱、富钾的特点，属于高钾钙碱性岩石系列；岩石具有较高的稀土元素总量；富集轻稀土元素，轻、重稀土元素分馏明显，δEu为0.89~0.97，具较弱的负异常；微量元素特征表明，岩石富集大离子亲石元素K、U、Th、Rb、Ba、Sr，亏损高场强元素Nb、Ta、P、Ti；大离子亲石元素的富集和Nb、Ta等高场强元素的亏损，说明岩浆主要来源于壳源物质的部分熔融。本区花岗岩形成于碰撞后与伸展构造的转换阶段，属于碰撞造山过程的产物。碰撞诱发的岩石圈地幔物质上涌导致地壳发生部分熔融作用，形成富钾的初始含矿岩浆，岩浆沿深大断裂上升侵位最终形成了格咱岛弧燕山晚期的构造-岩浆作用及成矿事件。
Abstract:The Geza tectonic-magmatic belt is an important part of Yidun arc and also an important ore concentration area of copper polymetallic deposits in the Sanjiang region of southwest China. Based on the study of petrologic and geochemical characteristics and zircon U-Pb geochronology of the Tongchanggou oreforming porphyry on the southern margin of Geza arc, the authors systematically analyzed the petrogenesis, tectonic environment, diagenesis and metallogenic geodynamic background of the granite. The rock type of Tongchanggou metallogenic porphyry is granodiorite porphyry, the research results of the zircon U-Pb geochronology show that the age of emplacement and crystallization of the magma is 87.62±0.59Ma (MSWD=0.52) by LA-ICP-MS method. Petrographic and lithogeochemical characteristics indicate that Tongchanggou granodiorite porphyry has characteristics of high silicon (63.02%~74.06%), high alkali (K2O+Na2O=6.97%~8.79%), and rich K (K2O/Na2O=0.71~2.13). The rocks belong to high-K calc-alkaline rock series enriched in light rare earth elements. The fractionation between LREE and HREE is obvious. δEu values are 0.89~0.97 with a weak negative anomaly. The analytical results of trace elements show that the rocks are enriched in large ion lithophile elements of K, U, Th, Rb, Ba, Sr, and depleted in high field strength elements of Nb, Ta, P, Ti, suggesting that the formation of magma might have been derived from the partial melting of crust materials. The granite was formed in the conversion stage of post-collision and extensional tectonics, belonging to the product of the collision orogenic process. The collision induced upwelling of the lithospheric mantle which led to partial melting of the crust and formed the potassium-rich and orebearing initial magma. Magma rising and intrusion along the deep faults eventually triggered the late Yanshanian tectonic-magmatic and metallogenic events in Geza area.
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表 1 铜厂沟花岗闪长斑岩岩石主量、微量和稀土元素分析结果
Table 1. Major, trace and rare earth elements analytical results of Tongchanggou granodiorite porphyry
样号 LBB-10 LBB-19 LBB-21 LBB-27 LBB-29 LBB-59 LBB-66 LBB-76 LBB-13 LBB-35 LBB-2 岩性 花岗闪长斑岩 辉钼矿化花岗闪长斑岩 SiO2 69.01 74.06 65.56 65.91 68.35 66.73 72.12 69.73 66.18 66.52 67.06 TiO2 0.64 0.49 0.69 0.57 0.56 0.55 0.41 0.43 0.73 0.58 0.61 Al2O3 14.82 12.19 16.07 15.08 14.77 14.77 11.43 12.50 15.9 15.82 15.00 Fe2O3 0.51 0.27 1.13 2.63 1.43 1.94 2.90 1.26 0.70 1.61 1.60 FeO 1.07 0.72 1.84 1.64 1.52 1.53 0.88 1.34 0.89 1.49 2.05 MnO 0.03 0.02 0.04 0.02 0.02 0.05 0.02 0.04 0.03 0.02 0.07 MgO 1.45 1.07 1.73 1.29 1.21 1.35 0.90 1.03 1.47 1.36 1.55 CaO 2.64 2.21 2.98 3.01 2.75 2.79 2.50 2.93 2.93 3.00 3.09 Na2O 3.08 2.29 3.74 4.06 3.6 3.71 2.62 3.09 3.33 4.43 3.80 K2O 4.95 4.87 4.42 3.11 3.63 3.59 3.00 4.24 5.46 2.83 3.64 P2O5 0.31 0.24 0.33 0.28 0.26 0.27 0.21 0.21 0.31 0.28 0.29 烧失量 1.26 1.34 1.27 2.22 1.70 2.57 2.81 3.01 1.87 1.83 1.07 总量 99.76 99.77 99.79 99.82 99.80 99.85 99.81 99.82 99.81 99.77 99.82 A/CNK 0.97 0.93 0.98 0.97 0.99 0.98 0.94 0.83 0.95 1.00 0.95 SI 13.09 11.62 13.46 10.23 10.62 11.22 8.89 9.43 12.43 11.60 12.30 DI 81.50 86.00 77.10 