B02_copertina_R_OK C August 20-28,2004 Florence -Italy

Field Trip Guide Book - B02 Pre-Congress Associate Leaders:G.Godard,Rebay Leader: S.Martin (ITALIAN WESTERN ALPS) IN THEAOSTA VALLEY THE SUBDUCTEDTETHYS 32 Volume n°1-fromPR01toB15 GEOLOGICAL CONGRESS nd INTERNATIONAL B02 28-05-2004, 18:50:22 The scientific content of this guide is under the total responsibility of the Authors

Published by: APAT – Italian Agency for the Environmental Protection and Technical Services - Via Vitaliano Brancati, 48 - 00144 Roma - Italy

Series Editors: Luca Guerrieri, Irene Rischia and Leonello Serva (APAT, Roma)

English Desk-copy Editors: Paul Mazza (Università di Firenze), Jessica Ann Thonn (Università di Firenze), Nathalie Marléne Adams (Università di Firenze), Miriam Friedman (Università di Firenze), Kate Eadie (Freelance indipendent professional)

Field Trip Committee: Leonello Serva (APAT, Roma), Alessandro Michetti (Università dell’Insubria, Como), Giulio Pavia (Università di Torino), Raffaele Pignone (Servizio Geologico Regione Emilia-Romagna, Bologna) and Riccardo Polino (CNR, Torino)

Acknowledgments: The 32nd IGC Organizing Committee is grateful to Roberto Pompili and Elisa Brustia (APAT, Roma) for their collaboration in editing.

Graphic project: Full snc - Firenze

Layout and press: Lito Terrazzi srl - Firenze

B02_copertina_R_OK D 20-05-2004, 9:14:49 Volume n° 1 - from PR01 to B15

32nd INTERNATIONAL GEOLOGICAL CONGRESS

THE SUBDUCTED TETHYS IN THE AOSTA VALLEY (ITALIAN WESTERN ALPS)

AUTHORS: S. Martin (Università dell’Insubria, Como - Italy) G. Godard (Université Paris VII, Paris - France) G. Rebay (Università di Pavia - Italy)

