Journal of the Virtual Explorer

The Monte Rosa unit marks the junction between the Central and the Western Alps. In terms of tectonic position, it lies at a level corresponding to the Suretta nappe to the east and to the Gran Paradiso and Dora Maira to the Southwest. In the Argand’s view, Monte Rosa forms the fifth recumbent fold-nappe of the Penninic system (Lugeon and Argand, 1905; see Dal Piaz, 2001a, for a historical review), which likely derived from the European margin. The Penninic domain includes several stacked nappes (Fig. 2):

(i) the upper Monte Rosa [MR], Gran Paradiso and Dora Maira units;

(ii) the mid unit of the Grand Saint-Bernard [SB];

(iii) the lower Lepontine units of the Ossola-Ticino;

(iv) the Valais domain (lower/outer Penninic), composed of flysch and minor meta-ophiolite (e.g., Ballèvre and Merle, 1993; Dal Piaz et al., 2001).

Figure 2. Cross section and lithospheric interpretation of the Western Alps

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Cross section and lithospheric interpretation of the Western Alps.

PL= Periadriatic Line; PF= Penninic frontal thrust; ophiolitic units are in black; MR= Monte Rosa unit; SB= Grand St. Bernard system; H= Helvetic zone; HF= Helvetic frontal thrust. After Cortiana et al., 1998.

The Monte Rosa “nappe” may be subdivided into two main units derived from the following pre-Alpine protoliths:

(i) pre-granitic basement composed of a Variscan high-grade gneissic complex intruded by granite-granodiorite plutons;

(ii) remnants of Permian-Mesozoic sedimentary cover and the composite Furgg zone (Bearth, 1956).

The pre-granitic basement is composed of paragneiss, micaschists, migmatite and interlayered metabasites (Bearth, 1952; Dal Piaz, 1966, 1971; see refs. in Dal Piaz, 2001a, and Engi et al., 2001), which have undergone high-T low-P metamorphic conditions during the Variscan orogeny, producing K-feldspar and cordierite-bearing migmatite and pegmatite (e.g., Engi et al., 2001). They were sharply intruded by Late Carboniferous granodiorites and Permian granites (Hunziker, 1970; Liati et al., 2001; Scherrer et al., 2001). The whole Variscan basement was deformed and metamorphosed during Alpine orogeny, under eclogite-facies conditions (typically 500°C and 1.6 GPa: Chopin and Monié, 1984; Dal Piaz and Lombardo, 1986). Recently, Le Bayon et al. (2001) obtained a pressure estimate as high as 2.3 GPa for some whiteschists of an Alpine shear zone in the Ayas valley. According to recent radiometric determinations, the eclogite-facies peak of the Monte Rosa is nearly contemporaneous to the peak of the Zermatt-Saas eclogites and coesite-bearing metasediments (Middle Eocene: Rubatto et al., 1998; Rubatto and Gebauer, 1999; Pawlig and Baumgartner, 2001). Finally, a mesoalpine (Late Eocene-Early Oligocene: 38-35 Ma) overprint from greenschist- to amphibolite-facies metamorphic conditions affected the whole Monte Rosa “nappe” (Bearth, 1958).

The Furgg zone has been defined as a high-strain zone with an heterogeneous association of paragneiss, leucocratic gneiss and melanocratic rocks, derived from Permian granitoids and/or volcanic rocks, Cambrian metagabbros (Liati et al., 2001), and mafic boudins in marble presumably derived from Mesozoic rocks intruded by basic dykes (e.g., Bearth, 1954; Keller and Schmid, 2001). For a few authors, this zone could represent the cover of the Monte Rosa basement, whereas others interpret it as a tectonic mélange (see review in Dal Piaz, 2001b, p. 295). Initially observed and mapped by Bearth at the northern border of the massif, it was successively recognised southwards and at various structural levels, closely associated to and folded together with the polymetamorphic basement (Dal Piaz, 2001a). The more-or-less retrogressed eclogite boudins occurring in the italian southern side of the Monte Rosa basement are representative of Variscan or older mafic granulite or amphibolite, derived in turn from continental tholeiitic basalts (Ferrando et al., 2002).

