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Geological Field Trips

Società Geologica Italiana

2014 Vol. 6 (1.2) ISPRA Istituto Superiore per la Protezione e la Ricerca Ambientale

SERVIZIO GEOLOGICO D’ITALIA Organo Cartografico dello Stato (legge N°68 del 2-2-1960) Dipartimento Difesa del Suolo

ISSN: 2038-4947

Valle d’ section of the : multi-stage HP and assembly of a rifted continental margin 10th International Eclogite Conference - (Aosta, ), 2013

DOI: 10.3301/GFT.2014.02 Valle d’Aosta section of the Sesia Zone: multi-stage HP metamorphism and assembly of a rifted continental margin R. Compagnoni - M. Engi - D. Regis GFT - Geological Field Trips geological fieldtrips2014-6(1.2) Periodico semestrale del Servizio Geologico d'Italia - ISPRA e della Società Geologica Italiana Geol.F.Trips, Vol.6 No.1.2 (2014), 44 pp., 9 figs, 5 tabs. (DOI 10.3301/GFT.2014.02) Valle d’Aosta section of the Sesia Zone: multi-stage HP metamorphism and assembly of a rifted continental margin 10th International Eclogite Conference, Courmayeur (Aosta, Italy) - Pre-conference excursions: September 5, 2013

Roberto COMPAGNONI(1), Martin ENGI(2), Daniele REGIS(3)

(1) Department of Sciences, University of Torino, via 35, 10125 , Italy (2) Institute of Geological Sciences, The University of Bern, Baltzerstrasse 3, 3012 Bern, (3) Department of Environment, Earth and Ecosystems, The Open University, Walton Hall, Milton Keynes MK7 6AA,

Corresponding Author e-mail address: [email protected]

Responsible Director Claudio (ISPRA-Roma) Editorial Board Editor in Chief M. Balini, G. Barrocu, C. Bartolini, 2 Gloria Ciarapica (SGI-) D. Bernoulli, F. Calamita, B. Capaccioni, Editorial Responsible W. Cavazza, F.L. Chiocci, Maria Letizia Pampaloni (ISPRA-Roma) R. Compagnoni, D. Cosentino, S. Critelli, G.V. Dal Piaz, C. D'Ambrogi, Technical Editor P. Di Stefano, C. Doglioni, E. Erba, publishing group Mauro Roma (ISPRA-Roma) R. Fantoni, P. Gianolla, L. Guerrieri, Editorial Manager M. Mellini, S. Milli, M. Pantaloni, Maria Luisa Vatovec (ISPRA-Roma) V. Pascucci, L. Passeri, A. Peccerillo, Convention Responsible L. Pomar, P. Ronchi, B.C. Schreiber, Anna Rosa Scalise (ISPRA-Roma) L. Simone, I. Spalla, L.H. Tanner, Alessandro Zuccari (SGI-Roma) C. Venturini, G. Zuffa. ISSN: 2038-4947 [online] http://www.isprambiente.gov.it/it/pubblicazioni/periodici-tecnici/geological-field-trips

The Geological Survey of Italy, the Società Geologica Italiana and the Editorial group are not responsible for the ideas, opinions and contents of the guides published; the Authors of each paper are responsible for the ideas, opinions and contents published. Il Servizio Geologico d’Italia, la Società Geologica Italiana e il Gruppo editoriale non sono responsabili delle opinioni espresse e delle affermazioni pubblicate nella guida; l’Autore/i è/sono il/i solo/i responsabile/i. DOI: 10.3301/GFT.2014.02 Manuscript received 17 October 2013; accepted 17 December 2013 Valle d’Aosta section of the Sesia Zone: multi-stage HP metamorphism and assembly of a rifted continental margin R. Compagnoni - M. Engi - D. Regis geological fieldtrips2014-6(1.2)

INDEX

Information

Safety ...... 4 Stop 2: Jadeite-bearing leucocratic orthogneiss quarry Hospitals ...... 5 (Stop 2a) and glacial terrace with a variety of basement Accomodation ...... 5 rocks (Stop 2b) ...... 23 Other useful addresses ...... 5 Stop 2a: Jadeite-bearing leucocratic orthogneiss Riassunto ...... 7 (trade name: “Verde Argento”) ...... 24 Abstract ...... 8 Stop 2b: A variety of paraschists, layers of leucocratic Program summary ...... 10 orthogneiss and metamorphic veins ...... 27 Stop 3a: Transposed intrusive contact ...... 23 Excursion notes Stop 3b: Deformed Permian granitoid and aplite dykes with mafic autoliths recrystallized at quartz eclogite-facies 1. Introduction ...... 11 conditions, cut by postmetamorphic Oligocene andesite 3 1.1 Sesia Zone ...... 12 dykes ...... 29 1.2 Piemonte Zone ...... 13 Stop 4: Pervasive late-Alpine greenschist-facies 2. The internal sub-units of the Sesia Zone ...... 14 metamorphism overprinting the earlier eclogitefacies 2.1 Eclogitic Micaschist Complex ...... 14 mineral assemblages ...... 31 2.2 Gneiss Minuti Complex ...... 14 Stop 5: Foliated serpentinite with metarodingite and 2.3 Second diorito-kinzigitic zone ...... 15 forsterite + Ti-clinohumite + Mg-chlorite veins ...... 33 3. Aim of the excursion ...... 16 References ...... 37 Itinerary

Stop 1: Variscan amphibolite-facies continental index crust recrystallized under quartz eclogite facies ...... 20 Main lithologies ...... 21 Evidence of fluid-rock interactions ...... 22 "Yo-Yo-" ...... 22

DOI: 10.3301/GFT.2014.02 geological field trips 2014 - 6(1.2)

4 information R. Compagnoni - M. Engi - D. Regis Regis R. Compagnoni - M. Engi D. : ca. 200 m Quarry of the jadeite-bearing orthogneiss : none : none : none : none : none – Valle d’Aosta section of the Sesia Zone: multi-stage HP metamorphism and assembly a rifted continental margin : ca. 300 m a.s.l. : from ca. 500 to 350 m a.s.l. : from c. 350 to ca. 300 m a.s.l. : ca. 900 m a.s.l. : c. 750 m a.s.l. : c. 320 m a.s.l. : c. 570 m a.s.l. DOI: 10.3301/GFT.2014.02 Safety and waterproof Mountain boots, sun glasses, and sun-tan lotion are strongly recommended, as well warm, wind resistant clothing. Stop 1: Quincinetto-Pramotton Altitude Altitude Altitude Note: Val de is called Gressoney on some maps Altitude Altitude Altitude Altitude Elevation change (on foot) Stop 2a: Montestrutto above Stop 2b: Glacial terrace Elevation change (on foot) for Stop 2a + 2b - Guillemore Gorge Fointainemore del Lys: Stop 3a: Val Elevation change (on foot) at the bridge across river Fointainemore del Lys: Stop 3b: Val Elevation change (on foot) consular road Stop 4: , Roman Elevation change (on foot) Stop 5: Hillock west of the Clapey house. Elevation change (on foot) geological field trips 2014 - 6(1.2)

5 information –Tel.: 115 –Tel.: - Tel.: (+39) - Tel.: - 1515 Tel.: rock garden with R. Compagnoni - M. Engi - D. Regis Regis R. Compagnoni - M. Engi D. - 118 Tel.: Other useful addresses Soccorso Alpino (Mountain Rescue) 800 319 Corpo Forestale (Forestry Corps) Pronto Soccorso (First Aid) geological information) Place” at , area) “Grand Area attrezzata (Park a few km downstream from Aosta. Corpo Vigili del Fuoco (Fire Brigade) Maria RATTO Sara Dr. Dirigente Responsabile Centro Funzionale Regionale d’Aosta Valle Regione 2/A, Via Promis - 11100 Aosta E-mail: [email protected]. and geologic map site: Topographic geologiavda.partout.it www. BERTOLO Davide Dr. geologiche Attività 33, Località Amérique (Aosta) - 11020 QUART 0165-776808 Tel.: E-mail: [email protected] IL GIARDINO DELLE ROCCE ( Valle d’Aosta section of the Sesia Zone: multi-stage HP metamorphism and assembly a rifted continental margin DOI: 10.3301/GFT.2014.02 Hospitals Aosta Ospedale Regionale - 11100 1, Viale Ginevra (+39) 0165 543266, 543315 Tel: Cross) C.R.I. (Italian Red 2, Via Grand’Eyvia -11100 (+39) 0165 551566 Tel.: Ospedale Civile di Ivrea 2, Piazza Credenza - 10015 (+39) 0125 4141 Tel.: Cross) C.R.I. (Italian Red - 10015 1, Piazza Croce Rossa Baltea, 3 Via Dora (+39) 0125 418111 Tel.: Accomodation Camping Mombarone Daniele n° 54 Torre Fraz. 10010 Settimo Vittone (Torino) 0125 757 907 Tel.:(+39) [email protected] www.campingmombarone.it geological field trips 2014 - 6(1.2)

6 information R. Compagnoni - M. Engi - D. Regis Regis R. Compagnoni - M. Engi D. Valle d’Aosta section of the Sesia Zone: multi-stage HP metamorphism and assembly a rifted continental margin Road map of the field trip with logistical Road points. Basemap ©2014 Google. DOI: 10.3301/GFT.2014.02 geological field trips 2014 - 6(1.2)

