Fluids in the South Armorican Shear Zone, France

Abstract

International audienceThe South Armorican Shear Zone (SASZ) is alithsopheric-scale shear zone from western Europethat was active during Variscan times. U–Pb analyseson zircon and monazite and 40Ar-39Ar analyses onmuscovite from mylonites and syntectonic granitesdefine a minimum duration of 20 Ma for thedeformation and magmato-hydrothermal historyalong the SASZ, between 320 Ma and 300 Ma. Giantquartz veins associated with the deformed zonesrecord important crustal-scale fluid circulations. Mostquartz veins have δ18O values between 10 and 16‰,indicating a crustal origin for the fluids.Microthermometry on fluid inclusions from euhedralquartz indicates that late fluids were mostly aqueouswith very low salinity (0–1.7 wt% eq.) and withhomogenization temperatures ranging between 150and 270 °C. Together with very low δ18O values ofsome euhedral quartz down to -2‰, these featuresargue for a surface origin for these fluids. Calculatedδ18Ofluid values of about -11‰ reflect surface-derivedfluids sourced at high elevation. Independantly, somemylonites from the SASZ have low δ18O silicatevalues compared to their undeformed protolith, whichhas to be related to the influx of surface-derivedwaters. The heat source necessary for this crustalscale downward fluid infiltration followed by upwardmotion was likely provided by the exhumation oflower crustal units in the South Armorican domain.New investigations are currently in progress, notablyδD measurements on micas from mylonites from boththe SASZ and associated detachments to betterconstrain the paleo-altitude reached during theorogeny. Various ore deposits were formed duringthese events, so that this integrated study should helpto draw the metallogenic crustal landscape of thisregio