The P-T path of the ultra-high pressure Lago Di Cignana and adjoining high-pressure meta-ophiolitic units: insights into the evolution of the subducting Tethyan slab

The Lago di Cignana ultra-high-pressure unit (LCU), which consists of coesite-eclogite facies metabasics and metasediments, preserves the most deeply subducted oceanic rocks worldwide. New constraints on the prograde and early retrograde evolution of this ultra-high pressure unit and adjoining units provide important insights into the evolution of the Piemontese-Ligurian palaeo-subduction zone, active in Paleocene-Eocene times. In the LCU, a first prograde metamorphic assemblage, consisting of omphacite + Ca-amphibole + epidote + rare biotite + ilmenite, formed during burial at estimated P < 1.7 GPa and 350 < T < 480 degrees C. Similar metamorphic conditions of 400 < T < 650 degrees C and 1.0 < P < 1.7 GPa have been estimated for the meta-ophiolitic rocks juxtaposed to the LCU. The prograde assemblage is partially re-equilibrated into the peak assemblage garnet + omphacite + Na-amphibole + lawsonite + coesite + rutile, whose conditions were estimated at 590 < T < 605 degrees C and P > 3.2 GPa. The prograde path was characterized by a gradual decrease in the thermal gradient from similar to 9-10 to similar to 5-6 degrees C km(-1). This variation is interpreted as the evidence of an increase in the rate of subduction of the Piemonte-Ligurian oceanic slab in the Eocene. Accretion of the Piemontese oceanic rocks to the Alpine orogen and thermal relaxation were probably related to the arrival of more buoyant continental crust at the subduction zone. Subsequent deformation of the orogenic wedge is responsible for the present position of the LCU, sandwiched between two tectonic slices of meta-ophiolites, named the Lower and Upper Units, which experienced peak pressures of 2.7-2.8 and < 2.4 GPa respectively.