JOURNAL OF PETROLOGY
VOLUME 40
NUMBER 7
PAGES 1151–1185
1999
Evolution of Mg/Fe Ratios in Late Variscan
Plutonic Rocks from the External Crystalline
Massifs of the Alps (France, Italy,
Switzerland)
FRANÇOIS DEBON∗ AND MARIE LEMMET
LABORATOIRE DE GÉODYNAMIQUE DES CHAÎNES ALPINES, UPRES-A CNRS 5025, INSTITUT DOLOMIEU, 15 RUE
MAURICE-GIGNOUX, 38031 GRENOBLE CEDEX, FRANCE
RECEIVED MARCH 18, 1998; REVISED TYPESCRIPT ACCEPTED FEBRUARY 1, 1999
Late Variscan plutonic bodies are widespread in the External
Crystalline Massifs of the Alps (Argentera, Pelvoux, Belledonne,
Grandes Rousses, Mont Blanc, Aiguilles Rouges, Aar, Gotthard).
They can be classified on the basis of their Mg/(Fe + Mg) ratio
and mafic mineral content (expressed by the B = Fe + Mg +
Ti parameter). Together with available ages of emplacement, this
classification highlights the existence of two plutonic suites, one
early, Viséan (~330–340 Ma), and highly magnesian, the other
later, mainly Stephanian (~295–305 Ma), and more ferriferous.
This evolution from a magnesian plutonism to a more ferriferous
one, which also occurs in other Variscan massifs (e.g. Corsica),
might be accounted for by a combination of factors related to the
nature of the source of the magmas, the physical and chemical
conditions of melting, and the Late Variscan geodynamic setting.
As a basis for these considerations a comprehensive review is
presented of all the External Crystalline Massifs and their
Late Variscan intrusions.
The External Crystalline Massifs of the Alps or ECM
belong to the Helvetic domain, bounded to the east by
the Frontal Penninic Thrust (von Raumer, 1984; von
Raumer et al., 1993; Ménot et al., 1994). From south to
north, they are the Argentera, Pelvoux, Belledonne,
Grandes Rousses, Mont Blanc and Aiguilles Rouges
massifs (Western Alps), and the Aar and Gotthard massifs
(Central Alps) (Fig. 1). They appear as dome-like structures of crystalline basement surrounded by a mostly
Mesozoic sedimentary cover. As a whole, they define an
arcuate structure, possibly inherited from the Variscan
orogeny (Bogdanoff et al., 1991).
During the Variscan orogeny, the ECM were part of
the Helvetic–Moldanubian terrane of the internal zone
of the Variscides (von Raumer & Neubauer, 1993).
Precambrian metamorphic units and Precambrian to
Palaeozoic sediments, interlayered with volcanic and
ultramafic rocks, display in most areas a polymetamorphic
history and underwent a highly complex pre-Variscan
and Variscan evolution. Although they share a number
of similarities, the ECM each have also specific characteristics (von Raumer et al., 1993).
In addition to orthogneisses deriving from Early Palaeozoic plutonic (and volcanic?) protoliths (Bussy & von
Raumer, 1993; Sergeev & Steiger, 1993; Barféty et al.,
1999), the ECM are characterized by a widespread group
of plutons, mainly emplaced during the Carboniferous
associated with Late Variscan strike-slip tectonics (Bonin
et al., 1993; von Raumer et al., 1993; Bonin, 1997).
Debon et al. (1994, 1998) have suggested the existence
of two suites of Late Variscan plutons in the ECM, one
early, Viséan (~330 Ma), and highly magnesian, the
∗Corresponding author. Present address: 3, Chemin du Magnit, 38490
Le Passage, France. Telephone: +33-04-74-88-70-44.
 Oxford University Press 1999
Alps; geochronology; Late Variscan plutonism; major and
trace element geochemistry
KEY WORDS:
INTRODUCTION
JOURNAL OF PETROLOGY
VOLUME 40
Fig. 1. Variscan basement in Central Europe [modified from von
Raumer et al. (1993)]. External Crystalline Massifs: AA, Aar; AG,
Argentera; AR, Aiguilles Rouges; BE, Belledonne; GO, Gotthard; GR,
Grandes Rousses; MB, Mont Blanc; PE, Pelvoux. Other Variscan
massifs: BF, Black Forest; BM, Bohemian massif; CM, French Massif
Central; CO, Corsica; MA, Maures–Tanneron; VO, Vosges.
other later, mainly Stephanian (~300 Ma), and more
iron rich. The aim of this study is better to constrain this
hypothesis on the basis of a large set of data taken from
the literature and to discuss its petrogenetic and tectonic
implications. Accordingly, the main geological, petrographical, geochemical (major and trace elements), and
chronological features of up to 68 plutonic bodies are
reviewed, allowing a comprehensive and renewed appraisal of the Late Variscan plutonism of the ECM.
GEOLOGICAL SETTING, TYPOLOGY
AND CHRONOLOGY OF THE LATE
VARISCAN PLUTONIC BODIES
In the ECM as a whole, Late Variscan plutonic bodies
occur as intrusions of various shapes (from roughly
elliptical to almost linear) and sizes (from <1 km2 up to
550 km2), displaying sharp contacts and a conformable
or a cross-cutting relationship with the country rocks.
They mainly consist of massive or foliated, light-coloured,
biotite monzo- and syenogranites (Table 1). Intermediate
and mafic rocks with biotite and amphibole are subordinate, and pyroxene-bearing types are almost completely absent except in the Argentera massif. Most of
the granites enclose mafic igneous enclaves, although
these are highly variable in proportion.
The average contents of quartz, mafic minerals and
feldspars, and the nomenclature of the main rock types
making up the studied plutonic bodies are recorded in
NUMBER 7
JULY 1999
Table 1. These features were determined from the average chemical compositions (Table 2), using the classification of Debon & Le Fort (1983, 1988). Because they
depend on the available chemical analyses, they may be,
in some cases, not really representative of the entire
plutonic bodies. In addition, the nomenclature thus defined sometimes differs from the name given in current
use to an intrusion. To make the discussion clearer, the
names ‘adamellite’ and ‘granite’ recommended by Debon
& Le Fort (1983) have been replaced by their approximate
equivalents, i.e. ‘monzogranite’ and ‘syenogranite’, respectively. Only those plutonic bodies for which chemical
data and/or radiometric ages of emplacement are available appear in Tables 1 and 2.
Some 900 chemical analyses (major and trace elements)
of Late Variscan plutonic rocks from the ECM were
compiled from the literature, and are available on the
Journal of Petrology Web site. The data include the granites
and, in some cases, their mafic igneous enclaves (average
compositions in Tables 2 and 4, below). Altogether, these
rocks display a wide compositional range, from gabbro
to syenogranite, and define a subalkaline [or alkali–calcic
(Peacock, 1931)] suite, i.e. intermediate between the calcalkaline and alkaline trends (Fig. 2). This suite clearly
differs from typical calc-alkaline series such as that defined
by the Late Variscan plutonic rocks of the Pyrenees
(Debon & Enrique, 1996). Actually, it is a composite
suite comprising also a few peraluminous bodies (some
of the Pelvoux plutons; Montenvers, Vallorcine and Cacciola granites).
The mg-number–B diagram of Debon & Le Fort (1988)
plots the Fe + Mg + Ti parameter (B), proportional to
the weight content in mafic minerals (de la Roche, 1964),
against the Mg/(Fe + Mg) ratio (mg-number). A ‘critical
line’ separates a magnesian igneous domain from a
ferriferous one (Fig. 3). The use of this diagram led us
to the recognition of three groups of plutonic bodies in the
ECM, namely, magnesian (Mg), magnesian–ferriferous
(Mg–Fe) and ferriferous (Fe), according to the positions
of their plots (average compositions) above, close to and
below the ‘critical line’, respectively. Highly ferriferous
granites remain, however, almost completely absent in
the ECM.
The ages of the Late Variscan plutonic bodies of
the ECM are recorded in Table 2. Only those results
considered to indicate the age of emplacement were
selected from the literature, namely, those obtained by
U–Pb on zircon, titanite or monazite, lead-evaporation
on single zircon, and Rb–Sr whole-rock isochron. Ar–Ar,
K–Ar and Rb–Sr mineral ages were discarded because
of isotopic disturbances during the Alpine orogenesis
(Hunziker et al., 1992; Schaltegger, 1994).
In the following sections, the studied plutonic bodies
are numbered as in Tables 1 and 2.
1152
DEBON AND LEMMET
EVOLUTION OF Mg/Fe RATIOS IN PLUTONIC ROCKS
Table 1: Geological setting and petrography of plutonic bodies from the External Crystalline
Massifs
Plutonic body
Approximate Geological
Characteristic
Quartz
Mafic
area
minerals
%
minerals spars
rock
%
type
setting
(km2)
Feld%
Major
Argentera massif
1: Malinvern–Argentera
?
sheets, dykes, plugs
bi, cpx, am, tit, all
metamonzonites
2: Central (or Argentera) granite
50
bi, ms, ± ga
pluton
7·6
41·2
51·2
± 7·8
± 9·6
± 1·9
33·6
4·0
62·4
± 13·4
± 3·3
± 16·1
qsy
ad
Pelvoux massif
3: Bans granite
4: Claphouse granite
4
9
5: Colle Blanche–Moutières
granite
bi, ± am, all
pluton
bi, ± ms
pluton
part of a plutonic complex
bi, ± ms
10
6: Colle Blanche–Moutières
part of a plutonic complex
bi, am, tit
quartz monzonite
7: Combe Guyon (or Alfrey)
2
pluton
bi, ms
granite
8: Combeynot granite
< 20
main part of a pluton
bi, all, mt
(coarse-grained type)
26·3
12·8
60·9
± 0·2
± 7·6
± 7·4
33·1
13·6
53·3
± 2·3
± 3·1
± 2·7
26·1
8·1
65·8
± 5·1
± 5·6
± 6·6
19·2
25·6
55·2
± 8·2
± 12·5
± 11·7
35·1
7·3
57·6
± 7·3
± 7·1
± 10·7
32·7
9·9
57·4
± 5·6
± 3·3
± 8·3
ad
gr
ad
qmz
ad
gr
9: Graou granite
1
pluton
bi
30·2
11·2
58·6
ad
10: Grun de Saint Maurice granite
6
pluton
bi, ms
29·7
8·5
61·8
ad
± 3·1
± 3·7
± 5·5
4
pluton
bi, ms
29·3
6·6
64·1
± 4·7
± 3·6
± 2·8
(porphyritic type)
11: Péou de Saint Maurice granite
12: Quatre Tours granite
13: Riéou Blanc granite
2
8
pluton
bi, ms
bi, ± ms
part of a pluton
(coarse-grained type)
14: Riéou Blanc granite
2
part of a pluton
bi, ms
(fine-grained type)
15: Rochail granite
< 34
bi, (± ms), ± tit
pluton
(dominant type)
16: Bérarde–Promontoire granite
17: Berches granite
18: Bourg granite
19: Cray granite
20: Etages granite
40
4
6
3
30
bi, ± ms
pluton
pluton
bi, ms
bi, ± am, all
pluton
bi, ms, cd, ± ga
pluton
bi, (± ms), ± ga
pluton
31·4
6·7
61·9
± 5·8
± 4·9
± 3·6
30·4
10·9
58·7
± 5·6
± 5·4
± 4·3
30·0
9·5
60·5
± 5·3
± 4·1
± 6·2
27·6
6·9
65·5
± 3·3
± 3·1
± 5·0
34·2
5·3
60·5
± 4·9
± 4·6
± 4·3
34·3
5·6
60·1
± 2·8
± 2·8
± 3·5
26·2
16·5
57·3
± 2·8
± 3·5
± 4·5
35·1
3·5
61·4
± 3·3
± 2·0
± 1·9
34·7
5·9
59·4
± 4·0
± 5·2
± 5·1
ad
ad
ad
gr
ad
ad
gr
gr
gr
gr
21: Gioberney granite
5
pluton
bi, ± ms
31·4
6·4
62·2
gr
22: Orgières granite
4
pluton
bi, ± am, all
24·4
17·2
58·4
ad
± 5·6
± 9·6
± 8·6
20
pluton
bi, (± ms)
33·6
8·2
58·2
± 3·5
± 3·8
± 4·4
23: Pelvoux–Pic de Clouzis
(or Ailefroide) granite
1153
gr
JOURNAL OF PETROLOGY
VOLUME 40
NUMBER 7
JULY 1999
Table 1: continued
Plutonic body
Approximate Geological
Characteristic
Quartz
Mafic
area
minerals
%
minerals spars
rock
%
type
setting
(km2)
Feld%
Major
24: Pétarel granite
5
pluton
bi, ms
37·9
8·3
53·8
gr
25: Pic de Valsenestre granite
6
pluton
bi, (± ms)
26·7
15·5
57·8
ad
± 3·5
± 3·0
± 2·3
26: Turbat–Lauranoure granite
33
pluton
bi, ± ms
32·8
8·0
59·2
± 5·6
± 5·7
± 2·7
gr
Belledonne massif
27: Sept Laux granite
bi, ± ms
pluton
95
28: Sept Laux granite
mafic enclaves
29: Saint Colomban granite
(porphyritic type)
am, bi, tit, all
main part of a pluton
bi, ± am, ± ms, tit, all
< 60
30: Saint Colomban granite
mafic enclaves
31: La Lauzière granite
am, bi, all, tit
part of a pluton
bi, (± ms), tit
25
32: La Lauzière quartz syenite
part of a pluton
bi, am, tit
26·9
6·9
66·2
± 5·9
± 4·1
± 4·1
12·6
52·2
35·2
± 10·6 ± 32·6
± 27·6
23·6
14·2
62·2
± 8·0
± 6·6
± 5·5
6·1
40·7
53·2
± 10·5 ± 22·0
± 21·5
28·7
7·5
63·8
± 10·7
± 7·3
± 6·1
8·5
29·1
62·4
± 7·9
± 17·1
± 16·5
ad
qmz
ad
mz
gr
qsy
Grandes Rousses massif
33: Alpetta granite
34: Roche Noire–La Fare granite
>8
>7
bi, (± ms), ± tit
pluton
bi, ± ms, ± tit
pluton(s)
23·9
13·1
63·0
± 6·5
± 10·1
± 7·4
30·1
6·5
63·4
± 7·7
± 7·1
± 5·3
ad
ad
Mont Blanc massif
35: Montenvers granite
8
36: Mont Blanc granite
pluton
bi, ms (?)
32·8
7·2
60·0
ad
main part of a pluton
bi, (± am), all
30·5
8·0
61·5
ad
± 4·3
± 1·9
± 4·4
part of a pluton
bi, all
34·3
6·9
58·8
± 3·9
± 1·4
± 3·8
(central type)
37: Mont Blanc granite
(border type)
< 225
38: Mont Blanc granite
mafic enclaves
39: Mont Blanc granite
bi, am, all, tit
mafic enclaves
bi, (± am), all, tit
16·5
33·7
49·8
± 9·1
± 15·8
± 9·7
18·3
22·8
58·9
± 13·8 ± 14·8
± 10·0
ad
qmzd
qmzd
Aiguilles Rouges massif
40: Pormenaz monzonite
41: Vallorcine granite
2
< 10
pluton
am, bi, tit, mt
13·1
27·1
59·8
qsy
main part of a pluton
bi, ms, ± (and, sil)
28·9
10·7
60·4
gr
± 7·5
± 7·0
± 7·3
(porphyritic type)
Aar massif
42: Giuv syenite
43: Punteglias granite
44: Punteglias diorite
45: Tödi granite
46: Brunni granite
47: Düssi diorite
48: Schöllenen diorite
6
10
<1
pluton
pluton
pluton
3
pluton
?
bi, am, tit, all
mafic stocks
<1
3
bi, am, tit, all
am, bi, all, tit
bi ?, all
plutonic complex
am, bi, tit, all
mafic enclaves
am, bi, tit
1154
8·7
33·5
57·8
± 7·5
± 16·4
± 12·7
24·1
13·7
62·2
± 9·9
± 13·9
± 4·1
11·3
71·2
17·5
± 8·1
± 83·5
± 91·6
28·1
5·1
66·8
± 18·0
± 6·0
± 11·9
5·4
53·3
41·3
± 15·6 ± 35·4
± 27·7
16·1
54·7
29·2
qsy
gr
qmz
ad
mzd
qmzd
DEBON AND LEMMET
Plutonic body
EVOLUTION OF Mg/Fe RATIOS IN PLUTONIC ROCKS
Approximate Geological
Characteristic
Quartz
Mafic
area
minerals
%
minerals spars
rock
%
type
setting
(km2)
49: Voralp granite
10
50: Central Aar granite
part of a plutonic complex
bi, all, tit
part of a plutonic complex
bi, all, tit, ± ga
(Central granite s.l.)
