JOURNAL OF PETROLOGY
VOLUME 38
NUMBER 12
PAGES 1767–1776
1997
Successive Granitic Magma Batches
During Pluton Emplacement: the Case of
Cabeza de Araya (Spain)
JEAN LOUIS VIGNERESSE1∗ AND JEAN LUC BOUCHEZ2
´
CREGU, BP 23, F-54501 VANDOEUVRE/NANCY CEDEX, FRANCE, AND ENS GEOLOGIE, BP 3,
1
F-54501 VANDOEUVRE/NANCY CEDEX, FRANCE
´
´
2
PETROPHYSIQUE, UNIVERSITE PAUL-SABATIER AND UMR CNRS NO. 5563, 38 RUE DES 36-PONTS,
F-31400 TOULOUSE CEDEX, FRANCE
RECEIVED JANUARY 1997; ACCEPTED AUGUST 1997
We present a dynamic scenario of granite emplacement in the upper
crust for the pluton of Cabeza de Araya (Extramadura, Spain),
based on interpretations of gravity data, internal structures and
geochemical variations. The three-dimensional shape of the pluton
is derived from the inversion of gravity data, and the magma flow
structures are obtained by mapping the rock anisotropy of magnetic
susceptibility. The deepest zones of the pluton, which also correspond
to vertical lineations, are interpreted as feeder zones from which
magma upwelled. These feeders form small, distinct areas connected
to the pluton’s floor 6 km below the present surface level. We propose
that the two main feeders of the pluton represent tension gashes that
formed at the base of the brittle crust and served as conduits for
the magma that was ready to be tapped. In the southern half of
the pluton, granite type A is cross-cut by type B in the magmatic
state. These magmas show a continuum in their chemistries attributed
to progressive differentiation. A fine-grained leucocratic granite, more
evolved chemically, cross-cuts the surrounding facies and crops out
above the southern feeder zone. It is interpreted as a late magma
batch, coming from a different source, that intruded into the
incompletely crystallized surrounding facies. The normal petrographic
zoning of the massif of Cabeza de Araya is therefore viewed as a
succession of continuous and discontinuous infillings, sequentially
permissive and forceful, of increasingly evolved magma batches into
a reservoir that opened at varying rates within the brittle crust.
INTRODUCTION
KEY WORDS: granite; gravity; magma emplacement; magnetic structures;
petrographic zoning
Various mechanisms have been suggested to account for
the emplacement of granitic bodies. They range from
forceful intrusion, via diapirism (Cruden, 1988) or dyking
(Clemens & Mawer, 1992), to permitted intrusion assisted
by deformation (Hutton, 1982; Castro, 1985; Guineberteau et al., 1987; Hutton et al., 1990). Between these
modes, ballooning (Brun et al., 1990), nested diapirs
(Bouchez & Diot, 1990; Paterson & Vernon, 1995),
stoping (Paterson & Fowler, 1993) and also recognition
of the ubiquity of syntectonic magmatism (Hutton, 1997)
call for a competition, depending on the crustal level
of observation, between mechanisms driven by magma
pressure and those involving tectonic accommodation.
Evidence for the original segregation and ascent of
felsic magmas (now frozen as plutons) is difficult to
observe in the field. As a consequence, these processes
have to be inferred from petrological and rheological
models of both the magma and country rock (e.g. Cruden,
1988; Weinberg & Podladchikov, 1994). Stages of magma
generation and transport have, however, little influence
on the mode of the final ponding of the magma, which,
in turn, is not directly observable in the field. In addition
to considerations from regional geology, information from
petrofabrics (Bouchez, 1997) and geophysics (Matthews,
1987; Vigneresse, 1990) can also provide patterns of
magma straining in its site of emplacement and the 3D
geometry of the magma reservoir at its emplacement
site. Combined petrofabric and gravity surveys have
∗Corresponding author. Present address: CREGU BP 23, F-54501
Vandoevre/Nancy Cedex, France. Telephone: 33 3 83 44 19 00. Fax:
33 3 83 44 00 29. e-mail: [email protected]
 Oxford University Press 1997
JOURNAL OF PETROLOGY
VOLUME 38
proved to be particularly efficient in first-order modelling
of magma emplacement (Guillet et al., 1985; Guineberteau et al., 1987; Ame´glio et al., 1997). In these
studies, however, the magma is viewed as a homogeneous
material in its physical and chemical properties. Further
refinement of emplacement modelling requires integration of magma compositions, among other geochemical considerations, that may help to distinguish, for
example, the succession of batches that entered their site
of final ponding.
