M Soret

P Agard

B Dubacq

A Vitale-Brovarone

P Monié

A Chauvet

H Whitechurch

B Villemant

Strain localization and fluid infiltration in the mantle wedge during subduction initiation: evidence from the base of the New Caledonia ophiolite

Despite decades of petrological and geochemical studies, the nature and setting of obducted ophiolites remain controversial: the influence of supra-subduction zone environments on pre-existing oceanic lithosphere is yet to assess, and the processes leading to subduction/obduction initiation are still poorly constrained. Our study documents successive influx of slab-derived fluids and progressive strain localization within the upper mantle in a supra-subduction environment during the first few My of the subduction history. We focus on strongly sheared mafic amphibolites intruding peridotites near the mantle-crust transition of the New Caledonia obducted ophiolite and ~50 to 100 m above the basal thrust contact of the ophiolite. These m-to hm-long and several m-thick shear bands are interpreted as inherited small-scale intrusions of mafic melts, probably dikes or sills, which were derived from a moderately refractory mantle source refertilized by supra-subduction zone fluids. 40 Ar/ 39 Ar age constraints on pargasite at ca. 90 Ma suggest that they could be inherited from the former Pacific west-dipping subduction.

Secondary deformation of these mafic intrusions is intimately associated to three major stages of fluid infiltration: (1) the first stage of deformation and metasomatism is marked by syn-kinematic growth of Ca-amphibole (at 700-800 ºC and 3-5 kbar) with a distinctive supra-subduction zone signature, and controlled later channelization of aqueous fluids. 40 Ar/ 39 Ar dating on magnesio-hornblende indicates that this deformation episode occurred at ca. 55 Ma, coincident with east-dipping subduction initiation; (2) the main metasomatic stage, characterized by the development of a phlogopite-rich matrix wrapping peridotites and amphibolite boudins, points to the percolation of alkali-rich aqueous fluids at still high temperature (650-750 ºC); (3) the last, low temperature (< 600 ºC) metasomatic stage results in the formation of deformed veinlets containing talc, chlorite and serpentine.

Introduction

Obduction emplaces fragments of dense oceanic lithosphere on top of continents over several hundreds of kilometers (e.g. Oman, Newfoundland, Turkey, New Caledonia; [START_REF] Coleman | Tectonic Eraplacement Upper Mantle Peridotires along Continental Edges[END_REF]. Despite numerous petrological and geochemical studies, the exact nature and setting of many obducted ophiolites remain unclear. The Oman ophiolite is a typical example where there are evidences for both a MORB-type signature [START_REF] Boudier | Shear zones, thrusts and related magmatism in the Oman ophiolite: Initiation of thrusting on an oceanic ridge[END_REF][START_REF] Ceuleneer | Mantle flow patterns at an oceanic spreading The Oman peridotites record[END_REF][START_REF] Nicolas | Accretion of Oman and United Arab Emirates ophiolite -Discussion of a new structural map[END_REF][START_REF] Godard | Relationships between geochemistry and structure beneath a palaeo-spreading centre: a study of the mantle section in the Oman ophiolite[END_REF][START_REF] Godard | Geochemical variability of the Oman ophiolite lavas: Relationship with spatial distribution and paleomagnetic directions[END_REF][START_REF] Le Mée | Mantle segmentation along the Oman ophiolite fossil mid-ocean ridge[END_REF]) and a supra-subduction zone (SSZ) geochemical imprint (Ernewein &al, 1988, Stern and[START_REF] Stern | Subduction zone infancy: Examples from the Eocene Izu-Bonin-Marinana and Jurassic California arcs[END_REF][START_REF] Shervais | Birth, death, and resurrection: The life cycle of suprasubduction zone ophiolites[END_REF][START_REF] Mcleod | Moist MORB" axial magmatism in the Oman ophiolite: The Evidence against a mid-ocean ridge origin[END_REF].

The origin of obducted ophiolites has long been debated (Coleman, 1971, Dewey and[START_REF] Dewey | Origin and Emplacement of the Ophiolite Suite: Appalachian Ophiolites in Newfoundland[END_REF][START_REF] Nicolas | Principes de Tectonique[END_REF][START_REF] Moores | Tethyan ophiolites, mantle convection, and tectonic "historical contingency": A resolution of the "ophiolite conundrum[END_REF], with recent studies focusing on subduction-obduction initiation and early emplacement of ophiolites [START_REF] Vaughan | Ophiolite obduction pulses as a proxy indicator of superplume events?[END_REF][START_REF] Agard | Plate acceleration: The obduction trigger?[END_REF][START_REF] Agard | Obduction: Why, how and where. Clues from analog models[END_REF][START_REF] Lagabrielle | Passive obduction and gravity-driven emplacement of large ophiolitic sheets: The New Caledonia ophiolite (SW Pacific) as a case study?[END_REF].

Most of our knowledge on the initiation of the subduction-obduction system comes from the amphibolite to granulite facies metamorphic soles found at the base of most large-scale non-metamorphosed ophiolites and from magmatic dikes emplaced at different levels of the mantle sequence. These dikes generally record partial refertilization of the mantle wedge by subduction-derived fluids and show little deformation (e.g. [START_REF] Pirard | Transfer of melts in the sub-arc mantle: Insights from high-pressure experiments and from the New Caledonia ophiolite[END_REF][START_REF] Xiong | Pyroxenite Dykes in Orogenic Peridotite from North Qaidam (NE Tibet, China) Track Metasomatism and Segregation in the Mantle Wedge[END_REF] whereas metamorphic soles are highly deformed portions of upper oceanic crust and sediments, heated and metamorphosed along the interface between the subducting slab and the hotter mantle wedge during early subduction (e.g. [START_REF] Wakabayashi | What constitutes "emplacement" of an ophiolite?: Mechanisms and relationship to subduction initiation and formation of metamorphic soles[END_REF]. Metamorphic soles typically predate the final emplacement of the ophiolite onto the continental margin by several millions of years (~20 My for Oman; [START_REF] Agard | Plate acceleration: The obduction trigger?[END_REF].

We document sheared mafic amphibolite and feldspar-rich veins outcropping over a scale of hundreds of meters, within the peridotites of the New Caledonia ophiolite. They occur near the crust-mantle transition zone (e.g. [START_REF] Prinzhofer | Structures in the new caledonia peridotites-gabbros: implications for oceanic mantle and crust[END_REF][START_REF] Pirard | Petrology and Geochemistry of the Crust-Mantle Boundary in a Nascent Arc, Massif du Sud Ophiolite, New Caledonia, SW Pacific[END_REF] and ~50 to 100 m above the basal contact of the ophiolite, south east of Noumea (Fig. 1). As these deformed, metamorphosed rocks occur at the base of the New Caledonia ophiolite, it is tempting to relate them to subduction initiation, as for metamorphic soles. These sheared amphibolites could represent metamorphic sole fragments (outcropping in the Thio area; [START_REF] Cluzel | Tectonic accretion and underplating of mafic terranes in the Late Eocene intraoceanic fore-arc of New Caledonia (Southwest Pacific): geodynamic implications[END_REF][START_REF] Cluzel | The metamorphic sole of New Caledonia ophiolite: 40Ar/39Ar, U-Pb, and geochemical evidence for subduction inception at a spreading ridge[END_REF] tectonically imbricated with basal peridotites, or be derived from melt products triggered by subduction-derived fluids (e.g. as the SSZ amphibole-bearing dikes in the

Geological setting

New Caledonia is located at the northern termination of the Norfolk Ridge, a stretched continental fragment rifted off the eastern Australian margin during the Early Cretaceous rollback of the Western Pacific subduction zone and the opening of New Caledonia and Loyalty basins [START_REF] Weissel | Evolution of the Tasman Sea reappraised[END_REF][START_REF] Van De Beuque | Transect sismique continu entre l'arc des Nouvelles-Hebrides de l'Australie: programme FAIJST (French Australian Seismic Transect) Continuous seismic transect between the New Hebrides Arc and the Eastern[END_REF][START_REF] Schellart | A Late Cretaceous and Cenozoic reconstruction of the Southwest Pacific region: Tectonics controlled by subduction and slab rollback processes[END_REF]).

The present-day structure of New Caledonia (Fig. 1a) results from a Paleogene episode of convergence. This event led to the closure of the South Loyalty Basin after the initiation of a north-east-dipping intra-oceanic subduction [START_REF] Cluzel | Tectonic accretion and underplating of mafic terranes in the Late Eocene intraoceanic fore-arc of New Caledonia (Southwest Pacific): geodynamic implications[END_REF][START_REF] Schellart | A Late Cretaceous and Cenozoic reconstruction of the Southwest Pacific region: Tectonics controlled by subduction and slab rollback processes[END_REF][START_REF] Matthews | Geologic and kinematic constraints on Late Cretaceous to mid Eocene plate boundaries in the southwest Pacific[END_REF] and was responsible for the obduction of the New Caledonia ophiolite rooted in the Loyalty Basin [START_REF] Collot | Overthrust emplacement of New Caledonia Ophiolite: Geophysical evidence[END_REF].

Geochronological and thermochronological data on mafic rocks from the metamorphic sole [START_REF] Cluzel | The metamorphic sole of New Caledonia ophiolite: 40Ar/39Ar, U-Pb, and geochemical evidence for subduction inception at a spreading ridge[END_REF]) and mafic dikes intruding the ultramafic ophiolite [START_REF] Cluzel | Earliest Eocene (53 Ma) convergence in the Southwest Pacific: evidence from pre-obduction dikes in the ophiolite of New Caledonia[END_REF][START_REF] Pirard | Transfer of melts in the sub-arc mantle: Insights from high-pressure experiments and from the New Caledonia ophiolite[END_REF] suggest that subduction inception occurred at ca. 56 Ma. At ca. 34 Ma, this intra-oceanic subduction led to the obduction of the South Loyalty Basin lithosphere toward the south-west onto the Norfolk Ridge (e.g. [START_REF] Paris | Sur l'âge Eocène supérieur de la mise en place de la nappe ophiolitique de Nouvelle-Calédonie[END_REF][START_REF] Prinzhofer | Structure et pétrologie d'un cortège ophiolitique: le Massif du Sud (Nouvelle Calédonie) : la transition manteau-croûte en milieu océanique[END_REF][START_REF] Collot | Overthrust emplacement of New Caledonia Ophiolite: Geophysical evidence[END_REF].

