Dating C/S structures in syntectonic granites by <i>in situ</i> Rb-Sr analyses on muscovite

The interplay of crystallization, deformation, and fluid-rock interactions makes deciphering geochronological information complex in syntectonic granites. In this study, we apply<i> in situ </i>Rb-Sr analyses to selected samples from the Questembert Variscan syntectonic granite in Western France, which was emplaced along the South Armorican Shear Zone as indicated by widespread C/S structures. The results show that the Rb-Sr system in muscovite records about 25–30 Ma of geological activity, from crystallization of the granite at ca. 320–315 Ma, recorded by muscovite porphyroclasts, to late-stage permeation of small quantities of fluids at 290 Ma along pre-existing shear planes that triggered localized recrystallization of fine-grained muscovite during the waning stages of regional deformation. These results, thus, show that microscale <i>in situ</i> Rb-Sr analyses allow recognizing discrete events long after the main phase of crystallization of the granite, emphasizing the need for careful petrographic analysis to inform interpretation of geochronological data.