52: Central Aar granite
part of a plutonic complex
bi, all, tit
(Central granite s.s., main type)
53: Central Aar granite
part of a plutonic complex
bi, ga
(Southern border granite)
54: Central Aar granite
<550
%
Major
pluton
(Grimsel granodiorite)
51: Central Aar granite
Feld-
part of a plutonic complex
bi, ga
(Central granite s.s., leucocratic
20·7
14·3
65·0
± 6·3
± 3·2
± 3·7
25·9
11·4
62·7
± 6·8
± 6·1
± 4·5
30·2
7·6
62·2
± 3·0
± 1·2
± 4·1
30·9
7·1
62·0
± 6·2
± 2·2
± 4·3
33·6
4·0
62·4
± 3·8
± 1·6
± 2·8
gd
gd
ad
ad
ad
type)
55: Central Aar granite
dykes and stocks
bi, flu, ga
(Aplite, fine-grained leucogranite)
56: Central Aar granite
part of a plutonic complex
bi, flu, ga
(Northern border granite)
57: Central Aar granite
part of a plutonic complex
bi, flu, ga
(Mittagflue granite)
58: Central Aar granite
part of a plutonic complex
bi, flu, ga
(Kessiturm aplite)
59: Gastern granite
35
pluton
33·9
3·9
62·2
± 3·0
± 2·9
± 3·5
34·3
4·4
61·3
± 3·7
± 3·8
± 3·1
34·1
3·2
62·7
± 4·6
± 1·2
± 5·2
34·7
3·2
62·1
± 4·1
± 1·5
± 3·1
gr
ad
gr
ad
bi, tit
Gotthard massif
60: Fibbia granite
61: Gamsboden granite
8
pluton
34·7
10·9
54·4
13
pluton
30·7
8·9
60·4
ad
part of a plutonic complex
33·1
10·7
56·2
ad
24·7
20·2
55·1
gd
36·7
7·0
56·3
ad
29·9
5·2
64·9
gr
62: Medel granite
gr
40
63: Cristallina granodiorite
part of a plutonic complex
64: Cacciola granite
2
pluton
65: Rotondo granite
26
pluton
66: Sädelhorn diorite
< 0.6
bi, ms, ga
7·4
38·6
54·0
mzd
67: Tremola granite
2
dyke
pluton
bi, tit, ga
33·3
8·8
57·9
gr
68: Winterhorn granite
1
pluton
31·8
2·5
65·8
ad
Characteristic minerals: all, allanite; am, amphibole; and, andalusite; bi, biotite; cd, cordierite; cpx, clinopyroxene; flu, fluorite;
ga, garnet; ms, muscovite; mt, magnetite; sil, sillimanite; tit, titanite.
Sources of data: 1, Lombardo et al. (1997), B. Lombardo (personal communication, 1997); 2, Faure-Muret (1955); 3–26, Le
Fort (1973), Debelmas et al. (1980), Barféty, Pêcher et al. (1984), Boisset (1986), Costarella (1987), Barféty et al. (1989); 27–32,
Debon et al. (1998, and references therein); 33–34, Giorgi (1979); 35, F. Bussy (personal communication, 1997); 36–39, Marro
(1986), Bussy (1990); 40, Bussy et al. (1998); 41, Poncerry (1981), Brändlein et al. (1994); 42–59, Schaltegger (1989, 1990a,
1993), Schaltegger & von Quadt (1990), Schaltegger et al. (1991), Schaltegger & Corfu (1992, 1995); 60–68, Oberli et al.
(1981), Bossart et al. (1986), S. A. Sergeev (personal communication, 1998).
Proportions by weight of quartz, mafic minerals and feldspars calculated from the chemical composition of the different
plutonic bodies (Table 2; La Roche, 1964; Debon & Le Fort, 1983, 1988). Standard deviations at ±2r. Rock types determined
from the average chemical compositions (Table 2) [refer to the classification of Debon & Le Fort (1983, 1988)]. Abbreviations:
ad, adamellite (~monzogranite); gd, granodiorite; gr, granite (~syenogranite); mz, monzonite; mzd, monzodiorite; qmz, quartz
monzonite; qmzd, quartz monzodiorite; qsy, quartz syenite.
Argentera massif
The Argentera massif (Fig. 4) is an elongate body extending N 125°E for ~55 km (Faure-Muret, 1955; Bogdanoff et al., 1991, and references therein). It comprises
four generally vertical metamorphic units, trending
N120–140°E, composed of migmatites, gneisses, orthogneisses (commonly of the augen type), mica schists and
amphibolites with eclogitic relics. Migmatites formed in
three successive stages, respectively pre-, syn- and postkinematic with regard to the Variscan thrusting phase of
1155
Plutonic
n
4
2
2
5
24
17
5
7
1
9
22
10
3
3
16
46
7
27
4
43
1
28
8
1
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
Pelvoux massif
3
1
Argentera massif
body
1156
75·00
0·15
0·37
±0·24
73·58
±0·26
±4·61
±1·58
0·54
0·22
66·01
73·30
0·15
±0·23
74·35
±3·31
0·10
±0·06
75·22
±2·37
0·54
±0·13
66·46
±2·13
0·19
±0·14
74·18
14·68
12·30
±1·40
13·15
±1·10
15·66
13·10
±1·28
13·08
±0·35
13·15
±0·86
15·49
±0·86
13·29
±1·58
12·85
±0·84
15·07
±0·40
14·80
±1·14
2·37
±0·40
1·99
±1·73
4·07
1·63
±1·37
1·57
±0·44
0·99
±0·72
3·90
±0·77
1·55
±1·08
1·41
±0·56
1·39
±1·36
2·15
±0·75
2·44
±0·97
1·28
±0·64
1·47
±0·78
1·96
2·38
±0·64
2·25
±1·33
1·56
±1·63
4·49
±1·12
1·77
±0·60
2·85
±1·53
2·81
±0·82
1·41
±1·55
6·63
Fe2O3∗
0·07
±0·08
0·04
±0·03
0·08
0·05
±0·04
0·04
±0·03
0·03
±0·02
0·08
±0·02
0·03
±0·05
0·05
±0·01
0·03
±0·08
0·04
±0·05
0·04
±0·02
0·04
±0·02
0·04
±0·02
0·04
0·05
±0·04
0·02
±0·03
0·04
±0·03
0·08
±0·02
0·04
±0·07
0·03
±0·08
0·07
±0·32
0·15
±0·02
0·10
MnO
0·59
±0·63
0·63
±1·21
1·52
0·49
±0·45
0·46
±0·37
0·23
±0·48
1·45
±0·26
0·37
±0·49
0·40
±0·41
0·74
±0·14
0·92
±0·85
1·02
±0·57
0·75
±0·57
0·63
±0·43
0·76
1·15
±0·40
0·96
±0·85
0·73
±1·90
3·18
±0·67
0·80
±0·86
1·34
±1·00
1·25
±0·27
0·10
±1·22
5·21
MgO
0·34
±0·68
0·48
±1·69
2·36
0·39
±0·77
0·50
±0·48
0·22
±1·12
1·90
±0·42
0·65
±0·58
0·43
±0·63
0·45
±0·87
0·51
±0·59
0·60
±0·80
0·30
±0·81
0·84
±0·87
1·29
1·34
±0·24
0·29
±1·10
0·55
±1·07
3·06
±0·79
0·67
±0·28
0·47
±0·45
1·59
±0·75
0·70
±1·38
4·63
CaO
3·40
±0·33
3·66
±0·47
3·70
3·67
±0·36
3·39
±0·12
3·31
±0·46
3·14
±0·50
3·22
±0·45
3·94
±0·72
4·26
±0·69
3·83
±0·50
3·88
±0·77
4·14
±0·67
4·08
±0·59
4·00
3·48
±0·97
3·68
±0·91
3·62
±0·58
3·61
±0·79
4·54
±0·17
3·48
±1·34
3·98
±2·10
4·28
±0·50
2·46
Na2O
4·34
±0·68
4·62
±1·07
3·92
5·15
±0·87
4·93
±0·16
5·33
±0·88
4·68
±0·73
5·17
±0·64
4·49
±0·57
4·76
±1·20
4·63
±0·31
4·28
±0·96
4·31
±0·95
4·47
±0·92
3·92
4·39
±0·65
4·67
±0·43
4·33
±1·20
4·16
±1·14
4·27
±0·35
4·28
±1·33
4·18
±1·32
4·00
±0·66
6·42
K2O
n.d.
n.d.
±0·18
0·31
n.d.
n.d.
n.d.
±0·08
0·31
n.d.
n.d.
±0·13
0·12
n.d.
n.d.
±0·16
0·07
±0·13
0·14
±0·07
0·08
n.d.
±0·03
0·06
±0·09
0·13
±0·25
0·44
±0·15
0·11
n.d.
n.d.
±0·06
0·12
±0·31
0·96
P2O5
LOI
1·32
±0·37
1·23
±0·80
1·45
0·90
±0·41
0·87
±0·31
0·78
±1·08
1·92
±0·32
0·86
±0·57
0·74
±0·49
1·07
±0·46
1·55
±0·47
1·50
±0·58
1·30
±0·55
1·00
±0·54
1·16
1·40
±0·29
1·17
±0·80
1·21
±0·95
1·99
±0·55
1·20
±0·29
1·73
±0·03
1·11
±0·34
0·55
±0·34
0·98
(or H2O)
99·88
±1·08
99·75
±1·03
99·63
98·90
±1·48
99·33
±1·87
99·37
±1·04
99·88
±1·27
99·51
±1·28
99·68
±0·69
99·65
±0·74
99·71
±0·91
99·43
±1·21
99·43
±0·81
99·86
±1·05
99·75
98·33
±0·14
99·74
±1·82
100·31
±1·20
99·49
±1·22
99·55
±1·03
99·71
±0·86
99·50
±0·26
100·08
±0·37
99·46
Total
n.d.
n.d.
n.d.
±124
575
±712
1480
n.d.
±423
523
±104
361
±195
1229
±468
449
±713
539
±538
1253
±300
961
±323
882
±414
1086
±514
1160
±344
1015
639
±251
417
±467
1089
±1139
2169
±990
1444
±190
999
1345
Ba
n.d.
n.d.
±47
155
n.d.
n.d.
n.d.
±66
206
n.d.
n.d.
±40
203
n.d.
n.d.
±67
181
±33
146
±42
142
n.d.
±100
237
±30
184
±63
178
±71
168
n.d.
n.d.
±177
317
±38
298
Rb
n.d.
±100
164
±275
594
n.d.
±84
100
±23
83
±269
453
±59
75
±112
65
±230
465
±204
371
±163
434
±251
337
±277
550
±118
415
474
±41
102
±265
291
±493
992
±330
584
±239
309
366
±34
37
±179
663
Sr
±0·6
0·8
±1·4
1·5
±0·7
1·3
±1·9
1·8
±0·6
0·8
±0·5
1·0
±4·2
7·0
±2·0
2·2
±0·3
0·5
±0·7
0·9
±55·9
33·3
±0·2
Sr
1·3
Rb/
86
87
202
±16
216
±31
203
228
±18
214
±5
220
±14
201
±11
214
±14
223
±19
238
±17
222
±10
216
±18
225
±9
227
±17
212
205
±23
218
±32
209
±36
205
±24
237
±8
203
±15
217
±68
223
±13
216
Na+K
0·46
±0·05
0·45
±0·07
0·41
0·48
±0·06
0·49
±0·01
0·51
±0·08
0·49
±0·07
0·51
±0·05
0·43
±0·06
0·42
±0·10
0·44
±0·05
0·42
±0·09
0·41
±0·09
0·42
±0·08
0·39
0·45
±0·10
0·46
±0·06
0·44
±0·07
0·43
±0·09
0·38
±0·03
0·45
±0·16
0·41
±0·18
0·39
±0·06
0·63
(Na+K)
K/
46
±21
45
±53
95
35
±29
33
±11
19
±20
92
±16
31
±26
29
±17
39
±23
53
±30
60
±27
37
±20
37
±20
47
62
±19
55
±39
40
±69
142
±31
45
±17
75
±42
71
±18
22
±53
229
B
0·61
0·33
±0·19
0·37
±0·12
0·41
0·37
±0·27
0·35
±0·43
0·27
±0·08
0·42
±0·14
0·32
±0·27
0·33
±0·11
0·51
±0·13
0·47
±0·19
0·44
±0·12
0·54
±0·20
0·44
±0·09
0·43
0·49
±0·05
0·46
±0·15
0·47
±0·08
0·58
±0·15
0·47
±0·17
0·48
±0·07
0·46
±0·22
0·10
±0·02
Mg–Fe
Mg–Fe
Mg–Fe
Mg–Fe
Mg–Fe
Mg–Fe
Mg–Fe
Mg–Fe
Mg–Fe
Mg
Mg
Mg
Mg
Mg
Mg
Mg
Mg
Mg
Mg
Mg
Mg
Mg
Fe
Mg
group
±11
343
±7
312
±?
295
±8
337
( Ma)
mg-no. Plutonic Age
ES, (XRF )
WA, MS
XRF
UZ
UZ
RS
SZ
Analytical methods
NUMBER 7
±2·34
0·12
±0·22
75·24
±3·43
0·24
±0·12
71·53
±1·49
0·24
±0·32
71·04
±2·44
0·36
±0·21
70·64
±1·59
14·50
±1·26
15·02
±0·93
15·54
13·73
±1·26
13·85
±1·83
14·06
±1·01
15·25
±0·83
15·63
±1·11
14·31
±0·04
15·32
±1·42
13·39
±0·66
14·35
Al2O3
VOLUME 40
±3·03
0·21
±0·14
72·55
±2·87
0·22
±0·14
71·95
±0·14
±2·20
±2·89
0·32
0·31
70·69
70·10
0·24
±0·10
72·56
±1·41
0·24
±0·21
73·86
±3·82
0·58
±0·21
62·65
±4·47
0·25
±0·15
70·27
±0·32
±1·30
±3·32
0·52
±0·13
±1·99
70·70
0·40
68·81
0·15
±0·27
75·24
±2·51
1·32
±0·34
56·40
TiO2
±5·53
SiO2
Table 2: Average chemical composition and age of plutonic bodies from the External Crystalline Massifs
JOURNAL OF PETROLOGY
JULY 1999
Plutonic
n
SiO2
17
26
21
37
10
21
21
28
29
30
31
32
1157
25
15
46
10
28
48
36
37
38
39
10
41
49
4
2
42
43
44
Aar massif
1
40
1·05
±0·42
50·95
±7·50
0·43
±0·44
68·63
±0·42
±6·25
±7·26
0·96
57·95
0·42
±0·26
70·00
±3·30
60·01
1·13
0·84
±0·55
62·91
±9·27
0·97
±0·33
59·12
±7·18
0·22
±0·06
74·22
±1·96
0·27
±0·07
72·45
±1·96
0·27
±0·27
±5·94
73·27
0·24
±0·20
72·56
±5·22
67·16
Aiguilles Rouges massif
1
35
Mont Blanc massif
34
33
0·40
1·09
±0·50
59·40
±5·94
0·28
±0·30
72·41
±5·96
1·04
±0·55
55·28
±7·84
0·42
±0·25
67·09
±4·95
0·80
±0·37
56·64
±0·15
±2·84
±6·77
0·23
±0·30
±5·03
70·91
0·27
±0·16
±2·35
72·73
0·58
TiO2
66·49
Grandes Rousses massif
35
27
Belledonne massif
6
25
Pelvoux massif (continued)
body
±9·19
13·65
±0·50
14·68
±1·45
15·00
±1·20
15·33
16·19
±2·16
15·94
±1·03
15·86
±0·84
12·99
±0·84
13·83
14·21
±1·87
14·46
±1·56
16·26
±2·49
15·80
±2·26
14·34
±3·61
15·78
±1·75
16·12
±3·02
12·49
±0·87
15·07
±1·85
13·61
±1·48
15·78
Al2O3
±2·99
8·21
±2·21
2·68
±2·33
5·73
±1·63
2·56
4·99
±3·68
6·12
±2·59
6·70
±0·39
1·96
±0·45
2·30
2·00
±1·36
1·32
±1·42
2·40
±2·06
4·77
±1·44
1·60
±2·95
6·77
±1·29
2·89
±1·63
6·00
±0·72
1·60
±1·38
2·04
±0·85
3·89
Fe2O3∗
±0·00
0·20
n.d.
