Journal of Structural Geology 28 (2006) 1734e1747
www.elsevier.com/locate/jsg
Analogue modelling of syntectonic leucosomes in migmatitic schists
Elena Druguet*, Jordi Carreras
Departament de Geologia, Universitat Autònoma de Barcelona, Unitat de Geotectònica, Edifici Cs, E-08193 Bellaterra (Barcelona), Spain
Received 28 February 2006; received in revised form 24 June 2006; accepted 30 June 2006
Available online 1 September 2006
Abstract
Migmatites from the Cap de Creus tectonometamorphic belt display a wide variety of structures, from those formed when the leucosomes
were melt-bearing, to those developed during solid-state deformation. The observed field structures have been modelled by means of analogue
experiments. The materials used in the models are layered plasticine as a schist analogue, and chocolate as analogue of the crystallizing leucosome. A model for the development of syntectonic migmatites is proposed in which initial melt-bearing patches, preferentially formed within
fertile pelitic layers, progressively evolve towards lens-shaped veins. Furthermore, heterogeneous deformation of anisotropic metasediments facilitates formation of extensional sites for further melt accumulation and transport. Melt crystallization implies a rapid increase in effective viscosity of leucosomes producing a reversal in competence contrast with respect to the enclosing schists. During the whole process, deformation
localizes around crystallizing veins, giving rise to different and contrasting structures for melt-bearing and for solid-state stages.
Ó 2006 Elsevier Ltd. All rights reserved.
Keywords: Analogue modelling; Deformation; Melt; Migmatite; Rheology
1. Introduction
Partial melting and deformation are spatially- and temporally-related processes. This is because deformation under
any tectonic regime increases melt mobility and facilitates
the space for melt accumulation, transport and final emplacement (Hutton, 1988; D’Lemos et al., 1992; Brown, 1994; and
numerous articles in Bouchez et al., 1997; Benn et al., 1998
and Castro et al., 1999). Furthermore, there is much evidence for melt controlled strain localization (McLellan,
1984; Hollister and Crawford, 1986; Davidson et al., 1994;
Grujic and Mancktelow, 1998). This double cause-and-effect
framework provides the feedback relations between migmatization (or broad magmatic processes) and deformation (Brown
and Solar, 1998; Brown, 2004).
Crystallization of migmatitic veins implies a dramatic increase in partial melt viscosity controlled by the volume fraction of melt in the solid-liquid aggregate. Since the work of
* Corresponding author. Tel.: þ34 9 3581 1163; fax: þ34 9 3581 1263.
E-mail address: elena.druguet@uab.es (E. Druguet).
0191-8141/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jsg.2006.06.015
Arzi (1978), this drastic rheological change has been studied
by theoretical and experimental means, being a subject of
long-standing debate in petrology and structural geology
(see review in Rosenberg and Handy, 2005, and references
therein). However, its consequences concerning strain localization and development of structures related to syntectonic
migmatites at different scales have been comparatively less investigated (McLellan, 1984; Brown and Solar, 1998). In the
present study particular attention is given to whether these rheological changes are in turn reflected in changes in geometry
and fabrics.
We explore the structural patterns displayed in syntectonic
leucosomes and in the adjacent host mesosome in order to gain
knowledge of the deformation and rheologic factors involved
in their development. This is a necessary approach, since
migmatite structures are often used as tools for resolving the
physical mechanisms that produce melt migration and emplacement and for determination of strain, kinematics and tectonic regimes. For this purpose, a combination of field and
experimental methods has been adopted. Field observations
in migmatitic outcrops from the Cap de Creus massif (Variscan
of the eastern Pyrenees) were essential for recognising the
E. Druguet, J. Carreras / Journal of Structural Geology 28 (2006) 1734e1747
initial geometries of leucosomes and to get first insights into
how are they modified to accommodate deformation. These
natural features were afterwards modelled by means of a series
of analogue experiments. The experimental results are highly
consistent with field hypothese and, moreover, provide more
clues on the development of syntectonic migmatites.
