The small island of Groix in southern Brittany, France, is well known for
exceptionally well-preserved outcrops of Variscan blueschists, eclogites, and
garnetiferous mica schists that mark a Late Devonian suture between Gondwana
and Armorica. The kinematics of polyphase deformation in these rocks is
reconstructed based on 3D microstructural analysis of inclusion trails
within garnet and pseudomorphed lawsonite porphyroblasts using differently
oriented thin sections and X-ray tomography. Three sets of inclusion trails
striking NE–SW, NNW–SSE, and WNW–ESE are recognized and interpreted to
witness a succession of different crustal shortening directions orthogonal
to these strikes. The curvature sense of sigmoidal and spiral-shaped
inclusion trails of the youngest set is shown to be consistent with
northwest and northward subduction of Gondwana under Armorica, provided
that these microstructures developed by overgrowth of actively forming
crenulations without much porphyroblast rotation. Strongly non-cylindrical
folds locally found on the island are reinterpreted as fold-interference
structures instead of having formed by progressive shearing and fold-axis
reorientation. Six samples of a lower-grade footwall unit of the Groix
ophiolitic nappe (Pouldu schists) were also studied. Inclusion trails in
these rocks strike E–W, similar to the youngest set recognized on Groix
island. They record Carboniferous N–S shortening during continental
collision. These new microstructural data from southern Brittany bear a
strong resemblance to earlier measured in inclusion-trail orientations in the
northwestern Iberia Massif. A best fit between both regions suggests not more than about
15∘ anticlockwise rotation of Iberia during the Cretaceous opening of
the Gulf of Biscay.
Introduction
Structural analysis in metamorphic terrains is traditionally based on a
combination of geological mapping, study of structures in outcrops, and
microstructures in thin sections that are commonly cut parallel to the
stretching lineation aimed at determining a shear sense. Not rarely does
this approach produce ambiguous or even paradoxical results (e.g. Zhang and
Fossen, 2020), apart from not allowing access to the kinematics of strongly
overprinted early formed fabrics. A major breakthrough in this state of
affairs was claimed by Bell (1985), Bell et al. (1986), and Bell and Johnson (1989) after recognizing the polyphase origin of inclusion-trail patterns
that were previously assumed to have formed by progressive shearing and
porphyroblast rotation (e.g. Zwart, 1962; Spry, 1963; Rosenfeld, 1970).
Sigmoidal and spiral-shaped inclusion trails were reinterpreted as
representing multiple crenulations that became sequentially fixed within
non-rotating porphyroblasts with episodic growth histories. The lack of
porphyroblast rotation would have resulted from the general
micro-partitioning of deformation between cleavage planes (accommodating
shear-strain components) and microlith domains and the preferential
nucleation of porphyroblasts within the latter (Bell, 1985; Aerden,
1995; Fay et al., 2008). Axes of inclusion-trail curvature in this view are
not controlled by shearing directions but by the intersection of two or more
foliations causing crenulation and folding and consequently referred to as
“foliation-intersection/inflexion axes” abbreviated as “FIA” (Bell et al.,
1992, 1995; Stallard et al., 2003). Another major difference
with the classic model is that the curvature sense (clockwise or anticlockwise) of inclusion trails being determined by the (S or Z)
asymmetry of overgrown crenulations indicates an opposite shear sense as
would have been originally deduced (see Fig. 1a, b).
(a) Traditional rotational interpretation of sigmoidal and
spiral-shaped inclusion trails and the range (purple) of inclusion-trail
strikes predicted by this model in rocks from Île de Groix. (b) “Non-rotational” interpretation of inclusion trails formed by overgrowth of
successive crenulation cleavages and the predicted range of inclusion-trail
strikes for Île de Groix. (c) Actually measured inclusion-trail strikes in
10 samples from Île de Groix. Note that they better match the predictions of
the non-rotation model.
The above conceptual changes regarding the significance of porphyroblast
inclusion trails prompted extensive collection of orientation data for these
microstructures in various metamorphic belts, which revealed their
regional-scale consistency and close relationship to large-scale tectonic
processes (e.g. Bell et al., 1998; Bell and Welch, 2002; Sayab, 2005; Ali,
2010; Shah et al., 2011; Aerden, 1994, 1998, 2004; Aerden et al., 2013; Kim and Ree, 2013;
Abu Sharib and Bell, 2011; Abu Sharib and Sanislav, 2013; Aerden and Sayab, 2017). The methodological novelty of this research resides in the
integration of quantitative microstructural data derived from precisely
oriented thin sections of different samples in an external (geographical)
reference frame. For this purpose, thin sections are systematically cut in
fixed orientations relative to geographic coordinates, independent of the
(variable) orientations of the macroscopic lineation and foliation in each
sample. Vertical and/or horizontal thin section orientations are usually
chosen as these represent small-scale true cross sections and maps that can
be directly compared with their larger-scale equivalents.
Aerden (2004) pioneered this approach in NW Iberia where he distinguished four sets of inclusion trails, each with a specific and regionally consistent
trend. The E–W trend of the oldest set was interpreted to record N–S-directed compression and subduction with unknown polarity. This was followed
by several changes in the direction of crustal shortening that produced the
three younger inclusion-trail sets. The youngest two sets, trending NE–SW and
WNW–ESE, were shown to correlate with regional-scale
fold-interference patterns developed throughout the Iberian Massif, and this
was key to the discovery of the partially blind Central Iberian Arc (Aerden,
2004; Martinez Catalán et al., 2011).
In this paper we report a similar microstructural study in the Armorican
Massif focusing on high-pressure metabasites of Île de Groix, but also
paying attention to a lower-grade footwall unit known as the Pouldu schists.
The island of Groix (ca. 15 km2) is a national reserve famous for
exceptionally well-preserved coastal outcrops of Variscan blueschists,
eclogites, and interlayered garnetiferous mica schists (Audren et al., 1993).
These are generally accepted to represent the remains of the floor of a
narrow ocean that, in the late Silurian and lower Devonian, separated
Gondwana in the south from the Armorica microplate to the north. Closure of
this ocean by subduction of the margin of Gondwana produced high-pressure
metamorphism dated 370–360 Ma on Île de Groix (Bosse et al., 2005). In
NW Iberia, a similar ophiolitic unit is recognized and correlated with the
one exposed on Groix island (Arenas et al., 1995; Diaz García et al.,
1999; Ballèvre et al., 2009, 2013).
The polarity and kinematics of ophiolite emplacement have remained poorly
constrained by (micro)structural data. Indeed, not much appears to have
changed since Quinquis et al. (1978) wrote
the bulk sense of shear on Groix has not yet been determined unequivocally, but may perhaps be deduced from systematic analyses of fold asymmetry and of microstructures in and around syntectonic garnets. The significance of glaucophane orientation in the basic rocks of Groix also needs to be studied: the orientation is extremely variable and may not be simply related to a shear direction.
