Argentera Massif
The Argentera Massif, the southernmost of the External Crystalline Massifs, straddles the boundary between Italy and France in the Maritime Alps of NW Italy and is largely composed of Variscan migmatites with abundant relicts of pre-anatectic rock types. It is an elliptical area of 60x30 km, trending WNW-ESE, with rugged relief and mountain tops of 2800-3000 m in the NW and 3100-3300 m in the centre (Figs. 2a and b). The Argentera Massif consists of the Gesso-Stura-Vésubie (GSV) Terrane to the NE and of the Tinée Terrane to the SW, which are separated by the Ferriere-Mollières shear zone (Fig. 3). The two terranes are characterized by distinct lithological associations and metamorphic evolutions, but both contain rare relicts of high-pressure (HP) and/or high-temperature (HT) mineral assemblages, which are commonly preserved within mafic rocks and exceptionally within metapelites.
Modern geological maps of the Argentera Massif (Faure-Muret, 1955; Malaroda et al., 1970; Malaroda, 1999) provide accurate lithological information, but fail to describe a coherent lithostratigraphy for the GSV Terrane. Such maps still use the now abandoned terminology of metamorphic rocks and migmatites of the French metasomatic school (Jung & Roques, 1952).
Gesso-Stura-Vésubie Terrane
This part of the Massif corresponds to the Malinvern-Argentera Complex of Malaroda et al. (1970) and comprises the Malinvern-Argentera and Chastillon-Valmasque complexes of Faure-Muret (1955).
The GSV Terrane (Fig. 3) mainly consists of migmatitic granitic gneiss—the “anatexites” of Malaroda & Schiavinato (1958) and Malaroda et al. (1970)—and of migmatitic paragneisses. Field relationships at the outcrop scale (Figs. 4a and b) suggest that at least a part of the “anatexites” were intrusive into the paragneiss sequence. The metasediments now appear as km-long bands in the granite gneiss, for example in the Rio Freddo valley (Bortolami & Sacchi,1968), but the intrusive contacts have been transposed by the Variscan foliation. Small bodies and boudins of mafic (amphibolite, eclogite, granulite) and ultramafic rocks are common in the “anatexites”, and are more rarely exposed in the paragneisses of the GSV Terrane. Intercalations of marble and calc-silicate rocks are rare, and may contain wollastonite in addition to diopside and grossular (Calisesi, 1971). Dacitic to rhyodacitic metavolcanic rocks with transitional contacts to the surrounding “granitoid” migmatites have been locally recognized in the central and southern GSV Terrane (e.g., Rubatto et al., 2001). The eastern GSV Terrane is characterized by the Bousset-Valmasque Complex (Rubatto et al., 2001), which consists of a close association of amphibolites, agmatites with amphibolite fragments, and migmatites with amphibole-bearing anatectic granite. Carboniferous to Permian granitoids are present in the GSV Terrane, the largest pluton being the Central Granite (Fig. 3).
In the following, we briefly describe, from the oldest to the youngest, the most significant lithologies of the GSV Terrane, and summarize the information available on its geological history.
Figure 2. Argentera Massif: panoramic views
Examples of young morphology on old rocks of the GSV Terrane. (a) The W face of the Argentera ridge, looking SE from the path to Colle di Valmiana, upper Gesso della Valletta valley. (b) The E side of the Argentera ridge and the Chiotas Reservoir from the path to Rifugio Genova, looking W. Upper Valle della Rovina.
Figure 3. Argentera Massif: geotectonic map
Geotectonic map of the Argentera Massif. The map is based on those of Faure-Muret (1955), Malaroda et.al. (1970), Malaroda (1999) and on sheet Gap of the 1:250’000 Geological Map of France. CG: Central Granite; VSL: Valle Stura leucogranite; FCSM: Fremamorta-Colle del Sabbione mylonite.
