Subduction plate margins have an intrinsically poor preservation potential for pre-subduction settings because most rocks sink and disappear into Earth’s mantle. However, a preserved pre-subduction setting has been reported in the Lower Penninic Units (Valaisan Basin) of the Italian Western Alps (Beltrando et al., 2012). Here, we review two main aspects of the Valaisan Basin cropping out in Italy: the nature of the syn-rift detrital sediments and the characteristics of the metabasalts in proximity to the continental crust metagranitoid. New fossil findings are reported, which confirm a Mesozoic age for the Valaisan rifting. Field and petrographic data from metabreccias and pillowed metabasalts near the fossil-rich high-pressure metasediments support the interpretation of the Valaisan Domain in the Breuil valley (Aosta) as a fossil ocean-continent transition zone.

scale of observation. Here we report a case from the Balmuccia peridotite massif, where microstructural observations on the websteritic dykes reveal that they recorded a two-stage deformation history. Lattice Preferred Orientations (LPO) measured in the peridotitic host rock indicate that the oldest deformation was characterized by sinistral simple shear while the youngest by pure shear flow regimes. However, the distribution of folded, stretched and folded-then-stretched dykes at the outcrop scale follows a pattern concordant with large strain accommodated by pure shear only. On the other hand, the only indication of a more complex evolution is provided by the uneven dyke distribution, which is inferred to be inherited from an older deformation event. In order to define the orientation of the dykes prior to the last deformation event, a backward restoration is presented. Integrating micro- and meso-structural observations, the relative orientation of the flow plane for the first deformation event and the strain ellipse for the second has been assessed. This approach resulted in the determination of a two-stage evolution for the Balmuccia peridotite. This local evolution allows discussing wider speculations on the late Paleozoic tectonics. The Balmuccia massif, while being deformed in a flow regime characterized by sinistral simple shear, presumably during a late-Variscan lithospheric-scale extensional event, was intruded by synkinematic Al-augite websterites. As extension continued, in the Early Permian, the massif was deformed by nearly pure shear flow and large horizontal stretching could bring the Balmuccia massif to lower crustal depths.

The direct observation and investigation of rift-related structures at the mesoscale is uncommon. Hence, detailed constraints on the evolution of the main faults and shear zones developed during crustal extension are not always available.The Ivrea-Verbano Zone, in the Italian Southern Alps, samples remnants of the former lower crust of the rifted margins surrounding the Alpine Tethys and therefore provides the opportunity to directly investigate rift-related tectonics. Here, several shear zones have been recognized and interpreted as related to Mesozoic rifting. However, even if there is a general agreement with this interpretation, the precise age of activity of many of those shear zones is not well constrained.In this paper we present a review of the Triassic-Jurassic geochronological and thermochronological data available for two sections of the Ivrea-Verbano Zone, the Strona di Omegna and Ossola valleys, where at least two extensional shear zones are exposed. Ductile deformation occurred under amphibolite-facies conditions and it was alternatively attributed to late Variscan deformation or to Triassic-Jurassic rifting-related tectonics.We discuss the available chronological data and the different interpretations provided for the shear zones considering also new geochronological studies on other lower crustal shear zones exposed in other sectors of the Ivrea-Verbano Zone. This review allows us to strengthen the more recent interpretations indicating that these shear zones are important tectonic structures related to Late Triassic-Jurassic deformation in the lower crust of the Adriatic margin.

The high-pressure (HP) continental Etirol-Levaz slice (ELS) is exposed on the right side of the Valtournenche (Aosta) at the contact between the overlying blueschist-facies Combin Zone and the underlying eclogite-facies Zermatt-Saas Zone. The ELS contains a layered metagabbro showing a complex evolution, which includes the pre-Alpine high-temperature (HT) recrystallization of the igneous mineralogies, and the polyphase Alpine metamorphism, initially at high-pressure (HP) quartz eclogite-facies, and greenschist-facies (GS) retrogression.In this paper, an unusual corundum-bearing ultramafic rocks associated with metagabbros are described. The studied sample is a Mg-chloritite with relict green spinel partly replaced by corundum, Mg-beltrandoite-2N3S (a new mineral of the hogbomite supergroup), Mg-chlorite and dolomite. The rock is crossed by mm- to cm-thick veins composed of coarse-grained corundum + Mg-chlorite + dolomite. P-T phase-diagram projections indicate that the corundum-bearing assemblages formed in the presence of a water-rich fluid (X(CO2) ≤ 0.04), during retrograde decompressional evolution. This stage follows the prograde HP-peak of the associated eclogites. The newly inferred prograde-to-retrograde P-T path suggests that the ELS and the underlying Zermatt-Saas Zone shared a common Alpine metamorphic evolution.The detailed study of the relict minerals preserved in the chloritite indicates its pre-Alpine protolith, a green spinel websterite, and its evolution, characterized by a high-T recrystallization of the original igneous asemblages. During the Alpine orogeny, the spinel websterite experienced metasomatic hydration that converted the original igneous rock into a chloritite.

