Marie Baïsset
email: marie.baisset.pro@gmail.com

Loïc Labrousse

Philippe Yamato

Alexandre Schubnel

Twinning and partial melting as early weakening processes in plagioclase at high pressure: insights from Holsnøy (Scandinavian Caledonides, Norway)

Keywords: eclogitization process, reaction-induced weakening, partial melting, plagioclase mechanical twinning, grain size reduction

published or not. The documents may come

Twinning and partial melting as early weakening processes in plagioclase at high pressure: insights from Holsnøy (Scandinavian Caledonides, Norway)

Introduction

Eclogitization is known to trigger important changes in mechanical properties of rocks (i.e., strength and density), and is therefore considered as a key process for the dynamics of convergence zones at short time scales (seismicity - [START_REF] Austrheim | Pseudotachylytes generated during seismic faulting and eclogitization of the deep crust[END_REF][START_REF] Lund | High-pressure metamorphism and deep-crustal seismicity: evidence from contemporaneous formation of pseudotachylytes and eclogite facies coronas[END_REF][START_REF] Hacker | Subduction factory 2. Are intermediate-depth earthquakes in subducting slabs linked to metamorphic dehydration reactions?[END_REF] as well as on the long-term (plate tectonics - [START_REF] Kirby | Intermediate-depth intraslab earthquakes and arc volcanism as physical expressions of crustal and uppermost mantle metamorphism in subducting slabs[END_REF][START_REF] Jung | Intermediate-depth earthquake faulting by dehydration embrittlement with negative volume change[END_REF][START_REF] Jackson | Metastability, mechanical strength, and the support of mountain belts[END_REF][START_REF] Hetényi | Metamorphic transformation rate over large spatial and temporal scales constrained by geophysical data and coupled modelling[END_REF].

Concomitance between eclogitization reactions and deformation localization in ductile

shear-zones or in pseudotachylytes has been extensively described [START_REF] Austrheim | Pseudotachylytes generated during seismic faulting and eclogitization of the deep crust[END_REF][START_REF] Früh-Green | Interdependence of deformation, fluid infiltration and reaction progress recorded in eclogitic metagranitoids (Sesia Zone, Western Alps)[END_REF][START_REF] Lund | High-pressure metamorphism and deep-crustal seismicity: evidence from contemporaneous formation of pseudotachylytes and eclogite facies coronas[END_REF][START_REF] John | Interrelations between intermediate-depth earthquakes and fluid flow within subducting oceanic plates: Constraints from eclogite facies pseudotachylytes[END_REF][START_REF] Steltenpohl | Retrograded eclogite-facies pseudotachylytes as deep-crustal paleoseismic faults within continental basement of Lofoten, north Norway[END_REF][START_REF] Scambelluri | Fossil intermediate-depth earthquakes in subducting slabs linked to differential stress release[END_REF]. Although eclogitization is acknowledged as a trigger for changes in the effective strength of rocks, how eclogitization reactions induce these rheological changes is still under investigation today. Indeed, eclogitization of rocks consists in a set of reactions of variable kinetics, that successively take place with increasing pressure and lead to the eventual formation of garnet and omphacite as index phases, but also secondary phases depending on the chemical system of the rock. These reactions can be responsible for grain size reduction, for the release or consumption of fluids, and can also lead to the nucleation of newly formed weaker or stronger phases, all these processes being able to strongly affect the mechanical properties of the rock.

However, as several progressive transformations are involved in the eclogitization process, the first reactions may generate transient phases and textures later obliterated by the subsequent reactions. In particular, within deformed zones where reactions usually reached completion, these transient steps are not visible anymore. It is therefore crucial to study undeformed and partially transformed zones where the first steps of mineral breakdown may have been preserved [START_REF] Wayte | A TEM study of disequilibrium plagioclase breakdown at high pressure: the role of infiltrating fluid[END_REF]. It is indeed possible that these first reaction steps play a major role in the ultimate localization of deformation [START_REF] Brodie | On the relationship between deformation and metamorphism, with special reference to the behavior of basic rocks[END_REF].

Eclogitization of the continental lower crust constitutes a perfect case study for depicting these first reaction stages, due to the low reactivity of dry plagioclase, one of its main rock-forming mineral [START_REF] Wain | Metastability of granulites and processes of eclogitisation in the UHP region of western Norway[END_REF][START_REF] John | Partial eclogitisation of gabbroic rocks in a late Precambrian subduction zone (Zambia): prograde metamorphism triggered by fluid infiltration[END_REF][START_REF] Young | Does continental crust transform during eclogite facies metamorphism?[END_REF][START_REF] Schorn | Details of the gabbro-to-eclogite transition determined from microtextures and calculated chemical potential relationships[END_REF][START_REF] Shi | Lower-crustal earthquakes in southern Tibet are linked to eclogitization of dry metastable granulite[END_REF][START_REF] Incel | Reaction-induced embrittlement of the lower continental crust[END_REF]. The breakdown of this phase with increasing pressure involves two chemical systems that present contrasted stoechiometries [START_REF] Holland | The reaction albite= jadeite+ quartz determined experimentally in the range 600-1200 C[END_REF][START_REF] Goldsmith | Plagioclase stability at elevated temperatures and water pressures[END_REF]) and decoupled kinetics due to slow interdiffusion of NaSi and CaAl [START_REF] Wayte | A TEM study of disequilibrium plagioclase breakdown at high pressure: the role of infiltrating fluid[END_REF]): (1) the hydrated Ca-system (zoisite formation from the reaction anorthite + H 2 O = zoisite + kyanite + quartz), and (2) the Na-system (jadeite formation from reaction albite = jadeite + quartz).

If reaction [START_REF] Liou | Analcime equilibria[END_REF] has often been considered of major importance in the eclogitization process and its mechanical consequences [START_REF] Shi | Lower-crustal earthquakes in southern Tibet are linked to eclogitization of dry metastable granulite[END_REF][START_REF] Incel | Reaction-induced embrittlement of the lower continental crust[END_REF], reaction (1) that initiates earlier along the prograde P-T path is still an underrated transformation. For instance, the crystallization kinetics and mechanical properties of epidote are still poorly known, although being one of the most ubiquitous minerals of metamorphism. However, because fluids are involved in its balance, reaction kinetics can be very fast as soon as fluids enter the system [START_REF] Wayte | A TEM study of disequilibrium plagioclase breakdown at high pressure: the role of infiltrating fluid[END_REF][START_REF] Rubie | Disequilibrium during metamorphism: the role of nucleation kinetics[END_REF][START_REF] Pattison | Petrological consequences of variations in metamorphic reaction affinity[END_REF]). This reaction is also characterized by a sensible volume change (~15%). It is therefore expected to generate fast changes of the state of stress inside the grains themselves, then mechanically destabilizing the rock at an early stage [START_REF] Stünitz | Weakening and strain localization produced by syndeformational reaction of plagioclase[END_REF].

The island of Holsnøy (Scandinavian Caledonides) is ideal for studying the initial steps of plagioclase breakdown at high pressure, because initial granulite is only partially transformed (20 -30% - [START_REF] Boundy | Structural development and petrofabrics of eclogite facies shear zones, Bergen Arcs, western Norway: implications for deep crustal deformational processes[END_REF][START_REF] Putnis | Metamorphic processes and seismicity: the Bergen Arcs as a natural laboratory[END_REF]. Zones of ongoing eclogitization of the granulite have been described and mapped [START_REF] Boundy | Structural development and petrofabrics of eclogite facies shear zones, Bergen Arcs, western Norway: implications for deep crustal deformational processes[END_REF]) in shear-zones [START_REF] Austrheim | Shear deformation and eclogite formation within granulite-facies anorthosites of the Bergen Arcs, western Norway[END_REF][START_REF] Kaatz | Widening of hydrous shear zones during incipient eclogitization of metastable dry and rigid lower crust-Holsnøy, western Norway[END_REF][START_REF] Bras | Transient weakening during the granulite to eclogite transformation within hydrous shear zones (Holsnøy, Norway)[END_REF], in pseudotachylytes and their damage zones [START_REF] Austrheim | Pseudotachylytes generated during seismic faulting and eclogitization of the deep crust[END_REF][START_REF] Petley-Ragan | Microstructural records of earthquakes in the lower crust and associated fluid-driven metamorphism in plagioclase-rich granulites[END_REF][START_REF] Centrella | The granulite-to eclogite-and amphibolite-facies transition: a volume and mass transfer study in the Lindås Nappe, Bergen Arcs, west Norway[END_REF], as well as in undeformed haloes spreading around a central vein, or in the form of diffuse patches (so-called eclogitic fingers - [START_REF] Jamtveit | Accelerated hydration of the Earth's deep crust induced by stress perturbations[END_REF][START_REF] Zertani | Modification of the seismic properties of subducting continental crust by eclogitization and deformation processes[END_REF][START_REF] Putnis | Preservation of granulite in a partially eclogitized terrane: Metastable phenomena or local pressure variations?[END_REF].

In order to discuss the potential importance of early plagioclase breakdown on the mechanical behavior of its host-rock, we here present a detailed petrological analysis of the first steps of the breakdown processes affecting plagioclase grains at their boundaries as well as at their interior, through a transect into the tip of an eclogite finger. Through the study of plagioclase crystallographic orientation patterns around newly formed zoisite, we elaborate on the stresses recorded within the partially reacted plagioclase grains that can be accommodated by the activation of twin systems although the granulite remains macroscopically undeformed. At plagioclase-plagioclase grain boundaries, transient products formed after plagioclase breakdown have been identified, such as kyaniteanalcime-bearing associations that may result from crystallization after a melt. We use thermodynamic modelling to state that partial melting is possible at pressure and temperature (P-T) conditions close to the one acknowledged for Holsnøy rocks.

Eventually, we elaborate on the importance of these initial stages of reaction and how they possibly contribute to mechanical alteration of the plagioclase matrix (grain size reduction, melt production) even before strain localization.

