Valentin Laurent
email: valentin.laurent@univ-orleans.fr

Benjamin Huet
email: benjamin.huet@univie.ac.at

Loïc Labrousse
email: loic.labrousse@upmc.fr

Laurent Jolivet
email: laurent.jolivet@univ-orleans.fr

Patrick Monie
email: patrick.monie@gm.univ-montp2.fr

Romain Augier
email: romain.augier@univ-orleans.fr

Extraneous argon in high-pressure metamorphic rocks: Distribution, origin and transport in the Cycladic Blueschist Unit (Greece)

The 40 Ar/ 39 Ar geochronological technique has significantly contributed to the study of crustal deformation. However, the frequent occurrence of extraneous argon in high-pressure terrains can make the interpretation of 40 Ar/ 39 Ar ages problematic. This contribution attempts to: i) characterize the distribution and origin of extraneous argon in the Cycladic Blueschist Unit (CBU, Greece) by applying the 40 Ar/ 39 Ar dating technique on three Cycladic islands, Syros, Tinos and Ios, which span a complete transect of the CBU and ii) discuss regional implications for the tectonometamorphic evolution of the CBU. Our results, combined with ages available in the literature, show that periods of peak-burial conditions (55-49 Ma) and syn-(45-38 Ma) to post-orogenic exhumation (37-21 Ma) are well constrained in the CBU by the 40 Ar/ 39 Ar method. Moreover, Variscan apparent ages are preserved in the Cycladic basement of Ios, indicating that the 40 Ar/ 39 Ar system was not completely reset during the Eocene metamorphic episode. Additionally, our 40 Ar/ 39 Ar data show that the occurrence of extraneous argon is heterogeneously distributed in the CBU. While high-pressure rocks of Syros and Tinos are only locally affected, extraneous argon is clearly evidenced in the CBU of Ios, yielding geologically meaningless ages up to 900 Ma. On each island, the most affected ages were obtained in microstructures where fluid circulation is enhanced such as shear bands or strain shadows around garnets. We suggest that the Cycladic Basement, a polymetamorphic continental unit, behaved as an argon reservoir that has been mobilized by fluid circulation at the Eocene. These argon-enriched fluids have then heterogeneously contaminated metamorphic rocks when percolating through the CBU. Finally, we claim that a minimum closure temperature of 550°C must be accepted for the phengites of the CBU and conclude that volume diffusion is not the main process resetting the argon isotopic system in high-pressure rocks compare to recrystallization enhanced by fluid circulation and deformation.

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Introduction

Extraneous argon is usually thought to represent a frequent drawback in dating highpressure and low-temperature (HP-LT) metamorphic rocks with the 40 Ar/ 39 Ar method. By definition, extraneous argon is the additional 40 Ar leading to measured 40 Ar/ 36 Ar ratios in excess of atmospheric initial values (see [START_REF] Kelley | Excess argon in K-Ar and Ar-Ar geochronology[END_REF] for a review). Two forms of extraneous argon have been identified in metamorphic rocks, i) inherited argon corresponds to in situ contamination of radiogenic 40 Ar deriving from K-rich minerals of the rock system (e.g.

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3 [START_REF] Giorgis | Distribution and significance of extraneous argon in UHP eclogite (Sulu terrain, China): insight from in situ 40 Ar/39 Ar UV-laser ablation analysis[END_REF][START_REF] Vincenzo | Comparison of 40Ar-39Ar and Rb-Sr data on phengites from the UHP Brossasco-Isasca Unit (Dora Maira Massif, Italy): implications for dating white mica[END_REF], ii) excess argon is additional to atmospheric argon and corresponds to 40 Ar incorporated into the mineral lattice and/or as fluid inclusion by other processes than in situ radioactive decay of 40 K (i.e. from outside the system), for example by incorporation of external argon-enriched fluids (e.g. [START_REF] Qiu | Paleozoic ages and excess 40 Ar in garnets from the Bixiling eclogite in Dabieshan, China: New insights from 40 Ar/39 Ar dating by stepwise crushing[END_REF]Warren et al., 2011). Units of oceanic affinity, which underwent a single HP metamorphic cycle in the subduction zone, are often less affected by the presence of inherited argon than polymetamorphic continental units [START_REF] Arnaud | Evidence for excess argon during high pressure metamorphism in the Dora Maira Massif (western Alps, Italy), using an ultra-violet laser ablation microprobe 40 Ar-39 Ar technique[END_REF][START_REF] Scaillet | Excess 40 Ar transport scale and mechanism in high-pressure phengites: A case study from an eclogitized metabasite of the Dora-Maira nappe, western Alps[END_REF]. The main reason is that felsic continental basement rocks do not easily record HP parageneses because they have often been dehydrated by earlier metamorphic reactions and thus need an external fluid infiltration to react and then to record metamorphism [START_REF] Proyer | Metamorphism of pelites in NKFMASH-a new petrogenetic grid with implications for the preservation of high-pressure mineral assemblages during exhumation[END_REF]. In the absence of a free aqueous fluid phase, phengite can trap inherited argon from the breakdown of precursor K-rich minerals (e.g. muscovite, K-feldspar), even if this breakdown occurred at temperature exceeding the range of commonly accepted closure temperature for argon diffusion in white micas (400-450 °C, e.g. Villa, 1998;[START_REF] Harrison | Diffusion of 40 Ar in muscovite[END_REF] of several hundred degrees (e.g. [START_REF] Giorgis | Distribution and significance of extraneous argon in UHP eclogite (Sulu terrain, China): insight from in situ 40 Ar/39 Ar UV-laser ablation analysis[END_REF]. On the contrary, mafic lithologies of oceanic affinity are often characterized by continuous dehydration reactions along the prograde path, releasing large amounts of fluids and allowing the system to evolve in open conditions [START_REF] Scaillet | Excess 40 Ar transport scale and mechanism in high-pressure phengites: A case study from an eclogitized metabasite of the Dora-Maira nappe, western Alps[END_REF][START_REF] Itaya | Evolution of the Sanbagawa and Shimanto highpressure belts in SW Japan: insights from K-Ar (Ar-Ar) geochronology[END_REF]. Inherited argon released from mineral precursors may be evacuated by fluid circulation along damaged zones, resetting the argon isotopic system of each mineral phase.

However, mafic lithologies characterized by small porosities maintained by low permeability during subduction can also retain in situ protolith-derived inherited argon under closed system conditions [START_REF] Smye | The signature of devolatisation: Extraneous 40 Ar systematics in high-pressure metamorphic rocks[END_REF].

The Cycladic Blueschist Unit (CBU) is a HP-LT metamorphic belt that experienced a single burial and exhumation cycle in the Hellenic subduction zone. Previous 40 Ar/ 39 Ar data [START_REF] Maluski | Dating the metamorphic events in the Cycladic area; 39 Ar/40 Ar data from metamorphic rocks of the Island of Syros (Greece)[END_REF][START_REF] Bröcker | 40Ar/39Ar and oxygen isotope studies of polymetamorphism from Tinos Island, Cycladic blueschist belt, Greece[END_REF][START_REF] Bröcker | The geological significance of 40Ar/39Ar and Rb-Sr white mica ages from Syros and Sifnos, Greece: a record of continuous (re) crystallization during exhumation?[END_REF][START_REF] Putlitz | Prograde mica 40 Ar/39 Ar growth ages recorded in high pressure rocks (Syros, Cyclades, Greece)[END_REF][START_REF] Lister | White mica 40Ar/39Ar age spectra and the timing of multiple episodes of high-pressure metamorphic mineral growth in the Cycladic eclogiteblueschist belt, Syros, Aegean Sea, Greece[END_REF] A C C E P T E D M A N U S C R I P T

show that this unit, largely composed of lithologies of oceanic affinity (metabasalts, metagabbros, metasediments; e.g. van der Maar, 1980;[START_REF] Melidonis | The geological structure and mineral deposits of Tinos island (Cyclades, Greece)[END_REF], is poorly affected by the presence of extraneous argon, making the CBU an ideal case study to better understand the significance of 40 Ar/ 39 Ar ages in HP metamorphic context.

Our study aims at applying the 40 Ar/ 39 Ar dating technique on three Cycladic islands, Syros, Tinos and Ios, which span a complete transect of the CBU, from the basal thrust over the Cycladic Continental Basement (CCB) observed on Ios [START_REF] Huet | Thrust or detachment? Exhumation processes in the Aegean: insight from a field study on Ios (Cyclades, Greece)[END_REF] to the upper detachments responsible for their exhumation, the Vari Detachment (Trotet et al., 2001;Soukis and Stöckli, 2013) and the North Cycladic Detachment System (Jolivet et al., 2010).

The combination of our results with data available in the literature shows that periods of peak P-T conditions and syn-to post-orogenic exhumation are well constrained by 40 Ar/ 39 Ar conventional step-heating experiments on phengite single grains or populations. This evolution helps interpreting in-situ 40 Ar/ 39 Ar ages on phengites, which show a more complex signal. Our results imply that the presence of extraneous argon in the CBU is heterogeneous and closely correlated to the vicinity of the CCB, a continental unit affected by a polymetamorphic history. Finally, a synthesis of petrostructural and geochronological studies conducted in the Aegean domain combined with our new data allows proposing a geodynamic scenario of the tectonometamorphic evolution of this domain from late burial in the subduction channel to final exhumation.

Geological setting

Aegean domain

The tectonometamorphic evolution of the Aegean domain can be summarized in two main stages [START_REF] Jolivet | Cenozoic geodynamic evolution of the Aegean[END_REF][START_REF] Ring | The Hellenic subduction system: highpressure metamorphism, exhumation, normal faulting, and large-scale extension[END_REF]. First, from late Cretaceous to early Eocene, subduction and collision of Africa with Eurasia formed the Hellenides-Taurides chain

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5 [START_REF] Bonneau | Subduction, collision et schistes bleus; l'exemple de l'Egee (Grece)[END_REF]. During this episode, the crust was thickened and HP-LT metamorphic units, such as the CBU, started to exhume between a top-to-the south thrust at the base and top-to-the east/northeast syn-orogenic detachments at the top (Trotet et al., 2001;[START_REF] Huet | Thrust or detachment? Exhumation processes in the Aegean: insight from a field study on Ios (Cyclades, Greece)[END_REF][START_REF] Augier | Exhumation kinematics of the Cycladic Blueschists unit and back-arc extension, insight from the Southern Cyclades (Sikinos and Folegandros Islands, Greece)[END_REF]. Then, as a consequence of the acceleration of African slab rollback, post-orogenic extension affected the previously thickened crust from ca. 35 Ma in the Aegean Domain while the frontal zones were still under compression in Crete and the Peloponnese [START_REF] Jolivet | Cenozoic geodynamic evolution of the Aegean[END_REF][START_REF] Ring | The Hellenic subduction system: highpressure metamorphism, exhumation, normal faulting, and large-scale extension[END_REF]. In the Cyclades, back-arc extension was accommodated by regional-scale detachments such as the North Cycladic Detachment System (NCDS) or the West Cycladic Detachment System (WCDS) (Fig. 1a; Jolivet et al., 2010;[START_REF] Grasemann | Miocene bivergent crustal extension in the Aegean: Evidence from the western Cyclades (Greece)[END_REF].

In this study, 40 Ar/ 39 Ar analyses were performed over three islands located in different positions of the Aegean domain: Tinos along the northern margin of the Cyclades, Syros in the centre and Ios in the south. They also show different structures recording different stages of the CBU evolution that is described in the following.

Geology of Syros

Located in the central part of the Aegean (Fig. 1a), most of Syros surface exposes the CBU except for the southeast where the Vari Unit occurs (Fig. 1b). This unit, correlated to the Upper Cycladic Unit (UCU) where HP rocks have not been recognized, is composed of greenschist rocks and the Vari orthogneiss (Fig. 1b; Soukis and Stöckli, 2013;[START_REF] Laurent | Strain localization in a fossilized subduction channel: Insights from the Cycladic Blueschist Unit (Syros, Greece)[END_REF]. The CBU stacks in Syros can be internally subdivided in three subunits, all bounded by top-to-the east shear zones and characterized by their lithology and predominant metamorphic facies that are from bottom to top (Fig. 1b, [START_REF] Laurent | Strain localization in a fossilized subduction channel: Insights from the Cycladic Blueschist Unit (Syros, Greece)[END_REF]:

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1) Posidonia Subunit, composed of the structurally lower gneiss of Komito overlain by albitic micaschists with intercalated boudins of metabasite and thin marble layers. This subunit has been pervasively overprinted in greenschist-facies conditions and only few blueschist-and eclogite-facies parageneses are observed within the core of metabasite boudins. The Achladi-Delfini Shear Zone separates the Posidonia Subunit from the upper Chroussa Subunit.

2) Chroussa Subunit, composed of a lithostratigraphic sequence of alternating micaschists, thick marble layers and metabasites. In this subunit, some areas are overprinted in the greenschist-facies, while other places show well-preserved eclogite-and blueschist-facies parageneses. The Kastri Shear Zone separates the Chroussa Subunit from the upper Kampos Subunit.

3) Kampos Subunit, which is mainly composed of a mélange of metabasites wrapped in serpentinites and minor metasediments. Eclogite-and blueschist-facies parageneses are spectacularly preserved and mostly escaped significant retrogression. The top-to-the east Vari Detachment caps the Kampos Subunit, and at larger scale separates the top of the CBU on Syros from the upper Vari Unit (Trotet et al., 2001;Soukis and Stöckli, 2013;[START_REF] Laurent | Strain localization in a fossilized subduction channel: Insights from the Cycladic Blueschist Unit (Syros, Greece)[END_REF].

Eclogite-facies parageneses are recognized within all three subunits of the CBU [START_REF] Laurent | Strain localization in a fossilized subduction channel: Insights from the Cycladic Blueschist Unit (Syros, Greece)[END_REF]. Despite their contrasting degree of retrogression, all subunits reached similar HP-LT metamorphic peak conditions at ca. 20 kbar and 550 °C (Trotet et al. 2001).

Exhumation of the CBU in Syros was accommodated along large-scale shear zones by a topto-the east/northeast sense of shear from the depth of the eclogite-facies all the way to the depth of the greenschist-facies (Trotet et al., 2001;[START_REF] Laurent | Strain localization in a fossilized subduction channel: Insights from the Cycladic Blueschist Unit (Syros, Greece)[END_REF]. Studies attempting to date the maximum burial led to ca. 53-42 Ma ages (Tomaschek et al., 2003;[START_REF] Putlitz | Prograde mica 40 Ar/39 Ar growth ages recorded in high pressure rocks (Syros, Cyclades, Greece)[END_REF][START_REF] Lagos | High precision Lu-Hf geochronology of Eocene eclogite-facies rocks from Syros, Cyclades, Greece[END_REF][START_REF] Lister | White mica 40Ar/39Ar age spectra and the timing of multiple episodes of high-pressure metamorphic mineral growth in the Cycladic eclogiteblueschist belt, Syros, Aegean Sea, Greece[END_REF]. The final, mostly syn-kinematic

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7 retrogression in the greenschist-facies conditions has been dated as late as at 25 to 21 Ma [START_REF] Bröcker | The geological significance of 40Ar/39Ar and Rb-Sr white mica ages from Syros and Sifnos, Greece: a record of continuous (re) crystallization during exhumation?[END_REF]. These ages are presented with more details in the discussion.

