Gran Paradiso - Università degli Studi di Torino

UNIVERSITÀ DEGLI STUDI DI TORINO 

Scuola di Dottorato in Scienze della Natura e Tecnologie Innovative 

Dottorato in Scienze della Terra 

Dipartimento di Scienze della Terra 

Seminario in ricordo di Ugo POGNANTE e Raffaella RUFFINI 

Torino, 12 novembre 2012 

Fragments of Hercynian continental crust in the Internal 

Western Alps: 

Pre-Alpine relics in the Internal Crystalline Massifs and in 

the Sesia Zone 

Roberto COMPAGNONI and Marco BELTRANDO 

Dipartimento di Scienze della Terra, Università di Torino

In red (5 in Legend) the Internal 

Crystalline Massifs: MR= Monte Rosa, 

GP= Gran Paradiso, DM= Dora-Maira. 

In yellow-orange (6 in Legend) the 

Austroalpine system: SZ= Sesia Zone, 

DB= Dent Blanche nappe

The blue colours 

show the HP/UHP 

Alpine metamorphic 

overprint

The two arrows 

indicate the GP and 

SZ, that will be 

considered in more 

detail in this 

presentation

Bonneval 

Bonneval 

0 2 4 

km 

Gran Paradiso Massif (GP) 

Valsavaranche 

Valsavaranche 

Valsavaranche 

Valsavaranche 

Valsavaranche 

Valsavaranche 

Valsavaranche 

Valsavaranche 

Valsavaranche 

La La Levanna 

Levanna 

3555 3555 m 

m 

Gr. Gr. Nomenon 

Nomenon 

3488 3488 m 

m 

Valsavaranche 

Valsavaranche 

Ceresole 

Ceresole 

Forno 

Forno 

Herbetet 

Herbetet 

3778 3778 m 

m 

Gran Gran Paradiso 

Paradiso 

4061 4061 m 

m 

Valle Valle Valle Valle Valle Valle Valle Valle Valle del del del del del del del Forno Forno Forno Forno Forno Forno Forno Forno Forno 

Valnontey Valnontey Valnontey Valnontey Valnontey Valnontey Valnontey Valnontey Valnontey 

Noasca 

Noasca 

Cogne 

Cogne 

Valeille Valeille Valeille Valeille Valeille Valeille Valeille Valeille Valeille 

San San Pietro 

Pietro 

3692 3692 m 

m 

Piantonetto 

Piantonetto 

Piantonetto 

Piantonetto 

Piantonetto 

Piantonetto 

Piantonetto 

Piantonetto 

Piantonetto 

Valle Valle d'Aosta 

d'Aosta 

Lillaz 

Lillaz 

Bardoney Bardoney Bardoney Bardoney Bardoney Bardoney Bardoney Bardoney Bardoney 

Valle Valle Valle dell'Orco dell'Orco dell'Orco dell'Orco dell'Orco dell'Orco dell'Orco dell'Orco dell'Orco 

Pt. Pt. di di A A cque cque Rosse 

Rosse 

2952 2952 m 

m 

Campiglia 

Campiglia 

Forzo 

Forzo 

M. M. Colombo 

Colombo 

2848 2848 m 

m 

Locana 

Locana 

GABUDIANU RADULESCU et al (2011): J. metam. Geol., 29, 851–874, Fig.1 

Simplified geological map of the GP massif : Variscan basement is shown in red and the augengneisses and the Permian relict 

porphyritic metagranitoids in light pink. Red star: location of the hornfels studied by Gabudianu et al. (2011) 

Soana Soana Soana Soana Soana Soana Soana Soana Soana

Relict intrusive contact between the Variscan metamorphic basement and 

the Permian granitoids on the eastern side of the Teleccio Lake.

Relict intrusive breccia between an earlier Permian light grey granite and 

a later dark grey granite. Eastern side of the Teleccio Lake.

GABUDIANU RADULESCU et al (2011): J. metam. Geol., 29, 851–874, Fig. 2 

Photomicrographs of typical relict hornfels from the mountain wall on the eastern side of the Piantonetto Lake. 

Images a) and b) illustrate the late-Variscan mylonitic fabric. Images c) and d) the progressive transformation of 

Variscan amphibolite facies garnet (Grt 1) into Permian contact metamorphic garnet (Grt 2). PPL


Chemical compositions of garnet 1 (Variscan), garnet 2 (Permian) and Alpine eclogitic garnet 3 from the Lake 

Teleccio hornfels


8.6 St ss ≈ 2.4 Crd ss + 2.7 Spl ss + 3.6 Sil + 0.08 H 2O 

8.4 St ss ≈ 1 Qtz + 3.5 Spl ss + 3.9 Sil + 1.35 H 2O: 

Dark layers (also visible in Fig.a, slide 8) of former cordierite and plagioclase, which include light nodules (redrawn 

enlarged on the right side). Nodules originally consisted of Variscan staurolite, which was converted to 

sillimanite + spinel + cordierite during Permian contact metamorphism: see the two possible reactions reported 

above. During the early Alpine eclogite-facies metamorphism sillimanite was converted to kyanite.