Geological setting and sampling strategy 2.1 Geological context

The area studied here is situated in the island of Holsnøy, Bergen Arcs, Western Norway, and is mostly composed of anorthosites an gabbros that underwent granulite facies metamorphism 930 Ma ago, during the Grenvillian orogeny [START_REF] Bingen | Zircon U-Pb geochronology in the Bergen arc eclogites and their Proterozoic protoliths, and implications for the pre-Scandian evolution of the Caledonides in western Norway[END_REF]). The Bergen Arcs (inset Fig. 1a) have been formed by the nappe stacking of different units, which accretion results from the collision between Baltica and Laurentia during the Caledonian orogeny [START_REF] Roberts | The Scandinavian Caledonides: event chronology, palaeogeographic settings and likely modern analogues[END_REF]. The island of Holsnøy exposure belongs to the Lindås nappe [START_REF] Andersen | Mineral-fluid-melt interactions in high-pressure shear zones in the Bergen Arcs nappe complex, Caledonides of W. Norway: Implications for the fluid regime in Caledonian eclogite-facies metamorphism[END_REF], composed of rocks that underwent high-pressure metamorphism during the collisional Scandian phase at 430 Ma [START_REF] Austrheim | Eclogite formation and dynamics of crustal roots under continental collision zones[END_REF][START_REF] Kühn | The Caledonian tectonometamorphic evolution of the Lindås Nappe: Constraints from U-Pb, Sm-Nd and Rb-Sr ages of granitoid dykes[END_REF][START_REF] Bingen | Trace element signature and U-Pb geochronology of eclogite-facies zircon, Bergen Arcs, Caledonides of W Norway[END_REF][START_REF] Glodny | Geochronology of fluid-induced eclogite and amphibolite facies metamorphic reactions in a subduction-collision system, Bergen Arcs, Norway[END_REF][START_REF] Jamtveit | The effects of earthquakes and fluids on the metamorphism of the lower continental crust[END_REF], and correlated to the Dalsford nappe in the Western Gneiss Complex, and the Jotun nappe within the Scandian nappe pile inland [START_REF] Andersen | Mineral-fluid-melt interactions in high-pressure shear zones in the Bergen Arcs nappe complex, Caledonides of W. Norway: Implications for the fluid regime in Caledonian eclogite-facies metamorphism[END_REF][START_REF] Jakob | Revised tectonostratigraphic scheme for the Scandinavian Caledonides and its implications for our understanding of the Scandian orogeny[END_REF]). This tectonostratigraphic complex has been attributed to the distal part of the Baltica margin (the Jotun microcontinent, [START_REF] Jakob | Revised tectonostratigraphic scheme for the Scandinavian Caledonides and its implications for our understanding of the Scandian orogeny[END_REF]) imbricated within the Scandian wedge and then dilacerated during the late orogenic Caledonian extension.

Grenvilian granulite facies pervasive metamorphism occurred at temperatures of 800-850°C and pressures lower than 1 GPa [START_REF] Austrheim | Shear deformation and eclogite formation within granulite-facies anorthosites of the Bergen Arcs, western Norway[END_REF]. Incomplete reaction during the Scandian burial event developed a prograde amphibolite facies (0.8-1.0 GPa ~ 600-650°C [START_REF] Glodny | Geochronology of fluid-induced eclogite and amphibolite facies metamorphic reactions in a subduction-collision system, Bergen Arcs, Norway[END_REF][START_REF] Centrella | Textural and chemical evolution of pyroxene during hydration and deformation: a consequence of retrograde metamorphism[END_REF][START_REF] Moore | Microstructurally controlled trace element (Zr, U-Pb) concentrations in metamorphic rutile: an example from the amphibolites of the Bergen Arcs[END_REF]) and peak eclogite facies imprint (2.2 GPa, 690°C;[START_REF] Bhowany | Phase equilibria modelling constraints on P-T conditions during fluid catalysed conversion of granulite to eclogite in the Bergen Arcs, Norway[END_REF]) disseminated inside a anorthositic and gabbroic complex. The two events (grenvilian and caledonian) were preserved from later retrogression, restrained to the km-size amphibolite shear zones that surround the granulite-eclogite massif [START_REF] Boundy | Structural development and petrofabrics of eclogite facies shear zones, Bergen Arcs, western Norway: implications for deep crustal deformational processes[END_REF]. For this reason, Holsnøy partly eclogitized granulites are a precious example of lower continental crust showing the onset of eclogitization reactions, considered therefore as a field analogue of the of lower continental crust portions buried at depth under present-day collisional orogens (like below Tibet for example, [START_REF] Austrheim | Pseudotachylytes generated during seismic faulting and eclogitization of the deep crust[END_REF][START_REF] Jackson | Metastability, mechanical strength, and the support of mountain belts[END_REF][START_REF] Labrousse | Initiation of crustal-scale thrusts triggered by metamorphic reactions at depth: Insights from a comparison between the Himalayas and Scandinavian Caledonides[END_REF] and references therein).

Eclogitization extent and significance

Mapping of lithologies present in Holsnøy have been described in several petrological studies since the 1980s [START_REF] Austrheim | Shear deformation and eclogite formation within granulite-facies anorthosites of the Bergen Arcs, western Norway[END_REF][START_REF] Austrheim | Eclogitization of lower crustal granulites by fluid migration through shear zones[END_REF][START_REF] Putnis | Metamorphic processes and seismicity: the Bergen Arcs as a natural laboratory[END_REF][START_REF] Putnis | Preservation of granulite in a partially eclogitized terrane: Metastable phenomena or local pressure variations?[END_REF][START_REF] Bhowany | Phase equilibria modelling constraints on P-T conditions during fluid catalysed conversion of granulite to eclogite in the Bergen Arcs, Norway[END_REF][START_REF] Bras | Transient weakening during the granulite to eclogite transformation within hydrous shear zones (Holsnøy, Norway)[END_REF][START_REF] Jamtveit | High pressure metamorphism caused by fluid induced weakening of deep continental crust[END_REF][START_REF] Jamtveit | Rapid fluid-driven transformation of lower continental crust associated with thrustinduced shear heating[END_REF][START_REF] Zertani | Reactive fluid flow guided by grain-scale equilibrium reactions during eclogitization of dry crustal rocks[END_REF] and the systematic relationships between eclogitization and strain led to the definition of: (1) up to km-wide eclogitic shear zones that constitute an anastomosed network inside the pristine granulite, surrounded by less reacted zones where the granulite becomes individualized into blocks by a network of thin m-wide eclogite facies shear zones (breccia zones - [START_REF] Boundy | Structural development and petrofabrics of eclogite facies shear zones, Bergen Arcs, western Norway: implications for deep crustal deformational processes[END_REF]), ( 2) zones of static eclogitization [START_REF] Jamtveit | Accelerated hydration of the Earth's deep crust induced by stress perturbations[END_REF][START_REF] Jamtveit | Disequilibrium metamorphism of stressed lithosphere[END_REF][START_REF] Zertani | Modification of the seismic properties of subducting continental crust by eclogitization and deformation processes[END_REF][START_REF] Zertani | Reactive fluid flow guided by grain-scale equilibrium reactions during eclogitization of dry crustal rocks[END_REF][START_REF] Putnis | Preservation of granulite in a partially eclogitized terrane: Metastable phenomena or local pressure variations?[END_REF] It has been proposed that transformations of granulites during the Caledonian metamorphism remained partial because of the limited amount of fluids involved in the reactions [START_REF] Austrheim | Eclogitization of lower crustal granulites by fluid migration through shear zones[END_REF][START_REF] Jamtveit | Fluid controlled eclogitization of granulites in deep crustal shear zones, Bergen arcs, Western Norway[END_REF]). Fluid supply is therefore a major controlling factor for the eclogitization reaction in Holsnøy [START_REF] Rubie | The catalysis of mineral reactions by water and restrictions on the presence of aqueous fluid during metamorphism[END_REF][START_REF] Putnis | Fluid-induced processes: metasomatism and metamorphism[END_REF][START_REF] Putnis | Metamorphic processes and seismicity: the Bergen Arcs as a natural laboratory[END_REF][START_REF] Centrella | The granulite-to eclogite-and amphibolite-facies transition: a volume and mass transfer study in the Lindås Nappe, Bergen Arcs, west Norway[END_REF][START_REF] Moore | Microstructurally controlled trace element (Zr, U-Pb) concentrations in metamorphic rutile: an example from the amphibolites of the Bergen Arcs[END_REF][START_REF] Zertani | Reactive fluid flow guided by grain-scale equilibrium reactions during eclogitization of dry crustal rocks[END_REF] and hence the observed associated heterogeneous strain pattern. It has also been proposed that pressure and stress distribution were actually not homogeneous at the time of metamorphism and strain [START_REF] Jamtveit | High pressure metamorphism caused by fluid induced weakening of deep continental crust[END_REF][START_REF] Moore | Stress orientation-dependent reactions during metamorphism[END_REF][START_REF] Putnis | Preservation of granulite in a partially eclogitized terrane: Metastable phenomena or local pressure variations?[END_REF], Moulas et al., 2022). The impact of the eclogitization reaction itself has eventually been proposed to explain the observed intimate relationship between strain and eclogite facies imprint [START_REF] Bras | Transient weakening during the granulite to eclogite transformation within hydrous shear zones (Holsnøy, Norway)[END_REF]. In this prospect the relative chronology between development of ductile eclogite facies shear zones, spreading of static eclogite facies halos, and brittle deformation (pseudotachylytes) as well as the ability of these localized strain zones to promote fluid percolation are the key evidence for the on-going debate.

