In this study, petro-geochemical, isotopic and geochronological data have been acquired in order to characterize and date fluid circulations within the South Armorican Shear Zone. These circulations have been dated by U-Pb on both zircon and monazite and by muscovite 40Ar/39Ar analyses, extracted from mylonites and syntectonic granites displaying evidence of hydrothermal alteration. At 315 Ma, the South Armorican Shear Zone localized the circulation of magmato-metamorphic fluids, responsible for the crystallization of giant quartz veins parallel to its main strike. Around 300 Ma, an influx of meteoric fluids is recorded in mylonites, as well as in some granitic samples and in quartz veins oblique to the South Armorican Shear Zone, interpreted as crustal-scale tension gashes. These results have also been used to study the behaviour of isotopic chronometers during fluid-rock interactions. Although zircons do not seem to be affected, fluid circulations induced recrystallization of monazite and muscovite and therefore disturbed and even reset the U-Pb and K-Ar chronometers. In the syntectonic Questembert massif for example, 40Ar/39Ar dates obtained on six samples cover a time span of 16 Ma linked to a pervasive infiltration of fluids. Such results therefore illustrate the need to associate geochronology with a detailed petro-geochemical study, from whole rock characterization to the textural and/or chemical study of the minerals selected for dating.
