Fabrice Jouffray

Maria Iole Spalla

Jean Marc Lardeaux

Marco Filippi

Gisella Rebay

Michel Corsini

Davide Zanoni

Michele Zucali

Guido Gosso

Variscan eclogites from the Argentera-Mercantour Massif (External Crystalline Massifs, SW Alps): a dismembered cryptic suture zone

Keywords: Argentera-Mercantour, southwestern Alps, Variscan eclogites, protoliths geochemistry, metamorphic evolution, 40 Ar/ 39 Ar dating

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

High pressure (HP) metamorphic rocks exposed in mountain belts provide first-order constraints for addressing evolution of orogens [START_REF] Ernst | High-and ultrahigh-pressure metamorphism: Past results and future prospects[END_REF] and mafic eclogites are of particular interest because they are records of old subduction zones [START_REF] Miyashiro | Evolution of Metamorphic Belts[END_REF][START_REF] Ernst | Metamorphic zonations on presumably subducted lithospheric plates from Japan, California, and the Alps[END_REF]). In the European Variscan belt, HP metamorphic rocks are widespread [START_REF] Medaris | Czech eclogites: terrane settings and implications for Variscan tectonic evolution of the Bohemian Massif[END_REF][START_REF] Ballèvre | Palaeozoic history of the Armorican Massif: models for the tectonic evolution of the suture zones[END_REF][START_REF] Schulmann | An Andean type Palaeozoic convergence in the Bohemian Massif[END_REF][START_REF] Martínez Catalán | A rootless suture and the loss of the roots of a mountain chain: the Variscan belt of NW Iberia[END_REF][START_REF] Lardeaux | The Moldanubian Zone in the French Massif Central, Vosges/ Schwarzwald and Bohemian Massif revisited: differences and similarities[END_REF] and have been extensively considered to provide information on pre-continental collision plate tectonics framework [START_REF] Matte | The Variscan collage and orogeny (480-290 Ma) and the tectonic definition of the Armorica microplate: a review[END_REF][START_REF] Schulmann | Chronological constraints on the pre-orogenic history, burial and exhumation of deep-seated rocks along the eastern margin of the Variscan Orogen, Bohemian Massif, Czech Republic[END_REF][START_REF] Schulmann | Anatomy of a diffuse cryptic suture zone: An example from the Bohemian Massif, European Variscides[END_REF][START_REF] Franke | The Palaeozoic Variscan oceans revisited[END_REF].

Mafic eclogites are also exposed in Variscan basement slices preserved in the Alpine belt (Fig. 1 and "Online Resource 10"; [START_REF] Spalla | P-T evolutions vs. numerical modelling: a key to unravel the Paleozoic to early-Mesozoic tectonic evolution of the Alpine area[END_REF][START_REF] Spalla | The transition from Variscan collision to continental break-up in the Alps: insights from the comparison between natural data and numerical model predictions[END_REF][START_REF] Roda | What drives Alpine Tethys opening? Clues from the review of geological data and model predictions[END_REF] with references therein). In the External Crystalline Massifs (ECMs) of the western Alps, eclogites that retain protolith geochemical signature and pre-alpine HP mineralogy are rare [START_REF] Raumer | The External Massifs, relics of Variscan basement in the Alps[END_REF][START_REF] Latouche | Evolution précoce du massif de l'Argentera: apport des eclogites et des granulites[END_REF][START_REF] Bogdanoff | Les massifs cristallins externes des alpes occidentales françaises, un fragment de la zone interne varisque[END_REF][START_REF] Ferrando | Metamorphic history of HP mafic granulites from the Gesso-Stura Terrain (Argentera Massif, Western Alps, Italy)[END_REF][START_REF] Rubatto | Carboniferous high-pressure metamorphism of Ordovician protoliths in the Argentera Massif (Italy), Southern European Variscan belt[END_REF]. The Argentera-Mercantour Massif (Fig. 1) is the southernmost of the ECMs, within which the metamorphic evolution of mafic eclogites remains poorly constrained. We document here the discovery of preserved omphacite (jadeite contents up to 42 mol%) within the retrogressed eclogites of this Massif. Our goal is to present and discuss structural, geochemical, mineralogical, petrological and 40 Ar/ 39 Ar geochronological data on these Variscan HP metamorphic rocks.

Geological setting

The ECMs belong to the Helvetic-Dauphinois domain of the Alpine belt [START_REF] Schmid | Tectonic map and overall architecture of the Alpine orogen[END_REF], where Alpine tectonic history has not significantly erased Variscan signatures [START_REF] Raumer | The Palaeozoic metamorphic evolution of the Alpine External Massifs[END_REF], 2009, 2013;Guillot and Menot 2009;[START_REF] Compagnoni | Paleo-European crust of the Italian Western Alps: Geological history of the Argentera Massif and comparison with Mont Blanc-Aiguilles Rouges and Maures-Tanneron Massifs[END_REF][START_REF] Spalla | The transition from Variscan collision to continental break-up in the Alps: insights from the comparison between natural data and numerical model predictions[END_REF][START_REF] Fréville | Structural, metamorphic and geochronological insights on the Variscan evolution of the Alpine basement in the Belledonne Massif (France)[END_REF]. The Argentera-Mercantour Massif, located at the French-Italian border, is a piece of European continental crust shortened in response to late Alpine collision [START_REF] Polino | Tectonic erosion at the Adria margin and accretionary processes for the Cretaceous orogeny of the Alps[END_REF][START_REF] Bogdanoff | Apatite fission track analysis in the Argentera massif: evidence of contrasting denudation rates in the External Crystalline Massifs of the Western Alps[END_REF][START_REF] Schwartz | Diachronous exhumation of HP-LT metamorphic rocks from south-western Alps: evidence from fission track analysis[END_REF][START_REF] Schreiber | 3-D modelling of Alpine Mohos in Southwestern Alps[END_REF].

Following the first investigations of [START_REF]Etudes géologiques sur le Massif de l'Argentera-Mercantour et sur ses enveloppes sédimentaires[END_REF], Malaroda andSchiavinato (1958, 1960) and [START_REF] Malaroda | Carta geologica del Massiccio dell'Argentera alla scala 1:50.000 e note illustrative[END_REF], this massif was systematically divided into two main units [START_REF] Bogdanoff | Les massifs cristallins externes des alpes occidentales françaises, un fragment de la zone interne varisque[END_REF][START_REF] Rubatto | Complex Paleozoic magmatic and metamorphic evolution in the Argentera Massif (Western Alps), resolved with U-Pb dating[END_REF][START_REF] Compagnoni | Paleo-European crust of the Italian Western Alps: Geological history of the Argentera Massif and comparison with Mont Blanc-Aiguilles Rouges and Maures-Tanneron Massifs[END_REF][START_REF] Carosi | Geology of the northwestern portion of the Ferriere-Mollieres Shear Zone, Argentera Massif, Italy[END_REF]) separated by a steeply dipping, NW-SE striking, km-scale shear zone, the « Ferriere-Mollières Shear Zone » (FMSZ, Fig. 2), also known as « Valletta Shear Zone ». The FMSZ, recognized since [START_REF]Etudes géologiques sur le Massif de l'Argentera-Mercantour et sur ses enveloppes sédimentaires[END_REF], is a dextral transpressive shear zone active in Late Carboniferous

Eclogites in the Argentera-Mercantour Massif: Localized or diffuse occurences?

In the Ibero-Armorican arc and French Massif Central, mafic eclogites are localized in specific horizons (Leptyno-Amphibolite Complexes) in association with ultramafic rocks and HP felsic rocks [START_REF] Catalán | Space and time in the tectonic evolution of the northwestern Iberian Massif: implications for the Variscan belt[END_REF][START_REF] Ballèvre | Palaeozoic history of the Armorican Massif: models for the tectonic evolution of the suture zones[END_REF][START_REF] Faure | A review of the pre-Permian geology of the Variscan French Massif Central[END_REF]Lardeaux 2014b). These horizons are localized at the base of the allochtonous nappes thrusted upon low-pressure units [START_REF] Matte | Sutures, thrusts and nappes in the Variscan arc of western Europe. Plate tectonics interpretation[END_REF][START_REF] Ledru | Où sont les nappes dans le Massif central français?[END_REF][START_REF] Ledru | The Velay dome (French Massif Central): melt generation and granite emplacement during orogenic evolution[END_REF][START_REF] Lardeaux | The Variscan French Massif Central-a new addition to the ultra-high pressure metamorphic 'club': exhumation processes and geodynamic consequences[END_REF][START_REF] Berger | New occurrence of UHP eclogites in Limousin (French Massif Central): age, tectonic setting and fluid-rock interactions[END_REF]. In Sardinia or Maures-Tanneron Massif (southern European Variscides) eclogites are recognized as lenses dispersed within migmatites [START_REF] Carmignani | The Hercynianchain in Sardinia (Italy)[END_REF][START_REF] Franceschelli | Metabasites with eclogite facies relics from Variscides in Sardinia, Italy: a review[END_REF][START_REF] Cruciani | P-T evolution of eclogite-facies metabasite from NE Sardinia, Italy: insights into the prograde evolution of Variscan eclogites[END_REF][START_REF] Schneider | Thermal and mechanical evolution of an orogenic wedge during Variscan collision: an example in the Maures-Tanneron Massif (SE France)[END_REF]).

