JOURNAL OF PETROLOGY
VOLUME 42
NUMBER 9
PAGES 1751–1772
2001
Partial Melting of Aluminous Metagreywackes
in the Northern Sierra de Comechingones,
Central Argentina
JUAN E. OTAMENDI1 AND ALBERTO E. PATIÑO DOUCE2∗
1
DEPARTAMENTO DE GEOLOGÍA, UNIVERSIDAD NACIONAL DE RÍO CUARTO, 5800 RÍO CUARTO, ARGENTINA
2
DEPARTMENT OF GEOLOGY, UNIVERSITY OF GEORGIA, ATHENS, GA 30602, USA
RECEIVED JUNE 29, 2000; REVISED TYPESCRIPT ACCEPTED MARCH 5, 2001
We describe a suite of metamorphic and migmatitic rocks from the
Sierra de Comechingones (Sierras Pampeanas of Central Argentina)
that include unmelted gneisses, migmatites and refractory granulites.
The gneisses are aluminous greywackes metamorphosed in the
amphibolite grade and are likely to have been the protoliths for the
higher-grade migmatites and granulites. Mineralogical characteristics
and major and trace element compositions show that metatexite
migmatites, diatexite migmatites and granulites are all melt-depleted
rocks. The migmatites (both metatexites and diatexites) have undergone H2O-fluxed melting and lost >20% melt, whereas the
granulites have lost as much as 60% melt, formed chiefly by
dehydration-melting of biotite. The granulites are rocks from which
essentially all the granitic components have been extracted, whereas
the migmatites cooled and solidified while still retaining a granitic
fraction. The only evidence for the presence of melt in the study
area, apart from migmatitic leucosomes, are dikes and sills of
leucogranite rich in cumulate K-feldspar. These leucogranite bodies
may represent the pathways along which melt was drained from the
exposed rocks. Our study shows that in the Sierra de Comechingones
both migmatites and granulites represent source regions of anatectic
magmas, which were ‘frozen in’ at different stages of development.
The widespread occurrence of migmatites attests to
the fact that crustal anatexis is a common phenomenon.
There is no consensus on whether or not migmatites
represent the source regions of anatectic magmas (e.g.
Brown, 1994; Sawyer, 1998), even though Sawyer
(1996) showed that diatexite migmatites have rheological
and chemical properties that suggest that they are
parental to granite magmas. It is also known that
granulites depleted in incompatible major and trace
elements are present in the lower continental crust
(e.g. Lambert & Heier, 1968). These rocks have been
interpreted as refractory residues of partial melting
complementary to granitic magmas (e.g. Clemens, 1990;
Vielzeuf et al., 1990), and there is ample theoretical
and experimental evidence that supports this concept
(e.g. Brown & Fyfe, 1970; Thompson, 1982; Vielzeuf
& Holloway, 1988; Patiño Douce & Johnston, 1991;
Patiño Douce & Beard, 1995). It is thus possible that
the source regions of some granitic magmas could
consist of a wide spectrum of migmatites and meltdepleted granulites.
In this paper we focus on a suite of metamorphic
and migmatitic rocks from the Sierras Pampeanas of
Argentina. The rocks in the study area include unmelted
gneisses, migmatites and refractory granulites. We
describe the fabrics, mineral assemblages and field
relations of the various rock types and present data
on major and trace element bulk-rock compositions.
We use petrographic characteristics and major and
trace element compositions to show that migmatites
and granulites are both residual rocks, which have
undergone different degrees of melt depletion. We
argue that the migmatites and the granulites represent
different sources of granitic magma in the same crustal
section.
∗Corresponding author. Telephone: 001-706-542-2394. Fax: 001-706542-2425. E-mail: [email protected]
 Oxford University Press 2001
KEY WORDS:
migmatites; granulites; anatexis; metagreywackes; granites
INTRODUCTION
JOURNAL OF PETROLOGY
VOLUME 42
GEOLOGICAL SETTING
The study area is centered on the Rı́o Santa Rosa, within
the Sierra de Comechingones (Fig. 1). The Sierra de
Comechingones is one of the southernmost ranges of the
Sierras Pampeanas of Central Argentina (Fig. 1). It
is composed of upper amphibolite to granulite facies
metamorphic and migmatitic rocks that were derived
from a predominantly clastic sedimentary sequence. The
most abundant protoliths were aluminous greywackes,
accompanied by volumetrically minor limestones (Gordillo, 1984; Otamendi et al., 1999). If shale intercalations
were present in the pre-metamorphic sequence they must
have been very thin, as they cannot be recognized in the
unmelted gneisses at scales of 1 m or less. Metamorphism
was the result of a regional Early Cambrian (Rapela et
al., 1998; Sims et al., 1998) tectono-thermal event called
the Pampean Orogeny. Peak metamorphism in the study
area attained temperatures ranging from 650 to 950°C
at nearly uniform pressures of >7–8 kbar (Otamendi et
al., 1999).
The Sierra de Comechingones was affected by four
major deformation phases (Martino et al., 1995). D1 is
poorly preserved as a gneissic foliation folded by rootless
D2 folds. D2 deformation is characterized by mineral
segregation and folding. It took place during prograde
metamorphism, probably close to the metamorphic peak
(Gordillo, 1984). This timing is suggested by the observation that metatexite leucosomes often nucleate on
the quartzo-feldspathic layers that define the S2 foliation.
Sigmoidal channels oblique to S2 foliation and filled with
discordant leucosomes indicate that the partially molten
rocks were subject to shear stress, but it is unclear whether
this deformation corresponds to D2 or to an as yet
unrecognized deformation episode. This is so because
the dominant structural features throughout most of the
Sierra de Comechingones correspond to the D3 phase
(Martino et al., 1995), which began at conditions close to
the metamorphic peak but continued during the cooling
stage (Otamendi el al., 1999). D3 deformation is partitioned into high- and low-strain domains, and is associated in the former with retrogression of garnet +
cordierite + K-feldspar assemblages to biotite + sillimanite (Otamendi et al., 1999). S3 fabric is penetrative
in NNW–SSE-trending high-strain zones that nucleated
along the boundaries of homogeneous lithologic blocks.
The fabric is weaker and becomes harder to distinguish
from S2 in the cores of homogeneous lithologic bodies,
such as the Cerro Pelado Dam diatexite massif (Fig. 1).
D3 kinematic indicators of shear sense are ambiguous,
but most of them seem to indicate that D3 structures
formed in response to dextral shearing associated with
SSE-directed extensional deformation (Martino et al.,
1994). Steeply dipping NNW–SSE-trending mylonitic
belts that overprint D3 structures are ascribed to a D4
NUMBER 9
SEPTEMBER 2001
event (Martino et al., 1994). D4 mylonite belts are spatially
associated with the granulites that record the highest
metamorphic conditions (Otamendi et al., 1999) and may
be responsible for juxtaposing these rocks against lowergrade migmatites. Final exhumation of the Sierra de
Comechingones occurred during the Famatinian Orogeny (>490–390 Ma) and was accommodated along
narrow regional-scale shear belts that are affected by
low-grade metamorphic recrystallization (Martino et al.,
1995; Rapela et al., 1998).
ROCK TYPES AND FIELD
RELATIONS
Several rock types can be distinguished in the Sierra de
Comechingones on the basis of their mineral assemblages
and morphological characteristics. The spatial distributions of the rock types and the sample localities are
shown in Fig. 1 (regional distribution) and Fig. 2 (detail
of the Rı́o Santa Rosa area, where the highest-grade
rocks occur). The lithological characteristics, mineral
compositions and metamorphic evolution of these rocks
were discussed by Otamendi et al. (1999). The most
important features, including the field relations and inferred melting reactions, are also summarized here. Phase
assemblages and modal compositions of representative
samples are given in Table 1. Detailed mineral compositions have been given by Otamendi et al. (1999, tables
2–6).
Biotite–garnet gneisses (Type I)
These are the most abundant rocks in the Sierra de
Comechingones (Gordillo, 1984). Type I gneisses have
a medium- to coarse-grained granoblastic texture and
are composed of the mineral assemblage Qtz + Pl +
Bt + Grt + Ap + Zrn + Rt ± Ilm ± Kfs [mineral
symbols after Kretz (1983)]. They have a gneissic fabric
given by the alternation of quartzo-feldspathic-rich and
biotite-rich layers (Fig. 3a). In places it is possible to
recognize the preservation of primary centimeter-scale
sedimentary bedding (Fig. 3b). This relic bedding is
defined by varying biotite/plagioclase ratios. Biotite-rich
beds are strongly foliated and are separated from less
foliated biotite-poor beds by millimeter-thick quartz-rich
layers. Garnet abundances do not vary noticeably across
this relic sedimentary bedding. The gneissic foliation is
overprinted by a continuous schistosity in D3 high-strain
domains. Muscovite and fibrolite commonly appear in
these highly strained Type I rocks, but these Al-rich
phases are otherwise notoriously absent from Type I
gneisses.
1752
OTAMENDI AND PATIÑO DOUCE
PARTIAL MELTING OF METAGREYWACKES
Fig. 1. Geological map of the northern Sierra de Comechingones, modified after Bonalumi & Gigena (1987). Sample locations refer to samples
discussed in the text. Insets show the location of the northern Sierra de Comechingones within the Sierras de Córdoba, which are the southernmost
ranges of the Sierras Pampeanas of central Argentina.
