Late-Carboniferous infiltration of meteoric water in Variscan shear zones

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Dusséaux, Camille | Gébelin, Aude | Boulvais, Philippe | Dubois, Michel | Ruffet, Gilles | Poujol, Marc | Branquet, Yannick | Barou, Fabrice | Mulch, Andreas

Edité par HAL CCSD

International audience. Crustal-scale shear zones represent sites of significant fluid circulation and are critical inter- faces where metamorphic, magmatic and surface-derived fluids can mix. The Armorican Massif (AM) and the French Massif Central (FMC) are part of the Variscan Belt of Western Europe and are both characterized by ductile shear zones that present spatial and temporal relationships with peraluminous granites emplaced during the late Carboniferous. Combining microstructural, hydrogen isotope (δD), geochronology (U/Pb and Ar/Ar) and fluid inclusion analysis of granitic mylonite samples, we aim at understanding the mechanisms of fluid flow and fluid-rock interac- tion that occurred during high-temperature deformation in these fossil hydrothermal systems. Based on the hydrogen isotope ratios of synkinematic muscovite (δDmuscovite) from mylonitic leucogranite, we determine the source of fluids that infiltrated different types of shear zones in the southern part of the AM. Combined with temperatures of hydrogen isotope exchange de- duced from quartz microstructures, EBSD and Ti-in-Ms thermometry, we calculate the hydrogen isotope ratios of water (δDwater) present in these shear zones during high-temperature defor- mation. Results indicate a variation of the δDwater values from -33%o in the lower part of the Sarzeau detachment footwall to -74%o in the upper part of the Quiberon and Piriac detachments footwall that we interpret to reflect a progressive interaction with meteoric fluids. Infiltration of surface-derived fluids is also supported by fluid inclusions in quartz grains from the same samples that are composed of low to very low salinity water (0.5 to 4.2 wt% eq. NaCl).In agreement with hydrogen isotope results obtained in the southern part of the AM, syntectonic leucogranites from the Millevaches massif (western part of the FMC) yield δDmuscovite values as low as -116%o that indicate an incontestable signature of meteoric fluids. Based on 40Ar/39Ar and U/Pb dating, we propose that penetration of such D-depleted fluids occurred between _~320 and 300 Ma through brittle normal faults developed during upper-crustal extension while the emplacement of high-grade metamorphic rocks in the middle-lower crust sustained fluid convection at depth. Calculated δDwater values provide paleoaltimetry estimates suggesting that this part of the Variscan orogen reached a moderately-high elevation during the late Carboniferous.

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