Geology of Tinos

Tinos Island is located in the northern part of the Cyclades, 15 km northeast of Syros (Fig. 1a). This island is mainly composed of the CBU except for the Upper unit that is composed of serpentinite, metagabbros and minor metasediments (Fig. 1c; [START_REF] Melidonis | The geological structure and mineral deposits of Tinos island (Cyclades, Greece)[END_REF][START_REF] Avigad | Low-angle faults above and below a blueschist belt-Tinos Island, Cyclades, Greece[END_REF]. The Upper unit belongs to the UCU that was metamorphosed in the amphibolite-and greenschist-facies at ca. 70 Ma and escaped significant record of the alpine HP-LT event [START_REF] Avigad | Low-angle faults above and below a blueschist belt-Tinos Island, Cyclades, Greece[END_REF][START_REF] Bröcker | Rb-Sr isotope studies on Tinos Island (Cyclades, Greece): additional time constraints for metamorphism, extent of infiltration-controlled overprinting and deformational activity[END_REF]Katzir et al., 1996). The Tinos Detachment, belonging to the NCDS, separates this unit from the lower CBU (Fig. 1c, e.g. [START_REF] Jolivet | Ductile extension and the formation of the Aegean Sea[END_REF].

CBU in Tinos is represented by a sequence of metapelites, marbles and metabasites (Fig. 1c). The structurally lowest rocks are dolomitic marbles, which were previously interpreted either as the base of the CBU (Mélidonis 1980;[START_REF] Parra | Relation between the intensity of deformation and retrogression in blueschist metapelites of Tinos Island (Greece) evidenced by chloritemica local equilibria[END_REF][START_REF] Bröcker | The base of the Cycladic blueschist unit on Tinos Island (Greece) re-visited: field relationships, phengite chemistry and Rb-Sr geochronology[END_REF] or as part of the external Gavrovo-Tripolitza Nappe also cropping out in Evia and continental Greece [START_REF] Avigad | Low-angle faults above and below a blueschist belt-Tinos Island, Cyclades, Greece[END_REF]. A gradient of greenschist-facies retrogression is observed when approaching the Tinos Detachment, from preserved eclogites and blueschists to the southwest toward strongly retrograded rocks to the northeast [START_REF] Jolivet | Ductile extension and the formation of the Aegean Sea[END_REF][START_REF] Parra | Relation between the intensity of deformation and retrogression in blueschist metapelites of Tinos Island (Greece) evidenced by chloritemica local equilibria[END_REF]. Additionally, a gradient of finite deformation is observed with evolution from a coaxial deformation at the base of the CBU toward a highly non-coaxial deformation at the top, associated with top-to-the-northeast sense of shear within the Tinos Detachment [START_REF] Jolivet | Ductile extension and the formation of the Aegean Sea[END_REF][START_REF] Mehl | From ductile to brittle: evolution and localization of deformation below a crustal detachment (Tinos, Cyclades, Greece)[END_REF]. Metamorphic peak P-T conditions in the CBU were estimated at 18-15 kbar for 500-550 °C, whereas initiation of

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greenschist-facies overprint is estimated at 7-9 kbar and 450-500 °C [START_REF] Bröcker | 40Ar/39Ar and oxygen isotope studies of polymetamorphism from Tinos Island, Cycladic blueschist belt, Greece[END_REF][START_REF] Parra | Relation between the intensity of deformation and retrogression in blueschist metapelites of Tinos Island (Greece) evidenced by chloritemica local equilibria[END_REF]. 40 Ar/ 39 Ar and Rb-Sr ages on phengites constrain the HP metamorphic episode between 55-40 Ma and the period of retrogression in the greenschist-facies between 25 and 18 Ma [START_REF] Bröcker | 40Ar/39Ar and oxygen isotope studies of polymetamorphism from Tinos Island, Cycladic blueschist belt, Greece[END_REF][START_REF] Bröcker | Rb-Sr isotope studies on Tinos Island (Cyclades, Greece): additional time constraints for metamorphism, extent of infiltration-controlled overprinting and deformational activity[END_REF].

Finally, a Miocene granodiorite (17-14 Ma; e.g. [START_REF] Altherr | A late Oligocene/early Miocene high temperature belt in the Attic-Cycladic crystalline complex (SE Pelagonian, Greece)[END_REF][START_REF] Bröcker | The contact aureole on Tinos (Cyclades, Greece): tourmalinebiotite geothermometry and Rb-Sr geochronology[END_REF][START_REF] Brichau | Extensional faulting on Tinos island, Aegean sea, Greece: How many detachments?[END_REF], observed in the east of Tinos (Fig. 1c), seals the ductile deformation. This late-tectonic intrusion through the Tinos Detachment induced contact metamorphism in both the CBU and the UCU [START_REF] Avigad | Low-angle faults above and below a blueschist belt-Tinos Island, Cyclades, Greece[END_REF][START_REF] Bröcker | The contact aureole on Tinos (Cyclades, Greece): tourmalinebiotite geothermometry and Rb-Sr geochronology[END_REF].

Geology of Ios

The geological structure of Ios is classically described as a dome structure cored by the lower CCB and mantled by the CBU (Fig. 1d, e.g. van der Maar, 1980;[START_REF] Huet | Thrust or detachment? Exhumation processes in the Aegean: insight from a field study on Ios (Cyclades, Greece)[END_REF].

The CCB is mainly composed of a variably deformed and metamorphosed Variscan granite core surrounded by garnet-micaschists (Fig. 1d, van der Maar, 1980). The amphibolite-facies metamorphism, dated at ca. 310 Ma in the garnet-micaschist series, relates to the pre-alpine history of the CCB (van der Maar, 1980;[START_REF] Henjes-Kunst | Isotopic dating of pre-alpidic rocks from the island of Ios (Cyclades, Greece)[END_REF][START_REF] Baldwin | Thermochronology of the South Cyclades Shear Zone, Ios, Greece: Effects of ductile shear in the argon partial retention zone[END_REF]. Even though HP-LT mineralogical index such as glaucophane or clinopyroxene-garnet association have been described (van der Maar, 1980;[START_REF] Henjes-Kunst | Isotopic dating of pre-alpidic rocks from the island of Ios (Cyclades, Greece)[END_REF], the peak P-T metamorphic conditions are poorly constrained with estimates ranging from 25 kbar -540 °C [START_REF] Perraki | New constraints for the Alpine HP metamorphism of the Ios basement, Cyclades, Greece[END_REF] to 11 kbar -475 °C [START_REF] Gupta | Ductile shearing, hydrous fluid channelling and high-pressure metamorphism along the basement-cover contact on Sikinos, Cyclades, Greece[END_REF].

The overlying CBU is composed of marble, metapelite and metabasite (Fig. 1d, van der Maar, 1980). HP-LT metamorphic rocks such as eclogite and blueschist are locally

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preserved and are also overprinted in the greenschist-facies conditions (van der Maar, 1980;van der Maar and Jansen, 1983). Peak metamorphic conditions of 18.5 ± 3 kbar for 510 ± 20 °C [START_REF] Huet | Rhéologie de la lithosphère continentale: L'exemple de la Mer Egée[END_REF] were constrained between 50 and 40 Ma by K-Ar and 40 Ar/ 39 Ar dating (van der Maar and Jansen, 1983;[START_REF] Baldwin | Thermochronology of the South Cyclades Shear Zone, Ios, Greece: Effects of ductile shear in the argon partial retention zone[END_REF]. Using the same radiochronometers, the greenschist retrogression was dated between 25-30 Ma and correlated to the regional Oligo-Miocene LP-HT event (van der Maar and Jansen, 1983;[START_REF] Baldwin | Thermochronology of the South Cyclades Shear Zone, Ios, Greece: Effects of ductile shear in the argon partial retention zone[END_REF].

The contact between the CCB and the CBU was first described as a top-to-the south extensional shear zone, the South Cycladic Shear Zone (SCSZ, [START_REF] Lister | Metamorphic core complexes of Cordilleran type in the Cyclades, Aegean Sea, Greece[END_REF]. More recently, two main tectonometamorphic events have been described [START_REF] Huet | Thrust or detachment? Exhumation processes in the Aegean: insight from a field study on Ios (Cyclades, Greece)[END_REF][START_REF] Augier | Exhumation kinematics of the Cycladic Blueschists unit and back-arc extension, insight from the Southern Cyclades (Sikinos and Folegandros Islands, Greece)[END_REF], i) a first penetrative, top-to-the south sense of shear was contemporaneous with overthrusting of the CBU on the CCB at ca. 35 Ma, ii) a second extensional top-to-the north shearing has been observed mainly in the CBU correlated with the Oligo-Miocene postorogenic extension stages [START_REF] Huet | Thrust or detachment? Exhumation processes in the Aegean: insight from a field study on Ios (Cyclades, Greece)[END_REF].

Analytical methods

40 Ar/ 39 Ar dating combined direct in-situ analyses of phengite on polished 0.5 mm thick rock-sections and conventional laser-probe step-heating technique performed on phengite single grains and concentrates. Single grains of mica were handpicked in fraction 0.5-1 mm. Only single grains with no apparent deformation and inclusion were kept for further processing (Fig. 2). For dating populations, phengites were concentrated from the fraction 0.25-0.5 mm using a Frantz magnetic separator. Separates were then washed in acetone, ethanol and distilled water and dried at 80 °C in oven before packing into aluminium foils for irradiation. All samples were irradiated at McMaster (Hamilton, Canada) and analysed at Géosciences Montpellier (Université de Montpellier 2, France, see Monié and [START_REF] Monie | Coeval blueschist exhumation along thousands of kilometers: Implications for subduction channel processes[END_REF] and [START_REF] Augier | Exhumation, doming and slab retreat in the Betic Cordillera (SE Spain): in situ 40 Ar/ 39 Ar ages and P-T-d-t paths for the Nevado-Filabride complex[END_REF] for analytical details). Irradiation flux was monitored using the sanidine FCT standard with an age of 28.02 ± 0.28 Ma [START_REF] Renne | Intercalibration of standards, absolute ages and uncertainties in 40 Ar/39 Ar dating[END_REF]. The 40 Ar/ 39 Ar isotopic data are presented in supplementary material.

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Grains that were not irradiated were mounted with double-stick conductive carbon tape. They were then imaged with a scanning electron microscope (SEM) and mineral chemical compositions were determined using a CAMECA SX100 electron probe microanalyser at ISTEP laboratory (Paris, France, see Yamato et al. (2007) for analytical details).

Cores and rims of eight grains per sample were analysed. The chemical composition of populations and in-situ dated phengites was analysed in thin-sections realised with the mirrorhalf of rock section used for in-situ dating, allowing correlation between microstructural position, chemistry and ages. The phengite chemistry is presented on X Mg -Si (per formula unit) diagrams.

Sampling strategy, samples description and phengite chemistry

Syros is the island where HP parageneses are best preserved from greenschist-facies retrogression and where peak metamorphic conditions were dated with U/Pb, Lu/Hf and Rb-Sr geochronology (Tomaschek et al., 2003;[START_REF] Lagos | High precision Lu-Hf geochronology of Eocene eclogite-facies rocks from Syros, Cyclades, Greece[END_REF][START_REF] Cliff | Geochronological challenges posed by continuously developing tectonometamorphic systems; insights from Rb-Sr mica ages from the Cycladic Blueschist Belt, Syros (Greece)[END_REF]. Samples from this island are therefore likely to provide constraints on the HP evolution of the CBU and the behaviour of the K/Ar system at HP conditions. Conversely, rocks from Tinos were selected along the greenschist-facies strain gradient record in the footwall of the NCDS to constrain the age of the syn-kinematic retrogression during back-arc extension. Sampling on Tinos therefore offers the opportunity to bring further time-constraints on the NCDS evolution, from initiation to deactivation, and better understand the K/Ar system in such an environment. Rocks from Ios were selected in the vicinity of the contact between the CBU and the CCB, allowing dating first the overthrusting of the CBU on the CCB and second,

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constraining its late-orogenic top-to-the north reactivation. Moreover, the CCB is mainly composed of continental basement rocks and thus the potential link between these lithologies and the presence of extraneous argon in phengites was studied in details. In the following, a petrostructural description of each dated sample is presented. Mineralogy of all analysed samples is presented in Table 1.

Syros

Eight samples were selected among rocks collected on Syros. These samples are all located on the geological map and on a synthetic cross-section (Figs. 1b,3a). S07-14 and S07-16 samples were selected in the northern part of Syros where HP parageneses are best preserved (Fig. 1b).

Samples S07-01, S07-02, S07-04, S07-04bis and S07-Jojo were sampled in the southern coast of Syros, near Cape Katerghaki (see location in Fig. 1b). Along this outcrop, retrogression of eclogite in blueschist-facies conditions is associated with top-to-the east shearing [START_REF] Laurent | Strain localization in a fossilized subduction channel: Insights from the Cycladic Blueschist Unit (Syros, Greece)[END_REF]. Phengite grains from S07-01 micaschist show dispersed chemical compositions (Fig. 3c). S07-02 is a blueschist-facies metabasite showing homogeneous phengite composition (Fig. 3c). S07-04 and S07-04bis are eclogites that are slightly retrogressed in blueschist-facies conditions. Phengite composition of S07-04 shows a systematic decrease of X Mg from core to rim (Fig. 3c). Sample S07-Jojo is an eclogite that is crosscut by blueschist-facies top-to-the east shear bands consisting of glaucophane, quartz, calcite and phengite (Fig. 3b, Table 1). Phengite composition is similar in the eclogitic foliation and shear bands (Fig. 3c).

Sample S07-17 is a metapelite collected near Komito (see location in Fig. 1b) and corresponds to the basal part of the CBU where greenschist-facies retrogression is pervasive

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(Fig. 3a). The analysed phengites show homogeneous chemical composition with some phengite cores showing lower Si value (Fig. 3c).

Tinos

Sixteen samples from Tinos were analysed and dated. Most of them were collected on the same outcrops as those studied by [START_REF] Parra | Relation between the intensity of deformation and retrogression in blueschist metapelites of Tinos Island (Greece) evidenced by chloritemica local equilibria[END_REF] for determining the P-T path of the CBU on Tinos. These samples are all located on a geological map and a synthetic crosssection of the island (Figs. 1c,4a).

T07-07 and T07-09, respectively a micaschist and a calcschist (Table 1), were sampled along the southwest coast of Tinos (Fig. 4a). In thin section, T07-07 shows discrete top-to-the northeast shear bands marked by chlorite and phengite. Phengite composition within the foliation and within shear bands differs significantly (Fig. 4d). Analysed single grains of T07-09 show homogeneous chemical composition from core to rim except for one core analysis (Fig. 4d).