±0·05
0·10
±0·02
0·05
0·06
±0·11
0·14
±0·10
0·13
±0·02
0·05
±0·02
0·05
0·05
±0·02
0·02
±0·05
0·04
±0·12
0·09
±0·03
0·03
±0·06
0·11
±0·03
0·04
±0·04
0·10
±0·02
0·02
±0·02
0·04
±0·02
0·06
MnO
1·85
CaO
1·23
1·47
5·28
2·32
5·30
0·55
3·26
1·63
0·78
1·31
1·32
1·23
5·12
3·05
1·03
3·02
5·24
1·83
6·60
±17·39 ±2·55
11·25
±1·77 ±1·39
1·50
±2·37 ±2·27
4·11
±0·66 ±0·81
0·89
2·98
±1·25 ±2·06
1·59
±2·19 ±2·53
3·66
±0·20 ±0·42
0·45
±0·21 ±0·47
0·48
0·47
±0·87 ±0·89
0·67
±1·51 ±1·40
1·52
±2·60 ±1·91
3·56
±0·82 ±0·70
0·74
±3·68 ±2·11
5·17
±0·78 ±1·55
1·50
±6·72 ±2·46
8·24
±0·51 ±0·83
0·62
±0·52 ±1·53
0·62
±0·22 ±0·97
1·22
MgO
±3·25
2·35
±1·02
3·63
±0·90
2·66
±0·74
3·39
3·19
±1·40
4·22
±0·87
3·41
±0·40
3·52
±0·35
3·60
3·50
±1·06
3·98
±0·98
3·98
±1·18
3·54
±0·53
3·86
±1·53
4·01
±0·65
3·98
±1·17
2·51
±0·60
4·20
±0·35
3·28
±0·45
3·44
Na2O
±0·57
3·10
±1·41
5·10
±1·10
5·91
±0·67
5·02
5·73
±1·75
3·53
±1·41
3·10
±0·91
4·40
±0·49
4·76
4·51
±1·55
4·61
±1·40
4·36
±1·85
6·11
±0·97
5·25
±1·39
3·80
±0·92
4·02
±2·31
4·75
±1·16
4·30
±0·58
4·76
±0·79
4·14
K2O
±0·28
0·50
±0·19
0·28
±0·49
0·87
±0·09
0·30
0·47
±0·26
0·29
±0·14
0·29
±0·02
0·06
±0·03
0·08
0·19
±0·13
0·08
±0·38
0·21
±0·33
0·50
±0·14
0·10
±0·54
0·91
±0·13
0·17
±0·56
0·85
±0·13
0·12
0·13
±0·17
0·30
P2O5
LOI
±0·57
1·70
±0·16
0·70
±0·75
1·33
±0·46
1·03
1·69
±1·04
1·08
±0·92
1·44
±0·28
0·52
±0·39
0·61
0·66
±1·01
0·96
±2·45
1·93
±0·73
1·23
±0·58
0·69
±0·70
1·60
±0·82
1·19
±0·73
1·57
±0·29
0·72
±0·51
1·01
±0·76
1·46
(or H2O)
±1·15
99·56
±0·69
99·43
±0·88
99·87
±0·90
100·01
99·46
±0·79
99·73
±0·69
99·80
±0·56
99·62
±1·18
99·75
100·45
±1·45
99·68
±1·48
99·89
±1·26
99·35
±1·13
99·85
±1·04
99·76
±1·18
99·74
±1·07
99·25
±1·18
99·26
±1·35
99·72
±1·08
99·21
Total
841
±1810
1610
±806
1813
±861
2508
n.d.
1971
±325
386
±238
591
±117
401
±182
505
375
±491
1286
±453
1147
±1277
2425
±499
497
±1682
2169
±602
848
±1147
1936
±680
1110
±469
455
±253
Ba
±106
153
±59
257
±116
340
±129
254
273
±220
373
±184
212
±60
220
±69
267
235
±81
192
±70
188
±83
238
±103
238
±57
178
±74
190
±129
249
±87
175
213
±44
162
Rb
±834
555
±144
574
±260
641
±71
115
604
±104
157
±97
273
±31
96
±34
109
93
±144
367
±134
421
±846
1066
±314
293
±836
1063
±373
400
±315
534
±289
517
±101
99
±74
217
Sr
Rb/
±0·9
1·0
±0·3
1·3
±1·2
1·6
±6·8
8·1
1·3
±8·1
7·7
±3·2
2·5
±3·0
6·8
±4·0
7·4
7·3
±1·3
1·6
±0·8
1·3
±0·7
0·8
±6·5
3·7
±0·4
0·6
±2·5
1·8
±1·4
1·5
±3·4
1·3
4·5
±1·0
2·2
Sr
86
87
±117
142
±9
225
±35
211
±28
216
225
±38
211
±32
176
±14
207
±14
217
209
±18
226
±24
221
±48
244
±25
236
±38
210
±26
214
±34
182
±19
227
±19
207
±10
199
Na+K
±0·33
0·50
±0·14
0·48
±0·11
0·60
±0·07
0·49
0·54
±0·17
0·36
±0·13
0·37
±0·07
0·45
±0·04
0·47
0·46
±0·14
0·43
±0·13
0·42
±0·13
0·53
±0·06
0·47
±0·16
0·39
±0·07
0·40
±0·21
0·55
±0·09
0·40
±0·04
0·49
±0·08
0·44
(Na+K)
K/
86
±464
395
±77
76
±91
186
±39
59
151
±82
127
±88
187
±8
38
±11
44
40
±40
36
±56
73
±95
162
±40
42
±122
226
±37
79
±181
290
±23
38
±32
44
±17
B
±0·29
0·69
±0·14
0·50
±0·06
0·58
±0·09
0·40
0·54
±0·08
0·33
±0·10
0·51
±0·10
0·31
±0·06
0·29
0·32
±0·25
0·46
±0·11
0·54
±0·09
0·59
±0·18
0·46
±0·12
0·59
±0·08
0·50
±0·13
0·71
±0·13
0·42
±0·15
0·36
±0·04
0·38
Mg
Mg
Mg
Mg–Fe
Mg
Fe
Mg–Fe
Mg–Fe
Fe
Mg–Fe
Mg
Mg
Mg
Mg
Mg
Mg
Mg
Mg
Mg–Fe
Mg–Fe
group
±2·5
334
±2·5
334
±2·5
334
±2
307
±2
332
±2
305
+ 5/–3
306
±3
304
+ 6/–5
307
±13
341
±14
343
±16
343
±13
335
±5
302
( Ma)
mg-no. Plutonic Age
XRF
XRF
XRF
XRF, ES
ES, XRF
ES, (XRF )
UZ
UZ
UZ
UZM
UZ
UZ
UZ
UZ
UZ
LZ
LZ
LZ
LZ
UZ
Analytical methods
DEBON AND LEMMET
EVOLUTION OF Mg/Fe RATIOS IN PLUTONIC ROCKS
4·39
Na2O
4·64
K2O
0·09
P2O5
0·83
(or H2O)
LOI
99·00
Total
Ba
770
173
Rb
232
Sr
2·4
Sr
86
Rb/
87
240
Na+K
0·41
(Na+K)
K/
B
28
0·34
group
1158
5
5
9
21
3
2
1
61
62
63
64
65
66
Gotthard massif
60
56·03
75·10
74·19
65·61
72·13
72·75
72·98
2·19
0·37
0·08
0·49
0·31
0·33
0·33
12·61
16·14
12·98
13·87
15·88
14·53
13·69
13·78
±1·65
0·95
8·14
1·70
2·01
4·78
2·52
2·72
2·76
±0·44
0·04
0·16
0·06
0·07
0·08
0·03
0·04
0·04
±0·02
0·05
±0·02
0·16
3·43
0·11
0·50
1·85
0·96
0·46
0·88
±0·14
0·18
±0·14
0·54
5·07
0·67
0·44
3·75
1·62
1·58
1·63
±0·64
0·63
±0·25
0·36
±1·18
4·01
4·55
4·12
3·55
3·49
3·25
3·86
2·82
±0·28
3·77
±0·64
3·84
±0·44
4·96
2·97
5·19
4·16
3·29
4·35
4·25
4·82
±0·24
4·70
±1·59
4·83
±0·48
4·40
±0·99
0·03
0·36
0·25
0·02
0·15
0·09
0·12
0·25
±0·02
0·02
±0·04
0·02
±0·04
0·02
±0·05
0·40
±0·22
0·29
±0·26
0·38
±0·25
0·35
±0·31
98·58
99·04
100·55
98·89
99·37
99·79
99·80
100·29
±1·22
99·24
±0·46
98·84
±1·27
99·38
±1·61
98·94
±1·81
318
130
14
437
466
379
318
±452
204
±198
145
±335
164
±547
435
±206
156
79
260
259
142
150
199
199
±102
284
±101
275
±107
220
±100
207
±55
239
±72
156
379
51
9
181
114
155
147
±79
43
±59
27
±100
43
±91
85
±42
67
±58
10·3
0·6
14·7
83·3
2·3
3·8
3·7
3·9
±79·8
43·8
±33·9
43·7
±50·5
31·5
±26·2
12·2
±8·5
11·4
±1·7
3·0
±24·8
210
243
203
182
197
215
193
±9
221
±15
226
±18
223
±15
221
±8
222
±8
216
±7
224
0·37
0·30
0·45
0·44
0·38
0·47
0·42
0·53
±0·03
0·45
±0·11
0·45
±0·04
0·42
±0·11
0·48
±0·03
0·46
±0·20
0·40
±0·05
0·41
±0·09
63
214
29
39
112
59
50
60
±8
18
±6
18
±21
25
±16
22
±9
22
±12
39
±7
42
±34
0·36
0·46
0·11
0·33
0·43
0·43
0·25
0·39
±0·15
0·27
±0·19
0·24
±0·21
0·24
±0·18
0·31
±0·09
0·25
±0·10
0·33
±0·15
0·32
±0·09
Fe
Fe
Mg–Fe
Mg–Fe
Mg–Fe
Fe
Mg–Fe
?
Mg–Fe
Fe
Fe
Mg–Fe
Fe
Mg–Fe
Mg–Fe
Mg–Fe
308
+ 5/–4
293
±1·1
294·3
±11
292
±20
303
±1·2
299·4
±1·2
299·4
±4
303
±2·5
296·5
±7
298
±2
299
±2
297
±2
299
±2
309
±3
310
±2
XRF
UZ
UZ
UZ
UZ
UZ
UZ
UZ
UZ
UZ
UZ
UZ
UT
UZ
UT
UZ
UZ
NUMBER 7
59
0·09
±0·04
75·95
±2·86
1·02
±0·41
0·25
±0·40
3·57
±0·90
0·39
±0·34
720
±408
92
±68
218
±12
0·34
±0·04
14
2·38
0·04
±0·03
0·66
±0·66
0·03
±0·02
99·04
±2·06
212
±84
2·1
±2·6
±18
58
12·51
±1·86
0·07
±0·01
75·62
1·38
±0·86
0·26
±0·32
4·75
±0·23
0·53
±0·64
546
±129
231
±152
±0·05
79
Mg
13
12·69
±2·12
0·03
±0·03
3·75
±0·32
0·08
±0·03
98·85
±1·21
141
±67
±11
0·33
0·74
Mg
57
0·10
±0·13
75·59
±3·04
1·08
±0·71
0·66
±0·30
4·04
±1·99
0·55
±0·23
912
±498
±1·0
221
303
±0·20
0·63
±0·15
17
12·73
0·21
±0·13
4·05
±1·44
0·07
±0·01
99·29
±1·96
±110
1·4
0·43
±196
296
±34
XRF
methods
Analytical
VOLUME 40
±1·34
0·04
±0·01
1·33
±0·52
4·36
±0·52
0·66
±0·37
±35
301
158
±0·12
0·37
±0·14
56
0·14
±0·16
75·07
±2·47
1·23
±0·47
0·48
±0·23
4·09
±0·31
0·15
±0·19
±403
133
0·6
±54
185
±47
22
12·41
±0·77
0·06
±0·02
0·89
±0·76
3·80
±0·94
±0·66
1111
545
±0·2
0·5
±2·5
55
0·13
±0·04
74·98
±2·68
1·92
±0·61
0·50
±0·30
4·24
±0·61
±0·35
98·86
122
±33
750
±204
13
13·53
±1·09
0·07
±0·03
1·96
±1·11
±0·05
0·82
1524
±61
127
±28
54
0·26
±0·12
72·77
±3·77
2·10
±0·28
0·85
±0·61
±0·57
0·18
99·47
±1396
1527
±1172
6
13·40
±0·23
0·07
±0·02
±0·44
3·39
n.d.
±1·67
100·86
±2·53
53
0·26
±0·04
72·56
±1·60
2·95
±1·42
±1·31
4·61
0·14
±1·62
2·37
±0·38
9
14·82
±1·76
±0·22
2·63
3·23
±0·85
0·71
±0·03
52
0·44
±0·24
69·34
±4·88
±0·01
0·99
2·77
±1·89
3·29
±0·62
39
±0·91
0·08
6·79
±0·79
3·57
±1·85
51
±1·30
3·80
8·78
±2·88
6·88
±0·78
Fe
0·57
±0·16
66·17
±3·39
15·61
0·16
±6·92
7·47
±0·58
15
6·22
±0·07
0·16
±0·07
50
11·66
±2·40
7·82
±1·27
?
0·62
±3·79
14·08
±0·83
49
59·10
1·01
±0·63
53·50
±9·11
0·16
±0·25
72·62
±7·91
±2
333
( Ma)
mg-no. Plutonic Age
1
0·61
CaO
48
0·39
MgO
3
0·03
MnO
47
1·32
Fe2O3∗
Mg–Fe
13·95
Al2O3
2
TiO2
46
SiO2
?
n
45
(continued)
Aar massif
body
Plutonic
Table 2 continued
JOURNAL OF PETROLOGY
JULY 1999
Plutonic
n
SiO2
3
68
75·76
74·98
0·02
0·32
TiO2
12·86
12·96
Al2O3
0·73
2·77
Fe2O3∗
0·08
0·04
MnO
0·17
0·42
MgO
0·26
1·01
CaO
4·55
3·56
Na2O
4·44
4·92
K2O
0·03
0·12
P2O5
LOI
(or H2O)
98·90
101·10
Total
15
414
Ba
362
222
Rb
10
81
Sr
Sr
86
104·8
7·9
Rb/
87
241
219
Na+K
K/
0·39
0·48
(Na+K)
14
49
B
0·31
0·23
Mg–Fe
Fe
group
±1·1
294·3
( Ma)
mg-no. Plutonic Age
XRF
UZ
Analytical methods
Average compositions calculated from a set of 866 analyses, representative of the major rock types making up the different plutonic bodies. Standard deviations
at ±2r. Raw data available on the Journal of Petrology Web site.
Plutonic bodies: (a) Argentera massif: 1, Malinvern–Argentera metamonzonites; 2, Central granite; (b) Pelvoux massif: 3, Bans granite; 4, Claphouse granite; 5,
Colle Blanche–Moutières granite; 6, Colle Blanche Moutières quartz monzonite; 7, Combe Guyon (= Alfrey) granite; 8, Combeynot granite (coarse-grained type);
9, Graou granite; 10, Grun de Saint Maurice granite; 11, Péou de Saint Maurice granite; 12, Quatre Tours granite; 13, Riéou Blanc porphyritic granite; 14, Riéou
Blanc fine-grained granite; 15, Rochail granite; 16, Bérarde–Promontoire granite; 17, Berches granite; 18, Bourg granite; 19, Cray granite; 20, Etages granite; 21,
Gioberney granite; 22, Orgières granite; 23, Pelvoux–Pic de Clouzis granite; 24, Pétarel granite; 25, Pic de Valsenestre granite; 26 Turbat–Lauranoure granite;
(c) Belledonne massif: 27, Sept Laux granite; 28, mafic enclaves in 27; 29, Saint Colomban granite (porphyritic type); 30, mafic enclaves in 29; 31, La Lauzière
granite; 32, La Lauzière quartz syenite; (d) Grandes Rousses massif: 33, Alpetta granite; 34, Roche Noire–La Fare granite; (e) Mont Blanc massif: 35, Montenvers
granite; 36, Mont Blanc granite (central type); 37, Mont Blanc granite (border type); 38 and 39, mafic enclaves in 36 (and 37); (f) Aiguilles Rouges massif: 40,
Pormenaz monzonite; 41, Vallorcine granite (porphyritic type); (g) Aar massif: 42, Giuv syenite; 43, Punteglias granite; 44, Punteglias diorite; 45, Tödi granite; 46,
Brunni granite; 47, Düssi diorite; 48, Schöllenen diorite; 49, Voralp granite; 50–58, Central Aar granite [50, Grimsel granodiorite; 51, Central granite s.l.; 52, Central
granite s.s. (main type); 53, Southern border granite; 54, Central granite s.s. (leucocratic type); 55, Aplite and fine-grained leucogranite; 56, Northern border
granite; 57, Mittagflue granite; 58, Kessiturm aplite]; 59, Gastern granite; (h) Gotthard massif: 60, Fibbia granite; 61, Gamsboden granite; 62, Medel granite; 63,
Cristallina granodiorite; 64, Cacciola granite; 65, Rotondo granite; 66, Sädelhorn diorite; 67, Tremola granite; 68, Winterhorn granite.
n, number of analysed samples. For P2O5, Ba, Rb and Sr, n is often less than the given value. Major elements in oxide %, trace elements in ppm. Fe2O3∗, total
iron as ferric oxide; LOI, loss on ignition; 87Rb/86Sr = Rb/Sr × 2·8956 (J. Sonet, personal communication, 1985); sum (Na + K) is in gram-atoms × 103 in 100 g
of rock; the B (= Fe + Mg + Ti) parameter, also in gram-atoms × 103, is proportional to the amount by weight of mafic minerals (de la Roche, 1964); mg-no. =
Mg/(Fe + Mg). Fe, total iron. Plutonic group defined through the mg-number–B diagram (Fig. 3): Mg, magnesian; Mg–Fe, magnesian–ferriferous; Fe, ferriferous.
