Chronology and geochemistry of the granitoids in Heri Town, Tongde County, Qinghai Province, and its tectonic significance
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摘要:
采用LA-ICP-MS锆石U-Pb同位素测年方法，在青海同德县和日乡一带黑云母花岗闪长岩中获得锆石U-Pb年龄为229.7±1.0 Ma。岩石地球化学特征表明，该花岗闪长岩属高钾钙碱性系列，具微弱的负Eu异常和右倾的稀土元素球粒陨石标准化配分特征，是下地壳部分熔融的产物。综合已有的研究成果，认为和日乡黑云母花岗闪长岩侵位时代为晚三叠世，属I型花岗岩类，形成于板块碰撞前的活动陆缘弧构造环境，其成因与阿尼玛卿-勉略洋的向北俯冲消减密切相关，为西秦岭印支期进入碰撞造山阶段提供了年龄上限约束。
Abstract:In this study, the author used LA-ICP-MS zircon U-Pb isotopic dating method and obtained weighted average zircon U-Pb age of 229.7±1.0 Ma from the biotite granodiorite in Heri Town, Tongde County, Qinghai Province.The geochemical characteristics show that it belongs to high-K calcium-alkali series, and resulted from partial melting of crustal components, with weak negative Eu anomalies and right-inclined chondrite-normalized REE patterns.Based on a comprehensive analysis of previous researched, the authors hold that the biotite granodiorite was formed in the Late Triassic, and belonged to I-type granite formed in the active continental margin environment before the plane collision.Its genesis was closely related to the northward subduction of the Ańyemagen-Mianlue Ocean.The results obtained by the authors provide a new age constraint for the entrance of West Qinling into the syn-collision stage in the Indosinian.
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Keywords:
- granodiorite /
- zircon U-Pb age /
- active continental margin /
- West Qinling
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图 1 研究区大地构造位置(a)和地质简图(b)
Figure 1. Tectonic location(a)and simplified geological map(b) of the study area
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图 2 和日乡黑云母花岗闪长岩野外及镜下照片
Q—石英；Pl—斜长石；Kf—钾长石；Bi—黑云母
Figure 2. Outcrop and microscopic photos of Heri biotite granodiorite
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图 4 黑云母花岗闪长岩锆石年龄频率直方图
Figure 4. Zircon age frequency histogram of biotite granodiorite
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图 5 黑云母花岗闪长岩LA-ICP-MS锆石U-Pb谐和图(a)和206Pb/238U表面年龄加权平均值(b)
Figure 5. Zircon LA-ICP-MS U-Pb concordia diagram (a) and weighted mean206Pb/238U age (b) of biotite granodiorite
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图 6 SiO2-(K2O+Na2O)(a)和SiO2-K2O(b)图解
Figure 6. SiO2-(K2O+Na2O)(a) and SiO2-K2O(b) plots
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图 7 微量元素原始地幔蛛网图(a)和稀土元素球粒陨石配分型式图(b)
Figure 7. Primitive mantle-normalized trace element spider diagrams (a) and chondrite-normalized REE patterns (b)
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图 8 成因类型和构造环境判别图解
a—ACF图解; b—Na2O-K2O图解; c—Y-Nb图解; d—(Y+Nb)-Rb图解; e—R1-R2图解(R1=4*Si-11*(Na+K)-2(Fe+Ti, R2=6*Ca+2Mg+Al)；WPG—板内花岗岩；VAG—火山弧花岗岩；syn-COLG—同碰撞花岗岩；ORG—大洋中脊斜长花岗岩
Figure 8. Discrimination diagrams of genetic type and tectonic environment
表 1 黑云母花岗闪长岩LA-ICP-MS锆石U-Th-Pb同位素分析结果
Table 1 LA-ICP-MS zircon U-Th-Pb isotopic composition of biotite granodiorite
测点 含量/10-6 同位素比值 年龄/Ma Pb U Th/U 207Pb/206Pb 1σ 207Pb/235U 1σ 206Pb/238U 1σ 207Pb/206Pb 1σ 207Pb/235U 1σ 206Pb/238U 1σ 0192190.45740.05050.00140.24490.00510.03520.00030.2190.622224 22320271730.33830.05080.00160.24430.00640.03490.0004233722225221203 10 232 0.5147 0.0562 0.0027 0.2801 0.0121 0.0362 0.0005 458 103 251 10 229 3 04 10 222 0.4061 0.0524 0.0014 0.2634 0.0049 0.0365 0.0003 302 58 237 4 231 2 05 6 154 0.3449 0.0490 0.0014 0.2444 0.0052 0.0362 0.0004 147 64 222 4 229 2 06 8 185 0.4233 0.0504 0.0021 0.2533 0.0091 0.0365 0.0004 