Geodynamic evolution of a subduction plate interface: constraints from the study of lawsonite-bearing eclogite in the mélange of the Voltri Massif (Ligurian Alps, Italy)

Lawsonite (lws) blueschists and eclogites are expected to be the prevailing lithotypes developing during deep subduction of the oceanic crust and should therefore be quite abundant in exhumed subduction complexes. Nevertheless, lws-bearing eclogites have been described only from few localities in the world. Moreover the occurrence of lws- bearing high-pressure metamorphic rocks, associated with eclogite facies rocks, could give important constraints on the coupling mechanisms between different tectonometamorphic slivers in subduction zones and on the interaction with fluids along the subducting slab. Here we present the structural and petrological study of a lws-bearing eclogitic metagabbro, cropping out in the north-western sector of the metaophiolitic Voltri Massif (Ligurian western Alps, Italy). The Voltri Massif occurs at the eastern end of the western Alps and in the study area is characterised by ocean-, continental- and mantle-derived slices of tectonometamorphic units, involved in the Alpine orogenesis. The metagabbro body is a 20-m sized lens and shows a peculiar alternation of Na-amphibole-rich blue and Na- pyroxene-rich green layers, with a mylonitic texture. Both layers include isoclinal intrafoliar folds, which are the oldest recognizable deformation. The metagabbro was affected by a superimposed folding event, testified by isoclinal folds, deforming all the previous structures and causing the alternating green and blue banding. This body is in contact with glaucophane-bearing metasediment and both are interlayered with serpentine schists. Ophicalcites and serpentinites with a variable degree of carbonation (till their complete transformation into listvenites) also crop out in the area. The thermodynamic modelling suggests that the metagabbro body reached metamorphic peak conditions at relatively low temperature (T = 465-477°C, P = 20.9-24.4 kbar), in a setting where H2O was continuously provided to a cold system. H2O under-saturated conditions, with the occurrence of both lawsonite and epidote, characterised the exhumation path. The occurrence of carbonated serpentinites suggests that an intense and long-lasting circulation of CO2-rich fluids affected the area. Further evidence of this circulation comes from an adjoining outcrop (La Pesca locality) some hundreds meters far; we therefore compared the two areas to constrain the timing of CO2 fluid circulation and its role in the lawsonite preservation. The metamorphic peak conditions recorded by the metagabbro body and the occurrence of variously carbonated serpentinites suggest that this area can be interpreted as a portion of the top of the subducted slab, coupled with a “cool” mantle wedge, where both H2O- and CO2-rich fluids were present; in particular we think that this area was nearby the slab-mantle interface. This sector was thus involved in a low viscosity serpentinite channel, where it was affected by a shear regime that brought these high-pressure rocks back to the surface.