The ophiolite complex of New Caledonia is composed of three allochtonous units of Cretaceous-Eocene age accreted over a calc-alkaline, island-arc derived basement (Fig. 1a) [START_REF] Aitchison | Eocene arc-continent collision in New Caledonia and implications for regional southwest Pacific tectonic evolution[END_REF][START_REF] Cluzel | Tectonic accretion and underplating of mafic terranes in the Late Eocene intraoceanic fore-arc of New Caledonia (Southwest Pacific): geodynamic implications[END_REF]. This basement was created from the Permian to the Early Cretaceous with the accretion of three major volcano-sedimentary terranes (i.e. Teremba, Boghen and Koh terranes) along the eastern Gondwana margin. Above this basement, the Tertiary subduction complex is composed of four main units:

1a Tertiary high-pressure metamorphic belt at the northern end of New-Caledonia (Fig. 1a) made of volcano-sedimentary and mafic blueschist and eclogite facies rocks, some of which are interbedded in a serpentinite matrix and minor metasediments (Clarke et al., 1997;[START_REF] Carson | Hydration of eclogite, Pam Peninsula, New Caledonia[END_REF][START_REF] Cluzel | Tectonic accretion and underplating of mafic terranes in the Late Eocene intraoceanic fore-arc of New Caledonia (Southwest Pacific): geodynamic implications[END_REF][START_REF] Vitale Brovarone | True metamorphic isograds or tectonically sliced metamorphic sequence? New high-spatial resolution petrological data for the New Caledonia case study[END_REF]. Mafic eclogites have Late Cretaceous to Eocene protolith ages and share geochemical affinities with the Poya terrane [START_REF] Cluzel | Tectonic accretion and underplating of mafic terranes in the Late Eocene intraoceanic fore-arc of New Caledonia (Southwest Pacific): geodynamic implications[END_REF][START_REF] Spandler | Late Cretaceous-Tertiary tectonics of the southwest Pacific: Insights from U-Pb sensitive, high-resolution ion microprobe (SHRIMP) dating of eclogite facies rocks from New Caledonia[END_REF] 2the Poya terrane, a weakly metamorphic mafic rocks unit (e.g. [START_REF] Eissen | Geochemistry and tectonic significance of basalts in the Poya Terrane, New Caledonia[END_REF][START_REF] Cluzel | The Poya Nappe, ex-"Formation des Basaltes" (New Caledonia, southwest Pacific): a Campanian to Upper Palaeocene oceanic plateau obducted in the late Eocene[END_REF][START_REF] Cluzel | Tectonic accretion and underplating of mafic terranes in the Late Eocene intraoceanic fore-arc of New Caledonia (Southwest Pacific): geodynamic implications[END_REF], outcrops along most of the west coast of New Caledonia and also forms several isolated exposures along the eastern side (Fig. 1a).

This terrane consists mainly of massive and pillow basalt, dolerite, and gabbro associated with chert and volcaniclastic rocks [START_REF] Eissen | Geochemistry and tectonic significance of basalts in the Poya Terrane, New Caledonia[END_REF]. The origin of the Poya Terrane is controversial. Geochemical associations of back-arc basin and enriched-MORB signatures constrain its formation to a back-arc environment [START_REF] Cluzel | The Poya Nappe, ex-"Formation des Basaltes" (New Caledonia, southwest Pacific): a Campanian to Upper Palaeocene oceanic plateau obducted in the late Eocene[END_REF][START_REF] Cluzel | Tectonic accretion and underplating of mafic terranes in the Late Eocene intraoceanic fore-arc of New Caledonia (Southwest Pacific): geodynamic implications[END_REF]. Paleontological studies date its formation between 83 and 55 Ma [START_REF] Cluzel | Tectonic accretion and underplating of mafic terranes in the Late Eocene intraoceanic fore-arc of New Caledonia (Southwest Pacific): geodynamic implications[END_REF]. This unit was interpreted as representing the crustal cover of the South Loyalty Basin [START_REF] Spandler | Late Cretaceous-Tertiary tectonics of the southwest Pacific: Insights from U-Pb sensitive, high-resolution ion microprobe (SHRIMP) dating of eclogite facies rocks from New Caledonia[END_REF][START_REF] Lagabrielle | Passive obduction and gravity-driven emplacement of large ophiolitic sheets: The New Caledonia ophiolite (SW Pacific) as a case study?[END_REF], with the above ages likely indicating the opening period of the basin.

3the ultramafic ophiolite (named "Nappe des Péridotites") found on the south-eastern portion of New Caledonia (Fig. 1a) is constituted mainly of a 2-3 km thick depleted mantle sequence (spinel-bearing harzburgite) above a 20 to 100 m thick porphyroclastic serpentine mylonite. In the northern part of New Caledonia, lherzolite is present in the Tiebaghi and the Poum massifs (e.g. [START_REF] Moutte | Le massif de Tiébaghi, Nouvelle Calédonie et ses gîtes de chromite[END_REF][START_REF] Ulrich | Multiple melting stages and refertilization as indicators for ridge to subduction formation: The New Caledonia ophiolite[END_REF]. This suite of ultramafic rocks is interpreted as representing upper mantle rocks of the South Loyalty Basin [START_REF] Lagabrielle | Passive obduction and gravity-driven emplacement of large ophiolitic sheets: The New Caledonia ophiolite (SW Pacific) as a case study?[END_REF] and documents a complex history with several supra-subduction zone settings, as attested by magmatic intrusions.

In the Massif du Sud, [START_REF] Marchesi | Migration and accumulation of ultra-depleted subduction-related melts in the Massif du Sud ophiolite (New Caledonia)[END_REF] and [START_REF] Pirard | Petrology and Geochemistry of the Crust-Mantle Boundary in a Nascent Arc, Massif du Sud Ophiolite, New Caledonia, SW Pacific[END_REF] reported gabbronorites and olivine gabbro sills at the top of ophiolitic sequence. These gabbros represent the lower crust of the ophiolite. They are separated from the underlying harzburgite by a dunite complex composed of dunite channels with the presence of dunite cumulates, wherlite, clinopyroxenite and websterite sills and dikes. Amphibole is commonly present in clinopyroxenite bodies but absent from the lower crust gabbronorites [START_REF] Pirard | Transfer of melts in the sub-arc mantle: Insights from high-pressure experiments and from the New Caledonia ophiolite[END_REF][START_REF] Pirard | Petrology and Geochemistry of the Crust-Mantle Boundary in a Nascent Arc, Massif du Sud Ophiolite, New Caledonia, SW Pacific[END_REF]. These authors interpret this complex as the crust-mantle transition with mafic sills and dikes being the result of percolating, hydrous primitive arc magmas within the peridotite during early stages of the subduction system and nascent arc crust formation.

The Massif du Sud is also extensively cross-cut by Early Eocene felsic, anhydrous and hydrous coarse grained dikes (leucodiorite, granitoid, micro-diorite, dolerite and hornblendite; [START_REF] Prinzhofer | Structure et pétrologie d'un cortège ophiolitique: le Massif du Sud (Nouvelle Calédonie) : la transition manteau-croûte en milieu océanique[END_REF][START_REF] Cluzel | Late Oligocene post-obduction granitoids of New Caledonia: A case for reactivated subduction and slab break-off[END_REF][START_REF] Cluzel | Earliest Eocene (53 Ma) convergence in the Southwest Pacific: evidence from pre-obduction dikes in the ophiolite of New Caledonia[END_REF][START_REF] Pirard | Transfer of melts in the sub-arc mantle: Insights from high-pressure experiments and from the New Caledonia ophiolite[END_REF][START_REF] Pirard | Petrology and Geochemistry of the Crust-Mantle Boundary in a Nascent Arc, Massif du Sud Ophiolite, New Caledonia, SW Pacific[END_REF] discordant with the harzburgite and/or the gabbro sills from the lower crust. These dikes are generally unmetamorphosed and undeformed with the exception of few diorite dikes which are boudinaged, locally micro-folded and where amphibole shows a preferred orientation [START_REF] Cluzel | The metamorphic sole of New Caledonia ophiolite: 40Ar/39Ar, U-Pb, and geochemical evidence for subduction inception at a spreading ridge[END_REF]. Upon geochemical and geochronological features, these dikes have been interpreted as formed in a suprasubduction zone setting. These dikes are essentially considered as synchronous or immediately posterior to magma injection in active faults that formed during the first stages of the 55 Ma subduction [START_REF] Cluzel | Earliest Eocene (53 Ma) convergence in the Southwest Pacific: evidence from pre-obduction dikes in the ophiolite of New Caledonia[END_REF][START_REF] Cluzel | The metamorphic sole of New Caledonia ophiolite: 40Ar/39Ar, U-Pb, and geochemical evidence for subduction inception at a spreading ridge[END_REF]. However, Prinzhofer (1981) obtained 2 different K-Ar ages (i.e. 90 ± 10 Ma and 42 ± 5 Ma) for dolerite dikes intruding the Massif du Sud. The youngest dikes coincide with the Eocene New Caledonia subduction. The Late Cretaceous dikes could be an evidence of the ocean spreading stage of the South Loyalty Basin [START_REF] Prinzhofer | Structure et pétrologie d'un cortège ophiolitique: le Massif du Sud (Nouvelle Calédonie) : la transition manteau-croûte en milieu océanique[END_REF][START_REF] Matthews | Geologic and kinematic constraints on Late Cretaceous to mid Eocene plate boundaries in the southwest Pacific[END_REF].