Based largely on previously published data, this paper
attempts at better understanding the emplacement mechanism of the granite pluton of Cabeza de Araya, Extramadura, Spain (Fig. 1). First, we correlate internal
structures and pluton shape at depth so as to locate
feeder zones. Then, we compare the structural patterns
with positions of feeders and variations in petrographical
types to infer and discuss the successions of magma pulses
and the continuous vs discontinuous character of their
supply.
THE CABEZA DE ARAYA GRANITE
COMPLEX
The pluton of Cabeza de Araya (Spain), about 70 km in
length and 18 km in width with an arcuate shape, was
selected for its large area (1000 km2), good exposure
and well-defined intrusive relationships within low-grade
metamorphic wall-rocks. Its general geology, partly summarized in Fig. 1, is well documented in past work and
includes petrography (Corretge´, 1971; Corretge´ et al.,
1985), geochemistry (Corretge´ et al., 1985; Perez del
Villar, 1988), regional structural geology (Castro, 1985;
Diez-Balda, 1986), internal fabric (Amice & Bouchez,
1989; Amice, 1990; Amice et al., 1991) and a gravity
survey (Audrain et al., 1989).
This late Variscan (~300 Ma) pluton intrudes late
Precambrian mica schists. The surrounding country
rocks, mostly meta-sandstones and schists, contain metreto kilometre-scale folds with vertical and N130°E-striking
axial planes, typified by the Caceres syncline, a structure
of 60–70 km width of Carboniferous age. Fold axial
planes are parallel to a pronounced regional schistosity
imprinted during the early Carboniferous (Diez-Balda,
1986). A second schistosity is observed, with the same
general orientation, but locally wrapping around the
granitic intrusions of the Extremadura (Castro, 1985);
hence this is also late Carboniferous in age. A metamorphic aureole, of 300–1000 m width (Corretge´, 1971),
and more or less synchronous with the latter schistosity,
is developed around the pluton.
Emplacement of the pluton has been modelled by
Castro (1985) as a mega-tension gash more or less parallel
to the regional schistosity, i.e. N130°E, and coeval with
NUMBER 12
DECEMBER 1997
a dextral regional shear zone. As the principal regional
compressive stress was roughly north–south at the time
of emplacement, as argued by Martinez-Catalan & DiezBalda (1987), a dextral strike-slip component might have
acted parallel to the tension gash (Amice, 1990; Amice
et al., 1991). Finally, NE–SW late faults have slightly
uplifted the northern and southern ends of the massif.
At the pluton centre, one of these faults may have guided
the setting of the Alentejo–Plasencia diabase dyke, which
cross-cuts most of the Spanish peninsula and is related
to the opening of the Atlantic Ocean.
Petrography and geochemistry
The main petrographic and geochemical features of
Cabeza de Araya have been described by Corretge´ et al.
(1985). The whole massif, like many other Variscan
granites of the Central Spanish System, is made of Stype granites. Three main petrographic types (A, B and
C) are recognized on the basis of texture and modal
composition. Their location is presented in Fig. 1e and
their average compositions are reported in Table 1 (Corretge´ et al., 1985). Group A granites occupy the largest
area. They are characterized by K-feldspar megacrysts
(up to 10 cm long), cordierite, biotite and subordinate
muscovite. Group B is made of coarse-grained, nonporphyritic two-mica granites that crop out in the southern part of the massif. Both groups appear to be related
by a fractional crystallization process as exemplified by
their Sr/Rb trend (Fig. 2). In addition, some two-mica
granites display high Na, Rb and Li contents attributed
to post-magmatic enrichment by hydrothermal fluids.
Group C contains fine-grained, two-mica leucogranites
which are located in the core of the pluton, following a
classical normal zoning pattern; i.e. becoming more felsic
from periphery to centre. Group C granites do not
necessarily represent the most evolved term of the above
magmatic differentiation trend, as shown by their discordant Sr/Rb trend (Fig. 2). Rather, they may correspond to an independent magmatic pulse originating
from a slightly different meta-sedimentary source, as also
suggested by their slightly higher Na, P and Sr contents
(Table 1).
