Geological Field Trips

Società Geologica Italiana

2014 Vol. 6 (1.3) 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

Crust-mantle interactions during subduction of oceanic & continental crust 10th International Eclogite Conference - Courmayeur (Aosta, Italy), 2013

DOI: 10.3301/GFT.2014.03 Crust-mantle interactions during subduction of oceanic & continental crust D. Castelli - R. Compagnoni - B. Lombardo - S. Angiboust - G. Balestro - S. Ferrando - C. Groppo - T. Hirajima - F. Rolfo GFT - Geological Field Trips geological fieldtrips2014-6(1.3) Periodico semestrale del Servizio Geologico d'Italia - ISPRA e della Società Geologica Italiana Geol.F.Trips, Vol.6 No.1.3 (2014), 73 pp., 27 figs, 3 tabs. (DOI 10.3301/GFT.2014.03) Crust-mantle interactions during subduction of oceanic & continental crust 10th International Eclogite Conference, Courmayeur (Aosta, Italy) - Post-conference excursions: September 9-10, 2013

Daniele CASTELLI(1,2), Roberto COMPAGNONI(1), Bruno LOMBARDO(2), Samuel ANGIBOUST(3), Gianni BALESTRO(1), Simona FERRANDO(1), Chiara GROPPO(1), Takao HIRAJIMA(4), Franco ROLFO(1,2)

(1) Department of Earth Sciences, University of Torino, Via Valperga Caluso 35, 10125 Turin, Italy (2) IGG – CNR, Via Valperga Caluso 35, 10125, Torino, Italy (3) Helmholtz-Zentrum Potsdam, Deutsches GeoForschungsZentrum (GFZ), Germany (4) Department of Geology and Mineralogy, Faculty of Science, Kyoto University, Kitashirakawa Oiwake-cho, Sakyo-ku, Kyoto 606-502, Japan

Corresponding Author e-mail address: [email protected]

Responsible Director Claudio Campobasso (ISPRA-Roma) Editorial Board Editor in Chief M. Balini, G. Barrocu, C. Bartolini, 2 Gloria Ciarapica (SGI-Perugia) 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.2014.03 Manuscript received 08 October 2013; accepted 19 December 2013 Crust-mantle interactions during subduction of oceanic & continental crust D. Castelli - R. Compagnoni - B. Lombardo - S. Angiboust - G. Balestro - S. Ferrando - C. Groppo - T. Hirajima - F. Rolfo INDEX geological fieldtrips2014-6(1.3) Information Itinerary

Safety ...... 4 Day 1: The Monviso meta-ophiolite complex: HP Hospitals ...... 4 metamorphism of oceanic crust & interactions with Accomodation ...... 4 ultramafics Other useful addresses ...... 4 STOP 1.1: General view of the Monviso Meta-ophiolite Riassunto ...... 7 complex and serpentinite of the basal serpentinite unit ...... 37 Abstract ...... 8 STOP 1.2: Serpentinite of the basal serpentinite unit ...... 38 Program summary ...... 10 STOP 1.3: Calcschists and micaschists (basal serpentinite unit); Cr-rich magnesiochloritoidbearing eclogite loose Excursion notes blocks (Lago Superiore unit) ...... 39 STOP 1.4: Serpentinite with tectonic inclusions of 1. Introduction...... 10 eclogitic rocks (basal serpentinite unit) ...... 40 1.1 Tectonic outline of the Western Alps ...... 10 STOP 1.5: Smaragdite metagabbro with transposed dykes of 1.2 High to ultra-high pressure metamorphic rocks in metabasalt (Lago Superiore unit) ...... 42 the western Alps ...... 12 STOP 1.6: Layered eclogitic sequence (Lago Superiore unit) ..44 3 2. The Monviso meta-ophiolite complex ...... 14 STOP 1.7: Prasinite (Viso Mozzo Unit) ...... 45 2.1 Tectonometamorphic andtectonostratigraphic units ..14 STOP 1.8: FeTi-oxide metagabbro boudins and eclogite-facies 2.2 Igneous features of the Monviso Meta-ophiolite shear zones and veins (Lago Superiore unit) ...... 46 complex ...... 19 2.3 The Alpine polyphase metamorphic evolution ...... 20 Day 2: Metasomatism from & to ultramafics: the UHP 2.4 Comparison with other Alpine Meta-ophiolites ...... 23 continental Brossasco-Isasca Unit 3. The UHP contintal Brossasco-Isasca Unit STOP 2.1: Metagranite recrystallized under UHP conditions (Dora-Maira Massif) ...... 24 with greenschistfacies shear zone ...... 50 3.1 Geological setting of the southern Dora-Maira Massif ..24 STOP 2.2: Metagranites with xenoliths of paraschists and 3.2 The coesite-bearing Brossasco-Isasca Unit ...... 26 metabasics: relict intrusive contact between Permian

3.2.1 The monometamorphic complex ...... 27 granitoids and the country metamorphic basement ...... 54 index 3.2.2 The polymetamorphic complex ...... 28 STOP 2.3: Typical whiteschist with pyrope megablasts ...... 57 3.3 The P-T evolution of the Brossasco-Isasca Unit ...... 32 3.4 The age of the Alpine UHP metamorphism ...... 34 References ...... 66

DOI: 10.3301/GFT.2014.03 geological field trips 2014 - 6(1.3)

4 information Accomodation Hotel Excalibur Piazza Cesare Battisti n.2 (CN) - 12036 Revello +39 349 1693945 / 348 0189937 Tel. Agriturismo “El Cavalier” Revello (CN) 4 - loc. San Firmino 12036 Revello Via Roncaglia, +39Tel: 348 7147007 / +39 347 0962865 Bar-Ristornate Natale - Paesana 13Via Roma, (CN) - 12034 Paesana +39 0175 94125 Tel.: La Locanda del Sale Via Segheria, 2 - 12030 Sanfront (CN) +39 349 7922428 Tel. Other useful addresses Museo del Piropo (CN) n. 25 - Martiniana Po Via Roma +39 0175 46505 Tel. Museo Naturalistico del Fiume Po Cuneese del Po Parco (CN) Piazza Denina n. 5 - Revello +39 0175 46505 Tel. Crust-mantle interactions during subduction of oceanic & continental crust D. Castelli - R. Compagnoni - B. Lombardo - S. Angiboust - G. Balestro - S. Ferrando - C. Groppo - T. Hirajima - F. Rolfo Rolfo - F. Hirajima - C. Groppo T. Ferrando Angiboust - G. Balestro S. Lombardo - S. Castelli - R. Compagnoni B. D. : ca. 500 m along an easy : less than 100 m : 2000-2500 m a.s.l. : 400-1000 m a.s.l. 2 DOI: 10.3301/GFT.2014.03 Safety Mountain boots, sun glasses, and sun-tan lotion are and water strongly recommended, as well warm, windproof clothes, since weather can change rapidly in Alpine settings. 1 Day Altitude Altitude Hospital Saluzzo Ospedale Civile di Saluzzo Via Spielberg, 58 – (CN) 12037 Saluzzo +39 0175 215111 Tel. Savigliano Annunziata Ospedale SS. Via Ospedali, 14 - (CN) 12038 Savigliano +39 0172 719111 Tel. Elevation gain (on foot) path Day Elevation gain (on foot) geological field trips 2014 - 6(1.3)

5 information Crust-mantle interactions during subduction of oceanic & continental crust D. Castelli - R. Compagnoni - B. Lombardo - S. Angiboust - G. Balestro - S. Ferrando - C. Groppo - T. Hirajima - F. Rolfo Rolfo - F. Hirajima - C. Groppo T. Ferrando Angiboust - G. Balestro S. Lombardo - S. Castelli - R. Compagnoni B. D. DOI: 10.3301/GFT.2014.03 geological field trips 2014 - 6(1.3)

6 information Crust-mantle interactions during subduction of oceanic & continental crust D. Castelli - R. Compagnoni - B. Lombardo - S. Angiboust - G. Balestro - S. Ferrando - C. Groppo - T. Hirajima - F. Rolfo Rolfo - F. Hirajima - C. Groppo T. Ferrando Angiboust - G. Balestro S. Lombardo - S. Castelli - R. Compagnoni B. D. di escursione propone un itinerario “petrografico” nel cuore della famosa unità Brossasco- “petrografico” di escursione propone un itinerario di escursione è dedicato alla visita delle sequenze tettono-metamorfiche del Massiccio di escursione è dedicato alla visita delle sequenze : Alpi Occidentali, Massiccio Ofiolitico del Monviso, Massiccio Dora-Maira, Unità Brossasco-Isasca, Massiccio Dora-Maira, : Alpi Occidentali, Massiccio Ofiolitico del Monviso, primo giorno secondo giorno DOI: 10.3301/GFT.2014.03 Riassunto tettono-metamorfiche le sequenze Questa escursione di due giorni in un settore delle Alpi Cozie vuole illustrare la subduzione alpina in durante con le ultramafiti di crosta oceanica e continentale le loro interazioni condizioni di alta e altissima pressione. Il ofiolitico del Monviso. Il Massiccio del Monviso è uno dei relitti di crosta oceanica meglio preservati nelle Alpi è uno dei relitti di crosta oceanica meglio preservati Il Massiccio del Monviso ofiolitico del Monviso. e che ha subito condizioni nel Mesozoico della Tetide l’apertura Occidentali, che si è formato durante proposta la subduzione alpina. L’escursione durante metamorfiche di alta pressione in facies eclogitica a quarzo tipicamente porzioni dell’antico oceano lungo un itineraio le diverse offre la possibilità quasi unica di osservare eclogiti, metabasiti, e Fe-Ti, serpentiniti, metagabbri Mg-Al osservare litotipi si potranno i vari Tra alpino. in un contesto di alta montagna, proprio si svolge metasedimenti e minori corpi di metaperidotiti. L’itinerario inizia alle sorgenti (3841 m), che è il simbolo delle Alpi Cozie. L’escursione del Monviso di fronte alla piramide tre spettacolari laghi di montagna (Lago Fiorenza, Lago Chiaretto e e raggiunge a Pian del Re del Po un paesaggio tipicamente glaciale. Superiore) attraversando Il Isasca di ultra-alta pressione (Massiccio Dora-Maira meridionale), e permette di osservare le evidenze più le evidenze meridionale), e permette di osservare pressione (Massiccio Dora-Maira Isasca di ultra-alta ed Alpina di questa unità tettono-metamorfica. Si potrà metamorfica pre-Alpina spettacolari dell’evoluzione fu scoperta dove (a sud di Case Parigi), di scisti bianchi a piropo Case Ramello visitare il famoso affioramento le che preserva la coesite in rocce di crosta continentale, uno spettacolare metagranito per la prima volta eccezionalmente pre-Alpino coronitiche di altissima pressione e un relitto contatto intrusivo paragenesi di basamento all’interno dei metagranitoidi). (xenoliti preservato Parole chiave pressione, eclogiti, coesite, scisti bianchi. crosta oceanica e continentale, metamorfismo di alta- ultra-alta geological field trips 2014 - 6(1.3)

