Abstract

This work investigates the interplay between fluids and metamorphic recrystallisation in the Austroalpine Dent Blanche Nappe (Western Alps, Italy). This nappe consists of the Arolla and Valpelline Units, respectively dominated by metagranitoids and paraschists with Pre-Alpine to Alpine polymetamorphic history. In the Arolla Unit the Alpine tectonitic to mylonitic foliation anastomoses around weakly deformed bodies preserving the pre-Alpine igneous textures. In the Valpelline Unit, the Alpine structures are concentrated within a narrow shear horizon at the contact with the Arolla Unit, which parallels the pre-Alpine foliation. This contrast in the Alpine deformation between the Arolla and the Valpelline Units suggests heterogeneous fluid distribution in the Dent Blanche system during the Alpine orogeny. The relict (meta)igneous Arolla rocks record a post-magmatic, pre-Alpine stage of amphibolite-facies hydration at relatively low pressure. A deeper crustal origin of the Valpelline Unit than the Arolla Unit can be envisaged due to the presence of high temperature and pressure assemblages (clinopyroxene, garnet, plagioclase, amphibole) in the mafic rocks. In Arolla granitoids igneous plagioclase records two hydration events recorded by aggregates of: (i) pre-Alpine amphibolite-facies andesine + epidote, and (ii) Alpine greenschist-facies albite + epidote. The quartz crystals associated with the altered magmatic plagioclase contain two-phase (liquid + vapour) and three-phase (liquid + vapour + chloride daughter crystals) inclusions. Isochores of the salt-saturated inclusions plot in the andesine + epidote stability field, pointing to pre-Alpine plagioclase alteration in response to infiltration of saline aqueous solutions at amphibolite-facies conditions. Most isochores of the liquid + vapour inclusions plot in the albite + epidote stability field, suggesting entrapment during the lower temperature greenschist-facies hydration of plagioclase. Preferential fluids influx in the Arolla Unit during pre-Alpine amphibolite-facies metamorphism enhanced the reaction softening of plagioclase. Alteration of plagioclase played a major role in promoting the deformation of granitoids during Alpine tectonics.

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