Quartz fabric transition in a cooling syntectonic granite (Hermitage Massif,France)

The Hermitage Granite, situated in the northwest Massif Central (France) is a syntectonic Hercynian leucogranite emplaced along an active transcurrent shear zone. During emplacement and cooling, the progressive deformation is marked by the development of a primary homogeneous foliation gradually affected by ductile shear bands (C-S mylonites). Increase in strain heterogeneity during cooling corresponds to a change of dominant deformation mechanism of the quartz phase from grain growth and migration recrystallization to intracrystalline 〈a〉 slip and rotation recrystallization. Migration recrystallization is characterized by preferred orientations of c axes close to the principal extension direction. We discuss relevant deformation mechanisms and rheological implications for syntectonic plutons. In particular, we argue that the transitions between homogeneous and heterogeneous accumulation of strain cannot generally be correlated with transition between magmatic and solid-state flow.