Ages given with 2r errors, apart from those obtained through the lead-evaporation method (1r). In the latter case, errors are calculated using all the measured
ratios of all the different steps considered for mean age determination (Cocherie et al., 1992), this leading to wider errors than the 2rm ones (Debon et al., 1998).
Age of the Argentera granite (295 Ma) recalculated with k87Rb = 1·42 × 10–11 year–1.
Analytical methods: ES, emission spectrometry [±ICP-MS (inductively coupled plasma emission mass spectrometry)]; MS, isotope dilution mass spectrometry;
WA, wet chemical analysis; XRF, X-ray fluorescence. Dating methods: LZ, lead-evaporation on single zircon; RS, Rb–Sr whole-rock isochron; SZ, U–Pb SHRIMP
microprobe analysis on zircon; UT, U–Pb on titanite; UZ, U–Pb on zircon; UZM, U–Pb on zircon and monazite.
Sources of geochemical data: 1, Colombo (1996), B. Lombardo (personal communication, 1997); 2, Faure-Muret (1955), Ferrara & Malaroda (1969); 3–26, Le Fort
(1973), Barféty, Pêcher et al. (1984), de Boisset (1986), Barféty et al. (1989), G. Banzet et al. (unpublished data, 1964–1997); 27–32, Gasquet (1979), Siméon (1979),
Poncerry (1981), Lacheny (1995), Debon et al. (1998), Barféty et al. (1999), G. Vivier (unpublished data, c. 1975–1985); 33, 34, Giorgi (1979), A. Ploquin & G. Vivier
(unpublished data, c. 1975–1985); 35, F. Bussy (personal communication, 1997); 36–39, Marro (1986), Bussy (1990); 40, Bussy et al. (1998); 41, Poncerry (1981);
42–44, 46–48, Schaltegger et al. (1991), U. Schaltegger (personal communication, 1997); 50–58, Schaltegger (1989) [see also Schaltegger (1990a)]; 60–68, S. A.
Sergeev (personal communication, 1998).
Sources of geochronological data: 1, Lombardo et al. (1997), B. Lombardo (personal communication, 1997); 2, Ferrara & Malaroda (1969); 8, Cannic et al. (1998);
15, 26, Guerrot (1998; personal communication, 1998); 27, 29–31, Debon et al. (1998); 35, Bussy & von Raumer (1993); 36, 38, 39, Bussy (1992); 40, Bussy et al.
(1998); 41, Bussy (1995); 42–44, 47–52, 57, Schaltegger & Corfu (1992); 45, Schaltegger & Corfu (1995); 56, Schaltegger & von Quadt (1990); 59, Schaltegger (1993);
60, 61, 65, 67, Sergeev et al. (1995); 62, Grünenfelder (1962); 64, Oberli et al. (1981); 66, Bossart et al. (1986).
4
67
Gotthard massif (continued)
body
Table 2: continued
DEBON AND LEMMET
EVOLUTION OF Mg/Fe RATIOS IN PLUTONIC ROCKS
1159
TiO2
Al2O3
Fe2O3∗ MnO
61·30
Average + 1r
Average – 1r
0·13
15·15
14·17
16·12
±0·97
2·87
1·04
4·69
±1·82
0·05
0·01
0·09
±0·04
0·14
3·38
±1·62
1·76
MgO
0·22
3·84
±1·81
2·03
CaO
1160
75·86
68·08
1r
Average + 1r
Average – 1r
13·87
12·56
15·19
±1·31
2·26
1·07
3·44
±1·19
0·05
0·03
0·08
±0·02
76·15
69·65
1r
Average + 1r
Average – 1r
13·55
12·35
14·74
±1·20
2·06
1·14
2·97
±0·91
0·06
0·02
0·09
±0·04
0·14
0·71
±0·29
0·43
0·19
1·22
1·14
0·37
1·94
±0·78
1·16
0·29
2·00
±0·86
3·66
3·44
4·31
±0·43
3·87
3·22
4·11
±0·44
3·90
5·10
±0·60
4·50
3·92
5·11
±0·60
4·51
3·83
5·67
±0·92
4·75
K2O
0·02
0·13
±0·06
0·08
0·04
0·30
±0·13
0·17
0·00
0·62
±0·32
0·30
P2O5
0·31
0·77
±0·23
0·54
0·40
1·43
±0·52
0·92
0·64
1·74
±0·55
1·19
(or H2O)
LOI
99·38
99·54
99·68
Total
216
Rb
191
141
292
249
240
132
153
806
178
302
±326 ±62
480
267
1147
±440 ±59
707
716
2205
±744 ±76
1460
Ba
18
197
±90
108
6
377
±186
192
270
831
±281
551
Sr
Rb
0·0
34·9
±18·9
16·0
0·0
28·5
±19·6
8·8
0·0
3·2
±1·6
1·6
/86Sr
87
211
230
±10
221
202
226
±12
214
205
240
±18
223
Na+K
K/
0·38
0·49
±0·06
0·43
0·39
0·50
±0·06
0·45
0·37
0·54
±0·09
0·45
(Na+K)
85
19
60
±20
39
20
79
±29
49
19
152
±66
B
0·50
0·19
0·35
±0·08
0·27
0·25
0·45
±0·10
0·35
0·41
0·59
±0·09
25·8
35·6
±4·9
30·7
26·3
35·7
±4·7
31·0
13·8
32·0
±9·1
22·9
%
mg-no. Quartz
Feldspars
3·5
10·7
±3·6
7·1
3·6
14·2
±5·3
8·9
3·4
27·4
±12·0
15·4
59·5
64·9
±2·7
62·2
57·1
63·2
±3·1
60·1
55·9
67·5
±5·8
61·7
minerals % %
Mafic
NUMBER 7
Plutonic bodies (numbered as in Tables 1 and 2) selected for average calculations: Mg group: 1, 3–15, 27, 29, 31–34, 40, 42, 43, 47; Mg–Fe group: 16–26, 35, 37,
41, 46, 51–53, 55, 58, 60, 62–64, 68; Fe group: 2, 36, 50, 54, 56, 57, 61, 65, 67. Enclaves and Sädelhorn ferriferous dyke discarded from calculations. Calculations
performed from individual sample compositions, except for the Gotthard plutons. Other explanations as in Tables 1 and 2.
0·08
0·41
0·24
±0·16
72·90
±3·25
Average
0·70
±0·51
3·11
4·44
±0·67
3·78
Na2O
VOLUME 40
Ferriferous (Fe) group (9 plutonic bodies/111 analyses)
0·09
0·48
0·29
±0·20
71·97
±3·89
Average
Magnesian–ferriferous (Mg–Fe) group (25 plutonic bodies/306 analyses)
Low-mg-number suite
73·39
1r
0·81
0·47
±0·34
67·34
±6·04
Average
Magnesian (Mg) group (24 plutonic bodies/338 analyses)
High-mg-number suite
SiO2
Table 3: Average chemical and mineralogical compositions of the three plutonic groups of the External Crystalline Massifs
JOURNAL OF PETROLOGY
JULY 1999
DEBON AND LEMMET
EVOLUTION OF Mg/Fe RATIOS IN PLUTONIC ROCKS
Fig. 2. Plot of Late Variscan plutonic rocks from the ECM on the R1–R2 diagram of de la Roche et al. (1980). Data available on the Journal
of Petrology Web site. For comparison, Late Variscan plutonic rocks from the Pyrenees are also shown (Debon et al., 1991). Mafic igneous enclaves
included in both compilations.
deformation. Several Late Variscan plutonic bodies, very
different from each other, occur in this massif.
The heterogeneous, biotite and/or amphibole Valmasque granite (~7 km2), loaded with xenoliths of country
rocks and mafic or ultramafic enclaves, was interpreted
as an anatectic body, linked to the last stage of migmatite
formation (Faure-Muret, 1955; Bogdanoff et al., 1991).
(1) The Malinvern–Argentera metamonzonites (Lombardo et al., 1997; B. Lombardo, personal communication, 1997) crop out as sub-vertical sheets up to
1 km long, dykes and plugs, emplaced into paragneisses
and amphibolites. These porphyritic rocks are variably
foliated and recrystallized, and were previously described
as pyroxene-bearing porphyritic gneisses or biotite and
amphibole augen migmatites. The abundance of pyroxene constitutes a distinctive feature (Table 1). Chemical
data show that these rocks are dark, subalkaline and
magnesian syenites and quartz syenites. Their dating,
through SHRIMP analysis on zircon, yielded an age of
337 ± 8 Ma.
(2) The Central (or Argentera) granite (~50 km2) is a
homogeneous pluton, the emplacement of which postdates the three migmatitic stages (Faure-Muret, 1955;
Bogdanoff et al., 1991). It mainly consists of a massive,
coarse-grained, porphyritic in places, garnet-bearing twomica leucocratic monzogranite, which is subalkaline and
ferriferous. Fine-grained leucogranites and microgranites
occur at its margins. A poorly constrained Rb–Sr wholerock isochron gave an age of 295 Ma (recalculated
with new constants) for this Central granite (Ferrara &
Malaroda, 1969).
Pelvoux massif
The Pelvoux massif (Figs 5 and 6) is a roughly circular
body, ~40 km in diameter. It comprises a crystalline
basement divided into an inner highly migmatitic domain
(‘noyau’) and an outer mesozonal domain (‘cortex’) (Le
Fort & Pêcher, 1971, 1981; Le Fort, 1973; Barféty,
Pêcher et al., 1984). The inner domain mainly consists
of gneisses and acidic migmatites including some amphibolites and old blastomylonites, overlain by banded
amphibolites and amphibole-bearing augen gneisses. It
also includes orthogneissic bodies (e.g. Crupillouse), probably derived from porphyritic granites emplaced in Early
Palaeozoic times. At least two migmatitic events occurred,
the latter, of the low-pressure–high-temperature type
(cordierite), pre-dating (or accompanying; Grandjean et
al., 1996) the intrusion of the Late Variscan granites. In
addition to detrital formations, the less metamorphic
outer domain is composed of carbonaceous mica schists,
marbles and leptynitic–amphibolitic formations.
The Pelvoux massif displays a net of variously orientated fractures, locally underlined by pinched remnants
of the Mesozoic sedimentary cover. Its particular isodiametric shape and complex structure when compared
with the other ECM might be accounted for by the
importance of Alpine sub-meridian and oblique shortening (Sue et al., 1997).
More than 20 Late Variscan plutonic bodies have been
distinguished in the Pelvoux massif, covering about onethird of its area (Fig. 6). They occur as intrusions of
various sizes (1–40 km2), emplaced into the inner domain
or cross-cutting the contact between the inner and the
outer domain. Based on data from the literature (Le Fort,
1973, and references therein; Barféty et al., 1976, 1989;
Debelmas et al., 1980; Barféty, Pêcher et al., 1984; de
Boisset, 1986; Banzet, 1987; Costarella, 1987; G. Banzet,
unpublished data, 1985–1991), their main field and petrographic characteristics are summarized hereafter and in
Table 1, following our two-fold partition into magnesian
1161
1162
2·25
99·74
Total
±0·14
±0·29
1·17
±0·03
0·06
±0·65
4·67
±0·97
3·68
±0·24
0·29
±0·40
0·96
±0·04
0·02
±0·64
(or H2O)
LOI
13·85
±1·26
99·82
1·53
0·14
4·51
4·31
0·93
0·68
0·04
1·29
14·85
0·21
±0·47
99·93
±2·26
2·10
±0·07
0·31
±0·48
4·66
±0·71
3·08
±0·76
2·00
±0·14
1·39
±0·01
0·08
±0·58
3·71
±0·24
15·42
±0·04
0·50
±3·85
66·70
2
18
±1·02
99·67
±0·25
1·18
±0·11
0·29
±0·44
4·26
±0·52
3·85
±1·75
2·95
±0·37
1·72
±0·01
0·09
±1·12
4·43
±1·58
15·53
±0·20
0·62
±2·69
64·77
2
22
99·78
0·78
0·13
4·74
3·17
1·61
0·56
0·03
2·30
14·35
0·26
71·85
1
26
±0·86
99·33
±0·27
0·73
±0·15
0·14
±0·97
4·51
±0·50
4·16
±0·62
1·51
±0·65
0·78
±0·02
0·02
±0·83
1·77
±0·79
14·91
±0·21
0·27
±2·52
70·53
6
27
99·67
1·30
0·17
4·04
3·75
2·65
1·66
0·05
3·30
16·49
0·48
65·78
1
29
±0·05
99·79
±0·79
0·74
±0·06
0·05
±0·45
5·15
±0·16
4·09
±0·23
0·56
±0·50
0·62
±0·02
0·01
±0·55
1·53
±1·73
14·06
±0·14
0·23
±3·93
72·74
3
31
±1·67
99·44
±0·17
0·75
±0·03
0·09
±0·38
4·92
±0·15
3·59
±0·40
1·31
±0·25
0·56
±0·02
0·06
±0·73
2·32
±0·56
14·01
±0·09
0·29
±2·51
71·55
3
36
±1·36
102·95
±0·72
1·18
±0·06
0·31
±0·62
4·85
±1·07
3·52
±1·18
1·03
±0·77
0·73
±0·03
0·04
±4·41
4·21
±1·46
14·42
±0·36
0·39
±5·50
72·28
6
41
±0·31
99·52
±0·71
1·60
±0·23
0·97
±1·93
5·54
±0·62
2·14
±1·27
5·96
±8·08
10·08
±0·02
0·10
±0·95
6·19
±3·03
11·54
±0·32
0·87
±6·08
54·52
7
28
99·28
1·33
0·66
4·84
2·85
4·97
7·02
0·12
6·42
13·09
0·68
57·30
1
30
±0·41
99·04
±0·45
1·28
±0·37
0·50
±0·81
6·61
±0·55
3·55
±1·87
3·04
±2·38
3·62
±0·04
0·06
±2·87
5·10
±1·22
15·92
±0·51
1·05
±7·93
58·33
2
32
±0·49
100·01
±1·01
1·64
±0·14
0·26
±1·71
3·02
±0·56
3·30
±3·00
5·42
±3·02
3·89
±0·06
0·11
±2·68
6·67
±0·98
15·76
±0·39
0·95
±8·84
58·99
6
38
±1·91
99·36
±1·08
1·39
±0·20
0·32
±2·31
4·19
±1·87
3·77
±1·25
2·99
±1·40
1·62
±0·12
0·15
±4·02
6·12
±1·30
15·74
±0·63
0·87
±10·80
62·20
3
39
99·46
1·69
0·47
5·73
3·19
3·02
2·98
0·06
4·99
16·19
1·13
60·01
1
40
±2·85
99·73
±0·31
0·72
±0·11
6·21
±0·65
2·96
±1·17
4·59
±1·28
3·40
±0·02
0·08
±1·30
4·62
±1·62
15·82
±0·23
0·85
±4·38
60·49
4
42
NUMBER 7
P2O5
K2O
Na2O
0·24
±0·10
71·33
1
15
Intermediate and mafic rocks
VOLUME 40
CaO
MgO
MnO
Fe2O3∗
Al2O3
TiO2
±1·41
72·56
7
n:
SiO2
8
body:
Plutonic
Felsic rocks
Table 4: Average chemical and mineralogical compositions of selected plutonic rocks from the External Crystalline Massifs
JOURNAL OF PETROLOGY
JULY 1999
417
1163
Zr
Y
V
U
Th
Ta
Sr
Rb
Ni
Nb
4·47
Hf
±38
149
±11·1
27·8
±6·8
21·5
±13·18
6·75
±25·5
24·6
±1·72
2·42
±41
102
±100
237
±0·7
1·7
±5·8
17·3
±0·78
3·0
Cr
±251
7
n:
Ba
8
body:
Plutonic
1
101
9·3
25·0
7·51
19·1
437
195
<10
9·6
14·0
1085
15
Felsic rocks
1272
1400
198
±1·6
25·2
±14·1
62·0
±3·28
6·41
±3·4
22·4
±337
468
±55
215
±2·8
13·0
20·4
295
±10·1
26·5
±21·2
81·5
±4·06
6·12
±12·5
23·7
±308
548
±13
171
±18·4
16·5
19·5
25·5
±7·1
20·0
±492
2
22
±2·8
±191
2
18
1
26
158
22·9
20·2
2·95
16·0
1·56
137
213
3·1
10·8
4·76
6·7
563
1124
±90
146
±4·1
9·2
±13·0
21·4
±9·67
6·07
±8·8
20·7
±1·27
1·50
±140
500
±54
180
±11·7
12·7
±7·4
12·6
±1·49
3·85
±28·9
28·9
±530
6
27
1
29
213
13·1
48·4
8·18
26·7
456
173
12·9
14·0
38·6
944
572
±82
175
±3·7
8·2
±15·9
18·4
±2·17
9·33
±10·5
49·3
±0·07
1·44
±203
233
±100
244
±6·9
12·3
±2·9
16·9
±0·24
6·41
±13·4
23·4
±566
3
31
511
±67
187
±12·0
39·0
±13·0
27·7
±0·76
6·73
±9·8
27·9
±0·80
2·38
±30
113
±125
284
±1·5
0·8
±4·6
14·3
±0·94
5·96
<14
±145
3
36
477
±102
144
±17·8
20·3
±38·4
28·2
±5·2
15·5
±98
107
±60
256
±26·7
59·5
±4·3
14·7
±10·5
31·8
±392
6
41
2058
±167
233
±4·0
16·8
±33·1
109·1
±5·06
9·93
±19·3
32·5
±223
477
±101
249
±453·4
311·1
±8·8
20·4
±648·1
686·4
±1526
7
28
1
239
18·3
111·0
9·15
48·7
579
185
198·0
18·8
544·0
3684
30
2588
545
84·2
±177
6
38
±58
502
±1·4
21·2
±54·3
90·8
±2·15
7·93
±37·4
47·0
±0·75
2·15
±600
953
±42
246
±36·3
74·5
±8·7
25·5
±0·14
14·05
±43
212
±6·0
36·8
±64·7
129·7
±2·18
3·43
±4·7
10·3
±0·35
1·08
±98
285
±92
179
±45·0
24·2
±4·9
13·5
±1·26
5·58
±96·2 ±104·6
145·0
±1386
2
32
Intermediate and mafic rocks
519
±66
301
±11·1
48·0
±85·5
98·3
±2·62
4·80
±8·8
13·1
±0·37