7 information Crust-mantle interactions during subduction of oceanic & continental crust D. Castelli - R. Compagnoni - B. Lombardo - S. Angiboust - G. Balestro - S. Ferrando - C. Groppo - T. Hirajima - F. Rolfo Rolfo - F. Hirajima - C. Groppo T. Ferrando Angiboust - G. Balestro S. Lombardo - S. Castelli - R. Compagnoni B. D. excursion proposes a “petrographic” itinerary in the core of world-famous UHP Brossasco- itinerary proposes a “petrographic” excursion is dedicated to the visit of the tectonometamorphic sequences of the Monviso meta-ophiolitic is dedicated to the visit of tectonometamorphic sequences Monviso : Western Alps, Monviso meta-ophiolitic Massif, Alps, Monviso : Western Unit, oceanic Brossasco-Isasca Massif, Dora-Maira first day second day DOI: 10.3301/GFT.2014.03 Isasca Unit (Dora-Maira Massif), and Isasca Unit (Dora-Maira the most spectacular evidence of both its pre-Alpine allowing to observe will visit the famous pyrope-bearing whiteschist outcrop of Case Ramello We Alpine metamorphic evolution. with in continental crust, a spectacular metagranite first discovered where coesite was (south of Case Parigi) igneous intrusive relict of pre-Alpine well preserved assemblage and an exceptionally UHP coronitic mineral within metagranitoids). contact (basement xenoliths Key-words metamorphism, eclogite, coesite, whiteschist. and continental crust, high- ultra-high-pressure massif. The Monviso Massif is one of the best preserved relics of oceanic crust in the western Alps that formed Massif is one of the best preserved The Monviso massif. and that underwent HP metamorphism during Alpine western Alpine Tethys during opening of the Mesozoic the almost unique chance to see and appreciate different portions of ancient subduction. This fieldtrip gives and Fe-Ti lithologies, we will see serpentinite, Mg-Al Among the various ocean floor along a mountain trail. eclogite, metabasite, metasediments and minor metaperidotite. metagabbro, (3841 m), of Monviso is located in a high mountain landscape front of the pyramid The fieldtrip itinerary and at Pian del Re river starts from the spring of Po which is the symbol of Cottian Alps. The excursion (Lago Fiorenza, Lago Chiaretto and Superiore), through a reaches three spectacular mountain lakes typical glacial landscape. The Abstract oceanic and continental tectonometamorphic sequences their is aimed at illustrating This excursion during Alpine subduction at (U)HP conditions. with ultramafics interactions The geological field trips 2014 - 6(1.3)

8 information excursion can be reached after a 45 minutes drive from can be reached after a 45 minutes drive excursion Crust-mantle interactions during subduction of oceanic & continental crust first day D. Castelli - R. Compagnoni - B. Lombardo - S. Angiboust - G. Balestro - S. Ferrando - C. Groppo - T. Hirajima - F. Rolfo Rolfo - F. Hirajima - C. Groppo T. Ferrando Angiboust - G. Balestro S. Lombardo - S. Castelli - R. Compagnoni B. D. excursion consists of three stops that are located on the right side southernmost Val excursion second day DOI: 10.3301/GFT.2014.03 Program summary of the The starting point (Pian del Re) Saluzzo or Revello through the Po Valley up to Crissolo and then to Pian del Re (Fig. 1). The field trip follows up to Crissolo and then Pian del Re Valley through the Po or Revello Saluzzo a circular path which rises from 2000 m up to about 2400 a.s.l. and consists of eight stops. Consider ca. 4- the whole itinerary. 5 hours to walk The Varaita (Stop 2.1), on the left side of Vallone di Gilba (Stop 2.2), and on the right side of Po Valley (Stop 2.3), Valley di Gilba (Stop 2.2), and on the right side of Po (Stop 2.1), on the left side of Vallone Varaita from Saluzzo- (Fig. 1). Stop 2.1 (Brossasco bridge) can be reached after a 30-45 minutes drive respectively which is a along the Gilba Valley, from Stop 2.1 to 2.2 requires a 30 minutes drive The transfer Revello. tributary of the main Varaita To to the outcrop. uphill to Case Bastoneri and then a 20 minute walk Valley, and then to and Revello back to Saluzzo reach Stop 2.3 from 2.2, consider at least one hour drive and uphill to Case Ramello. Martiniana Po geological field trips 2014 - 6(1.3)

