Geochemical characteristics and LA-ICP-MS zircon U-Pb age of Early Cretaceous granites in Menba area, Tibet
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摘要:
门巴地区早白垩世花岗岩位于冈底斯弧背断隆带东段，是研究冈底斯演化的重要组成部分。以门巴地区黑云母花岗岩和二云母花岗岩为研究对象，运用岩石地球化学和锆石U-Pb测年方法，分析了该区域早白垩世花岗岩的地球化学特征及构造背景。样品锆石具有明显的振荡环带，属岩浆成因锆石；测得LA-ICP-MS锆石206Pb/238U年龄为139.4~140.6Ma，为早白垩世。岩石地球化学特征表明，门巴地区早白垩世花岗岩具有富硅、高碱，贫FeO、MgO、CaO的特征，为强过铝质的高分异花岗岩，属高钾钙碱性系列，并具同源岩浆演化的趋势；稀土元素配分模式为右倾型，具Rb、Ba、Th、Nb、Ce等富集，Hf、Zr、Sm、Y、Yb等亏损的特征；门巴地区花岗岩形成于陆-陆碰撞造山环境，构造背景由同碰撞逐渐过渡为造山期后的构造环境，具有后碰撞伸展构造性质，是软流圈上涌或玄武岩底侵的岩浆响应。
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关键词:
- 早白垩世 /
- 花岗岩地球化学 /
- LA-ICP-MS锆石U-Pb年龄 /
- 门巴地区
Abstract:The Early Cretaceous granites in the Menba area are located in the eastern segment of the Gangdise belt, being an important part for the study of Gangdise evolution. With biotite granite and two mica granite in Menba area as the object of study, the author analyzed the geochemical characteristics and tectonic setting of the Early Cretaceous granites in this area by using the method of rock geochemistry and zircon U-Pb chronology. In this paper, the zircon samples for the test and analysis have obvious oscillation ring band, belonging to zircon of magma origin, and the LA-ICP-MS zircon 206Pb/238U age is 139.4~140.6Ma, suggesting Early Cretaceous. The geochemical characteristics of rocks indicate that the Early Cretaceous granites in Menba area are rich in silicon, high in alkali, and poor in FeO, MgO, CaO, implying strong aluminum high differentiation granite and high potassium calcium alkaline series, with the trend of homologous magma evolution; the model of rare-earth element allocation is the right type with Rb, Ba, Th, Nb, Ce enrichment and Hf, Zr, Sm, Y, Yb depletion. The granite in Menba area was formed in the land-land collision orogenic environment, the tectonic background was gradual transition from "the same collision" to the tectonic environment after "orogenic period", which had the extensional tectonic characteristics of "post collision", and was the magma response to the soft flow ring upwelling or the basalt bottom invasion.
