# Microstructural, compositional and textural inheritance in deformed mafic amphibolites

Laura Airaghi, Hugues Raimbourg, Jacques Précigout

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Laura Airaghi, Hugues Raimbourg, Jacques Précigout. Microstructural, compositional and textural inheritance in deformed mafic amphibolites. Journal of Structural Geology, 2026, 679, pp.119884. 10.1016/j.jsg.2026.105630. insu-05483199v2

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Microstructural, compositional and textural inheritance in deformed mafic amphibolites

Laura Airaghi, Hugues Raimbourg, Jacques Précigout

PII: S0191-8141(26)00016-7

DOI: https://doi.org/10.1016/j.jsg.2026.105630

Reference: SG 105630

To appear in: *Journal of Structural Geology*

Received Date: 10 June 2025

Revised Date: 12 January 2026

Accepted Date: 17 January 2026

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Please cite this article as: Airaghi, L., Raimbourg, H., Précigout, J., Microstructural, compositional and textural inheritance in deformed mafic amphibolites, *Journal of Structural Geology*, https://doi.org/10.1016/j.jsg.2026.105630.

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# Microstructural, compositional and textural inheritance in
deformed mafic amphibolites

Laura Airaghi¹, Hugues Raimbourg¹, Jacques Précigout¹

¹Institut des Sciences de la Terre d'Orléans (ISTO), UMR 7327, Univ. Orléans, CNRS, BRGM, OSUC, F-45071Orléans, France

Corresponding author: laura.airaghi@univ-orleans.fr

## ABSTRACT

This study investigates the scales of compositional equilibria and the role of early hydration for the development of microstructures and mineral fabric in mafic amphibolites, all deformed along a major tectonic plate boundary (Hokkaido, Japan). From fractured domains to mm-size shear bands and meter-size shear zones, element and cathodoluminescence mapping, as well as thermodynamic modelling show that both in plagioclase and in amphibole, strong compositional gradients were observed at the smallest (10 to 200 micron) scale, which in many instances are inherited from an early, static stage of fracturing, hydration and replacement of igneous minerals.

Phase distribution and grain sizes in shear bands and shear zones were also inherited this early metamorphic stage, giving rise to: (1) monomineralic amphibole layers derived from amphiboles replacing aggregates of Fe-Mg igneous minerals in metagabbros, (2) monomineralic plagioclase layers derived from plagioclase-rich domains of metagabbros that experienced limited early breakdown reactions and (3) intimately mixed and fine-grained amphibole-plagioclase layers developed only where symplectites after igneous minerals were previously formed.

While crystallographic and shape preferred orientations of amphibole were acquired from early fracture-driven reactions, and then strengthened in shear bands/zones by dissolution-precipitation, nucleation and oriented growth, the ones of plagioclase, whose reaction was incomplete in fractured domains, continued to evolve with increasing viscous strain and reaction progress. Where plagioclase experienced early, partial breakdown, viscous strain was further accommodated by dissolution-precipitation and phase nucleation. In contrast, where igneous plagioclase was largely preserved, crystal plastic deformation accompanied dissolution-precipitation. Heterogeneous early hydration of mafic rocks led therefore to a patchwork of local reacted domains, where inherited microstructures gave rise to heterogeneous phase distribution, grain sizes, fabrics and preconditioned the rock for strain
partitioning, and hence, strain localization at a scale of hundreds of microns. In particular, phase mixing was the product not of strain, but rather of an initial stage of chemical reactions.

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**Key words:** mafic middle crust, shear zones, local equilibria, deformation, inheritance, dissolution-reprecipitation, fracturing, phase mixing

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## 1 Introduction

Strain localization in the lower and middle mafic crust is controlled by the formation of shear zones that may accommodate up to hundreds of kilometers of displacement. Constraining the processes that lead to shear zones formation is therefore key to understand crustal deformation and dynamics. In these shear zones, rock weakening is promoted by grain size reduction that activates grain-size-sensitive (GSS) creep (e.g., Kruse and Stünitz, 1999; Kenkmann and Dresen, 2002; Baratoux et al., 2005; Kanagawa et al., 2008; Mehl and Hirth, 2008; Menegon et al., 2015; Okudaira et al., 2015) and lead to the formation of fine-grained mixed-phase layers (Rutter et al., 1985; Getsinger and Hirth, 2014; Stünitz and Tullis, 2001; Pearce et al., 2011; Mansard et al., 2020; Marti et al., 2018; Lee et al., 2022). At temperature conditions of the lower to middle crust (500-800°C), various deformation mechanisms have been recognized and used to explain grain size reduction and phase mixing, including dynamic recrystallization, dissolution-reprecipitation and phase nucleation. As a major mineral of mafic amphibolites, amphibole can accommodate strain by dissolution-reprecipitation (Imon et al. 2004; Baratoux et al., 2005; Soret et al., 2019; Lee et al., 2022; Giuntoli et al., 2018; Merit et al., 2025) or plastic deformation (Kruse and Stünitz, 1999; Cumbest et al. 1989; Skrotzki 1992; Siegesmund et al. 1994; Díaz Aspiroz et al. 2007; Pearce et al. 2011). Dissolution-reprecipitation and oriented growth may result in the formation of a strong fabric, despite a lack of crystal plasticity (Stokes et al., 2012; Getsinger and Hirth, 2014; Giuntoli et al., 2018; Carpenter et al., 2025). At similar conditions, plagioclase is also described to deform by either dominant crystal plasticity that drives dynamic recrystallization by sub-grain rotation, particularly in monomineralic layers (i.e. Kruse and Stünitz et al., 1999; Kruse et al., 2001; Stünitz et al., 2003; Baratoux et al., 2005; Svahnberg and Piazolo, 2010; Miranda et al., 2016), or by dissolution-reprecipitation that commonly involves both, oriented crystal growth and phase nucleation, irrespectively of the degree of phase mixing (Dimanov et al., 1999; Marti et al., 2018; Lee et al., 2022; Gardner et al., 2021; Moore et al., 2024; Merit et al., 2025). Recently, a number of studies have also underlined the importance of phase nucleation during syn-kinematic fluid-assisted metamorphic reactions to reduce the grain size and promote strain localization (Getsinger and Hirt, 2014; Marti et al., 2018; Mansard et al., 2020). The extent

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of such reactions is, in some cases proportional to viscous strain (Mancktelow and Pennacchioni, 2005; Goncalves et al., 2016), but in some others, it is hardly correlated to it, for example in rocks experiencing fracturing and related grain size reduction before strain localization on the damage zones (Pennacchioni and Mancktelow, 2018; Okudaira et al. 2015). In such instance, fabrics inherited from the fracturing-induced reaction stage have been observed in plagioclase (Moore et al., 2024), along with mineral segregation inherited from the distribution of former igneous minerals (Airaghi et al., 2024). These observations question how early stages of fracture-induced reactions affect phase distribution and mineral fabrics in shear zones and how they modify rock strength and subsequent viscous strain localization.

This work investigates the microstructures, fabrics and compositional heterogeneities in deformed metagabbros and amphibolites of the Poroshiri ophiolite. The latter lies within a major dextral tectonic plate boundary that accommodates hundreds of kilometers of displacement (Fig. 1a; Jolivet and Huchon, 1989). Within this zone, strain is heterogeneously distributed in shear bands, the thickness of which ranges from few millimeters to several meters. Deformation mechanisms and composition variations of plagioclase and amphibole – the major constituents of the studied lithologies – were investigated from the fractured domains to the shear zone using electron backscatter diffraction (EBSD), quantitative compositional mapping of major and trace elements, as well as optical cathodoluminescence (C.L.). Thermodynamic modelling was performed by varying the size of the reactive volume from tenths of microns to the entire thin section to establish the factors controlling the compositional variations and discuss the scale of equilibrium in high strain domains.

## 2 Geological setting and sampling

The Central Hokkaido, in Northeast Japan (Fig. 1a, b), comprises a large number of accreted ophiolitic bodies from a fast-spreading mid-oceanic ridge and composed of a thick sequence of ultramafic, mafic rocks and sedimentary protoliths (Miyashita et al., 2007; Kemp et al., 2007; Takahashi et al., 2021; Usuki et al., 2006). These bodies include the Poroshiri ophiolite, crosscut by a N-S trending metamorphic and deformation system, the Hidaka Belt (Fig. 1b and c) (Jolivet and Miyashita, 1985; Ueda and Miyashita, 2005; Ueda, 2016). The latter consists in mafic and sedimentary units bounded by N-S trending thrusts and shear zones (Fig. 1b), including the Hidaka Main Thrust (HMT) is a high-angle, east-dipping shear zone that separates the Poroshiri Ophiolite to the west from the Hidaka metamorphic belt to the east by a high-angle, east-dipping shear zone (Fig. 1b). The Poroshiri Ophiolite formed between the end of the Jurassic and Early Cretaceous by the obduction of the oceanic crust of a marginal basin during the collision of the Okhotsk block and Eurasia (Jolivet et al., 1988). From the Eocene to
Middle Miocene, the right-lateral strike-slip movement along Hokkaido resulted in the formation of a shear zone of hundreds of kilometers long, and finally in the opening of the Japan Sea as a pull-apart basin (Fig. 1a; Jolivet and Huchon, 1989). Along the ophiolite, the conditions of deformation vary from ~800 °C to the east to ~400 °C to the west (Fig. 1c; Grapes et al., 1976; Jolivet, 1984; Jolivet and Miyashita, 1985; Tanaka et al., 2012; Kato et al., 2016; Airaghi et al., 2024). Dextral strike-slip shear zones are wide-spread in the ophiolite, most of them being vertical and oriented N-S with a horizontal lineation. Sinistral shear zones with a normal component have also been described in the eastern units of the Poroshiri ophiolite, where foliated amphibolites have been transformed into mylonitic ones, during retrograde metamorphism (Kato et al., 2016).