9 information Crust-mantle interactions during subduction of oceanic & continental crust D. Castelli - R. Compagnoni - B. Lombardo - S. Angiboust - G. Balestro - S. Ferrando - C. Groppo - T. Hirajima - F. Rolfo Rolfo - F. Hirajima - C. Groppo T. Ferrando Angiboust - G. Balestro S. Lombardo - S. Castelli - R. Compagnoni B. D. Fig. 1 – Geographical location of the field trip areas and Stops. Fig. 1 – Geographical DOI: 10.3301/GFT.2014.03 geological field trips 2014 - 6(1.3) excursion notes 10 the (ii) the Helvetic-Dauphinois domain, the Helvetic-Dauphinois (i) the Southalpine domain or Southern Alps (Fig. 2). (iv) Crust-mantle interactions during subduction of oceanic & continental crust D. Castelli - R. Compagnoni - B. Lombardo - S. Angiboust - G. Balestro - S. Ferrando - C. Groppo - T. Hirajima - F. Rolfo Rolfo - F. Hirajima - C. Groppo T. Ferrando Angiboust - G. Balestro S. Lombardo - S. Castelli - R. Compagnoni B. D. , which is subdivided in a series of nappes, consists of a Variscan (or older) , which is subdivided in a series of nappes, consists Variscan the Austroalpine domain, and is a composite realm, which includes both continent- and ocean-derived units (Fig. 2). and ocean-derived is a composite realm, which includes both continent- (iii) The continental Briançonnais - Grand Saint Bernard zone consists of a Variscan crystalline basement, a consists of a Variscan Saint Bernard zone The continental Briançonnais - Grand amphibolite- They mainly consist of a Variscan The ICM are tectonic windows within the Piemonte zone. of Calcschists with meta-ophiolites” includes also known as “Zone Piemonte Zone, The ocean-derived DOI: 10.3301/GFT.2014.03 Unlike these domains, where both basement and cover rocks are exposed, the Jura chain (separated from the chain (separated rocks are exposed, the Jura these domains, where both basement and cover Unlike sequences (Fig. 2). molasse basin) only consists of deformed cover main Alpine chain by the Swiss and eastern Alps it is hidden exposed, while in central domain is extensively In the western Alps, Penninic by the nappes of Austroalpine domain and reappears only in two tectonic windows Engadine Hohe Tauern. domain The Helvetic-Dauphinois Penninic domain, Penninic crystalline basement and a post-Variscan, mainly Mesozoic sedimentary cover (Fig. 2). The crystalline sedimentary cover mainly Mesozoic crystalline basement and a post-Variscan, (ca. mainly Permian metamorphics intruded by post-Variscan or older, basement, which consists of Variscan, which are (from N to S): is exposed in the so called “External Crystalline Massifs”, 270 to 300 Ma) granitoids, and Argentera. Belledonne, Pelvoux Rouges, Mont Blanc-Aiguilles domain The Penninic 1. Introduction 1.1 Tectonic outline of the Western Alps domains of the Alps are: The main tectonic and palaeogeographic The continental units are the external Briançonnais Zone (or Grand Saint Bernard composite nappe), and the (or Grand The continental units are the external Briançonnais Zone The ICM are and Valosio. Dora-Maira Paradiso, Gran “Internal Crystalline Massifs” (ICM) of Monte Rosa, units of the Piemonte Zone. by the ocean-derived overlain (i) to Eocene cover. sequence and a Mesozoic Permo-Carboniferous (ii) to augen-gneisses by the converted granitoids, porphyritic facies metamorphic basement, intruded by Permian Alpine tectonics and metamorphisms. (iii) sedimentary cover, Ligurian-Piemontese oceanic lithosphere and its Mesozoic of both the Mesozoic fragments 1992; Dal Piaz, consisting of “calcescisti” (calcschists, or “schistes lustrés” in French) (see e.g.: Deville et al., 1999). geological field trips 2014 - 6(1.3) excursion notes 11 Crust-mantle interactions during subduction of oceanic & continental crust D. Castelli - R. Compagnoni - B. Lombardo - S. Angiboust - G. Balestro - S. Ferrando - C. Groppo - T. Hirajima - F. Rolfo Rolfo - F. Hirajima - C. Groppo T. Ferrando Angiboust - G. Balestro S. Lombardo - S. Castelli - R. Compagnoni B. D. Fig. 2 – Simplified structural sketch map of the western Alps with location tectonic maps reported in Figs. 3 and 8. sketch Fig. 2 – Simplified structural DOI: 10.3301/GFT.2014.03 geological field trips 2014 - 6(1.3) excursion notes 12 is exposed (Fig. 2), which was not involved in the Alpine not involved is exposed (Fig. 2), which was Sesia Zone and Dent Blanche nappe system, which includes Sesia Zone Crust-mantle interactions during subduction of oceanic & continental crust D. Castelli - R. Compagnoni - B. Lombardo - S. Angiboust - G. Balestro - S. Ferrando - C. Groppo - T. Hirajima - F. Rolfo Rolfo - F. Hirajima - C. Groppo T. Ferrando Angiboust - G. Balestro S. Lombardo - S. Castelli - R. Compagnoni B. D. DOI: 10.3301/GFT.2014.03 orogeny. It includes the Ivrea Zone, characterized by pre-Alpine granulite- to amphibolite-facies granulite- by pre-Alpine characterized It includes the Ivrea Zone, orogeny. metamorphics, and the Serie dei Laghi, which is composed of an amphibolite-facies basement, intruded by by Permian dei Laghi”) and overlain (“Graniti granites Permian sedimentary cover. and a Mesozoic rhyolites by the Austroalpine is overlain The Piemonte Zone To SE of the Canavese line, the Southalpine Domain SE of the Canavese To the main thrust sheets of Dent Blanche s.s., Mont Mary, Monte Emilius and several minor outliers. Monte Emilius and several Mont Mary, the main thrust sheets of Dent Blanche s.s., to the Southern Alps through Dent tectonic cross-section from the Pre-Alps From a simplified NW-SE it is evident that the orogenic wedge Blanche Nappe (with the top of Matternhorn) and Sesia Zone, the Alpine Line and, consequently, to the Canavese going from the Jura thicker becomes progressively is increasing from the external to internal side of western Alps. metamorphic grade domain west of Saluzzo to show part of a cross-section within the Penninic is devoted excursion This two-day (Fig. 2). 1.2 High to ultra-high pressure metamorphic rocks in the western Alps than the latter. the former being much rarer and Alpine eclogites occur, In the western Alps, both pre-Alpine eclogites were reported from the polymetamorphic basement of External Crystalline Massifs The pre-Alpine Saint Bernard Zone, from the Briançonnais/Grand Belledonne and Argentera, of Mont Blanc, Aiguilles Rouges, eclogites in the ascertained if the lack of pre-Alpine conclusively and from the Serie dei Laghi. It is not yet massifs is a primary feature or if it the and Dora-Maira Paradiso Gran Monte Rosa, internal Penninic Alpine polyphase recrystallization. The last interpretation could be consequence of the intense and pervasive Compagnoni Paradiso: in the Gran supported by the local occurrence in polymetamorphic basement (e.g., Alpine 1974) of meta-basics, where the presence rounded aggregates small euhedral & Lombardo, age. garnet of pre-Alpine eclogitic garnets suggest recrystallization after coarser-grained and Austroalpine domains of the the Penninic all over were recognized main Alpine metamorphic events Two low pressure (LP) UHP) metamorphism and a younger (HP, western Alps: an older high- to ultrahigh-pressure the older one. obliterates and extensively metamorphism, which overprints of the HP to UHP metamorphism is difficult define at large scale, because it The areal extent and zoning during subduction, long before the final nappe emplacement. Therefore, once considered developed geological field trips 2014 - 6(1.3) excursion notes 13 rocks are extensively exposed in the external Piemonte rocks are extensively Crust-mantle interactions during subduction of oceanic & continental crust D. Castelli - R. Compagnoni - B. Lombardo - S. Angiboust - G. Balestro - S. Ferrando - C. Groppo - T. Hirajima - F. Rolfo Rolfo - F. Hirajima - C. Groppo T. Ferrando Angiboust - G. Balestro S. Lombardo - S. Castelli - R. Compagnoni B. D. rocks are reported from only two localities: the continental crust of Penninic rocks are the most widespread. They occur in most of the Sesia Zone and Monte Emilius rocks are the most widespread. They occur in of Sesia Zone and lawsonite blueschist- facies and lawsonite blueschist- DOI: 10.3301/GFT.2014.03 Zone and in the Briançonnais Zone. In this area the carpholite-bearing assemblages are frequently found in and in the Briançonnais Zone. Zone rocks of appropriate bulk chemical composition (Goffé, 1984). isograds of the HP metamorphism are really tectonic contacts between nappes, which experienced different isograds the LP metamorphism postdates main nappe emplacement peak metamorphic conditions. On the contrary, cut across the tectonic contacts between nappes. and its isograds i.e. coesite-bearing eclogite-, quartz-bearing All facies typical of the HP to UHP metamorphism are observed, and lawsonite blueschist-facies. eclogite-, epidote blueschist- Coesite-eclogite facies of the Dent Blanche nappe system, in most of the internal Piemonte Zone, and in the Internal Crystalline of the Dent Blanche nappe system, in most internal Piemonte Zone, and Valosio. Dora-Maira Paradiso, Gran Massifs of Monte Rosa, Epidote blueschist- Brossasco-Isasca Unit, southern Dora-Maira Massif (Chopin, 1984) and the sedimentary cover of the Massif (Chopin, 1984) and the sedimentary cover Unit, southern Dora-Maira Brossasco-Isasca Val at Lago di Cignana, Upper Valtournenche, Zone metamorphic ophiolites of the Piemonte Zermatt-Saas Unit, recent petrological data from both 1989, 1991, 1998). As to the Brossasco-Isasca (Reinecke, d’Aosta 2007a) suggest that 2007; Groppo et al., samples (Castelli et al., experiments (Hermann, 2003) and natural diamond-eclogite facies conditions were attained at about 750°C and 4.0Sampeyre-Dronero As to the southern DMM (Fig. 8), its and the underlying “Pinerolo Unit”. Unit”, the “Polymetamorphic Unit”, pressure of coesite (Chopin, 1984) and other ultra-high become widely known after the discovery rocks have magnesio-staurolite, magnesio- assemblages, including pyrope, jadeite + kyanite, and mineral minerals dumortierite and the peculiar ellenbergerite-phosphoellenbegerite solid solution series (Chopin, 1987; Chopin are associated to the only 1986, 1993, 1994, 1995). Such unusual minerals et al., eclogites kyanite-bearing 2000). 