2.a. Continental crust in the Western Alps

In the Western Alps, several tectonic units contain slices of continental crust sampling three main continental blocks involved in the Alpine collision (Dal Piaz et al., Reference Dal Piaz, Bistacchi and Massironi2003; Schmid et al., Reference Schmid, Fügenschuh, Kissling and Schuster2004, Reference Schmid, Kissling, Dichl, van Hinsbergen and Molli2017; Dal Piaz, Reference Dal Piaz2010; Handy et al., Reference Handy, Schmid, Bousquet, Kissling and Bernoulli2010, Reference Handy, Schmid, Paffrath and Friederich2021; Agard & Handy, Reference Agard and Handy2021; Brunsmann et al., Reference Brunsmann, Rosenberg and Bellahsen2024). These slices are derived from:

(i) The European passive margin, exposed in the Helvetic-Dauphinois domain, where pre-Mesozoic basement rocks are exposed in the External Crystalline Massifs (Fig. 1), associated with a Meso-Cenozoic cover. In these massifs, Alpine metamorphism reaches only sub-greenschist to greenschist facies conditions (e.g. Bousquet et al., Reference Bousquet, Oberhänsli, Goffé, Wiederker, Koller, Schmid, Schuster, Engi, Berger and Martinotti2008; Bellanger et al., Reference Bellanger, Bellahsen, Jolivet, Baudin, Augier and Boutoux2014), and Variscan tectonics is largely preserved (Simonetti et al., Reference Simonetti, Carosi, Montomoli, Langone, D’Addario and Mammoliti2018; Reference Simonetti, Carosi, Montomoli, Cottle and Law2020, Reference Simonetti, Carosi, Montomoli, Law and Cottle2021).

Figure 1.

Tectonic sketch of the Western Alps (modified from Dana, Reference Dana2024). The geolocation of the map is shown in the upper left insert. Magmatic protoliths with ages around 500–525 Ma are reported (1: Liati et al., Reference Liati, Gervers, Froitzheim and Fanning2001; 2: Sartori et al., Reference Sartori, Gouffon and Marthaler2006; 3: Scheiber et al., Reference Scheiber, Berndt, Mezger and Pfiffner2014; 4: Bussy et al., Reference Bussy, Derron, Jacquod, Sartori and Thélin1996; 5: Guillot et al., Reference Guillot, Liégeois and Fabre1991; 6: Bertrand et al., Reference Bertrand, Pidgeon, Leterrier, Guillot, Gasquet and Gattiglio2000b, 7: Bertrand et al., Reference Bertrand, Guillot and Leterrier2000a; 8: Ménot et al., Reference Ménot, Peucat, Scarenzi and Piboule1988; 9: Thiéblemont et al., Reference Thiéblemont, Jacob, Lach, Guerrot and Leguérinel2023; 10: Balestro et al., Reference Balestro, Festa, Cadoppi, Groppo and Roà2022; 11: Filippi et al., Reference Filippi, Jouffray, Lardeaux, Tiepolo and Spalla2024). The tectonic subdivision of the Western Alps is largely based on Schmid et al. (Reference Schmid, Fügenschuh, Kissling and Schuster2004, Reference Schmid, Kissling, Dichl, van Hinsbergen and Molli2017) and Gouffon et al. Reference Gouffon, Bernoulli, Dall’agnolo, Fantoni, Jordan, Madritsch, Mosar, Picotti, Pfiffner, Schenker, Schlunegger and Schmid(2024b). The red box highlights the position of Fig. 2. Abbreviations of the tectonic units cited in the text: AM = Ambin Massif, DM = Dora-Maira, GP = Gran Paradiso, MR = Monte Rosa, R = Ruitor, SM = Siviez-Mischabel, ZH = Zone Houillère.

(ii) The Briançonnais microcontinent (Michard et al., Reference Michard, Schmid, Lahfid, Ballèvre, Manzotti, Chopin, Iaccarino and Dana2022), whose remnants are exposed in the Middle Penninic domain, separated from the Helvetic-Dauphinois by the Penninic Frontal Thrust (Ceriani et al., Reference Ceriani, Fügenschuh and Schmid2001).

(iii) The Adria extensional allochthons and the Adriatic continental plate. The extensional allochthons (Sesia and Dent Blanche Units; Fig. 1) underwent hyper-extension during the Latest Triassic to Early Jurassic rifting (Mohn et al., Reference Mohn, Manatschal, Müntener, Beltrando and Masini2010; Manzotti et al., Reference Manzotti, Ballèvre, Zucali, Robyr and Engi2014) and nowadays are exposed on top of the nappe pile in the Salassic domain (Marthaler et al., Reference Kroner and Romer2020; Gouffon et al., Reference Gouffon, Bernoulli, Dall’agnolo, Fantoni, Jordan, Madritsch, Mosar, Pfiffner, Picotti and Schenker2024a). These units experienced blueschist to eclogitic Alpine metamorphism (Compagnoni, Reference Compagnoni1977; Beltrando et al., Reference Beltrando, Compagnoni and Lombardo2010; Manzotti et al., Reference Manzotti, Ballèvre, Zucali, Robyr and Engi2014; Reference Manzotti, Rubatto, Zucali, Korh, Cenki-Tok, Ballevre and Engi2018). In contrast, an almost complete crustal section of the former Adriatic continental plate is exposed in the South Alpine domain (Fig. 1), where Alpine metamorphism is absent or weak and mostly related to deformation zones (Henk et al., Reference Henk, Franz, Teufel and Oncken1997; Handy et al., Reference Handy, Franz, Heller, Janott and Zurbriggen1999; Schmid et al., Reference Schmid, Kissling, Dichl, van Hinsbergen and Molli2017).