Sampling strategy

The relationships between eclogitization front and strain at the shear zone boundaries have been carefully documented in recent studies [START_REF] Bras | Transient weakening during the granulite to eclogite transformation within hydrous shear zones (Holsnøy, Norway)[END_REF][START_REF] Kaatz | How fluid infiltrates dry crustal rocks during progressive eclogitization and shear zone formation: insights from H 2 O contents in nominally anhydrous minerals[END_REF]), and show that (i) the fluid percolation front is further advanced in the granulite than the effective reaction front [START_REF] Kaatz | How fluid infiltrates dry crustal rocks during progressive eclogitization and shear zone formation: insights from H 2 O contents in nominally anhydrous minerals[END_REF]) and (ii) that the reactions developing at the outer edge of the strain zone actually involve only part of the bulk rock chemical system and induce a substantial solid volume change [START_REF] Bras | Transient weakening during the granulite to eclogite transformation within hydrous shear zones (Holsnøy, Norway)[END_REF]. The very first increment of reactions seem therefore to have a critical effect on the fluid budget and the mechanical alteration of the granulite. The aim of the present study is to complete the existing dataset with detailed petrological observations performed at the locus where those very first reactions increments are expected to be best preserved, i.e. at the tip of a "static" finger within an overall undeformed granulite block.

The rocks studied here were sampled on an outcrop previously described in [START_REF] Bras | Transient weakening during the granulite to eclogite transformation within hydrous shear zones (Holsnøy, Norway)[END_REF] at the tip of a meter-scale long and decimeter wide eclogite finger (Figs. 1c-d) that develops into the granulite along its foliation, underlined in the field by elongated coronas of garnet, orthopyroxene and clinopyroxene (red and black Fig. 1d; red zones Fig. 2b). This finger is rooted in a dextral eclogitic shear zone with an orientation to N110, oblique to the granulitic foliation, which strikes N020 (Fig. 1, SZ1 Bras et al., 2021). The orientation of this shear zone is consistent with the global orientation of the shear at the scale of the whole massif (i.e., parallel to the major Hundskjeften shear zone -- [START_REF] Boundy | Structural development and petrofabrics of eclogite facies shear zones, Bergen Arcs, western Norway: implications for deep crustal deformational processes[END_REF]Zertani et al. 2019 and Fig. 1a). Two thin sections have been prepared through the finger, from the less reacted granulite to more eclogitized zones (Figs. 2,3,4) and are localized in Fig. 1d and in the inset of Fig. 2a (transect T1 at the tip of the finger contains unreacted granulite whereas transect T2 is more internal, and therefore more eclogitized).

Methods

Thin sections were analyzed with scanning electron microscopy (SEM) to backscattered (BSE) images and energy dispersive spectroscopy (EDS) maps. Analyses were performed at ISTeP (Sorbonne Université, Paris) with a ZEISS Supra 55 variable pressure (VP) SEM-FEG (Field Emission Gun) and at ENS (Paris) with a ZEISS Sigma SEM-FEG, both equipped with backscattered electron (BSE) and energy dispersive (EDS) detectors. Operation conditions for EDS maps were set at 15 kV with a spot size of 2.27 μm for large-scale maps, and 10 kV with a spot size of 0.45 μm for small-scale maps, with a working distance of 15 mm. Quantitative mineral compositions were measured by electron probe microanalysis (EPMA) at CAMPARIS facility (Sorbonne Université, Paris) using a CAMECA SX-100 and SX-FIVE electron microprobes. Working conditions were set to 15 kV accelerating voltage, 10 nA beam current, 5 μm beam size and 10 s counting time for major mineral analysis. Albite, orthoclase, Fe2O3, MnTiO3, Cr2O3 and diopside were used as standards. For analcime analysis, they were set to 15 kV accelerating voltage, 4 nA beam current, a counting time of 5 s on the peak for Na, K and Si instead of 10 s for other elements, and a defocussed beam size of 5 μm in order to avoid Na and K losses that are expected while analyzing weakly crystallized phases or glasses (Morgan andLondon 1996, 2005). These two elements were measured first in that respect. The following reference glasses were used for alkali-loss correction: BCR-2G, BHVO-2G, BIR-1G. H 2 O concentrations in analcime and amorphous pools are calculated by difference of chemical analyses totals from 100 %.

For Electron back-scatter diffraction (EBSD) mapping, thin sections were additionally polished with colloidal silica. EBSD analyses were carried out at ISTeP (Sorbonne Université, Paris) using a ZEISS Supra 55 VP SEM-FEG and at ENS (Paris) using a ZEISS Sigma SEM-FEG, both equipped with an EBSD detector. Samples were tilted to 70° during acquisition, with electron beam set to 15 kV, and a beam current of 10 nA at a working distance of 15 mm. At ISTeP, data acquisition was performed with a Quantax Esprit 2.2 software and post-processing done with the same sample software, whereas at ENS data acquisition was performed with an Oxford Instrument Aztec software and post-processing done using Oxford Instruments Channel 5.12 software Tango. Postprocessing consisted in applying a wild spike and iterative nearest neighbour corrections.

EBSD data were processed with the Matlab Toolbox MTEX (version 5.3.1) for grain reconstruction and crystal orientation analysis [START_REF] Bachmann | Texture analysis with MTEXfree and open source software toolbox[END_REF]. Data sets were cleaned from points with mean angular deviation (MAD) index > 1. Grain boundaries are defined by misorientations ≥ 10°. Grains ≤ 3 pixels were removed. Grain size is calculated from grain areas (A) by the equivalent diameter equation . In order to process EBSD data, plagioclase crystal lattice has been defined from labradorite of the American Mineralogist's crystal structure database [START_REF] Wenk | The average structure of An 62-66 labradorite[END_REF] as follows: ! = 8.17 Å, ( = 12.87 Å, * = 7.10 Å, , = 93.46°, 2 = 116.05°, 4 = 90.48°, and space group is P-1. Twin boundaries are defined by misorientations > 178° between adjacent plagioclase grains, i.e., ± 2° away from the theoretical 180° around b-axis rotation associated to albite and pericline twinning (Figs. 5a,b). Preferential orientation of maximal finite shortening (Z) and stretching (X) directions for each plagioclase grain were plotted in stereodiagrams after calculation from plagioclase crystal theory [START_REF] Lawrence | Stress analysis based on albite twinning of plagioclase feldspars[END_REF][START_REF] Borg | Experimental deformation of plagioclases[END_REF] Egydio-Silva and Mainprice 1999) using pixel orientations in twinned and un-twinned regions (e.g., Fig. 5c). As albite and pericline twinning systems are orthogonal and contain each other Burgers vector, focal mechanism-like «beachballs » could be drawn after the aforementioned stereodiagrams by coloring compression quadrants in white and tension quadrants in black (e.g., inset Fig. 5c). Note that both twin planes express the same stress field, but none of them are required to infer from the best strain ellipsoid. Indeed, as their orientation in the crystal is unique, crystal orientation only is sufficient. Best stress orientation for activation of plagioclase twinning system can also be inferred from Schmid factor (SF) calculations. SF is the coefficient that links the component of externally applied

d = 2 A π
stresses normal to the slip plane (5 0 , Fig. 5a) to the resulting shear stresses acting on the slip plane in the slip direction (6, Fig. 5a), as expressed by the Schmid law: 6 = 5 0 × SF with SF = cos(,) × cos [START_REF] Liou | Analcime equilibria[END_REF], with ! the angle between 5 0 and the externally applied force, and β the angle between 6 and the externally applied force (Fig. 5a). Maximum value of SF is 0.5 at 45° of slip direction and slip plane pole, as inferred from geometrical considerations (Fig. 5a), and because twinning system not only has a direction but also a sense for gliding, minimum SF value is -0.5. For each plagioclase grain analyzed by EBSD, SF was color-plotted as a spherical function (e.g., Fig. 5c), by calculating the best stress tensor orientations for each pixel of un-twinned regions in order to activate albite or pericline twinning systems.

Raman analysis were performed at ENS with a Renishaw Invia Raman microspectrometer equipped with a 514 nm Cobolt laser. The Lithothèque ENS de Lyon database (https://lithotheque.ens-lyon.fr/Raman/raman.php) was used for mineral identification by peak comparison between reference mineral spectra and spectra that have been acquired

with a 50x objective between 175 cm -1 and 2062 cm -1 . Intensities between 2432 cm -1 and 3886 cm -1 were additionally collected for water band observation. Resolution was 1.8 cm - 1 , spectra were integrated over 90 s, and the laser power was 5 mW on the sample.

Acquired spectra were normalized to their maximum intensity.

Thermodynamic modelling was used to estimate possible P-T conditions for melting, for three different chemical systems and with variable water contents.

Perple_X_6.9.1 [START_REF] Connolly | Multivariable phase diagrams; an algorithm based on generalized thermodynamics[END_REF][START_REF] Connolly | The geodynamic equation of state: what and how[END_REF] was used for this purpose, with the internally consistent thermodynamic database hp62ver.dat [START_REF] Holland | An improved and extended internally consistent thermodynamic dataset for phases of petrological interest, involving a new equation of state for solids[END_REF]. The solution models are for ternary feldspar [START_REF] Holland | Activity-composition relations for phases in petrological calculations: an asymmetric multicomponent formulation[END_REF], epidote [START_REF] Holland | An improved and extended internally consistent thermodynamic dataset for phases of petrological interest, involving a new equation of state for solids[END_REF], garnet [START_REF] White | New mineral activity-composition relations for thermodynamic calculations in metapelitic systems[END_REF], clinopyroxene [START_REF] Green | An order-disorder model for omphacitic pyroxenes in the system jadeite-diopside-hedenbergite-acmite, with applications to eclogitic rocks[END_REF], clinoamphibole [START_REF] Green | Activity-composition relations for the calculation of partial melting equilibria in metabasic rocks[END_REF], white mica [START_REF] White | New mineral activity-composition relations for thermodynamic calculations in metapelitic systems[END_REF]) and melt [START_REF] Green | Activity-composition relations for the calculation of partial melting equilibria in metabasic rocks[END_REF]. We also performed calculations with the thermodynamic database hp11ver.dat [START_REF] Holland | An improved and extended internally consistent thermodynamic dataset for phases of petrological interest, involving a new equation of state for solids[END_REF] with the solution model for haplogranitic melt [START_REF] Holland | Calculation of phase relations involving haplogranitic melts using an internally consistent thermodynamic dataset[END_REF][START_REF] White | Calculation of partial melting equilibria in the system Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O (NCKFMASH)[END_REF] for comparison with the tonalitic melt solution model of [START_REF] Green | Activity-composition relations for the calculation of partial melting equilibria in metabasic rocks[END_REF]. This solution model for the melt is more consistent with our chemical system but not well constrained at high pressures. Consistent solution models for the other minerals are for ternary feldspar [START_REF] Fuhrman | Ternary-feldspar modeling and thermometry[END_REF], epidote [START_REF] Holland | An improved and extended internally consistent thermodynamic dataset for phases of petrological interest, involving a new equation of state for solids[END_REF], garnet [START_REF] Holland | An internally consistent thermodynamic data set for phases of petrological interest[END_REF], clinopyroxene [START_REF] Holland | Thermodynamics of order-disorder in minerals: II. Symmetric formalism applied to solid solutions[END_REF], clinoamphibole (Diener et al. 2011), and white mica [START_REF] Coggon | Mixing properties of phengitic micas and revised garnet-phengite thermobarometers[END_REF][START_REF] Auzanneau | Titanium in phengite: a geobarometer for high temperature eclogites[END_REF].

