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Periodico di Mineralogia (2015), 84, 3B (Special Issue), 591-614
PERIODICO di MINERALOGIA
established in 1930
DOI: 10.2451/2015PM0434
An International Journal of
MINERALOGY, CRYSTALLOGRAPHY, GEOCHEMISTRY,
ORE DEPOSITS, PETROLOGY, VOLCANOLOGY
and applied topics on Environment, Archaeometry and Cultural Heritage
High-temperature metamorphism and crustal melting:
working with melt inclusions
Omar Bartoli1,*, Antonio Acosta-Vigil1,2 and Bernardo Cesare1
Dipartimento di Geoscienze, Università di Padova, Via Gradenigo 6, 35131 Padova, Italy
Instituto Andaluz de Ciencias de la Tierra, Consejo Superior de Investigaciones CientíficasUniversidad de Granada, Avda. de Las Palmeras nº 4, Armilla 18100, Granada, Spain
*
Corresponding author: [email protected]
1
2
Abstract
The application of melt inclusion (MI) studies to migmatitic and granulitic terranes
is a recent, small-scale approach for a better understanding of melting in the continental
crust. In order to show the role of anatectic MI in providing a wealth of microstructural
and compositional information on high-temperature metamorphism and crustal anatexis, we
review a series of studies on the crustal footwall of the Ronda peridotites (Betic Cordillera,
S Spain), which consists of an inverted metamorphic sequence with granulite-facies rocks
showing extensive melting on top and amphibolites-facies rocks at the bottom. We studied the
microstructures and geochemistry of small (2-10 µm) primary MI hosted in peritectic garnet
of metatexites at the bottom of the migmatitic sequence and of mylonitic diatexites close to
the contact with the mantle rocks. The occurrence of MI is a proof that the investigated rocks
were partially melted at some time in their history, despite other microstructures indicating the
former presence of melt in diatexites were erased by deformation. MI show a variable degree
of crystallization ranging from totally glassy to fully crystallized (nanogranites), consisting
of Qtz+Pl+Kfs+Bt+Ms aggregates (often modal Kfs > Pl in diatexites). Piston cylinder
remelting experiments led to the complete rehomogenization of nanogranites in metatexites
at the conditions inferred for anatexis. Compositions of investigated MI are all leucogranitic
and peraluminous and differ from those of coexisting leucosomes and from melts calculated by
phase equilibria modeling. Systematic compositional variations have been observed between
MI in metatexites and diatexites: the former commonly show higher H2O, CaO, Na2O/K2O
and lower FeO. The compositions of MI in metatexites and diatexites are interpreted to
record the composition of the anatectic melts produced from a peraluminous greywacke i) on,
and immediately after crossing, the fluid-saturated solidus of this metasedimentary rock, and
ii) during anatexis via biotite dehydration melting at increasing temperature, respectively.
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Periodico di Mineralogia (2015), 84, 3B (Special Issue), 591-614
O. Bartoli et al.
While partial melting at the bottom of the migmatitic sequence likely started in the presence
of an aqueous fluid phase, MI data support the fluid-absent character of the melting event in
diatexites. Anatectic MI should therefore be considered as a new and important opportunity
to understand the partial melting processes.
Key words: crustal anatexis; migmatite and granulite; peritectic phase; melt inclusions;
nanogranite.
Introduction
High-temperature (HT) metamorphism and
partial melting (anatexis) of the mid- to lower
continental crust, together with extraction and
ascent of the magma to upper crustal levels,
represent the main agents of differentiation of the
continental crust (Brown, 2010; Sawyer et al.,
2011), and have profound effects on the rheology
of the lithosphere and, as a consequence, on its
geodynamics (e.g., Vanderhaeghe, 2001; Brown
et al., 2011). Anatexis of the metasedimentary
crust produces granitic magmas (Brown,
2013) that can segregate from the source
regions leaving high grade metamorphic rocks
like granulites as residuum (Vielzeuf et al.,
1990), and either form S-type granites in the
upper crust (e.g. Petford et al., 2000; Brown,
2013) or migrate to the surface originating
explosive volcanism (e.g., Pallister et al., 1992).
Migmatites in the middle crust represent both
zones of melt generation and zones of melt
transfer (Sawyer, 2008; Brown et al., 2011).
The wide spectrum of issues related to hightemperature metamorphism and crustal anatexis
justifies the increasing attention that these
topics have received in the last years (Brown
and Rushmer, 2006; Sawyer et al., 2011;
Brown, 2013; Brown and Korhonen, 2009; and
references therein). From a petrological and
geochemical point of view, there is the strong
urge for a better chemical characterization of
natural crustal melts (Sawyer et al., 2011). In fact,
despite the wealth of experimental studies on
crustal melting (Clemens, 2006; and references
therein), and the important information that
they provide, their direct application to natural
contexts is often not straightforward, and
sometimes problematic (see White et al., 2011).
Traditionally,
representative
examples
of natural anatectic melts produced by
metasedimentary protoliths were identified in
S-type granites and rhyolites, and in leucosomes
from migmatites (Brown and Rushmer, 2006;
Clemens and Stevens, 2012). However, the
reliability of the former has been challenged
in recent works that document an important
contamination of primary melt compositions
by entrainment of residual or peritectic material
(Stevens et al., 2007; Clemens et al., 2011;
Clemens and Stevens, 2012 and references
therein). In turn, leucosome chemistry is generally
affected by cumulus phenomena and fractional
crystallization, or presence of pre-anatectic
phases (Sawyer, 2008, 2014; Marchildon and
Brown, 2001). It is therefore apparent that the
composition of natural anatectic melts remains
one of the least constrained parameters in the
petrological modelling of high-temperature
metamorphism and crustal melting.
However “This situation is changing...” as
stated by Sawyer et al. (2011). By studying
the glassy melt inclusions (MI) hosted in
peritectic minerals of metapelitic enclaves from
the Neogene Volcanic Province (SE Spain),
Cesare (2008) and Acosta-Vigil et al. (2007,
2010) demonstrated that during their growth,
peritectic minerals can entrap small droplets
of the coexisting anatectic melt produced
during incongruent melting reactions. The reexamination of more conventional, regionally
metamorphosed migmatite and granulite
Periodico di Mineralogia (2015), 84, 3B (Special Issue), 591-614
terranes has recently proved the common
occurrence of MI also in peritectic phases of
these rocks (e.g. Cesare et al., 2009; Ferrero et
al., 2012; Bartoli et al., 2013b). In most cases
these inclusions totally crystallized upon slow
cooling and now contain cryptocrystalline
aggregates that have been named “nanogranite”
owing to their grain size, texture and chemical/
mineralogical composition (Cesare et al., 2009).
