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\DOI{10.5802/crgeos.317}
\datereceived{2025-07-15}
\daterevised{2025-10-15}
\dateaccepted{2025-11-07}
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\begin{noXML}

\CDRsetmeta{articletype}{research-article}

\TopicFR{P\'etrologie, p\'{e}trophysique}
\TopicEN{Petrology, petrophysics}

\title{Natural deformation processes of lower crustal prismatic
sillimanite under high to ultra-high temperature conditions highlighted
by EBSD investigation}

\alttitle{Processus de d\'eformation naturelle de la sillimanite
prismatique en conditions de haute \`a ultra-haute temp\'erature mis en
\'evidence par l'utilisation de l'EBSD}

\author{\firstname{Alain} \lastname{Vauchez}\CDRorcid{0000-0003-4127-8976}\IsCorresp}
\address{Geosciences Montpellier, Montpellier University and CNRS,
34095 Montpellier, France}
\email[A. Vauchez]{alain.vauchez@umontpellier.fr}

\author{\firstname{Jean-Marie} \lastname{Dautria}}
\addressSameAs{1}{Geosciences Montpellier, Montpellier University and
CNRS, 34095 Montpellier, France}
%\email{jean-marie.dautria@orange.fr}

\author{\firstname{Abdelkader} \lastname{El Maz}}
\address{Department of Geology, University of Moulay Isma\"{i}l,
Meknes, Morocco}
%\email{elmazabdel@yahoo.fr}

\shortrunauthors

\keywords{\kwd{Prismatic sillimanite}\kwd{High
temperature}\kwd{Deformation mechanisms}\kwd{Dislocation
creep}\kwd{Dynamic recrystallization}\kwd{Paragenesis control}}

\altkeywords{\kwd{Sillimanite prismatique}\kwd{M\'ecanismes de
d\'eformation}\kwd{Haute
temp\'erature}\kwd{Fluage-dislocation}\kwd{Recristallisation
dynamique}\kwd{Contr\^ole par la parag\'en\`ese}}

%\thanks{}

\begin{abstract}
Quartzo-feldspathic and restitic granulites from the Middle Atlas lower
crust (Morocco) contain prismatic sillimanite crystals up to $2 \times 1$ mm
large, deformed under high temperature ($T \geq 850$ {\textdegree}C)
and moderate pressure ($P \sim 0.9$ GPa) conditions. EBSD mapping reveals a
heterogeneous deformation of sillimanite crystals in the
quartzo-feldspathic samples depending on the minerals surrounding them.
When they are included in sillimanite aggregates or constrained by
large garnet grains, sillimanite deformation is strong and frequently
reach dynamic recrystallization. Contrastingly, sillimanite crystals
are significantly less deformed or even undeformed when isolated in the
strongly deformed quartzo-feldspathic matrix. In the restitic samples
almost all sillimanites display a homogeneous deformation but less
frequent recrystallization. Deformed sillimanites display
microstructures typical of dislocation-creep processes: misorientation,
subgrain boundaries and subgrains, which point to recrystallization
through subgrain rotation. Subgrain boundaries misorientation
dominantly results from rotation around [100]. Crystallographic
preferred orientation displays a concentration of [001] around the
lineation. It is stronger in the restitic than in the
quartzo-feldspathic samples, probably due to the heterogeneity of
deformation in the latter. These data substantiate (010)[001] as the
dominant dislocations slip system. This agrees with previous
experimental deformation and transmission electron microscope
observations. Our results differ from those obtained on fibrolitic
sillimanite, which support (001)[010] and (001)[100] as dominant slip
systems in large grains and a major effect of initial grain orientation
relative to lineation. This difference may result from the contrasted
$P$--$T$ conditions of deformation: ${\geq}$850 {\textdegree}C and 
${\sim}$0.9 GPa for the Middle Atlas granulites, ${\sim}$700
{\textdegree}C and \mbox{${\sim}$0.45 GPa} for the West Greenland ones.

