The Ligurian Tethys: Mantle processes and geodynamics

Abstract

Ophiolite massifs (i.e., Lanzo, Voltri, Ligurides, Corsica) of the Alpine–Apennine system represent lithosphere remnants of the Jurassic Ligurian Tethys oceanic basin, which separated the Europe and Adria continental margins. Ophiolitic mantle peridotites record structural and compositional features induced by tectonic and magmatic processes in the sub-continental lithosphere by passive rifting leading to continental breakup and sea-floor spreading in the Jurassic Ligurian Tethys.Field, structural, petrologic and geochemical studies of the lithospheric peridotites provide important tools to unravel the processes that drove extension and rifting of the continental Europe–Adria lithosphere towards breakup and oceanic spreading. Extension in the pre-Triassic Europe–Adria continental domain was a classic example of passive rifting, driven by far field tectonic forces. The early stage of rifting was a-magmatic (a-magmatic passive rifting). The far-field tectonic forces induced lithosphere stretching and thinning by means of melt-free extensional shear zones, that allowed passive upwelling of the asthenosphere until it reached melting conditions on decompression. Silica-undersaturated, isolated single melt increments, strongly depleted in trace elements, were formed by fractional melting. They infiltrated unmixed through the extending mantle lithosphere under spinel-facies conditions by diffuse and focused porous flow (magmatic passive rifting) and induced significant melt/peridotite interactions during upward percolation (i.e., thermo-chemical and mechanical erosion, asthenospherization and rejuvenation of the mantle lithosphere). The percolating liquids became silica-saturated by melt/peridotite interaction (pyroxene dissolution/olivine precipitation) and migrated to shallow lithospheric levels (i.e., plagioclase-peridotite facies conditions). There, increasing heat loss by conduction induced their stagnation, storage and progressive crystallization, that impregnated and refertilized the host peridotite (the hidden, non-extrusive magmatism).Melt thermal advection through the extending lithosphere, above the melting asthenosphere, strongly modified the compositional and rheological characteristics of the percolated mantle lithosphere. A wedge-shaped, softened and weakened zone was formed along the axial mantle lithosphere of the extensional system, between the future continental margins. This axial wedge represented a preferential zone where the underlying hotter and deeper asthenosphere upwelled and “intruded” the extending colder sub-continental mantle lithosphere. Further extension led to continental break-up and splitting, to formation of the extended Europe and Adria margins and to sea-floor exposure of the sub-continental lithospheric mantle.The hot upwelling asthenosphere column was characterized by higher degrees of partial melting on decompression, complete aggregation of the single fractional melt increments, and deepening of the melting sources (i.e., onset of partial melting under garnet-peridotite facies conditions). This partial melting event formed the aggregated MORB liquids (the oceanic magmatism) which migrated from the asthenosphere within high porosity dunite channels through the melt-reacted sub-continental peridotites, without significant interaction with the host peridotites. These aggregated MORBs formed olivine gabbro intrusions in the shallow mantle lithosphere and MOR pillow basalt flows and edifices, above the tectonically denudated and sea-floor exposed, lithospheric mantle peridotites.In this scenario, the divergent forces induced by the active upwelling asthenosphere may compete with far-field tectonic forces and even drive the system causing a change from passive rifting to active rifting and the installation of a ridge-type system.