The studied samples belong to a series of metagabbros and mafic amphibolites collected along the Chiroro river (Fig. 1c). The rocks experienced an initial stage of fracturing and hydration at granulite- to amphibolite-facies conditions (800-550 °C, Tanaka et al., 2012; Kato et al., 2016; Airaghi et al., 2024) marked by the opening of cm to µm-size amphibole-bearing fractures surrounded by reaction halos (Fig. 1d). Microfracture network had a direct control on the percolation depth of fluids and thus the extent of the amphibolitization reaction (Airaghi et al., 2024). In the adjacent volume of rocks, fractured metagabbros progressively changed into amphibolites, giving rise to fractures that evolved into cm to mm-scale shear bands with different degrees of amphibolitization (Fig. 1e). The most evolved stages of deformation and amphibolitization is observed in the macroscopic, meter-scale major shear zone mainly composed of foliated amphibolite (Fig. 1f), locally mylonitized along cm-scale sinistral shear bands to form mylonitic amphibolite (Fig. 1f; Kato et al., 2016). The series of rocks studied here -the most reactive ones- expose therefore a continuum of reaction and deformation enabling to analyze how the microstructures and crystallographic fabrics evolve with increasing strain and mineral transformation. For further information about the same rock series in terms of pressure-temperature path and fluid infiltration history, the readers may also refer to (Airaghi et al., 2024).
<figure><img src="image_3.png" /><figcaption>**Figure 1:** Geologic setting of the study area and macroscopic views of deformation structures. (a) Tectonic map of the Sakhalin-East Japan Sea strike-slip zone (modified from Fournier et al., 1994). (b) Regional emplacement of Hidaka metamorphic belt. (c) Geological map showing the Poroshiri ophiolite (modified from Miyashita and Arai, 1994; Tanaka et al., 2012) and the study area along the Chiroro river (red star). (d) Outcrop scale view of fractured metagabbros with reaction halos around fractures (white arrows). (e) Outcrop scale view of mm-scale amphibolitic shear bands in weakly deformed metagabbros (white arrows). (f) Outcrop scale view of m-scale shear zone containing foliated and mylonitic amphibolites.</figcaption></figure>

## 3 Methods

Backscattered electron images were obtained with a scanning electron microscope (SEM) Zeiss Merlin Compact at the *Institut des Sciences de la Terre d'Orléans* (ISTO) equipped with a Bruker XFlash 6/30 SDD detector, here using "internal standards". Local bulk compositions were obtained from SEM calibrated average composition analyses of subdomains of variable size. Quantitative point analyses were acquired with a CAMECA SX Five microprobe and a IHP200F JEOL FEG-SEM at ISTO and MACLE-CVL platform of the *Centre National de la Recherche Scientifique* (CNRS) in Orléans (Univ. Orléans, France). Mineral composition was quantified by point
analysis made at 15 kV and 10 nA, with a ~ 1 µm beam size, in the wavelength dispersive spectroscopy (WDS) mode, using diopside (Ca, Mg, Si), MnTiO₃ (Mn, Ti), orthoclase (K, Al), hematite (Fe), albite (Na), and Cr₂O₃ (Cr) as standards for measuring the elements written in brakets at the same conditions. WDS mapping was also performed at conditions of 15 kV and using a diaphragm of 120 µm. At the microprobe, mapping conditions were fixed at 15 kV, 100 nA, 150-200 ms as dwell time and a spot size of 1 µm. X-ray maps were then quantified with the software XMAPTOOLS following the procedure described in Lanari et al. (2019). Cathodoluminescence (C.L.) images were acquired with a Cathodyne instrument (by NewTec Scientific) at a voltage of 15 kV, beam current of 150 µA and using an exposure time of 10s.

Plagioclase and amphibole deformation textures were then investigated by electron backscatter diffraction (EBSD). EBSD data have been acquired at MACLE-CVL platform (ISTO, CNRS) on polished thin sections using a JEOL IT800SHL SEM coupled to an Oxford Instruments EBSD system with advanced Symmetry detector at ISTO and, respectively. Analytical conditions were set to 5 nA of probe current and 20 kV of accelerating voltage, at a working distance between 15 and 18 mm and using a step size of 1 µm for mapping. After acquisition, the two EBSD maps were cleaned using the Oxford AZtecCrystal 2.2 software applying the following “cleaning” procedure: (1) one step to replace the isolated pixels (wild spikes) fully surrounded by the correctly indexed ones, (2) ten iterations of zero solution removal considering 5 neighbors (level 4) and (3) one more iteration considering 4 neighbors (level 5). Their respective indexation rates were between 94.02 and 97.29 % before cleaning and between 98.42 and 99.43 % after cleaning. The data were then exported and treated/plotted using the open-source MTEX toolbox (version 5.8.2; https://mtex-toolbox.github.io) for Matlab (Hielscher and Schaeben, 2008; Bachmann et al., 2010). A rotation of 40° around [X] axis was performed for sample H16-33 and H16-45 to adjust the cut of drilled cores to the observed lineation. The crystallographic preferred orientation was obtained from the mean orientation of grains for amphibole and plagioclase, and both the texture (J; Bunge, 1982) and misorientation (M; Skemer et al., 2005) indices have been calculated to quantify the fabric strength for each map. Geometrically necessary dislocation (GND) densities were finally calculated from the lattice curvature gradients based on EBSD data (Pantleon, 2008). A grain was defined by a minimum area of 10 pixels (where 1 pixel is 1 µm) with a misorientation angle threshold of 10° to define grain boundaries, giving rise to a minimum ‘calculated’ grain size considered of 3-5 µm equivalent diameter.

Trace element mapping on amphibole was performed by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) using a RESOlation SE 193 nm laser ablation system with a Laurin Technique S155 ablation cell coupled to an Agilent 8900 QQQ quadrupole mass spectrometer. The system was tuned to high signal-
to-background ratios, while maintaining stable ablation, transport and ionization conditions with limited
fractionation (U/Th=1.00±0.02%), low oxide production rates (ThO/Th<0.2%), and low production of doubly
charged ions ($^{42}$Ca$^{2+}$/$^{42}$Ca+<0.3%). At these conditions, the samples cell was flushed with He (~400 ml/min) and
Ar (~0.5 l/min). Additional Ar (~0.994 l/min) was added to the torch area. The plasma was sustained at an Ar gas
flow of ~15 l/min and an RF power of 1480 W. For the compositional mapping, the laser was operated at 10 Hz
and with an on sample fluence of ~4 mJ/cm$^2$. Ablation was performed with a rectangular, 8 mm laser beam diameter,
which ablated the target zone in a horizontal raster scanning at speeds of 6 mm/sec from right to left and bottom
to top. Isotopes measured were $^{27}$Al, $^{29}$Si, $^{49}$Ti, $^{53}$Cr, $^{57}$Fe, $^{60}$Ni, $^{88}$Sr, $^{89}$Y, $^{137}$Ba, $^{139}$La, $^{140}$Ce, $^{147}$Sm, $^{153}$Eu, and $^{208}$Pb
with dwell times of 10 ms per isotope and a total sweep time of 0.1626 seconds. $^{27}$Al, $^{88}$Sr and $^{137}$Ba where selected
to best show plagioclase-induced compositional variations in amphibole. For each line scan, isotope concentrations
were measured for 15 seconds on the gas blank before and after ablation. The raw data were then treated in Iolite
(version 4.7.1; Paton et al. 2011; Petrus et al. 2017) with counts per second (cps) as input and output shown in
empirical cumulative distribution function (ECDF) mode to best highlight contrasting concentrations and
XMapTools (version 4.3, Lanari et al., 2019).

Equilibrium phase diagram for local reactive bulk rock was calculated in the NaCaTiFMASH system with
the Perple_X software version 6.9.1 (Connolly, 2009; Connolly and Kerrick, 1987) using the internally consistent
thermodynamic dataset (hp62ver.dat) of Holland and Powell (2011) and the following solution models: Green et
al. (2016) for melt, amphibole and clinopyroxene (augite), Holland and Powell (1998) for orthopyroxene and
olivine, and Holland and Powell (2003) (I1, HP) for plagioclase, ilmenite and spinel. Bingo Antidote software was
also used with the same thermodynamic dataset to estimate the effect of changing the size of local reactive bulk
composition on plagioclase and amphibole composition and proportions (Lanari and Hermann, 2021).

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181 **4 Results**

182 **4.1 Petrography and microstructures**

Four amphibolite samples (H16-25, H16-31, H16-33 and H16-45) were selected for detailed microstructural
and compositional analyses along the deformation and reaction gradient described in section 2 (Fig. 2a-k and Fig.
S1). They include: (1) metagabbros with a dense network of fractures and extensive amphibolitization, but without
or little evidence of viscous deformation (Fig. 2a-d and Fig. S1 for the area), (2) millimeter-scale (~5 mm) localized
shear bands sharply bounded by fractured domains with very weak, incipient viscous strain (Fig. 2e-g and Fig. S1
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for the area) and (3) amphibolites belonging to the macroscopic shear zones (foliated and mylonitic amphibolites; Fig. 2h-k).

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(1) **Fractured domains.** Fractured, amphibolitized metagabbros show three different mineralogical areas are observed: (i) amphibole (Amp) pseudomorphs (Ca-amphibole ± cummingtonite) after igneous pyroxene (mainly clinopyroxene, Cpx) and olivine (Ol), roughly aligned parallel to the shear bands (Fig. 2b-d and Fig. S1); (ii) 500 µm large relicts of igneous plagioclase (Pl) grains (Pl₁), enriched in oxide inclusions and cross-cut by micrometric fractures filled by fine grained Pl and/or Amp (Fig. 2b-d) and (iii) symplectite-like microstructures made of mixed fine-grained (20-40 µm) secondary plagioclase (Pl₂) and Amp at Pl₂ - Ol pseudomorphs grain boundaries (and minor Cpx; black square in Fig. 2b and Fig. 2c, d). The relative amount of Amp and Pl varies depending on the reactional domain: at Pl₁ rims, few Amp grains are observed within dominant Pl₂, while in symplectites, similar amount of Pl and Amp are produced (Fig. 2c and d respectively). Grain size is reduced by one order of magnitude from 500-1000 µm (size of igneous grains) to 50-100 µm at pseudomorphs rims or to ~15 µm in average in Ol pseudomorphs core, symplectites and microfractures (Fig. 2b-d).

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(2) **Mm-scale shear bands.** Monomineralic Amp layers alternate with elongated Pl aggregates and mixed-phase Pl-Amp layers to define the foliation. Efficient grain size reduction and phase mixing relative to fractured metagabbro are observed since all Pl₁ has reacted to Pl₂ (Fig. 2f and g). Pl and Amp are of similar grain size in mixed-phase layers and, similarly to fractured metagabbros, domains with different Pl and Amp proportions are observed (Fig. 2g). Grain boundaries show numerous triple junctions (Fig. 2g).

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(2) **Macroscopic shear zones.** Whitin these zones, both foliated and mylonitic amphibolites are recognized. Foliated amphibolite is mainly composed of layers dominated by Amp or mixed-phase layers made of Amp+Pl (Fig. 2h, j). Relicts of Pl₁ up to 1 mm in size are abundant, and mantled by a thin rim (10-100 µm) of smaller, inclusion-free grains of Pl (Pl₂) also observed in mixed Pl-Amp layers (Fig. 2j). Pl₁ porphyroclasts only show local evidence of intracrystalline deformation as undulatory extinction close to Pl-Pl grain boundaries. Ca-Na mineral phases (zeolites-like) can be locally observed associated to Pl₂. The alignment of Amp grains defines the lineation, but the sense of shear cannot be clearly recognized because of a lack of asymmetric microstructures. Mylonitic amphibolite shows similar microstructures than foliated amphibolite, although more abundant relicts of Pl₁ and rare phase mixed-phase layers (Pl + Amp or Amp + Cpx) are observed (Fig. 2i, k). Monomineralic-Pl layers exhibit a ribbon-like structure, formed by Pl₁ porphyroclasts showing extensive intracrystalline deformation and mantled by fine-grained, inclusion-free Pl₂ grains to form core-mantle structures (Fig. 2k). Pl₂ grains also occur in dilatant
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sites, such as fractures, and in asymmetric pressure shadows showing sinistral sense of shear (Figs. 2k). Sinistral shear bands oblique to mylonitic foliation are also recognized (red in Fig. 2k). In Amp-dominated layers, ~150 µm large Amp grains are elongated parallel to the stretching lineation; they lack intracrystalline deformation and are cross-cut by micrometric fractures (Fig. 2k).