T07-10a, T07-10b, T07-11, T07-13, T07-14 and T07-15bis were sampled along the road south of the Isternia Pass, in an outcrop where HP parageneses are well preserved (Figs.

4a, 4b, Table 1). Sample T07-10a is a vein transposed into the foliation and deformed by topto-the northeast shear bands. Chemical analyses of phengites indicate constant composition of single grains (Fig. 4d). Sample T07-10b is a micaschist hosting the T07-10a vein and is intensely affected by top-to-the northeast shearing. Chemical analyses indicate that phengite have homogeneous compositions from core to rim (Fig. 4d). Sample T07-11 is a blueschistfacies metabasite, in which phengite shows homogeneous composition except for one core analysis (Fig. 4d). T07-13 is a blueschist-facies metabasite showing minor shearing deformation. Chemical composition of phengite is constant in this sample except for one core

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13 analysis (Fig. 4d). T07-14 is also a blueschist-facies metabasite affected by strong top-to-thenortheast shear deformation. Garnet and glaucophane is partially replaced by chlorite,

showing retrogression in greenschist-facies conditions (see detailed mineralogy in Table 1).

Phengite grains show identical composition from core to rim (Fig. 4d). Sample T07-15bis is a micaschist where strain shadows around garnets indicate top-to-the NE sense of shear.

Chemical analyses show that Si content of phengite is higher in the foliation and pressure shadows than in shear bands (Fig. 4d).

T07-16 sample corresponds to a HP-micaschist partially overprinted in the greenschist-facies conditions collected at the boundary between well preserved and completely overprinted HP metamorphic rocks (Fig. 5d). Here again, the greenschist-facies retrogression appears coeval with top-to-the northeast shearing. Chemical composition of phengite grains is constant from core to rim. However, recrystallized phengites in micro-scale shear bands have lower Si and X Mg values (Fig. 5e).

T07-25 is a micaschist sampled near Kato Pyrgos (see location on Fig. 4a). Chemical analyses of phengite grains show scattered Si and X Mg values without clear core-rim zonation (Fig. 5e).

Samples T07-24, T07-22, T07-19 and T07-18 come from Planitis islet just below the contact between the CBU and Upper Unit (Fig. 5a). These samples were collected within the detachment zone and are affected by intense top-to-the northeast shearing. T07-24 is a micaschist sampled at the top of a thick marble layer (Fig. 5a). Rims of phengite grains have homogeneous composition while core compositions are more dispersed (Fig. 5e). T07-22 is an intensely sheared vein sampled a few meters below the detachment (Fig. 5a). Analysed phengites have homogeneous Si content and more dispersed X Mg values (Fig. 5e). T07-19 is a micaschist collected two meters below the detachment plane (Fig. 5a). Foliation-forming phengites or phengite inclusions in pyrite crystals have distinct composition from

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14 recrystallized newly-formed phengites in shear bands (Fig. 5e). T07-18 is a marble sampled in the Upper Unit, ca. 10 meters above the detachment (Fig. 5a). Slightly distinct compositions have been analysed for phengites in foliation and in shear bands (Fig. 5e).

Sample T07-27 is a micaschist collected near Kolympitra Bay (see location in Fig. 1c)

and is structurally located at ca. 100 meters below the detachment plane. It belongs to a level of interbedded metasediments within intensely folded marbles (Fig. 5b). Core-rim analyses of phengite grains show heterogeneous compositions (Fig. 5e).

Sample T07-30 was collected at the top of the granodioritic intrusion of Tinos (Fig. 1c). Here, aplitic sills are injected within the foliation of rocks of the Upper Unit and deformed by syn-kinematic normal faults (Fig. 5c). This sample shows no apparent shear deformation.

Ios

Four samples have been dated in Ios. They are located on a geological map of the island and a synthetic cross-section (Figs. 1d,6a).

I07-23 is a micaschist sampled from the CCB, a few meters below the contact with the CBU (Fig. 6a). In this outcrop, top-to-the south sense of shear is partly overprinted by late extensional top-to-the north shear bands. Analysed grains of white mica show variable chemical compositions defining two distinct groups, muscovites with a low Si content and phengites with higher Si content (Fig. 6b). Muscovites are likely to be inherited from the antealpine amphibolite-facies metamorphism recorded in the CCB [START_REF] Henjes-Kunst | Isotopic dating of pre-alpidic rocks from the island of Ios (Cyclades, Greece)[END_REF], while phengites are associated with alpine deformation [START_REF] Huet | Thrust or detachment? Exhumation processes in the Aegean: insight from a field study on Ios (Cyclades, Greece)[END_REF].

I07-122 is a micaschist collected in the CBU, tens of meters above the contact with the CCB, and affected by top-to-the south shear bands (Fig. 6a). Chemical composition of

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phengites is correlated to their microstructural position (Fig. 6b). Phengites with high Si content are located within the foliation and those with intermediate Si are observed within shear bands in association with paragonite.

Samples I07-15 and I07-18 were collected in the CBU, in blueschist-facies mafic boudins affected by top-to-the south shearing (Fig. 6a). Chemical analyses of these samples indicate that white micas have paragonite composition except for one core analysis of I07-15 that corresponds to phengite composition with low Na content (Fig. 6b).

40 Ar/ 39 Ar results

Syros

Two single grains of S07-14 were analysed by 40 Ar/ 39 Ar step-heating yielding reproducible age patterns (Fig. 7a). The age spectrum of the single grain S07-14 (1) shows a plateau age of 49.44 ± 2.62 Ma for 70% of 39 Ar released (Fig. 7a). Consistently, all temperature steps of the single grain S07-14 (2) yield a flat age spectrum characterized by a plateau age of 50.84 ± 0.84 Ma (Fig. 7a). 40 Ar/ 39 Ar step-heating experiments on a concentrate of S07-16 yield a decreasing age spectrum defining a flat portion at 55.04 ± 0.56 Ma for 75%

of 39 Ar released (Fig. 7a). The inverse isochron yields an age of 56.70 ± 1.30 Ma with an atmospheric initial 40 Ar/ 36 Ar of 312.3 ± 14.0 (MSWD = 26.27). The high MSWD value is the consequence of very low error on the different isotopic ratios and of the high radiogenic content. The presence of a significant atmospheric component released during the first heating steps is common in 40 Ar/ 39 Ar analyses, a component that is weakly bound to the mineral surface and possibly in fluid inclusions [START_REF] Qiu | Paleozoic ages and excess 40 Ar in garnets from the Bixiling eclogite in Dabieshan, China: New insights from 40 Ar/39 Ar dating by stepwise crushing[END_REF].

One single grain of S07-01, S07-04 and S07-04bis samples and two single grains of S07-02 were analysed by laser-heating experiments (Fig. 7a). While the first temperature steps of the age spectrum of S07-01 are discordant, a plateau age at 41.65 ± 0.95 Ma is

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defined for about 70% of 39 Ar released (Fig. 7a). The important error on final temperature steps is linked to a high atmospheric contribution from mineral inclusions. All temperature steps of single grain S07-02 (1) yield a flat age spectrum with a plateau age at 43.51 ± 0.56 Ma (Fig. 7a). The second single grain S07-02 (2) yields a similar flat age spectrum characterized by a plateau age at 41.95 ± 0.77 Ma for 90% of 39 Ar released (Fig. 7a). While the first temperature steps of the age spectrum of S07-04 are contaminated by weakly bound excess argon, the remaining 90% of 39 Ar released gives a plateau age of 44.89 ± 0.65 Ma (Fig. 7a).

The age spectrum of S07-04bis yields apparent ages ranging from 44 to 40 Ma with the last four steps defining a plateau age of 40.29 ± 0.73 Ma for 85% of 39 Ar released (Fig. 7a).

S07-Jojo sample was analysed by the 40 Ar/ 39 Ar in-situ laser ablation technique (Fig. 7b). Most of measured 40 Ar/ 39 Ar apparent ages are comprised between 50 and 47 Ma, without any distinction between phengites located in the eclogitic foliation or in the blueschist-facies shear band (Fig. 7b). Two apparent ages at 55.33 ± 3.48 Ma and 42.47 ± 2.03 Ma from the shear band fall out of this range. An inverse isochron, excluding these two points, yields an age of 48.5 ± 1.10 Ma with an atmospheric initial 40 Ar/ 36 Ar of 309.4 ± 42.2 (MSWD = 0.16, Fig. 7b).

One single grain separated from sample S07-17 and analysed by step-heating yields an age spectrum characterized by a significantly younger plateau age of 26.12 ± 0.52 Ma for 80% of 39 Ar released (Fig. 7a).

Tinos

One single grain of T07-10a and T07-10b, two distinct grains of T07-11 and T07-13 and three grouped grains of T07-14 were dated by 40 Ar/ 39 Ar step-heating technique (Fig. 8a).

Additionally, in-situ laser ablation was conducted on a rock-section of sample T07-15bis (Fig. 8a). The single grain of T07-10a yields a flat age spectrum characterized by a plateau age of

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34.48 ± 0.99 Ma for 100% of 39 Ar released in five steps (Fig. 8a). The single grain of T07-10b yields a slightly increasing age spectrum at the end of degassing and a plateau age of 44 Ma for about 65% of 39 Ar released (Fig. 8a). Two steps have a large analytical error due to an imprecise evaluation of 36 Ar and therefore of the atmospheric contribution in these steps. The resolution of the single phengite grain T07-11 (1) age spectrum is limited by degassing of 72% of the total 39 Ar released during a single step. However, a plateau age of 47.67 ± 0.77 Ma is calculated for the three last temperature steps (90% of 39 Ar released). From the same sample, the single grain T07-11 (2) yields a flat age spectrum characterized by a well-defined plateau age of 42.09 ± 0.60 Ma for 95% of the total 39 Ar released (Fig. 8a). These data show an age scatter of ca. 5 Ma between the two dated single grains of T07-11, probably reflecting the presence of different phengite generations in this sample. The single phengite grain of T07-13 (1) yields a flat age spectrum characterized by a plateau age of 40.43 ± 1.53 Ma defined by the seven last temperature steps (60% of the total 39 Ar released, Fig. 8a). Consistently, four temperature steps of the single grain T07-13 (2) define a plateau age of 38.89 ± 0.48 Ma for 70% of the total 39 Ar released (Fig. 8a). The three grouped grains of T07-14 yield a complex age spectrum with two flat segments and apparent ages ranging from 55 to 34 Ma (except the two first steps that show strong atmospheric contribution from the mineral surface, Fig. 8a).

The first low temperature step shows ages at 42 ± 2 Ma while the last temperature steps define a low-quality plateau age of 48.37 ± 1.35 Ma for 53% of the total 39 Ar released. T07-15bis sample was analysed by 40 Ar/ 39 Ar in-situ laser ablation technique (Fig. 8b). In strain shadows around garnet, apparent ages range from ca. 90 to 49 Ma, while within foliation apparent ages are comprised between 58 and 44 Ma. Additionally, apparent ages ranging from 49 to 40 Ma were measured within shear bands, showing that no clear correlation between measured age and microstructural location is observed in this sample (Fig. 8b). Excluding the three ages older than 50 Ma, an inverse isochron yields an age of 41.5 ± 1.5 Ma with an initial

A C C E P T E D M A N U S C R I P T contaminated the sample.
Two single grains of T07-16 were dated by40 Ar/ 39 Ar step-heating in addition to direct in-situ laser ablation on phengites. Each single grain yields a flat age spectrum and consistent plateau ages of 41.78 ± 0.86 Ma (90% of the total 39 Ar released) and 41.96 ± 0.83 Ma (100% of the total 39 Ar released, Fig. 8a). In-situ experiments yield apparent ages ranging from 53 to 41 Ma for phengites located in strain shadows of garnet and comprised between 50 to 41 Ma for phengites within foliation (Fig. 8c). In shear bands, in-situ apparent ages range from 50 to 35 Ma. No clear correlation between the microstructures and 40 Ar/ 39 Ar ages is observed (Fig. 8c), apart for younger apparent ages measured in shear bands. 40 Ar/ 39 Ar step-heating experiments on sample T07-07 and T07-09 were conducted respectively on a concentrate and one single grain (Fig. 8a). The concentrate yields a humpshaped age spectrum with apparent ages ranging from 27 to 23 Ma. Chemical compositions from this sample show two distinct groups (foliation and shear bands, Fig. 4d). The variation in 40 Ar/ 39 Ar apparent ages may reflect outgassing of phengite with different compositions (Wijbrans and McDougall, 1986). Due to this potential mixing, we interpret the maximum age (27 Ma) as providing a minimum age for phengite in foliation and minimum age (23 Ma) as providing maximum age for phengite in shear bands. The resolution of the single grain T07-09 age spectrum is limited by the first temperature step, which liberates a large gas amount corresponding to 80% of the total 39 Ar released (Fig. 8a). This anomalous behaviour could relate to high paragonite content as suggested by one chemical composition of core-analysed white mica within this sample (Fig. 4d). The low K-content and poor resolution of this age spectrum make questionable the age of 37.53 ± 1.30 Ma defined by the first temperature step (Fig. 8a) that is significantly older than ages obtained from the concentrate.

One single grain and a population from T07-25 were dated by 40 Ar/ 39 Ar step-heating

A C C E P T E D M A N U S C R I P T
(Fig. 9a). The single grain yields a slightly discordant concave age spectrum with an age of 31.84 ± 0.46 Ma integrated on 90% of 39 Ar released (Fig. 9a). The analysis of the concentrate yields a more complex age spectrum characterized by a sharp decrease of apparent ages over the first four temperature steps and then a progressive age increase from 34 to 26 Ma (Fig. 9a). A partially flat segment provides an age of 30.50 ± 0.34 Ma, consistent with the minimum of the single grain age spectrum. Analyses of phengite chemical compositions from this sample show distinct core-rim compositions and several phengite generations can also be expected (Fig. 5e).

Single grains from T07-24, T07-27 and T07-22 and one concentrate of T07-18 were dated by 40 Ar/ 39 Ar step-heating (Fig. 9a). Additionally, 40 Ar/ 39 Ar in-situ laser ablation experiments were conducted on 9c). The single grain of T07-24 yields a flat age spectrum characterized by a well-defined plateau age of 23.77 ± 0.94 Ma for 90% of total 39 Ar released (Fig. 9a). The single grain of T07-27 also yields a flat age spectrum with a plateau age of 31.47 ± 0.82 Ma for 95% of 39 Ar released (Fig. 9a). Degassing of the single grain T07-22 gives for the third step a large gas yield corresponding to 85% of the total 39 Ar released (Fig. 9a). The analysis of the concentrate of T07-18 yields a complex age spectrum characterized by a sharp decrease of apparent ages during the first temperature steps and then a progressive increase of apparent ages ranging from 26 to 41 Ma with an intermediate flat segment at 41.12 ± 0.42 Ma (60% of total 39 Ar released, Fig. 9a). Phengite chemical compositions from this sample show distinct values from foliation to shear bands showing the presence of two phengite generations (Fig. 5e). The variation in 40 Ar/ 39 Ar apparent ages may reflect outgassing of these two generations. 40 Ar/ 39 Ar in-situ experiment of T07-18 yields apparent ages ranging from 44 to 42 Ma for phengites within foliation and 66 to 14 Ma for phengites within shear bands. While in-situ apparent ages of phengites within foliation can be correlated with the plateau age of 41.12 ± 0.42 Ma obtained for this sample,

A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT 20
no clear correlation between microstructures and 40 Ar/ 39 Ar ages is observed (Fig. 9b). In-situ experiments of T07-19 yield apparent ages ranging from 40 to 21 Ma for phengites located within foliation and 44 to 28 Ma for phengites within shear bands (Fig. 9c). Again, no clear correlation between microstructures and 40 Ar/ 39 Ar ages is observed.