\end{abstract}

\begin{altabstract}
Les x\'enolites de granulites quartzo-feldspathiques et restitiques
remont\'es en surface depuis la cro\^ute inf\'erieure par les volcans
plio-quaternaires du Moyen Atlas (Maroc) contiennent des cristaux de
sillimanite prismatique pluri-millim\'etriques, d\'eform\'es \`a haute
temp\'erature ($T \geq 850$ {\textdegree}C) et sous pression
mod\'er\'ee ($P=\text{0,9}$ GPa). Dans les granulites
quartzo-feldspathiques, la cartographie EBSD r\'ev\`ele une
d\'eformation h\'et\'erog\`ene des sillimanites en fonction des
min\'eraux qui les entourent. Lorsquelles sont regroup\'ees en
agr\'egats ou comprim\'ees par de gros grains de grenat, la
d\'eformation des sillimanites est forte et atteint fr\'equemment la
recristallisation dynamique. Par contre, lorsquelles sont isol\'ees
dans la matrice quartzo-feldspathique fortement d\'eform\'ee, les
sillimanites sont significativement moins d\'eform\'ees, voire non
d\'eform\'ees. Dans les \'echantillons restitiques, la plupart des
sillimanites pr\'esentent une d\'eformation homog\`ene mais une
recristallisation moins fr\'equente. Dans les deux types de granulites,
les sillimanites d\'eform\'ees pr\'esentent des microstructures
typiques des processus de fluage-dislocation : extinctions ondulantes,
sous-joints et sous-grains, qui indiquent une recristallisation par
rotation des sous-grains. La d\'esorientation associ\'ee aux
sous-joints r\'esulte principalement dune rotation autour de [100].
Lorientation cristallographique est caract\'eris\'ee par une
concentration de [001] autour de la lin\'eation. Elle est plus forte
dans les \'echantillons restitiques que dans les \'echantillons
quartzo-feldspathiques, probablement en raison de
lh\'et\'erog\'en\'eit\'e de la d\'eformation dans ces derniers. Ces
donn\'ees confirment (010)[001] comme le syst\`eme de glissement des
dislocations dominant. Cela concorde avec les observations
ant\'erieures obtenues par d\'eformation exp\'erimentale et au
microscope \'electronique en transmission. Nos r\'esultats diff\`erent
de ceux obtenus sur la sillimanite fibrolitique, qui ont conduit \`a
consid\'erer (001)[010] et (001)[100] comme les syst\`emes de
glissement dominants dans les gros grains, ainsi qu'un effet majeur de
lorientation initiale des grains par rapport \`a la lin\'eation. Cette
diff\'erence peut r\'esulter des conditions $P$--$T$ contrast\'ees lors
de la d\'eformation entre les granulites du Moyen Atlas (${\geq}$850
{\textdegree}C, 0,9 GPa) et celles de louest Groenland (${\sim}$700
{\textdegree}C et ${\sim}$0,45 GPa).
\end{altabstract}

\maketitle

%\vspace*{4pt}

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\end{noXML}

\defcitealias{ElMazetal2025}{ibid.}
\defcitealias{PiazoloJaconelli2014}{ibid.}

\section{Introduction}

Sillimanite is one of the three natural polymorphs of aluminosilicate
(Al$_{2}$O[SiO$_{4}$]), the other two \mbox{being} andalusite and
kyanite. It usually occurs in regional metamorphic rocks derived from
Al-rich pelites and psammites or aluminous anatectic orthogneiss. More
seldom, it may also appear in High Temperature (HT) hornfels
\citep[e.g.,][]{Deeretal2013}. It is a nesosilicate which crystallizes
in the orthorhombic system and it occurs in two forms, either as
bundles of undulating and twisted fine fibers, called fibrolites, or as
slender elongated prisms with a nearly square basal section and a
well-marked cleavage parallel to \{010\}. The shape of sillimanite
crystals depends on the crystallization conditions, but in all case,
this mineral is an indicator of high-temperature metamorphism. The
fibrolite variety occurs in metamorphic rocks at the upper limit of
medium-grade amphibolite facies \citep[e.g.,][]{Guidotti1970}. The
prismatic variety characterizes rocks belonging to HT to UHT granulitic
facies. As shown in
\citet[][Figure~12]{Suzukietal2025}, in such conditions, sillimanite can
crystallize during prograde evolution (e.g., through the breakdown of
muscovite${+}$quartz) and retrograde one \citep[through the breakdown of
garnet; e.g.,][]{Kerrick1987, ElMazGuiraud2001, GuidottiJohnson2002}.