Results

Petrological evolution of a static eclogite facies finger

The granulite away from the finger

The less reacted granulite analyzed here consists of ferro-magnesian elongated coronas (~ 30%) that mark the granulitic foliation in an anorthositic matrix (Fig. 2c).

Chemical compositions are given in Table 1 and phase abbreviations are after [START_REF] Kretz | Symbols for rock-forming minerals[END_REF]. Coronas are composed of millimeter-sized euhedral grains of garnet (Alm 25 Grs 19 Py 56 ), slightly jadeitized clinopyroxene (Di 64 Hd 17 Jd 19 ) and amphibole. Garnet is pervasively fractured within a predominant network of ~N040 to N045 fractures (red arrows in T1, Fig. 2a). Fractures perpendicular to the finger elongation are also discernable on the more internal transect (red arrows in T2, Fig. S1a). Matrix is composed of millimeter-sized euhedral grains of plagioclase (An 41 Ab 58 Or 01 ) with minor isolated garnets. Other minor phases can be identified within the granulite as Fe-Ti oxides and apatite (bright minerals Figs. 2a,S1a). This low-transformed area constitutes the first 5 mm from the western part of transect T1, and is almost not present in transect T2.

Eclogitization in this area is limited and is marked by the nucleation of zoisite + K-feldspar at plagioclase-plagioclase grain boundaries that are 10-20 μm thick (orange arrows in Figs. 2a,2c,3a). Zoisite form sheaves of needles often elongated perpendicular to the grain boundary, inside diffuse patches of potassium feldspar. The initiation of the reaction is also characterized by the growth of a thin rim (20-30 μm) of omphacite at plagioclasegarnet grain boundaries (Figs. 2c,3d). Some μm-thick needles of zoisite and minor kyanite needles also nucleate inside the plagioclase grains.

Evolution toward the finger

The amount of zoisite and kyanite found inside plagioclase grains increases toward the center of the finger. Preferential growth of these minerals is guided by plagioclase clivages, as underlined by the non-random orientation of zoisite and kyanite long axis (Fig. 2d). Plagioclase in this region is more albitic (An 13 Ab 86 Or 01 ). Quartz is rarely observed.

The omphacite rim around garnet is thicker than in the less transformed area described in Section 4.1.1 and takes the form of an outer rim of hundreds of microns thick and an inner thinner rim, separated by kyanite grains that underline the former plagioclase-garnet grain boundary (Fig. 2d). These kyanite grains are therefore considered as the remnants of an early crystallization stage. A clinopyroxene-plagioclase symplectite at grain boundaries in both rims marks limited retrogression. Omphacite grain-size in these rims described in analogue lithologies [START_REF] Putnis | Preservation of granulite in a partially eclogitized terrane: Metastable phenomena or local pressure variations?[END_REF][START_REF] Zertani | Reactive fluid flow guided by grain-scale equilibrium reactions during eclogitization of dry crustal rocks[END_REF]; respectively), and are therefore not detailed here.

In the core of the finger

Complete breakdown of plagioclase is only reached in the center of the finger, in zones where the distance between garnet grains is limited (<1-2 mm) and therefore allowed diffusion of Na-Si and Ca-Al between garnet and plagioclase (Fig. S1a). In these zones (Fig. 2e), plagioclase has been totally replaced by an assemblage of omphacite (Di 35 Hd 10 Jd 49 Ts 06 ) and phengite + kyanite + zoisite + apatite + rutile as minor phases.

Kyanite grains underline former grain boundaries between plagioclase and ferromagnesian phases (green arrows in Fig. 2a).

Strain recorded inside plagioclase grains

Intragranular plastic deformation inside plagioclase grains in partially transformed zone was estimated by calculating the angular difference between average orientation of each plagioclase grain and individual orientation of pixels belonging to this grain (Mis2Mean, Fig. 5d). High misorientation angle domains (up to 6°) are observed within plagioclase grains and seem limited by zoisite and kyanite inclusions (Fig. 5d), implying that they arise from strain accumulated during or after inclusions development. Although albite and pericline twinning in plagioclase can arise both from growth and plastic deformation. [START_REF] Borg | Mechanical twinning and slip in experimentally deformed plagioclases[END_REF][START_REF] Marshall | Deformation mechanisms in experimentally deformed plagioclase feldspars[END_REF][START_REF] Smith | Twins. In Feldspar Minerals[END_REF], deformation twins can be identified by morphological criteria. They are usually irregular, present lenticular forms, and are limited by lattice defects in the grain (as cracks for example), in opposition to growth twins that have parallel boundaries and strike through the entire grain [START_REF] Incel | Partial melting and reaction along deformation features in plagioclase[END_REF]. In the studied plagioclase crystal, we can therefore infer that twins are related to strain because (i) they are limited by zoisite inclusions (Fig. 5e, 4b), (ii) they are of variable thickness and some are lenticular, and (iii) they correspond to low misorientation areas (Fig. 5d) and individualize domains of high misorientation, i.e., they also accommodate intragranular deformation. Most of the activated twins are albite twins. Some grains show pericline twins associated with albite twins, but none of the observed grains show pericline twinning activated only.

Plagioclase grains from poorly transformed areas were chosen for SF calculation (see location on T1 Fig. 6a). They present a various but limited quantity of zoisite and kyanite grains growing in their interiors. Best orientation for maximum shortening at the time of twinning formation was deduced from twinned and un-twinned regions orientation (as in the example of Figs. 5d-f, where deformation twins most probably formed in response to a shortening of NNE-SSW orientation). This orientation, approximatively N182, 42°, is visible on the focal mechanism-like «beachballs » stack (Fig. 6b), and quantitatively specified by the synthetic SF spherical function (Fig. 6c) that was calculated by stacking and averaging the SF of the regions of Fig. 6a. Orientation map for a plagioclase grain in a region where the reaction is more advanced (location on Fig. 6a) has been plotted over band contrast image in Fig. 7. We can see that in this region, plagioclase has been dismantled in small 10 μm-scale plagioclase and zoisite grains (Fig. 7b).

Analcime-bearing textures

Analcime pools have been identified at former plagioclase-plagioclase grain boundaries (Fig. 8) in partially transformed zones (white arrows Figs. 2a, S1a), as inferred from the association with zoisite sheaves of needles typical from former plagioclaseplagioclase grain boundaries (Fig. 8a). Analcime was identified by Raman analysis (Fig. 9, see also [START_REF] Velde | Raman spectra of analcime under pressure[END_REF] for spectra comparison), consistent with microprobe chemical analysis (Table 2) and EBSD indexing in some limited regions (Fig. S3). Nonindexing in other regions (Fig. 10) may be due to the high electron beam sensitivity of such zeolite crystals [START_REF] Greer | Electron diffraction and HRTEM imaging of beamsensitive materials[END_REF], or poor crystallinity. Analcime pools are often porous (red arrow Fig. 8c) and electron beam creates a hole inside them (red arrow Figs. 2) than plagioclase surrounding the analcime-kyanitebearing grain boundary (An 15 Ab 83 Or 02 , « plagioclase in matrix » in Table 2). Several kyanite grains associated with analcime contain 1-2 μm rounded inclusions of quartz (Figs. 8a,10c). In zones where analcime areas really form pools, we can observe the syntaxial crystallization of K-feldspar (Fig. 8b). Rarely, some euhedral quartz grains also grow inside the analcime pools (Fig. 8c). Where these particular grain boundaries reach a ferro-magnesian mineral as garnet or clinopyroxene, analcime pools are not present and the unique remnant of former grain boundary with plagioclase is kyanite inside the double omphacite rims (green arrows Fig. 2; red arrow Fig. 8e). The zones where analcime-bearing grain boundaries join omphacite rims are often affected by some retrogression under the form of a Ca-clinopyroxene+plagioclase symplectite, and analcime pools there appear surrounded by a thin layer (~4-5 μm) of the Caclinopyroxene, which indicates that the symplectite develops after formation of these pools (Fig. 8c). Analcime can also be found associated with zoisite needles (Figs. 8a,10a). In some of these regions, zoisite is directly in contact with analcime and presents straight grain boundaries (Fig. 8a). In other regions (Fig. 10), zoisite is surrounded by fine-grained plagioclase grains (<1 μm, as described above for kyanite). However, in contrast to kyanite which presents straight boundaries, plagioclase surrounding zoisite forms cusps pointing into it with low dihedral angles, so that zoisite boundaries appear tortuous (Fig. 10b). In some places, zoisite ultimately breaks down into anorthite-rich plagioclase. In these areas, analcime around kyanite progressively transforms into anorthite-rich plagioclase forming a continuous network from zoisite to kyanite, while analcime pools seem pseudomorphosed by quartz (Fig. 8f). Analcime also crystallized inside injectite-like veins that crosscut all the paragenesis (yellow arrows Figs. 2,8d), and even the symplectite after omphacite. Chemical composition of analcime in these veins is reported in Table 2. We did not find any textures indicating that a liquid of analcime composition could have been injected from the veins into the former plagioclase-plagioclase grain boundaries, rather the veins seem to crosscut these grain boundaries (Fig. 8d). Reaction between the veins and the minerals they crosscut is never observed.

