赣西北麦斜岩体锆石U-Pb年代学、地球化学以及Sr-Nd同位素研究及其岩石成因  

Zircon U-Pb Geochronology,Geochemistry,and Sr-Nd Isotopic Characteristics of the Maixie Biotite-Granodiorite Pluton in Northwestern Jiangxi Province and Their Implications for Petrogenesis

作　　者：王帅[1,2] 陶继华[1,3,4] 李武显 王安东[2] 吕璞良[3] WANG Shuai;TAO Jihua;LI Wuxian;WANG Andong;LU Puliang(School of Earth Science, East China University of Technology, Nanchang , 330013, China;State Key Laboratory Breeding Base of Nuclear Resources and Environment, East China Institute of Technology, Nanchang, 330013, Jiangxi, China;State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, 510640, China;School of Geosciences, The University of Sydney, Sydney, 2006, Australia)

机构地区：[1]东华理工大学,地球科学学院,南昌330013 [2]东华理工大学核资源与环境省部共建国家重点实验室培育基地,南昌330013 [3]中国科学院广州地球化学研究所,同位素地球化学国家重点实验室,广州510640 [4]悉尼大学,地质科学学院,悉尼2006

出　　处：《地质学报》2018年第4期747-768,共22页Acta Geologica Sinica

基　　金：国家自然科学青年基金(编号：41303020); 同位素地球化学国家重点实验室(中国科学院广州地球化学研究所)(编号：SKLIGKF-13-06); 东华理工大学核资源与环境国家重点实验室培育基地自主基金(编号：Z1606);东华理工大学博士科研启动基金(编号：DHBK2012208); 国家留学基金(CSC201608360054)资助成果

摘　　要：赣西北麦斜岩体以灰白色中细粒等粒黑云母花岗闪长岩为主。本文通过锆石LA-ICP-MS法测得黑云母花岗闪长岩的侵位年龄为433.4±7.1Ma(MSWD=0.82)和431.8±6.0 Ma(MSWD=0.43),表明其属于早古生代岩浆活动的产物。岩石含少量角闪石,地球化学特征显示高硅(SiO_2=62.0%~71.43%)、高钾(K_2O/Na_2O=0.88~2.42)、富碱[(K_2O+Na_2O)=5.39%~8.12%]、低Fe_2O_3(2.61%~5.70%)、MgO(0.96%~2.90%)、CaO(1.39%~5.11%)、TiO_2(0.27%~0.67%)、P_2O_5(0.06%~0.10%)含量,整体弱过铝质(A/CNK介于0.98~1.25之间,平均为1.08)特征,且P_2O_5含量与SiO_2含量呈明显的负相关关系,矿物学和地球化学特征显示其属于高钾钙碱性Ⅰ型花岗岩。微量元素特征上表现为富集Rb、Th、Pb和LREE,亏损Nb、Ta、Ti、P、Ba、Sr等元素,轻重稀土分馏明显[(La/Yb)_N=5.00~40.42,平均为17.17],弱到中等Eu负异常(δEu=0.63~1.00,平均为0.77)。岩石具有均一的Sr-Nd同位素组成[(^(87)Sr/^(86)Sr)_i=0.708444~0.708962,ε_(Nd)(t)=-7.48^-7.22,t_(2DM)(Nd)=1.75~1.87Ga,平均为1.81Ga],相对较高的(^(87)Sr/^(86)Sr)_i比值和低ε_(Nd)(t)值及较古老Nd模式年龄暗示其来自以古元古代地壳为主要成分的部分熔融作用产物。综合实验岩石学以及样品地球化学特征,表明麦斜黑云母花岗闪长岩形成于中、下地壳中-基性岩(如角闪岩)的水不饱和部分熔融,初始岩浆中可能有一定量的幔源物质混入,并经历了广泛的角闪石、黑云母、钛铁矿、榍石、斜长石、磷灰石等矿物的分离结晶作用,这些幔源物质是华南古元古代时期经历了俯冲流体的交代作用而形成富集地幔的部分熔融产物。结合区域地质背景以及岩石地球化学特征认为其可能形成于陆内后碰撞环境,由幔源基性物质底侵,导致华南早古生代中下地壳部分熔融而形成广泛分布的华南早古生代陆内岩浆岩。Maixie pluton is composed predominantly of light-gray,fine-to medium-grained,equigranular biotite granodiorite.LA-ICP-MS zircon U-Pb dating yielded two weighted mean 206 Pb/238 U ages of 433.4±7.1 Ma（MSWD=0.82）and 431.8±6.0 Ma（MSWD=0.43）for the Maixie pluton,indicating that it was the product of magmatic activities in the Early Paleozoic.The Maixie granodiorites contain minor amount of amphiboles.Geochemical analysis shows that the pluton is characterized by high SiO2（62.0%～71.43%）and high K2O/Na2O（0.88～2.42）（with a total alkali（K2O＋Na2O）=5.39%～8.12%）,and low Fe2O3（2.61%～5.70%）,MgO（0.96%～2.90%）,CaO（1.39%～5.11%）,TiO2（0.27% ～0.67%）and P2O5（0.06%～0.10%）,with weak peraluminium（A/CNK=0.98～1.25,with an average value of 1.08）.There exists a clear negative correlation between SiO2 and P2O5.All these features suggest that the Maixie granodiorites belong to high-K calc-alkaline I-type granite.The trace element studies show an enrichment of Rb,Th,LREE and depletion of Nb,Ta,Ti,P,Ba,Sr and other elements,with distinct LREE and HREE fractionation of（La/Yb）N=5.00～40.42（a mean of 16.97）and weak to medium Eu negative anomalies ofδEu=0.63～1.00（an average of 0.77）.Homogeneous Sr-Nd isotopic compositions{（^（87） Sr/^（86） Sr）i=0.708667～0.709175,ε（Nd）（t）=-7.57～-7.29,T（DM2）=1.75～1.87 Ga（a mean of 1.81 Ga）},relatively high（^（87） Sr/^（86） Sr）Iratios and negativeε（Nd）（t）values with two-stage Nd mode ages of Paleoproterozoic suggest that the Maixie granodiroites generated by partial melting of the Paleoproterozoic crust.Experimental petrology and geochemical characteristics of our studied samples show that the Maixie biotite granodiorites resulted from water-unsaturated partial melting of the intermediate-mafic rocks（e.g.,amphibolite）in the lower-to middle-crust,with addition of a certain content of mantle-derived materials at the initial stage of magmas,and extensive fractional crystallization

关 键 词：麦斜岩体 华南 早古生代 Ⅰ型花岗岩 SR-ND同位素 

分 类 号：P588.121[天文地球—岩石学]