In a recent paper \citep{ElMazetal2025}, we described a set of
prismatic sillimanite-bearing quartzo-feldspathic and refractory
metapelitic granulite xenoliths from a Moroccan Quaternary tuff ring
(Tafraoute, 33{\textdegree}31$'$29$''$N--4{\textdegree}41$'$35$''$W,
550~Ka) which belongs to the Middle Atlas alkaline volcanic district.
The setting of the collected xenoliths has been previously described in
details in \citet{ElMessbahietal2015,ElMessbahietal2020}
and \citet{ElMazetal2025}.\ In addition to their richness in prismatic
sillimanite (up to more than 50\% in the case of some refractory
samples) whose size reaches 2~mm long and 1~mm wide, these granulites
contain garnet, orthopyroxene, spinel, ilmenite, rutile, osumilite and
corundum are also present but in very minor amounts. The
\citetalias{ElMazetal2025} thermobarometric study of quartzo-feldspathic
granulites supports that the lower crust of Middle Atlas underwent
several successive metamorphic episodes. $P$--$T$ conditions of the
first event ($1.1 \pm 0.1$~GPa and 850--880~{\textdegree}C), likely
hercynian, were estimated in sample G3, selected for the present study,
using the composition of feldspars included in garnet and of the garnet
core. Then, temperature and pressure estimated using the Opx-Grt
geothermobarometer for Opx of coronas in contact with the rims of
garnets substantiate a progressive heating up to ultra-high
Temperatures ($T = 1050 \pm 50$~{\textdegree}C) under relatively
moderate Pressure ($P = 0.9 \pm 0.1$~GPa) that triggered anhydrous
partial melting. At the beginning of this heating, responsible for the
formation of prismatic sillimanites, the lower crust \mbox{underwent} a
transtensional deformation likely responsible for the deformation of
sillimanites \citepalias{ElMazetal2025}. In addition, temperatures between
720 and 830~{\textdegree}C have been estimated from the exchange of Ca,
Na and K between plagioclase and K-feldspars in the matrix of
quartzo-feldspathic samples. These lower temperatures likely correspond
to post-alpine cooling before the extraction of the xenoliths during
the volcanic episode. The sillimanite prisms from these granulites are
more or less deformed depending on the samples and their large sizes
make observation and study of this deformation with optic microscope
and Electron BackScattered Diffraction (EBSD) rather easy.

Sillimanite-bearing samples from various geodynamic contexts have been
studied.\ In most of them, prismatic sillimanite did not recorded
post-crystallization deformation.\ Therefore, only few studies have been
carried to characterize the deformation mechanisms of this mineral.\ 
Among them, \citet{DoukhanChristie1982} experimentally deformed
monocrystals of prismatic sillimanite under temperature of
900~{\textdegree}C, pressure of 1.1~GPa, and strain rate of
$10^{-5}~\mathrm{s}^{-1}$ and studied the deformed prisms in a
Transmission Electron Microscope (TEM). They concluded that [001]
glides in (100) and (010) are the easiest glide systems, even if the
deformed samples were cut with an orientation that favor activation of
the [100](010) and [010](100) slip systems. In 1990, Lambregts and Van
Roermund published a detailed TEM study of prismatic sillimanite from
South Norway, deformed under \mbox{$P$--$T$} conditions of 0.6~GPa and
750~{\textdegree}C. They showed that under such conditions, prismatic
sillimanite deforms plastically with (100)[00l] being the dominant and
(001)[l00] the subordinate slip-systems. More recently,
\citet{PiazoloJaconelli2014}, using the EBSD techniques, published a
detailed study of fibrolitic sillimanite deformation mechanisms in an
Archean paragneiss from southern West Greenland deformed at
${\sim}$700~{\textdegree}C and ${\sim}$0.45~GPa. They showed that the
(001)[010] and (001)[100] slip systems are dominant and that (010)[001]
and (100)[001] are also activated in needles initially oriented with
[001] parallel to the lineation. They also concluded that grain
boundary sliding was activated and was dominant in the outer parts of
fine-grained clusters.

Our study provides new data on natural deformation of prismatic
sillimanite under HT to UHT conditions.\ They can be compared to the
results of \mbox{experimental} deformation of
\citet{DoukhanChristie1982} as well as those of TEM studies of
monocrystals \citep[e.g.,][]{LambregtsVanRoermund1990}. Our data are
statistically better representative of the deformation of this mineral
at the rock scale.\ They also allow to consider the effect of the
surrounding mineral paragenesis on the behavior of prismatic
sillimanite crystals. The recent improvements of the EBSD technique and
of data processing, especially using the MTEX toolbox in MATLAB
\citep[][\url{https://mtex-toolbox.github.io/}]{HielscherSchaeben2008,
Bachmannetal2010, Bachmannetal2011, Mainpriceetal2014} allowed us to
better determine crystallographic orientations including those of tiny
grains and also intracrystalline misorientations and associated
microstrucures. In addition, we can compare dominant deformation
mechanisms of prismatic sillimanite at high temperature with those of
fibrolitic sillimanite at lower temperature revealed by
\citet{PiazoloJaconelli2014}.