Florence - Italy August 20-28, 2004

Pre-Congress B02

B02_R_OK A 20-05-2004, 9:25:51 Front Cover: Cervino/matterhorn

B02_R_OK B 20-05-2004, 9:25:53 THE SUBDUCTED TETHYS IN THE AOSTA VALLEY (ITALIAN WESTERN ALPS) B02

Leader: S. Martin Associate Leaders: G. Godard, G. Rebay

Introduction whereas the second group will come across the The fi ve-day excursion “The subducted Tethys in Zermatt-Saas meta-ophiolites towards the overlying the Aosta valley (Italian western Alps)” presents Sesia-Lanzo unit (i.e., Adria microplate), down to an overview of the eclogitized ophiolites of the Lago Gabiet (2342 m). Take away lunch to be eaten Zermatt-Saas unit, focusing on the relationships on the way. Return to Gressoney valley by cableway. and metamorphic features of different lithologies Bus to Lillianes (eclogite-facies rocks of the Sesia- (serpentinite, metagabbro and metabasalt, Mn- Lanzo unit), and then to Collegio Gervasone at rich quartzite, hydrothermal sulphide deposits and Châtillon (dinner and discussion). metasediments). These rocks underwent high- to Third day: Departure for the Saint-Marcel valley, by ultrahigh-pressure metamorphism during subduction minibus. Walk to the Servette Fe-Cu mine, with a visit of the Tethyan ocean, and developed peculiar high- of the ancient foundry and slag heaps, and then on to pressure peak to decompressional assemblages the Praborna Mn mine (high-pressure metamorphism according to their composition. of hydrothermal oceanic deposits). Return through the First, in order to acquire a general view of the main abandoned mine village of Chuc and the unusual Cu Alpine nappes and their lithologies, we will cross hydroxide deposit in the river at Acqua verde. Take the Tethys ocean suture (the Zermatt-Saas and away lunch; dinner at Gervasone (Châtillon). Combin units of the ophiolitic Piedmont Zone) from Fourth day: Departure by minibus for Cignana the former European passive continental margin (coesite-bearing occurrence in the Zermatt-Saas meta- (now represented by the Monte Rosa nappe) to the ophiolites). Walk from Lago di Cignana (2158 m) Adria (Africa) continental margin (Sesia-Lanzo and down to the eclogitized oceanic metagabbros Dent Blanche nappes). We will then visit some of of Crepin (1577 m). Take away lunch; dinner at the most typical outcrops of Alpine geology in the Gervasone (Châtillon). NW Alps: (i) the Saint-Marcel Fe-Cu hydrothermal Fifth day: Departure for Cervinia by bus and then sulphide deposits; (ii) the famous Praborna Mn by cableway up to Plateau Rosa, for a breathtaking mine with its unique high-pressure minerals; (iii) view of the Cervino (Matterhorn) peak and the the Cignana coesite site and the Crepin metagabbro Africa/Europe collisional zone. On 19th August in the (Valtournanche). Attention will be especially focussed afternoon, travel to Florence by bus. on the eclogitized ophiolites and hydrothermal oceanic deposits. In addition, historical aspects, Weather can be from warm to quite cold (near or Alpine views and regional setting will be taken in below 0°C). Bring warm, waterproof clothes and consideration. The fi eld trip will be concluded with mountain shoes; walks will be on marked paths in an the breathtaking view from Plateau Rosa, south alpine environment (up to 3500 m); altitude changes of Cervino (Matterhorn), where we will be able to of up to 900 m per day. summarise the regional relationships of the collisional nappe stack and the ophiolitic suture (remnants of the Regional geologic setting “lost Tethyan ocean”), and will have the opportunity On the northern side of the Aosta valley, the Alpine to enjoy some last eclogite outcrops. collisional wedge (the Penninic-Austroalpine nappe First day: Meeting on the 15th August 2004 at the stack) is characterized, from bottom to top, by: (i) railway station of Novara, from where we will leave the eclogite-facies Monte Rosa nappe (the European at 13:00 for Gressoney (arrival time at 15:00). From continental margin), (ii) the eclogitic Zermatt-Saas there, we will take a cableway up to Passo dei Salati meta-ophiolite, overlain in turn by (iii) a few eclogitic (2950 m; panorama). After a short walk across the Austroalpine slices (outliers) and/or a Permian- PR01 to B15 n° 1 - from Volume contact between the Monte Rosa and Zermatt-Saas Mesozoic decollement cover unit of debatable units (i.e., the European plate and Tethys ocean, continental origin (the Pancherot-Cime Bianche unit), respectively), we will reach Rifugio Città di Vigevano (iv) the blueschist-facies Combin meta-ophiolite, (2864 m) for dinner. and (v) the capping upper Austroalpine units (Adria/ Second day: One group will visit the Zermatt-Saas Africa continental margin), consisting of the eclogite- meta-ophiolites and the underlying Monte Rosa facies Sesia-Lanzo inlier and blueschist-facies Dent basement, up to Rifugio Città di Mantova (3500 m), Blanche-Mont Mary-Pillonet klippen (outliers)

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Volume n° 1 - from PR01 to B15 B02 - B02 Leader: S.Martin Ponton, VP= Valpelline unit,ZP=Piedmont zone. Rosa, Pi=Pillonet,SB=GranSanBernardonappe,S=Santanel,SC=Sion-Courmajeur, SL=Sesia-Lanzo,TP=Tour Gran Paradiso, SL=Sesia-Lanzo,IIDK=IIDiorite-kinzigitic zone,LS=Simplonline,ML=MonteLeone,MR= GR= Glacier-Rafray, FR=Fobello-Rimella unit,LA,LS= Simplonline,LC=Canavese line,MM=MontMary, GP= CH= Chatillonslice,CMB=Cossago-Mergozzo-Brissagoline,CR=Cremosina line,E=M.Emilius,EL=Etirol-Levaz, B=Berisal, line, meta-ophiolites, AR=Aosta-Ranzola slice, An=Antrona Legend: A=Antigorio,AB=Arcesa-Brusson Grey colour:ophioliticunits,whitebasementunits. Figure 1-Tectonic map andblock-diagram ofthe Aosta valleyand surroundingareas,NW Alps. 20-05-2004, 15:31:40 THE SUBDUCTED TETHYS IN THE AOSTA VALLEY (ITALIAN WESTERN ALPS) B02

Figure 2 - Cross section and lithospheric interpretation of the Western Alps, after Canepa et al., 1999. PL=Periadriatic Line; PF=Penninic frontal thrust; ophiolitic units are in black; MR= Monte Rosa nappe; SB=Grand St. Bernard system; H=Helvetic zone; HF=Helvetic frontal thrust.

