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