Episyenites within the Tauern Window metagranitoids: unpredictable?

The core of the Tauern tectonic window (Eastern Alps) consists of pre-Alpine granitoids (~ 295 Ma) metamorphosed and variably deformed during Alpine (~30 Ma) amphibolite facies metamorphism. Episyenites occur as local alteration haloes (as wide as a few meters) surrounding steeply dipping, strike-slip small faults (offset < 1 m) that crosscut the metamorphic deformation structures. Episyenites are well recognizable in the field for their macroscopic porosity which is in the range between 25-35 % volume (meta-granodiorite) and 13% (meta-aplite) and mainly derived from dissolution of mm-sized quartz. Glacier-polished outcrops allow the detailed investigation of the relationships between the episyenites and the structure of exploited faults. Field mapping indicates that episyenites: (i) are spatially linked to precursor faults and overprinted statically pre-existing structures; (ii) are discontinuous along faults; (iii) have a thickness (of as much as a few meters) that does not correlate with either the amount of fault slip or the density of the fracture network; (iv) developed with a similar extent in rocks with original conspicuous variations of quartz grain size and structure. The studied outcrop includes a relatively large volumes of episyenite associated to faults, but episyenites are rare in the Tauern meta-granitoids despite the pervasiveness of faulting. This localized occurrence of episyenite represents a section of a vertical pipe structure exploiting a portion of the fault network. Our study indicates that the location and the extent of episyenite alteration cannot be simply predicted from the geometry and fracturing patterns of the network of precursor cataclastic faults. Quartz dissolution during episyenitization was accompanied and/or followed by: (i) pervasive substitution of oligoclase and biotite/chlorite of the meta-granodiorite by albite and clay-minerals, respectively; and (ii) precipitation of adularia, albite, anatase, calcite, hematite and zeolite within pores. Isotopic data from the calcite filling the episyenite porosity suggest a meteoric source of the fluids ( ?18O (SMOW) ? -2 ?). In contrast, fluids syn-kinematic with previous episodes of fluid-rock interaction during faulting and ductile shearing had a deeper origin ( ?18O (SMOW) ? 8-9 ?). The episyenite mark the transition from diffuse deformation to almost rigid-block behaviour of the Tauern Window tectonic unit, during progressive exhumation and cooling, and the transfer of deformation to localized slip along the extensional detachment of the Brenner Fault.