JOURNAL OF PETROLOGY
VOLUME 55
NUMBER 8
PAGES 1535^1559
2014
doi:10.1093/petrology/egu033
The Relevance of Crystal Transfer to Magma
Mixing: a Case Study in Composite Dykes from
the Central Pyrenees
TERESA UBIDE1,2*, CARLOS GALE¤1, PATRICIA LARREA1,3,
ENRIQUE ARRANZ1, MARCELIANO LAGO1 AND PABLO TIERZ1,4
1
DEPARTMENT OF EARTH SCIENCES, UNIVERSIDAD DE ZARAGOZA, PEDRO CERBUNA 12, 50009 ZARAGOZA, SPAIN
2
SCHOOL OF NATURAL SCIENCES, DEPARTMENT OF GEOLOGY, TRINITY COLLEGE DUBLIN, DUBLIN 2, IRELAND
3
DEPARTMENT OF GEOLOGY AND ENVIRONMENTAL EARTH SCIENCE, MIAMI UNIVERSITY, 114 SHIDELER HALL,
OXFORD OH, 45056 USA
4
ISTITUTO NAZIONALE DI GEOFISICA E VULCANOLOGIA (INGV), SEZIONE DI BOLOGNA, VIA DONATO CRETI 12, 40128
BOLOGNA, ITALY
RECEIVED DECEMBER 26, 2013; ACCEPTED MAY 27, 2014
ADVANCE ACCESS PUBLICATION JULY 2, 2014
KEY WORDS: composite dyke; crystal transfer; geochemical modelling;
magma mixing; principal component analysis
Two composite dykes containing abundant mafic enclaves within a
felsic host crop out in the Maladeta Plutonic Complex, Pyrenees,
Spain. Field, petrographic and geochemical criteria reveal mixing between gabbroic and aplitic magmas, giving rise to a variety of
hybrid compositions. The rocks contain spongy plagioclase, quartz
ocelli and amphibole^biotite clots that are interpreted as early crystals destabilized by reaction with the hybrid melts. Spongy plagioclase and quartz ocelli were mechanically transferred from the felsic
to the mafic magma, whereas amphibole^biotite clots are former pyroxene crystals from the mafic magma. Multivariate statistics (principal component analysis) have been used to examine variations in
the mineral trace element compositions, which are best explained by
the crystal transfer process. This study shows that crystal transfer
represents a mixing mechanism that can overcome some of the physical limitations of interaction between rheologically contrasting
magmas and explain deviations of the hybrid compositions from the
theoretical mixed chemical composition. In particular, hybrid wholerock compositions show non-linear correlations in inter-element
variation diagrams for elements that are enriched or depleted in preferentially transferred crystals. This effect has been quantified by
extending magma mixing modelling to include crystal transfer. The
composite dykes studied could be regarded as scale models of the behaviour of larger-scale magmatic systems, so this investigation has
important implications for interpreting the petrogenesis of igneous
suites by magma mixing.
The mixing of magmas is a common process in nature. It is
one of the main differentiation mechanisms acting to produce compositional diversity in igneous rocks and it is
implied as a trigger for volcanic eruptions (Leonard et al.,
2002; Martel et al., 2006; Freeley et al., 2008; Kent et al.,
2010; Wiesmaier et al., 2011). Therefore, it has been the
focus of many studies worldwide, in both plutonic and volcanic environments. However, the physicochemical processes involved during magma mixing still remain poorly
understood (Perugini & Poli, 2012).
When two or more magmas mix a variety of hybrids can
develop. The composition of the end-member magmas has
a strong influence on the outcome of mixing; if they are
rheologically similar, mixing is extremely efficient and
produces a homogeneous hybrid, whereas if they have contrasting (e.g. mafic vs felsic) compositions the large thermal and rheological differences between them hinder
hybridization (e.g. Sparks & Marshall, 1986; Frost &
Mahood, 1987; Grasset & Albare'de, 1994; Bateman, 1995;
Perugini et al., 2003, 2008). From a chemical point of view,
* Corresponding author. Telephone: þ35318961244. Fax: þ35316711199.
E-mail: [email protected]
ß The Author 2014. Published by Oxford University Press. All
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I N T RO D U C T I O N
JOURNAL OF PETROLOGY
VOLUME 55
the mixing of two magmas has classically been considered
to generate linear correlations in inter-element variation
diagrams, with hybrid magmas plotting along the theoretical mixing line connecting the end-members. Recent studies, however, have shown that chaotic dynamics may
enhance efficient hybridization between contrasting
magmas and produce non-linear geochemical correlations
between the hybrids (Perugini et al., 2006, 2008; De
Campos et al., 2011).
Because magmas usually carry early crystals, these are
mechanically transferred to the hybrid magmas during
mixing. The transferred crystals typically develop disequilibrium features owing to reaction with the new host
magma composition (e.g. Barbarin & Didier, 1992;
Prelevic¤ et al., 2004; Kumar & Rino, 2006). Accordingly,
it is well established that the occurrence of xenocrysts
transferred from another coeval magmatic system provides evidence of magma mixing (e.g. Vernon, 1984,
1990; Castro et al., 1990; Neves & Vauchez, 1995; Wilcox,
1999; Waight et al., 2000; Baxter & Feely, 2002; Janousek
et al., 2004; Barbarin, 2005; Mu«ller et al., 2005; Renna
et al., 2006; Humphreys et al., 2009; Erku«l & Erku«l,
2012). Despite experimental data that seem to indicate
that crystals are relevant to magma mixing (e.g. Kouchi
& Sunagawa, 1985), the role of crystal transfer has
not been rigorously addressed (Tate et al., 1997;
Perugini et al., 2003; Wiesmaier et al., 2011; Perugini &
Poli, 2012) and recent numerical models and experiments
do not consider it in their approach (e.g. Perugini &
Poli, 2012).
In granitic complexes, the mixing of chemically and
physically contrasting, crust- and mantle-derived magmas
has been commonly called on to explain compositional
variability (e.g. Collins, 1996). However, this interpretation
has been challenged recently because different elements
commonly do not couple in coherent magma mixing
arrays (Stevens et al., 2007; Clemens et al., 2011; Clemens
& Stevens, 2012; Farina et al., 2012). Instead of magma
mixing the above-cited researchers have proposed that
compositional diversity in granitic suites is generated in
the source, mainly by variable degrees of entrainment of
the peritectic assemblage into the magma. Although crystal transfer is invoked in some of these studies, its role in
shaping magma composition is not generally taken into
consideration.
Composite dykes, formed by intrusion of contemporaneous mafic and felsic magmas, occur widely in many bimodal magmatic terrains (e.g. Wiebe, 1973; Snyder et al.,
1997; Katzir et al., 2007; Litvinovsky et al., 2012). They represent natural laboratories in which to study the arrested
interaction between contrasting magmas at the centimetre
scale. Thus, detailed studies carried out on these complex
intrusions can provide the key to a better understanding
of magma mixing.
NUMBER 8
AUGUST 2014
The aim of this contribution is to go deeper into the
understanding of crystal transfer during magma mixing,
based on a study of two composite dykes from the
Maladeta Plutonic Complex (Pyrenees, Spain). A comprehensive petrological, mineralogical and geochemical
study is used to unravel the formation of the dykes in the
context of mixing with crystal transfer and to quantify, for
the first time, the effect of crystal transfer on the composition of the hybrids. The final goal of this contribution is
to determine how and to what extent crystal transfer
modulates the composition of hybrid igneous rocks and,
potentially, entire igneous suites.
The term ‘magma mixing’ is used in the broad sense of
interaction between magmas without implying whether
the final product is homogeneous or not. ‘Mingling’ refers
to the physical dispersion of magmas. ‘Hybridization’
refers to the chemical interaction of magmas, the results
of which are ‘hybrids’. ‘Crystal transfer’ refers to the process
of mechanical exchange of crystals between magmas.
GEOLOGIC A L S ET T I NG
The Pyrenees constitute an Alpine, east^west-trending,
mountain chain that has joined the Iberian and Eurasian
domains since late Cretaceous times. As a result of postorogenic erosion and exhumation (Fitzgerald et al., 1999),
the Paleozoic basement crops out in the central part of the
chain in the so-called Pyrenean Axial Zone (Mattauer &
Seguret, 1971; Fig. 1a). This zone contains several late
Variscan granitic plutons (Castro et al., 2002; Arranz &
Lago, 2004). The Maladeta Plutonic Complex (Fig. 1b) is
one of the largest, exposed over an area of more than
400 km2.
The Maladeta Plutonic Complex was emplaced into
Cambro-Ordovician to Carboniferous metasediments
during the late Carboniferous (Evans et al., 1998). It is composed of two main units: the Aneto Unit to the west and
the Bo|¤ Unit to the east (Fig. 1b). The boundary between
these units is defined by a Variscan ductile shear zone,
reactivated as a fault system during the Alpine orogeny
(Poblet, 1991). This shear zone is responsible for placing different structural levels in the complex at a similar topographic level, with the Aneto Unit representing a deeper
level than the Bo|¤ Unit (Arranz, 1997). Thermobarometric
estimates for pelites of the contact metamorphic aureole
of the Aneto Unit (Delgado, 1993) indicate intrusion conditions of 625 258C (garnet^biotite and garnet^ilmenite
thermometry) and 7^9 km depth (300 50 MPa given the
paragenesis hercynite þ sillimanite below the univariant
equilibrium almandine^sillimanite^hercynite^corundum).
The Maladeta Plutonic Complex has a calc-alkaline affinity and exhibits concentric zoning with discontinuous
outcrops of mafic rocks in the periphery, granodiorites as
the main external facies and granites cropping out in the
central part. Late magmatic dykes cut the plutonic rocks
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UBIDE et al.
CRYSTAL TRANSFER AND MAGMA MIXING
Fig. 1. (a) Location of the Maladeta Plutonic Complex (rectangle) within the Pyrenean Axial Zone [modified from Zwart (1986)]. (b)
Simplified geological map of the Maladeta Plutonic Complex [modified from Arranz (1997)]. The rectangle indicates the study area in (c).
