Piccardo, GB; Rampone, E; Romairone, A; Scambelluri, M; Tribuzio, R; Beretta, C
Ophiolites exposed along the Western Alpine - Northern Apennine (WA-NA) orogenic chain represent the oceanic lithosphere of the Ligurian Tethys which separated, during Late Jurassic - Cretaceous times, the Europe and Adria plates. WA-NA ophiolites show peculiar compositional, structural and stratigraphic characteristics: 1) mantle peridotites are dominantly fertile, clinopyroxene(cpx)-rich lherzolites, while depleted, cpx-poor peridotites are subordinate; 2) gabbroic intrusives and basaltic volcanites have a MORB affinity; 3) gabbroic rocks were intruded into mantle peridotites. The Jurassic Ligurian Tethys was floored by a peridotite-gabbro basement, subsequently covered by extrusion of discontinuous basaltic fows and by sedimentation of radiolarian cherts, i.e. the oldest oceanic sediments. In the whole Ligurian Tethys the inception of the oceanic stage, that followed rifting and continental breakup, occurred during Late Jurassic. The Ligurian ophiolites (Voltri Massif of the Ligurian Alps (LA) and Liguride Units of the NA) are a representative sampling of the diversity of the oceanic lithosphere which floored the Jurassic Ligurian Tethys. In the WA-NA ophiolites the gabbroic rocks occur as km-scale bodies intruded in mantle peridotites. REE composition of computed liquids in equilibrium with their clinopyroxenes indicates a clear MORB affinity. Geochronological data on NA ophiolitic gabbros yield ages of intrusion in the range 185-160 Ma: Triassic ages of intrusion are documented for gabbroic rocks from the Montgenevre ophiolites (Wester Alps) (212-192 or 185 Ma). The intrusion ages of the ophiolitic gabbroic rocks are significantly older than the Late Jurassic (160-150 Ma) opening of the Ligurian Tethys and the basaltic extrusion. Basaltic volcanites are widespread in the WA-NA ophiolites: petrological and geochemical studies have provided clear evidence of their overall tholeiitic composition and MORB affinity. Zircon U/Pb dating on acidic differentiates yield ages in the range 160-150 Ma for the basaltic extrusion: these ages are consistent with the palaeontological ages of the radiolarian cherts (160-150 Ma). The Exteral Liguride (EL) mantle peridotites are fertile spinel lherzolites and display a complete recrystallization under spinel-facies conditions, that is interpreted as the stage of annealing recrystallization at the conditions of the regional geotherm, after accretion of the EL mantle section to the conductive lithosphere (i.e. isolation from the convective asthenospheric mantle). Nd model ages indicate Proterozoic times for the lithospheric accretion. The Internal Liguride (IL) mantle ultramafics are depleted peridotites, i.e. refractory residua after low-degree fractional melting on a MORB-type asthenospheric mantle source, producing MORB-type melts. The IL peridotites display a complete equilibrium recrystallization that is related to accretion of the IL residual mantle to the conductive lithosphere, after partial melting. Nd model ages indicate Permian times (275 Ma) for the depletion event. The Erro-Tobbio (ET) mantle peridotites of the Voltri Massif (LA) are spinel lherzolites, and represent refractory residua after variable degrees of incremental partial melting starting from a MORB-type asthenospheric mantle source: they show granular to tectonite-mylonite fabrics, these latter occur in km-scale shear zones where plagioclase- and amphibole-facies assemblages were developed during deformation. The Ligurian peridotites show records of a tectonic-metamorphic evolution, after the accretion to the conductive subcontinental lithosphere, i.e. 1) development of km-scale shear zones, 2) partial reequilibration at plagioclase-facies and amphibole-facies conditions, and 3) later sea-water interaction and partial serpentinization, which indicates their progressive upwelling from subcontinental lithospheric depths to the ocean foor. Plagioclase-facies reequilibration developed at 273-313 Ma in the ET peridotites and 165 Ma in the EL peridotites (Sm-Nd systematics). These data indicate that the decompress onal evolution of the lithospheric mantle of the Europe-Adria system was already active since Late Carboniferous - Permian times and continued till the Late Jurassic opening of the Ligurian Tethys. Further evidence of the extensional decompressional evolution of the Europe-Adria lithosphere in the Ligurian sector is given by the continental crust material (the gabbro-derived granulites): their gabbroic protoliths were intruded during Lower Carboniferous - Upper Permian times (about 290 Ma, Sr-Nd systematics), and underwent decompressional retrogression from granulite to amphibolite facies between Permian and Middle Triassic times. Geological-structural knowledge on the Western Alps indicates that the Europa-Adria system, following Variscan convergence, underwent Late Palaeozoic onset of lithosphere extension through simple shear mechanisms along deep low-angle detachment zones, evolving to asymmetric continental rift and Late Jurassic oceanic opening. This may account for the partial melting under decompression of the asthenospheric mantle and the gabbroic intrusions. This post-Variscan evolution is evidenced by: 1) the Late Carboniferous to Jurassic subsolidus decompressional evolution (spinel-to plagioclase-to amphibole-facies transition to late oceanic serpentinization) recorded by the subcontinental lithospheric mantle sections of the EL and ET peridotites; 2) the Permian decompressional partial melting of asthenospheric mantle sources recorded by the IL residual peridotites; 3) the post-Variscan Permian MORE-derived gabbroic bodies, which were intruded into the extending lithosphere of the Adria margin (Austroalpine Units of the Western Alps); 4) the Triassic-Jurassic ophiolitic MORB-type gabbros, intruded into the subcontinental mantle, which were exposed at the sea-floor during Late Jurassic opening of the Ligurian Tethys. The Ligurian ophiolites represent, therefore, the spatial association of: - Proterozoic and Permian subcontinental lithospheric mantle peridotites, which are locally (as in the EL Units) linked to continental crust granitoids and granulites; - Triassic to Jurassic gabbroic rocks, intruded in the peridotites; - Late Jurassic MORB volcanites, interlayered with radiolarian cherts. This peculiar association cannot be reconciled with present-day mature oceanic lithosphere, where the mantle peridotites and the associated gabbroic-basaltic crust are linked by a direct cogenetic relationship and are almost coeval. In addition, the large exposure of mantle peridotites to the sea-foor, and the long history of extensional upwelling recorded by peridotites agree with a geodynamic evolution driven by the passive extension of the Europe-Adria continental lithosphere. The passive extension of the lithosphere is the most suitable geodynamic process to account for the tectonic denudation at the sea-floor of large sectors of subcontinental mantle, as deduced from analogue geophysical modelling for mantle exhumation at continent-ocean boundary. Structural, metamorphic and magmatic features recorded by the Austroalpine (Sesia-Lanzo) and Southalpine (lvrea-Verbano) units (the marginal units of the future Adria plate) suggest that the lithosphere extension was asymmetric , with eastward dipping of the detachment zones. The subduction history of mafic-ultramafic associations of the Western NA and WA ophiolites was accompanied by prograde reactions, culminating in one main high pressure event. It caused eclogitization (i.e. development of metamorphic assemblages characterized by the association of sodic clinopyroxene and almandine-rich garet, in the absence of plagioclase) of mafic rocks and partial recrystallization and dewatering (i.e. formation of metamorphic olivine in equilibrium with antigorite, diopside, Ti-clinohumite and fluids) of ultramafites, previously variably hydrated (serpentinized) during the oceanic evolution. The high pressure ultramafic rocks still preserve oxygen isotope signatures of the oceanic settings, indicating that the fluid recycled at the eclogitic stage was the one incorporated during exp sure close to the oceanic floor.