213 92 229 7 231 3 07 12 275 0.4596 0.0544 0.0015 0.2725 0.0056 0.0364 0.0004 387 60 245 4 230 2 08 9 211 0.4543 0.0512 0.0014 0.2566 0.0050 0.0364 0.0004 249 60 232 4 230 2 09102390.53050.05000.00130.24220.00460.03510.0003195602204223210 16 387 0.4447 0.0508 0.0013 0.2545 0.0048 0.0364 0.0003 231 60 230 4 230 2 1192100.44140.05090.00150.24460.00590.03480.0004238682225221212 9 194 0.5645 0.0528 0.0014 0.2655 0.0053 0.0364 0.0004 322 60 239 4 231 2 13 8 184 0.4390 0.0524 0.0018 0.2595 0.0074 0.0359 0.0004 302 76 234 6 228 2 14 9 210 0.4688 0.0532 0.0014 0.2678 0.0052 0.0365 0.0004 337 59 241 4 231 2 154761.30680.05140.00190.24710.00800.03490.0004258842247221216 9 194 0.4306 0.0556 0.0014 0.2791 0.0049 0.0364 0.0003 436 55 250 4 231 2 17 7 174 0.4082 0.0510 0.0014 0.2561 0.0051 0.0364 0.0004 242 61 232 4 231 2 18 10 228 0.4580 0.0543 0.0015 0.2727 0.0056 0.0364 0.0004 385 61 245 5 230 2 19 15 195 0.3905 0.0515 0.0017 0.2573 0.0071 0.0362 0.0004 264 75 233 6 229 2 20 9 203 0.4620 0.0523 0.0016 0.2605 0.0063 0.0361 0.0004 299 69 235 5 229 2 21 8 193 0.3439 0.0512 0.0014 0.2571 0.0051 0.0364 0.0004 250 61 232 4 231 2 22 8 196 0.4697 0.0508 0.0014 0.2496 0.0050 0.0356 0.0003 233 62 226 4 226 2 23 9 206 0.3767 0.0522 0.0015 0.2600 0.0057 0.0362 0.0004 292 65 235 5 229 2 24 8 182 0.3355 0.0518 0.0021 0.2586 0.0091 0.0362 0.0004 277 89 234 7 229 3 25 6 137 0.3629 0.0511 0.0019 0.2561 0.0079 0.0364 0.0004 245 82 232 6 230 3 注：标注删除线的点号在本次统计分析中未被采用，原因见文中描述 
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表 2 黑云母花岗闪长岩主量、微量和稀土元素分析结果
Table 2 Major, trace elements and REE content of biotite granodiorite
样号 D2005/2 D2005/10 D2005/15 D2005/23 D2005/29 SiO2 65.33 68.48 68.32 68.12 68.34 TiO2 0.54 0.43 0.42 0.42 0.41 Al2O3 15.89 15.27 15.75 15.22 15.34 Fe2O3 0.81 0.54 1.10 1.05 0.69 FeO 3.10 2.54 2.00 1.93 2.36 MnO 0.07 0.05 0.04 0.04 0.04 MgO 1.12 0.81 0.86 0.91 0.86 CaO 4.02 3.04 3.06 3.15 2.94 Na2O 3.86 3.58 3.57 3.62 3.75 K2O 3.09 3.34 2.81 3.39 3.19 P2O5 0.12 0.10 0.11 0.10 0.10 烧失量 1.06 0.86 1.12 1.21 1.07 Mg# 0.34 0.32 0.34 0.36 0.34 K2O/Na2O 0.80 0.93 0.79 0.94 0.85 A/CNK 0.934 1.016 1.088 0.991 1.025 σ 2.12 1.86 1.59 1.93 1.88 Ti 3653 2521 2519 2579 2626 V 46.6 35.1 34 38.4 38.9 Cu 11.5 15.6 7.6 8 7.4 Zn 45.6 36 39.3 32.4 36.7 Ga 19.5 20.2 21.2 20.9 20.5 Sr 298 306 317 294 303 Ba 569 647 630 539 588 Ni 4.48 3.9 3.54 3.42 4.31 Cr 6.9 6.8 8.9 7.4 12.9 Zr 160 148 154 142 143 Pb 19 18.7 18.4 18.9 18.6 Rb 99.6 122 117 126 121 Cs 2.92 4.93 5.22 3.33 5.71 Th 3.62 1.58 1.99 1.93 4.02 U 0.38 0.51 0.47 1.02 0.8 Co 7.62 6.58 6.74 4.83 5.93 Nb 17.6 21.5 18.2 18.2 21.5 Hf 1.7 2 1.97 1.84 1.93 Ta 2.96 0.47 0.91 0.5 0.38 Mo 1.55 1.07 1.16 0.67 1.46 Ag 0.041 0.044 0.046 0.034 0.047 Sn 3.54 2.33 2.01 2.22 3.09 Sb 0.18 0.21 0.16 0.19 0.24 Bi 1.94 0.085 0.084 0.076 0.091 Au 3.19 1.67 2.57 2.77 2.59 La 33.7 30.2 25.8 26.6 30.5 Ce 72.1 62.9 56.9 54.3 63.6 Pr 7.38 6.84 6.02 5.97 6.66 Nd 27.7 27.1 22.1 22.7 25.2 Sm 5.39 5.35 4.23 4.21 4.77 Eu 1.39 1.32 1.15 1.11 1.27 Gd 4.25 3.96 3.01 3.14 3.56 Tb 0.73 0.63 0.46 0.49 0.53 Dy 3.10 2.30 1.61 1.90 1.76 Ho 0.75 0.45 0.35 0.38 0.39 Er 1.77 1.03 0.81 1.04 0.91 Tm 0.28 0.15 0.10 0.15 0.14 Yb 1.71 0.80 0.77 0.97 0.80 Lu 0.25 0.13 0.12 0.14 0.11 Y 18.10 11.30 8.34 10.40 9.68 ΣREE 160.50 143.16 123.43 123.10 140.20 LREE/HREE 11.50 14.15 16.08 13.99 16.10 (La/Yb)N 14.14 27.08 24.03 19.67 27.35 δEu 0.89 0.88 0.99 0.93 0.94 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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