The Briançonnais microcontinent, which was separated from the European margin by the Valaisan basin, likely formed during the Cretaceous (Frisch, Reference Frisch1979; Stampfli, Reference Stampfli1993; De Broucker et al., Reference De Broucker, Siméon, Stampfli, Thiéblemont, Lach and Marthaler2021; Boschetti et al., Reference Boschetti, Mouthereau, Schwartz, Rolland, Bernet, Balvay and Lahfid2025). The Briançonnais-derived units include (i) the Grand Saint Bernard nappe system, comprising detached cover units in the French-Italian Alps and basement units located west of the ophiolite-bearing units (e.g. Desmons, Reference Desmons1992; Sartori et al., Reference Sartori, Gouffon and Marthaler2006; Dumont et al., Reference Dumont, Schwartz, Guillot, Malusà, Jouvent, Monié and Verly2022; Pantet et al., Reference Pantet, Epard, Masson, Baumgartner-Mora, Baumgartner and Baumgartner2023; Dana et al., Reference Dana, Iaccarino, Schmid, Petroccia and Michard2023; Fig. 1), and (ii) the basement-dominated Internal Crystalline Massifs, which lack significant Mesozoic covers (Gasco et al., Reference Gasco, Gattiglio and Borghi2011; Reference Gasco, Gattiglio and Borghi2013; Ballèvre et al., Reference Ballèvre, Camonin, Manzotti and Poujol2020). The paleogeographic attribution of the Internal Crystalline Massifs (Dora-Maira, Gran Paradiso and Monte Rosa, Fig. 1) has been debated (e.g. Froitzheim et al., Reference Froitzheim2001, proposed a sub-Penninic origin, at least for the Monte Rosa Massif), but they are generally considered part of the pre-Triassic basement of the Briançonnais microcontinent (Michard, Reference Michard1967; Ballèvre et al., Reference Ballèvre, Manzotti and Dal Piaz2018, Reference Ballèvre, Camonin, Manzotti and Poujol2020; Michard et al., Reference Michard, Schmid, Lahfid, Ballèvre, Manzotti, Chopin, Iaccarino and Dana2022). The Briançonnais-derived units record peak Alpine metamorphism, varying from eclogite and blueschist facies conditions in the Internal Crystalline Massifs to blueschist, greenschist and sub-greenschist facies in more external Classic Briançonnais units (Bousquet et al., Reference Bousquet, Oberhänsli, Goffé, Wiederker, Koller, Schmid, Schuster, Engi, Berger and Martinotti2008; Beltrando et al., Reference Beltrando, Compagnoni and Lombardo2010). UHP conditions were reached in the Brossasco-Isasca and Rocca Solei (Southern Dora-Maira) and in the Chasteiran, Muret and Serre (Northern Dora-Maira) Units (Chopin, Reference Chopin1984; Manzotti et al., Reference Manzotti, Schiavi, Nosenzo, Pitra and Ballèvre2022, Reference Manzotti, Schiavi, Ballèvre and Nosenzo2025b; Groppo et al., Reference Groppo, Ferrando, Tursi and Rolfo2025).

2.b. Overview of the pre-Alpine evolution of the Western Alps

Intrusive rocks dated between c. 530 and 500 Ma represent the oldest magmatic rocks in the Western Alps (Fig. 1; e.g. Bertrand et al., Reference Bertrand, Pidgeon, Leterrier, Guillot, Gasquet and Gattiglio2000b; Thiéblemont et al., Reference Thiéblemont, Jacob, Lach, Guerrot and Leguérinel2023). Along with large outcrops of metabasic rocks of similar ages (540–500 Ma) found in the Lepontine and Austroalpine units, these rocks have been interpreted as allochthonous fragments of the Cadomian basement (von Raumer et al., Reference Von Raumer, Stampfli, Arenas and Sánchez Martínez2015; Siegesmund et al., Reference Siegesmund, Oriolo, Schulz, Heinrichs, Basei and Lammerer2021, Reference Siegesmund, Oriolo, Broge, Hueck, Lammerer, Basei and Schulz2023; Thiéblemont et al., Reference Thiéblemont, Jacob, Lach, Guerrot and Leguérinel2023). They could represent pieces of former Ediacaran to Cambrian magmatic arcs, which were accreted to the northern, active margin of Gondwana, an active continental margin at the time, involved with the subduction of the Iapetus Ocean (Linnemann et al., Reference Linnemann, Gerdes, Drost and Buschmann2007; Von Raumer & Stampfli, Reference Von Raumer and Stampfli2008; Kroner & Romer, Reference Kroner and Romer2013; Stampfli et al., Reference Stampfli, Hochard, Vérard and Wilhem2013; Couzinié et al., Reference Couzinié, Laurent, Poujol, Mintrone, Chelle-Michou, Moyen and Marko2017).

The future Alpine basements, located along the northern margin of Gondwana (von Raumer & Neubauer, Reference Von Raumer, Neubauer, von Raumer and Neubauer1993), underwent an event of bimodal magmatism during the Middle to Late Ordovician. The tectonic setting of this magmatism remains uncertain (e.g. Kroner & Romer, Reference Kroner and Romer2013; Von Raumer et al., Reference Von Raumer, Bussy, Schaltegger, Schulz and Stampfli2013; Bergomi et al., Reference Bergomi, Dal Piaz, Malusà, Monopoli and Tunesi2017). Some studies have proposed that short-lived (470–450 Ma) bimodal magmatism indicates a transtensional to extensional setting (Ballèvre et al., Reference Ballèvre, Fourcade, Capdevila, Peucat, Cocherie and Fanning2012; Von Raumer et al., Reference Von Raumer, Bussy, Schaltegger, Schulz and Stampfli2013; Villaseca et al., Reference Villaseca, Martínez, Orejana, Andersen and Belousova2016), whereas other models favour an Alaskan-type orogenic setting, known as Cenerian orogeny (Zurbriggen, Reference Zurbriggen2017; Siegesmund et al., Reference Siegesmund, Oriolo, Schulz, Heinrichs, Basei and Lammerer2021, Reference Siegesmund, Oriolo, Broge, Hueck, Lammerer, Basei and Schulz2023; Finger & Riegler, Reference Finger and Riegler2023). Ordovician orthogneisses in the pre-Variscan basement of the Central and Western Alps are closely associated with eclogitic mafic boudins and massive to banded amphibolite sills (Desmons, Reference Desmons1992; Poller, Reference Poller1997; Gaggero et al., Reference Gaggero, Cortesogno and Bertrand2004; Gauthiez et al., Reference Gauthiez, Bussy, Ulianov, Gouffon and Sartori2011). These bimodal magmatic features suggest that all these units were part of the same basement in the northern Gondwana margin during the Middle to Late Ordovician period (Bergomi et al., Reference Bergomi, Dal Piaz, Malusà, Monopoli and Tunesi2017).

The northern margin of Gondwana was later dismembered into different units during the Variscan orogeny (Late Devonian to Early Permian). The Variscan belt was assembled along the southern margin of Laurussia, including the Avalonia and Armorica microcontinents (Matte et al., Reference Matte, Maluski, Rajlich and Franke1990; Stampfli & Borel, Reference Stampfli and Borel2002). Several pulses of magmatism are related to the Variscan belt: an early pre-collisional pulse of medium- to high-K calc-alkaline melts (Late Devonian to Early Carboniferous, 380–345 Ma), followed by synorogenic high-K calc-alkaline to shoshonitic magmatism (345–330 Ma), and later post-collisional plutons intruded between 330 and 300 Ma (Bonin, Reference Bonin1988; Reference Bonin, Brändlein, Bussy, Desmons, Eggenberger, Finger, Graf, Marro, Mercolli, Oberhänsli, Ploquin, von Quadt, von Raumer, Schaltegger, Steyrer, Visonà and Vivier1993; Schaltegger, Reference Schaltegger1997), associated with Barrovian metamorphism and anatectic migmatites (Žák et al., Reference Žák, Verner, Janoušek, Holub, Kachlík, Finger and Trubač2014; Ballèvre et al., Reference Ballèvre, Manzotti and Dal Piaz2018). Evidence of this Variscan magmatic evolution in the Alps is preserved in the External Crystalline Massif and partially in the Briançonnais-derived basements, where the Grand Nomenon pluton provides a Late Devonian magmatic emplacement age (Bergomi et al., Reference Bergomi, Dal Piaz, Malusà, Monopoli and Tunesi2017; Bertrand et al., Reference Bertrand, Pidgeon, Leterrier, Guillot, Gasquet and Gattiglio2000b).

