A study of geochronology, geochemistry and genesis of Maqigang beschtauite pluton, south-eastern Guangxi
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摘要:
马其岗石英二长斑岩体出露于桂东南博白-梧州断裂带南东侧,对其开展了锆石U-Pb年代学、地球化学、Sr-Nd-Hf同位素及成因研究。结果表明,岩体LA-ICP-MS锆石U-Pb年龄为90.2±1.5Ma(MSWD=1.7)。岩体富碱(ALK=7.38%~8.14%)、富钾(K2O=4.41%~4.78%),稀土元素特征为轻稀土元素富集型,微量元素特征为富集大离子亲石元素(Rb、Th、U、K、Pb、LREE),亏损高场强元素(Nb、Ta、P、Ti、HREE),符合钾玄岩系列的岩石特点。岩体有较高的Mg#值(42.82~50.35),较低的Sr含量(268.00×10-6~304.00×10-6),以及较高的锆石饱和温度(860~883℃),同时Nb/Ta(平均值为11.24)、Zr/Hf(平均值为38.20),Th/La值(平均值为0.17)明显不同于大陆地壳特征,表明岩浆主要来自下部地壳或地幔,在上升侵位过程中受到地壳大规模混染程度较小,具有EMⅡ富集地幔端元的Sr-Nd同位素特征。二阶段Nd模式年龄(tDM2)变化于1.33~1.36Ga之间,二阶段Hf模式年龄(tDM2)主体为1.20~1.50Ga,两者模式年龄较一致,显示马其岗岩体可能是中元古代中期镁铁质岩石部分熔融的产物。石英二长斑岩形成于板内伸展环境,整个华南在90Ma左右存在一次大规模的伸展事件,其动力学机制与古太平洋板块低角度俯冲有关。
Abstract:Maqigang beschtauite pluton is located in the southeast of the Bobai-Wuzhou fault, southeastern Guangxi.The zircon UPb geochronology, elemental geochemistry and Sr-Nd-Hf isotopic components as well as petrogenesis of Maqigang beschtauite were investigated in this paper.The LA-ICPMS zircon U-Pb age is 90.2±1.5Ma (MSWD=1.7).The beschtauite is characterized by rich alkalis (ALK=7.38%~8.14%) and K (K2O=4.41%~4.78%).The rare earth elements exhibit the light REE enrichment type, with trace elements characterized by enrichment of LILE (e.g., Rb, Th, U, K, Pb, LREE) and depletion of HFSE elements (e.g., Nb, Ta, P, Ti, HREE).The geochemical characteristics of Maqigang beschtauite show shoshonitic features.The beschtauite has relatively high Mg# values (42.82~50.35) and zircon saturation temperatures (860~883℃) and low Sr values (268.00×10-6~304.00×10-6).The Nb/Ta ratios (11.24 on average), Zr/Hf ratios (38.20 on average) and Th/La ratios (0.17 on average) of the beschtauite are remarkably different from those of upper crustal rocks, indicating that the beschtauite originated from lower crustal or mantle source.In addition, the beschtauitic magma was less contaminated by the upper crust substance in the process of emplacement.The Sr-Nd isotopic com-positions show that they have the characteristics of the EMⅡ source.Nd isotopic two-phase model age (tDM2=1.33~1.36Ga) is similar to the Hf isotopic two-phase model principal age (tDM2=1.20~1.50Ga), inmplying that the beschtauite was derived dominantly from mafic rocks in the middle Proterozoic.The Maqigang beschtauite was formed in an intra-plate extensional environment.There was a huge stretching event throughout South China at 90Ma±.The dynamic mechanism of the event was connected with low angle sub-duction of the ancient Pacific plate.