Relict regional Variscan staurolite (determined with microraman analyses, shown in the diagram to the right) 

preserved in a contact spinel rimmed by corundum

GABUDIANU RADULESCU et al (2011): J. metam. Geol., 29, 851–874 

Relict cordierite armoured within a contact metamorphic garnet 2. Cf. slide 9.

Gran Paradiso Massif 

The Lago Teleccio metasedimentary rocks record: 

- Variscan regional metamorphism, characterized by amphibolite facies mineral 

assemblages including quartz, plagioclase, K-feldspar (Kfs 1), biotite, garnet (Grt 1) 

and staurolite; 

- late-Variscan mylonitization; 

- Permian contact metamorphism of the Variscan metamorphic rocks related to 

granitoid intrusion, resulting in static recrystallization and ⁄ or new growth of quartz, 

garnet (Grt 2), plagioclase, K-feldspar (Kfs 2), cordierite, green spinel, biotite 

and prismatic sillimanite. 

This thermal event occurred at P = 0.23–0.35 Gpa; T = 670–700 °C; a H2O = 0.75–1.0


Metamorphic evolution of the Piantonetto hornfels with the relevant mineral assemblages.


Pre- Alpine (regional Variscan and Permian contact) and Alpine P-T-paths recorded in the Gran Paradiso rocks.

Radiometric ages of granitoid protoliths 

BERTRAND, PAQUETTE, GUILLOT (2005): SMPM, 85, 15-29.

BERTRAND, PAQUETTE, GUILLOT (2005): Permian zircon U–Pb ages in the Gran Paradiso 

massif: revisiting post-Variscan events in the Western Alps. SMPM, 85, 15-29.

RING, COLLINS, KASSEM (2005) U-Pb SHRIMP data on the crystallization age of the Gran Paradiso augengneiss, Italian 

Western Alps: Further evidence for Permian magmatic activity in the Alps during break-up of Pangea. EgH, 98, 363–370.

RING, COLLINS, KASSEM (2005) U-Pb SHRIMP data on the crystallization age of the Gran Paradiso augengneiss, Italian 

Western Alps: Further evidence for Permian magmatic activity in the Alps during break-up of Pangea. EgH, 98, 363–370.

Monte Rosa

Monte Rosa Massif (MR) 

Gasco et al. (2011), Fig 1.

Relict intrusion breccia with Variscan basement 

xenoliths in a Permian metagranite 

Dal Piaz (2004), Figg. 13 and 14 

MR 

Relict Permian porphyritic granite with aplite dykes

MR 

The Monte Rosa nappe (Bearth, 1952; Dal Piaz, 2001 for a review) 

consists of: 

• pre-Alpine metamorphic basement, consisting predominantly of 

high-grade paragneiss with migmatite, with the following mineral 

assemblages (Bearth, 1957; Dal Piaz and Lombardo, 1986): 

- biotite-sillimanite-garnet-quartz-K-feldspar-plagioclase 

- garnet-biotite-muscovite-sillimanite-plagioclase. 

•a composite Late Paleozoic batholith with granitic to granodioritic 

composition, intrusive into the metamorphic basement. Intrusion 

ages gave been estimated at: 

- 310–330 Ma (Hunziker, 1970; Frey et al.,1976; Engi et 

al.,2001) 

- 260–270 Ma (attributed to a late-magmatic stage: Lange et al., 

2000; Engi et al., 2001)

MR: review of existing protolith ages 

Liati et al. (2001) dated zircon cores from a retrogressed (originally eclogitic) 

metagabbro included in leucocratic orthogneiss from north-west of 

Mattmark dam, upper Saas valley. 

-510 ± 5 Ma: crystallization of the gabbroic protolith 

-272 ±4 Ma: crystallization of leucocratic orthogneiss 

Lange et al. (2000): 

-270 ± 4 Ma ( zircon ) and 268 ± 2 Ma (monazite) from a 

granite in the upper Ayas valley 

Engi et al. (2001b): 

- 260 Ma: monazite from metapelite, interpreted as related to 

contact metamorphism 

Ring et al. (2005): 

- 272 ± 4 Ma interpreted as intrusion ages

Dora-Maira

The Dora-Maira Massif (DMM) 

Geological map of the Dora-Maira massif extracted 

from the 1 to 400,000 geologic map of the Western Alps. 