Discussion

Our results show that a reaction sequence can be inferred from successive observations from the pristine granulite (out of the finger) to the core of the « static » eclogite finger. Such spatial evolution can be interpreted as the time sequence of successive reactions, following a hydration process from the core to the boundaries of the eclogite finger [START_REF] Austrheim | Eclogitization of lower crustal granulites by fluid migration through shear zones[END_REF][START_REF] Jamtveit | Fluid controlled eclogitization of granulites in deep crustal shear zones, Bergen arcs, Western Norway[END_REF][START_REF] Jamtveit | Accelerated hydration of the Earth's deep crust induced by stress perturbations[END_REF][START_REF] Zertani | Reactive fluid flow guided by grain-scale equilibrium reactions during eclogitization of dry crustal rocks[END_REF]. As no significant deformation occurred in our static eclogite facies finger, the petrological record we describe here is interpreted as the earliest stage of the eclogitization process, prior any feedback of strain on reactions. Analcime-bearing textures and twinning observed within plagioclase could therefore correspond to remnants of the early eclogitization history. We here discuss this point in the light of previous work.

Reaction sequence preserved through the static eclogitic finger

Assuming homogeneous P-T conditions through the whole finger, it is very likely that the edge-to-core petrological evolution described in Section 4.1 corresponds to a temporal evolution of the eclogitization. The very beginning stages of reaction would then have been fossilized at the finger edge, whereas more advanced stages would have been preserved when approaching the center. This could be due to a two-step fluid diffusion in the granulite (first fast H + /H 2 , then slower H 2 O) as it has been proposed for eclogitic shear zones in Holsnøy [START_REF] Kaatz | How fluid infiltrates dry crustal rocks during progressive eclogitization and shear zone formation: insights from H 2 O contents in nominally anhydrous minerals[END_REF]. The observation of potassium being integrated into K-feldspar in the least transformed areas in opposition to the more transformed domains

where it is integrated into phengite is also in agreement with a gradient in water activity.

In that prospect, we can propose a reaction sequence for the static eclogitization of the granulite comparable to the ones proposed by [START_REF] Bras | Transient weakening during the granulite to eclogite transformation within hydrous shear zones (Holsnøy, Norway)[END_REF] and [START_REF] Kaatz | How fluid infiltrates dry crustal rocks during progressive eclogitization and shear zone formation: insights from H 2 O contents in nominally anhydrous minerals[END_REF] for eclogitic shear zones. Although reactions involving a solid solution such as plagioclase are by definition divariant, the extreme difference in reaction kinetics between the Na and Ca end-members reactions implies that reactions need to be assessed separately, which does not imply they are independent. The first reaction grain boundaries involves the Ca chemical system with the nucleation of zoisite needles intergrown with K-feldspar (Figs.

2c, 3a,b). Then, reaction starts inside plagioclase grains by the nucleation of zoisite and kyanite needles, which leads to an albitization of the former plagioclase (Fig. 2d). This first stage of reaction corresponds to the anorthite breakdown reaction that can be written under the form of:

4 CaAl 2 Si 2 O 8 + 2 H 2 O ⟶ 2 Ca 2 Al 3 Si 3 O 12 (OH) + H 2 Al 2 Si 2 O 8 4 An + 2 H 2 O ⟶ 2 Zo + H-Pl (1)
as proposed by [START_REF] Kaatz | How fluid infiltrates dry crustal rocks during progressive eclogitization and shear zone formation: insights from H 2 O contents in nominally anhydrous minerals[END_REF] for the first step of hydrogen influx into the nominally anhydrous plagioclase, followed then by the real hydration reaction: at plagioclase-clinopyroxene and plagioclase-garnet grain boundaries respectively [START_REF] Mattey | Isotopic constraints on fluid infiltration from an eclogite facies shear zone, Holsenøy, Norway[END_REF][START_REF] Schneider | Micro-scale element migration during eclogitisation in the Bergen arcs (Norway): a case study on the role of fluids and deformation[END_REF]). These two reactions can be catalyzed by the presence of water but do not involve water as a stoichiometric reactant. Grossular is only found in exsolutions inside clinopyroxene as already described by [START_REF] Raimbourg | Mécanismes d'éclogitisation et conséquences mécaniques pour l'exhumation des roches métamorphiques de haute pression[END_REF]. The double omphacite rims separated by a thin layer of kyanite observed around the ferromagnesian phases (Figs. 2,3), indicate an early event of kyanite nucleation at plagioclase-clinopyroxene and plagioclase-garnet grain boundaries that preceded the Nadiffusion related formation of omphacite. Undulose boundaries of the garnet (Fig. 3d-f) attest for destabilization of this phase at time of omphacite formation, while its rims evolve toward the almandine end-member (Table 1) governed by reaction (4). Reactions ( 2), (3), and (4) proceed until total breakdown of former plagioclase and Na-Ca equilibration in omphacite (Fig. 2e), which is rarely the case in our samples.

4

Twinning in plagioclase: a first deformation stage at low differential stress

The very first step of plagioclase breakdown during pressure increase and fluid influx seems to be the nucleation and growth of zoisite. We have shown that it is associated with strain-related twinning inside plagioclase grains (Fig. 5). Our mechanical analysis of these twins yields a significant anisotropy with a ~ N-S dipping to the south maximum compressive stress direction at the time of twinning (Fig. 6b). No clear direction of minimal compressive stress direction is defined, implying a pure shear regime. This maximum stress orientation deduced from twinning is close to the pole of the eclogite foliation (i.e., maximal principal shortening) measured for the nearby Hundskjeften zone [START_REF] Zertani | Modification of the seismic properties of subducting continental crust by eclogitization and deformation processes[END_REF], at the termination of which the studied finger has been sampled.

Moreover, such orientation of the stress is also in agreement with the orientation of shear along eclogite-facies shear zones described by [START_REF] Raimbourg | Kinematics of syneclogite deformation in the Bergen Arcs, Norway: implications for exhumation mechanisms[END_REF]. The well defined maximum compressive stress direction and poorly defined minimum compressive stress direction also echoes the axial compression-related finite strain deduced from Crystallographic Preferred Orientation (CPO) analysis in omphacite from Holsnøy sheared eclogites [START_REF] Bascou | EBSDmeasured lattice-preferred orientations and seismic properties of eclogites[END_REF]. It therefore seems that twinning was controlled by regional field stress state.

On the other hand, most of the twins show clear textural relationship with the zoisite inclusion network within the grains: most of them are bounded by zoisite-filled cracks, and their density fades from the cracks toward the inclusion free domains (Fig. 4), so that twinning seems to be associated to the zoisite inclusion network development. Twinning solely associated with grain-scale stresses is expected to yield a homogeneous isotropic SF density function, as no clear anisotropy is seen in the inclusion pattern. We therefore have both evidence of regional stress state and local process control on the twinning pattern, which suggests that the addition of both local and regional stress states has to be considered.

The complete reaction anorthite + H 2 O = zoisite + kyanite + quartz, has a -15% solid reaction change. The observed crystallization of zoisite or kyanite alone in their host grain and the necessary export of silica toward grain boundaries implies an even larger local contraction. In this prospect the development of zoisite inclusions could be seen as an anticrack system, with significant stress increase at its tips [START_REF] Sternlof | Anticrack inclusion model for compaction bands in sandstone[END_REF]). This model would then reconcile the two apparently contradicting observations above, but also implies that the regional differential stress was lower than the poorly constrained activation threshold of the albite twinning system. This requires that local stress disturbance due to the zoisite-in reaction was high enough for the addition of both stress states to yield a maximum stress above this threshold.

An alternative scenario would be to consider twinning as a late feature in regards to zoisite formation. It is indeed expected that inclusion-bearing plagioclase would also have its twinning domains bounded by the pre-existing inclusions, if they develop later. However, some recent studies [START_REF] Jamtveit | High pressure metamorphism caused by fluid induced weakening of deep continental crust[END_REF][START_REF] Putnis | Preservation of granulite in a partially eclogitized terrane: Metastable phenomena or local pressure variations?[END_REF]) argue for the amphibolite facies event in Holsnøy to be coeval with the eclogite facies state, depending on the pressure field in the granulites. Therefore, even if linked to the amphibolite facies event, twinning would have been activated in response to the same stress field than the one registered by the eclogitic shear zones. However, there is no argument allowing to state that the considered granulitic bloc would have reached its maximum stress only at the time of the amphibolitization, while intense deformation has been accommodated by the surrounding shear zones at the time of eclogitization.

In regions where the reaction is more advanced (Figs. 4,7, see location in Fig. 6a), plagioclase grain size has been reduced. If twin boundaries were to evolve into new grain boundaries, the resulting grain size would be even more reduced, to only a few micrometers (Fig. 7b,c). This evolution of the twins is consistent with previous studies that already emphasized the ability for twin boundaries to facilitate subgrain rotation [START_REF] Menegon | The effect of Dauphiné twinning on plastic strain in quartz[END_REF]. Twin planes in plagioclase are also prone to fluid infiltration [START_REF] Incel | Partial melting and reaction along deformation features in plagioclase[END_REF] or new phases nucleation [START_REF] Wayte | A TEM study of disequilibrium plagioclase breakdown at high pressure: the role of infiltrating fluid[END_REF] as high-angle grain boundaries are.

Partial melting of the partially eclogitized granulites

In order to assess an eventual partial melting stage in the eclogitization process developing at the edge of the studied finger, we develop here the textural evidences supporting the occurrence of a melt at grain boundaries, the P-T-X H2O conditions prone to melting compared to acknowledged P-T path for the Holsnøy granulites, the expected composition of melt, how it compares with paragenesis remaining at grain boundaries, and eventually to what reactions such a partial melting stage can be attributed.

Analcime pools: textural evidence for partial melting?

To our knowledge, it is the first time that analcime pools are described in Holsnøy.