1991; Nowlan et al., known in the western Alps (Kienast et al., 1990; 1991; Henry et al., 1990; Chopin et al., (e.g. Henry, investigations Detailed field and petrographic 1993; Compagnoni 1993; Turello, 1993, 1995; Sandrone et al., 1991; Michard et al., Compagnoni & Hirajima, 2004) 2002; Compagnoni et al., 2000; Groppo, 1995; Chopin & Schertl, 1999; Matsumoto Hirajima, et al., shown that the southern DMM is a pile of imbricated thrust sheets, resulting from Alpine tectonic have locally cover, continental crust and its Triassic juxtaposition and metamorphic reworking of slices Variscan from the these last derived of calcschists and meta-ophiolites the Piemonte Zone, by thin layers separated ocean floor. Tethys Mesozoic by Most of these slices are composed the same continental lithologic association, but characterized 2012b). with refs.; Compagnoni et al., 2003b, (e.g. Compagnoni & Rolfo, different Alpine climax overprints been distinguished in the southern DMM (Fig. 8),The following tectonometamorphic units have from the lower levels: to the upper structural (i) intercalations of quartzite and minor paragneiss including lenses of metabasite. During the Alpine orogeny intercalations of quartzite and minor paragneiss conditions (T ~ 400 °C, P 0.8 these rocks experienced peak metamorphism under epidote-blueschist-facies 2003). et al., 1991; Avigad Chopin et al., GPa: geological field trips 2014 - 6(1.3) excursion notes 25 the San Chiaffredo Unit is a Fig. 8 – Tectonic sketch map of sketch Fig. 8 – Tectonic Dronero-Sampeyre Unit. Dronero-Sampeyre the southern Dora-Maira Massif the southern Dora-Maira (modified from Compagnoni et al. (2012b) (see Fig. 1 for the location in Alps). of the Western the framework units of the Massif are listed Tectonic lowermost from the structurally highest Pinerolo Unit to the structurally (ii) continental portion of pre-Alpine crust consisting of a Variscan amphibolite-facies metamorphic basement (now micaschist and eclogite) intruded by Permian Crust-mantle interactions during subduction of oceanic & continental crust D. Castelli - R. Compagnoni - B. Lombardo - S. Angiboust - G. Balestro - S. Ferrando - C. Groppo - T. Hirajima - F. Rolfo Rolfo - F. Hirajima - C. Groppo T. Ferrando Angiboust - G. Balestro S. Lombardo - S. Castelli - R. Compagnoni B. D. the Brossasco-Isasca Unit (BIU) is a portion of pre-Alpine continental crust consisting of a Variscan Unit (BIU) is a portion of pre-Alpine the Brossasco-Isasca continental crust similar to the BIU and San Chiaffredo Unit, Solei Unit is a portion of pre-Alpine the Rocca DOI: 10.3301/GFT.2014.03 granitoids (now fine-grained orthogneiss and augen-gneiss). During the Alpine orogeny this unit experienced peak orthogneiss and augen-gneiss). During the Alpine orogeny (now fine-grained granitoids metamorphism under quartz eclogite-facies conditions (T ~ 550 °C, P = 1.5 GPa). (iii) micaschist, impure marble and eclogite) intruded amphibolite-facies metamorphic basement (now garnet-kyanite and pyrope-bearing whiteschist). Peak (now mainly augen-gneiss and minor metagranite granitoids by Permian metamorphic conditions in the coesite eclogite-facies within diamond stability field (T ~ 730 °C, P = 4.0-4.3 were experienced during the Alpine orogeny. GPa) (iv) and chloritoid- bearing micaschist amphibolite-facies metamorphic basement (now garnet- consisting of a Variscan orthogneiss and (now fine-grained granitoids and micaceous gneiss, marble eclogite) intruded by Permian this unit experienced peak metamorphism During the Alpine orogeny augen-gneiss with minor metagranitoid). 2000). 1991; Matsumoto & Hirajima, Chopin et al., facies conditions (T ~ 550 °C, P 1.5 GPa: under quartz-eclogite by a late Alpine greenschist - facies recrystallisation, been overprinted All these tectonometamorphic units have assemblages. the former HP or UHP metamorphic mineral obliterated and extensively reworked which pervasively geological field trips 2014 - 6(1.3) excursion notes 26 Crust-mantle interactions during subduction of oceanic & continental crust D. Castelli - R. Compagnoni - B. Lombardo - S. Angiboust - G. Balestro - S. Ferrando - C. Groppo - T. Hirajima - F. Rolfo Rolfo - F. Hirajima - C. Groppo T. Ferrando Angiboust - G. Balestro S. Lombardo - S. Castelli - R. Compagnoni B. D. Fig. 9 - Simplified geological map of the central portion of Fig. 9 - Simplified geological map of the central Fig. 8 for location). the coesite-bearing "Brossasco-Isasca Unit” and relationships the coesite-bearing "Brossasco-Isasca with the other adjoining units, labelled in map (modified 2007, see 2003b and Castelli et al., from Compagnoni & Rolfo, DOI: 10.3301/GFT.2014.03 3.2 The coesite-bearing Brossasco-Isasca Unit Unit is about The coesite-bearing Brossasco-Isasca and is tectonically sandwiched 10×4×1 km in size Solei Unit and the Rocca between the overlying by underlying Pinerolo Unit, which are characterized Alpine quartz eclogite-facies and epidote blueschist- Areal extent, internal facies conditions, respectively. mainly setting and geologic interpretation of the BIU, studies of from field and laboratory derived (1993), (1991), Turello Compagnoni & Hirajima Compagnoni et al. (1995), Groppo (2002), (2003b), Compagnoni et al. (2004) and & Rolfo Compagnoni et al. (2012b) are reported in Fig. 9. A new geologic map including the whole BIU has been recently 2012b). published (Compagnoni et al., been have complexes lithostratigraphic Two 1995): distinguished in the BIU (Compagnoni et al., from the a “Monometamorphic Complex” derived Alpine tectonic and metamorphic reworking of and a “Polymetamorphic granitoids, Permian from the Alpine reworking of a derived Complex”, geological field trips 2014 - 6(1.3) excursion notes 27 Fig. 10 - Pre-Alpine relationships among the BIU Fig. 10 - Pre-Alpine (protolith of the polymetamorphic complex). lithologies as inferred from the structural and lithologies as inferred from the structural relics escaped to the Alpine polyphase mineralogical deformation (modified from Compagnoni et pervasive (protolith of the granitoids 2012b). Permian al., monometamorphic complex) were emplaced into a amphibolite-facies metamorphic basement Variscan consisting of micaschists, marbles and amphibolites Crust-mantle interactions during subduction of oceanic & continental crust D. Castelli - R. Compagnoni - B. Lombardo - S. Angiboust - G. Balestro - S. Ferrando - C. Groppo - T. Hirajima - F. Rolfo Rolfo - F. Hirajima - C. Groppo T. Ferrando Angiboust - G. Balestro S. Lombardo - S. Castelli - R. Compagnoni B. D. DOI: 10.3301/GFT.2014.03 Variscan amphibolite-facies metamorphic basement. The two complexes roughly correspond to the “gneiss and amphibolite-facies metamorphic basement. The two complexes Variscan metamorphic of pre-Alpine formation” of Chopin et al. (1991). Relics and to the “varied metagranite” contacts (Fig. 9) intrusive assemblages and of pre-Alpine located all along the boundary between (Biino & Compagnoni, 1992; Compagnoni Hirajima, Monometamorphic and polymetamorphic complexes before the 1995), allowed inferring the relationships between two complexes 1991; Compagnoni et al., (Fig. 10). Alpine orogeny 3.2.1 The monometamorphic complex orthogneiss, which locally preserves to medium- fine-grained It mainly consists of augen-gneiss grading of pyrope-bearing whiteschist from a few protolith. This gneissic complex includes layers relics of the granitoid from the granitoid dm- to tens of m- thick (Fig. 9) that are considered be metasomatic rocks derived 2012); further details on the 2009; Ferrando, et al., 2001; Ferrando protolith (Compagnoni & Hirajima, in the description of Stop 2.3. whiteschist are given are also included within the orthogneiss (e.g. granofels of garnet-jadeite-kyanite-quartz layers Rare consists 2003b). The UHP peak assemblage in this granofels 1995; Compagnoni & Rolfo, Compagnoni et al., + hematite of coesite, pyrope-rich almandine, jadeite (often replaced by fibrous albite + paragonite colourless glaucophane. Coesite is found as inclusion in minor phengite and rare very magnetite), kyanite, et al. (1987) and Sharp This unusual lithotype has been considered by Schreyer garnet, jadeite and kyanite. et al. (1993) as the possible product of partial melting host whiteschist protolith, but convincing 2009). et al., evidence for this interpretation is still lacking (see also Grevel geological field trips 2014 - 6(1.3) excursion notes 28 Crust-mantle interactions during subduction of oceanic & continental crust D. Castelli - R. Compagnoni - B. Lombardo - S. Angiboust - G. Balestro - S. Ferrando - C. Groppo - T. Hirajima - F. Rolfo Rolfo - F. Hirajima - C. Groppo T. Ferrando Angiboust - G. Balestro S. Lombardo - S. Castelli - R. Compagnoni B. D. DOI: 10.3301/GFT.2014.03 The gradual transition from undeformed metagranitoid to foliated orthogneiss indicates that the latter derived from to foliated orthogneiss indicates that the latter derived from undeformed metagranitoid transition The gradual the Alpine tectonic and metamorphic reworking of former (Biino & Compagnoni, 1992), i.e. orthogneiss age of ca.275 (U/Pb SHRIMP on zircons) is the most probable intrusion are monometamorphic rocks. A Permian data). 