The findings of nanogranite in many anatectic
terranes worldwide (Cesare et al., 2009, 2011;
Ferrero et al., 2012; Barich et al., 2014; Bartoli
et al., 2013b; Darling 2013), along with the
novel experimental approach recently proposed
to successfully re-melt these inclusions in order
to obtain the original composition of the trapped
melt (Bartoli et al. 2013a), open the possibility
for the detailed geochemical characterization
of natural anatectic melts from different
geodynamic settings.
In this paper we review a series of MI studies
on the migmatitic terrane underlying the Ronda
peridotites (Betic Cordillera, S Spain), in order
to show how MI hosted in peritectic minerals
of migmatites and granulites can provide a
wealth of microstructural and compositional
information on HT metamorphism and crustal
anatexis.
Geological setting and sampling
The Betic Cordillera (S Spain) represents the
westernmost part of the peri-Mediterranean
Alpine orogen, formed during the N-S to NWSE convergence of the African and Iberian
plates from Late Cretaceous to Tertiary times
(Andrieux et al., 1971; Dewey et al., 1989). This
study focuses on former anatectic rocks located
structurally below and at the contact with the
Ronda peridotites (Figure 1), which represent
the largest known exposure of subcontinental
lithospheric mantle on the surface of the Earth
(≈ 300 km2; Obata, 1980). The peridotites
occur as km-thick slabs sandwiched in-between
High-temperature metamorphism and crustal…
593
mostly metasedimentary crustal rocks (Lundeen,
1978; Platt et al., 2013). These crustal units are
characterized by their increasing metamorphic
grade, extent of melting and intensity of
deformation towards the contact with the mantle
rocks (Loomis, 1972; Westerhof, 1977; TorresRoldán, 1981, 1983; Tubía et al., 1997, 2013;
Platt et al., 2003; Acosta-Vigil et al., 2001,
2014; Esteban et al., 2008). Mantle and crustal
rocks are separated by ductile shear zones (e.g.,
Tubía et al., 1997, 2013). Different authors
(see Sánchez-Rodríguez and Gebauer, 2000;
Esteban et al., 2011a; Acosta-Vigil et al., 2014;
Massonne, 2014 and references therein) ascribed
the timing of HT metamorphism and anatexis in
the crustal rocks around the Ronda peridotites to
either the Alpine or Variscan orogenies.
In the study area, the mantle rocks are
emplaced over the Ojen nappe (Figure 1c)
which, in general, is formed by ≈ 30 m of
strongly mylonitic rutile-bearing pelitic
gneisses at the very contact with the peridotites,
and ≈ 200 m of rutile-free mostly quartzofeldspathic gneisses, that grade downwards
into undeformed pelitic and quartzo-feldspathic
diatexites and metatexites (Figure 2) (Tubía,
1988; Acosta, 1998). The bottom of the sequence
is constituted by amphibolite-facies Sil-bearing
schists and marbles (Westerhof, 1977; Tubía,
1988). A low-temperature shear zone divides
retrograde migmatites and chlorite-rich schists
(Tubía et al., 1997). Decimetric to decametric
amphibolite lenses that preserve eclogitic
relicts (Figure 2) have been described within
the migmatites (Tubía and Gil-Ibarguchi, 1991;
Tubía et al., 1997), mostly included within the
quartzo-feldspathic metatexites (Acosta, 1998).
This study is primarily focused on MI in the
quartzo-feldspathic metatexite (sample ALP1)
and mylonitic diatexite (sample ALP13). These
samples were collected in the metamorphic
footwall of the Sierra Alpujata peridotite massif
(Figures 1, 2), roughly in correspondence of the
Los Villares transect of Tubía et al. (1997). These
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O. Bartoli et al.
Figure 1. (a) Location map of the study area in the S Spain. (b) Simplified geological map of the western sector
of the Betic Cordillera (modified after Esteban et al., 2011b). (c) Geological map of the Sierra Alpujata massif.
Blue and yellow stars show the location of the studied diatexite ALP13 and metatexite ALP1.
rocks have a bulk rock composition corresponding
to that of Ca-poor, Si-rich peraluminous
greywacke (Si2O ≈ 72-74 wt%, FeO ≈ 2-3
wt%, Na2O ≈ 2 wt%, K2O ≈ 4.5-5 wt%), with
lower alumina content (Al2O3 ≈ 14-14.5 wt%)
and aluminium saturation index (ASI ≈ 1.2-1.3)
than typical pelitic rocks (Bartoli, 2012). In the
field, the metatexite ALP1 shows a stromatic
structure (Sawyer, 2008) with thin (≤ 1 cm) and
discontinuous leucocratic layers surrounded by
a mesocratic matrix (Figure 2). Conversely, the
diatexite ALP13 appears as a deformed, gneissic
rock composed of alternating leucocratic bands
and mesocratic bands (Figure 2).
Petrography
The quartzo-feldspathic migmatites are
composed of varying modal amounts of Qtz, Pl,
Kfs, Bt, Sil and Grt, with minor amounts of Gr,
Ilm, Ap, Zrn and Mnz (mineral abbreviations
after Kretz, 1983). Graphite is randomly
distributed in the matrix of rocks.
Metatexites are fine- to medium-grained (≈ 0.2-
Periodico di Mineralogia (2015), 84, 3B (Special Issue), 591-614
High-temperature metamorphism and crustal…
595
Figure 2. Schematic section of the crustal footwall of the Ronda peridotites at Sierra Alpujata showing the location
of the studied samples (blue and yellow stars as in Figure 1). The field aspect images and photomicrographs
show the macro and microstructural evolution of migmatites as a function of distance to the bottom of the
Ronda peridotite slab. Red arrows show the location of peritectic garnets. Yellow lines show the traces of the
main foliation defined by biotite and/or sillimanite folia.
3.0 mm) rocks made of i) a mesocratic matrix
(containing Qtz+Pl+Kfs+Bt+Sil+Ilm+Ap) that
encloses Grt and Kfs porphyroblasts, and ii)
discontinuous, medium-grained leucocratic
bands (Figure 2). The main foliation (Sp) is
defined by abundant oriented Bt flakes (≈ 8-12
vol.%) generally clustered with fibrolitic Sil
(Figure 3a). Apatite is mostly included within
biotite and sillimanite aggregates. Along with
these minerals, rare (≈ 1vol.%) Ms crystals are
present in metatexites, and appear with resorbed
shapes, included in K-feldspar porphyroblasts
or associated with Bt and Sil (Figure 3b, c).
Notably, fibrolite apparently grew on primary
muscovite (Figure 3c). All these textures suggest
a prograde exhaustion of muscovite. Alkali
feldspar is often poikiloblastic, containing
inclusions of Qtz, Pl, Bt and Sil (Figure 3d).
Garnet (2-5 vol.%) occurs as small (≈ 50-200
μm in diameter) subhedral to euhedral crystals
(Figure 3a). The leucocratic layers (≈ 5 vol.%)
contain Qtz, Pl, Kfs, Bt and rare Grt. Here
feldspars may show euhedral shapes (Figure 3e).