Precisely along these lines
of suggested further research, we have performed detailed 3D microstructural
analyses of 10 garnetiferous blueschist samples of the Île de Groix
ophiolitic nappe, 4 samples of albite-porphyroblast, bearing greenschists
(Pouldu schists) of a footwall unit cropping out along the mainland coast,
plus 2 kyanite-staurolite schists collected further inland from the
North Armorican Zone (Fig. 2). Integrated with structural field data from
previous workers, the new microstructural data indicate a polyphase
deformation history masked by a seemingly simple L-S fabric. Implications
for the formation mechanism of sheath folds (Île de Groix is one of the main
type locations) are discussed and for variations in the direction of
Variscan subduction with time. A possible correlation is proposed between
inclusion-trail trends in NW Iberia and in the Armorican Massif, which if
correct, implies not more than 15∘ anticlockwise rotation of Iberia during
opening of the Gulf of Biscay.
(a) Simplified geological maps of southern Brittany and NW Iberia
showing the location of ophiolite outcrops and samples studied herein and by
Aerden (2004) in NW Iberia. (b) Schematic N–S cross section after
Ballèvre et al. (2013) showing the structural relationships between
tectonic units in the Variscan orogen.
Geological setting and previous work
The Variscan (or Hercynian) orogeny took place in the Devonian and
Carboniferous as a consequence of Gondwana–Laurussia convergence with the
Armorica and Avalonia microcontinents occupying intermediate positions (e.g.
Matte, 2001). The resulting closure of oceanic domains created multiple
ophiolitic sutures, whose precise location, timing, and correlation continue
to be a major research topic (e.g. Azor et al., 2008; Faure et al., 2008;
Ballèvre et al., 2009; Arenas et al., 2016). The high-pressure rocks of
Île de Groix are part of a partially submerged ophiolitic klippen of a
thrust nappe that separates Armorican crustal units in the hanging wall
(outcropping in the Central and North Armorican domains) from Gondwana-derived units in the footwall cropping out extensively in the South
Armorican Domain and NW Iberia (Fig. 2). Mineral and stretching lineations
have an average N–S trend in the east and southeast of the island, changing
gradually to NW–SE further north and west. Bosse et al. (2002) proposed a
thrust contact between garnetiferous eclogites, blueschists, and mica schists
outcropping in the eastern part of the island (Fig. 3a), overlying a
lower-grade (albite–epidote facies) unit outcropping in the west containing
scarce garnet porphyroblasts (Triboulet, 1974; Schulz et al., 2001). Peak
metamorphic conditions are estimated at 475 ∘C and 18–20 kbar in the
upper unit and 450 ∘C and 12–15 kbar in the lower unit (Bosse et al.
2002; Ballèvre et al., 2003).
(a) Stretching lineation pattern in Île de Groix and sample
locations. (b) Moving-average rose diagrams for inclusion-trail strikes.
Pie chart symbols in circles give the average trend and plunge direction of
FIAs in samples as determined from radial sets of thin sections. Small
spirals inside these pie charts indicate the curvature sense of
inclusion trails as seen when viewing down FIA plunge (anticlockwise in G14,
G12, G11; clockwise in G19 and G7). Pink, blue, yellow, and red trend lines
code for four different inclusion trail sets distinguished in this paper from
older to younger. (c) Inclusion-trail strikes in Pouldu schist samples and
Tréogat formation (AU1). Arrows represent fold axes measured in the outcrop.
(d) Data from two staurolite–kyanite schists of the Central Armorican Domain.
The general tectono-metamorphic zonation of the Armorican Massif has led
most workers to accept a north-dipping subduction zone associated with south-verging thrusting (e.g. Matte, 2001; Faure et al., 2008; Ballèvre et
al., 2009, 2013; Philippon et al., 2009; Fig. 2b). However,
structural data have not allowed the independent confirmation of this and, in fact,
have yielded conflicting results. Lagarde (1980) concluded NW-directed
thrusting from shear criteria in the Champtoceaux Complex (Fig. 2a),
suggesting southeast-directed subduction. A similar kinematic was deduced
on Île de Groix from rotated snowball garnets and quartz c-axis fabrics by
Quinquis (1980), Quinquis and Choukroune (1981), and Cannat (1985). To
reconcile this with north-dipping subduction, Quinquis and Choukroune (1981) proposed that this shear sense corresponds to back-thrusting and
ophiolite obduction, but little independent evidence has been presented to
support this model.
Quinquis (1980) noticed that shear bands (C and C′ planes) on Île de Groix
indicate opposite shear senses that could not be clearly linked to different
metamorphic conditions and were therefore interpreted as conjugate sets.
Shelley and Bossière (1999) endorsed this and showed that quartz fabrics
studied in 57 samples also record about equal amounts of opposite
shear senses. From this they concluded that the main foliation and lineation
of Île de Groix formed by coaxial vertical shortening in a period of crustal
extension and exhumation. In contrast, Philippon et al. (2009) recently
argued a consecutive origin of a first set of top-to-the-SW shear bands
related to prograde metamorphism, followed by a second top-to-the-north set
during retrogression. However, they did not resolve the conflict this poses
with respect to top-to-the-north shearing indicated by “rotated” garnets,
which clearly formed during prograde metamorphism (Bosse et al., 2002;
Ballèvre et al., 2003).
Microstructural analysis using vertical and horizontal thin sections
A total of 138 oriented thin sections were studied from 33 oriented samples:
20 from Île de Groix, 9 from the Pouldu schists and an equivalent unit in
the Baye d'Audièrne (Tréogat formation), and 4 from the Central
Armorican Zone (Fig. 2). Their precise location can be consulted in the
Supplement provided with this paper. Initially, a single
horizontal thin section was cut of each to evaluate the interest of the rock
for further study and to measure the strike of inclusion trails (relative to
geographic coordinates). The latter only proved possible in about half of
the samples, as the other half contain garnets or plagioclase porphyroblasts
that are too altered or have poorly developed inclusion trails. Seven Île de
Groix samples containing the most promising inclusion trails were studied
further in six vertical thin sections striking N0, N30, N60, N90, N120, and
N150 aimed at constraining the orientation of inclusion-trail curvature axes
(i.e. FIA; see Introduction) and record their curvature sense.
Strike of inclusion trails in Île de Groix blueschist samples
Inclusion-trail strikes were measured in garnet porphyroblasts of 10 samples
from three areas of Île de Groix, all pertaining to the high-grade eastern
domain of the island or “upper unit” of Bosse et al. (2002; Fig. 3a). The
data are plotted in moving-average rose diagrams (Fig. 3b) made with the
computer program “MARD” of Munro and Blenkinsop (2012). As pointed out by
these authors, moving-average rose diagrams reveal the distribution of modal
maxima more accurately than their conventional binned equivalents.
Stretching lineations in the high-grade domain are associated with a gently
dipping or subhorizontal composite foliation. Therefore, if porphyroblasts
rotated in the direction of the lineation during growth, then they can be
expected to have developed inclusion trails broadly striking orthogonal to
that lineation (Fig. 1a). Our measurements, however, show exactly the
opposite: a main NNE–SSW maximum subparallel to stretching lineations and
fold axes (Fig. 1c). This agrees better with a “non-rotational”' origin of
inclusion trails via overgrowth of successive crenulations, and this is
supported further by truncational relationships commonly seen between
inclusion trails in porphyroblast cores versus rims (Fig. 4a and b).