Finally, one single biotite grain of T07-30 was dated by 40 Ar/ 39 Ar step-heating in order to constrain cooling age of the granodioritic intrusion of Tinos. A plateau age of 13.90 ± 2.40

Ma is calculated for 90% of total 39 Ar released (Fig. 9a).

Ios

One single grain of I07-23 was dated by 40 Ar/ 39 Ar step-heating. The age spectrum has a hump-shaped profile subdivided in three distinct segments (Fig. 10a). The first three temperature steps yield apparent ages around 187 Ma, the next seven steps define a plateau age of 282.57 ± 3.79 Ma for 53% of total 39 Ar released and finally, the two last temperature steps give apparent ages ranging from 264 to 261 Ma. This kind of profile is reminiscent of that produced by Wijbrans and McDougall (1986) and suggests the preservation of inherited muscovite associated with the growth of Alpine fine-grained white micas. The minimum cooling age of 282 Ma obtained for the intermediate segment is reasonably consistent with the age of the Variscan events determined on Ios (305-295 Ma, Henjes-Kunst and Kreuzer, 1982).

40 Ar/ 39 Ar in-situ experiments were conducted on I07-122 and yield apparent ages ranging from 90 to 27 Ma for phengites within the foliation and 939 to 69 Ma for phengites within shear bands (Fig. 10b). Measured apparent ages in shear bands are generally older than in foliation, reflecting the presence of extraneous argon in phengites within shear bands. The origin of extraneous argon is discussed in the following section.

Single phengite grains of I07-15 and I07-18 were dated by 40 Ar/ 39 Ar step-heating (Fig.

A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT 21 10a
). The single grain I07-15 yields an age spectrum characterized by a plateau age of 41.00 ± 1.72 Ma for 83% of total 39 Ar released (Fig. 10a). Single grain I07-18 yields an age spectrum characterized by a plateau at 48.46 ± 0.82 Ma for 65% of total 39 Ar released and a slight decrease of apparent ages at the end of degassing (Fig. 10a).

Discussion

Internal consistency of the 40 Ar/ 39 Ar age results and comparison with previous geochronological studies

Timing of the tectonometamorphic evolution of the CBU, CCB and the UCU of Syros, Tinos and Ios have been investigated using mostly of the existing geochronological methods such as U/Pb [START_REF] Keay | The geological evolution of the Cyclades, Greece: constraints from SHRIMP U-Pb geochronology[END_REF][START_REF] Bröcker | U-Pb zircon geochronology of unusual eclogite-facies rocks from Syros and Tinos (Cyclades, Greece)[END_REF]Tomaschek et al., 2003;[START_REF] Bröcker | Ionprobe U-Pb zircon ages from the high-pressure/lowtemperature mélange of Syros, Greece: age diversity and the importance of pre-Eocene subduction[END_REF][START_REF] Brichau | Extensional faulting on Tinos island, Aegean sea, Greece: How many detachments?[END_REF], Lu/Hf [START_REF] Lagos | High precision Lu-Hf geochronology of Eocene eclogite-facies rocks from Syros, Cyclades, Greece[END_REF], Sm-Nd [START_REF] Dragovic | Pulsed dehydration and garnet growth during subduction revealed by zoned garnet geochronology and thermodynamic modeling, Sifnos, Greece[END_REF], 40 Ar/ 39 Ar [START_REF] Maluski | Dating the metamorphic events in the Cycladic area; 39 Ar/40 Ar data from metamorphic rocks of the Island of Syros (Greece)[END_REF][START_REF] Bröcker | 40Ar/39Ar and oxygen isotope studies of polymetamorphism from Tinos Island, Cycladic blueschist belt, Greece[END_REF][START_REF] Bröcker | High-Si phengite records the time of greenschist facies overprinting: implications for models suggesting mega-detachments in the Aegean Sea[END_REF][START_REF] Bröcker | The geological significance of 40Ar/39Ar and Rb-Sr white mica ages from Syros and Sifnos, Greece: a record of continuous (re) crystallization during exhumation?[END_REF][START_REF] Baldwin | Contrasting P-T-t Histories for Blueschists from the Western Baja Terrane and the Aegean: Effects of Synsubduction Exhumation and Backarc Extension[END_REF][START_REF] Baldwin | Thermochronology of the South Cyclades Shear Zone, Ios, Greece: Effects of ductile shear in the argon partial retention zone[END_REF]Tomaschek et al., 2003;[START_REF] Putlitz | Prograde mica 40 Ar/39 Ar growth ages recorded in high pressure rocks (Syros, Cyclades, Greece)[END_REF]Zeffren et al., 2005;[START_REF] Brichau | Extensional faulting on Tinos island, Aegean sea, Greece: How many detachments?[END_REF][START_REF] Forster | Core-complex-related extension of the Aegean lithosphere initiated at the Eocene-Oligocene transition[END_REF][START_REF] Rogowitz | Influence of high strain rate deformation on 40Ar/39Ar mica ages from marble mylonites (Syros, Greece)[END_REF][START_REF] Lister | White mica 40Ar/39Ar age spectra and the timing of multiple episodes of high-pressure metamorphic mineral growth in the Cycladic eclogiteblueschist belt, Syros, Aegean Sea, Greece[END_REF], Rb-Sr [START_REF] Altherr | A late Oligocene/early Miocene high temperature belt in the Attic-Cycladic crystalline complex (SE Pelagonian, Greece)[END_REF][START_REF] Henjes-Kunst | Isotopic dating of pre-alpidic rocks from the island of Ios (Cyclades, Greece)[END_REF][START_REF] Bröcker | Rb-Sr isotope studies on Tinos Island (Cyclades, Greece): additional time constraints for metamorphism, extent of infiltration-controlled overprinting and deformational activity[END_REF], 2000, 2005;[START_REF] Bröcker | Unusual bulk-rock compositions in eclogite-facies rocks from Syros and Tinos (Cyclades, Greece): implications for U-Pb zircon geochronology[END_REF][START_REF] Bröcker | High-Si phengite records the time of greenschist facies overprinting: implications for models suggesting mega-detachments in the Aegean Sea[END_REF][START_REF] Bröcker | The geological significance of 40Ar/39Ar and Rb-Sr white mica ages from Syros and Sifnos, Greece: a record of continuous (re) crystallization during exhumation?[END_REF][START_REF] Thomson | Timing and nature of formation of the Ios metamorphic core complex, southern Cyclades, Greece[END_REF], fission track and (U-Th)/He thermochronology [START_REF] Altherr | A late Oligocene/early Miocene high temperature belt in the Attic-Cycladic crystalline complex (SE Pelagonian, Greece)[END_REF]Hejl et al., 2008;[START_REF] Ring | Fast extension but little exhumation: the Vari detachment in the Cyclades, Greece[END_REF][START_REF] Brichau | Extensional faulting on Tinos island, Aegean sea, Greece: How many detachments?[END_REF][START_REF] Thomson | Timing and nature of formation of the Ios metamorphic core complex, southern Cyclades, Greece[END_REF]Soukis and Stöckli, 2013). Data are all compiled in the figure 11. When enough data exist, each 40 Ar/ 39 Ar age (t 0 ± σ 0 ) is represented by a curve defined by a normalized probability law following the equation:

𝑝(𝑡) = 1 𝜎 0√ 2𝜋 exp((𝑡 -𝑡 0 ) 2 𝜎 0 ⁄ 2 )
where t 0 is the absolute age and σ 0 is the uncertainty on the age. The corresponding frequencies are then summed to provide a single curve. Moreover, maximum and minimum ages given by hump-shaped 40 Ar/ 39 Ar spectra are represented by blue squares connected by

A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT 22
dashed lines (Fig. 11). Geochronological constraints available for Ios result mainly from 40 Ar/ 39 Ar studies on phengite, with few age constraints provided by other methods. Moreover, a majority of 40 Ar/ 39 Ar studies on concentrates yielded complex age spectra reflecting mixture of mineral generations (Fig. 11). Finally, samples used to date the overthrusting of the CBU onto the CCB on Ios show the presence of extraneous argon preventing any geological interpretation but improving our understanding of argon behaviour in HP metamorphic rocks (Fig. 10).

Time-constraints on peak-burial conditions during subduction

Three age ranges have been previously proposed for peak metamorphism (Fig. 11), i) between 80 and 75 Ma (U/Pb ages on zircon, [START_REF] Bröcker | U-Pb zircon geochronology of unusual eclogite-facies rocks from Syros and Tinos (Cyclades, Greece)[END_REF][START_REF] Bröcker | Ionprobe U-Pb zircon ages from the high-pressure/lowtemperature mélange of Syros, Greece: age diversity and the importance of pre-Eocene subduction[END_REF], ii) between 53 and 50 Ma (U/Pb ages on zircon, 40 Ar/ 39 Ar in phengites and Lu/Hf on garnet, Tomaschek et al., 2003;[START_REF] Lagos | High precision Lu-Hf geochronology of Eocene eclogite-facies rocks from Syros, Cyclades, Greece[END_REF][START_REF] Lister | White mica 40Ar/39Ar age spectra and the timing of multiple episodes of high-pressure metamorphic mineral growth in the Cycladic eclogiteblueschist belt, Syros, Aegean Sea, Greece[END_REF], iii) between 52 and 42 Ma ( 40 Ar/ 39 Ar ages on phengites, [START_REF] Putlitz | Prograde mica 40 Ar/39 Ar growth ages recorded in high pressure rocks (Syros, Cyclades, Greece)[END_REF].

Based on our results and studies of Tomaschek et al. (2003), [START_REF] Lagos | High precision Lu-Hf geochronology of Eocene eclogite-facies rocks from Syros, Cyclades, Greece[END_REF], and [START_REF] Lister | White mica 40Ar/39Ar age spectra and the timing of multiple episodes of high-pressure metamorphic mineral growth in the Cycladic eclogiteblueschist belt, Syros, Aegean Sea, Greece[END_REF] on Syros and [START_REF] Dragovic | Pulsed dehydration and garnet growth during subduction revealed by zoned garnet geochronology and thermodynamic modeling, Sifnos, Greece[END_REF] on Sifnos, we propose that timing of peak metamorphism in the Cyclades is comprised between 55 and 49 Ma. Indeed, the distribution of 40 Ar/ 39 Ar ages obtained on phengite single grains and populations shows two peaks at 55 and 49 Ma (Fig. 11). Moreover, dating of samples S07-14 and S07-16 from Syros with well-preserved HP assemblages (Table 1) was conducted in order to constrain the age of peak metamorphism in the CBU and yield consistent plateau ages at ca. 55 and 50 Ma (Fig. 7). Besides, Lu/Hf ages at 52.2 ± 0.3 Ma, 51.4 ± 0.4 Ma and 50 ± 2 Ma are also associated with eclogite parageneses and show no chemical and isotopic disturbance related to an earlier event [START_REF] Lagos | High precision Lu-Hf geochronology of Eocene eclogite-facies rocks from Syros, Cyclades, Greece[END_REF]. Cretaceous and Eocene U/Pb ages were determined on

A C C E P T E D M A N U S C R I P T
zircons of the same sample (Tomaschek et al., 2003). Cretaceous ages were correlated with pristine zircons interpreted to reflect the age of the magmatic protolith, while Eocene ages were correlated to skeletal zircons interpreted to date recrystallization at peak metamorphic conditions (Tomaschek et al., 2003). Finally, Sm-Nd age constraints on garnet from Sifnos, the other Cycladic island where HP parageneses are well preserved (e.g. Trotet et al., 2001;[START_REF] Dragovic | Using garnet to constrain the duration and rate of water-releasing metamorphic reactions during subduction: An example from Sifnos, Greece[END_REF][START_REF] Roche | Anatomy of the Cycladic Blueschist Unit on Sifnos Island (Cyclades, Greece)[END_REF], support this timing of peak metamorphism [START_REF] Dragovic | Pulsed dehydration and garnet growth during subduction revealed by zoned garnet geochronology and thermodynamic modeling, Sifnos, Greece[END_REF].

The age of peak metamorphism is more difficult to constrain on Tinos and Ios because HP parageneses are more overprinted than in Syros. 40 [START_REF] Bröcker | 40Ar/39Ar and oxygen isotope studies of polymetamorphism from Tinos Island, Cycladic blueschist belt, Greece[END_REF][START_REF] Bröcker | High-Si phengite records the time of greenschist facies overprinting: implications for models suggesting mega-detachments in the Aegean Sea[END_REF][START_REF] Bröcker | The geological significance of 40Ar/39Ar and Rb-Sr white mica ages from Syros and Sifnos, Greece: a record of continuous (re) crystallization during exhumation?[END_REF][START_REF] Bröcker | Rb-Sr isotope studies on Tinos Island (Cyclades, Greece): additional time constraints for metamorphism, extent of infiltration-controlled overprinting and deformational activity[END_REF]. Indeed, two distinct 40 Ar/ 39 Ar and Rb-Sr age clusters are observed (Fig. 11): i) 40 Ar/ 39 Ar ages at ca. 32

Ma and, ii) Rb-Sr ages comprised between 24 and 22 Ma. Most of Rb-Sr ages from Tinos were measured in intensely sheared samples located below the detachment, allowing constraining the final increment of deformation associated with the Tinos Detachment. These results are consistent with the plateau age at 26.12 ± 0.52 Ma in S07-17 and the youngest insitu 40 Ar/ 39 Ar ages in T07-19 ranging from 21 to 28 Ma (Fig. 9).

