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

摘要: A lower amphibolite Alpine shear zone from the Fibbia metagranite (Gotthard Massif, Central Alps) has been studied to better understand parameters controlling strain localization in granitic rocks. The gradient on metre-scale shows an evolution a weakly deformed (QtzI- KfsI-AbI-BtI ±P l II-ZoI-PhgI-Grt) fine banded ultramylonite (QtzII-KfsII-AbII-PlII-BtII- PhgII ± Grt-ZoII). Strain is coeval with dynamic recrystallization of quartzofeldspathic matrix and modal increase mica, at expense K-feldspar. continuous plagioclase during deformation into very fine-grained assemblage forming anastomosed ribbons interpreted as dominant process initiation development. initiated under closed-system conditions destabilization metastable AbI-ZoI porphyroclasts (20-50 lm sized) AbII-PlII aggregates, minor crystallization phengite development requires change state system, which becomes open externally derived fluids mass transfer. Indeed, balance calculations thermodynamic modelling show that characterized by gains CaO, FeO H2O. progressive input CaO drives metamorphic aggregate more PlII-rich finer aggregate. Input water favours K-feldspar form interconnected network weak phases. Thus, 50% bulk rock volume would induce decrease strength might contribute zone. This study emphasizes major role reactions particularly process. Plagioclase represents least one-third systems forms stress- supporting framework controls rheology. Therefore, due changes P-T ⁄ or composition must be taken account, together quartz K-feldspar, order processes granites.