7 information rifting R. Compagnoni - M. Engi - D. Regis Regis R. Compagnoni - M. Engi D. (CME), in particolare delle caratteristiche petrografiche, (CME), in particolare delle caratteristiche ” (Stop 2), ma non in altre porzioni adiacenti del CME. In yo-yo tectonics Valle d’Aosta section of the Sesia Zone: multi-stage HP metamorphism and assembly a rifted continental margin complesso dei micascisti eclogitici DOI: 10.3301/GFT.2014.02 uno degli affioramenti descritti (Stop 4) è evidente una parziale retrocessione in facies scisti verdi delle descritti (Stop 4) è evidente una parziale retrocessione in facies scisti verdi uno degli affioramenti di HP associata ad una deformazione da duttile a fragile. paragenesi pre-collisionale del margine continentale Adriatico e le relazioni della Zona Sesia con le adiacenti unità pre-collisionale del margine continentale Adriatico e le relazioni della Zona Piemontese. oceaniche della Zona scelti si collocano in uno scenario spettacolare e forniscono un’opportunità ideale per Gli affioramenti dalle eclogiti (contenenti onfacite del CME varia spettro litologico caratteristico L’ampio esaminare rocce di HP. ± + rutilo) a micascisti fengitici ± granato ± glaucofane zoisite ± fengite paragonite + granato onfacite/giadeite ± cloritoide, calcescisti e marmi, ortogneiss. Nonostante l’intensa deformazione polifasica e la ricristallizzazione metamorfica di HP, del protolite pre- possibile riconoscere caratteri localmente è ancora da uno derivato d’Aosta: due tipi di ortogneiss felsici possono essere riconosciuti in Valle esempio, Per Alpino. di contatti intrusivi, inclusioni cogenetiche, filoni aplitici, e altre permiani (con locale preservazione granitoidi ornamentale tipo la pietra pre-varisici, da leucomonzograniti strutture magmatiche: Stop 3), l’altro derivato che Argento”, presso Settimo Vittone/Montestrutto (Stop 2; nota come “Verde “Argentera” nella cava estratta e quarzo). contiene giadeite, fengite, K-feldspato P-T-D-t di una complessa evoluzione evidenze monociclici mostrano Metasedimenti policiclici e più raramente dagli scisti blu,alpina, che localmente contiene relitti della storia prograda a uno o più stadi in facies eclogitica. Sesia. Nell’area di HP della Zona studi petrocronologici hanno datato in dettaglio questa evoluzione Recenti evidenza di dell’escursione, in una scaglia tettonica di dimensione chilometrica è stata identificata chiara indice di “ ciclicità in condizioni di HP, Riassunto Sesia, è la descrizione della della Zona le parti centrali Lo scopo principale di questa escursione, che attraversa geologia del strutturali e geocronologiche. Il CME è un corpo di crosta continentale polimetamorfica con una pervasiva strutturali i descritti ben illustrano l’orogenesi Alpina. Gli affioramenti impronta di alta pressione (HP) acquisita durante subduzione e collisione coinvolgendo ai margini di placche convergenti, processi che si sono verificati esumazione continentale, metamorfismo e deformazione in condizione della facies eclogitica, seguiti da rapida e parziale retrocessione. Altri argomenti di questa escursione riguardano il magmatismo associato al geological field trips 2014 - 6(1.2)

8 information Eclogitic R. Compagnoni - M. Engi - D. Regis Regis R. Compagnoni - M. Engi D. derivati da originari filoni doleritici. Queste meta- derivati boudin , costituita dai calcescisti e dai termini ofiolitici della Zona “-Saas”. , costituita dai calcescisti e termini ofiolitici della Zona (EMC), notably on its petrographic, structural, and geochronological characteristics. The and geochronological characteristics. structural, (EMC), notably on its petrographic, Valle d’Aosta section of the Sesia Zone: multi-stage HP metamorphism and assembly a rifted continental margin Zona Sesia, Valle d’Aosta, eclogite, dinamica di zolle convergenti, polimetamorfismo, convergenti, eclogite, dinamica di zolle d’Aosta, Sesia, Valle Zona Zona Piemontese DOI: 10.3301/GFT.2014.02 rodingiti a grana fine e le associate vene metamorfiche ripiegate nelle serpentiniti contengono Mg-clorite, fine e le associate vene rodingiti a grana metamorfica alpina in condizioni di facies forsterite e titanclinohumite che indicano una ri-equilibrazione il picco di HP oltre 20 Ma dopo la più recente queste unità hanno raggiunto Tuttavia, eclogitica a quarzo. Sesia. impronta metamorfica in facies eclogitica della Zona secolo di numerosissimi studi dettaglio, Sesia sia stata oggetto ormai da quasi mezzo Nonostante la Zona la complessa giustapposizione dei suoi terreni di HP e le sue relazioni con parti più esterne delle Alpi alcuni degli lo scopo di mostrare Occidentali sono state solo in parte chiarite. Questa guida ha prevalentemente in essi contenuti, e pertanto non Sesia e discutere criticamente le evidenze più classici della Zona affioramenti riconosciuta. finora contiene i dettagli di ciascun stadio della complessa evoluzione Parole chiave: subduzione, collisione, cicli di pressione, esumazione. Micaschist Complex high-pressure (HP) imprint mass of polymetamorphic continental crust with a pervasive EMC is an impressive plate The outcrops visited offer insights into the processes at a convergent acquired during the Alpine orogeny. continental subduction and collision, metamorphism deformation under eclogite facies margin, involving exhumation and partial retrogression. Additional topics of this field trip address conditions, followed by rapid the magmatism associated with pre-collisional rifting of Adriatic continental margin and relation to the adjacent Piemonte oceanic units. Sesia Zone ample opportunities to examine HP rocks. The outcrops selected are set in a spectacular scenery and provide L’affioramento sulla sommità della collinetta dello Stop 5 mostra una serpentinite antigoritica foliata con relitti sulla sommità della collinetta dello Stop 5 mostra L’affioramento spinello) che contiene peridotitici (clinopirosseno, Oltre ai quattro stop della Zona Sesia, un affioramento finale illustra le rocce di HP dell’adiacente unità finale illustra Sesia, un affioramento Oltre ai quattro stop della Zona oceanica della Abstract is laid on the geology of parts of the Sesia Zone The main focus of this field trip through central geological field trips 2014 - 6(1.2)