2. Field observations at Cap de Creus
Variscan LP/HT metamorphism, magmatism and migmatization in the Cap de Creus (eastern Pyrenees) are syntectonic
with transpressional deformation. This has been largely verified in previous works (Druguet and Hutton, 1998; Druguet,
2001). The studied area is formed by locally migmatized upper
amphibolite facies schists together with small granitoid bodies
and a swarm of anatectic pegmatites (Fig. 1). Deformation
displays a heterogeneous distribution, producing a km-scale
sub-vertical E-W trending zone of high strain with a steep extension direction (Druguet et al., 1997; Druguet, 2001). The
migmatitic schists which are the object of this study occur in
two broad domains with regard to deformation gradients. One
is a domain of relative low strain where partial melting is only
incipient. The other is the high strain domain, which includes
zones of incipient partial melting but also zones of pervasive
migmatization and igneous activity (the so-called Cap Gros,
Serena, Punta dels Farallons and Tudela migmatite complexes,
Fig. 1). The migmatitic schists display distinctive features in
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these two domains as regard to occurrence, geometry and deformation structures, which will be described in next sections.
2.1. Structures in domains of relative low strain and
incipient partial melting
In these domains, alternating sub-vertical layers of metagreywackes and metapelites form the predominant lithology.
The main foliation (here S1) is sub-parallel to bedding and
more penetrative in the pelitic layers. S1 is a result of the D1
deformational event, which took place at the earliest stages
of the Variscan metamorphism (Druguet, 2001). Although
folded and crenulated to variable degrees during D2 transpressional deformation which developed at peak-metamorphic
conditions, the bedding-S1 fabric is not completely transposed
so that it defines the main anisotropy.
Small isolated leucosome patches appear systematically in
the pelitic layers (Fig. 2), suggesting the local onset of partial
melting in relatively more fertile layers (i.e. those with the
more suitable composition for generating an anatectic melt).
The grain size of the leucosomes varies between 0.5 and 8 mm
and their composition varies from leucogranitic to trondhjemitic
(Druguet et al., 1995), with quartz-plagioclase K-feldspar
cordierite almandine as the main mineral assemblage.
In sections perpendicular to layering, individual leucosomes
have a sub-circular (Fig. 2a), lenticular or very irregular,
patchy (Fig. 2b) shape. They are generally more elongate
Fig. 1. Simplified map of metamorphic and migmatitic zones in the NE Cap de Creus tectonometamorphic belt (after Ramı́rez, 1983; Druguet and Hutton, 1998;
Druguet, 2001). Abbreviations: Chl ¼ chlorite, Mu ¼ muscovite, Bt ¼ biotite, Crd ¼ cordierite, And ¼ andalusite, Sil ¼ sillimanite, Kfs ¼ potassium feldspar,
IPM ¼ incipient partial melting, LSD ¼ low strain domain, HSD ¼ high strain domain. The curved shape of the boundary between the LSD and the HSD is a result
of post-D2 folding and shearing.
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Fig. 2. Field photographs and related sketches of isolated leucosome patches from low strain domains. (a) Discordant, almost circular, pegmatitic patch developed
in a pelitic layer (Lighthouse area). Section is sub-perpendicular to the stretching lineation. Width of view 23 cm. (b) Discordant diffuse leucocratic patch developed in a pelitic layer and showing a weak foliation necking effect (southeast of Cala Serena). Section is sub-parallel to the stretching lineation. Width of
view 14 cm.
in sections perpendicular to layering and sub-parallel to the
L2 stretching lineation, such that leucosomes have a threedimensional roughly fusiform shape. In all cases their occurrence is markedly discordant with layering.
Deflection of bedding-S1 around discordant leucosomes is
another common feature in this zone. In Fig. 2b, the schistosity
in the pelitic layer around the leucocratic patch and the interphase pelite-greywacke are slightly deflected. This type of
structure has been observed in a range of strain intensities
from less to more deformed. In Fig. 3a metapelitic segments
are pinched adjacent to the leucosomes, such that the structures resemble leucosome-filled interboudin partitions. Incipient leucosome patches are associated with incipient necking
(detail shown in Fig. 3b) and larger patches are in turn usually
linked with more marked necking (detail shown in Fig. 3c).
This structure will be named ‘‘foliation necking effect’’.
There are numerous examples of similar structures in the
literature, found in migmatite areas (Ramberg, 1955; Kisters
et al., 1998; Grujic and Mancktelow, 1998, their Fig. 5a;
Ghosh and Sengupta, 1999, their Fig. 5f; Vanderhaeghe,
1999, his Fig. 4g; Bons, 1999; Labrousse et al., 2002, their
Fig. 4d; Marchildon and Brown, 2003, their Fig. 4b; Brown,
2004, his Fig. 2) but also around quartz or calcite veins
from lower grade terrains (Lacassin, 1988; Swanson, 1992).
The classical interpretation is that these structures result
from layer or foliation boudinage, with veins occupying the interboudin partitions (necks, fractures). This is a reliable assessment for areas of pervasive boudinage where boudins develop
in infertile layers of resistant material (e.g. amphibolite, calcsilicate rock). However, in the Cap de Creus, strain localization around soft, partially molten, material is envisaged as
the main process responsible for the pinching of foliation.
