The discovery of Middle-Late Ordovician syenogranite on the southern margin of Altun orogenic belt and its geological significance
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摘要:
出露于阿尔金造山带帕夏拉依档沟一带的正长花岗岩，LA-ICP-MS锆石U-Pb测年结果显示其形成年龄为455.1±3.6Ma，属中―晚奥陶世。地球化学结果显示，主量元素具有富硅、富铝、富钾，低钛、贫钙、贫镁的特点，为强过铝质花岗岩系列，具高钾钙碱性特征。稀土元素总量较高，轻稀土元素富集、重稀土元素亏损，稀土元素球粒陨石标准化配分曲线有右倾型特征和明显的负Eu异常，与典型壳源花岗岩配分曲线一致。Ba、Sr、Ti等具负异常，Rb、Th、K等大离子亲石元素具正异常，显示S型花岗岩特征。结合原岩判别图解，推断其源区物质主要来源于上地壳变泥质沉积岩类。结合区域资料，认为正长花岗岩形成于挤压体制向拉张体制转换的构造环境，属后碰撞花岗岩类，表明在中―晚奥陶世阿中地块和柴达木地块已由挤压碰撞阶段转为伸展后碰撞阶段。
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关键词:
- 阿尔金造山带 /
- LA-ICP-MS锆石U-Pb测年 /
- 正长花岗岩 /
- 地球化学 /
- 岩石成因
Abstract:The syenogranite is located in the Paxialayidang ditch of the Altun Mountains. The U-Pb dating of zircons from the syenogranite using LA-ICP-MS yielded a group age of 455.1±3.6Ma, indicating that the crystallization of the intrusion occurred in Middle-Late Ordovician period. The geochemical analysis shows that major elements are characterized by high SiO2, Al2O3 and K2O and low TiO2, CaO and MgO, which suggests that syenogranite belongs to the typical high-K calc-alkaline series with deeply peraluminous feature. In addition, the rocks are enriched in total REE. The samples are enriched in LREE (light rare earth elements) and depleted in HREE (heavy rare earth elements) with Eu anomalies. The chondrite-normalized REE patterns show right-oblique type. The syenogranite is enriched in large ion lithophile elements of Rb, Th, K and depleted in high field strength elements of Ba, Sr, Ti, with the characteristics of S-type granite. In combination with the diagrams for discriminating compositions of original rocks, the authors hold that the rocks were formed by the partial melting of meta-pelitic sedimentary rocks from the lower crust. Combined with the data of regional geological characteristics, the authors consider that the syenogranite was formed in the transitional tectonic setting from the compressional to the extensional regime, thus belonging to the post-collisional granites. It is shown that Azhong Block and Qaidam Block entered into a transformation period from compression to extension during Middle-Late Ordovician period.
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Keywords:
- Altun orogenic belt /
- LA-ICP-MS zircon U-Pb dating /
- syenogranite /
- geochemistry /
- petrogenesis
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致谢: 在成文过程中得到中国地质调查局西安地质调查中心李向民研究员的帮助，审稿专家提出宝贵的意见，在此表示衷心的感谢。
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图 1 阿尔金造山带地质构造图(a)及研究区地质简图(b, 据参考文献①修改)
TRB—塔里木盆地；QL—祁连山；QDB—柴达木盆地；WKL—西昆仑；EKL—东昆仑；HMLY—喜马拉雅山；INP—印度板块；Q—第四系；N2y—新近系油砂山组；J1—2dm—侏罗系大煤沟组；
Figure 1. Geological and tectonic map of Altun orogenic belt (a) and geological sketch map of the study area (b)
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图版Ⅰ
a、b、c.正长花岗岩宏观露头照片；d.正长花岗岩遥感影像特征；e.正长花岗岩中条纹长石；f.正长花岗岩中斜长石绢云母化、粘土化。Pt1A—古元古代阿尔金岩群；γδQb—盖里克片麻岩；ηγO2-3a—二长花岗岩；ξγO2-3c—正长花岗岩；Qhpal—全新统冲洪积物
图版Ⅰ.
