南秦岭老城花岗岩成因:锆石U-Pb年龄和Sr-Nd同位素的制约  

Petrogenesis of Laocheng Granite in South Qinling:Constraints from Zircon U-Pb Age and Sr-Nd Isotopic Composition

作　　者：方博文[1] 张贺[1] 叶日胜 王岩[1] 陈福坤[1] FANG Bo-wen ZHANG He YE Ri-sheng WANG Yan CHEN Fu-kun(School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, Anhui, Chin)

机构地区：[1]中国科学技术大学地球和空间科学学院,安徽合肥230026

出　　处：《地球科学与环境学报》2017年第5期633-651,共19页Journal of Earth Sciences and Environment

基　　金：国家自然科学基金项目(41372072)

摘　　要：老城岩体出露于南秦岭宁陕地区,属于五龙岩体群的重要组成部分。岩体的不同岩性单元均含有大量暗色微粒包体。锆石U-Pb年龄分析结果显示,寄主岩石形成年龄为213~219 Ma,其包体的锆石结晶年龄为211 Ma,属于早中生代五龙岩体群岩浆作用中期产物。岩石地球化学分析显示:寄主岩石的主体为花岗闪长岩和石英二长岩,具有低Si、弱过铝质、富碱和高Mg~#值(52~67)的地球化学特征,属于高钾钙碱性系列Ⅰ型花岗岩;暗色包体主要为闪长质成分,具有低SiO_2(53.64%~62.40%)、富碱和高Mg~#值(63~72)的地球化学特征,属于钾玄岩岩石系列。暗色包体与其对应的寄主岩石相比具有相对较低的初始N(^(87)Sr)/N(^(86)Sr)值(0.704 66~0.705 82)、较高的εNd(t)值(-4.3^-1.6)和高Mg#值,显示形成包体的原始岩浆可能源于幔源玄武质岩浆。SrNd同位素地球化学特征显示,寄主岩石和包体之间发生过明显的成分交换,使这些成分在二者中大体达到平衡。对比南秦岭基底岩石的初始Sr-Nd同位素比值,推测老城花岗岩岩浆源区主要是新元古代基性下地壳物质。包体和寄主岩石的关系显示,岩浆混合可能为基性岩浆注入到已经开始结晶的酸性岩浆,两种岩浆发生不完全混熔,形成了寄主岩石和暗色基性包体。Laocheng pluton exposed in Ningshan area of South Qinling,together with Wulong pluton,consist of an important part of Early Mesozoic granitoid magmatism in central South Qinling.Abundant mafic enclaves can be found in different units of Laocheng pluton.Analysis results of zircon U-Pb ages show that the host rock from Laocheng pluton is emplaced in 213-219 Ma,and its mafic enclaves are crystallized in 211 Ma,which are more or less simultaneous with the formation of Wulong pluton.Geochemical data show that the host rock from Laocheng pluton is granodiorite and quartz monzonite in composition with the characteristics of low Si,rich alkali,weak peraluminous and high Mg-#（52-67）,and belongs to high-K calc-alkaline I-type series;mafic enclaves are mainly dioritic,having the characteristics of low SiO2（52.6%-62.4%）,rich alkali and high Mg^#（63-72）,belonging to shoshonite series.Compared with the host rock,themafic enclaves have lower initial N（^87Sr）/N（^86Sr）（0.704 66-0.705 82）,highεNd（t）（-4.3--1.6）and higher Mg^#,suggesting that the primary magma of the enclaves might have originated from a basaltic magma.Sr-Nd isotope geochemistry suggests that the host rock and mafic enclaves have exchanged prior to the formation of Laocheng pluton,and an equilibrium has been reached.Compared with Sr-Nd isotope ratio of basement rocks exposed in South Qinling,it is suggested that the magma of Laocheng granite originates from Neoproterozoic mafic crustal material.Relationship between host rock and mafic enclaves indicates that a magma mixing between the crystallizing acid magma and the basic magma slightly later injects into the magma chamber;these two magmas are not completely miscible during the mixing process.

关 键 词：地球化学 花岗岩 寄主岩石 包体 早中生代 锆石U-PB年龄 SR-ND同位素 南秦岭 

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