Giovanni B. Piccardo*, Elisabetta Rampone, Anna Romairone, Marco Scambelluri,
Riccardo Tribuzio and Claudio Beretta Evolution of the Ligurian Tethys: inference from petrology and geochemistry of the Ligurian Ophiolites 147-192

ABSTRACT - 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 flows 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 (Western 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 External 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 floor. 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 decompressional 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 MORB-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-floor, 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 (Ivrea-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 garnet, 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 exposure close to the oceanic floor.

* Corresponding author

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Giovanni B. Andreozzi*, Ferdinando Bosi and Flavia Garramone Synthetic spinels in the (Mg,Fe²⁺,Zn) (Al,Fe³⁺)₂O₄ system - II. Preliminary chemical and structural data of hercynite and magnesioferrite samples 193-204

ABSTRACT - Flux-grown spinel samples belonging to spinel s.s. - hercynite and spinel s.s. - magnesioferrite solid solutions were investigated by electron microprobe and single-crystal X-ray diffraction. Results confirmed the compositional homogeneity of the samples, and structural refinements gave final disagreement factors R ranging from 1.33 to 3.71%. The solid solution between spinel s.s. and hercynite is complete. The progressive substitution of Fe²⁺ for Mg does not affect the M-O distance (1.93 Å), but causes a T-O enlargement (from 1.92 to 1.97 Å) which is responsible for increments in both a₀ (from 8.0855 to 8.1646 Å) and u (from 0.2621 to 0.2642). The solid solution between spinel s.s. and magnesioferrite is incomplete at temperatures below 1000°C. Because of this, most of the samples examined belong to the spinel s.s. side, but the magnesioferrite end-member is also included. As a consequence of increasing Fe³⁺, M-O increases linearly from 1.93 to 2.04 Å, whereas T-O remains almost constant. In this case, the a0 variation (from 8.0855 to 8.3841 Å) is essentially due to the M-O increase, although Fe3+ ´ Al substitution involves both T and M sites.

* Corresponding author

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Piero Comin-Chiaramonti*, Francesco Princivalle, Vicente A.V. Girardi, Celso B. Gomes, Angela Laurora and Alberto Zanetti Mantle xenoliths from Ñemby, Eastern Paraguay: O-Sr-Nd isotopes, trace elements and crystal chemistry of hosted clinopyroxenes 205-230

ABSTRACT - Clinopyroxenes from protogranular spinel-peridotite mantle xenoliths in Eocenic melanephelinite of Ñemby hill, central eastern Paraguay (Asunción-Sapucai-Villarrica graben), are studied for: 1) crystal-chemical relationships between hosted clinopyroxenes and O isotopes; 2) trace element behaviour; 3) Sr-Nd relationships between whole rock and hosted pyroxenes. The mantle xenoliths are distinguished into two main suites, LK (relatively low in K and incompatible elements, IE) and HK (high in K and IE), both ranging from lherzolite to dunite and showing trends of "melt extraction". Clinopyroxenes crystal chemistry shows equilibration pressure over a range of 12-18 kb, in both suites. The hosted clinopyroxenes display extremely variable enrichment/depletion behaviours, mainly in LREE. The enriched components were mostly trapped in clinopyroxene which had previously crystallized from depleted to quasi-chondritic mantle sources. Oxygen isotopes (clinopyroxene-olivine pairs) suggest that equilibration temperatures were higher in the HK suite than in LK suite. On the whole, the isotopic Sr-Nd data indicate that, prior to enrichment, the lithospheric mantle was dominated by a depleted component, isotopically resembling a MORB source or even more depleted. Model ages (Nd-TDM) of clinopyroxenes and host rocks confine the main enrichment, metasomatic, events to the Brasiliano cycle (i.e. 900-460 Ma). In contrast, Rb-Sr isotope systematics may be related with fluids induced by melting episodes occurred at Early Cretaceous times, during the different phases of lithospheric thinning in the area.

