The core of the Tauern tectonic window (Eastern Alps) consists of pre-Alpine granitoids (∼295 Ma) vari-ably deformed during Alpine (∼30 Ma) amphibolite-facies metamorphism. Episyenites occur as localalteration haloes (as wide as a few meters) surrounding steeply dipping, strike-slip faults, with offsets<1 m, overprinting the metamorphic deformation structures. Episyenites are well recognizable in the fieldby their macroscopic porosity, ranging between 25–35 vol% (meta-granodiorite) and 13% (meta-aplite),mainly derived from dissolution of quartz. Glacier-polished outcrops allow the detailed investigationof the relationships between the episyenites and the structure of the associated faults. Field mappingindicates that episyenites: (i) are spatially linked to pre-existing faults and statically overprinted thesestructures; (ii) are discontinuous along faults; (iii) have a thickness (of as much as a few meters) thatdoes not correlate with either the amount of slip along the pre-existing faults or the spatial density of thefracture network; (iv) developed with a similar extent in rocks with conspicuous variations of the origi-nal quartz grain size and structure. The studied outcrop includes a relatively large volume of episyeniteassociated with faults. However, despite the pervasiveness of faulting, episyenites are rare in the Tauernmeta-granitoids. This localized occurrence of episyenite is inferred to represent a section of a verticalpipe structure exploiting a portion of the fault network. Our study indicates that the location and theextent of episyenite alteration cannot be simply predicted from the geometry and the fracturing pat-terns of the pre-existing cataclastic faults. Quartz dissolution during episyenitization was accompaniedand/or followed by: (i) pervasive substitution of oligoclase and biotite/chlorite of the meta-granodioriteby albite and vermicular chlorite, respectively; and (ii) precipitation of adularia, albite, anatase, calcite,hematite and zeolites within pores. Isotopic data from the calcite filling of the pores suggest a surficialsource of fluids associated with this calcite precipitation (δ18O (SMOW) ≈−2‰ and −3‰). In contrast,fluids syn-kinematic with the older episodes of fluid-rock interaction, during faulting and ductile shear-ing, had a deeper origin (δ18O (SMOW) ≈8–9‰). In the structural history, episyenite marks the transitionfrom diffuse deformation to almost rigid-block behaviour of the Tauern tectonic unit, during progressiveexhumation and cooling (at T <300◦C). This transition reflects the transfer of deformation to localizedslip along the Brenner extensional detachment.
|Titolo:||Episyenites in meta-granitoids of the Tauern Window (Eastern Alps): Unpredictable?|
|Data di pubblicazione:||2016|
|Appare nelle tipologie:||2.1 Articolo su rivista |
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