\section{Methodology} \label{sec2}

\looseness=-1
Among the samples described in \citet{ElMazetal2025}, we have selected
two sillimanite-rich granulites for detailed study of the sillimanite
deformation mechanisms using Electron Backscattered Diffraction (EBSD):
one quartzo-feldspatic (G3) and one quartz and feldspar free restitic
(TAF501). The size of sample G3 (${\sim}$75~cm long) allowed to
accurately determine the macroscopic lineation and foliation; then to
cut two well-oriented thin sections, the first one (G3xz) parallel to
the lineation ($X$ structural axis) and to the pole to foliation plane
($Z$ structural axis), i.e., in the $XZ$ structural plane, and the
second (G3yz) perpendicular to the lineation, i.e., in the $YZ$
structural plane. For the refractory TAF501, the foliation and
lineation were more difficult to recognize and only one thin section
was cut almost in the $XZ$ plane.

After detailed observation under the optical microscope, carefully
polished thin sections of these samples have been analyzed using the
EBSD technique for detailed crystallographic mapping and measurements
to characterize intracrystalline microstructures and mechanisms active
during deformation. Indexing of EBSD patterns was performed in a
Camscan Crystal Probe X500FE scanning \mbox{electron} microscope (Geosciences
Montpellier, University of Montpellier, France) equipped with EBSD and
energy dispersive spectroscopy (EDS) detectors, both controlled by the
AZtecHKL acquisition software (Oxford Instruments). Acquisition of
diffraction patterns was performed with a working distance of 25~mm and
an acceleration voltage of 18 to 21~kV. To map the whole thin sections,
we use a grid step size of 10~$\upmu$m for the two G3 and 15~$\upmu$m
for TAF501. The two detailed maps on G3xz and G3yz were performed with
a grid step size of 5~$\upmu$m and 4~$\upmu$m respectively. These last
step sizes have been chosen to obtain a precise description of all
crystals, including the smallest ones, and of their internal structure.
The raw indexation rate varies from 75\% to 82\%. To improve the
indexation rate and rebuilt more crystals, we processed raw data using
Channel~5 software (Oxford Instruments) and the MTEX toolbox in MATLAB
\citep[\url{http://mtex-toolbox.github.io/},][]{HielscherSchaeben2008,
Bachmannetal2010, Bachmannetal2011, Mainpriceetal2014} following the
procedure described in detail in \citet{Baptisteetal2015}. With MTEX,
we also computed the orientation distribution functions (ODFs) using
the ``de la Vall\'{e}e Poussin'' kernel function (half-width of
10{\textdegree}), the Crystallographic Preferred Orientation (CPO) and
intragranular misorientations parameters for sillimanite. These
misorientations result essentially from the accumulation of
geometrically necessary dislocations; they were quantified using the
Kernel Average Misorientation (KAM, a measure of the mean
misorientation around a measured pixel) and the local misorientation
relative to the mean orientation of the grain (Mis2Mean).

\section{Microstructures} \label{sec3}

Detailed mineralogical and phases chemical composition of both G3 and
TAF501 were published in \citet{ElMazetal2025}. Figure~\ref{fig1} shows
the EBSD maps of the whole studied thin sections. In addition to
prismatic sillimanite, only major phases that may have influenced its
deformation are considered in this section. 

\begin{figure*}
\includegraphics{fig01}
\caption{\label{fig1}EBSD maps of the three thin sections used in this
study to determine deformation mechanisms in prismatic sillimanite. G3
is representative of the quartzo-feldspatic granulites and TAF501 of
the restitic ones. Each color corresponds to a mineral phase (color
chart under the maps) except the black color that identifies
not-indexed pixels. Light purple lines inside sillimanite crystals
(aqua blue) represent subgrain boundaries. The grid step size is
10~$\upmu$m for G3xz and G3yz and 15~$\upmu$m for TAF501. Better
quality images are available as Figures~S1--S3 in Supplementary
material.}
\end{figure*}

Both studied samples contain prismatic sillimanites up to 2~mm long. In
TAF501, most of them display subgrain boundaries and only few are
partially recrystallized while, in G3, a larger proportion of
sillimanite crystals are partially to totally recrystallized.

