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