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Keywords:
- geochronology /
- geochemistry /
- genesis /
- Maqigang beschtauite pluton /
- southeastern Guangxi
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致谢: 成稿过程中得到中国地质调查局武汉地质调查中心徐德明研究员的帮助指导,在此予以感谢。
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图 1 马其岗岩体地质图(据参考文献①修改)
Q—第四系;K2l1—罗文组第一段;K2x1—西垌组第一段;K2x—西垌组(未分段);TJ1t—天堂组;D2d—东岗岭组;S2l2—连滩组第二段;S2l1—连滩组第一段;Pt3s—射广组;1—石英二长斑岩;2—二长花岗岩;3—片麻状二长花岗岩;4—地质界线;5—角度不整合界线;6—断层;7—韧性断层;8—角岩化;9—采样点
Figure 1. Geological map of Maqigang beschtauite pluton
图 4 马其岗石英二长斑岩稀土元素配分曲线(a)及微量元素蛛网图(b)(球粒陨石标准化数值据参考文献[10])
Figure 4. Chondrite-normalized REE patterns(a)and primitive mantle-normalized trace elements patterns(b)of Maqigang beschtauite
表 1 马其岗石英二长斑岩LA-ICP-MS锆石U-Th-Pb同位素分析数据
Table 1 LA-ICP-MS zircon U-Th-Pb data of Maqigang beschtauite
点号 含量/10-6 同位素比值(经普通铅校正) 年龄(经普通铅校正)/Ma Pb Th U 207Pb/206Pb ±1σ 207Pb/235U ±1σ 206Pb/238U ±1σ 207Pb/206Pb ±1σ 207Pb/235U ±1σ 206Pb/238U ±1σ 1 11.09 379 654 0.0495 0.0028 0.0944 0.0052 0.0138 0.0002 172 131 91.6 4.9 88.3 1.3 2 12.26 333 737 0.0501 0.0024 0.0981 0.0046 0.0142 0.0002 211 111 95.0 4.2 91.2 1.0 3 38.28 1059 2383 0.0505 0.0013 0.0965 0.0026 0.0138 0.0001 217 61 93.6 2.4 88.3 0.7 4 198.7 391 376 0.1447 0.0020 7.188 0.123 0.3579 0.0041 2284 23 2135 15 1972 19 5 40.82 1076 2504 0.0500 0.0014 0.1006 0.0030 0.0145 0.0001 195 67 97.3 2.7 92.9 0.8 6 14.83 543 869 0.0482 0.0024 0.0936 0.0045 0.0142 0.0002 109 124 90.9 4.2 90.6 1.1 7 11.98 580 579 0.0558 0.0030 0.1181 0.0065 0.0153 0.0002 456 122 113 6 98.1 1.1 8 18.94 277 943 0.0515 0.0016 0.1269 0.0041 0.0179 0.0003 261 72 121 4 115 2 9 54.1 184 604 0.0558 0.0012 0.6149 0.0134 0.0794 0.0007 456 51 487 8 492 4 10 18.58 587 1123 0.0462 0.0015 0.0899 0.0029 0.0141 0.0001 9.36 74.1 87.4 2.7 90.0 0.7 11 41.4 1204 2552 0.0481 0.0014 0.0921 0.0025 0.0138 0.0001 106 69 89.4 2.4 88.6 0.6 12 50.8 1357 3102 0.0523 0.0012 0.1015 0.0024 0.0140 0.0001 298 54 98.1 2.2 89.8 0.7 13 27.94 575 1388 0.0487 0.0014 0.1182 0.0034 0.0178 0.0003 132 67 113 3 114 2 14 267.5 1135 3490 0.0530 0.0007 0.4946 0.0067 0.0674 0.0004 328 36 408 5 420 3 15 12.10 328 526 0.0644 0.0033 0.1593 0.0075 0.0182 0.0003 754 107 150 7 117 2 16 13.55 545 740 0.0505 0.0028 0.0995 0.0053 0.0145 0.0002 217 128 96.4 4.9 93.0 1.2 17 10.84 348 643 0.0536 0.0022 0.1014 0.0039 0.0140 0.0002 354 97 98.1 3.6 89.4 1.5 18 20.10 686 1124 0.0497 0.0018 0.0947 0.0032 0.0139 0.0001 189 81 91.9 3.0 88.9 0.7 表 2 马其岗石英二长斑岩主量、微量和稀土元素分析结果及参数
Table 2 Compositions and parameters of major, trace and rare earth elements in Maqigang beschtauite
样品 mqg-1 mqg-2 mqg-3 D3015-1 样品 mqg-1 mqg-2 mqg-3 D3015-1 D3013-1 样品 mqg-1 mqg-2 mqg-3 D3015-1 D3013-1 SiO2 64.76 64.04 64.26 64.00 La 70.30 68.70 69.20 71.31 67.58 Ba 1173.00 1049.00 1031.00 935.00 945.00 TiO2 0.94 1.07 1.08 1.06 Ce 128.00 127.00 128.00 138.60 135.40 Rb 176.00 170.00 155.00 160.00 164.00 Al2O3 14.66 14.69 14.68 14.68 Pr 14.20 14.30 14.30 15.93 15.55 Sr 268.00 280.00 304.00 283.00 268.00 Fe2O3 2.76 3.71 3.04 3.31 Nd 54.30 55.00 54.60 59.02 56.10 Y 47.50 56.20 47.80 39.22 39.35 FeO 3.38 2.80 2.99 2.25 Sm 10.30 9.98 9.96 10.87 10.77 Zr 265.00 207.00 386.00 429.00 457.00 