Automatic orientation analysis of the inclusion-trail patterns pictured in
these figures using the image analysis software package Fiji (Schindelin et al.,
2012) reveals their bimodal preferred orientations reminiscent of the
subvertical and subhorizontal preferred orientations of inclusion trails
previously reported in other metamorphic regions (e.g. Bell et al., 1992;
Hayward, 1992; Aerden, 1994, 1998, 2004; Mares, 1998; Stallard and Hickey,
2001; Bell and Sapkota, 2012; Sayab, 2005; Shah et al., 2011; Aerden and
Ruiz-Fuentes, 2020). These authors all interpreted this to reflect
alternations between orogenic shortening and gravitational collapse, but a
somewhat different explanation will be proposed in Sect. 4.4 based on
additional 3D data obtained by X-ray tomography.
(a, b) Line drawings of inclusion trails and associated truncations
(yellow lines) in differently striking vertical thin sections of G7 and G14.
Half arrows indicate the strike of each section and way up. (c) Binned rose
diagram plotting the angles of all dashes representing inclusion trails in (a) and (b) as measured with the “Analyse Particle” tool of Fiji. The
broadly bimodal distribution reflects the presence of two or more sets of
inclusion-trail planes, consistent with overgrowth of successive crenulation
cleavages rather than with continuous rotation of syntectonic
porphyroblasts.
Inclusion-trail curvature sense and genetic implications
Quinquis and Choukroune (1981) reported that out of a total of 29 thin
sections studied (presumably all cut parallel to the stretching lineation),
26 contained sigmoidal or spiral-shaped inclusion trails indicating
top-to-the-north shearing. Since these authors assumed the rotational
inclusion-trail model, it follows that the trails predominantly curve
anticlockwise when viewed in the westward direction. In effect, we found the
same predominance in our samples after counting the number of clockwise vs.
anticlockwise trails in all vertical thin sections striking N-0, N30, N120,
and N150, that is, in all thin sections making a small angles with the
regional stretching lineation. This resulted in 99 anticlockwise and 27
clockwise inclusion trails viewing westward – a ratio of about 4 : 1. The
classic rotational interpretation of inclusion trails predicts
top-to-the-north shearing from this asymmetry, which is paradoxical with
respect to the widely accepted northward subduction and southward thrusting
in the Armorican Massif. The non-rotational model resolves this paradox as
it predicts an opposite shear sense from the same inclusion-trail asymmetry,
consistent with top-to-the-south thrusting (Bell and Johnson, 1989; their
Figs. 16 and 17).
Average FIA trends in five Île de Groix samples
Hayward (1990) devised a method for determining the average orientation of
porphyroblast FIAs in a sample. The method exploits the fact that sigmoidal
or spiral-shaped inclusion trails, just like asymmetric folds, either
exhibit an S- or Z-asymmetry in cross section (thin section) depending on
the orientation of the section relative to the FIA. First, a radial set of
vertical thin sections is cut from the sample with regular angular spacing
around the compass. From these, the average FIA trend can be constrained to
the strike interval where the inclusion-trail asymmetry observed in the
different thin sections switches (e.g. Abu Sharib and Sanislav, 2013 –
their Fig. 2). Once the average FIA trend is known, the average FIA plunge
can be constrained by cutting a new radial set of thin sections, this
time about a horizontal axis oriented orthogonal to the FIA trend. The
method was refined by Bell et al. (1995) to potentially allow the
distinction of multiple FIA sets in a sample with different timing.
We successfully applied the method to five samples whose average FIAs are
represented in Fig. 3b as pie chart symbols. FIAs were constrained to within
30∘, but 10∘ for sample G7 thanks to cutting two extra
thin sections. The plunge direction could also be determined from the
asymmetry of inclusion trails as seen in horizontal thin sections, but we
did not determine plunge angles because of the large number of extra thin
sections needed. No results were obtained for samples G18, G19, and G20 as
these contain relatively few porphyroblasts whose inclusion trails show
inconsistent asymmetries in most of their thin sections. X-ray tomography
data for G20 presented in Sect. 4.6. suggest that this inconsistency is
due to a mixture of two different FIA sets in these samples that could not be
resolved using thin sections. Samples G3, G13, and G15 mostly contain garnets
with straight inclusion trails and too few asymmetric ones to confidently
apply the technique.
Mainland samples (Pouldu schists, Tréogat formation, Central Armorican Domain)
The “Pouldu schists” are a volcano-sedimentary unit metamorphosed in
greenschist- to amphibolite-facies conditions (380–650 ∘C and 5.5–6.5 kbar; Triboulet, 1992). They crop out in a ca. 50 km long band
along the southern coast of Brittany and contain abundant albite
porphyroblasts (Fig. 2). The macroscopic cleavage in the field strikes E–W
to NE–SW and generally dips steeply north or south. Horizontal thin sections
of seven oriented samples of the unit show a N060 striking foliation cut by
widely spaced E–W-striking dextral shear bands, likely related to the south
Armorican shear zone system. In sample PO5, the N060 striking main foliation
has the appearance of a narrowly spaced crenulation cleavage overprinting an
older more E–W-striking foliation.
The strike of inclusion trails in albite porphyroblasts was measured in three
greenschist samples of the Pouldu schists (PO2, PO3, PO5) and in a fourth
sample (AU1) of similar rocks cropping out in the Baye d'Audièrne
(Tréogat Formation; Lucks et al., 2002). Inclusion trails consistently
strike E–W to ESE–WNW in PO2, PO3, and PO5, but have a larger spread in AU1
(Fig. 3b). N–S-striking vertical thin sections of PO2 and PO3 show that the
main foliation (“S2” in Fig. 5a) crenulates an older “S1” included in syn-D2
porphyroblasts (Fig. 5b). Some later porphyroblast growth, probably linked
to the development of weak subhorizontal crenulations (“S3” in Fig. 5),
created younger inclusion trails of S2 (Fig. 5b). Nearby granites dated
330–320 Ma are deformed by S2 (Béchennec et al., 2012) and imply that
this cleavage post-dates the Late Devonian to early Carboniferous
metamorphism and associated polyphase deformation of Île de Groix
(365–345 Ma; Bosse et al., 2005).
(a) Microstructural line drawings traced on photographs of thin
sections of greenschist samples PO2 and PO3 containing albite
porphyroblasts. Three foliations can be recognized. S2 corresponds to the
macroscopic cleavage and crenulates an older S1. S3 corresponds to weak
subhorizontal crenulations. (b) Interpretation of porphyroblast growth
history. Most albite porphyroblasts form early during D2 when they overgrow
S1. Further growth early during D3 produces additional inclusion trails of S2.
Two staurolite–kyanite schists from the Central Armorican Domain collected
about 5 km NNE of Quimper were studied in horizontal thin sections only.
Their inclusion trails strike broadly N060 (Fig. 3d) and might correlate
with the equally N060-striking S2 in our Pouldu schist samples. The age of
metamorphism in these rocks is broadly constrained to the period 350–320 Ma
(Schulz et al., 1998).
