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

Abstract

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 two different metre-scale shear zones from this area. Within a few tens of centimetres of each shear zone, the granulite protolith (initially plagioclase + garnet + two pyroxenes) is transformed into an albite + zoisite + garnet + clinopyroxene assemblage. The outer edge of the shear zones consists in a fine-grained heterogeneous assemblage of omphacite + zoisite + kyanite + garnet + phengite ± albite ± quartz. The core of the shear zones is an homogeneous eclogite assemblage of coarser omphacite + kyanite + garnet + zoisite + phengite ± quartz. 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. Field and petrological observations suggest that this transient assemblage is mechanically weaker than both the starting strong granulite and the final eclogite. We here investigate the impact of transient weakening during syn-tectonic metamorphism using a one-dimensional numerical model of a fluid-fluxed, reacting shear zone. We show that transient weakening is required to explain our field and petrological observations. Furthermore, although the widening of the shear zones was primarily controlled by fluid infiltration, we show that strain hardening during the end of the eclogitisation reactions sequence had a noticeable widening effect on the shear zones.