In the Argentera-Mercantour Massif, the presence of rare eclogites was first reported by [START_REF]Etudes géologiques sur le Massif de l'Argentera-Mercantour et sur ses enveloppes sédimentaires[END_REF] and other limited occurences identified later [START_REF] Malaroda | Carta geologica del Massiccio dell'Argentera alla scala 1:50.000 e note illustrative[END_REF][START_REF] Latouche | Evolution précoce du massif de l'Argentera: apport des eclogites et des granulites[END_REF][START_REF] Colombo | Le rocce eclogitiche dei Laghi del Frisson (Argentera sudorientale, Alpi Marittime)[END_REF][START_REF] Rubatto | Complex Paleozoic magmatic and metamorphic evolution in the Argentera Massif (Western Alps), resolved with U-Pb dating[END_REF]. Ages of 459 ± 4 Ma and 486 ± 7 Ma have been obtained by U-Pb dating on zircons for protoliths of HP mafic rocks [START_REF] Rubatto | Complex Paleozoic magmatic and metamorphic evolution in the Argentera Massif (Western Alps), resolved with U-Pb dating[END_REF][START_REF] Rubatto | Carboniferous high-pressure metamorphism of Ordovician protoliths in the Argentera Massif (Italy), Southern European Variscan belt[END_REF]. We discovered a significant number of other occurrences (Fig. 2) with specific characters: First, eclogites occur as poorly deformed lenses, centimetric to metric thick, within surrounding migmatites (Fig. 3). Second, boudins of eclogites can be associated with mafic HP granulites and/or serpentinites. Up to now, an association with HP felsic granulites remains unknown. Third, eclogites are severely retrogressed under amphibolite facies conditions: preserved eclogite facies assemblages are exceptional (Fig. 4). Last, eclogites occur in both western and eastern units in all types of migmatites, i.e. within biotite-sillimanite-garnet banded M1 migmatites as well as within leucocratic cordierite bearing M2 migmatites, but also within migmatitic orthogneisses and migmatitic amphibolites.

Typical field relationships are illustrated by two lithological and structural maps (Figs. 5 and6).

In this massif, mafic granulites have been studied in detail [START_REF] Ferrando | Metamorphic history of HP mafic granulites from the Gesso-Stura Terrain (Argentera Massif, Western Alps, Italy)[END_REF][START_REF] Rubatto | Complex Paleozoic magmatic and metamorphic evolution in the Argentera Massif (Western Alps), resolved with U-Pb dating[END_REF][START_REF] Rubatto | Carboniferous high-pressure metamorphism of Ordovician protoliths in the Argentera Massif (Italy), Southern European Variscan belt[END_REF], but petrological and structural analyses on eclogites remain scarce, probably because deciphering preserved eclogite facies assemblages is rare.

Geochemistry of eclogites

Analytical conditions and results are given in "Online Resource 1".

Major elements

Our samples, displaying mid ocean ridge basalt (N-MORB) average composition [START_REF] Mckenzie | Partial melt distributions from inversion of rare earth element concentrations[END_REF], show a weak depletion in CaO and TiO2 and an enrichment in K2O. On the Al2O3 vs. TiO2 diagram [START_REF] Konzett | High pressure tourmaline formation and fluid activity in Fe-Ti-rich eclogites from the Kreuzeck Mountains, Eastern Alps, Austria[END_REF], they tend to lie in the basalt field and for a few samples in plagioclase-rich cumulates (Fig. 7), suggesting that their protoliths were basalts and gabbros.

In the Total Alkali versus Silica diagram (Online Resource 2) SiO2 content ranges from 40.51 to 52.01 wt% and our sample compositions are mainly in the field of basalts, while in the AFM diagram, they plot mainly in the fields of tholeiitic, and for a restricted number of samples in the calc-alkaline volcanic series (Fig. 7).

However, major elements are mobile during metamorphism [START_REF] Winchester | Geochemical discrimination of different magma series and their differentiation products using immobile elements[END_REF]. Taking into account that surrounding migmatites have undergone dehydration reactions coeval with retrogression of eclogites, the latter could have experienced chemical contamination from outside (see Štípská et al. 2014 for example).

Consequently K2O enrichment from surrounding migmatites is possible. We therefore consider the least mobile elements during alteration and metamorphism (i.e. HFS elements such as Sc, Y, Zr, Hf, Ti, Nb and Ta, or REEs) when using chemical diagrams for protoliths characterization.

Trace elements and REE

The eclogites have typical N-MORB compositions in the Zr-Ti-Nb-Y [START_REF] Pearce | (Ed) Trace Element Geochemistry of Volcanic Rocks: Applications for Massive Sulphide Exploration[END_REF] or in the Y-Nb-Zr [START_REF] Meschede | A method of discriminating between different types of mid-ocean ridge basalts and continental tholeiites with the Nb-Zr-Y diagram[END_REF]) diagrams, except for two samples displaying within-plate chemical signatures and corresponding to a foidite and a picrobasalt (Fig. 8a, see also Online Resource 2).

In discriminant diagrams (Fig. 8b, and Online Resources 3,4 and 5) sample compositions plot in the field of tholeitic basalt and MORB, whereas a few of them show affinity with calk-alkaline to alkaline basalts. This can be interpreted as the consequence of various tectonic contexts of protoliths formation or as the result of chemical contamination during protoliths formation and/or evolution. To face these uncertainties, the best tools are the diagrams proposed by [START_REF] Pearce | Geochemical fingerprinting of oceanic basalts with applications to ophiolite classification and the search for Archean oceanic crust[END_REF] or the chondrite normalized Th vs. Nb (normalization according to [START_REF] Sun | Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes[END_REF] proposed by [START_REF] Saccani | A new method of discriminating different types of post-Archean ophiolitic basalts and their tectonic significance using Th-Nb and Ce-Dy-Yb systematics[END_REF], within which the effects of crustal contamination on fractional crystallization and on the MORB-OIB array can be depicted. In these diagrams, a significant number of our samples plot outside the MORB-OIB array suggesting that these rocks have been contaminated by crustal components (Fig. 8c, see also Online Resource 6).

The Nb/La ratio is regarded as an index of crustal contamination of magmas [START_REF] Thompson | Continental flood basalts arachnids rule OK?[END_REF]). For our eclogites, this ratio ranges between 0.20 and 1.71 with a mean value of 0.7. For comparison, MORB and OIB typically show values of 0.93 and 1.3 respectively [START_REF] Sun | Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes[END_REF], while continental flood basalts display values of 0.14-1.4 [START_REF] Thompson | Continental flood basalts arachnids rule OK?[END_REF]. This indicates that our samples have undergone crustal contamination.

Contamination is also suggested by low and variable Zr/Hf ratios (mean value of 34.11) and Nb/Ta ratios (mean value of 12.47) in comparison with those of N-and E-MORB (respectively 36 and 17 according to [START_REF] Sun | Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes[END_REF]. Moreover, MORB and OIB have uniform value of 25 ± 5 for their Ce/Pb ratio whereas the mean value for continental crust is around 4 [START_REF] Hofmann | Nb and Pb in oceanic basalts: new constrains on mantle evolution[END_REF]). The Ce/Pb value of our samples is in the range of 0.03-15.7 (with a mean value of 5.12) and thus compatible with crustal contamination of MORB basalts.

According to [START_REF] Hofmann | Sampling mantle heterogeneity through oceanic basalts isotopes and trace elements[END_REF], the Nb/U and the Pb/Nd ratios are indicators for crustal contamination. Nb/U ratio of 47 ± 11 is typical for MORBs, while the mean value of continental crust is 8 [START_REF] Rudnick | Nature and composition of the continental crust: A lower crustal perspective[END_REF]. This ratio varies from 2 to 47.8 with a mean value of 12.6 for the studied eclogites. The Pb/Nd ratios are of 0.04, 0.07, 0.08, and 0.63 for N-MORB, E-MORB, OIB, and continental crust respectively [START_REF] Hofmann | Sampling mantle heterogeneity through oceanic basalts isotopes and trace elements[END_REF].