(Ballèvre et al., 1986; Bigi et al., 1990; Dal Piaz, main units derived from these pre-Alpine protoliths: (i) 1999; Dal Piaz et al., 2001). a pre-granitic basement composed of a Variscan, high- In this introduction, we present the features of grade, gneissic complex; (ii) Upper Carboniferous the main nappe systems to be visited during the and/or Lower Permian granite-granodiorite plutons; excursion, namely the Monte Rosa nappe (inner (iii) remnants of Permian-Mesozoic sedimentary Penninic domain, i.e., European continental crust), cover and the composite Furgg zone (Bearth, 1956). the Zermatt-Saas and Combin meta-ophiolites (i.e., The pre-granitic basement is composed of paragneiss, the Tethys ocean), and, fi nally, the Sesia-Lanzo and micaschists, migmatite and interlayered metabasites Dent Blanche nappes (Austroalpine domain, i.e., the (Bearth, 1952; Dal Piaz, 1966, 1971; see references Adria microplate). in Dal Piaz, 2001a, and Engi et al., 2001), which have undergone high-T low-P metamorphic conditions The European continental crust during the Variscan orogeny, producing K-feldspar (Monte Rosa unit) and cordierite-bearing migmatite and pegmatite The Monte Rosa nappe lies at the junction between (e.g., Engi et al., 2001). They were sharply intruded the Central and the Western Alps. In terms of tectonic by Upper Carboniferous granodiorites and Permian position, it is situated at a level corresponding to the granites (Hunziker, 1970; Liati et al., 2001; Scherrer Suretta nappe, to the east, and to the Gran Paradiso et al., 2001). The whole Variscan basement was and Dora Maira nappes, to the southwest. In Argand’s deformed and metamorphosed during Alpine orogeny, view, Monte Rosa forms the fi fth recumbent fold- under eclogite-facies high-P conditions (typically nappe of the Penninic system (Lugeon & Argand, 500°C and 1.6 GPa: Chopin & Monié, 1984; Dal 1905; see Dal Piaz, 2001a, for a historical review), Piaz & Lombardo, 1986). Recently, Le Bayon et al. which likely derived from the European margin. The (2001) obtained a pressure estimate as high as 2.3 Penninic domain includes several stacked nappes GPa for some whiteschists from an Alpine shear zone

(Figure. 2): (i) the upper Monte Rosa [MR], Gran of the Ayas valley. According to recent radiometric PR01 to B15 n° 1 - from Volume Paradiso and Dora Maira nappes; (ii) the mid nappe determinations, the eclogite-facies peak of the Monte of the Grand Saint-Bernard [SB]; (iii) the lower Rosa unit is nearly contemporaneous to the peak Lepontine nappes of the Ossola-Ticino; (iv) the Valais of the Zermatt-Saas eclogites and coesite-bearing domain (lower/outer Penninic), composed of fl ysch metasediments (Middle Eocene: Rubatto et al., 1998; and minor meta-ophiolites (e.g., Ballèvre & Merle, Rubatto & Gebauer, 1999; Pawlig & Baumgartner, 1993; Dal Piaz et al., 2001). 2001). Finally, a mesoalpine (Late Eocene-Early The Monte Rosa nappe may be subdivided into three Oligocene: 38-35 Ma) overprint from greenschist- to

