地质学报（英文版）2008，Vol.82Issue(1)：84-98,15.
Excess and Radiogenic Argon in High-Pressure Mylonites of the Tan-Lu Fault Zone, Eastern China
Excess and Radiogenic Argon in High-Pressure Mylonites of the Tan-Lu Fault Zone, Eastern China
摘要
Abstract
Bulk separates of porphyroclastic phengite, neoformed phengite and their mixtures from the Tan-Lu HP myionites overprinted on the Sulu UHP rocks were analyzed with the 40Ar/39 Ar step heating method. Two samples of the neoformed phengite from ultramylonite give 40Ar/39Ar plateau ages of 209.9±1.8 Ma and 214.3±1.8 Ma, which are interpreted as representing cooling times of the Tan-Lu sinistral faulting, and provide geochronological evidence for the syn-orogenic faulting of the Tan-Lu fault zone. The results show that the phengite formed during the retrograde eciogite-facies mylonitization was not contaminated with excess argon and can be used for dating the deformation.Argon closure in previous K-bearing minerals with excess argon under a retrograde HP dry condition is considered to be the reason for lack of excess argon incorporation in the neoformed phengite. Five porphyroclastic phengite samples yield 40Ar/39Ar plateau ages ranging from 666±12 Ma to 307.1±3.3 Ma, which are interpreted as being contaminated with excess argon. Two mixture samples with plateau ages of 239.4±2.1 Ma and 239.3±2.0 Ma show upward-convex age spectra caused by the mixture of older porphyroclastic phengite with excess argon incorporation and younger neoformed phengite without excess argon incorporation. It is demonstrated that excess argon introduced from the previous UHP metamorphism is still preserved in the pre-existing phengite after the Tan-Lu eclogite-facies mylonitization. The intense deformation under HT and HP conditions cannot erase excess argon in the previous phengite totally due to restricted fluid activities. These porphyroclastic phengite previously contaminated with excess argon cannot be used for dating the later HP deformation. This indicates that deformation under a HP dry condition does not play an important role in removing previous 40Are in phengite.关键词
excess argon, phengite, mylonite, Tan-Lu fault zone, 40Ar/39Ar datingKey words
excess argon, phengite, mylonite, Tan-Lu fault zone, 40Ar/39Ar dating分类
天文与地球科学引用本文复制引用
XIE Chenglong,ZHU Guang,WANG Yongsheng,HU Zhaoqi..Excess and Radiogenic Argon in High-Pressure Mylonites of the Tan-Lu Fault Zone, Eastern China[J].地质学报（英文版）,2008,82(1):84-98,15.基金项目
This study was funded by the National Natural Science Foundation of China (grant numbers 40272094,40672131). （grant numbers 40272094,40672131）