From the Late Carboniferous to the Early Permian, wrench tectonics, post-Variscan transtension and lithospheric thinning were active (Schuster & Stüwe, Reference Schuster and Stüwe2008; Von Raumer et al., Reference Von Raumer, Bussy, Schaltegger, Schulz and Stampfli2013; Spalla et al., Reference Spalla, Zanoni, Marotta, Rebay, Roda, Zucali and Gosso2014; Manzotti et al., Reference Manzotti, Poujol and Ballèvre2015). This transition caused the fragmentation of the Variscan belt through several dextral shear zones (Corsini & Rolland, Reference Corsini and Rolland2009; Guillot et al., Reference Guillot, di Paola, Ménot, Ledru, Spalla, Gosso and Schwartz2009; Carosi et al., Reference Carosi, Montomoli, Tiepolo and Frassi2012; Simonetti et al., Reference Simonetti, Carosi, Montomoli, Cottle and Law2020, Reference Simonetti, Carosi, Montomoli, Law and Cottle2021), with faulting and lithospheric extension associated with igneous intrusions and the opening of intramontane coal-bearing basins (Von Raumer, Reference Von Raumer1998; Ballèvre et al., Reference Ballèvre, Manzotti and Dal Piaz2018).

During the Early Permian, mafic melts intruded at depth in the future Alpine basement (Dal Piaz, Reference Dal Piaz1993; Marotta & Spalla, Reference Marotta and Spalla2007; Manzotti et al., Reference Manzotti, Ballèvre and Dal Piaz2017), associated with bimodal volcanism and granite intrusions in the upper crust (Dal Piaz & Martin, Reference Dal Piaz and Martin1998; Schaltegger & Gebauer, Reference Schaltegger and Gebauer1999; Bergomi et al., Reference Bergomi, Dal Piaz, Malusà, Monopoli and Tunesi2017). This widespread Early Permian magmatism is generally interpreted as a response to lithospheric thinning and strike-slip tectonics, followed by regional extension and K-rich volcanism in the Late Permian (Schaltegger & Brack, Reference Schaltegger and Brack2007; Dallagiovanna et al., Reference Dallagiovanna, Gaggero, Maino, Seno and Tiepolo2009).

The scarce Upper Carboniferous magmatism and the occurrence of Lower Permian intrusions and volcanism suggest that the Briançonnais units derive from the Gondwana foreland, in a more external position within the Variscan belt with respect to the basement of the External Crystalline Massifs (Bergomi et al., Reference Bergomi, Dal Piaz, Malusà, Monopoli and Tunesi2017; Ballèvre et al., Reference Ballèvre, Manzotti and Dal Piaz2018). According to Ballèvre et al. (Reference Ballèvre, Manzotti and Dal Piaz2018), the Permian event likely influenced the subsequent Triassic thermal subsidence, leading to the deposition of thick carbonate platforms in the Briançonnais s.l. domain (Mégard-Galli & Baud, Reference Mégard-Galli and Baud1977; Dumont et al., Reference Dumont, Schwartz, Guillot, Malusà, Jouvent, Monié and Verly2022; Michard et al., Reference Michard, Schmid, Lahfid, Ballèvre, Manzotti, Chopin, Iaccarino and Dana2022). In contrast, in the Helvetic Zone, where no such thermal subsidence occurred, Triassic sediments were deposited in the ‘Germanic Trias’ facies, with thicknesses not exceeding a few metres (von Gümbel, Reference von Gümbel1891; Hauschke & Wilde, Reference Hauschke and Wilde1999; Ziegler, Reference Ziegler, Raumer and Neubauer1993).

2.c. The Dora-Maira Massif

The Dora-Maira Massif consists of tectonic units largely made of pre-Alpine basement and partly detached Permo-Mesozoic covers (Novarese Reference Novarese1895; Vialon, Reference Vialon1966; Michard, Reference Michard1967; Henry, Reference Henry1990; Sandrone et al., Reference Sandrone, Cadoppi, Sacchi, Vialon and von Raumer J.F.1993). The main tectonic units of the Dora-Maira Massif (Chopin et al., Reference Chopin, Henry and Michard1991; Michard et al., Reference Michard, Henry and Chopin1993, Reference Michard, Henry, Chopin, Coleman and Wang1995, Reference Michard, Schmid, Lahfid, Ballèvre, Manzotti, Chopin, Iaccarino and Dana2022; Bonnet et al., Reference Bonnet, Chopin, Locatelli, Kylander-Clark and Hacker2022; Manzotti et al., Reference Manzotti, Schiavi, Nosenzo, Pitra and Ballèvre2022, Reference Manzotti, Millonig, Gerdes, Whitehouse, Jeon, Poujol and Ballèvre2025a, Reference Manzotti, Schiavi, Ballèvre and Nosenzo2025b; Nosenzo et al., Reference Nosenzo, Manzotti, Krona, Ballèvre and Poujol2024) from bottom to top are (Fig. 2):

Figure 2.

Tectonic sketch of the Dora-Maira Massif (compiled from Henry, Reference Henry1990; Sandrone et al., Reference Sandrone, Cadoppi, Sacchi, Vialon and von Raumer J.F.1993; Piana et al., Reference Piana, Fioraso, Irace, Mosca, d’Atri, Barale, Falletti, Monegato, Morelli, Tallone and Vigna2017; Michard et al., Reference Michard, Schmid, Lahfid, Ballèvre, Manzotti, Chopin, Iaccarino and Dana2022; Nosenzo et al., Reference Nosenzo, Manzotti, Krona, Ballèvre and Poujol2024; Groppo et al., Reference Groppo, Ferrando, Tursi and Rolfo2025 and our new observations). Magmatic photolith ages from the literature are given (a: Bussy & Cadoppi, Reference Bussy and Cadoppi1996; b: Nosenzo et al., Reference Nosenzo, Manzotti, Krona, Ballèvre and Poujol2024; c: Nosenzo et al., Reference Nosenzo, Manzotti, Poujol, Ballèvre and Langlade2022; d: Paquette et al., Reference Paquette, Montel and Chopin1999; e: Chen et al., Reference Chen, Zhou, Zheng and Schertl2017; f: Balestro et al., Reference Balestro, Festa, Cadoppi, Groppo and Roà2022; g: Gebauer et al., Reference Gebauer, Schertl, Brix and Schreyer1997). Tectonic units abbreviations: VG = Val Grana, VMS = Val Maira-Sampeyre, CLSZ = Cima Lubin Shear Zone, DU = Dronero, VSZ = Valmala Shear Zone, RU = Ricordone, SE = Serre, MU = Muret, RS = Rocca Solei, GU = Grimbassa, CU = Chasteiran, SC = San Chiaffredo, BIU = Brossasco-Isasca, PSU = Pinerolo–Sanfront, MV = Monviso, OR = Orsiera-Rocciavrè, SU = Susa, LA = Lanzo. The yellow stars show the location of the studied samples; the black square is the area covered by the Geological Map of Italy sheet 172 ‘Pinerolo’.