The star indicates the location of the quartz gabbro sample used for comparison. (c) Geological map of the study area (the length and width
of some dykes is exaggerated). Composite dyke 1 is located mostly below the water level of the lake, whereas composite dyke 2 is situated next
to the eastern side of the dam wall. The star indicates the location of the aplite sample used for comparison. Legend is as in (b).
and the metasedimentary country-rocks. Two mafic^felsic
composite dykes were described by Charlet (1979) and
Arranz (1997) from the Bo|¤ Unit at the Cavallers dam
(Fig. 1c). These composite dykes have never been studied
in detail and are the focus of this contribution.
F I ELD A PPEA R A NC E OF T H E
COM POSI T E DY K ES
In the Cavallers dam area (Fig. 1c) several dykes cut
through the granodiorite. The commonest and thickest
are aplite and pegmatite intrusions. Among them, two
mafic^felsic composite dykes occur; one crops out for
more than 100 m on the western side of the lake, mostly
below the water level, so that it can only be observed
when the lake is empty (dyke 1) and the other is tens of
metres long and located on the eastern side of the dam
wall (dyke 2). Dyke 1 has a N050 mean orientation, parallel to that of the host aplite dyke, and is 5^6 m thick
(Fig. 2a); dyke 2 is thinner (2^3 m) and has a N170 strike
(Fig. 2b). Both dykes are subvertical. Some centimetrethick aplite dykes cross-cut dyke 2. Thin lamprophyre
dykes represent the youngest intrusions, as they cut into
all the aplite, pegmatite and composite dykes (Fig. 1c).
The composite dykes contain abundant mafic enclaves
set in a felsic aplite matrix (Fig. 2). The proportion of
mafic enclaves relative to the aplite in each composite
dyke was determined by means of digital image analysis
of several photographs of the best exposed areas, using the
ImageJ 1.45s free software (http://imagej.nih.gov/ij). On
average, mafic enclaves make up 82 vol. % of composite
dyke 1 and 74 vol. % of composite dyke 2.
In both composite dykes, the mafic enclaves have
rounded morphologies and lobate contacts (Fig. 2c and d);
no chilled margins were observed. They are frequently
intruded by felsic veins, which eventually divide the larger
enclaves into smaller ones (Fig. 2c). The margins of neighbouring enclaves commonly fit each other (Fig. 2d); and
mafic protrusionss interpreted as droplets separating from
the enclaves into the aplite are sometimes recognized
(Fig. 2e). The size of the enclaves is variable, with the largest ones usually occurring in the central part of each
dyke. In dyke 1 the enclaves reach metre-scale diameters,
whereas in dyke 2 they are normally centimetre sized.
Detailed examination of the enclaves reveals differences
in colour and grain size (Fig. 2c, f and g). These changes
can be recognized even within the same enclave (Fig. 2c
and f). In contrast, the aplitic portion of the composite
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AUGUST 2014
Fig. 2. Photographs showing the field relationships between the mafic enclaves and the felsic aplite in the composite dykes; in (c)^(h) a pen or a
pencil indicates the scale. (a) and (c) correspond to dyke 1, whereas (b) and (d)^(h) correspond to dyke 2. (a) General view of composite
dyke 1. (b) General view of composite dyke 2, marked with dashed lines. (c) The aplite (FEL) divides (see black arrows) the mafic enclaves,
which have different tones (MAF and INT). (d) The morphology of a disaggregated enclave fits in with the border of the neighbouring one.
(e) Mafic droplet separating from an enclave. (f) The three main rock types: MAF, mafic; INT, mafic^intermediate; FEL, felsic. (g) Detail of
(f); the presence of large plagioclase crystals in the INT-type enclave, apparently transferred from the aplite (see white arrow), and the occurrence of large mafic clots in both enclaves should be noted. (h) Plagioclase crystal from the aplite captured at the border of an enclave.
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UBIDE et al.
CRYSTAL TRANSFER AND MAGMA MIXING
dykes is macroscopically more homogeneous, although it
shows a finer crystal size in the first few millimetres surrounding the mafic enclaves. Three main rock types were
recognized within the composite dykes (Fig. 2c, f and g):
the mafic rock type (MAF) represents dark, fine-grained
enclaves with sharp contacts with the aplite; the mafic^
intermediate rock type (INT) represents lighter-coloured,
coarser-grained enclaves with more diffuse contacts; the
felsic rock type (FEL) represents the aplite surrounding
the enclaves. INT-type enclaves are scarcer, especially in
dyke 1, and they frequently host MAF-type cores (Fig. 2f).
There is no relationship between the size of the enclave
and the enclave type.
A possible mechanical exchange of crystals, or crystal
transfer, has been identified between the contrasting
magma types. Plagioclase and minor quartz crystals
occur inside the mafic enclaves; their dimensions are identical to those occurring in the aplite and larger than the
enclave crystal size (see white arrows in Fig. 2g and h).
They are more abundant in INT-type enclaves, where
they appear more widely distributed. In contrast, they are
normally located close to the enclave margin in MAFtype enclaves. They are also identified in the microcrystalline aplite surrounding the mafic enclaves. Rounded mafic
polycrystalline aggregates (clots) are also observed within
the mafic enclaves (Fig. 2g). They are unevenly distributed
among the enclaves, are very abundant in some of them
(e.g. Fig. 2g) but scarce in others. They have not been
recognized inside the aplite.
S A M P L E S A N D A N A LY T I C A L
M ET HODS
We collected samples covering all the rock types in the
composite dykes. The samples were named first after the
dyke (1 and 2), then after the rock type (FEL, INT and
MAF) and then correlatively with letters (Table 1). For
comparison, felsic and mafic rocks were sampled outside
the composite dykes (samples APL and GAB; Table 1): we
selected the aplitic country rock of composite dyke 2
(Fig. 1c) and a quartz-gabbro located at the border of the
Maladeta Plutonic Complex (Fig. 1b), as no other mafic
rocks crop out near the composite dykes. The classification
and UTM coordinates of all the samples are given in
Table 1. After careful petrographic examination of the
samples, we determined major and trace element concentrations at the whole-rock and mineral scales, with special
emphasis on minerals to test the process of crystal transfer.
A total of 12 samples were selected for whole-rock chemical analysis, including 10 samples covering the diversity
and frequency of rock types in the composite dykes
(Table 1). Contact areas between rock types were sawn off
to avoid contamination, so that each analysed sample is
petrologically homogeneous. Given the presence of
transferred phases, almost all the sample material was
used up for whole-rock analyses to ensure that the analysed
powders were representative. Samples were crushed in
a manganese steel jaw-crusher and milled in an agate
vibrating cup mill at the Servicio General de Apoyo a la
Investigacio¤n (SAI) of the University of Zaragoza
(Spain). Major and trace element concentrations were
determined at the Centro de Instrumentacio¤n Cient|¤ fica
(CIC) of the University of Granada (Spain). Major elements were determined by X-ray fluorescence (XRF;
Philips Magix Pro PW 2440) and trace elements were
determined by inductively coupled plasma-mass spectrometry (ICP-MS; Perkin^Elmer Sciex Elan 5000), following
the methods described by Bea et al. (1999).
For mineral analyses, 35 thin sections were prepared
covering all samples. Major element compositions were
determined on 30 mm thick polished sections by electron
microprobe (EMP) at the Servicios Cient|¤ fico-Te¤cnicos of
the University of Oviedo (Spain), using a CAMEBAX
SX-50 electron microprobe equipped with four wavelength-dispersive spectrometers. Analyses were performed
using an accelerating voltage of 15 kV and an electron
beam current of 15 nA, with a beam diameter of 3 mm.
Counting times were 20 s for Na and 10 s for the rest of the
elements. Analyses were corrected using a ZAF procedure.
Precision was 0·5^6% for oxides with concentrations
41·5 wt % and 510% for oxides with concentrations
51·5 wt %. Trace element concentrations were determined
on 100 mm thick sections cut from the same rock slices as
used for preparing EMP sections by laser ablation (LA)ICP-MS at the CIC of the University of Granada (Spain),
using a 213 mm Mercantek Nd-YAG laser coupled to a
quadrupole Agilent 7500 ICP-MS system with a shielded
plasma torch. The ablation was carried out in a He atmosphere, using a laser beam with a diameter fixed at 80 mm,
a repetition rate of 10 Hz and output energy of 1mJ per
pulse. The ablation time was 60 s and the spot was preablated for 45 s with a laser output energy of 0·3 mJ per
pulse. NIST-610 glass was employed as an external
standard and silicon concentrations obtained by electron
microprobe as an internal standard. Further details on the
technique have been given by Bea et al. (2005).
P E T RO G R A P H Y
The three rock types in the composite dykes (FEL, felsic;
INT, mafic^intermediate; MAF, mafic), defined on the
basis of their field appearance show systematic differences
under the microscope (Figs 3 and 4; Table 1).
The aplitic portion of the composite dykes (FEL rock
type) has an equigranular texture mainly composed of
plagioclase, quartz, K-feldspar and biotite with an average
crystal size of 600 mm (Fig. 3a). Most biotite crystals have
bladed habits and there is accessory acicular apatite.