The G3 microstructure in both sections (G3xz and G3yz,
Figure~\ref{fig1}) is heterogeneous, especially garnets and
sillimanites display local concentrations in layers elongated parallel
to the foliation. In addition, plagioclase form monomineralic narrow
layers (up to 1~mm thick) in which grains display re-equilibrated
boundaries with straight segments and 120{\textdegree} triple
junctions.\ These plagioclase layers alternate with fine-grained quartz
layers (0.1 to 0.4~mm thick) that contain many elongate rectangular
crystals (platten-quartz). Fine-grained K-feldspar is more dispersed
through the thin sections and some grains display \mbox{interstitial} shape or
form films between grains of other phases. In both planes, garnet shape
is ovoid with its maximum length parallel to the direction of the
foliation. The garnet size is slightly smaller in the $YZ$ (up to
$5\times 3$~mm) than in the $XZ$ (up to $6\times 4$~mm) planes. All
crystals are surrounded by narrow coronas constituted of undeformed
tiny grains of Opx ${\pm}$ spinel. In both G3xz and G3yz, sillimanite
is dispersed through the rock but it locally forms aggregates
preferentially associated with garnet. In G3xz, the shape of most
sillimanite crystals (up to $2\times 1$~mm) is elongated parallel to
the lineation with an ovoid tendency (Figure~\ref{fig1}). These
crystals display intracrystalline misorientations associated to
subgrain boundaries perpendicular to their elongation. However, a small
proportion of sillimanite crystals isolated in the quartzo-feldspathic
matrix have a rather rounded shape and are free of subgrain boundaries
(Figures~\ref{fig2} and~S4 Supplementary material). In G3yz
(Figure~\ref{fig1}), sillimanite crystals shape is less elongate and
rather smaller than in G3xz. In addition, the orientation of elongated
crystals is more dispersed, and they display subgrain boundaries and a
clear intracrystalline misorientation. In both thin sections,
sillimanite crystals, either incorporated in monomineralic aggregates
or in garnet-sillimanite aggregates, are partially to totally
recrystallized (Figure~\ref{fig2}). The new grains have a size between
${\sim}$20 and ${\sim}$80~$\upmu$m and an angular equidimensional shape, except
in some places where they are slightly elongated parallel to the
foliation\break (Figure~\ref{fig2}).

\begin{figure}
\vspace*{3pt}
\includegraphics{fig02}
\vspace*{2pt}
\caption{\label{fig2}Detailed EBSD maps of two areas of concentration
of partially recrystallized prismatic sillimanite (aqua) and garnet
(brown) partly replaced by Opx (dark blue) \textbf{in} both the G3xz
and G3yz thin sections. The grid step size is 5~$\upmu$m for G3xz and
4~$\upmu$m for G3yz. The color chart is the same as in
Figure~\ref{fig1}.}
\end{figure}

Sample TAF501 (Figure~\ref{fig1}, TAF501xz) is almost entirely
constituted of garnet and sillimanite. Most garnets are elongated (up
to ${\sim}$1~cm long and 0.4~cm wide), but their long axis is frequently
slightly oblique to the lineation. All garnet crystals are more or less
disrupted and they are wrapped by Opx ${\pm}$ spinel coronas resulting
from garnet breakdown, which are significantly larger than in G3. At
the thin section scale sillimanite crystals are jointed and forms a
kind of network including garnets and their Opx coronas
(Figure~\ref{fig1}). The sillimanite crystals shape tends to be
rectangular with a larger size than in G3 (up~to~2.5~mm) and a long
axis parallel to the lineation, {except} when they wrap large
garnets. Almost all crystals of sillimanite contain well-developed
subgrain boundaries and only few of them are partially recrystallized
(Figure~\ref{fig1}).

\section{Crystallographic preferred orientations and intracrystalline
misorientations} \label{sec4}

The crystallographic preferred orientations of samples G3 and TAF501
are rather similar (Figure~\ref{fig3}): they are characterized by a
clear, but not strong concentration of [001] parallel to the lineation
and a dispersion of [100] and [010] within and around the $YZ$
structural plane, with weak concentrations of both axis around the pole
of the foliation ($Z$~structural axis). In TAF501, the concentration of
[100] is significantly stronger than the [010] one. This suggests that
(100)[001] was the dominant slip system in TAF501 while in G3 the CPO
favors an equivalent activation of both (100)[001] and (010)[001] slip
systems.