(i) The Pinerolo–Sanfront Unit (‘Complesso Grafitico del Pinerolese’ of Franchi & Novarese, Reference Franchi and Novarese1895 and Borghi et al., Reference Borghi, Cadoppi, Porro, Sacchi and Sandrone1984, and ‘Ensemble de Pinerolo’ of Vialon, Reference Vialon1966) mainly consists of monocyclic metasediments (metaconglomerates, meta-arkoses and metapelites characterized by the presence of graphite) of Permo-Carboniferous age (Manzotti et al., Reference Manzotti, Ballèvre and Poujol2016; Carosi et al., Reference Carosi, Montomoli, Iaccarino, Dana, Corno, De Cesari and Spina2025) and Upper Paleozoic magmatic bodies (Fig. 2; Bussy & Cadoppi, Reference Bussy and Cadoppi1996). Alpine metamorphic conditions are generally referred to as the blueschist or eclogite-blueschist transition facies (Borghi et al., Reference Borghi, Cadoppi, Porro and Sacchi1985; Groppo et al., Reference Groppo, Ferrando, Gilio, Botta, Nosenzo, Balestro, Festa and Rolfo2019).

(ii) The ‘Basement Complex’ (Chopin et al., Reference Chopin, Henry and Michard1991; Michard et al., Reference Michard, Henry and Chopin1993, Reference Michard, Henry, Chopin, Coleman and Wang1995; Bonnet et al., Reference Bonnet, Chopin, Locatelli, Kylander-Clark and Hacker2022), a set of different tectono-metamorphic units with comparable lithologies and metamorphic Alpine evolution at eclogitic facies (in some cases up to UHP conditions, Chopin, Reference Chopin1984; Manzotti et al., Reference Manzotti, Schiavi, Nosenzo, Pitra and Ballèvre2022, Reference Manzotti, Schiavi, Ballèvre and Nosenzo2025b; Groppo et al., Reference Groppo, Ferrando, Tursi and Rolfo2025). These units have been distinguished primarily based on ‘peak’ P-T estimates (Compagnoni et al., Reference Compagnoni, Rolfo, Groppo, Hirajima and Turello2012; Groppo et al., Reference Groppo, Ferrando, Gilio, Botta, Nosenzo, Balestro, Festa and Rolfo2019, Reference Groppo, Ferrando, Tursi and Rolfo2025) and consist of a pre-Alpine basement with rarely preserved Permo-Mesozoic cover (Fig. 2; Franchi, Reference Franchi1898; Sacchi et al., Reference Sacchi, Balestro, Cadoppi, Carraro, Delle Piane, Di Martino, Enrietti, Gallarà, Gattiglio, Martinotti and Perello2004; Caron, Reference Caron1977; Sacchi et al., Reference Sacchi, Balestro, Cadoppi, Carraro, Delle Piane, Di Martino, Enrietti, Gallarà, Gattiglio, Martinotti and Perello2004; Marthaler et al., Reference Marthaler, Fudral, Deville and Rampnoux1986; Gasco et al., Reference Gasco, Gattiglio and Borghi2013; Nosenzo et al., Reference Nosenzo, Manzotti, Krona, Ballèvre and Poujol2024). The pre-Alpine basement is mainly made of pre-Carboniferous metasediments (garnet-chloritoid-bearing micaschist, paragneiss and marble; Vialon, Reference Vialon1966; Cadoppi, Reference Cadoppi1990; Wheeler, Reference Wheeler1991) associated with several metabasite and orthogneiss bodies. These meta-intrusive rocks have been regarded as late Variscan (Hunziker et al., Reference Hunziker, Desmons and Hurford1992; Sandrone et al., Reference Sandrone, Cadoppi, Sacchi, Vialon and von Raumer J.F.1993; Bussy & Cadoppi, Reference Bussy and Cadoppi1996), although few studies have addressed their dating (e.g. Bussy & Cadoppi, Reference Bussy and Cadoppi1996; Paquette et al., Reference Paquette, Montel and Chopin1999; Chen et al., Reference Chen, Schertl, Zheng, Huang, Zhou and Gong2016, Reference Chen, Zhou, Zheng and Schertl2017; Nosenzo et al., Reference Nosenzo, Manzotti, Poujol, Ballèvre and Langlade2022, Reference Nosenzo, Manzotti, Krona, Ballèvre and Poujol2024).

(iii) The uppermost Dronero Unit (Michard et al., Reference Michard, Henry and Chopin1993), cropping out in the southern portion of the Massif (Fig. 2), consists of polycyclic garnet ± chloritoid-bearing micaschist, metagranitoid and metabasite bodies (Michard & Vialon, Reference Michard and Vialon1966; Michard, Reference Michard1967; Henry, Reference Henry1990), with monocyclic ankerite-chloritoid-bearing micaschist attributed to the Permian-Carboniferous (Michard, Reference Michard1967; Henry, Reference Henry1990). The Dronero Unit reached blueschist facies Alpine metamorphic conditions (Groppo et al., Reference Groppo, Ferrando, Gilio, Botta, Nosenzo, Balestro, Festa and Rolfo2019; Bonnet et al., Reference Bonnet, Chopin, Locatelli, Kylander-Clark and Hacker2022). This unit is separated by the underlying units by the Ophiolitiferous Band of Henry et al. (Reference Henry, Michard and Chopin1993) or Valmala Shear Zone (Fig. 2; Balestro et al., Reference Balestro, Nosenzo, Cadoppi, Fioraso, Groppo and Festa2020; Michard et al., Reference Michard, Schmid, Lahfid, Ballèvre, Manzotti, Chopin, Iaccarino and Dana2022), consisting of highly deformed tectonic slices of continental and oceanic rocks.