9 information Piemonte R. Compagnoni - M. Engi - D. Regis Regis R. Compagnoni - M. Engi D. Valle d’Aosta section of the Sesia Zone: multi-stage HP metamorphism and assembly a rifted continental margin , specifically calc-schists and ophiolite members of the “Zermatt-Saas” zone. The hilltop outcrop zone. and ophiolite members of the “Zermatt-Saas” , specifically calc-schists Sesia Zone, , eclogite, convergent dynamics, polymetamorphism, subduction, eclogite, convergent Aosta Valley, Sesia Zone, DOI: 10.3301/GFT.2014.02 oceanic unit spinel) containing lenses foliated antigorite schist with relics (clinopyroxene, (Stop 5) displays containing Mg-chlorite metarodingites and the folded veins These fine-grained from doleritic dykes. derived eclogite under low-temperature and titanoclinohumite within serpentinite once again indicate equilibration eclogite these units reached that HP stage more than 20 Ma after the youngest facies conditions. However, in the Sesia Zone. facies imprint recognized and the complex assembly of its HP-terranes Despite nearly half a century of intense study in the Sesia Zone, remains incompletely understood. This field guide their relation to more external parts of the Western merely introduces a few of the classic outcrops and discusses some critical evidence they contain, but so far. recognized details on each stage of the evolution it could not incorporate Key-words: exhumation. collision, pressure cycling, The spectrum characteristic of the EMC ranges from eclogites (containing omphacite and/or jadeite, garnet, of the EMC ranges The spectrum characteristic chloritoid, rutile) to phengite schists, calcschists, and marbles, as well a phengite, glaucophane, zoisite, of orthogneisses. Despite the intense polyphase deformation and HP-metamorphic recrystallization, it variety instance, two in some of the protoliths. For characteristics pre-Alpine is possible in some locations to recognize (with granitoids from Permian one derived types of felsic orthogneiss can be distinguished in the Aosta Valley, and other magmatic structures; veins contacts, magmatic inclusions, leucocratic of intrusive local preservation such as the building stone mined at leuco-monzogranite, from pre-Variscan Stop 3), the other derived Argento” contains jadeite, quarry near Settimo Vittone / Montestrutto (Stop 2; so-called “Verde “Argentera” quartz). phengite, K-feldspar, metasediments contain evidence of a complex Alpine PTDt-evolution, monocyclic and more rarely Polycyclic history from blueschist, one or more stages at eclogite facies. Recent locally including relics of their prograde in some detail. In the area visited, of the Sesia Zone dated this HP-evolution petrochronological studies have tectonic slice (Stop 1), but not in adjacent parts has been identified in one km-size clear evidence of HP-cycling retrogression and attendant ductile to brittle deformation of Partial tectonics”. of the EMC, indicating “yo-yo the HP-rocks is evident in one of outcrops (Stop 4). a final outcrop introduces HP-rocks of the adjacent Apart from the four localities in Sesia Zone, geological field trips 2014 - 6(1.2) information 10 R. Compagnoni - M. Engi - D. Regis Regis R. Compagnoni - M. Engi D. can be reached driving Stop 3b ) are located between the hamlets of ). Then, walk back to the Argentera quarry and back to the Argentera ). Then, walk Stop 3a Stop 2b ). Leaving the quarry along a dirt road down to a paved minor the quarry along a dirt road down to paved ). Leaving Stop 2a requires a 40 minutes’ up to the top SS26 along the National Road drive is reached walking a few hundred meters further on, to the end of road. is reached walking Stop 5 Stop 1 Valle d’Aosta section of the Sesia Zone: multi-stage HP metamorphism and assembly a rifted continental margin ) (ca. 35 minutes’ Road. parking lot right next to the Roman Use the sizeable drive). Stop 4 DOI: 10.3301/GFT.2014.02 to the parking. An alternative way to reach Stop 2 is to drive from Quincinetto to the hamlet of Montestrutto to reach Stop 2 is drive way to the parking. An alternative (15 minutes’ (SS26) along the National Road path leads up From the village of Montestrutto a paved drive). quarry (Stop 2a), following the same route (Stop 2b) and from there to the Argentera to the glacial terrace change (on foot) is some 200 m. the elevation sense. Either way, but in the reverse explained above, (Stop 3) requires a 40-45 minutes’ from /Montestrutto to The transfer along the drive The Guillemore Gorges ( (i.e. Gressoney valley). del Lys Val stream. and , immediately below the small dam on Lys Fontainemore locality Espaz. at Fontainemore, River downstream from Stop 3a to the small bridge that crosses Lys up to SS26 the National Road St. Martin, and take to Pont del Lys) (Val back down the valley Then, drive Donnas ( from Stop 4 to The transfer and drive the small road to Saint Germain and Château de , hillslope; then take of the Montjovet up to the path Clapey (Sappé) house. After two minutes’c. 500 m southward to the Clapey house, in walk right up to the top. climb the small hill to your road is preserved, front of which a portion the Roman road (10 minutes’ is reached ( a glacial terrace walk), Program summary a small town located near the entrance field trip is located at Quincinetto, The starting point of this one-day (ca. 45 minutes along the A5 Highway (Fig. 1). Quincinetto can be reached from Torino of the Aosta Valley at the south end of bridge, enter village on exit from the highway, time). Immediately after you travel right; on your a right and follow the road skirting upstream, with river left, then after 100 meters take your parking after 700 meters. The gate to the powerstation is locked. To visit the outcrops beyond, you may need to make arrangements. need to make may you visit the outcrops beyond, To The gate to the powerstation is locked. of the some 200 meters before the end of road, show many giant blocks on the riverbank, Alternatively, same features and can be visited freely. (SP72) Road and then to Nomaglio along the Provincial to Borgofranco drive reach Stop 2 from Quincinetto, To quarry. of c. 575 m a.s.l.) with the country road leading to Argentera up to the junction (at elevation road, a few down the steep gravel Consider that the road is narrow and place for parking limited. Walk hundred meters into the large quarry ( geological field trips 2014 - 6(1.2) excursion notes 11 R. Compagnoni - M. Engi - D. Regis Regis R. Compagnoni - M. Engi D. Briançonnais domain, which originated from a continental ribbon located along the distal European margin; these units are not visited on this field trip. of widespread eclogites in the The discovery continental basement rocks of the Sesia of an era ago sparked some 35 years Zone of HP rocks. Owing to intense investigation spectacular assemblages and metamorphic of the initial studies textures, many focussed on petrological aspects (see Table that discovered 1 for references). It was most of the rock types studied are imprint (under a strong Paleozoic polycyclic: upper amphibolite- to granulite-facies is common (Lardeaux & Spalla, grade) HT relics are particularly 1991). Pre-Alpine common in dry and weakly deformed rock of the types. On the other hand, many eclogites show partial retrogression to assemblages. greenschist-facies Valle d’Aosta section of the Sesia Zone: multi-stage HP metamorphism and assembly a rifted continental margin Fig. 1 – Geographical location of the field trip area and stops. Fig. 1 – Geographical DOI: 10.3301/GFT.2014.02 1. Introduction Alps, of the Italian Western (Fig. 1), one of the major valleys d’Aosta field trip runs through the Valle This day d’Aosta of the Valle a small village located at the entrance starts at Quincinetto, c. 100 km long. The excursion Most of the near the town of Ivrea, which is nestled in an amphitheatre Pleistocene glacial moraines. starting Further up the Aosta valley, stops are within continental basement units of the Sesia Zone. excursion dominate, and the last stop of field trip is ophiolitic units of the Piemonte Zone near the town of Verres, basement rocks of the mostly Paleozoic traverses d’Aosta further upstream, the Valle Even in this zone. geological field trips 2014 - 6(1.2) excursion notes 12 R. Compagnoni - M. Engi - D. Regis Regis R. Compagnoni - M. Engi D. Valle d’Aosta section of the Sesia Zone: multi-stage HP metamorphism and assembly a rifted continental margin Fig. 2 - Geotectonic map of the Sesia Zone and the Fig. 2 - Geotectonic map of the Sesia Zone (2011), Kapferer et al. (2012), Regis et al. (2012). et al. (2012), Regis (2011), Kapferer adjoining units, based on maps by Hermann (1937), Gosso et al. (1981), Gosso (1989a), Passchier (1977), Pognante et al. al. (1982), Williams & Compagnoni (1983), Pognante (1987), Biino & Compagnoni (1988), Castelli (1991), et al. (1994), Venturini Lardeaux & Spalla (1991), Venturini & Spalla (2001), Servizio (1995), Steck et al. (1999), Rebay (2002, 2006), Zucali (2002), Geologico d'Italia/APAT & Compagnoni (2003), Spalla Zulbati Ferraris & Messiga (2007), Kaczmarek et al. (2004), Rebay Ferrando et al. (2008), Gasco al., & Muntener (2008), Zanoni et al. (2010), Zanoni (2009), Delleani et al. (2013), Zanoni (2010), Dal Piaz (2011), Zucali & Spalla DOI: 10.3301/GFT.2014.02 No evidence of UHP-conditions has been but UHP-relics reported from the Sesia Zone, metasediments been found in tectonized have coesite (Reinecke, of the Piemonte Zone: 1991) and quite recently microdiamonds been reported 2011) have et al., (Frezzotti from the Lago di Cignana area (to be visited during the pre-conference field trip). 1.1 Sesia Zone (also known as the Sesia-Lanzo The Sesia Zone is the most easterly unit of axial belt Zone) Alps. It is bounded by the Insubric the Western Line) to the Line (locally called External to the west, and east, the Piemonte Zone Massif to the south (Fig. 2). ultramafic Lanzo geological field trips 2014 - 6(1.2) excursion notes 13 R. Compagnoni - M. Engi - D. Regis Regis R. Compagnoni - M. Engi D. Valle d’Aosta section of the Sesia Zone: multi-stage HP metamorphism and assembly a rifted continental margin DOI: 10.3301/GFT.2014.02 The Sesia Zone is a composite unit that consists predominantly of Palaeozoic continental basement similar to is a composite unit that consists predominantly of Palaeozoic The Sesia Zone by Permian is characterized block in the . The Ivrea zone which is an equivalent the Ivrea Zone, a Mesozoic Within the Sesia Zone, metamorphism and lacks Alpine overprint. amphibolite- to granulite-facies 1994); it underwent Alpine metamorphism together with et al., is locally found (Venturini sedimentary cover the basement rocks. of Oligocene age crops out close to the Insubric Line (Ahrendt, cover volcano-sedimentary A post-metamorphic 1994; Babist et al., et al., been suggested (e.g. Venturini have subdivisions of the Sesia Zone 1969). Various 2006), but the classical three sub-units, as defined by Compagnoni et al. (1977) on grounds of rock types, are used below: Eclogitic Micaschists Complex (EMC), Gneiss and prevalent metamorphic grade Minuti Complex (GMC), and Second Diorite-Kinzigit (2DK) (see Section 2). 1.2 Piemonte Zone Alps, has been referred to under which crops out along the whole length of Western The Piemonte Zone, Ligurian–Piemontese Zone, Ligurian–Piemonte Zone, different names, including Piemonte– Zone, and by the It is bounded by the Sesia Zone dei calcescisti con pietre verdi”. and “Zona Schistes Lustrés Zone Plain to the southeast and by Briançonnais units west (Fig. 2). Po that was samples remnants of the Piemonte–Liguria Ocean and sedimentary cover The Piemonte Zone 1992). On the base of deposited along its margins (e.g. Bearth, 1967; Lemoine, 1985; Deville et al., lithological associations and metamorphic imprints two main ensembles, first described in the northwestern Alps: an eclogite facies Alps by Bearth (1967), can be distinguished all along the arc of Western (Fig. 2; from north to south, see Servizio Geologico and a blueschist facies Combin Zone Zone Zermatt–Saas 1983; Servizio Geologico d’Italia, 2002; Agard et al., et al., 2008; Perotto et al., d’Italia, 2006; Beltrando 2002). 1982; Schwartz, 2001; Lombardo & Pognante, geological field trips 2014 - 6(1.2) excursion notes 14 R. Compagnoni - M. Engi - D. Regis Regis R. Compagnoni - M. Engi D. Valle d’Aosta section of the Sesia Zone: multi-stage HP metamorphism and assembly a rifted continental margin DOI: 10.3301/GFT.2014.02 2. The internal sub-units of the Sesia Zone 2.1 Eclogitic Micaschist Complex minor basement comprising paragneisses, The Eclogitic Micaschists Complex (EMC) consists of a polycyclic metamafic rocks, orthogneisses and impure marbles. This basement is intruded by abundant Carboniferous to are dominant in the 1998). Granitoids 1998; Rubatto, and minor gabbros (Bussy et al., granitoids Permian 1972). A labelled “Sesia Gneiss” (Dal Piaz et al., they are traditionally northeastern part of the Sesia Zone; carbonate schist and locally with monometamorphic paragneiss, sedimentary cover, probably Mesozoic 1994). The EMC et al., part of the EMC (Venturini manganiferous impure quartzite, is found in the central & Essene, 2002; lower than 600 °C (Tropper at temperatures reached maximum pressures less than 2 GPa assemblage greenschist-facies of the Gneiss Minuti Complex, a pervasive 2002). In the proximity Zucali et al., 1977) 1972, Compagnoni et al., of the EMC (e.g. Dal Piaz et al., the eclogite-facies parageneses overprints (Fig. 2). 2.2 Gneiss Minuti Complex rich in The Gneiss Minuti Complex (GMC) mainly consists of orthogneisses interbedded with more mafic layers exposures, such as in the lower Aosta Valley, albite, white mica, chlorite, epidote, and actinolite. In several age with associated aplitic dyke of probable Permian from granitoids derived the orthogneisses appear to have in other locations, such as basement. In contrast, polycyclic which intruded into a locally preserved swarms, of pre-Alpine trace the Gneiss Minuti Complex does not show any and the southern Sesia Zone, the Orco Valley sedimentary sequence comprising at the expense of a Mesozoic metamorphism and probably developed 1925; Dal 1922; Gennaro, marble, calcschist, and metachert (Zambonini, and rare predominant meta-arkose 1989b). 1971; Minnigh, 1977; Pognante, Piaz et al., geological field trips 2014 - 6(1.2) excursion notes 15 R. Compagnoni - M. Engi - D. Regis Regis R. Compagnoni - M. Engi D. 2.3 Second diorito-kinzigitic zone (2DK) consists of The second diorito-kinzigitic zone amphibolite- pre-Alpine of well-preserved slivers micaschists with facies garnet–sillimanite–biotite local migmatitic leucosomes, minor amphibolite, marble, and a small harzburgite body (Dal Piaz et under blueschist- 1971). Alpine re-equilibration al., facies conditions is restricted to the margins of (Dal Piaz et al., or to narrow shear zones slivers 1981; et al., 1979; Passchier 1971; Gosso et al., Williams & Compagnoni, 1983). The 2DK crops out discontinuously along the contact between EMC and the GMC (Fig. 2). and evolution (formerly Sesia-Lanzo Zone) (formerly Sesia-Lanzo B. Emphasis on structural geology Valle d’Aosta section of the Sesia Zone: multi-stage HP metamorphism and assembly a rifted continental margin studies on the Sesia Zone Hy (1984) Ridley (1989a) * Koons (1986)Koons Wheeler & Butler (1993) Castelli (1991) et al. (1999) Tropper Reinsch (1979)Reinsch et al. (1994) Avigad Minnigh (1977)Minnigh (1979) (1984) Rubie (1989b) Pognante Stöckhert (1985) Stünitz (1989) Pognante (1989a)Pognante (1997) Rebay Zucali et al. (2004) Babist et al. (2006) Compagnoni (1977) Lardeaux et al. (1982) Rubatto et al. (2011)Rubatto et al. (2010) Zanoni Dal Piaz et al. (1978) Williams & Compagnoni (1983) Zucali & Spalla (2011) Zucali & Spalla (2011) Pognante et al. (1988)Pognante (1997) Inger & Ramsbotham Oberhänsli et al. (1985) Ridley (1989b) Regis et al. (2012, 2013) Regis Lardeaux & Spalla (1991) & Spalla (2001) Rebay Spalla et al. (1991, 1996) Zucali (2002) Select Pelletier & Müntener (2006)Pelletier Gosso et al. (2010) Dal Piaz et al. (1971, 1972) et al. (1980) Pognante Compagnoni & Maffeo (1973) et al. (1981) Passchier Ferraris & Compagnoni (2003)Ferraris Zucali et al. (2002) Konrad-Schmolke et al. (2006)Konrad-Schmolke (2006) Konrad-Schmolke Table 1 - Select references to studies on HP/UHP metamorphism Table A. Emphasis on HP-petrology Vuichard & Ballèvre (1987, 1988)Vuichard Spalla et al. (1996) in the internal . in the internal Western Konrad-Schmolke et al. (2011a, b)Konrad-Schmolke Meda et al. (2010) DOI: 10.3301/GFT.2014.02 * Includes articles in English only, no abstracts * Includes articles in English only, geological field trips 2014 - 6(1.2) excursion notes 16 R. Compagnoni - M. Engi - D. Regis Regis R. Compagnoni - M. Engi D. Valle d’Aosta section of the Sesia Zone: multi-stage HP metamorphism and assembly a rifted continental margin DOI: 10.3301/GFT.2014.02 3. Aim of the excursion timing of concerns the relative across the Piemonte Zone transect d’Aosta An intriguing aspect of the Valle Alpine imprint. Geochronology of the HP-rocks indicates that largely continental units now exposed in were subducted in the Upper Cretaceous, some 25-30 Ma earlier than HP-imprint oceanic Sesia Zone at 1998). Plate-tectonic evolutions et al., which is Eocene in age (e.g. Rubatto units in the Piemonte Zone, sequence, i.e. oceanic lithosphere is typically margins normally show the opposite temporal convergent Alps, despite some In the Western follow subsequently. subducted first and accretion of continental slices may between the continental and more external oceanic units is very confusing age data, the gap in HP-age at a evolution 2 for references). It is interpreted to reflect the convergent clearly established (see Table 2001), leading to the subduction of an previously rifted and thinned continental margin (e.g. Dal Piaz et al., 2010a,b). et al., (Dal Piaz, 1999; Beltrando (OCT) zone ocean-continent transition presents an opportunity to appreciate the progress made in understanding d’Aosta This field trip through Valle petrological and geochronological work – structural, by integrating this evolution of spanning some forty years has been documented in detail, deformation related to the Alpine tectonic evolution Polyphase intense study. 1981; Zucali, 2002; Babist et al., Alps (e.g. Passchier both in the external and internal parts of Western but it comprises at least three continental exhumed as a coherent HP-terrain, was 2006). The Sesia Zone et al., nappes. The evidence gathered strongly suggests that the pre-orogenic structure at lithospheric scale unexpected discovered studies have fundamentally affects where, when and how HP-rocks are formed. Recent 2012), with some 2011; Regis, et al., (Rubatto of the Sesia Zone complexities in the kinematic evolution and tectonics”). The scales, rates (“yo-yo experienced substantial pressure cycles tectonic slices having are targets of ongoing studies. within the Sesia Zone of tectonic mixing by differential movements duration margins remains a challenge, and type and quantifying the internal dynamics at convergent Characterizing Alps are a critical reality check for numerical of the internal Western examples such as the HP-terrains et al. 2010 and references therein). modelers who aim to understand Earth dynamics (e.g. Roda