\begin{figure*}
\includegraphics{fig03}
\caption{\label{fig3}Sillimanite crystallographic preferred orientation
from EBSD measurements on the three thin sections. Lower-hemisphere
stereographic projections of one mean orientation per grain with
contours at multiple of a uniform distribution (MUD). Both samples
display a clear, although not strong, concentration of [001] parallel
to the lineation.}
\end{figure*}

\begin{figure*}
\includegraphics{fig04}
\caption{\label{fig4}Crystallographic preferred orientation of
sillimanite from the strongly recrystallized domains shown on
Figure~\ref{fig2}. These CPO are more dispersed than those obtained for
the whole thin section; this dispersion is probably due to dynamic
recrystallization. Similar projection and contours characteristics than
in Figure~\ref{fig3}.\ 1~ppg ${=}$ One point per grain, i.e., an average
orientation was plotted for each grain.}
\end{figure*}

\begin{figure*}            
\includegraphics{fig05}
\vspace*{-1pt}
\caption{\label{fig5}Miss2Mean map of sillimanite in samples TAF501~(a)
and G3xz~(b). The variation of color represents the misorientation of
each measured point inside a crystal relative to the crystal mean
orientation. The misorientation between subgrains is up to
${\sim}$15{\textdegree}. The white lines limiting color variations inside
sillimanite crystals are subgrain boundaries. In TAF501, almost all
grains display subgrains domains with a variation of orientation up to
15{\textdegree}. In G3xz, grains isolated in the quartzo-feldspathic
matrix display only weak or even no intragranular misorientation. Grey
color is Garnet and white is the color of all other minerals.}
\vspace*{-1pt}
\end{figure*}

The CPOs measured in the detailed maps of Figure~\ref{fig2} are less
informative (Figure~\ref{fig4}). The statistics are perturbated by
either the big size of porphyroclasts relative to the new grains one
when plotted with the whole measurements in each grain, or by the large
number of new grains recrystallized from the same parent grain when
plotted with one mean value per grain (one point per grain). However,
these CPOs illustrate a dispersion of the crystallographic axes'
orientation, inclusive of [001], but [001] and [100] are the only axes
showing a concentration around the $X$ and the $Z$ structural axes
respectively. 

Detailed mapping of intragranular crystallographic orientation (step
of~5 and 4~$\upmu$m) reveal intragranular misorientations, their
organization and their intensity. For instance, Figure~\ref{fig5}
(and~S5) shows the misorientations associated to subgrain boundaries
and the misorientation angle up to ${\sim}$15{\textdegree} relative to
the mean orientation of the grain (Miss2Mean map) in samples TAF501 and
G3xz. Interestingly, the CPO in G3xz is slightly weaker than in TAF501,
although more sillimanite grains have been partially to totally
dynamically recrystallized than in TAF501. In the latter almost all
sillimanite crystals display subgrain boundaries and rather strong
intragranular misorientation while in G3, a significant proportion of
sillimanite grains are isolated in the quartzo-feldspathic matrix and
most of them only display a faint or even no deformation. This
heterogeneity may have dispersed the CPO in G3 and explain why its
fabric is weaker than the\break TAF501 one.

\begin{figure*}
\includegraphics{fig06}
\vspace*{-1pt}
\caption{\label{fig6}Crystallographic orientation of the rotation axes
for misorientations between 2 and 15{\textdegree} in G3xz and TAF501.
Both samples display a strong concentration of rotation axes around
[100] and a few ones parallel to [010] and none around [001]. This
supports that the main slip direction of dislocation was [001].}
\vspace*{-1pt}
\end{figure*}

In the three thin sections (Figure~\ref{fig6}), a large majority of
misorientation axis with rotation in the range 2{\textdegree}--15{\textdegree}
(subgrain boundaries) are parallel or close to [100] and only a
minority to [010]. These subgrain boundaries are perpendicular to both
the slip plane and the slip direction, thus, combined with the CPO
(Figure~\ref{fig3}) this supports that the dominant slip system is
(010)[001] and that (100)[001] is a subsidiary one.

\looseness=-1
Partially recrystallized sillimanites frequently display subgrain
boundaries that progressively become new grain boundary
(Figure~\ref{fig7}). In several sillimanite grains, a few subgrain
boundaries are predominantly orthogonal or less frequently oblique to
the main (001) ones. These different systems interact to form small
subgrains of similar size than the new grains~(Figure~\ref{fig7}). The
misorientation axes of these subgrain boundaries are frequently [010]
or [100]. This points to the participation of subsidiary slip systems
with [100] or [010] slip directions to the recrystallization process.

