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 spine] 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.