X-ray tomography of five Île de Groix samplesData acquisition and processing method
X-ray-computed tomography (XCT) scans were acquired at the University of
Granada with an Xradia 510 (Versa Zeiss) microtomographer at resolutions of
13–15 µm, using 140 kV voltage and 2500–3200 projections. Four thin section
blocks, each measuring 10–15 cm3, of samples G3, G11, G12,
and G14 plus a more irregular piece of G20 of similar volume were scanned. Geographic
orientation arrows made of metal wire were stuck on the samples to aid
reorientation of the generated tiff image stacks such that geographic north
is parallel with the y axis and true vertical with the z axis. Image stacks
were processed with the Fiji software package (Schindelin et al., 2012).
After reorientation, the spatial orientation of all straight
inclusion trails or inclusion-trail segments visible in the image stacks
was determined by measuring their strike and pitch angles on xy, yz, and
xz slices and fitting those angles to great circles on a stereonet.
Furthermore, the curvature axes of individual sigmoidal or spiral-shaped
inclusion trails (FIA) were measured in an analogous manner as the
thin-section-based technique of Hayward (1990; see Sect. 3.3), and as
previously done by Huddlestone-Holmes and Ketcham (2010), Aerden and Ruiz-Fuentes (2020), and Sayab et al. (2021). First, the FIA trend is
constrained by interactively rotating a vertical slice through the
porphyroblast and recording where the inclusion-trail curvature sense
switches. Then, a horizontal slice is interactively rotated about a
horizontal axis oriented normal to the previously determined FIA trend to
constrain the FIA plunge.
The image stacks also allowed us to study the preferred orientation of
garnet porphyroblasts as well as relatively large opaque minerals present in
all five samples. The BoneJ plugin of Fiji (Doube et al., 2010) was used for this
purpose. This tool enables automatic calculation of best-fit ellipsoids for
a large number of “objects” in a binary (black and white) image stack. To
apply this, X-ray scans were first segmented by thresholding, only leaving
voxels with grey-scale values within the range corresponding to garnet
crystals or opaque minerals and then binarizing the stack (i.e. setting all
grey values to black). Subsequently, a size filter was applied to remove
small particles and in some cases the “dilate” tool to re-join objects
belonging to the same garnet crystal that became separated during
thresholding due to the presence of fractures and related alteration. In all
samples except G20, well-developed preferred orientations of opaque minerals
were detected, but not of garnets. In G20, an opposite situation was found: only
well-preferred orientation of garnets was found. In the following subsections
microstructural results are described for each sample and presented in
stereoplots made with the program “Stereonet” of Rick Allmendinger. The
original files of these stereoplots are provided in the Supplement.
Sample G11
G11 is a blueschist from “Amer” in the SE of the island (Fig. 3a). It
contains garnet porphyroblasts as well as rectangular pseudomorphs probably
after lawsonite composed of a mixture of white mica, chlorite, albite, and
epidote (Cogné et al., 1966; Felix and Fransolet, 1972; Ballèvre et
al., 2003). Lawsonite relics have never been found, though, and some authors
have argued that the replaced mineral could have been plagioclase (Shelley
and Bossière, 1999). Lawsonite is a high-pressure mineral and should
have grown partially synchronous with garnet on a prograde path, whereas
plagioclase is more likely to have formed during retrogression. Thus, the
relative timing of the pseudomorphed mineral relative to garnet growth and
inclusion trails is relevant to this question.
Inclusion trails in garnet porphyroblasts of G11 vary from simple to
sigmoidal to spiral shaped, but they never curve more than about 90∘
from the centre to the rim (Figs. 6a, b, c, g and 7). The tomographic images
further revealed the presence of relatively large elongate crystals with
high X-ray attenuation (higher than garnet), which in thin section were
identified as magnetite, partially or completely replaced by brown-reddish
goethite (Fig. 6d, h). Some of the opaques attain porphyroblastic sizes and
contain scarce silicate inclusions. Best-fit ellipsoids calculated for these
crystals with BoneJ (see Sect. 4.1) have long axes (x) well aligned with the
macroscopic mineral–stretching–intersection lineation and short axes normal
to the foliation (Fig. 7).
Photographs of G11. (a, b) Garnet porphyroblast (parallel and
crossed polars) with sigmoidal trails in a N–S-striking vertical section.
Barb of N-arrow points upward. Note top-to-the-south shear sense suggested
by asymmetric strain shadows consistent with a non-rotational interpretation
of the inclusion trails. (c) Garnet with spiral-shaped inclusion trails with
a truncation surface between the porphyroblast core and rim. (d) Opaque
mineral with elongate shape replaced by goethite. (e, f) Lawsonite
pseudomorphs (parallel and crossed polars) showing weakly sigmoidal
inclusion trails oriented oblique to the matrix foliation. (g) Tomographic
image of spiral inclusion trails garnet in G12. (h) Tomographic image of an
elongate lawsonite pseudomorph, a garnet porphyroblast (Gt) and an opaque
mineral.
Stereoplots (equal angle, lower hemisphere; made with the program
“Stereonet”) for internal foliations and FIAs preserved within garnet
crystals and lawsonite pseudomorphs in samples G11 and G12, as well as long
and short axes of best-fit ellipsoids calculated for opaque minerals
present in the matrix of both samples (contoured using a blue ramp). See
legend and main text for detailed description of these data. The stereoplot
data can be colour matched to representative oriented maps (note north
arrow) of inclusion trails shown at the bottom of the figure, traced on
high-resolution photographs of horizontal thin sections. A lawsonite
pseudomorph (Laws.) is also drawn.
Sigmoidal or spiral-shaped inclusion trails were measured both in the
centre and in the rims, where they become sharply deflected or truncated by
younger inclusion trails. The porphyroblast-rim measurements tightly define
a steeply SW-dipping plane, whereas foliations within porphyroblast cores
have more variable orientations between steeply NE-dipping and shallowly
W-dipping. The intersection lines of core and rim inclusion-trail planes
agree well with 10 FIAs measured directly with the asymmetry-switch
technique described in the previous section. Simple (straight) inclusion
trails have variable orientations that broadly coincide with the foliations
measured in porphyroblast cores and rims associated with sigmoidal and
spiral-shaped trails.
The above data allow confident interpretation of the bimodal inclusion-trail
strike pattern exhibited by the rose diagram for G11 in Fig. 3b as
corresponding to two age sets of inclusion trails: an older set striking
NNW–SSW and a younger one striking WNW–ESE. Note that the 10 individual
porphyroblast FIAs determined from the X-ray scan agree well with the
average FIA initially determined from the radial set of thin sections (Fig. 3b).
Some of the lawsonite pseudomorphs in G11 also have visible inclusion trails
in the X-ray scan (Fig. 6e, f, and h), although less finely defined as in
garnets. They are straight to weakly sigmoidal, oblique to the matrix
foliation, but subparallel to inclusion trails in garnet rims. The latter
suggests that the pseudomorphed mineral grew synchronous with late-stage
garnet, and this supports that the pseudomorphed mineral was lawsonite and
grew towards the end of the prograde path as concluded by Ballèvre et
al. (2003). Interestingly, the curvature sense of inclusion trails in the
pseudomorphs is mostly opposite to that in garnets when viewed in N–S
vertical sections. Assuming a non-rotational origin of the trails, this
implies a change from top-to-the-south shearing to top-to-the-north shearing
that is further discussed in Sect. 6.3.
