南岭地区万洋山岩体锆石LA-ICP-MS U-Pb年龄和地球化学特征及其构造意义
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本文为中国地质调查局地质大调查项目(编号: 1212011120804、12120114084501和12120114020701)的成果。


LA-MC-ICP-MS Zircon U-Pb Age and Geochemical Characteristics of Wanyangshan Batholith in Nanling Area,South China, and Tectonic Implications
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    摘要:

    万洋山岩体位于湘赣两省交界地带,为加里东期多阶段岩浆活动的复式岩体,花岗岩主要岩石类型有黑云母二长花岗岩、黑云母花岗闪长岩和二云母二长花岗岩,以黑云母二长花岗岩分布面积最广。对岩体中黑云母二长花岗岩中的锆石样品进行激光剥蚀—多接收器电感耦合等离子体质谱(LAICPMS) UPb定年,锆石的16个测点 206Pb/238U的加权平均年龄为(446.0±34)Ma (n=16,MSWD= 015)(95%置信度),反映该岩体形成于晚奥陶世至早志留世。岩石地球化学表明岩体中SiO2的含量为6591%~7335%,K2O的平均含量为420%,Na2O+K2O为590%~788%,K2O/ Na2O平均值为164,Al2O3 平均值为1381%,ASI平均值为109%,总体属于高钾钙碱性过铝质花岗岩。微量元素组成中Ba、K、Sr、P、Ti表现出明显的亏损,Rb、Th、U、Nb、Zr、Hf等则相对富集,稀土元素总量中等(159.71×10-6~262.78×10-6) ,轻稀土富集LREE/HREE=6.16~10.01,(La /Yb)N=6.37~12.17,具明显的负Eu异常(δEu = 0.30~0.59) 。岩体的[n(87Sr)/n(86Sr)]i值为0.71223~0.72509,εSr(t)值为117.5~299.9Ma,εNd (t) 值为-9.39~-7.30,两阶段Nd 模式年龄(TDM2)为177~194 Ga。根据上述岩石地球化学特征表明万洋山岩体为来源于地壳的S型花岗岩,花岗岩氧化物和微量元素构造环境判别图解指示岩体形成于后碰撞构造环境。结合前人对华南加里东期岩体的研究成果,推断华南加里东期花岗岩岩体的具体形成机制为:在全球板块构造的影响,华夏板块与扬子板块拼接后,板块间的强烈挤压应力相对松弛、压力降低的后碰撞构造环境下,因地壳增厚而升温的中上地壳岩石减压熔融并向上侵位。

    Abstract:

    Wanyangshan batholith is an important part of Caledonian magmatism in south China. Though some work have been performed to reveal petrology, geochemistry, UPb dating and tectonic setting of the batholith, but lack of work on tectonic setting of Caledonian granitoids. We discuss tectonic setting of Caledonian granitoids and provide new data for early Late Paleozoic granite and crustal evolution in south China.Methods:We apply methods including zircon LAICPMS UPb dating, main elements, trace elements, rare earth elements and Sr, Nd isotopic analysis and testing. We analyze the interior structures of zircons by the CL technology and insitu zircon UPb dating of a biotite twomica monzogranite sample. For main elements analysis and testing, we apply burning method for Si summation, volumetric method for Al and Fe2+, spectrophotometric method for Fe3+, Ti and P, atomic absorption spectrometry for K, Na, Ca, Mg and Mn. Trace elements and rare earth elements have been tested on the X series Thermo 2 type inductively coupled plasma mass spectrometry. Sr and Nd isotope analysis has been performed on the MAT261 multi receiver mass spectrometer.Results:We obtained the 16 points of zircon 206Pb/238U weighted average ages of (446.0±3.4)Ma(MSWD= 0.15, 95% confidence degree), suggesting crystallization age of the batholith. The result indicates it formed during late Ordovician to Early Silurian the Southern China Caledonian magmatism. Geochemical analyses showed that: the Wanyangshan batholith has a SiO2 content of 65.91 to 73.35%, an average K2O content of 4.20%,(Na2O+K2O) content of 5.90 to 7.88%.the average of K2O/Na2O, Al2O3 and ASI are 1.64, 13.81% and 1.09%. Which indicates granitic pluton is highK calcalkaline peraluminous. The contents of Rb, (Th+U), (Nb+La+Ce), Nd,(Zr+Hf+Sm) and (Y+Yb+Lu) are relatively higher and the granite intrusion has moderate ∑REE (159.71×10-6~262.78×10-6). It is enriched in LREE and depleted in HREE(LREE/HREE=6.16~10.01), (La/Yb)N=6.37~12.17,with obvious negative Eu anomaly (δEu = 0.30~0.59).The Wanyangshan granite has a range of initial strontium from 0.71223 to 0.72509, and a range of εSr(t) from 117.5 to 299.9, εNd (t) ranges from -9.39 to -7.30, with the depleted mantle model age of 1.77~1.95 Ga.Conclusions:Those methods above have been successfully applied to formation mechanism of Caledonian Wanyangshan batholith, providing some new evidences for understanding the tectonic setting of Southern China Caledonian granites.Acknowledgements: This study was financially supported by the Chinese Geological Surveey(No. 1212011120804, No.12120114084501 and No.12120114020701). We would like to express our thanks to Xia Jinlong from Wuhan Center of China Geological Survey for explanation of isotopic dating. We would also like to exprese our thanks to reviewers and editors for valuable advice on this paper.

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季文兵,路远发,付建明,程顺波,卢友月.2016.南岭地区万洋山岩体锆石LA-ICP-MS U-Pb年龄和地球化学特征及其构造意义[J].地质论评,62(5):1329-1343,[DOI].
JI Wenbing, LU Yuanfa, FU Jianming, CHENG Shunbo, LU Youyue.2016. LA-MC-ICP-MS Zircon U-Pb Age and Geochemical Characteristics of Wanyangshan Batholith in Nanling Area, South China, and Tectonic Implications[J]. Geological Review,62(5):1329-1343.

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  • 收稿日期:2016-01-12
  • 最后修改日期:2016-08-24
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  • 在线发布日期: 2016-09-22
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