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图 2 中―晚奥陶世正长花岗岩锆石阴极发光(CL)图像及U-Pb年龄
Figure 2. Zircon CL images and U-Pb ages of the syenogranite in Middle-Late Ordovician period
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图 3 中―晚奥陶世正长花岗岩LA-ICP-MS锆石U-Pb年龄谐和图
Figure 3. LA-ICP-MS zircon U-Pb concordia diagram for the syenogranite in Middle-Late Ordovician period
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图 5 中―晚奥陶世正长花岗岩球粒陨石标准化稀土元素分布型式(a)和原始地幔标准化微量元素蛛网图(b) [15]
Figure 5. Chondrite-normalized REE patterns(a) and primitive mantle-normalized trace element patterns (b) of the syenogranite in Middle-Late Ordovician period
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图 6 中―晚奥陶世正长花岗岩10000Ga/Al-FeO/MgO(a)和10000Ga/Al-Y图解(b)
Figure 6. Diagrams of 10000Ga/Al-FeO/MgO (a) and 10000Ga/Al-Y (b) of the syenogranite in Middle-Late Ordovician period
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图 7 中―晚奥陶世正长花岗岩ACF图解
Figure 7. ACF diagram of the syenogranite in Middle-Late Ordovician period
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图 8 中―晚奥陶世正长花岗岩Rb/Sr-Rb/Ba图解
Figure 8. Rb/Sr-Rb/Ba diagram of the syenogranite in Middle-Late Ordovician period
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表 1 正长花岗岩(PM030-25) 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 analyses of the syenogranite(PM030/25-1)
点号 同位素比值 年龄/Ma 207Pb/206Pb 207Pb/235U 206Pb/238U 208Pb/232Th 207Pb/206U 207Pb/235U 206Pb/238Th 208Pb/232Th 比值 误差
(1σ)比值 误差
(1σ)比值 误差
(1σ)比值 误差
(1σ)年龄 误差
(1σ)年龄 误差
(1σ)年龄 误差
(1σ)年龄 误差
(1σ)01 0.05707 0.00171 0.57749 0.00941 0.07342 0.00112 0.05629 0.0011 494 65 463 6 457 7 1107 21 02 0.05687 0.00168 0.57305 0.00885 0.0731 0.00111 0.07703 0.00145 486 65 460 6 455 7 1500 27 03 0.05535 0.00164 0.55877 0.00869 0.07323 0.00112 0.02783 0.00069 426 64 451 6 456 7 555 14 04 0.05688 0.00174 0.57085 0.00989 0.0728 0.00113 0.05736 0.00174 486 67 459 6 453 7 1127 33 05 0.06285 0.00188 0.58845 0.00961 0.06792 0.00105 0.14796 0.00256 703 63 470 6 424 6 2789 45 06 0.0543 0.0016 0.54623 0.00841 0.07297 0.00112 0.0405 0.00066 383 65 443 6 454 7 803 13 07 0.07356 0.00224 0.68855 0.0118 0.0679 0.00106 0.27561 0.00532 1029 60 532 7 424 6 4920 84 08 0.05507 0.00162 0.55805 0.00866 0.0735 0.00114 0.04739 0.00075 415 64 450 6 457 7 936 15 09 0.0534 0.00173 0.52987 0.01078 0.07197 0.00114 0.0417 0.00171 346 72 432 7 448 7 826 33 10 0.05618 0.00166 0.56795 0.00899 0.07332 0.00114 0.04302 0.00086 459 65 457 6 456 7 851 17 11 0.08164 0.0024 0.70473 0.01088 0.0626 0.00098 0.37524 0.00498 1237 56 542 6 391 6 6440 73 12 0.05635 0.00166 0.56979 0.00893 0.07332 0.00115 0.06536 0.00104 466 64 458 6 456 7 1280 20 13 0.05457 0.00161 0.55501 0.0087 0.07375 0.00116 0.06298 0.00094 395 64 448 6 459 7 1235 18 14 0.05535 0.00163 0.55715 