Our eclogites present a Pb/Nd ratio ranging between 0.14 and 1.81 with a mean value of 0.41. This confirms crustal contamination.

The total REE content indicates that most samples show an enrichment, 10-40 times greater than average N-MORB. The REE patterns of the Argentera-Mercantour eclogites are presented in Fig. 9 and the patterns of OIB, N-MORB and E-MORB are shown for comparison.

Primitive mantle normalized trace element pattern shows contrasting trends: a first group of samples show enrichment in LREE with respect to HREE, whereas the second is characterized by slightly depletion, compatible with E-MORB and N-MORB trends, respectively. Some samples are characterized by strong Th depletion, similar to average N-MORB values, whereas a few others have Th positive anomaly. All samples show positive Rb, Pb and U anomalies and trends with Sr and Ba depletions, and most of them Nb, Ta and Mo depletions. These trends are relevant markers for contaminated basaltic sources [START_REF] Donnelly | Origin of enriched ocean ridge basalts and implications for mantle dynamics[END_REF]). Finally, the (La/Sm)N ratio is highly variable (0.42-3.09), indicating various degrees of fractionation (Carlson 2003) .

Petrography of eclogites

The Argentera-Mercantour eclogites experienced severe retrogression and eclogite facies assemblage is rarely preserved. The cores of the boudins consist of fine-grained rocks with reddish millimeter to centimeter-sized garnets but their margins are totally transformed into garnet-free amphibolites.

The peculiar features of these eclogites are the occurrence of clinopyroxene-plagioclase symplectitic domains and of plagioclase-brown/green amphibole symplectitic coronas around garnets (Fig. 10a, b, c, d andf). The development of clinopyroxene-plagioclase symplectites results from omphacite destabilization and is a discontinuous precipitation explained in terms of decreasing pressure with or without temperature increase [START_REF] Boland | Mechanisms of exsolution in omphacites from high temperature, type B, eclogites[END_REF][START_REF] Joanny | La transition omphacite => diopside + plagioclase dans les éclogites du Rouergue (Massif central français): un exemple de précipitation discontinue[END_REF][START_REF] Joanny | The clinopyroxene/plagioclase symplectite in retrograde eclogites: a potential geothermometer[END_REF]. Some of our samples show an evolution of the symplectite texture, with thinner clinopyroxene lamellae developed at the expense of the coarser ones (Fig. 10b).

Garnet occurs as grains up to 5 millimeter in size containing inclusions of quartz, amphibole, rutile, zircon and in some samples small sized brown biotite, epidote, ilmenite, apatite and monazite (Fig. 10e). In one sample (Lac Niré), an inclusion of omphacite, associated with amphibole, was observed within garnet. Amphibole, the most abundant phase, occurs in different textural sites: inclusions within garnets, large green to light-brown amphibole grains in the matrix, brown amphibole rims near the plagioclase-clinopyroxene symplectites, discontinuous droplets in association with plagioclase within coronas around garnets, large poikiloblastic green-brown grains, in association with biotite, fully replacing the symplectites or invading totally the garnets grains, and pale green amphibole filling late veins.

Plagioclase occurs together with clinopyroxene in symplectites replacing omphacite, but also together with the dark-brown amphibole rimming garnet or amphibole 1, close to the symplectites or together with droplets of amphibole around garnet, and in late veins associated with titanite, chlorite, white mica, biotite and pale green amphibole.

In the matrix, rutile grains are usually rimmed by ilmenite. Quartz is common and biotite and K-feldspar are interstitial with respect to garnet, clinopyroxene and amphibole 1 and 2. In the most retrogressed samples, chlorite is developed at the expense of biotite and amphibole as well as in the fractures of garnet.

Mineral chemistry

Mineral analyses were performed on 3 samples (Lac Long, Lac Niré and Pas des Ladres) in the French part of the massif, and 3 samples (upper Valasco Valley) in the Italian part. Analytical conditions and results are presented in "Online Resource 7".

Garnet from French samples is almandine-rich (almandine 49-56 mol%) and spessartine-poor with pyrope, grossular contents ranging from 20 to 32 and 17 to 24 mol% respectively. In the diagram of [START_REF] Coleman | Eclogites and eclogites -their differences and similarities[END_REF], garnets are distributed between the fields of B and C eclogite types (Fig. 11b). The zoning pattern is limited to the mineral rims, with an increase in almandine content correlated to a decrease of pyrope and grossular contents.

In Valasco Valley, garnet is Fe-rich (almandine 61-54 mol%) with low spessartine (3-6 mol% at core, 1-3 mol% at rim), pyrope and grossular contents 8-12 mol% and 21-32 mol%, respectively. They plot in the field of type C eclogites (Fig. 11b) showing a constant pyrope content and grossular enrichment from core to rim. Indeed Mn and Fe 2+ decrease from 0.16 to 0.01 and 1.83 to 1.70 apfu, respectively, whereas Ca slightly increases from 0.75 to 0.96 apfu, from core to rim.

Clinopyroxene was identified in different sites. In the Lac Niré sample, the clinopyroxene inclusion within garnet is omphacite with jadeite content around 32 mol%. In Lac Niré as well as in Pas des Ladres eclogites, clinopyroxene 1 is replaced by clinopyroxene 2 + plagioclase symplectites. In the coarser lamellas, the jadeite component is 42 mol%, while the thinner ones have jadeite contents of 15-27 mol% (Fig. 11a).

In Valasco Valley, clinopyroxene 1 is not preserved, but its composition has been estimated, by mass balance calculations of symplectite domains, at a jadeite content from 24 to 38 mol%. Pyroxene in symplectite (Cpx 2) is always diopside with jadeite content < 5 mol% in all localities.

Plagioclase lamellae from the symplectites are oligoclase (anorthite 20-32), while plagioclase forming coronitic shells around garnet in equilibrium with amphiboles is andesine (Fig. 11c). Plagioclase in veins with actinolite is anorthite 8-13. Amphiboles, from Pas des Ladres and Lac Niré are calcic (Fig. 11d) according to the classification of [START_REF] Leake | Nomenclature of amphiboles: additions and revisions to the International Mineralogical Association's amphibole nomenclature[END_REF] and [START_REF] Hawthorne | Nomenclature of the amphibole supergroup[END_REF]. The amphiboles in inclusions within garnets are magnesio-hornblendes with Mg content of about 0.22 to 0.31 apfu. The amphiboles from coronas around garnet and the amphiboles replacing clinopyroxene of the symplectites, or invading garnets are magnesio-hornblendes with Mg content of about 0.30 to 0.38 apfu.

In Valasco Valley, amphibole 1 and 2 are mostly pargasite. Amphibole 3a is ferro-pargasite, amphibole 3b is mostly ferro-edenite or magnesio-hornblende. Amphibole 4 is actinolite. Amphibole 1 and 2 have Al IV between 1.53-2.04 apfu and Al VI between 0.37 and 0.74 apfu. Ti content in amphibole 1 varies between 0.12 and 0.20 whereas amphibole 2 is Ti-richer (from 0.20 to 0.31 apfu). Na content decreases from 0.55 apfu in amphibole 1 to 0.49 apfu in amphibole 2. Amphibole 3a, in rims around garnet, is particularly Al-rich (Altot from 2.55 to 3.74 apfu, clustering at 3.06 apfu), with XMg < 0.5 and Ti lower than 0.14 apfu. Amphibole 3b, mainly replacing clinopyroxene, has Altot between 0.75 and 2.07 apfu and Ti lower than 0.07 apfu. Amphibole 4 has Altot <0.70 apfu and Ti< 0.04 apfu everywhere.

Metamorphic history and P-T evolution

The Argentera-Mercantour eclogites preserve evidences of a first eclogitic stage documented by the occurrence of almandine-rich garnet and by the preservation of omphacite (jadeite 42 mol%) within symplectites and as inclusion within garnet (jadeite 32 mol%). Mineral inclusions in garnet suggest that the stable mineral assemblage during this stage was garnet + omphacite + calcic amphibole + quartz + rutile + zircon ± epidote ± biotite.

A post-eclogitic stage is evidenced by the development of clinopyroxene (jadeite 27-5 mol%)-plagioclase symplectites at the expense of omphacites and plagioclase-calcic amphibole-ilmenite coronas around garnets.

