Different PT paths recorded in a tectonic mélange (Voltri Massif, NW Italy): implications for the exhumation of HP rocks

The Cascine Parasi Mélange (CPM) of the high-pressure, meta-ophiolitic Voltri Massif (Ligurian Western Alps), consists of a foliated chlorite-actinolite greenschist matrix enclosing lenses of metabasites and metasediments. The surrounding units consist of serpentinites not enclosing these metamorphic rocks. The matrix records three sets of folds: (i) Dm1/Dm2 (blueschist to greenschist-facies conditions), which can be correlated to folds in the metasedimentary blocks; (ii) Dm3, which are the most obvious in the field and which partially re-orient the previous structures. The metabasite lenses preserve internal High-Pressure (HP) schistosities unrelated to the matrix foliation. The lenses equilibrated at different peak metamorphic conditions (ranging from eclogite- to blueschist-facies) and some recorded the prograde transition from lawsonite-bearing assemblages to garnet blueschists. Individual lenses display different segments of typical subduction PT paths which apparently converge in the blueschist facies. A late stage greenschist-facies re-equilibration is particularly widespread at the rims of the HP lenses. These structural and metamorphic features suggest that the mélange was active during early phases of the structural evolution of the area, at least through the exhumation and emplacement of the HP blocks into shallower crustal levels at conditions transitional from blueschist- to greenschist-facies; the older history is only preserved inside the blocks.