Biotite–garnet–K-feldspar–cordierite
migmatites (Type II)
These are migmatites that display a wide variety of
morphologies, among which metatexites and diatexites
[nomenclature after Ashworth (1985)] occur as endmember types. The migmatites are distinguished from
Type I gneisses on the basis of mineral assemblages
(widespread coexistence of garnet and/or cordierite with
K-feldspar) and structural evidence of partial melting
(e.g. Sawyer, 1999). Relative to Type I gneisses, biotite
in Type II migmatites (see Otamendi et al., 1999) is
distinctly richer in Ti (>0·15 vs >0·25 Ti atoms p.f.u.)
and poorer in octahedral Al (>0·38 vs >0·22 Al6 atoms
p.f.u). Type II migmatites contain accessory apatite,
zircon, monazite and rutile. Monazite occurs as unzoned
euhedral grains, either next to or included in garnet, but
almost never as isolated crystals in the leucosomes.
Contacts between gneisses and migmatites correspond
in most cases to high-strain D3 shear zones, but grad-
ational contacts are observed in some places south of the
Rı́o Santa Rosa (Fig. 1). In such cases, migmatites with
layer-parallel leucosomes are intercalated with packages
of unmelted gneiss. The transition from one rock type
to the other is completed over distances of the order of
100 m.
Metatexites are characterized by a well-developed textural and mineralogical layering that results from the
alternation of Qtz + Grt + Bt + Pl ± Crd melanosomes
with Qtz + Kfs ± Grt ± Crd leucosomes. Some
metatexites preserve remnants of the sedimentary bedding observed in the gneisses but others show no vestiges
of pre-migmatization fabrics. Metatexites that appear to
represent incipient melting contain only layer-parallel
leucosomes <1 cm thick (sample SCP24). As melt contents
increase layer-parallel leucosomes become thicker
(1–2 cm) and a second set of leucosomes oblique to the
metamorphic layering appears (Fig. 3c, sample OCP10).
Leucosomes eventually coalesce to form discordant dikes
1753
and concordant sills of leucogranite (described below).
The leucogranite bodies are spatially associated with
stromatic migmatites in which melanosomes are interlayered with Kfs + Qtz ± Pl leucosomes <3 mm thick
(sample RSR31b). These stromatic migmatites are depleted in Qtz and Pl and enriched in Bt and Grt relative
to metatexites that are not associated with leucogranite
bodies.
Diatexites (sample RSR32) have a coarse-grained equigranular fabric with disseminated alkali feldspar megacrysts (Fig. 3d). Melanosome and leucosome are less
distinct than in the metatexites, but it is nevertheless
1754
RSR23
RSR31b
RSR32
RSR31a
RSR37
RSR16
RSR04
RSR24
RSR17
RSR18
I
IIM
IID
L
L
III
III
III
IV
IV
18·6
18·8
22·7
17·9
13·7
1·5
3·2
25·9
—
—
15·3
17·5
14·2
60·8
58·4
10·2
24·3
—
—
(%)
K-feldspar
3·3
3·1
16·2
13·2
16·4
0·6
0·6
15·1
16·4
19·2
25·0
(%)
Biotite
24·9
20·2
4·8
6·1
5·7
3·2
4·8
7·0
7·6
6·7
4·1
(%)
Garnet
25·9
25·7
2·5
13·2
16·3
—
—
1·5
3·6
—
—
(%)
Cordierite
0·3
0·5
4·5
1·3
2·2
—
—
—
0·3
—
—
(%)
Sillimanite
—
—
—
—
—
0·7
1·3
—
—
—
—
(%)
Muscovite
2·8
4·9
—
—
—
—
—
—
—
—
—
(%)
Anthophyllite
2·0
3·0
1·0
0·9
0·9
—
—
0·1
—
0·3
0·4
(%)
0·4
1·3
0·2
0·9
0·4
—
—
0·1
0·3
1·1
0·6
(%)
Fe–Ti oxides Accessories
Modal proportions were obtained by counting between 1800 and 2100 points per sample. Rock types: I, Type I gneisses; IIM, Type II metatexite; IID, Type II
diatexite; L, leucogranites; III, Type III migmatites; IV, Type IV granulites. Sample locations are shown in Figs 1 and 2.
21·8
22·5
32·8
29·0
30·2
33·2
31·4
40·1
25·6
33·7
32·4
(%)
Plagioclase
NUMBER 9
21·9
39·0
37·5
QU19
I
Quartz
(%)
Sample
type
VOLUME 42
Rock
Fig. 2. Geological map of the Rı́o Santa Rosa area (outlined in Fig.
1), showing the distribution of the highest-grade rocks in the northern
Sierra de Comechingones. Sample locations refer to samples discussed
in the text.
Table 1: Modal mineralogical compositions of representative samples
JOURNAL OF PETROLOGY
SEPTEMBER 2001
OTAMENDI AND PATIÑO DOUCE
PARTIAL MELTING OF METAGREYWACKES
Fig. 3. (a), (b) and (c).
possible to recognize a faint foliation and to identify
biotite-rich schlieren oriented roughly parallel to this
foliation. The diatexites also contain metatexite rafts,
refractory xenoliths of mafic and calcareous rocks and
leucogranite sills. The transition from metatexite to
diatexite occurs over scales of a few meters.
Leucogranites
Tabular leucogranite bodies generally thicker than 5 cm
and ranging in length from 10 to 100 m are found
emplaced within Type II migmatites, but cannot be
mapped as independent units at the scale of Figs 1 and
2. They include discordant dikes, lensoidal sills and bodies
with irregular borders and variable thickness (Fig. 3e
and f ). The contacts with the migmatites are commonly
sharp. The leucogranite bodies are chiefly composed
of alkali feldspar and quartz. Alkali feldspar occurs as
subhedral crystals, sometimes megacrysts a few centimeters long, forming an adcumulus texture with interstitial (intercumulus) anhedral quartz. Plagioclase occurs
only as inclusions in poikilitic alkali feldspar crystals.
Garnet is a common accessory mineral. Its composition
is indistinguishable from that of garnet in Type II migmatites. Muscovite occurs only as mats growing on corroded biotite crystals and is thought to be a retrograde
phase.
Biotite–garnet–K-feldspar–cordierite–
sillimanite migmatites (Type III)
Type III migmatites are homogeneously foliated rocks
with a ‘reversed’ schlieren structure in which leucosome
streaks occur inside the dominant melanosome (e.g.
sample RSR24). These rocks thus contain a greater
1755
JOURNAL OF PETROLOGY
VOLUME 42
NUMBER 9
SEPTEMBER 2001
Fig. 3. (a) Type I gneiss displaying its characteristic metamorphic fabric. The hammer is 40 cm long. (b) Compositional banding in Type I
gneisses. The strongly foliated bands are much richer in biotite than the poorly foliated ones. Towards the top of the photograph is a leucogranite
dike that can be followed in the field into Type II metatexites. The lens cap is 56 mm in diameter. (c) Type II metatexite displaying both layerparallel and oblique leucosomes. The lens cap is 56 mm in diameter. (d) Type II diatexite. Included in the diatexite are quartz boudins (gneiss
relics?) and metatexite rafts. The lens cap is 56 mm in diameter. (e) Leucogranite bodies displaying both concordant and discordant orientations
relative to the foliation of Type II metatexites. The body marked with the arrow is 40 cm wide. (f ) Leucogranite sills with variable thickness
and irregular margins. The hammer is 40 cm long.
melanosome/leucosome proportion than Type II metatexites. Large leucogranite bodies such as those associated
with Type II migmatites are never found in Type III
migmatites. They differ mineralogically from the former
in having greater cordierite abundances and in the fact
that they always contain abundant sillimanite and some
hercynitic spinel (Table 1). Monazite in unzoned euhedral
grains is abundant in Type III leucosomes, in contrast
to Type II metatexites in which this accessory phase is
mostly restricted to the melanosomes. Type III migmatites
acquire stromatic structures in D3 high-strain belts
(samples RSR04 and RSR16), suggesting that this deformation stage outlasted the presence of melt. Contacts
with Type II migmatites always correspond to these highstrain belts.
Garnet–cordierite granulites (Type IV)
Granulites are distinguished by the absence of prograde
biotite and alkali feldspar. These rocks are homogeneous
and massive at the outcrop scale, but hand specimens
reveal a well-developed foliation defined by compositional
segregation accompanied by a stretching lineation. Garnet, cordierite, and Fe–Ti oxides are concentrated in
poorly defined melanocratic bands that alternate with
plagioclase- and quartz-rich leucocratic domains. Plagioclase and garnet in these rocks display noticeable Ca
enrichment towards the rims (An45 to An50 and Grs3 to
Grs6, respectively). Zoned subhedral crystals of zircon
are very abundant in all Type IV granulites but monazite
is conspicuously absent. Anthophyllite occurs in variable
1756
OTAMENDI AND PATIÑO DOUCE
PARTIAL MELTING OF METAGREYWACKES
Table 2: Bulk major and trace element compositions
Type:
I
I
I
I
I
IIM
IIM
IIM
IID
Sample:
QU19
QU13
SCP13
SCP15
RSR23
SCP24
OCP10
RSR31b
RSR32
65·41
SiO2
70·23
70·53
68·96
72·49
69·81
69·91
70·06
61·10
TiO2
0·95
0·91
0·84
0·68
1·04
0·73
0·80
1·19
1·00
Al2O3
12·86
12·94
13·57
12·01
12·13
13·60
12·99
17·61
15·27
FeOT∗
5·54
5·65
5·72
4·72
6·06
4·76
5·48
8·52
7·28
MnO
0·09
0·01
0·08
0·07
0·10
0·08
0·09
0·15
0·15
MgO
2·57
2·54
2·75
2·22
2·76
2·20
2·74
3·55
3·15
CaO
2·12
1·99
2·14
2·02
2·16
2·02
2·19
1·26
1·83
Na2O
2·11
2·06
2·64
2·52
2·00
2·76
2·42
1·95
2·03
K2O
2·63
2·61
2·40
2·39
2·02
2·52
2·17
3·85
2·98
P2O5
0·18
0·18
0·07
0·13
0·19
0·09
0·19
0·07
0·08
LOI
0·70
0·68
0·71
0·89
0·32
0·92
0·87
0·84
0·63
99·98
100·10
99·88
100·14
98·59
99·59
100·00
100·09
99·81
Total
ASI
1·26
1·32
1·25
1·15
1·29
1·24
1·26
1·82
1·54
Cr
85
97
63
57
72
50
60
79
113
Ni
43
41
30
26
35
23
26
46
52
Co
32
31
82
78
14
96
56
28
38
Sc
17
16
14
12
13
12
12
14
25
V
125
128
99
82
115
76
89
126
160
Zn
74
81
70
82
106
73
89
116
91
Rb
121
122
107
88
90
92
103
72
148
Cs
3
3
3
2
5
1
1
4
2
Ba
542
527
300
422
413
367
429
526
550
Sr
202
206
180
192
113
183
173
122
198
Y
27
24
29
29
36
27
27
29
44
Th
12
14
19
13
13
14
11
10
19
Zr
n.d.