MnO 0.09 0.10 0.09 0.07 Eu 2.45 2.30 2.32 2.45 2.52 Nb 37.80 37.20 37.80 31.40 32.00 MgO 1.42 1.49 1.45 1.28 Gd 9.19 9.27 9.20 9.62 9.73 Th 18.30 18.20 17.10 20.20 19.90 CaO 2.50 2.88 3.33 3.33 Tb 1.51 1.60 1.54 1.45 1.43 Pb 27.10 25.20 24.30 24.50 26.50 Na2O 3.36 3.27 3.08 2.95 Dy 8.80 9.48 8.94 8.19 8.17 Ga 23.80 23.10 22.50 17.50 17.60 K2O 4.78 4.52 4.41 4.43 Ho 1.60 1.75 1.56 1.57 1.60 Zn 101.00 139.00 102.00 87.00 97.00 P2O5 0.28 0.28 0.33 0.35 Er 4.68 5.56 4.92 4.26 4.25 Cu 17.30 15.70 15.70 14.90 15.90 H2O+ 0.42 0.81 0.81 1.58 Tm 0.77 0.84 0.70 0.68 0.66 Ni 10.10 10.30 8.73 12.00 14.90 CO2 0.17 0.15 0.15 0.09 Yb 4.53 5.18 4.53 4.24 4.19 V 55.80 52.90 60.00 87.60 82.90 灼失 0.78 0.84 0.90 Lu 0.68 0.82 0.67 0.62 0.63 Cr 14.50 13.10 13.50 7.10 13.30 总量 100.30 100.65 100.60 99.38 ∑REE 311.31 311.78 310.44 328.81 318.58 Hf 7.40 6.07 9.61 10.70 11.20 ALK 8.14 7.79 7.49 7.38 LREE 279.55 277.28 278.38 298.18 287.92 Cs 9.37 8.47 10.60 11.20 10.70 K2O/Na2O 1.42 1.38 1.43 1.50 HREE 31.76 34.50 32.06 30.63 30.66 Sc 8.20 8.29 9.40 2.40 2.10 ACNK 0.94 0.94 0.93 0.94 ∑L/∑H 8.80 8.04 8.68 9.73 9.39 Ta 2.15 2.16 2.15 15.90 17.20 σ 3.05 2.88 2.64 2.59 (La/Yb)n 11.13 9.51 10.96 12.06 11.57 Co 11.50 12.10 11.80 30.90 17.20 Mg# 42.82 48.68 46.36 50.35 δEu 0.25 0.24 0.24 0.24 0.25 U 3.63 3.44 3.29 74.30 60.60 注:石英二长斑岩样品D3015-1、D3013-1据参考文献①;主量元素含量单位为%,微量和稀土元素为10-6 表 3 马其岗石英二长斑岩Sr-Nd同位素组成分析结果
Table 3 Sr-Nd isotope data of Maqigang beschtauite
样号 Sm/10-6 Nd/10-6 147Sm/144Nd 143Nd/144Nd 2σ εNd(t) t/Ma Rb/10-6 Sr/10-6 87Rb/86Sr 87Sr/86Sr 2σ (87Sr/86Sr)i tDM2/Ga mqg-1 9.359 51.17 0.1113 0.512320 0.000002 -5.22 90.2 168.8 253.1 1.880 0.71158 0.00001 0.70916 1.33 mqg-2 9.419 51.36 0.1116 0.512312 0.000003 -5.38 90.2 165.0 274.6 1.694 0.71133 0.00003 0.70915 1.34 mqg-3 9.941 53.80 0.1124 0.512302 0.000004 -5.59 90.2 154.8 301.9 1.446 0.71097 0.00003 0.70911 1.36 表 4 马其岗石英二长斑岩锆石原位Hf同位素组成
Table 4 LA-MC-ICP-MS zircon Hf isotope data of Maqigang beschtauite
点号 176Yb/177Hf 176Lu/177Hf 176Hf/177Hf 2σ t/Ma εHf(t) 2σ tDM1/Ga tDM2/Ga 1 0.031335 0.000883 0.282641 0.000014 88 -2.7 0.5 0.86 1.33 2 0.024523 0.000692 0.282593 0.000013 91 -4.4 0.5 0.93 1.44 3 0.038007 0.001082 0.282694 0.000010 88 -0.9 0.3 0.79 1.21 4 0.028841 0.000795 0.281523 0.000014 1911 -2.5 0.5 2.40 2.70 5 0.037295 0.001070 0.282638 0.000019 92.9 -2.8 0.7 0.87 1.33 6 0.027701 0.000782 0.282609 0.000014 91 -3.8 0.5 0.90 1.40 7 0.044030 0.001216 0.282678 0.000019 97 -1.3 0.7 0.82 1.24 8 0.025466 0.000742 0.282573 0.000012 115 -4.6 0.4 0.95 1.47 9 0.057336 0.001613 0.282495 0.000015 492 0.5 0.5 1.09 1.43 10 0.026640 0.000762 0.282606 0.000012 90 -3.9 0.4 0.91 1.41 11 0.031684 0.000962 0.282626 0.000012 88.6 -3.3 0.4 0.89 1.36 12 0.045690 0.001275 0.282681 0.000011 89.8 -1.3 0.4 0.81 1.24 13 0.022608 0.000672 0.282619 0.000012 114 -3.0 0.4 0.89 1.36 14 0.036568 0.001099 0.282274 0.000010 420 -8.7 0.4 1.38 1.96 15 0.025953 0.000755 0.282707 0.000012 117 0.2 0.4 0.77 1.16 16 0.036549 0.001034 0.282666 0.000016 93 -1.8 0.6 0.83 1.27 17 0.022785 0.000658 0.282656 0.000011 89 -2.2 0.4 0.84 1.30 18 0.028315 0.000830 0.282663 0.000012 88.9 -1.9 0.4 0.83 1.28 -
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