闽西南宣和加里东期花岗岩锆石U-Pb年龄、地球化学特征及对华南造山的启示

Zircon U−Pb geochronology,geochemical characteristics and tectonic implications of Caledonian granites from the Xuanhe area,Southwestern Fujian Province

龙岩宣和岩体是闽西南地区呈北东向弧形出露,最大的燕山期—加里东期复式岩体,但是有关该岩体的形成时代及成岩环境的认识仍存在分歧,进而制约了对闽西南地区构造环境的探讨。文章以出露于闽西南地区的宣和正长花岗岩为研究对象,在详细野外地质调查基础上,开展了岩石学、LA−ICP−MS锆石U−Pb地质年代学、岩石地球化学及Sr−Nd同位素测试等工作,在此基础上探讨了宣和岩体的岩石类型、岩浆来源和成岩构造环境。宣和岩体主要由含斑细粒、少斑中细粒、似斑状中粒、似斑状中粗粒及斑状细粒正长花岗岩等组成,4个样品的锆石LA−ICP−MS U−Pb年龄为(424.6±2.8)Ma、(426.7±2.6)Ma、(435.5±2.4)Ma及(447±3.6)Ma,表明其形成时代不是原来认为的印支期,而是加里东期。地球化学特征上,具高硅、高钾(K2O>Na2O),低TiO2、FeO、MgO的特征,A/CNK平均值1.23,属高钾钙碱性过铝质系列岩石;稀土元素含量高,稀土配分曲线右陡倾,轻重稀土分馏明显,Eu中等负异常;富集大离子亲石元素K、Rb、Pb、Nd及高场强元素Th、U,相对亏损Sr、Ba、Nb、Ce,显示高分异的S型花岗岩的特征。其Sr同位素初始比值介于0.71291~0.71399,εNd(t)值变化于−9.1~−13.4,对应的二阶段Nd模式年龄为1.91~2.27 Ga,推测宣和岩体是由与麻源群变质岩相当的地壳物质部分熔融形成的,为华南加里东期板内造山作用的产物。

The NE−trending Xuanhe granitic mass is the largest Yanshanian-Caledonian complex rock mass in southwestern Fujian,which is exposed in southwestern Fujian Province in the arc form.However,there are still different opinions in the formation age of rock mass and its petrogenetic environment,which restricts the discussion of the tectonic environment of southwestern Fujian.Based on detailed field geological survey,the authors investigated the Xuanhe syenogranites,mainly on the basis of the LA−ICP−MS zircon U−Pb geochronology,petrochemistry and Sr−Nd isotope analysis,and discussed the rock types,magma source and tectonic environment.The Xuanhe granites are mainly composed of syenogranites in the forms of speckle fine grain,oligophyric mediumfine grain,porphyritic fine grain,porphyritic fine coarse grain and porphyritic fine grain.The LA−ICP−MS zircon U−Pb age of four samples from the granites are(424.6±2.8)Ma,(426.7±2.6)Ma,(435.5±2.4)Ma and(447±3.6)Ma respectively,indicating that the formation age of Xuanhe granite is Caledonian instead of Indosinian as previously held.The rocks belong to high-K calc alkaline peraluminous rock series,characterized geochemically by high Si,high K(K2O>Na2O),low TiO2,low FeO and low MgO,with average A/CNK value being 1.23.The rocks are S-type grantie,with the characteristics of high content of rare earth elements,obvious right steeply inclined REE fractionation curve,middle negative Eu anomaly,enrichment of large ion lithophile elements such as K,Rb,Pb and Nd and high field strength elements of Th,U and Ba,and relative depletion of Sr,Nb and Ce.The initial Sr isotope ratios are from 0.71291 to 0.71399,the values ofεNd(t)vary from−9.1 to−13.4,and two stage Nd model ages correspond to 1.91−2.27 Ga.It is inferred that the granites were formed by partial melting of crustal material of Mayuan Group metamorphic rocks,being probably a product of intraplate Caledonian orogeny in South China.