Some plagioclase crystals have inclusion-rich cores
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Table 1: Summary of studied rocks
Emplacement:
Composite dyke 1
Composite dyke 2
Aplite dyke
Qz-gabbro
UTM coord.:
31T 323894
31T 324095
31T 324080
31T 300973
4717544
4716908
4716863
4724874
Rock type:
FEL
INT
MAF
FEL
INT
MAF
APL
GAB
Samples:
1FEL-a,
1INT
1MAF-a,
2FEL
2INT-a,
2MAF-a,
APL
GAB
2INT-b
2MAF-b
Aplite
Tonalite
Qz-diorite
Aplite
Qz-gabbro
1FEL-b
Classification:
Aplite
1MAF-b
Qz-diorite
Qz-gabbro
Mineral assemblage (averages)
% Pl
38
54
50
30
65
50
40
58
% Kfs
25
—
—
25
—
—
25
—
% Qz
30
10
10
40
12
5
30
6
% Amp
—
15
18
—
5
25
—
15
5
20
20
5
18
20
2
12
51
1
2
51
51
51
51
51
% Bt
% Opq
% Cpx
—
—
—
—
—
—
—
3
% Opx
—
—
—
—
—
—
—
3
% Ms
2
—
—
—
—
—
2
—
Disequilibrium textures
% Spongy Pl
51*
0–5
0–1
51*
0–6
0–2
—
—
% Ocellar Qz
51*
1–4
1–3
51*
3–6
1–4
—
—
% Amp in clots
—
0–1
2–4
—
0–6
10–20
—
—
% Bt in clots
—
1–8
0–1
—
5–18
1–5
—
—
% Ap in clots
—
51
51
—
51
51
—
—
Modes were calculated on thin sections by visual estimation under the microscope. UTM datum: WGS84. Mineral
abbreviations are after Whitney & Evans (2010).
*Present only close to the contact with mafic enclaves.
overgrown by an inclusion-free rim, implying two clearly
distinct crystallization stages. Close to the contact with
mafic enclaves, biotite becomes more abundant and has a
platy habit; in addition, the texture of the rock usually becomes microporphyritic (Fig. 3b), with 20^80% volume
fraction of plagioclase and quartz crystals with sizes similar to those in the normal areas, set in a finer-grained
groundmass (with an average crystal size of 100 mm). For
the sake of simplicity, we continue to refer to the felsic
host material as aplite, keeping in mind its textural variability. The larger crystals have rounded cores overgrown
by a rim in optical continuity, which commonly includes
groundmass microcrysts; the core and rim areas are separated by an inclusion-rich boundary. In the case of plagioclase, the cores sometimes show signs of resorption and
occasionally spongy cellular textures (Fig. 4a) as defined
by Hibbard (1995; hereafter referred to as ‘spongy textures’) and the overgrowths are idiomorphic to subidiomorphic. In the quartz crystals, the inclusion-rich border
is frequently thicker and rich in biotite crystals, sometimes
defining quartz ocelli. The contact between the aplite and
the mafic enclaves is not sharp, but defined by a continuous
interlocking and exchange of crystals (Fig. 4a).
The mafic^intermediate enclaves (INT rock type) comprise rocks of quartz-diorite to tonalite composition. They
have a microporphyritic texture (Fig. 3c) defined by large
(c. 600 mm) plagioclase crystals, quartz ocelli, clusters or
clots of amphibole^biotite crystals and bladed biotite crystals set in a finer-grained (c. 100 mm) groundmass that is
mainly composed of plagioclase, biotite and quartz. The
cores of the large plagioclase crystals show resorption features, usually developing spongy textures; in contrast to
the FEL rock type, spongy plagioclase cores in INT-type
enclaves frequently include acicular biotite and apatite
crystals and an accumulation of epidote microcrysts at the
core border. Regarding the quartz ocelli (Figs 3c and 4c),
their biotite-rich border is thicker (up to 200 mm) than in
the microporphyritic areas of the FEL rock type. Spongy
plagioclase and quartz ocelli form up to 12 vol. % of the
rock (Table 1). The clots (Fig. 3c) are polycrystalline aggregates composed of subidiomorphic to allotriomorphic, prismatic, light green amphibole crystals. Their central areas
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CRYSTAL TRANSFER AND MAGMA MIXING
Fig. 3. Photomicrographs of the studied samples. Parallel (left) and crossed (right) polars, transmitted light. For comparison, the same magnification was used in all the images. (a) FEL rock type. (b) FEL rock type; the continuous white line marks the contact between the main equigranular area (right), equivalent to (a), and a microporphyritic area (left) developed close to a mafic enclave; the large plagioclase and
quartz crystals transferred from the equigranular area into the microporphyritic area (see white arrow) should be noted. (c) INT rock type.
(d) MAF rock type. (e) Aplite sample used for comparison. (f) Quartz gabbro sample used for comparison. In this and subsequent figures mineral abbreviations follow the recommendations of Whitney & Evans (2010).
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Fig. 4. Detailed photomicrographs of the studied samples. Transmitted light; (a) and (b) with crossed polars and (c)^(f) with parallel polars.
(a) Contact between the aplite and an MAF-type enclave, defined by a continuous interlocking of crystals (see white arrow); the spongy cellular
texture of the plagioclase crystal in the aplite in contact with the mafic enclave should be noted. (b) Spongy core of plagioclase in an MAFtype enclave; in contrast to the spongy crystal in (a), this crystal has mafic inclusions and an accumulation of epidote microcrysts at the core
border. (c) Ocellar quartz in an INT-type enclave; the rim is rich in biotite. (d) Ocellar quartz in an MAF-type enclave; the rim is rich in
amphibole and thicker than in (c). (e) Biotite clot rich in apatite inclusions in an INT-type enclave. (f) Amphibole clot with a small cluster of
biotite crystals in an MAF-type enclave; it should be noted that the prismatic morphology of this clot resembles the shape of a previous phase
and is better defined than that of the clot in (e).
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CRYSTAL TRANSFER AND MAGMA MIXING
are usually replaced by clusters of biotite crystals, which, in
some cases, replace the whole clot (Fig. 4e). Single crystals
of opaque minerals are in some cases related to biotite
inside the clots. Apatite inclusions are frequent in the biotite crystals, especially in clots where amphibole is mostly
or completely replaced by biotite. Quartz crystals commonly occur at the border of biotite-rich clots. The clots
display prismatic to rounded sections, indicating that they
probably represent pseudomorphs of previous phases.
The mafic enclaves (MAF rock type) are of quartzgabbro to quartz-diorite composition. They are broadly
similar in texture and mineral assemblage to INT-type enclaves, but show certain distinct features (Fig. 3d). In
MAF-type enclaves, amphibole predominates over biotite
(Table 1). This implies that the clots (Fig. 4f) and the borders of quartz ocelli (Fig. 4d) are mainly composed of
amphibole and include little or no biotite. It also implies
that both amphibole and biotite are present as mafic
phases in the rock groundmass. Large plagioclase crystals
are scarcer and in all cases they have spongy cores rich in
inclusions of mafic minerals (Fig. 4b). No bladed biotite is
observed. The groundmass is frequently finer-grained
with an average crystal size of 50 mm. Amphibole clots
occur in variable proportions and are especially frequent
in dyke 2, where they can form up to 20 vol. % of the rock
(Table 1).
The aplitic country rock selected for comparison
(sample APL) has an equigranular texture composed of
quartz, plagioclase, K-feldspar and muscovite with an
average grain size of 600 mm (Fig. 3e). Plagioclase crystals
are usually the largest and have a subidiomorphic habit;
they are commonly albitized. Quartz crystals, in contrast,
are usually allotriomorphic. The sizes of both plagioclase
and quartz crystals are similar to those of spongy plagioclase and quartz ocelli present in the composite dykes.
This sample is very similar in mineralogy and grain size
to the equigranular areas of the FEL rock type (compare
with Fig. 3a); however, sample APL lacks the characteristic
bladed biotite and acicular apatite of the FEL rock type.
The quartz-gabbro sample used for comparison (GAB)
has an equigranular texture composed of plagioclase,
amphibole, biotite, quartz, clinopyroxene and orthopyroxene with an average grain size of 600 mm. Most of the
amphibole is observed replacing pyroxene crystals, especially clinopyroxene, as indicated by the remnant cores
(Fig. 3f). The size of the pyroxene crystals is similar to the
size of the amphibole^biotite clots occurring in the
enclaves.
W H O L E - RO C K C H E M I S T RY
The major and trace element whole-rock compositions of
the composite dykes and the aplitic country-rock are
given in Table 2. This table also includes the composition
of the reference quartz-gabbro sample, taken from Arranz
(1997). The samples have low loss on ignition (LOI)
values, which indicate a low degree of alteration, especially
for the felsic rocks.
The whole-rock compositions are plotted together with
fields of mineral compositions in binary major and trace
element diagrams (Fig. 5). The reference samples (APL
and GAB) show the most extreme whole-rock compositions for many major and trace elements. Samples from
the composite dykes plot between these, with the aplite
compositions (FEL) close to APL. The enclave compositions (INT and MAF) are either close to GAB or intermediate between GAB and APL. For CaO, Al2O3, Fe2O3,
TiO2, Na2O, K2O, Sr, Y, Nb, Ta, Zr and Th, enclave compositions plot between APL and GAB and the mineral
compositions are also relatively close by. In contrast, for
MgO, Pb and the transition elements V, Cr, Co, Ni and
Zn, mineral compositions plot away from APL and GAB
and enclave compositions plot displaced towards amphibole compositions. For mobile elements such as the large
ion lithophile elements (LILE) Ba, Rb, Cs and Be, and
Cu and U, enclave compositions are not aligned between
APL and GAB, nor are they close to mineral compositions.
Regarding the rare earth elements (REE), enclave compositions vs La are progressively more enriched relative to
APL^GAB from Ce, where correlation with La is good
and minerals plot close to the whole-rocks, to Lu, where
the minerals plot away from the rocks and the enclaves
are displaced towards amphibole (Fig. 5).
Primitive mantle-normalized patterns of REE are similar in shape for all rock types in the composite dykes
(Fig. 6), with La/LuN values ranging from 4·0 to 6·6 and
negative Eu anomalies in all cases. The mafic enclaves are
more REE enriched than the aplite, consistent with mafic
rocks (e.g. sample GAB) and aplites (e.g. sample APL)
from the Maladeta Plutonic Complex. In each composite
dyke, MAF-type enclaves are more enriched in light REE
(LREE) compared with INT-type enclaves, whereas
both enclave types have similar heavy REE (HREE)
abundances.