Other facies have been described in addition to this
simple petrographic sketch, namely, a subordinate nonporphyritic border facies along the northwestern termination of the massif (Amice, 1990) and a heterogeneous
mingling zone (H in Fig. 1e) with a strong hydrothermal
imprint (Perez del Villar, 1988; Perez del Villar & Mingarro, 1988). A late garnet porphyry dyke (not shown in
Fig. 1e), emplaced along the northern border of the latter
heterogeneous zone, attests to the high Al content of the
late S-type magmas.
From observations of the contacts, type B intrudes type
A with gradational contacts marked by enclaves of A
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VIGNERESSE AND BOUCHEZ
SUCCESSIVE GRANITIC MAGMA BATCHES
Fig. 1. The multi-disciplinary approach to the study of the Cabeza de Araya granite pluton. (a) Bouguer gravity anomaly map and stations of
gravity measurements. (b) Sites of structural measurements performed both by classical field methods (except in the fine-grained facies C) and
with the anisotropy of magnetic susceptibility technique. (c) Simplified depth contours obtained after gravity data inversion; contours at 3 km
intervals point to pronounced deepening along elongate zones (shaded at depth [6 km) in both the north and south of pluton. (d) Magnetic
lineation map (230 measurements) showing the two main families: NW–SE trends with low plunges, and subvertical plunges mainly into
petrographic type C, at least in the south. The petrographic types (see e) are shaded. (e) Main petrographic types (see text for details) and
cleavage trajectories in the country rocks. Satellite granite bodies: ZM, Zarza la Major; ES, Estorninos; BR, Brozas. Locality: CA, Caceres. (f )
Magnetic susceptibility magnitudes (in 10–5 SI units) corrected for diamagnetism: the three selected domains compare favourably with the
petrographic types (contoured).
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Table 1: Chemical compositions of the three main facies of Cabeza de Araya
(from Corretge´ et al., 1985)
Type:
A
B
C
n:
43
27
16
r
x
r
x
r
x
wt %
SiO2
TiO2
72·67
0·254
2·23
0·14
74·76
0·195
1·29
75·85
0·13
0·09
0·05
0·84
Al2O3
14·10
0·95
13·77
0·78
13·18
1·23
Fe2O3T
2·14
0·56
1·49
0·80
1·11
0·40
MgO
0·45
0·17
0·38
0·19
0·25
0·20
MnO
0·034
0·01
0·026
0·01
0·023
0·01
CaO
0·81
0·22
0·58
0·22
0·54
0·12
Na2O
3·45
0·38
3·36
0·14
3·69
0·34
K 2O
4·78
0·35
4·40
0·28
4·21
0·42
P2O5
0·22
0·05
0·17
0·10
0·26
0·07
1·02
0·54
0·97
0·49
1·04
0·48
MV
Total
99·93
100·09
100·14
ppm
Li
147
46
212
68
175
104
Rb
261
42
315
50
327
64
Sr
51
15
33
11
37
15
Ba
726
282
517
131
529
167
MV, mean void = loss on ignition.
within B over distances of a few metres to tens of metres.
Types C and H clearly cross-cut type A in the north,
and locally display a transitional facies that is usually fine
grained and rich in K-feldspar megacrysts (Amice, 1990).
In the south, type C cross-cuts type B. No solid-state
deformation has been observed at any of the A–B–C–H
contacts. In short, as argued by Amice (1990), mutual
relations between A, B and C strongly suggest that the
corresponding magmas were emplaced in that order, each
before the previous magma had finished crystallizing.
Finally, cataclastic zones observed along the garnet porphyry dyke attest to its very late emplacement with
respect to the remainder of the pluton.
Gravity data
We used a 3D iterative technique, derived from Cordell
& Henderson (1968), adapted to small-scale gravimetric
investigations (Vigneresse, 1990; Ame´glio et al., 1997),
which is simpler to run when the geometry and distribution of the sources of mass are constrained by surface
data. The assigned contour line between the granite and
its country rocks, as well as specific density contrasts at
each node of the gridded Bouguer anomaly map, constitute the initial input data to the inversion process.