Only rare studies already mentioned zeolite in irregularly shaped pools replacing a former melt [START_REF] Kiseeva | Mantle melting versus mantle metasomatism-"The chicken or the egg" dilemma[END_REF] in mantle xenolites). The recent paper of [START_REF] Liu | High-pressure granulite facies re-equilibration and zoisite-biotite dehydration melting during decompression of an ultrahigh-pressure garnet clinopyroxenite from the island of Fjørtoft, Norway[END_REF] constitutes the first study that truly associates analcime pools to crystallization of a nepheline normative melt, in a garnet clinopyroxenite of the Western Gneiss Region, at around 600-650°C. Several characteristics of analcime pools described in Section 4.3 constitute arguments for the presence of a former melt, and appear to be very similar to those described by [START_REF] Liu | High-pressure granulite facies re-equilibration and zoisite-biotite dehydration melting during decompression of an ultrahigh-pressure garnet clinopyroxenite from the island of Fjørtoft, Norway[END_REF]. Indeed, the pools have wedge-like offshoots, i.e., they form cusps with low dihedral angles toward the host plagioclase (Figs.

S4a, 10b,c), sometimes dendritic morphologies (Fig. 10 that grew in some of the analcime pools (Fig. 8c), (ii) the thin rim of micro-metric plagioclase that recrystallized between analcime and kyanite (Fig. 10), (iii) the rim of diopside that surround the analcime pools when they are in contact with the symplectite after omphacite (Fig. 8c), and (iv) the euhedral K-feldspar crystals that are growing from the host plagioclase-pools interface toward the center of analcime pools (Figs. 8b,S4a,b).

This last texture between associated analcime, K-feldspar and albite is common for magmatic crystallization of hydrous Na-rich melts as described in natural case studies (e.g., Markl 2001; Analcime I from Pe-Piper et al. 2019) and also stated by experimental studies that estimated the conditions of analcime crystallization at <650°C, 0.5-1.3 GPa [START_REF] Peters | The melting of analcite solid solutions in the system NaAlSiO4-NaAlSi3O8-H2O[END_REF][START_REF] Newton | Jadeite, analcite, nepheline, and albite at high temperatures and pressures[END_REF][START_REF] Boettcher | Phase relationships in the system NaAlSiO 4-SiO 2-H 2 O to 35 kilobars pressure[END_REF]Kim andBurley 1971a,b,c, 1980;[START_REF] Liou | Analcime equilibria[END_REF][START_REF] Thompson | Analcime: Free energy from hydrothermal data. Implications for phase equilibria and thermodynamic quantities for phases in NaAIO2-SiO2-H2O[END_REF][START_REF] Roux | Primary igneous analcite-an experimental study[END_REF][START_REF] Johnson | Thermodynamic studies of zeolites: analcime and dehydrated analcime[END_REF]. Moreover, a subsolidus assemblage analcime + albite + vapor was observed after crystallization of melt produced at 2 GPa, 750°C in the NASH system [START_REF] Boettcher | Phase relationships in the system NaAlSiO 4-SiO 2-H 2 O to 35 kilobars pressure[END_REF]. All these aforementioned characteristics of analcime as found in our samples allow us to exclude a metasomatic replacement origin of this mineral after albite or jadeite as it has been observed in some Na-rich environment [START_REF] Harlow | Jadeitites, albitites and related rocks from the Motagua Fault Zone, Guatemala[END_REF][START_REF] Harlow | Jadeitite from Guatemala: new observations and distinctions among multiple occurrences[END_REF][START_REF] Tsujimori | Petrogenetic relationships between jadeitite and associated high-pressure and low-temperature metamorphic rocks in worldwide jadeitite localities: a review[END_REF]. The close association of kyanite surrounded by analcime (Figs. 8,10) and quartz surrounded by a thin film of K-feldspar in some plagioclase-plagioclase grain boundaries (Fig. S4b) is an additional argument for crystallization from a former melt. Similar quartz-K-feldspar textures have indeed been characterized as melt filled pores pseudomorphs in migmatites by several studies [START_REF] Holness | On the pseudomorphing of melt-filled pores during the crystallization of migmatites[END_REF][START_REF] Vernon | Microstructures of melt-bearing regional metamorphic rocks[END_REF]. We suggest that the rim of kyanite that separates the double omphacite rim around ferro-magnesian phases could be the remnant of analcime-kyanite bearing grain boundaries as described between plagioclaseplagioclase grains.

P-T-XH2O conditions prone to partial melting

To elaborate on the possible conditions for partial melting of our transforming granulite, we calculated P-T and P-X H2O pseudosections with the tonalitic melt solution model from [START_REF] Green | Activity-composition relations for the calculation of partial melting equilibria in metabasic rocks[END_REF] which is the only one constrained at pressures up to 1.3

GPa and often used for calculations at these pressures (e.g., [START_REF] Putnis | Preservation of granulite in a partially eclogitized terrane: Metastable phenomena or local pressure variations?[END_REF][START_REF] Liu | High-pressure granulite facies re-equilibration and zoisite-biotite dehydration melting during decompression of an ultrahigh-pressure garnet clinopyroxenite from the island of Fjørtoft, Norway[END_REF]. Results of the calculations are presented in Fig. 11 and Fig. S6 (see Method section for details). Calculations with the haplogranitic melt solution model from [START_REF] Holland | Calculation of phase relations involving haplogranitic melts using an internally consistent thermodynamic dataset[END_REF] and [START_REF] White | Calculation of partial melting equilibria in the system Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O (NCKFMASH)[END_REF] have also been performed (Fig. S5, S6) but considered as poorly constrained for our samples in the eclogite P-T range. We performed the calculation for three different effective compositions (Table 3) in order to examine the possibility for partial melting of our samples at different stages of equilibration during the ongoing eclogitization. For local equilibrium at different stages of plagioclase breakdown in matrix domains, we use (1) the pristine plagioclase composition of our granulite facies area (Granulite Table 1, and Pl 0 Table 3) and ( 2) the more albite-rich composition of plagioclase in partially eclogitized area (Table 1 and Pl 1 Table 3) in the CKNASH system. Depending on the effective local chemical system considered, thermodynamic modelling indicates that partial melting is possible near the peak P-T-X H2O conditions experienced by our samples provided that: (i) the local water content is ≥0.4 wt% for albitized plagioclase chemical system, or (ii) pressure drops down to 1.6

GPa or temperature rises to 730°C for a fully hydrated (1 wt% H 2 O) initial plagioclase chemical system. As already asserted by [START_REF] Liu | High-pressure granulite facies re-equilibration and zoisite-biotite dehydration melting during decompression of an ultrahigh-pressure garnet clinopyroxenite from the island of Fjørtoft, Norway[END_REF] according to experimental studies [START_REF] Peters | The melting of analcite solid solutions in the system NaAlSiO4-NaAlSi3O8-H2O[END_REF][START_REF] Roux | Primary igneous analcite-an experimental study[END_REF]) analcime can coexist with melt at temperatures ≤ 650°C at eclogite facies pressures and could therefore have crystallized just below 650°C at the time of symplectite crystallization. The observed injectite-like veins filled by analcime as described in Section 4.3, crosscut all the paragenesis, including the analcime-kyanite grain boundaries as well as the symplectite after omphacite (yellow arrows Figs. 2,8d). Neither the temporal nor the genetic relationships between these analcime veins and analcime-kyanite grain boundaries can be unequivocally determined, but it is obvious that injection of the veins corresponds to a late event, at least post-dating the retrogression into symplectite, while the analcimekyanite grain boundaries constitute an early stage in the rock evolution. Veins could on one hand correspond to a late injection of a melt of analcime composition, formed at higher pressures and maintained liquid during the retrograde path. Volume increase caused by the melt decompression could have fractured the host rock. It is on the other hand unlikely that injection veins came from a low pressure pseudotachylyte further away in the outcrop that we did not see while sampling since no analcime-bearing pseudotachylyte remnants has ever been described in Holsnøy, and melt produced in pseudotachylytes have initial compositions close to their host-rock [START_REF] Zhong | Lower crustal earthquake associated with highly pressurized frictional melts[END_REF].

Expected composition of the melt

Melt compositions from thermodynamic calculations are roughly constant on the whole range of P-T-X H2O conditions, and regardless of the solution model chosen for melt.

All compositions cluster close to 58 SiO 2 ; 14 Al 2 O 3 ; 1 CaO; 6 Na 2 O; 1 K 2 O; 20 H 2 O in wt % so they are quartz and corundum normative (9 and 1 wt% respectively) instead of beingnepheline normative as expected if considering analcime pools as a crystallized former melt. This inconsistency was also highlighted by [START_REF] Liu | High-pressure granulite facies re-equilibration and zoisite-biotite dehydration melting during decompression of an ultrahigh-pressure garnet clinopyroxenite from the island of Fjørtoft, Norway[END_REF] but as they stated, very few studies already described this type of partial melting phenomena, particularly in K-poor high-pressure rocks like those described here. Moreover, melt models are not well constrained at such high pressures so compositions extracted from thermodynamic modelling should be cautiously handled. It is also highly possible that analcime only represents the final crystallization of the last liquid phase and that initial melt composition was different. In addition, compilation from both natural and experimental data [START_REF] Labrousse | Continental exhumation triggered by partial melting at ultrahigh pressure[END_REF]) emphasized that high-pressure melts evolve toward more Na-rich compositions (i.e., trondhjemitic) with increasing pressure. Hence, it is probable that a melt formed at 2 GPa would be Na-rich, and possibly enough to lead to the eventual crystallization of such a Na-rich phase as analcime.