M I N E R A L C H E M I S T RY
The various petrographic types of plagioclase, biotite and
amphibole in the composite dykes were analysed for their
major and trace element compositions. For comparison,
plagioclase was also analysed in sample APL and amphibole and pyroxene were analysed in sample GAB.
Major elements
Representative electron microprobe analyses are given in
Supplementary Data: Table I (all supplementary data are
available for downloading at http://www.petrology.oxfordjournals.org). Plagioclase structural formulae were calculated on an 8-oxygen basis; compositions vary from
anorthite to oligoclase (Fig. 7a). The most extreme
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Table 2: Major (wt %) and trace (ppm) element composition of the analysed whole-rock samples
Sample:
1FEL-a
IFEL-b
SiO2
72·76
73·93
TiO2
0·12
0·09
Al2O3
14·69
Fe2O3T
MnO
1MAF-a
1MAF-b
2INT-a
2INT-b
2MAF-a
2MAF-b
55·17
55·99
56·18
0·81
1·05
1·02
73·99
65·38
62·54
55·33
54·11
74·20
0·09
0·46
0·55
0·75
0·78
0·07
14·53
17·12
17·97
18·27
13·89
15·62
15·96
16·50
16·48
13·85
1·41
1·06
7·41
0·04
0·03
0·20
7·17
6·99
1·15
4·26
4·95
7·14
7·43
0·71
8·85
0·14
0·13
0·03
0·08
0·11
0·14
0·16
0·02
MgO
0·48
0·32
0·18
5·79
4·60
4·65
0·29
3·58
4·29
7·06
7·40
0·16
CaO
1·31
4·95
1·53
8·06
8·02
7·19
1·16
4·45
5·30
7·05
7·06
0·71
Na2O
9·11
3·14
3·89
1·90
1·38
1·04
3·01
2·51
2·10
1·68
1·50
2·81
1·57
K2O
4·69
3·63
1·91
1·87
2·72
5·27
2·15
2·17
2·72
3·00
5·57
0·99
P2O5
0·16
0·18
0·13
0·19
0·20
0·15
0·13
0·12
0·13
0·13
0·10
0·24
LOI
0·42
0·42
0·88
1·11
1·00
0·21
0·80
1·46
1·17
1·20
0·79
1·80
99·22
99·61
99·39
99·49
99·39
99·23
99·42
99·55
99·66
99·25
99·00
99·70
Total
1INT
Rb
259
161
147
144
269
Cs
34
10
28
34
48
Be
7
4
8
2
3
2FEL
189
9·2
15
126
128
168
188
21
14
29
25
12
9
4
5
APL
197
GAB*
53·1
1·11
17·8
42
6·3
3·9
8
3
Sr
71
94
250
312
253
99
190
179
225
193
86
283
Ba
107
164
223
367
354
128
218
163
191
169
131
170
V
19
10
Cr
164
b.d.l.
194
177
163
8
98
105
178
173
316
193
156
134
243
240
390
472
b.d.l.
15
154
170
Co
3
b.d.l.
26
20
17
1
16
18
30
30
b.d.l.
18
Ni
30
b.d.l.
75
13
5
5
69
67
124
129
b.d.l.
15
Cu
6
6
11
10
8
5
27
27
32
19
b.d.l.
12
Zn
22
18
135
73
88
13
47
53
69
78
b.d.l.
88
Ga
19
16
21
21
20
15
18
16
18
17
15
Y
10
12
26
27
28
13
23
24
23
23
12
Nb
15
14
Ta
Zr
Hf
Pb
4·6
33
1·6
70
2·6
32
1·4
41
7·9
0·8
63
10
0·9
81
9·8
0·8
97
2·4
2·5
2·6
7·6
5·3
4·5
9·9
1·8
24
1·2
70
7·8
1·3
60
2·2
25
6·9
1·0
67
2·1
17
9·4
0·9
69
2·4
17
7·3
0·8
73
2·1
15
4·9
0·9
31
1·2
55
21
27
11
0·9
81
2·2
10
U
4·6
5·2
3·8
2·8
3·0
3·6
7·4
5·4
4·9
4·3
2·5
3·4
Th
3·1
2·4
9·2
7·6
7·2
3·4
5·7
5·6
6·8
6·3
3·0
7·9
La
Ce
7·1
15
5·7
12
20
23
24
45
50
48
5·4
16
17
35
36
1·3
4·9
Sm
1·9
1·7
4·7
5·5
5·4
2·4
3·5
3·6
4·0
4·1
1·8
Eu
0·37
0·39
1·2
1·5
1·2
0·58
0·94
0·87
0·99
0·92
0·47
1·37
Gd
2·0
1·7
4·9
5·5
5·0
2·5
4·2
3·8
4·3
3·9
2·0
5·2
Tb
0·34
0·27
0·80
0·83
0·80
0·44
0·69
0·62
0·70
0·64
0·37
0·8
Dy
1·9
1·8
5·2
5·2
4·8
2·5
4·4
4·0
4·5
4·1
2·2
4·6
Ho
0·37
0·36
1·0
1·1
1·0
0·49
0·82
0·84
0·91
0·85
0·37
0·90
Er
0·96
0·95
2·9
2·9
2·7
1·3
2·3
2·2
2·4
2·3
0·98
2·7
Tm
0·16
0·15
0·44
0·42
0·42
0·18
0·35
0·35
0·38
0·36
0·14
0·4
Yb
0·97
0·93
2·8
2·6
2·6
1·2
2·1
2·1
2·3
2·2
0·92
2·4
Lu
0·15
0·15
0·43
0·39
0·37
0·16
0·32
0·31
0·37
0·33
0·14
0·36
13
3·7
14
4·2
17
*Sample EA-214 from Arranz (1997).
Fe2O3T, total Fe expressed as Fe2O3. LOI, loss on ignition; b.d.l., below detection limit.
1544
4·4
18
1·6
24
50
6·8
7·4
3·3
6·7
14
1·8
24
2·0
16
32
Nd
25
5·8
13
28
Pr
21
6·0
8·5
18
5·7
5·9
24
5·5
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CRYSTAL TRANSFER AND MAGMA MIXING
Fig. 5. Whole-rock compositions of the studied samples. The fields represent mineral compositions in the studied samples; the triangles represent mineral compositions used in the modelling (see Discussion and Table 3).
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Fig. 6. Primitive mantle (McDonough & Sun, 1995) normalized
REE patterns of the samples from the composite dykes. Shaded areas
represent the composition of aplites (A) and mafic rocks (B: gabbros,
diorites, tonalites and lamprophyres) in the Maladeta Plutonic
Complex (Arranz, 1997).
compositions were observed in samples APL and GAB
(An21^10 and An93^46 respectively). A good correlation between the rock type and the anorthite content is observed
in the composite dykes. In dyke 1, plagioclase varies from
An23 to An16 in the aplite (FEL rock type), from An62 to
An46 in INT-type enclaves, and from An85 to An62 in
MAF-type enclaves. In dyke 2, the compositional ranges
are An36^13 (FEL rock type), An63^22 (INT rock type) and
An63^30 (MAF rock type). Plagioclase compositions in the
mafic rock types (INT and MAF) are similar to those of
sample GAB, whereas in the aplite (FEL) they are similar
those of sample APL.
Single plagioclase microcrysts are generally unzoned. In
contrast, spongy crystals show wider major element compositional variability. From core to rim, they usually show
slightly irregular flat profiles and a final abrupt anorthite
enrichment at the rim (see Supplementary Data: Table I).
The final rim An enrichment is larger for spongy plagioclase crystals hosted in mafic enclaves compared with
those in the aplitic portion of the composite dykes (see
Supplementary Data: Table I).
Biotite structural formulae were calculated on the basis
of 11 equivalent oxygen atoms per formula unit; all the
compositions plot above the Mg:Fe ¼ 2:1 division defined
by Deer et al. (1966) (Fig. 7b). Biotite composition also correlates with rock type, defining a variation trend where
the most Fe- and AlIV-rich compositions are those characteristic of biotite from the aplitic portion of the composite
dykes. The variation range is larger for dyke 2 than for
dyke 1. In both dykes, there is a compositional gap between
biotite in the aplite and in the enclaves. In contrast, the
compositions in the various enclave types partly overlap.
No significant compositional differences have been identified between the biotite types recognized petrographically
such as groundmass microcrysts, bladed crystals and crystals within clots.
Amphibole compositions were recalculated on the
basis of 23 equivalent oxygen atoms per formula unit and
Fig. 7. Major element chemistry of (a) plagioclase, (b) biotite and (c)
amphibole from the studied samples.
are classified as magnesiohornblende and actinolite
according to Leake et al. (1997) (Fig. 7c). Amphibole compositions are all similar regardless of the dyke or enclave
type they are from, and there is no correlation between
amphibole composition and crystal type (groundmass
microcrysts or crystals in clots). Some of the compositions are similar to those of amphibole from the GAB
sample.
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CRYSTAL TRANSFER AND MAGMA MIXING
Trace elements
Representative LA-ICP-MS analyses are summarized in
Supplementary Data: Table II. REE concentrations have
been normalized to the composition of the whole-rock
(Fig. 8) to check relative enrichments or depletions of the
mineral phases relative to the rock they appear in.
Plagioclase was analysed in sample APL and in the composite dykes, including crystals from the equigranular
areas of the aplite, spongy crystals from the aplite close to
the contact with the enclaves, and spongy crystals and
microcrysts from INT-type enclaves. In MAF-type enclaves, large plagioclase crystals are scarce and too modified by resorption, and groundmass microcrysts are too
small, to provide reliable spot analyses. All the analyses
have HREE-depleted normalized patterns with strong Eu
enrichment (Fig. 8a), as is typical for plagioclase. In contrast to the major elements, trace element concentrations
show differences between the plagioclase types, especially
in the case of the LREE. Groundmass microcrysts have
lowest values, whereas spongy crystals have compositions
similar to those of crystals in the aplitic portion of the composite dykes and in the APL sample. The positive anomaly
in Eu is especially marked for spongy plagioclase.