Rutile is replaced by ilmenite in the matrix. Garnet rims are in equilibrium with amphibole-plagioclase coronas containaing in some samples also diopside. This last assemblage is typical for granulite-facies conditions.

An amphibolite-facies stage is constrained by the replacement of symplectitic clinopyroxenes by amphiboles and the development of poikiloblastic amphiboles, in association with biotites and ilmenite, at the expense of all previous minerals.

A late stage is documented by formation of micro-cracks filled with albite, actinolite, titanite and chlorite and by replacement of biotites and calcic amphiboles by chlorite. The replacement of ilmenite by titanite can also be related to this late stage (greenschist facies conditions).

A first approach for estimation of eclogitic P-T conditions is the use of experimentally determined domains for the observed mineralogy. For samples with chemical compositions of N-MORB, we use the grid determined by [START_REF] Schmidt | Experimentally based water budgets for dehydrating slabs and consequences for arc magma generation[END_REF]. With H2O in excess, the association garnet-omphacite-amphibole-quartz is stable in the range 600-700°C for 1.4-2.4 GPa. This estimation is in agreement with the field of amphibole bearing eclogite facies for basaltic compositions [START_REF] Ernst | High-and ultrahigh-pressure metamorphism: Past results and future prospects[END_REF] Thermobarometry is a second approach and Fe 2+ -Mg exchange between garnet and clinopyroxene is well adapted for eclogites. The calibration established by [START_REF] Krogh-Ravna | The garnet-clinopyroxene Fe 2+ -Mg geothermometer: an updated calibration[END_REF] integrates the effects of Ca and Mn components and requires clinopyroxene with a sodic content in the range 0-0.51 to work. Because of severe retrogression, this thermometer was only applied in the Lac Niré sample using the preserved inclusion of omphacite (Na content of 0.33) within garnet. Application of this geothermometer, for pressure from 1.2 to 1.4 GPa, showed temperatures of 710 -720°C.

The garnet-hornblende Fe-Mg thermometer [START_REF] Graham | A garnet-hornblende geo-thermometer: calibration testing and application to the Pelona schist, southern California[END_REF], applied to Lac Long samples showing inclusions of magnesio-hornblende with rational grain boundaries with garnet and in equilibrium with quartz and rutile, gives a range of 640-690 °C.

However, the reliability of Fe-Mg exchange thermometry is dependent on uncertainties in estimation of the Fe 3+ /Fe tot ratios in minerals [START_REF] Carswell | Metamorphic evolution, mineral chemistry and thermobaromety of schists and orthogneisses hosting ultra-high pressure eclogites in the Dabieshan of central China[END_REF]. As established by [START_REF] Štípská | Constraining the P-T path of a MORB-type eclogite using pseudosections, garnet zoning and garnet-clinopyroxene thermometry: an example from the Bohemian Massif[END_REF], if Fe 3+ content is not taken into account for garnet-clinopyroxene thermometry the calculated temperatures are overestimated, therefore the proposed values (710-720°C) are considered as maximum temperatures.

We used also the Zr-in-rutile thermometry [START_REF] Watson | Crystallization thermometers for zircon and rutile[END_REF], Thomkins et al. 2007) relevant for quartz and zircon-bearing rocks. In the Lac Niré eclogite, the Zr content in rutiles in inclusion within eclogitic garnet is between 221 and 458 ppm. Application of the Zr content in rutile thermometer for pressure of 1.2 and 1.5 GPa showed temperatures of 634-694 and 646-706°C respectively.

The minimal pressure of the eclogitic stage is estimated at 1.4-1.6 GPa using the maximal values of the jadeite content (32-42 mol%) of omphacites [START_REF] Gasparik | Phase equilibria at high pressure of pyroxenes containing monovalent and trivalent ions. In: Prewitt CT (Ed) Pyroxenes. Reviews in mineralogy 7[END_REF][START_REF] Holland | Activities of components in omphacite solid solutions[END_REF]). Ti-content in amphibole thermometer [START_REF] Otten | Metamorphism and deformation during and after cooling of the Artfjallet gabbro[END_REF], using the core compositions of in amphibole 1 grains, gives temperatures of 680-715°C in Valasco Valley eclogites. [START_REF] Ernst | Experimental phase-equilibrium study of Al-and Ti-contents of calcic amphibole in MORB-A semiquantitative thermobarometer[END_REF] calibration suggests T= 700-740°C, using the same data, for which Al-content indicates P = 1.1-1.7 GPa.

In summary, application of various thermo-barometers, to all the studied samples, yields to a P-T range of 640-740°C and 1.1-1.7 GPa (Online Resource 8).

Thermodynamic calculations were performed with the free energy minimization program THERIAK/DOMINO [START_REF] De Capitani | The computation of equilibrium assemblage diagrams with Theriak/Domino software[END_REF]Petrakakis 2010, version 2015). Pseudosections were calculated for two samples from Valasco Valley and Lac Long (Fig. 12). Compositions were chosen starting from the whole rock analysis, considering H2O in excess, and not considering the presence of a melt, as the mafic boudins are enclosed in migmatitic felsic rock, but without evidence of internal melting. For Valasco Valley, we have removed K2O from the composition, as biotite and K-feldspar occur as interstitial grains or filling late veinlets suggesting that boudins were contaminated during country rocks partial melting. For Lac Long sample, because biotite was observed in equilibrium with eclogitic garnet, K2O was considered in the system. Finally, we have evaluated the effect of O and H2O on our pseudosection, as suggested by [START_REF] Guiraud | H2O in metamorphism and unexpected behaviour in the preservation of metamorphic assemblages[END_REF] and [START_REF] Rebay | Calculated phase equilibria for a MORB composition in a P T window, 450-650°C and 18-28 kbar: the stability of eclogite[END_REF] to evaluate their effect on the stability and preservation of assemblages. We used the following mineral a-x relations: plagioclase [START_REF] Holland | Activity-composition relations for phases in petrological calculations: an asymmetric multicomponent formulation[END_REF], garnet [START_REF] White | Progress relating to calculation of partial melting equilibria for metapelites[END_REF], ilmenite [START_REF] White | The effect of TiO2 and Fe2O3 on metapelitic assemblages at greenschist and amphibolite facies conditions: mineral equilibria calculations in the system: K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3[END_REF], amphibole [START_REF] Diener | A new thermodynamic model for clino-and orthoamphiboles in the system Na2O-CaO-FeO-MgO-Al2O3-SiO2-H2O-O[END_REF], clinopyroxene [START_REF] Green | An order-disorder model for omphacitic pyroxenes in the system jadeite-diopside hedenbergite-acmite, with applications to eclogitic rocks[END_REF], orthopyroxene [START_REF] White | The interpretation of reaction textures in Fe-rich metapelitic granulites of the Musgrave Block, central Australia: constraints from mineral equilibria calculations in the system K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3[END_REF], biotite, [START_REF] White | Progress relating to calculation of partial melting equilibria for metapelites[END_REF]) and white mica [START_REF] Coggon | Mixing properties of phengitic micas and revised garnetphengite thermobarometers[END_REF]. The thermodynamic database is ds55 [START_REF] Holland | An internally consistent thermodynamic dataset for phases of petrological interest[END_REF].

Valasco Valley pseudosection is represented in Fig. 12a where the observed assemblage garnet, amphibole 1, omphacite, rutile and quartz, is stable in a field that spans from 625 to 740 °C and 1.2 to 1.75 GPa. This pentavariant field is limited by the appearance of plagioclase towards high temperatures, the disappearence of amphibole towards higher pressure and temperatures, the appearance of a jadeitic pyroxene at higher pressures and lower temperatures, the presence of clinozoisite at lower temperatures and the presence of titanite towards lower temperatures. The compositions of clinopyroxene and amphibole in the field are close to those detected in Valasco Valley eclogites. In addition thermobarometric estimates performed on eclogite facies minerals from Valasco Valley rocks perfectly fit with the HP assemblage stability field. Addition of Fe2O3 would shift epidote stability towards higher temperature, up to close the penta-variant field we are interested in for around 10 mol% of FeOt as F2O3. Conversely, plagioclase-in and amphibole-out lines would be affected only by minor shifting towards higher temperatures and pressures, respectively, of the order of four tens of Celsius degrees and less than 0.1 GPa. A similar configuration would result from taking into account zoisite instead of clinozoisite in a Fe2O3-free system.

In the Lac Long pseudosection (Fig. 12b) the assemblage amphibole, omphacite, garnet, rutile, quartz and biotite is stable in a P-T range of 600 to 780°C and 1.25 to 1.80 GPa, in agreement with thermobarometric estimates.