In the Maritime Alps (NW Italy - SE France), the Middle Triassic-Berriasian platform carbonates of the Provencal Domain are locally affected by an intense hydrothermal dolomitization. This dolomitization resulted from a large-scale hydrothermal circulation related to deep-rooted faults, and is indirect evidence of a significant earliest Cretaceous fault activity in this part of the Alpine Tethys European palaeomargin. New carbonate U-Pb dating and geochemical (stable isotope and noble gases on inclusion-hosted water, 87Sr/86Sr, clumped isotopes) data allowed a better understanding of the timing and mechanisms of the hydrothermal circulation. Hydrothermal fluids probably originated from seawater, which was involved in a deep circulation within the underlying crystalline basement, undergoing heating to more than 200°C and substantial compositional modification by prolonged interaction with basement rocks. Thin cement rims rich in carbonaceous material, locally alternated with hydrothermal dolomite cements, are interpreted as remnants of ephemeral microbial communities that could colonize the upper part of hydrothermal conduits during periods of reduced hot fluid flow and contemporaneous downward seawater infiltration.

The Charlie Gibbs offsetting by ~ 340 km the Mid Atlantic Ridge (MAR) axis at 52°-53° N is one of the main transform systems of the North Atlantic. Located between long mid-ocean ridge segments influenced from the south by the Azores and from the north by the Iceland mantle plumes, this transform system has been active since the early phases of North Atlantic rifting. Object of several surveys in the ’70 and ’80, Charlie Gibbs received great attention for its unique structure characterized by two long-lived right-lateral transform faults linked by a short ~ 40 km-long intra-transform spreading centre (ITR) with parallel fracture zone valleys extending continuously towards the continental margins. In October 2020 expedition S50 of the R/V A.N. Strakhov surveyed an area of 54,552 km2 covering the entire Charlie Gibbs transform system and the adjacent MAR spreading segments. We collected new bathymetric, magnetic and high-resolution single channel seismic data, along with basaltic, gabbroic and mantle rocks from 21 dredges. This work contains preliminary data from cruise S50 and discusses the large-scale architecture of this unique, long-lived transform system.

The Cheshmeh-Bid ophiolitic massif in the Khajeh-Jamali district (Southern Iran) is dominated by harzburgite-dunite tectonites locally intruded by orthopy-roxenite dikes. These latter are composed of dominant coarse orthopyroxene with minor olivine, Cr-spinel, clinopyroxene and amphibole. Estimated equilibrium temperatures for Mg-hornblende and edenitic amphibole reveal a late stage magmatic origin. The Cheshmeh-Bid orthopyroxenites are characterized by very low AlO, CaO, NaO and TiO abundances coupled to relatively high MgO and SiO contents. They display U-shaped REE patterns, selective LILE enrichment and positive Pb and Sr anomalies. The host harzburgites are highly refractory mantle residues resulting from fluid-assisted melting. Field observations and mineral assemblages suggest that the pyroxenites formed by melt injection along fractures within rather cold ambient harzburgites and chromitites at moderate pressure (P > 1 GPa). Based on bulk-rock compositions and mineral chemistry, we infer that the Cheshmeh-Bid orthopyroxenites originated from the intrusion and crystallization of hydrous Si-rich, low-Ca melts with a boninite signature in a supra-subduction environment. Fine-grained neoblastic domains developed in the pyroxenites in response to subsolidus ductile deformation and recrystallization, which were most likely related to the exhumation of the Cheshmeh-Bid ophiolite massif.

Identifying the depositional redox conditions is useful to evaluate the interplay between climate changes, biological feedbacks and de-oxygenation processes in the oceans during the Oceanic Anoxic Events (OAEs). Here, we focus on the about 56 m-thick Albian-Turonian Fontana Valloneto stratigraphic section cropping out in Southern Italy (Potenza, Basilicata), belonging to the “Flysch Rosso” Formation, and containing an equivalent of the Bonarelli Horizon (globally called OAE2 which occurred at~ 94 Ma). Inorganic geochemical compositions and Total Organic Carbon contents obtained from this section are here used to assess depositional environment and redox conditions. The paucity of carbonates within the entire sequence and a gradual decrease in Y, Zr and Al contents along the section suggest a deep depositional environment (below the Calcite Compensation Depth) and an overall decrease in the terrigenous supply. Samples within the Bonarelli Horizon (BH-e) show highly variable TOC contents (~ 0 to ~ 30 wt%) that, mirrored by variations in redox sensitive and nutrient-related elements (e.g. V, Mo and U) and Mn, suggest variation of the seawater primary productivity associated to changes of the local redox conditions between suboxic to strongly euxinic. We infer that during OAE2 the accumulation of the black shales was associated to high Organic Matter (OM) productivity, high biogenic silica production and fine-grained sedimentation (mainly aeolian dust and illite) in a period of “sluggish” oceanic circulation and stagnant conditions. These periods were alternated by moments of more active oceanic circulation and enhanced runoff, leading to the local deposition of ra-diolarites with very low TOC contents. Finally, a comparison with other section from the proto-Atlantic Ocean and the Mesozoic Tethys sustains the idea that the drawdown of redox-sensitive elements (V, Mo and U) was a global process during the deposition of OAE2, providing a link between the environmental changes detected in our section with the global perturbations developed during this oceanic anoxic event.