The subduction-related early Alpine foliation (S1), discontinuously recorded and marked by eclogite- or blueschist-facies mineral assemblages, was followed by a S2 foliation developed during the Monte Rosa exhumation-related shearing, accompanied by retrogression (Wheeler and Butler, 1993). These two foliations were further folded by large folding during the mesoalpine event (e.g., the Vanzone antiform and Antrona synform).

The meta-ophiolites of the composite Piedmont nappe extend along the entire arc of the Western Alps up to the Central Alps (Bigi et al., 1990). They form numerous metamorphic units, which are scattered at different structural levels of the Alpine nappe pile, from the uppermost Platta-Arosa unit (Central Alps) to the lowest and outermost Versoyen unit (in the Western Alps).

Argand (1916) attributed these meta-ophiolites to the Penninic domain. Since the development of the plate tectonics, they are considered as derived from the oceanic lithosphere of the Liguro-Piedmont branch of the Tethys that opened in the Middle-Late Jurassic between Europe and Adria (Africa) passive continental margins (see historical review in Dal Piaz, 2001b). This oceanic lithosphere was sliced and dismembered during the plate margin convergence that led to the subduction of the oceanic and continental crusts under the Adriatic margin and to their partial exhumation. The occurrence of either a single Piedmont ocean or two separated oceanic branches (South-Penninic / Piedmont and North-Penninic / Valais basins) has been envisaged since the first work of Sturani (1973) to the last contribution of Rubatto et al. (1998), who tried to date both oceans.

On the basis of the metamorphic evolution of these meta-ophiolites, Dal Piaz (1965, 1974), Bearth (1967) and Kienast (1973) distinguished two main units, namely the Combin (blueschist-facies) and Zermatt-Saas (mainly eclogite-facies) units. This discrimination has been accepted by many authors (e.g., Elter, 1971), who also used lithostratigraphic and structural criteria to distinguish between these two units. Here, we only emphasise the features of the lower Zermatt-Saas meta-ophiolite, whereas those of the upper Combin unit are described in a section below.

The rocks of Zermatt-Saas unit are serpentinite with locally preserved mantle peridotite relics, abundant ophicalcitic breccias, minor metagabbro with magmatic mineral or textural relics (e.g., Allalin, Mellichen, Crepin), metabasalts with N-MORB affinity (Dal Piaz et al., 1979b; Dal Piaz et al., 1981; Beccaluva et al., 1984; Pfeiffer et al., 1989) and metasediments derived from the internal part of the Tethys (e.g., Bearth, 1967; Ernst and Dal Piaz, 1978).

The meta-ophiolites originally closer to oceanic hydrothermal out-flow zones show occurrences of Fe-Cu sulphide and Mn ore deposits (e.g., Dal Piaz and Omenetto, 1978). The formers are located within metabasalts, the latter in siliceous sediments (metacherts). The most important Cu-Fe sulphide deposits of the Zermatt-Saas unit are within high-pressure metabasalts with strong oceanic alteration (garnet glaucophanite, chloriteschists and talcschists). They are located in the southern Aosta valley, noticeably at Saint-Marcel (see Stop 2.5) and Champ-de-Praz, but also in the Täsch area (Zermatt-Saas, Switzerland, Widmer et al., 2000). The Mn deposits occur mainly as metamorphosed boudinaged quartzites rich in braunite, piemontite, spessartine (Castello, 1981). At Praborna (Saint-Marcel; see Stop 2.6 below), which is by far the most important and famous occurrence, the ore deposit includes very peculiar Mn-bearing silicates (e.g., Martin-Vernizzi, 1982; Martin and Kienast, 1987; Mozgawa, 1988). It is thought to derive from an oceanic hydrothermal system, or accumulation of Mn-rich oceanic nodules and “umbers”, as evidenced by high Sb-, Sr- and Ba-contents (Perseil, 1988; Perseil and Smith, 1995; Tumiati, pers. comm.).

The effect of oceanic hydrothermalism and alteration on the mafic rocks is also evidenced by abnormal contents in various elements (Na, OH, Mg, Ca) (Beccaluva et al., 1984; Barnicoat and Bowtell, 1995; Martin and Cortiana, 2001) and the scattering of d18O values (Cartwright and Barnicoat, 1999).