geological field trips 2014 - 6(1.2) excursion notes

17

HP-max YoYo R. Compagnoni - M. Engi - D. Regis Regis R. Compagnoni - M. Engi D. Decompression? Prograde HP1 stage: 540-550 °C 1.9-2.0 GPa HP D3 stage? 9 . 5 0 3 4 . . . ± 7 1 1 5 1 1 3 1 ± ± ± ± . ± ± ± 8 3 6 5 4 3 3 5 6 6 7 6 64.2±2.5 D3 stage? 7 69.2±2.7 71.0±0.8 63.6±0.7 min. age of D2 6 6 6 s s m m i i r r

n n n n n o o o o o c c c c c roxene r r r r r i i i i i z white mica – whole rock zircon rim1 78.6±0.9zircon rim2zircon rim3 76.8±0.9 HP1 (prograde) allanite core 73.7±0.9 75.6±0.8 LP (decompression) allanite rim1 HP2 (retrograde?) 69.8±0.8 HP1: ~540 °C, 1.7 GPa (D1/D2) allanite rim2 Decompression to MP 65-62 HP2: ~550 °C, 1.4-2.0 GPa (D3) z z garnet and phengite – whole rock z allanite 85.8±1.0 py phengite – albite phengite – whole rock z r

r r

A U-Th-Pb SHRIMP U-Th-Pb U-Th-Pb SHRIMP U-Th-Pb S S - r - - b b A

R R Rb-Sr Rb-Sr & e r e y t i h s r p k o o i c r d o p - r n z

i t t e t m c e t s s i r a i i i v l s l a - h h - e e a e u e t t c c e a l i i b p t q t s s i b g g a a e a a r t t e o c c l e a e i i d m c a m m m + e eclo j Meta m micaschist (garnet- glaucophane bearing) HP gneiss: phengite- sodic amph-garnet m micaschist monocyclic phengite- quartzite ) ) 1 9 0 ) 9 0 6 9 2 9 1 ( ( 9

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l b g a u n Valle d’Aosta section of the Sesia Zone: multi-stage HP metamorphism and assembly a rifted continental margin R I Duchêne et al. (1997) meta-intrusive / eclogite Lu-Hf Rubatto et al. (2011) eclogitic micaschist Ruffet et al. (1995) D Rubatto et al. (1999) Regis et al. (2013) ) n i e t n r o a r e s c r M z

a u n . t o M M

S B

e e t t t a n n n Table 2 - Select references to geochronological studies of HP/UHP metamorphism in the Internal Western Alps. 2 - Select references to geochronological studies of HP/UHP metamorphism in the Internal Western Table o m o o i P DOI: 10.3301/GFT.2014.02 EMC: Fondo slice EMC: Druer slice Eclogitic Micaschist Complex Quincinetto M C Marine (Pont St. Martin) - Fontainemore Mombarone Tectonic domain Reference Rock type Method Mineral Age/Ma Interpretation and conditions QuinseinaMarine Inger et al. (1996) monocyclic marble U-Pb titanite 66.3±0.4 ( Val Chiusella Babist et al. (2006) Monte Mucrone Montestrutto Cima Bonze Regis et al. (2013) M geological field trips 2014 - 6(1.2) excursion notes 18 t n i r p m i s

e s i c e i a c f

a t f

s R. Compagnoni - M. Engi - D. Regis Regis R. Compagnoni - M. Engi D. i t s h e i c d h ? s c a x n r s a e g e e o u m r l r

P P B G Greenschist facies overprint Eclogite facies imprint Eclogite facies . Rb-Sr. g 1 6 4 . . . 2 2 0 8 5 7 0 5 ± ± ± 1 3 4 3 4 4 8 6 2 4 . . . - - - ± - - 4 8 0 6 9 7 5 7 9 7 stems, e. 4 4 4 3 4 4 75-73 ~ 4 4 4 y ic s p e t i t g e n n e r a h g p zircon 44.1±0.7 zircon rims 47.5±1.0 Eclogite facies imprint phengite – whole rock phengite – whole rock d f r r N S S H - - - - b b u m U-Pb U-Pb L R R Rb-Sr S & Ar-Ar SHRIMP SHRIMP e t i arable results from other isoto e g t p i o l g c o e l - c e e

n , t t a s s s s i i h s s i i h h orted com e e e p e e t t t c c p o i i i s s n n c g g g - - g g u c c o o o

l l l l l a P P l c c c a a e e g e c c gneiss minuti Rb-Sr H H ) ) 8 1 ) ) ) ) 9 0 ) 9 9 9 3 9 0 6 9 9 9 0 1 2 9 9 ( 9 9 0 (

