北秦岭东段二郎坪群火山岩锆石的LA-ICP-MS U-Pb定年及其地质意义  

Zircon LA-ICP-MS U-Pb dating of basic volcanics from Erlangping Group of the North Qinling,eastern Qinling Mountains and its geological implications

作　　者：赵姣[1,2] 陈丹玲[1,2] 谭清海 陈淼[1,2] 朱小辉 郭彩莲[1,2] 刘良[1,2] 

机构地区：[1]西北大学大陆动力学国家重点实验室 [2]西北大学地质学系,陕西西安710069

出　　处：《地学前缘》2012年第4期118-125,共8页Earth Science Frontiers

基　　金：国家重点基础研究发展计划"973"项目(2009CB825003);国家自然科学基金项目(41072051);西北大学大陆动力学国家重点实验室基金项目

摘　　要：利用阴极发光(CL)技术和LA-ICP-MS原位分析方法,对北秦岭东段军马河、湾潭和湍河3个地区的二郎坪群基性火山岩进行了系统的锆石U-Pb年代学研究。CL图像分析结果显示,三地火山岩中的锆石皆呈自形-半自形柱状形态和微弱的、宽的振荡环带结构。微量元素分析揭示,所有锆石都具有较高的稀土总量、重稀土和Th、U含量(ΣREE为(397.7～7 941.6)×10-6;ΣHREE为(376.7～7 774.2)×10-6;Th为(107.9～5 824.2)×10-6;U为(89.3～3 841.9)×10-6)以及轻稀土亏损、重稀土明显富集的左倾稀土配分曲线型式,所有锆石的Th/U比值皆大于0.4,显示典型岩浆锆石特征。LA-ICP-MS定年结果获得3地基性火山岩的形成年龄分别为(463±1.8)Ma、(475±1.5)Ma和(473±1.3)Ma,三者在误差范围内一致,表明北秦岭二郎坪群基性火山岩的形成时代为463～475Ma。系统的岩石学和地球化学研究显示,二郎坪群火山岩具有弧后盆地火山岩的地球化学特征。研究获得的二郎坪基性火山岩463～475Ma的结晶年龄明显晚于其南侧北秦岭超高压榴辉岩的原岩年龄((791±6)Ma)以及变质年龄((502±11)Ma),而且榴辉岩的地球化学特征显示为板内玄武岩,也与二郎坪火山岩不同,因而指出北秦岭超高压榴辉岩与二郎坪火山岩没有直接成因联系。Cathodoluminescence and LA-ICP-MS analysis were conducted on zircon grains from three basic vol-canic rock localities of the Erlangping Group,namely,in Junmahe,Wantan and Tuanhe-Tuanyuan regions,eastern Qinling Mountains.CL investigations show that the zircon grains from the three areas have the same be-havior of euhedral to subhedral prismatic shape with weak,broad oscillatory zoning internal structure.Trace elementanalyses revealed that the analyzed zircon grains are high in contents of REE,HREE,Th and U（ΣREE=（397.7-7941.6）×10-6,ΣHREE=（376.7-7774.2）×10-6,Th=（107.9-5824.2）×10-6,U=（89.3-3841.9）×10-6）,high Th/U ratios（0.4）and show left-dipping REE patterns depleted in LREE and strongly enrichedin HREE.All of these characters suggesting a typical magmatic zircon origin.LA-ICP-MS zircon dating yieldcrystallization ages of（463±1.8）Ma,（475±1.5）Ma and（473±1.3）Ma for basic volcanic rocks from Jun-mahe,Wantan and Tuanhe-Tuanyuan sections,respectively.These ages are well consistent with each otherwithin errors,indicating that the basic volcanics of the Erlangping Group in the North Qinling Orogen wereformed during 463-475Ma.This age is obviously younger than the protolith age（791±6）Ma as well as themetamorphic age（502±11）Ma of the ultrahigh pressure（UHP）metamorphic eclogite next to the south in theQinling Group.Together with their distinct geochemical affinities,Erlangping volcanics are of back-arc basinbasalt and the protolith of the UHP eclogite is of within plate basalt.Therefore,the UHP eclogite in theNorth Qinling has no direct relationship with the basic volcanics in the Erlangping Group.

关 键 词：LA-ICP-MS锆石定年 基性火山岩 二郎坪群 北秦岭 

分 类 号：P588.145[天文地球—岩石学] P597.3[天文地球—地质学]