Thus, the foliation necking effect does not necessarily correspond to boudinage but it is the main way for a system with
a strong mechanical contrast (molten material and schists) to
accommodate deformation. The main evidence supporting
this interpretation are:
(1) Leucosome patches are restricted to pelitic layers (Figs. 2
and 3), suggesting that sites of initial melting were
controlled by lithology and that subsequent segregation
and accumulation processes were constrained to the pelitic
protolith.
(2) The presence in outcrops that are located close to each other
of a gradient of structures, from small circular blobs to patches bounded by foliation necking patterns (Figs. 2 and 3).
(3) No foliation necking patterns or boudins are observed in
layers without leucosomes and far away from these. In
fact, the competence contrast between metapelitic and
metagreywackic layers during D2 deformation is considered to be low, the metagreywackes being slightly stiffer.
This is probably because the greater amount of quartz in
the greywackes is balanced by the coarser grains in the
metapelites (due to more profuse porphyroblast growth)
but without evidence of a strength reversal such as that described in some other high-grade metamorphic areas (e.g.
Vernon et al., 2003).
(4) The regional maximum stretch is close to 1 in horizontal
sections from low to medium strain domains where
many of the described structures are found, and pegmatites
emplaced at a low angle to the XY plane of finite strain are
not boudinaged and may even be slightly shortened (Bons
et al., 2004).
Progressive deformation in the solid-state (i.e. after full
crystallization of melt-bearing patches) may lead to modification of former structures. This is suggested by the presence of
some isolated leucosomes showing a lensoid shape subconcordant with layering. Also the foliation necking effect is
overprinted by the opposite effect of viscous flow around leucosomes, hereafter named ‘‘foliation wrapping effect’’. Leucosomes at this stage represent competent inclusions with regard
to the enclosing schists. An example of a leucosome which
shows evidence of having been weakly deformed in the solid
state is depicted in Fig. 3d.
2.2. Structures in the high strain domain
The domain of relative high D2 strain is characterized by
close to isoclinal folds of bedding-S1 and a penetrative S2
E. Druguet, J. Carreras / Journal of Structural Geology 28 (2006) 1734e1747
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Fig. 3. Photographs of deformed leucosome patches and schists from the Lighthouse area. Outcrop surfaces are sub-perpendicular to the stretching lineation. (a)
Discordant patches developed in a pelitic layer. There is a necking effect of foliation adjacent to leucosomes and weak solid-state stretching of the patches themselves. Note that the surrounding metagreywackes are less deformed. Width of view 66 cm. (b) and (c) Close-up views of the structures shown in (a). (d) Isolated
pegmatitic leucosome showing a lensoid shape, sub-concordant with layering in the surrounding pelitic schists. Width of view 45 cm.
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Fig. 4. Field photographs of deformed leucosomes. (a) Sub-concordant and concordant leucosomes are preferentially developed in pelitic layers. Note the development of some foliation wrapping effect, more marked in the small lenses at the right of the photograph. Section is sub-parallel to the stretching lineation. Width
of view 24 cm. (b) Detail of a sub-concordant lens-shaped vein probably developed by subsequent solid-state deformation of a discordant patch. Section is subparallel to the stretching lineation. Width of view 14 cm. (c) Stromatic migmatite with some lens-shaped leucosomes. Section is sub-perpendicular to the stretching
lineation. Width of view 33 cm. (a) and (b) are from the Tudela migmatite complex, and (c) is from the Cala Serena migmatite complex.
axial planar foliation. In consequence, the layering defined by
bedding-S1 is less recognisable due to D2 transposition.
Migmatites in this domain range from schists with isolated
leucosome patches (in zones of incipient partial melting) to
more widespread stromatic and nebulitic migmatites (in the
migmatite complexes).
In zones of only incipient partial melting, leucosomes are
restricted to the here transposed metapelitic layers. They exhibit foliation necking patterns developed prior to full crystallization of melt which are similar although generally more
marked with increasing strain than those described above for
the low strain domains.