To achieve high resolution, 760 gravity stations were
deployed (Fig. 1a) on the massif and its immediate
surroundings (Audrain et al., 1989). The closely spaced
isovalue contour lines of the Bouguer anomaly (Fig. 1a)
define a steep inward gradient and indicate that the
massif is bounded by inward dipping surfaces. Two
regions of pronounced minima, with amplitudes of –11
mgal relative to the exterior of the body, are present in
the north and south of the pluton. They are not connected
to each other and are slightly different in their mean
trend and shape.
Density measurements were performed on 72 rock
samples from all granite types and surrounding schists.
Densities of types A and B are taken as 2·68±0·01,
and of types C and H as 2·67±0·01 and 2·66±0·01,
respectively, contrasting with 2·79 for the average density
of the country rocks. However, because the density
contrasts at depth cannot be determined directly, the
depth of the pluton’s floor calculated at each point is not
unique. Hence, during the iterative computation of the
depth values, different density contrasts with depth in
the range of the possible ones, i.e. corresponding to the
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VIGNERESSE AND BOUCHEZ
SUCCESSIVE GRANITIC MAGMA BATCHES
northern tip does not coincide with the lowest-density
rock types (C and H) observed at the surface.
Structural data
Fig. 2. Sr vs Rb diagram [compiled after Corretge´ et al. (1985) and
Perez del Villar (1988)], showing a classical differentiation trend for
types A and B, and a discordant trend for C.
different rock-types, have been tested, to minimize, in
map view, the difference between the calculated zero
depth and the actual limit of the granite body (see
Ame´glio et al., 1997). Inappropriate density contrasts are
easily detected as they result in anomalous depth values.
The residual values between the measured and recalculated gravity fields give an idea of the precision of
the calculated depths. A good first-order picture of the
pluton’s shape probably results from this inversion, with
a bulk uncertainty in the calculated depths not exceeding
±20% (Ame´glio et al., 1997). Because of the good coverage of gravity measurements, the shape of the pluton
is certainly better estimated than the absolute depth
values themselves.
After data inversion, controlled by several tests on the
sensitivity of the results to the density contrasts, a map
of the calculated depth of the pluton’s floor is obtained
(Fig. 1c). The pluton is not very thick, as 75% of its
volume lies within 4·5 km below the surface. Although
the two deeper regions roughly correspond to the Bouguer
gravity lows (Fig. 1a), it should be noted that the deepest
areas of the floor do not necessarily correlate with the
largest density contrasts. For example, although the –10
km depth contour corresponds to the low-density, ironpoor type C leucogranite, the –12 km contour of the
Samples have been cored at 230 stations regularly spaced
throughout the pluton (Fig. 1b). At each site, the bulk
magnetic susceptibility and the magnitudes and orientations of the three magnetic ellipsoid axes have been
determined. Obviously, the magnetic susceptibility and
its anisotropy depend on the magnetic mineralogy (Rochette et al., 1992). As in many granite massifs (see Archanjo
et al., 1992; Darrozes et al., 1994; Bouchez, 1997) the
magnetic fabrics of Cabeza de Araya are roughly aligned
with the preferred orientation of crystals (Amice, 1990;
Amice et al., 1991).
In the absence of magnetite at Cabeza de Araya,
the magnitude of the magnetic susceptibility is directly
proportional to the weight percent of iron in the rock
(Rochette, 1987; Rochette et al., 1992), thus allowing
correlation of bulk susceptibility with petrography (compare Fig. 1e with Fig. 1f ), and ultimately with the magma
chemical evolution (see Gleizes et al., 1993). The long
axis of the magnetic susceptibility ellipsoid is called the
magnetic lineation, and the short axis is normal to
the magnetic foliation plane. These linear and planar
structures mimic those that are recorded by the average
orientation of biotite, the main iron-bearing silicate in
Cabeza de Araya; they are also parallel to the structures
marked by the K-feldspar megacrysts of facies A (Amice
et al., 1991). On the basis of theory and experiments, the
origin and significance of the magnetic and magmatic
fabrics have been thoroughly discussed by Bouchez
(1997). As a good first approximation, the foliation normal
and the lineation respectively equate to the shortening
direction and stretching direction of the finite strain
undergone by the deforming magma in its site of emplacement.