Dark brown: monometamorphic Pinerolese unit with 

metadioritoids in dotted red. 

Darker reddish: Polymetamorphic basement 

Pink: orthogneiss derived from Alpine recrystallization 

of Permian granitoids. 

Green: main ophiolitic massifs of the Piemonte Zone 

Purple: Piemonte zone metasediments (=“calcescisti”)

Northern Dora-Maira Massif : 

Dora-Maira Massif 

-a polymetamorphic basement of probable pre-Permian age (Vialon, 

1966; Borghi et al., 1984) 

-a monometamorphic complex of probable Permo-Carboniferous age 

(Franchi, 1906; Vialon, 1966; Borghi et al., 1984) 

-an igneous intrusive complex of late-Palaeozoic age of granitoids 

and minor dioritoids 

The polymetamorphic basement consists of: 

- Grt-Chd-micaschist + Eclogites 

-Pre-Variscan orthogneiss (“gneiss glanduleux” of Vialon, 1966; 

Compagnoni & Sandrone, 1981; Borghi et al., 1984, 1985; 

Sandrone et al., 1986)

The monometamorphic complex 

Northern DMM 

[“Complesso grafitico di Pinerolo” of Franchi & Novarese (1895); 

“Ensemble de Pinerolo” of Vialon (1966)] consists of: 

- metaconglomerate 

- meta-arkose 

- metapelite (Grt-rich) 

“Ensemble de Dronero et Sampeyre” of Vialon (1966), 

- fine-grained gneiss and micaschist 

- volcanosedimentry rocks (“Porphyroïdes arkosique” of Vialon (1966) 

The igneous intrusive complex includes a variety of orthogneiss derived from: 

- granitic to granodioritic protoliths 

- minor dioritoids (“Dioritic gneiss of Malanaggio”)

Northern DMM 

BUSSY & CADOPPI (1996) and BERTRAND et al. (2000) 

The main results are as follows: 

- 457 ± 2 Ma (Caledonian) crystallization of Punta Muret augengneiss, 

which is a polymetamorphic peraluminous granite of 

anatectic origin 

- 304 ± 2 Ma: crystallization of the Cavour leucogranite, which is an evolved 

granite of probably calc-alkaline affiliation 

- 290 ± 2 and 288 ± 2 Ma: crystallization of the dioritic and granodioritic 

gneiss of Malanaggio, which are typical calc-alkaline rocks 

- 267-279 Ma: Sangone granite 

- 268-283 Ma: Freidour granite 

Intrusion ages

Southern DMM 

Southern Dora-Maira massif with the UHP Brossasco-Isasca Unit (ruled)- Darker red: Polymetamorphic basement; 

Pink: orthogneisses derived from Alpine tectonometamorphic reworking of Permian granitoids, originally intruding 

the basement

Lithological composition of the Southern DMM : 

-a polymetamorphic basement of probable pre-Permian age 

• Alm-Ky-Jd-Phg micaschist + marble + Ky-eclogite (in 

Brossasco-Isasca unit, BIU) 

• Grt-Chd-micaschist + marble + eclogite (in other units) 

-an igneous intrusive complex of late-Palaeozoic age, consisting of 

granitoids 

Intrusion age (BIU): 

304 ± 10 Ma: metagranite (U-Pb on zircon; Tilton et al., 1991) 

c. 300 Ma : metagranite (U-Pb on zircon; Tilton et al., 1997) 

c. 275 Ma: zircon in whiteschist, interpreted as possible 

crystallization age of the protolith (Gebauer et al., 1997)

Monte 

Rosa 

Gran 


Dora- 

Maira 

Internal Crystalline Massifs: summary 

Pre- 

Variscan 

intrusives 

(dated) 

Gabbro 

510 Ma 

Granitoids 

450 Ma 

Variscan 

metamorphism 

Amphibolitefacies 

with 

migmatites 


(Stt + Grt) 


(Stt + Grt) 

with migmatites 

Post- 

Variscan 

intrusives 

(dated) 


c. 270 Ma 


c. 270 Ma 


& 

dioritoids 

c. 300-260 

Ma 

Post-Variscan 

metamorphism 

Post- 

Variscan 

cover 

Alpine metamorphic 

overprint 

HT-hornfels ? Eclogite-facies GS 

Contact 

metamorphism 

P = 0.23-0.35 GPa 

T = 670-700 °C 

Y Eclogite-facies GS 

HT-hornfels Y Eclogite-facies GS

Sesia-Lanzo Zone (SL)

