南秦岭略阳鱼洞子岩群磁铁石英岩形成时代的锆石U-Pb年代学约束  

Constraints from Zircon U-Pb Chronology of Yudongzi Group Magnetite-Quartzite in the Lueyang Area,Southern Qinling,China

作　　者：王洪亮[1,2] 徐学义[1] 陈隽璐[1] 闫臻[3] 李婷[1] 朱涛[1] 

机构地区：[1]中国地质调查局西安地质调查中心,西安710054 [2]西北大学大陆动力学国家重点实验室,西安710069 [3]中国地质科学院地质研究所,北京100037

出　　处：《地质学报》2011年第8期1284-1290,共7页Acta Geologica Sinica

基　　金：国土资源大调查项目(编号1212010610319);国家自然科学基金项目(编号40773044和40472044);国家科技支撑计划项目(编号2006BAB01A11));西北大学大陆动力学国家重点实验室开放课题基金(编号06LCD16)资助成果

摘　　要：鱼洞子杂岩是秦岭造山带出露的重要太古宙基底,也是该地区最古老的复杂地质体。鱼洞子杂岩主要由变质地层(鱼洞子岩群)和规模不等的花岗质变质侵入体(花岗片麻岩)组成。各地质体间多以构造面理接触。鱼洞子岩群是重要的铁矿产出层位,铁矿及其赋矿围岩的形成时代一直没有可靠的依据。本次选择小沟山矿区磁铁石英岩分选出大量锆石。CL影像特征显示锆石普遍具有清晰的幔-边结构,幔部发育较清晰的震荡环带,显示典型的岩浆锆石特征;边部为灰白色后期变质环带。依据锆石CL影像特征,结合锆石成因分析,通过锆石微区U-Pb同位素LA-ICPMS(激光剥蚀等离子体质谱)测定,获得鱼洞子小沟山矿区磁铁石英岩岩浆锆石不一致线上交点年龄为2645±25 Ma(MSWD=1.2),207Pb/206Pb加权平均年龄为2645±24 Ma,二者极为一致,代表石英岩原岩的形成时代。同时获得变质锆石较好的不一致线上交点年龄为2555±24 Ma(MSWD=1.4),与207Pb/206Pb加权平均年龄2527±27 Ma在误差范围内较为一致,应为原岩发生变质形成磁铁石英岩的时代,是太古宙一次重要的区域变质事件信息的反映。Yudongzi complex is a significant Archean basement exposed in the Qinling orogenic belt and also the oldest geological mass in the Qinling area.It is composed mainly of metamorphic strata（Yudongzi Group） and granitic metamorphic intrusions with various sizes,most of which contact with structural foliation.The Yuzidong Group is an important iron-hosting horizon,but there has been no reliable evidence for formation ages of iron deposit and ore-hosting country rocks.The study picked lots of zircons from magnetite quartzite at that Xiaogoushan mining area.CL images of zircons show a distinct mantle-edge structure which most zircons have,oscillatory zoning developed within mantle indicating magmatic in origin,and late-stage metamorphic,grayish zoning edge.According to CL imaging features and genetic analysis of zircons,we obtained an upper intercept age of 2645±25 Ma（MSWD=1.2） and a weighted mean 207Pb/206Pb age of 2645±24 Ma for the magnetite-quartzite magmatic zircons in Xiaogoushan mine area using microdomain U-Pb isotope LA-ICP-MS（laser ablation ICP-MS） determination.Two ages are consistent to each other,representing the formation age of protolith of quartzite.The upper intercept age of 2555±24 Ma（MSWD=1.4） for the metamorphic zircons is consistent with the weighted mean 207Pb/206Pb age of 2527±27 Ma within the error range,and should indicate the metamorphic age of quartzite.This age reflects an important regional metamorphic event occurring in the Archean.From above we can say that the age reflects an important regional metamorphism event in Archean.

关 键 词：秦岭造山带 鱼洞子岩群 磁铁石英岩 锆石 年代学 

分 类 号：P542[天文地球—构造地质学]