南岭西段加里东期苗儿山岩体锆石SHRIMPU-Pb年龄、地球化学特征及其构造意义  

Zircon SHRIMP U-Pb dating and geochemistry of Caledonian Miao'ershan pluton in the western part of the Nanling Mountains and their tectonic significance

作　　者：柏道远 钟响 贾朋远 熊雄 黄文义 

机构地区：[1]湖南省地质调查院,湖南长沙410016

出　　处：《岩石矿物学杂志》2014年第3期407-423,共17页Acta Petrologica et Mineralogica

基　　金：中国地质调查局资助项目(1212011120793;1212010811032)

摘　　要：苗儿山岩体位于南岭西段,主体为加里东期花岗岩,少量印支期和早燕山期花岗岩。加里东期花岗岩具块状构造,由早期黑云母花岗闪长岩、中期斑状黑云母二长花岗岩和晚期细粒黑(二)云母二长花岗岩组成,以斑状黑云母二长花岗岩为主。对中期斑状黑云母二长花岗岩和晚期细粒二云母二长花岗岩各进行了1个样品的锆石SHRIMP U-Pb年龄测试,分别得到428.5±3.8 Ma、409±4 Ma的年龄值,反映出早志留世末、志留纪末-泥盆纪初2期岩浆事件。中期主体花岗岩具有富硅(SiO2=70.09%~76.59%)、中铝(Al2O3=12.71%~14.72%)、高钾(K2O=4.48%~5.73%)、中碱(Na2O+K2O=7.24%~7.91%)、高ASI(平均1.15)的特点,总体属高钾钙碱性系列过铝质花岗岩类。主体花岗岩微量元素中Ba、Nb、Sr、P、Ti表现为明显亏损,Rb、(Th+U+K)、(La+Ce)、Nd、(Zr+Hf+Sm)、(Y+Yb+Lu)等则相对富集,稀土总量中等(179.8×10-6~270.6×10-6),轻稀土富集[(La/Yb)N=5.07~14.33],具明显的负Eu异常(δEu=0.15~0.46)。岩体具有较高的ISr值(0.70660~0.72082)和较低的εNd(t)值(-8.29^-7.94),两阶段Nd模式年龄(t2DM)为1.81~1.84 Ga。C/MF-A/MF图解显示源岩为变质泥质岩和碎屑岩。上述地球化学特征表明花岗岩为S型花岗岩,源岩主要为中、上地壳岩石。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。基于岩石成因、构造环境判别以及区域构造演化过程,推断加里东期苗儿山花岗岩的具体形成机制为:在陆内强挤压之后挤压应力相对松弛、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。结合区域资料分析,苗儿山地区在奥陶纪末-志留纪初北流运动和志留纪后期的广西运动中均产生过强烈的陆内挤压和地壳增厚。The Miao＇ ershan pluton in the western segment of the Nanling Mountains is mainly composed of Caledonian granites with minor Indosinian and Yanshanian granites.The Caledonian plutons are composed of early stage biotite granodiorite,middle stage porphyritic biotite monzogranite （principal part） and late stage finegrained biotite/two-mica monzogranite,with all the rocks having massive structure.The authors carried out U-Pb zircon dating of a porphyritic biotite monzogranite sample and a fine-grained two-mica monzogranite sample and obtained weighted average ages of 428.5 ± 3.8 Ma and 409 ± 4 Ma respectively,suggesting two magrnatic events that occurred in the late episode of Early Silurian and in the late episode of Silurian to the early stage of Devonian respectively.Geochemically,the middle stage rock body is a high-K calc-alkaline peraluminous granitic pluton and is characterized by higher SiO2（70.09%～76.59%）,moderate Al2O3（12.71%～14.72%）,higher K2O （4.48%～5.73%） and moderate alkali （Na2O ＋ K2O=7.24% ～7.91%） content as well as higher ASI （1.15 on average）.The granite is depleted in Ba,Nb,Sr,P and Ti but enriched in Rb,（Th＋ U＋ K）,（La＋ Ce）,Nd,（Zr ＋ Hf ＋ Sm）and （Y ＋ Yb ＋ Lu）.It displays moderate total REE content （∑REE =179.8 × 10-6～ 270.6 × 10-6）,enrichment of LREE [（La/Yb）N =5.07～ 14.33] and negative Eu anomalies （δEu =0.15 ～0.46）.It also has slightly higher Isr（0.706 60～0.720 82） but lower εNd（t） （-8.29～-7.94） with Nd model ages （t2DM） being 1.81～1.84 Ga.The C/MF-A/MF diagram indicates that the Caledonian Miao＇ ershan granites came from mudstones and clastic rocks.All these geochemical characteristics point to S-type granitiod and suggest that the granites might have come from rocks of middle-upper crust.Multiple oxide and trace element diagrams for discrimination of structural environment show that the granites were formed in a post-collisional tectonic setting.Based on petrogenesis and discrimina

关 键 词：锆石 SHRIMP U-PB定年 地球化学特征 S型花岗岩 后碰撞构造环境 北流运动 广西运动 苗儿山 湘西南 

分 类 号：P597.3[天文地球—地球化学] P595[天文地球—地质学]