<figure><img src="image_4.png" /></figure>

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**Figure 2:** Microscopic views of studied rocks. (a) Fracture with alteration halos in a metagabbro. (b) images of an olivine (Ol, left) pseudomorphically replaced by amphibole (Amp) and of plagioclase (Pl)-Amp symplectite at the rims between Ol pseudomorph and Pl₁ (black square). (c) Backscattered electron (BSE) image showing incipient formation of symplectite at the rims between Amp pseudomorphs after pyroxene (Cpx) and Pl₁ (domain with low proportion of Pl₂ within Amp). (d) BSE image of Ol pseudomorph replaced by Amp and associated symplectite (domain with equal proportion of Pl₂ and Amp). Note the compositional heterogeneity of Amp in the pseudomorph revealed by variations of brightness. (e) mm-
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scale shear bands forming on former fractures. (f) Alternating Amp monomineralic layers and Pl-Amp
mixed-phase layers in the mm-scale shear band. (g) BSE enlarged views of layers with different Pl and
Amp proportions within the mm-scale shear band. (h) Foliated amphibolite in the macroscopic shear zone.
(i) Mylonitic amphibolite in the macroscopic shear zone. (j) BSE image of Pl₁ porphyroclasts, Amp
monomineralic layers and Pl+Amp layers in foliated amphibolite. (k) BSE image of Amp monomineralic
layers with zoned Amp grains (see variations of brightness), Cpx-Amp mixed-phase layers and ribbons
of recrystallized Pl₁ in mylonitic amphibolite. Red lines: sinistral shear bands oblique to the foliation.

## 4.2 Grain size, textures and CPO of plagioclase and amphibole

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(1) **Fractured domains.** Textures for fractured domains were analyzed from the same thin section than for mm-scale shear zones to allow direct comparison of fabrics and textures evolution within and outside the shear band. In fractured domains of amphibolitized metagabbros, textures were investigated along a reaction profile running from Ol pseudomorph to Pl₁, through symplectites (Fig. 3a and b). Amp shows a variety of grain size with smallest grains in symplectites (~14 µm), cracks within Pl₁ (~8 µm) and cores of the pseudomorphs (20-30 µm, Table 1). In contrast, the largest grains are observed at pseudomorphs rims (50-150 µm, Fig. 3a and Table 1). Amp also shows a variety of aspect ratio. While large Amp grains at rims of Ol pseudomorphs and small Amp grains in symplectites exhibits in average an aspect ratio (AR) of 1.8-1.9, Amp grains within pseudomorphs and in Pl₁ cracks exhibit a higher AR of 2-2.2, locally up to 3-4 (Table 1). In all microstructural domains along the reaction profile, Amp grains also show a clear shape preferred orientation (SPO) and a crystallographic fabric (J=1.9-2.6, M=0.04-0.07), with the [001] axes oriented subparallel – or slightly oblique (~20°) – to the dominant direction of microfractures in Pl₁. This is also the case for Amp grains within Pl₁ microcracks, which are elongated parallel to the walls of microfractures (mostly parallel to the main outcrop-scale fracture and to the direction of the mm-scale shear bands; Fig. 3a-d, Fig. S2). Furthermore, while only two poles are observed in the pole figure of Amp CPO within symplectites, an additional pole is observed for Amp within the pseudomorph as well as in Pl₁ cracks, suggesting a partial control of parental grains on the orientation of Amp grains (Ol and Pl₁; Fig. 3c). Pl₁ is largely preserved and shows very weak intra-grain misorientation. The fabric of Pl₂ is weak (but partially controlled by the main fracture direction), except for Pl₂ in microfractures that preserves the same CPO as parent Pl₁ and thus a strong fabric (Fig. 3c). The crystallographic fabric of Pl₂ progressively randomizes in symplectites with increasing distance to the Pl₁ (Fig. 3c, points 3 and 4) and SPO appears more widespread than that of Amp (Fig. 3c and d)
<table><thead><tr><th rowspan="3"></th><th colspan="5">AMPHIBOLE</th><th colspan="5">PLAGIOCLASE</th></tr><tr><th colspan="4">GRAINSIZE (eq. Diameter in µm)</th><th rowspan="2">Grains</th><th colspan="4">ASPECT RATIO</th><th colspan="4">ASPECT RATIO</th></tr><tr><th>Mean</th><th>Median</th><th>Mode</th><th>Std</th><th>Mean</th><th>Median</th><th>Mode</th><th>Std</th><th>Mean</th><th>Median</th><th>Mode</th><th>Std</th><th>Grains</th></tr></thead><tbody><tr><td colspan="14"><strong>Fractured domains</strong></td></tr><tr><td>In pseudomorphs</td><td>14.5</td><td>11.6</td><td>1.1</td><td>11</td><td>90</td><td>17.41</td><td>2.3</td><td>2.0</td><td>1.2</td><td>1.2</td><td>839</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Pseudomorphs rims</td><td>20.8</td><td>13.1</td><td>3.5</td><td>20</td><td>155</td><td></td><td>1.8</td><td>1.7</td><td>1.5</td><td>0.6</td><td>525</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Symplectites</td><td>14.0</td><td>11.1</td><td>1.0</td><td>9.7</td><td>48</td><td>15.83</td><td>2.0</td><td>1.7</td><td>1.4</td><td>1.0</td><td>136</td><td>14.8</td><td>12.2</td><td>8.7</td><td>10.2</td></tr><tr><td>Plagioclase fractures</td><td>8.8</td><td>6.9</td><td>3.5</td><td>5.6</td><td>36</td><td>11.09</td><td>2.1</td><td>1.8</td><td>1.5</td><td>1.1</td><td>533</td><td>8.4</td><td>6.8</td><td>3.7</td><td>4.8</td></tr><tr><td colspan="14"><strong>Mm-scale shear band</strong></td></tr><tr><td>Monomineralic layers (zone A)</td><td>18.5</td><td>12.8</td><td>3.7</td><td>21</td><td>181</td><td>18.86</td><td>2.0</td><td>1.8</td><td>1.2</td><td>0.8</td><td>448</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Mixed-phase layers (zone B)</td><td>14.5</td><td>9.5</td><td>3.9</td><td>15</td><td>130</td><td>17.14</td><td>2.0</td><td>1.8</td><td>1.4</td><td>0.7</td><td>771</td><td>9.9</td><td>7.5</td><td>4.1</td><td>7.9</td></tr><tr><td>Mixed-phase layers (zone C)</td><td>11.0</td><td>9.4</td><td>1.0</td><td>7.3</td><td>59</td><td>8.95</td><td>1.9</td><td>1.7</td><td>1.4</td><td>0.8</td><td>198</td><td>8.9</td><td>6.6</td><td>4.1</td><td>6.4</td></tr><tr><td colspan="14"><strong>Macroscopic shear zone</strong></td></tr><tr><td>Foliated amp</td><td>29.9</td><td>11.9</td><td>5.0</td><td>49</td><td>374</td><td>34.7</td><td>2.2</td><td>2.0</td><td>1.7</td><td>0.9</td><td>402</td><td>14.5**</td><td>10.8</td><td>1.8</td><td>13.6</td></tr><tr><td>Foliated amp</td><td>28.2</td><td>9.0</td><td>17.8</td><td>31</td><td>248</td><td>41.0</td><td>2.1</td><td>1.9</td><td>1.1</td><td>0.7</td><td>756</td><td>18.4**</td><td>12.7</td><td>7.7</td><td>17.8</td></tr><tr><td>Mylonitic amp</td><td>20.0</td><td>12.0</td><td>3.9</td><td>25</td><td>266</td><td>27.0</td><td>2.4</td><td>2.1</td><td>1.8</td><td>1.1</td><td>1014</td><td>12.5**</td><td>8.5</td><td>3.9</td><td>14.1</td></tr><tr><td>Mylonitic amp</td><td>16.7</td><td>10.8</td><td>3.7</td><td>19</td><td>218</td><td>21.5</td><td>2.5</td><td>2.3</td><td>1.4</td><td>1.0</td><td>562</td><td>12.6**</td><td>8.6</td><td>3.6</td><td>11.9</td></tr></tbody></table>

<figure><img src="image_5.png" /><figcaption>**Figure 3:** Microstructural and textural features of Amp and Pl in fractured domains. (a) and (b) Inverse pole figure map (IPF) of Amp and Pl respectively. Arrow indicates the reaction front along which microdomains (1-5) were identified for differential microtextural analysis. 1: Amp within Ol pseudomorphs, 2: Amp at rims of Ol pseudomorphs, 3: symplectites, 4: symplectites close to Pl₁ rims, 5: fractured Pl₁. (c) Lower-hemisphere, equal-</figcaption></figure>

<footnote>* mean grainsize for grains with aspect ratio > 3</footnote>

<footnote>**included Pl₁ relics</footnote>
267 area pole figures showing the crystallographic preferred orientations (CPO) of Amp (black) and Pl (blue) along the transect 1 to 5. Number of points is equal to the number of grains (mean grain orientation used). (d) Shape preferred orientation (SPO) of Amp and Pl.
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(2) **Mm-scale shear bands.** Mm-scale shear bands are composed of microstructural domains that are very reminiscent of the ones defined in fractured zone (4.2.1). This is the case for monomineralic Amp layers (zone A in Fig. 4a) that exhibit characteristics similar to Amp pseudomorphs after Ol, and fine-grained Amp-Pl mixed-phase domains (zone C in Fig. 4a) exhibiting features similar to symplectites in terms of grain size, SPO and grain aspect ratio. (Table 1). Amp locally shows larger grain size (zone B in Fig. 4a; Table 1) and preserves a clear CPO, which is slightly strengthened compared to fractured domains ($J = 3.01-3.3, M = 0.08-0.12$) and transposed into the stretching lineation (Fig. 4c and d). Although showing an overall weak fabric and widespread axis orientation, Pl preserves micro-domains with stronger fabrics (zone C in Fig. 4a with $J=2.33$) made of fine-grained aggregates with similar orientations, as for grains of $Pl_2$ nucleating in $Pl_1$ microfractures (white arrows in Fig. 4b). In contrast to fractured domains, mm-scale shear also show complete recrystallization of $Pl_1$ into fine grained $Pl_2$.
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<figure><img src="image_6.png" /><figcaption>**Figure 4:** Microstructural and textural features of Amp and Pl in a mm-scale shear band. (a-b) IPF maps of Amp (a) and Pl (b). (c) Lower-hemisphere, equal-area pole figures showing CPOs of Amp (black) and Pl (blue) for the domains indicated in panel a. Number of points is equal to the number of grains (mean orientation) (d) SPOs of Amp and Pl.</figcaption></figure>

**(3) Macroscopic shear zone.** In foliated and mylonitic amphibolite of the macroscopic shear zones, microstructures and textures similar to the ones observed in mm-scale shear bands are recognized, except for layers or aggregates made of relicts of Pl₁ (Figs. 5a-1 and 6a-1). This is the case for very weak intra-grain deformation in Amp (2-4°) with Amp [001] axis parallel to the stretching lineation, regardless of the degree of Amp-Pl phase mixing (Fig. 5a-b, 6a-b and S4a-b). However, new textural and microstructural features are also observed compared to previously described domains, including:
293 (i) larger Amp grain size in Amp monomineralic layers (mean equivalent diameter of ~20 µm, but 294 standard deviation of 25-31, Table 1), with several grains exceeding 240 µm (Fig. 5a, 6a S4a 295 and S5a; Table 1);

296 (ii) higher aspect ratio for Amp. While in many respects Amp aspect ratio is similar to 297 microstructural domains described above, the size of the grains with the highest aspect ratios is 298 higher than in fractured domains. We also observe some grain larger than 100 µm with aspect 299 ratios > 4 (Fig. S4c and S5b; Table 1).

