Variscan deformation phases in the southwestern Bohemian Massif: new constraints from sheared granitoids [Variszische Deformationsphasen in der s�dwestlichen B�hmischen Masse: neue Randbedingungen aus zerscherten Granitoiden.]

The subsolidus Variscan evolution of several small granitic bodies from the southern Bayerischer Wald (Bavarian Forest) indicates the presence of a new deformation phase referred to as D3. The principal compression direction was N–S to NNW–SSE during a long period of time and in a wide temperature range, leading to the development of the Bayerischer Pfahl shear zone system and to the deformation by dextral shear of the migmatites located in the surroundings of the Fürstenstein Pluton. The “D3shear zone system” consists of an array of sinistral shear zones trending ENE to ESE, which are commonly found in granitoids. D3took place under upper greenschist to lower amphibolite facies conditions, as supported by the observed fabrics and quartz textures. Both, the intrusion and the deformation of the granitoids affected by D3occurred at deep to intermediate levels of the crust. New and pre-existing geochronological data constrain the age of D3to about ∼315 Ma, whereas the intrusion of at least some of the sheared granitoids was pre-kinematic with respect to D3, as exemplified by the Saunstein dyke (324.4 ± 0.8 Ma). D3was triggered by rotation of the principal compression direction towards a NE–SW direction. This rotation was either related to the late-Variscan switch in the convergence direction between Laurussia and Gondwana, or was caused by local perturbations of the regional stress field. Those perturbations may be related either to the intrusion of large stocks, such as the Saldenburg granite, or to the existence of ephemeral releasing bends at the Bayerischer Pfahl shear zone.