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Dipartimento per lo Studio del Territorio e delle sue Risorse, University of Genova, Corso Europa 26, I-16132, Genova, Italy (e-mail: piccardo{at}dipteris.unige.it)
Alpine–Apennine ophiolites derive from the Jurassic Ligurian Tethys oceanic basin formed by lithosphere extension and failure in the pre-Triassic Europe–Adria system. The basin was floored by mantle peridotites and was characterized by along-axis alternation of avolcanic and volcanic segments. Lithosphere extension and thinning caused asthenosphere adiabatic upwelling and decompressional melting. Mid-ocean ridge basalt (MORB)-type melts diffusely percolated through and reacted with the overlying lithospheric peridotites, which were strongly modified, both depleted (harzburgites and dunites) and enriched (plagioclase peridotites), by melt–peridotite interaction and melt refertilization. The stratigraphic–structural features (mantle at the sea floor and alternation of avolcanic and volcanic segments) coupled with petrological features (presence of alkaline melts and strongly heterogeneous, melt-modified peridotites) allow us to interpret the Ligurian Tethys as a Jurassic analogue of modern ultraslow-spreading oceans. The Liguria Mode for the inception of an oceanic basin consists of: (1) the rifting (continental) stage, dominated by extension of continental lithosphere and tectonic exhumation of lithospheric mantle; (2) the drifting (transition) stage, characterized by melt-related processes (i.e. inception of asthenosphere partial melting and MORB melt percolation through the overlying mantle lithosphere); (3) the spreading (oceanic) stage, characterized by failure of the continental crust, sea-floor exposure of mantle peridotites and discontinuous MORB extrusion.