\begin{figure*}
\vspace*{2pt}
\includegraphics{fig07}
\vspace*{3pt}
\caption{\label{fig7}Excerpts from KAM maps of sample G3 showing the
intracrystalline structure of two partially recrystallized sillimanite
grains. Black lines are grain boundaries (misorientation
${>}$15{\textdegree}) and red ones are subgrain boundaries
(misorientation of 2{\textdegree}--15{\textdegree}). These maps display
examples of progressive transition from subgrain boundary to grain
boundary (e.g., arrow on both images).}
\vspace*{2pt}
\end{figure*}

\section{Discussion and conclusions} \label{sec5}

Data and observations obtained from the Middle Atlas lower crustal
granulites using the EBSD technique strengthen the determination of the
dominant \mbox{processes} active in prismatic sillimanite during
\mbox{natural} deformation under high temperature \mbox{conditions}
\mbox{(${\geq}$850~{\textdegree}C)}.\ Deformation of prismatic
\mbox{sillimanite} \mbox{crystals} in the studied granulites is
characterized by intragranular microstructures (subgrains boundaries to
subgrains) typical of dislocations creep leading to recrystallization
by subgrains rotation. Most subgrains boundaries have misorientation
axis parallel or close to [100].

The dominant slip systems during the deformation of the studied
prismatic sillimanite, statistically determined from EBSD data, are
(010)[001] and (100)[001]. They are similar to those evidenced by TEM
characterization carried out on single-crystals experimentally deformed
\citep{DoukhanChristie1982} and on naturally deformed garnet-migmatite
\citep{LambregtsVanRoermund1990}. Interestingly, EBSD mapping of the
G3yz thin section shows that many crystals of sillimanite are elongate
obliquely or less frequently perpendicularly to the foliation. In
addition, they display subgrain boundaries perpendicular to [001] with
[100] as main misorientation rotation axis. This supports that they
deformed through dislocation creep with [001] as dominant slip
direction. Some of them, initially misoriented relatively to the
deformation axes, were still deformed mostly through dislocation glide
parallel to [001], suggesting a flattening strain component in addition
to dominant shearing. The behavior of these crystals agrees with
\citet{DoukhanChristie1982} observations that even initially
misoriented grains deformed through dislocations creep parallel\break to
[001].

In contrast with prismatic sillimanites, in the fibrolite sillimanites
studied by \citet{PiazoloJaconelli2014}, the slip systems (001)[010]
and (001)[100] are dominant, at the exception of needles that were
initially oriented with [001] parallel to the lineation, and for which
[001] is the main slip direction.\ This \mbox{difference} may result
from several causes in addition to the initial orientation of
sillimanite crystals relative to the lineation. The most likely are the
$P$--$T$ conditions under which deformation occurred:
${\sim}$700~{\textdegree}C and 0.45~GPa, in the biotite stability field,
for the fibrolites studied by \citetalias{PiazoloJaconelli2014}, versus
${>}$850~{\textdegree}C and ${\sim}$1~GPa, outside the biotite stability
field, for the Middle Atlas prismatic sillimanites. In the first case,
the three slip systems may be equally activated depending on the
initial orientation of the crystals when, under higher temperature
conditions, the [001] slip direction is more easily activated than the
other two and become largely dominant. However, an effect of the strain
regime cannot be ruled out since deformation occurred through simple
shear for the fibrolites \citepalias{PiazoloJaconelli2014} versus
transtensional shear for the Middle Atlas granulites.

\begin{figure*}
\includegraphics{fig08}
\caption{\label{fig8}Detail from the EBSD map of G3xz thin section
showing an example of the contrast of deformation between sillimanite
involved in an aggregate of garnet and sillimanite (e.g.,~$A$) and
sillimanite grains surrounded by quartz and plagioclase (e.g.,~$B$).
Black lines are grain boundaries and purple ones are subgrain
boundaries (misorientation ${<}$15{\textdegree}). The color chart is
the same than on Figure~\ref{fig1}.}
\end{figure*}

EBSD data from the quartzo-feldspathic granulite G3 highlight that the
deformation of sillimanite is highly heterogeneous depending on the
nature of the minerals surrounding the sillimanite \mbox{crystals}
(Figures~\ref{fig8}, \ref{fig5} and~S2). Deformation is stronger when
sillimanite grains are surrounded by either other sillimanites or
garnet or both. In these cases, deformation may extend to dynamic
recrystallization.\ In contrast, sillimanite grains isolated in the
quartzo-feldspatic matrix are less deformed or even escaped
deformation, even those that were well oriented for the dominant slip
systems to be activated. Comparing in G3 the microstructures of
sillimanite with those of surrounding quartz and plagioclase highlights
that under high temperature conditions prismatic sillimanite is stiffer
than quartz and feldspars and, thus, may have escaped
\mbox{deformation} because strain was fully accommodated by the
quartzo-feldspathic matrix.