E. Druguet, J. Carreras / Journal of Structural Geology 28 (2006) 1734e1747
In the migmatite complexes, which are restricted to the
high strain domain, leucosomes are more abundant than in
domains of incipient partial melting. Owing to D2 transposition and pervasive migmatization, a new migmatitic layering
is in general well developed, with alternating irregular bands
of leucosome, melanosome and mesosome (Fig. 4a,c). Leucosomes rarely occur as isolated bodies, but as a series of
patches, lenses or veins. Moreover, several generations of
leucosomes have been recognised on the basis of structural
and crosscutting criteria with regard to the main migmatitic
layering. Leucosome patches also exhibit foliation necking
patterns. However, they have more irregular shapes, with
veins emanating from central pods and cross-cutting the
migmatitic banding. These often leads to the development
of complex vein networks consisting of cracks and discrete
shear domains filled with leucosome (Fig. 5a) which resemble the diktyonitic structures described by McLellan (1988)
or the interconnected leucosome networks of Brown and
Solar (1999).
Often, leucosomes may have recorded deformation in the
solid state, as shown by lenses stretched to different degrees
towards parallelism with layering (Fig. 4b,c and Fig. 6). In
this situation, the foliation wrapping patterns developed in
solid-state conditions prevail over relics of foliation necking
patterns. Also due to solid-state deformation, veins initiated
at a high angle to layering (i.e. the leucosome-filled fractures)
may subsequently become folded (Fig. 5b).
Hence, the structures present in both low and high strain
domains suggest a syn-deformational reversal in competence
contrast between leucosomes and schists. There is a range of
structures around leucosomes whose end members are: (i) foliation necking effect, developed in a partial molten state of
the leucosomes; and (ii) foliation wrapping effect, developed
during solid-state deformation conditions. Their distinction
is of particular interest because it may provide information
on the changes in rheological and deformational behaviour
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of melt e host rock systems during the transition from partial
melting to crystallization. The next section deals with these aspects through experimental approaches.
3. Analogue modelling
The aim of the experimental part of this work is to model
the structures observed in the field and to test the hypothesis
on the development of syntectonic leucosomes. The experiments were performed using the deformation apparatus
BCN-stage, a strain rate controlled prototype which allows
variable temperature operation (Carreras et al., 2000a).
Previous work by other authors on analogue modelling of
partial melts and magmas include microstructural models to
analyse the effect of liquid fraction on the deformation mechanisms at the grain-scale and to gain understanding on melt
migration and segregation processes (Means and Park, 1994;
Park and Means, 1996; Rosenberg and Handy, 2000; Walte
et al., 2005). In their experiments, organic crystalline compounds, such as norcamphor, were used as analogues for the
solid phases and different liquid compounds as melt analogues. Other experiments focused on melt segregation and ascent have used diverse melt analogues, such as in situ
fermentation gas in wet sand (Bons et al., 2001) or partially
molten waxes (Barraud et al., 2004). Grujic and Mancktelow
(1998) explored the influence of weak inclusions (vaseline in
a matrix of paraffin wax) in shear domain development. At
larger scales, magma ascent processes (diapirism and dyking)
and pluton emplacement have been extensively modelled since
the famous work by Ramberg (1967). Among more recent
studies which have modelled syntectonic intrusions are those
by Román-Berdiel et al. (1997) whose experiments were based
on the injection of a low viscosity silicone putty into a sand
pack, and Galland et al. (2003) who used vegetable oil as
magma analogue.
Fig. 5. Field photographs of migmatite structures from the Cala Serena migmatite complex. (a) Network of irregular patches interconnected throughout leucosomefilled fractures. Section is oblique to the stretching lineation. Width of view 28 cm. (b) Photograph of a hand specimen showing a folded and stretched leucosome
vein. Note the presence of both foliation necking and foliation wrapping effects in the adjacent schist. Section is normal to the stretching lineation.
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Fig. 6. Photograph and sketch of leucosome lenses within a stretched melanosome layer (Cala Serena migmatite complex). Outcrop surface is sub-parallel to the
stretching lineation. Pencil for scale is 15 cm long. Note the strong foliation wrapping effect. Boudinage is partly apparent. Incorrect assumption of boudinage of an
initially tabular leucosome vein would give around 200% extension parallel to foliation.
3.1. Analogue materials
As the present study focuses on structures developed in
both the leucosomes and the schists at the transition from
melt-bearing to solid-state system, we have investigated analogue materials to represent: (i) the ductile flow of anisotropic/layered rocks; and (ii) the deformational behaviour of
a crystallizing melt.
A plasticine-vaseline-confetti mixture as an analogue of the
schists: we used plasticine (from the company Oclu-Plast S.A.,
Barcelona) and 9 wt.% vaseline. This mixture was made anisotropic by adding paper flakes (confetti) in different proportions. Carreras et al. (2000b) and Gómez-Rivas et al. (2005)
used similar mixtures to model shear domain development in
anisotropic materials. Plasticine, a non-linear elastoviscous
material, has been widely used as viscous rock analogue and
its rheological properties are well established (McClay,
1976; Zulauf and Zulauf, 2004 and references within).
