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g eoLogicaL setting

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EASTERN CENTRAL ALPS)

2. g eoLogicaL setting

The Austroalpine domain, located in SE Switzerland and N Italy (Fig. III-1ab), consists essentially of a Late-Cretaceous nappe-stack sampling remnants of the Adriatic passive margin formed during the Jurassic rifting (e.g. Trümpy, 1975). Notably, Jurassic rifting processes led to the exhumation of pre-rift Permian upper, middle, lower crust and sub-continental mantle rocks to shallow levels, as indicated by their P–T–t evolution and their contacts with Mesozoic sediments (Mohn et al., 2011; Mohn et al., 2010; Müntener et al., 2000). The portions of the Permian lithosphere exhumed during the Jurassic rifting were subsequently incorporated in the Austroalpine nappe stack. The low imprint of the Alpine, pressure-dominated metamorphism observed in the Austroalpine units enables us to investigate and characterize different portions of the basement affected by Variscan and post-Variscan events.

2.1 Evidence for Variscan orogeny

The pressure-dominated Variscan metamorphism in the Austroapline domain is testified by the presence of eclogites in the Upper Tonale unit (Ulten zone) at 15-25 kbar for 750-800°C (Hauzenberger et al., 1996; Tumiati et al., 2003) associated to Grt-peridotites preserving UHP

58 Formation et exhumation des granulites permiennes

evidence reaching 27 kbar for 850°C (Nimis & Morten, 2000). These HP-rocks are associated in the field to Grt–Ky migmatites to a P-peak at 11 kbar/750 °C at 351–343 Ma (U-Pb on Mnz), followed by a T-peak at 750–800 °C for 8.5–12.5 kbar at 330–326 Ma (U-Pb on Mnz;

Braga et al., 2007; Braga & Massonne, 2008; Langone et al., 2011). Variscan amphibolite facies metamorphism is documented for Grt–St micaschists of the Campo unit at ~5–9 kbar and 550–600°C (Bianchi Potenza et al., 1978a; Braga et al., 2001; Notarpietro & Gorla, 1981; Pace, 1966), in the Err and Bernina units (Büchi, 1994; Halmes, 1991), and inferred in the Margna unit (Guntli & Liniger, 1989). Calc-alkaline diorites and granodiorites were emplaced in the Bernina unit at 338–324 Ma (Rageth, 1984; Spillmann & Büchi, 1993; Von Quadt et al., 1994)

Tirano

Fig. III-1: (a) Tectonic map of the Alpine realm based on Schmid et al. (2004) with Permian to Triassic mafic intrusions modified after Spalla et al. (2014): 1–Monte Ragola (Meli et al., 1996); 2–Corio/

Monastero (Rebay & Spalla, 2001); 3–Versoyen unit (Beltrando et al., 2007); 4–Sassa (Baletti et al., 2012); 5–Sermenza (Bussy et al., 1998); 6–Mont Collon (Monjoie et al., 2007); 7–Ivrea main gabbro (e.g. Pin, 1986); 8– Finero (Zanetti et al., 2013); 9–Val Biandino (Pohl et al., 2014; Thöni et al., 1992);

10–Fedoz/Braccia (Hansmann et al., 2001); 11–Sondalo (Tribuzio et al., 1999); 12–Monzoni–Predazzo (Borsi et al., 1968; see references in Mundil et al., 2010); 13–Bressanone/Brixen (Del Moro & Visonà, 1982); 14–Eisenkappel (Miller et al., 2011) ; 15–Bärofen/Gressenberg (Miller & Thöni, 1997; Thöni

& Jagoutz, 1992). I.L. refers to the Insubric Line, P.F. to the Pennic Front. (b) Litho-tectonic map of the Austroalpine and upper-Penninic nappes in N-Italy and SE-Switzerland compiled from Del Moro

& Notarpietro (1987), Del Moro et al. (1999), Gosso et al. (2004), Mohn et al. (2011), Staub (1946), the 1:25,000 geological maps of Switzerland, the 1:10,000 and 1:25,000 geological maps of Italy and personal observations. Black rectangle in (b) reports location of the map in Fig. III-2a. Main Permian intrusives are reported and correspond to Fe, Fedoz gabbro; Br; Braccia gabbro; So, Sondalo gabbro;

Se, Serottini intrusives; and Ma, Martell granite.