n.d.
63
n.d.
66
n.d.
n.d.
La
38·1
33·2
Ce
93
81
Pr
10·7
Nd
40
9·4
35
68
49·7
101
11·9
44
58
36·1
39·0
38·6
32·2
25·8
91
92
80
83
62
9·1
36
10·5
39
9·9
37
8·3
33
7·6
28
44·4
111
13·1
49
Sm
7·5
6·6
7·9
6·7
8·3
7·5
6·7
5·7
9·9
Eu
1·6
1·5
1·5
1·3
1·4
1·6
1·2
1·3
2·0
Gd
6·2
5·6
6·3
5·6
7·6
6·4
6·0
5·2
8·6
Tb
0·8
0·8
0·9
0·9
1·2
0·9
0·8
0·9
1·3
Dy
4·8
4·4
5·5
5·0
6·9
5·0
5·1
5·9
7·7
Ho
0·9
0·8
1·2
1·1
1·4
1·1
1·0
1·2
1·5
Er
2·6
2·4
3·5
3·2
4·2
3·4
3·0
3·7
4·6
Tm
0·4
0·3
0·5
0·6
0·6
0·5
0·4
0·5
0·7
Yb
2·3
2·2
3·3
3·1
4·0
3·3
2·8
3·5
4·4
Lu
0·3
0·3
0·5
0·2
0·6
0·5
0·4
0·5
0·6
1757
JOURNAL OF PETROLOGY
VOLUME 42
NUMBER 9
SEPTEMBER 2001
Table 2: continued
Type:
L
L
L
L
III
III
III
IV
IV
IV
Sample:
RSR31a
RSR37
RSR41
RSR48
RSR04
RSR24
RSR16
RSR17
RSR18
RSR30
TM85†
SiO2
73·57
74·70
73·23
73·40
65·30
64·83
63·79
62·23
57·57
62·69
TiO2
0·07
0·07
0·04
0·05
1·15
1·32
1·23
1·48
1·66
1·47
0·5
Al2O3
13·34
13·44
14·15
15·87
16·07
16·13
15·70
17·35
16·43
14·0
4·2
14·3
70·1
FeOT∗
1·45
1·24
0·94
0·82
6·43
7·53
9·02
10·52
12·94
10·85
MnO
0·03
0·03
0·02
0·04
0·09
0·13
0·19
0·24
0·27
0·21
MgO
0·43
0·39
0·14
0·23
3·19
2·71
3·13
4·47
6·21
4·29
CaO
0·27
0·27
0·50
0·60
2·01
1·43
1·20
2·50
1·87
2·43
2·5
Na2O
2·10
2·14
2·33
1·94
1·82
1·97
1·47
1·15
0·82
1·04
3·7
K2O
7·12
7·36
8·17
7·74
3·23
3·83
3·07
0·14
0·19
0·25
1·8
P2O5
0·18
0·15
0·19
0·2
0·08
0·04
0·07
0·02
0·03
0·02
LOI
Total
0·77
0·50
0·60
0·75
0·70
0·66
0·67
0·90
0·78
0·61
99·33
100·29
100·31
100·07
99·87
100·52
99·97
99·35
99·69
100·29
ASI
1·15
1·12
1·04
1·13
Cr
7
8
3
5
67
1·56
60
1·61
76
2·04
64
2·38
73
3·50
59
2·57
Ni
5
7
2
3
47
35
43
32
57
42
Co
12
15
16
14
17
22
29
25
23
23
Sc
3
3
2
2
14
11
16
20
21
15
V
8
11
2
4
76
99
119
102
69
93
Zn
15
16
4
7
63
109
100
41
56
44
Rb
115
166
104
101
106
88
80
3
6
4
Cs
2
3
2
1
2
3
3
1
1
1
Ba
1193
1380
959
1017
429
378
397
19
20
24
Sr
143
203
150
178
137
87
93
55
36
58
48
Y
5
6
8
4
36
27
35
63
68
Th
1
2
1
1
13
15
16
2
5
Zr
46
34
33
27
n.d.
n.d.
n.d.
n.d.
La
3·1
Ce
7
Pr
0·8
1·2
1·3
0·5
10·6
Nd
3
4
4
2
43
5·3
11
8·1
13
3·3
40·1
34·7
37·4
5
86
79
84
9·4
36
8·6
18
10·1
2·1
37
8
n.d.
12·7
23
2·9
11
2·3
3
n.d.
8·8
20
2·2
8
38
80
8·9
32
Sm
0·7
0·8
0·8
0·4
9·9
7·0
7·0
2·1
2·7
2·1
5·6
Eu
1·5
2·0
1·2
1·5
1·3
1·3
1·2
0·6
0·5
0·6
1·1
Gd
0·7
0·9
0·8
0·4
6·7
6·0
6·1
4·4
5·1
3·9
4·7
Tb
0·1
0·2
0·2
0·1
0·9
0·9
1·0
1·2
1·3
1·0
0·77
Dy
1·2
1·2
1·3
0·7
6·9
5·6
6·6
10·3
11·2
8·4
4·4
Ho
0·2
0·2
0·3
0·2
1·5
1·2
1·4
2·5
2·7
2·0
1
Er
0·7
0·6
0·8
0·5
3·9
3·7
4·5
8·1
8·2
5·9
2·9
Tm
0·1
0·1
0·2
0·1
0·6
0·5
0·7
1·2
1·2
0·9
0·4
Yb
0·7
0·5
0·8
0·5
3·6
3·6
4·5
7·7
7·2
5·4
2·8
Lu
0·1
0·1
0·1
0·1
0·5
0·5
0·7
1·1
1·0
0·7
0·43
Rock types as in Table 1. Sample locations are shown in Figs 1 and 2. Typical uncertainties in major element contents (1
SD) are: SiO2, 0·99 wt %; TiO2, 0·06 wt %; Al2O3, 0·21 wt %; FeO(t), 0·02 wt % ; MnO, 0·01 wt %; MgO, 0·04 wt %; CaO,
0·06 wt %; Na2O, 0·08 wt %; K2O, 0·07 wt %; P2O5, 0·01 wt %. Uncertainties in trace element abundances were estimated
with the absolute value of the ratio: 100 × [(sample − standard)/standard]. These value is typically in the range 5–10 for
all trace elements. ASI, alumina saturation index [Al2O3/(CaO + Na2O + K2O)], in molar proportions. n.d., data not determined.
∗Total Fe content reported as FeO.
†TM85 shows the average major element composition of quartz-intermediate Phanerozoic greywackes and average REE
abundances in Post-Archaean Australian shales, both taken from Taylor & McLennan (1985).
1758
OTAMENDI AND PATIÑO DOUCE
PARTIAL MELTING OF METAGREYWACKES
modal proportions and tends to be associated with melanocratic segregations. Type IV granulites also contain
a small amount of Ti-poor phlogopite that is clearly a
retrograde phase (Otamendi et al., 1999).
PETROLOGIC EVOLUTION
The transition from Type I gneisses to Type II metatexites
is marked by the first appearance of K-feldspar-bearing
layer-parallel leucosomes that contain ovoid grains of
cordierite and by increases in the modal proportions of
garnet and ilmenite [Table 1; see also Otamendi et al.
(1999)]. The migmatites are thus inferred to have been
produced by anatexis at the amphibolite-to-granulite
transition, according to an incongruent melting reaction
of the general form
Qtz + Pl + Bt ± Sil ± H2O = melt (± Kfs) +
Grt/Crd + Ilm.
(1)
The participation of at least a small amount of free H2O
in this melting reaction is inferred on the basis of the
fact that Type II metatexites are enriched in K and
depleted in Na and Ca relative to Type I gneisses [see
below, and also Patiño Douce (1996) and Patiño Douce
& Harris (1998)]. Sillimanite is absent from most Type
I rocks but is required by this melting reaction, which is
constrained by the observed mineral assemblage of Type
II migmatites. Otamendi et al. (1999) argued that the
sillimanite necessary to stabilize the peritectic assemblage
garnet ± cordierite in Type II metatexites was present
not as a discrete phase but rather as a dioctahedral Al
component in biotite, [Al2R−3]bt [with R = Fe2+ or Mg;
see Patiño Douce et al. (1993)], in Type I gneisses.