Figure 4. GSV Terrane: ortho and para migmatitic gneisses
Migmatitic gneisses. (a) Lithologic contact between migmatitic granitic gneiss (lighter rocks and scree on the left) and migmatitic paragneisses (reddish rocks and scree on the right). GSV Terrane. Looking N from Lago del Claus, upper Gesso della Valletta valley. (b) Detail of the contact between migmatitic granitic gneiss (on the left) and migmatitic paragneiss (on the right). GSV Terrane. Laghi della Sella, upper Valle della Meris.
Relicts of HP mafic rocks
The most significant mafic bodies of the GSV Terrane are exposed in three localities (Fig. 3): 1) Laghi del Frisson, where a varied mafic sequence mainly consists of HP granulites (Figs. 5a and b); 2) Val Meris, where a large body of eclogite and retrogressed eclogite is surrounded by “anatexites” (Colombo, 1996; Rubatto et al., 2001); 3) Lago di Nasta, where amphibolitized eclogite and possibly HP granulite are both exposed (Fig. 5c). U-Pb dating on zircons suggests protolith ages for the mafic intrusions ranging between 459 ± 4 Ma (Meris eclogite, Rubatto et al., 2001) and 486 ± 7 Ma (Laghi del Frisson, Rubatto et al., 2010). Ordovician (480-460 Ma) mafic rocks, usually overprinted by low-P metamorphism, are disseminated within the External Crystalline Massifs (Ménot & Paquette, 1993; Rubatto et al., 2001; Guillot & Ménot, 2009) and, as in the Argentera, occur as relatively small bodies within the migmatitic basement.
The Laghi del Frisson mafic sequence (Fig. 5a), exposed in a tectonic window within the Helvetic-Dauphinois sedimentary cover (Fig. 3), forms a lens-shaped body, about 200 m thick and 500 m long, surrounded by biotite-bearing migmatites of broadly granitic composition (‘‘biotite anatexite’’of Malaroda et al., 1970). First mapped and described as "garnet–pyroxene gneiss" by Campanino (1962 in Malaroda et al., 1970), the sequence is characterized by a compositional layering that mainly consists of alternating mm- to dm-thick mafic layers of green-brownish Pl-poor and gray-whitish Pl-rich HP granulite. The rocks of the sequence are medium- to fine-grained, do not show evidence of partial melting, and are characterized by a pervasive mylonitic foliation parallel to the compositional layering (Colombo et al., 1994; Ferrando et al., 2008; Fig. 5b). The sequence shows a high degree of crustal contamination (Rubatto et al., 2010).
This chemical signature is common to other Ordovician mafic rocks exposed in the other External Crystalline Massifs, though these are often associated with Si-rich magmas, which are lacking in the Laghi del Frisson sequence. The systematic and repeated interlayering of the two main compositional types and their geochemical characteristics are inherited from a magmatic protolith, most likely a mafic layered intrusion. The original igneous layers were strongly reworked and folded by a pervasive mylonitic deformation, locally observed all through the GSV Terrane and predating widespread migmatization in the Late Carboniferous (see below). Lack of migmatisation of the mafic sequence is attributed to its more refractory composition when compared to the surrounding migmatites (Ferrando et al., 2008).
Figure 5. GSV Terrane: HP mafic rocks
(a) Lago Frisson and the ridge between the upper Val Grande di Vernante and Val Gesso. The banded HP granulites are exposed in the small hill by the lake and in the ridge to the right of the saddle (Passo della Mena). (b) Lago Frisson HP granulites. Typical compositional banding parallel to a pervasive mylonitic foliation. Banding consists of grey-whitish Pl-rich HP granulite and reddish Pl-poor HP granulite. (c) Banded amphibolite with portions containing remnants of eclogitic garnet. GSV Terrane. Lago di Nasta, upper Gesso della Valletta valley.