The Late Cretaceous age of 85 Ma was also measured in zircon cores from the highpressure Pouebo unit [START_REF] Spandler | Late Cretaceous-Tertiary tectonics of the southwest Pacific: Insights from U-Pb sensitive, high-resolution ion microprobe (SHRIMP) dating of eclogite facies rocks from New Caledonia[END_REF]. In addition, [START_REF] Cluzel | Late Oligocene post-obduction granitoids of New Caledonia: A case for reactivated subduction and slab break-off[END_REF] report granitoid dikes intruding the Massif du Sud at 27 and 24 Ma. They interpret these dikes, coeval with the final obduction stages, as related to the inception of a new subduction along the west coast of New Caledonia in response to the blocking of the previous Eocene subduction. Altogether these dikes appear to record several subduction-related events, suggesting that the ophiolite witnesses several and distinctive stages of migration, accumulation and re-equilibration of different melts (including hydrous melts) in a supra-subduction environment (e.g. [START_REF] Marchesi | Migration and accumulation of ultra-depleted subduction-related melts in the Massif du Sud ophiolite (New Caledonia)[END_REF][START_REF] Ulrich | Multiple melting stages and refertilization as indicators for ridge to subduction formation: The New Caledonia ophiolite[END_REF][START_REF] Pirard | Transfer of melts in the sub-arc mantle: Insights from high-pressure experiments and from the New Caledonia ophiolite[END_REF], rather than being the result of a fractional crystallization from a single magma-type during oceanic spreading [START_REF] Prinzhofer | Residual peridotites and the mechanisms of partial melting[END_REF].

4an amphibolite to granulite facies metamorphic sole is found in places between the ophiolite and the Poya and Pouebo underlying terranes [START_REF] Cluzel | The metamorphic sole of New Caledonia ophiolite: 40Ar/39Ar, U-Pb, and geochemical evidence for subduction inception at a spreading ridge[END_REF]. This metamorphic sole is interpreted to have been welded to the ophiolite during subduction inception. Using various thermobarometers, [START_REF] Cluzel | The metamorphic sole of New Caledonia ophiolite: 40Ar/39Ar, U-Pb, and geochemical evidence for subduction inception at a spreading ridge[END_REF] estimate peak crystallization conditions around 5 kbar and 800 to 950 °C.

Thermochronology on hornblende ( 40 Ar/ 39 Ar), zircon and sphene (U-Pb) indicate that these mafic rocks crystallized at ca. 56 Ma [START_REF] Cluzel | The metamorphic sole of New Caledonia ophiolite: 40Ar/39Ar, U-Pb, and geochemical evidence for subduction inception at a spreading ridge[END_REF]. This result and inferred pressure-temperature conditions for the New Caledonia metamorphic sole give a geothermal gradient greater than 40 ºC/km, consistent with subduction inception occurring at or near the spreading ridge of the South Loyalty Basin.

Outcrop-scale deformation patterns and mineralogy

The study area is exposed at low tide on the foreshore of Plum Beach (southeast of Noumea), across a ~150x100 m surface (Fig. 1b). This area is bounded to the west by the New Caledonia Basin and by a densely vegetated cliff to the east. It is located near the base of the ophiolite, 50 to 100 m above the north-east dipping contact with the underlying Poya unit, which outcrops 2 km to the north.

The study area is a major deformation zone characterized by large-scale mafic shear bands (up to 100 m long in places) and centimetric to decimetric orthopyroxenite veins away from the deformed area. The shear bands and veins crosscut the peridotite, which is constituted mainly of highly serpentinized harzburgite and rare dunite. One to two meters away from the shear bands, the peridotite is massive and has only a tectonite porphyroclastic texture with a high temperature foliation steeply dipping to the NE (parallel to the paleo-Moho according to [START_REF] Prinzhofer | Structures in the new caledonia peridotites-gabbros: implications for oceanic mantle and crust[END_REF]. The orientation of this foliation is discordant with respect to the deformation observed in the mafic shear bands.

Ten major shear bands have been recognized in the study area. Shear bands #1 to #7 are 30 to 50 m long and strike ~N150 (Fig. 1b). They disappear under the eastern cliff but likely extend further to the south-east, where similar shear bands are found in apparent continuity along the coast (#8 to #10). The overall length of these shear bands is thus greater than 150 m.

The shear bands are m-to several m-thick and commonly outlined by 10-50 cm thick amphibolite bands, mostly found at the core of the shear bands (and, when found on the margins, only on their south-west side; Fig. 2).

Peridotite and amphibolite are strongly boudinaged within the amphibolitebearing shear zones (Fig. 3a,b,c). Strain increases towards the core of the shear bands, as shown by the marked decrease in boudin size, from the m-to the cm-scale. Both ultramafic and mafic boudins are sigmoidal and indicate consistent sinistral-reverse, top to the north-west shear senses across the whole outcrop (Fig. 2,3a). These boudins are embedded in a phlogopite-rich matrix described below.

A network of distributed leucocratic feldspar-rich veins also cross-cuts the peridotite. This network is mainly found on the margins of the high strain bands.

Mutual cross-cutting relationships between the dark amphibolites and the feldsparrich veins suggest that they formed coevally. The vein network is developed only on one side of the amphibolite bands (to the NE); the density, thickness and length of the veins decrease away from the amphibolites (Fig. 3c,d,f,g). The mafic shear band #4bis is a dm-thick dark amphibolite without feldspar-rich veins (Fig. 2). Amphibolite bands (Fig. 3e) chiefly comprise calcic amphibole with 10 to 50 vol.% plagioclase, variably altered to reddish-to-yellowish low-grade oxidized aggregates. Pyroxene (always orthopyroxene, as detailed below) amounts to 15-30 vol.%.

Leucocratic feldspar-rich veins are characterized by a predominance of plagioclase with up to 20 vol.% coarse-grained orthopyroxene crystals (Fig. 4a,b).

Most orthopyroxenes are rimmed by calcic-amphibole.

A light colored, green to grey matrix comprising phlogopite, chlorite, talc and serpentine is observed within the shear bands and appears to form at the expense of both the mafic and ultramafic rocks (Fig. 3c,d,e,f). Late, milky white cm-thick serpentine veinlets rim peridotite boudins and also cross-cut the boudins and the matrix. Phlogopite is also observed concentrated in m-scale veins next to some of the shear bands (#9).

Sampling strategy and analytical techniques

4.1 Sample selection 60 samples (Table 1) were collected in the field across the whole range of textures and mineralogical assemblages of the shear bands (both for mafic -M-and ultramafic -UM-protoliths; Fig. 2). 15 samples are dark amphibolites, either within deformation bands (e.g. M4d; Fig. 2,5a) or isolated in undeformed peridotite (e.g. M8a), whereas 5 samples are deformed feldspar-rich mafic veins (e.g. M8e; Fig. 5b), including an undeformed feldspar-rich veinlet isolated in peridotite (sample M9).

Ultramafic samples comprise 17 peridotite boudin samples surrounded by deformed feldspar-rich veins and phlogopite-rich matrix (e.g. UM13, UM10; Fig. 5c,d), 14 undeformed peridotite samples (e.g. UM12, UM14) and one orthopyroxenite vein sample.

Electron probe microanalysis (EPMA)

EPMA was carried out at CAMPARIS (UPMC-IPGP, Paris, France) on amphibolites (n=7), feldspar-rich veins (n=5), peridotite boudins (n=5), undeformed peridotites (n=2) and on the biotite-rich matrix (n=7), using classical analytical conditions for spot analyses (1-2 m spot size; 15kV, 10nA, wavelength-dispersive spectroscopy mode), using Fe 2 O 3 (Fe), MnTiO 3 (Mn, Ti), diopside (Mg, Si), CaF 2 (F), orthoclase (Al, K), anorthite (Ca) and albite (Na) as standards for bracketed elements.

Representative analyses are given in Table 2.

Thermobarometry

Pressure and temperature conditions of crystallization have been estimated using the hornblende-plagioclase geothermobarometer of [START_REF] Holland | Non-ideal interactions in calcic amphiboles and their bearing on amphibole-plagioclase thermometry[END_REF] modified by [START_REF] Anderson | The effects of temperature and fO 2 on the Al-in-hornblende barometer[END_REF] in three olivine-bearing amphibolites and in six olivine-free amphibolites. This geothermobarometer is based on the reaction: edenite + albite = richterite + anorthite. The absence of Ti-rich phases in amphibolites invalidates the use of Ti-based geothermobarometers. Representative pressuretemperature estimates are given in Table 2.

The Fe 3+ estimation method in amphibole given in [START_REF] Holland | Non-ideal interactions in calcic amphiboles and their bearing on amphibole-plagioclase thermometry[END_REF] provides the minimum (Fe 3+ /Fe tot ) ratio for which sufficient Na is allocated to the Asite. Fe 3+ /Fe tot here averages around 0.4 in late amphibole (i.e. magnesio-hornblende).

With a (Fe 3+ /Fe tot ) ratio = 1, estimated after the method of [START_REF] Leake | Nomenclature of amphiboles[END_REF], the estimated temperature is ~ 70 ˚C higher than that calculated with Fe 3+ /Fe tot = 0.4.

ICPMS and LA-ICPMS:

Whole-rock major element analysis was performed on 7 samples (Table 3) at EOST in Strasbourg, France, by ICP-AES and ICP-MS (see Table 2; analytical procedure given in [START_REF] Omrani | Arc-magmatism and subduction history beneath the Zagros Mountains, Iran: A new report of adakites and geodynamic consequences[END_REF] and at UPMC Univ. Paris 6 (following the procedure of [START_REF] Salaün | Cannibalism of olivine-rich cumulate xenoliths during the 1998 eruption of Piton de la Fournaise (La Réunion hotspot): Implications for the generation of magma diversity[END_REF]).