300 (iii) stronger fabric for Amp (J = 4.29-6.33, M ~ 0.2 in foliated amphibolite, J = 3.09-4.21, M = 0.12-301 0.17 in mylonitic amphibolite), regardless of the degree of mixing between Pl and Amp (Figs. 302 5h and 6h).

303 In Amp-Pl mixed-phase layers of both foliated and mylonitic amphibolites, the CPO of Pl is always relatively 304 weak (J = 2.39 and 2.31 respectively; Fig. 5k, Fig. 6k). In contrast, differences are observed for the texture and 305 fabric of Pl in Pl-monomineralic layers. In foliated amphibolite, Pl₁ porphyroclasts show weak intragrain 306 misorientation, except locally at Pl-Pl grain boundaries (Fig. 5d, c and S4d). Pl₁ porphyroclast are mantled by a 307 rim of small Pl grains along Pl-Pl grain boundaries (Fig. 5d). Such small grains show similar crystallographic 308 orientation to Pl₁ and are associated to intra-grain misorientation in Pl₁ of 10-15° (Fig. 5e and f respectively). The 309 SPO of Pl is only slightly stronger than in previously described microstructures (J = 2.44, M = 0.05, Fig. 5j). In 310 mylonitic amphibolite, Pl₁ porphyroclasts exhibits instead systematic intra-grain deformation (Fig. 5e and S5e). 311 Within porphyroclasts, polygonal misoriented domains with similar size of recrystallized grains and with high 312 densities of Geometrically Necessary Dislocations (GDN) are observed (~10¹³-10¹⁴ m⁻²; Figs. 6c-f). Pl 313 porphyroclasts largely recrystallize in small (10-30 µm) Pl grain in pressure shadows of Pl₁ and in zones of high 314 dislocation densities, forming microstructures typical of dynamic recrystallization elongated in ribbons parallel to 315 the foliation (Fig. 6d). Small Pl grains show either similar grain orientation and dislocation density to parent Pl₁ 316 (white arrows in Fig. 6d-f), despite a lack of intra-grain defects (red arrows in Fig. 6d-f), or a strong misorientation 317 with respect to Pl₁, but still with no evidence of intra-grain deformation (black arrows in Fig. 6d-f; Table 1). The 318 SPO and CPO of Pl in these viscously deformed monomineralic layers are stronger than in foliated amphibolite 319 (J=2.85-3.43, M=0.05-0.07; Fig. 6j and S3f). Recrystallized Pl₂ exhibits [010] axis roughly aligned parallel to the 320 lineation and clusters in CPO pole figure partly controlled by the orientation of the parent Pl₁ grain (Fig. 6c, j).
Macroscopic shear zones : foliated amphibolite

<figure><img src="image_7.png" /><figcaption>**Figure 5:** Microstructures and textures of Amp and Pl in the foliated amphibolite of the macroscopic shear zone.</figcaption></figure>

All acquired EBSD maps for foliated amphibolite are reported in figure S3a-c with related Amp and Pl CPOs. (a) IPF map of Amp in phase mixed-phase layers. (b) Intra-grain lattice misorientation in Amp (from area indicated by the white rectangle in panel a). (c) IPF map of Pl in mixed-phase layers. (d) IPF maps of Pl in Pl monomineralic layers. (e) Enlarged view of the black square of panel d (small recrystallized Pl grains). (f) Intra-grain lattice misorientation in Pl grains (same area of panel e). (g) Lower-hemisphere, equal-area pole figures showing an example CPO of Amp in monomineralic layers (from map of Fig. S3c and S4a). Number of points is equal to the number of grains (h) CPO of Amp in mixed-phase layers (from the entire map of panel a, see also Fig. S3b). (i) SPO of Amp. (j) Example of

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CPO of Pl in monomineralic layers (from the entire map in panel d, see also Fig. S3a). (k) Example of

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CPO of Pl in phase mixed-phase layers (from the entire map in panel c, Fig. S3b). (l) SPO of Pl.

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<figure><img src="image_8.png" /></figure>

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**Figure 6:** Microstructures and textures of Amp and Pl in mylonitic amphibolite. All acquired EBSD maps for foliated amphibolite are reported in figure S3d-g with related CPOs for both amphibole and plagioclase.

(a) IPF map of Amp in monomineralic layers (upper left) and Amp+Cpx layers (bottom right). Related IPF for Pl grains, intragrain misorientation and CPOs are reported in Fig. S3f and S5d-e. (b) Intra-grain lattice misorientation in Amp (same area of panel a). (c) IPF map of Pl in monomineralic layers. (d) Enlarged view of the black frame of panel c. (e) Intragrain lattice misorientation in Pl (same area of panel d). (f) Geometrically necessary dislocation densities in Pl (same area of panel d). (g) Lower-hemisphere,

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equal-area pole figures showing an example of the CPO of Amp in monomineralic layers (from the entire map of Fig. S3d and S5a). Number of points is equal to the number of grains. (h) Example of CPO of Amp in phase mixed-phase layers (from map of Fig. S3g and S5f). (i) SPO of Amp. (j) Example of CPO of Pl in monomineralic layers (from the entire map in panel c and Fig. S3e). Yellow dots: 6 grains of Pl₁ (equivalent diameter > 100 µm). (k) Example of CPO of Pl in phase mixed-phase layers (from map in Fig. Fig. S3g and S5g). (l) SPO of Pl.

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## 4.3 Mineral composition and cathodoluminescence signature

The chemical compositions of Pl and Amp grains varies at the grain scale and according to the microstructural position within the rock, in fractured domains, mm-scale shear bands and within the macroscopic shear zone (Figs. 7 and 8).

(1) **Fractured domains.** Amp composition evolves from actinolite and Mg-hornblende (Cr-rich) to Al-poor (and Cr-poor) pargasite/edenite from the core of Cpx and Ol pseudomorphs (where cummingtonite is also observed) to the symplectitic microstructures (Fig. 7a, b; Fig. S6a, Table S1). Amp in Pl₁ cracks show similar chemical composition to Amp in symplectites (Fig. 7b). Igneous Pl₁ is a labradorite (~An₆₀) with an intragrain homogeneous composition (Fig. 8a, b) and yellow/reddish color in cathodoluminescence (Fig. 9a). Pl in symplectites shows a higher anorthite content than Pl₁ (up to An₈₀⁻₉₂, Fig. 8a, b; Table S1) and green to reddish color in C.L. (Fig. 9a). Pl₁ microfractures are filled with Pl₂ grains with anorthite contents similar to symplectites (yellow arrow in Fig. 8b) or with Ab-richer Pl₂ (red arrow in Fig. 8b). Small Pl₂ grains in microfractures of Pl₁ exhibit red to pink colors in C.L. (yellow arrows in Fig. 9a).

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<figure><img src="image_9.png" /><figcaption>**Figure 7:** Chemical composition of Ca-Amp. (a) Si⁴⁺ apfu vs. (Na+K) a.p.f.u. in Amp. (b) Al₂O₃ (wt %) content in Amp in fractured domains (Ol pseudomorphs and symplectites, see black frame in Fig. 3a for location). Cumm: cummingtonite. (c) Al₂O₃ map in mm-scale shear band (see black frame in Fig. 4b for location). (d) Al₂O₃ map in mm-scale shear band for areas with different proportion of Amp and Pl. (e) Al₂O₃ map of Amp in foliated amphibolite of the macroscopic shear zone (white rectangle in Fig. 5a). The inset indicates an Amp grain with zoning truncated perpendicular to shortening (arrows). (f) Map of ⁸⁸Sr content in Amp (see dashed black frame in panel e for location). (g) Al₂O₃ map of Amp in mylonitic amphibolite (same area of Fig. 6a).</figcaption></figure>

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(2) Mm-scale shear bands. In mm-scale shear bands, the local bulk chemical composition obtained from Amp monomineralic layers is close to that of Ol (or Cpx) pseudomorphs estimated from fractured domains. Mixed-phase layers with similar amount of Pl and Amp to symplectites (as zone C in Fig. 4a) exhibits instead local bulk composition close to that of symplectites (Table 2). Such similarities in the local bulk composition between different mineralogical layers and their respective igneous or fracture-induced altered precursors are also observed in mm-scale shear bands formed at higher temperature conditions (Fig. S8, Table S2; see also Airaghi et al., 2024).

At grain scale, Amp in monomineralic layers preserve to some extent the compositional zoning and heterogeneities observed in pseudomorphs of fractured domains, including heterogeneous concentration of chromium (Fig. 7c, d and Fig. S6b). In mixed layers, Amp composition is more homogeneous, but it varies among different domains over a length of ~200 µm, as a function of the modal abundance of Pl (Fig. 7d). In fine-grained mixed-phase domains made of similar amounts of Amp and Pl (as in symplectites), Amp is Al-rich as in symplectites, while in fine-grained mixed-phase domains dominated by Pl (78 vol%), Amp is Al-poorer (Fig. 7d and Table S1). Composition of fine-grained Pl evolves from An-rich (as in symplectites and Pl₁ microfractures) to Ab-richer (~An₄₀) at the very grain boundaries, regardless of the relative abundance of Pl and Amp (Fig. 8c). However, differences are observed in the C.L. signature of Pl grains among different domains: while in fine-grained Amp-Pl areas (zone C of Fig. 4a-b), Pl₂ shows green to red color as in symplectites (Fig. 9b), Pl₂ grains are larger and all green in Pl₂-dominated mixed-phase layers, a feature not observed in fractured domains (Fig. 9c).