Both the Pl-poor and Pl-rich HP granulites contain several generations of minerals which, coupled with thermobarometric data, define four metamorphic stages (Fig. 6; Ferrando et al., 2008). The HP granulite-facies peak (stage A: 735 ± 15 °C, ~ 1.38 GPa) is recorded in the core of porphyroclastic garnet and omphacite in association with plagioclase, rutile ± amphibole ± quartz. The first decompression (stage B: ~ 710 °C and 1.10 GPa) corresponds to the rim of porphyroclastic garnet and omphacite in equilibrium with a second generation of plagioclase, rutile ± amphibole ± quartz. Mylonitization (stage C) is marked by neoblastic garnet, diopside, plagioclase, titanite ± amphibole ± quartz, and occurred at amphibolite-facies conditions, i.e. pressures of 0.85 GPa and still relatively HT (665±15 °C). Finally, during stage D (500 < T < 625 °C; P < 0.59 GPa) plagioclase + amphibole symplectites replaced the rim of garnet and clinopyroxene. The evolution and peak metamorphic conditions recorded by the Laghi del Frisson mafic sequence are similar to those recorded by the Meris eclogite (T ~ 750°C and P ~ 1.5 GPa; Colombo, 1996; Rubatto et al., 2001). Based on the common metamorphic conditions, Ferrando et al. (2008) concluded that the Laghi del Frisson HP granulites and the Meris eclogite underwent the same metamorphic cycle and that the two rock types preserve different peak assemblages only because of their different bulk chemical composition.
Geochronology of HP rocks from the Argentera was first attempted by Paquette et al. (1989) using the zircon isotope-dilution TIMS technique on garnet amphibolites with relict eclogitic garnet from the Argentera Massif and eclogites from the Belledonne and Aiguilles Rouges Massifs. They obtained mainly discordant data, whose upper and lower intercepts are of difficult interpretation. In most samples, no age constraints on the HP metamorphism were obtained, but for the Argentera Massif a lower intercept of 424±4 Ma from an amphibolite was proposed as the age of HP metamorphism. Notably, a second mafic rock from the same area returned an upper intercept at ~350 Ma with a meaningless lower intercept. More recently, Carboniferous ages at ~340 Ma were obtained for zircon rims in the Frisson Pl-rich HP granulite and sector zoned zircons of the Pl-poor HP granulite (340.7 ± 4.2 and 336.3 ± 4.1 Ma, respectively: Rubatto et al., 2010). Several lines of evidence constrain the formation of the Carboniferous zircon rims to before mylonitization stage C and, possibly, during stage A: 1) the HREE depletion in the zircon rims is in line with formation, before or during zircon crystallization, of metamorphic garnet that sequestrated HREE from the reactive bulk rock (Rubatto, 2002); 2) the zircon rims lack a significant negative Eu anomaly, which is also absent in the other peak metamorphic minerals such as omphacite, garnet and plagioclase; 3) Ti-in-zircon thermometry indicates temperatures of at least 700–770 °C, which are within that reported for the HP peak of stage A, but generally higher than those of the first retrogression of stage B; 4) the Carboniferous zircon domains show evidence of intense deformation, likely related to the mylonitic stage.
Figure 6. P-T path of the GSV Terrane
P–T path of the GSV Terrane and metamorphic stages A to D of the Lago Frisson HP granulites, from Ferrando et al. (2008). Phase relations for Al2SiO5 are after Holdaway and Mukhopadhyay (1993) and the wet granite solidus is after Aranovich and Newton (1996).
Metavolcanic rocks
The presence of dacitic to rhyodacitic metavolcanic rocks at Cima Ghiliè-Testa della Rovina and in Val Meris (Fig. 3) is reported by Romain (1982, p.30), Ghiglione (1990), Bierbrauer (1995), Colombo (1996) and Rubatto et al. (2001). The Cima Ghiliè metadacite is undeformed, preserves a porphyritic structure and contains granulite-facies xenoliths. This rock type shows progressive deformation to foliated metadacite (Fig. 7), and transitional contacts to the surrounding migmatites. The undeformed metadacite preserves phenocrysts of quartz, plagioclase, K-feldspar, biotite and altered cordierite (Colombo et al., 1993). The microcrystalline matrix consists of quartz, feldspars and aggregates of biotite + quartz that probably derived from former xenocrysts of granulite-facies orthopyroxene. The metadacite contains pink, euhedral zircons with composite cores, with variable zoning, and oscillatory-zoned overgrowths. The cores yielded concordant U-Pb ages at 560 Ma, 600 Ma and 700 Ma, respectively. The SHRIMP analyses of the oscillatory-zoned overgrowths yielded a mean 206Pb/238U age of 443 ± 3 Ma (Rubatto et al., 2001).