76.50 79.50 78.20 80.80 82.80 80.50 77.50 76.80 σ 2.50 1.60 2.90 2.20 2.00 2.20 1.10 2.00 3.30 2.20 2.30 La 53.86 46.25 59.48 74.46 56.61 60.51 52.31 46.85 59.97 57.41 58.38 Ce 99.33 80.82 106.10 119.10 96.41 104.90 86.55 81.50 112.50 94.29 102.20 Pr 10.67 8.43 11.00 11.66 9.82 10.76 8.57 8.36 12.09 9.43 10.41 Nd 35.76 28.82 37.32 37.93 32.27 35.51 27.43 27.28 41.29 31.08 33.78 Sm 5.71 4.54 5.81 5.49 5.18 5.44 4.00 3.98 6.48 4.75 5.04 Eu 1.52 1.16 1.56 1.45 1.24 1.49 1.02 1.07 1.73 1.31 1.32 Gd 4.73 3.80 4.80 4.72 4.13 4.53 3.33 3.30 5.33 4.07 4.23 Tb 0.63 0.50 0.66 0.63 0.56 0.59 0.45 0.44 0.72 0.54 0.56 Dy 3.01 2.48 3.25 2.93 2.65 2.88 2.14 2.10 3.50 2.60 2.62 Ho 0.55 0.43 0.57 0.52 0.47 0.52 0.35 0.39 0.61 0.45 0.49 Er 1.38 1.12 1.46 1.34 1.18 1.42 1.00 1.01 1.58 1.11 1.29 Tm 0.20 0.16 0.23 0.19 0.17 0.21 0.15 0.15 0.24 0.17 0.19 Yb 1.21 0.97 1.38 1.16 1.07 1.24 0.91 0.93 1.41 1.05 1.19 Lu 0.16 0.14 0.19 0.16 0.14 0.17 0.13 0.14 0.20 0.15 0.17 Y 14.08 11.49 14.91 13.76 12.45 14.06 10.16 10.16 16.40 12.09 13.01 Ba 1553 1381.00 1368.00 1009.00 1182.00 1193.00 823.00 1501.00 1529.00 1032.00 1505.00 Cu 98.40 19.10 95.80 300.20 108.40 257.30 63.70 167.60 50.10 317.00 31.60 F 676.00 505.00 799.00 505.00 526.00 597.00 622.00 393.00 766.00 572.00 648.00 Ga 20.70 16.30 22.70 22.30 20.50 21.90 16.20 16.60 22.70 23.20 22.30 Hf 8.50 5.70 7.40 5.80 7.10 5.70 4.90 5.40 9.70 7.10 6.60 Mo 1672.00 1258.00 59.30 9.10 30.60 55.90 271.70 117.40 409.90 4.20 3.40 Nb 30.20 26.00 31.20 29.20 27.00 36.30 15.30 26.50 39.10 25.90 35.30 Pb 23.10 21.40 25.50 17.10 20.00 24.70 20.80 25.90 22.50 24.40 35.90 Rb 96.00 83.00 80.00 66.00 96.00 83.00 105.00 104.00 123.00 62.00 96.00 Sr 819.00 557.00 711.00 592.00 780.00 609.00 473.00 356.00 966.00 813 623.00 Ta 2.03 1.62 2.02 1.97 1.74 2.31 0.93 1.69 3.24 1.71 2.21 Th 15.40 12.10 16.20 17.20 16.70 25.10 18.40 20.20 16.80 19.30 22.00 W 2.50 1.60 1.20 7.10 4.10 4.00 42.90 3.70 4.40 5.30 2.60 Zn 36.00 25.00 38.00 27.00 33.00 50.00 27.00 42.00 40.00 29.00 61.00 Zr 205.00 144.00 176.00 155.00 198.00 151.00 131.00 134.00 239.00 193.00 163.00 注：主量元素含量单位为%，微量和稀土元素含量为10-6 
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表 2 铜厂沟花岗闪长斑岩岩体LA-ICP-MS锆石U-Th-Pb同位素分析
Table 2. U-Th-Pb composition of zircons from Tongchanggou granodiorite porphyry measured by LA-ICP-MS technique
测点 Pb/10-6 Th/10-6 U/10-6 Th/U 207Pb/206Pb ±1σ 207Pb/235U ±1σ 206Pb/238U ±1σ 207Pb/235U年龄/Ma ±1σ 206Pb/238U年龄/Ma ±1σ 1 8.2 139 577 0.24 0.0521 0.0054 0.0972 0.010 0.014 0.0004 94 9 89 3 2 8.0 175 553 0.32 0.050 0.0051 0.090 0.009 0.014 0.0003 87 8 87 2 3 7.3 107 529 0.20 0.053 0.0026 0.100 0.005 0.0134 0.0002 95 4 86 1 4 6.7 179 451 0.40 0.0474 0.0031 0.090 0.006 0.014 0.0002 87 5 89 1 5 9.8 689 519 1.33 0.0654 0.0071 0.1242 0.014 0.014 0.0003 119 13 88 2 6 10.2 181 740 0.25 0.050 0.0024 0.092 0.0044 0.014 0.0002 89 4 87 1 7 25.5 2695 1170 2.30 0.0515 0.0034 0.097 0.006 0.014 0.0002 94 5 88 1 9 5.3 75 403 0.19 0.050 0.003 0.0911 0.006 0.014 0.0002 89 5 87 1 10 8.0 117 608 0.19 0.0473 0.0024 0.088 0.0043 0.014 0.0002 86 4 88 1 12 14.5 343 1039 0.33 0.0477 0.002 0.090 0.0033 0.014 0.0002 87 3 88 1 13 9.0 187 641 0.29 0.051 0.003 0.0953 0.0051 0.014 0.0002 93 5 88 1 14 6.6 100 488 0.20 0.0492 0.0031 0.093 0.0063 0.014 0.0003 90 6 88 2 17 9.0 181 646 0.28 0.0462 0.003 0.088 0.006 0.014 0.0002 85 5 88 1 18 8.8 191 634 0.30 0.050 0.0031 0.0914 0.0063 0.014 0.0002 89 6 87 1
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