Zircon U-Pb geochronology and petro-geochmistry of the Late Triassic coarse grained porphyaceous monzonitic granite in Cuolong Co, Tibet
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摘要:
对出露于冈底斯中带偏西部错龙错地区原划为早白垩世的粗粒巨斑二长花岗岩进行了较系统的岩石学、LA-ICP-MS锆石U-Pb定年、地球化学研究。结果表明，该地区粗粒巨斑二长花岗岩的成岩年龄为212.6±2.7Ma，属晚三叠世。岩石为高钾钙碱性-钾玄岩系列，具高SiO2、K2O的特征。Al2O3含量为13.18%~16.26%，铝饱和指数A/CNK为0.96~1.33（平均值为1.11），为准铝质-过铝质花岗岩。稀土元素总量在182.99×10-6~324.34×10-6之间，轻稀土元素明显富集，重稀土元素相对亏损，δEu值变化于0.38~0.57之间，具明显的负Eu异常。微量元素富集K、Rb、Th和相对亏损Ba、Nb、Sr、P、Ti，具有类似造山后花岗岩的微量元素特征。该类岩石的地球动力学背景应为班公湖-怒江洋壳南向俯冲有关的由挤压向伸展转变的同碰撞向后碰撞的过渡阶段。
Abstract:Lithology, LA-ICP-MS zircon U-Pb age, and geochemistry of coarse grained porphyaceous monzonitic granite from Lower Cretaceous Cuolong Co area in the west of the middle Gangdise granitoid belt were studied in detail in this paper. The results show that magma zircons from coarse grained porphyaceous monzonitic granite yielded a weighted mean age of 212.6±2.7Ma, indicating Late Triassic. The rock is shoshonitic or high K calc-alkaline and relatively rich in Si2 and K2O. Al2O3 content is between 13.18% and 16.26%, A/CNK ratios vary from 0.96 to 1.33 (1.11 on average), displaying peraluminous characteristics. Total rare earth element values are between 182.99×10-6 and 324.34×10-6. Trace element ratio spider diagram displays apparent enrichment of light rare earth elements, depletion of heavy rare earth elements, δEu variation between 0.38 and 0.57, and sharp negative Eu anomalies. Trace elements are characterized by enrichment of K, Rb, Th and depletion of Ba, Nb, Sr, P, Ti, similar to features of orogenesis granite. Geodynamic analysis of rock background shows that it was most likely associated with a back-arc setting in response to conversion from compression to stretching related to the southward subduction of the Bangong Co-Nujiang oceanic lithosphere.
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致谢: 感谢中国地质科学院国家地质实验测试中心对锆石U-Pb测年工作所作出的努力；感谢湖北省地质实验测试中心对本文地球化学数据进行分析；感谢野外工作及论文写作过程中提供指导和帮助的四川省地质调查院贾小川教授级高工、杨学俊和刘卫新高级工程师。
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图 1 研究区地质简图
a—青藏高原冈底斯带构造单元划分；b—错龙错地区地质简图。Ⅰ-1—冈底斯-下察隅晚燕山-喜山期岩浆弧带；Ⅰ-2—隆格尔-念青唐古拉复合古岛弧带；Ⅰ-3—革吉-申扎弧后盆地带；Ⅰ-4—它日错-班戈-那曲前陆盆地
Figure 1. Simplified geological map of the study area
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图版Ⅰ
a.粗粒巨斑二长花岗岩野外特征；b.粗粒巨斑二长花岗岩钾长石斑晶；c、d.粗粒巨斑二长花岗岩显微结构特征。Qz—石英；Pl—斜长石；Kfs—钾长石；Bi—黑云母
图版Ⅰ.