The subduction and exhumation history of the Zermatt-Saas unit is signed by prograde relics as pseudomorphs after lawsonite, eclogite-facies assemblages and by greenschist-facies assemblages indicating retrogression. The Zermatt-Saas rocks that crop out north of the Aosta-Ranzola fault (i.e., in the northern part of the Aosta valley), gave the highest P-T estimates for the peak metamorphism (e.g., Meyer, 1983; Van der Klauw et al., 1997; Reinecke, 1998), with values as high as 2.7-2.9 GPa and 600-630°C for the coesite-bearing metasediments of Cignana (Reinecke, 1998; see Stop 3.3), whereas metabasites from the southern part yielded relatively lower P-T conditions (e.g., Mottana, 1986; Martin and Tartarotti, 1989), typically 2.0±0.3 GPa and 550±50°C (Servette: Martin et al., 2005; see Stops 2.5).

The formation of the Zermatt-Saas oceanic crust is attributed to the Jurassic (164-153 Ma: Rubatto et al., 1998). Geochronology yielded a range of ages between 52 and 43 Ma (Eocene) for its high-pressure metamorphism, depending on the technique used (Botwell et al., 1994; Barnicoat et al., 1993; Rubatto et al., 1998; Mayer et al., 1999; Dal Piaz et al., 2001). The different results may correspond to different steps of the P-T path between the peak conditions and the retrogression below 500°C.

The structure of the Zermatt-Saas meta-ophiolite in the Saint-Marcel valley and Monte Avic massif is generally characterized by a N-S-trending lineation parallel to the axes of isoclinal folds, and related to a D2 deformation phase that occurred under eclogite facies (Tartarotti, 1988; Martin et al., 2004). Relics of an earlier prograde deformation (D1) have been recognised only in the core of garnet crystals. The D2 foliation is further folded by a D3 deformation phase with axes still oriented N-S. An E-W-trending D4 regional tectonic phase developed under greenschist facies (e.g., Elter, 1960; Ballèvre, 1988). South dipping fault planes belonging to the Aosta-Ranzola normal fault system (Bistacchi and Massironi, 2000), locally reactivated as N-vergent thrusts, represent the last deformation episode D5 (Martin and Tartarotti, 1989).

On the northern side of the Aosta valley, the ophiolitic Zermatt-Saas (below) and Combin (above) units are discontinuously separated by a thin slice of a Permo-Mesozoic sedimentary sequence (Pancherot-Cime Bianche: Dal Piaz, 1999, and refs. therein). This slice is composed of albite-bearing quartzitic schists (Permian), Verrucano conglomerate, tabular quartzite (Lower Triassic), limestone and dolostone (Middle-Upper Triassic), polygenic sedimentary breccias (Jurassic) and calcschists (Cretaceous?), which have been interpreted as deposited on a thinned continental margin or on an extensional allochton (Mt Emilius?) trapped inside the ocean (Dal Piaz, 1999, and refs. therein). These extra-oceanic sediments have been metamorphosed during the Alpine events, but they still conserve some fossils (Kienast, pers. comm.).

Moreover, some slices of cover-free eclogite-facies continental crust are known along the tectonic contact between the Zermatt-Saas and Combin meta-ophiolites, in the lowered northern hangingwall of the Aosta-Ranzola normal fault system (Bistacchi et al., 2001). The most important are the Etirol-Levaz (Kienast, 1983; Ballèvre et al., 1986), Châtillon and Saint-Vincent slices. They are also known as lower Austroalpine outliers (eclogite-facies) because they are located at a structural level lower than that of the Sesia-Lanzo inlier and Dent Blanche-Mont Mary-Pillonet upper Austroalpine outliers that override the Combin unit.

In the southern side of the Aosta-Ranzola fault system, the overlying Combin unit was eroded and the preserved top units are represented by numerous eclogite-facies lower Austroalpine outliers (or intermediate basement slices), which occur over (Mt Emilius) or inside (Glacier-Rafray, Tour Ponton, Acque Rosse) the Zermatt-Saas meta-ophiolite. There, only the Santanel slice seems to be interleaved between the Zermatt-Saas and Combin units.