0.00873 0.07298 0.00115 0.06047 0.00084 426 64 450 6 454 7 1187 16 15 0.05469 0.00161 0.55186 0.00871 0.07316 0.00116 0.02667 0.00039 400 63 446 6 455 7 532 8 16 0.05455 0.00161 0.55219 0.00877 0.07339 0.00117 0.03629 0.00059 394 64 446 6 457 7 721 12 17 0.05348 0.00158 0.54063 0.00861 0.07329 0.00117 0.02897 0.00048 349 65 439 6 456 7 577 9 18 0.0608 0.00179 0.52975 0.00842 0.06316 0.00101 0.11114 0.00152 632 62 432 6 395 6 2130 28 
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表 2 中―晚奥陶世正长花岗岩主量、微量和稀土元素分析结果
Table 2 Major, trace and rare earth elements analyses of the syenogranite in Middle-Late Ordovician period
样号 PM030/25-1 PM030/25-2 PM030/27-1 PM030/27-2 SiO2 73.53 73.49 72.15 72.21 TiO2 0.14 0.15 0.11 0.13 Al2O3 13.96 13.68 14.49 14.52 Fe2O3 0.68 0.70 0.65 0.65 FeO 0.76 0.72 0.86 0.85 MnO 0.02 0.02 0.02 0.02 MgO 0.32 0.30 0.29 0.29 CaO 0.52 0.51 0.55 0.50 Na2O 2.36 2.18 1.99 2.01 K2O 6.68 6.59 8.20 8.17 P2O5 0.06 0.05 0.06 0.06 烧失量 0.54 0.66 0.40 0.52 总计 99.57 99.05 99.77 99.93 σ 2.68 2.52 3.56 3.55 K2O/Na2O 2.83 3.02 4.12 4.06 AR 1.97 1.89 1.72 1.73 SI 2.96 2.86 2.42 2.42 FL 94.56 94.50 94.88 95.32 MF 81.77 82.56 83.93 83.84 R1 2455.31 2537.80 2138.28 2141.96 R2 345.35 337.79 357.47 352.70 AIK 0.80 0.78 0.84 0.84 A/CNK 1.16 1.17 1.10 1.11 K2O+Na2O 9.04 8.77 10.19 10.18 Na2O/K2O 0.35 0.33 0.24 0.25 m/f 0.41 0.39 0.35 0.35 La 25.4 23.66 15.1 18.23 Ce 54.6 50.38 32.3 36.66 Pr 7.23 7.42 4.22 5.64 Nd 25.8 23.66 15.1 17.23 Sm 7.17 6.98 4.24 4.67 Eu 0.598 0.61 0.635 0.63 Gd 6.66 5.99 4 4.26 Tb 1.08 1.02 0.665 0.85 Dy 5.77 4.99 3.68 3.95 Ho 0.966 0.95 0.632 0.72 Er 2.33 2.16 1.53 1.73 Tm 0.318 0.29 0.208 0.26 Yb 1.78 1.78 1.14 1.28 Lu 0.264 0.2 0.164 0.19 Y 30.2 28.76 19.1 21.93 ∑REE 170.17 158.85 102.71 118.23 LREE 120.80 112.71 71.60 83.06 HREE 49.37 46.14 31.12 35.17 LREE/HREE 2.45 2.44 2.30 2.36 δEu 0.26 0.28 0.46 0.42 δCe 0.94 0.89 0.94 0.84 (La/Yb)N 9.64 8.98 8.95 9.62 (La/Sm)N 2.23 2.13 2.24 2.46 (Gd/Yb)N 3.03 2.73 2.84 2.70 Cu 14.10 13.99 13.50 13.65 Pb 66.90 66.40 65.80 66.10 Zn 35.50 34.60 26.00 28.20 Co 1.88 1.94 2.59 2.42 Ni 2.13 2.45 3.57 3.31 Cr 17.10 16.96 15.30 16.10 V 6.60 8.20 11.90 10.30 Ga 17.20 17.10 16.20 16.80 Sr 86.10 88.20 92.30 91.40 Ba 255.70 266.00 295.80 289.00 Rb 319.20 327.00 339.50 338.00 Nb 28.70 26.80 11.10 13.70 Ta 3.42 3.32 0.94 1.45 Zr 67.80 65.90 26.40 27.10 Hf 7.46 7.32 1.77 1.89 U 7.23 7.13 6.40 6.41 Th 16.70 15.10 9.24 10.30 Ag 0.03 0.03 0.04 0.04 Au/10-9 3.17 3.16 3.12 3.16 Cs 7.47 7.58 7.87 7.79 Mg# 29.43 28.37 26.30 26.43 Rb/Sr 3.71 3.71 3.68 3.70 K/Rb 173.65 167.23 200.42 200.57 Ba/Sr 2.97 3.02 3.20 3.16 Th/Ta 4.88 4.55 9.82 7.10 K 55429.79 54682.98 68042.55 67793.62 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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