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 in Mesozoic times. Most of the
peridotites represent former subcontinental mantle which was: (i)
isolated from the convective mantle at different times (from
Proterozoic to Permian); (ii) accreted to the thermal lithosphere,
where it cooled along a conductive geothermal gradient under spinel
peridotite facies conditions.
Our investigations reveal important records of melt/peridotite
interaction and melt impregnation (i.e. formation of plagioclase
peridotites), which were related to asthenosphere/lithosphere
interaction occurred during lithospheric extension leading to rifting
and drifting of the Jurassic Ligurian Tethys. The early
asthenosphere/lithosphere interaction was caused by the reactive
percolation of asthenospheric melts, which induced significant
depletion, refertilization and heating of the lithospheric mantle
peridotites. The plagioclase peridotites of the Alpine-Apennine
ophiolites mostly derive from melt impregnation, whereas part of the
depleted spinet peridotites result from reactive percolation of
depleted MORB-type melts, rather than being solely 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, the
asthenospheric melts reached the surface, both intruding as MORB
gabbroic bodies or extruding as MORB lava flows.
These peridotites record two magmatic cycles: (1) An early magmatic,
non-volcanic stage: the early diffuse porous flow percolation and
impregnation by single melt increments, focused percolation in dunite
channels and intrusion of MORB-type melts; (2) A late magmatic,
volcanic stage: the late intrusion and extrusion of magmas deriving
from aggregated MORB liquids.
The sequence of periods characterized by absence (non-volcanic or
a-magmatic or magma-starved stages) and presence (volcanic or magmatic
stages) of volcanism represents one of the most peculiar feature of
slow and very-slow spreading ridges. Melt stagnation in the oceanic
lithospheric mantle has been proposed as the dominant mechanism of
peridotite impregnation and plagioclase peridotite formation along slow
spreading systems. It could be that amagmatic periods of (ultra-) slow
spreading ridges are characterized by melt stagnation in the thermal
lithosphere, leading to plagioclase peridotite formation. Our results
evidence the great variability in terms of melt composition and regime
of melt percolation during the rift evolution. They provide, moreover,
a mechanism to explain non-volcanic and volcanic stages during the rift
evolution of the Ligurian Tethys and might be equally applicable to
modem slow spreading ridges, which are characterized by variable
magmatic (volcanic) and amagmatic (non-volcanic) stages.