Geochemistry and zircon U-Pb age of granite porphyry in the Yabunuma region close to the southern margin of the Asuo structural melange belt, Gangdise, Tibet
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摘要:
西藏中冈底斯北部尼玛县阿索乡亚布努马地区东侧出露一处花岗斑岩岩脉,LA-ICP-MS锆石U-Pb测年结果显示,该花岗斑岩的形成时代为晚侏罗世(161.2±5.9Ma)。全岩地球化学数据显示其高硅、富碱、富铝的特征,属于碱性准铝质花岗斑岩; 富集轻稀土元素,轻、重稀土元素分异明显,具有明显的负Eu异常,富集Rb、Pb等大离子亲石元素,亏损Ba、Sr元素及Nb、Ta、Ti、U等高场强元素,形成于岛弧环境。其源区可能为来自俯冲带增厚下地壳的深熔作用,结合区域上同时代的岩浆事件,亚布努马花岗斑岩应该形成于以班公湖-怒江洋南向俯冲为动力背景的陆缘弧环境。
Abstract:The granite porphyry vein in the Yabunuma area is located in Asuo and Nyima area of northern Central Gangdise. LAICP-MS zircon U-Pb age of 161.2±5.9Ma indicates emplacement during the Late Jurassic. The bulk rock geochemical analyses indicate the characteristics of high silica, rich alkali and Al, suggesting alkali metaluminous granite porphyry. The granite porphyry samples are rich in LREE, the fractionation between LREE and HREE is strong and shows obvious negative Eu anomalies, with the characteristics of enrichment of Rb and Pb, depletion of Ba, Sr, Nb, Ta, Ti and U, suggesting an island-arc setting. The granite porphyry is likely to root in anatexis of thickened lower crust. Considering coeval magma event in the vicinity, the authors hold that granite porphyry from Yabunuma region should have formed in an active epicontinental environment on the background of the southern subduction of Bangong Co-Nujiang Tethyan Ocean crust.
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Key words:
- central Gangdise /
- granite porphyry /
- southern subducton /
- anatexis /
- active epicontinental environment
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图 4 亚布努马花岗斑岩SiO2-K2O图解[9]
Figure 4.
图 8 亚布努马花岗斑岩成因类型判别图解(底图据参考文献[18])
Figure 8.
图 9 亚布努马花岗斑岩构造判别图解[29]
Figure 9.
表 1 亚布努马地区花岗斑岩样品主量、微量和稀土元素分析结果
Table 1. Major, trace elements and REE data of the granite porphyry in the Yabunuma region
样品号 SiO2 TiO2 Al2O3 TFe2O3 MnO MgO CaO Na2O K2O P2O5 烧失量 总计 N16T18H1 68.45 0.10 13.96 0.80 0.01 0.47 3.68 4.12 5.53 0.09 2.77 99.98 N16T18H2 74.05 0.04 13.47 0.72 0.02 0.20 0.23 4.02 6.23 0.06 0.46 99.51 N16T18H4 65.96 0.08 17.39 0.81 0.01 0.45 1.72 5.64 6.37 0.06 1.50 99.98 N16T18H5 68.42 0.14 16.31 1.38 0.01 0.91 0.99 5.90 4.48 0.06 1.08 99.68 样品号 Li P K Sc Ti V Cr Mn Co Ni Cu N16T18H1 3.02 412.8 37820 3.072 547.8 4.646 0.09072 129.48 1.0356 1.835 1.8172 N16T18H2 0.18036 288.8 39540 3.184 238.6 4.276 0.4062 163 0.729 2.174 3.996 N16T18H4 1.6922 289.4 43420 4.56 428 5.06 0.4146 118.94 0.9524 1.4962 1.9416 N16T18H5 7.132 254 27620 3.206 732.6 6.724 13.75 108.3 1.0418 8.836 1.958 样品号 Zn Ga Rb Sr Y Zr Nb Cs Ba La Ce N16T18H1 12.262 11.478 183.64 82.8 24.36 67.16 9.164 2.524 188.78 15.31 32.3 N16T18H2 15.036 11.824 213.4 72.92 10.99 23.66 7.44 5.442 168.32 5.23 10.74 N16T18H4 12.702 14.226 198.8 78.36 22.96 66.72 8.88 1.9046 150.66 21.1 42.6 N16T18H5 17.604 14.31 120.78 63.82 22.58 108.94 10.718 1.3062 89.2 26.68 53.36 样品号 Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb N16T18H1 3.686 13.824 2.97 0.5246 3.172 0.5824 4.204 0.8722 2.69 0.4194 2.844 N16T18H2 1.3616 5.226 1.2528 0.288 1.3784 0.2774 2.052 0.4176 1.3512 0.2304 1.7374 N16T18H4 4.456 15.55 3.242 0.2394 3.27 0.618 4.444 0.8974 2.764 0.4424 3.148 N16T18H5 5.674 19.98 3.69 0.3664 3.542 0.6174 4.248 0.8616 2.662 0.4218 3.052 样品号 Lu Hf Ta Pb Th U LREE HREE ∑REE LREE/HREE Eu/Eu* N16T18H1 0.374 1.85136 0.9378 17.992 7.988 4.834 68.6146 15.158 83.7726 4.526626 0.52 N16T18H2 0.2498 0.92466 1.1622 36.78 4.788 2.454 24.0984 7.6942 31.7926 3.132021 0.67 N16T18H4 0.4342 2.1964 1.086 22.68 12.252 7.938 87.1874 16.018 103.2054 5.443089 0.22 N16T18H5 0.436 3.0514 0.921 24.04 15.926 7.736 109.7504 15.8408 125.5912 6.928336 0.31 注:主量元素含量单位为%,微量和稀土元素含量为10-6 表 2 亚布努马花岗斑岩LA-ICP-MS锆石U-Th-Pb同位素测定结果
Table 2. U-Th-Pb composition of the zircons from the granite porphyry in the Yabunuma region as measured by LA-ICP-MS
测点 Pbrad 232Th 238U Th/U 207Pb/206Pb 1σ 207Pb/235U 1σ 206Pb/238U 1σ 年龄/Ma /10-6 207Pb/206Pb 1σ 207Pb/235U 1σ 206Pb/238U 1σ NT18-07 4.7 83 168 0.49 0.04922 0.00455 0.16917 0.01554 0.02492 0.00037 158 179 159 13 159 2 NT18-09 2.8 54 96 0.57 0.04947 0.00824 0.17227 0.02852 0.02525 0.00053 170 295 161 25 161 3 NT18-11 4.8 98 171 0.57 0.04932 0.00512 0.16841 0.01741 0.02476 0.00037 163 205 158 15 158 2 NT18-12 4.7 82 158 0.52 0.04931 0.00441 0.1805 0.01603 0.02654 0.00039 163 172 168 14 169 2 NT18-15 9.7 152 278 0.55 0.04781 0.00365 0.16469 0.01239 0.02498 0.00033 90 171 155 11 159 2 注:中国地质大学(北京)地学实验中心测试 -
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