Zircon U-Pb age and geochemical characteristics of Qinlingliang rapakiwi granite from Western Qinling orogenic belt in Shaanxi
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摘要:
对秦岭梁岩体进行了地质学、岩石学、地球化学及年代学研究。秦岭梁岩体主要岩石类型有似斑状黑云环斑花岗岩、似斑状角闪黑云环斑花岗岩、中粒黑云二长花岗岩，普遍发育环斑结构，LA-ICP-MS锆石U-Pb测年结果为215.1±2.5Ma(MSWD=0.29，n=17)，显示秦岭梁环斑花岗岩形成于晚三叠世。秦岭梁环斑花岗岩属于高钾钙碱性系列，具有准铝质-弱过铝质的特点，富集Rb、Th、K、Pb、Nd等，贫Ba、Nb、Ta、La、P、Eu等，明显亏损Nb、Ta、P。稀土元素含量中等偏低，轻、重稀土元素分异程度高，稀土元素标准化配分曲线呈明显的右倾模式。秦岭梁环斑花岗岩岩浆主要来源于地壳熔融，同时具有壳幔混合特点。结合区域地质背景，认为秦岭梁环斑花岗岩形成于造山后伸展环境。
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关键词:
- 西秦岭 /
- 秦岭梁环斑花岗岩 /
- 地球化学 /
- LA-ICP-MS锆石U-Pb测年
Abstract:Based on geological, petrological and geochemical studies, the authors investigated Qinlingliang rapakiwi granite intrusion, porphyritic biotite rapakivi granite, porphyritic hornblende biotite rapakivi granite and medium-grained biotite monzonitic granite. LA-ICP-MS zircon U-Pb analysis suggests that Qinlingliang rapakiwi granite intrusion was formed in late Triassic(215.1±2.5Ma). Qinlingliang rapakiwi granite belongs to high-K calc-alkaline series with aluminous-peraluminous nature, and is characterized by enrichment of Rb, Th, K, Pb and Nd, depletion of Ba, Nb, Ta, La, P and Eu, and significant loss of Nb, Ta and P. In addition, this intrusion has moderate-low REE concentration and right-inclined distribution curve. The magma probably mainly originated from partial melting of the crust, and also had the characteristics of crust-mantle mixing. In combination with regional geological background, the authors hold that Qinlingliang rapakiwi granite intrusion probably originated from a post orogenic extension environment.
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图 4 秦岭梁环斑花岗岩锆石阴极发光(CL)图像及年龄
Figure 4. Zircon CL images and ages of Qinlingliang rapakiw granite
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图 5 秦岭梁环斑花岗岩LA-ICP-MS锆石U-Pb年龄谐和图
Figure 5. LA-ICP-MS zircon U-Pb concordia diagrams of Qinlingliang rapakiwi granite
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图 6 秦岭梁环斑花岗岩体K2O- SiO2(a)和A/CNK-A/NK图解
Figure 6. K2O- SiO2 diagram (a) and A/CNK-A/NK (b) diagram of Qinlingliang rapakiwi granite
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图 7 秦岭梁环斑花岗岩体球粒陨石标准化稀土元素配分模式图(a)和原始地幔标准化微量元素蛛网图(b)
(标准化数据据参考文献[33])
Figure 7. Chondrite-normalized of REE patterns (a) and primitive mantle- normalized spidergrams of trace elements (b) of Qinlingliang rapakiwi granite
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图 9 秦岭梁环斑花岗岩体构造环境判别图
ORG—洋脊花岗岩；VAG—火山岛弧花岗岩；WPG—板内花岗岩；COLG—碰撞花岗岩；Syn-COLG—统碰撞花岗岩；Post-COLG—后碰撞花岗岩
Figure 9. Tectonic discrimination diagrams of Qingling rapakiwi granite
表 1 秦岭梁岩体LA-ICP-MS锆石U-Th-Pb同位素分析结果
Table 1 LA-ICP-MS zircon U-Th-Pb dating results of Qinlingliang rapakiwi granite samples
