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, trendingN120-140°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 post-kinematic with regard to the Variscan thrusting phase of deformation. Several Late Variscan plutonic bodies, very different from each other, occur in this massif.

Figure 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.)

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 post-dates the three migmatitic stages (Faure-Muret, 1955; Bogdanoff et al., 1991). It mainly consists of a massive, coarse-grained, porphyritic in places, garnet-bearing two-mica leucocratic monzogranite, which is subalkaline and ferriferous. Fine-grained leucogranites and microgranites occur at its margins. A poorly constrained Rb-Sr whole-rock 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.

Figure 5. Sketch map of the Pelvoux, Belledonne, Grandes Rousses, Mont Blanc and Aiguilles Rouges massifs [after Vivier et al., (1987)].

Figure 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.)

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 one-third 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 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 g-atoms * 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

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 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 fine-grained 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 (2[sgr]) 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).

Magnesian-ferriferous plutonic bodies

The average compositions of these plutonic bodies plot close to or slightly above the `critical line' of the mg-number-B diagram (Fig. 3).

(16) Bérarde-Promontoire granite (~40 km²): a fine- to 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 muscovitesyenogranite. 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 coarse-grained, 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, 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).

Figure 7. Geological sketch map of the northeastern domain of the Belledonne massif [modified from Vivier et al., (1987)]. (See Table 2 for chronological data.)

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 (2[sgr]) 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 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.

Figure 8. Geological sketch map of the Grandes Rousses massif [modified from 1:50 000 geological maps of France, and Giorgi, (1979)].

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, light-coloured monzogranites, with biotite ± rare muscovite. Mafic igneous enclaves are common. The La Faremonzogranite 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, 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 rhyoliticformation, dated at 307 ± 2 Ma (Bussy & von Raumer, 1993); (iii) two Late Variscan plutonic bodies (Montenvers, Mont Blanc).

Figure 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.)

(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, 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 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-bearingleucogranites 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).

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 emplaced. Three successive intrusive suites, compositionally different, have been distinguished in the Aar massif by Schaltegger, (1994, and references therein).

Figure 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.)

`Shoshonitic-ultrapotassic 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 coarse-grained 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).

(45) Tödi granite (<1 km²): a fine-grained, strongly deformed, porphyritic granite (Schaltegger & Corfu, 1995), probably belonging to the same suite (Schaltegger, 1994).

`High-K calc-alkaline suite'

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 calc-alkaline (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.

(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.

Gotthard massif

The Gotthard massif (Fig. 10) extends for ~80 km, parallel to the Aar massif. It is separated from the latter 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 * <= 0·2 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).

Two plutonic suites

The Late Variscan plutonic bodies of the ECM can be divided into three groups on the basis of their average 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) high-mg-number suite corresponding to the magnesian plutonic bodies, and a mainly Stephanian (~295-305 Ma) low-mg-number 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).

Figure 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.

Figure 12. Ages of emplacement of Late Variscan plutonic bodies from the ECM. Data from Table 2. Error bars at 2[sgr], 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.

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 calc-alkaline suites' are followed by `Late Carboniferous near-alkaline 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 high-mg-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).

Figure 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.

Figure 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).

Figure 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.

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 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 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

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 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 low-mg-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 ([epsilon]_Nd) 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 ([epsilon]_Hf) 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 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.

Figure 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.

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 ⁸⁷Sr/⁸⁶Sr isotope ratios (0·704-0·708) higher than primary mantle values; (4) their generally negative initial [epsilon]_Nd (-3 to -5) and [epsilon]_Hf 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-mg-number one (Table 3; Figs 11 and 13); (2) the similar low contents in HREE and in most compatible elements (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 [epsilon]_Hf 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 fO₂ 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 fO₂ 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 fO₂ 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 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, fO₂ 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 fO₂ 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-mg-number to the low-mg-number suite. Consequently, this increase of aH₂O might have induced an increase of fO₂ (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 fO₂ 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 fO₂ and aH₂O at the level of the protoliths might be alternative means of accounting for the transition from the high-mg-number to the low-mg-number suite.

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 short-lived episode of extension, between ~345 and 340 Ma (Viséan), marked by the development of a large volcano-sedimentary basin and the exhumation of adjacent high-grade 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 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-mg-number 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-mg-number 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 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-S-directed 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).

Figure 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.

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).