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图 3 粗粒巨斑二长花岗岩锆石阴极发光(CL)图像
Figure 3. CL image of zircon from coarse grained porphyaceous monzonitic granite
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图 4 粗粒巨斑二长花岗岩锆石U-Pb年龄谐和图
Figure 4. Zircon U-Pb concordia diagram of coarse grained porphyaceous monzonitic granite
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图 6 粗粒巨斑二长花岗岩稀土元素配分图(a)和微量元素蛛网图(b)[23]
Figure 6. Chondrite-normalized REE patterns(a) and primitive mantle-normalized trace element spidergram(b) of coarse grained porphyaceous monzonitic granite
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图 8 粗粒巨斑二长花岗岩的Rb/Sr-Rb/Ba关系[29]
Figure 8. Relation of Rb/Sr-Rb/Ba of coarse grained porphyaceous monzonitic granite
表 1 粗粒巨斑二长花岗岩LA-ICP-MS锆石U-Th-Pb同位素分析结果
Table 1 LA-ICP-MS zircon U-Th-Pb data of coarse grained porphyaceous monzonitic granite
序号 207Pb*/206Pb* 207Pb*/235U 206Pb*/238U 208Pb/232Th 年龄/Ma Th/U 比值 ±1σ 比值 ±1σ 比值 ±1σ 比值 ±1σ 207Pb/
206Pb±1σ 207Pb/
206Pb±1σ 206Pb/
238U±1σ 208Pb/
232Th±1σ 1 0.05266 0.00115 0.2367 0.0058 0.03352 0.00073 0.01076 0.00021 314 51 216 5 213 5 216 4 0.19 2 0.05140 0.00124 0.2375 0.0064 0.03353 0.00073 0.01107 0.00024 259 57 216 5 213 5 223 5 0.18 3 0.05135 0.00122 0.2397 0.0064 0.03347 0.00073 0.01030 0.00022 257 56 218 5 212 5 207 4 0.19 4 0.05616 0.00121 0.2806 0.0067 0.03857 0.00084 0.0106 0.00021 459 49 251 5 244 5 213 4 0.17 5 0.05222 0.00119 0.2306 0.0058 0.03266 0.00072 0.01092 0.00022 295 53 211 5 207 4 220 4 0.23 6 0.05149 0.00147 0.2522 0.0083 0.03279 0.00073 0.01028 0.00025 263 67 228 7 208 5 207 5 0.21 7 0.06011 0.00146 0.2989 0.0082 0.03557 0.00079 0.01138 0.00022 608 54 266 6 225 5 229 4 0.68 8 0.05206 0.00123 0.2425 0.0064 0.03440 0.00076 0.01094 0.00022 288 55 220 5 218 5 220 4 0.26 9 0.05058 0.00123 0.2356 0.0065 0.03385 0.00075 0.01117 0.00024 222 58 215 5 215 5 225 5 0.19 10 0.05380 0.00112 0.2605 0.0061 0.04132 0.00091 0.00923 0.00018 363 48 235 5 261 6 186 4 0.16 11 0.05205 0.00119 0.2471 0.0064 0.03397 0.00076 0.01157 0.00024 288 53 224 5 215 5 233 5 0.18 12 0.05126 0.00116 0.2447 0.0063 0.03706 0.00083 0.00971 0.00022 253 53 222 5 235 5 195 4 0.12 13 0.05254 0.0011 0.2500 0.0059 0.03918 0.00087 0.01157 0.00022 309 49 227 5 248 5 233 4 0.16 14 0.05181 0.00122 0.2384 0.0064 0.03344 0.00075 0.01061 0.00022 277 55 217 5 212 5 213 4 0.22 15 0.05446 0.00117 0.2656 0.0064 0.03785 0.00084 0.01083 0.00021 390 49 239 5 239 5 218 4 0.15 16 0.05144 0.00124 0.2405 0.0066 0.03345 0.00075 0.00593 0.00012 261 57 219 5 212 5 120 2 0.41 17 0.05333 0.00157 0.2419 0.0082 0.03311 0.00076 0.01042 0.00024 343 68 220 7 210 5 210 5 0.31 18 0.05096 0.00116 0.2322 0.0060 0.03372 0.00076 0.01093 0.00022 239 54 212 5 214 5 220 4 0.18 19 0.05150 0.00185 0.2407 0.0099 0.03447 0.00081 0.01023 0.00033 263 84 219 8 218 5 206 7 0.18 注：Pb*代表放射成因铅 