<table><caption>**Table 2:** Local bulk composition for different reaction areas in fractured domains, mm-scale shear bands and macroscopic shear zone. "psudom": pseudomorphs, "sym": symplectite. "mono": monomineralic layers. Same colors and numbers ((1), (2) etc.) are used for domains that shares similar local bulk compositions.</caption><thead><tr><th rowspan="3">Area<br>Label</th><th colspan="3">Fractured domains</th><th colspan="2">Mm-shear bands</th><th colspan="2">Macroscopic shear zone</th></tr><tr><th>Ol pseudom</th><th>Cpx pseudom</th><th>Pl-Amp symp</th><th>Amp-mono</th><th>Amp-Pl</th><th>Amp mono</th><th>Mixed layers</th></tr><tr><th>(1)</th><th>(2)</th><th>(3)</th><th>(1)</th><th>(3)</th><th>(2)</th><th>(4)</th></tr></thead><tbody><tr><td>SiO2</td><td>43.87</td><td>45.90</td><td>41.03</td><td>40.37</td><td>41.75</td><td>49.44</td><td>46.03</td></tr><tr><td>TiO2</td><td>0.21</td><td>0.24</td><td>0.00</td><td>0.33</td><td>0.00</td><td>0.78</td><td>0.91</td></tr><tr><td>Al2O3</td><td>12.71</td><td>6.66</td><td>22.27</td><td>14.40</td><td>23.20</td><td>5.68</td><td>16.00</td></tr><tr><td>FeO</td><td>13.44</td><td>10.71</td><td>7.44</td><td>7.97</td><td>3.71</td><td>8.85</td><td>8.42</td></tr><tr><td>MnO</td><td>0.23</td><td>0.48</td><td>0.31</td><td>0.40</td><td>0.27</td><td>0.00</td><td>0.28</td></tr><tr><td>MgO</td><td>13.84</td><td>15.55</td><td>9.48</td><td>12.36</td><td>5.49</td><td>14.64</td><td>9.21</td></tr><tr><td>CaO</td><td>11.16</td><td>9.07</td><td>11.36</td><td>9.66</td><td>11.68</td><td>13.47</td><td>11.60</td></tr><tr><td>Na2O</td><td>1.99</td><td>1.22</td><td>2.02</td><td>1.86</td><td>1.73</td><td>0.65</td><td>5.97</td></tr><tr><td>K2O</td><td>0.10</td><td>0.10</td><td>0.16</td><td>0.00</td><td>0.00</td><td>0.04</td><td>0.12</td></tr></tbody></table>
<figure><img src="image_10.png" /><figcaption>**Figure 8:** Chemical composition of Pl. (a) Al<sup>IV</sup> apfu vs anorthite (XAn) in Pl. (b) XAn in Pl of fractured domains (same area of black frame in Fig. 3a and 7b). (c) XAn in the mm-scale shear band (same area of Fig. 4a-b and 7c). (d) XAn in Pl in foliated amphibolite of macroscopic shear zone (white rectangle in Fig. 5c and 7e). (e) XAn in Pl of mylonitic amphibolite (same area of Figs. 6a, 7g and S4d). (f) XAn in Pl of mylonitic amphibolite (same area of Fig. 6d). Black arrows: fragments of Pl<sub>1</sub> (Pl<sub>1</sub> composition) overgrown by Ab-rich Pl. Red arrows: Ab-rich microfractures and Ab-rich Pl<sub>2</sub> grains with local oscillatory zoning. Yellow arrows: An-rich microfractures and Pl<sub>2</sub> (see Fig. 6d-f for comparison).</figcaption></figure>

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<figure><img src="image_11.png" /><figcaption>**Figure 9:** Optical cathodoluminescence images of Pl in fractured domains (a), in mixed-phase layers of mm-scale shear bands (b), in Pl-dominated domains of mm-scale shear bands (c), in foliated amphibolite (d) and mylonitic amphibolite (e) of the macroscopic shear zone. Yellow arrows: healed cracks in Pl₁. The white rectangle highlights an area with symplectites.</figcaption></figure>

**(3) Macroscopic shear zone.** In foliated amphibolite, Amp exhibits Mg-hornblende composition in both monomineralic and mixed-phase layers (0.4 < Na a.p.f.u. < 0.6, 1.5 < Al a.p.f.u. < 2.2; Table S1). Higher Al contents are observed in Amp of mixed-phase layers than Amp of monomineralic layers (mean of 2.07 and 1.75 respectively; Table S1), as observed in mm-scale shear bands (Fig. 7d). However, the overall Al and Na content also expands towards values lower than that of shear bands (Fig. 7a). Amp exhibits a patchy or oscillatory zoning, truncated perpendicular to the foliation (white arrows in Fig. 7e and related inset). In Amp grains with high aspect ratios (> 3) Al-rich overgrowths develop parallel to the lineation (black arrows in Fig. 7e). These Al-rich domains correspond to domains enriched in Sr and to a lesser extent in Ba (Fig. 7f and Fig. 7b). Al-rich compositions are also observed in micrometric fractures that cross-cut Amp zoning (red arrows in Fig. 7e and S5c). In mylonitic amphibolite, Amp is also a Mg-hornblende, but with lower contents of Na and Al (0.20 < Na a.p.f.u. < 0.42 and 1
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In both foliated amphibolite and mylonitic amphibolites, Pl exhibits a variety of chemical composition (Fig. 8d-f). Composition of relict porphyroclasts is very close to that of relict Pl₁ in metagabbros, with bimodal anorthite components of labradorite (An₅₈-₆₃) and andesine (An₄₀-₅₀) forming zonation truncated perpendicular to the shortening direction (white arrow in Fig. 8e). More albitic Pl (An₄₅-₅₀; black arrows in Fig. 8d-f and Fig. S4g), reddisher in C.L. (Fig. 11d) is observed replacing or overgrowing fragments of Pl₁, either in crystallographic continuity with Pl₁ (see Fig. 7c-h for comparison) or as fine-grained Pl₂ mantling Pl₁ also exhibiting oscillatory zoning (black arrows in Fig. 8f and S4e and g). These grains correspond to the dislocation-free grains highly misoriented relative to Pl₁ observed at the EBSD. Locally, grains with bytownite composition are observed (XAn₈₀-₉₀), a composition close to that of Pl in symplectites. Such small scale (20-30 µm in size) compositional heterogeneities, including relicts of Pl₁, are preserved even with increasing degree of phase mixing (compare Fig. 8d and e).

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## 5 Discussion

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### 5.1 Local compositional equilibria from fractured domains to macroscopic shear zones

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Several lines of evidence indicate that compositional equilibria were only achieved at short distances, both in fractured domains and shear bands/shear zone, despite (i) high-temperatures conditions (~650°C), (ii) significant strain and (iii) the presence of H₂O (Airaghi et al., 2024). In fractured domains of metagabbros, local compositional equilibria are attested by the widespread preservation of Pl₁, as well as local enrichment of Al in Amp from the cores of Cpx/Ol pseudomorphs to symplectites. The latter may be explained by different proportions of igneous Fe-Mg minerals and Pl involved in the Amp-forming reaction (Santos et al., 2019; Airaghi et al., 2024 and references therein). In mm-scale shear bands, although Amp and Pl are chemically more homogeneous, at least at the scale of the microstructure (e.g., former symplectites), they are heterogeneous at the scale of different domains of 100 to 500 µm thick (Fig. 7c and d). Intra-grain compositional heterogeneities, such as irregular or oscillatory zoning in Pl₁ and Amp, are also present in the highest strained rocks, the mylonitic amphibolite of the macroscopic shear zone. In Pl, such compositional patterns have been interpreted either as the expression of immiscibility between a calcic and sodic Pl at T < 500-600 °C (Mukai et al., 2014) or the activity of grain boundary sliding accommodated by dissolution and reprecipitation (Svahnberg and Piazolo, 2013). Temperature estimates
in this sample is equal to or higher than 600 °C (Airaghi et al., 2024) and Pl lacks the anorthitic or albitic end-members, such as expected in the case of immiscibility after the breakdown of labradoritic Pl₁ (Bøggild or Huttenlocher intergrowth, Carpenter, 1994). Furthermore, Mukai et al. (2014) associate the micrometer-thick zoning to exsolutions normally limited to nanoscale layers that have coarsened under the effect of stress. However, in the samples examined here, grains with oscillatory or complex zoning are the poorest in defects. Instead, replacement textures between Ab-rich Pl₂ over An-rich Pl₂ and relicts Pl₁ indicate that different Pl compositions reflect different P-T metamorphic stages, along with possible compositional variations of the local reactive volume. The same reasoning is applied to Amp for which variations of pressure and temperature can explain the general compositional trend of amphibole from fractured areas to shear zones, but not the small-scale compositional variations within the same microstructure (e.g. macroscopic shear zone). H₂O content in the rock may have an effect on Pl and Amp composition, but only at undersaturated conditions, where Opx and Cpx are also expected to be stable (Airaghi et al., 2024). Instead, as observed in fractured domains, Amp composition may depend on small-scale compositional variations due to varying proportion of Pl and Amp undergoing dissolution and reprecipitation during deformation. This hypothesis is supported by the fact that Al-enriched overgrowths of Ca-Amp are also enriched in Sr and Ba normally hosted by Pl, which points to Amp growth coeval to the breakdown of Pl (Marks et al., 2004). The contemporaneous formation of Al-rich Amp and albitic Pl from Pl₁ implies coeval crystallization of calcic phases locally observed along microcracks. In order to evaluate to which extent variable amount of dissolved Pl₁ may affect Amp composition, we modelled the variation of mineral composition in both foliated and mylonitic amphibolites by changing the composition of the reactive bulk volume. This was achieved in two ways: (i) by arbitrarily modifying the area considered as the bulk rock from compositional maps (Fig. 10a), using the software XMapTools (version 4.3) and add-on Bingo Antidote (Lanari and Duesterhoeft, 2019; Lanari and Hermann, 2021), and then estimating the difference between the measured and calculated mineral composition (*Quality factor of predicted mineral composition* in Fig. 10b-c); and (ii) by considering the bulk composition of two end-member: one made of 100 % of Al-depleted Amp, the other made by 100 % of Ab-rich Pl considered to be in equilibrium in the macroscopic shear zone (Fig. 10d). The participation of Pl₁ to the reaction is expected to further promote the growth of Al-richer Amp. In both cases, Amp composition becomes Al- and Na-rich with the increasing amount of Pl in the reactive volume. The best fit between measured and modelled Amp composition is obtained for ~40 vol. % of Amp in the assemblage (Fig. 10b and c). The or oscillatory zoned overgrowths in elongated Amp grains of macroscopic shear zones were therefore interpreted as due to local variations of the composition of the reactive volume during Amp growth. These features, along with
the preservation of relict compositions, indicate that the extent of compositional equilibrium was as small as 20 µm and no more than 100-200 µm, even in the major shear zones. Such heterogeneities were preserved at grain scale even in the phase-mixed layers. It is of note that the Ca content of modelled Pl also increases with increasing volume of reacting Pl₁ (from 10 to 30 vol % for T > 600°C; Fig. 10d). However, no Pl with An₈₅ was modelled in amphibolite-facies conditions, confirming that An-rich Pl₂ formed at higher temperature conditions (granulite-facies; Airaghi et al., 2024).

