Meteoric water infiltration in the Variscan crust during the orogenic collapse : O isotope characterization and U-Th-Pb dating along the South Armorican shear zone

National audienceThe South Armorican Shear Zone (SASZ; Jégouzo, 1980) is a major dextral wrench zone active during the Carboniferous. It is located in the Armorican Massif, within the French Variscan orogenic belt. The localization of the deformation along this feature has enhanced the local permeability, which in turn, channelled fluid circulations. Oxygen isotope characterization of mylonitized rocks along the SASZ evidenced meteoric water circulation in two of the sampled quarries (d18OWR down to 1.5). Fluid-rock interactions locally led to noticeable silica leaching, indicating downward motion of meteoric fluids. In the altered samples, zircon and monazite LA-ICP-MS U-Th-Pb analyses have been performed in-situ on separated loose grains and in-context in thin section. In both cases, zircon UPb data show the presence of both common Pb contamination and continuous Pb-loss, and yield late Variscan ages around 300 Ma. In one sample, monazites were also analysed and yield a consistent 207Pb-235U mean age of 299.6 ± 3.6 Ma. Numerous giant quartz veins are located along the SASZ (Lemarchand et al., submitted). Veins parallel to the SASZ mostly derive from metamorphic fluids, whereas veins oblique to the SASZ are characterized by d18O as low as -2, and are interpreted as crustal-scale tension gashes corresponding to highly-permeable zones capable of draining large quantities of fluids returning to the surface. Their sub-meridian orientation is compatible with a N-S shortening direction (Gumiaux et al., 2004). Combined O isotope and U-Th-Pb studies demonstrate that a meteoric water convective cycle existed in this portion of the Variscan orogen during the orogenic collapse. The mylonitized SASZ acted as a drain for the descending meteoric fluids while quartz veins are the witnesses of upgoing paths. Gumiaux, C., et al. (2004). Geodinamica Acta 17, 289-307. Jégouzo, P., (1980). J. of Struct. Geol. 2, 39-47. Lemarchand, J., et al. Submitted to the J. Geol. Soc. London