浙江嵊泗正长花岗岩的形成时代与成因:来自锆石U-Pb年代学、岩石地球化学和Hf同位素的证据  

The formation age and origin of syenite granite in Shengsi,Zhejiang Province:Constraints from zircon U-Pb ages,rock geochemistry and Hf isotopes

作　　者：潘少军 张建芳 陈小友 马俊祥 蔡晓亮 黄国成 PAN Shaojun;ZHANG Jianfang;CHEN Xiaoyou;MA Junxiang;CAI Xiaoliang;HUANG Guocheng(Zhejiang Institute of Geosciences,Hangzhou 311200,China)

机构地区：[1]浙江省地质院,浙江杭州311200

出　　处：《地学前缘》2025年第2期430-444,共15页Earth Science Frontiers

基　　金：浙江省级基础性公益性战略性地质工作专项资金项目(2022012,2025004);中国矿产地质志项目(DD20221695,DD20190379,DD20160346)。

摘　　要：嵊泗岛地处浙闽岩浆带最北端,为海陆转换带,受古太平洋板块俯冲影响较大。本文以嵊泗正长花岗岩岩体为研究对象,开展了岩相学、锆石U-Pb测年、锆石Lu-Hf微区分析和岩石地球化学等工作。该岩体主要由钾长石(含量为50%~65%)、石英(含量为20%~25%)、斜长石(含量为20%~25%)和黑云母(含量为5%~10%)等组成,呈中细粒花岗结构。获得了高精度测年数据,其加权平均年龄为(95.4±1.2)Ma,表明该岩体为晚白垩世时期岩浆活动的产物。Lu-Hf微区分析结果显示,ε_(Hf)(t)值均为负值(-6.2~-4.0),变化范围较小,但均远高于地壳平均值,锆石二阶段模式年龄为1547~1405 Ma(平均值为1465 Ma),均低于华夏基底变质岩(桃溪)的二阶模式年龄。岩石地球化学特征表明:该岩体具有高硅、富碱、富铝和富钾的特征,镁、钙、磷、钛和铁等元素含量极低,属于准铝质弱过铝质高钾钙碱性花岗岩系列;Rb、K等大离子亲石元素和Th、U、Zr、Hf等高场强元素富集,Nb、Sr和P等元素亏损,稀土元素总量较高,轻稀土元素分馏作用强于重稀土元素,且具有明显的Eu负异常,分异指数高;具有低Sr、高Yb花岗岩的特征,反映了嵊泗正长花岗岩形成时压力较小。通过对岩石类型和构造环境的探讨,认为嵊泗正长花岗岩属于A型花岗岩,为晚白垩世时期古太平洋板块后撤伸展拉张背景下的产物。岩石圈拉张减薄,地幔物质底侵、上涌导致地壳物质熔融,壳幔混合作用下形成的以壳源物质为主、少量幔源物质参与的混合岩浆会在地壳浅部侵位并形成该类岩石。Shengsi Island,located at the northernmost end of the Zhejiang-Fujian magmatic belt,represents a transitional zone between land and sea,significantly influenced by the subduction of the ancient Pacific Plate.Petrological analysis,zircon U-Pb geochronology,zircon Lu-Hf isotopic microanalysis,and geochemical characterization were conducted on the Shengsi syenite granite pluton in this study.The Shengsi pluton primarily consists of potassium feldspar(50%-65%),quartz(20%-25%),plagioclase(20%-25%),and biotite(5%-10%),exhibiting a medium-to fine-grained granite texture.High-precision zircon U-Pb dating yielded a weighted average age of 95.4±1.2 Ma,indicating that the pluton formed during the Late Cretaceous.Zircon Lu-Hf isotopic analysis revealed the consistently negative ε_(Hf)(t)values(-6.2 to-4.0)with a narrow range of variation,significantly higher than the average crustal value.The two-stage zircon model ages range from 1405 Ma to 1547 Ma(mean 1465 Ma),which are younger than the second-stage model ages of the Huaxia basement metamorphic rocks(Taoxi).Geochemical analysis indicates that the Shengsi syenite granite is characterized by high silicon,alkali,aluminum,and potassium contents,with extremely low levels of Mg,Ca,P,Ti,and Fe.It belongs to the metaluminous to weakly peraluminous,high-potassium calc-alkaline granite series.The rock is enriched in large-ion lithophile elements(e.g.,Rb,K)and high-field-strength elements(e.g.,Th,U,Zr,Hf),while elements such as Nb,Sr,and P are depleted.The total rare earth element(REE)content is relatively high,with stronger fractionation of light REEs compared to heavy REEs,pronounced negative europium anomalies,and high fractionation indices.The low Sr and high Yb content of the granite suggests relatively low pressure during its formation.The Shengsi syenite granite is classified as A-type granite based on the petrological and geochemical characteristics,as well as the tectonic setting.It formed during the Late Cretaceous in a tectonic environment associated with the retreat an

关 键 词：正长花岗岩 岩石成因 构造背景 嵊泗 晚白垩世 

分 类 号：P588.121[天文地球—岩石学] P597[天文地球—地质学] P581