Emplacement of the syn-kinematic granodioritic intrusion exposed in Tinos was dated at 14.5 ± 0.3 Ma by U/Pb analyses on zircon [START_REF] Keay | The geological evolution of the Cyclades, Greece: constraints from SHRIMP U-Pb geochronology[END_REF][START_REF] Brichau | Extensional faulting on Tinos island, Aegean sea, Greece: How many detachments?[END_REF]. Besides, cooling is constrained by mid-to low-temperature thermochronology with ages comprised between 14 and 8 Ma (Fig. 11, [START_REF] Altherr | A late Oligocene/early Miocene high temperature belt in the Attic-Cycladic crystalline complex (SE Pelagonian, Greece)[END_REF]Hejl et al., 2008;[START_REF] Ring | Fast extension but little exhumation: the Vari detachment in the Cyclades, Greece[END_REF][START_REF] Brichau | Extensional faulting on Tinos island, Aegean sea, Greece: How many detachments?[END_REF][START_REF] Thomson | Timing and nature of formation of the Ios metamorphic core complex, southern Cyclades, Greece[END_REF]Soukis and Stöckli, 2013). The ca. 14 Ma 40 Ar/ 39 Ar age obtained in this study, consistent with earlier studies yielding similar to younger ages (Figs. 9, 11, Altherr et al., 1982;Avigad, 1998), therefore record an increment of fast cooling.

A C C E P T E D M A N U S C R I P T

Keys for the interpretation of in-situ 40 Ar/ 39 Ar ages

All in-situ 40 Ar/ 39 Ar apparent ages obtained in this study are summarized in Fig. 12.

These ages were obtained from phengites located in three different microstructural positions: i) within foliation, ii) within strain shadows of garnets and iii) within shear bands. A major observation is the lack of obvious correlation between apparent ages and microstructural location. For example, in S07-Jojo and T07-16, apparent ages correspond to well-defined isochrons (Figs. 7,8). While in S07-Jojo homogeneous 40 Ar/ 39 Ar apparent ages are consistent with homogeneous chemical composition of phengite, for T07-16 phengite composition within shear-bands can be distinguished from within foliation and strain shadows of garnets (Fig. 8). In I07-122, T07-15bis, T07-18 and T07-19, oldest apparent ages were obtained within shear bands or garnet strain shadows, contradicting the relative chronology established in the field and by thin-section observation (Figs. 8,9,10). This discrepancy is even more clearly marked in I07-122 sample where meaningless apparent ages of 413 Ma and 940 Ma were obtained within shear bands (Fig. 10). In this case, it is therefore difficult to establish unambiguous relationship between microstructures, phengite chemical composition and insitu 40 Ar/ 39 Ar apparent ages at centimetre to millimetre-scale.

Two interpretations can explain the homogeneous distribution of apparent ages and compositions in S07-Jojo, i) measured ages are meaningful and phengites that crystallized the second hypothesis was preferred. In addition, the outcrop where sample S07-Jojo was sampled shows an east-west gradient of retrogression from eclogite to blueschist-facies associated with top-to-the east sense of shear (Fig. 3, [START_REF] Laurent | Strain localization in a fossilized subduction channel: Insights from the Cycladic Blueschist Unit (Syros, Greece)[END_REF]. This interpretation explains the overall range of apparent 40 Ar/ 39 Ar ages (42-55 Ma, Fig. 7) as the consequence of heterogeneous fluid circulation and resetting of 40 Ar/ 39 Ar isotopic system rather than by the presence of excess argon, which is not identified in Syros. Preservation of old ages due to inefficient removal of grain boundary Ar by the grain boundary fluids has been also described in the Alps (Warren et al., 2012). Heterogeneous fluid circulation and recrystallization can also be involved in T07-16 for which 40 Ar/ 39 Ar apparent ages, ranging from 53 to 38 Ma, have no consistent distribution (Fig. 8). Following this interpretation, older 40 Ar/ 39 Ar apparent ages constrained the period of peak metamorphism at 55-49 Ma while the youngest estimate the age of syn-blueschist retrograde deformation between 45 and 38 Ma.

Apparent 40 Ar/ 39 Ar ages of phengite older than 60-55 Ma, with a questionable geological significance in the CBU, seem to be the consequence of the presence of excess argon. These old apparent ages are correlated with microstructures that favoured fluid circulation, such as shear bands or strain shadows around garnets (Figs. 10,12). Following this observation, fluid circulation seems to be able to transport excess argon in significant quantities.

Finally, the chemical composition of micaschists in the CBU is characterized by high Na-content evidenced by the systematic presence of Na-rich phengites, paragonite or albite (Trotet et al., 2001;[START_REF] Parra | Relation between the intensity of deformation and retrogression in blueschist metapelites of Tinos Island (Greece) evidenced by chloritemica local equilibria[END_REF]. This specific composition could involve a more complex behaviour of argon isotopes trapping and transport, explaining why other in-situ 40 Ar/ 39 Ar studies on non-sodic HP-LT micaschist report a clearer relationship between microstructures and 40 Ar/ 39 Ar apparent ages (e.g. [START_REF] Agard | Exhumation of the Schistes Lustrés complex: in situ laser probe 40 Ar/ 39 Ar constraints and implications for the Western Alps[END_REF][START_REF] Augier | Exhumation, doming and slab retreat in the Betic Cordillera (SE Spain): in situ 40 Ar/ 39 Ar ages and P-T-d-t paths for the Nevado-Filabride complex[END_REF].

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Origin of extraneous argon

Extraneous argon is heterogeneously distributed in phengites from the CBU. Our ,12). The most contaminated samples were found on Ios because of the proximity with the CCB (Fig. 6a). In addition, our data show that

Variscan apparent ages at 282.57 ± 3.79 Ma are preserved in the CCB (Fig. 10a), indicating that the 40 Ar/ 39 Ar system was not completely reset during the Eocene HP-LT metamorphic episode.

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ACCEPTED MANUSCRIPT 29 6.4. Closure temperature of the K/Ar system in high-pressure phengites

Considering reconstructed P-T paths of the CBU in Syros (Trotet et al., 2001;[START_REF] Lister | White mica 40Ar/39Ar age spectra and the timing of multiple episodes of high-pressure metamorphic mineral growth in the Cycladic eclogiteblueschist belt, Syros, Aegean Sea, Greece[END_REF] or in Sifnos [START_REF] Groppo | Glaucophane schists and associated rocks from Sifnos (Cyclades, Greece): New constraints on the P-T evolution from oxidized systems[END_REF][START_REF] Dragovic | Using garnet to constrain the duration and rate of water-releasing metamorphic reactions during subduction: An example from Sifnos, Greece[END_REF], the maximum temperature (Tmax) reached by HP-LT metamorphic rocks is ca. 550 °C. Moreover, the P-T evolution followed by the CBU during first exhumation steps is rather isothermal, at least for the most retrograded units (Trotet et al., 2001;[START_REF] Groppo | Glaucophane schists and associated rocks from Sifnos (Cyclades, Greece): New constraints on the P-T evolution from oxidized systems[END_REF][START_REF] Dragovic | Using garnet to constrain the duration and rate of water-releasing metamorphic reactions during subduction: An example from Sifnos, Greece[END_REF].

Using the closure temperature concept, rocks derived from Syros should all yield a consistent age for a given grain size corresponding to the time span elapsed from the bulk cooling through ca. 400-450 °C (Villa, 1998;[START_REF] Harrison | Diffusion of 40 Ar in muscovite[END_REF] preventing the discrimination of former events. Our results and previously published data (Tomaschek et al., 2003;[START_REF] Putlitz | Prograde mica 40 Ar/39 Ar growth ages recorded in high pressure rocks (Syros, Cyclades, Greece)[END_REF][START_REF] Lagos | High precision Lu-Hf geochronology of Eocene eclogite-facies rocks from Syros, Cyclades, Greece[END_REF][START_REF] Lister | White mica 40Ar/39Ar age spectra and the timing of multiple episodes of high-pressure metamorphic mineral growth in the Cycladic eclogiteblueschist belt, Syros, Aegean Sea, Greece[END_REF] show that this is not the case and argon diffusion at these HP conditions seems to be minor. Therefore, phengites of the CBU must have been more retentive to argon diffusion and a minimum closure temperature of 550 °C corresponding to Tmax should be accepted for phengites of the CBU. Our results suggest that reset of the argon isotopic system in HP metamorphic context, at least in the CBU, is controlled by recrystallization enhanced by fluid circulation and deformation (Villa, 1998;[START_REF] Vincenzo | Comparison of 40Ar-39Ar and Rb-Sr data on phengites from the UHP Brossasco-Isasca Unit (Dora Maira Massif, Italy): implications for dating white mica[END_REF][START_REF] Lister | White mica 40Ar/39Ar age spectra and the timing of multiple episodes of high-pressure metamorphic mineral growth in the Cycladic eclogiteblueschist belt, Syros, Aegean Sea, Greece[END_REF].

Another argument showing that the closure temperature of argon in HP phengites could be higher than traditionally accepted is the comparison between 40 Ar/ 39 Ar and Rb-Sr ages. Indeed, on each Cycladic Island studied here, Rb-Sr ages are a few millions years younger than 40 Ar/ 39 Ar ages (Fig. 11). A similar situation has been reported in different HP-LT metamorphic belts (e.g. [START_REF] Li | Excess argon in phengite from eclogite: Evidence from dating of eclogite minerals by Sm Nd, Rb Sr and 40Ar39Ar methods[END_REF][START_REF] Vincenzo | Comparison of 40Ar-39Ar and Rb-Sr data on phengites from the UHP Brossasco-Isasca Unit (Dora Maira Massif, Italy): implications for dating white mica[END_REF][START_REF] Bröcker | The geological significance of 40Ar/39Ar and Rb-Sr white mica ages from Syros and Sifnos, Greece: a record of continuous (re) crystallization during exhumation?[END_REF] and it was also observed at sample-scale on Tinos [START_REF] Bröcker | High-Si phengite records the time of greenschist facies overprinting: implications for models suggesting mega-detachments in the Aegean Sea[END_REF]. It is often interpreted as an argument for the presence of extraneous argon in phengites. Our study shows that in the

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CBU, particularly on Syros, this argument cannot be used. Older 40 Ar/ 39 Ar ages are better explained by a minimum closure temperature of argon in HP phengites of 550 °C, equivalent or even higher than the one accepted for the Rb-Sr dating method in phengite (ca. 500 ± 50 °C; [START_REF] Blanckenburg | Time calibration of a PT-path from the Western Tauern Window, Eastern Alps: the problem of closure temperatures[END_REF]. Consequently, inferring 40 Ar/ 39 Ar ages in HP phengites as geologically meaningless because Rb-Sr ages are younger is an interpretation that should be regarded with great care.

Regional implications: the tectonometamorphic evolution of the CBU

From examples of Syros, Tinos and Ios, and integrating data from the literature of the whole Aegean domain, we suggest geochronological constraints on the P-T-t-d evolution of metamorphic units, mainly focalized on the CBU. These time-constraints confirm the distinction between a first syn-orogenic step of exhumation within the subduction zone followed by post-orogenic exhumation contemporaneous of the formation of the Aegean Sea and accommodated by a crustal-scale system of detachments. The following discussion is illustrated by a series of geodynamic reconstructions in the form of N-S oriented crosssections (Fig. 13).

From late-burial to peak metamorphic conditions

Peak-pressure P-T conditions of the CBU are rather homogeneous through the Cyclades. In Tinos [START_REF] Parra | Relation between the intensity of deformation and retrogression in blueschist metapelites of Tinos Island (Greece) evidenced by chloritemica local equilibria[END_REF], Andros [START_REF] Huet | Coupled phengite 40 Ar-39 Ar geochronology and thermobarometry: PTt evolution of Andros Island (Cyclades, Greece)[END_REF], Syros (Trotet et al., 2001), Sifnos (Trotet et al., 2001;[START_REF] Groppo | Glaucophane schists and associated rocks from Sifnos (Cyclades, Greece): New constraints on the P-T evolution from oxidized systems[END_REF][START_REF] Dragovic | Using garnet to constrain the duration and rate of water-releasing metamorphic reactions during subduction: An example from Sifnos, Greece[END_REF], Sikinos [START_REF] Augier | Exhumation kinematics of the Cycladic Blueschists unit and back-arc extension, insight from the Southern Cyclades (Sikinos and Folegandros Islands, Greece)[END_REF] and Ios [START_REF] Huet | Thrust or detachment? Exhumation processes in the Aegean: insight from a field study on Ios (Cyclades, Greece)[END_REF], these conditions are all comprised between 18-22 kbar and 500-550 °C. The period of peak-pressure in the CBU has been constrained in Syros, Sifnos,

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Tinos and Ios at 55-49 Ma [START_REF] Bröcker | 40Ar/39Ar and oxygen isotope studies of polymetamorphism from Tinos Island, Cycladic blueschist belt, Greece[END_REF][START_REF] Baldwin | Contrasting P-T-t Histories for Blueschists from the Western Baja Terrane and the Aegean: Effects of Synsubduction Exhumation and Backarc Extension[END_REF]Tomaschek et al., 2003;[START_REF] Lagos | High precision Lu-Hf geochronology of Eocene eclogite-facies rocks from Syros, Cyclades, Greece[END_REF][START_REF] Dragovic | Pulsed dehydration and garnet growth during subduction revealed by zoned garnet geochronology and thermodynamic modeling, Sifnos, Greece[END_REF][START_REF] Lister | White mica 40Ar/39Ar age spectra and the timing of multiple episodes of high-pressure metamorphic mineral growth in the Cycladic eclogiteblueschist belt, Syros, Aegean Sea, Greece[END_REF].

Different conditions were determined for the peak-pressure of the CCB with about 15 kbar -550 °C in Naxos (Avigad, 1998;[START_REF] Duchene | Pressure-temperature-time evolution of metamorphic rocks from Naxos (Cyclades, Greece): constraints from thermobarometry and Rb/Sr dating[END_REF], 19 kbar -530 °C in Samos [START_REF] Feenstra | Variscan relicts in Alpine high-P pelitic rocks from Samos (Greece): evidence from multi-stage garnet and its included minerals[END_REF], 11 kbar -475 °C for Sikinos [START_REF] Gupta | Ductile shearing, hydrous fluid channelling and high-pressure metamorphism along the basement-cover contact on Sikinos, Cyclades, Greece[END_REF]) and ca. 16.5 kbar -500 °C for Ios [START_REF] Huet | Thrust or detachment? Exhumation processes in the Aegean: insight from a field study on Ios (Cyclades, Greece)[END_REF]. This dispersion contrasts with the homogeneity of conditions in the CBU albeit the highest peak conditions determined in Samos and Ios are close to those determined for the CBU. Few geochronological constraints exist for the age of peak-pressure in the CCB. In Naxos, U/Pb radiochronometric ages in zircon between 55 and 45 Ma were interpreted as dating peak metamorphism [START_REF] Martin | The isotopic composition of zircon and garnet: a record of the metamorphic history of Naxos, Greece[END_REF]. In Ios, the period of peak pressure in the CCB is difficult to constrain by 40 Ar/ 39 Ar method because of excess argon (Fig. 10). Nevertheless, ages similar to those determined in Naxos [START_REF] Martin | The isotopic composition of zircon and garnet: a record of the metamorphic history of Naxos, Greece[END_REF] and in the CBU (50-55 Ma) were measured and can therefore be considered as a first approximation.