Compared with metatexite, the diatexite
ALP13 is richer in Grt (≈ 5-10 vol.%) and
poorer in Bt (≈ 2-5 vol.%). The fine-grained
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Periodico di Mineralogia (2015), 84, 3B (Special Issue), 591-614
O. Bartoli et al.
Figure 3. Microstructures in
the stromatic metatexite ALP1
(a-e) and diatexite ALP13 (fi). (a) Photomicrograph of the
mesocratic matrix in which
biotite and sillimanite grains
define the main foliation.
Red arrows: graphite lamellae
(b) Resorbed muscovite
armoured
in
K-feldspar
porphyroblast. (c) Primary
muscovite partially replaced
by fibrolite. (d) K-feldspar
poikiloblast with inclusions
of quartz, plagioclase and
biotite. (e) Euhedral faces of
feldspar, suggesting crystal
growth from melt (Vernon,
2011) in a leucocratic band.
(f) Mesocratic matrix showing
fabric-forming
sillimanite.
Red arrows: graphite lamellae.
(g) Garnet crystal partially
replaced by Bt formed
as product of retrograde
reactions during cooling.
(h, i) Photomicrographs of
two leucocratic bands. In
(h) feldspars and quartz are
deformed and elongated,
whereas they display euhedral
shape with planar faces in (i).
Periodico di Mineralogia (2015), 84, 3B (Special Issue), 591-614
(grain size of ≈ 20-200 μm), quartzo-feldspathic
matrix includes porphyroclasts of Grt (0.5-3
mm in diameter) and Kfs (up to 2 cm in size)
(Figure 2). A mylonitic foliation is defined
by alignment of Sil folia and minor elongate
crystals of Bt and Ilm, oriented ribbons of Qtz
and quartzo-feldspathic layers (Figures 2, 3f).
Ms is absent, whereas Sil is both fibrolitic and
prismatic (Figure 3f, g). Bt and minor Bt+Qtz
and Bt+Pl+Qtz intergrowths often grew in the
strain shadows partially to totally replacing Grt
crystals (Figure 3g). Quartz microstructures,
such as chessboard texture, subgrains, irregular
grain boundaries and undulose extinction,
indicate the occurrence of high strain and
dynamic recrystallization at temperatures ≥
650 °C (Stipp et al., 2002). These mylonitic
diatexites are also characterized by, compared
to the metatexites, a greater abundance and
greater thickness (up to 20 cm) of leucocratic
bands (Figure 2), which are mainly composed of
Qtz+Kfs+Pl often showing rounded or elongate
shapes mantled by finer-grained trails (Figure
3h). Locally, leucocratic bands may contain
feldspars displaying euhedral shapes with
straight boundaries (Figure 3i).
Microstructures indicating the presence of
melt such as mineral pseudomorphs after melt
films and pools (Sawyer, 2001; Vernon, 2011)
are abundant in the matrix of metatexite, but
are very rare in the diatexite (Figure 4). The
leucocratic bands of both metatexites and
diatexites, mainly composed of Qtz, Pl and Kfs,
showing a granitic composition (see below) and
containing some igneous microstructures such as
euhedral minerals (Figure 3e, i), are interpreted
as anatectic leucosomes. The crystallization
of these leucocratic portions from an anatectic
melt is also supported by the presence of a more
albitic, euhedral plagioclase (Sawyer, 2001).
Biotite and Bt+Pl+Qtz intergrowths replacing
garnet (Figure 3g) are likely to have formed as
a result of melt-consuming retrograde reactions
(Kriegsman and Hensen, 1998).
High-temperature metamorphism and crustal…
597
Figure 4. Photomicrographs of pseudomorphs
after melt films in metatexite. (a) Plagioclase that
crystallized as a melt pseudomorph around rounded
quartz. (b) K-feldspar with cuspate outlines that
has probably crystallized from a pool of melt. The
reactant minerals, quartz and plagioclase, are rounded
and resorbed. Crossed polars with l plate.
Microstructural characterization of MI
MI have been recognized within garnet in both
types of quartzo-feldspathic migmatites (Figure
5). In the diatexite, MI-bearing garnets are less
abundant (~ 20% of the garnet population) than
in the metatexite (~ 90%). In general, MI are
clustered, forming groups of tens of inclusions
which are often characterized by a similar size.
Clusters, generally displaying a subspherical
geometry, are preferentially located at the
core of small garnets in the metatexite (Figure
5a), whereas they do not have a preferential
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Periodico di Mineralogia (2015), 84, 3B (Special Issue), 591-614
Figure 5. Photomicrographs of the MI-bearing garnets
in metatexite (a) and diatexite (b, c). MI clusters
may show a subspherical geometry (red arrows) or a
spiral-like arrangement (red dotted lines).
arrangement within garnets of the diatexite
(Figure 5b, c). In the latter they may occur both
at the core and close to rim, sometimes showing
O. Bartoli et al.
a sigmoidal to spiral-like geometry (Figure 5c).
No compositional discontinuities have been
observed between MI-rich and MI-free portions
of the garnets, and MI do not form trends along
linear discontinuities of the host crystal.
In transmitted light under the optical
microscope, most MI appear dark-brownish
(Figure 6a) and contain a polycrystalline
aggregate of birefringent crystals under cross
polarized light (Figure 6b, c). Other MI are
transparent in transmitted light and contain
a homogeneous isotropic phase, often along
with a bubble (Figure 6d). Raman spectroscopy
indicated that these bubbles are empty and
therefore represent shrinkage bubbles. In the
diatexite, some dark-brownish MI mantle
fibrolite needles (Figure 6e). The shape of both
types of inclusion is isometric and their size
does not exceed 15 μm (average size ~ 5 μm).
Owing to the small size of MI, their
microstructures
can
be
successfully
characterized with back-scattered electron
(BSE) imaging, using the new generation of
Field Emission Gun (FEG)-based scanning
electron microscopes (SEM) and a working
distance in the range 7-15 mm. MI-forming
crystals, have been identified by acquiring EDS
and Raman spectra, and X-ray maps of the major
elements (see also Ferrero et al., 2012; Bartoli
et al., 2013b). Under SEM investigation, MI
appear typically facetted, and often with a welldeveloped negative crystal shape (Figure 7).
They show a variable degree of crystallization,
even in the same cluster, ranging from totally
(i.e. nanogranites), to partially crystallized, and
down to crystals-free (glassy) MI (Figure 7).
Glassy MI, common in the diatexite, are very
rare in the metatexite. No systematic difference
in size between the different types of inclusions
is observed. Indeed, the size of glassy MI is
often equal to (Figure 7a), and sometimes even
larger than (Figure 7b), that of the partially
crystallized or nanogranite inclusions.
