Deeply subducted continental fragments: II. Insight from petrochronology in the central Sesia Zone (Western Italian Alps)

Abstract. Continental granulite terrains subducted to mantle depths commonly show only partial and localized eclogitization. The Sesia Zone (NW Italy) is exceptional as eclogitic micaschists predominate in large parts of this terrane, and Alpine high-pressure (HP) assemblages almost completely replaced the Permian granulite protoliths. This study documents when and at what conditions this extensive HP-equilibration took place. Results constrain the main stages of mineral growth and deformation, associated with fluid influx that occured during continental subduction. Our study comprises both main complexes of the Sesia terrane and covers some of the recently recognized tectonic subunits involved in its assembly, hence our data constrain the HP-tectonics that formed the Sesia Zone. In the Internal Complex (IC), pulses of fluid percolated at eclogite facies conditions, between 77 and 55 Ma with the HP-conditions reaching ~ 2 GPa and 600–670 °C. By contrast the External Complex (EC) records a lower pressure peak of ~ 0.8 GPa for 500 °C, at ~ 63 Ma. The juxtaposition of the two complexes occurred during exhumation, at ~ 0.8 GPa and 350 °C; the timing is constrained between 46 and 38 Ma. Mean vertical exhumation velocities are constrained between 0.9 and 5.1 mm/year for the IC, up to its juxtaposition with the EC. Exhumation to the surface occurred before 32 Ma, as constrained by the overlying Biella Volcanic Suite, at a mean vertical velocity between 1.6 and 4 mm/year. These findings constrain the processes responsible for the exhumation and assembly of high pressure continental units.