Natural annealing of dynamically recrystallised quartzite fabrics: Example from the Cévennes, SE French Massif Central

Quartzite samples from the SW-Cévennes (French Massif Central) have been investigated in order to reveal the effects of annealing on a previously dynamically recrystallised quartz fabric. The studied quartzite is interlayered with turbiditic micaschist series, and after regional deformation the whole sequence was intruded by the St-Guiral granodiorite at 1–3 kbar and up to 650–700 °C. Recorded crystallographic preferred orientation (CPO) patterns of the dynamically recrystallised quartzite are classified as type I with a monoclinic symmetry and type II with an orthorhombic symmetry. Type I is related to simple shear-dominated deformation and type II to apparent constriction. These two fabric types are consistent with structural observation that a constrictive and coaxial deformation regime controlled the formation of non-cylindrical and refolded folds. In the metamorphic aureole, which produced essentially static annealing, CPO patterns intensify slightly and grain size increases towards the granodiorite until the temperature at which inversion from low- to high quartz is triggered. This inversion caused a volume increase and subsequent intergranular stresses activated slip systems fitting the temperature and water content.This study shows that annealing of dynamically recrystallised quartz crystals can activate previously inactive slip systems, reactivate slip systems and amplify older CPO fabrics. This conclusion identifies limits for the use of annealed quartz fabrics in reconstructing structural histories when earlier dynamic recrystallisation has occured.

Research highlights
► Natural example of dynamically recrystallised quartz fabric annealing.
► Increased temperature during annealing triggers quartz inversion.
► New slip systems activated by quartz inversion related volume increase.
► Annealing intensifies slightly quartz fabrics.