High-temperature deformation of a granitoid from the Zone of Erbendorf-Vohenstraß (ZEV)

Abstract

Microfabrics and textures indicate that progressive mylonitization of granitoids from the vicinity of the KTB site occurred under high temperature and probably prograde conditions by pure shear deformation. Diffusion-controlled deformation mechanisms and diffusional mass transfer for all rock-forming minerals were prominent processes which also contributed to the development of compositional layering. Typical chessboard patterns in quartz suggest that even the latest increments of plastic deformation took place at high temperatures within the stability field of high quartz. Largely constant chemical bulk-rock compositions and lack of retrogressive reactions are evidence of a dry and closed system during mylonitization. Only a late post-mylonitic fluid influx is documented by secondary fluid inclusions along healed microcracks. Taking recent geochronological data into account, different geological scenarios are most suitable: (a) intrusion of the protolith between 525 and 475 Ma into Cadomian basement and subsequent mylonitization in a related extensional regime; and (b) Early Variscan mylonitization in a subduction zone environment between 475 and 380 Ma.