Evolution of the lithospheric mantle during passive rifting: Inferences from the Alpine–Apennine orogenic peridotites

• Alpine-Apennine peridotites record melt percolation through the lithospheric mantle during rifting of Ligurian Tethys
• Melt thermal advection by percolation induced significant rheology softening and weakening of the lithospheric mantle
• Rheology weakening destabilized lithospheric mantle between future continental margins and promoted continental break-up

This paper presents an updated review of recent field/structural and petrologic/geochemical studies on orogenic peridotites from the Alpine–Apennine ophiolites (NW Italy). Results provide determinant constraints to the evolution of the lithospheric mantle during passive rifting of the fossil Ligurian Tethys oceanic basin.The pre-rift, spinel lherzolites precursors, preserved in the mantle section of the Ligurian ophiolites, were resident in the lithosphere along an intermediate geothermal gradient (T about 1000 °C, P compatible with spinel-peridotite facies). Passive rifting by far-field tectonic forces induced whole-lithosphere extension and thinning (the a-magmatic stage). After significant thinning of the lithosphere, the passively upwelling asthenosphere underwent decompression melting along the axial zone of extension. Silica-undersaturated melt fractions infiltrated via diffuse/focused porous-flow through the lithospheric mantle under extension (the magmatic stage) and underwent pyroxenes-dissolving/olivine-crystallizing interaction with the percolated host peridotite.Pyroxenes assimilation and olivine deposition modified the melt compositions into silica-saturated. These derivative liquids migrated to shallower, plagioclase-peridotite facies levels, where they stagnated and impregnated/refertilized the lithospheric mantle. Melt thermal advection by melt infiltration heated to temperatures higher than 1200 °C the lithospheric mantle column above the melting asthenosphere.The syn-rift magmatic and tectonic processes induced significant rheological softening/weakening that destabilized the lithospheric mantle of the Europe–Adria plate along the axial zone of extension. The presence of destabilized lithospheric mantle between the future continental margins played a determinant role in promoting the geodynamic evolution from pre-oceanic rifting to oceanic spreading.The active upwelling of hotter/deeper asthenosphere inside the destabilized axial zone promoted transition to active rifting, enhancing continent break-up. Asthenosphere underwent partial melting and formed aggregated MORB liquids that migrated inside high-porosity dunite channels. The MORB liquids formed olivine-gabbro intrusions and pillowed lava flows (the oceanic crustal rocks).This paper evidences the primary role of mantle destabilization by melt infiltration in the geodynamic evolution of the Ligurian Tethys rifting.

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