1997 with discussion on previous radiometric protolith (Gebauer et al., age for the granitoid 3.2.2 The polymetamorphic complex 1995, 2004), with eclogite and marble intercalations (e.g. Compagnoni et al., This complex includes paragneiss and structures (Fig. 9). A detailed description of different mineralogies locally preserving relics of pre-Alpine 1995; here we focus on a few in Compagnoni et al., lithologies of the polymetamorphic complex is given relics. assemblages and/or pre-Alpine lithologies preserving the most interesting UHPM mineral Kyanite-almandine paraschist of The most typical lithology of the polymetamorphic complex is a brownish micaschist with porphyroblasts the rock consists of phengite, garnet, kyanite, and almandine, up to 2 cm across. Microscopically, kyanite tourmaline. A second generation quartz, jadeite, abundant accessory rutile and aggregates of yellow-greenish decompression, locally occurs at the rim of kyanite during retrograde developed most likely of acicular kyanite, intergrowths of albite + paragonite, fine-grained replaced by very Jadeite is generally porphyroblasts. and locally is partly replaced by paragonite almandine is partly altered to biotite and/or chlorite, and kyanite by an earlier margarite corona. inclusions, which locally define a and almandine are usually crowded with mineral kyanite Porphyroblastic jadeite, quartz rough planar internal foliation. Almandine usually includes chloritoid, rutile, staurolite, kyanite, in the garnet. be found everywhere inclusions may and coesite, chlorite white micas. Kyanite staurolite, chloritoid and quartz occur only in the garnet core mantle, whereas coesite is On the contrary, (increasing inclusions in garnet and its chemical zoning found only in the garnet rim. Distribution of mineral from the quartz to trajectory P-T pyrope content from core to rim) point a growth during prograde coesite stability field. the occurrence of 2000) revealed et al., Ferraris 3T-polytype: of the phengite (a zoned Detailed investigations formed during exhumation in the quartz stability field, by a talc + quartz nano-scale “exsolutions” that likely reaction which produced a less celadonite-rich phengite. geological field trips 2014 - 6(1.3) excursion notes 29 Crust-mantle interactions during subduction of oceanic & continental crust D. Castelli - R. Compagnoni - B. Lombardo - S. Angiboust - G. Balestro - S. Ferrando - C. Groppo - T. Hirajima - F. Rolfo Rolfo - F. Hirajima - C. Groppo T. Ferrando Angiboust - G. Balestro S. Lombardo - S. Castelli - R. Compagnoni B. D. DOI: 10.3301/GFT.2014.03 Eclogite from which appear to derive numberless small bodies of eclogites occur, Within the basement paraschist, contact, cm-thick discontinuous brownish- the eclogite-paraschist At boudinage of original concordant layers. and usually contains idioblastic garnet, fine-grained The eclogite is very red garnetites frequently occur. preferred orientation, quartz (as polygonal granoblastic lineated omphacite, phengite with well-developed accessory rutileaggregates), local zoisite, and minor apatite, zircon, sulphides local graphite. Most eclogites exhibit a faint foliation defined by eclogites also contain kyanite. Some light green quartz-rich The occurrence of small coesite inclusions in the omphacite suggests that the alignment of rutile grains. eclogites, omphacite is from former coesite. In the retrograded quartz aggregates derive the granoblastic and albite ± green symplectitic intergrowth of diopsidic clinopyroxene typically replaced by a fine-grained amphibole, and phengite is partially replaced by symplectitic aggregates of biotite + oligoclase (Groppo et al., consisting of idioblastic omphacite, discordant veins 2007a). Often, the eclogites are cut by coarse-grained minor interstitial quartz and accessory apatite. In of coarse- or pockets other eclogites, discontinuous layers from folding and derived quartz, omphacite and accessory apatite are common, which appear to have grained of former discordant metamorphic veins. transposition Marble of locally with thin discontinuous interlayers The marble consists of both calcite- and dolomite-rich layers types mylonitic fine-grained is usually coarse, but very micaschist and boudins of eclogite. The grain-size reflecting different stages of the polyphase Alpinelocally occur, (e.g. tectonometamorphic evolution Costa Monforte (where the largest marble lens occurs: Fig. 9), most 2003b). At Compagnoni & Rolfo, highly- common type is a pure phengite-bearing calcite marble, in which white mica chemically-zoned, by silicate-rich (garnet, 2005). This marble is locally characterized et al., ordered 3T phengite (Ferraris to the regional UHP foliation. phengite, rutile, epidote, and titanite) domains stretched parallel clinopyroxene, data show that the marbles underwent same Alpine petrological and crystallographic Microstructural, 2007). 2005; Castelli et al., et al., (Ferraris of the country paragneiss tectonometamorphic evolution geological field trips 2014 - 6(1.3) excursion notes 30 Crust-mantle interactions during subduction of oceanic & continental crust D. Castelli - R. Compagnoni - B. Lombardo - S. Angiboust - G. Balestro - S. Ferrando - C. Groppo - T. Hirajima - F. Rolfo Rolfo - F. Hirajima - C. Groppo T. Ferrando Angiboust - G. Balestro S. Lombardo - S. Castelli - R. Compagnoni B. D. -undersaturated sites, pseudomorphs consisting of corundum -undersaturated rutile 2 DOI: 10.3301/GFT.2014.03 Relics of pre-Alpine paragneiss and paraschist relict of the pre-Alpine The most common mineralogical rare. relics are generally pre-Alpine In the paraschists, by a thin corona of small idioblastic overgrown garnet up to 2 cm in diameter, history is a porphyroblastic 2006; (Groppo et al., Alpine garnets, these last including small needles of rutile with a “sagenitic” arrangement 2007). Spiess et al., by pseudomorphous and metamorphic rock, mostly characterised remnant of a pre-Alpine The best-preserved between Brossasco and Isasca. at Case Garneri, halfway coronitic reactions, has been recognized light dark and includes irregular cm- to dm-sized rock is very well preserved this exceptionally Macroscopically, portions of metatects. Under the microscope, dark portions consist of relict garnet, nematoblasts former sillimanite aggregates, red-brown biotite partly replaced by phengite ±pseudomorphically replaced by kyanite rutile and ± albite + zoisite aggregates of patchy surrounded by new coronitic garnet, lens-like with jadeite relics kyanite relics and pseudomorphs quartz aggregates. Such mineral after former plagioclase, and polygonal granoblastic ± garnet + kyanite are embedded in a dark grey matrix, mainly consisting of fine-grained phengite ± biotite, abundant: are very from former contact metamorphic cordierite. Accessory minerals interpreted as derived ilmenite completely replaced by rutile and surrounded a garnet corona, apatite, zircon, monazite graphite, and tourmaline. In some SiO aggregates, surrounded by an inner kyanite and an outer garnet corona occur, which may have derived from derived have which may and an outer garnet corona occur, aggregates, surrounded by an inner kyanite flakes rock foliation is defined by the preferred orientation of graphite former spinel. The rough pre-Alpine and sillimanite nematoblasts by the preferred dimensional orientation of former plagioclase blasts. sillimanite, and accessory apatite, garnet contains inclusions of plagioclase, quartz, biotite, rare Pre-Alpine usually defines a closely spaced internal foliation graphite This fine-grained tourmaline, and dusty graphite. garnet, which usually exhibits a strongly often discordant with respect to the external foliation. Pre-Alpine into contact cordierite, is in turn replaced by a dark grey transformed corroded shape and appears extensively 1). (Table aggregate of garnet and minor kyanite an igneous microstructure and consist of plagioclase replaced by jadeite The light portions locally preserve partly replaced by phengite + quartz intergrowths, granoblastic K-feldspar and zoisite, (usually albitized) and minor biotite polygonal quartz aggregates, cordierite completely replaced by garnet and kyanite, Around the leucocratic is ubiquitous and locally occurs as large deformed flakes. 1). Graphite (Table muscovite geological field trips 2014 - 6(1.3) excursion notes 31 s s s u u u o o o h h h p p p r r r o o o m m m o o o d d d coronitic u u u e e e s s s p p pseudomorphous p Wm r r z p t e e E S S Q Bt + Chl pseudomorphous Ab/Olg + Czo Alpine retrogression Type of replacement ] e y y p Phg ± Qtz Jd + Qtz o E K K Kfs microperthite pseudomorphous C → → [ Zo + Ky O) O) 2 2 Alpine UHP metamorphism (+ H (+ H ] ] d z t n Q A Crust-mantle interactions during subduction of oceanic & continental crust [ [Crd] Grt + Ky Ser + Chl pseudomorphous [ [Spl?] Grt + Crn Ser pseudomorphous metamorphism Permian contact D. Castelli - R. Compagnoni - B. Lombardo - S. Angiboust - G. Balestro - S. Ferrando - C. Groppo - T. Hirajima - F. Rolfo Rolfo - F. Hirajima - C. Groppo T. Ferrando Angiboust - G. Balestro S. Lombardo - S. Castelli - R. Compagnoni B. D. ] ] z l z i t n Bt Bt Grt + Phg Bt + Chl coronitic S Ms Ms Kfs Kfs Grt Q [Pl] [Pl] M [ [Ilm] [Ilm] Rt + Grt Ttn + Chl coronitic [ Variscan Regional Tab. 1 – Metamorphic evolution of the paraschist from the polymetamorphic complex. of the paraschist 1 – Metamorphic evolution Tab. Ap, Gr, Zrn, Tur metamorphism DOI: 10.3301/GFT.2014.03 portions, biotite aggregates and kyanite aggregates after a former prismatic contact andalusite locally occur. portions, biotite aggregates and kyanite polygonal quartz aggregates, the ubiquitous presence of granoblastic Coesite has not been found: however, single quartz crystal, is considered to be the unambiguous evidence locally pseudomorphous after a pre-Alpine coesite. of the former occurrence early-Alpine includes: (i) a compositions and microstructure, the inferred polymetamorphic evolution On the basis of mineral (in the sillimanite stability field), which medium-P regional metamorphic event Variscan, most likely pre-Alpine, static low-P metatects; (ii) a pre-Alpine the leucocratic reached the upper amphibolite-facies and developed related to the thermal metamorphism connected intrusion of Permian recrystallization, most likely of contact biotite, andalusite, cordierite; (iii) a peak Alpine which led to the widespread development granitoids, the growth of by pseudomorphous and coronitic reactions involving UHP static recrystallization, characterised coesite, jadeite, garnet, phengite, kyanite, zoisite, and rutile; (iv) a late Alpine greenschist facies retrogression, responsible for the complete coesite to polygonal inversion quartz and granoblastic the replacement of jadeite 1). by albite (Table geological field trips 2014 - 6(1.3) excursion notes 32 Crust-mantle interactions during subduction of oceanic & continental crust D. Castelli - R. Compagnoni - B. Lombardo - S. Angiboust - G. Balestro - S. Ferrando - C. Groppo - T. Hirajima - F. Rolfo Rolfo - F. Hirajima - C. Groppo T. Ferrando Angiboust - G. Balestro S. Lombardo - S. Castelli - R. Compagnoni B. D. (OH)], margarite and rutile also occur. The relict bluish-green spinel occurs as medium- (OH)], margarite and rutile also occur. 