湖南铜山岭花岗闪长岩中榍石微区原位Nd同位素和微量元素组成及其岩石成因的指示意义  

Petrogenesis of the Mesozoic Tongshanling Granodiorite in Southern Hunan Province,South China:Clues from in-situ Nd Isotopes and Elements of the Titanite

作　　者：王丹 付勇[1] WANG Dan;FU Yong(College of Resource and Environmental Engineering,Guizhou University,Guiyang 550025,China)

机构地区：[1]贵州大学资源与环境工程学院,贵州贵阳550025

出　　处：《岩矿测试》2023年第2期282-297,共16页Rock and Mineral Analysis

基　　金：贵州省创新人才基地项目(RCJD-2018)。

摘　　要：与热液矿床形成有关的花岗质岩石普遍遭受热液蚀变,且全岩成分仅代表均一化某一时间点的信息,采用全岩成分分析难以有效地揭示花岗质岩石的形成与演化。花岗质岩石中副矿物稳定不容易蚀变,近年原位测试技术的快速发展和日趋成熟,可以准确获取副矿物原位元素和同位素组成,通过副矿物元素和同位素组成可以有效地揭示岩浆来源和演化信息,显著提高了岩浆作用过程的空间分辨率,成为探讨岩石成因的新手段。本文以湘南与铜铅锌多金属成矿密切相关的铜山岭岩体为研究对象,利用电子探针(EPMA)、激光剥蚀等离子体质谱(LA-ICP-MS)和激光剥蚀多接收等离子体质谱(LA-MC-ICP-MS)等原位测试技术,对花岗闪长岩和暗色包体两类岩石样品中的副矿物榍石开展了原位元素和Nd同位素分析。结果表明:所有榍石中Al+Fe与Ti具有明显的负相关关系且稀土元素含量较高,稀土元素与Al和Fe一起主要通过(Al,Fe 3+)+REE=Ti 4++O 2-方式替换榍石的Ti位和Ca位而进入晶格。球粒陨石标准化稀土配分模式上,大部分榍石显示Eu正异常。榍石中微量元素对Zr/Hf、Nb/Ta、Y/Ho比值变化范围分别为21.0~31.5、10.4~13.9、27.4~35.0,未发生明显分异。暗色包体中榍石的ε_(Nd)(t)值为-3.5~-8.9,平均值为-7.2±2.4,花岗闪长岩中榍石的ε_(Nd)(t)值为-5.4~-9.9,平均值为-6.9±2.4,所有榍石中Nd同位素组成均一且具有一致的负的初始Nd同位素组成,与华南大陆中下地壳Nd同位素演化趋势一致。元素地球化学特征表明,榍石中微量元素只与其晶体结构有关,不受热液蚀变作用的影响,能有效地示踪岩浆起源和性质。通过元素和同位素地球化学特征研究,本文认为铜山岭花岗质岩浆具有高温、高氧逸度特征,很可能由镁铁质角闪岩相中下地壳脱水熔融产生的水不饱和岩浆形成。BACKGROUND:The compositions of the bulk-rocks are commonly modified by hydrothermal alteration or merely represent the magmatic information of snapshot during evolution.Hence,use of the chemical compositions of bulk-rocks makes it difficult to trace the source and evolution of granitic magmas.The petrogenesis of the granitic rock is therefore difficult to decipher.In the face of these difficulties,an alternative new approach is to trace the magmatic source and evolution by in-situ analyzing elements and isotope compositions of accessory minerals in granitic rocks.This new approach can significantly improve the spatial resolution of the magmatic processes.Titanite(CaTiSiO_(5))is a common accessory mineral of granitic rocks and contains a large amount of elements,including Al,Fe,Nb,Ta,Zr,Cr,V,Sn in the Ti site,and rare earth elements(REEs)Y,Na,Mn,Pb,U,Th,Sr and Ba in the Ca site.REEs and high field strength elements(HFSEs,i.e.,Nb,Ta,Zr,Th and U)in titanite are sensitive to the changes of temperature,pressure,oxygen fugacity(f O_(2)),water fugacity and melt composition.Additionally,titanite has high Nd concentration and low Sm/Nd ratio and is suitable for in-situ Nd isotope analyses.Therefore,titanite is an ideal accessory mineral to be used to investigate the magmatic source and evolution of granitic rocks.OBJECTIVES:To trace the magmatic source and evolution,and to decipher the petrogenesis of the granitic rock.METHODS:Titanite from mafic microgranular enclave(MME)and hosted granodiorite of the Tongshanling granitic pluton were checked by transmission light and back-scattered electron(BSE).BSE images were performed at the Institute of Geochemistry,Chinese Academy of Sciences(IGCAS)in Guiyang,on carbon-coated,polished epoxy blocks using JSM-7800F field emission scanning electron microscopy(SEM)operated at 20kV accelerating voltage and a beam current of 10nA.Their in-situ major element compositions were analysed by electron probe microanalyzer(EPMA)at IGCAS.An accelerating voltage of 25kV and a probe current of 10nA

关 键 词：LA-MC-ICP-MS 原位同位素分析 ND同位素 微量元素 榍石 花岗闪长岩 湘南地区 

分 类 号：O657.63[理学—分析化学] O562.6[理学—化学]