Résumé
Samples from the Cap de Creus shear zone network were investigated to constrain the timing of the basement's polycyclic evolution by combining structural and microstructural analyses, electron imaging chemical investigations, and laser ablation mass spectrometry in situ Rbsingle bondSr and Th-U-Pb geochronology. Two generations of muscovite (Ms I and Ms. II) yield in situ Rbsingle bondSr isochron ages of 290–270 Ma and 60–50 Ma, consistent with Usingle bondPb ages of c. 290 Ma in monazite cores and 50–30 Ma rims. Microstructure study shows that Ms. II grew during dynamic recovery of quartz ribbon precursors (Qz I). Fe-enrichment in Ms. II, localized in mica fish tails and C′ shear planes, along with Fe-rich veins marking Qz II grain boundaries, evidence that the Rbsingle bondSr system in muscovite recorded an episode of dynamic fabric recovery of quartz in presence of fluids. We thus demonstrate that rocks fabrics were annealed during late Variscan HT-LP metamorphism and deformation in the Early Permian, followed by ductile-plastic reactivation of the shear zones during the Eocene. This reactivation was facilitated by external fluid influx, drained by Ms. II neocrystallization and Qz II boundary migration. The Cap de Creus shear zones, usually interpreted as Variscan, also record a major Eocene reactivation during the formation of the Pyrenean orogenic prism through underthrusting of basement slices. These zones acted as early accommodation structures for crustal shortening. This study highlights the critical role of fluid-induced rheological softening in ductile reactivation of polycyclic basements and provides a context-dependent framework for interpreting the behavior of the Rbsingle bondSr geochronometer in muscovite during deformation.