Multidisciplinary characterization of fluid sources in ductileshear zones (Armorican Massif, Variscan belt, France)

National audienceDuctile shear zones are sites of significant fluid circulation and hydrothermal alteration wheremetamorphic, magmatic and surface-derived fluids meet. Characterization of the meteoricsource of crustal fluids can be used to better understand ore deposition and mineralization atthe orogen scale or for stable isotope paleoaltimetry reconstructions. Microstructural,thermometry, geochronological and hydrogen (δD) and oxygen (δ18O) isotope data fromsyntectonic peraluminous granites from ductile shear zones in the Armorican Massif permit todetermine the source of fluids present during deformation. Furthermore, they allow to study thespatial and temporal evolution of localized deformation and fluid-rock interaction across theseVariscan fossil hydrothermal systems, including the timing and duration of fluid flow and watermineralisotope exchange.At the regional scale, a 41‰ difference in δDwater values amongst mylonitic leucograniteemplaced along strike-slip and detachment zones highlights a mixing relationship betweenmetamorphic/magmatic fluids (δDwater ~ -33‰) and meteoric fluids with δDwater values aslow as -74‰. The mixing between surface-derived and deep fluids is further supported by fluidinclusions aligned along synkinematic structural planes in quartz grains from detachmentfootwalls. They contain very low to medium salinity water (0 to 7 wt% eq. NaCl) and haveintermediate δD and δ18O values.We focus on the Quiberon detachment zone (QDZ) where synkinematic muscovite andtourmaline crystallized and equilibrated with deuterium-depleted surface-derived fluids duringhigh-temperature deformation supported by titanium-in-muscovite thermometry andmicrostructures. 40Ar/39Ar data on muscovite and U(-Th)/Pb geochronology on zircon,monazite and apatite from syntectonic leucogranites, together with microstructural data,suggest that meteoric fluid-rock-deformation interactions started at ~320 Ma and played amajor role in leaching uranium at ~305 Ma. U-Th/Pb data from migmatites located belowthe QDZ strengthen the idea that meteoric fluids infiltration, detachment activity, syntectonicleucogranite emplacement and migmatization were coeval and allowed the development of asustained hydrothermal system