LA-ICPMS was performed on amphibole in at IRD Bondy using a 7500 cx AGILENT NI Standards coupled with a laser NWR-213 nm. Analyses were carried out using a 10 Hz laser repetition rate and 50 µm beam diameter for amphibole. All analyses were conducted at fixed beam position. For internal standardization 43 Ca was used for amphibole. For external standardization, NIST SRM 610 and 612 glasses were used after [START_REF] Pearce | A Compilation of New and Published Major and Trace Element Data for NIST SRM 610 and NIST SRM 612 Glass Reference Materials[END_REF]. Raw data were processed using GLITTER (GEMOC, Macquarie University, Australia). The following masses were analyzed (

Radiometric dating

The selected samples were crushed and sieved; single grains of amphibole were handpicked under binocular microscope and cleaned in ultrasonic bath with acetone and distilled water. They were packaged in Al foils and irradiated for 40 hours in the core of the Triga Mark II nuclear reactor of Pavia (Italia) with several aliquots of the Fish Canyon sanidine standard (28.03 ± 0.08 Ma; [START_REF] Jourdan | Age calibration of the Fish Canyon sanidine 40 Ar/ 39 Ar dating standard using primary K-Ar standards[END_REF] as flux monitor. Argon isotopic interferences on K and Ca were determined by irradiation of KF and CaF 2 , pure salts from which the following correction factors were obtained:

( 40 Ar/ 39 Ar) K = 0.00969 ± 0.00038, ( 38 Ar/ 39 Ar) K = 0.01297 ± 0.00045, ( 39 Ar/ 37 Ar) Ca = 0.0007474 ± 0.000021 and ( 36 Ar/ 37 Ar) Ca = 0.000288 ± 0.000016. Argon analyses were performed at Géosciences Montpellier (France) with two analytical devices that each consist of: (a) an IR-CO2 laser of 100 kHz used at 5-15% during 60 sec, (b) a lenses system for beam focusing, (c) a steel chamber, kept at 10 -8 -10 -9 bar, with a drilled copper plate, (d) an inlet line for purification of gases including two Zr-Al getters, (e) a multi-collector mass spectrometer (Argus VI from Thermo-Fisher) or an MAP215-50 single collector mass spectrometer depending on the used device. A custom-made software controls the laser intensity, the timing of extraction/purification and the data acquisition. To measure the Ar background within the system, one blank analysis was performed every three sample analyses. ArArCalc© v2.5.2 was used for data reduction and plotting. The one-sigma errors reported on plateau, isochron and total gas ages include the error on the irradiation factor J. Atmospheric 40 Ar was estimated using a value of the initial 40 Ar/ 36 Ar of 295.5. Data are portrayed as age spectra on figure 11 and summarized in Table 5.

Petrography

Mafic rocks

Dark mafic bands (Fig. 6a,b,c) contain abundant amphibole (40-70 vol.%), plagioclase (10-50 vol.%), orthopyroxene (10-20 vol.%) ± olivine (0-25 vol.%).

Olivine is only found in some mafic layers (i.e. shear band #4bis; M8a, Fig. 6c). No clinopyroxene is found. Sulphide inclusions in amphibole, iron oxides and rare apatite (samples M8a and M8b) are also present.

Amphibole surrounds all other mineral phases and appears texturally as a late crystallization product with respect to olivine, orthopyroxene (Fig. 6d,e,g,7a,b,c,d).

Sample M8a show two distinct amphibole-bearing layers as an olivine-free amphibolite vein cross-cut a boudinaged olivine-bearing amphibolite band (Fig 6c). In olivine-bearing amphibolites (shear band #4bis), amphibole (referred to as early amphibole) is strongly pleochroic from blue to green under the microscope with a sub-automorph habitus and marked cleavages (Fig. 6d,g). By contrast, amphibole in olivine-free amphibolite (referred to as late amphibole) shows a distinctly paler pleochroism and presents a higher degree of deformation (Fig. 6e).

Textural relationships between plagioclase, orthopyroxene and olivine are obscured by the amphibolitization stage. Olivine appears as fine-grained clusters closely associated with, and generally included in, orthopyroxene (Fig. 6g,7a,b). In sample M4e, olivine was replaced by orthopyroxene before the crystallization of large amphibole grains (Fig. 7c,d). Olivine is also partially destabilized by tremolite.

Primary Cr-rich spinel crystals are present in olivine (Fig. 7a). In olivine-bearing samples, plagioclase is rare and partially to totally replaced by amphibole (Fig. 6g).

Leucocratic feldspar-rich veins are mainly composed of plagioclase (70-100%) with subordinate orthopyroxene (0-20%) and rare amphibole (0-10%). Undeformed leucocratic veins show euhedral amphibole in a groundmass of large plagioclase crystals (Fig. 6f). Close to the shear bands, veins of plagioclase evidence deformation and grain size reduction. Orthopyroxene is only partially preserved yet retaining textural evidence of former equilibrium with plagioclase. Orthopyroxene is included in amphibole or rimmed by amphibole pressure shadows (Fig. 6e), again suggesting the partial destabilization of coarse grained primary assemblage of orthopyroxene (± olivine) and plagioclase to form various generations of amphibole. A network of late serpentine, chlorite and talc is observed in orthopyroxene fractures (Fig. 6d).

As for ultramafic rocks, mafic bands are locally replaced by a phlogopite-rich matrix (in which phlogopite is in turn replaced by chlorite). Orthopyroxene and calcic-amphibole appear only as relicts into layers of phlogopite and acicular orthoamphibole. Plagioclase is destabilized and rimmed by a Ba-rich K-feldspar.

Ultramafic rocks (UM) and matrix

The mineralogy of undeformed peridotites is dominated by an isotropic network of serpentine (75-90%), with rare relicts of brown orthopyroxene, olivine phenocrysts and black chromiferous spinels. These peridotites belong to the highly depleted harzburgites of the Massif du Sud (i.e. [START_REF] Prinzhofer | Residual peridotites and the mechanisms of partial melting[END_REF][START_REF] Ulrich | Multiple melting stages and refertilization as indicators for ridge to subduction formation: The New Caledonia ophiolite[END_REF][START_REF] Pirard | Petrology and Geochemistry of the Crust-Mantle Boundary in a Nascent Arc, Massif du Sud Ophiolite, New Caledonia, SW Pacific[END_REF]. Few deformation textures are observed, where the extensive late serpentinization may have erased former deformation textures.

Peridotites boudinaged in shear bands (Fig. 7b) show a similar mineral assemblage. However, a reaction rim around peridotite boudins contains coarsegrained hornblende pseudomorphing orthopyroxene, and containing chromite inclusions (Fig. 7g). Light colored phlogopite (Fig. 7b,f,h) and acicular ferromagnesian ortho-amphibole (Fig. 5f) co-crystallized around peridotite boudins as a result of the partial destabilization of ultramafic orthopyroxene and secondary hornblende. Therefore, this assemblage postdates the boudinage of the peridotite and the first hydration recorded by the crystallization of hornblende. Phlogopite increases in modal proportion towards the amphibole-rich mafic bands, along with a grain size decrease but shows no preferred orientation. Chlorite is interlayered with phlogopite and crystallizes as a late stage alteration product responsible for the macroscopically greenish to greyish color of the matrix. Millimetric veinlets of talc-chloriteserpentine, slightly sheared in places, also cross-cut peridotites boudins (Fig. 7h).

These veinlets are themselves cross-cut by a late isotropic serpentine network.

Mineralogy

Orthopyroxene

Orthopyroxene from harzburgites displays a typical Mg# [Mg/(Mg+Fe)] of 0.92 ± 0.02 (Fig. 8a). In mafic rocks, the Mg# of orthopyroxene shows lower values ranging between 0.77 and 0.85.

Cr 2 O 3 and CaO contents (Fig. 7a) show small yet systematic variations between the mafic bodies, with higher values in olivine-bearing rocks (< 0.20 wt% Cr 2 O 3 ; 0.35-0.65 wt% CaO, except for three points with 0.82 and 0.97 wt% CaO) than in leucocratic feldspar-rich rocks (< 0.04 wt% Cr 2 O 3 ; 0.15-0.35 wt% CaO). Cr 2 O 3 and CaO contents are higher in ultramafic rocks, with large intra-sample variations (0.40-0.80 wt% Cr 2 O 3 ; 0.40-1.80 wt% CaO). Al contents are low (~0.02 p.f.u.) and show no systematic variation.

Orthopyroxene in harzburgite falls within the composition domain of orthopyroxene-poor harzburgite described by [START_REF] Prinzhofer | Structures in the new caledonia peridotites-gabbros: implications for oceanic mantle and crust[END_REF] and [START_REF] Pirard | Petrology and Geochemistry of the Crust-Mantle Boundary in a Nascent Arc, Massif du Sud Ophiolite, New Caledonia, SW Pacific[END_REF]Fig. 8a). Orthopyroxene in olivine-bearing and olivine free amphibolites overlaps the range of orthopyroxene in amphibolite and hornblendite dikes described by [START_REF] Pirard | Transfer of melts in the sub-arc mantle: Insights from high-pressure experiments and from the New Caledonia ophiolite[END_REF]. Orthopyroxene in mafic bands have similar Mg# orthopyroxene from gabbronorite sills of [START_REF] Pirard | Petrology and Geochemistry of the Crust-Mantle Boundary in a Nascent Arc, Massif du Sud Ophiolite, New Caledonia, SW Pacific[END_REF], but show lower Ca content.

Olivine

The patterns of Mg# of olivine (Fig. 8b) in harzburgite and in olivine-bearing mafic rocks mirror those of orthopyroxene described above (Fig. 8b). Mg# in ultramafic olivine is 0.91-0.92 and the MnO content varies between 0.06 and 0.22 wt% (Fig. 8b), in the same range as those of nearby harzbugites [START_REF] Pirard | Petrology and Geochemistry of the Crust-Mantle Boundary in a Nascent Arc, Massif du Sud Ophiolite, New Caledonia, SW Pacific[END_REF].

Olivine in mafic bands shows similar Mg# (0.78-0.85 wt%) to amphibolite and hornblendite dikes of [START_REF] Pirard | Transfer of melts in the sub-arc mantle: Insights from high-pressure experiments and from the New Caledonia ophiolite[END_REF]. The Mg# is lower and the MnO content (0.20-0.39 wt%) is higher than reported for mafic cumulates by [START_REF] Pirard | Petrology and Geochemistry of the Crust-Mantle Boundary in a Nascent Arc, Massif du Sud Ophiolite, New Caledonia, SW Pacific[END_REF]Fig. 8b).