1·43
±33
158
±171
372
±12·8
6·7
±4·0
17·0
±2·08
7·28
<14
±668
3
39
1
349
40·0
109·0
9·60
25·9
604
273
39·0
22·0
111·0
1971
40
2528
±27
320
±3·8
19·5
±36·1
126·3
±9·98
29·75
±31·3
93·0
±71
685
±40
384
±9·5
28·5
±38·7
84·0
±323
4
42
DEBON AND LEMMET
EVOLUTION OF Mg/Fe RATIOS IN PLUTONIC ROCKS
1164
2·07
±0·03
0·31
±0·04
2·03
±0·14
4·07
±0·59
41·9
±11·2
7·75
±1·95
1·61
±0·52
5·69
±1·17
48·7
±11·6
90·7
±24·2
18
2
2·07
±0·71
0·32
±0·10
2·20
±0·98
4·56
±1·94
49·4
±27·9
9·14
±5·84
1·85
±0·79
6·41
±3·49
57·1
±26·4
108·8
±61·6
22
2
0·37
2·25
0·35
2·10
0·91
4·15
0·75
4·78
0·80
6·14
28·9
7·58
67·1
33·3
26
1
29·9
±12·7
58·0
±27·4
6·40
±3·90
23·1
±14·8
4·03
±2·54
0·95
±0·50
2·66
±1·96
0·35
±0·25
1·78
±1·04
0·33
±0·22
0·85
±0·42
0·12
±0·06
0·84
±0·33
0·13
±0·03
27
6
0·20
1·11
0·17
1·25
0·56
2·92
0·55
4·57
1·36
5·82
34·6
9·57
85·8
43·9
29
1
39·9
±15·4
70·8
±32·7
6·80
±3·35
22·7
±13·2
3·47
±2·30
0·63
±0·43
2·06
±0·95
0·27
±0·17
1·44
±0·82
0·27
±0·14
0·76
±0·35
0·11
±0·04
0·84
±0·21
0·14
±0·03
31
3
26·0
±18·7
56·1
±40·2
41
6
4·32
±0·32
0·73
±0·06
1·37
±1·32
0·19
±0·17
1·54
±1·47
(28)
26·2
±(34)
±29·3
6·67
5·01
±1·47 ±3·32
0·92
0·75
±0·09 ±0·65
4·19
±3·06
0·89
±0·09
3·24
±2·62
38·5
±8·3
81·3
±18·5
36
3
56·4
±30·7
120·7
±58·1
15·07
±7·19
66·0
±33·4
12·67
±5·82
2·51
±0·86
7·32
±2·88
0·84
±0·25
3·82
±0·92
0·63
±0·14
1·67
±0·43
0·20
±0·07
1·33
±0·40
0·20
±0·07
28
7
0·23
1·60
0·22
1·64
0·68
3·82
0·78
5·84
1·94
9·15
51·3
12·73
116·3
60·9
30
1
64·8
±6·3
151·1
±19·4
18·39
±0·16
76·1
±8·9
13·38
±1·15
2·68
±1·02
7·04
±0·96
0·95
±0·06
4·77
±0·07
0·80
±0·04
1·99
±0·11
0·26
±0·01
1·72
±0·45
0·26
±0·08
32
2
37·3
±14·3
83·8
±22·8
39
3
3·20
±0·63
0·51
±0·11
0·94
±0·19
4·10
±0·53
0·67
±0·05
1·03
±0·12
(29)
(32)
±(29)
±(9)
6·52
7·87
±1·68 ±0·31
1·67
1·23
±0·44 ±0·66
35·9
±11·9
75·4
±26·7
38
6
0·45
3·00
0·50
3·00
1·36
6·80
1·50
11·00
3·07
16·40
89·1
23·00
168·0
75·0
40
1
1·13
±0·19
0·10
±0·00
3·00
±1·63
<1
46·5
±6·0
7·75
±4·12
2·15
±0·58
46·8
±7·5
103·5
±14·0
42
4
Standard deviations at ±2r. Complete data set available on the Journal of Petrology Web site. Plutonic bodies numbered as in Tables 1 and 2.
Sources of data: (a) felsic rocks: 8, Combeynot granite, coarse-grained type [F. Debon, unpublished data, 1997; samples mainly collected by Costarella (1987) but
reanalysed]; 15, 18 and 22, Rochail, Bourg and Orgières granites, respectively (G. Banzet, unpublished data, 1985–1986); 26, Turbat–Lauranoure granite ( F. Debon,
unpublished data, 1997); 27, 29 and 31, Sept Laux, Saint Colomban and La Lauzière granites, respectively (Debon et al., 1998); 36, Mont Blanc granite, central
type (Bussy, 1990); 41, Vallorcine granite, porphyritic type (Brändlein et al., 1994); (b) intermediate and mafic rocks: 28, vaugneritic–durbachitic enclaves from the
Sept Laux granite (Lacheny, 1995; Debon et al., 1998); 30, mafic enclave from the Saint Colomban granite (Debon et al., 1998); 32, La Lauzière quartz syenite
(Debon et al., 1998); 38 and 39, magnesian–ferriferous and ferriferous enclaves, respectively, from the Mont Blanc granite (Bussy, 1990); 40, Pormenaz monzonite
(Bussy et al., 1998); 42, Giuv syenite (Schaltegger et al., 1991).
Analytical methods: 8, 15, 18, 22, 26–32: emission spectrometry (ICP ± ICP-MS); 36, 38, 39: XRF and INAA (instrumental neutron activation analysis); 41: XRF and
ICP; 40: XRF and ICP-MS; 42: XRF.
Other explanations as in Table 2.
0·07
0·74
0·70
1·49
1·95
0·65
2·69
14·1
29·9
16·6
15
1
NUMBER 7
Lu
Yb
Tm
29·3
±12·8
63·1
±27·2
6·23
±2·14
22·5
±7·4
4·68
±0·98
0·53
±0·21
4·05
±0·50
0·67
±0·14
4·12
±1·34
0·88
±0·36
2·65
±1·00
0·45
±0·22
3·20
±1·62
0·53
±0·27
8
7
Intermediate and mafic rocks
VOLUME 40
Er
Ho
Dy
Tb
Gd
Eu
Sm
Nd
Pr
Ce
La
Plutonic
body:
n:
Felsic rocks
Table 4: continued
JOURNAL OF PETROLOGY
JULY 1999
DEBON AND LEMMET
EVOLUTION OF Mg/Fe RATIOS IN PLUTONIC ROCKS
Fig. 3. Plot of Late Variscan plutonic bodies (average compositions) from the ECM on the mg-number–B diagram of Debon & Le Fort (1988).
Data from Table 2. Numbering of the plutonic bodies as in Tables 1 and 2. The B parameter, expressed in gram-atoms × 103 in 100 g of rock,
is proportional to the weight content in mafic minerals (de la Roche, 1964). This content, in weight percent, is calculated by dividing B by 5·55.
Fe is total iron. For comparison, the two fields defined by the Pelvoux plutons are shown in the different diagrams. Reference system of mean
compositions: ad, adamellite (~monzogranite); gd, granodiorite; go, gabbro; gr, granite (~syenogranite); qd, quartz diorite; to, tonalite. The
‘critical line’ passing through the gr, ad, gd, to, qd and go reference points is used to distinguish between magnesian (Mg), magnesian–ferriferous
(Mg–Fe) and ferriferous (Fe) plutonic rocks (according to the positions of their plots above, close to and below this line, respectively).
1165
JOURNAL OF PETROLOGY
VOLUME 40
Fig. 4. Geological sketch map of the Argentera massif [modified from
Bogdanoff et al. (1991) and von Raumer et al. (1993)]. (See Table 2 for
chronological data.)
NUMBER 7
JULY 1999
and magnesian–ferriferous bodies (Figs 3 and 6; Table 2).
Apart from the Colle Blanche–Moutières quartz monzonite, the Mg group clusters around a B value of 52 gatoms × 103 (i.e. 9·3% of mafic minerals), whereas the
Mg–Fe group displays a bimodal distribution around B
values of 94 and 35 g-atoms × 103 (i.e. 17% and 6·3%
of mafic minerals), and includes the most leucocratic
plutons. In addition to predominant subalkaline plutonic
bodies, at times with alkaline affinity (Combeynot; Costarella, 1987), each group contains some peraluminous
plutons (Berches, Claphouse, Cray, Grun de Saint Maurice, Pétarel, Riéou Blanc). The Mg/Fe typology thus
defined reveals a crude concentrically zoned arrangement
of the two groups, with the magnesian plutons located
at the periphery of the massif (Fig. 6). However, because
the overall geometry was disturbed by important Alpine
sub-meridian and oblique shortening (Sue et al., 1997),
caution is urged in interpreting such a zonation (Bonin
et al., 1993).
Because of Alpine tectonic and metamorphic events,
the alteration of biotite and plagioclase to chlorite and
sericite is common in the Pelvoux granites. Except in
some plutons [Combe Guyon, Grun and Péou de Saint
Maurice, Quatre Tours, Riéou Blanc (fine-grained type),
Berches, Cray, Pétarel], muscovite is rare or completely
absent and the distinction between primary and secondary muscovite often uncertain.
Mafic igneous enclaves occur in most of the granites,
although these are highly variable in proportion. On the
whole, their abundance is greater in the magnesian
granites. Vaugnerite–durbachite enclaves are restricted
to some magnesian plutons, specifically Colle Blanche–
Moutières, Péou de Saint Maurice, Quatre Tours and
Rochail (Banzet, 1987; Barféty et al., 1989).
Magnesian plutonic bodies
Fig. 5. Sketch map of the Pelvoux, Belledonne, Grandes Rousses,
Mont Blanc and Aiguilles Rouges massifs [after Vivier et al. (1987)].
The average compositions of these plutonic bodies plot
in the magnesian field of the mg-number–B diagram,
markedly above the ‘critical line’ (Fig. 3).
(3) Bans granite (~4 km2): a medium-grained, weakly
foliated, dark monzogranite with biotite ± amphibole,
rich in mafic igneous enclaves.
(4) Claphouse granite (~9 km2): a fine-grained, cataclastic, dark syenogranite with biotite ± muscovite,
highly chloritized and sericitized.
(5, 6) Colle Blanche–Moutières granite and quartz
monzonite (~10 km2): a complex made up of two similar,
extremely heterogeneous intrusions, separated by a screen
of gneisses and migmatites. Two types of rock occur: a
fine-grained, foliated, biotite ± muscovite monzogranite
(5), mainly located at the margins of the intrusions, and
a dominant complex of mafic and intermediate rocks (6),
massive or foliated, mainly composed of medium-grained
quartz monzonite with biotite and amphibole. Both types
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Fig. 6. Geological sketch map of the Pelvoux massif [modified from Le Fort & Pêcher (1971, 1981), Barféty, Pêcher et al. (1984) and G. Banzet
(unpublished data, 1985–1991)]. (See Table 2 for chronological data.)
enclose mafic enclaves (especially of the vaugnerite type).
(7) Combe Guyon (or Alfrey) granite (~2 km2): a
coarse-grained, quartz-rich, two-mica monzogranite,
with a porphyritic tendency.
(8) Combeynot granite (~20 km2): a roughly concentrically zoned subvolcanic complex comprising two
granitic units affected by a strong brittle deformation.
The inner unit forms the main part of the complex. It
is composed of a massive, homogeneous, coarse-grained
and porphyritic, alkali-feldspar syenogranite, with highly
chloritized biotite (Costarella, 1987). As shown by seven
new analyses, this granite is magnesian, although with
some alkaline affinities. Its recent U–Pb zircon dating
yielded an upper intercept age of 312 ± 7 Ma (Cannic
et al., 1998). Pb loss and inherited components hampered
precise age determination. This age agrees with those
obtained by the total-lead zircon method (310 ± 14 Ma,
320 ± 13 Ma; Barbieri, 1970). In addition to the finegrained leucocratic syenogranite making up the outer
unit, sheets and dykes of rhyolites and microgranites
occur outside the complex. These highly differentiated
rocks are ferriferous.
(9) Graou granite (~1 km2): a porphyritic biotite monzogranite with K-feldspar megacrysts up to 10 cm.
(10) Grun de Saint Maurice granite (~6 km2): is composed of several bodies of porphyritic, two-mica monzogranite, strongly deformed in many places.
(11) Péou de Saint Maurice granite (~4 km2): a
medium- and fine-grained, generally foliated, two-mica
monzogranite, rich in mafic enclaves (vaugnerites, etc.).
(12) Quatre Tours granite (~2 km2): a fine-grained,
light-coloured, two-mica monzogranite, enclosing mafic
enclaves (durbachites, etc.).
(13, 14) Riéou Blanc granite (~10 km2): this is composed
of two granitic units. The main unit, a coarse-grained,
generally porphyritic, weakly foliated, biotite ± muscovite monzogranite (13), is cross-cut by a fine-grained,
locally microgranular, two-mica syenogranite (14).
(15) Rochail granite (~34 km2): comprises two types
of granites. The dominant type is a medium-grained,
weakly foliated, biotite ± rare muscovite monzogranite.
To the north, this merges into a coarser and darker
foliated biotite monzogranite with a porphyritic tendency.
Both monzogranites enclose mafic igneous enclaves, especially of the durbachite type (the so-called ‘syénite du
Lauvitel’) (de Boisset, 1986; Banzet, 1987). A recent
U–Pb zircon dating has yielded an age of 343 ± 11 Ma
for the dominant type (Guerrot, 1998, and personal
communication, 1998; C. Guerrot & F. Debon, in preparation), whereas an age of 331 ± 32 Ma (2r) was
ascribed to the northern type from a questionable Rb–Sr
whole-rock isochron (initial 87Sr/86Sr ratio of 0·7047 ±
0·0013) (Demeulemeester, 1982).
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Magnesian–ferriferous plutonic bodies
The average compositions of these plutonic bodies plot
close to or slightly above the ‘critical line’ of the mgnumber–B diagram (Fig. 3).
(16) Bérarde–Promontoire granite (~40 km2): a fineto coarse-grained, massive, light monzogranite with biotite ± muscovite.
(17) Berches granite (~4 km2): a fine-grained (aplitic),
two-mica syenogranite.
(18) Bourg granite (~6 km2): a fine-grained, homogeneous, variably altered, biotite ± amphibole monzogranite, rich in mafic minerals. Small microgranular
enclaves lie locally along the foliation.
(19) Cray granite (~3 km2): a leucocratic, fine-grained
syenogranite, with muscovite, biotite, cordierite, ± garnet.
(20) Etages granite (~30 km2): a coarse-grained, variably porphyritic, foliated, biotite ± rare muscovite
syenogranite. Fine-grained mafic enclaves occur locally.
(21) Gioberney granite (~5 km2): coarse-grained, porphyritic in places, strongly deformed, biotite ± muscovite
syenogranite.
(22) Orgières granite (~4 km2): fine-grained, weakly
foliated, biotite ± amphibole monzogranite, rich in mafic
minerals, enclosing a variety of enclaves.
(23) Pelvoux–Pic de Clouzis (or Ailefroide) granite
(~20 km2): a coarse-grained, biotite ± rare muscovite
syenogranite.
(24) Pétarel granite (~5 km2): a medium-grained, porphyritic, weakly foliated, quartz-rich, two-mica syenogranite.
(25) Pic de Valsenestre (~6 km2): a porphyritic, commonly deformed and altered, biotite ± rare muscovite
monzogranite, rich in mafic minerals.
(26) Turbat–Lauranoure granite (~33 km2): a coarsegrained, variably porphyritic and foliated, biotite ±
muscovite syenogranite, enclosing rare microgranular
enclaves, and recently dated at 302 ± 5 Ma by U–Pb
on zircon (Guerrot, 1998, and personal communication,
1998; C. Guerrot & F. Debon, in preparation).