Figure 7. GSV Terrane: metadacite
Slightly foliated metadacite with xenoliths of metamorphic rocks including Grt-Opx granulite. GSV Terrane. Cima Ghilié, upper Gesso della Valletta valley.
The rocks described in the SE GSV Terrane as “pseudoporphyric gneisses” by Malaroda (1991; 1999), though more deformed and recrystallized, appear to be similar to the Ghiliè metadacite both in structure and composition, and are indicated with the same symbol in Fig. 3. Part of the “leptynites”, reported especially in the NW of the GSV Terrane in the map of Malaroda et al. (1970), are possibly also derived from recrystallized acid metavolcanics.
Migmatitic paragneisses and migmatitic granitic gneiss
A striking feature of the GSV Terrane is the abundance and variety of migmatites (Malaroda, 1968), formed from granitic (“anatexites” Auct.), pelitic (Fig. 8a) and mafic protoliths. The bulk composition of the “anatexites” is homogeneously granitic (Faure-Muret, 1955; Malaroda & Schiavinato, 1957a; De Pol, 1966; Blasi & Schiavinato 1968). These rocks generally lack structural relicts of the igneous protolith. A notable exception are the “augen anatexite” and “augen embrechite” (Fig. 8b; Malaroda et al., 1970), which are likely derived from the Late Ordovician porphyritic granitoids that have been extensively documented in the pre-Variscan basement of the External Crystalline Massifs (e.g. Aiguilles Rouges: Bussy & von Raumer, 1993).
Figure 8. GSV Terrane: Migmatites I
(a) Thinly banded paragneiss showing a discordant leucosome with concordant branches developed along an incipient dextral shear zone. GSV Terrane, Laghi della Sella, upper Valle della Meris. (b) Migmatite with K-feldspar porphyroclasts derived from a granitoid of probable Ordovician age. GSV Terrane. Ridge between Valrossa and Valmiana, Valle di Valasco, upper Gesso della Valletta valley.
Figure 9. GSV Terrane: Migmatites II
(a) Thinly banded “anatexite” with both concordant and discordant granitoid leucosomes. GSV Terrane. Path from Rifugio Genova to Colle Fenestrelle, upper Valle della Rovina. (b) “Anatexite” showing cm-thick veins of pinitized cordierite ± quartz corresponding to the latest stages of the anatectic melt crystallisation. GSV Terrane. Upper Valle della Meris.
Figure 10. GSV Terrane: Bousset-Valmasque Complex
(a) “Granite de la Valmasque”. “Agmatite” with angular melanocratic fragments (“enclaves”) of hornblendite in a leucogranite matrix. The white patches in the hornblendite are plagioclase-diopside aggregates. GSV Terrane. Upper Valmasque, Pas de la Fous. This outcrop is the same as that described by Faure-Muret (1955, p.100 and Fig.1, Plate XI). (b) Ultramafic fragments are enclosed in an anatectic matrix of granitoid composition. The ultramafite-matrix contact is marked by a continuous rim of radially arranged amphibole, whereas the larger ones still have a serpentinized core. The smaller fragments are completely replaced by amphibole. GSV Terrane. Path between Rifugio Genova and Colle Fenestrelle,upper Valle della Rovina. (c) A larger ultramafic block in a granitoid matrix with sub-idioblastic plagioclase crystals (“perlgneiss”). Path between Rifugio Genova and Colle Fenestrelle, upper Valle della Rovina.
