Mantle Evolution from Continental Rifting to Ocean Birth: The Record of Alpine-Apennine Ophiolitic Peridotites

Western Alps - Northern Apennines (AA) ophiolites are lithosphere remnants of the Ligurian Tethys oceanic basin (the Ligurian-Piemontese basin) that separated the Europe and Adria continental blocks during Middle Jurassic – Cretaceous times. AA ophiolites stratigraphy evidences that the Ligurian Tethys was floored by a gabbro-peridotite basement discontinuously covered by basaltic lava flows, underlying the oceanic sediments. Present-day oceanic analogues are: (i) the ocean-continent transition zones (e.g. the paired Galicia-Newfoundland margins of North Atlantic); (ii) the slow - ultra-slow spreading ridges (e.g. the Polar Ridges and SWIR), (iii) the oceanic core complexes along MAR. AA ophiolitic peridotites derive from the sub-continental Europe-Adria lithospheric mantle that was exhumed during passive lithosphere extension and was exposed at the sea-floor of the Ligurian Tethys after Middle Jurassic continental break-up. Mantle peridotites show different structural/compositional characteristics depending on their palaeogeographic settings within the ancient basin. Peridotites from more marginal, peri-continental settings consist of pristine fertile spinel lherzolites, showing spinel (garnet) pyroxenite layers, that underwent extensional shearing and metamorphic evolution from spinel- to plagioclase- to amphibole-chlorite-peridotite facies conditions to near sea-floor serpentinization. Peridotites from more internal, oceanic settings show highly variable compositions, ranging from depleted spinel harzburgites and dunites to plagioclase-enriched peridotites. This extreme heterogeneity was formed by porous flow melt percolation, melt-rock interaction and refertilization of the pristine sub-continental lithospheric mantle by MORB-type asthenospheric melts, that occurred during exhumation and preceding shallow level emplacement or sea-floor exposure. Present knowledge evidences that pre-oceanic rifting in the Ligurian-Piemontese domain was accomplished by: 1) tectonic-metamorphic stages, active during Triassic times, dominated by extension and thinning of the continental lithosphere and exhumation of the sub-continental mantle; 2) magmatic stages, starting from Early Jurassic times, characterized by decompression melting of the adiabatically upwelling asthenosphere and melt percolation through the lithospheric mantle along the axial zone of the future oceanic basin.