<figure><img src="image_12.png" /><figcaption>**Figure 10:** Thermodynamic modelling with varying local bulk composition. (a) Al map in macroscopic shear zone (mylonitic amphibolite, from area in Fig. 8e) where the varying size of the areas considered as reactive volume for calculation in panels b and c (local bulks A and B) is indicated by black arrows. (b) and (c)</figcaption></figure>
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Calculated Amp and Pl composition vs. volume of Amp and Pl (vol %) observed in the bulk area A and B from the map. (d) Variations in Amp and Pl composition with varying amount of Pl or Amp involved in the reaction for foliated amphibolite. Red star: observed composition of Ab-rich Pl₂. Black star: observed composition of Al-poor Amp. Blue star: observed composition of Al-rich Amp overgrowths.

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## 5.2 Deformation mechanisms, grain size reduction and fabric formation

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### 5.2.1 Fractured domains: fracturing, breakdown reactions and symplectite formation

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Microstructural and textural observations show that micro-fracturing was an active mechanism of grain size reduction in both the fractured domains and viscously deformed domains (shear bands and shear zone) formed at temperature conditions ≥ 600°C (Kato et al., 2016; Airaghi et al., 2024). In all structures, fracturing of Pl is attested by Amp filling fractures and Pl₁ porphyroclasts, which are cross-cut by alignments of small Pl₂ grains with similar characteristics to parent grains, although slightly misoriented (Moore et al. 2024). Pl₁ exhibits similar or identical orientation on each side of the fracture. Micro-fractures oriented parallel to shear bands suggest that brittle precursors acted as a source for nucleation of ductile bands. The formation of a microfracture network within the alteration halos parallel to the major outcrop-scale fracture is also compatible with the accommodation of volume changes as amphibolitization reaction progresses from the fracture to the host rock (Grund et al., 2025). Fracturing allows the grain size to be reduced from 500-1000 µm (igneous minerals) to 150-300 µm (Fig. 3b). Metamorphic reaction triggered by fluid percolating in the network of microfractures then led to the nucleation of Amp and Pl₂ in microcracks, as well as to the production of symplectite, giving rise to further grain size reduction to 8-15 µm. This confirms the role of micro-fracturing for triggering local retrogressive reaction, even at high temperature conditions (Getsinger et al., 2013; Airaghi et al., 2024; Carpenter et al., 2025).

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An interesting feature is the lack of any intra-grain substructures in Amp, regardless of the intensity of viscous strain. Strong SPO, aspect ratio and CPO with [001] axes sub-parallel to the direction of the main fracture system, which characterize Amp in microfractures of Pl₁, appears to be instead controlled by the direction of fracture walls. High aspect ratio and strong CPO in Amp replacing Cpx and Ol, as indicated by their [001] axes sub-parallel to the rough alignment of former igneous minerals parallel to the direction of the main fracture system, may be acquired during pseudomorphic replacement reactions (Airaghi et al., 2024; Lee et al., 2022; Mérit et al., 2025). Amp in symplectites shows a fabric similar to the one observed in pseudomorphs, suggesting that textural inheritance from the topotactic replacement of Fe-Mg igneous minerals survived the breakdown reaction and phase
mixing, as observed in Cpx-Pl symplectites after omphacite (Zertani et al., 2024). Secondary clusters observed in the pole figure of Amp CPO within symplectites and Pl₁ cracks may be interpreted as fingerprints of parent grains (either Ol or Pl₁ according to the microstructural position, as in point 5 of Fig. 3c). Pl in the fractured domains lack evidence of plasticity, as well, but it shows a strong CPO in Pl₂ within microcracks due to the crystallographic inheritance from the parent Pl₁ grain (point 5 in Fig. 3c). In contrast, the CPO of Pl₂ in symplectitic microstructures progressively weaken with increasing distance to Pl₁ grain boundary, reconciling observations done in different types of symplectites where Pl may either preserve the CPO of the parent grain or exhibit a very weak CPO (Heidelbach and Terry, 2013; Zertani et al., 2024).

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### 5.2.2 Mm-scale shear bands: breakdown reactions, grain boundary sliding and phase nucleation

Both Amp and Pl show composition and C.L. heterogeneities in mm-scale shear bands, such as observed in the fractured domains. They are therefore interpreted as due to metamorphic reactions replacing Fe-Mg igneous minerals by Amp and giving rise to a complete recrystallization of Pl₁ to Pl₂ (as zone B in Fig. 4a). However, while Amp inherited a clear fabric from symplectites that further strengthen with increasing viscous strain, the fabric of plagioclase weakened as the reaction progressed (Fig. 4c). A stronger CPO of Amp (with respect to the one in fractured domains) and Amp [001] axes oriented roughly parallel to the stretching lineation are both compatible with the activation of (100) <001> dislocation slip system (Díaz Aspiroz et al. 2007; Elyaszadeh et al., 2018). However, the weak intra-grain misorientation and patchy, truncated compositional variations in Amp are better explained by the dissolution of Amp coupled to oriented growth, as well as by phase nucleation. Amp nucleation at Amp-Pl grain boundaries during the formation of shear bands increased the frequency of Amp-Amp grain contacts and allowed grain growth, as well as the alignment of crystallographic axes with the stretching lineation. Amp nucleation in between Pl₂ grains is also required to explain the occurrence of phase mixing (as in Fig. 3e). As already observed experimentally for Ol (Précigout and Stünitz, 2016), dominant grain-size-sensitive (GSS) creep – such as diffusion creep – may explain the lack of plasticity in Pl and partial to complete randomization of the CPO during shearing. The complete phase mixing and lack of grain elongation during GSS creep indicate that grain boundary sliding is significantly involved – and probably dominant – to accommodate strain (Langdon, 2006).

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### 5.2.3 Macroscopic shear zone: dissolution-precipitation and plastic deformation
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In the macroscopic shear zones, differences between foliated amphibolite and mylonitic amphibolite in many aspects including the degree of viscous deformation of Pl and the metamorphic conditions (708 °C vs. 550-678 °C respectively, Kato et al., 2016; Airaghi et al., 2024) led to propose that foliated amphibolite evolved into mylonitic amphibolite during the retrograde metamorphic path (Kato et al., 2016). Decreasing temperature conditions are also consistent with a decrease in Al and Na contents of Amp from foliated to mylonitic amphibolites. Despite these differences, similar textures are observed in monomineralic Amp layers of both microstructures. As in mm-scale shear bands, Amp shows a strong CPO but weak intra-grain deformation, and the preservation of undeformed, straight cracks indicate an overall lack of plasticity (e.g. Stokes et al., 2012). Truncated zoning and lobate Amp grain boundaries suggest instead that dissolution occurred, together with, although less evident, oriented growth. This is attested by (i) Al-rich or oscillatory zoned overgrowth oriented parallel to the stretching lineation in grains with high aspect ratio, (ii) fractured filled with Al-rich Amp cross cutting the truncated oscillatory and (iii) larger Amp grain size than the one described in the other microstructural domains (Fig. 7e and g; Table 1). Amp therefore experienced grain growth in a specific direction (as observed by Stokes et al., 2012; Svahnberg and Piazolo, 2013; Giuntoli et al., 2018, Lee et al., 2022; Mérit et al., 2025), despite the fact that the formation of large grains is expected to favor crystal plasticity (Brodie and Rutter, 1987; Paterson, 1990). With increasing phase mixing, grain size is kept small by phase nucleation coupled to grain boundary pinning, and hence, limited growth, as attested by evidence of compositional changes at the very rims of grains (Olgaard and Evans, 1986; Herwegh et al., 2011; Bercovici and Mulyukova 2018). These observations have been also documented for the sheared Kågen gabbros in Norway (Mérit et al., 2025).

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In contrast, different deformation mechanisms are documented for Pl in the foliated and mylonitic amphibolites. In foliated amphibolite, rare evidence of crystal plasticity in Pl were observed, but the breakdown of Pl₁ to Pl₂ (An- and Ab-rich Pl in mm-thick and foliated amphibolites respectively) with Pl₁ showing lobated rims, particularly perpendicular to the shortening direction. This points to dissolution-precipitation and nucleation of Pl. Dissolution of Pl is also interpreted to be enhanced by fracturing (Stünitz, 1998; Gardner et al., 2021; Moore et al. 2024), which is indicated here by compositional changes in zones of high dislocation densities, as well as the formation of small fragments with similar dislocation densities, orientation and composition of Pl₁, although slightly misoriented (Petley-Ragan et al. 2020; Petley-Ragan et al. 2018; Pittarello et al. 2008).

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Mylonitic amphibolite partly preserved the microstructures observed in the foliated amphibolites but rare evidence of symplectite-derived plagioclase points to limited early fluid rock interactions leading to Pl breakdown. In contrast to foliated amphibolites, in mylonitic amphibolites, monomineralic Pl layers exhibit evidence of plastic

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deformation and polygonal subgrains of similar shape and size than recrystallized grains, as well as asymmetric tails that resemble dynamically recrystallized Pl domains. Plastic deformation in Pl is also supported by a strong CPO, stronger than the one documented for the other microstructural domains (J up to 3.43). However, dynamic recrystallization of Pl, here by subgrain rotation cannot account by itself for both a weak CPO and sharp compositional change of up to 20 % with respect parent grains, i.e. from An-rich Pl₁ to Ab-rich Pl₂ (Kruse et al. 2001; Stünitz et al. 2003; Stünitz, 1998; Rosenberg and Stünitz, 2003), even when considering diffusion along high-angle grain boundaries (Yund and Tullis, 1991). Solid volume inter-diffusion of NaSi and CaAl between grains is also too slow to account for these compositional differences and should result in intermediate composition for recrystallized grains or smooth zoning rather than oscillatory one (Grove et al., 1984). Grain boundary migration may result in such a patchy zoning in some, larger grains (Fig. 8f). However, it can hardly account for the crystallization of new, Ab-rich grains formed in dilatant sites (Fig. 8e) and hence, it needs in any case to be assisted by diffusion processes for grain boundary to migrate towards new, Ab-rich compositions. Furthermore, Ab-rich grains do not show any crystallographic relationships with parent grains (Fig. 6d). In addition, evidence of dissolution, such as local truncated zoning in Pl can be observed in monomineralic layers coupled with widespread nucleation of small, strain-free albitic Pl in pressure shadows and dilatant sites (Fig. 8e and f). A strong SPO and a weak CPO, together with aligned and straight grain boundaries, further indicate a contribution of grain boundary sliding (GBS) to accommodate Pl plasticity, suggesting a shift from grain size insensitive to grains size sensitive creep (Miranda et al., 2016 and references therein). Hence, in the main shear zone, which represents the most deformed rocks of the series, both plastic deformation, dynamic recrystallization and dissolution-precipitation contribute to strain accommodation and reconstruction of Pl. Considering the mylonitic amphibolites as deriving from the foliated one, reacted plagioclase (i.e. Pl₂) layers might be partly inherited from the latter, while higher viscous strain allowed the remnant Pl₁ to deform plastically.

