Metamorphic re-equilibration, deformation and rheology along a nascent plate boundary: the case study of the Bay of Islands Ophiolitic Complex, Newfoundland

Strain localization and softening in metastable crustal rocks involve complex feedbacks between deformation mechanisms, metamorphic reactions and fluid circulation, as long pointed out for shear zones by previous authors. These feedbacks, however, have rarely been scrutinized and documented precisely at grain-scale. Furthermore, while recent studies have shown that high-grade metamorphic rocks (T°C>550°C) deform through a combination of dislocation creep (DC), diffusion creep and dissolution-precipitation creep (DPC), available creep laws only account for dislocation creep and/or solid-state diffusion processes. Deciphering the role and contribution of DPC to strain accommodation at grain-scale is therefore important to better understand the rheological behavior of rocks, as well as of plate boundaries (for example, deep mechanical coupling in subduction zones likely occurs when/where DC takes over fluid-assisted DPC).

This study investigates the evolution of deformation mechanisms and metamorphic reequilibration of the metamorphic sole of the Bay of Islands ophiolitic complex (BOIC, Newfoundland) and its associated overlying mantle, which jointly preserve evidence for deformation-reaction-fluid feedbacks leading to gradual strain localization at plate boundary scale. Detailed patterns and chronology of deformation-reaction-fluid interactions are constrained by structural, textural, chemical and microstructural data acquired in both the metamorphic sole and the basal mantle. A new method based on EPMA and EBSD maps overlay was also used to track and quantify grain-scale deformation mechanisms, as well as the interplay between grain size reduction, mineral reactions and material transfer. Results show progressive cooling of the mantle associated with increasing deformation (from protomylonitic to ultramylonitic stages) and fluid-related metasomatization of peridotites towards the contact. Fluids are interpreted as coming from the metamorphic sole in which activation of dissolution-precipitation processes are dominant.