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Keywords:
- Early Cretaceous /
- granites geochemistry /
- LA-ICP-MS zircon U-Pb age /
- Menba area
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图 1 门巴地区早白垩世花岗岩分布简图
Q—全新统冲积物；E2P—始新统帕那组火山碎屑岩；C2P1l—上石炭统-下二叠统来姑组含砾砂质板岩、千枚状板岩、含砾石砂质板岩；γβK2—晚白垩世黑云母花岗岩；γJ3K1—早白垩世-晚侏罗世二云母花岗岩；γβJ3—晚侏罗世黑云母花岗岩；ηγJ3—晚侏罗世二长花岗岩；γJ3—晚侏罗世花岗岩；γδJ1—早侏罗世花岗闪长岩
Figure 1. The sketch map of Early Cretaceous granites distribution in Menba area
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图 2 门巴地区早白垩世花岗岩SiO2-K2O图解（a）和A/CNK-A/NK图解（b）
Figure 2. SiO2-K2O(a)and A/CNK-A/NK(b)plots for the Early Cretaceous granites in Menba area
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图 3 门巴地区早白垩世花岗岩稀土元素球粒陨石标准化配分图（a）和微量元素原始地幔标准化蛛网图（b）
Figure 3. Chondrite-normalized REE patterns (a) and primitive mantle-normalized trace earth element patterns (b) of the Early Cretaceous granites in Menba area
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图 4 门巴地区早白垩世花岗岩锆石U-Pb谐和图（a、c）和阴极发光（CL）图像（b、d）
Figure 4. The zircon U-Pb concordia diagrams(a, c)and CL images(b, d)from the Early Cretaceous granites in Menba area
表 1 门巴地区早白垩世花岗岩主量元素分析结果
Table 1 Whole-rock major element data of the Early Cretaceous granites in Menba area
% 元素 黑云母花岗岩（巨斑） 黑云母花岗岩（中粒） 二云母花岗岩（中粒） MB-1 MB-2 MB-3 MB-4 MB-5 MB-6 MB-7 MB-8 MB-9 SiO2 75.59 78.42 77.55 74.18 72.22 71.17 72.76 71.92 73.36 Al2O3 12.6 11.09 11.54 12.57 13.94 13.02 14.38 14.34 13.63 Fe2O3 0.14 0.26 0.24 0.64 0.75 0.97 0.19 0.36 0.37 FeO 1.29 1.09 1.14 1.65 1.31 2.39 1.47 1.75 1.57 Na2O 2.75 2.71 2.72 2.83 2.82 2.84 2.72 2.78 2.83 K2O 5.46 4.74 4.95 4.33 4.95 3.91 5.01 4.83 4.77 MnO 0.02 0.03 0.03 0.06 0.02 0.08 0.02 0.03 0.04 MgO 0.28 0.22 0.22 0.57 0.48 1.45 0.51 0.79 0.52 CaO 0.63 0.12 0.27 1.51 1.18 2.01 1.24 1.46 1.33 TiO 0.22 0.13 0.15 0.25 0.75 0.41 0.22 0.27 0.23 P2O5 0.06 0.05 0.08 0.06 0.11 0.08 0.17 0.19 0.13 烧失量 0.98 1.07 1.12 1.21 1.06 1.02 0.79 0.87 0.93 Na2O+K2O 8.21 7.45 7.67 7.16 7.77 6.75 7.73 7.61 7.60 A/NK 1.53 1.49 1.50 1.76 1.79 1.93 1.86 1.88 1.79 A/CNK 1.43 1.46 1.45 1.45 1.56 1.49 1.60 1.58 1.53 
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表 2 门巴地区早白垩世花岗岩微量元素分析结果
Table 2 The trace element data of the Early Cretaceous granites in Menba area