Biotite compositions were determined in the three rock
types of the composite dykes. Groundmass microcrysts,
bladed crystals and crystals inside clots were analysed.
However, the concentrations obtained are very low and
only three analyses yield reliable (40·01) normalized REE
concentrations (Fig. 8b). The abundances increase towards
the HREE, with differences in La^Ce and Eu^Gd.
Striking differences can be identified for the transition
elements V, Cr, Co, Ni and Cu, related to the type of rock
in which the biotite was analysed (Fig. 9). The concentration of transition elements in biotite increases from the
aplite to the enclaves and from INT- to MAF-type enclaves. In the enclaves biotite is enriched in these elements
compared with amphibole or pyroxene.
Amphibole is present in only the mafic enclaves (MAF
and INT rock types). Amphibole clots and microcrysts
were analysed in both mafic rock types from both dykes.
The normalized REE patterns (Fig. 8b) are HREE enriched and show a negative Eu anomaly. Clear differences
between amphibole microcrysts and clots are not recognized. Instead, slight differences are related to the type of
rock in which the analysed amphibole is hosted (Ubide
et al., 2009): higher REE contents and deeper Eu anomalies
are observed in amphibole from less mafic enclaves.
Clinopyroxene and orthopyroxene were analysed in
sample GAB. Their normalized REE abundances increase
towards the HREE and show a negative Eu anomaly
(Fig. 8b). Orthopyroxene has low REE concentrations
similar to biotite in the composite dykes; however, the biotite patterns are highly irregular and their HREE contents
are lower. Clinopyroxene patterns are similar to those of
Fig. 8. REE patterns normalized to corresponding whole-rock abundances in (a) plagioclase and (b) biotite, amphibole and pyroxene
from the studied samples. Plagioclase and biotite analyses with normalized values below 102 have been screened out.
Fig. 9. Variation of Ni vs Cr in biotite from the composite dykes. The
fields represent the composition of amphibole from the composite
dykes and pyroxenes from sample GAB.
amphibole and different from those of orthopyroxene and
biotite.
Principal component analysis
To evaluate the complete trace element dataset obtained by
LA-ICP-MS we have applied multivariate statistics, using
principal component analysis (PCA). PCA reduces the
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dimensionality of a compositional dataset by transforming
the original variables (element contents) into a new set of
variables (the principal components, PCs), which are
uncorrelated and ordered so that the first few retain most
of the variation present in all of the original variables
(Joliffe, 2002). It follows that PC2 vs PC1 plots can be used
to statistically group or discriminate the compositions, as
well as to identify possible variation trends. The significance of the distribution of data around PC1 and PC2
axes is evaluated based on the statistical load of each element on each PC. In this way, PCA allows interpretations
based on the whole analytical dataset (Ubide et al., 2012,
2014).
Using PAST (Hammer et al., 2001) statistical free software, we carried out two PCA evaluations; one for plagioclase compositions and the other for amphibole and
pyroxene compositions. Biotite was excluded because the
number of analyses is too low to be statistically representative and they exhibit very low concentrations and irregular
patterns for some geochemically meaningful elements
(e.g. REE; see Fig. 8b). All the analysed trace elements
were included in each PCA. The compositions were evaluated according to the mineral types recognized petrographically, as for the REE in Fig. 8.
The results of the PCA are presented in Fig. 10 and
Supplementary Data: Tables III and IV. Plagioclase compositions define two groups with clearly different trends
(Fig. 10a). One group corresponds exclusively to plagioclase microcrysts, which show a subhorizontal variation
trend for PC140 and negative values on PC2. The other
group is composed of spongy plagioclase, plagioclase from
the FEL rock type and plagioclase from sample APL.
These compositions define a subvertical dispersion at relatively constant values of PC1 (close to zero). PC1 explains
35·7% of the total variance and reflects a greater contribution of the transition elements Sc to Zn, together with Li,
Cs, Nb, Ta and Th. PC2 explains 27·7% of the variance
and has greater loads for Y, Pb and some REE (see
Supplementary Data: Table III).
The subhorizontal group of microcrysts close to the direction of PC1 is mainly dependent on two analyses, and
the elements reflected in PC1 are incompatible in plagioclase. In contrast, the distribution of compositions along
PC2 in the subvertical group is noteworthy. Plagioclase
from sample APL plots in the middle of the group, closely
surrounded by plagioclase from the FEL rock type, which
already defines a subvertical pattern. Spongy plagioclase
compositions are the most dispersed, although they maintain the subvertical distribution, defining the outer limits
of the group. In the FEL rock type three spongy crystals
were analysed, yielding three groups of compositions: the
four analyses obtained from one of the crystals cluster at
PC2 around two; the three analyses obtained from another
crystal cluster at PC2 around 0·5; and the two analyses
NUMBER 8
AUGUST 2014
Fig. 10. Principal component analysis (PCA) for 38 trace elements in
(a) plagioclase and (b) amphibole and pyroxene from the studied
samples, showing component 2 (PC2) against component 1 (PC1).
Convex hulls (polygons) link three or more analyses of the same
type, defining their distribution. Details can be found in
Supplementary Data Tables III and IV.
obtained from the third crystal plot at PC2 below zero, in
agreement with the more homogeneous spongy compositions from INT-type enclaves.
In the amphibole^pyroxene PCA (Fig. 10b), orthopyroxene compositions appear significantly isolated along PC1.
Clinopyroxene and amphibole, in contrast, show coincident, mainly positive, PC1 values; however, they display
differences on PC2. A closer look at the loads of the PC1
variable reveals a greater contribution of Yand most REE
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UBIDE et al.
CRYSTAL TRANSFER AND MAGMA MIXING
to the observed variability, within a general context of relative uniformity. On PC2, in contrast, the contribution of
some LILE and Th is especially high (see Supplementary
Data: Table IV). PC1 explains 42·5% of the global variance, whereas PC2 explains 14·3%.
In contrast to the normalized REE patterns (Fig. 8b),
differences can be observed between the types of amphibole (Fig. 10b). Amphibole clots define an independent
polygon, distinct from amphibole microcrysts. The latter
are separated into two groups: microcrysts from MAFtype enclaves and those from INT-type enclaves.
Particularly striking is the observation that the distribution
polygon of clinopyroxene is similar in shape to that of the
amphibole clots and located within it. This means that the
amphibole clots are closer compositionally to clinopyroxene from sample GAB than to amphibole microcrysts
from the same samples. The large field of amphibole clots
points to a greater compositional variation for this group
compared with clinopyroxene.
PCA highlights differences in the compositional dataset.
For plagioclase, the two main groups distinguished by
PCA in Fig. 10 agree with the differences observed for the
LREE in Fig. 8a. In addition, PCA detects differences between crystals. In the case of amphibole, the various
amphibole types are indistinguishable according to REE
contents (Fig. 8b) but are clearly recognized using PCA
(Fig. 10). In the same way, the differences already recognized based on REE contents (differences between orthopyroxene and the rest) are reproduced by PC1, which has
high loads for the REE (see Supplementary Data: Table
IV).
DISCUSSION
Crystal transfer during magma mixing
The composite dykes consist of pillow-like mafic enclaves
with rounded shapes and lobate contacts, surrounded by a
felsic aplite (Fig. 2). Field relationships such as the fitting
between neighbouring enclaves suggest that the mafic and
felsic magmas coexisted in a partly liquid state and interacted mechanically (mingled) before solidification (e.g.
Barbarin & Didier, 1991). At the microscopic scale, the contacts between the enclaves and the aplite are characterized
by continuous crystals crossing the contact (Fig. 4a), as
would be expected of coeval mafic and felsic magmas
(Barbarin & Didier, 1991). All rocks forming the dykes
show mineralogical evidence of chemical disequilibrium
conditions owing to hybridization, such as bladed biotite,
acicular apatite, plagioclase crystals with two clearly distinct crystallization stages or spongy cores, quartz ocelli
with ferromagnesian rims, and amphibole and biotite as
pseudomorphs in clots (e.g. Hibbard, 1995; Figs 3 and 4).
The increasing abundance of biotite in the aplite close to
the contact with the enclaves also reveals hybridization.
In consequence, the various rocks forming the composite
dykes (MAF, INTand FEL types) are themselves hybrids,
with INT-type enclaves representing the greatest degree
of hybridization.
The large size and disequilibrium textures of spongy
plagioclase, quartz ocelli and amphibole^biotite clots
(Figs 3 and 4) suggest that they might be transferred xenocrysts (Baxter & Feely, 2002; Perugini et al., 2003; Prelevic¤
et al., 2004). They occur in the mafic enclaves in the composite dykes; spongy plagioclase and quartz ocelli also
occur locally in the aplitic portion of the composite dykes
close to the contact with the enclaves.
The size of spongy plagioclase and ocellar quartz crystals is similar to the size of plagioclase and quartz crystals
in the equigranular parts of the aplite and in the aplitic
country-rock (sample APL). Spongy textures in plagioclase are progressively more developed in more mafic
rocks (compare Fig. 4a and b). Furthermore, spongy cores
usually have more Ca-rich overgrowths and the anorthite
content of the overgrowths is higher for crystals hosted in
the enclaves than in the aplite (Supplementary Data:
Table I). The rims of quartz ocelli are thicker in the enclaves than in the aplite and are biotite rich in INT-type
enclaves and amphibole rich in MAF-type enclaves (compare Fig. 4c and d). Taken together, all these lines of evidence indicate that spongy plagioclase and quartz ocelli
are plagioclase and quartz crystallized in the felsic
magma and transferred to the mafic magma, where they
developed resorption textures and crystallized rims in
equilibrium with the new host magma composition.
On the other hand, the size and shape of amphibole^
biotite clots resemble those of the crystals of pyroxene in
the reference quartz gabbro (sample GAB). Despite the
lack of pyroxene relicts in the amphibole^biotite clots, the
relicts observed in the quartz-gabbro sample indicate that
amphibole mostly replaces pyroxene and especially clinopyroxene (Fig. 3f). The clots have a better defined shape
and lower fractions of biotite in MAF-type enclaves than
in INT-type enclaves (compare Fig. 4e and f).
