Fluid infiltration along the South Armorican shear zone
Résumé
Giant quartz veins associated with the South Armorican Shear Zone (SASZ) record huge fluid circulation during the Hercynian period. A regional-scale mapping of veins allows identifying two groups of veins on the basis of their geometric relationship with the SASZ (fig. 1). Veins from the fist group are parallel to the SASZ and associated with zones of intense deformation, veins from the second group developed on the southern edge of the SASZ, in an oblique meridian direction. The former likely record infiltration of fluids along permeable pathways; the latter may represent crustal-scale tension gashes associated with East-West ó3 direction in the regional context (North-South shortening and dextral working of the SASZ along a N120 direction). Microthermometry on fluid inclusions indicate that fluids were mostly aqueous with very low-salinity. Together with very low ä18O values of hydrothermal quertz, down to -2., these features argue for a surface origin. Scarce but significant CO2 fluid inclusions indicate a metamorphic contribution for some fluids, the origin of which probably lying in the exhumed metamorphic HT basement in the southern part of the region. Veins represent SiO2 sinks and we propose through a geochemical characterisation of deformed granites that mylonitic zones developed after syntectonic granites may represent SiO2 sources.