Similar shear zones with both oceanic and continental rocks (Michard et al., Reference Michard, Schmid, Lahfid, Ballèvre, Manzotti, Chopin, Iaccarino and Dana2022) occur above the Dronero Unit: (i) the San Damiano Shear Zone, which separates the Pre-Piemonte Val Maira-Sampeyre Unit from the Dronero Unit (Michard et al.,Reference Michard, Henry and Chopin1993; Michard et al., Reference Michard, Schmid, Lahfid, Ballèvre, Manzotti, Chopin, Iaccarino and Dana2022; Dana, Reference Dana2024), and (ii) Cima Lubin Shear Zone (CLSZ) separating the Val Maira–Sampeyre and Val Grana Units in the south and the continental units of the Dora-Maira Massif from the oceanic-derived Monviso Unit in the north (Fig. 2; Michard, Reference Michard1967; Piana et al., Reference Piana, Fioraso, Irace, Mosca, d’Atri, Barale, Falletti, Monegato, Morelli, Tallone and Vigna2017; Michard et al., Reference Michard, Schmid, Lahfid, Ballèvre, Manzotti, Chopin, Iaccarino and Dana2022).

In Fig. 2, a simplified, updated tectonic sketch of the entire Dora-Maira Massif, compiled from various sources, illustrates the current state of knowledge. It is worthy to emphasize that only the northern and southern portions of the Dora-Maira Massif have been mapped and studied intensively (e.g. Michard et al., Reference Michard, Henry and Chopin1993; Cadoppi et al., Reference Cadoppi, Castelletto, Sacchi, Baggio, Carraro and Giraud2002; Sacchi et al., Reference Sacchi, Balestro, Cadoppi, Carraro, Delle Piane, Di Martino, Enrietti, Gallarà, Gattiglio, Martinotti and Perello2004; Gasco et al., Reference Gasco, Gattiglio and Borghi2011; Compagnoni et al., Reference Compagnoni, Rolfo, Groppo, Hirajima and Turello2012; Groppo et al., Reference Groppo, Ferrando, Gilio, Botta, Nosenzo, Balestro, Festa and Rolfo2019; Reference Groppo, Ferrando, Tursi and Rolfo2025; Bonnet et al., Reference Bonnet, Chopin, Locatelli, Kylander-Clark and Hacker2022; Manzotti et al., Reference Manzotti, Ballèvre and Poujol2016, Reference Manzotti, Schiavi, Nosenzo, Pitra and Ballèvre2022, Reference Manzotti, Millonig, Gerdes, Whitehouse, Jeon, Poujol and Ballèvre2025a, Reference Manzotti, Schiavi, Ballèvre and Nosenzo2025b; Nosenzo et al., Reference Nosenzo, Manzotti, Poujol, Ballèvre and Langlade2022, Reference Nosenzo, Manzotti and Robyr2023, Reference Nosenzo, Manzotti, Krona, Ballèvre and Poujol2024; Balestro et al., Reference Balestro, Gattiglio, Roà and Festa2025), whereas the central portion (comprised between the Pellice and Po Valleys, Fig. 2) is still relatively poorly known.

The rock samples studied in this contribution were collected from a large area (Fig. 2), among the Pellice, Germanasca and Chisone Valleys, including the Pinerolo–Sanfront, the ‘Basement Complex’ Units and CLSZ, investigated in the frame of the Geological Map of Italy CARG project – Sheet 172 ‘Pinerolo’.

2.d. The Dora-Maira orthogneisses and metadiorites

Orthogneisses are a very common lithology in the Dora-Maira Massif (Fig. 2). Based on petrographic and geochemical data, and particularly protolith ages, several groups of orthogneisses have been recognized in the past (Sandrone et al., Reference Sandrone, Cordola and Fontan1986; Reference Sandrone, Cadoppi, Sacchi, Vialon and von Raumer J.F.1993; Borghi et al., Reference Borghi, Cadoppi, Poli, Sacchi and Sandrone1989; Paquette et al., Reference Paquette, Montel and Chopin1999; Bussy & Cadoppi, Reference Bussy and Cadoppi1996; Nosenzo et al., Reference Nosenzo, Manzotti, Poujol, Ballèvre and Langlade2022, Reference Nosenzo, Manzotti, Krona, Ballèvre and Poujol2024). In Table 1, we present an overview of all the different types of orthogneiss, showing a revised nomenclature based on our own original observations, compared and integrated with the existing literature, in order to provide an updated summary of their main features (mineral abbreviations used in the tables, figures and text are after Whitney & Evans (Reference Whitney and Evans2010) except for Wm = white mica).

Table 1.

Meta-intrusive rocks of the Dora-Maira Massif and associated regional shear zones (excluding meta-mafic rocks), with their main features (MR = magmatic relics, MM = metamorphic minerals, ACC = accessory minerals) and the proposed or published absolute ages available in the literature prior to this study. Reference: (1) Franchi (Reference Franchi1898), Franchi & Novarese (Reference Franchi and Novarese1895), Novarese (Reference Novarese1895; Reference Novarese1896; Reference Novarese1898) and Stella (Reference Stella1895; Reference Stella1896); (2) Vialon (Reference Vialon1966); (3) Borghi et al. (Reference Borghi, Cadoppi, Porro, Sacchi and Sandrone1984); (4) Sandrone et al. (Reference Sandrone, Cordola and Fontan1986); (5) Sandrone et al. (Reference Sandrone, Sacchi, Cordola, Fontan and Villa1988); (6) Henry (Reference Henry1990); (7) Cadoppi (Reference Cadoppi1990); (8) Wheeler (Reference Wheeler1991); (9) Bussy & Cadoppi (Reference Bussy and Cadoppi1996); (10) Gebauer et al. (Reference Gebauer, Schertl, Brix and Schreyer1997); (11) Paquette et al (Reference Paquette, Montel and Chopin1999); (12) Chen et al. (Reference Chen, Zhou, Zheng and Schertl2017); (13) Balestro et al. (Reference Balestro, Festa, Cadoppi, Groppo and Roà2022); (14) Nosenzo et al. (Reference Nosenzo, Manzotti, Poujol, Ballèvre and Langlade2022); (15) Nosenzo et al. (Reference Nosenzo, Manzotti, Krona, Ballèvre and Poujol2024)

In the following, we will describe in detail the different orthogneiss groups recognized in the Dora-Maira Massif and their main features.