Petrographic evidence therefore suggests that amphibole
clots replace former pyroxene originally crystallized in the
mafic magma and destabilized to amphibole owing to
mixing with the felsic, water-rich magma (e.g. Vernon,
1984, 1990; Castro & Stephens, 1992; Choe & Jwa, 2004).
The replacement of pyroxene and amphibole with biotite
as a result of magma mixing (e.g. Tate et al., 1997, and references therein) represents further hybridization of the
mafic magma in the studied system. This process requires
considerable local-scale mass transfer, notably of H2O and
K2O (Lavaure & Sawyer, 2011) and these components can
be supplied by the felsic magma.
Given the petrographic connection between spongy
plagioclase and ocellar quartz in the composite dykes and
plagioclase and quartz in the aplitic country-rock, and
also between amphibole^biotite clots in the composite
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dykes and pyroxene in the reference quartz-gabbro, the
aplitic country rock APL and the quartz-gabbro GAB
might represent the mixing end-members. Whole-rock
and mineral compositions also indicate this could be the
case, because they are most extreme for APL and GAB
and intermediate for the hybrids (Figs 5 and 7a).
Major element compositions in minerals yield similar
results for transferred versus groundmass types of plagioclase, amphibole and biotite occurring in the composite
dykes. In terms of trace elements, however, the differences
are clear and the transferred phases can be linked to their
source in the end-members. Spongy plagioclase compositions are similar to plagioclase compositions in APL and
in the equigranular parts of the aplitic portion of the composite dykes; they are different from those of plagioclase
microcrysts in the groundmass of the enclaves (Figs 8a
and 10a). The composition of amphibole in clots is similar
to the composition of clinopyroxene in GAB and different
from the composition of amphibole microcrysts in the
groundmass of the enclaves, which varies depending on
the enclave type (Fig. 10b). Hence, transfer of plagioclase
and clinopyroxene from the end-members is geochemically
supported; interaction between the transferred crystals
and the new host magma is reflected in a greater compositional dispersion compared with their source minerals in
the end-members (Figs 8 and 10). Replacement of orthopyroxene by amphibole, however, is not supported
(Fig. 8b and 10b). This process involves considerably
larger mass transfers than the clinopyroxene^amphibole
replacement, and therefore amphibole replaces clinopyroxene preferentially as observed in the mafic end-member
(Fig. 3f).
Replacement of pyroxene and amphibole with biotite requires further hydration and additional mass transfer.
Biotite in clots contains inclusions of apatite and is commonly associated with opaque minerals and quartz. These
associations are especially frequent in clots where amphibole is mostly or completely replaced by biotite. Biotite
contains lower abundances of SiO2 than pyroxene or
amphibole (e.g. see Fig. 5). Accordingly, in the replacement
process there is a surplus of silica, which is probably
hosted by quartz. On the other hand, clinopyroxene and
amphibole have higher CaO,Yand REE contents than biotite and orthopyroxene (Figs 5 and 8b) and thus the replacement of clinopyroxene or amphibole by biotite has to
involve the formation of Ca-, Y- and REE-bearing accessory phases. Some opaque minerals and especially apatite
(Fig. 11) are characterized by high concentrations of these
elements (e.g. Rollinson, 1993; Sha & Chappell, 1999). As
shown in Fig. 11, as little as 2 wt % of apatite in biotite reproduces the REE composition of clinopyroxene and
amphibole. Considering the similar average densities of
apatite and biotite (see Supplementary Data: Table V),
this proportion is equivalent to 1·9 vol. % of apatite and
NUMBER 8
AUGUST 2014
Fig. 11. Primitive mantle (McDonough & Sun, 1995) normalized
REE patterns of minerals: amphibole, pyroxene and one biotite clot
analysis in the studied samples and apatite in S-type granites from
Sha & Chappell (1999). Considering a combination between the biotite clot composition and analysis in sample CB-84 from Sha &
Chappell (1999), only 2 wt % of apatite is needed to reproduce the
REE pattern of clinopyroxene and amphibole.
this is consistent with the volume fraction of apatite in the
biotite-rich clots (Fig. 4e). Biotite has the highest concentrations of transition elements, with variations correlating
with the type of rock that hosts the biotite (Fig. 9). This is
probably because biotite re-equilibrates very quickly with
the host composition and can lose some compositional
traits of the minerals it has replaced (Lavaure & Sawyer,
2011).
Given that plagioclase and quartz xenocrysts in the
mafic enclaves come from the felsic end-member, efficient
crystal exchange mechanisms must operate to overcome
the rheological obstacles to the process. Crystal transfer
from the aplite to the enclaves most probably takes place
while the mafic magma is less viscous than the felsic
magma (Waight et al., 2000), before the viscosities and temperatures of the two magmas become equal. Therefore,
the process is probably related to the strain induced by
the injection and relative flow of the mafic magma against
the felsic magma, which may have facilitated the capture
of crystals by the more liquid mafic magma, plucked from
the viscous aplitic crystal^melt mush. In the same way,
the high viscosity of the aplitic magma may have prevented the incorporation of pyroxene crystals from the
less viscous mafic end-member. Pyroxene crystals from the
mafic magma were unevenly distributed among the separating enclaves during mingling with the felsic magma.
The influence of crystal transfer on the
efficiency of mixing
In the composite dykes, mafic^felsic interface morphologies are complex and viscous fingering processes are
observed (see Fig. 2e), revealing a high viscosity contrast
between the magmas (Perugini & Poli, 2005). Large thermal and rheological differences between contrasting
magmas are considered to hinder hybridization (e.g.
Sparks & Marshall, 1986; Frost & Mahood, 1987; Grasset
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CRYSTAL TRANSFER AND MAGMA MIXING
& Albare'de, 1994; Bateman, 1995; Perugini et al., 2003,
2008). However, Kouchi & Sunagawa (1985) observed that
the presence of crystals increases considerably the efficiency of mixing. More recently, Perugini et al. (2003) suggested that crystal transfer during mixing may be
accompanied by transfer of melt, inducing dilution of the
host magma. The possible influence of crystal transfer on
the efficiency of mixing should therefore be evaluated.
An interesting field observation is that the enclave type
(MAF or INT) correlates with the amount and distribution of plagioclase and quartz xenocrysts transferred from
the aplite. In MAF-type enclaves the transferred felsic
phases are scarcer and usually located close to the enclave
margin (Fig. 2h). In contrast, in INT-type enclaves transferred plagioclase and quartz crystals are more frequent
(Table 1) and widely distributed inside the enclave
(Fig. 2g). These differences are consistent with the proposal
that crystal transfer from the felsic end-member enhanced
hybridization of the mafic enclaves; the transfer of felsic,
volatile-rich melt with the crystals may have diluted and
hydrated the mafic magma, lowering its solidus temperature and delaying crystallization (e.g. Grasset &
Albare'de, 1994; Wiebe et al., 2001). In line with this hypothesis, there is a high volume fraction of hydrous minerals in
the enclaves (e.g. amphibole and biotite) and there are no
chilled margins around them, implying that quenching of
the mafic magma played a minor role (e.g. Snyder & Tait,
1995, 1998). Moreover, INT-type enclaves are slightly coarser-grained than MAF-type enclaves, which show sharper
contacts with the aplite. In summary, INT-type enclaves
remained longer in a liquid state and became more
evolved because they captured more crystals from the
aplite. The MAF-type cores recognized in some INT-type
enclaves (Fig. 2f) would represent areas not reached by
the transfer process, which therefore chilled more rapidly.
The influence of crystal transfer on the
composition of the hybrids
In the aplitic portion of the composite dykes amphibole^
biotite clots do not occur and spongy plagioclase and
quartz ocelli occur only locally in the first millimetres surrounding the mafic enclaves. Because aplite^enclave contact areas were avoided when preparing the samples for
whole-rock analysis, the aplitic whole-rock compositions
should not record any influence of crystal transfer.
Indeed, aplite compositions are very close to the felsic
end-member (Fig. 5).
Unlike the aplite, the enclaves contain varying volume
fractions of spongy plagioclase, ocellar quartz and amphibole^biotite clots (Table 1). As shown in Fig. 5, when mineral compositions plot close to rock compositions, the
composition of the enclaves usually aligns between the
two end-members. In contrast, when mineral compositions
plot away from rock compositions, enclave compositions
plot displaced towards mineral compositions, especially
amphibole. Strikingly, amphibole is the transferred mineral that reaches highest modal proportions in the enclaves
(Table 1). These observations suggest that preferential
transfer of minerals may modify significantly the wholerock composition of the enclaves.
To test if deviations from ideal mixing are due to crystal
transfer we have modelled the accumulation of 20 wt %
of each transferred phase in the mafic end-member. In the
models, we have included spongy plagioclase, ocellar
quartz, amphibole in clots, biotite in clots and also apatite
in clots because this accessory mineral is an important repository of trace elements in the system (e.g. Fig. 11).
Because apatite is an accessory mineral, we have added
only 2 wt % to the mafic end-member. The selected, representative mineral compositions are summarized in Table 3
and plotted in Fig. 5; for apatite we have considered a representative composition from S-type granites (Sha &
Chappell, 1999) and for quartz we have considered an
ideal composition solely composed of silica with zero concentrations of other elements.
Modelled compositions are illustrated as vectors (Fig. 12),
so that the orientation and length of each vector indicates
the effect of mixing each component with the mafic endmember. The ideal APL^GAB mixing line is also depicted,
with a vector indicating linear mixing of GAB with 20 wt
% APL (Fig. 12). Amphibole vectors point away from theoretical mixing for Mg, V, Pb and the HREE (e.g. Lu).
Biotite is especially relevant for V and apatite affects Lu
particularly. Plagioclase and quartz vectors show smaller
deviations from ideal mixing; the former has greatest
impact on Sr and the latter produces enrichment in Si and
dilution in the other elements.