Belledonne massif
The Belledonne massif (Fig. 5) is an elongate composite
body extending N 30°E for ~100 km. It comprises three
domains (terranes) with distinct lithological, metamorphic, tectonic and magmatic features: an outer domain and two inner domains, namely, a southwestern
inner domain and a northeastern one. These domains
were juxtaposed during Early Carboniferous times as a
result of late-orogenic strike-slip faulting (Ménot, 1988a,
1988b). Late Variscan granites are restricted to the northeastern inner domain (Fig. 7). They intrude a gneissic,
Fig. 7. Geological sketch map of the northeastern domain of the
Belledonne massif [modified from Vivier et al. (1987)]. (See Table 2
for chronological data.)
amphibolitic and migmatitic basement displaying a polyphase metamorphic evolution and including lenses of
Early Palaeozoic orthogneisses (~490 Ma; Barféty et al.,
1999). Green and black schists associated with metavolcanic rocks of uncertain age also occur in this domain
(Vivier et al., 1987).
The Late Variscan granites form three almost linear
intrusions (Vivier et al., 1987; Debon et al., 1998, and
references therein). From west to east, they are the Sept
Laux, Saint Colomban and La Lauzière plutons. The
three plutons are subalkaline, with either calc-alkaline
(Saint Colomban) or alkaline (La Lauzière) affinities, and
magnesian. Their dating, through lead-evaporation on
single zircon, yielded ages of 335 ± 13 Ma (Sept Laux
granite), 343 ± 16 Ma (Saint Colomban granite), 343
± 14 Ma (mafic enclave of the Saint Colomban granite),
and 341 ± 13 Ma (La Lauzière granite) (Debon et al.,
1998). Previously (Demeulemeester, 1982), an age of 322
± 43 Ma (2r) was obtained for the Sept Laux granite
from an Rb–Sr whole-rock isochron (initial 87Sr/86Sr ratio
of 0·7066 ± 0·0005).
(27, 28) Sept Laux granite (~95 km2): a concentrically
zoned pluton, made up of fine- and medium-grained,
porphyritic in places, foliated, biotite ± muscovite granites (27), mainly of the light monzogranitic composition
(Table 1). Mafic igneous enclaves (vaugnerites, durbachites, etc.) (28), with amphibole and biotite, are locally
abundant in the outer zone. The average composition
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corresponds to a quartz monzonite, rich in mafic minerals.
(29, 30) Saint Colomban granite (~60 km2): the main
rock type (29) is a medium-grained, porphyritic, often
strongly foliated and partly recrystallized (‘orthogneissic’),
dark-coloured monzogranite, with biotite ± rare amphibole or muscovite. Mafic igneous enclaves (30), with
amphibole and biotite, are locally abundant in its southern part. The average composition corresponds to a
monzonite, rich in mafic minerals.
(31, 32) La Lauzière granite and quartz syenite (~25
km2): a heterogeneous pluton comprising two groups of
medium- to coarse-grained, massive or foliated rocks.
The dominant group consists of biotite ± secondary
muscovite syenogranites (31). Quartz syenites rich in
biotite and amphibole predominate in the other group
(32).
Grandes Rousses massif
The small Grandes Rousses massif (Figs 5 and 8) extends
N 20°E for some 25 km. It comprises a crystalline
basement composed of gneisses, locally migmatitic or of
the augen-type, mica schists, schists and rare amphibolites
(Giorgi, 1979; Bogdanoff et al., 1991). In addition to these
formations of highly variable metamorphic grade, Late
Carboniferous sediments also occur.
Late Variscan intrusions (Alpetta, Roche Noire–La
Fare) are common in the western part of the massif
(Giorgi, 1979; Bogdanoff et al., 1991). They are subalkaline and magnesian. Because the Alpetta intrusion is
probably part of the Saint Colomban pluton (see Belledonne massif; Bogdanoff et al., 1991), it would be ~340
my old.
(33) Alpetta granite (>~8 km2): a medium-grained,
porphyritic, strongly foliated and partly recrystallized,
dark monzogranite with biotite ± rare muscovite. Intercalations of amphibole and biotite ‘gneisses’ probably
represent deformed mafic igneous enclaves.
(34) Roche Noire–La Fare granite (> ~7 km2): a
composite complex cross-cutting the Alpetta pluton,
made up of two elongate intrusions. Roche Noire consists
of medium-grained, foliated, locally porphyritic, lightcoloured monzogranites, with biotite ± rare muscovite.
Mafic igneous enclaves are common. The La Fare
monzogranite differs from Roche Noire by less biotite and
the presence of microgranular textures. It is a subvolcanic
body that merges upwards into a metavolcanic complex
(Fig. 8).
Mont Blanc massif
The Mont Blanc massif (Figs 5 and 9), extending N 40°E
for 55 km, comprises three main formations (von Raumer,
Fig. 8. Geological sketch map of the Grandes Rousses massif [modified
from 1:50 000 geological maps of France, and Giorgi (1979)].
1987; Bussy, 1990; Bogdanoff et al., 1991, and references
therein): (i) a N 20–25°E-trending subvertical crystalline
basement, mainly composed of gneisses, migmatites, mica
schists with intercalations of eclogites, orthogneisses dated
at 453 ± 3 Ma and locally remelted at 317 ± 2 Ma
(Bussy & von Raumer, 1993), and Devonian–Dinantian(?)
cordierite migmatites; (ii) a Late Variscan rhyolitic
formation, dated at 307 ± 2 Ma (Bussy & von Raumer,
1993); (iii) two Late Variscan plutonic bodies
(Montenvers, Mont Blanc).
(35) The Montenvers granite (~8 km2) is an elongate
body, composed of foliated two-mica(?) monzogranites
enclosing microgranular enclaves (Bussy, 1990). It is
a peraluminous body (Bussy & von Raumer, 1993),
magnesian–ferriferous (according to only one analysis),
dated at 307 + 6/– 5 Ma by U–Pb on zircon (Bussy &
von Raumer, 1993).
(36–39) The large Mont Blanc granite (~225 km2)
is mainly made up of a medium- to coarse-grained,
porphyritic, generally foliated, biotite monzogranite, with
relics of amphibole in places (central type; 36) (Marro,
1986; Bussy, 1990). Outwards, the central type merges
locally into an even-grained, quartz-rich, light monzogranite (border type; 37), up to 3–4 km wide. In addition,
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age of 304 ± 3 Ma for the central monzogranite, and
of 306 + 5/– 3 Ma and 305 ± 2 Ma for the mafic
enclaves (Bussy, 1992; Bussy & von Raumer, 1993).
Aiguilles Rouges massif
The Aiguilles Rouges massif (Figs 5 and 9) extends for
~45 km, parallel to the Mont Blanc massif. It is separated
from the latter by the ‘Chamonix–Martigny zone’ made
up of deformed Mesozoic formations. Following Bogdanoff et al. (1991, and references therein), the composition and tectonic–metamorphic evolution of the
crystalline basement are roughly similar in both massifs.
In addition, the Aiguilles Rouges massif comprises sedimentary and volcanic rocks of Viséan and Westphalian
ages (Bellière & Streel, 1980) and a few Late Variscan
plutonic bodies mainly represented by the Pormenaz and
Vallorcine intrusions.
(40) The Pormenaz monzonite (~2 km2) intrudes
gneisses and metagraywackes (Bussy et al., 1998). This
funnel-shaped intrusion is made up of foliated, porphyritic
rocks, rich in amphibole, biotite and titanite, enclosing
durbachitic enclaves. Leucocratic granites and aplitic
dykes occur at the periphery. Subalkaline and magnesian
quartz syenites, rich in mafic minerals, may be the main
rock type (only one analysis). They were dated at 332
± 2 Ma by U–Pb on zircon (Bussy et al., 1998).
(41) The Vallorcine granite (~10 km2) crops out as an
elongate body resembling the Montenvers granite (Fig. 9).
It mainly consists of a coarse-grained, massive or foliated,
porphyritic, dark syenogranite, with biotite, muscovite,
± rare andalusite or sillimanite, enclosing fine-grained
biotite-rich igneous enclaves (Poncerry, 1981; Brändlein
et al., 1994). Aplitic granites and cordierite-bearing
leucogranites occur locally, in particular at the periphery
of the intrusion. This peraluminous and magnesian–
ferriferous granite was dated by U–Pb on zircon and
monazite at 307 ± 2 Ma (Bussy, 1995).
Fig. 9. Geological sketch map of the Mont Blanc and Aiguilles Rouges
massifs [modified from von Raumer et al. (1993)]. (See Table 2 for
chronological data.)
conspicuous dykes of leucocratic granites and aplites
occur. Mafic igneous enclaves are common in the central
type. They were divided by Bussy (1990) into two groups,
a ‘magnesian’ group with biotite and amphibole (38),
the other ‘ferriferous’ with biotite alone (39). Quartz
monzodiorites form their main rock type. The Mont
Blanc granite represents an alkali–calcic (subalkaline)
association (Bussy, 1990). The dominant central monzogranite and its mafic enclaves with biotite alone are
ferriferous, whereas the border granite and the biotite–
amphibole enclaves are, on average, magnesian–
ferriferous (Fig. 3). U–Pb datings on zircon yielded an
Aar massif
The Aar massif (Fig. 10) extends N 60°E for ~110 km. It
comprises a pre-Variscan and Variscan polymetamorphic
basement, made up of several units with different metamorphic histories, separated by mylonite zones (Schaltegger, 1990a, 1993; von Raumer et al., 1993, and
references therein). From north to south, they are the
Innertkirchen–Lauterbrunnen, Erstfeld, Guttanen and
southernmost Ofenhorn–Stampfhorn units. The first unit
is migmatitic whereas the others mostly consist of gneisses
and schists, with some calcsilicate and mafic or ultramafic
lenses. Late Variscan volcaniclastic rocks were deposited
during Viséan(?) and Stephanian times (Schaltegger &
Corfu, 1995) while widespread plutonic bodies were
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EVOLUTION OF Mg/Fe RATIOS IN PLUTONIC ROCKS
Fig. 10. Geological sketch map of the Aar and Gotthard massifs [modified from von Raumer et al. (1993), Schaltegger (1994) and Schaltegger
& Corfu (1995)]. Plutonic groups (A, B, C, D) refer to the chronological classification of Schaltegger (1994): A, 331–337 Ma; B, 306–312 Ma;
C, 296—~303 Ma; D, ~290–295 Ma. (See Table 2 for chronological data.)
emplaced. Three successive intrusive suites, compositionally different, have been distinguished in the Aar
massif by Schaltegger (1994, and references therein).
(45) Tödi granite (<1 km2): a fine-grained, strongly deformed, porphyritic granite (Schaltegger & Corfu, 1995),
probably belonging to the same suite (Schaltegger, 1994).
‘Shoshonitic–ultrapotassic suite’
‘High-K calc-alkaline suite’
This suite comprises several small plutonic bodies (Schaltegger et al., 1991; Schaltegger & Corfu, 1992). These
subalkaline and magnesian bodies were dated by U–Pb
on zircon at 334 ± 2·5 Ma (Giuv syenite, Punteglias
granite and diorite; Schaltegger & Corfu, 1992) and 333
± 2 Ma (Tödi granite; Schaltegger & Corfu, 1995).
(42) Giuv syenite (~6 km2): is made up of a coarsegrained central facies and a medium- to fine-grained
marginal facies, both porphyritic and foliated, with biotite
and amphibole. Quartz syenites rich in mafic minerals
could be the main rock type.
(43) Punteglias granite (~10 km2): a lens-shaped intrusion consisting of various porphyritic and foliated
granites and granodiorites. Syenogranites rich in mafic
minerals could be the main rock type.
(44) Punteglias diorite (<1 km2): a complex of some
10 small stocks displaying abundant mingling features,
associated in the field with the Punteglias granite. It
comprises a variety of biotite and amphibole rocks (e.g.
quartz monzonites rich in mafic minerals).
This suite is composed of four small plutonic bodies:
Brunni granite, Düssi diorite, Schöllenen diorite and
Voralp granite (Schaltegger et al., 1991; Schaltegger &
Corfu, 1992). Most of them are subalkaline and magnesian. However, the Schöllenen diorite could be calcalkaline (only one analysis), and the Brunni granite is
magnesian–ferriferous. U–Pb zircon or titanite datings
yielded an age of 308 ± 2 Ma for the Düssi diorite (and,
possibly, to the associated Brunni granite; Schaltegger et
al., 1991), 310 ± 3 Ma for the Schöllenen diorite, and
309 ± 2 Ma for the Voralp granite (Schaltegger &
Corfu, 1992).
(46) Brunni granite (~3 km2): a locally porphyritic,
foliated, light monzogranite, with allanite and epidote.
(47) Düssi diorite (~3 km2): a heterogeneous complex
associated in the field with the Brunni granite, displaying
abundant mingling phenomena, made up of meladiorites
to granodiorites with amphibole or biotite, locally porphyritic. Monzogabbros rich in mafic minerals could
represent the main rock type.
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(48) Schöllenen diorite: a complex of enclaves, probably
reaching km3 size (Schaltegger & von Quadt, 1990),
enclosed within the Central Aar granite, made up of
medium-grained, massive rocks rich in biotite and amphibole, at least in part quartz monzodioritic in composition.
(49) Voralp granite (~10 km2): a subvolcanic pluton
of unknown typology which might belong to the same
suite (Schaltegger & Corfu, 1992).
‘Calc-alkaline to subalkaline granitic suite’
This suite comprises the Central Aar and Gastern granites
(Schaltegger, 1994).
(50–58) The Central Aar granite (~550 km2) is a large
elongate complex made up of coarse- to fine-grained,
massive to strongly foliated, granodiorites to leucocratic
granites, most of them enclosing variable amounts of
mafic enclaves (Schaltegger, 1990a). All of these rocks
contain biotite, whereas garnet and fluorite can occur in
the most leucocratic members. Nine granitic bodies, with
transitional or sharp contacts, have been distinguished
along two main profiles (Grimsel pass and Reuss valley)
[Schaltegger (1990a) and references therein]. On the
basis of increasing SiO2 content (Table 2), they are as
follows (Table 1): (50) Grimsel granodiorite (granodiorite);
(51) Central granite sensu lato (light granodiorite); (52)
Central granite sensu stricto (main type; monzogranite);
(53) Southern border granite (light monzogranite); (54)
Central granite sensu stricto (leucocratic monzogranite);
(55) Aplite, fine-grained leucogranite (dykes and stocks
of leucocratic syenogranite); (56) Northern border granite
(leucocratic monzogranite); (57) Mittagflue granite
(leucocratic monzogranite); (58) Kessiturm aplite (intrusion of leucocratic monzogranite, 0·2 km × 0·8 km
in size). These nine granitic bodies define a typical
subalkaline association. Some are ferriferous (50, 54, 56,
57) and the others magnesian–ferriferous (51–53, 55, 58),
without any gap between them. Five out of these bodies
were dated by U–Pb on zircon or titanite (Schaltegger
& von Quadt, 1990; Schaltegger & Corfu, 1992; review
by Schaltegger, 1994), yielding the following results: 299
± 2 Ma (50), 297 ± 2 Ma (51), 299 ± 2 Ma (52), 298
± 7 Ma (56) and 296·5 ± 2·5 Ma (57), leading to a
mean age of 298 ± 2 Ma for the whole Central Aar
granite (Schaltegger, 1994).
(59) The Gastern granite (~35 km2), at the western
end of the Aar massif, is a porphyritic biotite granite. It
was dated at 303 ± 4 Ma by U–Pb on zircon (Schaltegger,
1993), but its chemical typology is unknown.
NUMBER 7
JULY 1999
by the ‘Urseren–Garvera zone’ made up of deformed
Permo-Carboniferous and Mesozoic formations. Its crystalline basement consists of monotonous gneisses including lenses of amphibolites, eclogites, mafic or
ultramafic rocks, and marbles (von Raumer et al., 1993,
and references therein). Unlike the Aar massif, this basement also comprises widespread orthogneisses (‘Streifengneis’), derived from Late Ordovician intrusions (439 ± 5
Ma; Sergeev & Steiger, 1993). Widespread Late Variscan
plutons occur. They can be divided into two groups on
the basis of structural and chronological data (Oberli et
al., 1981; Schaltegger, 1994, and references therein).
Most of them were dated by U–Pb on zircon.
An older group, composed of strongly deformed
(gneissic) granites, mainly comprises: (60) the Fibbia granite (~8 km2), a coarse-grained porphyritic syenogranite
(299·4 ± 2 Ma; Sergeev et al., 1995); (61) the Gamsboden
granite (~13 km2), a monzogranite similar to the Fibbia
granite (301 ± 2 Ma, Guerrot & Steiger, 1991; 299·4
± 2 Ma, Sergeev et al., 1995); (62, 63) a complex (~40
km2) comprising the Medel monzogranite (303 ± 20
Ma; Grünenfelder, 1962) and the Cristallina granodiorite.
