Redetermination of the age of granitic stocks in Silingtou area, Xiuning Country, southern Anhui Province
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摘要:
前人对皖南休宁县石岭头花岗质小岩株进行了K-Ar测年，结果为133.4Ma，属于早白垩世。笔者近期的野外地质调查发现，这些花岗质小岩株含过铝矿物堇青石，岩性特征与休宁花岗闪长岩体非常相近，为中粗粒花岗闪长岩，且见有变形，局部片理化明显。分别在休宁花岗闪长岩体内部和北部外叶村河中采集测年样品进行测定，获得休宁花岗闪长岩体内部835.4±32Ma和外叶小岩株841±29Ma的年龄，二者在误差范围内基本一致，表明它们属于同期岩体。岩石地球化学数据显示，花岗质小岩株具有与休宁花岗闪长岩体类似的地球化学特征，亏损Nb、Ta、Sr等高场强元素，富集K、Ru、Ba、U、Th，且均为强过铝质。结合前人所测休宁岩体数据对比分析，认为这些原先被认为早白垩世的花岗岩是新元古代的产物，为晋宁期休宁岩体的一部分，同时休宁岩体北界应北移约2km。其上覆盖的南华系休宁组砂岩应为超覆沉积形成。
Abstract:The small granitic stocks in Xiuning granodiorite have K-Ar age of 133.4Ma, suggesting that these stocks were formed in Early Cretaceous. During recent field geological survey, the authors found that these small stocks contain aluminum mineral cordierite, and their lithologic characteristics are very similar to those of Xiuning granodiorite. They are all coarse grained granodiorite with local deformation and obvious schistosity. Samples collected from both internal Xiuning granodiorite and Waiye stock yielded LA-ICP-MS zircon U-Pb age of 835.4±32Ma for Xiuning granodiorite and 841±29Ma for Waiye rock, showing that they were formed during the same period. In addition, these small granitic stocks have similar geochemical characteristics to Xiuning granodiorite. Whole-rock geochemical data show that both samples are characteristics by lower HFSE Nb, Ta, Sr and higher K, Ru, Ba, U, Th, and thus belong to the peraluminous S-type granite. By comparative analysis of lithology characteristics, U-Pb age and geochemical data from previous study and this paper, the authors confirm that these stocks should intruded during the Neoproterozoic as a part of the Jinningian Xiuning pluton, not in the Early Cretaceous as previously thought. And the northern boundary of Xiuning pluton must have migrated northward for about 2km. The sandstone (no conglomerate) of Xiuning Formation resulted from the transgressive deposition on these stocks.
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致谢: 锆石U-Pb同位素的测试得到中国地质大学(武汉)地质过程与矿产资源国家重点实验室(GPMR)胡兆初教授的支持和帮助, 在此表示衷心的感谢。
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图 1 皖南休宁岩体地区地质简图(据参考文献①修改)
a, b-研究区大地构造位置图; c-黄山市休宁县休宁花岗闪长岩体地质图及D052采样位置; d-石岭头地区外叶花岗闪长岩地质图及D252采样位置
Figure 1. Geological map of Xiuning rock body area
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图 2 D252休宁兰渡外叶村花岗闪长岩野外照片
a-外叶(D252)花岗闪长岩内部; b-外叶(D252)花岗闪长岩边部片理化; Hbl-角闪石; Pl-斜长石
Figure 2. Field features of Waiye granodiorite(D252)in the north of Landu, Xiuning County
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图 3 休宁岩体和外叶花岗质小岩株稀土元素球粒陨石标准化配分曲线(a)及微量元素原始地幔标准化蛛网图(b)(标准化数据据参考文献[10])
Figure 3. REE patterns (a) and trace element spider diagram (b) for Xiuning grandiorite and the Waiye granodiorite
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图 4 休宁岩体(D052)和外叶花岗质小岩株(D252)锆石阴极发光(CL)图像及微区U-Pb年龄分析点位
