The Evolution of the Lithospheric Mantle during Mesozoic Rifting in the Ligure-Piedmontese Domain

Giovanni B. Piccardo

Available structural, petrological, geochemical and geochronological data on the Alpine-Apennine ophiolitic peridotites allow to reconstruct from a mantle perspective the composite scenario of the evolution of the lithospheric mantle from the early continental extension to the formation of the Jurassic Ligure-Piemontese oceanic basin.

Continental extension by far field tectonic forces was already active during Triassic times and caused the progressive exhumation of the Europe-Adria sub-continental lithospheric mantle. Extension caused necking of the mantle lithosphere and adiabatic upwelling of the asthenosphere which underwent decompression melting along the axial zone of the extensional system.

Melt fractions from the asthenosphere percolated through, and interacted with the lithospheric mantle under spinel-facies conditions and formed reactive spinel peridotites. The percolating melts maintained their geochemical signature during reactive percolation and modified their composition from olivine- to orthopyroxene-saturated. Under plagioclase-facies conditions the rising melts impregnated by interstitial crystallization the percolated peridotites. The percolating melts were entrapped and stored into the shallow lithospheric mantle. These melts never reached the surface since lava flows with similar compositional characteristics did never erupt at the sea-floor of the basin.

Subsequently, aggregated MORBs migrated within replacive dunite channels and reached shallow levels forming gabbroic intrusions and basaltic extrusions which represented the Late Jurassic oceanic crustal rocks of the basin.

Mantle processes and geodynamics were strictly interconnected during the evolution of the Ligure-Piemontese realm. Mantle processes recorded in the mantle lithosphere are good indicators of the geodynamic evolution of the extensional continental system towards the opening of the oceanic basin.