NiO content in mafic olivine varies between 0.16 and 0.21 wt%, which is consistent with the most evolved olivine found in olivine-bearing cumulates [START_REF] Pirard | Petrology and Geochemistry of the Crust-Mantle Boundary in a Nascent Arc, Massif du Sud Ophiolite, New Caledonia, SW Pacific[END_REF].

Compositional trends in NiO and MnO vs. Mg# match the fractional crystallization trends evidenced by [START_REF] Pirard | Petrology and Geochemistry of the Crust-Mantle Boundary in a Nascent Arc, Massif du Sud Ophiolite, New Caledonia, SW Pacific[END_REF].

Feldspar

Plagioclase occurs only in mafic rocks, as veins in leucocratic feldspar-rich rocks (with a composition of a norite), or as scarce, isolated crystals in olivine-bearing rocks. In noritic rocks, albite content [Na# = Na/(Na+Ca)] ranges from 0.46 to 0.66 (from labradorite to andesine; Fig. 8c), in the domain of amphibolite dikes described by [START_REF] Pirard | Transfer of melts in the sub-arc mantle: Insights from high-pressure experiments and from the New Caledonia ophiolite[END_REF]. In olivine-bearing rocks, plagioclase corresponds to anorthite (Na#: 0.02-0.07; Fig. 8c), in the range of amphibolite dikes of [START_REF] Pirard | Transfer of melts in the sub-arc mantle: Insights from high-pressure experiments and from the New Caledonia ophiolite[END_REF]. Secondary feldspar rimming norite veins are rich in alkalis (~5.5 wt% Na 2 O; ~3.5 wt % K 2 O; ~12.5 wt% BaO) and have intermediate composition between anorthoclase and hyalophane.

Ca-amphibole

Amphibole shows Mg# comparable to that of orthopyroxene and olivine in both mafic (0.90-0.74; Fig. 9a) and ultramafic rocks (~0.91 ± 0.01; Fig. 9a), and Na < 0.5 p.f.u. and (Ca+Na) B > 1.0 p.f.u. typical of calcic-amphibole [START_REF] Leake | Nomenclature of amphiboles[END_REF].

In olivine-bearing mafic bodies, early amphibole is pargasitic with (Na+K) A ≥ 0.5 p.f.u and Si ranging from 6.12 to 6.30 p.f.u. (Fig. 9b). Late amphibole destabilizing this pargasite hornblende has (Na+K) A ≤ 0.5 p.f.u. and a higher Si content (6.50-7.00 p.f.u.; Fig. 9b), and belongs to the magnesio-tschermakite hornblende [START_REF] Leake | Nomenclature of amphiboles[END_REF]. In olivine-free samples (i.e M3c), late amphibole shows a decrease in (Na+K) A and an increase of the Si content (6.37-6.88 p.f.u.; Fig. 9b). This evolution is also observed with the content in Ti (0.15-0.05 p.f.u.). This composition is similar to the composition of amphibole from undeformed mafic dikes described by [START_REF] Pirard | Transfer of melts in the sub-arc mantle: Insights from high-pressure experiments and from the New Caledonia ophiolite[END_REF], and partially fall in the domain of the metamorphic sole amphibolites from Thio considering their Ti and (Na+K) A contents, yet with a much lower Mg# (see [START_REF] Cluzel | The metamorphic sole of New Caledonia ophiolite: 40Ar/39Ar, U-Pb, and geochemical evidence for subduction inception at a spreading ridge[END_REF].

Early amphibole in olivine-bearing amphibolite has the lowest Ti (0.01 and 0.05 p.f.u) and does not fall in the domains of amphibole-bearing dikes [START_REF] Pirard | Transfer of melts in the sub-arc mantle: Insights from high-pressure experiments and from the New Caledonia ophiolite[END_REF] or of the metamorphic sole [START_REF] Cluzel | The metamorphic sole of New Caledonia ophiolite: 40Ar/39Ar, U-Pb, and geochemical evidence for subduction inception at a spreading ridge[END_REF] (Fig. 9c). Amphibole with the highest Si content is again the poorest in (Na+K) A content (Fig. 9b). Tremolite is only present in olivine-bearing rocks and texturally associated to mafic olivine (Fig. 9b).

Calcic-amphibole destabilizing ultramafic orthopyroxene are magnesiohornblende with high Si content (7.19-7.46 p.f.u.), (Na+K) A between 0.25 to 0.12 p.f.u. (Fig. 9b) and Ti < 0.10 p.f.u. (Fig. 9c).

Late acicular amphibole crystallizing within the phlogopite-rich matrix belongs to the ferro-magnesian anthophyllite series (Si ~8 p.f.u; Al tot ≤ 0.23 p.f.u.; (Ca+Na) B ≤ 0.15 p.f.u.; [START_REF] Leake | Nomenclature of amphiboles[END_REF].

Phlogopite

Phlogopite presents a Mg# of 0.89, regardless of its structural position in the shear band (Table 1). Only few crystals display the K 2 O content of a regular mica owing to retrogression by chlorite. Cr 2 O 3 (< 0.02 wt%) and TiO 2 (0.05 wt%) are present in small amounts.

Accessory minerals

Early amphibole grains (i.e. pargasite) host various inclusions (e.g. spinel, magnetite, sulfides). Magnetite inclusions derive from the oxidation of iron-sulphide and spinel. Spinel in harzburgite is a chromite with a Cr# [Cr# = (Cr/(Cr+Al+Fe 3+ )] of 0.72 ± 0.05 and a Mg# of 0.35 ± 0.05 (Fig. 9d). Spinel is consistently more aluminous in mafic rocks, with a Cr# from 0.18 to 0.38 for a Mg# varying between 0.25 and 0.32 (Fig. 9d). TiO 2 in mafic and ultramafic spinel is below the detection limit. Cu-and Ni-rich Fe-sulphides (i.e. pentlandite-pyrrhotite) are found as inclusions in early amphibole in olivine-bearing mafic rocks. 10a). Large Ion Lithophile Elements (LILE; Pb, U, Th, Ba, Rb, Cs) show large enrichments relative to REE, increasing with their degree of mobility and incompatibility [(Cs/Lu) N = 62 ± 39] (Fig. 10a).

Geochemistry

The compositions range of metamorphic sole [START_REF] Cluzel | The metamorphic sole of New Caledonia ophiolite: 40Ar/39Ar, U-Pb, and geochemical evidence for subduction inception at a spreading ridge[END_REF] display a similar pattern to amphibolites though slightly shifted towards higher values by a factor of ~2 (Fig. 10a). Gabbronorite sills [START_REF] Pirard | Petrology and Geochemistry of the Crust-Mantle Boundary in a Nascent Arc, Massif du Sud Ophiolite, New Caledonia, SW Pacific[END_REF] are highly depleted in all incompatible elements. However, they display a pattern roughly similar to that of amphibolites, with a mean relative depletion factor of ~10 but with larger Pb and Sr relative anomalies (Fig. 10a). Arc tholeiitic dikes [START_REF] Cluzel | Earliest Eocene (53 Ma) convergence in the Southwest Pacific: evidence from pre-obduction dikes in the ophiolite of New Caledonia[END_REF] have incompatible elements compositions overlapping with those amphibolites (Fig. 10a).

It strongly suggests that tholeiitic dikes and amphibolites have a same origin.

In phlogopite-rich rocks, concentrations in HREE are similar to those of olivinefree and olivine-bearing amphibolite (Fig. 10a). However, phlogopite-rich rocks have a larger enrichment in LREE relative to HREE [(La/Lu) N = 3.55 ± 1.77]. These rocks are depleted in HFSE relative to REE but less depleted than olivine-bearing and olivine-free amphibolite (with the exception of the sample M4f). Sr shows a large depletion relative to REE and LILE display a large enrichment especially in Cs.

7b. In-situ trace elements

Incompatible trace elements compositions of early and late amphibole, respectively found in olivine-bearing and olivine-free amphibolites display similar N-MORB normalized patterns (Fig. 10b) with however slightly higher content in late amphibole (i.e. magnesio-hornblende) compared to early amphibole (i.e. pargasite). anomalies in LILE) are observed in whole-rock data on amphibolites and in-situ data on amphibole, suggesting that the trace element budget in the rocks is mainly controlled by amphibole. However, higher HREE content in late amphibole compared to that in early amphibole is difficult to explain with a fluid mediated recrystallization. This higher content more likely mirrors the more differentiated nature of the norite dikes compared to the troctolite dikes (prior to metamorphic hydration).

Amphibole in mafic amphibole-bearing dikes analyzed by [START_REF] Pirard | Transfer of melts in the sub-arc mantle: Insights from high-pressure experiments and from the New Caledonia ophiolite[END_REF] displays a pattern similar to that of the amphibole reported here (grey overlay; Fig. 10b): it shows a similar enrichment in Pb and depletions in Ti-Sr-Nb-Ta with comparable absolute values, including in the most mobile and incompatible trace elements, advocating for similar origins. Ba-content is similar in both analyses series and Rb-content is lower in the amphibole of [START_REF] Pirard | Transfer of melts in the sub-arc mantle: Insights from high-pressure experiments and from the New Caledonia ophiolite[END_REF] than all our early and late amphiboles (Fig. 10b). Cs has not been measured.

40

Ar/ 39 Ar constraints on mafic rocks formation 40 Ar/ 39 Ar laser probe step-heating dating was carried out on 20 isolated single amphibole crystals from 11 different rocks, using the procedure described above.

These samples are mainly olivine-free and olivine-bearing amphibolites from the Plum beach. They also include amphibolites from the Thio metamorphic sole (sample C10) and undeformed gabbroic dikes from the Massif du Sud (samples SNC03 and SNC04, col des Deux Tétons). Age spectra and inverse isochron plots are reported on figure 11 for some of these samples. The closure temperature for the 40 Ar/ 39 Ar method in hornblende is around 550 ± 50 ºC (Harrison, 1981). For most samples, errors are relatively large due to the fact that only one grain was analyzed each time and that the K content (and therefore the amount of radiogenic Ar) is low in these amphiboles.