Crystallized inclusions contain aggregates
Periodico di Mineralogia (2015), 84, 3B (Special Issue), 591-614
High-temperature metamorphism and crustal…
599
Figure 6. Photomicrographs of melt inclusions. (a) Plane-polarized image of a MI cluster in metatexite. (b, c)
Plane-polarized and crossed polars images respectively of a crystallized inclusion in metatexite. (d) Glassy
MI containing a shrinkage bubble (red arrow) in diatexite, plane-polarized light. (e) Crystallized MI with a Sil
needle (white arrow) that is likely to have favored the entrapment of melt, plane-polarized light.
Figure 7. SEM-BSE images of coexisting crystallized
and preserved glassy MI in metatexite (a) and
diatexite (b).
of Qtz, Bt, Ms, Pl and Kfs with equigranular,
hypidiomorphic to allotriomorphic texture
(Figure 8). Crystal size ranges from hundreds
of nm to a few μm. The SEM investigation
of tens of MI highlights some differences in
the mineral mode of nanogranites from the
two samples. In the metatexite, MI generally
contain Qtz+Bt+Ms+Pl and rare Kfs (Figure 8a,
b), whereas the assemblage Qtz+Bt+Ms+Kfs
and minor Pl is common in garnets from the
diatexite (Figure 8c, d). The largest grains within
crystallized MI generally consist of subhedral
to euhedral micas, ≤ 2 μm in size, which often
grew starting from the inclusion walls (Figure
8) and are likely to be the first phases to have
crystallized (see also Ferrero et al., 2012).
Feldspars form subhedral to anhedral crystals,
whereas Qtz occurs as an interstitial phase.
Sometimes granophyric to microgranophyric
intergrowths of Qtz and feldspars are present
(Figure 8b), mostly in MI from the diatexite.
Some nanogranites display a variable microto nano-porosity that is more developed in the
samples from metatexite (Figure 8b). Here,
micro-Raman mapping of some crystallized
MI located below the Grt surface documented
the presence of micro- and nano-pores filled
with liquid H2O (Figure 9), suggesting H2O
exsolution during crystallization of hydrous
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O. Bartoli et al.
Figure 8. SEM-BSE images of nanogranites and partially crystallized inclusions in metatexite (a, b, e) and
diatexite (c, d, f). Red arrows: primary nanoporosity.
melts to nanogranites, and preservation of the
H2O within the inclusion.
Partially
crystallized
inclusions
are
indistinguishable from nanogranites under the
optical microscope because of their small size.
The presence of glass together with crystals
is only revealed by SEM investigation. Glass
occupies different area percentages of the MI,
and commonly coexists with Ms, Bt and Qtz in
the partially crystallized MI from the metatexite
(Figure 8e). In the diatexite, rare partially
crystallized MI generally contain only Ms and/
or Bt together with the glass (Figure 8f).
Periodico di Mineralogia (2015), 84, 3B (Special Issue), 591-614
Chemical characterization of MI
The large variability in microstructures and
grain size within nanogranites, coupled with
their small diameters, does not allow a reliable
estimate of their modal composition based on
image analysis. Only glassy inclusions may
be analyzed directly by electron microprobe,
but they are very rare in the metatexite. To
recover complete and meaningful compositional
data, including the volatile contents of melt,
nanogranites and partially crystallized MI in
metatexite must be remelted to a homogeneous
liquid, reversing the phase changes that occurred
during natural cooling.
High-temperature metamorphism and crustal…
601
Experimental re-homohenization of nanogranites
The first attempts to remelt nanogranites
using the routine technique in igneous petrology
(i.e. the one atmosphere heating stage, see
Esposito et al., 2012) resulted in extensive
inclusion decrepitation and interaction with the
host mineral (Cesare et al., 2009). Remelting
experiments try to reverse the phase changes
(crystallization and exsolution of fluids) that
occurred in MI along the cooling path after
entrapment (see Figure 9 in Bartoli et al.,
2013a), and by using a ‘conventional’ heating
stage to conduct these experiments, the internal
pressure of inclusions largely exceeds the
external pressure (i.e., ambient pressure),
Figure 9. Raman mapping of liquid H2O distribution within a crystallized melt inclusion in metatexite. (a)
Representative Raman spectrum obtained from mapping: the peaks at 3620 and 3691 cm-1 correspond to main
OH stretching vibrations in muscovite and biotite, respectively. (b) Investigated inclusion below garnet surface.
(c) Raman map in the 3200-3400 cm-1 stretching region (bounded by red dotted lines in a) of liquid H2O. The
inclusion contains both hydroxylated minerals and free H2O in the pores.
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Periodico di Mineralogia (2015), 84, 3B (Special Issue), 591-614
producing decrepitation of MI volatile loss.
As a consequence of H2O loss, the complete
remelting of nanogranite inclusion occurs at
higher temperature than that of entrapment,
favoring dissolution of host crystal and melt
contamination (Bartoli et al., 2013a). To prevent
these drawbacks, we remelted MI in metatexite
at high-pressure conditions using a singlestage piston cylinder apparatus. Experiments
were run at 700, 750 and 800 °C and 5 kbar,
both under dry conditions and with excess
H2O (see Bartoli et al., 2013a for details on the
experimental procedure). After the experiments
at 700 °C, MI reached a complete melt + vapour
homogenization (Figure 10a). MI still preserve
the original negative crystal shape, suggesting
that the host garnet did not dissolve into the melt
during heating, and therefore that the trapping
temperature was not significantly exceeded.
Higher experimental temperatures (750 and
800 °C) resulted in dissolution of the host into
the melt, as suggested by irregular walls, and
by formation of (i) ≈ 5 μm long decrepitation
cracks extending into the host garnet and (ii)
one or more bubbles (Figure 10b).
Chemical composition of MI
The micrometre scale of the MI required the
use of a focused EMP beam with size of ≈ 1 μm
and analytical conditions chosen to minimize
volatile and alkali loss (see details in Bartoli et
al., 2013c). All the analyzed glassy MI contain
a SiO2-rich, leucocratic and peraluminous melt
(SiO2 ≈ 70-78 wt%, FeOTot+MgO+MnO+TiO2
< 3 wt%, ASI ≈ 1.10-1.20) (Table 1). Glassy
MI in the metatexite have rather constant
composition, with Na2O/K2O from 0.7 to 0.8
and K# ≈ 0.47 [K# = mol. K2O/(Na2O+K2O)].