Syn-orogenic exhumation

Exhumation within the subduction channel in a high P-T ratio environment allowing partial retrogression of eclogites in the blueschist-facies is well documented on Syros, Tinos and Ios. On Syros and Tinos, syn-orogenic exhumation is controlled by top-to-the east/northeast sense of shear associated with the Vari Detachment (Trotet et al., 2001;[START_REF] Parra | Relation between the intensity of deformation and retrogression in blueschist metapelites of Tinos Island (Greece) evidenced by chloritemica local equilibria[END_REF]Soukis and Stöckli, 2013;[START_REF] Laurent | Strain localization in a fossilized subduction channel: Insights from the Cycladic Blueschist Unit (Syros, Greece)[END_REF] and is constrained between 49 and 38 Ma (Fig. 11). On Ios, exhumation in the blueschist-facies is controlled by top-to-the south shearing associated with overthrusting of the CBU onto the CCB, which is constrained between a minimum age of 50 Ma and a maximum age of 25 Ma (Fig. 11, Forster and Lister,

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2009; [START_REF] Huet | Thrust or detachment? Exhumation processes in the Aegean: insight from a field study on Ios (Cyclades, Greece)[END_REF]. This crustal-scale structure extends over nearly 500 km from Olympos to Western-Turkey [START_REF] Ring | Early exhumation of high-pressure rocks in extrusion wedges: Cycladic blueschist unit in the eastern Aegean, Greece, and Turkey[END_REF][START_REF] Lacassin | Syn-and post-orogenic exhumation of metamorphic rocks in North Aegean[END_REF]. Such HP-LT exhumation model of the CBU, accommodated by a general extrusion structure between a top-to-the northeast detachment at the top and top-to-the south thrust at the base, has also been proposed for Samos Island [START_REF] Ring | Early exhumation of high-pressure rocks in extrusion wedges: Cycladic blueschist unit in the eastern Aegean, Greece, and Turkey[END_REF] or the Cyclades in a broad sense [START_REF] Huet | Thrust or detachment? Exhumation processes in the Aegean: insight from a field study on Ios (Cyclades, Greece)[END_REF][START_REF] Jolivet | Cenozoic geodynamic evolution of the Aegean[END_REF][START_REF] Ring | The Hellenic subduction system: highpressure metamorphism, exhumation, normal faulting, and large-scale extension[END_REF]. On Samos, the activity of syn-orogenic structures in the blueschist-facies has been constrained between 42 and 32 Ma by 40 Ar/ 39 Ar and Rb-Sr geochronological methods on phengite, partly coeval with structures observed in the north and south of the Cyclades [START_REF] Ring | Early exhumation of high-pressure rocks in extrusion wedges: Cycladic blueschist unit in the eastern Aegean, Greece, and Turkey[END_REF]. Furthermore, tectonic structures with similar kinematics and P-T conditions were observed in Evia (Xypolias et al., 2012). Rb-Sr ages on HP-LT phengites of these structures are comprised between 33 and 21 Ma, which is diachronous compared to Samos and the rest of the Cyclades. However, it was shown in Tinos that the Rb-Sr phengite system could be partially or totally reset in the greenschistfacies without important reequilibration of the major element chemistry [START_REF] Bröcker | High-Si phengite records the time of greenschist facies overprinting: implications for models suggesting mega-detachments in the Aegean Sea[END_REF].

It is therefore possible that HP tectonic structures observed in Evia are older than 33 Ma.

Finally, the CBU are also exposed in the Olympos-Ossa-Pelion ranges (north Aegean Domain), between a top-to-the northeast detachment at the roof and a basal top-to-the southwest thrust, which were active in HP-LT conditions between 45 and 35 Ma [START_REF] Lacassin | Syn-and post-orogenic exhumation of metamorphic rocks in North Aegean[END_REF].

The global structure of syn-orogenic exhumation of the CBU was similar in the whole Aegean domain with crustal-scale tectonic contacts initiated between 50-45 Ma and deactivated around 35-30 Ma (Fig. 13). In the Cyclades, these structures have accommodated about 30-25 km of vertical exhumation, exhuming the CBU at ca. 30 km deep into the crust, below the upper Pelagonian unit (Fig. 13).

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Post-orogenic exhumation

A drastic change in kinematic boundary conditions occurs at ca. 35 Ma with a decrease of the absolute northward motion of Africa and the southward retreat of the subducting slab (Fig. 13, [START_REF] Jolivet | Mediterranean extension and the Africa-Eurasia collision[END_REF]. This drastic change marks the transition from syn-orogenic exhumation to post-orogenic extension and is recorded by a decreasing exhumation rate and an isobaric warming in Tinos [START_REF] Parra | Relation between the intensity of deformation and retrogression in blueschist metapelites of Tinos Island (Greece) evidenced by chloritemica local equilibria[END_REF] and Andros [START_REF] Huet | Coupled phengite 40 Ar-39 Ar geochronology and thermobarometry: PTt evolution of Andros Island (Cyclades, Greece)[END_REF]. The crustal geometry of tectonic structures accommodating the initiation of post-orogenic exhumation is poorly constrained. Little retrogression in greenschist-facies conditions is observed in Syros below the Vari Detachment [START_REF] Laurent | Strain localization in a fossilized subduction channel: Insights from the Cycladic Blueschist Unit (Syros, Greece)[END_REF]. Therefore, at least part of the Vari Detachment ceased to be active in HP-LT conditions, accommodating only the syn-orogenic exhumation (Fig. 13). Being a major inherited structure, the base of the UCU was reactivated by post-orogenic detachments such as the NCDS (Fig. 13, Jolivet et al., 2010).

In the western Cyclades, post-orogenic exhumation is accommodated by the WCDS (Fig. 13; [START_REF] Grasemann | Miocene bivergent crustal extension in the Aegean: Evidence from the western Cyclades (Greece)[END_REF]. The initiation of post-orogenic exhumation in the Cyclades is therefore diachronous, the NCDS (30-35 Ma, Jolivet et al., 2010) being activated before the WCDS (26-22 Ma, Grasemann et al., 2012). However, the last increments of ductile deformation recorded by 40 Ar/ 39 Ar and Rb-Sr radiochronometers are dated at 18-20 Ma both for the NCDS and WCDS (this study, [START_REF] Grasemann | Miocene bivergent crustal extension in the Aegean: Evidence from the western Cyclades (Greece)[END_REF][START_REF] Cossette | Lithological, rheological, and fluid infiltration control on 40Ar/39Ar ages in polydeformed rocks from the West Cycladic detachment system[END_REF]. The latest ductile increments of activity of these two detachment systems is therefore synchronous, showing that for the last 5-8 Ma, top-to-the northeast and southwest shear directions coexist in the Cyclades (Fig. 13).

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Syn-kinematic intrusions and final exhumation

Ductile detachments do not explain the final exhumation of the CBU above the brittleductile transition. Once plutonic intrusions were emplaced below the NCDS at about 12-16

Ma, deformation jumped on a structurally higher position along secondary detachments as observed in Tinos [START_REF] Jolivet | Ductile extension and the formation of the Aegean Sea[END_REF][START_REF] Brichau | Extensional faulting on Tinos island, Aegean sea, Greece: How many detachments?[END_REF], Mykonos [START_REF] Lecomte | Geometry and kinematics of a low-angle normal fault on Mykonos island (Cyclades, Greece): Evidence for slip at shallow dip[END_REF], Serifos [START_REF] Rabillard | Interactions between plutonism and detachments during Metamorphic Core Complex formation[END_REF] and Ikaria [START_REF] Beaudoin | The Ikaria high-temperature Metamorphic Core Complex (Cyclades, Greece): Geometry, kinematics and thermal structure[END_REF][START_REF] Laurent | Interrelations between extensional shear zones and synkinematic intrusions: The example of Ikaria Island (NE Cyclades, Greece)[END_REF].

The vertical exhumation until a few kilometres deep is therefore associated with upward migration of the actively deforming zone, which is a consequence of intrusions that lifts up the ductile-brittle transition (Fig. 13, [START_REF] Jolivet | Ductile extension and the formation of the Aegean Sea[END_REF].

Finally, the lack of clasts from the CBU and CCB in continental Miocene sedimentary basins (8-10 Ma) exposed in Mykonos, Naxos and Paros [START_REF] Sanchez-Gomez | Geochronology of clasts in allochthonous Miocene sedimentary sequences on Mykonos and Paros islands: implications for back-arc extension in the Aegean Sea[END_REF] shows that the deeper metamorphic units have not reached the surface before ca. 8 Ma, corresponding to the youngest fission tracks ages measured in the Cyclades [START_REF] Ring | The Hellenic subduction system: highpressure metamorphism, exhumation, normal faulting, and large-scale extension[END_REF]Soukis and Stöckli, 2013). The final vertical exhumation was accommodated by brittle highangle normal faults observed throughout the Aegean domain (Fig. 13). Pliocene paleogeographic reconstructions, after the Messinian salinity crisis, show that the current morphology in horst and grabens of the Cyclades would be posterior to 4 Ma (Hejl et al., 2008).

Conclusion

Our study shows that the timing of the tectonometamorphic evolution of the HP-LT CBU is well constrained by the 40 Ar/ 39 Ar dating technique, allowing the identification of phengites contaminated by extraneous argon. The distribution of extraneous argon is heterogeneous over the CBU, and seems to be linked with the vicinity of the Cycladic For single grain analyses, both core and rim chemical compositions have been measured. Table 2. 40 Ar/ 39 Ar isotope results of concentrates.

A C C E P T E D M A N U S C R I P T

A C C E P T E D M A N U S C R I P T

  either in the foliation or in shear bands are coeval recording different thermochemical equilibria (in the presence and absence of fluids for instance) ii) Deformation and fluid circulation, channelized within shear bands during blueschist-facies deformation, homogenized compositions and reset 40 Ar/ 39 Ar isotopic record in phengites within shear bands and within the close eclogitic foliation. Because 40 Ar/ 39 Ar step-heating experiments on single grain and bulk phengites clearly distinguish eclogite-facies peak metamorphism (55-49 A C C E P T E D M A N U S C R I P T Ma) from retrograde blueschist-facies deformation (49-38 Ma) at the island-scale (Fig. 11),

  results suggest that metamorphic rocks of Syros do not seem affected by extraneous argon while HP units in Tinos are only locally affected, with maximum 40 Ar/ 39 Ar apparent ages of 90 Ma measured in strain shadows around garnets(Figs 8, 12). Conversely, phengites are most affected by extraneous argon in Ios, with meaningless apparent ages measured in shear bands up to 900 Ma (Fig.10). Two hypotheses may explain this observation: i) an open fluid circulation system allowed evacuating inherited argon from the CBU of Syros and to a lesser extent in Tinos, while a closed system in Ios has preserved inherited argon in the CBU, ii) during mylonitization of the orthogneiss in the CCB, recrystallization of the Variscan Kfeldspar and/or its transformation to phengite[START_REF] Huet | Thrust or detachment? Exhumation processes in the Aegean: insight from a field study on Ios (Cyclades, Greece)[END_REF] released 40 Ar and the CCB thus behaved as a reservoir of inherited argon mobilized by fluid circulation during the Alpine metamorphic episode. These argon-enriched fluids have then heterogeneously contaminated metamorphic rocks when percolating the CBU. In our interpretation, the heterogeneous presence of extraneous argon in the CBU is the consequence of the second hypothesis because our results show evidences of contamination by excess argon preferentially located in domains of large fluid infiltration (Figs. 10

  inherited argon has been preserved from the Variscan metamorphic episode. The main evidences of excess argon in the CBU were identified within microstructures where fluid circulation is favoured, showing that excess argon derived from the circulation of external argon-enriched fluids. Additionally, assuming a relatively isothermal exhumation of the CBU, we suggest a minimum closure temperature of 550 °C for phengite of the CBU and conclude that volume diffusion is a minor process resetting the argon isotopic system in high-pressure rocks compare to recrystallization enhanced by fluid circulation and deformation. Finally, our results support that HP units of oceanic affinity with a simple metamorphic history are less affected by the presence of extraneous argon than polymetamorphic continental units. Indeed, due to continuous dehydration reactions along the prograde path, oceanic units released large quantities of fluids allowing an evolution in open conditions resetting the argon isotopic system. These new ages and conclusions in terms of their geological significance are then used to describe the sequence of events that led to the formation and exhumation of the CBU. .,Agard, P., Goffé, B., De Andrade, V., Vidal, O., Jolivet, L., 2007. New, high- precision P-T estimates for Oman blueschists: implications for obduction, nappe stacking and exhumation processes. Journal of Metamorphic Geology 25, 657-682. Zeffren, S.,Avigad, D., Heimann, A., Gvirtzman, Z., 2005. Age resetting of hanging wall rocks above a low-angle detachment fault: Tinos Island (Aegean Sea). Tectonophysics 400, 1-25.

Figure captions : Figure 1 :

 :1 Figure captions:

Figure 2 :

 2 Figure 2: BSE secondary electron images of three phengite single grains. a, b) Phengite single

Figure 3 :

 3 Figure 3: Description and phengite chemistry of Syros samples. a) Location of samples on a

Figure 4 :

 4 Figure 4: Outcrop conditions and phengite chemistry of Tinos samples. a) Location of

Figure 5 :

 5 Figure 5: Description and phengite chemistry of Tinos samples. a) Synthetic 3D sketch of the

Figure 6 :

 6 Figure 6: Description and phengite chemistry of Tinos samples. a) Location of samples on a

Figure 8 :

 8 Figure 8: 40 Ar/ 39 Ar ages from Tinos. a) Age spectra obtained on single grain and populations

Figure 9 :

 9 Figure 9: 40 Ar/ 39 Ar ages from Tinos. a) Age spectra measured on single grains and

Figure 10 :

 10 Figure 10: 40 Ar/ 39 Ar ages from Ios. a) Age spectra obtained on single grains and populations

Figure 11 :

 11 Figure 11: Synthetic diagram compiling geochronological data from Syros, Tinos and Ios

Figure 12 :Figure 13 :

 1213 Figure 12: Summary of 40 Ar/ 39 Ar in-situ apparent ages measured in phengites. Grey-scale

Figure 1 A

 1 Figure 1

Figure 8 A

 8 Figure 8

2 Time-constraints on exhumation in the subduction and back-arc zones

  Figs. 1, 3, 4).40 Ar/ 39 Ar apparent ages from phengite single grains in blueschist-facies rocks of Syros range from 45 to 40 Ma(Figs. 3, 7). In Tinos, 40 Ar/ 39 Ar ages ranging from 45 to 38 Ma correspond in a large majority to single grains and populations of phengites from , which have a closure temperature higher than peak metamorphic temperature, constrain recrystallization during exhumation in the blueschist-facies P-T conditions between 42 and 30 Ma. Our results confirm this interpretation and suggest that ages determined in HP rocks of the CBU ranging from 45 to 38 Ma are meaningful, constraining the first exhumation steps from eclogite to blueschist-facies P-T conditions, accommodated by top-to-the northeast extensional shearing on top of the CBU and top-to-the south thrusting over the CCB.In this study, 40 Ar/ 39 Ar ages associated with greenschist-facies parageneses range between 37 and 23 Ma except for T07-16 and T07-22 samples from Tinos, which preserve ages ranging from 53 to 38Ma (Figs. 7, 8, 9). These results are consistent with previous Rb-Sr and 40 Ar/ 39 Ar studies

	blueschist-facies rocks (Figs. 4, 8). This range of ages has been also found just below and
	above the Tinos detachment (Figs. 4, 9). Finally, not excluding the possibility of extraneous
	argon, the two blueschist-facies samples I07-18 (garnet-bearing blueschist) and I07-15
	(albite-epidote blueschist) from the CBU in Ios show consistent plateau ages at 48.46 ± 0.82
	Ma and 41.00 ± 1.72 Ma constraining respectively the transition from eclogite-to blueschist-

Ar/ 39

Ar ages ranging from 50 to 48 Ma were obtained in the less retrograded metamorphic rocks both on phengite single grains and populations, which can be interpreted as the age of peak metamorphism or the early exhumation path in Tinos and Ios

(Figs. 8, 10)

. This age seems robust because it overlaps within error the age measured in Syros for the peak of metamorphism. However, additional radiochronologic constraints using different isotopic methods should be conducted to better constrain this age on Tinos and Ios.

