中-浅正变质岩锆石SHRIMP法与TIMS法测年结果不一致处理——以滇东南南温河花岗岩为例  

A Study on SHRIMP and TIMS Zircons Dating on Low-to Medium-Grade Ortho-Metamorphic Rocks: Example on the Nanwenhe Granites, Southeastern Yunnan Province,China

作　　者：谭洪旗[1,2] 刘玉平[1] 徐伟 郭利果[4] 叶霖[1] 李朝阳[4] 

机构地区：[1]中国科学院地球化学研究所矿床地球化学国家重点实验室,贵州贵阳550002 [2]中国地质科学院矿产综合利用研究所,四川成都610041 [3]胜利油田股份公司,山东东营557000 [4]中国科学院广州地球化学研究所,广东广州410640

出　　处：《矿物学报》2011年第1期62-69,共8页Acta Mineralogica Sinica

基　　金：973项目(批准号:G2007CB411408);国家基金项目(批准号:40302026;40972129);矿床地球化学国家重点实验室自主选题项目(编号:KCZX20090106)

摘　　要：变质岩中的锆石大多遭受变质热事件的改造,但中-浅变质岩一般不甚发育新生锆石或变质增生边,不易通过现有的锆石测年技术获得该岩石的变质年龄。中-浅正变质岩的锆石为岩浆锆石与变质增生锆石的混合物,其U-Pb同位素组成可以采用二端元混合模式来表达。利用同位素稀释法(TIMS)可以获得混合锆石U-Pb年龄及对应的同位素组成;结合CL等内部结构分析,利用离子探针法(SHRIMP)可以获得岩浆锆石U-Pb年龄及对应的同位素组成,进而推算出变质增生锆石U-Pb年龄对应的同位素组成。在对滇东南南温河花岗岩锆石U-Pb年代学的研究中,我们发现同一样品的SHRIMP与TIMS U-Pb法测年结果不一致,根据上式推算出后期主变质年龄约为230 Ma,与前人利用其它测年方法获得的结果基本一致,符合研究区主变质期为印支期的区域地质背景。该方法为中-浅正变质岩年代学研究提供了一种新思路。Zircons in metamorphic rocks have generally undergone thermal alteration.As new growth zircons and metamorphic accretional rims are weakly developed in the low-to medium-grade metamorphic rocks,it is difficult to date them by using ordinary zircon U-Pb isochronal techniques.In these rocks,the isotopic compositions of zircons with magmatic core and metamorphic accretional rims represent the mixing of two compounds different genesis,which could be expressed by the mixture model of two isotope end-members.The isotopic composition of mixed zircon could be obtained by TIMS technique,and the isotopic composition of magmatic zircon could be dated by using SHRIMP technique on the basis of the internal fabric analysis such as CL.Thus,the isotopic composition of metamorphic zircon could be calculated.The zircons U-Pb geochronology of the same samples exhibit the discordance between SHRIMP and TIMS U-Pb dating results from the Nanwenhe Granites,Southeastern Yunnan Province,P.R.China.The age of peak metamorphic event（～230 Ma）was obtained through the above mentioned calculating method,in accordance with the indosinian metamorphic ages obtained by other dating methods.This study provides a new way to date the low-to medium-grade metamorphic event.

关 键 词：中-浅正变质岩 锆石 U-PB年龄 SHRIMP法 TIMS法 南温河花岗岩 滇东南 

分 类 号：P579[天文地球—矿物学] P597[天文地球—地质学]