Excess and Radiogenic Argon in High-Pressure Mylonites of the Tan-Lu Fault Zone, Eastern China

Bulk separates of porphyroclastic phengite,neoformed phengite and their mixtures from the Tan-Lu HP mylonites overprinted on the Sulu UHP rocks were analyzed with theAr/Ar step heating method.Two samples of the neoformed phengite from ultramylonite give ~)Ar/Ar 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 eclogite-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 yieldAr/Ar 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 previousArin phengite.