Role of plagioclase and reaction softening in a metagranite shear zone at mid‐crustal conditions (Gotthard Massif, Swiss Central Alps)

Abstract A lower amphibolite Alpine shear zone from the Fibbia metagranite (Gotthard Massif, Central Alps) has been studied to better understand the parameters controlling strain localization in granitic rocks. The strain gradient on the metre‐scale shows an evolution from a weakly deformed metagranite (Qtz I –Kfs I –Ab I –Bt I  ± Pl II –Zo I –Phg I –Grt) to a fine banded ultramylonite (Qtz II –Kfs II –Ab II –Pl II –Bt II –Phg II  ± Grt–Zo II ). Strain localization is coeval with dynamic recrystallization of the quartzofeldspathic matrix and a modal increase in mica, at the expense of K‐feldspar. The continuous recrystallization of plagioclase during deformation into a very fine‐grained assemblage forming anastomosed ribbons is interpreted as the dominant process in the shear zone initiation and development. The shear zone initiated under closed‐system conditions with the destabilization of metastable Ab I –Zo I porphyroclasts into fine‐grained (20–50 μm sized) Ab II –Pl II aggregates, and with minor crystallization of phengite at the expense of K‐feldspar. The development of the shear zone requires a change in state of the system, which becomes open to externally derived fluids and mass transfer. Indeed, mass balance calculations and thermodynamic modelling show that the ultramylonite is characterized by gains in CaO, FeO and H 2 O. The progressive input of externally derived CaO drives the continuous metamorphic recrystallization of the fine‐grained Ab II –Pl II aggregate into a more Pl II ‐rich and finer aggregate. Input of water favours the crystallization of phengite at the expense of K‐feldspar to form an interconnected network of weak phases. Thus, recrystallization of 50% of the bulk rock volume would induce a decrease of the strength of the rock that might contribute to the development of the shear zone. This study emphasizes the major role of metamorphic reactions and more particularly plagioclase on strain localization process. Plagioclase represents at least one‐third of the bulk rock volume in granitic systems and forms a stress‐supporting framework that controls the rock rheology. Therefore, recrystallization of plagioclase due to changes in P–T conditions and/or bulk composition must be taken into account, together with quartz and K‐feldspar, in order to understand strain localization processes in granites.