ZIRCON U-Pb DATING AND GEOCHEMISTRY OF THE RHYOLITES FROM XINMIN FORMATION IN SANGENHE FOREST FARM, NORTHERN DAXINGANLING MOUNTAINS
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摘要:
三根河林场新民组火山岩主要由流纹岩、流纹质熔结凝灰岩组成,U-Pb定年结果表明流纹岩形成于165.71±0.83 Ma的中侏罗世.岩石地球化学分析结果显示,SiO2含量71.05%~76.77%,Na2O+K2O含量6.66%~8.49%,铝饱和指数(A/CNK)1.02~1.17,相对富硅、碱、过铝质,稀土元素总量中等,轻、重稀土元素分馏明显,具有较强的Eu负异常(δEu=0.39~0.66)和Dy负异常,大离子亲石元素Ba、Sr相对亏损,高场强元素Th、U相对富集,Nb、Dy、Ti相对亏损,岩石具有A型流纹岩的特征.岩石地球化学研究表明,岩浆来源于地壳物质的部分熔融作用.空间上,新民组沿大兴安岭火山岩带中北段均有分布,显示大兴安岭火山岩带于中侏罗世处于张性构造环境.
Abstract:The volcanic rocks of Xinmin Formation in Sangenhe forest farm are mainly composed of rhyolites and rhyolitic ignimbrites. The U-Pb dating results indicate the rhyolites were formed in Middel Jurassic, with the age of 165.71±0.83 Ma. The petrogeochemical analysis shows that the rhyolites are characterized by relatively high silicon, alkali and peraluminous, with the SiO2 content of 71.05%-76.77%, Na2O+K2O of 6.66%-8.49% and A/CNK of 1.02-1.17. The rocks present medium total REE content, significant fractionation of HREEs and LREEs, strong negative Eu anomaly (δEu=0.39-0.66) and negative Dy anomaly, relative depletion of LILEs (Ba and Sr), enrichment of HSFEs (Th and U) and relative depletion of Nb, Dy and Ti, with the characteristics of A-type rhyolites. The study shows the magma is derived from partial melting of crustal materials. The Xinmin Formation is spatially distributed along the middle-north section of Daxinganling volcanic belt, revealing the volcanic belt was in a tensional tectonic environment in Middle Jurassic.
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Key words:
- Xinmin Formation /
- rhyolite /
- zircon U-Pb age /
- geochemistry /
- tectonic setting /
- Daxinganling Mountains
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表 1 流纹岩样品(P9LT171)LA-ICP-MS锆石U-Pb同位素测试结果
Table 1. LA-ICP-MS zircon U-Pb isotopic test results of rhyolite sample P9LT171
测点 含量/10-6 同位素比值 年龄/ Ma Pb U 207Pb/206Pb 207Pb/235U 206Pb/238U 208Pb/232Th 232Th/238U 207Pb/206Pb 207Pb/235U 206Pb/238U 1 5 158 0.0496±0.0030 0.1801±0.0111 0.0264±0.0003 0.0100±0.0001 1.1154±0.0033 174±143 168±10 168±2 2 18 500 0.0501±0.0011 0.1777±0.0040 0.0257±0.0003 0.0099±0.0000 1.3623± 0.0055 197±50 166±4 164±2 3 3 99 0.0492±0.0041 0.1782±0.0152 0.0262±0.0003 0.0109±0.0002 0.6136±0.0052 159±193 166±14 167±2 4 9 286 0.0488±0.0013 0.1785±0.0048 0.0265±0.0003 0.0109±0.0001 0.6127±0.0017 138±62 167±4 169±2 5 3 87 0.0494±0.0059 0.1782±0.0214 0.0262±0.0003 0.0096±0.0002 1.2715±0.0030 165±281 167±20 167±2 6 9 321 0.0492±0.0017 0.1752±0.0061 0.0258±0.0003 0.0097±0.0001 0.6105±0.0037 160±80 164±6 164±2 7 6 188 0.0494±0.0025 0.1792±0.0094 0.0263±0.0003 0.0103±0.0001 0.8876±0.0041 167±119 167±9 167±2 8 19 656 0.0495±0.0010 0.1765±0.0038 0.0259±0.0003 0.0086±0.0000 0.8055±0.0008 170±48 165±4 165±2 9 9 297 0.0497±0.0016 0.1795±0.0058 