Séminaire de Francesco GIUNTOLI (Plymouth University, UK)

Unravelling the metamorphic and deformative history of subducted crust

Séminaire de Francesco GIUNTOLI, 2017

The role of deeply subducted crustal domains is of primary importance in the understanding of orogenic processes. How such domains behave in the subduction channel, which metamorphic and deformative processes they experience and how these affect the general architecture of the orogen are some of the fundamental concepts that render subducted crustal domains valuable targets. These processes can be directly studied in orogens that contain exhumed crustal HP-rocks.
The Sesia Zone in the Western Alps is one of such worthy domains. Complex zoned garnet from micaschists preserve traces of two orogenic cycles: cores suggest Permian HT conditions and several Alpine rims show eclogite facies conditions. These garnet textures involve resorption, atoll garnet formation and diffusion in proximity of fractures and are the effect of fluid influxes occurring at eclogite facies conditions in mostly anhydrous lithotypes.
Plagioclase and amphibole microtextures of mylonites from the COSC-1 drill core (Seve Nappe, Scandinavian Caledonides) suggest that fracturing and dissolution- precipitation creep processes are responsible for the development of this fabric. Amphibole and plagioclase thermobarometry constrains the growth of these minerals at epidote amphibolite facies conditions, but no evidence of crystal plasticity is present in the analysed samples, although deformation occurred at pressure and temperature conditions at which plagioclase is expected to deform by dislocation creep. As a result, at middle crustal levels the presence of water at the grain boundaries enhances chemical reaction and is responsible for deformation through dissolution and precipitation processes that decrease considerably the rheology of the rock volume.