Conversely, the composition of glassy MI in the
diatexite is much more variable, richer in FeO,
MgO and P2O5 and lower in CaO. In particular,
these inclusions are highly variable in Na and
K contents, such that we have classified them
into two groups: type I, with K# ≥ 0.6 and
O. Bartoli et al.
Na2O/K2O < 0.5, and type II, with K# ≤ 0.5
and Na2O/K2O > 0.6 (Table 1). Type II MI have
been found only in 2 of the 20 investigated
garnets. The analyzed MI correspond to
granites based on their normative compositions
(Figure 11a,b). When plotted in the Qtz-Ab-Or
normative ternary diagram, MI data define two
different clusters according to their K# and the
composition of type II MI in diatexite overlaps
that of glassy MI from metatexite (Figure 11b).
All MI plot in the Qtz field, close to the 5 kbar
cotectic curve and at some distance from the
eutectic melt compositions of the haplogranite
system. It should be noted that the involvement
of Fe, Ti and Ca moves eutectic points and
Figure 10. SEM-BSE images of MI in metatexite after
remelting experiments at 700 °C (a) and 800 °C (b).
Periodico di Mineralogia (2015), 84, 3B (Special Issue), 591-614
cotectic curves toward quartz-richer, albitepoorer compositions, as recently demonstrated
by Wilke et al. (2015) at a pressure of 2 kbar.
High-temperature metamorphism and crustal…
603
Normative compositions of MI re-homogenized
at 700 °C overlap those of coexisting glassy
MI in metatexite (Figure 11c, Table 1). The
Table 1. Major element composition (wt%) of melt inclusions in metatexite ALP1 (data from Bartoli et al.,
2013b) and diatexite ALP13 (data from Bartoli et al., submitted), and of coexisting leucosomes (data from
Bartoli, 2012). Analyses are shown on an anhydrous basis. Numbers in parentheses refer to 1σ standard
deviation.
Melt inclusions
Metatexite
No.
Analyses
76.79
(0.68)
TiO2
0.09
Al2O3
12.98
Diatexite
remelted*
Type I
Type II
37
31
8
3
SiO2
Leucosomes
76.45
(1.08)
78.34
(0.78)
(0.15)
0.05
(0.15)
13.00
(0.08)
0.05
(0.08)
(0.84)
11.66
(0.42)
Metatexite
Diatexite
3
1
78.88 (1.70) 76.35
0.07 (0.07)
(4.48)
75.48
0.13
(0.04)
0.07
11.74 (0.83) 13.28
(2.60)
14.32
FeOT
1.32
(0.12)
1.89
(0.25)
1.56
(0.42)
1.40 (0.36)
0.82
(0.24)
0.64
MnO
0.10
(0.10)
0.18
(0.11)
0.06
(0.06)
0.08 (0.08)
0.02
(0.00)
0.02
MgO
0.08
(0.03)
0.13
(0.08)
0.15
(0.10)
0.15 (0.16)
0.24
(0.05)
0.21
CaO
0.43
(0.21)
0.49
(0.13)
0.07
(0.05)
0.15 (0.18)
0.99
(0.04)
0.82
Na2O
3.40
(0.32)
3.09
(0.45)
2.02
(0.36)
3.17 (0.52)
2.21
(0.20)
2.35
K2O
4.62
(0.27)
4.45
(0.35)
5.92
(0.39)
4.13 (0.34)
5.74
(1.39)
5.90
P2O5
0.20
(0.30)
0.26
(0.22)
0.18
(0.22)
0.23 (0.29)
0.13
(0.02)
0.13
ASIa
1.15
(0.09)
1.20
(0.09)
1.19
(0.10)
1.19 (0.12)
1.13
(0.05)
1.22
Na2O/ K2O
0.74
(0.09)
0.70
(0.13)
0.34
(0.06)
0.77 (0.11)
0.40
(0.06)
0.40
0.63
(0.03)
0.62
41.56 (5.06) 39.07
(8.86)
37.09
K#
0.47
(0.03)
0.49
(0.05)
0.66
(0.04)
0.46 (0.03)
H2O by diff
9.24
(1.61)
8.67
(2.16)
2.42
(1.60)
3.63 (1.08)
35.70
(3.05)
41.26
(2.68)
b
Calculated normative mineralogy
Qtz
34.23
(2.86)
Crn
1.87
(0.63)
2.50
(0.90)
1.86
(0.89)
1.92
(0.72)
2.87
Or
24.77
(1.34)
24.05
(2.3)
34.02
(2.15)
23.50 (1.95) 33.58
1.91 (1.02)
(6.57)
34.43
Ab
26.11
(2.07)
23.85
(3.7)
16.56
(2.98)
25.84 (4.29) 18.53
(1.37)
19.60
An
0.83
(0.72)
0.91
(0.80)
0.13
(0.22)
(0.23)
3.15
0.18 (0.23)
Alumina Saturation Index [= mol. Al2O3/(CaO+Na2O+K2O)]
K# = mol. K2O/(Na2O+K2O)
* experimental run at 700 °C, 5 kbar, 24 h (for details see Bartoli et al., 2013a)
a
b
4.04
604
Periodico di Mineralogia (2015), 84, 3B (Special Issue), 591-614
O. Bartoli et al.
composition of glass from remelted MI at
750 and 800 ºC shows much more scatter
than glassy MI and glass from the 700 ºC
experiments (Figure 11c). In particular, glasses
showing the greatest scatter are those showing
clear microstructural and chemical evidence of
interaction with the host garnet (e.g. irregular
inclusion walls, FeOT > 2.5 wt% and ASI >
1.5; see Bartoli et al., 2013a). The average H2O
content estimated by difference (i.e. 100-EMP
totals) is much lower in MI from diatexite (≈
2.4-3.6 wt%) than in MI from metatexite (≈ 9.2
wt%) (Table 1). NanoSIMS and Raman analyses
on MI re-homogenized at 700 °C confirmed
the high H2O content of these low-temperature
melts (up to 9.8 and 7.6 wt% H2O, respectively;
Bartoli et al., 2013b, 2014).
Melt inclusions in other migmatites
Figure 11. Pseudoternary diagrams showing the
normative An, Qtz, Or and Ab compositions of
all analyzed melt inclusions. Compositions of
leucosomes in metatexite (white squares) and
diatexite (black square) are plotted for comparison.
(a) An-Or-Ab diagram (after O’Connor, 1965). (b)
Qtz-Ab-Or diagram. Black triangle and lines show
eutectic point and cotectic lines for the subaluminous
haplogranite system at 0.5 GPa and aH O = 1; black
2
stars are eutectic points at aH O = 0.6 and 0.4 (Becker
2
et al., 1998). (c) Qtz-Ab-Or diagram for re-melted
MI in metatexite. Data from dry and wet experiments
overlap at 700, 750 and 800 °C (not shown; for details
see Bartoli et al., 2013a).
Although this paper focuses on MI in quartzofeldspathic metatexites and diatexites, MI are
virtually present in all migmatites outcropping
below the Ronda peridotite at Sierra Alpujata.
Here, we briefly report some interesting
microstructural features of MI in other samples
from the same anatectic terrane.