2.d.1. Meta-intrusives of the Pinerolo–Sanfront Unit

Three different types of acid to intermediate meta-intrusives have been distinguished in the Pinerolo–Sanfront Unit based on petrography, geochemistry and U–Pb dating (Novarese, Reference Novarese1895; Zanettin-Lorenzoni, Reference Zanettin-Lorenzoni1967; Borghi et al., Reference Borghi, Cadoppi, Porro, Sacchi and Sandrone1984; Cadoppi, Reference Cadoppi1990; Sandrone et al., Reference Sandrone, Cadoppi, Sacchi, Vialon and von Raumer J.F.1993; Bussy & Cadoppi, Reference Bussy and Cadoppi1996), namely: the Cavour orthogneiss, the Malanaggio metadiorite and the Freidour orthogneiss (Tab. 1, Fig. 2). U–Pb dating on zircons from these three types of meta-intrusives returned Upper Carboniferous to Lower Permian ages (Bussy & Cadoppi, Reference Bussy and Cadoppi1996). Magmatism in the Pinerolo–Sanfront Unit evolved from calc-alkaline to more alkaline melts, starting from the oldest Cavour orthogneiss and progressing to the younger Freidour orthogneiss (Table 1). The Cavour orthogneiss crops out in a small outcrop (<0.08 km²) in the Rocca di Cavour inselberg (Fig. 2; Vialon, Reference Vialon1966; Bussy & Cadoppi, Reference Bussy and Cadoppi1996), whereas the metadioritic to meta-granodioritic Malanaggio metadiorite crops out as several km-scale bodies in the lower Chisone Valley (Fig. 2; Franchi & Novarese, Reference Franchi and Novarese1895; Novarese, Reference Novarese1895; Borghi et al., Reference Borghi, Cadoppi, Porro, Sacchi and Sandrone1984; Sandrone et al., Reference Sandrone, Sacchi, Cordola, Fontan and Villa1988, Reference Sandrone, Cadoppi, Sacchi, Vialon and von Raumer J.F.1993; Bussy & Cadoppi, Reference Bussy and Cadoppi1996; Vialon, Reference Vialon1966). Original magmatic textures and mineral assemblages (Tab. 1) of the Malanaggio metadiorite are well preserved, mainly in the more acidic ‘quartz-diorite’ facies (Bussy & Cadoppi, Reference Bussy and Cadoppi1996). Two samples have been dated by Bussy and Cadoppi (Reference Bussy and Cadoppi1996), yielding crystallization ages of 290 ± 2 Ma for the ‘quartz-diorite’ facies and 288 ± 2 Ma for the ‘granodioritic’ facies. These ages, slightly older than the one obtained for the Freidour orthogneiss (268–283 Ma; Bussy & Cadoppi, Reference Bussy and Cadoppi1996), are further supported by the field evidence of dykes and sills of the latter cross-cutting the Malanaggio metadiorite (Section 4).

The Freidour orthogneiss, with a granitic composition, makes up a c. 120 km² body in the study area (Fig. 2). Given the large areal extension, we have studied a sample collected in a different location (Fig. 2; Tab. 2) than the sample studied by Bussy & Cadoppi (Reference Bussy and Cadoppi1996). Orthogneiss bodies in the southern portion of the Pinerolo–Sanfront Unit (Monte Bracco area; Fig. 2) have not been studied in detail, but have been referred to the Freidour orthogneiss (Piana et al., Reference Piana, Fioraso, Irace, Mosca, d’Atri, Barale, Falletti, Monegato, Morelli, Tallone and Vigna2017).

Table 2.

Meta-intrusive samples studied in this contribution. Coordinates are given in WGS84 system. Mineral abbreviations after Whitney & Evans (Reference Whitney and Evans2010), except for Wm = white mica

4.a. Malanaggio metadiorite

The Malanaggio metadiorite of the Pinerolo–Sanfront Unit is composed of fine-grained dioritic gneisses with a typical grey colour and a banded appearance (Fig. 3a), locally containing centimetre to decimetre-sized mafic microgranular enclaves (MME). The collected samples (C168, C175) have a spaced foliation defined by the presence of microlithons alternating with cleavage domains, with quartz-albite domains alternating with actinolite, biotite and chlorite layers (Fig. 3b). Rare relicts of magmatic hornblende are preserved. Quartz exhibits undulose extinction, and it is recrystallized by the subgrain rotation (SGR) mechanism (Stipp et al., Reference Stipp, StuÈnitz, Heilbronner and Schmid2002a, Reference Stipp, Stünitz, Heilbronner and Schmid2002b).

Figure 3.

Selected field images and photomicrographs of the studied lithologies. (a) Outcrop of Malanaggio metadiorite with the typical banded appearance and microgranular mafic enclaves. (b) Crossed polarized light (CPL hereafter) microphoto of the Malanaggio metadiorite. (c) Fine-grained facies of the Freidour orthogneiss. (d) Dyke of Freidour orthogneiss cross-cutting the Malanaggio metadiorite. (e) CPL photomicrograph of a large K-feldspar porphyroclast wrapped by the phengite foliation in the Freidour orthogneiss. (f) Field image of the Ferrera orthogneiss felsic facies, W of Bobbio Pellice village. (g) Field image of the Ferrera orthogneiss leucocratic facies (100 m from the outcrop shown in f). (h) CPL photomicrograph of the Ferrera orthogneiss (sample SD2) showing the spaced foliation defined by white mica.

4.b. Freidour orthogneiss

The Freidour orthogneiss sample (DD112, Pinerolo–Sanfront Unit) is a fine-grained metagranite (Fig. 3c), locally containing decimetre-sized MME in the coarser-grained portions. Aplitic, fine-grained sills and dykes, connected to the main orthogneiss body, are observed in the host micaschist at the contact with the Freidour orthogneiss, and they also crosscut the Malanaggio metadiorite (Fig. 3d). In the collected sample, K-feldspar porphyroclasts (microcline) are generally centimetre or sub-centimetre in length, whereas in the biotite-rich coarser-grained portions, they can reach 3–4 cm in length (Fig. 3e). The main foliation is a spaced one (Passchier & Trouw, Reference Passchier and Trouw2005) and it is defined by white mica (phengite), chlorite and rare biotite, alternating with fine-grained quartz-feldspathic microlithons. Quartz exhibits undulose extinction, and it is recrystallized by the SGR mechanism. Biotite generally grows at the edges of white mica and is partially replaced by chlorite.

4.c. Ferrera orthogneiss

The Ferrera orthogneiss samples SD2 and DD245 are greenish-grey felsic orthogneisses (Fig. 3f), whereas sample SD244 comes from a more leucocratic facies (Fig. 3g; 100 m from the latter samples). Intrusive relationships at the cartographic scale are observed between the Ghiandone orthogneiss and the host Ferrera orthogneiss, which is likely older. At the microscale, the samples are characterized by a continuous to spaced foliation defined by white mica (mainly phengite). Euhedral titanite and fine-grained chlorite are also observed along the foliation planes. Plagioclase is subhedral, showing polysynthetic twinning, whereas K-feldspar forms delta-type porphyroclasts with simple twinning, and quartz and white mica inclusions are often concentrated in the core of crystals (Fig. 3h). Feldspars show intense synkinematic recrystallization in the BLG regime. Rare mica fish, occurring in the fine-grained quartz-rich matrix, suggest a top-to-W sense of shear. Quartz shows microstructures pointing to a grain-boundary migration (GBM) recrystallization mechanism (Stipp et al., Reference Stipp, StuÈnitz, Heilbronner and Schmid2002a, Reference Stipp, Stünitz, Heilbronner and Schmid2002b), with local evidence for annealing and overprinting of SGR.