Amphibole from the Thio metamorphic sole provides an age of 56.9 ± 2.6 Ma (Fig. 11a,12) consistent with the age reported by [START_REF] Cluzel | The metamorphic sole of New Caledonia ophiolite: 40Ar/39Ar, U-Pb, and geochemical evidence for subduction inception at a spreading ridge[END_REF] from a nearby exposure. Similar ages are obtained on the two undeformed gabbros from the Massif du Sud for which intercept ages are in the range 51-52 ± 4 Ma (Fig. 11a,12; see the Online Fig. S1 for isotope correlation plot), with initial 40 Ar/ 36 Ar values close to the present-day atmospheric ratio (see Table 5). Large amphibole from a partially weathered pegmatite dike yields more scattered results with some evidence of excess argon as a result of fluid infiltration and only a maximum age of 60 ± 5 Ma can be attributed to this sample.

At Plum beach, 8 hornblende crystals from 4 olivine-free amphibolites (M3d, M4d, M8a-Z2, M8f) and 5 hornblende crystals from two weakly deformed olivinebearing amphibolite (M15 and M8a-Z1) yield ages ranging between 54 and 93 Ma, with the exception of one crystal (M15) that displays a strongly discordant age pattern and an integrated age of 167 ± 21 Ma likely related to trapped excess argon. For M8f, 4 amphiboles were analyzed and representative age spectra are shown on figure 11b, with plateau dates ranging from 54.9 ± 2.0 Ma to 59.2 ± 2.7 Ma and a similar age in the isotope correlation plot with an initial atmospheric ratio (reported in the Online Fig. S1). The consistency of the results on 4 grains from M8f and the lack of evidence of excess argon for this sample suggest that amphibolitisation and related deformation occurred at ca. 55 Ma, which is supported by the age of 54.1 ± 3.4 Ma obtained on amphibolite M3b from another shear zone. According to microprobe analyses and textural observations, it is likely that this age of ca. 55 Ma corresponds to the crystallization of secondary magnesio-hornblende that partially replaced pargasite and orthopyroxene. This age range coincides with the formation of the metamorphic sole at Thio (sample C10 and [START_REF] Cluzel | The metamorphic sole of New Caledonia ophiolite: 40Ar/39Ar, U-Pb, and geochemical evidence for subduction inception at a spreading ridge[END_REF], the emplacement of some gabbros (samples SNC03 and SNC04) and pre-obduction felsic dikes (53 Ma; [START_REF] Cluzel | Earliest Eocene (53 Ma) convergence in the Southwest Pacific: evidence from pre-obduction dikes in the ophiolite of New Caledonia[END_REF]. 8 amphiboles from 5 olivine-bearing samples yield older ages ranging from 73 to 93 Ma (Table 5) with relatively discordant age spectra (Fig. 11c) and large experimental errors due to the low amount of K. However, the isotope correlation diagram and the flat K/Ca ratios do not provide evidence of excess argon (reported in the Online Fig. S1), which could indicate that ages around 90 Ma are meaningful. This suggests that early amphibole is preserved in the dated rocks, which is consistent with the chemical data that point to the coexistence of primary pargasite predating synkinematic magnesio-hornblende developed around it.

Interpretations and discussion

Nature of the protolith and subduction signature

The sharp contact between mafic rocks and surrounding peridotites (preserved from metasomatic alteration in places) suggests that these mafic rocks were emplaced, prior to amphibolitization, as 100 m-scale intrusions, most likely as dikes or melt lenses. The lack of pervasive deformation in the peridotite a few meters away from the shear bands, and the presence of olivine and orthopyroxene in mafic rocks, rule out the possibility that the mafic bands represent sheared metamorphic sole amphibolites imbricated within mantle wedge peridotites. The protolith of the mafic bands is therefore interpreted as emplaced in the form of small-scale melt intrusions at temperatures where peridotites were comparatively brittle.

Partially preserved orthopyroxene, anorthitic plagioclase and high Mg# olivine indicate that the protolith of the amphibolite bands was a norite or a troctolitic norite.

Late hornblende (i.e. magnesio-hornblende) and Ca-Na plagioclase, which abound in amphibolite bands, may have recrystallized from orthopyroxene, olivine and anorthitic plagioclase (such as found in the "noritic" pods) during amphibolitisation and/or later fluid infiltration. Clinopyroxene was either absent or is now completely replaced (undeformed hornblendite dikes from Rivière des Pirogues show late amphibole replacing large clinopyroxene crystals; [START_REF] Pirard | Transfer of melts in the sub-arc mantle: Insights from high-pressure experiments and from the New Caledonia ophiolite[END_REF].

Olivine-bearing domains cross-cut by olivine-free domains (Fig. 6c) point to evolving compositions of the magma intruding the peridotite. The positive correlation between Cr# of spinel and Mg# of olivine in olivine-bearing amphibolites suggests that the mafic protolith formed through melting of harzburgite with a suprasubduction signature followed by fractional crystallization [START_REF] Marchesi | Migration and accumulation of ultra-depleted subduction-related melts in the Massif du Sud ophiolite (New Caledonia)[END_REF].

The presence of orthopyroxene, plagioclase and early amphibole (i.e. pargasite) is consistent with peridotite melting triggered by aqueous slab-derived fluids [START_REF] Pirard | Transfer of melts in the sub-arc mantle: Insights from high-pressure experiments and from the New Caledonia ophiolite[END_REF]. Amphibolites display flat N-MORB normalized patterns with lower REE abundances than N-MORB (REE Amphibolite / REE N-MORB ~0.5; Fig. 10). A simple melting model shows that these amphibolites are too enriched, however, to derive directly from melting of the host depleted harzburgite and require a less depleted ultramafic source (either refertilized by fluids or silica-rich melts). Trace elements patterns of amphibolites in fact closely fit those of unmetamorphosed microdioritic dikes found in the Massif du Sud, interpreted as subduction-related magma intrusions by [START_REF] Cluzel | Earliest Eocene (53 Ma) convergence in the Southwest Pacific: evidence from pre-obduction dikes in the ophiolite of New Caledonia[END_REF].

The slab signature is best recorded by HFSE and LILE. HFSE in the bulk rock display negative anomalies relative to REE (Fig. 10). These are characteristic of subduction zone magmas, as these elements are retained in rutile and zircon in the subducting slab (e.g. [START_REF] Coltorti | Amphiboles from suprasubduction and intraplate lithospheric mantle[END_REF][START_REF] Zack | An evaluation of reactive fluid flow and trace element mobility in subducting slabs[END_REF]. The enrichment in LILE (and to a lesser extent in LREE) is also consistent with a subduction signature, since LILE are mobilized during the dehydration of the upper part of the slab (e.g. [START_REF] Pearce | Characteristics and tectonic significance of suprasubduction zone ophiolites[END_REF]. Such enrichments are observed in metamorphic soles for Rb, Ba and also Pb [START_REF] Ishikawa | Trace element characteristics of the fluid liberated from amphibolite-facies slab: Inference from the metamorphic sole beneath the Oman ophiolite and implication for boninite genesis[END_REF]. These enrichments cannot result from late subsurface alteration, since LILE and HFSE are mainly concentrated in high temperature amphiboles which impose their geochemical signature in the whole rock (Fig. 10b; Table 4).

Petrological evolution, deformation and fluid ingression

Field relationships and mineralogy suggest that the dark amphibolite bands and the more leucrocratic noritic veins are cogenetic, with percolation apparently favored towards the north. This direction is compatible with upward melt migration expected at plate boundary (e.g. [START_REF] Matthews | Geologic and kinematic constraints on Late Cretaceous to mid Eocene plate boundaries in the southwest Pacific[END_REF] within the mantle wedge (Fig. 2,13a) and close to the nascent crust (e.g. [START_REF] Pirard | Petrology and Geochemistry of the Crust-Mantle Boundary in a Nascent Arc, Massif du Sud Ophiolite, New Caledonia, SW Pacific[END_REF]. Progressive aqueous fluid infiltration and amphibolitization of the preexisting mafic bands would have preferentially localized deformation in the former dikes during the first stages of the Eocene subduction. Fig. 4c and Fig. 5c exemplify mafic veins with peridotite boudins.

Deformation localization is here closely tied with fluid infiltration, consistently with the weaker rheology of amphibole and plagioclase compared to pyroxene [START_REF] Homburg | Investigation of the strength contrast at the Moho: A case study from the Oman Ophiolite[END_REF] and olivine.

Three main hydration stages can be seen in these rocks and related to deformation events, as schematized in Fig. 13:

-(a) amphibolitization stage:

Replacement of orthopyroxene by late amphibole is the second event of hydrous fluid intrusion and the major deformation stage (Fig. 13b). Whereas magnesiohornblende in undeformed leucocratic veins has an euhedral shape and are associated to a groundmass of large plagioclase crystals (Fig. 6f), magnesio-hornblende within the shear bands makes up the foliation, together with plagioclase (Fig. 6e), and presents syn-kinematic texture (e.g. pressure shadow around orthopyroxene; Fig. 6e).

Euhedral late amphibole could be of magmatic origin, but syn-kinematically deformed late amphibole has likely crystallized from an aqueous fluid. Nonetheless, evolving late amphibole (and possibly plagioclase) compositions (Fig. 8c,9b,c) indicate that mafic bands underwent progressive recrystallization during fluid infiltration and deformation (localized within the shear bands).

Although mafic bands are strongly boudinaged and folded, amphibole crystals themselves are only mildly deformed. This is consistent with plagioclase being weaker than amphibole [START_REF] Getsinger | Amphibole fabric formation during diffusion creep and the rheology of shear zones[END_REF] and with some static recrystallization after the peak of deformation. Peridotites in contact with mafic dikes are also boudinaged during this stage and record hydration through the crystallization of amphibole with a Mg# around 0.92 (Fig. 9a).

Magnesio-hornblende (i.e. late amphibole) compositions yield pressuretemperature conditions 3-5 kbar and 750 ± 50 ºC (Table 2). These estimations are similar to that calculated for leucodiorite dikes in the uppermost harzburgite (i.e. 3.5 ± 0.6 kbar and 800 ± 45 ˚C; [START_REF] Pirard | Transfer of melts in the sub-arc mantle: Insights from high-pressure experiments and from the New Caledonia ophiolite[END_REF]. As for metamorphic sole, the pressure estimates for the dikes cannot be explained by the present ophiolite thickness.