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l r d d - p g a o e e m a n Valle d’Aosta section of the Sesia Zone: multi-stage HP metamorphism and assembly a rifted continental margin R L Rubatto et al. (1998) eclogite A D R I C es are shown onl g y e a s r n z u on a f i o a l a g i v Z a R

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n o m r l l a i l o Table 2 (continued) Table l o a t i DOI: 10.3301/GFT.2014.02 Gneiss Minuti Complex Klippen, outliers a top Piemonte-Liguria units Piemonte-Liguria units G Etirol-Levaz Beltrando et al. (2010) eclogite P Note - Ar Lago Cignana Zermatt-Saas Zone C Tectonic domainVal d'Ayas – ReferenceSoana – Val d'Orco Rock type Method Mineral Age/Ma Interpretation and conditions V M E geological field trips 2014 - 6(1.2) itinerary 19 R. Compagnoni - M. Engi - D. Regis Regis R. Compagnoni - M. Engi D. Itinerary Valle d’Aosta section of the Sesia Zone: multi-stage HP metamorphism and assembly a rifted continental margin Fig. 3 - Enlargement of the central part of Fig. 2 Fig. 3 - Enlargement of the central with location of Stops 1 to 5 described in the text. DOI: 10.3301/GFT.2014.02 The lower Valle d’Aosta exposes a complete d’Aosta The lower Valle section (about 20 km long) across the and adjoining ophiolitic continental Sesia Zone low elevation, Albeit the rather Piemonte zone. good due to extensive exposures are very erosion and polishing by the Quaternary Valle glacier. d’Aosta Locations of the outcrops visited are shown in Fig. 1 and 3. Stops to 4 are within the Eclogitic Micaschist Complex. With respect to overprint, the subsequent greenschist-facies Stops 1 to 3 are in the Eclogitic Micaschist Complex where the eclogite-facies mineral minerals: (key assemblages are well preserved omphacite/jadeite and garnet, Na-pyroxene glaucophane), whereas Stop 4 is close to the tectonic contact with the Gneiss Minuti Complex, in an area with significant where jadeite has overprint, greenschist-facies been completely replaced, but glaucophane is Stop 5 is within the still locally preserved. meta-ophiolites of the eclogite-facies Zermatt- (Fig. 3). of the Piemonte zone Saas zone geological field trips 2014 - 6(1.2) itinerary 20 R. Compagnoni - M. Engi - D. Regis Regis R. Compagnoni - M. Engi D. Folded glaucophane-garnet micaschist. Scalaro above Folded (a) Valle d’Aosta section of the Sesia Zone: multi-stage HP metamorphism and assembly a rifted continental margin Omphacite-garnet rock. Mombarone area. (b) Fig. 4 – Outcrop appearance of representative lithologies of Stop 1. of representative Fig. 4 – Outcrop appearance Quincinetto. Quincinetto. DOI: 10.3301/GFT.2014.02 STOP 1: Variscan amphibolite-facies continental crust recrystallized under quartz eclogite facies Quincinetto – Bric Vert to Pramotton (Coord. 45°33’47.52” N , 7°48’29.34” E, elev. c. 300 m a.s.l.) Most lithologies typical of the Eclogitic Micaschist Complex can be collected from large blocks that fell used such large locals have Along the path leading from Chiapetti to Pramotton, towering face of Bric Vert. in their cabin constructions. blocks as outside walls crystals of omphacite (up to 1 cm long) were (Fig. 4b); euhedral coarse-grained are very minerals Early-Alpine is minor. Retrogression recently found in HP-veins. geological field trips 2014 - 6(1.2) itinerary 21 R. Compagnoni - M. Engi - D. Regis Regis R. Compagnoni - M. Engi D. Valle d’Aosta section of the Sesia Zone: multi-stage HP metamorphism and assembly a rifted continental margin DOI: 10.3301/GFT.2014.02 Main lithologies The most frequent lithologies are: ± glaucophane rutile); - Phengite micaschist (with garnet ± Na-pyroxenes and garnet glaucophanite - Glaucophane eclogite, omphacitite (with phengite ± ferroan dolomite zoisite), (± phengite); omphacite ± garnet); marble (with phengite, zoisite, - Calcite dolomite ± ankerite titanite, (calcite and dolomite) schists with accessory glaucophane, Fe-sulphide, - Quartz-phengite-carbonate a few cm wide, nodules of columnar white calcite are interpreted as Characteristic, apatite and graphite. pseudomorphic after HP aragonite. rich some eclogites are relatively Dolomite appears to be in equilibrium with the other high-pressure minerals; from marly sediments. derived in carbonate, suggesting that they are para-eclogites Case crosscutting the eclogitic micaschists in face above minette dyke Blocks of a post-metamorphic Chiapetti can also be sampled. The minette consists of phenocrysts augite, phlogopite, and apatite in a 1958). amphibole (De Marco, groundmass of orthoclase with minor quartz and small needles arfvedsonitic metabasics exposed in The road cuts a small body of banded and deformed eclogite, which is one several complex. A similar megaboudin, c. 800 m deriving from a dismembered layered most likely the paraschists mapped in detail by Zucali & Spalla (2011). was d’Aosta, on the other side of Valle long, exposed at Ivozio amphibolites; this is suggested by the occurrence of colourless from pre-Alpine These rocks probably derive as brown patches. The preserved replacing brown hornblende, which is rarely amphibole porphyroclasts determined to be Early Carboniferous (352 ± 8 Ma SHRIMP magmatic protolith age of such metabasics was et al. 1994, 1996) that (Venturini et al. 1999); the earlier hypothesis Rubatto U–Pb zircon at Cima di Bonze; these metabasics are monometamorphic is thus refuted. rocks of a different type are exposed some hundred meters up along the road south Interesting eclogitized Baltea. The dominant lithology is here an omphacite-glaucophane-garnet micaschist containing Dora the river 2002). Skeletal marbles (Castelli, 1991; Castelli & Rubatto, of polycyclic small eclogite boudins and layers the hornblende. This stage of initial garnet and less abundant glaucophane, both of Alpine age, overgrow the large amphibolite body of Croix Corma the Sesia Zone; eclogitization in metabasics is common all over Donnas being a clear example (Williams & Compagnoni, 1983). ridge above geological field trips 2014 - 6(1.2) itinerary 22 R. Compagnoni - M. Engi - D. Regis Regis R. Compagnoni - M. Engi D. O ratio, salinity). O ratio, 2 -H 2 O). The localization of deformation, e.g. mylonite bands in O). The localization of deformation, e.g. mylonite 2 Valle d’Aosta section of the Sesia Zone: multi-stage HP metamorphism and assembly a rifted continental margin DOI: 10.3301/GFT.2014.02 orthogneiss layers or mineral growth in brittle/ductile mixing zones of metasediments riddled with eclogite growth in brittle/ductile mixing zones or mineral orthogneiss layers in fluid abundance or composition (CO clasts, appears to reflect variations In the same outcrop (where this peculiar amphibolite to eclogite stage occurs) other rock types are found like with relict dyke, quartz - carbonate micaschists containing small eclogitic boudins. A metamorphic leucocratic igneous biotite, unconformably cuts these lithologies. Boudins of amphibolite also occur in the marbles exposed further to north along suspended path, now of metagranitoid is built. Dykes collapsed, originally skirting the spur on which ancient tower of Pramotton and repeatedly folded with marbles. In calcschist, rocks and eclogitic micaschists are interlayered pseudomorphs) occur again; they boudins containing coarse white aggregates of columnar calcite (aragonite been in equilibrium with omphacite. and appear to have from veins derive may Evidence of fluid-rock interactions at eclogite facies conditions are lithotypes of this outcrop sequence, effects fluid-rock interaction In several HP-fluid are observed. of reactive prominently visible. Hydrothermal reaction fronts attesting to the infiltration instance, omphacite-rich bands in calcschist are commonly rimmed by a reaction front of siderite. For to vein and at least one common type is linked generations, formed in several Glaucophane appears to have probably reflecting reaction can be observed, formation; glaucophane replacing omphacite and the converse in the fluid composition (activity of H variations “Yo-Yo-tectonics” 2011; et al., recent studies (Rubatto exposed at this locality near Quincinetto, the part of Sesia Zone For 2). In these Table at eclogite-facies conditions (cf. reported evidence of pressure cycling 2012) have Regis, that polycyclic rocks, but it is noteworthy placed on samples from monocyclic studies much emphasis was instance, two stages of evidence as well. For samples from the Quincinetto locality were found to contain key and from high-pressure metamorphism interrupted by a lower pressure stage were identified petrographically within zircon in a sample of eclogitic micaschist (Rubatto signatures of distinct growth-zones the trace-element 2011). Detailed analysis in a larger suite of samples allowed these stages to be quantified terms et al., When spot analysis of allanite and zircon overgrowths). conditions and to be dated by SHRIMP (U-Th-Pb P-T 2013), et al., stages of deformation (Regis evidence of several with the meso- and micro-structural integrated geological field trips 2014 - 6(1.2) itinerary 23 R. Compagnoni - M. Engi - D. Regis Regis R. Compagnoni - M. Engi D. Valle d’Aosta section of the Sesia Zone: multi-stage HP metamorphism and assembly a rifted continental margin DOI: 10.3301/GFT.2014.02 these petrochronological results provide a very detailed account of the internal tectono-metamorphic a very these petrochronological results provide 2013), for which the most recent data et al., slice (Regis dynamics. The present location is part of the Fondo subduction (to ~55 km, ~80-75 Ma), followed by exhumation ~40 involved indicate that pressure cycling for operated appears to have km, ~75-70 Ma), and again subduction (to ~50 ~65 Ma ago). Thus Yo-Yo slice of 2-3 mm/a. While the Fondo rate (vertical) an average at least 15 Ma (from ~80 to 65 Ma) and involved cannot at present be well delimited in the field, it should stressed that samples showing a similar P-T-t- to Valchiusella. d’Aosta km, from Valle span an area of several and local evidence of cycling evolution from more internal has been found in a sample suite analyzed no evidence of pressure cycling By contrast, earlier, 2012). There, the onset of subduction was (e.g. near Mombarone: Regis, parts of the Sesia Zone Samples consistently indicate that a depth of at least 60 km were a bit higher. pressures (and temperatures) this more internal portion of the Based on these data and observations, reached as early ~85 Ma ago. was et al., is now regarded as a different tectono-metamorphic slice, termed the Druer slice (Regis Sesia Zone in terms of the subdivision 2013); note that both slices are part of the Eclogitic Micaschist Complex (or, 2006, the Mombarone nappe). Babist et al., in order Ongoing work is aiming to refine the “details” of such tectono-metamorphic slices in Sesia Zone, slices with the record paths for the Druer and Fondo Comparing the P-T-t their shape and size. to constrain Pognante and southern Sesia Zone, reported by previous studies for different areas of the EMC (e.g. Ivozio by as a whole is characterized 1989b; Zucali & Spalla, 2011), it is increasingly evident that the Sesia terrane which were independently mobile (in space and time) during the Cretaceous tectonic fragments, several details of the kinematics in subduction channel are still missing, scale While many convergence. been partially of such mixing processes have component) and the rates tectonic mobility (at least its vertical documented. geological field trips 2014 - 6(1.2) itinerary 24 R. Compagnoni - M. Engi - D. Regis Regis R. Compagnoni - M. Engi D. Valle d’Aosta section of the Sesia Zone: multi-stage HP metamorphism and assembly a rifted continental margin DOI: 10.3301/GFT.2014.02 STOP 2: Jadeite-bearing leucocratic orthogneiss quarry (Stop 2a) and glacial terrace with a variety of basement rocks (Stop 2b) East of Montestrutto Argento”) name: “Verde orthogneiss (trade Stop 2a: Jadeite-bearing leucocratic Settimo Vittone – “Argentera” Quarry (Coord.: 45°32’25.70’’ N, 7°50’40.27’ E’, 450 m a.s.l.) by m-scale that is thickened orthogneiss layer This quarry has been opened in a ca. 10 m thick leucocratic axial plane. The fold axis trends folds, which are part of a large scale fold structure with flat-lying parasitic where a similar and thus the same structure is evident on opposite flank of valley, SW across the valley, . quarry has been opened above from Permian with orthogneiss bodies derived had long been considered coeval These orthogneiss layers 1973; (Compagnoni & Maffeo, such as the well-known Monte Mucrone metagranitoids Intrusives granitoids. 