Melt-related processes and thermo-chemical erosion in the mantle lithosphere of the Ligurian Tethys: the geodynamic relevance.
Giovanni B. Piccardo
Dipartimento per lo Studio del Territorio e delle sue Risorse, Università di Genova, Italy
The Alpine-Apennine ophiolitic peridotite massifs, which derive from the mantle lithosphere of the Jurassic Ligurian Tethys, comprise km-scale volumes having structural and compositional characteristics which point to interaction with MORB-type melts ascending by porous flow mechanisms. Melt-peridotite interaction strongly modified the pristine spinel peridotites forming “reactive” spinel harzburgites, “impregnated” plagioclase peridotites and “replacive” spinel harzburgites and dunites. The melt-related processes were active during lithosphere extension and mantle exhumation towards shallower crustal levels, and preceded intrusion of aggregate MORB.
Ongoing extension, recorded in the lithospheric mantle by the presence of km-scale extensional shear zones, was followed by the near-adiabatic upwelling and decompressional melting of the underlying asthenosphere. MORB-type fractional melt increments migrated from the top of the molten asthenosphere through the overlying mantle lithosphere.
An important effect of the asthenosphere/lithosphere interaction was a significant heating of the lithospheric mantle which brought it close to asthenospheric temperatures. Accordingly, the lithospheric mantle underwent thermo-chemical erosion i.e. it was asthenospherized.
Distribution and abundance of these modified peridotites within the ophiolitic peridotites of the Ligurian Tethys evidence that a substantial part of the mantle lithosphere was asthenospherized by melt reactive percolation along the axial zone of the future oceanic basin.
In conclusion, during extension of the Europe-Adria lithosphere, leading to the opening of the Jurassic Ligurian Tethys, asthenospherization of the mantle lithosphere should have been a controlling factor in the rapid transition from lithosphere extension by distributed continental deformation to localized oceanic spreading.