样品编号 同位素比值 同位素年龄/Ma 207Pb/206Pb 1σ 207Pb/235U 1σ 206Pb/238U 1σ 208Pb/232Th 1σ 207Pb/235U 1σ 206Pb/238U 1σ 208Pb/232Th 1σ 207Pb/235U 1σ QLL-1 0.05101 0.00122 0.2344 0.0061 0.03336 0.00079 0.00991 0.00034 241 54 214 5 212 5 199 7 QLL-2 0.04980 0.00112 0.2291 0.0057 0.03343 0.00078 0.00979 0.00030 186 52 210 5 212 5 197 6 QLL-3 0.05092 0.00117 0.2406 0.0061 0.03433 0.00080 0.01018 0.00032 237 52 219 5 218 5 205 6 QLL-4 0.05395 0.00163 0.2476 0.0078 0.03335 0.00081 0.01032 0.00050 369 66 225 6 212 5 208 10 QLL-5 0.05146 0.00119 0.2424 0.0062 0.03423 0.0008 0.0105 0.00034 261 52 220 5 217 5 211 7 QLL-6 0.05002 0.00116 0.2298 0.0058 0.03339 0.00078 0.01005 0.00030 196 53 210 5 212 5 202 6 QLL-7 0.05076 0.00115 0.2369 0.0059 0.03395 0.00078 0.01047 0.00031 230 52 216 5 215 5 211 6 QLL-8 0.05111 0.00115 0.2425 0.0061 0.03453 0.0008 0.01057 0.00033 246 52 221 5 219 5 213 7 QLL-9 0.05173 0.00126 0.2437 0.0064 0.03428 0.0008 0.01081 0.00040 273 55 221 5 217 5 217 8 QLL-10 0.05124 0.00129 0.2374 0.0064 0.03373 0.00078 0.01007 0.00038 252 57 216 5 214 5 202 8 QLL-11 0.05068 0.00129 0.2314 0.0062 0.03322 0.00077 0.01087 0.00044 226 58 211 5 211 5 219 9 QLL-12 0.04922 0.00125 0.2344 0.0063 0.03469 0.0008 0.01015 0.00039 158 58 214 5 220 5 204 8 QLL-13 0.05090 0.00123 0.2373 0.0061 0.03398 0.00078 0.0104 0.00036 236 55 216 5 215 5 209 7 QLL-14 0.05120 0.00143 0.2417 0.0070 0.03441 0.0008 0.01065 0.00050 250 63 220 6 218 5 214 10 QLL-15 0.05031 0.00117 0.2342 0.0059 0.03393 0.00077 0.01126 0.00036 209 53 214 5 215 5 226 7 QLL-16 0.05062 0.00122 0.2367 0.0061 0.03409 0.00078 0.01093 0.00039 223 55 216 5 216 5 220 8 QLL-17 0.05094 0.00124 0.2386 0.0062 0.03415 0.00078 0.01093 0.00039 238 55 217 5 217 5 220 8 
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表 2 秦岭梁环斑花岗岩主量元素分析结果
Table 2 The main elements analytical results of samples from Qinlingliang rapakiwi granite
% 样品号 SiO2 TiO2 Al2O3 TFe2O3 MnO MgO CaO Na2O K2O P2O5 烧失量 总计 A/CNK Mg# QLL-5 75.76 0.11 13.15 1.34 0.05 0.22 0.55 3.71 4.09 0.04 0.40 99.42 1.14 24.29 QLL-6 72.25 0.50 13.50 3.09 0.09 0.82 2.00 4.30 3.70 0.19 0.21 100.65 0.92 34.43 QLL-11 68.58 0.56 15.23 3.29 0.07 0.89 2.20 4.01 4.41 0.14 0.23 99.61 0.99 34.79 QLL-14 73.55 0.15 14.27 1.51 0.07 0.21 1.21 3.91 4.37 0.05 0.03 99.32 1.07 21.52 QLL-18 72.67 0.24 14.13 2.25 0.06 0.53 1.82 3.65 4.55 0.11 0.42 100.44 0.99 31.94 注：A/CNK=Al2O3/(CaO+Na2O+K2O)；Mg#=Mg2+/(Mg2++Fe2+(全铁))×100
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表 3 秦岭梁环斑花岗岩稀土和微量元素分析结果
Table 3 The rare earth elements analytical results of samples from Qinlingliang rapakiwi granite
10-6 样品号 La Ba Ce Rb Pr Sr Nd Y Sm Zr Eu Nb Gd Th Tb Ga Dy Ni Ho Cr Er Hf Tm Sc Yb Ta Lu Co U Pb Cs QLL-5 18.30 417.00 43.20 249.00 3.92 108.00 12.40 14.40 2.26 108.00 0.48 24.50 2.03 26.72 0.36 17.00 2.12 2.16 0.42 13.40 1.36 6.96 0.23 2.88 1.66 2.67 0.31 1.30 6.07 48.00 8.08 QLL-6 37.30 778.00 85.60 172.00 10.50 340.00 35.60 35.50 6.64 145.00 1.07 39.30 5.48 28.33 1.00 20.00 5.54 6.81 1.06 12.80 3.31 6.64 0.52 6.11 3.38 6.66 0.54 5.82 4.92 35.70 8.17 QLL-11 43.70 1450.00 85.30 151.00 8.72 322.00 26.80 7.84 3.80 197.00 0.82 15.60 3.07 27.92 0.39 18.40 1.54 3.32 0.25 7.46 0.78 6.04 0.11 2.05 0.71 1.41 0.12 2.41 2.49 42.30 3.97 QLL-14 33.00 1435.00 73.50 150.00 7.84 336.00 24.40 7.32 3.54 205.00 0.81 16.30 2.77 33.47 0.36 18.20 1.42 4.25 0.24 3.95 0.72 7.28 0.12 4.23 0.64 1.77 0.10 2.74 3.22 43.20 4.15 QLL-18 71.50 875.00 139.00 112.00 14.80 295.00 46.20 24.40 7.38 228.00 1.41 26.00 6.30 18.63 0.98 18.70 4.75 9.40 0.83 19.50 2.32 3.76 0.32 5.65 2.01 2.73 0.31 6.38 3.88 61.20 3.97
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