1985) are widespread in the EMC (e.g. Zucali, 2011), as related orthogneisses. Oberhänsli et al., 3), whereas the in composition (see Table the orthogneisses quarried here are leucomonzogranitic However, 4), and they differ in age. Liermann et al. (2002) dated (see Table are granodiorites metagranitoids Permian two samples collected from the mentioned quarries and obtained well-defined single zircon U-Pb discordia with upper intercepts at 435 ± 8 Ma for the Settimo Vittone sample and 396 21 Tavagnasco sample; both ages refer to the magmatic protolith. et al. (1999). Argento” are described in Fiora technique and processing of “Verde The excavation phengite ± minor garnet and orthogneiss (Fig. 5a) consists of quartz, jadeite, K-feldspar, The leucocratic glaucophane, with accessory Al-rich titanite, metamictic allanite, zircon, apatite, and local fluorite, Fe-sulphide the jadeite is surrounded by a thin retrogression corona mainly consisting of albite. and carbonate. Locally, quartz (KJQ) the jadeite- K-feldspar- et al. (1999) to calibrate This rock has been used by Tropper and pelites. geobarometer used to determine pressure in eclogite-facies granitoids with The orthogneiss in the quarry locally hosts narrow (mm to cm) shear bands of dark ultramylonite, to the main foliation (S1/S2 composite) (Fig. 5b). These bands contain ultra-fine-grained orientations parallel geological field trips 2014 - 6(1.2) itinerary 25 R. Compagnoni - M. Engi - D. Regis Regis R. Compagnoni - M. Engi D. 0,2 0,18 0,19 1,51 2,15 2,7 0,76 0,581,35 0,56 0,36 0,4 1,77 1,02 0,7 3,06 3,54 3,5 n.d. n.d. 0,05 64,83 66,615,81 16,31 66,6 16,4 3 3 5 O 2 2 O 2 O 2 O 2 O O 2 2 2 2 SiO Al CO P K TiO Fe H Na Table 4 - Major element composition (wt. %) of Table MEC188). orthogneiss and metagranitoid samples from orthogneiss and metagranitoid (MEC 84) (Callegari et al., Lilianes/Fontainemore 1) and from Monte Mucrone (MEC97, 1976, Table 0,10,2 0,14 0,2 n.d. 0,26 0,9 0,86 0,98 4,87 4,39 4,67 0,17 0,120,71 0,13 0,68 0,16 3,22 3,31 3,38 73,82 76,1113,24 74,43 13,01 13,44 Valle d’Aosta section of the Sesia Zone: multi-stage HP metamorphism and assembly a rifted continental margin 3 3 Major element composition (wt. %) of jadeite- 5 O 2 2 O - 2 O 2 O 2 O O 2 2 2 2 P H SampleSiO HPG1 HPG2 SL664 Sample MEC 84 MEC 97 MEC 188 CO Al FeOMnOMgO 0,99CaO 0,03Na 0,28 0,89 0,03 0,83 0,24 1,36 0,05 0,52 0,93 0,56 FeO 3,54 MnO MgO 3,99 CaO 4,65 0,07 4,2 2,1 3,93 0,07 1,22 3,7 0,07 1,4 Total 99,36 100,5 100,35 Total 99,56 99,95 100,5 TiO Fe K Table 3 Table and Santa Maria (SL664: Lombardo et al., 1977). and Santa Maria (SL664: Lombardo et al., bearing leucocratic orthogneiss samples from the Settimo bearing leucocratic quarries (HPG2) (Liermann, Vittone (HPG1) and Tavagnasco 1994), and from a roadside outcrop between Quincinetto DOI: 10.3301/GFT.2014.02 quartz+jadeite and local pseudotachylite; the bands may reflect rapid post-seismic stress release features, as post-seismic reflect rapid the bands may quartz+jadeite and local pseudotachylite; (Küster & TEM studies in this high yield strength material indicate differential stresses of 0.3-0.5 GPa & Stöckhert, 2003). Stöckhert, 1999; Trepmann geological field trips 2014 - 6(1.2) itinerary 26 R. Compagnoni - M. Engi - D. Regis Regis R. Compagnoni - M. Engi D. Layers of leucocratic orthogneiss (whitish) within of leucocratic Layers Photograph showing the typical appearance of the showing the typical appearance Photograph (c, d) (a) Stop 2b: Ultramylonite/pseudotachylite veins in the leucocratic jadeite-bearing in the leucocratic veins Ultramylonite/pseudotachylite (b) (c, d, e, f) Detail of roundish megablasts of jadeite (Jd) in the leucocratic orthogneiss at Detail of roundish megablasts jadeite (Jd) in the leucocratic (e) Small boudin of glaucophane eclogite within the paraschist. (f) Valle d’Aosta section of the Sesia Zone: multi-stage HP metamorphism and assembly a rifted continental margin Stop 2a: Jadeite-bearing leucocratic orthogneiss: Stop 2a: Jadeite-bearing leucocratic (a, b) Fig. 5 – the contact with the paraschist; the contact with paraschist; eclogite-facies paraschists at Montestrutto; eclogite-facies paraschists polished jadeite-bearing leucocratic orthogneiss; polished jadeite-bearing leucocratic Settimo Vittone; Quarry, orthogneiss. “Argentera” DOI: 10.3301/GFT.2014.02 geological field trips 2014 - 6(1.2) itinerary 27 R. Compagnoni - M. Engi - D. Regis Regis R. Compagnoni - M. Engi D. , ca. 5 cm across) occur, particularly close to the clear- , ca. 5 cm across) occur, 95 Valle d’Aosta section of the Sesia Zone: multi-stage HP metamorphism and assembly a rifted continental margin DOI: 10.3301/GFT.2014.02 cut contact with the paraschist (Fig. 5e). These unusual rocks resemble “metaconglomerates” because of the (Fig. 5e). These unusual rocks resemble “metaconglomerates” cut contact with the paraschist standing out on the surface. The best examples were described by Andreoli et roundish jadeite porphyroblasts ridge. d’Aosta - Valle along the Valchiusella al. (1976) from Monte Le Colme on the opposite side of Valley, with of a boudinaged jadeite + quartz phengite rock (possibly eclogite-facies veins) A few dm-thick layers pressure shadows indicate not only the sense of shear but also amount finite strain. asymmetric quartz-filled (several with minor phengite, cm-long rutile prisms and local carbonate rhombohedra quartz veins Refolded dm thick) are common, especially on the northern side of terrace. (Stop 2b) to the hamlet of Montestrutto meets “Francigena path connecting the glacial terrace The paved has represented the route followed by Way Since the early Middle Ages Francigena of Sigerico Way”. is known What today the seat of Papacy. and northern Europe aiming to reach , pilgrims of central is a route some 1600 km in length. The Archbishop Sigerico the year of Sigerico Way” as “The Francigena of the Archiepiscopal after the investiture to return Canterbury from Rome 80 legs in 79 days 990 covered by Itinerary“ declared a “Great European Cultural was In 2004 this way XV. John (Giovanni) from Pope Pallium of Santiago de Compostela” in Spain. the Council of Europe, similar to “Walk Stop 2b: A variety of paraschists, layers of leucocratic orthogneiss and metamorphic veins of leucocratic layers of paraschists, Stop 2b: A variety Glacial terrace above Montestrutto (Coord.: 45°32’17.18’’ N, 7°50’38.27’’ E, c. 300 m a.s.l.) Looking west, the point, c. 500 m east of the village Montestrutto. The outcrops are located near a vantage plain is visible in the distance; the south, Po To of Bard stands prominently in the middle valley. Fort surface at the foot of Quaternary moraine and in a few points the “roches moutonnées” of Ivrea Zone (Fig. 6). cover consisting of eclogite-facies assemblages with garnet, Na- banded lithologies prevail, On the glacial terrace, ± ± zoisite (omphacite/jadeite) ± quartz glaucophane white mica (phengite and/or paragonite) pyroxene modal proportions of relative carbonate and accessory rutile, apatite zircon. The bands reflect a wide range with small boudins of garnet omphacitite and which includes paraschist lithology, The overall sizes. and grain glaucophane eclogite (Fig. 5f), are responsible for the Isoclinal folding and transposition dips gently westward. (Fig. 5c,d). The leucocratic paraschists orthogneiss with darker of leucocratic association of layers nearly parallel leucocratic orthogneiss is similar in chemical composition to those described Stop 2a. Within the thickest roundish megablasts of jadeite (~ Jd orthogneiss layer, geological field trips 2014 - 6(1.2) itinerary 28 R. Compagnoni - M. Engi - D. Regis Regis R. Compagnoni - M. Engi D. Valle d’Aosta section of the Sesia Zone: multi-stage HP metamorphism and assembly a rifted continental margin Fig. 6 – Geologic view from Stop 2b towards the Valle d’Aosta outlet with the Quaternary moraine, the city of Ivrea, and outlet with the Quaternary moraine, d’Aosta the Valle Fig. 6 – Geologic view from Stop 2b towards the Cottian Alps in background. DOI: 10.3301/GFT.2014.02 geological field trips 2014 - 6(1.2) itinerary 29 R. Compagnoni - M. Engi - D. Regis Regis R. Compagnoni - M. Engi D. Valle d’Aosta section of the Sesia Zone: multi-stage HP metamorphism and assembly a rifted continental margin DOI: 10.3301/GFT.2014.02 Stop 3a: Transposed intrusive contact intrusive Stop 3a: Transposed Val del Lys: Fointainemore - Guillemore Gorge («Gouffre de Guillemore») (Coord.: 45°40’4.43’’ N, 7°51’31.01’’ E, c. 900 m a.s.l.) of by the waters of Mont Mars has been carved Nature Reserve The Gouffre de Guillemore within the Regional stream into rocks of the Eclogitic Micaschist Complex Sesia Zone. the Lys show an outstanding example of a deformed The polished rocks, exposed on both sides of the riverbed, orthogneiss, and now a weakly foliated fine grained granitoid, Permian contact between a most likely intrusive with boudins (Fig. 7a). The orthogneiss is a polymetamorphic basement mainly consisting of paragneiss due to to fresh eclogite, most likely of metabasics (Fig. 7b), now converted crowded with rounded xenoliths Oligocene, dm- A post-metamorphic to the paragneiss. of mafic bodies relative character the more refractory contact. cuts across the deformed paragneiss-granitoid thick, andesite dyke at quartz eclogite- with mafic autoliths recrystallized and aplite dykes granitoid Stop 3b: Deformed Permian Oligocene andesite dykes facies conditions, cut by post-metamorphic Val del Lys: Fontainemore - bridge across the River (Coord: 45°38’35.78’’ N, 7°51’20.63’’ E, c. 730 m a.s.l.) stream, exposes a nice example of deformed metagranodiorite, on the western side of Lys This outcrop, (Figs. 7c,d). The metagranodiorite crowded with mafic autoliths (or cognate inclusions) and aplitic dykes + small consists of the eclogite-facies assemblage quartz + omphacite phengite aggregates zoisite garnets + minor glaucophane accessory rutile, zircon, apatite and local relict igneous allanite. The retrogression is limited. greenschist-facies with small plagioclase phenocrysts and visible The eclogite-facies rocks are cut by undeformed andesite dykes intrusions, consisting of andesite and Alps, such post-metamorphic chilled margins (Fig. 7e). In the Western and related magmatism 1979). This ultrapotassic crosscut the Alpine nappe pile (Dal Piaz et al., lamprophyre, by partial melting of a metasomatised mantle with some crustal contamination (see probably generated was 1960; 1984). The age is 34-30 Ma (Krummenacher & Evernden, et al., 1979; Venturelli e.g. Dal Piaz et al., 1999; Owen, 2008 with ref. et al., 1984; Pettke et al., 1974; Venturelli 1968; Hunziker, & Ferrara, Carraro late-Oligocene magmatism in the southern parts of with the peak of post-collisional therein) and thus coeval imprint with albite, chlorite and epidote replacing 1998). Minor hydrothermal et al., Blanckenburg the Alps (von along sparse brittle fractures. the eclogite assemblage is localized geological field trips 