Although a discussion on the protoliths and genesis of the GSV migmatites is outside the scope of this review, we note that leucosomes in the migmatitic paragneiss and migmatitic granitic gneiss have mineral assemblages compatible with the main amphibolite-facies metamorphic stage recorded in the whole GSV Terrane. Specifically, the mineral assemblage observed in metapelites of the upper Val Gesso (Compagnoni et al., 1974; Bierbrauer, 1995; Prever, 1997)—i.e. quartz-plagioclase (An20-30)-biotite-fibrolitic sillimanite-cordierite with scarce K-feldspar and muscovite—and the relict kyanite and garnet rarely included in plagioclase suggest P-T conditions of the upper amphibolite-facies (650–700°C; 0.4–0.6 GPa) and decompression from higher P, respectively. In the migmatitic granitic gneiss, more rarely in migmatitic paragneisses, the end products of partial melting are pockets and larger bodies of a fine-grained muscovite-biotite granite [“granito aplitico microgranulare di anatessi” (fine grained aplitic anatectic granite) of Malaroda et al. (1970)] and quartzo-feldspathic pods and dykes that commonly contain large crystals of pinitized cordierite (Fig. 9).
The LP metamorphism and partial melting in the GSV Terrane are not directly dated, but a Late- to Mid-Carboniferous age for migmatization (≤ 323 ± 12 Ma) has been proposed on the basis of a zircon lower intercept age obtained from the Meris eclogite (Rubatto et al., 2001).
The Bousset-Valmasque Complex
The Bousset-Valmasque Complex was named “Entracque-Tenda Granite” by Roccati (1925) and “Granite à enclaves de la Valmasque” by Faure-Muret (1955). It consists of amphibole migmatite and amphibole-bearing anatectic granite that contains enclaves of amphibolite and ultramafics of angular or rounded shape, with size from a few cm to a few meters (Fig. 10). The Complex comprises two E-W directed subvertical bands separated by the Alpine Fremamorta-Colle del Sabbione shear zone (Fig. 3). The N band extends to a maximum thickness of ~ 5 km between Lago Brocan and the Bousset valley, whereas the S band extends to a maximum thickness of 1 km in the Gelas-Clapier ridge between the Gesso della Barra valley and the upper Valmasque. As noted by Roccati (1925) and Faure-Muret (1955), a noteworthy feature of the Bousset-Valmasque Complex is the great variety of mafic-ultramafic enclaves (including rare eclogite and metagabbro) (Fig. 10a), some of which were studied by Faure-Muret (1955) and Boucarut (1969). The igneous appearance and the lack of deformation of the Valmasque granite suggest either a peak anatectic origin linked to the Late Carboniferous metamorphic event or a post-metamorphic emplacement. However, no geochronological or modern petrological data exist to constrain the age and igneous/metamorphic history of the Bousset-Valmasque Complex.
Carboniferous-Permian magmatic rocks
Late Variscan plutonic bodies are widespread in the External Crystalline Massifs of the Alps. The age of emplacement, Mg/(Fe + Mg) ratio, and mafic mineral content allow to recognise two plutonic suites: an earlier, Visean (~ 330–340 Ma) and Mg-rich suite, and a later suite, mainly Stephanian (~ 295–305 Ma) and richer in Fe (Debon & Lemmet, 1999).
In the Argentera Massif, rocks belonging to the Visean plutonic suite occur as small bodies and dykes of metamonzonite in the central part of the GSV Terrane, e.g. on the N slope of the Cima dei Gélas and in the upper Valmasque (Figs. 3 and 11). Such rocks were first reported by Roccati (1925) as “pyroxene-bearing porphyritic gneiss” and were later mapped as “biotite-amphibole embrechites” by Malaroda et al. (1970). One of the largest metamonzonite outcrops is exposed near the Muraion glacier, N of Cima dei Gelas. Here the plutonic rock, though showing evidence of incipient migmatization, well preserves the original igneous fabric and mineralogy. The magmatic assemblage consists of cm-sized euhedral crystals of K-feldspar and minor plagioclase in a medium-grained mineral aggregate, which contains relicts of magmatic clinopyroxene that are partly replaced by amphibole and biotite. U-Pb SHRIMP analyses of zircons from this rock yielded a concordant age of 332 ± 3 Ma (Rubatto et al., 2001).