0.0262±0.0003 0.0087±0.0001 0.7715±0.0044 180±74 168±5 167±2 10 86 3129 0.0491±0.0007 0.1785±0.0026 0.0264±0.0003 0.0138±0.0002 0.2867±0.0029 1541±33 167±2 168±2 11 12 379 0.0506±0.0208 0.1775±0.0270 0.0255±0.0003 0.0135±0.0005 0.7355±0.0050 222±952 166±25 162±2 12 7 238 0.0495±0.0023 0.1766±0.0086 0.0259±0.0003 0.0086±0.0001 0.8678±0.0015 172±108 165±8 165±2 13 11 347 0.0499±0.0022 0.1781±0.0081 0.0259±0.0003 0.0078±0.0000 1.0935±0.0018 193±104 166±8 165±2 14 6 192 0.0496±0.0024 0.1788±0.0086 0.0261±0.0003 0.0084±0.0001 0.7051±0.0014 177±111 167±8 166±2 15 13 414 0.0494±0.0017 0.1781±0.0078 0.0262±0.0003 0.0090±0.0002 0.8292±0.0015 167±82 166±7 166±2 16 9 330 0.0494±0.0017 0.1789±0.0063 0.0263±0.0003 0.0088±0.0001 0.5742±0.0006 165±80 167±6 167±2 17 14 388 0.0501±0.0009 0.1783±0.0033 0.0258±0.0003 0.0106±0.0001 1.4567±0.0048 200±41 167±3 164±2 18 39 930 0.0494±0.0008 0.1808±0.0034 0.0265±0.0003 0.0269±0.0002 0.7435±0.0046 167±36 169±3 169±2 19 8 232 0.0497±0.0015 0.1794±0.0055 0.0262±0.0003 0.0129±0.0001 0.7799±0.0038 181±68 168±5 167±2 20 17 477 0.0493±0.0014 0.1780±0.0052 0.0262±0.0003 0.0131±0.0001 1.0189±0.0021 164±66 166±5 167±2 21 5 132 0.0493±0.0038 0.1784±0.0138 0.0262±0.0003 0.0147±0.0002 1.0506±0.0026 164±180 167±13 167±2 22 68 1879 0.0498±0.0006 0.1795±0.0025 0.0262±0.0003 0.0149±0.0000 0.9556±0.0034 184±29 168±2 167±2 23 17 509 0.0506±0.0017 0.1767±0.0060 0.0253±0.0003 0.0169±0.0001 0.7345±0.0027 224±77 165±6 161±2 24 8 179 0.0503±0.0020 0.1767±0.0072 0.0255±0.0003 0.0210±0.0002 1.0671±0.0033 207±91 165±7 162±2 25 26 697 0.0496±0.0014 0.1765±0.0049 0.0258±0.0003 0.0168±0.0001 0.9449±0.0036 176±64 165±5 164±2 测试单位:中国地质调查局天津地质调查中心实验室. 表 2 流纹岩主量元素和微量元素组成
Table 2. Contents of major and trace elementsin rhyolite samples
样品编号 P9LT187B2 P9LT171 P9LT238 P8LT20 SiO2 76.77 71.05 72.35 76.38 TiO2 0.19 0.2 0.4 0.13 Al2O3 11.41 12.46 13.58 12.17 Fe2O3 1.24 1.18 1.82 0.62 FeO 0.23 0.12 0.24 0.3 MnO 0.03 0.12 0.03 0.03 MgO 0.22 0.27 0.3 0.16 CaO 0.3 1.73 0.26 0.14 Na2O 3.68 2.06 3.98 2.99 K2O 4.24 4.6 4.51 4.83 P2O5 0.03 0.04 0.1 0.03 LOI 1.15 5.67 1.73 1.75 A/CNK 1.02 1.08 1.14 1.17 Rb 133.98 131.6 87.3 140.62 Sr 37.25 65.27 13.1 81.74 Ba 379.92 725.15 168 367.97 Nb 8.57 10.18 13.1 14.21 Zr 211.72 229.31 648 236.07 Th 19.92 20.01 11 8.96 Hf 7.03 7.69 15.1 9.5 Ga 17.29 23.25 23.1 25.23 Ti 1140 1200 2400 780 U 2.6 4.08 4.12 2.23 La 31.5 36.9 59.5 25.3 Ce 64.3 71.1 119 50 Pr 6.48 6.94 14.3 6.85 Nd 25.4 27 54.7 21.8 Sm 4.14 4.8 9.04 4.63 Eu 0.71 0.89 1.72 0.55 Gd 3.5 3.63 6.62 3.86 Tb 0.55 0.55 0.94 0.61 Dy 1.38 1.46 2.45 1.52 Er 1.46 1.67 2.87 1.72 Tm 0.28 0.3 0.43 0.33 Yb 1.63 1.68 2.92 1.88 Lu 0.21 0.26 0.44 0.29 Y 12.6 14.4 23.8 15.9 ΣREE 154.58 172.05 299.62 135.76 (La/Yb)N 19.32 21.98 20.37 13.47 δEu 0.56 0.63 0.66 0.39 测试单位:自然资源部呼和浩特矿产资源监督检测中心.含量单位:主量元素为%,微量元素为10-6. -
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