Enclave compositions show deviations from the ideal
mixing line (Fig. 12) that can be correlated with modes of
transferred phases (Table 1). Focusing first on the Mg, V
and Pb diagrams, MAF-type enclaves from composite
dyke 2 clearly deviate following the amphibole vector, and
this correlates with the highest volume fractions of transferred amphibole in these samples. MAF-type enclaves
from composite dyke 1, on the other hand, have the lowest
proportions of transferred phases and compositions most
similar to the mafic end-member. Transferred biotite,
quartz and plagioclase are abundant in INT-type enclaves,
and these deviate from theoretical mixing following combinations of biotite, quartz and plagioclase vectors. These
observations also apply to Cr, Co, Ni and Zn (not shown).
In the Ca and Sr diagrams, crystal transfer vectors have
an orientation similar to the ideal mixing line, or overall
compensate each other; accordingly, enclave compositions
are fairly close to the ideal mixing line (Fig. 12). This also
applies to Al, Ti, Na, K, Y, Nb, Ta, Zr and Th (not shown).
On the other hand, Ba is unusually enriched in MAFtype enclaves from composite dyke 1 (Fig. 12). Other
mobile elements (Rb, Cs, Be, Cu and U) also remain
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Table 3: Selected mineral compositions for the vector model
and the mixing with crystal transfer model
Mineral:
Pl*
Amp*
Bt*
Type:
spongy
clot
clot
Rock type:
INT
MAF
MAF
SiO2
62·82
53·98
Apy
Qzz
S-type
S-type
granite
granite
37·74
100
TiO2
0·03
0·16
2·37
0
Al2O3
23·15
1·59
16·84
0
MnO
0·02
0·74
0·36
0
MgO
0·00
15·73
13·31
0
FeOT
0·01
12·95
15·26
0
CaO
4·57
11·93
0·00
Na2O
8·90
0·20
0·21
K2O
0·16
0·05
9·48
P2O5
Total
99·66
97·33
Rb
6
4
Cs
0·2
1·1
Be
8
1
95·57
53·48
0
0
95·53
100
0
0
0
0
275
7
4
291
11
68
81
0
0
V
0
233
537
0
Cr
0
127
4483
0
Co
0
31
84
0
Ni
1
21
474
0
Cu
1
7
13
0
Zn
4
211
322
0
Ga
36·8
11·2
37·2
6·23
0
Y
1·41
38·5
Nb
0·0
1·2
48·3
Ta
0·0
0·1
4·6
0
Zr
0
4
0
0
Hf
0·0
0·3
0·0
Pb
25·8
2·32
19·9
1550
0
0
0
7
0
U
0·04
1·44
0·39
88
0
Th
0·07
0·54
0·08
5
0
La
2·61
1·70
0·52
143
0
Ce
2·96
8·44
2·31
526
0
Pr
0·22
1·79
0·39
90
0
Nd
0·70
2·09
439
0
Sm
0·20
5·11
0·49
207
0
Eu
1·23
1·31
0·34
6
0
Gd
0·15
6·39
0·60
263
0
Tb
0·03
1·08
0·08
48
0
Dy
0·19
7·05
0·66
337
0
Ho
0·04
1·52
0·18
57
0
Er
0·09
4·44
0·50
143
0
11·9
Table 3: Continued
Mineral:
Pl*
Amp*
Bt*
Type:
spongy
clot
clot
Rock type:
INT
MAF
MAF
Apy
Qzz
S-type
S-type
granite
granite
0
Tm
0·02
0·65
0·09
23
Yb
0·09
4·10
0·55
161
0
Lu
0·02
0·65
0·11
22
0
0
42·05
15·0
Ba
AUGUST 2014
*Data in this study (see Supplementary Data: Tables I
and II).
yAnalysis in sample CB-84 from Sha & Chappell (1999).
zConsidered pure SiO2.
Major elements expressed as wt % and trace elements as
ppm. FeOT, total Fe expressed as FeO. Mineral abbreviations are after Whitney & Evans (2010).
0
124
Sr
NUMBER 8
(continued)
unexplained by the crystal transfer vectors. Because the
rocks are not altered, some other process must be called
on. Given that mobile elements diffuse much more rapidly
than other elements, the observed enrichments could be
explained by selective diffusion linked to a release of volatiles from the aplite (Michaud, 1995, and references
therein). In this sense, the selective diffusion of mobile
elements and H2O away from the felsic to the mafic
magma has been proposed in a number of mixing studies
(e.g. Wiebe, 1973; Watson & Jurewicz, 1984; Michaud, 1995;
Wiebe et al., 2002).
Differences in diffusion could also affect other elements,
producing non-linear correlations in binary diagrams for
elements having large differences in diffusion coefficients.
This effect has been called ‘diffusive fractionation’, related
to chaotic dynamics producing advection (stretching and
folding) of the interacting magmas (e.g. Perugini et al.,
2006, 2008). For instance, diffusivities of the REE are low
and within the same order of magnitude, but increase
with increasing ionic radii (Nakamura & Kushiro, 1998).
Progressively lower correlation between the REE for
increasing differences in ionic radii (compare the Ce^La
and Lu^La diagrams in Fig. 12) could therefore be related
to diffusive fractionation. However, these differences are
also consistent with the accumulation of amphibole^biotite
clots (Fig. 12) and aplite compositions that do not record
crystal transfer can be reproduced by linear mixing for all
the REE (see Ce^La and Lu^La diagrams in Fig. 5). In
addition, chaotic fingerprints are mostly recognizable in
glassy volcanic rocks (Perugini & Poli, 2012) and filaments
typical of stretching and folding are not observed in the
composite dykes (Fig. 2). Therefore, diffusive fractionation
appears to play a minor role in the studied system and the
deviations of the hybrids from the theoretical composition
are mainly correlated with crystal transfer.
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CRYSTAL TRANSFER AND MAGMA MIXING
Fig. 12. Vectors from the mafic end-member (GAB) towards 20 wt % of the felsic end-member (APL) and 20 wt % of the transferred phases
plagioclase, quartz, amphibole and biotite; for apatite the vector points to an addition of 2 wt %. The mineral compositions used for modelling
are given in Table 3 and plotted in Fig. 5. The whole-rock compositions of the mafic hybrids (enclaves) are plotted for comparison, as well as
the whole-rock composition of the felsic end-member and the theoretical mixing line between the end-members (dotted line); the concentration
of V in the felsic end-member is below the detection limit, and has been considered to be zero for the purpose of drawing the mixing line. The
plotted elements are the same as in Fig. 5.
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Extending the magma mixing equation for crystal transfer
A quantitative approach to demonstrate that crystal transfer produces non-ideal chemical behaviour is to model the
composition of a particular enclave taking into account
the transferred phases. We have developed a specific geochemical model for whole-rock sample 2MAF-a, as it contains high volume fractions of transferred crystals
(Table 1). We have modelled hybrid compositions between
the mafic and felsic end-members extending the classical
two end-member model (e.g. Langmuir et al., 1978) to include crystal transfer; that is, taking into account
the modes of transferred phases in this sample
(Supplementary Data: Table V) and their composition
(Table 3). To transform volume fractions measured by petrography into mass fractions we have used the average
density of each mineral (plagioclase, quartz, magnesiohornblende, biotite and apatite; see Supplementary Data:
Table V) and the average density of quartz-diorite according to Petrov et al. (2005) for the groundmass. The change
from volume to mass fractions is not significant but slightly
enhances the effect of amphibole, biotite and apatite over
less dense plagioclase, quartz and groundmass
(Supplementary Data: Table V). The results of the model
for major and trace elements are presented in
Supplementary Data: Table VI for mass fractions of the
mafic end-member (sample GAB) increasing from 10% to
90% and mass fractions of the felsic end-member (sample
APL) decreasing accordingly.
The classical two end-member model (linear mixing)
and our mixing with crystal transfer model are compared
in Fig. 13. The former generates normalized REE patterns
parallel to those of the end-members (Fig. 13a). These patterns are consistent with the composition of sample
2MAF-a for the LREE, but are less enriched in Eu and
the HREE. In contrast, the mixing with crystal transfer
model generates hybrids that fit the composition of sample
2MAF-a better and are depleted in LREE and enriched
in Eu and HREE compared with the end-members
(Fig. 13b). This is because spongy plagioclase is enriched
in Eu over the enclaves (Fig. 8a), amphibole in clots is enriched in HREE over the enclaves (Fig. 8b), biotite in
clots is particularly depleted in LREE compared with the
enclaves (Fig. 8b), and the roles of quartz (REE depleted)
and apatite (REE enriched) overall compensate each
other and introduce no significant modifications to the
shape of the REE patterns. In consequence, sample
2MAF-a is better reproduced by mixing of high fractions
of the mafic end-member and low fractions of the felsic
end-member with the fractions of spongy plagioclase, ocellar quartz and amphibole^biotite-apatite clots observed in
the sample. As an example, considering 70% of the mafic
end-member in the mixture, mixing with crystal transfer
reproduces the REE concentrations better than classical
two end-member mixing (Fig. 13c).
Fig. 13. Modelling the composition of hybrid 2MAF-a from endmembers APL and GAB. (a, b) Primitive mantle (McDonough &
Sun, 1995) normalized REE patterns of the end-members and the
hybrid (as in Fig. 6), together with mixtures modelled according to
(a) two end-member mixing or (b) mixing with transfer of spongy
plagioclase, ocellar quartz and clots including amphibole, biotite and
apatite (see Supplementary Data: Table VI). The modelled compositions correspond to APL^GAB mixtures at intervals of 10%. For
modelling in (b), the amount of transferred phases is kept constant
and according to their modes in sample 2MAF-a (see
Supplementary Data: Table V) and the composition of the transferred
phases is as in the vector models (Table 3; Fig. 5). (c) Measured REE
concentrations in the hybrid vs expected REE concentrations according to two end-member mixing or mixing with crystal transfer, considering 30% APL and 70% GAB in both cases. It should be noted
that the coefficient of determination, R2, is closer to unity using
mixing with crystal transfer.