Sample G12
G12 is a blueschist similar to G11 from the same outcrop, but only one
possible lawsonite pseudomorph could be identified in the X-ray scan, which
does not show an internal fabric. The tomography of this sample reveals an
early NNW–SSE-striking foliation preserved within garnet porphyroblasts,
overprinted by a steeply S-dipping foliation included in some garnet rims
(Fig. 7). The garnet-rim foliation is in turn deflected or truncated by a
subhorizontal crenulation cleavage in the matrix that is responsible for the
flat-lying macroscopic cleavage. The same opaque minerals as found in G11
are present in the matrix but now also as relatively large inclusions
inside garnets, often occupying a central position suggesting that the
garnets nucleated on those grains. Best-fit ellipsoids for the matrix
opaques demonstrate their strong elongation in the NW–SE direction parallel to
the macroscopic lineation and parallel to the strike of inclusion trails in
garnet.
The above data allow the correlation of the different peaks in the strike rose
diagrams for G11 and G12 in Fig. 3b as (i) an early NE–SW-striking foliation
preserved in the cores of spiral-shaped inclusion trails, (ii) a younger
NNW–SSE-striking foliation included in the cores of sigmoidal inclusion
trails with variable dip angles, and (iii) a still younger ESE–WNW foliation
preserved in garnet rims and in lawsonite pseudomorphs.
Sample G14
G14 is another blueschist from Amer containing numerous relatively small
(1–2 mm) garnets with straight, sigmoidal, or spiral-shaped inclusion trails.
The spiral patterns exhibit significantly greater curvature (up to
180∘; Fig. 4b) than in G11 and G12. Unfortunately, only a few
garnets had visible inclusion trails in the tomography because of their very
fine grain size and profuse fracturing of garnets. A few inclusion-trail
planes could be measured plus five FIAs plunging steeply in different
directions (Fig. 8a).
Tomographic images and microstructural data for sample G14. (a) Stereoplot for internal foliation planes, FIAs (grey boxes), long and short
axes of opaque minerals, and axial plane of centimetre-scale fold. (b, c) Map
and cross section views of a fold outlined by an epidote-rich layer. Its
axial trace trends NNE–SSW and is transected by N–S-trending cleavage zones
also visible in (d). Tomographic cross section showing refolding with
subhorizontal axial planes of the fold in (b) implying a component of
vertical flattening.
Opaque crystals are elongated in the N–S to NE–SW directions parallel to the
modal maximum defined by 62 inclusion-trail strikes measured in two horizontal
thin sections (Fig. 3b). A centimetre-scale fold outlined by an epidote-rich layer
was found, whose axial plane also strikes NW–SE (Fig. 8a), hence suggesting
a genetic relationship with the inclusion trails. Vertical sections oriented
at a high angle to the fold axes (Fig. 8c) show refolding with subhorizontal
axial planes related to the subhorizontal crenulation cleavage observed in
thin sections of sample G7 (Fig. 4a). In plan view horizontal sections, the fold is transected
by N–S-striking cleavage zones and quartz lenses (Fig. 8d).
Based on their orientations, the five measured inclusion-trail planes can be
correlated with the three sets already distinguished in G11 and G12. The sets are
(i) an early NE–SW set responsible for the strike maximum of 62
inclusion trails measured in two horizontal thin sections of G14 (Fig. 3b),
(ii) a NNW–SSE-striking set, and (iii) a late WNW–ESE set. The lack of
subhorizontal internal foliations in G14 or in any other of the samples
studied with X-ray tomography implies that the roughly bimodal distribution
of pitch angles of inclusion trails in G7 and G14 (Fig. 4c) does not reflect
a superposition of subvertical and subhorizontal foliations associated with
compression–collapse cycles as shown in other metamorphic regions (Aerden
and Ruiz-Fuentes, 2020, and references cited therein) but rather two sets
of moderately to steeply dipping foliations with different strikes.
Sample G3 and G7
G3 was collected close to Fort Nosterven on the east coast of the island,
about 1 km south of G7. Garnets in this glaucophane–epidote schist have well-developed straight and sigmoidal inclusion trails, whose strikes were
measured in horizontal thin sections. The average N020 FIA trend in G7 was
determined from eight vertical thin sections with different strikes. G3 was
studied further with X-ray tomography, but its inclusion trails are poorly
visible in the scan. The few that could be measured all dip steeply NW with
N020 strike, parallel to the average FIA in sample G7 and a set of gently
SSW-plunging folds measured by Claude Audren near G7 at Plage du Trech (1974,
unpublished data; Fig. 9b). A single porphyroblast FIA was also measured,
defined by sigmoidal inclusion trails that curve into a subvertical N120-striking position in the porphyroblast rim. It is most likely related to one
of Audren's fold axes which has an anomalous N120 trend. Thus, this fold
axis is probably younger than the main group of SSW-trending fold axes. All
the above data are consistent with the orientations and relative
timing of three main sets of inclusion trails and related macroscopic structures
in Île de Groix, coded pink, blue, and orange in our figures.
(a) The 3D microstructural data for sample G3. See legend and Sect. 4.5. for detailed description of these data. (b) Field data collected by
Claude Audren near sample G7 at Plage du Trech and the average FIA trend
(pink pie chart) we determined for this sample from radial thin sections. The
FIA trend is parallel to fold axis measurements. Note that this conflicts
with progressive shearing, fold-limb rotation, and porphyroblast rotation.
Stretching lineations vary significantly, reflecting polyphase deformation.
(c) Microstructural data for G20. See legend and text of Sect. 4.5 for
detailed description.
Sample G20
G20 was collected near Port Lay on the central-north coast of Groix (Fig. 3a). The macroscopic cleavage dips 50∘ NE here and is associated
with a subhorizontal mineral lineation parallel to small-scale tight to
isoclinal fold axes. Six garnet FIAs defined by sigmoidal inclusion trails
were measured using X-ray tomography plus a larger number of straight
inclusion trail planes. Five FIAs plunge moderately NE and are caused by the
intersection of an older set of NE–SW-striking subvertical inclusion trails
with a younger set of steeply NE- or SW-dipping ones. Again, this allows
both sets to be correlated with similarly striking inclusion trails in the
other samples (Fig. 3b). The macroscopic cleavage is conspicuously parallel
to a subset of the younger (NNW–SSE-striking) inclusion trails, suggesting a
genetic relationship. The sixth FIA that was determined plunges shallowly
SSE and probably formed after the others by overgrowth of the younger
inclusion trail set (blue great circles in Fig. 9c).
Abundant opaque minerals in the sample conspicuously cluster around garnet
crystals, but ellipsoid best-fitting did not reveal significant preferred
orientations. Axial ratios of the opaques are also much lower as in the
earlier described samples (2.0 versus 4.0). Best-fit ellipsoids for garnet
porphyroblasts, however, revealed their preferred elongation parallel to the
lineation and fold axes and an average axial ratio of 2.1 (Fig. 9c). Aerden
and Ruiz-Fuentes (2020) recently showed that garnets commonly grow elongated
either parallel or perpendicular to their FIAs due to preferential
nucleation in actively forming microlithon domains and growth controlled by
the pre-existing cleavage within these domains. The six FIAs measured in G20
are all approximately normal to the maximum elongation axes
(X_Gt) of their host garnets (Fig. 9c).