2014 - 6(1.2) itinerary 30 Detail (d) R. Compagnoni - M. Engi - D. Regis Regis R. Compagnoni - M. Engi D. Deformed metagranodiorite, . The orthogneiss is crowded (a) (c) Detail of an undeformed andesite dyke Stop 3b: (e) (c,d,e) , now converted to fresh eclogite. , now converted (b) Valle d’Aosta section of the Sesia Zone: multi-stage HP metamorphism and assembly a rifted continental margin Stop 3a: deformed intrusive contact between a most likely Permian granitoid, now a weakly foliated fine granitoid, Permian contact between a most likely Stop 3a: deformed intrusive (a, b) Fig. 7 – cutting the metagranodiorite, with small plagioclase phenocrysts and evident chilled margins. cutting the metagranodiorite, grained orthogneiss, and a polycyclic basement consisting of paragneiss with eclogite boudins basement consisting of paragneiss orthogneiss, and a polycyclic grained with rounded xenoliths of metabasics with rounded xenoliths crowded with eclogitized mafic autoliths and aplitic dykes. The metagranodiorite consists of the eclogite-facies assemblage The metagranodiorite mafic autoliths and aplitic dykes. crowded with eclogitized andesite dykes. quartz + omphacite phengite. The eclogite-facies rocks are cut by undeformed post-metamorphic and deformed mafic autoliths an aplitic dyke. of (c), showing the eclogitized DOI: 10.3301/GFT.2014.02 geological field trips 2014 - 6(1.2) itinerary 31 R. Compagnoni - M. Engi - D. Regis Regis R. Compagnoni - M. Engi D. Valle d’Aosta section of the Sesia Zone: multi-stage HP metamorphism and assembly a rifted continental margin DOI: 10.3301/GFT.2014.02 STOP 4: Pervasive late-Alpine greenschist-facies metamorphism overprinting the earlier eclogite- facies mineral assemblages Donnas, Roman consular road to the Galliae (Coord: 45°36’8.03” N, 7°45’40.91” E, c.320 m a.s.l.) consular road is will stop at a famous archaeological site, where 221 m long portion of the Roman The excursion Alps, on the northern side of Western to the Rhone Valley built to link Rome work was exposed. This Cyclopean an impressive it preserves out by slaves; carved road was The relic Roman ). the Galliae (i.e. present-day as a a rock fall. The arch also served left to prevent arch, 4 metres in width and height, which was (XXXVI) indicates the distance in miles (ca. 50 km) between characters milestone on which the number 36 in Roman the furrows left by carriages city of Aosta. On the road pavement the present-day Donnas and Augusta Praetoria, Saint Martin, whose depressed arch spans 31 m across, are evident. The stone bridge in the nearby Village of Pont mountain stream. used during the Middle Ages to cross Lys road and was is part of this Roman road has been dug into the most external portion of Eclogitic Micaschist Complex Sesia The Roman the previous eclogite- obliterated extensively overprint (Fig. 3), where the late Alpine greenschist-facies Zone long rutile crystals, typical of the Eclogitic relics, several-cm assemblages. Among the HP mineral facies mineral at the core of Occasional garnet relics are preserved in quartz veins. Micaschist Complex, are locally preserved chlorite masses, the outline of which indicates they are pseudomorphs after garnet porphyroblasts. Fe-rich very Green amphibole laths locally retain a bluish core visible under the hand-lens. defeated the local Celtic of Bard is visible. Built in 25 BC after the Romans Looking northwest, the Fortress in 1800 by the French troops under command of , but destroyed population, the fortress was and remained almost intact till now as one of the best examples of rebuilt in 1827 by Carlo Felice early 1800 military strongholds. the mountain of Bard houses the permanent Museum Alps that illustrates present the Fortress At exhibitions and conferences. and temporary multimedia and multidisciplinary journey, through an interactive, on a spur the fortress is built in narrowest place of Aosta Valley, From a geological point of view, of corresponding to a complex fold structure (Williams & Compagnoni, 1983) where the three main complexes join (Fig. 8b). the Sesia Zone N.B. Collecting rock samples is not allowed at the archaeological site! geological field trips 2014 - 6(1.2) itinerary 32 ams Block diagram of the Block diagram R. Compagnoni - M. Engi - D. Regis Regis R. Compagnoni - M. Engi D. (b) Valle d’Aosta section of the Sesia Zone: multi-stage HP metamorphism and assembly a rifted continental margin View of the outcrops exposed along the Roman consular road at Bard, not far from Stop 4. The Roman road consular road at Bard, not far from Stop 4. The Roman View of the outcrops exposed along Roman (a) Fig. 8 – & Compagnoni, 1983, their Fig. 5). Units I, II and III correspond to the GMC, IIDK EMC, respectively. area of Bard, showing the F4 folds in contacts between units I, II and III, F2 F3 within unit III (from Willi has been dug into the most external portion of the Eclogitic Micaschist Complex of the Sesia Zone. has been dug into the most external portion of Eclogitic Micaschist Complex Sesia Zone. DOI: 10.3301/GFT.2014.02 geological field trips 2014 - 6(1.2) itinerary 33 R. Compagnoni - M. Engi - D. Regis Regis R. Compagnoni - M. Engi D. Valle d’Aosta section of the Sesia Zone: multi-stage HP metamorphism and assembly a rifted continental margin DOI: 10.3301/GFT.2014.02 STOP 5: Foliated serpentinite with metarodingite and forsterite + Ti-clinohumite Mg-chlorite veins Hillock west of the Clapey (Sappé) house (Coord: 45°43’37.78” N, 7°40’2.56” E, c.570 m a.s.l.) d’Aosta i.e. a roundish hill polished by the Quaternary Valle The outcrop consists of a “roche moutonnée”, from the granites boulders of porphyritic Upon retreat of the ice mass, a number erratic glacier. massif remained on top. (see Panseri (3006 m a.s.l.) ultramafics The hillock is the northeastern tip of large body Mont Avic fault della Ranzola trending fault, known as the Aosta-Col de Joux-Col 2008) that are cut by a E-W et al., system (Fig. 2). The Clapey serpentinite is a strongly foliated and crenulated antigorite rock with accessory magnetite. It metamorphic olivine and diopside that includes cm- to dm-thick folded of fine grained contains thin layers consisting of Mg-chlorite + minor magnetite ± antigorite forsterite Ti- boudinaged chloritites veins assemblage formed during Alpine eclogite-facies ± apatite. This mineral alloys clinohumite ± diopside Ni-Fe indicates that at least two olivine and Ti-clinohumite metamorphism. Microscopic examination of these veins and extinction, subgrain cm across with wavy up to several (i) porphyroclasts be recognized: may generations from porphyroclast derived granoblasts mortar microstructure (Figs. 9a,b), and (ii) fine grained show dusty patches rich in fluid recrystallization (Figs. 9a,b). Olivine and Ti-clinohumite porphyroclasts include an internal foliation (Si) that is defined inclusions and opaque phases. Locally olivine porphyroclasts by diopside needles. On the top of hillock, a folded and boudinaged rodingite, 20-30 cm thick, is discontinuously exposed within during peridotite the hosting serpentinites. The rodingite formed by metasomatism of an original basaltic dyke garnet and diopside and mainly contains a pinkish grossular-rich serpentinization. The rodingite is fine grained From a careful examination of the outcrop it is evident selvages. at the dyke with Mg-chlorite concentrated of Mg-chlorite ± the rodingite, only selvages that, where the metasomatic process completely dissolved contain only a small amount of titanium Since the fresh lherzolites forsterite and Ti-clinohumite are preserved. from basaltic (or (c. 0.2 wt.%), the Ti necessary for growth of abundant Ti-clinohumite must derive originally intruding the peridotite. A similar scenario has been proposed by Scambelluri et al. gabbroic) dykes Group. of the Ligurian Voltri bearing chloritite veins (1991) for the olivine-Ti-clinohumite geological field trips 2014 - 6(1.2) itinerary 34 XPL (d) XPL (Sample PPL, (c) (b) Sheared antigorite PPL, R. Compagnoni - M. Engi - D. Regis Regis R. Compagnoni - M. Engi D. (a) (a,b) Ultramafic rock interlayered with rock interlayered Ultramafic Fig. 9 – (Sample OF 272). OF 270). (c,d) the antigorite serpentinite consisting of forsterite, magnetite and antigorite ± Ti- clinohumite. The rock foliation is marked by the alignments of magnetite aggregates and the olivine preferred dimensional orientation. serpentinite with transposed vein of vein serpentinite with transposed forsterite + Ti-clinohumite coarse-grained (+ diopside + magnetite Mg-chlorite Note in the top alloys). antigorite ± Fe-Ni center a relict of the peridotitic clinopyroxene. Going back to the Clapey house and road, the Roman following southward serpentinites show a sub-vertical Valle d’Aosta section of the Sesia Zone: multi-stage HP metamorphism and assembly a rifted continental margin DOI: 10.3301/GFT.2014.02 banding with more massive layers of “dunitic” composition. They consist of fine-grained olivine, minor of “dunitic” composition. They consist fine-grained layers banding with more massive magnetite Ti-clinohumite. The foliation is defined by antigorite flakes, antigorite and magnetite, rare from olivine derived aggregates (Fig. 9c), and by the dimensional preferred orientation of fine-grained and consist of small of the original peridotite are rare (Fig. 9d). Relics recrystallization of porphyroclasts The magnetite + chlorite pseudomorphs after the original Cr-spinel. and lens-like porphyroclasts clinopyroxene Furthermore, the from an original mantle lherzolite. of these relics indicates that the rock derives preservation and in particular the inclusions of magnetite metamorphic olivine occurrence of the “dunitic” layers prior to the Alpine eclogite-facies in part at least already serpentinized was indicates that the lherzolite metamorphism. geological field trips 2014 - 6(1.2) itinerary 35 R. Compagnoni - M. Engi - D. Regis Regis R. Compagnoni - M. Engi D. Table 5 - Alpine metamorphic Table eclogite-facies assemblages. stages recorded in the Saint Germain albite-epidote amphibolite with relict Please, do not collect samples on the top of hillock where ice polished rocks were exposed about 10’000 years ago at the end of the last glaciation! Valle d’Aosta section of the Sesia Zone: multi-stage HP metamorphism and assembly a rifted continental margin ) Minerals Stage I Stage II Stage III Vein stage CKNOWLEDGEMENTS DOI: 10.3301/GFT.2014.02 Omphacite Garnet Rutile Quartz Apatite Glaucophane White mica Clinozoisite Green amphibole Albite Titanite Chlorite Carbonate Opaque ores (Ilm and/or Mt A The contact with metabasic rocks is exposed a few tens of m southwards along the Roman road. These along the Roman The contact with metabasic rocks is exposed a few tens of m southwards from metamorphic recrystallization castle is built, derive metabasics, on which the Saint Germain (or Montjovet) of original ophiolitic basalts. The banded, dark green albite-epidote amphibolites are partly re-equilibrated eclogites, consisting of omphacite + garnet, and accessory interstitial quartz, rutile apatite. The amphibolite main foliation is defined by lineated relict of pale glaucophane that rimmed with dark green amphibole, minor chlorite and accessory titanite. Albite only occurs with green amphibole as medium grained clinozoisite, replacement. On the basis of mineral symplectites after omphacite, indicating a static Na-pyroxene 5. in Table as summarized be recognized, relationships three Alpine metamorphic stages may microstructural Groppo for their careful editing and expert advice. and Chiara to Gisella Rebay grateful The Authors are very Valle d’Aosta section of the Sesia Zone: multi-stage HP metamorphism and assembly of a rifted continental margin R. Compagnoni - M. Engi - D. Regis g e o l o g i c a l