Figure 4. CL images and 206Pb/238U age plot of the zircon in Xiuning granodiorite (D052) and Waiye granodiorite(D252)
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图 5 休宁花岗闪长岩锆石U-P谐和图(a、c)及年龄加权平均值示意图(b、d)
Figure 5. Zircon U-Pb concordia diagrams (a, c) and 206Pb/238U age plots (b, d) of the zircon in the Xiuning granodiorite
表 1 外叶和休宁花岗闪长岩主量、微量和稀土元素地球化学数据
Table 1 Petrochemical and geochemical data of Waiye and Xiuning granodiorite
样品 03WN33* 03WN 35* 03HN49* 03HN51* WN350** D052 D252-y1 D252-y2 D252-y3 SiO2 68.95 68.61 70.24 70.96 75.71 69.6 68.91 66.73 68.47 TiO2 0.42 0.41 0.34 0.29 0.71 0.5 0.46 0.58 0.56 Al2O3 15.63 15.77 15.21 14.73 12.61 15 15.37 16.37 15.23 FeO- 3.28 3.4 2.79 2.63 2.68 3.74 3.33 3.81 3.89 MnO 0.06 0.05 0.05 0.05 0.02 0.06 0.066 0.065 0.068 MgO 1 1.24 0.97 1.05 0.5 1.17 1.09 1.34 1.30 CaO 1.36 0.8 0.8 0.97 0.15 0.68 0.72 0.89 1.03 Na2O 2.47 2.73 2.63 2.88 0.14 2.94 3.20 3.24 3.03 K2O 4.18 3.93 4.12 3.68 4.21 3.96 4.63 4.45 3.49 P2O5 0.21 0.24 0.16 0.19 0.06 0.2 0.18 0.19 0.19 烧失量 1.88 2.31 2.2 2.11 2.6 1.73 1.86 2.16 2.62 总计 99.44 99.49 99.51 99.54 99.39 99.73 99.83 99.81 99.88 A/CNK 1.41 1.55 1.49 1.4 2.56 1.45 1.40 1.49 1.57 Mn 413.8 383.7 397.2 419.2 154.93 464.79 514.74 503.79 525.15 Ni 13.07 16.49 10.4 11.82 9.56 - 13.4 16.2 16.9 Ga 17.62 19.58 17.46 17.59 17.4 18.8 16.6 18.1 17.7 Rb 133.8 151 130 132.1 142 139 138 111 127 Ba 808.3 620.3 894.8 670.9 466 724 879 910 380 Th 9.31 9.11 8.84 7.07 - 11.2 9.09 8.75 10.7 Nb 8.67 9.87 7.17 7.23 12 9 9.34 10.7 8.92 Ta 0.67 0.72 0.58 0.49 0.96 0.7 0.61 0.70 0.60 Hf 4.58 4.96 4.54 3.72 3.8 5 6.94 6.50 7.83 U 2.52 2.52 1.53 1.04 2.54 1.78 2.06 1.28 1.96 Sr 161 189.4 196.3 179 29.2 184.5 180 175 135 Zr 164.7 186.1 144.9 142.4 128 171 169 186 203 Y 24.26 33.68 25.64 25.07 26.2 22.9 21.8 20.8 29.6 La 24.49 27.39 24.32 22.45 33.3 28.2 21.2 29.3 41.1 Ce 51.8 56.85 50.91 47.97 70.2 64 51.0 63.5 82.8 Pr 6.73 7.42 6.67 6.01 8.17 7.15 6.08 8.46 10.3 Nd 26.05 28.5 25.51 22.39 32.2 27.1 25.0 34.6 41.3 Sm 5.83 6.27 5.53 4.97 6.31 6.03 5.35 7.20 8.68 Eu 1 1.07 1.07 0.89 1.11 1.16 1.12 1.43 1.20 Gd 5.48 5.62 4.97 4.21 4.94 5.91 4.58 5.82 7.34 Tb 0.88 0.92 0.8 0.71 0.88 0.93 0.84 0.95 1.21 Dy 4.53 5.14 4.46 3.69 5.44 4.42 4.52 4.48 6.02 Ho 0.9 0.95 0.89 0.68 1.02 0.89 0.80 0.76 1.07 Er 2.4 2.43 2.38 1.72 2.83 2.26 2.03 1.80 2.73 Tm 0.36 0.39 0.39 0.26 0.42 0.33 0.35 0.27 0.45 Yb 2.44 2.56 2.41 1.65 2.72 0.93 2.05 1.54 2.68 Lu 0.4 0.35 0.39 0.23 0.37 0.31 0.30 0.24 0.41 注:03WN33*、03WN35*、03WN49*、03WN51*四组数据来自于吴荣新等[7]; WN350**数据来自于薛怀民等[1]; 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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表 2 休宁和外叶花岗闪长岩 LA-ICP-MS锆石 U-Th-Pb分析结果
Table 2 LA-ICP-MS zircon U-Th-Pb analytical results of Xiuning and Waiye granodiorite
注：带删除线数据不参与计算
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