Finally, no absolute age data allows estimating the period of peak metamorphism in the CCB. A large age range between 60 and 40 Ma has been proposed

[START_REF] Baldwin | Thermochronology of the South Cyclades Shear Zone, Ios, Greece: Effects of ductile shear in the argon partial retention zone[END_REF]

. Paleogeographic reconstructions show that ages obtained in the CBU allow constraining maximal age for peak metamorphism in the CCB (e.g.

[START_REF] Huet | Thrust or detachment? Exhumation processes in the Aegean: insight from a field study on Ios (Cyclades, Greece)[END_REF][START_REF] Jolivet | Cenozoic geodynamic evolution of the Aegean[END_REF]

.

6.1.

On Syros and Tinos, distribution of 40 Ar/ 39 Ar and Rb-Sr ages in the CBU shows a clear correlation between the preservation of HP parageneses and ages older than ca. 35 Ma A C C E P T E D M A N U S C R I P T (facies and blueschist-to greenschist-facies P-T conditions (Figs. 6, 10).

The meaning of 40 Ar/ 39 Ar ages younger than peak metamorphism is debated (e.g.

[START_REF] Bröcker | The geological significance of 40Ar/39Ar and Rb-Sr white mica ages from Syros and Sifnos, Greece: a record of continuous (re) crystallization during exhumation?[END_REF]

. They can be interpreted as cooling ages controlled by Fick diffusion law or as recrystallization ages. Previous studies in the CBU argued that 40 Ar/ 39 Ar ages younger than peak metamorphism constrain recrystallization or neo-crystallization during exhumation and not solely cooling ages (e.g.

[START_REF] Baldwin | Thermochronology of the South Cyclades Shear Zone, Ios, Greece: Effects of ductile shear in the argon partial retention zone[END_REF][START_REF] Cossette | Lithological, rheological, and fluid infiltration control on 40Ar/39Ar ages in polydeformed rocks from the West Cycladic detachment system[END_REF][START_REF] Rogowitz | Influence of high strain rate deformation on 40Ar/39Ar mica ages from marble mylonites (Syros, Greece)[END_REF][START_REF] Lister | White mica 40Ar/39Ar age spectra and the timing of multiple episodes of high-pressure metamorphic mineral growth in the Cycladic eclogiteblueschist belt, Syros, Aegean Sea, Greece[END_REF]

. For example,

[START_REF] Cossette | Lithological, rheological, and fluid infiltration control on 40Ar/39Ar ages in polydeformed rocks from the West Cycladic detachment system[END_REF]

provided in situ Ar/Ar laser ablation analyses on white micas collected below a crustal-scale detachment exposed on Kea and Serifos islands (CBU, Greece). Both calcite-and quartz-rich lithologies were analysed in order to evaluate whether a correlation can be drawn between Ar/Ar ages, deformation and lithology. Their results show that older ages of 44-36 Ma are obtained in non-deformed phengites from calcite-rich samples, where deformation was partitioned in calcite, are consistent with the timing of HP metamorphism. On the contrary, younger ages of 25-18 Ma are measured in quartz-rich samples, where phengites appear completely newlycrystallized. This age is correlated with the timing of the Miocene ductile extension related to the activity of the detachment. Additionally, the recent work of Cliff et al. (2016) on Syros argued that Rb-Sr dating in phengites microsampled from blueschist-facies extensional A C C E P T E D M A N U S C R I P T fabrics

Table 1 :

 1

Lithology and mineralogy of dated samples. Mineral abbreviations are after Whitney and Evans (2010).

Table 3 .

 3 40 Ar/ 39 Ar isotope results of single grains. Table 4. 40 Ar/ 39 Ar isotope results of in-situ laser ablation experiments.

Table 5 .

 5 Synthesis of geochronological data measured on Syros.

Table 6 .

 6 Synthesis of geochronological data measured on Tinos.

Table 7 .

 7 Synthesis of geochronological data measured on Ios.

Table 2

 2

	Temperature	40 Ar/	39 Ar	38 Ar/	39 Ar	37 Ar/	39 Ar	36 Ar/	39 Ar	39 Ar	% 39 Ar % 40 Ar	* 40 Ar	* / Age	±1σ
								×10 -3 mol ×10 -14 mol released		39 Ar K (Ma) (Ma)
	SO7-16			Phg			J= 0.009989
	700	35.780	0.199	0.00001	99.931	0.037	0.15	17.41	6.23 108.97 6.91
	750	26.521	0.079	0.00005	39.764	0.011	0.20	55.62 14.75 248.09 21.67
	800	15.164	0.043	0.04232	11.534	0.021	0.29	77.41 11.74 200.16 6.03
	833	12.945	0.004	0.00003	0.006	0.021	0.38	99.83 12.92 219.10 2.12
	866	6.498	0.026	0.00001	1.178	0.072	0.68	94.34	6.13 107.28 1.09
	900	4.063	0.030	0.00319	1.718	0.178	1.43	87.02	3.54	62.73	0.71
	933	4.689	0.021	0.00651	4.125	0.449	3.33	73.58	3.45	61.16	0.64
	966	3.796	0.014	0.00167	1.625	1.796	10.89	86.83	3.30	58.54	0.20
	1000	3.426	0.013	0.00133	0.660	1.912	18.93	93.73	3.21	56.97	0.15
	1033	3.264	0.013	0.00154	0.462	3.668	34.38	95.21	3.11	55.22	0.13
	1066	3.235	0.013	0.00157	0.420	5.279	56.60	95.55	3.09	54.87	0.15
	1100	3.223	0.013	0.00245	0.350	4.562	75.81	96.18	3.10	55.05	0.14
	1200	3.245	0.013	0.00201	0.527	4.948	96.64	94.59	3.07	54.52	0.17
	1400	11.845	0.021	0.01214	30.953	0.799	100.00	22.61	2.68	47.69	0.81
	T07-07			Phg			J= 0.009989
	700	21.264	0.102	0.00001	64.131	0.077	0.19	10.78	2.29	40.83	6.47
	750	8.732	0.036	0.00001	23.757	0.075	0.38	19.37	1.69	30.22	3.24
	800	4.475	0.019	0.00000	7.158	0.114	0.67	52.28	2.34	41.71	2.01
	833	2.924	0.014	0.00929	3.326	0.150	1.04	65.72	1.92	34.29	1.23
	866	2.394	0.016	0.01442	2.294	0.206	1.56	70.90	1.70	30.39	0.79
	900	2.097	0.015	0.00617	2.710	0.722	3.37	60.86	1.28	22.93	0.42
	933	1.944	0.014	0.00299	1.635	0.717	5.16	74.13	1.44	25.78	0.44
	966	1.616	0.013	0.00105	0.686	1.409	8.68	86.23	1.39	24.89	0.20
	1000	1.579	0.013	0.00050	0.501	2.569	15.11	89.37	1.41	25.25	0.09
	1033	1.667	0.013	0.00108	0.470	5.179	28.07	90.48	1.51	27.02	0.08
	1066	1.625	0.013	0.00109	0.484	7.010	45.61	89.98	1.46	26.13	0.13
	1100	1.576	0.013	0.00121	0.391	7.277	63.82	91.40	1.44	25.78	0.12
	1200	1.482	0.013	0.00123	0.378	12.563	95.25	91.11	1.35	24.18	0.10
	1400	4.867	0.016	0.00769	12.173	1.897	100.00	25.68	1.25	22.40	0.55
	T07-18			Phg			J= 0.009989
	700	38.043	0.255	0.02595	113.615	0.001	0.21	11.71	4.46	78.56 11.51
	750	15.708	0.098	0.00004	21.130	0.001	0.32	60.15	9.45 162.70 9.52
	800	6.431	0.034	0.04326	8.679	0.001	0.55	59.93	3.85	68.15	3.84
	833	4.043	0.023	0.02410	4.286	0.002	0.94	68.33	2.76	49.12	1.88
	866	21.966	0.063	0.39098	64.584	0.002	1.26	13.18	2.90	51.45	5.14
	900	2.064	0.014	0.23186	2.006	0.037	7.70	71.37	1.47	26.35	0.20
	933	2.249	0.014	0.08909	1.801	0.061	18.48	75.93	1.71	30.51	0.15
	966	2.079	0.012	0.00641	0.252	0.076	31.81	95.69	1.99	35.49	0.09
	1000	2.353	0.012	0.00194	0.126	0.090	47.71	97.76	2.30	40.98	0.07
	1033	2.335	0.012	0.00217	0.190	0.111	67.22	96.93	2.26	40.34	0.06
	1066	2.397	0.012	0.00148	0.125	0.134	90.75	97.82	2.35	41.77	0.05
	1100	2.523	0.013	0.00661	0.154	0.036	97.07	97.59	2.46	43.84	0.14
	1200	3.107	0.014	0.01343	0.456	0.014	99.51	95.19	2.96	52.53	0.30
	1400	91.499	0.072	0.09171	287.170	0.003	100.00	7.25	6.63 115.72 11.78
	T07-25			Phg			J= 0.009989
	700	10.353	0.021	0.06329	13.823	0.001	0.17	60.44	6.26 109.38 5.61
	750	22.200	0.062	0.27173	33.797	0.000	0.20	55.03 12.22 207.77 24.22
	800	8.207	0.023	0.08183	3.719	0.001	0.39	86.50	7.10 123.60 2.86
	833	5.034	0.019	0.15816	7.479	0.003	0.94	56.02	2.82	50.12	1.99
	866	4.016	0.017	0.24385	5.082	0.004	1.64	62.67	2.52	44.80	1.53
	900	3.320	0.012	0.30718	4.912	0.004	2.40	56.51	1.88	33.51	1.32
	933	2.769	0.015	0.08513	4.407	0.034	8.47	52.63	1.46	26.07	0.28
	966	1.976	0.013	0.00325	1.246	0.083	23.35	80.59	1.59	28.48	0.12

Table 3

 3

	Step No.	40 Ar/	39 Ar	38 Ar/	39 Ar	37 Ar/	39 Ar	36 Ar/	39 Ar	39 Ar	% 39 Ar % 40 Ar	* 40 Ar	* / Age	±1σ
								×10 -3 mol ×10 -14 mol released		39 Ar K (Ma) (Ma)
	S07-01			Phg			J= 0.009901
	1	1.255	0.000	0.00000	0.002	0.002	0.24	98.72	1.24	21.99	2.32
	2	1.183	0.009	0.00000	0.002	0.002	0.46	98.63	1.17	20.72	2.19
	3	3.348	0.002	0.00000	0.001	0.005	0.98	99.53	3.33	58.56	2.20
	4	3.288	0.016	0.22079	0.000	0.014	2.49	99.53	3.29	57.81	1.28
	5	2.355	0.029	0.12570	0.001	0.006	3.18	99.34	2.35	41.47	1.18
	6	2.374	0.012	0.00000	0.000	0.096	13.44	99.34	2.36	41.64	0.34
	7	2.415	0.011	0.00196	0.241	0.426	58.76	96.41	2.33	41.12	0.67
	8	2.412	0.011	0.00764	0.000	0.109	70.40	99.35	2.40	42.32	0.39
	9	2.395	0.012	0.00000	0.000	0.037	74.35	99.35	2.38	42.01	0.52
	10	2.589	0.018	0.00000	0.979	0.039	78.49	88.23	2.28	40.35	6.40
	11	2.399	0.010	0.01928	0.211	0.099	89.08	96.81	2.32	41.02	2.57
	12	2.446	0.013	0.03059	0.345	0.071	96.60	95.29	2.33	41.17	3.66
	13	2.490	0.008	0.06179	0.000	0.032	100.00	99.37	2.48	43.74	0.64

