Giant quartz veins associated with the South Armorican Shear Zone record important fluid circulation during the Hercynian period. Regional-scale mapping of veins allows two groups of veins to be identified, on the basis of their geometric relationship with the South Armorican Shear Zone. Veins in the first group are parallel to the shear zone, whereas those in the second group developed in a direction oblique to it. The former probably record infiltration of fluids along permeable pathways in highly deformed zones; the latter may represent crustal-scale tension gashes in the regional context. Most quartz veins have δ18O values between 10 and 16‰ indicating a mid-crustal origin for the fluids. Microthermometry on fluid inclusions from euhedral quartz indicates that late fluids were mostly aqueous with very low salinity (0–1.7 wt% eq.) and with homogenization temperatures ranging between 150 and 270 °C. Together with very low δ18O values of some euhedral quartz, down to −2‰, these features argue for a surface origin. Corresponding δ18Ofluid values estimated near −11‰ are probably related to the high palaeo-elevation of meteoric precipitation. Scarce, but significant, H2O–CO2 fluid inclusions in euhedral quartz indicate a metamorphic contribution. These were probably sourced from the exhumed metamorphic basement in the southern part of the region.