Fluid Pulses During Stepwise Brecciation at Intermediate Subduction Depths (Monviso Eclogites, W. Alps): First Internally Then Externally Sourced

Eclogite-facies breccias from the Monviso metaophiolite complex (N-Italy and W-Alps) represent a unique opportunity to study the triggering mechanisms for transient brittle deformation at eclogite-facies conditions in ductilely deforming subducted oceanic lithosphere. Here the intact foliation of Fe-Ti-rich and Mg-Al-rich metagabbro clasts, cemented by unfoliated eclogitic matrices (paragenesis: omphacite ± garnet ± lawsonite), demonstrates pristine brecciation at eclogite-facies conditions. Successive generations of high-pressure veins and eclogitic matrices reveal multiple brittle rupture events. Pseudosection modeling, textural observations, and geochemical data suggest that minerals of both prograde veins and first brecciation event (M1 matrix) crystallized in presence of fluids buffered by the surrounding metagabbro minerals, while M2 matrix composition suggests an incipient infiltration of external fluids. Trace element composition of the third M3 matrix, associated to its impressive lawsonite pseudomorphs, clearly points to the massive ingression of external, serpentinite-derived fluids. Therefore, these rocks record the progressive increase in the scale of fluid circulation along multiple, stepwise eclogitic brecciation events, from locally released fluids (closed system) to kilometer-scale fluid infiltration. The successive brecciation steps promoted permeability creation (at least transiently), which resulted in progressive opening of the system to large-scale sustained fluid circulation. Eclogite-facies brecciation also controlled the initial stages of strain localization, which highlights the importance, in ductilely deforming low-permeability eclogite-facies rocks, of brittle deformation events for both strain localization initiation and creation of large-scale fluid circulation pathways. Thus, Monviso breccia blocks record the complete, stepwise development of a major shear zone, via progressive strain localization, permeability creation, and increasing scale of fluid circulation.