The proposed field is bounded by plagioclase at high temperature, amphibole at high pressure, phengite at low temperature, and titanite and clinozoisite at low pressure. If we consider zoisite instead of clinozoisite, the lower temperatures field boundary would be shifted to T at around 690°C, still in agreement with other P-T determinations made for this assemblage. Addition of Fe2O3 would shift both epidote stability towards higher temperature, and phengite towards higher temperature and lower pressure, until the closing of the penta-variant field of interest for slightly more than 10 mol% of FeOt as Fe2O3. The effects on the other field boundaries are negligible.

Geochronological constraints on eclogite facies metamorphism

Ages of eclogite facies metamorphism previously obtained in the eclogites from the ECMs, as well as from other segments of the European Variscan belt, are not well constrained [START_REF] Raumer | The Variscan evolution in the External massifs of the Alps and place in their Variscan framework[END_REF]Paquette et al. 2017[START_REF] Lotout | Timing and duration of variscan highpressure metamorphism in the French Massif Central: A multimethod geochronological study from the Najac Massif[END_REF]. The geochemical signatures of the studied eclogites are totally consistent, particularly regarding multi-elements abundance patterns, with those obtained for HP mafic granulites sampled in the Argentera-Mercantour Massif [START_REF] Rubatto | Complex Paleozoic magmatic and metamorphic evolution in the Argentera Massif (Western Alps), resolved with U-Pb dating[END_REF][START_REF] Rubatto | Carboniferous high-pressure metamorphism of Ordovician protoliths in the Argentera Massif (Italy), Southern European Variscan belt[END_REF] suggesting that these HP mafic granulites could be considered as markers of the incipient stage of eclogites retrogression. In such a case the age of eclogite facies metamorphism is probably close to that obtained for HP granulite facies, i.e. 340 ± 4 Ma [START_REF] Rubatto | Carboniferous high-pressure metamorphism of Ordovician protoliths in the Argentera Massif (Italy), Southern European Variscan belt[END_REF]) by U/Pb zircon dating. So far, we have not enough zircons from eclogites to try to get an age of the eclogite facies metamorphism. However, we performed 40 Ar/ 39 Ar dating on amphiboles from these eclogites. We selected a sample from the Lac Long eclogites. This sample (LL09-12J) is closely associated with the (LL13) sample used for the previously presented pseudosection and has a similar geochemistry. In this (LL09-12J) sample, magnesio-hornblendes are in textural equilibrium with plagioclase, ilmenite and garnet. The inferred age corresponds thus to the re-equilibration of eclogites under amphibolite facies conditions.

Analytical conditions and results are available as "Online Resource 9". Amphibole single grain of Lac Long sample (LL09-12J) yields a plateau age (Fig. 13) at 339.7 ± 12 Ma corresponding to 93.77 % Ar released and to four steps. The inverse isochron for the plateau steps gives a concordant age at 339.8 ± 12 Ma (MSWD = 2.17).

The plateau-age at 339.7 ± 12 Ma is considered as the best age estimate for the record of amphibolite facies imprint.

Discussion and conclusions

Origin of eclogites protoliths

Trace elements and REE patterns establish that the Argentera-Mercantour eclogites are MORBs contaminated by continental crust components. Crustal contamination can be the consequence of contamination of basaltic liquids, i.e. eclogite protoliths, prior to HP metamorphism or contamination of eclogites by surrounding migmatites during or after their emplacement.

In the first hypothesis, the multi-element diagrams of incompatible elements display Pb, U and Large Ion Lithophile Element (LILE) enrichments, and also Nb and Ta depletions. These patterns are characterisric of arc lavas and back-arc basalts [START_REF] Hofmann | Sampling mantle heterogeneity through oceanic basalts isotopes and trace elements[END_REF][START_REF] Labanieh | Martinique: a clear case for sediment melting and slab dehydration as a function of distance to the trench[END_REF]. In that case, the geochemical signatures of the studied eclogites are more compatible with those of supra-subduction zone basalts rather than those oceanic domains.

Considering the second hypothesis, during high temperature hydrothermalism as well as during high-temperature crustal melting, significant K, Pb, U and more generally LILEs influxes by crust-derived fluids migration can be envisaged [START_REF] Michard | The REE content of some hydrothermal fluids[END_REF][START_REF] Pattison | Infiltration-driven anatexis in granulite facies metagabbro, Grenville Province, Ontario, Canada[END_REF][START_REF] Klinkhammer | Geochemical implications of rare earth element patterns in hydrothermal fluids from mid-ocean ridges[END_REF][START_REF] Štípská | Intermediate granulite produced by transformation of eclogite at a felsic granulite contact, in Blansky les, Bohemian Massif[END_REF]). In such a case, the eclogite protoliths could be standard MORBs later contaminated during orogenic processes.

Paleo-gradient and context for eclogites formation

The eclogite-facies conditions were quantified by independent methods as comparison between natural and experimentally determined mineral assemblages, well calibrated independent thermobarometers and thermodynamic modeling. The obtained results are consistent with temperatures comprised between 640 and 740°C for pressure of 1.5  0.25 GPa.

To infer their geodynamic significance, we compare these P-T estimates with metamorphic gradients [START_REF] Miyashiro | Evolution of Metamorphic Belts[END_REF][START_REF] Ernst | Metamorphic zonations on presumably subducted lithospheric plates from Japan, California, and the Alps[END_REF][START_REF] Spear | Metamorphic Phase Equilibria and Pressure-Temperature-Time Paths[END_REF]) and with geotherms proposed by [START_REF] Cloos | Lithospheric buoyancy and collisional orogenesis: subduction of oceanic plateaus, continental margins, island arcs, spreading ridges, and seamounts[END_REF] for cold and warm subductions (Fig. 14).

The eclogitic stage fits the conditions predicted by the the different models calculated for warm subduction of oceanic crust and therefore they represent the traces of a suture zone.

The Argentera-Mercantour Massif: record of a dismembered suture zone

The Argentera-Mercantour eclogites are witnesses of an oceanic crust formed in a back-arc basin or in an oceanic domain. Whatever is the selected pre-orogenic configuration, these rocks are the exhumed remnants of subducted oceanic crust, therefore localizing a paleo-suture zone transposed after continental collision in a migmatitic complex. However this paleo-suture zone is totally dismembered and eclogites are mixed with various types of migmatites. The original suture is totally overprinted by mass transfer of partially molten continental crust, giving rise to a "diffuse cryptic suture zone" in the sense of [START_REF] Schulmann | Anatomy of a diffuse cryptic suture zone: An example from the Bohemian Massif, European Variscides[END_REF].

The mechanism leading the extensive dismembering of the suture zone remains unclear and different models can be considered. We can envisage dismembering, during continental collision, of previously stacked ophiolitic unit in response to vertical transfer (i.e. extrusion) of hot partially molten orogenic crust during ongoing shortening.

This results in large-scale gravity driven migmatitic dome-like structures favouring mixing of eclogites and migmatites as proposed in Variscan Bohemian Massif [START_REF] Štípská | Vertical extrusion and middle crustal spreading of omphacite granulite: a model of syn-convergent exhumation (Bohemian Massif, Czech Republic)[END_REF][START_REF] Schulmann | Chronological constraints on the pre-orogenic history, burial and exhumation of deep-seated rocks along the eastern margin of the Variscan Orogen, Bohemian Massif, Czech Republic[END_REF][START_REF] Schulmann | An Andean type Palaeozoic convergence in the Bohemian Massif[END_REF][START_REF] Schulmann | Anatomy of a diffuse cryptic suture zone: An example from the Bohemian Massif, European Variscides[END_REF] or in the Maures-Tanneron Massif [START_REF] Schneider | Thermal and mechanical evolution of an orogenic wedge during Variscan collision: an example in the Maures-Tanneron Massif (SE France)[END_REF][START_REF] Gerbault | Crustal exhumation during ongoing compression in the Variscan Maures-Tanneron Massif, France-Geological and thermo-mechanical aspects[END_REF]. We can also consider dismembering of subducted crust during oceanic subduction and/or incipient collision leading the formation of a "hot subduction channel" composed of subducted, partially molten, upper crustal rocks and lithospheric hydrated mantle rocks as proposed by et al. ( 2008). The result is a " hot subduction mélange", localized at the plates interface, producing fast exhumation of mafic HP rocks within molten meta-sediments.