10 O 8 (Al,Ti) 3 ) 2+ DOI: 10.3301/GFT.2014.03 to coarse-grained, fractured and corroded, porphyroclasts which locally include roundish crystals of ilmenite. and corroded, porphyroclasts fractured to coarse-grained, calcite exsolution, but is only partially replaced by at the rim dolomite does not display Porphyroclastic and along fractures. mineral pre-Alpine relationships and petrological data allow inferring a partially preserved Microstructural assemblage consisting of spinel + ilmenite dolomite calcite (+ plagioclase, inferred on the basis into a re-equilibrated assemblage was and low-P 2007b). This high-T petrological modelling) (Groppo et al., (now peak Alpine assemblage consisting of corundum + chlorite (clinochlore) rutile dolomite aragonite pseudomorphically replaced by to calcite). During the UHP metamorphism, spinel was completely inverted at the corundum locally developed aggregates of bluish corundum + clinochlore; coarser-grained fine-grained replaced by aggregates of rutile. The ilmenite was rim of Crn + Chl pseudomorphs after spinel. Pre-Alpine but no evidence of this phase has been replaced by UHP zoisite, likely plagioclase was inferred pre-Alpine reactions. These reactions, which consumed by lower-P in the marble, possibly because it was observed and the UHP Alpine assemblages, were responsible for pre-Alpine at the expense of both high-T developed of ilmenite after spinel + högbomite and a new generation from aragonite, the growth of calcite inverted aggregates of margarite around the after clinochlore and radial corundum + rutile, sheridanite overgrowth aggregates of corundum + clinochlore. fine-grained 3.3 The P-T evolution of the Brossasco-Isasca Unit of Fig. 11. diagram in the P-T Unit is summarized of the Brossasco-Isasca The whole metamorphic evolution polyphase of the polymetamorphic complex, is a pre-Alpine in the paraschists recognized The oldest event, in age. The occurrence of Variscan amphibolite-facies regional metamorphic recrystallization, most likely metamorphism, T higher than the wet suggests that during the regional Variscan metatects in paragneiss Relics of pre-Alpine assemblages in marble of dolomite marble retains scanty silicate-rich domains broadly In the Costa Monforte marble lens, a layer only partially paragenesis relics of a pre-Alpine to the regional foliation that still preserves aligned parallel by 2007). The fabric is characterized by the Alpine metamorphic assemblages (Castelli et al., overprinted irregular to lens-shaped aggregates (up a few cm-thick) of spinel + ilmenite dolomite + corundum chlorite + calcite ± dolomite; minor högbomite chlorite ± calcite, set in a matrix of fine- to medium-grained [(Mg,Fe geological field trips 2014 - 6(1.3) excursion notes 33 ) > 0.60 has been 2 Crust-mantle interactions during subduction of oceanic & continental crust Fig. 11 - Synopsis of selected Alpine P-T estimates and inferred Alpine pre- of selected Alpine P-T Fig. 11 - Synopsis D. Castelli - R. Compagnoni - B. Lombardo - S. Angiboust - G. Balestro - S. Ferrando - C. Groppo - T. Hirajima - F. Rolfo Rolfo - F. Hirajima - C. Groppo T. Ferrando Angiboust - G. Balestro S. Lombardo - S. Castelli - R. Compagnoni B. D. path of the BIU from Groppo et al. (2007b). Alpine (blue curve: pA) P-T paths for the BIU polymetamorphic complex (modified pA) P-T Alpine (blue curve: path proposed by is the Alpine P-T curve 2007). The solid, orange from Castelli et al., estimates by Groppo P-T the peak and retrograde Castelli et al. (2007) that integrates et al. (2007a: circles labelled 1 to 3). Solid ellipses A E are the P-T estimated respectively, stages of the BIU, peak and retrograde conditions of prograde, et al. (2009), whereas dotted ellipses are those estimated by Hermann by Ferrando estimates of (2003). The error bars of ages refer to the geochronologically dated P-T P-T & Hermann (2001). The dotted-dashed blue path (pA) is the pre-Alpine Rubatto modeled for high-T decarbonation reaction(s) along its prograde pre- decarbonation reaction(s) along its prograde modeled for high-T 2007b). Alpine path (Groppo et al., granite solidus were reached (T~700°C at P~0.3 GPa: Compagnoni et solidus were reached (T~700°C at P~0.3 GPa: granite contact low-P by a high-T, is overprinted 1995). This regional event al., recrystallization connected to the emplacement of Permian 1995; 1991; Compagnoni et al., (Compagnoni & Hirajima, granitoids 2003b). Compagnoni & Rolfo, are compatible with the events conditions of the pre-Alpine The P-T in the spinel-ilmenite-bearing assemblage preserved relict mineral dolomite marble, for which a binary fluid at X(CO DOI: 10.3301/GFT.2014.03 From whiteschists, three prograde Alpine stages have been inferred, corresponding to: ca. 1.6 GPa, ≤600°C; been inferred, corresponding to: ca. 1.6 GPa, Alpine stages have From whiteschists, three prograde 2009). As to the et al., (Ferrando 590Valle Po and Val Po Unit, between Valle Brossasco-Isasca DOI: 10.3301/GFT.2014.03 Metasomatism from & to ultramafics: the UHP continental Brossasco-Isasca Unit we will focus on three During the excursion stops that are located on the right side of (Stop 2.1), on Varaita the southernmost Val di Gilba (Stop 2.2), the left side of Vallone (Stop 2.3), Po and on the right side of Valle (Fig. 19). respectively STOP 2.1: Metagranite recrystallized under UHP conditions with greenschist- facies shear zone Brossasco, hillrock near the bridge on Torrente Varaita (Val Varaita) (Coord: 44°34’00’’ N, 7°22’05’’ E, elev. 590 m a.s.l.) is exposed along the cut of The metagranite parallel a country road running westwards (Fig. 19). Varaita) (Torrente to the river geological field trips 2014 - 6(1.3) itinerary 51 and a detail (a) Crust-mantle interactions during subduction of oceanic & continental crust at the northern vertical cliff of the hillrock near bridge on Torrente at the northern vertical Fig. 20 – Stop 2.1: Sketch of the metagranite outcrop of the metagranite Fig. 20 – Stop 2.1: Sketch transitional zone 1-3); Sm: incipient greenschist-facies foliation. 1-3); Sm: incipient greenschist-facies zone transitional (b) by an 1995), characterized Brossasco (from Michard et al., Varaita, (2) metagranite; heterogeneous deformation; (1) undeformed porphyritic (4) and minor folded quartz veins orthogneiss with passively mylonitic S/C structures in the (top-to-the-SW isoclinal folds (detail); (3) ultramylonite D. Castelli - R. Compagnoni - B. Lombardo - S. Angiboust - G. Balestro - S. Ferrando - C. Groppo - T. Hirajima - F. Rolfo Rolfo - F. Hirajima - C. Groppo T. Ferrando Angiboust - G. Balestro S. Lombardo - S. Castelli - R. Compagnoni B. D. DOI: 10.3301/GFT.2014.03 The best preserved metagranite is metagranite The best preserved (Fig. 21a) and locally contains porphyritic cognate aplitic and pegmatitic dykes, and centimetric enclaves microgranular of biotite-rich schists. The xenoliths is dissected by a metagranite massive which shows a dm-thick shear zone, foliation and a mylonitic well-developed reduction, significant grain-size pervasive accompanied by a general recrystallization (Fig. greenschist-facies from the massive 20). The transition to the surrounding metagranite and occurs augengneiss is locally gradual a distance from few centimetres over metres by progressive to several of a metamorphic foliation. development is surrounded on all The metagranite sides by the augengneiss of monometamorphic complex, which is by a greenschist-facies characterized acquired during This suggests either that most of the augen- gneiss deformation was metamorphic overprint. reworked penetratively eclogitic foliation was early-Alpine, most likely or that an earlier, event the late-Alpine In the surrounding augengneiss only evidence for former UHP during the greenschist event. wrapped metamorphism is the local occurrence of a high-celadonite core in phengite porphyroclasts, around by the main Alpine greenschist facies foliation, and probable original occurrence of assemblage 1991). garnet + rutile (now titanite: Chopin et al., grossular-rich microstructure (Fig. 21b), only exhibits an igneous hypidiomorphic Under the microscope, metagranite by the coronitic and pseudomorphous reactions of Alpine metamorphic partially obliterated geological field trips 2014 - 6(1.3) itinerary 52 Tab. 2 – Metamorphic Tab. complex. evolution of the metagranite evolution from the monometamorphic s s s s s u u u u u o o o o o h h h h h p p p p p r r r r r o o o o o m m m m m o o o o o d d d d d coronitic coronitic Type of u u u u u e e e e e replacement s s s s s p p p p p . r g g e t a i