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Volume n° 1 - from PR01 to B15 B02 - B02 Leader: S.Martin sliced anddismemberedduringtheplatemargin in of Europeand Adria (Africa)(seehistoricalreview Jurassic betweenthepassive continentalmargins branch ofthe Tethys thatopened intheMiddle-Late the oceaniclithosphereofLiguro-Piedmont plate tectonics,they are consideredasderived from the Penninicdomain.Sincedevelopment ofthe Argand (1916)attributed thesemeta-ophiolitesto Western Alps). to thelowest andoutermost Versoyen unit(inthe from theuppermostPlatta-Arosaunit(Central Alps) different structurallevels ofthe Alpine nappe pile, numerous metamorphicunits,whicharescatteredat Cambrian metagabbros(Liati from Permiangranitoidsand/orvolcanic rocks, leucocratic gneissandmelanocraticrocks,derived zone withaheterogeneousassociationofparagneiss, The Furgg zonehasbeendefi the wholeMonteRosanappe(Bearth,1958). amphibolite-facies metamorphicconditionsaffected up totheCentral Alps (Bigi Nappe extend alongtheentirearcof Western Alps The meta-ophiolitesofthecompositePiedmont The Zermatt-Saasmeta-ophioliteunit Antrona synform). the mesoalpineevent ( foliations werefurtherfoldedbylarge foldingduring retrogression (Wheeler&Butler, 1993). These two exhumation-related shearing,accompaniedby by aS2foliationdeveloped duringtheMonteRosa blueschist-facies mineralassemblages,was followed discontinuously recordedandmarked byeclogite-or The subduction-relatedearly Alpine foliation(S1), al. turn fromcontinentaltholeiiticbasalts(Ferrando or oldermafi Monte Rosabasement,arerepresentative of Variscan boudins, occurringonthesouthernItaliansideof Piaz, 2001a). The eclogitictoretrogressedmafi together withthepolymetamorphicbasement(Dal levels ofthenappe,closelyassociatedtoandfolded identifi the northernborderofmassif,itwas subsequently p. 295).Initiallyobserved andmappedbyBearthat a tectonicmelange(seereview inDalPiaz,2001b, Monte Rosabasement,whereasothersinterpretitas authors, thiszonemightrepresentthecover ofthe Bearth, 1954;Keller &Schmid,2001).For afew Mesozoic rocksintrudedbybasicdykes) ( boudins inmarbles(presumablyderived from DalPiaz,2001b). This oceaniclithospherewas , 2002). ed southwards, aswellatvarious structural c granulitesoramphibolites,derived in e.g., the Vanzone antiformand et al. ned asahigh-strain et al. , 1990). They form , 2001),mafi e.g., et c c

Beccaluva affi Mellichen, Crepin),metabasaltswithN-MORB with magmaticmineralortextural relics( abundant ophicalciticbreccias,minormetagabbro serpentinite fromlocallypreserved mantleperidotite, The rocksoftheZermatt-Saasunitarepredominantly section below. those oftheupperCombinunitaredescribedina the lower Zermatt-Saasmeta-ophiolites,whereas two units.Here,weonlyemphasizethefeaturesof and structuralcriteriatodistinguishbetweenthese ( discrimination hasbeenacceptedbymany authors Zermatt-Saas (mainlyeclogite-facies) units. This units, namelytheCombin(blueschist-facies) and (1967) andKienast(1973)distinguishedtwo main meta-ophiolites, DalPiaz(1965,1974),Bearth On thebasisofmetamorphicevolution ofthese Rubatto fi Penninic / Valais basins)hasbeenenvisaged sincethe branches (South-Penninic/PiedmontandNorth- a singlePiedmontoceanortwo separatedoceanic to theirpartialexhumation. The occurrenceofeither and continentalcrustsunderthe Adriatic margin and convergence thatledtothesubductionofoceanic Tethyan ocean( metasediments derived fromtheinternalpartof 1978). thought toderive fromanoceanichydrothermal Kienast, 1987;Mozgawa, 1988). This depositis silicates ( the oredepositincludesvery peculiarMn-bearing is byfar themostimportantandfamous occurrence, Praborna (Saint-Marcel;seeStop3.7below), which braunite, piemontite,spessartine(Castello,1981). At metamorphosed boudinagedquartzitesrichin et al. the Täsch area(Zermatt-Saas,Switzerland: Widmer 3.4 and3.5below) andChamp-de-Praz,but alsoin Aosta valley, noticeablyatSaint-Marcel(seeStops Zermatt-Saas unit. They arelocatedinthesouthern glaucophanite, chloriteschistsandtalcschists)inthe metabasalts withstrongoceanicalteration(garnet Fe sulphidedepositsarewithinhigh-pressure sediments (metacherts). The mostimportantCu- located withinmetabasalts,thelatterinsiliceous ( occurrences ofFe-CusulphideandMnoredeposits to oceanichydrothermalout-fl The meta-ophiolitesthatwereoriginallycloser rst work ofSturani(1973)tothelastcontribution of e.g., e.g., nity(DalPiaz Elter, 1971),whoalsousedlithostratigraphic DalPiaz&Omenetto,1978). The formerare , 2000). The Mndepositsoccurmainlyas et al. e.g., et al. (1998),whotriedtodatebothoceans. Martin-Vernizzi, 1982;Martin & e.g., , 1984;Pfeiffer et al. Bearth,1967;Ernst&DalPiaz, , 1979b;DalPiaz ow zonesshow et al. , 1989)and e.g., et al. 20-05-2004, 15:31:44 Allalin, , 1981; THE SUBDUCTED TETHYS IN THE AOSTA VALLEY (ITALIAN WESTERN ALPS) B02