These intermediate or lower Austroalpine continental slivers are mainly made of pre-Alpine high-grade paragneiss, marble, granitoids and continental gabbro bodies (Mt Emilius, Etirol-Levaz), which have undergone an eclogite-facies metamorphism and greenschist-facies retrogression (see Dal Piaz, 1999; Dal Piaz et al., 2001). Kienast (1983) and Ballèvre et al. (1986) obtained an estimate of 550°C and 1.6-1.7 GPa for the peak metamorphism of the Etirol-Levaz slice. Although the metamorphic history of these basement slivers is more-or-less comparable to that of the Sesia-Lanzo Zone (see below), the age of their high-P metamorphism (40-49 Ma: Dal Piaz et al., 2001) is 20-25 Ma younger than that in the Sesia-Lanzo domain, but roughly the same as the Zermatt-Saas meta-ophiolite.

The Combin unit consists of calcschists, impure marble, quartzitic schists and mafic to ultramafic meta-ophiolitic rocks. It displays a pervasive greenschist-facies overprint and scatterly preserved epidote-blueschist-facies relics, without traces of eclogite-facies assemblages that, in contrast, are well known in the underlying Zermatt-Saas unit. An oceanic hydrothermalism is documented by whole rock geochemistry and the presence of Mn-rich metacherts and disseminated ore deposits (Cu-Fe-oxides, sulphides and tourmaline) (Dal Piaz et al., 1979a; Castello, 1981; Martin and Cortiana, 2001). The P-T conditions for the development of the blueschist-facies paragenesis have been estimated at 0.5-0.7 GPa and 350-400°C (Sperlich, 1988; Martin and Cortiana, 2001).

A crystallisation or cooling age of 43.0±0.3 Ma was obtained for a Combin sodic amphibole of Grand Tournalin by the technique of 40Ar/39Ar total fusion (Martin and Cortiana, 2001). Cooling ages below 400°C are very scattered, ranging from 49 to 30 Ma (Delaloye and Desmons, 1976; Ayrton et al., 1982). These ages are quite similar to those of the Zermatt-Saas unit.

The Austroalpine domain is represented, in the Aosta Valley, by two first-rank units, namely (i) the Sesia-Lanzo internal zone, a 90-km long and 25-km large belt bounded to the east by the Canavese fault, and (ii) numerous external slices of continental crust, traditionally grouped as Dent Blanche nappe (Argand, 1916; Stutz and Masson, 1938; Compagnoni et al., 1977). Since the development of plate tectonics, these units have been widely interpreted as slices of the Adria microplate or African promontory.

The Sesia-Lanzo zone mainly consists of

(i) the internal Micascisti eclogitici complex (Stella, 1894; Franchi, 1900, 1902),

(ii) the external greenschist-facies Gneiss minuti complex (Gastaldi, 1871-1874) and

(iii) some klippen of the Adria lower crust (e.g., II zona Dioritico-Kinzigitica).

The Dent Blanche nappe (s.l.) is subdivided into two main groups of units, which are characterized by a contrasting subduction metamorphism and a different structural position:

(i) the blueschist-facies Dent Blanche (s.s.), Mont Mary and Pillonet thrust units (Diehl et al., 1952; Ayrton et al., 1982; Dal Piaz and Martin, 1988; Canepa et al., 1990; Dal Piaz et al., 2001), which, like the Sesia-Lanzo zone, override the entire ophiolitic Piedmont Zone and therefore may be reported as upper Austroalpine outliers (Dal Piaz, 1999);

(ii) the eclogite-facies lower Austroalpine outliers, described in section 2.3.

The pre-Alpine Adriatic crust was mainly composed of felsic and mafic granulites (e.g., Nicot, 1977; Lardeaux and Spalla, 1991), kinzigitic gneiss (e.g., Diehl et al., 1952; Canepa et al., 1990), a few slices of serpentinized mantle peridotite (e.g., Cesare et al., 1989), marble, abundant Late Palaeozoic granitoids (e.g., Gneiss Arolla) and gabbros (e.g., base of the Cervino/Matterhorn: Dal Piaz et al., 1977). A Mesozoic sedimentary cover, composed of quartzite, marble, dolostone, sedimentary breccia, conglomerate etc. is locally preserved in the Dent Blanche-Mont Mary system (Roisan zone, Mont Dolin) and has been metamorphosed together with the basement (Ayrton et al., 1982; Canepa et al., 1990). Similarly, a metamorphosed sedimentary cover has been described in the Sesia-Lanzo zone (Venturini, 1995).

