Per. Mineral. (2001), 70, 2, 147-192 http: //go. to/permin An InternationalJournal of PERIOD!CO di MINERALOGIA MINERALOGY, CRYSTALLOGRAPHY, GEOCHEMISTRY, established in 1930 ORE DEPOSITS, PETROLOGY, VOLCANOLOGY and applied topics on Environment, ArchaeometJ)' and Cultural Heritage Evolution of the Ligurian Tethys: inference from petrology and geochemistry of the Ligurian Ophiolites GIOVANNI B. PICCARD01*, ELISABETTA RAMPONE2, ANNA ROMAIRONE1, MARCO SCAMBELLURI1, RICCARDO TRIBUZI03 and CLAUDIO BERETTA1 ' Dipartimento per lo Studio del Territorio e delle sue Risorse. Universita di Genova, Corso Europa 26, I- 16132 Genova, Italy 2 Dipartimento di Scienze Geologiche e Geotecnologie. Universita di Milano- Bicocca, Piazza della Scienza 4, I-20126 Milano, Italy 3 Dipartimento di Scienze della Terra. Universita di Pavia, Via Ferrata 1, I-27 100 Pavia, Italy Submitted, October 2000 -Accepted, March 2001 ABSTRACT. - Ophiolites exposed along the equilibrium with their clinopyroxenes indicates a Western Alpine - Northern Apennine (WA-NA) clear MORB affinity. Geochronological data on NA orogenic chain represent the oceanic lithosphere of ophiolitic gabbros yield ages of intrusion in the the Ligurian Tethys which separated, during Late range 185-160 Ma: Triassic ages of intrusion are Jurassic - Cretaceous times, the Europe and Adria documented for gabbroic rocks from the plates. W A-NA ophiolites show peculiar Montgenevre ophiolites (Western Alps) (212-192 or compositional, structural and stratigraphic 185 Ma). The intrusion ages of the ophiolitic characteristics: 1) mantle peridotites are dominantly gabbroic rocks are significantly older than the Late fertile, clinopyroxene(cpx)-rich lherzolites, while Jurassic (160-150 Ma) opening of the Ligurian depleted, cpx-poor peridotites are subordinate; 2) Tethys and the basaltic extrusion. gabbroic intrusives and basaltic volcanites have a Basaltic volcanites are widespread in the WA-NA MORB affinity; 3) gabbroic rocks were intruded into ophiolites: petrological and geochemical studies mantle peridotites. The Jurassic Ligurian Tethys was have provided clear evidence of their overall floored by a peridotite-gabbro basement, tholeiitic composition and MORB affinity. Zircon subsequently covered by extrusion of discontinuous U/Pb dating on acidic differentiates yield ages in the basaltic flows and by sedimentation of radiolarian range 160-150 Ma for the basaltic extrusion: these cherts, i.e. the oldest oceanic sediments. In the ages are consistent with the palaeontological ages of whole Ligurian Tethys the inception of the oceanic the radiolarian cherts (160-150 Ma). stage, that followed rifting and continental breakup, The External Liguride (EL) mantle peridotites are occurred during Late Jurassic. fertile spinel lherzolites and display a complete The Ligurian ophiolites (Voltri Massif of the recrystallization under spinel-facies conditions, that Ligurian Alps (LA) and Liguride Units of the NA) is interpreted as the stage of annealing are a representative sampling of the diversity of the recrystallization at the conditions of the regional oceanic lithosphere which floored the Jurassic geotherm, after accretion of the EL mantle section to Ligurian Tethys. the conductive lithosphere (i.e. isolation from the In the W A-NA ophiolites the gabbroic rocks convective asthenospheric mantle). Nd model ages occur as km-scale bodies intruded in mantle indicate Proterozoic times for the lithospheric peridotites. REE composition of computed liquids in accretion. The Internal Liguride (IL) mantle ultramafics are depleted peridotites, i.e. refractory residua after low-degree fractional melting on a * Corresponding author, E-mail: [email protected]. it MORB-type asthenospheric mantle source, 148 G.B. PICCARDO, E. RAMPONE, A. ROMAIRONE, M. SCAMBELLURI, R. TR!BUZIO and C. BERETIA producing MORB-type melts. The IL peridotites decompressional partial melting of asthenospheric display a complete equilibrium recrystallization that mantle sources recorded by the IL residual is related to accretion of the IL residual mantle to the peridotites; 3) the post-Variscan Permian MORE­ conductive lithosphere, after partial melting. Nd derived gabbroic bodies, which were intruded into model ages indicate Permian times (275 Ma) for the the extending lithosphere of the Adria margin depletion event. The Erro-Tobbio (ET) mantle (Austroalpine Units of the Western Alps); 4) the peridotites of the Voltri Massif (LA) are spinel Triassic-Jurassic ophiolitic MORB-type gabbros, lherzolites, and represent refractory residua after intruded into the subcontinental mantle, which were variable degrees of incremental partial melting exposed at the sea-floor during Late Jurassic starting from a MORB-type asthenospheric mantle opening