南岭地区万洋山岩体年代学、地球化学特征及其地质意义

发布时间：2018-03-19 06:37

  本文选题：锆石LA-ICP-MS　切入点：U-Pb年龄　出处：《长江大学》2017年硕士论文　论文类型：学位论文

【摘要】：万洋山岩体位于湘赣两省交界地带,出露面积约1400km2,呈岩基产出,岩体主体具有块状构造,斑状似斑状结构,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。针对万洋山岩体缺乏高质量的年龄数据和高精度的地球化学分析数据,导致对华南地区动力学背景认识存在一定差异,本文在野外系统地质调查的基础上,采用先进的测试技术和方法,对万洋山岩体的岩相学、同位素地质学、地球化学、地质年代学等方面进行深入研究,并且探讨华南加里东期花岗岩形成的地质背景。对岩体中黑云母二长花岗岩中样品中的锆石进行激光剥蚀-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±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。花岗岩岩体的主要矿物成分有斜长石、钾长石、石英,次要矿物为黑云母。岩石地球化学表明岩体中Si O2的含量为65.91%~73.35%(平均值为69.50%),K2O的平均含量为3.19%~5.36%(平均值为4.20%),Na2O+K2O为5.90%~7.88%(平均值为6.76%),K2O/Na2O平均值为1.64,Al2O3平均值为13.81%,ASI平均值为1.09%。总体的TFeO(平均4.55%)含量偏高,镁铁比值(F/F+M)变化范围较小,为0.67~0.73,集中在0.70左右,碱钙指数(MALI)为2.72%~6.90%,变化范围较大。万洋山岩石样品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,具有较高的Rb/Sr比值(平均值为1.91%)和Rb/Nb比值(平均值为14.48%)。稀土元素总量中等(159.71×10-6~262.78×10-6),轻稀土富集明显(LREE/HREE=6.16~10.01),具有明显的右倾曲线,(La/Yb)N=6.37~12.17,具明显的负Eu异常,δEu值为0.30~0.59,平均值为0.45。岩体的[n(87Sr)/n(86Sr)]i值为0.71223~0.72509,与S型花岗岩[n(87Sr)/n(86Sr)]i值范围一致。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,与湘桂内陆带花岗岩的背景值(1.8~2.4 Ga)基本一致。岩石地球化学和同为素地球化学以均表明万洋山岩体为来源于地壳的S型花岗岩,源岩为中上地壳变质碎屑岩。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:受到全球板块构造活动的影响,扬子板块和华夏板块趋于闭合,主要是沿着钦—杭断裂带。结果导致华南地区以浅海盆地为主的环境关闭,两个板块拼贴到一起导致地壳大面积的增厚。由于地壳的增厚可以导致产热高的元素产生地球化学迁移,并且集中在增厚的区域。在足够的温度压力下,结果导致中上地壳的局部熔融。在后碰撞时期,发生陆内环境的伸展作用,岩石圈相对减薄,岩浆上涌并且侵位,最后形成该地区分布面积比较广的壳源花岗岩。
[Abstract]:WANYANGSHAN Hunan pluton is located in the junction of two provinces area, the exposed area of about 1400km2, a batholith, rock body has a massive structure, porphyritic porphyritic structure, complex rock mass of Caledonian polystage magmatic activities, the main types of granite rock is biotite two feldspar granite, biotite granodiorite and two mica two granite, biotite granite with two long distribution area is the most widely used. According to the data analysis of age data of high quality and high precision of the geochemical WANYANGSHAN lack of rock mass, leading to the dynamic background of Southern China area there exist certain differences, based on the investigation of field geology on the test using advanced technology and methods, to 10000 the rock mass of isotope geology, petrography, geochemistry, geochronology and other aspects of in-depth research, and discuss the geological background of Southern China CALEDONIAN GRANITE formation of. Zircon samples biotite granite two in rock by laser ablation multi collector inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb dating of zircon, the 16 points 206Pb/238U weighted average age (446 + 3.4) Ma (n=16, MSWD=0.15) (95% confidence), the rock formed in the late Ordovician to Early Silurian. The main mineral composition of granite are plagioclase, K-feldspar, quartz, biotite. Secondary mineral content in rock Si O2 indicate that the rock geochemical 65.91%~73.35% (average of 69.50%), the average content of K2O is 3.19%~5.36% (average 4.20%), Na2O+K2O 5.90%~7.88% (average of 6.76%), the average K2O/Na2O value was 1.64, the average Al2O3 value was 13.81%, the average ASI 1.09%. overall TFeO (average 4.55%) high content of magnesium, iron ratio (F/F+M) range of small, 0.67~0.73, concentrated in about 0.70, calcium alkali index (M ALI)涓，

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