Age constraints on amphibole reveal an age cluster at 55 ± 2 Ma (Fig. 12), which is well defined for late amphibole. Although the closure temperature for the 40 Ar/ 39 Ar method in hornblende is ~200 ºC lower than our temperature estimates for amphibole formation, we interpret these ages as crystallization ages in a context of fast cooling. Considering available constraints on the latest oceanic magmatic events, pressure-temperature estimates and the age of the New Caledonia metamorphic sole (Cluzel et al., 2012 and this study), high temperature metamorphism and cooling below the closure temperature must have occurred in a very short lapse of time after the Eocene subduction initiation.

-(b) formation of the alkali-rich matrix:

The alkali-rich matrix is composed of an association of Ba-rich feldspar, phlogopite and ortho-amphibole crystallized at the expense of pyroxene, olivine and Ca-amphibole, from both the mafic and ultramafic protoliths, with less deformation than during the main amphibolitization stage (Fig. 13c). This matrix points to an episode of fluid ingression postdating amphibolitization, with a markedly distinct fluid composition (i.e. richer in K, Na and Ba). Ortho-amphibole (anthophyllite) indicates temperatures of ~700 ± 50 °C [START_REF] Chernosky | The stability of anthophyllite in the presence of quartz[END_REF]Autio, 1979, Nozaka, 2011).

-(c) Late reaction with serpentine, talc, chlorite:

Finally, veinlets composed of hydrous phases (serpentine, talc and chlorite) form mm to cm-scale rims around mafic and peridotite boudins or cross-cut the matrix (Fig. 13d). With temperatures lower than 500-600 °C, according to presence of the talc-chlorite assemblage, this stage reflects the latest and coldest episode of reequilibration of these shear bands.

Geodynamics implications

Both petrology and geochemistry (through the similarity to undeformed amphibole-bearing dikes reported in the Massif du Sud; [START_REF] Cluzel | Earliest Eocene (53 Ma) convergence in the Southwest Pacific: evidence from pre-obduction dikes in the ophiolite of New Caledonia[END_REF][START_REF] Pirard | Transfer of melts in the sub-arc mantle: Insights from high-pressure experiments and from the New Caledonia ophiolite[END_REF] suggest that mafic shear bands correspond to former hm-to km-scale mafic intrusions emplaced in the highly refractory host harzburgite, deriving from the melting of a moderately depleted mantle source fluxed by subduction-derived fluids (see § 9.1).

Trace element geochemistry (i.e. HFSE depletion and LILE enrichment) and age constraints at ca. 55 Ma (Fig. 12) show that secondary amphibolitization coincides with the east-dipping subduction initiation preluding to obduction (Spandler et al., 1995;[START_REF] Cluzel | Tectonic accretion and underplating of mafic terranes in the Late Eocene intraoceanic fore-arc of New Caledonia (Southwest Pacific): geodynamic implications[END_REF][START_REF] Cluzel | The metamorphic sole of New Caledonia ophiolite: 40Ar/39Ar, U-Pb, and geochemical evidence for subduction inception at a spreading ridge[END_REF]. Fluid infiltration and at least part of the trace element enrichment could result from dehydration of the upper part of the slab (as for metamorphic sole dehydration; [START_REF] Ishikawa | Trace element characteristics of the fluid liberated from amphibolite-facies slab: Inference from the metamorphic sole beneath the Oman ophiolite and implication for boninite genesis[END_REF]. In this interpretation, the shear bands reported here are a direct record of early subduction processes in New Caledonia, deformation and mantle metasomatism in an active subduction setting.

By contrast, the magmatic origin of the mafic protoliths remains uncertain. The high degree of amphibolitization affecting these mafic dikes hampers modeling the degree of partial melting of the peridotites and to constrain the exact origin of the mafic protoliths.

Age constraints on amphiboles at both ca. 90 Ma (early pargasite) and ca. 55 Ma (magnesio-hornblende) can be accounted for in two different ways:

(1) amphibole-bearing troctolite to norite mafic intrusions would have formed at ca. 55 Ma as a result of the east-dipping subduction preluding to obduction, as for the relatively abundant dikes interpreted as supra-subduction and dated at ca. 55 Ma by [START_REF] Cluzel | Earliest Eocene (53 Ma) convergence in the Southwest Pacific: evidence from pre-obduction dikes in the ophiolite of New Caledonia[END_REF] or dikes found in the nearby Massif du Sud exposures (also interpreted as Eocene, based on structural data; Pirard, 2012), both interpreted as formed in a supra-subduction setting at the expense of a metasomatized, HFSEdepleted mantle. In this view, ages at ca. 90 Ma would be attributed to excess argon.

(2) mafic intrusions would represent inherited mafic melts from the Cretaceous South Loyalty basin accretion (dating back to ≥ 90 Ma; e.g. [START_REF] Cluzel | The metamorphic sole of New Caledonia ophiolite: 40Ar/39Ar, U-Pb, and geochemical evidence for subduction inception at a spreading ridge[END_REF] this study), as several early amphiboles yield an age of ca. 90 Ma (Fig. 12). This hypothesis is supported by the similarity with ages found in amphibole-bearing dikes in the Massif du Sud by [START_REF] Prinzhofer | Structure et pétrologie d'un cortège ophiolitique: le Massif du Sud (Nouvelle Calédonie) : la transition manteau-croûte en milieu océanique[END_REF]. Hydrous mafic melt intrusions would have been emplaced possibly as a result of the west-dipping Pacific subduction, and later been fluxed by fluids leading to late amphibole crystallization and ages at ca. 55 Ma (during the second, east-dipping subduction initiation). This secondary high temperature event has only partially reset the ca. 90 Ma age in pargasite.

The second interpretation is favored by the lack of evidence for excess argon, early amphibole (pargasite) compositions that are distinct from those of [START_REF] Pirard | Transfer of melts in the sub-arc mantle: Insights from high-pressure experiments and from the New Caledonia ophiolite[END_REF], and the peculiar deformation of these dikes (those of Pirard and Cluzel are not or only mildly deformed).

Deformation and fluid infiltration later localized on these preexisting mafic intrusions. Shear bands developed as a result of a threefold stage of fluid infiltration from slab-derived fluids. These mafic bands (and the later metasomatic matrix) are good witnesses of the subduction initiation processes, both in terms of fluid influx and progressive deformation near the base of the obducted New Caledonia ophiolite, from temperatures of 800-700 °C down to less than 600 °C. Localization of deformation along inherited structures such as mafic dike swarms also supports the view that subduction initiation is generally triggered near a preexisting weakness zone (i.e. [START_REF] Agard | Obduction: Why, how and where. Clues from analog models[END_REF].

Conclusions

This study focuses on strongly sheared mafic amphibolites within the uppermost mantle of the New Caledonia obducted ophiolite, near the transition zone and ~50-100 m above the basal thrust contact. Using a combination of petrological, geochemical, and geochronological data, we show that these amphibolites record melting and polyphase metasomatism within the mantle wedge during the early history of the subduction.

Shear bands indicate that deformation and high temperature fluid infiltration (from ~ 800 to ~500 °C) localized along preexisting hm-scale mafic intrusions emplaced within the upper mantle of the New Caledonia ophiolite and derived from peridotites refertilized by slab-derived fluids in a supra-subduction zone environment.

The timing of these melt intrusions remains uncertain. Age constraints on early amphibole at ca. 90 Ma and the lack of evidence for argon excess suggest that they could be inherited from the former west-dipping Pacific subduction.

Secondary deformation and amphibolitization of these mafic dikes were the result of a threefold infiltration of slab-derived fluids. The age cluster of amphibole at ca. 55 Ma indicates that amphibolitization of the mafic protoliths (as well as later alkali-metasomatism) relate to the onset of the Eocene east-dipping subduction initiation preluding to the obduction of the New Caledonia ophiolite.

These outcrops bring important information on the processes and timing of fluid influx, progressive deformation and peridotites refertilization during subduction initiation, showing how m-to hm-scale heterogeneities (inherited or early-emplaced magmatic dikes) localize deformation and metasomatism in the oceanic lithosphere.

In turn this suggests that similar small-scale heterogeneities may have controlled the development of larger thrusts, ultimately leading to the tectonic slicing (and later emplacement) of the New Caledonia ophiolite. 1). Shear band (SB) #2 is not represented at this scale. Abbreviations after [START_REF] Whitney | Abbreviations for names of rock-forming minerals[END_REF]. Abbreviations after [START_REF] Whitney | Abbreviations for names of rock-forming minerals[END_REF]. [START_REF] Marchesi | Migration and accumulation of ultra-depleted subduction-related melts in the Massif du Sud ophiolite (New Caledonia)[END_REF] (3) Horblendite and amphibolite dikes [START_REF] Pirard | Transfer of melts in the sub-arc mantle: Insights from high-pressure experiments and from the New Caledonia ophiolite[END_REF]. [START_REF] Sun | Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes[END_REF]. Data for the metamorphic sole are from [START_REF] Cluzel | The metamorphic sole of New Caledonia ophiolite: 40Ar/39Ar, U-Pb, and geochemical evidence for subduction inception at a spreading ridge[END_REF] and for pre-obduction dikes (53 Ma) from [START_REF] Cluzel | Earliest Eocene (53 Ma) convergence in the Southwest Pacific: evidence from pre-obduction dikes in the ophiolite of New Caledonia[END_REF].

