Quartz and zircon in garnet elastic geobarometry of HP rocks from the Sesia Zone

The Sesia zone is a rifted portion of the Adriatic Margin that subducted to high-pressure conditions during the Alpine Orogeny. It consists of two main complexes: the Internal Complex (IC) and the External Complex (EC). The IC is made up of polymetamorphic micaschists, eclogites, and ortho-gneisses equilibrated at eclogite facies conditions; the EC consists of alpine monometamorphic orthogneiss with minor paragneiss and quartzites metamorphosed at epidote blueschist facies conditions and intensely retrogressed at greenschist facies conditions. The question is therefore to understand if we can trace the Alpine metamorphic history through methods that do not rely only on chemical equilibration.

To tackle this objective, we used elastic geobarometry to derive pressure and temperature (P-T) conditions reached by three micaschists from the IC and one garnet-orthogneiss from the EC. Entrapment P obtained for the quartz inclusions in garnet in the IC range from 1.5-2 GPa at 600-650°C, in agreement with the P-T estimates determined through thermodynamic modelling. Coupled quartz and zircon in garnet geobarometry in the garnet-orthogneiss from the EC also display P-T conditions of 1.8 GPa and 650°C. These estimates disagree with the greenschist facies mineral assemblage of the rock (Ttn + Grt + Phg + Chl) and with the results of thermodynamic modelling (0.6-0.8 GPa and 500°C). The misfit in P-T estimates between elastic geobarometry and thermodynamic modelling might be due to an elastic reset of the quartz and zircon host-inclusion pairs at HP conditions. The use of coupled elastic geobarometry and thermodynamic modelling can help to unravel complex tectonometamorphic histories.