Seminar: “Eclogitic breccias from the Monviso metaophiolite: the unexpected witnesses of brittle deformation at 80 km depth” by Michele Locatelli, UPMC

The Monviso meta-ophiolite complex (Northern Italy, Western Alps) represents an almost intact fragment of Tethyan oceanic lithosphere metamorphosed at ~80 km depth (~2.6GPa–550 °C) during the Alpine subduction. Here, inside two major shear zones cutting across this slab fragment, are embedded variously brecciated metagabbro blocks. Textural and petrographic observations, coupled to thermodynamic modelling and geochemical data on breccoa domains suggests that:

• The unfoliated omphacite + garnet ± lawsonite matrices cementing the mylonitic metagabbro clasts crystallized at eclogite-facies condition.
• The successive brecciation steps were promoted by permeability creation (at least transiently) and resulted in the progressive opening of the system to large-scale sustained fluid circulation.
• Eclogite-facies brecciation can control initial strain localization within HP shear zones

Thus, Monviso metagabbros show that pristine brecciation at eclogitic condition may occur and exhaustively record the stepwise development of a major shear zone, via progressive strain localization, permeability creation and increasing scale of fluid circulation.

November 6th 2019 Earth and Environmental Science, room B7