Chocolate as an analogue of the crystallizing melt: chocolate is a multiphase suspension based on cocoa butter, sugar
and other minor particles. Largely because of its interest to
the food industry, the rheology of chocolate is a matter of scientific investigation. It is also an ideal material for research
into the effects of crystallization and liquid-solid phase
change. Molten chocolate (36e38 C) can be idealized as
a Bingham plastic to a pseudoplastic fluid (Shackleton and
Green, 1993; Beckett, 2000). Below 20e25 C cocoa butter
(which is the melt fraction in chocolate) crystallizes and chocolate behaves as a crystalline solid.
Chocolate is a solid at room temperature, but melts into
a smooth viscous liquid at body temperature (‘‘melts in
your mouth, .’’). What other food has this property?
I can’t think of any (Kovac, 2002).
Its low melting temperature and low temperature interval
for phase change are in the range of temperatures of the experimental device used in this work. Moreover, its viscoplasticBingham behaviour fits with the rheology of most crustal partial
melts (Petford, 2003). At the solid state, chocolate behaves
more rigidly than plasticine, as do granitic veins which are
usually more competent than their metamorphic host rocks.
Despite substantial differences in crystallization process compared to silicate melts, the described properties make chocolate an appropriate analogue of crystallizing melt for the
purpose of this study. This work represents the first attempt
to use chocolate in experimental geology. For the experiments
we used commercial dark couverture chocolate tablets.
Average values of viscosity in nature for both leucosomes
and schists and their equivalent analogues for the experiments
are detailed in Table 1.
Table 1
Values of strain rates and effective viscosities of mid crustal materials and their analogues used in the experiments
Parameter
Strain rate
14 1
Mid crust
10
Laboratory
2.5 105 s1
s
Viscosity of host
18
19
10 e10 Pa s
(schist at 700e500 C)
107 Pa s (plasticine)
Viscosity of partial melt
5
12
10 e10 Pa s
(granitic melt at 700 C)
2e60 Pa s (molten chocolate)
Viscosity of solid vein
1020e1021 Pa s
(granitic vein at 500 C)
1010e1012 Pa s (solid chocolate)
The strain rate value for the mid crust is taken from Pfiffner and Ramsay (1982). Viscosity values are taken from Davidson et al. (1994) for schists, Petford (2003)
for granitic melts and rocks, Gómez-Rivas et al. (2005) for plasticine and Shackleton and Green (1993) for molten chocolate, and extrapolated from data in Barnes
(1999) for solid chocolate.
E. Druguet, J. Carreras / Journal of Structural Geology 28 (2006) 1734e1747
3.2. Model design and deformation conditions
All models consist of 5 mm thick layers of anisotropic plasticine mixture. These layers were assembled in groups of low
anisotropy plasticine (grey plasticine layers with 4.3 wt.%
confetti) and high anisotropy plasticine (white plasticine
layers with 12 wt.% confetti), to represent a cm-alternance
of greywacke and pelitic schists respectively (Fig. 7). The
resulting effective viscosity contrast between high and low
anisotropic layers is very low, w2 (Gómez-Rivas et al., 2005),
the high anisotropic layers being slightly more viscous due
to their higher confetti fraction.
In the high anisotropic layers, several (4 or 6) holes were
perforated with the geometry of cylinders and rectangular
prisms. The holes were subsequently filled with molten chocolate (melted over a bain-marie) before the start of the experiment (Fig. 7). The chocolate filled cavities represent melt
accumulations in the fertile pelitic layers. Former processes
of melt segregation migration were omitted in these models.
The experiments were performed under pure shear, plane
strain boundary conditions at a constant strain rate of
2.5 105 s1, most under layer normal shortening (Figs.
7e10 and 11a) and some under layer parallel shortening
(Fig. 11b). The cylindrical three-dimensional geometries of
the chocolate-filled holes were adopted for simplicity in
view of the limitations of the experiments to plane-strain conditions. All experiments were run at decreasing temperature
although at different thermal conditions by varying initial
(T0) and final (Tf) cell temperatures. Experiments can be
grouped into three main sets according to cooling rates: low
1741
(T0 ¼ 28 C; Tf ¼ 26 C), intermediate (T0 ¼ 28 C; Tf ¼
20 C) and high (T0 ¼ 23 C; Tf ¼ 20 C) cooling rate experiments. Temperature of molten chocolate at the onset of
deformation was constant circa 38 C. Details on experimental
parameters important for the scaling of the models are given
in Table 1.