Captions

Gabbronorite sills from the crustal cumulates are from [START_REF] Pirard | Petrology and Geochemistry of the Crust-Mantle Boundary in a Nascent Arc, Massif du Sud Ophiolite, New Caledonia, SW Pacific[END_REF]. (b) Trace elements spider-diagrams of hornblende in olivine-free and olivine-bearing amphibolite M8a. Normalization values for the NMORB are from [START_REF] Sun | Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes[END_REF]. Grey area represents the trace elements values for amphibole in hornblendite and diorite dikes described by [START_REF] Pirard | Transfer of melts in the sub-arc mantle: Insights from high-pressure experiments and from the New Caledonia ophiolite[END_REF]. Plots of 40 Ar/ 39 Ar plateau age on amphibole from the different mafic units of the New Caledonia ophiolite in comparison of the timing of intrusion of pre-obduction dikes from [START_REF] Cluzel | Earliest Eocene (53 Ma) convergence in the Southwest Pacific: evidence from pre-obduction dikes in the ophiolite of New Caledonia[END_REF] and the metamorphic sole is from [START_REF] Cluzel | The metamorphic sole of New Caledonia ophiolite: 40Ar/39Ar, U-Pb, and geochemical evidence for subduction inception at a spreading ridge[END_REF]. Highpressure rocks are from [START_REF] Spandler | Late Cretaceous-Tertiary tectonics of the southwest Pacific: Insights from U-Pb sensitive, high-resolution ion microprobe (SHRIMP) dating of eclogite facies rocks from New Caledonia[END_REF] and Cretaceous gabbroic dikes crosscutting the peridotite are from [START_REF] Prinzhofer | Structure et pétrologie d'un cortège ophiolitique: le Massif du Sud (Nouvelle Calédonie) : la transition manteau-croûte en milieu océanique[END_REF]. White-colored data are for 40 Ar/ 39 Ar plateau ages. Black-colored data are for 40 Ar/ 39 Ar preferred ages (plateau age or intercept age). See also Table 5. b.d.l b.d.l b.d.l b.d.l TiO2 b.d.l b.d.l b.d.l b.d.l b.d.l 0.12 b.d.l b.d.l Al2O3 b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l K2O b.d.l b.d.l b.d.l b.d.l K2O b.d.l b.d.l b.d.l b.d.l b.d.l b.d b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l Na2O 0.21 0.54 6.47 6.61 0.46 K2O b.d.l b.d.l b.d.l b.d.l K2O b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l K2O b.d.l b.d.l b.d.l b.d b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l Abreviations after [START_REF] Whitney | Abbreviations for names of rock-forming minerals[END_REF]

  7a. Whole rock geochemistry Rare earth elements (REE) normalized to N-MORB of both olivine-bearing and olivine-free amphibolites display values lower than 1 [(REE) N ~ 0.4], with the exception of the sample M4f [(REE) N ~ 1)] (Fig. 10a). They display a slight enrichment in light REE (LREE) relative to heavy REE (HREE) [(La/Lu) N = 1.89 ± 0.35]. High field strength elements (HFSE; Ti, Hf, Zr, Nb, Ta) are slightly depleted relative to REE [(Nb/Lu) N = 0.41 ± 0.05] (Fig.

  REE patterns from heavy REE (HREE) to light REE (LREE) are flat and close to N-MORB values, with (La/Lu) N = 0.87 ± 0.39. HFSE are slightly depleted relative to REE [(Nb/Lu) N = 0.21 ± 0.07]. LILE show positive anomalies [(Cs/Lu) N = 12.1 ± 5.9]. Overall, similar behaviors (i.e. negative anomalies in HFSE and positive

Figure 1 a

 1 Figure 1

Figure 2

 2 Figure 2

Figure 3

 3 Figure 3

Figure 4

 4 Figure 4

Figure 5

 5 Figure 5

Figure 6

 6 Figure 6

Figure 7

 7 Figure 7

Figure 8 EPMA

 8 Figure 8

Figure 9 EPMA

 9 Figure 9

Figure 11

 11 Figure 11

Figure 12

 12 Figure 12

Figure 13

 13 Figure 13

Table 4 )

 4 : 7 Li, 11 B, 47 Ti, 85 Rb, 88 Sr, 89 Y, 90 Zr, 93 Nb, 133 Cs, 138 Ba, 139 La, 140 Ce, 141 Pr, 146 Nd, 147 Sm, 153 Eu, 157 Gd, 159 Tb, 169 Dy, 165 Ho, 166 Er, 172 Yb, 175 Lu, 178 Hf, 181 Ta, 208 Pb, 238 U, 232 Th.

Table 1 :

 1 Mineral occurrence for selected samples.

	Unit	Sample ol opx fsp Ba-fsp hbl	ath	phl	chl/tlc	srp
	Undeformed peridotite	M12 M14	x x	x x							l l
		M9	x	x	v			o	o	l	l
	Peridotite boudin	M10	x	x	v	o	o	o	o	l	l
		M16	x	x	v		o	o	o	l	l
		M8a	x	x	x		o			l
	Ol-bearing amphibolite	M8b M4e	x x	x x	x x		o o			l
		M15	x	x	x		o
		M3c		x	x		o
		M3b		x	x		o
		M4c		x	x		o
	Ol-free amphibolite	Mx		x	x		o
		M4d		x	x		o		l	l
		M4g		x	x		o		l	l
		8e		x	x		o

x: primary phases, o: secondary phases; l: late phases; v: vein wrapping peridotite boudin. Mineral abreviations after

[START_REF] Whitney | Abbreviations for names of rock-forming minerals[END_REF]

. Ba-fsp: celsian.

Table 2 :

 2 Selected representative EPMA analyses for olivine, orthopyroxene, plagioclase and amphibole. Fe3+ for amphibole calculated after[START_REF] Leake | Nomenclature of amphiboles[END_REF]

	Min.	Olivine	Min.	Orthopyroxene
	Samp. M4e M8a1 M8b UM10 Samp. M3c M4e M8a1 M8a2 M8b M8e UM10 UM16
	SiO2	40.05 39.56 40.24 41.62 SiO2	55.85 55.02 54.51 54.69 56.04 55.86 57.44 58.08
	TiO2

Table 2 :

 2 Selected representative EPMA analyses for olivine, orthopyroxene, plagioclase and amphibole. Fe3+ for amphibole calculated after[START_REF] Leake | Nomenclature of amphiboles[END_REF]

	.l	b.d.l	b.d.l

Table 3 :

 3 Whole rock geochemistry for representative amphibolite dikes. ) mafic amphibolite altered by 1-the phlogopite-rich matrix and 2-late chlorite-rich matrix b.d.l: below detection limit

	.l	7.92

Table 4 :

 4 : LA-ICP-MS geochemistry for representative Ca-amphibole in amphibolite dikes.

	Sample	M8a1	M8a1	M8a1	M8a2	M8a2	M8a2
	Mineral	prg	prg	prg	mhb	mhb	mhb
	SiO2 (wt%)	43.12	45.46	43.98	48.14	43.82	46.32
	TiO2	0.30	0.09	0.21	0.53	0.77	0.85
	Al2O3	13.99	12.79	14.11	10.48	13.13	11.82
	Cr2O3	0.07	0.15	0.24	0.17	0.10	0.07
	FeO	8.97	6.53	7.85	7.70	9.29	8.76
	MnO	0.16	0.07	0.10	0.13	0.08	0.06
	MgO	15.84	17.43	16.28	16.80	15.24	16.10
	CaO	11.83	12.37	12.12	11.64	11.89	12.01
	Na2O	2.79	2.39	2.44	1.82	2.48	2.14
	K2O	0.07	0.04	0.05	0.04	0.07	0.07
	Sum	97.13	97.31	97.37	97.47	96.87	98.20
	Cs (ppm)	b.d.l.	b.d.l.	b.d.l.	b.d.l.	0.24	0.07
	Rb	b.d.l.	b.d.l.	b.d.l.	b.d.l.	3.72	b.d.l.
	Ba	2.18	2.05	2.77	2.68	5.21	3.79
	Nb	0.36	0.31	0.40	0.68	0.65	0.87
	Ta	b.d.l.	b.d.l.	b.d.l.	b.d.l.	0.11	0.15
	La	2.23	1.90	2.26	2.57	2.67	2.81
	Ce	6.31	5.76	6.63	7.95	7.09	9.64
	Pb

  .

	Pr	0.95	0.92	1.23	1.60	1.21	1.67
	Sr	73.11	68.75	67.36	41.47	46.78	48.35
	Nd	4.05	4.59	5.67	7.81	8.66	10.30
	Zr	24.49	23.12	30.51	48.31	38.45	56.43
	Hf	1.13	0.85	1.04	1.95	1.62	2.08
	Sm	1.50	b.d.l.	1.64	3.23	2.96	2.74
	Eu	0.87	0.76	0.81	1.08	1.09	1.46
	Gd	b.d.l.	1.41	3.34	3.10	3.62	6.02
	Tb	0.33	0.32	0.62	0.82	0.69	1.20
	Dy	2.76	2.86	3.86	5.97	6.50	8.42
	Ho	0.67	0.68	0.93	1.31	1.18	2.03
	Er	2.36	1.60	2.80	4.20	3.43	5.65
	Yb	2.26	1.44	2.53	4.72	4.13	5.58
	Lu	0.22	0.36	0.49	0.62	0.67	0.76
	See appendix for spider diagrams.

Acknowledgments

We thank M. Ulrich (IPGS, France) for interesting discussions. We also thank B. Caron and O. Boudouma (ISTeP-UPMC, France), M. Fialin and N. Rividi (CAMPARIS), F. Le Cornec (IRD Bondy, France) and D. Deldicque (ENS Paris, France) for analytical support, M. Moroni (ISTeP-UPMC) for the scanning of thinsections. C. Pirard and J. Hermann are warmly thanked for their constructive reviews and suggestions, and Marco Scambelluri for his editorial handling. This work was essentially funded through the ONLAP project (ANR blanche, SIMI6; 2010 BLAN 615 01). Additional support was provided by an IUF grant (Institut universitaire de France) to Philippe Agard.

Online appendix

Figure S1

Corresponding reverse isochron plots ( 36 Ar/ 40 Ar vs. 39 Ar/ 40 Ar) are shown for some of the measurements, with intercept age, 36 Ar/ 40 Ar ratio and final reduced chi square (MSWD). Samples: SNC04: undeformed gabbroic dikes cross-cutting the Massif du Sud ophiolite; M15: olivine-bearing amphibolite; M8f and M3b: olivine-free amphibolite.