During the Alpine orogeny, the high-grade metamorphic and igneous basement was metamorphosed under eclogite-facies (Sesia-Lanzo and lower Austroalpine outliers) or blueschist-facies (upper Austroalpine outliers: Dent Blanche, Mont Mary, Pillonet) conditions, giving rise, respectively, to Micascisti eclogitici (auct.) and chloritoid-bearing micaschists (Dent Blanche: Kienast and Nicot, 1971), phengite-jadeite orthogneiss and Na-amphibole mafic boudins (Pillonet: Dal Piaz, 1976). As for the leucocratic granitoids that intruded the Adria basement during the Permian, they were transformed into Gneiss minuti (Sesia-Lanzo) and Gneiss Arolla (Dent Blanche s.l.).

The metagranitoids and metapelites of the Sesia-Lanzo zone are of particular interest, because they have preserved rather well the eclogite-facies paragenesis. The Micascisti eclogitici, first defined and studied by Stella (1894) and Franchi (1900, 1902), are coarse-grained micaschists, with quartz, phengite, paragonite, large garnet crystals, omphacite, glaucophane, chloritoid and rutile (e.g., Lillianes, see Stop 1.11). Some granites gave rise to the famous eclogite-facies rocks, with jadeite, quartz, phengite, garnet (e.g., Monte Mucrone: Compagnoni and Maffeo, 1973; Compagnoni et al., 1977; Oberhänsli et al., 1982, 1985; Rubbo et al., 1999).

In the Dent Blanche nappe, the blueschist-facies P-T conditions were estimated at 400-550°C and 0.7-0.8 GPa (Kienast and Nicot, 1971; Cortiana et al., 1998), whereas higher P-T-values have been obtained for the Sesia-Lanzo eclogite-facies rocks (e.g., 550±50°C and 1.4-2.1 GPa: Oberhänsli et al., 1985; Inger et al., 1996; Tropper et al., 1999).

The peculiarity of the Sesia-Lanzo nappe and of the upper Austroalpine outliers is the relative old age of their high-P metamorphism, which is dated as Late Cretaceous (110±13 Ma, Jäger et al., 1990; 69.2±2.7 Ma, Duchêne et al., 1997; 65±5 Ma, Rubatto et al., 1998). A similar age was obtained for the Pillonet klippe (75-73 Ma: Cortiana et al., 1998). These ages have been interpreted as the crystallisation age of the eclogite-facies paragenesis at a depth of 50-100 km.

In the Sesia-Lanzo zone, Gosso (1977) and Gosso et al. (1979) identified four Alpine deformation phases (D1-D4), on the basis of field interference structures and microstructures. The D1 phase was coeval to the eclogite-facies event and produced an almost complete transposition, whereas D2-D4 are post-nappe deformation phases, which occurred at different steps of the retrogression or during the late mesoalpine greenschist-facies overprint.

The tectonic contact between the Austroalpine domain (i.e., Sesia-Lanzo and Dent Blanche nappes) over the Piedmont meta-ophiolites (i.e., Zermatt-Saas and Combin units) is locally outlined by Permian gabbro bodies (e.g., Dal Piaz et al., 1977; Zanella, 1992) that were strongly deformed under blueschist or greenschist-facies conditions during accretion or exhumation (e.g., Monte Pinter, Monte Tantané, Cervino/Matterhorn, Moussallion). Therefore, it seems that the thrust plane reworked pre-existing normal shear zones of the Adria crust, along which the gabbro bodies had been exhumed and laterally juxtaposed to coeval shallower intrusions (Dal Piaz, 1993). In contrast with the Dent Blanche/Combin contact, which is generally straight and regular, the Monte Rosa/Zermatt-Saas contact is folded.