In the pelitic metatexite ALPA35.2 composed of Qtz, Pl, Kfs, Bt, Sil, Crd, Ilm, Gr
and rare Grt - MI may occur in ilmenite (100200 μm across) scattered in the melanosome.
MI (1.5-20 μm in diameter) are grouped with
no preferred microstructural location and appear
typically facetted, sometimes with a welldeveloped negative crystal shape (Figure 12a).
MI are totally crystallized and contain quartz,
plagioclase, biotite and rare apatite.
In the pelitic gneiss ALP14 - composed of
Qtz, Kfs, Bt, Sil, Grt, Crd, Rt, Gr and rare
Pl - outcropping at the very contact with the
peridotite (Figure 2), rutile needles are present
within many of the partially crystallized and
glassy MI (2-6 μm in diameter) hosted in garnet
(Figure 12b). The EMP analysis of the phase
Periodico di Mineralogia (2015), 84, 3B (Special Issue), 591-614
enclosed between the rutile needle and the
host garnet in Figure 12b provided a granitoid
composition with SiO2 = 66.2 wt%, Al2O3 = 12.3
wt%, TiO2 = 2.2 wt%, FeO = 4.1 wt%, Na2O =
0.6 wt% and K2O = 6.7 wt%. Such a composition
strongly suggests that this phase is a glass (i.e.,
the former melt), even though the anomalously
high Ti and Fe contents indicate that the EMP
measurement has been contaminated by the
neighboring rutile and garnet.
Figure 12. (a) SEM-BSE image of an ilmenite crystal
in the pelitic metatexite ALPA35-2, containing
primary MI. (b) SEM-BSE image of rutile-bearing
MI hosted in garnet of the pelitic granulite ALP14.
High-temperature metamorphism and crustal…
605
Discussion
Nature of the studied melt inclusions
In the studied quartzo-feldspathic rocks, MI
have been found in garnet, that is considered to
be a typical peritectic mineral in anatectic Alrich metasedimentary rocks (Thompson, 1982).
The observed zonal arrangement (Figure 5) is
a strong indicator of the primary nature of the
MI (i.e., they were trapped when garnet was
growing, Roedder, 1984). Hence, these MI
represents small samples of the melt coexisting
with the garnet during the prograde anatexis of
the studied rocks. This mode of occurrence (MI
hosted in peritectic phases) is different from that
of MI hosted by phenocrysts in lavas, where the
host mineral crystallized “from” the melt during
cooling. In migmatites, instead, peritectic
minerals and melt form at the same time during
the prograde history, i.e. the peritectic garnet
grows “with” the melt (see Figure 1 in Bartoli et
al., 2014 for details).
As in other migmatitic terranes (Ferrero et al.,
2012; Cesare et al., 2009; Barich et al., 2014),
three type of inclusions were identified in the
investigated quartzo-feldspathic migmatites:
nanogranites, partially crystallized inclusions
and preserved glassy inclusions. The phase
assemblage in crystal-bearing MI (quartz,
plagioclase, K-feldspar, muscovite and
biotite) indicates that the trapped melt likely
had a granitic composition, as confirmed by
the analyses of remelted MI. In the partially
crystallized MI, the glass represents the residual
melt after the partial crystallization of the former
trapped melt. During the crystallization of melts
to nanogranites in the garnets of metatexite, H2O
exsolved in micro- and nano-bubbles (Figure 9),
but it was also consumed by the crystallizing
biotite and muscovite.
Since the composition of nanogranites and
glassy MI in the metatexite is comparable,
their contrasting behaviour upon cooling is
unexpected. Glassy inclusions are common
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Periodico di Mineralogia (2015), 84, 3B (Special Issue), 591-614
in phenocrysts from volcanic rocks which
undergo sudden (minutes to days) cooling
from suprasolidus temperatures. Based on
the statistical study of the size of MI found in
garnets from granulites of the Kerala Khondalite
Belt, Cesare et al. (2009) showed a difference
between the mean diameter of the preserved
glassy inclusions (smaller) and the crystallized
nanogranites (larger), and proposed that
crystal nucleation was inhibited in the smaller
inclusions. The pore size effect is related to
the higher interfacial energy (and consequent
lower stability) of the crystals in smaller pores
with respect to those in larger ones (Holness
and Sawyer, 2008). However, in the present
study the apparent size of glassy MI is often
equal to, and sometimes even larger than, that
of the nanogranites (Figure 7). This suggests
that this range of dimensions (5-10 μm) may
represent a threshold at which additional factors
such as the heterogeneous distribution of
nucleation sites among inclusions (e.g. presence
or absence of irregularities on inclusion walls
or trapped minerals) may significantly affect
nucleation and melt crystallization (Cesare et
al., 2011; Ferrero et al., 2012). In addition, one
of the main compositional differences between
MI in metatexite and diatexite is their H2O
content. High H2O concentrations increase the
diffusivities in MI (Lowenstern, 1995), favoring
the partial or total crystallization of the majority
of MI in metatexite. Conversely, the higher
viscosity of the melt in MI from diatexites
(due to low H2O content) likely inhibited the
crystallization, with the formation of many
glassy MI.
Melting conditions and reactions along the
migmatitic terrane
The occurrence of MI represents a proof that
the investigated rocks were partially melted at
some time in their history (Cesare et al., 2011).
The abundance of MI in the studied rocks clearly
indicates the former occurrence of melt in them,
O. Bartoli et al.
even though deformation and high temperature
annealing in the diatexite have erased most
of the classic microstructures indicating the
former presence of melt, such as subhedral
microstructure and mineral pseudomorphs after
melt films and pools (see Holness and Sawyer,
2008; Holness et al., 2011).
The P-T conditions at which MI were trapped,
and in turn at which rocks melted, have been
investigated by means of thermodynamic
modelling of phase equilibria. The reader may
refer to Bartoli et al. (2013c) for details regarding
the model chemical systems, thermodynamic
databases and solution models used in the
calculation of the phase diagrams. The relevant
compositional isopleths for MI-bearing garnets
cross consistently at ≈ 660-700 °C, ≈ 4.5-5
kbar for the metatexite ALP1, and at ≈ 820-830
°C and ≈ 5.5-6.5 kbar for the diatexite ALP13
(Figure 13), indicating that the temperature and,
to a lesser extent, pressure of melting increase
across the migmatitic terrane towards upper
structural levels.
The entrapment of primary MI may be
promoted by the presence of fine-grained
minerals at the rims of the growing host, which
may act as surfaces to which the melt droplet
could cling (Roedder, 1984), and such “trapped
minerals” may help to constrain the P-T
conditions of anatexis (see Barich et al, 2014).
In the investigated quartzo-feldspathic rocks,
the presence of trapped sillimanite in MI (Figure
6e) supports the inferences obtained from phase
equilibria modeling (i.e. garnet and melt were
produced in the stability field of sillimanite).