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Chapitre III : Métamorphisme post-varisque dans les Alpes

but do not present evidence of Variscan deformation (Spillmann & Büchi, 1993). In the Margna unit, lithologies geochemically similar to intrusive rocks of the Bernina unit (Guntli & Liniger, 1989) are pervasively deformed (orthogneiss and mylonites) during the Alpine orogenic cycle (Spillmann & Büchi, 1993). The surface expression of these intrusive rocks is likely related to the undated calc-alkaline volcanics documented in the Err unit (Mercolli, 1989).

2.2 Evidence for the post-Variscan evolution in the upper and lower crust During the Permian and after the Variscan orogeny, the complete lithosphere was strongly affected by magmatism and metamorphism. At upper crustal levels, alkaline felsic magmatic rocks were emplaced. In the Bernina unit, the Carboniferous calc-alkaline suite is intruded by an alkaline syenite-granite (Spillmann & Büchi, 1993) at 295–292 Ma (Von Quadt et al., 1994) at P < 3 kbar (Al in magmatic hornblende, Büchi, 1987) and a granodiorite at 285 Ma at P < 4.5 kbar (Brusio granite with andalusite-bearing contact aureole; Boriani et al., 1982). Both calc-alkaline and alkaline intrusive rocks and their host rocks are unconformably overlain by younger alkaline rhyolites of 288 ± 7 Ma (Rageth, 1984; Staub, 1946; Von Quadt et al., 1994) occurring also as discordant dykes in few areas (Spillmann & Büchi, 1993). The overall rock association is cross-cut by basic dykes of unknown age but affected by a strong Alpine recrystallization (Rageth, 1984; Spillmann & Büchi, 1993).

Notably, the Permian lower crust was intruded by magmas of tholeiitic affinity such as the Fedoz and the Braccia gabbros in the Malenco unit (Fig. III-1b) at 281–278 Ma (U-Pb on Zrn, Hansmann et al., 2001; Spillmann & Büchi, 1993). The latter emplaced at the crust-mantle transition zone around 10 kbar (Hermann et al., 2001; Hermann et al., 1997).

The Braccia gabbro is overlain by HT-MP felsic granulites in the Malenco unit (Hermann et al., 1997). The felsic granulites consist of Grt–Ky bearing restites equilibrated at 10 kbar for 800-850°C (Hermann et al., 1997; Müntener et al., 2000). Monazite dating shows that HT conditions lasted ~20 Ma between 280 and 257 Ma (Hermann & Rubatto, 2003).

Crystallization of hydrous phases at ~9 kbar and 600°C (Müntener et al., 2000) was dated to late-Triassic early-Jurassic times (Villa et al., 2000), related to the onset of rifting leading to its exhumation (Müntener & Hermann, 2001; Trommsdorff et al., 2005).

60 Formation et exhumation des granulites permiennes

2.3 Evidence for the post-Variscan evolution in the middle crust

The middle Austroalpine Campo unit (following the nomenclature of Mohn et al., 2011) was thrust westwards over the lower Austroalpine units and was overprinted at its southern part by the Tertiary Insubric line. The Campo unit is separated from the overlying Grosina unit along a Jurassic shear zone (Fig. III-2a; Eita shear zone; Mohn et al., 2012). Both Grosina and Campo were thrust during Late-Cretaceous by the Filladi di Bormio unit (e.g. Gorla & Potenza, 1975).

The Campo unit consists of amphibolite-facies micaschist, paragneiss and minor amphibolite and calc-silicate of Ordovician protolith age (Bergomi & Boriani, 2012) presenting a variable eo-Alpine and Alpine overprint at its borders (Gazzola et al., 2000; Schmid & Haas, 1989).