The Argentera-Mercantour eclogites in the southern European Variscides framework

Considering field relationships between eclogites and surrounding migmatites, their geochemistry, and petrological evolutions, the Argentera-Mercantour eclogites show remarkable similarities with Sardinian eclogites [START_REF] Giacomini | Petrology and geochronology of metabasites with eclogite facies relics from NE Sardinia: constraints for the Palaeozoic evolution of Southern Europe[END_REF][START_REF] Franceschelli | Metabasites with eclogite facies relics from Variscides in Sardinia, Italy: a review[END_REF][START_REF] Cruciani | Re-equilibration history and P-T path of eclogites from Variscan Sardinia, Italy: A case study from the medium-grade metamorphic complex[END_REF][START_REF] Cruciani | Nature and age of pre-Variscan eclogite protoliths from the Low-to Medium-Grade Metamorphic Complex of north-central Sardinia (Italy) and comparisons with coeval Sardinian eclogites in the northern Gondwana context[END_REF]Fig. 14), with eclogites from the Maures-Tanneron Massif [START_REF] Bard | Texture et minéralogie d'une éclogite à disthène-saphirine-hypersténe-quartz en inclusion dans les gneiss migmatitites des Cavaliéres, massif de Ste Maxime (Maures, Var, France)[END_REF][START_REF] Briand | Felsic (A-Type)-Basic (Plume-Induced) Early Palaeozoic bimodal magmatism in the Maures massif (southeastern France)[END_REF][START_REF] Corsini | Late evolution of the southern European Variscan belt: exhumation of the lower crust in a context of oblique convergence[END_REF][START_REF] Schneider | Thermal and mechanical evolution of an orogenic wedge during Variscan collision: an example in the Maures-Tanneron Massif (SE France)[END_REF]) and finally with Corsican eclogites [START_REF] Palagi | Identification d'un complexe leptynoamphibolique au sein des "gneiss de Belgodère" (Corse occidentale)[END_REF][START_REF] Lardeaux | Corsica and Sardinia in the Variscan chain[END_REF][START_REF] Faure | Variscan orogeny in Corsica: new structural and geochronological insights, and its place in the Variscan geodynamic framework[END_REF]).

The European Variscan belt is the result of tectonic accretion of Gondwana derived micro-plates colliding with Laurussia during the Devonian [START_REF] Torsvik | A review of Paleozoic paleomagnetic data from Europe and their paleogeographic implications[END_REF][START_REF] Torsvik | Continental break-up and collision in the Neoproterozoic and Palaeozoic-a tale of Baltica and Laurentia[END_REF][START_REF] Tait | Geodynamic evolution of the European Variscan fold belt: palaeomagnetic and geological constraints[END_REF][START_REF] Matte | The Variscan collage and orogeny (480-290 Ma) and the tectonic definition of the Armorica microplate: a review[END_REF][START_REF] Raumer | Organization of pre-Variscan basement areas at the north-Gondwana margin[END_REF][START_REF] Cocks | The Palaeozoic geography of Laurentia and western Laurussia: a stable craton with mobile margins[END_REF][START_REF] Stampfli | The formation of Pangea[END_REF]. The continuity of this process led to the formation of a huge collision belt and to the amalgamation of the Pangea supercontinent in the late Paleozoic [START_REF] Franke | Variscan plate tectonics in Central Europe -current ideas and open questions[END_REF][START_REF] Franke | The mid-European segment of the Variscides: tectono-stratigraphic units, terrane boundaries and plate tectonic evolution[END_REF][START_REF] Matte | Terrane boundaries in the Bohemian Massif: result of largescale Variscan shearing[END_REF][START_REF] Mckerrow | The Late Palaeozoic relations between Gondwana and Laurussia[END_REF]Domeir and Torsvik 2014). Among the various micro-plates involved in this evolution, paleomagnetic constraints combined with geochronological data and regional geologic investigations have identified, in the southern Variscides, the so-called Maures-Esterel-Corsica-Sardinia ("MECS") block (see [START_REF] Edel | Early Permian 90° clockwise rotation of the Maures-Estérel-Corsica-Sardinia block confirmed by new palaeomagnetic data and followed by a Triassic 60° clockwise rotation[END_REF][START_REF] Edel | Late Palaeozoic palaeomagnetic and tectonic constraints for amalgamation of Pangea supercontinent in the European Variscan belt[END_REF]. If we consider that the occurrences and the petrochemical characteristics of eclogites are critical for deciphering the main suture zones in orogenic systems, the similarities between HP metamorphic rocks recognized in Argentera-Mercantour, Sardinia, Maures-Tanneron and Corsica suggest that the southernmost ECMs possibly linked with the MECS block. Such a link is also consistent with various paleo-plates tectonic reconstructions proposed by [START_REF] Stampfli | The formation of Pangea[END_REF]. In the framework of Gondwana derived micro-plates these authors distinguish, during upper Devonian times, northern blocks (Iberian Massif, Armorican Massif, French Massif Central, Bohemian Massif), located close to the Rheic (or Rheno-Hercynian) ocean and southern blocks (Sardinia, Corsica, ECMs,…) located close to the Paleo-Tethys. In such a scheme, the ages of eclogite facies metamorphism are different with a decrease of geological ages of HP metamorphism from northern to southern blocks. Geochronological data are therefore first-order constraints to discuss these models.

In the MECS block, late Cambrian-Ordovician to lower Silurian protolith ages of HP rocks are constrained by U/Pb zircon dating [START_REF] Innocent | U-Pb zircon age of 548 Ma for the leptynites (high-grade felsic rocks) of the central part of the Maures Massif. Geodynamic significance of the so-called leptynoamphibolitic complexes of the Variscan belt of western Europe[END_REF][START_REF] Palmeri | SHRIMP dating of zircons in eclogite from the Variscan basement in north-eastern Sardinia (Italy)[END_REF][START_REF] Giacomini | Petrology and geochronology of metabasites with eclogite facies relics from NE Sardinia: constraints for the Palaeozoic evolution of Southern Europe[END_REF][START_REF] Giacomini | Exhumation of a Variscan orogenic complex: insights from the composite granulitic-amphibolitic metamorphic basement of South-East Corsica (France)[END_REF][START_REF] Rossi | A restored section of the ''southern Variscan realm'' across the Corsica-Sardinia microcontinent[END_REF][START_REF] Schneider | Thermal and mechanical evolution of an orogenic wedge during Variscan collision: an example in the Maures-Tanneron Massif (SE France)[END_REF][START_REF] Cruciani | Nature and age of pre-Variscan eclogite protoliths from the Low-to Medium-Grade Metamorphic Complex of north-central Sardinia (Italy) and comparisons with coeval Sardinian eclogites in the northern Gondwana context[END_REF]. This time span is coherent with ages of 459 ± 4 Ma and 486 ± 7 Ma obtained in the Argentera-Mercantour Massif [START_REF] Rubatto | Complex Paleozoic magmatic and metamorphic evolution in the Argentera Massif (Western Alps), resolved with U-Pb dating[END_REF][START_REF] Rubatto | Carboniferous high-pressure metamorphism of Ordovician protoliths in the Argentera Massif (Italy), Southern European Variscan belt[END_REF] but also with as all protoliths ages obtained in the ECMs [START_REF] Raumer | Organization of pre-Variscan basement areas at the north-Gondwana margin[END_REF][START_REF] Schultz | Neoproterozoic to Early-Palaeozoic magmatic evolution in the Gondwana-derived Austroalpine basement to the south of the Tauern Window (Eastern Alps)[END_REF]). In the MECS block, the timing of crustal anatexis is constrained in the range 345-310 Ma (Di Vicenzo et al. 2004;[START_REF] Giacomini | Petrology and geochronology of metabasites with eclogite facies relics from NE Sardinia: constraints for the Palaeozoic evolution of Southern Europe[END_REF][START_REF] Giacomini | Exhumation of a Variscan orogenic complex: insights from the composite granulitic-amphibolitic metamorphic basement of South-East Corsica (France)[END_REF][START_REF] Demoux | Variscan evolution of the Tanneron massif, SE-France, examined through U-Pb monazite ages[END_REF][START_REF] Rossi | A restored section of the ''southern Variscan realm'' across the Corsica-Sardinia microcontinent[END_REF][START_REF] Corsini | Exhumation processes during post-collisional stage in the Variscan belt revealed by detailed 40Ar/39Ar study (Tanneron massif, SE France)[END_REF][START_REF] Faure | Variscan orogeny in Corsica: new structural and geochronological insights, and its place in the Variscan geodynamic framework[END_REF][START_REF] Li | From crustal anatexis to mantle melting in the Variscan orogen of Corsica (France): SIMS U-Pb zircon age constraints[END_REF][START_REF] Cruciani | Re-equilibration history and P-T path of eclogites from Variscan Sardinia, Italy: A case study from the medium-grade metamorphic complex[END_REF][START_REF] Oliot | Variscan crustal thickening in the Maures-Tanneron massif (South Variscan belt, France): new in situ monazite U-Th-Pb chemical dating of high-grade rocks[END_REF]. The age of 323 ± 12 Ma obtained for crustal partial melting in the Argentera-Mercantour Massif [START_REF] Rubatto | Complex Paleozoic magmatic and metamorphic evolution in the Argentera Massif (Western Alps), resolved with U-Pb dating[END_REF]) as well as our age at 339.7 ± 12 Ma for eclogite re-quilibration under amphibolite facies conditions are in agreement with these results. A characteristic signature of continental collision in the MECS block is the development of a transpressive regime in the range 330-310 Ma leading the development of ductile strike-slip shear zones in Sardinia [START_REF] Elter | Late Hercynian shear zones in Sardinia[END_REF][START_REF] Carosi | Transpressional deformation in NW Sardinia (Italy): insights on the tectonic evolution of the Variscan belt[END_REF][START_REF] Carosi | The structural evolution of the Asinara Island (NW Sardinia, Italy)[END_REF][START_REF] Carosi | Deformation during exhumation of medium-and high-grade metamorphic rocks in the Variscan chain in northern Sardinia (Italy)[END_REF] and Maures-Tanneron Massif [START_REF] Corsini | Late evolution of the southern European Variscan belt: exhumation of the lower crust in a context of oblique convergence[END_REF][START_REF] Corsini | Exhumation processes during post-collisional stage in the Variscan belt revealed by detailed 40Ar/39Ar study (Tanneron massif, SE France)[END_REF]). In the Argentera-Mercantour Massif, the FMSZ transpressive shear zone active in the range 330-310 Ma [START_REF] Simonetti | Kinematic and geochronological constraints on shear deformation in the Ferriere-Mollières shear zone (Argentera-Mercantour Massif, Western Alps): implications for the evolution of the Southern European Variscan Belt[END_REF]) is a record of a similar tectonic regime.

