Elastic thermobarometry of ultrahigh-pressure metapelites from the Brossasco-Isasca Unit (Dora-Maira Massif, Western Alps)

We applied zircon-in-garnet (ZiG) and chemical thermobarometry to garnet-bearing metapelites from the ultrahigh-pressure (UHP) Brossasco-Isasca Unit in the Dora-Maira Massif (Western Alps, Italy). The presence of chloritoid and chlorite inclusions in garnet cores and mantles and the presence of quartz aggregates from former coesite inclusions at the garnet rim suggest that most of the garnet volume formed during an early prograde path and around 20% of the volume under UHP conditions. The residual pressure from 32 zircon inclusions entrapped in the garnet rim was compared to phengite–garnet thermometry and previously published thermodynamic modelling to further constrain the UHP metamorphic conditions of these rocks. Nevertheless, calculated ZiG isomekes do not converge with the UHP conditions obtained from chemical thermobarometry. On the other hand, the inclusion residual-strain data suggest that the ZiG system was reset along the retrograde P-T path, at crustal depths. In particular, we suggest that the last thermal event commonly reported to occur during exhumation of the Brossasco-Isasca Unit may have reached higher T than presently thought, about 650–700 °C at 0.5 GPa. We conclude that, while ZiG thermobarometry should be applied with care to constrain UHP metamorphic conditions, it may provide new insights about the exhumation history of these rocks.