Prograde and retrograde history of eclogites from the Voltri Massif (Ligurian Alps, Italy): structural and petrological constraints.

The ophiolitic rocks of the Voltri Group (Ligurian Alps) represent exhumed portions of the Liguro-Piemontaise oceanic domain, which were subducted during the formation of the Alpine belt in Tertiary times. Eclogites occur as interlayered lenses within ultramafic sequences of the Beigua Unit. They are primarily constituted by garnet (rim-core: Alm78Grs11Pyr10Sps1-Alm46Grs23Pyr1Sps30), clinopyroxene (Jd19-50), and rutile. These eclogitic lenses retain evidence of the prograde metamorphic path by inclusions in garnet cores. The inclusions consist of quartz, Ca-amphibole, rutile, ilmenite, epidote (XCa=1.97) and paragonite. The retrograde evolution is attested by Na-amphibole overgrowth on early matrix garnet and clinopyroxene. All of that is overprinted by a texturally late amphibolite/greenschist facies assemblage. Quantitative thermobarometry was evaluated using THERMOCALC3.21 and TWQ2.02. Thermobarometry related to inclusion sets in different garnets record different conditions, but all are representative of the amphibolite facies (0.6-0.9 GPa and 400°-550°C). The Fe2+/Mg exchange reaction between clinopyroxene and garnet rims yields temperatures of approximately 500°C for the eclogitic stage. A minimum pressure of 1.3-1.5 GPa at 500°C was estimated by applying the jadeite-in-omphacite geobarometer. The occurrence of glaucophane as a first retrogressive product indicates retrogression through the blueschist facies. Jadeitic mole fraction (Jd36) in clinopyroxene in association with Na-amphibole provides pressure conditions of about 1.25 GPa. The blueschist retrogression is associated to the development of a non-coaxial fabric, with dominant top-to-the-N sense of shear. Zoning of Na-amphiboles, together with Na-Ca-amphibole in association with chlorite and albite, suggests a decompressional path with a final re-equilibration within the amphibolite/greenschist facies. This re-equilibration is associated with a top-to-the-W sense of shear, as deduced from the enveloping mafic bodies. The combined P-T data show a general counter-clockwise path running from the amphibolite facies through the eclogite and blueschist facies to the greenschist facies. This path provides a complete P-T-deformation history for the Voltri eclogites recording events during and after subduction processes. The rocks also imply modalities of exhumation from the deep-seated roots of the Alpine orogen.