10-6 元素 黑云母花岗岩（巨斑） 黑云母花岗岩（中粒） 二云母花岗岩（中粒） MB-1 MB-2 MB-3 MB-4 MB-5 MB-6 MB-7 MB-8 MB-9 Ba 305 92 152 501 489 492 353 366 417 Rb 313 368 368 203 114 178 312 290 269 Sr 51 29 34.4 88 150 107.3 122 124.8 108.8 Ga 29.5 28.3 28.5 17.5 16.9 17.3 23.1 22.5 20.9 Ta 2.4 2.5 2.5 1.1 1.47 1.14 1.2 1.24 1.12 Nb 15.7 17.3 16.97 15.8 15 15.56 22.5 21.75 19.82 Hf 4.5 4.3 4.37 4.8 9.7 6.27 4.3 4.84 4.5 Zr 120 97 102 142 193 154.5 125 131.8 130.2 Ti 1295 797 932 1498 4616 2439 1329 1647 1394 Y 34.46 58.29 51.46 39.92 33 37.41 12.05 14.14 22.95 Th 33 40.28 38.67 42 15.1 33.93 31.61 29.96 35.77 U 4.4 4.7 4.6 4.4 2.18 3.734 7.11 6.62 6.03 La 36.53 33.3 34.25 42.27 31 39.21 36.89 36.3 39.23 Ce 77.37 78.28 78.94 82.76 62 76.53 70.31 69.48 75.29 Pr 9.15 9.47 9.51 9.49 7 9.058 8.71 8.54 9.21 Nd 29.95 31.53 30.28 36.25 28 33.81 28.29 28.26 31.49 Sm 6.35 7.36 7.13 7.46 6.1 7.18 5.79 5.82 6.53 Eu 0.58 0.22 0.34 0.93 1.2 1.02 1 1.02 0.98 Gd 5.77 7.75 7.03 6.77 5.7 6.45 4.7 4.8 5.53 Tb 1.06 1.48 1.36 1.17 1.01 1.13 0.7 0.73 0.9 Dy 6.35 9.27 8.67 7.07 6 6.75 3.08 3.37 4.68 Ho 1.34 2.02 1.81 1.51 1.2 1.42 0.49 0.56 0.9 Er 3.57 6.11 5.43 4.13 3.6 4.10 1.03 1.29 2.35 Tm 0.61 0.95 0.85 0.68 0.59 0.65 0.14 0.19 0.36 Yb 3.89 5.92 5.31 4.23 3.7 4.06 0.73 1.03 2.13 Lu 0.56 0.89 0.81 0.65 0.62 0.63 0.11 0.16 0.33 ∑REE 183.08 194.55 191.72 205.37 157.72 192.00 161.97 161.55 179.91 LREE 159.93 160.16 160.45 179.16 135.30 166.81 150.99 149.42 162.73 HREE 23.15 34.39 31.27 26.21 22.42 25.19 10.98 12.13 17.18 LREE/HREE 6.91 4.66 5.13 6.84 6.03 6.62 13.75 12.32 9.47 (La/Yb)N 6.74 4.03 4.63 7.17 6.01 6.92 36.25 25.28 13.21 δEu 0.29 0.09 0.15 0.39 0.61 0.45 0.57 0.57 0.49 δCe 1.01 1.07 1.05 0.97 0.99 0.96 0.93 0.93 0.94
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表 3 门巴地区早白垩世花岗岩LA-ICP-MS锆石U-Th-Pb分析结果
Table 3 LA-ICP-MS zircon U-Th-Pb analytical data of the Early Cretaceous granites in Menba area
测点 U Pb 同位素比值 年龄/Ma 10-6 206Pb/238U ±1σ 207Pb/235U ±1σ 207Pb/206Pb ±1σ 207Pb/235U ±1σ 207Pb/206Pb ±1σ Pb/206U238 ±1σ 巨斑黑云母花岗岩(MB-2) 1 608 12 0.0211 0.0002 0.148 0.005 0.0508 0.0015 142 4 238 76 138 2 2 574 14 0.0209 0.0003 0.146 0.004 0.0505 0.0013 146 3 221 59 141 2 3 428 9 0.0213 0.0003 0.153 0.005 0.0511 0.0015 144 5 254 71 139 2 4 613 15 0.0215 0.0002 0.149 0.004 0.0506 0.0014 143 4 217 62 142 2 5 517 13 0.0212 0.0003 0.147 0.005 0.0509 0.0014 147 4 229 62 140 2 中粒二云母花岗岩(MB-9) 1 612 17 0.0212 0.0003 0.149 0.004 0.0506 0.0015 142 4 228 77 136 2 2 511 12 0.0211 0.0003 0.148 0.004 0.0511 0.0015 149 4 234 64 141 2 3 432 11 0.0215 0.0002 0.151 0.005 0.0513 0.0017 151 5 242 77 143 2 4 566 14 0.0216 0.0003 0.151 0.005 0.0507 0.0015 144 4 231 75 139 2 5 552 13 0.0212 0.0002 0.146 0.004 0.0508 0.0014 148 5 219 65 138 2
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