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《地学前缘(英文版)》 2014-04
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Dating of zircon from high-grade rocks:Which is the most reliable method?

Alfred Krner;Yusheng Wan;Xiaoming Liu;Dunyi Liu;Beijing SHRIMP Centre,Institute of Geology,Chinese Academy of Geological Sciences;State Key Laboratory of Continental Dynamics,Department of Geology,Northwest University;  
Magmatic zircon in high-grade metamorphic rocks is often characterized by complex textures as revealed by cathodoluminenscence (CL) that result from multiple episodes of recrystallization,overgrowth,Pb-loss and modifications through fluid-induced disturbances of the crystal structure and the original U-Th-Pb isotopic systematics.Many of these features can be recognized in 2-dimensionaI CL images,and isotopic analysis of such domains using a high resolution ion-microprobe with only shallow penetration of the zircon surface may be able to reconstruct much of the magmatic and complex postmagmatic history of such grains.In particular it is generally possible to find original magmatic domains yielding concordant ages.In contrast,destructive techniques such as LA-ICP-MS consume a large volume,leave a deep crater in the target grain,and often sample heterogeneous domains that are not visible and thus often yield discordant results which are difficult to interpret.We provide examples of complex magmatic zircon from a southern Indian granulite terrane where SHRIMP Ⅱ and LA-ICP-MS analyses are compared.The SHRIMP data are shown to be more precise and reliable,and we caution against the use of LA-ICP-MS in deciphering the chronology of complex zircons from high-grade terranes.
【Fund】: funded by the German Science Foundation(DFG) grant KR590/94-1 to AK
【CateGory Index】: P597.3;P588.3
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