A bulk finite strain axial ratio RX/Z ¼ 4 (50% shortening
along Z axis) was attained in all tests in a time interval between 7 and 8 h. Photographs of the upper surface of the
models were taken at 10 min intervals. At the end of the experiments, the deformed models were cut into 1 cm thick slices
perpendicular to layering and scanned.
4. Modelling results
Interpretation of the modelling results is based on two procedures. One is the analysis of photographs taken at different
deformation stages (Figs. 8 and 9) to construct the deformational history of individual experiments and to compare the results of different experiments. The second approach comprises
the analysis of finite structures from different scanned slices
(Figs. 10 and 11). This allows a more detailed examination
of the developed structures and also the detection of possible
internal variations.
4.1. Experiments under layer/foliation normal shortening
From the first strain increments, deformation concentrates
around melt inclusions (molten chocolate). This is shown in
Fig. 8 which illustrates four strain stages of an experiment
Fig. 7. (a) Sketch of a model used for experimental deformation under layer normal shortening. Models prepared to deform under layer parallel shortening differ
from this in orientation of the layers at 90 . (b) Photograph of the upper surface of a model specimen at the onset of an experiment under layer normal shortening.
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E. Druguet, J. Carreras / Journal of Structural Geology 28 (2006) 1734e1747
Fig. 8. Photographs of four deformation stages of an experiment performed under layer normal shortening and intermediate cooling rate. Note the progressive
change in shape of the molten phase and the development of foliation necking effect in the layered matrix (see text for further description).
E. Druguet, J. Carreras / Journal of Structural Geology 28 (2006) 1734e1747
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Fig. 9. Photographs of three deformation stages of an experiment performed under layer normal shortening and low cooling rate. Notice the escape of chocolate
through tension cracks and larger shear bands.
performed at an intermediate cooling rate. First, chocolate
bodies are flattened in the XY plane and a set of small radial
cracks form in the high anisotropic layers around chocolate inclusions, which are filled with the chocolate itself. At about
20% shortening, a foliation necking effect is perfectly visible
while flattening and crack propagation around molten chocolate proceeds. The resulting structures are analogous to those
observed at Cap de Creus and characterized by: (i) irregular,
spider-like (Wegmann, 1965) melt patches; and (ii) apparent
boudinage of the high anisotropic layers that embed the melt
patches. Apparent boudins clearly die-out with distance from
the melt patches, the last resembling melt-filled interboudin
necks. Between 40% and 50% shortening, the reversal in viscosity contrast between chocolate and plasticine mixture is attained. Then, there is a halt in foliation necking and chocolate
deforms in the solid state, stretches at a lower rate than plasticine layers and may even boudinage.
In the experiments performed at lower cooling rates all deformation takes place at the molten stage of chocolate, i.e.
chocolate does not solidify. A significant difference with regard to the high-temperature structures developed at the intermediate cooling rate is that for lower cooling rates the
foliation necking effect evolves towards tension cracks and
discrete shear bands through which molten chocolate migrates.
In these situations, different melt patches are interconnected
throughout the new fracture pattern which also cross-cuts the
white low anisotropic layers (Fig. 9).
Finally, fast cooling experiments give rise to a completely
different deformation history and also different finite structures compared to those developed in slow cooling experiments (Fig. 10). In this set of experiments, reversal of
viscosity contrast is attained at 20e25% bulk shortening,
with chocolate first deforming as a weak inclusion (stretching
at a higher rate than plasticine layers). The circular sectional
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Fig. 10. Details of internal sections of two initially identical specimens deformed up to a 50% layer normal shortening and (a) at a low rate of cooling
and (b) at a high rate of cooling. Notice the development of foliation necking
in (a) and foliation wrapping effect in (b).
shapes of the initial chocolate inclusions develop into elliptical
shapes, whereas the rectangular shapes, initially oriented with
their long axes parallel to the shortening axis Z, experience
a decrease in aspect ratio. After the reversal of viscosity contrast, chocolate behaves rigidly with respect to the surrounding
layers, so that solid-state finite structures prevail over moltenstate ones and are characterized by the development of a foliation wrapping effect (Fig. 10b).