Partial melting reactions

Textural observations as described in Section 5.3.1 emphasized the association of kyanite, zoisite, plagioclase and analcime in the partial melting textures. In regions where analcime pools are present, zoisite grain boundaries are tortuous, and they are only found at initial plagioclase-analcime contact zones but totally absent from the center of these regions (Fig. 10). Thus, it appears highly possible that zoisite was consumed by the partial melting reaction. While textures evidence that analcime and plagioclase recrystallized grains around kyanite result from crystallization after a melt, it is more difficult to decipher the implication of kyanite in the partial melting process. A possible melting reaction would be Zo + Ab + Qtz → Melt + Ky, which constitutes a plagioclase-involving version of the reaction Zo + NaAl-Cpx + Qtz → Melt + Ky + CaMg-Cpx proposed by [START_REF] Skjerlie | The fluid-absent partial melting of a zoisitebearing quartz eclogite from 1• 0 to 3• 2 GPa; Implications for melting in thickened continental crust and for subduction-zone processes[END_REF] for zoisite dehydration melting at high pressures. In this reaction quartz is consumed. This could constitute an explanation to the general lack of quartz in our sample. Moreover, this is in agreement with experimental studies in the NASH system [START_REF] Boettcher | Phase relationships in the system NaAlSiO 4-SiO 2-H 2 O to 35 kilobars pressure[END_REF] which proposed that metastable plagioclase could be involved in melting reactions in the presence of free fluid. The resulting melt can in a second time crystallize into Anl + Qtz + Pl + Fluid which in turn could explain the association of analcime with quartz and kyanite, and also the crystallization of small plagioclase grains around kyanite.

Zoisite dehydration melting at water absent conditions is reached at temperatures much higher than the one reached by our samples [START_REF] Skjerlie | The fluid-absent partial melting of a zoisitebearing quartz eclogite from 1• 0 to 3• 2 GPa; Implications for melting in thickened continental crust and for subduction-zone processes[END_REF]Cao et al. 2018). However, at water present conditions, melting of zoisite has been described

at reasonable temperatures at 2 GPa (~800°C for zoisite + kyanite + quartz + vapor = melt, [START_REF] Schmidt | Magmatic epidote[END_REF], depending on the chemical system involved. [START_REF] Schmidt | Magmatic epidote[END_REF] also showed that in the presence of a K-bearing phase, zoisite melting reactions would take place at even lower temperatures, e.g., the reaction zoisite + white mica + quartz + vapor = kyanite + melt at 725°C -2 GPa.

As a conclusion, we propose that textural evidence point toward a limited partial melting stage, due to zoisite and metastable Na-rich plagioclase melting in the local presence of a free fluid, at conditions close the acknowledged peak in the eclogite facies provided a transient hydration level higher than 0.4 wt%, a limited temperature increase or a limited pressure decrease.

Implications for the early mechanical alteration of the reacting granulite

Our structural analysis of twins in plagioclase with no shape preferred orientation (i.e., not significantly strained) implies that differential stress for twinning was lower than the creep strength of plagioclase. Twinning activation threshold in plagioclase is poorly documented but experiments indicate a plastic yield close to 100 MPa at 800°C, 1 GPa [START_REF] Borg | Mechanical twinning and slip in experimentally deformed plagioclases[END_REF]. A renewed estimate would ultimately constitute a missing paleopiezometer for deformation of rocks at high confining pressure strain in the lower crust.

Partial melting at high pressure in the presence of H 2 O-rich fluids has already been described in the Western Gneiss Region [START_REF] Labrousse | Crustal-scale boudinage and migmatization of gneiss during their exhumation in the UHP province of western Norway[END_REF][START_REF] Ganzhorn | Structural, petrological and chemical analysis of syn-kinematic migmatites: Insights from the Western Gneiss Region, Norway[END_REF][START_REF] Liu | High-pressure granulite facies re-equilibration and zoisite-biotite dehydration melting during decompression of an ultrahigh-pressure garnet clinopyroxenite from the island of Fjørtoft, Norway[END_REF]. Major implications on the rheology and strength of the continental crust have been highlighted, like the important weakening the presence of such a melt could induce [START_REF] Rosenberg | Experimental deformation of partially melted granite revisited: implications for the continental crust[END_REF][START_REF] Vanderhaeghe | Migmatites, granites and orogeny: Flow modes of partiallymolten rocks and magmas associated with melt/solid segregation in orogenic belts[END_REF][START_REF] Brown | Melting of the continental crust during orogenesis: the thermal, rheological, and compositional consequences of melt transport from lower to upper continental crust[END_REF][START_REF] Wang | Partial melting of deeply subducted eclogite from the Sulu orogen in China[END_REF]. This weakening can be responsible for the decoupling between the lower crust and the rest of the subducting slab, and the following exhumation. In the island of Holsnøy, felsic pegmatite veins intruding the high-pressure eclogites have already been described and associated with the crystallization after partial melting of the eclogites (of felsic to intermediate composition) at peak P-T conditions and in the presence of a H 2 Orich fluid [START_REF] Andersen | Mineral-fluid-melt interactions in high-pressure shear zones in the Bergen Arcs nappe complex, Caledonides of W. Norway: Implications for the fluid regime in Caledonian eclogite-facies metamorphism[END_REF]. A recent study of Jamtveit et al. ( 2021) also described pegmatites and dykes intrusions in the Lindås nappe and proposed the anorthositic granulites of Holsnøy as possible source for the melts that crystallized in these intrusions.

1.3-1.5 GPa, 710-750°C and 2-4 wt% H 2 O conditions were proposed for partial melting with temperature increase driven by shear heating at the base of the Lindås nappe.

Melting is possible in plagioclase after zoisite formation at the same temperature as proposed by [START_REF] Jamtveit | Rapid fluid-driven transformation of lower continental crust associated with thrustinduced shear heating[END_REF], even at pressures and water contents that have been estimated for the eclogite peak conditions. A slight increase in temperature due reaction enthalpy release [START_REF] Zertani | Reactive fluid flow guided by grain-scale equilibrium reactions during eclogitization of dry crustal rocks[END_REF] or a transient pressure drop cannot be excluded.

Indeed, both stress and pressure field at the tip of the propagating eclogite finger are expected to be transiently disturbed by (i) the volume change associated with plagioclase breakdown, as at the tip of an elongated inclusion [START_REF] Sternlof | Anticrack inclusion model for compaction bands in sandstone[END_REF][START_REF] Yamato | Reaction-induced volume change triggers brittle failure at eclogite facies conditions[END_REF], or (ii) strength drop due to eclogitization [START_REF] Mancktelow | Tectonic pressure: Theoretical concepts and modelled examples[END_REF][START_REF] Moulas | Stress field associated with elliptical inclusions in a deforming matrix: Mathematical model and implications for tectonic overpressure in the lithosphere[END_REF], the combination of the two being also possible. The part of the rock located at the tip of the finger (Fig. 12) may transiently pass from an over-pressurized domain to an underpressurized domain while the finger propagates. The estimated 0.4 GPa decompression necessary for partial melting at constant temperature (Fig. 11) can therefore be reached at a background pressure of 2.0 GPa. It is to be noted that while the over-pressurized domain remains limited to the very tip of the propagating finger, under-pressure is expected to affect a much wider area along the finger walls. Once initiated, partial melting, if occurring at fluid present conditions (as proposed in Section 5. facies shear zones, as currently acknowledged [START_REF] Austrheim | Eclogitization of lower crustal granulites by fluid migration through shear zones[END_REF][START_REF] Austrheim | Fluid and deformation induced metamorphic processes around Moho beneath continent collision zones: Examples from the exposed root zone of the Caledonian mountain belt, W-Norway[END_REF][START_REF] Jamtveit | Fluid controlled eclogitization of granulites in deep crustal shear zones, Bergen arcs, Western Norway[END_REF]), such processes could also be part of the transient weakening mechanisms needed to explain widening of eclogite facies shear zones during their development [START_REF] Bras | Transient weakening during the granulite to eclogite transformation within hydrous shear zones (Holsnøy, Norway)[END_REF][START_REF] Kaatz | Widening of hydrous shear zones during incipient eclogitization of metastable dry and rigid lower crust-Holsnøy, western Norway[END_REF]).

Conclusions

Eclogitization of rocks is a continuous process that involves multiple breakdown reactions of different kinetics and mechanical implications, during increasing pressure. In anorthosites where plagioclase constitutes the principal reactive system, these reactions are often delayed because of the sluggishness of NaSi-CaAl interdiffusion in water-limited environments. Transient parageneses and textures can therefore persist metastably through the whole P-T history of the rock, and control its mechanical behavior. Early stages of these plagioclase breakdown reactions have been preserved within the undeformed partially eclogitized granulites studied. Transformation proceeds differently inside the protolith plagioclase grains and at their boundaries. In both cases, zoisite crystallization has a first order impact. Indeed, zoisite nucleation and growth inside the grains distort the plagioclase crystal lattice acting as defects for the activation of twinning in response to far field stresses. At former grain boundaries, kyanite-analcime association attests of a partial melting event that could have affected the rocks near the peak P-T conditions in response to water infiltration, local P-T changes, or transient changes in the effective chemical system due to zoisite crystallization. Grain size reduction took place in the whole transforming rock via the crystallization of small grains from a melt, as well as after an event of twinning under stress that triggered the creation of new grain boundaries.

This may contribute to mechanical alteration of the reacting granulite over probably short timescales, not mentioning effective viscosity drop induced by even small volumes of melt production at grain boundaries. Rheological contrasts generated between partially transformed and preserved granulite then predispose the rock to potential further localization of the deformation. Orientation map of a former plagioclase grain from a reacted area (see location in Fig. 6a), plotted with a colormap (inset) corresponding to the angle between X direction, here defined as parallel to the [100] direction, and the thin section plane. Y perpendicular to the plane (010). (b) Grain size histogram of "grains" delimited by grain-and twin-boundaries.

Figure captions

(c) Enlargement showing grain size reduction induced by zoisite formation and twinning.

Twin boundaries are indicated in red and low angle grain boundaries in white. 3) are, from top to bottom: the initial granulitic plagioclase composition and the albitized plagioclase composition (Fig. 2d). Calculations have been performed using the melt solution model of [START_REF] Green | Activity-composition relations for the calculation of partial melting equilibria in metabasic rocks[END_REF], see Method section for details. Melt modes (in volume %) are indicated by the red lines, and X An by the yellow lines. Zones where melt is predicted are highlighted in red.

Fig.12 Finger Propagation.

Pressure field evolution at the tip of a reacting and propagating finger (modified from [START_REF] Yamato | Reaction-induced volume change triggers brittle failure at eclogite facies conditions[END_REF]. ΔP corresponds to variation of pressure P relative to background pressure P bg such as ΔP = (P-P bg ) / P

Table captions

Table 1 Representative mineral composition (wt%) of granulite to eclogite parageneses.