Tectonic interpretationChanges in subduction direction
Evidence has been presented for three regionally developed sets of
inclusion trails with consistent orientations that successively developed in
blueschist–eclogite facies rocks of Île de Groix. All three sets probably formed
on a single prograde metamorphic path reaching 18–20 kbar and 450 ∘C according to detailed petrological work (Bosse et al., 2002; Ballèvre et al.,
2003). Schulz et al. (2001), however, proposed a superposition of two
(Variscan) metamorphic cycles based on complex chemical zoning in
amphiboles, whose relationship with the different generations of inclusion
trails distinguished herein merit further research.
The three sets of inclusion trails have moderate to subvertical orientations and
can be linked to three periods of crustal shortening perpendicular to their
NE–SW, NNW–SSE, and WNW–ESE strikes. The earlier mentioned predominance of
anti-clockwise inclusion trails observed in thin vertical thin sections that
strike parallel to or at a low angle with the regional mineral lineation is
likely determined by the oldest (NE-striking) and youngest (E–W-striking)
inclusion-trail sets as these intersect the thin section planes at a high
angle. The intermediate-age inclusion trail set strikes subparallel to the
regional mineral lineation and hence can be expected to produce inconsistent
curvature senses in the same thin sections (see Hayward, 1990). This may be
why about 25 % of all counted porphyroblasts exhibit an opposite curvature
sense (i.e. clockwise).
Consequently, we interpret the oldest NE–SW-striking inclusion trails to
witness a NW-directed subduction. The polarity of an intermediate subduction
stage, corresponding to NNE–SSW-striking inclusion trails, remains
undetermined by our data, but was probably towards the WSW according to
other geological evidence (e.g. Martinez Catalán et al., 1997). The
asymmetry of the youngest WNW–ESE-striking trails corresponds to the latest
stages of NNE-directed subduction, followed by exhumation.
Inclusion trails in the Pouldu schists have a similar orientation to those of the
youngest set of Île de Groix (see Sect. 3.4). They record continued N–S
compression in the Carboniferous that still generated a late set of
post-metamorphic chevron-type folds with E–W-trending axes on Île de Groix
(see Sect. 6.1.).
A gravitational spreading high-grade thrust nappe?
The average dip of all 103 inclusion-trail planes measured with X-ray
tomography in five samples from Île de Groix is 57∘ (σ=21∘). The average plunge of all 33 measured FIAs is
43∘ (σ=15∘). These relatively steep dips and
plunges imply a limited role, if any, of intermittent gravitational collapse
stages during prograde metamorphism. This is because compression–collapse
cycles should generate subhorizontal FIAs formed by the intersection of
alternating subvertical and subhorizontal foliations. This situation has
been shown in the Appalachians, the European Alps (Bell and Bruce, 2006;
their Fig. 18), and Variscan NW Iberia (Aerden, 2004; his Fig. 4b). The
inclusion trails of Île de Groix, however, formed in the context of a
Late Devonian subduction, before continental collision could have
sufficiently thickened the crust to start off gravitational collapse phases.
A notable difference between eastern and western Île de Groix, apart from
the higher metamorphic grade of the former, is the attitude of the main
foliation. This is particularly clear from structural data of Cogné et
al. (1966) compiled and re-plotted in Fig. 10a. Whereas in eastern Groix
the main foliation dips gently east to south, in the west it dips moderately
to steeply NE or SW. This difference was previously attributed to
large-amplitude folding of a single main foliation with NW–SE axial planes
(see Bosse et al., 2002 – their Fig. 12). We alternatively propose that the
flat-lying foliation in the high-grade eastern domain is more weakly
developed in the lower-grade western domain, where consequently, older
foliations and folds are more widely preserved as shown conceptually in Fig. 10b. This model is consistent with the extrusion and gravitational spreading
of a thrust nappe over a lower-grade footwall (Fig. 10b) as famously
modelled by Bucher (1956) and Merle (1989) and as proposed earlier for
thrust nappes in the Montagne Noire (Aerden, 1998; Aerden and Malavieille,
1999). The model accounts for vertical shortening components associated with
the flat-lying main foliation of the high-grade domain indicated by strain
shadows, shear bands, and quartz fabrics all showing inconsistent shear
senses (Shelley and Bossière, 1999) and by steeply dipping foliations
preserved within porphyroblasts documented here.
(a) Main foliation and lineation data from Cogné et al. (1966) re-plotted in equal-area, lower-hemisphere stereoplots for
higher-grade eastern Groix and lower-grade western Groix. The high-grade
domain has a gently E–S-dipping foliation. The lower-grade area has
moderately to steeply SW- and NE-dipping foliations. The original plot files
are included in the Supplement. (b) Proposed interpretation of
the structural relationship between both domains separated by a thrust
cutting pre-existing folds. (c) Re-drafted field sketch of Boudier and Nicolas (1976; their Fig. 2) of an outcrop at Vallon du Lavoir showing
upright folds overprinted by a subhorizontal crenulation cleavage and
associated refolding. (d) Re-drafted field sketch of Cogné et al. (1966;
their Fig. 5) at Vallon du Lavoir, which we interpret as a tight anticline
overprinted by a horizontal cleavage, in turn overprinted by a SW-dipping
crenulation cleavage. The dashed line has been added .
Detailed field observations of Cogné et al. (1966) and Boudier and Nicolas (1976) at Vallon du Lavoir (central south coast; Fig. 3a) also
corroborate an important role of vertical shortening in the lower-grade
western domain. Both works describe a decametre-scale upright anticline at
this location overprinted by a horizontal crenulation cleavage and related
metre-scale refolding (Fig. 10c and d). This geometry is directly comparable
with the centimetre-scale vertically flattened fold found in sample G14 (Fig. 8c).
Samples G18, G19, and G20 come from the same outcrop near Port Lay (Fig. 3a) where
the main cleavage dips steeply (50∘) NE, despite still belonging
to the high-grade domain and located close to the inferred basal thrust. The
first explanation that comes to mind is that the foliation was originally
flat lying but was later steepened by folding. However, the following
observations suggest otherwise. Firstly, the foliation is parallel to and
partially continuous with a set of NNW–SSE-striking inclusion trails in G20
(Fig. 9c – blue great circles). In contrast, the flat-lying transposition
cleavage at Amer (samples G11, G12, and G14) formed after a younger set of
WNW–ESE-striking inclusion trails (Fig. 7 – orange great circles). Thus, the
main foliations at both locations do not appear to be the same generation.
Secondly, the main foliation in G20 is sharply deflected towards the
horizontal at matrix–garnet boundaries, suggesting the sample was affected by
vertical shortening, although only weakly. This leads us to interpret the
studied outcrop at Port Lay as forming part of a low-strain lens (Fig. 10b).
