Ductile deformation and Ar-40/Ar-39 dating of the Changle-Nanao ductile shear zone, southeastern China

An investigation of ductile deformation, mylonitization and Ar-40/Ar-39 geo chronology in the Changle-Nanao ductile shear zone, southeastern China, is reported in this paper. At least two phases of ductile deformation occurred in the shear zone in the Mesozoic. The first is a NE-trending foliation wi dely developed in metasedimentary rocks and orthogneisses, and is associate d with metamorphism of amphibolite facies at 165-178 Ma. The second is a st rike-slip-related ductile deformation developed both in the metamorphic roc ks and the Mesozoic volcanics and includes folds, shear foliation or cleava ge, S-C fabrics, NE-striking stretching lineation, and mylonitic microstruc tures. The ductile deformation and dislocation microstructures of quartz an d the brittle deformation microstructures of plagioclase in the mylonites f rom the shear zone indicate that the second phase of ductile deformation an d mylonitization occurred under greenschist facies conditions at temperatur es of 300-350 degrees C. Ar-40/Ar-39 plateau ages of five minerals from def ormed and mylonitized rocks in the shear zone are in the range of 118-107 M a. The plateau ages record the time of ductile deformation and mylonitizati on associated with strike-slip movement along the Changle-Nanao ductile she ar zone. (C) 2000 Elsevier Science Ltd. All rights reserved.