Under the same conditions, our model approaches the
whole-rock composition of sample 2MAF-a for most
major and trace elements other than the REE (see
Supplementary Data: Table VI). For elements such as Ca
the modelled compositions are very similar to those
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UBIDE et al.
CRYSTAL TRANSFER AND MAGMA MIXING
obtained with the classical two end-member model (i.e. the
ideal mixing line in Fig. 12). In contrast, for elements particularly enriched or depleted in the transferred phases
compared with the mixing line, such as Mg, V and Pb,
mixing with crystal transfer models the composition of
sample 2MAF-a better than two end-member mixing
(Supplementary Data: Table VI). For mobile elements neither model reproduces the composition of sample 2MAFa, probably because mobile elements are enriched owing
to selective diffusion as explained above. These results support the proposal that accumulation of transferred crystals
has a direct chemical impact on the hybrids and therefore
both chemical exchanges (hybridization) and crystal exchanges (preferential crystal transfer) shape the composition of the enclaves in the composite dykes.
Formation of the composite dykes
The mafic^felsic distribution in composite dykes provides
information about the relative time of emplacement of the
magmas. When the mafic magma intrudes first, the resulting composite dykes typically have mafic margins and
felsic interiors, with contacts between the two rock types
that are nearly planar and parallel to the dyke walls. In
contrast, when the felsic magma intrudes first, the resulting composite dykes typically contain mafic pillows surrounded by felsic margins (Snyder et al., 1997). The
composite dykes studied here are consistent with the latter
(Fig. 2), so they most probably formed by injection of a
mafic magma into a crystallizing aplite dyke (Vernon,
1984; Barbarin & Didier, 1992; Wiebe et al., 2001; Barbarin,
2005).
Taking all the available data into account, the formation
of the composite dykes studied is interpreted as follows
(Fig. 14). The mafic magma, which carried early pyroxene
crystals, was injected into the felsic magma, which was
cooler and rich in plagioclase and quartz crystals
(Fig. 14a). The two magmas mingled, triggering
disaggregation of the mafic magma into enclaves (Fig.
14b). During injection and mingling, plagioclase and
quartz crystals were incorporated into the mafic magma
from the more viscous felsic magma. Meanwhile, early
pyroxene crystals carried by the mafic magma were unevenly distributed among the disaggregating enclaves. The
contact area between the two magmas strongly increased
owing to mingling, favouring the occurrence of chemical
exchange between them (Fig. 14c). Hybridization was especially efficient in those enclaves capturing greater amounts
of plagioclase and quartz crystals from the aplite. The
transferred plagioclase and quartz crystals were resorbed
and overgrown by rims in equilibrium with the hybrid
magma. Pyroxene crystals were destabilized to amphibole
and biotite, becoming clots composed of microcrysts of
these minerals. Therefore, transferred plagioclase, quartz
and pyroxene crystals occur as destabilized xenocrysts in
the hybrids. The original end-members of mixing do not
occur in the composite dykes and the composition of the
hybrids is significantly affected by the accumulation of
transferred xenocrysts.
Implications for the interpretation of
magma mixing
Magmas are not homogeneous media as they are composed of crystals (solid phase), melt (liquid phase) and
volatiles (gas phase). During magma mixing, melt and
volatiles are miscible, whereas crystals are transferred in a
solid state and might not be homogeneously distributed in
the hybrids. As shown in the studied composite dykes, preferential transfer and accumulation of crystals has strong
implications for the composition of the hybrid magmas.
Mixing between two end-member magmas has classically been considered to produce linear correlations between whole-rock compositions in binary diagrams.
However, crystal-free experimental and numerical
approaches have shown that non-linear correlations may
Fig. 14. Schematic illustration of the interaction processes involved in the formation of the composite dykes studied. (a) Injection of crystal-poor
mafic magma into crystal-rich felsic magma. (b) Mingling between the mafic and felsic magmas. Crystal transfer takes place in (a) and (b):
plagioclase crystals and quartz crystals are transferred from the felsic magma into the mafic enclaves and pyroxene crystals are unevenly distributed among the enclaves. (c) Hybridization: generation of hybrid minerals and destabilization of the transferred phases: plagioclase into
spongy plagioclase with more primitive overgrowth rims, quartz into quartz ocelli surrounded by amphibole and biotite and pyroxene into
amphibole^biotite clots. Those enclaves capturing more felsic crystals undergo greater hybridization.
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be observed if elements with different diffusivities are compared (Perugini et al., 2006, 2008; De Campos et al., 2011).
Adding the effect of the crystals, we have shown that if
crystals are preferentially transferred to certain hybrids,
the composition of these will deviate significantly from
the theoretical hybrid composition towards the composition of the transferred mineral/s. It follows that non-linearity within a suite of igneous rocks does not necessarily
imply non-consanguinity by mixing.
In the plutonic environment, magma mixing has traditionally been considered to be a relevant process in the
genesis of granitic rocks (e.g. Clarke, 1992; Pitcher, 1993).
Mixing of mantle-derived mafic magma and crustalderived felsic magma is commonly called on to explain
compositional diversity at the pluton and batholith scale
(e.g. Xiong et al., 2012; Clausen et al., 2014). Some researchers, however, have argued against large-scale
magma mixing, given that inter-elemental variations do
not fit the correlations expected by mixing (e.g. Clemens
et al., 2011; Farina et al., 2012).
In the system studied here, petrological and geochemical
characteristics (particularly oxygen isotope signatures) indicate that the mafic end-member was derived from partial
melting of the lithospheric mantle, whereas the felsic endmember was derived from anatexis of metasedimentary
rocks (Arranz, 1997). At the batholith scale, granodiorites
are volumetrically dominant (Fig. 1) and have been proposed to represent hybrid magmas between the same endmembers (Michard-Vitrac et al., 1980; Arranz, 1997). It follows that mixing with crystal transfer could potentially be
extrapolated as a relevant process in the genesis of the plutonic complex. The larger thermal and time constraints at
the batholith scale allow for more complex processes and
greater homogenization of the mixing products, so many
of the features that can be observed in composite dykes
may be obscured in larger-scale intrusions. A regional petrological and geochemical study would be necessary to assess the extent of magma mixing and preferential crystal
transfer in the genesis of the plutonic complex as a whole.
In general, the role of magma mixing in igneous petrogenesis could be revisited considering the physicochemical
effects of the crystal cargo. In particular, we have shown
that crystal transfer helps in mixing the end-members and
produces non-linear compositional trends between them.
Therefore, interpretation of whole-rock geochemical variations should be preceded by a deep understanding of mineral assemblages and disequilibrium textures to evaluate
the origin of the crystals and the role of processes such as
crystal transfer during magma mixing.
CONC LUSIONS
Detailed field, petrographical and geochemical studies on
two mafic^felsic composite dykes cropping out in the
NUMBER 8
AUGUST 2014
Maladeta Plutonic Complex (Pyrenees, Spain) yield the
following conclusions.
(1) Crystal transfer is a significant process during magma
mixing, as mixed magmas are rarely devoid of crystals. In the studied case, plagioclase and quartz crystals were incorporated from the felsic magma into
the mafic enclaves and early pyroxene crystals carried
by the mafic magma were unevenly distributed
among the separating enclaves. The transferred
phases were altered into xenocrysts of spongy plagioclase, quartz ocelli and amphibole^biotite clots,
respectively.
(2) Crystal transfer can be tracked through mineral trace
element compositions. The transferred crystals are
compositionally similar to early crystals in the endmembers and different from hybrid microcrysts.
Statistical tools such as principal component analysis
are very useful for establishing interpretations based
on the whole analytical dataset.
(3) Crystal transfer enhances hybridization of rheologically contrasting magmas. The transfer of crystals
from the felsic to the mafic magma is accompanied
by a transfer of felsic melt that dilutes and hydrates
the mafic magma.
(4) Crystal transfer affects the composition of the hybrids,
producing deviations from theoretical hybrid compositions (e.g. non-linear correlations in inter-element
diagrams). This effect can be quantified with geochemical modelling and should be taken into consideration when interpreting whether or not igneous
suites are generated by magma mixing.
AC K N O W L E D G E M E N T S
We are grateful to Robert A. Wiebe, Frank J. Tepley and an
anonymous reviewer, as well as to Editors Richard Price
and Marjorie Wilson, for very constructive comments that
helped us improve this paper. We thank Olga Cazalla
(University of Granada, Spain) for assistance during LAICP-MS analyses. We are indebted to Juan Rofes
(University of the Basque Country, Spain) for his help
with multivariate statistics. We thank Carlos Ubide
(University of the Basque Country, Spain) and Francisco
Javier Lo¤pez-Moro (University of Salamanca, Spain) for
discussions on geochemical modelling of crystal transfer,
Balz S. Kamber (Trinity College Dublin, Ireland) for discussions on the implications of this work to igneous geochemistry, and Federico Farina (University of
Stellenbosch, South Africa) for his comments on an earlier
version of the paper. We thank Richard Stephenson for improving the wording. We would like to acknowledge the
use of Servicio General de Apoyo a la Investigacio¤n
(SAI), Universidad de Zaragoza.
1556
UBIDE et al.
CRYSTAL TRANSFER AND MAGMA MIXING
FUNDING
This work was supported by the Ministerio de Ciencia e
Innovacio¤n (CGL2008-06098/BTE) and the Instituto de
Estudios Altoaragoneses (XXIII Concurso de Ayudas
para Proyectos de Investigacio¤n 2007). We also acknowledge the Gobierno de Arago¤n, supporting the research
group Geotransfer. This work was performed as part of
the PhD work of T.U. on a grant from the Gobierno Vasco
(BFI06.189).
S U P P L E M E N TA RY DATA
Supplementary data for this paper are available at Journal
of Petrology online.
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