Conversely, pre-existing rutile crystals within
MI from the pelitic granulites located structurally
above the quartzo-feldspathic diatexites (Figure
12b) strongly supports the presence of anatectic
melt at higher pressure, in agreement with
independent geothermobarometric estimates
from previous authors (Figure 13).
In the calculated pseudosections for the
quartzo-feldspathic migmatites, after crossing
Periodico di Mineralogia (2015), 84, 3B (Special Issue), 591-614
the fluid-saturated solidus the garnet and melt
modes increase towards higher temperatures,
supporting the peritectic nature of MI-bearing
garnets (Figure 13). Conversely, the amount
of biotite decreases (not shown) indicating that
anatexis in the migmatitic terrane underlying
the Ronda peridotites at Sierra Alpujata largely
occurred by a continuous melting reaction
consuming biotite, up to the Bt-out curve (Figure
13). Metatexites at the bottom of the anatectic
sequence experienced low-temperature melting,
and peritectic garnet in these rocks trapped
the anatectic melt generated immediately
after entering supersolidus conditions (Figure
13). Some microstructures reflect a prograde
exhaustion of muscovite (Figure 3b, c) and
the melt-in line coincides with the Ms-out
curve at the pressure of interest (Figure 13). It
Figure 13: Inferred P-T conditions of equilibration
for the studied metatexite ALP1 and diatexite ALP13
(with data from Bartoli et al., 2013c). P-T estimates
for pelitic granulites (i.e.rutile-bearing diatexites)
are reported for comparison (data from Tubía et
al., 1997). (b) Simplified section reported in Figure
2, showing the relative stratigraphic position of the
rocks reported in (a). See text for explanation.
High-temperature metamorphism and crustal…
607
follows that muscovite could have participated
as reactant to the melting reaction. However,
the trace element contents of MI are needed
to understand in detail the role of muscovite
during anatexis, and in general the nature of
melt-producing reaction in the metatexites (see
Acosta-Vigil et al., 2010).
The high H2O contents (up to 9.8 wt%)
observed in MI from the metatexite ALP1 and
the constraints from phase equilibria modeling
(i.e. that melt was produced immediately after
crossing the fluid saturated solidus; Figure 13),
indicate that partial melting at the bottom of
the migmatitic sequence started in the presence
of an H2O-rich fluid, likely produced by the
subsolidus devolatilization of hydroxylated
phases. The H2O contents (≈ 2-4 wt%) of MI
from diatexite ALP13, however, are close to
the values predicted for H2O-undersaturated
granitic melts at conditions of interest (Holtz et
al., 2001) and support the fluid-absent character
of the melting event at higher structural levels.
An intriguing aspect of our study is that
peritectic garnet in metatexite formed at ≈ 660700 °C, in contrast with results from melting
experiments that predict peritectic garnet
generally developing above 800 °C by the
fluid-absent melting of biotite (see Clemens,
2006 and references therein). Notably, these
low temperatures are consistent with the
complete experimental re-homogenization of
nanogranites at 700 °C (Figure 10a). The results
of our research indicate, therefore, that small
amounts of peritectic garnet may be produced
in natural metasediments starting from as low as
660-700 °C, well below Bt-out conditions. All
the above observations renew the importance
of studying MI to better constrain melting
processes in crystalline basements.
Evolution of melt composition during prograde
melting
A point to be addressed is whether MI are
representative of the bulk anatectic melt in
608
Periodico di Mineralogia (2015), 84, 3B (Special Issue), 591-614
the system at the time of entrapment. Detailed
studies on the geochemistry of MI in migmatites
are just in their infancy (Cesare et al., 2011;
Ferrero et al., 2012; Bartoli et al., 2013b, 2014),
and there is little information available on the
significance of MI compositions yet. On the
other hand, MI hosted in minerals of anatectic
metapelitic enclaves hosted in the felsic
peraluminous lavas of the Neogene Volcanic
Province of SE Spain, have been widely
characterized from the geochemical point of
view during the last 15 years (e.g. Cesare et
al., 2003, 2007; Cesare, 2008; Acosta-Vigil
et al., 2007, 2010, 2012; Ferrero et al., 2011).
On the basis of a huge dataset of major and
trace elements Acosta-Vigil et al. (2010, 2012)
demonstrated that MI compositions do not
represent exotic boundary layers, except for
trace elements compatible with respect to the
host phase, but rather reflect a melt reservoir
with a well-defined geochemical signatures.
Likewise, the analyses of MI presented
here correspond to regular leucogranitic
peraluminous compositions, similar to those of
the MI in the El Hoyazo enclaves or to those
of experimental glasses reported in the literature
(see Clemens 2006 and references therein). Sirich (SiO2 > 70%), peraluminous compositions
have been also reported for MI in xenoliths from
Vulcano Island (S Italy), and were interpreted
as primary anatectic melts formed by melting
of basement metamorphic rocks (Frezzotti et
al., 2004). In addition the compositions of MI
from metatexites and diatexites have a limited
variability in the Qtz-Ab-Or normative diagram
(Figure 11b) and vary as expected during
increasing temperature along the prograde
supersolidus path of these rocks (see below).
Therefore, it is concluded that the major element
compositions of MI presented here represent the
bulk composition of the primary anatectic melt
at the time of MI entrapment.
In order to compare the different types
of anatectic melts observed in the studied
O. Bartoli et al.
rocks (melt inclusions and leucosomes) and
to highlight compositional similarities and
differences, a series of bivariant diagrams have
been created (Figure 14). Compositions of type
II MI in diatexite often overlap those of glassy
MI in metatexite (Figure 11, 14), suggesting
that these rocks have garnet crystals/domains
initially formed at lower T that trapped the
earliest low-temperature melts produced in
the mylonitic diatexites during their prograde
melting. Higher Na2O/K2O values in type II MI
of the diatexite and in those of the metatexite
may also reflect higher aH2O values at the
onset of melting, because the increasing a H2O
depresses the plagioclase + quartz solidus more
strongly than the stability of micas (Conrad
et al., 1988; Patiño-Douce and Harris, 1998),
consuming plagioclase in greater proportion than
biotite during melting. The higher K contents of
diatexite type I MI (Figures 11b, 14a) can be
explained by the progressive consumption of
biotite with rising temperature that produces
an increase in the K2O content of the melt
(Patiño-Douce and Johnston, 1991; Gardien et
al., 2000). Although the FeO content of glassy
MI increases from the metatexite to the diatexite
(Figure 14b) in agreement with the increasing
melting temperature (e.g. Patiño-Douce and
Johnston, 1991), type I MI in the diatexite show
a large spread in Fe concentrations (0.8-2.4
wt%) and seem to define two different clusters
according to their Fe content (Figure 14b). The
lack of a clear positive correlation between
ASI, Mg# and FeO content argues against
any significant melt-host garnet interaction
(Figure 14c, d). Despite the major elements of
MI provide important information on melting
reactions (see above), a detailed characterization
of the trace elements contents is needed to shed
light on the controls on MI compositions and
the mechanisms of crustal anatexis (e.g. melting
reactions, role of accessory phases, equilibrium
versus disequilibrium melting; see Acosta-Vigil
et al., 2010, 2012).