All facies at Cabeza de Araya have most foliations
with steep dips (Fig. 3a), particularly along the periphery
of the pluton and near the border of subtype C in the
south. Their average strike is close to N130°E, i.e. parallel
to the elongation of the pluton in map view. This direction
is also parallel to the zone-axis (triangles of Fig. 3a)
around which the foliations of facies A and B have a
tendency to rotate. In contrast, in facies C the zoneaxis is rather subvertical. The shapes of the magnetic
susceptibility ellipsoids can be used, to a first order, as
markers of strain regimes. Prolate to neutral ellipsoids
are mostly observed in subtypes A and B, in accordance
with their tendency toward zonal orientations of the
foliation planes. Oblate ellipsoids often correlate with
proximity to the borders of the pluton, or with limits
between subtypes, particularly between facies B and C
(Amice, 1990).
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Fig. 3. Orientation diagrams for magnetic foliation poles and magnetic lineations in facies A, B and C, and R=A+B+C. R, the average
foliation plane, has a NW–SE strike and subvertical dip; the lineation has a variable plunge from horizontal to vertical. The other diagrams
show how these orientations distribute into facies A, B and C. Μ, poles of best fitting planes showing the zone axes of the foliations; Χ, bestfitting poles showing the subhorizontal and subvertical plunges of lineations, respectively, in facies B and C.
The lineations show two main families of orientations
(Figs 1d and 3b). One family has steep plunges (>60°)
whereas the other has lineations close to horizontal
(<30°). Intermediate plunges of lineations mostly belong
to regions between steep and shallow plunge zones. The
subvertical lineations mainly correspond to subtype C
(Fig. 3b) in the southern part of the pluton; they also
form clusters in the north, within subtype A. The subset
with low plunges mainly corresponds to subtypes A and
B; its mean azimuth at N137°E is close to the zone-axis
of foliations, and is parallel to the elongation of the pluton
in map view. However, several lineations observed to be
at different angles on both sides of the A–B contacts
point to flow discontinuity between these magmas and
confirm our field observations of cross-cutting relationships.
DISCUSSION
Shape of the magma reservoir and feeder
zones
The main characteristics of the pluton can be summarized
as follows. In transverse cross-section the massif is rather
narrow and limited by walls dipping inwards at 30–70°.
With respect to the present topography, 75% of its volume
lies above the 4·5 km depth contour. The northeastern
contact wall of the pluton is concave toward the inside
of the pluton and is steeply dipping, up to 70°. By
contrast, the northwestern wall has much shallower dips
(Fig. 4a). The same pattern, but symmetrically disposed
with respect to the pluton centre, holds for the southern
half of the pluton, where the southwestern wall is concave
and very steep, contrasting with the gentle dip of the
opposite wall (Fig. 4b). In both pluton halves, the gently
dipping walls steepen abruptly at depths greater than 4
km. At the floor of the pluton, two narrow and elongated
zones extend to depths greater than 6 km.
The average trend of the magmatic lineation parallels
the bulk elongation of the pluton and two main zones of
steeply plunging lineations are observed. The large zone
of subvertical lineations in the south correlates with the
deepest part of the pluton (compare Fig. 1c with Fig. 1d)
and mostly with the late, more leucocratic and possibly
more evolved subtype C (compare Fig. 1d with Fig. 1e).
In the north, within facies A, a correlation between
lineation plunge and depth to the floor also exists, although less clearly defined, and with no particular correlation with late facies C or H. These regions of deep
floor and vertical lineations are interpreted as feeder
zones for the magma. Their walls are >70° in dip at
depths >6 km, and look like flat cylindrical conduits with
their short horizontal dimension <3 km at >8 km depth
(Figs. 1c and 4).
Model of gash opening
Castro (1986) invoked the filling of a regional extensional
gash, a model of emplacement that is useful as a
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SUCCESSIVE GRANITIC MAGMA BATCHES
Fig. 4. Transverse cross-sections of the pluton of Cabeza de Araya, obtained from gravity data inversion. The asymmetrical shape of the
reservoir with respect to the central axis, with a steep eastern wall in the north (I–I′), and a steep western wall in the south (II–II′), should be
noted. Further steepening of the walls at ~6 km±1 km may correspond to the brittle–ductile transition at time of emplacement. Sections at
depth [6 km (shaded) are interpreted as the magma feeder zones. The interpreted limits of facies are traced.
