Ophiolitic peridotites of the Alpine-Apennine system: mantle processes and geodynamic relevance.

Ophiolites exposed in the Alpine-Apennine mountain range represent the oceanic lithosphere of the Ligurian Tethys, a small oceanic basin separating the Europe and Adria plates during Mesozoic time. Most of the peridotites represent former subcontinental mantle which was: (a) isolated from the convective mantle at different times (from Proterozoic to Permian); and (b) accreted to the thermal lithosphere, where it cooled along a conductive geothermal gradient under spinel-peridotite facies conditions. These peridotites record two magmatic cycles: (1) early diffuse porous flow percolation and impregnation by single-melt increments, focused percolation in dunite channels, and intrusion of MORB-type melts; and (2) late intrusion and extrusion of magmas deriving from aggregated MORB liquids. The early lithosphere/asthenosphere interaction by melt percolation induced significant depletion/refertilization and heating of mantle peridotites, leading to the thermochemical erosion of lithospheric mantle. Plagioclase-bearing peridotites of the Alpine-Apennine ophiolites were derived from melt impregnation, whereas part of the depleted spinel peridotites resulted from reactive percolation of depleted melts, rather than being refractory residua after near-fractional melting. The presence of large areas of impregnated peridotites indicates that significant volumes of melts were trapped in the lithospheric mantle; subsequently, asthenospheric melts reached the surface, both intruding as MORB gabbroic bodies or extruding as MORB lava flows. Our results provide a mechanism to explain nonvolcanic and volcanic stages during rift evolution of the Ligurian Tethys, and might be equally applicable to modern slow-spreading ridges, which are characterized by variable magmatic (volcanic) and amagmatic (nonvolcanic) stages.