4.d. Ghiandone orthogneiss

The Ghiandone orthogneiss samples (36A and DC51) are characterized by the presence of abundant (i) large polycrystalline aggregates resembling K-feldspar megacrysts (up to 10 cm), hosted in a fine-grained matrix (Fig. 4a), and (ii) mafic microgranular enclaves. Because of Alpine metamorphisms, the K-feldspar is almost entirely replaced by fine-grained aggregates of albite and quartz. Fresh K-feldspar is preserved only in very low-strain domains, such as the Punta Muret outcrop (Novarese, Reference Novarese1895; Bouffette et al., Reference Bouffette, Lardeaux and Caron1993; the Muret orthogneiss of Nosenzo et al., Reference Nosenzo, Manzotti, Poujol, Ballèvre and Langlade2022). The main foliation is spaced, and a crenulation cleavage is rarely present. The foliation is defined by abundant white mica (mainly phengite), associated with minor biotite and chlorite along the cleavage domains. Small garnet (up to 2–3 % in volume), epidote and titanite are found along the foliation (Fig. 4b). Microlithons are mainly made of fine-grained aggregates of quartz and plagioclase.

Figure 4.

Selected field images and photomicrographs of the studied lithologies. (a) Ghiandone orthogneiss showing the distinctive large polycrystalline aggregates porphyroclasts and microgranular mafic enclaves. (b) CPL photomicrograph of the Ghiandone orthogneiss. (c) Field image of the Granero orthogneiss with cm-sized K-feldspar porphyroclast. (d) CPL photomicrograph of Granero orthogneiss (sample DD43) showing K-feldspar porphyroclast enveloped by the main foliation defined by white mica and quartz-felspathic levels. (e) Typical banding of the Cialancia orthogneiss, with alternating dark and leucocratic levels. (f) Aplitic dyke associated to the Cialancia orthogneiss intruding the host micaschist. (g) CPL photomicrograph of Cialancia orthogneiss (sample Y6) showing a K-feldspar porphyroclast surrounded by a matrix of plagioclase, quartz and phengite. (h) Giasset metadiorite dyke cross-cutting the Ghiandone orthogneiss, suggesting a younger protolith age for the metadiorites.

4.e. Granero orthogneiss

The Granero orthogneiss sample (DD43) is a white to grey augengneiss (Fig. 4c), collected in the same outcrop dated by Nosenzo et al. (Reference Nosenzo, Manzotti, Krona, Ballèvre and Poujol2024). It displays a spaced foliation defined by fine-grained layers of quartz and the shape-preferred orientation of white mica (Fig. 4d). K-feldspar porphyroclasts (mainly microcline) up to cm-sized are wrapped by the main foliation and contain inclusions of epidote and white mica. Quartz is dynamically recrystallized in the GBM/SGR regime.

4.f. Cialancia orthogneiss

The Cialancia orthogneiss displays different facies grain sizes and deformation features, ranging from portions where the magmatic fabric is still preserved to a banding with alternations of leucocratic and darker levels with different mineralogy (Fig. 4e). Leucocratic aplitic sills, which are boudinaged and folded, are associated with the main Cialancia orthogneiss (Fig. 4f). Samples SC2, DC75, C211 and Y6 were collected within the dominant strongly deformed leucocratic facies with K-feldspar porphyroclasts, reaching up to 4–5 cm, wrapped by the main spaced foliation defined by white mica and biotite. Sample Y18 was collected in the dark layered facies of the Cialancia orthogneiss. Magmatic biotite relicts are rare in all samples. Quartz often occurs in ribbons, associated with plagioclase, and it is dynamically recrystallized in the GBM/SGR regime (Fig. 4g). K-feldspar is commonly microcline and more rarely orthoclase (with the typical Carlsbad twinning).

4.g. Giasset metadiorite

The Giasset metadiorite is fine-grained and variably deformed (Fig. 4h and 5a). Where intensively deformed, a continuous to spaced foliation, defined by white mica and amphibole, is observed. Fine aggregates of biotite and amphibole could represent pseudomorphs after original magmatic amphibole. Garnet is observed in very small grains, partially overgrown by biotite rims. Sample SC58 was collected from a hundred-metre-sized metadiorite body associated with several dykes. In the main body, microgranular mafic enclaves with dark colours and elongated shape are common (Fig. 5a). Sample SC59 was collected from one of the aforementioned dykes, which crosscuts the Ghiandone orthogneiss (Tab. 1), suggesting that Giasset metadiorite is younger (Fig. 4h).

Figure 5.

Selected field images and photomicrographs of the studied lithologies. (a) Giasset metadiorite containing cm-sized mafic microgranular enclaves. (b) CPL photomicrograph of the Giasset metadiorite showing fine-grained Alpine mineral crystallization (sample SC58). (c) Field image of the Luserna orthogneiss, with associated quartz-phengitic micaschist. (d) CPL photomicrograph of the Luserna orthogneiss (sample DD109) showing a K-feldspar porphyroclast, wrapped by the main foliation. (e) Field view of the Giulian orthogneiss body sampled within the Cima Lubin Shear Zone, view from the Pellice side of the Germanasca-Pellice watershed ridge. (f) CPL photomicrograph of the Giulian orthogneiss sample (AB21) showing quartz-rich microlithons and a continuous to spaced foliation defined by white mica.

4.h. Luserna orthogneiss

Collected samples (DD109, DC76, DC77 and SC41) are felsic orthogneiss with mm-sized K-feldspar porphyroclasts, frequently associated with quartz-phengitic (Fig. 5c) micaschist and quartzite levels. In all the samples, at the microscale, porphyroclasts are surrounded by a spaced foliation defined by alternation of micaceous cleavage domains and quartz-feldspathic-rich microlithons (Fig. 5d). K-feldspar is locally recrystallized by bulging (BLG), whereas quartz shows recrystallization mechanisms by SGR, locally overprinted by a later BLG mechanism.

4.i. Giulian orthogneiss

Based on our field mapping, we report the presence of several lenses of leucocratic orthogneiss, no more than 10 m thick, dismembered within a thick band of mylonitic calcschist separating the Dora-Maira Massif from the overlying oceanic Monviso Unit in the Pellice Valley. These lenses (Fig. 5e), among other tectonic slices of dolostone, serpentinite, metabasite and micaschist, are particularly widespread in the Colle Giulian area (Pellice Valley, Fig. 2), where sample AB21 was collected. This high-strain zone, with both tectonic slices of oceanic and continental rocks, was previously considered part of the continental Giulian-Sea Bianca Unit of Balestro et al. (Reference Balestro, Fioraso and Lombardo2011) and is interpreted here as the northern extension of the CLSZ (Piana et al., Reference Piana, Fioraso, Irace, Mosca, d’Atri, Barale, Falletti, Monegato, Morelli, Tallone and Vigna2017), defined further south by Michard et al. (Reference Michard, Schmid, Lahfid, Ballèvre, Manzotti, Chopin, Iaccarino and Dana2022).