However, the ages of eclogite facies metamorphism remains poorly constrained in the MECS block as well as in the ECMs [START_REF] Raumer | The Variscan evolution in the External massifs of the Alps and place in their Variscan framework[END_REF][START_REF] Schneider | Thermal and mechanical evolution of an orogenic wedge during Variscan collision: an example in the Maures-Tanneron Massif (SE France)[END_REF][START_REF] Cruciani | Re-equilibration history and P-T path of eclogites from Variscan Sardinia, Italy: A case study from the medium-grade metamorphic complex[END_REF][START_REF] Spalla | The transition from Variscan collision to continental break-up in the Alps: insights from the comparison between natural data and numerical model predictions[END_REF][START_REF] Regorda | How many subductions in the Variscan orogeny? Insights from numerical models[END_REF]) and new radiometric datings are necessary to discriminate between the different geodynamic scenarios.

The U/Pb zircon age obtained on HP granulites in Corsica at ca. 360 Ma is still debated [START_REF] Li | From crustal anatexis to mantle melting in the Variscan orogen of Corsica (France): SIMS U-Pb zircon age constraints[END_REF], therefore the only available geochronological constraints are the age of HP mafic granulites at ca. 340 ± 4 Ma (Rubatto et al. 2010) and the the time span of 344-330 Ma. for retrogression of eclogites under amphibolite facies.

In the framework of the Alpine belt, the identification of which and how many Variscan suture zones (e.g. [START_REF] Matte | Tectonics and plate tectonics model for the Variscan belt of Europe[END_REF][START_REF] Matte | The Variscan collage and orogeny (480-290 Ma) and the tectonic definition of the Armorica microplate: a review[END_REF][START_REF] Franke | The mid-European segment of the Variscides: tectono-stratigraphic units, terrane boundaries and plate tectonic evolution[END_REF][START_REF] Faure | Paleozoic tectonic evolution of medio-Europa from the example of the French Massif Central and Massif Armoricain[END_REF]Guillot et al. 2009;[START_REF] Lardeaux | The Moldanubian Zone in the French Massif Central, Vosges/ Schwarzwald and Bohemian Massif revisited: differences and similarities[END_REF]) occur in the Alpine chain is still debated [START_REF] Regorda | How many subductions in the Variscan orogeny? Insights from numerical models[END_REF]. The evolution of this Palaeozoic lithosphere in terms of geodynamic scenario has been rarely explored (e.g. von [START_REF] Raumer | Gondwana-derived microcontinents -the constituents of the Variscan and Alpine collisional orogens[END_REF][START_REF] Raumer | Pre Mesozoic-Alpine basements -their place in the European Paleozoic framework[END_REF][START_REF] Spiess | The crystalline basement of the Adria microplate in the eastern Alps: a review of the palaeo-structural evolution from the Neoproterozoic to the Cenozoic[END_REF]. The discussion on the provenance of the Alpine basement units in terms of palaeogeography and influence of Palaeo-Tethys evolution on the Variscan dynamics is still open. For this purpose, the comparison of the thermo-mechanical evolution of the Argentera-Mercantour eclogites with those of other Variscan eclogites from the Alps is a key starting point to shed light on their geodynamic significance (Fig. 14 and "Online Resource 10"). PT conditions inferred from eclogites and HP-granulites of Helvetic Domain indicate a thermal state higher than that of warm subduction zones [START_REF] Cloos | Lithospheric buoyancy and collisional orogenesis: subduction of oceanic plateaus, continental margins, island arcs, spreading ridges, and seamounts[END_REF], mainly compatible with the initial unperturbed geotherm or higher, with the only exception of Valasco Valley and Lac Niré rocks. The P-T conditions of the latter lie on the warm subduction geotherm, fully comprised in the Franciscan metamorphic gradient, like some of the Sardinian eclogites. For the first time this indicate that oceanic subduction-related metamorphism developed also in the Helvetic Variscan crust. This makes the investigated rocks more similar to the Variscan eclogites from other Alpine structural domains, which mainly show a thermal state between warm and cold subduction gradients or locally colder, as the case of some Austroalpine eclogites from Oetztal, Tonale and Silvretta nappes.

The previously presented constraints demonstrate the coherence of the Variscan geological signatures depicted in the Argentera-Mercantour Massif and those of the MECS block, which share the same tectono-metamorphic evolution. Hence we envisage that the southernmost ECM is a part of the MECS block in the framework of the European Variscan belt. To test such a hypothesis it will be now critical to obtain high-resolution chronological constraints on the age of eclogite facies metamorphism, particularly in Argentera-Mercantour and Maures-Tanneron Massifs.

Figure captions

Fig. 1: The Argentera-Mecantour Massif in the Alpine arc (after [START_REF] Spalla | The transition from Variscan collision to continental break-up in the Alps: insights from the comparison between natural data and numerical model predictions[END_REF][START_REF] Regorda | How many subductions in the Variscan orogeny? Insights from numerical models[END_REF] [START_REF]Etudes géologiques sur le Massif de l'Argentera-Mercantour et sur ses enveloppes sédimentaires[END_REF], Malaroda et al. 1970, Corsini et al., 2004[START_REF] Carosi | Geology of the northwestern portion of the Ferriere-Mollieres Shear Zone, Argentera Massif, Italy[END_REF]and Gosso et al. 2019). FMSZ -Ferrere-Mollières Shear Zone. CG -Central Granite. 1 Eclogite occurrences and sampling localities with yellow stars; 2 Migmatitic paragneisses; 3 Migmatitic orthogneisses and metavolcanics; 4 Amphibolites; 5 Syn-tectonic diorites; 6 leucocratic cordieritebearing anatexites; 7 Amphibolite to greenschistfacies mylonites; 8 Granitoids; 9 Carbonaceous schists, conglomerates, and siliciclastic sandstones (late Carboniferous); 10 -Permian to Eocene sequences; 11 late-to post-Alpine faults; 12 Penninic Front; 13 Greenschist-facies mylonites. White contours labelled a and b locate the maps of Fig. 5 and6, respectively. Baragar, 1971 (7b). clinopyroxene,clinozoisite,rutile,quartz;amphibole,clinopyroxene,titanite,clinozoisite,rutile,quartz;amphibole,clinopyroxene,titanite,clinozoisite,quartz;amphibole,titanite,clinozoisite,quartz;amphibole,clinopyroxene,titanite,quartz;amphibole,clinopyroxene,titanite,rutile,quartz. For pseudosection (b), numbered fields match with the following assemblages: 1 -garnet, white mica, 2 amphiboles, clinopyroxene, quartz, rutile; 2 -garnet, white mica, 2 amphiboles, clinopyroxene, quartz, rutile, clinozoisite; 3 -garnet, biotite, white mica, amphibole, 2 clinopyroxenes, quartz, rutile; 4 -garnet, biotite, amphibole, 2 clinopyroxenes, quartz, rutile; 5 -garnet, biotite, white mica, amphibole, clinopyroxene, quartz, rutile; 6 -garnet, biotite, white mica, amphibole, clinopyroxene, quartz, rutile, clinozoisite; 7 -garnet, biotite, amphibole, clinopyroxene, quartz, rutile, titanite; 8 -garnet, biotite, amphibole, quartz, rutile, sphene; 9garnet, biotite, amphibole, clinopyroxene, quartz, rutile, clinozoisite; 10 -plaglioclase, garnet, biotite, amphibole, quartz, rutile; 11 -plagioclase, garnet, biotite, amphibole, quartz, rutile, titanite; 12 -garnet, biotite, clinopyroxene, quartz, rutile; 13 -plagioclase, garnet, biotite, clinopyroxene, quartz, rutile. Some very small fields have not been numbered. [START_REF] Miyashiro | Evolution of Metamorphic Belts[END_REF][START_REF] Ernst | Metamorphic zonations on presumably subducted lithospheric plates from Japan, California, and the Alps[END_REF][START_REF] Spear | Metamorphic Phase Equilibria and Pressure-Temperature-Time Paths[END_REF] and geotherms after [START_REF] Cloos | Lithospheric buoyancy and collisional orogenesis: subduction of oceanic plateaus, continental margins, island arcs, spreading ridges, and seamounts[END_REF]