DiscussionInclusion trail sets vs. folding sequences in the field
Cogné et al. (1966) distinguished three deformation phases. Their first phase
corresponds to tight to isoclinal folds (called “fundamental folds”) with
NW–SE to N–S trends. The second phase caused refolding of the fundamental
folds associated with a SW-dipping crenulation cleavage striking N130–140 as
shown in their field sketch (Fig. 10d). A third set of E–W-trending
chevron-type folds were considered post-metamorphic. The authors describe
variable relationships between the mineral lineation (defined by glaucophane
and epidote) and fold axes as being commonly parallel to each other and
apparently coeval but locally oblique indicating a younger age of the
folding and still elsewhere associated with the second-phase folds. In the
latter case, two sets of oblique glaucophane lineations were reported (their
Fig. 12, p. 70).
Boudier and Nicolas (1976) distinguished four deformation phases (D1–D4), the
youngest of which corresponds to the third-phase (post-metamorphic)
structures of Cogné et al. (1966). D1 refers to the mineral lineation
(L1), and D2 and D3 refer to NNW–SSE-trending folds that partially reoriented
L1. Orientation data of these authors show a good match with the strike
directions of our successive inclusion trail sets (Fig. 11b, c). However,
our new microstructures indicate an opposite relative timing of N165- versus
N120-trending fabrics in better agreement with Cogné et al. (1966) and
Quinquis and Choukroune (1981). Additional evidence for younger
N120 structures was found in a sketch by Claude Audren in 1974
(unpublished as far as we know) kept at the “Maison de la Reserve Naturelle
Le Bail” on Île de Groix. The sketch (redrafted in Fig. 11a) depicts folding
of a N160-trending lineation around the hinge of a N120 fold.
(a) Accurately re-drawn sketch by Claude Audren (1974; unpublished
as far as we know) of isoclinal folds at Vallon de Kérigant and
corresponding structural data. Refolding of a N165-trending lineation around
the nose of a N120-trending fold is indicated. (b) Lineations (L1) measured
at nearby location Vallon du Lavoir by Boudier and Nicolas (1976) showing a
N120 trend maximum of L1 (yellow trend lines) oblique to B2 fold axes (blue
trend lines). (c) Structural data from the same authors collected across the
island. Note the bimodal pattern of L1 reflecting the strikes of the three sets
of inclusion trails distinguished in Figs. 3, 6, 8, and 9.
Formation mechanism of sheath folds
Based on a detailed study of quartz fabrics in 57 samples, Shelley and
Bossière (1999) concluded that most folds of Île de Groix island
nucleated with their axes immediately parallel to the maximum stretching
direction (x), instead of first parallel to the y axes and then rotating
towards x during progressive shearing (see Cobbold and Quinquis, 1980;
Quinquis and Choukroune, 1981). However, they still interpreted a single
kinematic frame with a component of shortening parallel to the intermediate
strain axes (y). The polyphase character of the main (composite) foliation
and lineation demonstrated herein places the significance of fold-axis-parallel stretching and the origin of sheath folds in a different light.
Fold axes could have nucleated with their axes parallel to x due to vertical
shortening of vertical foliations and upright folds whose axes were already
subparallel to x (Fig. 12). Likewise, non-cylindrical folds and sheath folds
may have formed by vertical flattening of precursor folds with subvertical
axial planes but steeply plunging or subvertical axes (Fig. 12). Thus,
depending on the geometry of the precursor folds, new folds nucleated either
with straight axes parallel to x or with strongly curved axes oblique to x.
This model agrees well with the detailed analysis of Audren and Triboulet (1993)
of a sheath from Groix in which they concluded that the fold started to form
towards the end of the prograde path but developed further during the
retrogression.
Conceptual models showing how vertical shortening and horizontal
stretching can have produced highly variable fold geometries depending on the
original orientations of pre-existing folds. Note how strongly curved fold
axes can form without need of extremely large shear trains. Adding a
(horizontal) shearing component in the direction of x and/or y can be
expected to have further modified the fold-interference patterns.
Inclusion trail data vs. shear bands
Predominantly clockwise curvature of inclusion trails in lawsonite
pseudomorphs studied in sample G11 versus anticlockwise in garnets suggests
a change from top-to-the-south to top-to-the-north shearing, provided that
the inclusion trails formed by overgrowth of crenulations (the non-rotation
model). Interestingly, Philippon et al. (2009) deduced a similar switch in
regional-tectonic transport from two sets of shear bands studied along the
south coast of the island. They claimed that high-grade rocks conserving
well-shaped lawsonite pseudomorphs only contain top-to-the-south shear
bands, whereas rocks lacking such pseudomorphs also contain top-to-the-north
criteria or exclusively so. Based on this, it was concluded that prograde
metamorphism was associated with top-to-the-south shearing and was followed
by top-to-the-north shearing, retrogression, and partial destruction of
pseudomorphs. However, our lawsonite-bearing sample G11 cast doubts on this
model as it exhibits mainly top-to-the-north shear-sense criteria in N–S
sections (cross Fig. 13) despite coming from a location where, according to
Fig. 6 of Philippon et al. (2009), top-to-the-south criteria should be
found. Moreover, the authors excluded from their statistical analysis shear
bands occurring close to or within inverted limbs of tight to isoclinal
folds, perhaps because they concluded that the folding post-dates the
shear bands and might have completely overturned them. In the absence of
less ambiguous evidence for different metamorphic conditions of both
shear-band sets and for their timing relative to folding, we consider a
synchronous origin of opposite shear-band sets in bulk coaxial deformation
accompanied by retrogression more likely following Shelley and Bossière (1999). The local predominance of one set may simply reflect the
partitioning of bulk coaxial deformation in zones with opposite
shear senses.
Asymmetric strain shadows and shear bands indicating mainly
top-to-the-north but some top-to-the south shearing as well in a N–S-striking vertical section of G11. These criteria are associated with a
horizontal transposition cleavage that post-dates garnet porphyroblasts.
Note anticlockwise curvature sense of inclusion trails. The FIAs of these
garnets plunge 45∘ west (see Fig. 7) highly oblique to the
(horizontal) matrix foliation.
Comparison with inclusion-trail data from NW Iberia
Figure 14 compares the inclusion-trail and field data presented herein from
southern Brittany with that of Aerden (2004) for the “Basal Unit” of the
allochthonous complexes of NW Iberia. This unit is composed of orthogneisses
and high-pressure mica schists retrogressed to greenschist facies and
represents the subducted margin of Gondwana (Arenas et al., 1995;
Martínez-Catalán et al., 1996; Fig. 2a). Thus, it occupies a similar
structural position as the Pouldu schists below an ophiolitic unit. Three
sets of inclusion trails striking E–W, NE–SW, and NNW–SSE were distinguished
in this unit preserved within plagioclase and garnet porphyroblasts. The
oldest of these has an E–W trend and can be linked to a high-pressure event
dated 370–360 Ma (Li and Massonne, 2017), hence synchronous with prograde metamorphism in Île de
Groix (Bosse et al., 2005). The two younger internal foliations sets formed at
lower pressures, and their age is only loosely constrained to pre-320 Ma.