of the Ligurian Tethys. source: they show granular to tectonite-mylonite The Ligurian ophiolites represent, therefore, the fabrics, these latter occur in km-scale shear zones spatial association of: where plagioclase- and amphibole-facies - Proterozoic and Permian subcontinental assemblages were developed during deformation. lithospheric mantle peridotites, which are locally (as The Ligurian peridotites show records of a in the EL Units) linked to continental crust tectonic-metamorphic evolution, after the accretion granitoids and granulites; to the conductive subcontinental lithosphere, i.e. 1) - Triassic to Jurassic gabbroic rocks, intruded in development of km-scale shear zones, 2) partial the peridotites; reequilibration at plagioclase-facies and amphibole­ - Late Jurassic MORB volcanites, interlayered facies conditions, and 3) later sea-water interaction with radiolarian cherts. and partial serpentinization, which indicates their This peculiar association cannot be reconciled progressive upwelling from subcontinental with present-day mature oceanic lithosphere, where lithospheric depths to the ocean floor. Plagioclase­ the mantle peridotites and the associated gabbroic­ facies reequilibration developed at 273-313 Ma in basaltic crust are linked by a direct cogenetic the ET peridotites and 165 Ma in the EL peridotites relationship and are almost coeval. In addition, the (Sm-Nd systematics). These data indicate that the large exposure of mantle peridotites to the sea-floor, decompressional evolution of the lithospheric mantle and the long history of extensional upwelling of the Europe-Adria system was already active since recorded by peridotites agree with a geodynamic Late Carboniferous Permian times and continued evolution driven by the passive extension of the till the Late Jurassic ·opening of the Ligurian Tethys. Europe-Adria continental lithosphere. The passive Further evidence of the extensional decompressional extension of the lithosphere is the most suitable evolution of the Europe-Adria lithosphere in the geodynamic process to account for the tectonic Ligurian sector is given by the continental crust denudation at the sea-floor of large sectors of material (the gabbro-derived granulites): their subcontinental mantle, as deduced from analogue gabbroic protoliths were intruded during Lower geophysical modelling for mantle exhumation at Carboniferous - Upper Permian times (about 290 continent-ocean boundary. Structural, metamorphic Ma, Sr-Nd systematics), and underwent and magmatic features recorded by the Austroalpine decompressional retrogression from granulite to (Sesia-Lanzo) and Southalpine (lvrea-Verbano) amphibolite facies between Permian and Middle units (the marginal units of the future Adria plate) Triassic times. suggest that the lithosphere extension was Geological-structural knowledge on the Western asymmetric, with eastward dipping of the Alps indicates that the Europa-Adria system, detachment zones. following Variscan convergence, underwent Late The subduction history of mafic-ultramafic Palaeozoic onset of lithosphere extension through associations of the Western NA and WA ophiolites simple shear mechanisms along deep low-angle was accompanied by prograde reactions, detachment zones, evolving to asymmetric culminating in one main high pressure event. It continental rift and Late Jurassic oceanic opening. caused eclogitization (i.e. development of This may account for the partial melting under metamorphic assemblages characterized by the decompression of the asthenospheric mantle and the association of sodic clinopyroxene and almandine­ gabbroic intrusions. This post-Variscan evolution is rich garnet, in the absence of plagioclase) of mafic evidenced by: 1) the Late Carboniferous to Jurassic rocks and partial recrystallization and dewatering subsolidus decompressional evolution (spinel- to (i.e. formation of metamorphic olivine in plagioclase- to amphibole-facies transition to late equilibrium with antigorite, diopside, Ti-clinohumite oceanic serpentinization) recorded by the and fluids) of ultramafites, previously variably subcontinental lithospheric mantle sections of the hydrated (serpentinized) during the oceanic EL and ET peridotites; 2) the Permian evolution. The high pressure ultramafic rocks still Evolution of the Ligurian Tethys: inference from petrology and geochemistJy of the Ligurian Ophiolites 149 preserve oxygen isotope signatures of the oceanic e interpretata come lo stadia di equilibratura alle settings, indicating that the fluid recycled at the condizioni del locale gradiente geotermico, dopo eclogitic stage was the one incorporated during 1' accrezione del mantello delle EL alla litosfera exposure close to the oceanic floor. conduttiva (cioe l'isolamento dal mantello