The younger group is made up of massive rocks, nearly
devoid of any deformation. It mainly comprises: (64) the
Cacciola granite (~2 km2), a medium- to fine-grained,
biotite and muscovite monzogranite (292 ± 11 Ma;
Oberli et al., 1981); (65) the Rotondo syenogranite (~26
km2) (294·3 ± 1·1 Ma; Sergeev et al., 1995); (66) the
Sädelhorn diorite, outcropping as a sigmoidal dyke,
~3 km × Ζ0·2 km in size, composed of a fine-grained
quartz-bearing monzodiorite (Table 1) with biotite, epidote and titanite (293 + 5/ – 4 Ma; Bossart et al., 1986);
(67) the Tremola syenogranite (~2 km2), similar to the
Rotondo granite (294·3 ± 1·1 Ma; Sergeev et al., 1995);
(68) the leucocratic Winterhorn monzogranite (~1 km2)
(Oberli et al., 1981).
Judging from average compositions (S. A. Sergeev,
personal communication, 1998), these plutonic bodies are
most probably subalkaline apart from the peraluminous
Cacciola monzogranite and the possibly calc-alkaline
Cristallina granodiorite. They may be divided into a
magnesian–ferriferous group [Fibbia, Medel (?)–
Cristallina, Cacciola, Winterhorn] and a ferriferous group
(Gamsboden, Rotondo, Sädelhorn, Tremola) (Fig. 3).
The fact that this Mg/Fe typology is based on ratios
directly calculated from average compositions makes it
unsteady, and might account for some discrepancies with
previous classifications (see above).
Gotthard massif
DISCUSSION
Two plutonic suites
The Gotthard massif (Fig. 10) extends for ~80 km,
parallel to the Aar massif. It is separated from the latter
The Late Variscan plutonic bodies of the ECM can be
divided into three groups on the basis of their average
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EVOLUTION OF Mg/Fe RATIOS IN PLUTONIC ROCKS
Mg/(Fe + Mg) ratio and mafic mineral content (Fig. 3).
The chemical and mineralogical compositions of the
three groups (Table 3; mafic enclaves and Sädelhorn
dyke excluded from calculations) variably overlap one
another, with a general tendency for an increase of the
Si (and quartz) content and 87Rb/86Sr ratio and a decrease
in Ti, Al, Fe, Mg, Ca, P, Ba, Sr and mafic mineral
content on going from the Mg group to the Fe group,
through the Mg–Fe group. In contrast, chemical parameters such as the alkalis [Na, K, sum Na + K, K/
(Na + K) ratio], commonly used for the classification of
igneous rocks, or the Rb and feldspar contents, remain
almost similar in the three groups.
Judging from average compositions, a significant gap
separates the magnesian plutonic bodies from the others
in mg-number–B diagrams (Figs 3 and 11). In contrast,
there is no distinct gap between the magnesian–ferriferous
bodies and the ferriferous ones, which, in addition, can
coexist within a single pluton (Mont Blanc and Central
Aar granites). Apart from those obtained for three magnesian bodies (Combeynot, Düssi, Schöllenen; see below),
ages recorded among the magnesian, magnesian–
ferriferous and ferriferous intrusions vary from 343 to
332 Ma, 307 to 292 Ma and 305 to 293 Ma, respectively
(Table 2; Figs 11 and 12). They corroborate the Mg/Fe
typology: a gap separates most of the magnesian bodies
from the others, whereas ages obtained for the magnesian–ferriferous and the ferriferous intrusions overlap
each other. Altogether, these data are interpreted to
indicate that the three plutonic groups form only two
separate suites, namely a Viséan (~330–340 Ma) highmg-number suite corresponding to the magnesian plutonic
bodies, and a mainly Stephanian (~295–305 Ma) low-mgnumber suite comprising both the magnesian–ferriferous
and the ferriferous bodies. The boundary between the
two suites does not coincide with the ‘critical line’ of the
mg-number–B diagram, but is situated significantly above
it (Fig. 11).
Thus, as suggested by previous studies (Debon et al.,
1994, 1998), the Late Variscan intrusions of the ECM
display a remarkable, discontinuous evolution in the
course of time, from magnesian to more ferriferous
compositions. Accordingly, the mg-number is regarded,
in the ECM, as a first-rank discrimination criterion.
Bonin et al. (1993) and Bonin (1997) have proposed an
overall evolution of the entire Late Variscan plutonic
rocks of the Alps towards increasingly alkaline compositions: ‘Lower to Middle Carboniferous high-K calcalkaline suites’ are followed by ‘Late Carboniferous nearalkaline associations’ and then by A-type Mid- to Late
Permian plutonic–volcanic complexes. This point of view
is questionable in the case of the ECM. The Viséan highmg-number and the mainly Stephanian low-mg-number
plutonic suites display remarkably similar high contents
in alkalis (Na + K) and K/(Na + K) ratios (Table 3),
and both are mainly subalkaline (Figs 2 and 13), whatever
their Mg/Fe typology and age of emplacement may be.
On the R1–R2 diagram (de la Roche et al., 1980), of
common use for the classification of igneous rocks and
their magmatic associations (e.g. de la Roche, 1979;
Batchelor & Bowden, 1985; Rollinson, 1993), the two
suites are indistinguishable from each other, apart from
a higher proportion of mafic rocks in the high-mg-number
suite and of quartz-rich granites in the low-mg-number
suite (Figs 11 and 13). In addition, both Viséan and
Westphalian ages were obtained for two granites of
distinct alkaline affinity, namely La Lauzière (341 ± 13
Ma; Debon et al., 1998) and Combeynot (312 ± 7 Ma;
Costarella, 1987; Bonin, 1997; Cannic et al., 1998).
Actually, the subvolcanic Combeynot granite exhibits
atypical features relative to the other plutonic bodies of
the ECM. Its dominant coarse-grained type is clearly
magnesian (Figs 3 and 11) but displays REE and spiderdiagram patterns similar to those of ferriferous granites
(Figs 14 and 15). It is younger by some 20 my than the
other magnesian plutons (332–343 Ma), but remains
significantly older than most of the plutonic bodies of
the low-mg-number suite (292–308 Ma) (Fig. 12). The
reasons for these discrepancies are unclear. In particular,
although this granite is separated from the Pelvoux massif
(Fig. 6) by a westward-vergent thrust zone (e.g. de Gracianski, 1993), it cannot represent a fragment of the
Internal Alps thrust onto the ECM by Alpine tectonics,
because its Mesozoic cover is undoubtedly of the Helvetic
type (Barféty, 1988, and personal communication, 1998).
Except for Ba, Rb and Sr (Table 2), trace element
data remain rather scarce for the Late Variscan plutonic
rocks of the ECM (Table 4). REE and spiderdiagram
patterns obtained from available data seem hardly pertinent to discriminate between the plutonic bodies, maybe
except for Eu, HREE, Sr and highly compatible elements
(Figs 14 and 15; see also Vittoz et al., 1987; Bonin et al.,
1993).
Discriminating criteria other than the mg-number—
structural, mineralogical or geochemical—were used in
some specific massifs (Pelvoux, Mont Blanc, Aiguilles
Rouges, Aar, Gotthard). They led to classifications of the
Pelvoux plutons somewhat different from ours, indicating,
for example, an E–W-directed evolutionary trend for this
plutonism (Vittoz et al., 1987, and references therein).
Many criteria can actually be used and there is no reason
why they should lead to a unique classification. In the
scope of this paper, the most worthwhile criteria are
those liable to display an evolution through time, as the
mg-number does at the scale of all the ECM. Using, in
addition to the above-mentioned criteria, precise ages of
emplacement, Bussy & Hernandez (1997) and Bussy et
al. (1998) distinguished, in the Mont Blanc and Aiguilles
Rouges area, a 330 Ma magnesian plutonic event, followed by a peraluminous event at 307 Ma, and then by
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Fig. 11. Plot of Late Variscan plutonic bodies (average compositions) from the ECM on the mg-number–B diagram of Debon & Le Fort (1988).
Tödi (333 ± 2 Ma), Voralp (309 ± 2) and Gastern (303 ± 4 Ma) granites are not shown because of unknown chemical typology. Other
explanations as in Fig. 3.
of the ECM remain uncertain. A variety of interacting
factors are suitable to account for it, depending, in
particular, on the nature of the source materials, the
physical and chemical conditions of melting, and the
geodynamic setting.
Source materials
Fig. 12. Ages of emplacement of Late Variscan plutonic bodies from
the ECM. Data from Table 2. Error bars at 2r, except for ages
obtained through the lead-evaporation method (see Table 2). 8,
Combeynot granite; 47, Düssi diorite; 48, Schöllenen diorite. Tödi
(333 ± 2 Ma), Voralp (309 ± 2 Ma) and Gastern (303 ± 4 Ma)
granites are not shown because of unknown chemical typology.
the intrusion of the ferriferous Mont Blanc granite. In
the same way, the distinction of up to four intrusive
events was proposed in the Aar and Gotthard massifs
[for a review, see Schaltegger (1994)], with the first two
events mainly corresponding to the high-mg-number suite
and the other two to the low-mg-number suite (Fig. 10).
This shows that our partition into two major suites can
be, at least locally, made more accurate.
The reasons for the evolution from magnesian to more
ferriferous compositions in the Late Variscan plutonism
As indicated by their mafic enclaves, the Late Variscan
granites of the ECM are most probably hybrid rocks,
deriving from at least two source materials (Didier &
Barbarin, 1991, and references therein). A number of
studies on mafic enclave–host granite pairs (Debon, 1991,
and references therein), specifically in the ECM (Banzet,
1987; Debon et al., 1998), have shown that: (1) the
enclaves and their host granites share compositional
characteristics indicating their close relationship (e.g. Figs
14 and 15); (2) the two groups of rocks, however, are not
cogenetic and their relationship was probably acquired
through pervasive mechanical and chemical interaction
(especially differential interdiffusion) between two originally independent magmas.
Isotopic data which may provide constraints on the
source of the Late Variscan plutons are rather scarce.
The initial 87Sr/86Sr isotope ratios of plutonic rocks (mafic
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Fig. 13. Plot of Late Variscan plutonic bodies (average compositions) from the ECM on the R1–R2 diagram of de la Roche et al. (1980). Data
from Table 2. Other explanations as in Fig. 2.
Fig. 14. Chondrite-normalized REE diagrams for Late Variscan plutonic bodies (average compositions) from the ECM. Data (Table 4)
normalized to chondritic values of Evensen et al. (1978).
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Fig. 15. Chondrite-normalized element variation diagrams (spiderdiagrams) for Late Variscan plutonic bodies (average compositions) from the
ECM. Data (Table 4) normalized to chondritic abundances (Thompson, 1982), except for Rb, K and P, which are normalized to primitive
terrestrial mantle values (Sun, 1982). Symbols as in Fig. 14.
enclaves included) from the high-mg-number suite (Colle
Blanche–Moutières, Péou de Saint Maurice, Rochail,
and Sept Laux plutons; Giuv syenite) vary from 0·7038
to 0·7077 (Demeulemeester, 1982; Banzet, 1987; Schaltegger et al., 1991). Apart from an old value of 0·712
given by Ferrara & Malaroda (1969) for the Argentera
Central granite, those obtained for rocks from the lowmg-number suite (Mont Blanc and Central Aar granites)
range from 0·7049 to 0·7058 (Bussy et al., 1989; Schaltegger, 1990a, 1990b, 1994) with, in addition, a higher
value of 0·7074 for the Sädelhorn diorite from the
Gotthard massif (Bossart et al., 1986). Negative initial Nd
isotopic compositions (eNd) were obtained for rocks from
both the high-mg-number (–2 and –5 for the Combeynot
and Sept Laux granites, respectively; Cannic et al., 1998;
Debon et al., 1998) and the low-mg-number suite (–2·7
for the Sädelhorn diorite; Bossart et al., 1986; –3 and
–5 for the Gamsboden–Fibbia and Medel–Cristallina
granites, respectively; Guerrot & Steiger, 1991). Finally,
the initial Hf isotopic compositions (eHf ) determined for
rocks from the high-mg-number and the low-mg-number
suite range, in the Aar massif, from –8 to 0 and from –5
to +3·5, respectively (Schaltegger & Corfu, 1992, 1995),
whereas a value of +4·47 was obtained for the ferriferous
Sädelhorn diorite (Stille et al., 1989).
The large isotopic overlap between the high-mg-number and the low-mg-number suite is likely to indicate that
both suites were derived from common source materials.
This is corroborated by the fact that the two suites display
the same mainly subalkaline typology (Figs 2 and 13) as
well as similar REE and spiderdiagram patterns (distinctive positive spike at Th, but generally negative at
Ba, Nb, Sr, P and Ti, both for felsic and mafic or
intermediate rocks) (Figs 14 and 15). A subcontinental
enriched mantle and a continental crust might constitute
these common source materials (e.g. Banzet, 1987; Schaltegger et al., 1991; Stille & Steiger, 1991; Schaltegger &
Corfu, 1992; Schaltegger, 1994; Debon et al., 1998).
The involvement of an enriched-mantle component is
supported in particular by:
(1) the presence, in many magnesian plutons (Colle
Blanche–Moutières, Péou de Saint Maurice, Quatre
Tours, Rochail, Sept Laux, Pormenaz), of vaugneritic or
durbachitic enclaves, i.e. of isotopically heterogeneous
mafic rocks, rich in both compatible and incompatible
elements (e.g. Mg, K), and of lamproitic or lamprophyric
affinity (Banzet, 1987, and references therein; Sabatier,
1991). As shown by many studies, most workers agree
that the ultimate source of such K-rich mafic magmas
lies in an enriched upper mantle [for reviews, see Foley
et al. (1987), Wilson (1989) and Mitchell & Bergman
(1991)].
(2) The marked similarities displayed by the REE and
spiderdiagram patterns of felsic, intermediate and mafic
rocks from the two plutonic suites with those of both the
Sept Laux vaugnerites and certain ultrapotassic rocks
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Fig. 16. REE and spiderdiagram patterns of the Sept Laux vaugnerites (Table 4), high-K lamprophyres from the Northwestern Alps (Venturelli
et al., 1984a), lamproites from Southeastern Spain (Nixon et al., 1984; Venturelli et al., 1984b; Nelson et al., 1986), and potassic lavas from Central
Italy (Rogers et al., 1985). Other explanations as in Figs 14 and 15.
from Groups I (e.g. lamproites from Southeastern Spain;
high-K lamprophyres from the Northwestern Alps) and
III (e.g. basanites and leucitites from central Italy) of
Foley et al. (1987) [Figs 14–16 (see also Banzet (1987)].
(3) The ‘shoshonitic–ultrapotassic suite’ of the Aar
massif (Schaltegger et al., 1991), of vaugneritic affinity.
A continental contribution to the genesis of the Late
Variscan plutonic rocks from the ECM is suggested
by several lines of evidence: (1) the mainly granitic
composition of these rocks (Tables 1 and 2; Fig. 13); (2)
their positive anomaly in Th (Fig. 15); (3) their initial
87
Sr/86Sr isotope ratios (0·704–0·708) higher than primary
mantle values; (4) their generally negative initial eNd (–3
to –5) and eHf values (–8 to +4·5); (5) the presence of
(rare) zircons inherited from the lower or the upper crust
in some of these rocks (Bossart et al., 1986; Banzet, 1987;
Schaltegger & Corfu, 1992, 1995; Schaltegger, 1993;
Sergeev et al., 1995). Some of these features (2–4), however, are ambiguous because they can also characterize
ultrapotassic rocks such as many lamproites (e.g. Mitchell
& Bergman, 1991) and, thus, might also be accounted for
by a contribution from an enriched mantle. In addition,
Schaltegger (1994) considered that the general lack of
inherited lead in zircons from the Aar plutonic rocks
would indicate a predominantly mantle origin.
Although dated at 308 and 310 Ma, the Düssi and
Schöllenen diorites (numbers 47 and 48) belong to the
high-mg-number suite (Figs 3, 10–12). The former is
associated with the magnesian–ferriferous Brunni granite,
whereas the latter is an enclave within the ferriferous
Central Aar granite. Interpreting their ages is therefore
made difficult because mechanical or chemical interaction with the associated granites cannot be excluded.
More field data would be of prime importance to go
further in discussing these two particular cases, hence
supporting the fact that the magnesian magmatism of
lamproitic affinity was still active during the emplacement
of the low-mg-number plutonic suite. This is corroborated
by the magnesian–ferriferous group of enclaves (number
38) dated at 306 Ma that occurs in the ferriferous Mont
Blanc granite, possibly by the 307 Ma gabbroic enclaves
(U–Pb zircon dating; Bussy & Hernandez, 1997) enclosed
in the Fully migmatites (Aiguilles Rouges massif ), and
by volcanic rocks of shoshonitic affinity dated by U–Pb
on zircon at 308 ± 15 Ma in the Grandes Rousses
massif (Banzet et al., 1985; Cannic et al., 1998). However,
apart from the coarse-grained Combeynot granite, felsic
magnesian plutonic bodies are only Viséan in age. The
most probable perenniality of a mafic magnesian activity
during Late Variscan times is consistent with the conspicuous scarcity of strongly ferriferous granites in the
ECM (Fig. 3).