The Stephanian magmatic suite is represented in the GSV Terrane by the Central Granite, a large pluton straddling the boundary between Italy and France in the upper Gesso and Vésubie valleys, and by a minor leucogranite body in the upper Valle Stura (Brondi, 1958; Malaroda et al., 1970, and references therein). The Central Granite is a shallow pluton cutting across the regional metamorphic foliation of country rocks. It was emplaced through magmatic stoping at shallow crustal levels after the Variscan amphibolite-facies metamorphism and anatexis (Boucarut, 1967). Contact metamorphism of the surrounding rocks produced both andalusite and sillimanite (Compagnoni et al., 1974). The Valle Stura leucogranite, on the other hand, is parallel to the regional foliation trend of country rocks, though probably discordant at the outcrop scale.
Figure 11. GSV Terrane: metamonzonite
Deformed metamonzonite where are evident porphyroclasts of the relict igneous K-feldspar and an anatectic dykelet of granitoid composition (centre left), which is cutting across the pre-migmatitic metamorphic foliation . GSV Terrane. Upper Valmasque, just below the Pas de la Fous.
Compositionally, the Central Granite is a biotite-muscovite monzogranite in which four structural and compositional varieties have been distinguished and mapped (Malaroda & Schiavinato, 1957b, Malaroda et al., 1970): 1) medium-grained granite, 2) granite with large crystals of K-feldspar, 3) fine-grained aplitic granite, and 4) fine-grained porphyritic granite. Both variety 1 and 3 are locally garnet-bearing. Rounded enclaves of a quartz microdiorite (“enclaves microgrenues plagioclasiques” of Boucarut, 1969) with acicular apatite crystals are common in the central part of the pluton between Lac Nègre and Colle di Fremamorta.
The emplacement age of the Central Granite is constrained by Rb-Sr ages on magmatic muscovites (Ferrara & Malaroda, 1969) at 292 ± 10 Ma (av. of 4 analyses), whereas a cooling age at ca 296-299 Ma was proposed on the basis of 40Ar/39Ar single grains muscovite analyses from the granite (Corsini et al., 2004). The emplacement age of the Valle Stura leucogranite is only constrained by a single Rb/Sr muscovite age from a dyke from Bagni di Vinadio at 299 ± 10 Ma (Ferrara & Malaroda, 1969).
A probably younger suite of intrusive rocks is represented by quartz-porphyry dykes with large crystals of K-feldspar exposed on the W ridge of Cima del Dragonet, E of Terme di Valdieri (Franchi 1894; Malaroda et al., 1970). Early Permian-Late Carboniferous basalt-andesite pyroclastics and lava flows are reported from the SE of the GSV Terrane (Romain, 1978), in particular from the Vallon de Figuière Unit (Malaroda ,1999). The widespread swarms of basalt-andesite rocks [“porfiriti anfibolico-plagioclasiche, talora quarzifere” (locally quartz-bearing plagioclase-amphibole porphyrites) of Malaroda et al. (1970)] that cut across the Central Granite and its country rocks are possibly related to this young volcanism.
Carboniferous sedimentary sequences
Late Carboniferous sediments (conglomerates, sandstones, fossiliferous black schists) are reported from synforms in the GSV gneisses of the upper Vésubie valley (Fig. 3) by Faure-Muret (1955, and references therein) and Haudour et al. (1958). Plant fossils indicate a Westphalian D-Stephanian A age (Faure-Muret, 1955; Haudour et al., 1958). Deposition of these sediments marks the final exhumation of the GSV Terrane after the Variscan orogeny.
Unfossiliferous metasediments of presumed Late Carboniferous age (mollieresite Auct.) are described in the area between the Mollières valley and Saint Martin Vésubie (Faure-Muret, 1955; Malaroda et al., 1970). The mollieresite is lithologically similar to the fossiliferous Late Carboniferous—as it consists of conglomerates, sandstones and graphitic schists—but is affected by low-grade metamorphism (Bortolami et al., 1974).