4.2. Experiments under layer/foliation
parallel shortening
These experiments also show the effects of strain localization around crystallizing material. In these cases, during deformation of molten chocolate, tension cracks parallel to foliation/
layering nucleate in the plasticine layers adjacent to melt
inclusions, and these are subsequently filled with chocolate,
developing into chocolate veins. These veins, together with
the chocolate remaining in the initial inclusions become progressively deformed, their initial shapes (circles and rectangles)
being modified into irregular, patchy lenses. Deformation of the
enclosing plasticine layers is controlled by chocolate inclusions
during the whole experiment, with folds nucleating around them
(Fig. 11b). Folds are disharmonic, showing large variation in
axial plane orientation, wavelength and amplitude both at
molten and solid states. It is inferred that the irregular fold
patterns are imposed by the irregular melt patches. These folds
have similarities with the ‘‘viscous folds’’ described by McLellan
(1984) as typically developed in partially molten systems.
Fig. 11. Photographs of internal sections of two model specimens after (a) 50%
layer normal shortening and (b) 50% layer parallel shortening, both performed
at a intermediate cooling rate (see text for further description).
5. Discussion
The results of analogue models have many similarities with
the structures observed in migmatite terrains at Cap de Creus.
Information from both natural and experimental syntectonic
migmatites has been integrated, and their significance and
main implications will be treated in the following discussion.
5.1. Role of melt in strain localization
The natural and experimental examples shown in this work
evidence the strong influence of melt in strain localization.
Thus, these can be added to other indicators of melt-enhanced
deformation reported in the literature (McLellan, 1984; Davidson
et al., 1994; Brown and Solar, 1998; Grujic and Mancktelow,
1998; Mancktelow, 2002). In these experiments, the necking
of layering/anisotropy around melt accumulations is the most
distinctive structure developed before cooling of the melt
phase. These structures are geometrically similar to the boudin
neck-like structures observed at Cap de Creus. Foliation
necking effect develops if the molten material stretches faster
than the embedding layer, regardless of the ratio of stretching
between different layers. As demonstrated experimentally,
the presence in anatectic areas of necking structures around
leucosomes does not necessarily imply that the leucosomes
fill interboudin partitions. It could instead be the result of
an opposite effect, with boudinage-like structures localizing
in sites of melt accumulation. This has implications for
E. Druguet, J. Carreras / Journal of Structural Geology 28 (2006) 1734e1747
tectonic interpretations of anatectic areas due to strain overestimations or erroneous structural analysis that would derive
from the assumption of boudinage. To avoid incorrect interpretations particular attention should be paid to those cases
where the foliation necking effect could apply, i.e. where leucosomes are developed in fertile pelitic layers and/or where
equivalent undeformed patches are preserved.
In addition, foliation necking effect may be enhanced by
extension parallel to layer/foliation, as observed in the experiments under layer normal shortening and inferred from the
Cap de Creus structures, or, as deduced from other natural
migmatite structures, may also be favoured by volume loss
due to melt extraction (Bons, 1999). The strong control of
melt pods in strain localization is persistent in the experiments
performed at low cooling rates, in which the foliation necking
effect is followed by the development of discrete shear bands
interconnecting adjacent leucosomes. Grujic and Mancktelow
(1998) and Mancktelow (2002) have already demonstrated that
conjugate shear domains nucleate on weak inclusions and that
they link up to form anastomosing networks of high strain
domains.
Whereas boudin neck-like structures and shear bands are
typical results of strain localization around melt patches under
layer normal shortening, the equivalent effect under layer
1745
parallel shortening is the folding of the anisotropic layers.
As described before, fold geometries reflect a strong melt
phase control on fold nucleation and evolution, as any small
inclusion of molten material will act as an instability which
triggers folding of the layers.
5.2. Reversal of rheological contrast
A unique feature of the experiments presented here is that
they include the effects of phase transition and changing rheology of the inclusions (patches) during deformation. The effective viscosity ratio between inclusions and layered matrix
becomes inverse in a small temperature interval. This is
thought to occur in syntectonic migmatites involving a crystallizing leucosome and a gneissic or schistose mesosome.
As stated before, deformation tends to localize around the
melt-bearing phase, and, as also shown in these experiments,
continues developing around the solid leucosomes after their
crystallization. This gives rise to the development of a range
of structures that can be correlated with different mechanical
behaviours of migmatites, with a rheological threshold marked
by the reversal of competence contrast between leucosome
and host rock. Fig. 12 illustrates a model deduced from the
combination of field and experimental data to explain the
Fig. 12. Idealized sketch of a model for the development of syntectonic leucosomes and related structures based on field observations at Cap de Creus and experimental results. Qualitative relations between time, temperature and strain are shown. During regional deformation, initial melt-bearing pods (formed in pelitic
layers) evolve towards irregular patches and lensoid veins. (a) and (b) refer to two areas with rather different tectonothermal evolution. (a) Structural evolution in
domains of incipient partial melting and low strain. (b) Structural evolution in anatectic domains of high strain. The model is applicable to the Cap de Creus area
and potentially to other migmatitic zones.