The transition between the mantle section and the oceanic crust in the Maqsad area (Oman ophiolite) is mainly made of variably impregnated dunites locally associated with chromitite ore bodies. There, the dunitic transition zone (DTZ) developed above a mantle diapir that fed with MORB the former oceanic spreading centre. However, orthopyroxene and amphibole impregnations in dunites from the DTZ are witnesses of a hydrated magmatism that looks restricted to this interface. The main other piece of evidence is the nature of silicate minerals included in chromite grains scattered in dunite (e.g., amphibole, orthopyroxene, mica), which are mostly issued from a hydrated and silica-rich melt or fluid. Here, we report on a study of such inclusions along a section sampled in detail in the Maqsad DTZ. It brings critical information on the processes involved in the fluid-melt-peridotite reaction below oceanic spreading centres, complementary to the one provided by the interstitial silicates forming the matrix of the dunite. We first show that both the nature and the composition of the inclusions are well-correlated to those of the impregnations in the host dunites, then that the chemical evolution along the cross-section for all materials correlate to the presence of faults that developed at an early, syn-magmatic stage. This confirms that the early tectonics in the deep oceanic lithosphere primarily controls the fluid-melt-rock reactions and can condition chemical cycling, including for halogens (Cl, F), in oceanic spreading centre setting.\n

The New Caledonia ophiolite (Peridotite Nappe) hosts one of the largest and best-exposed mantle section worldwide, providing an exceptional insight into upper mantle processes. The Peridotite Nappe is mostly dominated by harzburgites, locally overlain by mafic-ultramafic cumulates, but also includes minor spinel and plagioclase lherzolites, cropping out in the northern part of the island. The New Caledonia harzburgites are low-strain tectonites, showing dominant porphyroclastic textures. The main mantle paragenesis is constituted by olivine (~ 75-85 vol%), orthopyroxene (~ 15-25 vol%) and spinel (< 1 vol%), while primary clinopyroxene is notably absent. An important textural feature of these mantle rocks is represented by the common occurrence of spinel-pyroxene symplectitic aggregates.In this work, we present a petrographical, textural and major element chemical characterization of the spinel-pyroxene symplectitic intergrowths occurring in the New Caledonia harzburgites (Kopeto, Central Massif, and Yate, Massif du Sud). Based on textures, size and relationships with the other mineral phases, these spinel-pyroxene clusters have been divided into two types, named type-A and type-B.Type-A symplectites occur in the Kopeto harzburgites and are composed of spinel-orthopyroxene (± clinopyroxene) intergrowths. In type-A symplectites, symplectitic spinel (Spl2) occurs as abundant vermicular shaped grains, ranging in size from ~ 0.5 to 2 mm. By contrast, spinel of the porphyroclastic assemblage (Spl1) shows smaller size (in the range of few m) and notably lower abundances (< 1%). Type-A symplectites develop exclusively on porphyroclastic olivine, which in turn displays evidence of chemical disequilibrium and corroded outlines. Bulk major element composition reconstructed for type-A symplectites rule out a derivation from a pre-existing garnet phase, as the model garnet compositions do not satisfy garnet stoichiometry, being characterized by Si deficiency. By contrast, major element chemical variations of the symplectitic phases, coupled with the high abundance of Spl2 and olivine resorption, suggest an origin from reactive percolation of opx-saturated hydrous melts or slab-derived fluids in a subduction zone setting.Type-B symplectites are found in Yate sample and consist of spinel-orthopyroxene (± clinopyroxene). They are characterized by smaller size (few hundreds of m, i.e. “micro-symplectites”) and different shapes compared to type-A symplectites, growing as vermicular, “myrmekite-like” intergrowths at the rims of porphyroclastic orthopyroxene. Major element chemical compositions of type-B symplectites are consistent with an origin as “cooling textures”. These structures may derive from unmixing of a high-T, Al-Cr rich, orthopyroxene due to the decreased solubility of the Cr-Al component (CrMgTs) during post-melting lithospheric cooling at T < 900°C.