Dioctahedral Al in biotite is liberated with rising temperature and/or decreasing pressure according to the
following reaction (see Patiño Douce et al., 1993; Otamendi et al., 1999):
Grt + [Al2R−3]bt = 2 As (Sil or Ky) +
Qtz (with R =Fe2+ or Mg).
(2)
The fact that biotite in the migmatites is strongly depleted
in Al6 relative to biotite in the gneisses is consistent with
the operation of reaction (2), and the observed growth
of euhedral sillimanite and quartz at the interface between
resorbed garnet and biotite in Type II metatexites provides clear petrographic evidence for it (see Otamendi et
al., 1999, fig. 3f ). The feasibility of this melting mechanism [i.e. reaction (2) supplying normative Al2SiO5 to
reaction (1)] from the point of view of mass balance is
further discussed below, in the context of partial melting
models.
The transition from Type II to Type III rocks is
characterized by increases in the modal proportions of
cordierite and sillimanite (Table 1). Otamendi et al. (1999)
attributed these changes solely to a rise in the peak
metamorphic temperature from Type II to Type III
migmatites that shifted reaction (2) to the right and
increased the degree of melting, thus producing more
cordierite by reaction (1). In addition to this temperature
effect, it is also possible that extraction of a plagioclaserich melt, formed by melting in the presence of a small
amount of free H2O, contributed further to Al enrichment
in the residue-rich Type III migmatites (see also below).
Type IV granulites are thought to represent refractory
residues formed by extraction of relatively H2O-poor
melts at extreme metamorphic temperatures (Otamendi
et al., 1999; see also below). In addition to their refractory
mineral assemblage, the fact that garnet and plagioclase
in the granulites are simultaneously enriched in Ca
from core to rim is consistent with these rocks having
undergone melt extraction (see Otamendi et al., 1999).
Further geochemical arguments that support the residual
nature of the granulites are discussed below.
BULK-ROCK CHEMICAL
COMPOSITIONS
Sample descriptions and analytical
procedures
Bulk-rock chemical analyses (Table 2) were carried out
on gneisses, leucogranite bodies (width >2 cm), migmatites and granulites. Samples of Type I gneisses were
taken along profiles 1–2 m long oriented perpendicular
to the compositional banding, so as to obtain bulk compositions averaged over the length scales characteristic of
lithological variability in the gneisses. Migmatite samples
were taken so as to be large enough to be representative
of the bulk migmatite composition, including mesosome,
melanosome and leucosome. This sampling technique
was adopted to determine whether the migmatites as a
whole are rocks enriched or depleted in melt, or rocks
that partially melted and in which the melt underwent
only local redistribution and solidified essentially in situ.
Each sample consisted of 5–12 kg of material collected
in the field and crushed. After crushing, samples were
split to obtain representative fractions of >300 g. Each
sample was finally pulverized in a tungsten-carbide mill.
The container was always pre-contaminated with the
sample about to be pulverized. A 100 mg fraction of
powder was digested for 24 h in an HNO3 and HF
solution in a Teflon vessel at >120°C. After evaporation
to dryness each sample was diluted in 100 ml of 2%
HNO3 solution. Gneisses and migmatites were analyzed
for major elements by atomic absorption spectrometry
and for trace elements by inductively coupled plasmamass spectrometry (ICP-MS), both at the University of
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Huelva’s Central Research Services facility (Spain). Major
element analyses were duplicated. Average precision and
accuracy for most elements is 5–10% relative, determined
by repeated analysis of the SARM-1 international rock
standard (see also Table 2). Leucogranites were analyzed
for major elements and Zr by X-ray fluorescence (XRF) at
Activation Laboratories Ltd, Ancaster, Ontario, Canada,
and for trace elements by ICP-MS at the University of
Huelva.
Major element compositions
Type I gneisses that do not show evidence of anatexis
and that we assume to represent the protolith composition
for the Sierra de Comechingones anatectic rocks have
major element compositions that are within the range of
quartz-intermediate greywackes (see Taylor & McLennan, 1985). Relative to the average of quartz-intermediate
greywackes (Fig. 4a), Type I gneisses are somewhat
depleted in Al2O3, CaO and Na2O, have similar contents
of SiO2, and are slightly enriched in K2O, FeOtotal and
MgO. Despite these differences, which may reflect lower
plagioclase and higher chlorite contents in the protoliths
of Type I gneisses compared with average quartz-intermediate greywackes, it is clear that the regionally dominant protolith in the Sierra de Comechingones was an
immature clastic sediment.
The major element compositions of some Type II
metatexites from the central Sierra de Comechingones
(samples OCP10 and SCP24; see Fig. 1) are virtually
indistinguishable from those of Type I gneisses (Table
2, Figs 4b and 5). These metatexites contain layer-parallel
leucosomes and also oblique leucosomes (e.g. Fig. 3c),
but are not spatially associated with larger leucogranite
bodies. They are found in the central Sierra de Comechingones, several kilometers to tens of kilometers south
of the exposure of the highest-grade rocks along the Rı́o
Santa Rosa (Fig. 1; see also Otamendi et al., 1999). These
metatexites with unmodified major element compositions
are important in two respects. In the first place, they
provide chemical evidence supporting the lithological
continuity between the regional gneisses and the migmatites. Second, they suggest that some migmatites in
the Sierra de Comechingones have undergone little or
no melt extraction.
In contrast to these rocks, the compositions of other
Sierra de Comechingones migmatites differ from those
of Type I gneisses. This group of migmatites with modified
bulk compositions includes stromatic Type II metatexites
associated with leucogranite bodies, Type II diatexites
and all Type III migmatites. All of these rocks are
enriched in K2O and in major elements contained in
ferromagnesian minerals (FeO∗, MgO, TiO2 and MnO),
and depleted in CaO and Na2O, relative to Type I
Fig. 4. (a) Major element compositions of Type I gneisses normalized
to the average composition of quartz-intermediate greywackes (after
Taylor & McLennan, 1985). (b) Major element compositions of migmatites, granulites and leucogranites normalized to the average composition of Type I gneisses.
gneisses (Table 2, Figs 4b and 5). These compositional
characteristics argue against a model in which the migmatites are the products of biotite dehydration-melting
of the gneisses followed by partial melt extraction, as
such processes would leave behind residues enriched in
Ca and depleted in K. They also argue against the
migmatites being the products of injection of melts formed
by dehydration-melting of other lithologies, as this model
would not lead to enrichment in ferromagnesian components, but rather the opposite. We argue that the most
plausible explanation for these migmatites is that they
are the products of partial melting of Type I gneisses
(aluminous metagreywackes) in the presence of a small
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OTAMENDI AND PATIÑO DOUCE
PARTIAL MELTING OF METAGREYWACKES
Fig. 5. Relative Na2O, K2O and CaO contents (wt %) of the various
rocks types in the Sierra de Comechingones.
amount of free H2O (H2O-fluxed melting, as opposed to
H2O-saturated melting; see Patiño Douce & Harris,
1998), followed by melt extraction. Compared with dehydration-melting, H2O-fluxed melting consumes a
greater proportion of plagioclase relative to micas. Extraction of such melts therefore leaves behind residues
enriched in biotite components (K2O, FeO, MgO, TiO2)
and depleted in plagioclase components (CaO, Na2O)
relative to the residues of dehydration-melting (Figs 4b
and 5). Owing to the low solubility of excess Al2O3 in
H2O-rich, relatively low-temperature melts (e.g. Patiño
Douce & Harris, 1998), H2O-fluxed melting is also able
to explain the noticeable Al enrichment in the migmatites
relative to their putative metagreywacke precursors.
Compared with Type I gneisses, the leucogranites are
strongly enriched in K, have similar Na and Al contents,
and are depleted in Ca (Figs 4b and 5). The relative
contents of these elements are such that the leucogranites
are less peraluminous than Type I gneisses, with alumina
saturation indices [ASI = Al2O3/(CaO + Na2O +
K2O) molar] of 1·12 and 1·2 for leucogranites and
gneisses, respectively (see also Table 2). Removal of melts
with the ASI of leucogranites is thus consistent with Al
enrichment of the residues. It should be noted that the
migmatites in which melt extraction appears to have
been insignificant (i.e. samples OCP10 and SCP24) are
not enriched in Al relative to Type I gneisses (Table 2,
Fig. 4b). Because the leucogranites are strongly enriched
in K2O and depleted in CaO relative to Type I gneisses,
however, they cannot represent unmodified melts formed
by H2O-fluxed melting of the gneisses (see Patiño Douce
& Harris, 1998). In fact, the very high K contents of the
leucogranites (7–8 wt % K2O, Table 2) strongly suggest
that they do not represent melt compositions at all
[compare experimental melt compositions from metagreywackes (e.g. Patiño Douce & Beard, 1995; Patiño
Douce, 1996) and metapelites (e.g. Patiño Douce &
Harris, 1998); see also below]. Magmas in the leucogranite bodies may have undergone K-feldspar accumulation and removal of residual melt, for which there
is petrographic evidence in the adcumulus texture of Kfeldspar megacrysts. An alternative explanation for K
enrichment in the leucogranites is the production of
peritectic Kfs, formed by melting in a system with high
H2O/K2O ratio (e.g. Carrington & Watt, 1994), and
subsequent incorporation of some of this Kfs into the
extracted magmas. Although we cannot rule out the
operation of this process, we believe that this could
not have been the dominant process, because both the
leucogranites and the associated migmatites are enriched
in K relative to the gneisses (Fig. 5).