The Campo unit was sporadically intruded by Permian granitoids (Bockemühl &

Pfister, 1985; Gazzola et al., 2000; Guglielmin & Notarpietro, 1997; Notarpietro & De Capitani, 1985a; Notarpietro & De Capitani, 1985b) at ~285-259 Ma based on Rb–Sr dating on muscovite and biotite (Boriani et al., 1982; Del Moro & Notarpietro, 1987; Del Moro et al., 1981). Their emplacement occurred at P < 4.5 kbar indicated by the presence of andalusite and sillimanite in the contact aureole (e.g. Del Moro & Notarpietro, 1987). The Campo unit is crosscut by pegmatites bearing Qtz–Pl–Ms–Bt–Grt–Ap±Dum±Chryso-Brl±Tur (Fig. III-4c; Linck, 1899) of 255–250 Ma Sm–Nd on garnet ages, 257-251 Rb–Sr on muscovite ages and 217–187 Ma K–Ar on muscovite ages (Hanson et al., 1966; Sölva et al., 2003; Thöni, 1981). The pegmatites have a common origin with the Tur-bearing Martell granite (Fig. III-1b), emplaced between 275 and 260 Ma (Bockemühl, 1988; Grauert et al., 1974a; Mair & Schuster, 2003). K–Ar ages of pegmatites coincide with the 40Ar/39Ar ages on biotite and muscovite from the Campo unit ranging between 184 and 189 Ma (Meier, 2003; Mohn et al., 2012; Thöni, 1981) interpreted to be related to Jurassic exhumation and cooling during rifting (Mohn et al., 2012). Rare trachytic dykes crosscut the Campo unit at ca. 32 ± 1 Ma (K–Ar on groundmass, Bianchi Potenza et al., 1985) and are related to the Periadriatic intrusion suite (e.g. Callegari & Brack, 2002; von Blanckenburg, 1992).

The Sondalo gabbroic complex is a ~40 km² mafic pluton derived from tholeiitic parental liquids (Tribuzio et al., 1999) emplaced at 4–6 kbar in the Campo unit (Braga et al., 2003; Braga et al., 2001). The pluton is concentrically zoned with an Ol-gabbro core surrounded by Opx-Cpx-bearing gabbro with a dioritic to granodioritic rim of a variable thickness (Campiglio &

Potenza, 1964; Campiglio & Potenza, 1966; Campiglio & Potenza, 1967; Koenig, 1964; Potenza, 1973). The pluton incorporated roof pendants (kinzigitic and granulitic metapelitic xenoliths) up to 300m in thickness and 1km in length during its emplacement whereas migmatites are localized in the contact aureole. Sm–Nd mineral-isochron dating performed on troctolite and

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Chapitre III : Métamorphisme post-varisque dans les Alpes

A

Filladi di Bormio - phyllite

Grosina unit - orthogneiss & metapelite pre-rift upper crust

Syn-rift (Middle Jurassic) Pre-rift (Triassic)

Sedimentary cover

Campo unit - micaschist & paragneiss pre-rift middle crust

Location of samples used for pictures and petrology

Fig. III-2: Geological and structural maps of the study area. (a) The study area of the Campo/Grosina.

Location of field photographs (Figs III-4 & III-5), samples used for petrology (labelled BPA) and location of the A–B transect presented in Fig. III-3 are indicated. (b) The Sondalo gabbro. Maps are compilation of Mohn et al. (2011), Campa et al. (1997), Koenig (1964), Braga et al. (2003), 1:10,000 geological maps of Italy and personal observations. (c) Stereonets of planar structures Sc1, Sc2 from the host-rock and from xenoliths of the Sondalo gabbro, Sc3 and magmatic foliation (equal-area, lower hemisphere projection).

62 Formation et exhumation des granulites permiennes

norite from the Sondalo gabbro s.l. gave respectively 300 ± 12 and 280 ± 10 Ma whereas Rb–Sr isochron ages are 266 ± 10 and 269 ± 16 Ma (Tribuzio et al., 1999) identical to a U–Pb zircon age from a diorite (~270 Ma; Bachmann & Grauert, 1981).

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