1746
E. Druguet, J. Carreras / Journal of Structural Geology 28 (2006) 1734e1747
deformational evolution of syntectonic migmatites. Before full
crystallization, the leucosome is much softer than the mesosome (Table 1) and foliation necking patterns develop at their
interphase. With progressive deformation, initial melt inclusions (preferentially formed in pelitic layers) develop into irregular, discordant patches, with veins emanating from the
partially melted inclusions (Fig. 12). After full crystallization,
the leucosome becomes stiffer than the mesosome and the foliated mesosome wraps around the veins. This foliation wrapping effect may partially or completely overprint the previous
necking patterns. With regard to the inclusions, the irregular
patches continue deforming in the solid state to form elongate
lenses sub-concordant with layering or foliation and, since
they are stiffer than matrix, they may be folded or boudinaged.
Incorrect assumptions concerning the initial sheet-like shape
of leucosomes may lead to errors in estimation of strain
from deformed migmatites.
The reversal of rheological contrast and its consequences in
terms of deformational behaviour are thought to be easily extrapolated to magmatic veins and dykes intruded in mid to low
crustal domains.
5.3. Role of tectonothermal history
The here presented experimental studies show that, under
a constant bulk strain rate, the thermal history has a strong influence on the structural development of syntectonic migmatites. In particular, the role of the cooling rate has been
investigated. In the experiments performed under rapid temperature drop, most deformation occurs after the molten phase
has solidified and thus solid-state structures prevail. On the
contrary, experiments subjected to a slow temperature drop
show the dominance of structures developed at the molten
stage of leucosomes.
On the other hand, migmatite structures from Cap de Creus
reveal that deformation and migmatization processes were
spatially and temporally related. Fig. 12 summarizes these relationships and integrates to some extent the field evidence and
experimental results. Domains of relative low strain and low
temperature are characterized by the presence of isolated
leucosome patches, with foliation necking patterns preserved
or weakly overprinted by late solid-state deformation
(Fig. 12a). In contrast, migmatites that are restricted to domains of relative high strain and pervasive partial melting
(migmatite complexes) exhibit distinct structural patterns.
There, since higher temperatures were likely achieved, partial
melts would have remained longer in the system, several leucosome generations would form and reversal of competence
contrast would have been attained later (Fig. 12b), compared
to the zones of incipient partial melting. Furthermore, high
strain and high temperature conditions would enable the development of shear zones and melt migration along them, similar
to the situation observed in the experiments performed at low
cooling rate. The latest generations of leucosomes would
record the smallest amount of deformation in the partially
molten stage, rapidly deforming in the solid state (Fig. 12b)
as the situation observed in the experiments performed at
high cooling rate.
6. Conclusions
In high grade crustal domains of coeval deformation and
partial melting, deformation tends to localize around syntectonic melts, giving rise to a range of particular structures
that can be correlated with different rheological stages of leucosomes, with a rheological threshold marked by the reversal
of competence contrast between the leucosome and host rock.
This has been investigated by means of field studies and experimental modelling. The experimental results show that the
reversal of viscosity contrast is associated with an increase
in effective viscosity of the leucosomes due to crystallization.
The similarity and coherence between natural and analogue
migmatite structures proves that chocolate is a good analogue
of crystallizing melt at the meso- (and macro-) scale.
The common presence in anatectic areas of foliation/layering deflections around leucosome patches or veins does not
necessarily imply that melt filled boudin necks or extensional
fractures, but it could be the result of an opposite effect: boudinage-like structures localized in sites of melt accumulation,
with consequent implications for strain estimations in migmatite terrains.
Acknowledgements
This article is dedicated to the memory of Elena’s parents,
deceased in 2005. We wish to thank Y.-T. Takeda, P.D. Bons
and A. Griera for helpful discussions on various aspects related to this work, and to L.M. Castaño, and again to A. Griera
and Y.-T. Takeda for laboratory support. We gratefully acknowledge M. Brown and S. Sengupta, whose constructive
reviews greatly improved the manuscript. Many thanks to
I. Alsop for helpful comments and English corrections. This
work was financed by the Spanish Ministry of Education
and Science (projects BTE2001-2616 and CGL2004-03657/
BTE).
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Elena Druguet