Abstract
In the Austroalpine Mont Mary nappe (Italian Western Alps) discrete zones of mylonites–ultramylonites developed from coarse‐grained, upper amphibolite facies metapelites of pre‐Alpine age. The syn–mylonitic mineral assemblage is quartz–biotite–muscovite–plagioclase–garnet–sillimanite–ilmenite–graphite, and formed via the model hydration reaction: Grt1+Kfs+H2O=Bt2+Ilm2+Qtz+Ms± Sil .Grain‐size reduction of about three orders of magnitude was accompanied by extensive recrystallization of all minerals except sillimanite, and by compositional changes of garnet and biotite. Deformation took place at temperatures of 510–580 °C under low‐pressure conditions (0.25–0.45 GPa) and corresponds to the latest stages of pre‐Alpine metamorphic evolution. The pre‐Alpine mylonitization conditions were close to the brittle‐ductile transition, as indicated by syn–mylonitic generation of pseudotachylytes and high differential stress inferred from quartz grain‐size piezometry. The brittle‐ductile behaviour at a relatively high temperature, and the absence of annealing textures in quartz aggregates, are suggestive of water‐deficient conditions during mylonitization. These were accomplished through progressive consumption of water by syn–kinematic hydration reaction and by adsorption onto the greatly increased grain boundary area resulting from dynamic recrystallization.
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.