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## 5.3 Reaction-controlled inherited microstructures in ductile shear bands/zones

In terms of microstructures, CPO, fabric, phase distribution, grain size, aspect ratios and mineral (or local) bulk compositions, the similarities between the fractured domains (pseudomorphs, symplectites, microcracks) and shear bands/zone (as zone A and C in Fig. 4a) indicate that textural, compositional features and phase mineral distribution were – at least partly – inherited from local reactional domains resulting from fracture-induced fluid infiltration before or during incipient viscous deformation (as indicated by alignment of Cpx and OI pseudomorphs
in fractured domains). Monomineralic Amp layers in mm-scale shear bands and in macroscopic shear zone were derived from Amp-rich areas replacing Ol and Cpx of metagabbros with small-scale chemical variations preserved (Fig. 11). This feature is attested by Al zoning in Amp that mimics the variations observed from core to rims of the pseudomorphs, as well as the heterogeneous distribution of Cr sensitive to local reservoirs as Ol (Fig. S6). In contrast, mixed Amp-Pl layers with variable amount of Pl and Amp derived from symplectites (Fig. 2b-c and f-g). This is attested by similar Amp and Pl composition, grain size and degree of phase mixing, as well as similarities in the local bulk compositions between different microstructural layers in the mm-scale shear bands and identified precursors of fractured domains (Table 2 and S2).

Phase distribution or segregation is therefore controlled by microstructural inheritance of a previous, heterogeneously distributed reactional stage, rather than strain-related mechanisms resulting from the sole coalescence of Pl and Amp through viscous processes and rheological contrasts (i.e., Kruse and Stünitz, 1999). Because the reaction progressed with increasing viscous strain, within these inherited domains, new reactional domains formed by (i) further nucleation of Amp in former Pl-rich areas, (ii) nucleation of Pl in Amp-dominated reactional domains (Fig. 4e) and (iii) dissolution and oriented growth of Amp in fine-grained mixed layers from symplectitic precursors. Such new reactional domains exhibit a local bulk composition different from all other microstructural precursors, as observed in the macroscopic shear zone (4) in Table 2).

It is of note that inheritance do not only encompass phase distribution and composition. Microstructural and textural observations from the fractured domains to the macroscopic shear zone indicate that the crystallographic fabrics, grain size reduction and SPO of both Amp and Pl were controlled at the first order by metamorphic reactions (Fig. 11). These observations indicate that textural and microstructural characteristics were mainly controlled by phase nucleation and subsequent grain growth; only minor modifications due to viscous strain solely are observed afterward.

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## 5.4 Implication for phase mixing and strain localization

In mm-scale shear bands and foliated amphibolites where evidence of early fracturing, fluid-induced reaction and symplectite formation is observed, phase mixing is also observed. In contrast, in the mylonitic amphibolite where no or little evidence of precursors are observed, intimate phase mixing is hardly achieved despite high viscous strain (Fig. 11). This suggests that phase mixing in the studied rocks needs to be initiated by fluid-induced symplectite formation, and hence that plastic deformation or syn-kinematic phase nucleation solely
637 without any breakdown reaction of igneous minerals was not efficient enough to form mixed-phase layers at
638 millimeter scale. Lack of symplectite precursors in mylonitic amphibolite may also be due to lower amounts of Ol
639 in the protolith, the main precursor of symplectites. Fine-grained, mixed-phase symplectites may thus promote
640 grain-size-sensitive creep mechanisms when sheared, giving rise to further phase nucleation in between existing
641 phases, possibly through grain boundary sliding and creep cavitation (Fusseis et al., 2009; Précigout et al., 2017).

642 This favours strain localization on reacted, damaged zones of metagabbros and amphibolites. Such a mechanisms
643 has been also observed in granitic mylonites where pre-kinematic hydration pre-conditioned the distribution of
644 subsequent viscous strain (Airaghi et al., 2020; Alaoui et al., 2023) and where the presence of fine-grained mixed-
645 phase myrmekite promoted strain localization in shear bands (Ceccato et al., 2018).

<figure><img src="image_13.png" /><figcaption>**Figure 11:** Sketch summarizing the macrostructural, microstructural and textural evolution from fractured and reacted domains to the major shear zones, from outcrop- to micrometric-scales.</figcaption></figure>

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**6 Conclusions**

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From the fractured domains to mm-scale shear bands and meter-scale shear zones, mafic metagabbros, foliated
and mylonitic amphibolites from the Poroshiri ophiolite (Hokkaido, Japan) preserve heterogeneous
microstructures and mineral composition far from bulk equilibrium, despite high-temperature conditions (~650
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°C) and high strain. Combined compositional, microstructural and textural observations point to the following conclusions:

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* In all microstructures, irrespective of viscous strain, compositional equilibria occurred over 100-200 µm length, as attested by the preservation of igneous Pl₁ and granulitic An-rich Pl₂, Amp compositional variations among different reactional domains and simple or oscillatory zoned overgrowths oriented parallel to the stretching lineation. Trace element analyses (Sr, Ba) and thermodynamic modelling on local reactive bulk compositions indicate that such a zoning pattern in Amp was due to varying proportion of coevally reacting Pl.

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* Microstructural and compositional domains in shear bands and shear zone were inherited from local reactional domains formed during fracturing-induced hydration, before or during incipient viscous strain. Monomineralic amphibole layers were derived from amphiboles replacing aggregates of Fe-Mg minerals in metagabbros, with small-scale chemical variations preserved. Mixed-phase layers were derived from symplectites and monomineralic plagioclase layers from plagioclase-rich domains that experienced limited early plagioclase breakdown.

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* In monomineralic layers, Amp accommodated strain mainly by dissolution, precipitation and oriented growth. The fabric of Amp was acquired during early hydration of metagabbros and was then slightly modified with increasing viscous strain (transposition of the CPO and SPO into the stretching lineation). The fabric of plagioclase, whose reaction was incomplete in fractured domains, continued to evolve with increasing strain and reaction progress in phase-mixed layers. In monomineralic layers, Pl accommodates strain by dominant dissolution and precipitation in amphibolites experiencing early plagioclase breakdown (foliated amphibolites) and by coeval plastic deformation and dissolution-precipitation in amphibolites where igneous plagioclase was largely preserved and viscous strain was high (mylonitic amphibolites).

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* Intimate phase mixing in shear bands/zone was achieved only where relicts of symplectites were observed, indicating that phase mixing was initiated by reaction-induced, fine-grained symplectites. Symplectites then further promoted GSS creep and grain boundary sliding when sheared in localized shear bands/zones.

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* Fracturing precursors and the formation of symplectite before and during viscous strain were key to transform the protolith into a heterogeneous patchwork of locally reacting domains with different
683 reactivities. They gave rise to different grain size and degree of phase mixing, prior to major viscous

684 deformation, initiating GSS creep and subsequent strain localization on reacted and/or damaged zones.

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686

687 Author contribution

688 L. Airaghi: conceptualization, data acquisition, formal analysis, data interpretation, writing with contribution from

689 all co-authors. H. Raimbourg: conceptualization, formal analysis. J. Précigout: data acquisition, validation.

690 Conflicts of interest

691 The authors declare that they have no conflict of interest.

692

693 Data availability

694 EBSD data and composition maps used for this study are available from Airaghi (2025)

695

696 Acknowledgements

697 We acknowledge the support from the LabEx VOLTAIRE (ANR-10-LABX-100-01). This project

698 benefited from the expertise and the facilities of the Platform MACLE - CVL which was co-funded by

699 the European Union and Centre-Val de Loire Region (FEDER). This work also benefitted of the

700 Axioscan equipement in BRGM (Orléans, France), supported by a grant of the French government,

701 managed by the National Research Agency (ANR), under the *Investissements d'Avenir* program as part

702 of the IMAGINE² project (reference: ANR-21-ESRE-0043). We sincerely acknowledge Kyu

703 Kanagawa, anonymous reviewer and editor for the constructive comments and reviews. We also

704 acknowledge Ida di Carlo (ISTO, Orléans) and Saskia Erdmann for their help with microprobe, SEM

705 and SLA-ICPMS as well as Laurent Arbaret, Guillaume Richard (ISTO) and Benoît Bévillard for their

706 implication in the field.

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708

709 Supplementary material
710

**Figure S1** Microscopic view of amphibolites with mm-scale shear bands H16-45 (a-b) of the foliated H16-33 (c-d) and mylonitic amphibolite H16-31 (e-f), either observed under plane-polarized light (a, c and e) or cross-polarized one (b, d, f) at the optical microscope.

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**Figure S2** Crystallographic preferred orientation (CPO) of Amp and Pl nucleating in microcracks of Pl₁ (enlarged view of Pl₁ in fractured domains of Fig. 3a-b). (a) Phase map around a microfracture perpendicular to the main direction of the fracture network. (b) Phase map around a micro-fracture parallel to the main direction of the fracture network. (c) CPO of Amp (black) and Pl (blue) within each type of microcrack.

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**Figure S3** Phase maps for all analyzed areas in the foliated and mylonitic amphibolites and related CPOs of plagioclase and amphibole. Dashed white area in panel a indicates the grains considered to produce the CPO of plagioclase.

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**Figure S4** Microstructures, textures and chemical composition of foliated amphibolite H16-33. (a) IPF map of Amp in a monomineralic layer. (b) Intra-grain lattice misorientation in Amp (same area of panel a). (c) Aspect ratio of Amp (same area of panel a). (d) Intra-grain misorientation for Pl monomineralic layers. (e) XAnorthite in Pl (see location in panel d). Black arrows show grains with oscillatory zoning. (f) Geometrically necessary dislocation densities in an area of Pl₁ recrystallization (same area of Fig. 5e of the main text). (g) Anorthite content in Pl (same area of panel e).

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**Figure S5** Microstructures, textures and chemical composition of mylonitic amphibolite H16-31. (a) IPF map of Amp in monomineralic layers. (b) Amp aspect ratio (white rectangle in panel a). (c) Al₂O₃ (wt.%) content in Amp (same area of panel b). White arrows: Al-rich domains. Red arrows: micro-fracture crosscutting Amp zonation, filled with Al-rich Amp. (d) IPF of Pl in Pl monomineralic-layers (same area of Fig. 6a). (e) Intra-grain lattice misorientation in Pl (same area of panel d). (f) IPF map of Amp in mixed Amp-Pl layers. (g) IPF map of Pl (same area of panel f).