* Corresponding author.

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Maurizio Mazzucchelli*, Giorgio Rivalenti, Angela B. de Menezes Leal, Vicente A. V. Girardi, Benjamim B. de Brito Neves and Wilson Teixeira Petrology of metabasaltic dykes in the Diamantina region, Minas Gerais, Brazil 231-254

ABSTRACT - A sequence of mafic dykes is found in the Diamantina region (Minas Gerais, Brazil) at the eastern border of the southern São Francisco Craton. The dykes have been distinguished into four groups on the basis of petrography and geochemical characteristics. Groups 1 to 3 cut the rock sequence older than Meso-Proterozoic, but do not cut the Neo-Proterozoic rocks, thus constraining their emplacement age. Group 1 is constituted by fine grained, sometimes foliated metabasites, which only exceptionally preserve relics of the primary mineral assemblage, and are located only in the lowest stratigraphic units of the Espinhaço Supergroup, suggesting an older emplacement age with respect to the other groups. Group 2 and 3 metabasites are better preserved than those of Group 1. Group 4 represents a single igneous body virtually non metamorphic. Its age is not constrained, but it is similar to Mesozoic dykes occurring further south in this region.
In all groups, composition is basaltic with tholeiitic affinity. Metamorphic element mobility substantially affected only the LILE, whereas igneous variation trends are preserved for all the other elements. The various groups differ for their incompatible trace element composition and ratios. These ratios are more similar to OIB (Ocean Island Basalt) values rather than to any other magma type. Geochemical evidence rules out any important influence of crustal contamination, fractional crystallisation, or variable degrees of melting of a common source material as an explanation for the inter-group variability. It is inferred that the geochemistry of the different groups reflects complementary characteristics and differences of their mantle sources. These latter are attributed either to a metasomatic enrichment of a variably depleted premetasomatic mantle by a component with OIB characteristics, or, alternatively (our preferred interpretation), to the melting of the metasomatised mantle at different depths. The metasomatised sources underwent extensive melting, producing tholeiitic melts retaining OIB-like geochemical characteristics. By analogy with OIB, the metasomatic component may be plume-related. Dyke emplacement may be controlled by passive crustal rifting induced by plume-related mantle diapirs. In such a scenario, the Group 1 samples could be related to the initial rifting phase and plume impingement in the lithosphere, whereas Groups 2 and 3 could represent advanced stages of crustal thinning and melting of the plume head source. The youngest Group 4 dykes presumably represent a limited and local occurrence of the Mesozoic mafic magmatism which affects the area of the Serra do Espinhaçho.

* Corresponding author.

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Piero Atzori, Rosolino Cirrincione*, Paolo Mazzoleni, Antonio Pezzino and Antonella Trombetta A tentative pre-Variscan geodynamic model for the Palaeozoic basement of the Peloritani mountains (Sicily): evidence from meta-igneous products 255-267

ABSTRACT - Although the metavolcanics interbedded in the Palaeozoic basement of the tectonic units constituting the Peloritani belt show various and contrasting geodynamic affinities, these rocks are the only record allowing us to establish the chronology of the extensive and compressive events which developed prior to the Hercynian orogenesis. For this reason, their study becomes very important in reconstructing a pre-Hercynian geodynamic model for the Peloritani area. This paper examines two of the most important metavolcanic successions, outcropping in the southern sector of the Peloritani belt. Geochemical analyses, carried out on these two metavolcanic successions, reveal different magmatic affinities: the first succession, Early Ordovician in age, has an alkaline basaltic composition and within-plate affinity; the second, Late Ordovician in age, consists of metavolcanics with orogenic affinity. Taking into account these different geochemical features, the present paper proposes a preliminary geodynamic model for the Palaeozoic basement of the Peloritani plate, spanning an interval from Late Cambrian- Early Ordovician to Late Ordovician.

* Corresponding author.

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