In the restitic granulite TAF501, only few sillimanite crystals are
partially recrystallized. Almost all sillimanite grains display
transverse subgrain boundaries that limit subgrains generally
displaying moderate misorientation. This suggests that dislocations
climb and associated recovery of the crystal lattice in the subgrains
were active during deformation. The restitic sample TAF501 and the
quartzo-feldspathic G3 display contrasted characteristics: the
deformation of sillimanite crystals is more homogeneous and weaker in
the first one and only few grains are partially recrystallized while,
in the second one, it is more heterogeneous and locally strong enough
to trigger full recrystallization. However, the CPO of TAF501 is
paradoxically stronger than the G3 one. This is likely related to their
mineralogical compositions, the restitic granulites are almost entirely
composed of garnet and sillimanite and deformation was more
homogeneously distributed than in the quartzo-feldspathic ones in which
a large proportion of sillimanite grains are dispersed in more
deformable quartzo-feldspatic layers that accommodated the deformation
that did not, or only slightly, affected sillimanite crystals.

\looseness=-1
A remaining question is ``During which stage of the studied granulites
evolution did the deformation of prismatic sillimanites occur?''.
Several convergent microstructural observations described in more
details in \citet{ElMazetal2025} allow to provide a reliable answer.\ 
The destabilization coronas around garnets do not display evidence of a
significant deformation, even when they are in contact with strongly
deformed sillimanites. The microstructures inside the coronas are well
preserved although these aggregates of very small grains of
orthopyroxene and spinel are rather easy to deform. The temperature of
garnet destabilization was estimated at \mbox{$1050 \pm
50$}~{\textdegree}C using Opx and garnet thermobarometers.\ The
quartzo-feldspathic granulites, G3 for instance (Figure~\ref{fig1}),
contain interstitial crystals of quartz, \mbox{K-feldspars} and, more
seldom plagioclase, related to an episode of anhydrous partial melting
that occurred at the peak temperature. These crystals have retained
their interstitial habitus and do not display any evidence of post
crystallization deformation.\ In G3, \mbox{platten-quartz} are
present~in monomineralic layers between \mbox{sillimanites}.
Such~quartz~grains, typical of static annealing under HT conditions and
slow cooling, do not display evidence of subsequent deformation. These
structural characteristics all together support that: (1)~the
deformation of sillimanite occurred before garnet destabilization,
i.e., during the prograde stage of the metamorphic evolution of the
studied granulites, and (2)~in the quartzo-feldspathic granulites
(e.g., G3), the formation of platten-quartz, the crystallization of
interstitial grains, and the subsequent cooling occurred under static
conditions, so after sillimanite deformation. No evidence of
significant deformation during the retrograde stage of the metamorphic
evolution has been detected. This led \citetalias{ElMazetal2025} to consider
that the deformation of the prismatic sillimanites occurred at the
beginning of the episode of heating to UHT responsible for the
destabilization of garnet and the anhydrous partial melting.

Finally, the studied deformed prismatic sillimanites, especially in
sample G3, have only recorded an incipient post-deformation annealing,
although they have been submitted to ultra-high temperatures \mbox{after}
deformation \citepalias{ElMazetal2025}. This points to the high thermal
stability of their crystallographic structure, even under temperature
around 1000~{\textdegree}C \citep[e.g.,][]{HollandCarpenter1986,
Igamietal2017}.

\section*{Acknowledgements}

We warmly acknowledge Doriane Delmas and Christophe Nevado for the
preparation of high-quality polished thin sections, Fabrice Barou for
the time devoted to the acquisition of high-grade EBSD data, Andrea
Tommasi for her support during the processing and interpretation of
EBSD data. We thank the anonymous reviewer for her/his suggestions that
helped us to improve our manuscript. We also thank Houssa Ouali and
Hicham El Messbahi for their help during samples collection.

\section*{Declaration of interests}

The authors do not work for, advise, own shares in, or receive funds
from any organization that could benefit from this article, and have
declared no affiliations other than their research organizations.

\section*{Supplementary materials}

Supporting information for this article is available on the journal's
website under \printDOI\ or from the author.

\CDRsupplementaryTwotypes[application/zip]{supplementary-material}{\cdrattach{crgeos-317-suppl.zip}}

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