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**Figure S6** Map of the chromium content in Amp of fractured domain (a) and of mm-scale shear band (b).

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**Figure S7** Trace element maps (Sr, Ba) compared to major element distribution (Ca, Al) in Amp for (a) a mm-scale shear band and (b) the macroscopic shear zone (i.e., Amp monomineralic layers).

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**Figure S8** Layers of different mineralogy and grain size in a highly-strained mm-scale shear band (Fig. 2d; see also Airaghi et al., 2024) inherited from the distribution of former igneous minerals (as olivine) now composed of breakdown reaction products.

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737
738 **Table S1** Representative mineral composition for Pl and Amp in different samples and microstructural positions
739 (modified from Airaghi et al., 2024).

740 **Table S2** Local bulk composition of different mineralogical domains (1 to 3 of Fig. S8b) in an olivine-rich, high-grade mm-scale shear band (Fig. S8a-b) compared to local bulk compositions of reaction domains in fractured areas (example for olivine pseudomorphs and symplectites).

743
744
745 **References**
746
747 Airaghi, L. (2025). Microstructural, compositional and textural inheritance in deformed mafic amphibolites [Dataset]. Entrepot.recherche.data.gouv-fr. 10.57745/9VPEE1

748
749 Airaghi L, Bellahsen N, Dubacq B, et al. Pre-orogenic upper crustal softening by lower greenschist facies metamorphic reactions in granites of the central Pyrenees. *J Metamorph Geol.* 2020; 38: 183–204.
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752 Airaghi, L., Raimbourg, H., Toyoshima, T., Jolivet, L., & Arbaret, L. (2024). Hydration of mafic crustal rocks at high temperature during brittle-viscous deformation along a strike-slip plate boundary. *Journal of Metamorphic Geology*, 42(8), 1035–1067. https://doi.org/10.1111/jmg.12791

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767 Bunge, H.J. (1982). Texture Analysis in Materials Science: Mathematical Methods. Elsevier

768 Carpenter, M. A. (1994). Subsolidus phase relations of the plagioclase feldspar solid solution. In: Parsons, I. (ed.)
769 Feldspars and their Reactions. Dordrecht: Kluwer Academic, pp. 221-269.

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1006
grain size and aspect ratio for Amp and Pl in different locations in the three structural domains: fractured domains, mm-scale shear bands and macroscopic shear zone (both foliated and mylonitic amphibolites). Zones A, B and C for mm-scale shear band referres

<table><thead><tr><th rowspan="3"></th><th colspan="10">AMPHIBOLE</th><th colspan="10">PLAGIOCLASE</th></tr><tr><th colspan="5">GRAINSIZE (eq. Diameter in µm)</th><th colspan="5">ASPECT RATIO</th><th colspan="5">GRAINSIZE (eq. Diameter in µm)</th><th colspan="5">ASPECT RATIO</th></tr><tr><th>Mean</th><th>Median</th><th>Mode</th><th>Std</th><th>Max</th><th>AR>3*</th><th>Mean</th><th>Median</th><th>Mode</th><th>Std</th><th>Grains</th><th>Mean</th><th>Median</th><th>Mode</th><th>Std</th><th>Max</th><th>Mean</th><th>Median</th><th>Mode</th><th>Std</th><th>Grains</th></tr></thead><tbody><tr><td><strong>Fractured domains</strong></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>In pseudomorphs</td><td>14.5</td><td>11.6</td><td>1.1</td><td>11</td><td>90</td><td>17.41</td><td>2.3</td><td>2.0</td><td>1.2</td><td>1.2</td><td>839</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Pseudomorphs rims</td><td>20.8</td><td>13.1</td><td>3.5</td><td>20</td><td>155</td><td></td><td>1.8</td><td>1.7</td><td>1.5</td><td>0.6</td><td>525</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Symplectites</td><td>14.0</td><td>11.1</td><td>1.0</td><td>9.7</td><td>48</td><td>15.83</td><td>2.0</td><td>1.7</td><td>1.4</td><td>1.0</td><td>136</td><td>14.8</td><td>12.2</td><td>8.7</td><td>10.2</td><td>56</td><td>2.1</td><td>1.8</td><td>1.8</td><td>1.0</td><td>278</td></tr><tr><td>Plagioclase fractures</td><td>8.8</td><td>6.9</td><td>3.5</td><td>5.6</td><td>36</td><td>11.09</td><td>2.1</td><td>1.8</td><td>1.5</td><td>1.1</td><td>533</td><td>8.4</td><td>6.8</td><td>3.7</td><td>4.8</td><td>30</td><td>2.3</td><td>1.9</td><td>1.1</td><td>1.5</td><td>745</td></tr><tr><td><strong>Mm-scale shear band</strong></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Monomineralic layers (zone A)</td><td>18.5</td><td>12.8</td><td>3.7</td><td>21</td><td>181</td><td>18.86</td><td>2.0</td><td>1.8</td><td>1.2</td><td>0.8</td><td>448</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Mixed-phase layers (zone B)</td><td>14.5</td><td>9.5</td><td>3.9</td><td>15</td><td>130</td><td>17.14</td><td>2.0</td><td>1.8</td><td>1.4</td><td>0.7</td><td>771</td><td>9.9</td><td>7.5</td><td>4.1</td><td>7.9</td><td>67</td><td>2.0</td><td>1.8</td><td>1.2</td><td>1.0</td><td>1436</td></tr><tr><td>Mixed-phase layers (zone C)</td><td>11.0</td><td>9.4</td><td>1.0</td><td>7.3</td><td>59</td><td>8.95</td><td>1.9</td><td>1.7</td><td>1.4</td><td>0.8</td><td>198</td><td>8.9</td><td>6.6</td><td>4.1</td><td>6.4</td><td>46</td><td>1.9</td><td>1.7</td><td>1.2</td><td>0.9</td><td>510</td></tr><tr><td><strong>Macroscopic shear zone</strong></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Foliated amp Monomineralic layers</td><td>29.9</td><td>11.9</td><td>5.0</td><td>49</td><td>374</td><td>34.7</td><td>2.2</td><td>2.0</td><td>1.7</td><td>0.9</td><td>402</td><td>14.5**</td><td>10.8</td><td>1.8</td><td>13.6</td><td>126</td><td>1.8</td><td>1.6</td><td>1.3</td><td>0.6</td><td>1370</td></tr><tr><td>Foliated amp Mixed-phase layers</td><td>28.2</td><td>9.0</td><td>17.8</td><td>31</td><td>248</td><td>41.0</td><td>2.1</td><td>1.9</td><td>1.1</td><td>0.7</td><td>756</td><td>18.4**</td><td>12.7</td><td>7.7</td><td>17.8</td><td>214</td><td>1.8</td><td>1.7</td><td>1.5</td><td>0.6</td><td>1016</td></tr><tr><td>Mylonitic amp Monomineralic layers</td><td>20.0</td><td>12.0</td><td>3.9</td><td>25</td><td>266</td><td>27.0</td><td>2.4</td><td>2.1</td><td>1.8</td><td>1.1</td><td>1014</td><td>12.5**</td><td>8.5</td><td>3.9</td><td>14.1</td><td>233</td><td>2.0</td><td>1.8</td><td>1.4</td><td>0.8</td><td>2130</td></tr><tr><td>Mylonitic amp Mixed-phase layers</td><td>16.7</td><td>10.8</td><td>3.7</td><td>19</td><td>218</td><td>21.5</td><td>2.5</td><td>2.3</td><td>1.4</td><td>1.0</td><td>562</td><td>12.6**</td><td>8.6</td><td>3.6</td><td>11.9</td><td>98</td><td>2.4</td><td>2.1</td><td>1.4</td><td>1.1</td><td>733</td></tr></tbody></table>

<footnote>* mean grainsize for grains with aspect ratio > 3</footnote>

<footnote>**included Pl₁ relicts</footnote>
<table><caption>**Table 2.** Local bulk composition for different reaction areas in fractured domains, mm-scale shear bands and macroscopic shear zone. "psudom": pseudomorphs, "sym": symplectite. "mono": monominerali layers. Same colors and numbers ((1), (2) etc.) are used for domains that shares similar local bulk compositions.</caption><thead><tr><th rowspan="3">Area<br>Label</th><th colspan="3">Fractured domains</th><th colspan="2">Mm-shear bands</th><th colspan="2">Macroscopic shear zone</th></tr><tr><th>Ol pseudom</th><th>Cpx pseudom</th><th>Pl-Amp symp</th><th>Amp-mono</th><th>Amp-Pl</th><th>Amp mono</th><th>Mixed layers</th></tr><tr><th>(1)</th><th>(2)</th><th>(3)</th><th>(1)</th><th>(3)</th><th>(2)</th><th>(4)</th></tr></thead><tbody><tr><td>SiO2</td><td>43.87</td><td>45.90</td><td>41.03</td><td>40.37</td><td>41.75</td><td>49.44</td><td>46.03</td></tr><tr><td>TiO2</td><td>0.21</td><td>0.24</td><td>0.00</td><td>0.33</td><td>0.00</td><td>0.78</td><td>0.91</td></tr><tr><td>Al2O3</td><td>12.71</td><td>6.66</td><td>22.27</td><td>14.40</td><td>23.20</td><td>5.68</td><td>16.00</td></tr><tr><td>FeO</td><td>13.44</td><td>10.71</td><td>7.44</td><td>7.97</td><td>3.71</td><td>8.85</td><td>8.42</td></tr><tr><td>MnO</td><td>0.23</td><td>0.48</td><td>0.31</td><td>0.40</td><td>0.27</td><td>0.00</td><td>0.28</td></tr><tr><td>MgO</td><td>13.84</td><td>15.55</td><td>9.48</td><td>12.36</td><td>5.49</td><td>14.64</td><td>9.21</td></tr><tr><td>CaO</td><td>11.16</td><td>9.07</td><td>11.36</td><td>9.66</td><td>11.68</td><td>13.47</td><td>11.60</td></tr><tr><td>Na2O</td><td>1.99</td><td>1.22</td><td>2.02</td><td>1.86</td><td>1.73</td><td>0.65</td><td>5.97</td></tr><tr><td>K2O</td><td>0.10</td><td>0.10</td><td>0.16</td><td>0.00</td><td>0.00</td><td>0.04</td><td>0.12</td></tr></tbody></table>

<figure><img src="image_14.png" /></figure>
## Highlights

* Compositional equilibria occur over ~200 µm regardless of viscous strain intensity

* Mineral fabrics and grain size are acquired during local metamorphic reaction

* Reactional domains formed during early fracturing are preserved in shear bands

* Intimate phase mixing in shear bands requires formation of symplectites beforehand

<figure><img src="image_15.png" /></figure>
Conflict of interest

The authors have no conflicts of interest to declare