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DOI: 10.3301/GFT.2014.02 geological field trips 2014 - 6(1.2) references 37 . itt., om a ntinental R. Compagnoni - M. Engi - D. Regis Regis R. Compagnoni - M. Engi D. Valle d’Aosta section of the Sesia Zone: multi-stage HP metamorphism and assembly a rifted continental margin polymetamorphic eclogitic marble of the internal Sesia Zone (Western Alps). Contrib. Mineral. Petrol., 142, 627-639. Petrol., Mineral. Alps). Contrib. (Western polymetamorphic eclogitic marble of the internal Sesia Zone 33, 335-375. Petrol., Soc. Ital. Mineral. margin. Rend. 33, Soc. It. Min. Petr., Italian Alps. Rend. assemblages in the Western crust with alpine high pressure–low temperature 281–334. 48, 75–80. 9, 61-77. metam. Geol., Alps. J. Italian Western continental crust: the Sesia Zone, western Alps. Tectonics, 25(6), 1-25. western Alps. Tectonics, continental crust: the Sesia Zone, 18, 147-166. Res., Gondwana perspective. 30, 194-219. Struct. Geol., Alps): Evidence for a dynamic orogen. J. (Western the Urtier Valley 38, 559-562. High-Pressure metamorphism. Geology, and (Ultra-) zones Transition 82, 413-427. geol. Helv., 78, 163–168. Mitt., petrogr. Unit. Schweiz. mineral. Dent–Blanche–Sesia 32, 97-111. Soc. It. Min. Petr., Rend. Sesia-Lanzo. nella zona exhumation of HP and UHP rocks in the Western Alps. Bull. Soc. Géol. Fr., 172, 617–636. Alps. Bull. Soc. Géol. Fr., exhumation of HP and UHP rocks in the Western 171, 425-438. Earth Planet. Sci. Lett., geochronology. high precision Sm-Nd and Rb-Sr 32, 681-698. Soc. It. Min. Petr., Rend. DOI: 10.3301/GFT.2014.02 Compagnoni R. (1977) - The Sesia-Lanzo Zone: high pressure-low temperature metamorphism in the Austroalpine continental high pressure-low temperature Zone: Compagnoni R. (1977) - The Sesia-Lanzo a slice of co zone, (1977) - The Sesia-Lanzo & Williams P.F. Lombardo B. Gosso G., J.C., Hunziker Dal Piaz G.V., Compagnoni R., Castelli D. (1991) - Eclogitic metamorphism in carbonate rocks; the example of impure marbles from the Sesia-Lanzo Zone, (1991) - Eclogitic metamorphism in carbonate rocks; the example of impure marbles from Sesia-Lanzo Castelli D. fr (2002) - Stability of Al- and F-rich titanite in metacarbonate: petrologic isotopic constraints D. & Rubatto Castelli D. Babist J., Handy M.R., Konrad-Schmolke M. & Hammerschmidt K. (2006) - Precollisional, multistage exhumation of subducted Konrad-Schmolke Handy M.R., Babist J., Bearth P. (1967) - Die Ophiolithe der Zone von Zermatt-Saas Fee. Beitr. geol. Karte Schweiz., N.F. 132, 130 p. N.F. Schweiz., geol. Karte Beitr. Fee. Zermatt-Saas von (1967) - Die Ophiolithe der Zone Bearth P. High-pressure metamorphism and orogenesis: An Alpine (2010a) - (Ultra-) Compagnoni R. & Lombardo B. M., Beltrando units of M. & Compagnoni R. 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