  Fig.1: The Argentera-Mecantour Massif in the Alpine arc(after Spalla et al., 2014;[START_REF] Regorda | How many subductions in the Variscan orogeny? Insights from numerical models[END_REF]. Black stars: Eclogites known in the pre-Alpine basements from different alpine zones; star labels individuate Alpine and Sardinian eclogites as listed in "Online Resource 10". Inset shows the Alpine arc in the framework of the European Variscan belt. Inset labels: Arm -Armorican Massif; BCBF -Bristol Channel-Bray Fault; BM -Bohemian Massif; Ca -Cantabrian terrane; Cib -Central Iberian; Co -Corsica; FMC -French Massif Central; GSBS -Galicia-Southern Brittany Suture; MT -Maures-Tanneron Massif; OM -Ossa Morena; Py -Pyrenees; Sa -Sardinia; Si -Sicilian-Apulian basements; SP -South Portuguese Zone; WL -West Asturian-Leonese.

Fig. 2 :

 2 Fig.2: Geological map of the Argentera-Mercantour Massif (after[START_REF]Etudes géologiques sur le Massif de l'Argentera-Mercantour et sur ses enveloppes sédimentaires[END_REF], Malaroda et al. 1970,

Fig. 3 :

 3 Fig. 3: Eclogitic boudins. (a) Col de Fenestre-Pas des Ladres (France) eclogitic boudin in metatexites; (b) Balma Ghiliè valley (Italy) eclogite within paragneisses; (c) Monte Matto (Italy) boudin of eclogite in cordierite bearing diatexites; (d) Lac Niré (France) eclogite within paragneisses; (e) Lac Long eclogitic boudin in metatexites.

Fig. 4 :

 4 Fig. 4: Retrogressed eclogites. (a) Lago Brocan (Italy) coronas of amphibole around garnets; (b) Lac Niré (France) eclogite with plagioclase around garnets.

Fig. 5 :

 5 Fig. 5: Geological map of the high Gordolasque valley at the Italian-French border. (a) Structural map. (b) Detailed map of eclogite boudin outcropping north of the Lac Long and cross-section.

Fig. 6 :

 6 Fig. 6: Structural map of upper Valasco Valley (Italy). In the inset detailed form-surface map located with a black star in the structural map, close to Lago Inferiore di Valscura.

Fig. 7 :

 7 Fig.7: Geochemistry of Argentera-Mercantour eclogites in the Al2O3 vs. TiO2 diagram afterKonzett et al. 2012 (7a) and in the AFM diagram after Irvine andBaragar, 1971 (7b).

Fig. 8 :

 8 Fig. 8: Chemical compositions of eclogites in the Y-Nb-Zr (8a after Meschede, 1986), the Th/Yb versus Ta/Yb (8b after Pierce, 1982) and in the N-MORB normalized Th-Nb (8c after Saccani, 2015) diagrams. Fields hatched in grey represent available chemical compositions of Sardinian eclogites (see text for discussion and references).

Fig. 9 :

 9 Fig. 9: Primitive mantle normalized REE (a) and LILEs HFSEs (b) patterns of eclogites. N-MORB, E-MORB and OIB compositions after[START_REF] Sun | Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes[END_REF].

Fig. 10 :

 10 Fig. 10: Photomicrographs of eclogites. (a) Clinopyroxene-plagioclase symplectite (crossed polars, lac Niré, France). Progressive decrease of lamellae size from center to rim; (b) Evolution of the symplectite, from coarse clinopyroxene lamellaes (with Jadeite content of 42 mol%) to thinner lamellaes (with jadeite content of 15 mol%), crossed polars, Pas des Ladres (France). Gt -Garnet; Pl -Plagioclase; Amp -Amphibole. (c) Reequilibrated eclogites with diopside-plagioclase symplectites. Relict amphibole 1 green cores are rimmed by brown Ti rich amphibole 2 that together with plagioclase occurs in coronas around garnets (plane polarized light, Lago Brocan, Italy); (d) Detail on the plagioclase-amphibole 2 coronas around garnet; (e) Inclusions of quartz, amphibole 1 and rutile in garnet (BSE image, Brocan Lake, Italy); (f) Diopside-plagioclase symplectite replacing omphacite (BSE image, sample 0878, Valasco Valley, Italy).

Fig. 11 :

 11 Fig. 11: Composition of clinopyroxenes (a), garnets (b), feldspaths (c) and amphiboles (d) from Argentera eclogites. In all the diagrams minerals from Lac Niré, Lac Long and Pas de Ladres eclogites are represented by stars. Italian eclogites are circles. Different colors have been used for minerals from Valasco Valley: in the diagram 11(a) of Morimoto (1988) black and grey circles represent the recalculated compositions of omphacites and clinopyroxene 2 respectively. In comparison, omphacite of Lac Niré appears in black stars. In Na/Na+Ca vs Al/Al+Si diagram 11(d), stars locate the end-member compositions labelled as follows: ACT -Actinolite, BAR -Barroisite, ED -Edenite, GL -Glaucophane; KTP -Katophorite, PRG -Pargasite, SAD -Sadanagaite, TS -Tschermakite, WIN -Winchite.

Fig. 12 :

 12 Fig.12: P-T pseudosections calculated from Valasco Valley (a, CARA884) and Lac Long (b, LL13). The red line divides the P-T field of plagioclase bearing assemblages, towards higher temperatures, from assemblages without plagioclase, towards higher pressures. The shaded light blue fields represent the P-T conditions determined using classic geothermobarometry for the eclogite-facies assemblage. Univariant lines are thick.For pseudosection (a), assemblages in the numbered fields are as follow: 1 -garnet, 2 clinopyroxenes, rutile, quartz; 2 -garnet, amphibole, 2 clinopyroxenes, rutile, quartz; 3 -garnet, amphibole, 2 clinopyroxenes, rutile, clinozoisite, quartz; 4 -garnet, 2 amphiboles, 2 clinopyroxenes, rutile, quartz; 5 -garnet, 2 amphiboles,

Fig. 13 :

 13 Fig. 13: 40Ar/39Ar age spectrum diagram performed in amphibole from Lac Long sample.

Fig. 14 :

 14 Fig.14: P-T conditions (violet square) of studied eclogites compared with metamorphic gradients (after

  : (1) near spreading ridge or volcanic arc, (2) normal gradient of old plate interior, (3) warm subduction zones, and (4) cold subduction zones. Vo represents the initial unperturbed geotherm. (a) P-T conditions of Argentera eclogites compared with eclogites from Helvetic-Dauphinois zone and Sardinian eclogites. (b) P-T conditions of Argentera eclogites compared with eclogites from Penninic and Austro-Alpines zones. Box labels refer to locations in Figure 1 and descriptions in "Online Resource 10".

FIGURE

  FIGURE 1

Italian authors acknowledge funding by PSR2019-IBOLLATI (University of Milano). We thank Pavla Štípská for her editorial work.