Type IV granulites are strongly enriched in FeO∗,
MgO, MnO and TiO2, and to a lesser extent in Al2O3,
relative to Type I gneisses (Fig. 4b). CaO is also enriched
in the granulites but K2O and Na2O are strongly depleted
in them (Fig. 5). The major element compositions of the
granulites reflect their refractory and residual mineralogical composition. The strong K depletion, however,
shows that the granulites must be the products of a
melting process different from that responsible for the
formation of the migmatites (see Fig. 5). Biotite dehydration-melting is the most likely explanation for the
origin of the granulites.
Trace element abundances
Amphibolite-grade Type I gneisses have rare earth
element (REE) contents that are similar to those of
post-Archaean sediments [Table 2, Fig. 6a; see also
Taylor & McLennan (1985)]. This is in good agreement
with our inference that these rocks represent a metamorphosed clastic sedimentary sequence. Type I gneisses
show weak negative Eu anomalies [Eu/Eu∗ >0·7, where
Eu∗ is the expected Eu value estimated as described by
Taylor & McLennan (1985)] and absolute abundances
of La and Ce around 100× chondrite. In contrast, the
heavy REE (HREE) abundances are less uniform, ranging
from about 10× chondrite for gneisses in the southern
sector of the study area (Rı́o Quillinzo, see Fig. 1) to
15× for the Rı́o Santa Rosa gneisses (Fig. 6a). This
difference almost certainly reflects the greater modal
proportion of garnet in the Rı́o Santa Rosa gneisses
compared with the Rı́o Quillinzo gneisses (Table 1).
Relative to Type I gneisses, Type II metatexites with
modified major element compositions and all Type III
migmatites have slightly lower Eu concentrations (EuN =
16–18 in Type I rocks, 15 in Type II rocks and 14 in
Type III rocks, where EuN denotes chondrite-normalized
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JOURNAL OF PETROLOGY
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SEPTEMBER 2001
Fig. 6. Chondrite-normalized REE plots [chondrite values taken from Taylor & McLennan (1985)]. (a) Type I gneisses. (b) Type II migmatites.
(c) Type III migmatites and Type IV granulites. (d) Leucogranites.
values) and are also depleted in Sr (Figs 6b and c and
7a). These trace element characteristics support H2Ofluxed melting, in which plagioclase is a chief reactant,
followed by melt extraction. These rocks are also generally
depleted in incompatible elements [large-ion lithophile
elements (LILE) and light rare earth elements (LREE)]
relative to Type I gneisses (Figs 6b and 7a). In contrast,
Type II metatexites with major element compositions
similar to those of the gneisses (samples OCP10 and
SCP24) are also generally similar to the latter in their
trace element abundances (Figs 6b and 7a, Table 2), in
agreement with our hypothesis that melt loss from these
rocks has been negligible. Type II diatexites have distinct
trace element patterns that are not depleted in Eu nor
Sr relative to Type I gneisses, and that are enriched in
LILE and LREE relative to the latter (Fig. 7a). These
characteristics would appear to contradict the interpretation that the diatexites are melt-depleted rocks, yet
some degree of melt loss is required by their major
element compositions (Figs 4b and 5). The explanation
for this discrepancy may reside in the fact that metatexites
and diatexites are the products of different melt migration
and homogenization length scales.
Type IV granulites have very low LREE and LILE
concentrations (Figs 6c and 7b), even compared with
other granulites that are considered to be residual products of partial melting and large-scale melt extraction.
Examples of such granulites (from the Phanerozoic Ivrea
Zone, the Late Proterozoic Pan-African rocks from Yaoundé and the Archaean Limpopo Belt) are compared
with the Rı́o Santa Rosa granulites in Fig. 7b. The very
low abundances of Ba and Rb in the latter rocks reflect
the low abundances of host phases for these elements,
such as biotite and alkali feldspar. Breakdown of feldspars
during melting also liberated Eu and Sr, which were thus
depleted in the residual granulitic assemblages (Fig. 7b).
The strong depletion of LREE in the granulites compared with the migmatites (Fig. 6b and c) reflects a
fundamental change in the behavior of monazite with
rising melting temperature. Monazite is present as unzoned euhedral crystals in Types II and III migmatite
and is the dominant LREE reservoir in these rocks. In
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OTAMENDI AND PATIÑO DOUCE
PARTIAL MELTING OF METAGREYWACKES
examples of residual granulites (Fig. 7b). This enrichment
is consistent with the observed accumulation of residual
garnet and zircon in the Rı́o Santa Rosa granulites.
The leucogranites are very rich in Ba and have positive
Eu anomalies (Eu/Eu∗ >6·7, Figs 6d and 7c). In contrast,
the trace elements that are partitioned by ferromagnesian
minerals (HREE, V, Cr, Sc, Co and Ni) are strongly
depleted in these rocks. Zr abundances in the leucogranites (<50 ppm) are much lower than those required
to saturate peraluminous granitic melts (e.g. Watson
& Harrison, 1983) at the granulite-facies temperatures
recorded in the Rı́o Santa Rosa migmatites (800–850°C;
see Otamendi et al., 1999). Strong Zr undersaturation in
melts can be explained by the fact that almost all zircon
in the gneisses and migmatites occurs as inclusions in
biotite and, given that biotite tends to stay in the residue
during H2O-fluxed melting (Patiño Douce & Harris,
1998), zircon is effectively shielded from equilibrating
with the melt. Crystallization of peritectic monazite would
have prevented LREE enrichment of the melts, and the
HREE would also have been trapped in the residues by
peritectic and/or residual garnet. The trace elements
in the melts would then chiefly derive from feldspar
breakdown, explaining the strong enrichment in Ba and
Eu (Watt & Harley, 1993). The leucogranites, however,
are undoubtedly cumulate rocks with some amount of
trapped interstitial melt, as attested by their textures and
by their major element compositions. The cumulate
nature of the rocks can also explain all of the trace
element characteristics of the leucogranites. The observed
trace element abundances in these rocks could thus be
the result of two distinct causes that act in concert: the
behavior of accessory phases during melting and the
accumulation of K-feldspar during crystallization. Identifying the relative contributions of each process may not
be feasible.
Fig. 7. Multi-element variation diagrams normalized to the average
trace element abundances of Type I gneisses. (a) Migmatites (Types II
and III). (b) Rı́o Santa Rosa granulites (Type IV) compared with other
residual granulites: garnet-rich rocks from the Yaoundé series (Barbey
et al., 1990), high-grade Limpopo metapelites (Taylor et al., 1986)
and average of stronalites from Ivrea Zone (Schnetger, 1994). (c)
Leucogranites.
contrast, monazite is absent from Type I gneisses, in
which apatite appears to be the chief LREE reservoir,
and from Type IV granulites, in which there is no LREErich accessory phase. These petrographic relationships
are consistent with crystallization of peritectic monazite
(Wolf & London, 1995) during initial H2O-fluxed melting
of apatite-bearing metasediments (at T >800°C, Otamendi et al., 1999), followed by dissolution of monazite
as melting temperatures rose above >850°C (Rapp et
al., 1987; see also Otamendi et al., 1999).
In contrast to the LREE, the HREE are enriched in
the Rı́o Santa Rosa granulites (Fig. 6c) as in other
MODELING OF PARTIAL MELTING
OF METAGREYWACKES IN THE
SIERRA DE COMECHINGONES
Our purpose in this section is to estimate the degree of
melting and melt extraction that is represented by the
various migmatites and granulites in the Sierra de Comechingones. We model partial melting in the Sierra de
Comechingones in two stages, each with a different
modeling strategy. This is required because of the differences in our degree of confidence in the identity of the
protolith, the melting reaction and the extent of melting
represented by each stage. In the first stage we model
formation of the migmatites from limited partial melting
of aluminous metagreywackes (Type I gneisses). In the
second stage we assess whether the granulites represent
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JOURNAL OF PETROLOGY
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widespread melting of the same aluminous metagreywackes or of already melt-depleted migmatites.
The modeling focuses on the systematics of three
distinct groups of trace elements. The first group consists
of Rb, Sr, Ba and Eu. The bulk of these elements is
contained in essential mineral phases (feldspars and
biotite) which are crucial participants in the melting
reactions. Because experiments have shown that equilibrium between these phases and melt is attained in very
short times (of the order of days, e.g. Patiño Douce &
Beard, 1995) we argue that it is reasonable to treat the
distribution of Rb, Sr, Ba and Eu as an equilibrium
process (e.g. Harris & Inger, 1992). The second group
of elements consists of the HREE, which are chiefly
contained in garnet and zircon. The behavior of these
elements is likely to be more complex than that of the
first group, because garnet is an essential participant of
the melting reactions and zircon is not, and also because
HREE in garnet have high KD values and low chemical
diffusivities (e.g. Hickmott & Shimizu, 1990; Schwandt et
al., 1996; Otamendi et al., 2001). As a first approximation,
however, we also model distribution of the HREE as an
equilibrium process, and then discuss the consequences
of this assumption. Finally, the third group consists of
the LREE, for which the principal host phases in the
Sierra de Comechingones rocks are either apatite (in Type
I gneisses) or monazite (in Types II and III migmatites).