Periodico di Mineralogia (2015), 84, 3B (Special Issue), 591-614
The crustal sequence of Sierra Alpujata
represents the first migmatitic terrane in which
both MI and coexisting leucosomes have been
analyzed and can be compared: although the
compositions of leucosomes are leucogranitic
and approach those of MI, there are differences.
In the Qtz-Ab-Or normative diagram (Figure
11b), three leucosomes have a peraluminous
leucogranitic compositions and plot close to
the Qtz-Or cotectic, suggesting that they may
approach unmodified (i.e. primary) anatectic
melts. A fourth leucosome, far from the cotectic
line, likely represents a composition modified
by accumulation or fractionation processes. The
High-temperature metamorphism and crustal…
609
observation that the compositions of leucosomes
are located away from the eutectic and closer
to the Qtz-Or side when compared to the
coexisting MI may indicate that MI record the
evolution of melt composition during the early
stages of anatexis, whereas leucosomes mostly
reflect the composition of melt at, or closer to,
the peak metamorphic conditions. However, all
the investigated leucosomes show remarkably
similar compositions in the variation diagrams
of Figure 14, often not consistent with primary
melts produced at higher temperatures than
MI. Leucosome chemistry may be commonly
affected by i) cumulus phenomena (Marchildon
Figure 14: Bivariant diagrams showing the compositions of MI and coexisting leucosomes. Symbols as in
Figure 11. The compositions calculated by thermodynamic modeling (red and green asterisks) are plotted for
comparison. (a) CaO vs. K# [K# =mol. K2O/(Na2O+K2O)]. (b) FeOT vs. K# (with all iron treated as FeO and
reported as FeOT). (c) FeOT vs. ASI [ASI = mol. Al2O3/(CaO+Na2O+K2O)]. (d) FeOT vs. Mg# [Mg# =mol.
MgO/(FeOt+MgO)].
610
Periodico di Mineralogia (2015), 84, 3B (Special Issue), 591-614
and Brown, 2001), ii) fractional crystallization
(Sawyer, 2008), iii) entrainment of peritectic
phases (Stevens et al., 2007), iv) entrainment
of crystals from the pre-anatectic framework
(Sawyer, 2014), and v) diffusion of components
towards the residue (White and Powell, 2010).
The high CaO concentrations (Figure 14a, b)
suggest that the investigated leucosomes likely
contain xenocrysts of An-rich plagioclase from
the rock matrix. Indeed, Sawyer (2014) has
recently demonstrated that leucosomes may
enclose entrained minerals as a consequence
of their growth mechanism (i.e. rupture of the
bridges of matrix). In addition, one leucosome
could have been affected by the presence of
residual quartz, as testified by the high content
of normative Qtz (Figure 11a).
In the framework of thermodynamic
modeling of phase equilibria, the calculation
of melt compositions and their reintegration
into the analyzed bulk composition has become
a routine approach to study the prograde P-T
evolution of anatectic rocks that have undergone
melt loss (see White et al., 2004). We notice,
however, that the melt model compositions
calculated at the P-T conditions of interest
differ in terms of K2O, Na2O, FeO, MgO and
CaO when compared to MI, and that although
the compositional departure decreases towards
higher temperatures, model compositions
never match the analyzed MI compositions
(Figure 14). As recently suggested by White
et al. (2011), the current melt model needs
to be improved, and we suggest that such
refinement may take advantage of the analytical
database that is being obtained from anatectic
MI (e.g. Cesare et al., 2011). In any case,
and for those cases where MI are present in
the residual anatectic rock, a more precise
protolith composition (and therefore more
constrained P-T estimations) may be obtained
by reintegrating the composition of the MI
hosted in peritectic phases.
O. Bartoli et al.
Concluding remarks
Peritectic phases of migmatites and granulites
may contain droplets of the coexisting melt
that was being produced during incongruent
crustal melting. The microstructural and
compositional characterization of glassy and
nanogranite inclusions hosted in peritectic
garnet of stromatic metatexites and mylonitic
diatexites from Sierra Alpujata (Betic
Cordillera, S Spain) indicate that partial
melting at the bottom of the migmatitic
sequence started in the presence of an aqueous
fluid phase, immediately after entering
supersolidus conditions, and continued,
particularly towards higher structural levels,
largely under H2O-undersaturated conditions
by progressive consumption of biotite, at
increasing temperatures and, to a lesser extent,
pressure. Notably, compositions of MI differ
from those of leucosomes in the host rocks
and of melts calculated from phase equilibra
modeling.
Based on the study of MI in natural samples,
our work confirms the conclusions of previous
experimental and theoretical studies that
partial melting of the metasedimentary crust
produces peraluminous leucogranitic melts.
However, our results also document that MI
in peritectic minerals represent a unique tool
to obtain in situ quantitative information on
crustal anatexis, making accessible the precise
melt composition for any anatectic terrane. It is
increasingly important that petrologic studies
of partially-melted crystalline basements
integrate classic petrologic tools with results
from MI investigation to better constrain
melting processes. We believe that many
occurrences of MI have been overlooked
because they simply were not searched for,
and that they will be uncovered by careful
re-investigation of migmatites and granulites
worldwide.
Periodico di Mineralogia (2015), 84, 3B (Special Issue), 591-614
Acknowledgements
The authors thank Eugenio Fazio, Patrizia
Fiannacca, Gaetano Ortolano, Rosalda Punturo
(University of Catania) Davide Zanoni and
Michele Zucali (University of Milano) for
the invitation to participate in the workshop
entitled “The art of deciphering structures
and compositions: research advancements
and investigation strategies in the study of
crystalline basements” at the Congresso
congiunto SGI-SIMP (2014), and to contribute
to this special issue. The authors are grateful to
Robert S. Darling, Stefan Jung and to the guest
editor Patrizia Fiannacca for their comments,
which improved this contribution. This research
benefitted from funding from the Italian Ministry
of Education, University, Research (grant PRIN
2010TT22SC to BC), from Padova University
(Progetti per Giovani Studiosi 2013 to OB and
Progetto di Ateneo CPDA107188/10 to BC) and
from the Ministerio de Ciencia e Innovación
of Spain (grant CGL2007-62992 to AAV). The
research leading to these results has received
funding from the European Commission,
Seventh Framework Programme, under Grant
Agreement nº 600376.
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