Insights into the post-emplacement history of the Saunstein granitic dyke showing heterogeneous deformation and inconsistent shear-sense indicators (Bavarian Forest, Germany)

The recent mapping survey undertaken in the Bavarian Forest led to the discovery of several E–W trending sinistral shear zones, usually affecting granitic dykes or areas intruded by dykes. We studied a granitic dyke affected by one of these shear zones. The deformation in the dyke, as well as in other dykes of the region, is not homogeneous and concentrates mainly next to the contact to the country rock. The differences in mineralogy between dyke and host probably controlled the distribution of strain, favoring the deformation of the lithology richer in quartz, i.e. the granite dyke, next to a rheological boundary, i.e. the contact to the host rock. As inferred from microfabrics and quartz textures, quartz deformation occurred by dislocation creep accommodated by grain boundary migration and subgrain rotation recrystallization. Intracrystalline glide on prism planes in the <a> direction indicate a temperature of ∼500–550 °C. Only in the most strained, fine grained bands was deformation achieved by grain boundary sliding. A late shear-sense reversal after the principal sinistral deformation in the dyke led to the development of a dextral quartz shape fabric. This reflects on the one hand how quartz is able to record late, low-strain overprints, and on the other hand, how oblique quartz foliations may lead to wrong interpretations when being used without corroboration of other shear-sense indicators.