Modeling of LREE distribution is problematic regardless
of whether this is done as an equilibrium or disequilibrium
process, chiefly because the LREE are essential structural
components of monazite and thus do not follow Henry’s
law, and also because estimates of the modal abundances
of accessory phases carry large uncertainties. We have
modeled the behavior of the LREE assuming equilibrium
between melt and essential mineral phases only. This
assumption predictably leads to significant discrepancies
between measured and modeled LREE abundances. We
show, however, that these discrepancies can be explained
in a straightforward way on the basis of the observed
petrographic characteristics of monazite and apatite and
of the reaction relationship between these two phases
during melting of aluminous metasediments, which was
demonstrated experimentally by Wolf & London (1995).
Formation of the migmatites
We used a generalized mixing model (Le Maitre, 1979)
to estimate the stoichiometry of the partial melting reaction on the basis of mass balance of major elements.
The procedure solves for the coefficients of the melting
reaction by multiple linear least-squares regression. Mineral compositions used for this calculation are shown in
Table 3 (data from Otamendi et al., 1999), and were
taken from Type I gneisses for reactant minerals (quartz,
NUMBER 9
SEPTEMBER 2001
plagioclase and biotite) and from the migmatites for
peritectic minerals (garnet, ilmenite and cordierite). The
data were fitted to the incongruent melting reaction
Qtz + Pl + Bt ± Sil ± H2O=leucogranite melt +
Grt + Ilm ± Crd.
(3)
Two different melt compositions were used to perform
the calculations (Table 4). Model I uses the average of
all glass compositions produced by dehydration-melting
of a model metagreywacke (SBG) at 7 kbar by Patiño
Douce & Beard (1995). Another set of calculations (Model
II) was performed using the average glass compositions
obtained by Patiño Douce (1996) by melting of the same
model metagreywacke as used by Patiño Douce & Beard
(1995) at 7–10 kbar but with 1–2 wt % added H2O. The
melts produced by Patiño Douce (1996) by H2O-fluxed
melting have lower K2O and higher Na2O and CaO
contents than the melts produced by Patiño Douce &
Beard (1995) by dehydration-melting of the same starting
material (Table 3). These differences reflect the increased
participation of plagioclase relative to biotite in the H2Ofluxed melting reaction, compared with dehydrationmelting.
Cordierite in the Rı́o Santa Rosa migmatites and
granulites could have formed both as a peritectic phase
and as a product of the breakdown of the assemblage
garnet + sillimanite + quartz during decompression
(see Otamendi et al., 1999). The appearance of cordierite
in the rocks cannot be unambiguously related to the
melting process and we therefore computed two sets of
models (Table 4), one which does not include cordierite
(model A), and another in which cordierite is included
as a peritectic phase (model B). The combination of
cordierite-present and cordierite-absent models with two
different melt compositions leads to four sets of stoichiometric coefficients. Table 5 shows the stoichiometric
coefficients obtained for each of these models. The predicted melt compositions are compared with the target
melt compositions in Table 4.
Regardless of the melt composition used in the calculations, models that include cordierite as a peritectic
phase yield much better statistical fits than those in which
cordierite is absent (Table 4). Among cordierite-bearing
models, the sum of the squares of the residuals is 0·11 for
model IB [experimental melt compositions from Patiño
Douce & Beard (1995), dehydration-melting] and 0·08
for model IIB [experimental melt compositions from
Patiño Douce (1996), H2O-fluxed melting]. The difference between the two models may not be statistically
significant, and a choice between them is best made on
petrological and geochemical grounds. Given that the
migmatites are enriched in K2O and depleted in CaO,
Na2O, Eu and Sr relative to their putative Type I
precursors, we choose the stoichiometry of model IIB
1764
OTAMENDI AND PATIÑO DOUCE
PARTIAL MELTING OF METAGREYWACKES
Table 3: Compositions of phases used to calculate reaction stoichiometry
Plagioclase
Biotite
Garnet
Ilmenite
Sillimanite
Cordierite
Melt I
Melt II
PDB95
PD96
72·93
SiO2
59·80
37·82
37·80
0·00
36·70
50·41
73·77
TiO2
0·00
4·31
0·00
51·84
0·00
0·00
0·42
0·32
Al2O3
25·66
18·12
22·00
0·00
62·70
33·50
14·42
14·73
FeO∗
0·03
16·32
31·00
47·04
0·00
6·11
1·68
1·88
MgO
0·00
13·13
7·50
0·04
0·00
9·88
0·59
0·46
CaO
6·50
0·05
0·95
0·00
0·00
0·00
1·23
2·03
Na2O
7·70
0·04
0·00
0·00
0·00
0·10
1·88
2·66
K2O
0·30
10·20
0·00
0·00
0·00
0·00
5·84
4·88
Mineral compositions taken from Otamendi et al. (1999). Plagioclase: average of Type I gneisses. Biotite: average of sample
RSR23, recalculated to 100 wt % H2O-free. Garnet: average of core and rim compositions in sample RSR31b. Ilmenite: from
sample RSR31b. Sillimanite: unpublished data from RSR04. Cordierite: average of crystals in matrix in samples RSR31b and
RSR32, recalculated to 100 wt % H2O-free. PDB95: average of glasses obtained by dehydration-melting of synthetic biotite
gneiss at 7 kbar (Patiño Douce & Beard, 1995). PD96: average of the glasses experimentally generated by water fluxed
melting of synthetic biotite gneiss at 7 and 10 kbar (Patiño Douce, 1996).
∗Total Fe given as FeO.
Table 4: Modeled melt compositions
Melting model
(I) PDB95
IA
IB
(II) PD96
IIA
IIB
measured
calculated
calculated
measured
calculated
calculated
72·93
SiO2
73·77
73·77
73·76
72·93
72·93
TiO2
0·42
0·98
0·43
0·32
0·80
0·33
Al2O3
14·42
14·42
14·42
14·73
14·73
14·73
FeO∗
1·68
1·04
1·67
1·88
1·33
1·87
MgO
0·59
3·20
0·60
0·46
2·70
0·47
CaO
1·23
1·46
1·47
2·03
2·21
2·23
Na2O
1·88
1·76
1·65
2·66
2·57
2·47
K2O
5·84
3·27
5·84
4·88
2·67
4·88
14·21
0·11
10·48
0·08
Sum of squares
of residuals
(I) PDB95 and (II) PD96 are experimental melt compositions I and II from Table 3. IA and IIA melting models assume no
crystallization of peritectic cordierite. IB and IIB melting models assume crystallization of peritectic cordierite.
(H2O-fluxed melting with production of peritectic cordierite) as representative of the initial melting reaction in
the Sierra de Comechingones (Table 5). Our hypothesis is
that this reaction was responsible for partial melting
of (Type I) aluminous metagreywackes, and that the
migmatites (Type II and Type III) are the products of
varying degrees of extraction of these melts.
The major element compositions of the leucogranites
do not match those of the melts formed by either
dehydration-melting or H2O-fluxed melting of metagreywackes (Fig. 8). The leucogranites are thus unlikely
to represent melts extracted from the migmatites, but
they could be rocks rich in cumulate K-feldspar derived
from parental melts formed by H2O-fluxed melting of
the migmatites (Fig. 8).
The stoichiometric coefficients for the preferred reaction (IIB, Table 5) show that sillimanite must be
consumed by the melting reaction, in a proportion
1765
JOURNAL OF PETROLOGY
VOLUME 42
Melting model
IB
IIA
IIB
Quartz
0·49
0·61
0·44
0·54
Plagioclase
0·23
0·23
0·34
0·33
Biotite
0·33
0·59
0·26
0·49
Sillimanite
0·08
0·33
0·05
0·26
Melt
1·00
1·00
1·00
1·00
Garnet
0·12
0·06
0·08
0·03
Ilmenite
0·01
0·04
0·01
0·03
Cordierite
—
0·66
—
0·56
Reactants
Products
CL = C0/[D(1—F ) + F ]
approximately half that of biotite. Comparison of the
modal proportion of biotite in the purported protolith
(Type I gneisses) with that in the migmatites indicates
that melting in the transition from gneisses to migmatites
consumed >6% biotite. Because sillimanite is very scarce,
or is altogether absent, in the gneisses, there must be
some mechanism that can supply the >3% of sillimanite
that is required by the melting reaction. The net-transfer
reaction (see Patiño Douce et al., 1993; Otamendi et al.,
1999)
Grt + (Al2R−3)Bt = 2 Al2SiO5 + SiO2
(with R =Fe2+ or Mg)
SEPTEMBER 2001
is a mechanism that can supply normative sillimanite.
On average, Type I gneisses have >25% modal biotite.
The transition from Type I gneisses to Type II migmatites
is accompanied by a decrease in the Al2O3 content of
biotite of >2·5 wt % (Otamendi et al., 1999). According
to reaction (4), this change in biotite composition can
supply 2 wt % of normative sillimanite, even if no
biotite breaks down. Breakdown of biotite did take place,
however (see Table 1), and could have supplied an
additional amount of normative sillimanite. We conclude
that the proposed cordierite-forming melting reaction
(model IIB, Table 5) is possible in aluminous metagreywackes with low or negligible modal proportions of
aluminous minerals (muscovite, sillimanite, etc.), and in
which the excess Al is contained in biotite.
Equilibrium trace element distributions were modeled
using the batch melting equation (Hanson, 1978)
Table 5: Calculated stoichiometric
coefficients for melting reaction
IA
NUMBER 9
(4)
(5)
where CL is the concentration of a trace element in the
liquid, C0 is the initial concentration of the trace element
in the protolith, F is the melt fraction at which a batch
of melt is removed, and D is the bulk distribution coefficient of the trace element for the mineral assemblage
present at the time of separation of melt and residue.
The concentration of the trace element in the residue
(CS) is then given by
CS = CLD.