o n h n z b t z t t t r A T C T

Q

t

e

/ l + B Bt coronitic p

+ + Qtz coronitic

a

o t t m n r b l B c B o I A i g m y l o p Crust-mantle interactions during subduction of oceanic & continental crust g s f h D. Castelli - R. Compagnoni - B. Lombardo - S. Angiboust - G. Balestro - S. Ferrando - C. Groppo - T. Hirajima - F. Rolfo Rolfo - F. Hirajima - C. Groppo T. Ferrando Angiboust - G. Balestro S. Lombardo - S. Castelli - R. Compagnoni B. D. P a K

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p → g d Grt + dactylitic Qtz corona J M O) 2 asymmetrically zoned Grt corona Bt is locally preserved as a textural relic, but its Mg-rich chemical composition indicates a as a textural is locally preserved Phg + dactylitic Qtz corona (after Kfs) (+H ] e s a h p

h ] z l t c t i P B r Kfs Q [ - Bt / Pl ) Bt / Kfs Bt / Qtz e F Ap, Zrn, Tur ( - Grt (xenocryst) i T [ Igneous minerals Alpine UHP peak metamorphism Alpine retrogression DOI: 10.3301/GFT.2014.03 metamorphic equilibration (Biino & Compagnoni, 1992; Bruno et al., 2001). Usually, biotite is 2001). Usually, (Biino & Compagnoni, 1992; Bruno et al., metamorphic equilibration pseudomorphically replaced by a high-celadonite Ti-rich phengite + rutile and surrounded continuous the contact with original igneous plagioclase site, coronitic garnet is garnet corona (Fig. 21c, d). At with pyrope- and almandine-rich composition on the biotite side grossular-rich asymmetrically zoned composition on the plagioclase side. recrystallizations. The original igneous assemblage consisted of quartz, plagioclase, K-feldspar, biotite and recrystallizations. The original igneous assemblage consisted of quartz, plagioclase, K-feldspar, the for K-feldspar, Except are locally observed. accessory apatite, tourmaline and zircon. Garnet xenocrysts 2). were completely replaced by Alpine metamorphic phases (Table igneous minerals The igneous biotite geological field trips 2014 - 6(1.3) itinerary 53 tz, n is r peak na and Igneous DM1086. (c) Hand specimen of metagranite: the Hand specimen of metagranite: (a) Fine-grained granoblastic aggregate of quartz, statically granoblastic Fine-grained (b) Crust-mantle interactions during subduction of oceanic & continental crust D. Castelli - R. Compagnoni - B. Lombardo - S. Angiboust - G. Balestro - S. Ferrando - C. Groppo - T. Hirajima - F. Rolfo Rolfo - F. Hirajima - C. Groppo T. Ferrando Angiboust - G. Balestro S. Lombardo - S. Castelli - R. Compagnoni B. D. Igneous biotite included in K-feldspar, pseudomorphically replaced by a single phengite crystal and surrounded thin garnet Igneous biotite included in K-feldspar, (d) Fig. 21 – Stop 2.1: Metagranite recrystallized under UHP conditions. See also Table 2. under UHP conditions. See also Table recrystallized Fig. 21 – Stop 2.1: Metagranite coesite, developed after primary euhedral igneous quartz. Sample DM 60. XPL. Modified from Compagnoni et al. (2004). after primary euhedral coesite, developed only evidence of the UHP metamorphic overprint is a very rough foliation and the sugary appearance of the original igneous quar rough foliation and the sugary appearance is a very only evidence of the UHP metamorphic overprint from former coesite. The coi from inversion polygonal aggregate of metamorphic quartz, derived now consisting of a granoblastic microstructures of the metagranites. 20 mm across. (b-d) Representative of coesite which replaced the igneous quartz during UHP metamorphism. Sample DM 496. XPL. from inversion derived biotite replaced by a Mg-richer + phengite. The biotite/phengite domain is surrounded first continuous garnet coro dactylitic intergrowth of phengite and quartz. Sample fine-grained which consists of a very a second corona against K-feldspar, PPL. corona and a dactylitic phengite + quartz intergrowth. In the upper right, a granoblastic polygonal quartz aggregate from forme corona and a dactylitic phengite + quartz intergrowth. In the upper right, granoblastic DOI: 10.3301/GFT.2014.03 geological field trips 2014 - 6(1.3) itinerary 54 Ca- 90 Crust-mantle interactions during subduction of oceanic & continental crust D. Castelli - R. Compagnoni - B. Lombardo - S. Angiboust - G. Balestro - S. Ferrando - C. Groppo - T. Hirajima - F. Rolfo Rolfo - F. Hirajima - C. Groppo T. Ferrando Angiboust - G. Balestro S. Lombardo - S. Castelli - R. Compagnoni B. D. , up to 3 cm long, are preserved. They include pseudomorphs after plagioclase, , up to 3 cm long, are preserved. is pseudomorphically replaced at the UHP metamorphic peak by jadeite (up to Jd is always replaced by a polygonal granoblastic aggregate (Fig. 21b, d), interpreted as aggregate (Fig. 21b, replaced by a polygonal granoblastic is always ) (Bruno et al., 2002) + zoisite + coesite (now quartz) + kyanite + K-feldspar (Fig. 21c). Because of the + K-feldspar + coesite (now quartz) kyanite 2002) + zoisite ) (Bruno et al., 10 DOI: 10.3301/GFT.2014.03 general pervasive greenschist-facies overprint, jadeite is mostly retrogressed to albite + white mica. Fresh overprint, greenschist-facies pervasive general only in the pseudomorphs after igneous plagioclase, armoured within jadeite is usually preserved instead of zoisite, idioblastic garnet is developed, Within such pseudomorphs, a small grossular-rich K-feldspar. close to the biotite site. of igneous K-feldspar Relics Esk derived from coesite inversion (Biino & Compagnoni, 1992; Compagnoni et al., 1995). (Biino & Compagnoni, 1992; Compagnoni et al., from coesite inversion derived 2 (Biino & in Table is summarized in the metagranite recognized The polyphase metamorphic evolution 2003b). 1995; Compagnoni & Rolfo, Compagnoni, 1992; Compagnoni et al., biotite and quartz (Fig. 21d). Along deformation bands or around mineral inclusions, K-feldspar recrystallizes inclusions, K-feldspar biotite and quartz (Fig. 21d). Along deformation bands or around mineral a second reaction rim aggregates. Around the biotite included in K-feldspar, to polygonal granoblastic (dactylitic) quartz and highly surrounds the garnet corona, which consists of an intergrowth vermicular (Fig. K-feldspar celadonitic phengite growing from the garnet corona towards 21c, d). The igneous quartz The igneous plagioclase STOP 2.2: Metagranites with xenoliths of paraschists and metabasics: relict intrusive contact between Permian granitoids and the country metamorphic basement North of Case Bastoneri, left side Vallone di Gilba (Coord: 44°35’01’’ N, 7°20’27’’ E, elev. 885 m a.s.l.) basement (now the polymetamorphic contact between the Variscan intrusive A poorly deformed pre-Alpine The blocks consist (now the monometamorphic complex) is preserved. granitoids complex) and the Permian of banded which includes numberless xenoliths K-feldspars, with cm-sized metagranite of a porphyritic are composed of both metapelites and metabasics, which locally metamorphic rocks (Fig. 22). The xenoliths and are finely dismembered, fabric, mineralogy a pre-intrusion folded foliation. Where the xenoliths preserve mode of the igneous rock were modified: this suggests that a significant assimilation country metamorphic rocks by the intruding magma did occur. geological field trips 2014 - 6(1.3) itinerary 55 (d) (a) Detail of the porphyritic structure of the metagranite. Detail of the porphyritic (b) Contact metamorphosed xenoliths of the country Variscan Contact metamorphosed xenoliths Fig. 22 – Stop 2.2: Metagranites with xenoliths of paraschists. of paraschists. with xenoliths Fig. 22 – Stop 2.2: Metagranites Detail of a xenolith of a banded metapelite. Detail of a xenolith (c) Note metagranite. porphyritic included in the late-Variscan paraschists metamorphic foliation. a clear Variscan preserve that the xenoliths Relict intrusive contact between Permian granitoids and the country granitoids contact between Permian intrusive Relict in a block near Case preserved metamorphic basement (paragneiss), Bastoneri. Crust-mantle interactions during subduction of oceanic & continental crust D. Castelli - R. Compagnoni - B. Lombardo - S. Angiboust - G. Balestro - S. Ferrando - C. Groppo - T. Hirajima - F. Rolfo Rolfo - F. Hirajima - C. Groppo T. Ferrando Angiboust - G. Balestro S. Lombardo - S. Castelli - R. Compagnoni B. D. DOI: 10.3301/GFT.2014.03 Metagranitoids consisted of metagranite The massive plagioclase, biotite, quartz, poikilitic euhedral phenocrysts (Fig. 22b), late K-feldspar interstitial quartz, and accessory apatite, tourmaline. Close to the zircon and rare (see below), biotite-rich metapelite xenoliths plagioclase and a rim enriched in euhedral minor quartz, usually occurs. The igneous exhibit the same pseudomorphous minerals and coronitic microstructures (Fig. 23a) (see described for the Brossasco metagranite Stop 2.1). Xenoliths of UHP a variety display The xenoliths and mineral metamorphic fabrics, grain-sizes in spite of the assemblages. However, Alpine recrystallization, in many extensive and ghost coronitic samples relict minerals and pseudomorphous microstructures still which are useful to infer the pre-Alpine occur, and evolution. mineralogy is similar to that The inferred evolution the relict lithologies of all over recognized polymetamorphic complex (Compagnoni & 1995; 1991; Compagnoni et al., Hirajima, 2003b). Compagnoni & Rolfo, geological field trips 2014 - 6(1.3) itinerary 56 a (d) are rix of Contact metamorphosed Variscan (b) Crust-mantle interactions during subduction of oceanic & continental crust D. Castelli - R. Compagnoni - B. Lombardo - S. Angiboust - G. Balestro - S. Ferrando - C. Groppo - T. Hirajima - F. Rolfo Rolfo - F. Hirajima - C. Groppo T. Ferrando Angiboust - G. Balestro S. Lombardo - S. Castelli - R. Compagnoni B. D. Relict Variscan paraschist showing kyanite pseudomorphs after contact metamorphic showing kyanite paraschist Variscan Relict (c) Metagranitoid collected near to the contact with country metamorphic basement (“granitoid Metagranitoid (a) Fig. 23 – Stop 2.2: Representative microstructures of the metagranite and of the contact metamorphosed Variscan amphibolite- and of the contact metamorphosed Variscan microstructures of the metagranite Fig. 23 – Stop 2.2: Representative fine-grained aggregate of Alpine garnet + kyanite. Sample DM495. PPL. Modified from Compagnoni et al. (2004). aggregate of Alpine garnet + kyanite. fine-grained marginal facies”). Euhedral crystals of former igneous plagioclase (now a fine-grained aggregate of jadeite+zoisite±kyanite), crystals of former igneous plagioclase (now a fine-grained marginal facies”). Euhedral and biotite (partly replaced by phengite) are set in a mat aggregate of kyanite+garnet±biotite), cordierite (now a fine-grained from coesite. Sample DM 1116. PPL. from inversion polygonal quartz derived granoblastic aggregate, sillimanite, pseudomorphically replaced by a UHP kyanite Bundles of regional Variscan amphibolite-facies paraschist. UHP contact metamorphic cordierite, now mainly consisting of fine-grained from late-Variscan embedded in a dark matrix derived garnet aggregates. Sample DM493. PPL. mica pseudomorphs after contact metamorphic cordierite. Sample DM1143. PPL. andalusite, and garnet+kyanite+white contact metamorphic cordierite, now replaced by garnet partly replaced by late-Variscan Metapelite showing a corroded Variscan facies paraschist (see Tables 1, 2). (see Tables facies paraschist DOI: 10.3301/GFT.2014.03 geological field trips 2014 - 6(1.3) itinerary 57 Crust-mantle interactions during subduction of oceanic & continental crust D. Castelli - R. Compagnoni - B. Lombardo - S. Angiboust - G. Balestro - S. Ferrando - C. Groppo - T. Hirajima - F. Rolfo Rolfo - F. Hirajima - C. Groppo T. Ferrando Angiboust - G. Balestro S. Lombardo - S. Castelli - R. Compagnoni B. D. DOI: 10.3301/GFT.2014.03 Most xenoliths are banded metapelites (Fig. 22c, d) which consist of layers rich in fine-grained randomly rich in fine-grained are banded metapelites (Fig. 22c, d) which consist of layers Most xenoliths including: oriented phengite alternating with layers aggregates pseudomorphous after prismatic sillimanite (Fig. 23b); - kyanite - coronitic garnet with local “sagenitic” rutile inclusions after biotite; garnet surrounded by thin coronas of Alpine garnet; porphyroblastic - pre-Alpine after primary ilmenite; - rutile aggregates rimmed with a garnet corona, most likely crystals after pre-Alpine products) with small zoisite (or its albite-bearing retrogressive - jadeite-rich pyroxene plagioclase; quartz aggregates after coesite; - polygonal granoblastic contact metamorphic, after pre-Alpine, - aggregates of small garnets with minor phengite, most likely cordierite (Fig. 23c, d). a grain-size during the Alpine peak metamorphic event, recrystallized In the samples more pervasively textures: the rock is, the pre-Alpine obliterates occurs, which progressively coarsening of the UHP minerals quartz and with accessory rutile and local Na-pyroxene, to a phengite-garnet granofels therefore, converted zoisite. and phengite- bearing eclogites with abundant to quartz- are metabasics, entirely converted Minor xenoliths partly replaced relics are large titanite grains, pre-Alpine accessory rutile. In the eclogite, only recognized by a rutile ± omphacite intergrowth. In the garnet, common occurrence of “sagenitic” suggests its biotite. from pre-Alpine derivation STOP 2.3: Typical whiteschist with pyrope megablasts Case Ramello, Martiniana (Valle Po) (Coord: 44°36’54’’ N, 7°22’00’’ E, elev. 860 m a.s.l.) This is the type locality where Chopin (1984) first described occurrence of regional metamorphic coesite in continental crust rocks. The BIU whiteschist occurs the orthogneiss of monometamorphic complex as or boudins from a few centimetres to ca. 20 m in thickness and metres hundreds of layers by a few decimetres to in length. The contact between the whiteschist and hosting orthogneiss is marked metres of a phengite-rich, banded gneiss locally preserving relics the igneous K-feldspar several phenocrysts. geological field trips 2014 - 6(1.3) itinerary 58 (b) Zoned ellenbergerite in association with kyanite, Zoned (d) Pyrope megablast, ca. 20 cm across. Note that pyrope is (a) Crust-mantle interactions during subduction of oceanic & continental crust D. Castelli - R. Compagnoni - B. Lombardo - S. Angiboust - G. Balestro - S. Ferrando - C. Groppo - T. Hirajima - F. Rolfo Rolfo - F. Hirajima - C. Groppo T. Ferrando Angiboust - G. Balestro S. Lombardo - S. Castelli - R. Compagnoni B. D. Radial kyanite aggregate in pyrope. Sample DM450. XPL. kyanite Radial (c) Fig. 24 – Stop 3: Whiteschists with pyrope megablasts. phengite, and rutile armoured in a pyrope megablast. Sample DM450. PPL. Modified from Compagnoni et al. (2004). Relics of coesite included in a small pyrope porphyroblast and partially converted to quartz along rims and fractures. Radial to quartz along rims and fractures. and partially converted of coesite included in a small pyrope porphyroblast Relics in the hosting pyrope. Sample are developed increase during the coesite to quartz inversion, consequent to volume fractures, DM507. PPL. crowded of inclusions of kyanite, rutile, etc. (b-d) Representative microstructures of the pyrope-bearing whiteschists. rutile, etc. (b-d) Representative crowded of inclusions kyanite, DOI: 10.3301/GFT.2014.03 geological field trips 2014 - 6(1.3) itinerary 59 ) constitutes the core of megablasts and ) occurs as rare irregular and corroded relics ) occurs as rare 16-26 Whiteschist consists of pyropes, usually habit, included in a exhibiting a trapezohedron from coesite, foliated matrix of quartz inverted that is high-celadonite phengite (polytype 3T, type under more stable than monoclinic poly- HP-UHP conditions; Amisano Canesi et al., talc 2000, 2005), kyanite, et al., 1994; Ferraris and accessory rutile, zircon monazite. On two the basis of modal amount minerals, (1) Prp- kinds of whiteschist can be recognized: rich whiteschist mainly consisting of pyrope megablasts (from 10 to 20 cm in diameter; Fig. (from 2 to 10 24a and Fig. 25) or porphyroblasts cm in diameter; Fig. 25) with minor amount of talc, phengite and quartz; (2) Qtz-rich kyanite, whiteschist that consists of quartz, kyanite, phengite, pyrope neoblasts (<2 cm in diameter; 2009). et al., Fig. 25) and talc (Ferrando of the Up to more than 50 per cent in volume pyrope blasts are commonly crowded with 3), some of which are inclusions (Table mineral not found in the matrix (Chopin, 1984; Schertl 1991; Hermann, 2003). et al., of pyrope four generations Near Case Ramello, are recognisable by microstructural relationships, chemical composition and 58-70 Alm Alm 69-81 25-37 Crust-mantle interactions during subduction of oceanic & continental crust e e m ] n t e i 7 i u ] t r ) t i 3 i d 4 r ) a h n t 4 d p s u e r p n g o a e D. Castelli - R. Compagnoni - B. Lombardo - S. Angiboust - G. Balestro - S. Ferrando - C. Groppo - T. Hirajima - F. Rolfo Rolfo - F. Hirajima - C. Groppo T. Ferrando Angiboust - G. Balestro S. Lombardo - S. Castelli - R. Compagnoni B. D. c s ] ] 28 8 |O|(SiO 3 ) O 4 8 |((P,As,S)O 3 |Si 10 |(OH) |(SiO |CO 3 6 12 |O [BO 4 4 [(OH) 6 [(OH) Al 2 Al (OH) 14 2 2 ) ] [(OH) 4 □ 18 New minerals Al 3 (Mg,Ti) Al[PO 6 2 (Mg,Fe, e 2 Ca Mg n H i l (OH,F) e 4 Ubiquitous primary inclusions a t i (Mg,Ti)(Al,Mg) r m PO e e r y t 2 t i i u p t (Mg,Li) e o o a t d n