A decreasing contribution from an enriched mantle of
lamproitic affinity in the course of time might account
for the replacement of the magnesian granites by more
ferriferous ones. This is suggested by: (1) the overall
greater abundance of mafic igneous enclaves (see Pelvoux
massif ) in the magnesian granites than in the more
ferriferous ones, and of mafic or intermediate plutonic
bodies in the high-mg-number suite than in the low-mgnumber one (Table 3; Figs 11 and 13); (2) the similar
low contents in HREE and in most compatible elements
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(Tb, Y, Tm, Yb) displayed by the vaugnerites and many
magnesian plutonic bodies, a feature that does not appear
in ferriferous bodies (Figs 14 and 15); (3) the continuance
of a typical, although fading, magnesian magmatic activity during the emplacement of the low-mg-number
plutonic suite (e.g. Düssi and Schöllenen diorites; see
above). In the Aar massif, however, the overall increase
of initial eHf values from magnesian to ferriferous rocks
was interpreted to reflect a decreasing crustal contribution
to magma generation in the course of crustal thinning
or to an increasing influence of an asthenospheric–mantle
component (Schaltegger & Corfu, 1992, 1995; Schaltegger, 1994).
Physical and chemical conditions of
melting
Assuming that both plutonic suites originated from similar
source materials, another possibility, not exclusive of the
preceding one, is that they were generated under different
physical and chemical conditions.
There are several experimental studies dealing with
the composition and especially with the Mg/Fe ratio of
primary granitic melts generated from various protoliths
( Johannes & Holtz, 1996, and references therein). These
are helpful in highlighting the possible effects, on this
ratio, of oxygen fugacity, temperature, pressure and water
activity.
Besides the chemical and mineralogical compositions
of the protolith, the f O2 conditions prevailing in the
source have dramatic effects on the iron content and
Mg/Fe ratio of the melts ( Johannes & Holtz, 1996).
However, only few systematic investigations of the effects
of f O2 were carried out because this parameter is difficult
to control and to monitor experimentally (F. Holtz,
personal communication, 1998). Oxygen fugacity exerts
a strong control on the nature of the ferromagnesian
phases of the source rocks and, therefore, on the composition of the melts themselves (Patiño Douce & Beard,
1996). The solubility of ferromagnesian minerals in melts
is significantly higher under reducing conditions ( Johannes & Holtz, 1996); however, the effects of f O2 on
the Mg/Fe ratio of the melts remain poorly known.
Depending on a number of factors, such as the composition of both the source rock and the coexisting melt,
this ratio might increase (Truckenbrodt et al., 1997) or
decrease (Scaillet et al., 1995; Johannes & Holtz, 1996;
Patiño Douce & Beard, 1996) under increasingly reducing
conditions.
Most studies show that the Mg and Fe contents of the
melts display a general tendency to increase with rising
temperature, dependent on the nature and stability of
the phases involved in the melting reactions ( Johannes
& Holtz, 1996). In contrast, pressure and water activity
NUMBER 7
JULY 1999
seem to have little direct effect on the iron and magnesium
contents of the melts. Pressure can have, however, important indirect effects on the Mg/Fe ratio because the
stability of some minerals is strongly dependent on P
( Johannes & Holtz, 1996), and the ferromagnesian content of the melt can decrease with decreasing aH2O
(Patiño Douce, 1996).
Among the above-mentioned physical and chemical
parameters, T, f O2 and P might be, a priori, the most
relevant to account for a significant change in the Mg/
Fe ratio of the melts. In particular, it seems likely that
a rise in temperature can trigger the involvement of
increasingly magnesium-rich minerals in the melting processes. The presence of comagmatic (although not cogenetic) vaugneritic enclaves in many magnesian granites
of the ECM implies the involvement of a high-temperature mafic magma [~1000°C for the emplacement
temperature ascribed to vaugnerites studied by Montel
& Weisbrod (1986)]. Such mafic magmas crystallized
under low f O2 conditions (Sabatier, 1980; Rossi, 1986;
Rossi & Cocherie, 1991).
In Corsica (Fig. 1), probably a part of the External
Alps (Lemoine, 1984), the Late Variscan granites also
comprise an early magnesian suite [(U1); 337–339 Ma]
and a younger more ferriferous suite [(U2); 288–307 Ma]
(Rossi, 1986; Rossi & Cocherie, 1995; Ménot et al., 1996).
Here again, the two suites are considered as being derived
from similar protoliths (namely, an Austro-Alpine continent) but under different conditions of melting; namely,
under granulite-facies conditions with pCO2 > pH2O for
the magnesian suite, and then under amphibolite-facies
conditions with lower pressure but higher aH2O for the
ferriferous suite (Rossi, 1986; Rossi & Cocherie, 1991).
In this case, P and aH2O would be the main factors
liable to account for the transition from the high-mgnumber to the low-mg-number suite. Consequently, this
increase of aH2O might have induced an increase of f O2
(Baker & Rutherford, 1996; F. Holtz, personal communication, 1998). In addition, the fact that magnesian
and ferriferous granites crystallized under reducing, near
the Ni–NiO buffer, and oxidizing conditions, respectively,
also suggests a possible increase of f O2 at the places of
melting with time. Just as in Corsica, the generation of
the magnesian granites from the Pelvoux massif might
be related to the early granulitic stage of metamorphism
(P = 5 ± 1 kbar, T = 800 ± 50°C) described by
Grandjean et al. (1996), and that of the magnesian–
ferriferous granites to the later amphibolitic stage (P =
3 ± 1 kbar, T = 700 ± 50°C) (C. Guerrot & F. Debon,
in preparation).
Altogether, the above data suggest that a decrease of
T and P, and an increase of f O2 and aH2O at the level
of the protoliths might be alternative means of accounting
for the transition from the high-mg-number to the lowmg-number suite.
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Geodynamic setting
The Variscan orogenic belt of Europe evolved during
the Palaeozoic convergence of Gondwana and Laurasia
and was consolidated in the Late Palaeozoic (e.g. Finger &
Steyrer, 1990, and references therein). The Late Variscan
plutonic bodies of the ECM were emplaced in an intracontinental environment, subsequent to the Devonian–
Early Carboniferous major stage of collision and crustal
thickening (Bonin et al., 1993; von Raumer & Neubauer,
1993; von Raumer et al., 1993; Bonin, 1997). Unlike the
South Alpine realms (Finger & Steyrer, 1990; Bonin,
1997), there is no evidence for an emplacement of these
bodies in an active continental margin, despite their
negative anomalies in Ba, Nb, Sr, P and Ti (Fig. 15),
similar to those displayed by subduction-related magmas.
In the ECM, these anomalies might be related to the
involvement of an upper-mantle component of lamproitic
affinity (see before), enriched in incompatible elements
by inferred pre- (or syn-)collisional subduction(s) (Banzet,
1987; Stille, 1987; Schaltegger & Corfu, 1992).
Extension, crustal thinning and wrench tectonics characterize the post-collisional evolution of the Variscan belt
(e.g. Ménard & Molnar, 1988). Two main successive
extensional events have been recognized by Burg et
al. (1994) in the western European Variscides: (1) the
predominantly Late Viséan–Westphalian extension was
a diachronous event, beginning in the inner, thickest
parts of the belt, in association with wrench tectonics
reactivating thrust zones during escape tectonics controlled by still active compressional forces. These processes induced an extension almost parallel to the
Variscan belt, at a time when thermal relaxation was
already occurring. (2) The second period of extension,
namely, the Late Stephanian to Early Permian event, is
characterized by major stretching and thinning with an
extension direction mainly transverse to the Variscan
belt. It was induced by the gravity collapse of the entire
belt.
Accordingly, it appears that the two plutonic suites of
the ECM, namely, the Viséan high-mg-number suite
and the mainly Stephanian low-mg-number suite, were
intruded at the beginning of each of the two major
extension events recognized by Burg et al. (1994), either
the Late Viséan–Westphalian event or the Late Stephanian to Early Permian event. This remarkable coincidence strongly suggests that magma generation was
triggered by drastic and abrupt changes in the tectonic
setting, including in the direction of extension, as pointed
out in many collisional domains (e.g. Boullier et al., 1986;
Liégeois & Black, 1987; Debon & Zimmermann, 1993)
and emphasized, in the Alps, by the following studies.
The high-mg-number suite forms part of the ‘Lower to
Middle Carboniferous high-K calc-alkaline suites’ recognized by Bonin et al. (1993) throughout the Variscan
Alps and which were emplaced during a stage of uplift
and erosion in a short-lived transpressional and/or transtensional regime. In the Aar massif, the intrusion of the
magnesian plutonic rocks marks the beginning of Late
Variscan strike-slip tectonics and coincides with a first
period of extension or transtension associated with the
formation of volcano-sedimentary basins (Schaltegger et
al., 1991; Schaltegger & Corfu, 1995). In the Belledonne
massif, the magnesian granites support a (short-lived?)
Early Viséan period of opening, in a tectonic regime that
was in this area, however, more probably transpressional
than transtensional (Debon et al., 1998).
Magnesian plutonic rocks have also been reported
from other massifs of the Moldanubian zone of the
Variscides [e.g. Central Bohemia: 340–343 Ma (Holub
et al., 1997); Southern Vosges: 339–342 Ma (Schaltegger
et al., 1996); Corsica: 337–339 Ma (Rossi & Cocherie,
1995; Ménot et al., 1996) (Fig. 1)]. In the Southern
Vosges, these rocks are linked to an extremely shortlived episode of extension, between ~345 and 340 Ma
(Viséan), marked by the development of a large volcanosedimentary basin and the exhumation of adjacent highgrade gneissic rocks (Schaltegger et al., 1996). In Corsica,
their generation is related to an abrupt adiabatic uplift
of the crust under extensional conditions (Rossi, 1986;
Ferré, 1989; Rossi & Cocherie, 1991).
The low-mg-number suite belongs to the ‘Late Carboniferous near-alkaline suites’ of Bonin et al. (1993),
which were emplaced in a major distensional regime.
According to Bussy & Hernandez (1997), the Vallorcine,
Montenvers and Mont Blanc granites (Mont Blanc and
Aiguilles Rouges massifs) were emplaced in an uplifting
basement subjected to crustal-scale strike-slip faulting, in
an overall extensional regime of ‘Basin and Range’ type.
Previously (Schulz & von Raumer, 1993), pull-apart
mechanisms have been proposed for the emplacement
of the Vallorcine granite. In the Aar massif, the intrusion
of the ferriferous granites (e.g. Central Aar granite),
accompanied by the formation of volcano-sedimentary
basins, coincided with a second period of Late Variscan
‘Basin and Range’-like extension or transtension (Schaltegger & Corfu, 1995). In Corsica, the ferriferous suite
(U2) was intruded in a still uplifting basement under
extensional conditions (Rossi, 1986; Rossi & Cocherie,
1991; Rossi et al., 1992; Thevoux-Chabuel et al., 1995).
Thus, it is likely that generation and emplacement of
the Late Variscan granites of the ECM were closely
linked to regional tectonics. They most probably represent
a discontinuous phenomenon, triggered by at least two
distinct tectonic events, dominated by extensional or
transtensional processes occurring in a general framework
of large-scale uplift and crustal thinning.
The two suites are variably represented among the
different ECM (Fig. 17). Granites of the high-mg-number
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Fig. 17. Synthetic map showing the Mg/Fe typology and the ages of emplacement of Late Variscan plutonic bodies from the ECM. Data from
Table 2. Magnesian–ferriferous and ferriferous bodies of the Mont Blanc and Central Aar granites are not distinguished from each other.
suite occur almost everywhere, with a conspicuous maximum in the Belledonne, Grandes Rousses and Pelvoux
massifs, i.e. at the point of inflexion of the arc defined
by the ECM. The ferriferous granites of the low-mgnumber suite, widely developed in the Argentera, Mont
Blanc, Aar and Gotthard massifs, are absent elsewhere,
whereas the magnesian–ferriferous granites also occur in
the Pelvoux and Aiguilles Rouges massifs. In other words,
the early high-mg-number suite and the younger low-mgnumber suite tend to develop within distinct massifs.
This implies that the conditions propitious for magma
generation were not simultaneously realized in the different ECM. Pre-Mesozoic reconstructions of the Alps remain a matter for speculation and thus the respective
positions of the ECM in Upper Carboniferous times are
poorly constrained (von Raumer & Neubauer, 1993;
Bonin, 1997), although Bogdanoff et al. (1991) considered
the ECM arc as mainly inherited from the Variscan
orogeny. Burg et al. (1994) showed that the two Late
Variscan extensional events were, respectively, nearly
parallel and mainly transverse to the belt. Because of
probable differences in their structural orientations, the
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ECM might have been predominantly affected by either
the first or the second of these two events, thus promoting
a greater development of either magnesian (in N–Sdirected massifs) or ferriferous granites (in E–W-directed
massifs). Differences in the original situation (more or
less internal) of the ECM relative to the Late Variscan
belt might also account for the contrasted distribution of
the two suites. This has been pointed out in Corsica–
Sardinia and, more generally, in the French Variscan,
where, as a whole, the high-mg-number plutons are
located in more internal parts of the belt than the
low-mg-number ones (Orsini, 1979, 1980; R. P. Ménot,
personal communication, 1998). The composite Belledonne massif, made up of three distinct domains (terranes), shows that Late Variscan strike-slip tectonics was
actually able to juxtapose basement pieces of very different origins (Ménot, 1988a, 1988b).
The ECM were part of the inner zone of the Variscides
(Bogdanoff et al., 1991; von Raumer et al., 1993), i.e. of
a domain propitious for early development of extensional
processes (Burg et al., 1994), as early as ~340 Ma by the
ages obtained for the Belledonne granites (Debon et al.,
1998). In addition, as the presence of vaugnerites or
durbachites implies a contribution from a subcontinental
mantle component, it is likely that, as emphasized by
Burg et al. (1994), Late Variscan strike-slip faults were
lithospheric scale, allowing mantle melts to rise into the
upper crust (Schaltegger et al., 1991).
The overall evolution of Mg/Fe ratios might be accounted for by the combination of a number of interacting
factors related to the nature of the source of the magmas,
the physical and chemical conditions of melting, and the
Late Variscan geodynamic setting.
On the basis of geochemical and isotopic data available
in the literature, it is suggested, in agreement with previous studies, that both suites are composed of hybrid
rocks originating from similar source materials, namely
a subcontinental enriched mantle of lamproitic affinity
and a sialic crust. A decreasing contribution from the
enriched mantle in the course of time, possibly related
to a decrease in temperature and pressure conditions
and to an increase of the oxygen fugacity and water
activity at the places of melting, might be the main
factors responsible for the evolution towards increasingly
ferriferous granites.
Changes in the physical conditions of melting were
probably linked to extensional processes occurring in a
general framework of strike-slip tectonics, large-scale
uplift and thinning. Emplacement of the Late Variscan
granites of the ECM was a discontinuous phenomenon,
composed of at least two distinct steps, the former Viséan
and the latter mainly Stephanian. Each step took place
at the beginning of the two major extension periods
recognized by Burg et al. (1994) in the Variscan belt of
Western Europe. This strongly suggests that magma
generation was triggered by drastic and abrupt changes in
the tectonic conditions, as pointed out in many collisional
domains.
CONCLUSION
ACKNOWLEDGEMENTS
Use of the mg-number–B diagram of Debon & Le Fort
(1988) led us to distinguish two suites of Late Variscan
intrusions in the External Crystalline Massifs of the Alps
(ECM), namely a high-mg-number and a low-mg-number
suite. With few exceptions, each suite is characterized by
a specific age of emplacement, namely, Viséan (~330–340
Ma) and mainly Stephanian (~295–305 Ma), respectively.
This demonstrates the existence of an evolution with
time from magnesian granites to more ferriferous ones
and therefore makes the Mg/Fe ratio a first-rank criterion
to approach the Late Variscan magmatic processes in
the ECM, at that time a part of a wide post-collisional
orogenic belt. In contrast, criteria commonly used to
discriminate between igneous suites (alkali content and
ratio, REE and spiderdiagram patterns) seem hardly
pertinent to evidence an evolution in the ECM. More
precise chronological data, however, would allow the
proposed partition into two major suites to be refined,
specifically regarding the composite low-mg-number suite.
In addition, although fading, the high-mg-number magmatism was still going on during the emplacement of the
low-mg-number suite.
Early drafts of the manuscript were greatly improved by
thorough reviews from Marjorie Wilson (Executive Editor
of this journal), R. P. Ménot (University of Saint-Etienne),
B. Bonin (University of Paris-Sud), and an anonymous
referee. The authors gratefully acknowledge F. Holtz
(CNRS, Orléans and University of Hannover), G. Banzet
(CNRS, Nancy), Kirsten Nicholson (University of Auckland) and A. Pêcher (University of Grenoble) for fruitful
comments, as also G. Banzet, F. Bussy (University of
Lausanne), Catherine Guerrot (BRGM, Orléans),
B. Lombardo (CNR, Torino), A. Pêcher, U. Schaltegger
(ETH, Zürich), S. A. Sergeev (ETH, Zürich) and G. Vivier (CNRS, Nancy) for providing them with unpublished
chemical or chronological data, K. Govindaraju (CRPG–
CNRS, Nancy) for high-quality chemical analyses,
K. Furness for his help with the English manuscript, and
Pam Stuart for editorial handling.
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