Table 4

 4

	Analyse No.	40 Ar/	39 Ar	38 Ar/	39 Ar	37 Ar/	39 Ar	36 Ar/	39 Ar	39 Ar	% 40 Ar	* 40 Ar	* /	Age	±1σ
								×10 -3 mol ×10 -14 mol		39 Ar K	(Ma)	(Ma)
	S07-Jojo			Phg			J= 0.009758
	1	2.972	0.095	0.00000	0.527	0.071	94.23	2.80	48.65	2.79
	2	3.275	0.052	0.00000	1.882	0.053	82.55	2.70	46.98	3.46
	3	3.656	0.167	0.06754	2.830	0.054	76.84	2.81	48.79	3.40
	4	3.992	0.046	0.00892	3.987	0.058	70.12	2.80	48.61	3.24
	5	4.386	0.027	0.01433	5.060	0.077	65.58	2.88	49.94	2.82
	6	3.262	0.022	0.04950	0.199	0.055	97.83	3.19	55.33	3.48
	7	3.788	0.026	0.09397	3.628	0.056	71.47	2.71	47.04	3.51
	8	4.536	0.037	0.02412	5.532	0.044	63.66	2.89	50.13	4.49
	9	3.353	0.018	0.05614	1.445	0.060	86.93	2.92	50.60	3.21
	10	3.367	0.022	0.03595	3.090	0.102	72.50	2.44	42.47	2.03
	T07-15bis			Phg			J= 0.009758
	1	4.029	0.034	0.00000	4.641	0.008	65.58	2.64	45.62	26.00
	2	2.502	0.015	0.00000	0.419	0.122	94.42	2.36	40.84	1.70
	3	7.203	0.020	0.03301	6.429	0.079	73.45	5.29	90.22	2.59
	4	4.031	0.015	0.01014	4.031	0.110	70.08	2.83	48.74	1.73
	5	3.499	0.020	0.00504	3.048	0.086	73.83	2.58	44.62	2.26
	6	5.708	0.020	0.00871	9.503	0.045	50.54	2.89	49.76	4.62
	7	4.986	0.020	0.01547	7.176	0.114	57.18	2.85	49.17	2.13
	8	13.733	0.021	0.05281	30.376	0.026	34.55	4.75	81.12	8.45
	9	4.407	0.024	0.00000	3.388	0.067	76.93	3.39	58.33	2.90
	T07-16			Phg			J= 0.009758
	1	5.677	0.027	0.08906	11.631	0.037	39.30	2.23	38.59	5.01
	2	3.803	0.035	0.00000	5.915	0.021	53.63	2.04	35.31	10.07
	3	3.467	0.016	0.01456	2.067	0.077	81.96	2.84	49.01	2.20
	4	3.573	0.018	0.00201	1.451	0.061	87.57	3.13	53.89	2.81
	5	2.561	0.012	0.00000	0.178	0.093	97.33	2.49	43.06	1.85
	6	3.066	0.014	0.00000	1.234	0.083	87.60	2.69	46.37	2.09
	7	2.862	0.021	0.00000	0.550	0.095	93.78	2.68	46.33	1.85
	8	2.852	0.014	0.00000	1.412	0.103	84.82	2.42	41.81	1.77
	9	3.004	0.018	0.00986	1.863	0.096	81.18	2.44	42.14	2.04
	10	2.862	0.021	0.00000	1.457	0.078	84.41	2.42	41.76	2.46
	11	3.715	0.047	0.11295	2.610	0.026	79.04	2.94	50.63	7.61
	T07-19			Phg			J= 0.009758
	1	1.931	0.020	0.00000	1.530	0.074	75.78	1.46	25.41	2.64
	2	2.357	0.023	0.00092	2.351	0.078	69.86	1.65	28.56	2.54
	3	2.974	0.037	0.00017	1.222	0.073	87.33	2.60	44.85	2.81
	4	2.195	0.025	0.00000	3.285	0.079	55.06	1.21	21.01	2.54
	5	2.365	0.032	0.01104	0.385	0.062	94.57	2.24	38.69	3.21
	6	2.530	0.030	0.06742	1.348	0.093	83.84	2.12	36.72	1.99
	7	2.671	0.025	0.00000	1.200	0.074	86.14	2.30	39.79	2.39
	T07-18			Phg			J= 0.009758
	1	4.446	0.018	0.03794	6.208	0.110	58.45	2.60	44.88	2.22
	2	4.848	0.016	0.02992	10.305	0.066	36.91	1.79	31.02	2.99
	3	2.537	0.015	0.01001	2.539	0.111	69.83	1.77	30.71	1.69
	4	3.382	0.015	0.00000	8.604	0.090	24.36	0.82	14.35	2.24
	5	4.155	0.021	0.00000	0.851	0.114	93.57	3.89	66.73	8.37
	6	2.574	0.017	0.05904	0.301	0.098	96.11	2.47	42.75	2.04
	7	2.958	0.014	0.00000	0.799	0.080	91.49	2.71	46.71	2.22
	I07-122							J= 0.009758
	1	3.799	0.114	0.00000	7.508	0.064	41.19	1.56	27.34	2.98
	2	3.486	0.062	0.03792	1.160	0.063	89.80	3.13	54.28	3.00
	3	3.364	0.025	0.00000	1.310	0.126	88.03	2.96	51.39	1.60
	4	14.19	0.039	0.00123	30.589	0.130	36.19	5.14	88.22	3.76

Table 5

 5

	Unit	Sample Lithology	Method	Analyzed fractions	Age (Ma)	Uncertainty (Ma)	Reference
	CBU	Ag31	Gln-Ep-Ph-eclogite Lu/Hf	WR-Grt	52.2	0.3	Lagos et al., 2007
	CBU	Ag85	Gln-Ep-Ph-eclogite Lu/Hf	WR-Grt-Omp	51.4	0.4	Lagos et al., 2007
	CBU	Ap21	Gln-Ep-Ph-eclogite Lu/Hf	WR-Grt-Omp	50.2	2	Lagos et al., 2007
	CBU	1081	omphacitite	Rb/Sr	WR-Ph	49.4	0.7	Brocker & Enders, 2001
	CBU	1083	omphacitite	Rb/Sr	WR-Ph	46.3	0.4	Brocker & Enders, 2001
	CBU	5243	calcschist	Rb/Sr	WR-Ph-Cal	28.7	2.4	Bröcker et al., 2013
	CBU	5244	greenschist	Rb/Sr	WR-Ph-Ep-Ab	20.5	1.3	Bröcker et al., 2013
	CBU	5246	greenschist	Rb/Sr	WR-Ph-Ep-Ab	25.3	0.9	Bröcker et al., 2013
	CBU	5267	calcschist	Rb/Sr	WR-Ph-Cal	33.5	3.2	Bröcker et al., 2013
	CBU	5831	greenschist	Rb/Sr	Ph-Ab-Cal-Ep	39.2	3.2	Bröcker et al., 2013
	CBU	SYR015 greenschist in marble Rb/Sr	Pg-Ph-Ep-Ab	30.8	2.9	Bröcker et al., 2013
	CBU	63287a calcschist	Rb/Sr	Ph	52.5	0.2	Cliff et al., 2016
	CBU	63287a calcschist	Rb/Sr	Ph	28.5	0.6	Cliff et al., 2016
	CBU	63287a calcschist	Rb/Sr	Ph	52.1	1.1	Cliff et al., 2016
	CBU	63287a calcschist	Rb/Sr	Ph	46.9	2.3	Cliff et al., 2016
	CBU	63287b calcschist	Rb/Sr	Ph	48.6	0.5	Cliff et al., 2016
	CBU	63287b calcschist	Rb/Sr	Ph	47.1	0.6	Cliff et al., 2016
	CBU	63287b calcschist	Rb/Sr	Ph	46.2	1.3	Cliff et al., 2016
	CBU	63286	calcschist	Rb/Sr	Ph	35.2	1	Cliff et al., 2016
	CBU	63286	calcschist	Rb/Sr	Ph	26	2.3	Cliff et al., 2016
	CBU	63286	calcschist	Rb/Sr	Ph	37.4	0.8	Cliff et al., 2016
	CBU	63286	calcschist	Rb/Sr	Ph	37.4	0.8	Cliff et al., 2016
	CBU	63286	calcschist	Rb/Sr	Ph	37.5	1.7	Cliff et al., 2016
	CBU	63297	calcschist	Rb/Sr	Ph	40.5	1.1	Cliff et al., 2016
	CBU	63297	calcschist	Rb/Sr	Ph	35.6	0.5	Cliff et al., 2016
	CBU	63297	calcschist	Rb/Sr	Ph	32.2	0.3	Cliff et al., 2016
	CBU	63297	calcschist	Rb/Sr	Ph	34.6	1.1	Cliff et al., 2016
	CBU	63297	calcschist	Rb/Sr	Ph	34.4	0.5	Cliff et al., 2016
	CBU	63300	calcschist	Rb/Sr	Ph	37.1	0.4	Cliff et al., 2016
	CBU	63300	calcschist	Rb/Sr	Ph	40.5	0.4	Cliff et al., 2016

Table 6

 6

	Unit	Sample	Lithology	Method	Analyzed fractions	Age (Ma)	Uncertainty (Ma)	Reference
	CBU	3513	impure marble	Rb/Sr	Ph-Cal	24.2	0.2	Brocker & Franz, 2005
	CBU	3514	impure marble	Rb/Sr	Ph-Cal	25	0.3	Brocker & Franz, 2005
	CBU	3515	impure marble	Rb/Sr	Ph-Cal	23.5	0.2	Brocker & Franz, 2005
	CBU	3519	calcschist	Rb/Sr	Ph-Cal	22	0.2	Brocker & Franz, 2005
	CBU	3520	impure marble	Rb/Sr	Ph-Cal	22.9	0.9	Brocker & Franz, 2005
	CBU	3522	calcschist	Rb/Sr	Ph-Cal	21.9	0.2	Brocker & Franz, 2005
	CBU	3521	impure marble	Rb/Sr	Ph-Cal	24	0.2	Brocker & Franz, 2005
	CBU	2016	calcschist	Rb/Sr	Ph-Cal	21.5	0.2	Brocker & Franz, 2005
	CBU	1221	phyllite	Rb/Sr	Ph-WR	23.3	0.4	Brocker & Franz, 2005
	CBU	1222	quartzite	Rb/Sr	Ph-WR	23.8	0.3	Brocker & Franz, 2005
	CBU	1418	phyllite	Rb/Sr	Ph-WR	24.1	0.2	Brocker & Franz, 2005
	CBU	1421	phyllite	Rb/Sr	Ph-WR	24.1	0.2	Brocker & Franz, 2005
	CBU	3518	quartzite	Rb/Sr	Ph-Cal	24	0.2	Brocker & Franz, 2005
	CBU	1076	greenschist-facies calcschist Rb/Sr	WR-Ph	21.7	0.2	Brocker & Franz, 1998
	CBU	318	metasediment	Rb/Sr	WR-Ph	22.00	0.2	Bröcker & Franz, 2000
	CBU	305	metasediment	Rb/Sr	WR-Ep-Ph	26.2	0.3	Bröcker & Franz, 2000
	CBU	461	metasediment	Rb/Sr	WR-Ph	21.2	0.2	Bröcker & Franz, 2000
	CBU	1163	metasediment	Rb/Sr	WR-Ph	21.8	0.3	Bröcker & Franz, 2000
	CBU	526	metasediment	Rb/Sr	WR-Ph	25.6	0.3	Bröcker & Franz, 2000
	CBU	348	metasediment	Rb/Sr	WR-Ph	32.1	0.3	Bröcker & Franz, 2000
	CBU	348	metasediment	Rb/Sr	WR-Bt	14.1	0.2	Bröcker & Franz, 2000
	CBU	28	metasediment	Rb/Sr	WR-Bt	8.2	0.1	Bröcker & Franz, 2000
	CBU	275	metasediment	Rb/Sr	WR-Bt	14.8	0.2	Bröcker & Franz, 2000
	CBU	453	metasediment	Rb/Sr	WR-Bt	14.3	0.1	Bröcker & Franz, 2000
	CBU	492	metasediment	Rb/Sr	WR-Bt	10.00	0.1	Bröcker & Franz, 2000
	CBU	3008	impure marble	Rb/Sr	Ph-Cal	41.1	0.4	Bröcker et al., 2004
	CBU	3009	impure marble	Rb/Sr	Ph-Cal	37.2	0.5	Bröcker et al., 2004
	CBU	3012	impure marble	Rb/Sr	Ph-Cal	24.2	0.3	Bröcker et al., 2004
	CBU	3013	impure marble	Rb/Sr	Ph-Cal	23.9	0.3	Bröcker et al., 2004
	CBU	3014	impure marble	Rb/Sr	Ph-Cal	24.5	0.3	Bröcker et al., 2004

Table 7

 7

	Unit Sample Lithology	Method	Analyzed fractions	Age (Ma)	Uncertainty (Ma)	Reference
	CCB 77-227 orthogneiss	Rb/Sr	WR-Ph	13.2	0.4	Henjes-Kunst & Kreuzer, 1982
	CCB IOS04-1 orthogneiss	Rb/Sr	Fsp-Ap-Ph	18	1	Thomson, et al., 2009
	CBU IOS03-2 impure marble	Rb/Sr	Ph-Cal	34.5	2.5	Thomson, et al., 2009
	CBU IOS04-6 impure marble	Rb/Sr	Ph-Cal	18.8	0.3	Thomson, et al., 2009
	CBU IOS04-7 quartzite	Rb/Sr	Ph-Fsp	27	8.0	Thomson, et al., 2009
	CCB AG03-10 Grt-micaschist	Ar/Ar concentrate Ph	30-40		Forster & Lister, 2009
	CCB AG03-08 Grt-micaschist	Ar/Ar concentrate Ph	29-33.5		Forster & Lister, 2009
	CCB AG03-09 Grt-micaschist	Ar/Ar concentrate Ph	29-32		Forster & Lister, 2009
	CCB AG03-01 leucogranite	Ar/Ar concentrate Ph	42.5-44		Forster & Lister, 2009
	CCB AG03-03 orthogneiss	Ar/Ar concentrate Ph	70-74		Forster & Lister, 2009
	CCB AG03-04 orthogneiss	Ar/Ar concentrate Ph	62.5-65.5		Forster & Lister, 2009
	CCB AG03-05 orthogneiss	Ar/Ar concentrate Ph	68.5-73		Forster & Lister, 2009
	CCB AG03-06 orthogneiss	Ar/Ar concentrate Ph	55.5-57		Forster & Lister, 2009
	CCB 88607 orthogneiss	Ar/Ar concentrate Ph	59-63		Baldwin & Lister, 1998
	CCB 90368 orthogneiss	Ar/Ar concentrate Ph	52-57		Baldwin & Lister, 1998
	CBU 88606 schiste à q-Ph	Ar/Ar concentrate Ph	30.5-42		Baldwin, 1996
	CBU 88610 Gln-schist	Ar/Ar concentrate Ph	25.6-39.8		Baldwin, 1996
	CBU 88638 Gln-schist	Ar/Ar concentrate Ph	25-44		Baldwin, 1996
	CBU 88639 Gln-schist	Ar/Ar concentrate Ph	23.5-42		Baldwin, 1996
	CBU 88641 Gln-Grt-schist	Ar/Ar concentrate Ph	25.3-49.3		Baldwin, 1996
	CCB 90350 Grt-micaschist	Ar/Ar concentrate Ph+Pg	53	2	Baldwin & Lister, 1998
	CCB 90354 orthogneiss	Ar/Ar concentrate Ph	31	1	Baldwin & Lister, 1998
	CCB 88609 orthogneiss	Ar/Ar concentrate Ph	20	1	Baldwin & Lister, 1998
	CCB AG03-07 orthogneiss	Ar/Ar concentrate Ph	24.9	0.1	Forster & Lister, 2009
	CBU I07-15 Ab-Ep-blueschist Ar/Ar single grain Ph	41	1.72	this study
	CBU I07-18 Grt-blueschist	Ar/Ar single grain Ph	48.46	0.82	this study
	CBU I07-122 Grt-Cld-micaschist Ar/Ar in situ	Ph	46.4	2	this study
	CBU I07-122 Grt-Cld-micaschist Ar/Ar in situ	Ph	77.3	3.7	this study
	CBU I07-122 Grt-Cld-micaschist Ar/Ar in situ	Ph	90.8	3.6	this study
	CBU I07-122 Grt-Cld-micaschist Ar/Ar in situ	Ph	90.8	3.7	this study

Ar/ 36 Ar of 368.8 ± 24.7 (MSWD = 0.53, Fig.8b) suggesting that some excess argon

Acknowledgments

This work has received funding from the ANR EGEO Project, from the European Research Council (ERC) under the seventh Framework Programme of the European Union (ERC Advanced Grant, grant agreement No 290864, RHEOLITH) and from the Institut Universitaire de France. It is a contribution of the Labex VOLTAIRE.

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