This sample consists of a leucocratic fine-grained orthogneiss, with a continuous to spaced foliation defined by alternating white mica (mainly phengite), titanite and quartz-felspathic domains. It also contains very small feldspar porphyroclasts, mainly microcline, and minor albite. Quartz is recrystallized in the GBM regime (Fig. 5f).

6.a. Freidour orthogneiss

Zircon crystals from sample DD112 are prismatic and display oscillatory zoning in CL (Fig. 8a). Texturally older cores have not been observed. Th and U concentrations can be very high: Th is mainly between 52 and 587 µg/g with some analyses as high as 1000, 2530 and 3730 µg/g, and U concentrations range from 301 to 20910 µg/g. Th/U ranges from 0.08 to 0.54. Most of the analyses yield dates in the range of 210–286 Ma, with considerable scatter along Concordia. The oldest group of dates defines a Concordia age of 267.2 ± 2.7 Ma (MSWD = 2.4; n = 13; Fig. 8a), which is taken as the minimum protolith emplacement age. This interpretation, however, should be treated with caution due to the scatter in the data.

Figure 8.

U–Pb zircon geochronology of orthogneiss samples: (a) Freidour orthogneiss; (b) Ferrera orthogneiss; (c) and d) Cialancia orthogneiss. Cathodoluminescence images of zircon crystals are shown on the left side, with the LA-ICP-MS spot position (red circles) and the relative date indicated. The Concordia diagrams on the right side report Concordia ages. Empty dashed ellipses represent dates excluded from the calculation.

6.b. Ferrera orthogneiss

Zircon grains from sample SD2 are either euhedral or subrounded with simple zoning (Fig. 8b). The concordant zircon analyses have Th and U concentrations in the range of 62–3180 and 141– 3648 µg/g, with Th/U between 0.09 and 0.98. One concordant zircon analysis yields a 1051.1 ± 16 Ma ²⁰⁶Pb/²³⁸U date, whereas the remaining concordant analyses yield Ediacaran–Lower Cambrian dates that define a Concordia age of 540.0 ± 5.4 Ma (MSWD = 0.94, n = 7, Fig. 8b). Two zircon rims yield concordant Ordovician dates, whereas two others provide Permian dates (of which only one is concordant at 267.9 ± 1.4 Ma). These zircons could reflect later metamorphic events; however, since they do not meet the minimum statistical requirements to define an age (n > 3), they are not considered geologically significant.

6.c. Cialancia orthogneiss

Zircon grains in the three studied Cialancia Orthogneiss samples (Y6, Y18 and SC2, Table 1) exhibit common features in all samples. Crystals are prismatic and show weak to marked oscillatory zoning. Zircon cores with a rounded or prismatic shape are less common in samples Y6 and SC2 but more common in sample Y18, which represents the darker portion of the Cialancia orthogneiss. Th and U concentrations are strongly variable in the range of 30–1110 and 373–13570 µg/g, respectively, with Th/U mainly between 0.01 and 0.17. Analyses obtained on inherited cores yield concordant dates around 1 Ga or between 500 and 600 Ma. Most of the remaining analyses provide concordant dates in the range of 416–481 Ma. The main cluster defines a Concordia age at 452.0 ± 4.5 Ma (MSWD = 2.3; n = 11; Fig. 8c) with a subset of data scattering along Concordia to lower values. In Sample Y18, zircon cores have Th/U comprised between 0.40 and 0.50, whereas the rims can reach very high U contents (596–6500 µg/g) and have lower Th/U between 0.03 and 0.17. A few inherited cores yield concordant dates of 1649 ± 21, 1366 ± 18 Ma and two between 600 and 650 Ma. A cluster of core analyses gives a Concordia age of 538.0 ± 5.4 Ma (MSWD = 1.9; n = 6; Fig. 8d). The rim analyses define a Concordia age of 452.0 ± 4.5 Ma (MSWD = 1.8; n = 5; Fig. 8d). In sample SC2, a few U-rich inherited zircon cores have dates older than 1 Ga (three concordant ²⁰⁷Pb/²⁰⁸Pb dates at 1914 ± 33, 2442 ± 30 and 2911 ± 29 Ma), and the majority is between 500 and 550 Ma. An isolated zircon dated at c. 393 Ma is not interpreted as geologically significant. The youngest analyses define a significant cluster with a Concordia age of 445.3 ± 4.5 Ma (MSWD = 1.7; n = 9; Fig. 9a).

Figure 9.

U–Pb zircon geochronology of orthogneiss samples: (a) Cialancia orthogneiss; (b) Giasset metadiorite; (c) Luserna orthogneiss; (d) Giulian orthogneiss (tectonic slice in Cima Lubin shear zone). Cathodoluminescence images of zircon crystals are shown on the left side, with the LA-ICP-MS spot position (red circles) and the relative date indicated. The Concordia diagrams on the right side report Concordia ages. Empty dashed ellipses represent dates excluded from the calculation.

6.d. Giasset metadiorite

Zircon crystals extracted from sample SC59 are colourless subhedral crystals with mainly oscillatory zoning (Fig. 9b). Th and U concentrations are in the range of 87–3000 µg/g and 378–4770 µg/g, respectively, with Th/U between 0.13 and 0.86. U–Pb analyses scatter along Concordia without defining significant clusters (Fig. 9b), preventing the estimation of a geologically meaningful age. Concordant analyses are mainly around 500–600 Ma and 300 Ma.

6.e. Luserna orthogneiss

Zircon crystals from sample DD109 are prismatic and display oscillatory zoning in CL (Fig. 9c). An inherited core with a dark CL emission occurs in one grain only. Th and U concentrations of concordant analyses range between 30–80 and 407–6040 μg/g, respectively, with a variable Th/U ratio from 0.01 to 1.19. The inherited core yields a concordant ²⁰⁶Pb/²³⁸U date of 379.1 ± 6.3 Ma. Most of the remaining analyses yield dates in the range of 208–297 Ma, with a cluster defining an age of 265.3 ± 2.7 (MSWD = 1.7; n = 11; Fig. 9c).

6.f. Giulian orthogneiss

Zircon crystals extracted from sample AB21 are subhedral with oscillatory zoning (Fig. 9d). Th concentrations are in the range of 28–760 µg/g with three analyses between 2500 and 3310 µg/g, while U concentrations range from 97 to 5630 µg/g. The Th/U of the concordant analyses is between 0.24 and 0.59, whereas the remaining grains scatter widely between 0.02 and 2.34. Five discordant analyses yield dates older than 550 Ma; the oldest concordant dates define a Concordia age at 548.0 ± 5.5 Ma (MSWD = 1.6; n = 4; Fig. 9d), whereas younger dates between 300 and 200 Ma are generally discordant. Despite being defined by only four analyses, the age of the oldest zircon population concurs with that found in the Ferrera sample, and so it can be taken as geologically significant.