Cold subduction zone in northern Calabria (Italy) revealed by lawsonite-clinopyroxene blueschists

Lawsonite blueschists are important markers of cold subduction zones, subjected to intense fluid circulation. This is because lawsonite preservation in exhumed blues- chists and eclogites is typically linked to cold exhumation paths, accompanied by hy- dration. In the Catena Costiera (Calabria, southern Italy), lawsonite-clinopyroxene blueschists of the Diamante-Terranova Unit, affected by ductile shearing and retro- gression, are exposed. Blueschists contain zoned clinopyroxene crystals, showing core-rim compositional variation from diopside to omphacite and hosting primary inclusions of lawsonite and titanite. Thermodynamic modelling of phase equilibria in the NCKFMASHTO system revealed peak metamorphic conditions of 2.0-2.1 GPa and 475-490°C for the Alpine subduction in Calabria. The subsequent post-peak metamorphic evolution mainly proceeded along a decompression and cooling path up to ~1.1 GPa and ~380°C. The final exhumation stages are recorded in the sheared blueschists where a mylonitic to ultramylonitic foliation developed at ~0.7 GPa and 290-315°C. Therefore, the P-T evolution of the Diamante-Terranova blueschists mostly occurred in the stability field of lawsonite, sustained by H2O-saturated condi- tions during the exhumation path. The results of this study indicate that the blueschists underwent peak metamorphic conditions higher than previously thought, reaching a maximum depth of ~70 km under a very cold geothermal gradient (~6.6°C/km), dur- ing the Eocene subduction of the Ligurian Tethys oceanic crust in Calabria.