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表 2 粗粒巨斑二长花岗岩主量、微量和稀土元素分析结果
Table 2 Major, trace and rare earth element analyses of coarse grained porphyaceous monzonitic granite
样品号 pm401-18H1 pm401-34H1 pm401-45H1 pm402-91H1 pm402-99H1 pm402-142H1 SiO2 70.35 69.9 71.33 69.63 70.84 72.11 TiO2 0.42 0.43 0.5 0.48 0.25 0.37 Al2O3 13.72 14.93 13.93 14.42 16.26 13.18 Fe2O3 0.74 0.59 0.64 0.8 0.5 0.56 FeO 1.7 1.85 2.15 2.07 0.98 1.8 MnO 0.05 0.04 0.05 0.04 0.02 0.03 MgO 0.67 0.73 0.86 0.79 0.47 0.61 CaO 2.34 1.4 1.18 1.75 0.45 0.9 Na2O 2.55 2.67 2.6 3.29 2.59 3.12 K2O 5.35 5.74 4.97 5.03 6.57 5.54 P2O5 0.15 0.17 0.15 0.17 0.09 0.16 H2O+ 1.04 1.26 1.36 1.09 0.72 1.16 CO2 0.73 0.08 0.08 0.11 0.04 0.11 烧失量 1.61 1.15 1.23 1.05 0.67 1.2 总量 101.41 100.94 101.03 100.73 100.45 100.84 La 64.54 68.05 76.31 76.31 64.54 68.05 Ce 123.26 130.43 143.92 143.92 123.26 130.43 Pr 13.52 14.14 15.46 15.46 13.52 14.14 Nd 47.81 50.18 54.85 54.85 47.81 50.18 Sm 8.68 8.87 9.2 9.2 8.68 8.87 Eu 1.04 1.23 1.11 1.11 1.04 1.23 Gd 7.51 7.57 8.15 8.15 7.51 7.57 Tb 1.24 1.13 1.22 1.22 1.24 1.13 Dy 6.53 5.39 6.16 6.16 6.53 5.39 Ho 1.24 0.92 1.12 1.12 1.24 0.92 Er 3.41 2.37 3.1 3.1 3.41 2.37 Tm 0.53 0.36 0.48 0.48 0.53 0.36 Yb 3.25 2.24 2.87 2.87 3.25 2.24 Lu 0.47 0.33 0.39 0.39 0.47 0.33 Y 37.6 26.69 33.63 33.63 37.6 26.69 ΣREE 283.04 293.21 324.34 324.34 283.04 293.21 (La/Yb)N 14.22 21.76 19.05 19.05 14.22 21.76 δEu 0.38 0.45 0.38 0.38 0.38 0.45 δCe 0.97 0.98 0.97 0.97 0.97 0.98 Co 5.15 5.6 5.77 5.76 2.52 4.62 Cu 2.17 3.21 12.07 6.91 5.58 4.43 Zn 56.45 32.96 54.83 55.02 33.31 46.62 Rb 305.1 312.9 271.1 295.2 335.4 317.5 Zr 223.99 231.01 261.14 265.3 90.39 223.9 Nb 19.73 20.5 20.7 14.38 8.52 12.53 Hf 5.83 6.05 7.02 6.1 2 5.2 Ta 2.15 2.49 2.37 1.37 1.2 1.5 Mo 0.39 0.38 0.29 0.59 0.35 0.41 W 2.56 1.29 1.84 1.58 1.22 1.55 Pb 65.41 66.89 57.56 55.38 80.77 68.85 Th 50.35 50.66 51.59 66.58 35.98 55.18 U 3.85 2.54 2.59 3.67 2.86 3.75 Cr 12.67 15.25 15.18 21.26 14.45 14.18 BaO 548 730 604.8 417.8 258.1 400.5 Ni 5.73 5.27 6.51 5.41 3.25 3.93 Sr 140.1 159.2 148.8 111.2 85.96 97.89 V 33.14 36.48 40.66 37.98 13.59 27.91 As 2.46 1.55 1.42 1.47 1.01 0.86 Sb 0.21 0.29 0.23 0.21 0.14 0.16 Sn 8.28 6.76 10.26 4.79 3.66 6.26 Ag 0.052 0.041 0.039 0.045 0.028 0.102 Au 0.3 0.3 0.3 0.2 0.5 0.3 注：主量元素含量单位为%，稀土和微量元素单位为10-6；分析测试单位为湖北省地质实验测试中心
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