(a) Inclusion trail and strike measures for the present study and
those of Aerden (2004) for 18 samples of the “Basal Unit” of the
allochthonous complexes of NW Iberia. The microstructures are correlated as
four sets marked pink, blue, yellow, and red from older to younger. Field
data from Boudier and Nicolas (1976), Engels (1972), and van Zuuren (1969)
correlated with the inclusion trails also show a good match. (b)The three rose diagrams plot all blue and yellow inclusion-trail trend lines shown in (a) for 0, 20, and 35∘ Iberia back rotation. The small bar graph gives the test statistic of Watson's U2 two-sample test for 5∘ increments of Iberia back rotation. The lower the test statistic, the better the fit. The analysis suggests no more than 15∘ relative rotation between NW Iberia and Armorica.
The palaeogeographic reconstruction of Fig. 14a fits the northern continental
margin of Iberia to the conjugate margin of south Brittany, which requires
20∘ clockwise back-rotation of Iberia. This amount of
back-rotation produces a remarkably good match of inclusion-trail
orientations in both regions, yet raises a relative timing
problem. Aerden (2004) interpreted that NNW–SSE-striking inclusion trails in
the Basal Unit (marked blue in Fig. 14a) post-date the E–W ones marked
yellow, which is opposite to what has been concluded in this paper for Île
de Groix. Since the relative timing of Aerden (2004) was based on only one sample
(sample 1 – Fig. 14a), we believe there is scope to further test this
chronology by studying new samples with now available 3D techniques. The
timing of NE–SW-striking inclusion trails in NW Iberia (marked red in Fig. 14a) is based on more abundant microstructural and field criteria, ruling
out a correlation with the also NW–SE-striking but much older
inclusion-trail set in Île de Groix. A correlation is possible, though, with
the internal foliations of the two samples from the Central Armorican Domain
and the crenulation cleavages observed in some of the Pouldu schist samples
(Fig. 3c and d).
In order to further assess the goodness of fit of the microstructural data in both regions, we applied a statistical test to all yellow and blue inclusion-trail trend lines in Fig. 14a considered for 5∘ increments of Iberia back-rotation between 0 and 40∘. Watson's U2 test for uniformity of two samples of circular data is appropriate here (Mardia et al. 2009; Agostinelli and Lund, 2017). The results of this test applied to our data (Fig. 14b) show that back rotations of 0–15∘ produce roughly equally good fits, but that above 15∘ the degree of fit worsens rapidly. Significantly, this supports recent plate kinematic reconstructions of the North Atlantic involving only 10–20∘ anticlockwise rotation of Iberia (Jammes, et al., 2009; Nirrengarten et al., 2018; Barnett-Moore et al., 2018).
We checked the curvature sense of rotational inclusion trails in seven of the
Aerden (2004) samples associated with the subduction related E–W- to NW–SE-trending FIAs (marked yellow in Fig. 14a). In samples 3, 4, 10, 14, and 20,
inclusion trails curve anticlockwise viewing west; in samples 2 and 19 they
curve clockwise. The larger number of anticlockwise trails is the same as
observed in Île de Groix, although we realize that this coincidence needs to
be further tested.
Conclusions
Garnet and pseudomorphed lawsonite porphyroblasts in blueschist–eclogite
facies rocks of Île de Groix preserve a succession of three sets of steeply
dipping inclusion trails striking NW–SE, NNW–SSE, and WNW–ESE. They are
interpreted to record three episodes of differently oriented compression
orthogonal to these strikes during subduction-related metamorphism dated
365–355 Ma (Bosse et al., 2005).
The consistency of inclusion-trail orientations across Groix island
implies limited porphyroblast rotation and hence an origin of sigmoid and
spiral inclusion trails by overgrowth of successive crenulation cleavages
(e.g. Bell et al., 1986; Aerden, 1995; Stallard and Hickey, 2001; Aerden and Ruiz-Fuentes, 2020) rather
than progressive shearing and porphyroblast rotation. This re-interpretation
reconciles a previous conflict between the dominant curvature sense of
inclusion trails on Île de Groix and north- or northwestward-directed
subduction of Gondwana under Armorica dictated by the general
tectono-metamorphic zonation of the Ibero-Armorican Arc.
Porphyroblast growth was followed by the development of a subhorizontal
crenulation cleavage accompanying retrogression and exhumation (Shelley and Bossière, 1999) and dated 355–345 Ma (Bosse et al., 2005). This foliation
fully transposed earlier steeply dipping fabrics preserved within
porphyroblasts and produced inconsistent (opposite) shear-sense criteria,
such as asymmetric strain shadows, shear bands, and quartz c-axis fabrics.
Together with centimetre- to decametre-scale fold-interference patterns, this
indicates vertical sub-coaxial shortening, which we tentatively relate to
gravitational spreading of a thrust nappe during continuous
plate convergence.
Fold axis parallel stretching and variable fold geometries ranging from
cylindrical to sheath-like did not result from progressive shearing parallel
to a single foliation (see Cobbold and Quinquis, 1980) but from vertical
shortening and horizontal shearing of pre-existing folds with subvertical
axial planes.
Inclusion trails studied in four greenschist samples from the Pouldu
schists (part of the footwall of the Île-de-Groix ophiolitic nappe) strike
WNW–ESE, subparallel to the youngest set of inclusion trails on Groix
island. They record continued N–S compression in the Carboniferous possibly
synchronous with gravitational spreading in the ophiolitic hanging wall, but
also afterwards when a late set of E–W-trending chevron-style folds
developed on Groix island.
The three sets of inclusion trails documented in southern Brittany can be
tentatively matched to three similar sets in NW Iberia (Aerden, 2004) in a
palaeogeographic reconstruction that places the northern margin of
Iberia back against the conjugate margin of southern Brittany. The alignment
of inclusion trails in this reconstruction suggests that Iberia experienced
not more than 15∘ anticlockwise rotation during the opening of the Gulf of
Biscay.
The supplement related to this article is available online at: https://doi.org/10.5194/se-12-971-2021-supplement.
Author contributions
DA and AF collected the samples. The laboratory work was carried out by DA assisted by ARF for the acquisition and study of X-ray tomographies and by MS for the preparation and study of radial sets of thin sections. The paper was written by the first author, but all authors contributed to the presented interpretation of data.
Competing interests
The authors declare that they have no conflict of interest.
Special issue statement
This article is part of the special issue “The Iberian Massif in the frame of the European Variscan Belt”. It is not associated with a conference.
Acknowledgements
The first author wishes to thank José Ramón Martínez Catalán for his guidance and friendship during a postdoc in Salamanca
(1996–1999). We thank Michel Ballèvre for helping us obtain permission
from the Préfecture du Morbihan to collect samples on Île de Groix and
for suggesting we include the Pouldu schists in our study. Île de Groix
national park guide Catherine Robert (and her dog) provided Domingo Aerden with helpful
information and pleasant company during fieldwork. We thank Fátima Linares-Ordoñez for X-ray scanning our samples and Bernhardt Schulz for
clarifying various aspects about the petrology of the study area. The
authors are very grateful to the two anonymous reviewers, who provided constructive
comments, and the handling editors of Solid Earth, Ícaro Dias da Silva and Federico Rossetti.
Financial support
The research was
supported by Spanish government project CGL2016-80687-R AEI/FEDER and projects RNM148, P18-RT-3275 and B-RNM-301-UGR18 of the Andalusia Autonomous Government.
Review statement
This paper was edited by Ícaro Dias da Silva and reviewed by two anonymous referees.
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