Polymetamorphic basement: granulite + amphibolite (migmatite) 

facies metamorphics (Lardeaux & Spalla, 1991) 

- medium to low-P granulite facies (Stage a) 

• mafics: Opx + Grt + Pl + brown Ho + Ilm 

• felsics: Grt + Kfs + Pl + Crd + Sil + Bt + Ilm 

- amphibolite-facies re-equilibration (Stage b) 

• mafics: Amp + Pl 

• felsics: Bt + Sil + Crd + Spl 

Monometamorphic sequence: Scalaro unit 

Granitoids and minor gabbros

Relict late-Variscan amphibolite-facies micaschist (“kinzigite”) from Monte Mucrone 

with whitish quartzo-feldspathic metatects and yellowish sillimanite (now kyanite) - 

rich + biotite layers

Photomicrograph of a meta-“Kinzigite” from the Monte Mucrone area with Grt 

porphyroclast and alternating biotite-sillimanite layer and metatects - PPL

Detail of a Variscan sillimanite (“fibrolite”) bundle pseudomorphically replaced 

by alpine kyanite. Note that sillimanite is still preserved within quartz crystals 

(lower right) - PPL

Intrusion age of metagranitoids and metabasics 

352 ± 8 Ma: Cima di Bonze: metagabbro (SHRIMP-Rubatto et 

al., 1999) 

285 ± 7 Ma: Monte Mucrone eclogite (SHRIMP-Rubatto et al., 1999) 

289 ± 2 Ma – 293 ± 3 Ma: Monte Mucrone metagranitoid (conv. 

U/Pb: Bussy et al., 1998) 

297 ± 18 Ma: Monte Mucrone metagranitoid (SHRIMP-Rubatto 

et al., 1999) 

287 ± 8 Ma: Monte Mucrone metagranitoid (206Pb/238U allanite 

age: Cenki-Tok et al., 2011)

Monte Mucrone metagranite. Note that the original igneous quartz (bluish) is still 

preserved.

Photomicrograph of the Monte Mucrone metagranite. Note the igneous euhedral 

plagioclases replaced by a high relief (dark grey) aggregate of Jd+Zo (+Qtz) - PPL.

Southern SL 

Beltrando (2012)

Grt-Pl-Ttn granulite

PLEASE NOTE: SLIDES REFERRING TO THE 

NEW UNPUBLISHED STUDY OF THE 

SOUTHERN SESIA ZONE BY BELTRANDO ET 

AL. (IN PREP.) HAVE NOT BEEN INCLUDED IN 

THIS ONLINE VERSION OF THE 

PRESENTATION

Monte 

Rosa 

Gran 


Dora- 

Maira 

Sesia 

Zone 

Internal Crystalline Massifs and Sesia Zone 

Pre-Variscan 

intrusives 

(dated) 

Gabbro 

510 Ma 


450 Ma 

Gabbros 

350 Ma 

Variscan 

metamorphism 


with 

migmatites 


(Stt + Grt) 


with 

migmatites 

Granulitefacies 

Post- 

Variscan 

intrusives 

(dated) 


c. 270Ma 


c. 270Ma 


dioritoids 

c. 300- 

260Ma 


c. 290 Ma 

and minor 

gabbros 

Post-Variscan 

metamorphism 

Post- 

Varisc 

an 

cover 

HT-hornfels ? 

Contact 

metamorphism 

P =0.23-0.35 GPa 

T = 670-700 °C 

Y 

HT-hornfels Y 

Granulite to 


265-273Ma 

Y 

Alpine 

metamorphic 

overprint 

Eclogitefacies 




GS 

GS 

GS 

GS

Variscan metamorphism (dated and inferred) 

Permian‐Triassic metamorphism (dated and inferred) 

Spalla & Marotta, 2007

Strona-Ceneri 

S Sesia Zone 

Ulten 

Variscan metamorphism in Western and Central Alps (dated) 

modified after Spalla & Marotta, 2007

Strona-Ceneri 

S Sesia Zone 

Ulten 

Variscan metamorphism in Western and Central Alps (dated) 

modified after Spalla & Marotta, 2007

CONCLUSIONS 

In the Western Alps, two different sectors may be 

distinguished, which are characterized by different 

Variscan metamorphism: 

• VARISCAN 

Gran Paradiso: amphibolite facies (Stt+Grt) 

(600°C

Ca. 3000 km of dextral strike slip motion between Laurasia and Gondwana at 280‐250 Ma 

(Muttoni et al., 2009)

(Muttoni et al., 2009)

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