Dating C/S Structures in Syntectonic Granites by In Situ Rb‐Sr Analyses on Muscovite

ABSTRACT The interplay of crystallisation, deformation and fluid–rock interactions makes deciphering geochronological information complex in syntectonic granites. In this study, we apply in situ 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 crystallisation of the granite at ca . 320–315 Ma, recorded by muscovite porphyroclasts, to late‐stage permeation of small quantities of fluids along pre‐existing shear planes at 290 Ma that triggered localised recrystallization of fine‐grained muscovite during the waning stages of regional deformation. These results, thus, show that microscale in situ Rb‐Sr analyses allow recognising discrete events long after the main phase of crystallisation of the granite, emphasising the need for careful petrographic analysis to inform interpretation of geochronological data.