Coupling Phase Equilibrium Modelling and Apatite-in-Garnet Elastic Barometry to Unravel the P–T Path of the Micaschist Unit From the Zicavo Metamorphic Complex (Corsica, France)

Abstract

To gain insights into the pressure–temperature evolution of a scarcely studied portion of the Variscan basement of Corsica in southern Europe, we combined phase equilibrium modelling, Zr-in-rutile thermometry and apatite-in-garnet elastic barometry. We investigated Grt-St micaschist rocks containing garnet grains of several millimetres in size hosting chloritoid + rutile inclusions. Garnet was found to be wrapped by a foliated matrix made up of muscovite, staurolite, chlorite, biotite and ilmenite. Intersecting compositional isopleths for garnet core, chloritoid and muscovite yielded P–T conditions for garnet core formation of 1.05–1.35 GPa and 430°C–535°C. These conditions were further constrained by Zr-in-rutile thermometry (470°C) and apatite-in-garnet elastic barometry, which intersect at 1.10 GPa and 455°C. We interpreted these conditions to be the beginning of garnet growth coinciding with the D₁ deformation. After decompression and heating, the thermal peak was reached during the main D₂ deformation, coeval with a dextral shearing; these conditions, constrained through garnet rim and matrix mineral isopleths, were found to be at 0.5–0.9 GPa and 540°C–580°C in the staurolite stability field. After (retrograde) D₃ deformation, isothermal decompression associated with granitoid intrusion caused contact metamorphism at shallower crustal levels. Overall, the combination of this P–T path with available ones from southern Corsica and NE Sardinia allows us to hypothesize a possible new geodynamic scenario. We hypothesize that Corsica and Sardinia were on opposite sides of the Lower Palaeozoic Mid-Variscan Ocean before the Variscan Orogeny occurred, after which they welded, leading to the current configuration of the Corsica–Sardinia block.