Transient weakening during the granulite to eclogite transformation within hydrous shear zones (Holsnøy, Norway)

In Holsnøy (Bergen Arcs, Norway), metastable granulite facies anorthosite rocks are partially eclogitised within hydrous shear zones, that have been interpreted as widening over time with fluid influx and strain. We here present a detailed petrological description of metre-scale shear zones from this area. The granulite protolith (originally plagioclase + garnet + two pyroxenes) is transformed into an albite + zoisite + garnet + clinopyroxene assemblage within a few tens of centimetres of the shear zones. The outer edge of the shear zones consists in a fine-grained heterogeneous assemblage of omphacite + zoisite + kyanite + garnet + phengite ± albite ± quartz. An eclogite composed of coarser omphacite + kyanite + garnet + zoisite + phengite quartz forms the core of the shear zones. As the shear zones widened over time, this lateral evolution from the edge to the core of the shear zones reflects the temporal evolution of the granulite from the beginning to the end of the eclogitisation reaction. The outer omphacite + zoisite + kyanite + garnet + phengite ± albite ± quartz assemblage therefore represents a transient eclogite facies assemblage. This transient assemblage appears to be mechanically weaker than both the starting granulite and the final eclogite, based on field and petrological findings. We investigate the impact of transient weakening during syn-tectonic metamorphism using a one-dimensional numerical model of a fluid-fluxed, reacting shear zone. Our numerical model shows that transient weakening is required to explain the field and petrological data. Furthermore, we show that, while fluid infiltration was predominantly responsible for the widening of the shear zones, strain hardening during the end of the eclogitisation reactions sequence had a noticeable widening effect on the shear zones.