first-order approximation. However, this model may be
refined using our data set. Instead of a single and very
long gash forming the whole pluton we suggest the
existence of two smaller, unconnected, en e´chelon tension
gashes. In principle, each tension gash formed in a
manner similar to the gash-opening model of Hutton
(1982) for the Donegal pluton. Gash opening is based
on the presence of a blocking point along a fault which
has a strike-slip component, and which opens progressively along with injection of magma. Our data satisfy
the double gash-opening model (Fig. 5) if we consider
that during dextral shear parallel to the regional foliation,
two gashes opened symmetrically with respect to the
centre of the pluton (which acted as a relay zone), the
blocking points being located at the gash tips. The
asymmetrical wall dips already described (Fig. 4) may
reflect the mechanism of progressive bending of the gash
walls. The feeders themselves, with their slightly arcuate
shapes in map view, large radii of curvature (35–50 km)
compared with their length (20 km), and very steep
borders (>70°), represent the traces at depth of the en
e´chelon faults. The changes in wall dips below –6 km
with respect to present surface (Fig. 4) are interpreted as
reflecting the rheological transition between the brittle
upper crust and the more ductile middle crust (see
Brun et al., 1990). In the brittle crust, large-scale crustal
dilatancy is allowed and provides the necessary volume
for granite emplacement. In contrast, the ductile crust
accommodates strain more readily around the upwelling
magma. Taking T [400°C for this rheological transition
and a thermal gradient of 30–25°C/km, this transition
probably took place 6–10 km deeper than its present
depth level.
Succession of magma batches and
formation of normal zoning
Emplacement of the pluton cannot be viewed as a rising
diapir. Rather, it is proposed that the magma progressively issued from the feeder zones so that the reservoir
progressively increased in volume, aided by the continuing transcurrent regional tectonics.
In the south, where the petrographic zoning is well
defined, facies A was the first magma emplaced at the
onset of gash opening (Fig. 5). It should be noted that a
time delay probably took place before the intrusion of
facies B, as indicated by the well-marked A–B contacts
across which clear differences exist in both petrographic
facies and susceptibility magnitudes. As facies B is slightly
more differentiated than A and undoubtedly genetically
related to it (Fig. 2; Corretge´ et al., 1985), magma differentiation probably continued at depth during the time
delay when no intrusion took place. However, no distinction between A and B can be made on the basis of
our structural data, as, in both facies, we observe N130°Estriking subvertical foliations and mostly subhorizontal
lineations with common continuation of trends across
contacts. This implies pluton-parallel stretching of both
facies A and B at their site of emplacement and crystallization. Facies C has been tapped from a different
source, and its intrusion into B is indicated by the clearcut
contacts between these two facies. Again, a possible
quiescence of the regional transcurrent tectonics occurred
before the intrusion of C, as indicated by the abrupt
increase in lineation plunges in facies C. In addition,
subvertical lineations unambiguously indicate that the
corresponding magma has been frozen-in while it was
upwelling above its feeder, and hence retained its vertical
linear structures.
In the northern tip of the pluton, the relationships
between the petrographic, structural and geophysical
characteristics are less obvious. There is little doubt that
the magma(s) that gave facies H (by mixing between A
and C?) upwelled after facies A (clearcut contacts), and
issued from a well-defined feeder zone. The linear fabric
of the area occupied by facies H was probably vertical
initially and has been reoriented toward parallelism with
the rest of the surrounding magma. Above the northernmost root-zone, facies A displays high-plunge lineations that call for its freezing during ascent. This had
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Fig. 5. Progressive opening and infilling of the pluton of Cabeza de Araya. (a) En e´chelon gashes more or less parallel to the regional schistosity,
at an angle with respect to the regional dextral shear zone (arrows) of Castro (1985); (b)–(d) progressive infilling by facies A, B, then H and C,
along with possible clockwise rotation of the dilating upper crust (light arrows in b).
to be the same for facies C, which clearly cross-cuts facies
A, but no lineation pattern in the small outcrop of facies
C has been determined. In our understanding, ubiquity
of high plunge lineations in this northern part of the
pluton indicates that no substantial lateral stretching
occurred in this tip zone of magma infilling.