p a c e j a i s t i r v a a r d magnesiochloritoid wagnerite Mg Tab. 3 – Review of mineral inclusions in pyrope blasts from BIU of mineral 3 – Review Tab. whiteschists. Less common primary inclusions Probable retrograde minerals DOI: 10.3301/GFT.2014.03 Bearthite Ellenbergerite Phosphoellenbergerite Magnesiodumortierite Magnesiostaurolite kyanite >> rutile > zircon, monazite, phengite, talc, clinochlore, paragonite, glaucophane phlogopite (usually retrogressed to K-deficient or vermiculite), coesite Data from Chopin (1984), Schreyer (1985), Chopin (1986), Chopin & Klaska et al. Data from Chopin (1984), Schreyer (1985), et al. (1995), Simon al (1997), & (1986, 1991, 1994), Schertl et al. (1991), Compagnoni Chopin (2001). Chopin et al. (1993) Chopin et al. (1986) Brunet et al. (1998) Chopin et al. (1995), Ferraris (1995) Chopin et al. (2003) at the core of porphyroblasts (Fig. 25) and includes staurolite, chloritoid, kyanite, chlorite and paragonite (Fig. 25) and includes staurolite, chloritoid, kyanite, at the core of porphyroblasts (Compagnoni & Prp 2001). Prograde Hirajima, I (Prp mineral inclusions (Ferrando et al., 200). Prp et al., inclusions (Ferrando mineral 0 (Prp geological field trips 2014 - 6(1.3) itinerary 60 ) 1-16 Alm 82-98 ) constitutes the 4-11 Alm 86-67 constitutes the inner rim of megablasts and porphyroblasts and the core of neoblasts (Fig. 25), and it contains inclusions of coesite phengite, (Fig. 24b), kyanite, rutile, zircon, monazite and rare Mg-chlorite and talc, but never Prp III phlogopite. Early retrograde (Prp in Prp I are rarely observed, and observed, in Prp I are rarely only within porphyroblasts. Prp II (Prp Peak outer rim (Fig. 25) and usually it is pseudomorphically replaced by and phengite, biotite, talc, kyanite chlorite. It includes phengite, rutile and zircon. kyanite, Similar pyrope-bearing whiteschist been and/or quartzite have other reported from several localities within the BIU (e.g. in In addition to the Gilba Valley). inclusions described for prograde an locality, the Case Ramello almost pure magnesiochloritoid (up to 97 mole % end-member) Crust-mantle interactions during subduction of oceanic & continental crust D. Castelli - R. Compagnoni - B. Lombardo - S. Angiboust - G. Balestro - S. Ferrando - C. Groppo - T. Hirajima - F. Rolfo Rolfo - F. Hirajima - C. Groppo T. Ferrando Angiboust - G. Balestro S. Lombardo - S. Castelli - R. Compagnoni B. D. Fig. 25 - Simplified sketch (modified from Ferrando et al., 2009) showing the et al., (modified from Ferrando Fig. 25 - Simplified sketch (c) neoblast. Mineral abbreviations after Fettes & Desmons (2007). abbreviations after Fettes (c) neoblast. Mineral four generations of pyrope recognized in the UHP whiteschists near Case Ramello, of pyrope recognized four generations inclusions: (a) megablast, (b) porphyroblast, and mineral their chemical zoning DOI: 10.3301/GFT.2014.03 porphyroblasts (Fig. 25) and includes kyanite (Fig. 20c) (locally oriented), talc, phlogopite (usually (Fig. 25) and includes kyanite porphyroblasts 1986; Chopin & Langer, ellenbergerite (Fig. 24d) (Chopin, 1986; Chopin et al., retrogressed to vermiculite), phengite. Coesite/quartz 1988), Mg-chlorite, Mg-dumortierite, rutile, zircon, apatite and monazite, but never geological field trips 2014 - 6(1.3) itinerary 61 Crust-mantle interactions during subduction of oceanic & continental crust D. Castelli - R. Compagnoni - B. Lombardo - S. Angiboust - G. Balestro - S. Ferrando - C. Groppo - T. Hirajima - F. Rolfo Rolfo - F. Hirajima - C. Groppo T. Ferrando Angiboust - G. Balestro S. Lombardo - S. Castelli - R. Compagnoni B. D. and the virtual absence of Ca Na. 2 DOI: 10.3301/GFT.2014.03 was reported as prograde inclusion within pyrope by Simon et al. (1997). Rare corundum, enstatite, inclusion within pyrope by Simon et al. (1997). Rare reported as prograde was most likely been also described by Simon & Chopin (2001) to fill pyrope cracks sapphirine, and gedrite, have during early decompression. developed Origin of the whiteschist: metasomatism from ultramafics ratio, by an extremely high Mg/Fe The chemical composition of whiteschist is unusual because characterized high SiO Chopin (1984) considered the BIU whiteschists similar to “the highly magnesian, metaevaporitic whiteschists” Chopin (1984) considered the BIU whiteschists similar to “the highly magnesian, metaevaporitic been sedimentary and the (1977) and, then, its origin should have described and reviewed by Schreyer a metasomatic Nevertheless, been “a mixture of quartz, Mg-chlorite and illite”. probable protolith should have by many has been demostrated within the (meta)granitoid along ductile shear zones origin by fluid infiltration 2001; Schertl & Schreyer, 1997; Compagnoni & Hirajima, 1993; Gebauer et al., Sharp et al., authors (e.g., 2009). et al., 2009; Grevel et al., 2008; Ferrando the chemical composition of BIU whiteschist and its field relationships with host orthogneiss Moreover, 2012 for a review). are similar to those of other Mg-metasomatic rocks occurring in the Alps (see Ferrando, massifs (Chopin, Paradiso micaschists” occurring in the orthogneiss of Gran example, the “silvery For 1981; Chopin & Moniè, 1984) are considered by Compagnoni Lombardo (1974) as the HP metasomatic to the because of their close field association and systematic transition product of sheared granites surrounding orthogneiss. to timing of metasomatism and origin the metasomatic fluid. Sharp et al. From protolith, the debate moved (1993) and Sharp & Barnes (2004) interpreted the whiteschists as metasomatic rocks that originated by metasomatism influx of fluids from the subducting oceanic lithosphere. Gebauer et al. (1997) proposed a low P–T in the BIU whiteschists of rifting of the Piemonte-Liguria Basin. The discovery occurred during the Permo-Triassic pyrope- garnets with almandine-rich core—interpreted as relic of a former pelitic xenolith—and superzoned rare (2001) to propose that the metasomatic process occurred in presence rich rim allows Compagnoni & Hirajima Alpine metamorphism, close to the UHP climax. fluid phase during prograde of a hydrous a model for et al. (2009) constrained Ferrando In a recent petrological, geochemical and fluid inclusion study, the metasomatic formation of whiteschists and their subsequent UHP history (Fig. 26). An original post- metamorphic basement, undergoes an Alpine of the country Variscan including xenoliths granitoid, Variscan geological field trips 2014 - 6(1.3) itinerary 62 Crust-mantle interactions during subduction of oceanic & continental crust D. Castelli - R. Compagnoni - B. Lombardo - S. Angiboust - G. Balestro - S. Ferrando - C. Groppo - T. Hirajima - F. Rolfo Rolfo - F. Hirajima - C. Groppo T. Ferrando Angiboust - G. Balestro S. Lombardo - S. Castelli - R. Compagnoni B. D. Fig. 26 - Origin and evolution of the BIU Fig. 26 - Origin and evolution dehydration reactions involving talc and phlogopite. reactions involving dehydration whiteschists up to UHP peak (modified from Ferrando of garnet are 2009). The different generations et al., most Fig. 25). An original post-Variscan, shown (cf. including a paraschist granitoid, Permian, likely metamorphic of the country Variscan xenolith HP to UHP basement, undergoes an Alpine prograde stage A, an almandine-rich garnet metamorphism. At stages B and C, the At grows in the xenolith. high-Mg of high-salinity, along shear zones infiltration of antigorite in from dehydration fluids, generated subducting serpentinites, metasomatised the the to generate orthogneiss and the xenoliths the same time, metamorphic whiteschist. At and talc, muscovite reactions – involving dehydration Mg-chlorite – also occurred within the whiteschists. At assemblage that formed in the peak mineral stage D, the presence of an alumino-silicate aqueous solution originated by likely enriched in LILE, Th and U, DOI: 10.3301/GFT.2014.03 prograde metamorphism (P=1.6 GPa and metamorphism (P=1.6 GPa prograde T<600 °C) with the growth of almandine-rich A first garnet (Prp 0) in the xenoliths. occurred at metasomatic event prograde and 560–590°C, due to the 1.7–2.1 GPa influx of external fluids originated from the promoted the increase in Mg and antigorite breakdown in subducting oceanic serpentinites: this event a whiteschist composition. During the following prograde decrease of alkalies and Ca in the orthogneiss toward 590

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