(6)
The bulk distribution coefficient (D) is calculated from
D = i X iKd i, where Kd i is the partition coefficient between mineral i and felsic melt (values given in Table 6)
and X i is the weight fraction of mineral i in the mineral
assemblage that coexists with melt at a given value of F.
The weight fractions of the residual minerals for any
Fig. 8. Na2O–K2O–CaO contents (wt %) of experimental melts formed by dehydration-melting (Patiño Douce & Beard, 1995; PDB) and H2Ofluxed melting (Patiño Douce, 1996; PD) of model metagreywackes (SGB starting material), compared with the leucogranites from the Rı́o Santa
Rosa and with the melt compositions predicted from modeled melting stoichiometry (see text). It should be noted that the leucogranites are
more potassic than both experimental and model melts, but that their compositions are consistent with accumulation of alkali feldspar
(orthoclase–albite solid solution), for which there is petrographic evidence (see text).
1766
Sources for partition coefficients: (a) Bea et al. (1994); (b) Nash & Crecraft (1985); (c) Arth (1976). n.d., data not available or poorly constrained, assumed to be
zero.
∗Anthophyllite/felsic melt Kd assumed to be equal to orthopyroxene/melt Kd, given by Nash & Crecraft (1985).
0·02(b)
2·25(b)
2·2(b)
0·04(b)
n.d.
n.d.
0·825(b)
2·32(a)
0·03(b)
1·6(b)
0·93(b)
0·04(b)
0·08(b)
0·78(b)
n.d.
0·77(a)
3·77(a)
2·85(a)
K-feldspar
Anthophyllite∗
n.d.
n.d.
n.d.
n.d.
4·43(a)
1·2(b)
1·47(b)
n.d.
1·01(b)
2·833(b)
1·64(b)
1·223(b)
29·6(c)
Ilmenite
n.d.
1·77(a)
39·9(c)
35(c)
0·99(a)
0·01(a)
0·03(a)
2·66(c)
0·1(a)
0·07(a)
0·35(c)
n.d.
0·06(a)
0·12(a)
0·01(a)
0·01(a)
0·06(a)
0·08(a)
Garnet
Cordierite
0·02(a)
0·09(b)
1·62(b)
1·47(b)
0·09(b)
0·13(b)
1·23(b)
0·05(a)
2·99(a)
0·17(b)
0·06(a)
0·05(a)
0·27(b)
0·38(b)
0·06(a)
0·01(a)
1·25(a)
0·19(a)
0·59(a)
0·06(a)
6·98(a)
Plagioclase
Biotite
La
Sr
Ba
Rb
Table 6: Partition coefficients used in melting models
Ce
Sm
Eu
Y
Yb
Lu
OTAMENDI AND PATIÑO DOUCE
PARTIAL MELTING OF METAGREYWACKES
Table 7: Calculated mineralogical compositions
of residues of partial melting
F
0
0·1
0·2
0·3
0·4
Quartz
0·36
0·33
0·29
0·25
0·18
Plagioclase
0·31
0·30
0·28
0·27
0·23
Biotite
0·25
0·22
0·18
0·13
0·07
Garnet
0·07
0·08
0·09
0·10
0·11
Cordierite
0·00
0·06
0·12
0·21
0·36
Fe–Ti oxide
0·01
0·01
0·02
0·03
0·04
Weight fractions of minerals coexisting with a given weight
fraction of melt (F ), calculated with reaction stoichiometry
IIB (see text and Table 5).
given value of F (shown in Table 7) were computed as
follows. The weight percent composition of the initial
mineral assemblage was estimated from the mode of
Type I rocks and mineral densities taken from Deer et
al. (1966). These initial weight fractions of Qtz, Pl, Bt,
Grt and Ilm were then varied according to the preferred
stoichiometry for the melting reaction (model IIB in
Table 5), yielding the values shown in Table 7. We
assumed that sillimanite was consumed by melting reaction (3) at the same rate as it was produced by reaction
(4), resulting in sillimanite-free residues. Given the very
low partition coefficients of most trace elements in sillimanite, the effect of this assumption on the model results
is likely to be negligible. The trace element abundances in
the source were obtained by averaging the trace element
abundances in Type I gneisses (data taken from Table
2).
The Rı́o Santa Rosa migmatites contain >15% biotite
(Table 1), which corresponds to residues formed by
somewhat more than 20% melting (Table 7). Complete
consumption of biotite requires >40% melting (Table 7).
Trace element abundances in the different varieties of
migmatites are compared in Fig. 9a with modeled abundances in residues formed by extraction of 20% and 40%
melt. The measured LREE abundances in all migmatite
types are significantly higher than the modeled abundances. This discrepancy obviously arises from neglecting
monazite in the melting models, but it is consistent with
the migmatites being residual rocks in which peritectic
monazite accumulated during melt extraction. There is
ample petrographic evidence for this process, in the form
of abundant euhedral and unzoned monazite crystals.
Agreement between measured and modeled abundances
is generally good for HREE and LILE, except for Ba
(for which abundances are higher in the migmatites
1767
JOURNAL OF PETROLOGY
VOLUME 42
NUMBER 9
SEPTEMBER 2001
associated with leucogranite bodies (sample OCP10). In
any event, it is clear that the trace element signatures of
the migmatites support the inference derived from major
element compositions that many of the Sierra de Comechingones migmatites (both metatexites and diatexites)
are melt-depleted residue-rich rocks, rather than meltdominated rocks.
The model trace element abundances in melts generated by 20–40% melting of Type I gneisses are compared in Fig. 9b with measured abundances in the
leucogranites. As with the migmatites, the greatest discrepancy occurs in the LREE, which are strongly depleted
in the leucogranites compared with the model results.
This discrepancy is consistent both with accumulation of
peritectic monazite in the residue of partial melting and
with accumulation of K-feldspar in the leucogranite
magmas.
Formation of the granulites
Fig. 9. Modeled trace element abundances generated by equilibrium
H2O-fluxed melting of Type I gneisses (see text), normalized to the
composition of primitive mantle suggested by Taylor & McLennan
(1985). (a) Trace element abundances in residual solid assemblages
formed by 20 and 40% melting, compared with the composition of
the various migmatite types. LREE enrichment in the migmatites
relative to the modeled abundances is consistent with crystallization of
peritectic monazite during initial melting of Type I gneisses in the
Sierra de Comechingones (see text). (b) Trace element abundances in
melts generated by 20 and 40% melting, compared with trace element
abundances in the leucogranites. The leucogranites are strongly depleted
in LREE relative to the expected melt compositions, which is consistent
both with their cumulate-rich nature and with crystallization of peritectic
monazite in the source.
than in the model residues, perhaps reflecting a poorly
constrained mineral–melt partition coefficient for Ba).
Although the resolution of the models is rather poor,
they suggest that Type II metatexites associated with
leucogranite bodies (sample RSR31b) and Type III migmatites (samples RSR16 and RSR24) may represent
residues of >20–40% melt extraction. The model results
are also consistent with our qualitative conclusions that
these rocks have undergone greater melt extraction than
Type II diatexites and than Type II metatexites from
the southern end of the study area, which are not
The stoichiometry of the melting reaction estimated for
the migmatites (Table 5), which represents H2O-fluxed
melting, is not necessarily applicable to formation of the
granulites. This is so because, at the high temperatures
recorded by the granulites ([900°C, Otamendi et al.,
1999), dehydration-melting of biotite is likely to have
been the overriding melting mechanism, and also because
it is not clear whether the protoliths for the granulites
were Type I gneisses or already melt-depleted migmatites.
In these circumstances, tying to base the melting model
on the stoichiometry of a specific melting reaction would
be unconstrained. We thus adopted an alternative strategy, which consisted of finding the degree of melting
required to minimize the difference between the abundances of LILE and REE calculated in a residual assemblage with the modal composition of Type IV
granulites and the abundances actually measured in the
granulites.
Trace element abundances were modeled with equations (5) and (6), using the observed modal composition
of Type IV granulites to calculate the value of the bulk
distribution coefficient, D (mineral partition coefficients
were those listed in Table 6). We generated two sets of
models using different source materials (Type I gneisses
or Type III migmatites) to set the initial concentration
values, C0. Equations (5) and (6) were then solved for
the value of F that would simultaneously minimize the
difference between each of the calculated values of CS
and the measured abundance of the same element in
Type IV gneisses.
The best fit models were obtained for melt fractions
of >50% and using Type III migmatites as the source
material (Fig. 10a). The granulites contain a small amount
(>3%) of Mg-rich biotite which, on the basis of mineral
1768
OTAMENDI AND PATIÑO DOUCE
PARTIAL MELTING OF METAGREYWACKES
Fig. 10. (a) Modeled trace element abundances in solid assemblages
formed by 50% equilibrium batch melting and equilibrated with a
granulite residual mineral assemblage consisting of: Qtz (22%), Pl
(16%), Grt (31%), Crd (23%), Ath (4%) and Ilm (4%). Models were
calculated using either Type I gneisses or Type III migmatites as the
protolith. These model trace element abundances are compared with
the trace element abundances in the Rı́o Santa Rosa Type IV granulites.
(b) Calculated trace element compositions of the melts modeled as in
(a), compared with the Rı́o Santa Rosa leucogranites and with strongly
peraluminous granitoids emplaced at higher structural levels of Sierra
de Cordoba. Juan XXIII peraluminous granite compositions are taken
from Rapela et al. (1998). Cerro Pelado granitoids ( J. E. Otamendi,
unpublished data, 1999) are associated with a diatexite massif located
>25 km south of the Rı́o Santa Rosa, where the metasedimentary
protolith is similar to the Rı́o Santa Rosa aluminous metagreywackes
(see Fig. 1).
chemistry, we consider to be a retrograde phase [see
discussion by Otamendi et al. (1999)]. Inclusion of even
this small amount of biotite in the trace element models
results in calculated residues with Rb and Ba contents
significantly higher than those measured in the granulites.
