甘肃北山音凹峡南花岗岩体的锆石LA-ICP MS定年及其构造意义  

LA-ICP MS zircon U-Pb ages of the granites from the south of Yin'aoxia and their tectonic significances.

作　　者：张文[1] 冯继承[1] 郑荣国[1] 吴泰然[1] 罗红玲[1] 贺元凯[1] 荆旭[1] 

机构地区：[1]造山带和地壳演化教育部重点实验室,北京大学地球与空间科学学院,北京100871

出　　处：《岩石学报》2011年第6期1649-1661,共13页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(40672146)资助

摘　　要：本文对位于甘肃北山南带的音凹峡南花岗岩体进行了锆石LA-ICP-MS定年和Hf同位素分析,结果表明该花岗岩体的形成年龄为281.7±2.9Ma,εHf(t)=+4.4～+7.8,平均+5.9;Hf同位素的模式年龄为613～771Ma。音凹峡南花岗岩体以黑云母花岗岩为主,偏铝质-过铝质,中钾钙碱性特征,具有较高的SiO2(72.84%～74.59%)、Na2O+K2O(6.54%～9.06%),富钠(Na2O/K2O>1);球粒陨石标准化分配模式图上,具相对平缓的富集轻稀土元素稀土配分模式,重稀土元素分馏不明显且相对亏损,Eu具有弱负异常,δEu=0.58～0.81。在微量元素原始地幔标准化蛛网图上,亏损Nb、Ba、P、Ti,富集Rb、Pb、K。结合区域地质背景,可以认为音凹峡南花岗岩体是壳幔混合成因的,形成于早二叠世后碰撞伸展体制下,进而认为音凹峡地区可能在晚石炭世洋盆闭合进入碰撞作用阶段,可以确定在早二叠世晚期进入后碰撞的裂谷伸展发育阶段。This paper reports LA-ICP MS zircon U-Pb ages and Hf isotopic compositions from the granites in south of Yin＇aoxia,southern Beishan Mountain,Gansu Province.The results show that the pluton was emplaced with the age of 281.7±2.9Ma.Its εHf（t） values are ＋4.4~＋7.8 with the average of ＋5.9 and Hf modal ages are 613~771Ma.The granitic body mainly consists of biotite granites which belong to middle-K cal-alkaline series with metaluminous-peraluminous characteristics and high contents of SiO2 （72.84%~74.59%）,Na2O＋K2O （6.54%~9.06%） and Na2O （Na2O/K2O1）.In addition,they invariably exhibit relatively gentle light rare earth elements （LREE） enrichment with flat heavy rare earth elements （HREE） and weak negative Eu anomalies （δEu=0.58~0.81） in the chondrite-normalized REE patterns,depletion of Nb,Ba,P,Ti and enrichment of Rb,Pb,K in the spidergram.Based on the regional geology,the above characteristics reflect that the Yin＇aoxia granites were the mixing products of crustal and mantle derived magmas and formed under the tectonic setting of post-collisional extension in Early Permian.This implies that the area of Yin＇aoxia may enter into the stage of collision after the closure of the ocean basin in Late Carboniferous and we could safely say that it turned to the stage of rift in post-collisional extensional period during Early Permian.

关 键 词：后碰撞 花岗岩 二叠纪 锆石U-Pb年龄 HF同位素 音凹峡 北山 

分 类 号：P588.121[天文地球—岩石学]