Composition of granulite-facies minerals results from analyses in the core of the grains, while analyses in minerals from the partially-transformed area have been performed at grain rims. The structural formulae of plagioclase, garnet and pyroxene were calculated based on 8 oxygens, 12 oxygens and 6 oxygens, respectively. b.d.l = below detection limit. n.a = not analysed. All Fe is counted as Fe2+ Granulite (as in Figure 2a) Partially eclogitized (as in Figure 2b)

Click here to access/download;Table ;Table2.pdf

  appearing as diffuse halos sometimes from a central fracture, but also (3) pseudotachylytes and their damage zones that recrystallize into high-pressure paragenesis (1.5 GPa ~ 680°C Bhowany et al. 2018; Petley-Ragan et al. 2018), or into lower pressure paragenesis (1.0 GPa ~ 600°C, Petley-Ragan et al. 2019).

  is ~ 100 μm(Figs. 2d, 3f). Slight zoning in garnet is visible in BSE images(Figs. 2d, 3e) with a Fe-enriched / Mg-depleted rim (Alm 43 Grs 17 Py 40 ) free of inclusions and with a tortuous boundary toward the inner omphacite rim. Although not further elaborated, it is to be noted that garnet in this area re-equilibrated to eclogite facies conditions as described by[START_REF] Raimbourg | Garnet reequilibration and growth in the eclogite facies and geodynamical evolution near peak metamorphic conditions[END_REF] (equivalent to re-equilibrated garnet II) while the growth of a Ca-richer eclogitic garnet (garnet III from same authors) is not observed, except as exsolutions along clinopyroxene crystal planes (as described by[START_REF] Raimbourg | Mécanismes d'éclogitisation et conséquences mécaniques pour l'exhumation des roches métamorphiques de haute pression[END_REF]. Jadeite component in clinopyroxene increased to Di 34 Hd 10 Jd 53 Ts 03 . Chemical evolution of clinopyroxene as well as fluid flow through the finger have been precisely

  8b, S4a).Analcime pools which have a diameter of ~10-80 μm, present different morphologies within the sample, but are always associated with kyanite. Indeed, analcime can form a thin rim (< 5 μm) around kyanite (10-20 μm grain size) that crosscut initial plagioclase grains forming an alignment of minerals (Figs.8a,b). When in contact with plagioclase, analcime pools form cusps with low dihedral angles pointing into plagioclase (red arrows Figs.10b,c). Sometimes fine-grained plagioclase (< 1 μm) crystallized between kyanite grains and analcime pools (Figs.8a, 10b-e). These late grains are more calcic (An 26-29 Ab 70-72 Or 01-02 ,Table

  3.4), can also enhance the negative volume change already initiated by the eclogitization reaction (Clemens and Droop 1998) and further amplify the stress/pressure field disturbance at the tip of the finger. Even though observations and thermodynamic modelling in the present study point toward limited partial melting extent, well below any Rheologically Critical Melt Percentage (RCMP ~ 7%, Rosenberg and Handy 2001), the transient presence of a melt phase in the plagioclase matrix at grain boundaries may also activate deformation mechanisms, such as melt-assisted switch from dislocation creep to diffusion creep (Dimanov et al. 2000; Nasipuri and Bhattacharya 2007). If fluid infiltration precedes the development of eclogite

Fig. 1 Figure 2 Fig. 2

 122 Fig.1 Sample location. a Geological map of northern Holsnøy, Norway, modified from Bras et al. (2021) after Austrheim et al. (2013) and Zertani et al. (2019). Red star: location of the outcrop studied. b Lithological map of the outcrop studied. The blue rectangle corresponds to photograph displayed in c. c Field photograph of the eclogite finger sampled. d Detailed field photograph of the finger (enlargement from c). Blue rectangles: location of the thin sections. T1 and T2 black rectangles: studied transects presented in Figure 2 and Figure S1

Fig. 1d )

 1d Fig.1d). Red zones in (b): corona structures that define the granulitic foliation. Orange arrows: plagioclase-plagioclase grain boundaries in low transformed areas. White arrows: analcime-kyanite decorated grain boundaries. Yellow arrows: analcime-bearing injectite vein. Green arrows: intersection of kyanite-bearing ferromagnesian rims and analcimekyanite decorated plagioclase grain boundaries. Red arrows: fractures inside garnet. BSE image (e) is localized in Figure S1a

Fig. 3 Fig. 4 Fig. 5 Fig. 6 Fig. 7

 34567 Fig.3 Successive textures at grain boundaries. BSE images illustrating plagioclaseplagioclase grain boundaries (a-c) and plagioclase-garnet grain boundaries (d-f). (f) minor retrogression of omphacite into a fine-grained symplectite (s)

Fig. 8 Fig. 9

 89 Fig.8 Chronological and textural relations in analcime-bearing grain boundaries. N020 orientation in (b) is valid for all other panels of this figure. Red arrow in (b): hole created by the EPMA electron beam. Red arrow in (c): porosity in an analcime pool. Red arrow in (e): intersection of kyanite-bearing ferromagnesian rims and analcime-kyanite decorated plagioclase grain boundaries

  Fig. 2a,b Fig. S1

Figure 1

 1 Figure 1 Click here to access/download;Figure;Figure1.pdf

  Figure 2 Click here to access/download;Figure;Figure2.pdf

Figure 4

 4 Figure 4Click here to access/download;Figure;Figure4.pdf

Fig. 5

 5 Fig.5

  Fig.S2

  Figure 11Click here to access/download;Figure;Figure11.pdf

Figure S1 .

 S1 Figure S1. Transect T2 through the eclogitic finger. BSE image (a) and photomicrograph in unanalyzed polarized light (b), see location in Figure 1d. Red zones in (b): corona structures that define the granulitic foliation. Orange arrows: plagioclase-plagioclase grain boundaries in low transformed areas. White arrows: analcime-kyanite decorated grain boundaries. Red arrows: fractures inside garnet.

Figure S2 .

 S2 Fig. 2e

2 O

 2 Figure S5. P-T pseudosections at X H 2 O = 1 wt % (left) and P-X H 2 O pseudosections at T = 700°C (right) for the initial plagioclase composition (Pl 0 ) and the partially reacted plagioclase composition (Pl 1 ), as indicated in Table 3. Calculations have been performed using the melt solution model of Holland & Powell 2001, see Method section for details. Zones where melt is predicted are highlighted in red. Melt modes (in volume %) are indicated by the red lines, and anorthite content in plagioclase by the yellow lines.

Figure S6 .

 S6 Figure S6. P-T pseudosections at X H 2 O = 1 wt % (left) and P-X H 2 O pseudosections at T = 700°C (right) for the whole rock composition of the studied sample, as indicated in Table 3. Calculations have been performed using the melt solution model of Green et al. (2016) (top) and Holland & Powell (2001) (bottom), see Method section for details. Zones where melt is predicted are highlighted in red. Melt modes (in volume %) are indicated by the red lines, and anorthite content in plagioclase by the yellow lines.

  CaAl 2 Si 2 O 8 + H 2 O ⟶ 2 Ca 2 Al 3 Si 3 O 12 (OH) + Al 2 SiO 5 + SiO 2[START_REF] Wayte | A TEM study of disequilibrium plagioclase breakdown at high pressure: the role of infiltrating fluid[END_REF], when the solubility of hydrogen in plagioclase is reached. Na diffusion at plagioclase-ferromagnesian phases grain boundaries then triggers the formation of omphacite grains, intergrown with kyanite and minor zoisite (Figs.

	(Golsdmith 1982; 2d, 3d,e,f), following the reactions:
	3 CaAl2SiO6 + 2 NaAlSi3O8 ⟶ 2 NaAlSi2O6 + 2 Al2SiO5 + Ca3Al2Si3O12
	3 Ca-ts + 2 Ab ⟶ 2 Jd (in Cpx) + 2 Ky + Grs	(3)
	3 CaAl 2 Si 2 O 8 + NaAlSi3O8 + Mg3Al2Si3O12 ⟶ 3 CaMgSi2O6 + NaAlSi2O6 + 4 Al2SiO5
	3 An + Ab + Prp ⟶ 3 Di + Jd + 4 Ky	(4)
	4 An + H 2 O ⟶ 2 Zo + Ky + Qtz	(2)

Table 2

 2 Representative mineral composition (wt%) of analcime, matrix plagioclase, and plagioclase around kyanite in analcime-kyanite grain boundaries. The structural formulae of plagioclase were calculated based on 8 oxygens. b.d.l = below detection limit. n.a = not analysed

Table 3

 3 Chemical compositions (wt%) used for thermodynamic calculations. Analyses for the whole rock composition were performed at ALS Dublin (OMAC Laboratories) by ICP-

	AES (major elements) and ICP-MS (trace elements). Loss on Ignition (LOI)
	measurements indicate a value of 0.29 wt%. The whole rock composition given here was
	recalculated considering 0 wt% H 2 O for a total of 100 wt% oxides.

Table 1

 1 Click here to access/download;Table;Table1.pdf

	!

Table 3 .

 3 Calculations have been performed using the melt solution model of Holland & Powell 2001, see Method section for details. Zones where melt is predicted are highlighted in red. Melt modes (in volume %) are indicated by the red lines, and anorthite content in plagioclase by the yellow lines.

	20
	25	30	35	40

Acknowledgements

We thank E. Bras for his help in sampling and mapping in the field. M.B. thanks D.

Deldicque and O. Boudouma for the EBSD analyses performed at ENS Paris and

Sorbonne Université, respectively. We thank W. Cao, S. Schorn and an anonymous reviewer for their constructive suggestions that truly helped improve that manuscript. J. Hermann and L. Menegon are especially thank for fruitful discussions on petrological

respectively. A.S. acknowledges the European Research Council grant REALISM (2016grant 681346). P.Y. acknowledges the Institut Universitaire de France for financial support.

Data availability

The data underlying this article will be shared on request to the corresponding author.