Ferriere–Mollières shear zone
The Ferriere–Mollières shear zone represents the main shear zone cutting the Paleozoic basement in the western portion of the Argentera Massif. It strikes NW–SE and extends from Ferriere (Valle Stura) to the northwest, to Mollières to the southeast. Mylonitic rocks, which are unconformably covered by the Triassic sediments in the Ferriere area, almost continuously crop out along the Ferriere–Mollières shear zone with a thickness ranging from 100 to 1000 m (Fig. 3). These mylonites have been interpreted by Bogdanoff (1986) as a sequence including both metasedimentary rocks (“Micaschistes de La Valetta”) and mylonites derived from high-grade metamorphic rocks of both the Tinée and GSV terranes.
In a typical section of the Ferriere–Mollières shear zone between Colle di Stau and Rocco Verde (Valle Stura), mylonites reach a thickness of nearly 1 km (Musumeci & Colombo, 2002). Foliations strike N140–150E and show upright attitude or dip strongly toward northeast or southwest. Mineral lineations are defined by elongated grains of quartz or feldspar, which have a dominant subhorizontal trend, or gently plunge (20°) toward northwest. The mylonites include small bodies of foliated, mica-rich quartzites [“Quarziti del Pebrun” (“Pebrun quartzites”) of Malaroda et al., 1970], amphibolites and marbles. In the Rocco Verde section, the Ferriere–Mollières mylonites are interleaved with dm- to m-thick ultramylonitic and phyllonitic layers. In mylonites and phyllonites, foliations dip strongly toward southwest and mineral lineations or slickenside striae are subhorizontal or gently dipping toward northwest. Mylonitic mineral assemblages are indicative of low- to very low-grade metamorphic conditions, whereas the porphyroclasts are derived from high-grade gneiss.
Mylonitic muscovite-bearing leucogranites crop out as subvertical northwest-trending intrusions in the northern portion of the Ferriere–Mollières shear zone. A Rb/Sr muscovite – whole rock age of 327±3 Ma on foliated leucogranite gives a lower limit for the age of mylonitic deformation for which kinematic indicators indicate a dextral sense of shear. This strike-slip tectonics is compatible with the extensional regime that occurred during Carboniferous across western Europe (Musumeci & Colombo, 2002).
Tinée Terrane
The Tinée terrane has been subdivided by Faure-Muret (1955) into three metamorphic formations (Fig. 3): the metasedimentary Varélios-Fougieret, the Anelle-Valabres (see also: Prunac, 1976) and the Rabuons lithological Formations. Part of the Rabuons Formation is exposed on the Italian side of the Argentera Massif in the upper part of side valleys joining the Stura River from Ferriere to Bagni di Vinadio (Malaroda et al., 1970). Lithologies in this area were grouped into two sequences, the Corborant - and Laroussa Series (Sacchi, 1961; De Pol ,1963), equivalent to the Rabuons and Anelle Formations of the French side, respectively.
Figure 12. Lithologies from Tinée Terrane
(a) Two-micas plagioclase gneiss typical of the Anelle Formation. A biotite-rich foliation with quartz, plagioclase and minor muscovite, sillimanite and garnet wraps around elongated quartz-plagioclase aggregates. Saint Etienne de Tinée, Vallon d’Assuéros, c. 1200 m a.s.l. Faure-Muret collection. Musée d’ Histoire Naturelle de Nice, France. Sample 139/46 and Plate II, Fig. 3 of Faure-Muret (1955). (b) Iglière granodioritic to tonalitic gneiss with a biotite foliation wrapping around quartz and feldspar aggregates. Road from Tinée Valley to Roya. Sample 289/45: Faure-Muret collection. Musée d’Histoire Naturelle de Nice, France. (c) Typical Rabuons gneiss, characterized by folded quartz-plagioclase-K-feldspar leucosomes in a quartz-oligoclase-K-feldspar-two micas-sillimanite matrix. Saint Etienne de Tinée, Route de l’Energie, Vallon du Lusernier, 2440 m a.s.l. Faure-Muret collection. Musée d’Histoire Naturelle de Nice, France. Sample 222/45 and Plate IV, Fig. 3 of Faure-Muret (1955). d) Augen gneiss of the Rabuons Formation with a mylonitic foliation. Porphyroclasts of K-feldspar and plagioclase and quartz-feldspar augen enclosed in a closely spaced foliation defined by biotite-rich layers and discontinuous quartz-feldspars layers. Saint Etienne de Tinée, Route de l’Energie, S of Mont Garnet, 2420 m a.s.l. Faure-Muret collection. Musée d’Histoire Naturelle de Nice, France. Sample 348/45 and Plate IV, Fig. 1 of Faure-Muret (1955).