This is consistent with biotite in the granulites being a
low-temperature retrograde phase rather than a hightemperature residual phase, and for this reason biotite
was excluded from the assemblage used to model trace
elements in the granulites.
The greatest discrepancy between measured and calculated trace element abundances is in the LREE. These
elements are noticeably less abundant in the granulites
than in the model calculations (Fig. 10a). This discrepancy
reflects complete dissolution of monazite, which is absent
from Type IV granulites. Using the monazite solubility
model of Montel (1993), we estimate that complete
dissolution of monazite in the Sierra de Comechingones
rocks required temperatures in the range 830–860°C
(assuming H2O contents between 1 and 4 wt %). We
have estimated peak temperatures of >800°C for the
migmatites and [900°C for the granulites (Otamendi et
al., 1999). The strong contrast in LREE contents between
migmatites and granulites is thus consistent with accumulation of peritectic monazite in the residues during
the relatively low-temperature H2O-fluxed melting stage
that gave rise to the migmatites, followed by dissolution
of monazite during the higher-temperature melting stage
that gave rise to the granulites.
The complementary anatectic melts extracted from the
granulites must have been rich in Rb, Ba and LREE
and poor in Sr and HREE (Fig. 10b). The predicted
LREE enrichment does not match the composition of
the leucogranites from the study area. However, peraluminous granitoids with trace element signatures that
resemble closely those that could be expected of anatectic
melts that have left behind residues such as the Rı́o Santa
Rosa granulites are known throughout the southern
Sierras Pampeanas (Fig. 10b, data after Rapela et al.,
1998; J. E. Otamendi, unpublished data, 2000). Many
of these granitoids are intruded in metamorphic sequences that are similar to those of the Sierra de Comechingones. These relationships suggest that aluminous
greywackes may have been an important crustal source
in the generation of peraluminous granitoid magmas
during the Early Cambrian Pampean Orogeny.
DISCUSSION AND CONCLUSIONS
The geochemical characteristics of the migmatites in the
Sierra de Comechingones show that they are residuerich rocks. Because pelitic lithologies are uncommon in
the region, the most likely protoliths for the migmatites
are the aluminous metagreywackes (Type I gneisses),
which are the only regionally abundant quartzo-feldspathic rocks in the Sierra de Comechingones. It is important to emphasize that, whereas migmatitic lithologies
in the Sierra de Comechingones extend over distances
of the order of kilometers (e.g. Figs 1 and 2), pelitic
intercalations in the sub-solidus Type I gneisses are not
recognized at scales of 1 m or less. The generation
of residual assemblages that contain aluminous phases
(cordierite and sillimanite) from protoliths that contain
Al-rich biotite but no muscovite or sillimanite is seen in
this context as the result of two contributing factors:
the decrease of the Al solubility in biotite with rising
temperature (Patiño Douce et al., 1993; Otamendi et al.,
1999), which can lead to saturation of aluminous phases,
1769
JOURNAL OF PETROLOGY
VOLUME 42
and the removal of melts that are less peraluminous
than their protoliths. Because the solubility of normative
corundum in anatectic melts is small (commonly <3 wt %
Al2O3, e.g. Patiño Douce & Johnston, 1991; Patiño
Douce & Harris, 1998) partial melting of even slightly
peraluminous sources can lead to strongly peraluminous
residues. This effect may be particularly important during
H2O-fluxed melting, as these melts are rich in normative
plagioclase (Patiño Douce, 1996; Patiño Douce & Harris,
1998), which has ASI = 1.
Significantly, the migmatites in the Rı́o Santa Rosa
area are enriched in K and depleted in Na, Ca, Sr and
Eu relative to the metagreywackes. These geochemical
characteristics indicate that the migmatites could not
have formed by dehydration-melting of the aluminous
metagreywackes and subsequent extraction of those melts.
Rather, they suggest H2O-fluxed melting, with subordinate participation of biotite relative to plagioclase in
the melting reaction (see Patiño Douce & Harris, 1998).
This initial melting stage took place at temperatures of
the order of 800°C (Otamendi et al., 1999). Breakdown
of residual biotite in the migmatites became the dominant
melting mechanism at a later stage, when temperature
rose to >900°C (Otamendi et al., 1999) and free aqueous
fluids were perhaps no longer available. Incongruent
dehydration-melting of biotite was thus the dominant
process in the generation of the cordierite granulites. The
dramatic drop in LREE contents from migmatites to
granulites is consistent with destabilization of monazite
resulting from the substantial rise in melting temperature
that we estimated independently on the basis of phase
equilibrium (Otamendi et al., 1999).
Geochemical modeling suggests that the Rı́o Santa
Rosa migmatites are the source regions of anatectic
granites after extraction of at least 20% melt and perhaps
more. Similar conclusions have been reached for other
migmatitic terranes (e.g. Sawyer, 1996, 1998; Carson et
al., 1997), suggesting that at least in some cases there is
a link between migmatites and granitic magmas. Geochemical modeling also suggests that the refractory cordierite granulites from the Rı́o Santa Rosa are the
products of extraction of an additional 50% of melt,
starting from already melt-depleted migmatites. Assuming 20% melt extraction from gneisses to migmatites
followed by 50% melt extraction from migmatites to
granulites we can conclude that the granulites are the
residues of a total melt extraction of the order of 60%.
It is significant that many experiments have shown that
this is approximately the maximum amount of granitic
melt that can be obtained by melting a wide range of
crustal rocks (e.g. Vielzeuf & Holloway, 1988; Patiño
Douce & Johnston, 1991; Patiño Douce & Beard, 1995,
1996). The Rı́o Santa Rosa granulites are thus likely to
NUMBER 9
SEPTEMBER 2001
embody the ultimate residue of crustal anatexis, explaining their extreme trace element patterns compared
with other residual granulites.
The Rı́o Santa Rosa area illustrates the roles of migmatites and granulites in the origin of anatectic granites.
Geochemically depleted granulites are the ultimate residues left by almost complete extraction of anatectic
melts. Migmatites (both metatexites and diatexites) are
also residual rocks that have undergone partial melt
extraction, but that were caught in the process of becoming depleted granulites. They represent intermediate,
lower-temperature, steps in the process of differentiation
of the crust into a fusible granitic fraction and a refractory
aluminous and ferromagnesian fraction.
In the Sierra de Comechingones some Type II metatexites are more strongly depleted in melt than Type II
diatexites, even though both have crystallized at comparable P–T conditions (see Otamendi et al., 1999). This
emphasizes that the generation of distinct migmatitic
fabrics is critically dependent on the relationship between
rate of melt generation and rate of melt extraction, which
is in turn a function of strain rate (see Sawyer, 1994).
Metatexites may thus represent partially molten domains
from which melt is actively being removed perhaps as
fast as it forms, so that melt proportions large enough to
allow a melt-dominated rheology never build up (see also
Sawyer, 1994). These partially molten rocks thus retain
a metamorphic appearance. Diatexites, in contrast, may
represent domains from which melt extraction is more
sluggish than in metatexites (perhaps as a consequence
of lower strain rates), so that melt fraction builds up,
allowing the transition to a melt-dominated rheology to
take place. The partially molten rock can then undergo
a significant degree of homogenization, losing much of
its metamorphic appearance (e.g. Brown, 1994; Sawyer,
1996). The difference between metatexites and diatexites
is thus not necessarily one of temperature nor of degree
of melting, but is rather determined by how fast melt is
removed from the partially molten rock relative to how
fast it forms.
A puzzling aspect of the Sierra de Comechingones
high-grade terrane is that, although the migmatites and
granulites are melt-depleted rocks, the melts themselves
cannot be positively identified in the study area. The
leucogranites could be cumulate-rich rocks formed from
parental melts such as those extracted from the migmatites and granulites, but their volume is too small to
account for the amount of ‘missing melt’. The leucogranite dikes and sills could conceivably represent melt
pathways through which the partially molten rocks were
being drained. The absence of unmodified melt products
in the neighborhood of melt depleted rocks is thus an
indication of the fact that anatectic melts are capable of
migrating over long distances and undergoing fractional
crystallization in the process. Whether such melts can
1770
OTAMENDI AND PATIÑO DOUCE
PARTIAL MELTING OF METAGREYWACKES
give rise to large granitic intrusions in the absence of
mass influx from the mantle is still an open question (see
Castro et al., 1999; Patiño Douce, 1999).
ACKNOWLEDGEMENTS
Geochemical data were obtained at the Servicios Centrales de Investigación y Desarrollo of the University of
Huelva (Spain). We thank J. de la Rosa for performing,
and often repeating, whole-rock geochemistry, and A.
Demichelis for helping in the field and for drafting Figs
1 and 2. Financial support was provided by NSF Grants
EAR-9316304 and EAR-9725190 to A.E.P.D. and FONCYT Grant PICT99 to J.O. This work was initiated
while J.O. was at the University of Georgia supported
by a postdoctoral fellowship from CONICET (Argentina), and was completed with the support of a research fellowship from the same agency. Comments
from journal reviewers Gordon Watt, John Kaszuba and
Fernando Bea, and editorial suggestions from George
Bergantz, helped us to greatly improve the paper, and
are most appreciated.
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