Transient weakening during the granulite to eclogitetransformation within hydrous shear zones (Holsnøy, Norway

International audienceIn Holsnøy (Bergen Arcs, Norway), metastable granulite facies anorthosite rocks are partiallyeclogitised within hydrous shear zones, that have been interpreted as widening over time withfluid influx and strain. We here present a detailed petrological description of metre-scale shearzones from this area. The granulite protolith (originally plagioclase + garnet + two pyroxenes) istransformed into an albite + zoisite + garnet + clinopyroxene assemblage within a few tens ofcentimetres of the shear zones. The outer edge of the shear zones consists in a fine-grainedheterogeneous assemblage of omphacite + zoisite + kyanite + garnet + phengite ± albite ± quartz.An eclogite composed of coarser omphacite + kyanite + garnet + zoisite + phengite quartz formsthe core of the shear zones. As the shear zones widened over time, this lateral evolution from theedge to the core of the shear zones reflects the temporal evolution of the granulite from thebeginning to the end of the eclogitisation reaction. The outer omphacite + zoisite + kyanite +garnet + phengite ± albite ± quartz assemblage therefore represents a transient eclogite faciesassemblage. This transient assemblage appears to be mechanically weaker than both the startinggranulite and the final eclogite, based on field and petrological findings. We investigate the impactof transient weakening during syn-tectonic metamorphism using a one-dimensional numericalmodel of a fluid-fluxed, reacting shear zone. Our numerical model shows that transient weakeningis required to explain the field and petrological data. Furthermore, we show that, while fluidinfiltration was predominantly responsible for the widening of the shear zones, strain hardeningduring the end of the eclogitisation reactions sequence had a noticeable widening effect on theshear zones