The Anelle-Valabres and Rabuons Formations are less migmatitic than the GSV terrane (Fig. 12). The Rabuons migmatites (Fig. 12c) in particular contain a significantly smaller volume of neosome than the migmatites of the GSV. This neosome forms “augen” or ribbons of K-feldspar and plagioclase or muscovite-bearing granite (Sacchi, 1961; Malaroda, 1968, Tables I, II, III). Both Formations have relicts of more or less retrogressed eclogite (Faure-Muret, 1955; Malaroda et al., 1970). The Anelle-Valabres Formation also includes mafic granulites, coronitic metagabbros (Latouche & Bogdanoff, 1987; Fig. 3) and marbles (Prunac, 1976; Bogdanoff, 1980). In addition to muscovite related to regional metamorphism, the Anelle-Valabres Formation also contains sillimanite and late kyanite (Fig. 12a). The Rabuons Formation contains muscovite and sillimanite, with scarce kyanite present as relicts in gneisses (Faure-Muret, 1955; Bogdanoff, 1980, 1986) and in quartz veins (Pierrot et al., 1974).
Peculiar amphibole migmatites [amphibole-bearing gneiss and dioritic gneiss (“gneiss anfibolici” and “gneiss dioritici”) of Malaroda et al., 1970] that are somewhat similar to the Valmasque Granite, were reported by Franchi (1894, 1933) and Franchi & Stella (1930) in the Ferriere valley, at the NW tip of the Tinée Terrane. Similar migmatites also crop out further to the SE in the upper Bagni di Vinadio valley (Sacchi,1961).
The Tinée terrane is generally lacking large intrusive bodies. Minor intrusions are 1) the Iglière metagranodiorite (Fig. 12b), which was emplaced into the Anelle-Valabres Formation before the Variscan metamorphism and deformations (Bogdanoff & Ploquin 1980), and 2) a small body of aplitic granite and some discordant garnet-tourmaline-muscovite pegmatite dykes within the Rabuons Formation (Piccoli, 2002).
From his structural work in the upper Tinée valley, Bogdanoff (1980,1986) concluded that the Tinée Terrane records a long deformation history, with complex deformation in Late Proterozoic-Early Paleozoic times (D1). This phase was followed early in the Variscan cycle by HP metamorphism responsible for the formation of HP granulites and eclogites at the expense of basic rocks. A second deformation phase (D2) developed the regional foliation S2. The metamorphic overprint at amphibolite-facies conditions was accompanied by migmatisation. This metamorphism was synchronous with two deformation events at the megascopic scale: nappe stacking and recumbent folding of the Anelle and Rabuons Formations (D3), followed by verticalization of the earlier structures and upright folding at the hectometric scale (D4). A final deformation (D5) consisted of folds of variable style from metric to decametric scale and vertical shear zones, and did not modify significantly the earlier structures.
Time constraints on the evolution of the Tinée terrane are limited to 40Ar/39Ar data (Monié & Maluski 1983): muscovites from the Rabuons Formation yielded and age of 342 ± 7 Ma, the biotites from the Iglière metagranodiorite are as old as ~ 312-316 Ma, and amphiboles and biotites from the Rabuons gneiss were dated at ~ 299 and 283 Ma, respectively.