The three facies of Cabeza de Araya form an overall
normal zonation, i.e. are more silicic toward the pluton
centre. Among the several mechanisms that may explain
the petrographic zoning of plutons (Pitcher, 1993; see
also Hecht et al., 1997), whether the zoning formed
in situ or at depth is a first-order question. Wall-rock
assimilation, one of the possible mechanisms of in situ
zoning, is ruled out as it is not reflected in the magma
chemistries. The high level of emplacement of the pluton,
hence its rapid cooling, and the high viscosities of such
magmas call for limited differential movements of melt
with respect to the solid fraction in the site of emplacement, ruling out in situ fractionation. Magmatic
differentiation at depth (facies A and B) and tapping of
a different source of magma (facies C) are therefore
preferred. As already argued, the order of magma emplacement was A then B then C, as attested by the
contacts and cross-cutting relationships in the field.
Therefore, and at least in the south where there is an
intimate association between magma type C and the
feeder zone, emplacement of a normally zoned pluton
may be seen as the progressive feeding of a chemically
more differentiated magma, issuing from a feeder preferentially located at pluton centre in map view, and
‘pushing’ aside the previously emplaced batches of magma.
In conclusion, contrary to the static in situ differentiation
model of Corretge´ et al. (1985), our dynamical model of
continuous feeding (progressive petrographical evolution,
continuous structural trends) vs discontinuous feeding
(cross-cutting contacts, discordant structural patterns) implies that magma fractionation occurred within the source
rocks, or at least before final emplacement, but not in
the ultimate reservoir.
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VIGNERESSE AND BOUCHEZ
SUCCESSIVE GRANITIC MAGMA BATCHES
Infilling of the gash
The two root zones were not connected to each other
and operated independently, as attested by the separate
zoning patterns in the south and north, each pattern
being centred above a root zone. This does not change,
however, the very mechanism of gash infilling. Our model
of tectonically driven gash opening basically calls for
a permissive rather than forceful process for magma
emplacement. We think that magma pressure accompanying intrusion was rather low on average during
pluton emplacement, as it had just to be sufficient in
magnitude to feed the gashes. However, as already discussed by Paterson & Fowler (1993) for extensional pluton
emplacement, the rate of crust opening by country rock
displacement, hence the room available in the crust, was
critical. If the tectonic rate was fast enough for a given
production of magma in the source region, the magma
pressure during each intrusion (A, B, C) was probably
steady state. To account for the differences in structural
patterns on opposing sides of facies contacts, either the
rate of tectonic opening drastically decreased between
the batches and stopped magma infilling, or the rate of
magma production itself became null. In both cases, the
time elapsed between batches was short, as the magma
did not fully crystallize before the next injection occurred.
As thermal considerations favour a constant rate for
magma production, transition between facies was probably governed by the rate of tectonic opening. Hence,
the magma pressure must have increased in the source
region, or somewhere between the source and the reservoir, creating sufficient pressure to intrude the reservoir.
The infilling was therefore discontinuous.
CONCLUSION
The granitic massif of Cabeza de Araya is an example of
felsic magmas emplaced in the brittle crust and displaying
petro-structural features that allow us to trace the different
magma pulses. We have correlated the shape of the
pluton at depth obtained from a gravity data inversion
with surficial structural data. The feeder zones of the
pluton were determined where vertical lineations occur
above zones of a deeper floor. Two unconnected arcuate
zones, with a pronounced asymmetry in the dips of their
walls, delineate the feeder zones. Along with the bulk
3D shape of the massif, internal structure and regional
deformation, it is argued that magma was mostly permissively emplaced within extensional zones of the crust.
The successive pulses of magma are recorded in their
structural and chemical evolutions. Although intrusion
of a given facies was not necessarily synchronous in the
north and south, the succession of batches always occurred in the same order, and within a not yet fully
crystallized (i.e. fully consolidated) magma. From this
case study, normal zoning of a granite pluton is viewed
as the result of the continuous or discontinuous emplacement of progressively more evolved magma batches
issuing from the same feeder and pushing aside the
former batches.
ACKNOWLEDGEMENTS
We thank Marc Amice and Jack Audrain for their detailed
fieldwork and data of remarkable quality obtained while
preparing their thesis in 1987–1989. The paper also
benefited from discussions with A. Ne´de´lec (Toulouse),
K. Benn (Ottawa), W. E. Stephens (St Andrews), C.
Miller (Nashville) and L. Hecht (Mu¨nchen), and detailed
reviews by A. Cruden (Toronto), S. Paterson (Los Angeles) and J. Lowenstern (Menlo Park). CNRS–INSU
DBT program (Theme 5), UMR CNRS 5563 (Toulouse)
and CREGU (Nancy) are thanked for financial support.
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