system, or accumulation of Mn-rich oceanic nodules Tartarotti, 1989). and “umbers”, as evidenced by high Sb, Sr and Ba contents (Perseil, 1988; Perseil & Smith, 1995; The intermediate continental slices Tumiati, personal communication). On the northern side of the Aosta valley, the ophiolitic The effect of oceanic hydrothermalism and alteration Zermatt-Saas (below) and Combin (above) units on the basic rocks is also evidenced by abnormal are discontinuously separated by a thin slice of a contents of various elements (Na, OH, Mg, Ca) Permo-Mesozoic sedimentary sequence (Pancherot- (Beccaluva et al., 1984; Barnicoat & Bowtell, 1995; Cime Bianche: Dal Piaz, 1999, and refs. therein). Martin & Cortiana, 2001) and the scattering of δO18 This slice is composed of albite-bearing quartzitic values (Cartwright & Barnicoat, 1999). schists (Permian), conglomerate (Verrucano), tabular The subduction and exhumation history of the quartzite (Lower Triassic), limestone and dolostone Zermatt-Saas unit is marked by such prograde relics (Middle-Upper Triassic), polygenic sedimentary as pseudomorphs after lawsonite, eclogite-facies breccias (Jurassic rift) and calcschists (Cretaceous?), assemblages and decompressional retrogression. which have been interpreted as being deposited on The Zermatt-Saas rocks that crop out north of the a thinned continental margin or on an extensional Aosta-Ranzola fault (i.e., in the northern part of the allochthon (Mt. Emilius?) trapped inside the ocean Aosta valley), gave the highest P-T estimates for (Dal Piaz, 1999, and references therein). These extra- the peak metamorphism (e.g., Meyer, 1983; van der oceanic sediments were metamorphosed during the Klauw et al., 1997; Reinecke, 1998), with values as Alpine events, but they still conserve some fossils high as 2.7-2.9 GPa and 600-630°C for the coesite- (Kienast, personal communication). bearing metasediments of Cignana (Reinecke, 1998; Moreover, some slices of cover-free eclogite-facies see Stop 4.2), whereas metabasites from the southern continental crust are known along the tectonic contact part yielded relatively lower P-T conditions (e.g., and metamorphic gap between the Zermatt-Saas and Mottana, 1986; Martin & Tartarotti, 1989), typically Combin meta-ophiolites, in the lowered northern 2.0±0.3 GPa and 550±50°C (Servette: Martin et al., hangingwall of the Aosta-Ranzola normal fault 2004; see Stop 3.4). system (Bistacchi et al., 2001). The most important The formation of the Zermatt-Saas oceanic crust is are the Etirol-Levaz (Kienast, 1983; Ballèvre et al., attributed to the Jurassic (164-153 Ma: Rubatto et 1986), Châtillon, and Saint-Vincent slices. They are al., 1998). Geochronology yielded a range of ages also known as lower Austroalpine outliers (eclogitic) between 52 and 43 Ma (Eocene) for its subduction because they are located at a structural level that metamorphism, depending on the technique used is lower than both that of the Sesia-Lanzo inlier (Botwell et al., 1994; Barnicoat et al., 1993; Rubatto and the Dent Blanche-Mont Mary-Pillonet upper- et al., 1998; Mayer et al., 1999; Dal Piaz et al., 2001). Austroalpine outliers that override the Combin unit. The different results may correspond to different steps In the southern footwall of the Aosta-Ranzola fault of the P-T path between the peak conditions and the system, the Combin unit has been eroded and the retrogression below 500°C. preserved top units are represented by numerous The structure of the Zermatt-Saas meta-ophiolite in eclogite-facies lower-Austroalpine outliers (or the Saint-Marcel valley and Monte Avic massif is intermediate basement slices), which occur over generally characterized by a N-S-trending lineation (Mt. Emilius) or inside (Glacier-Rafray, Tour Ponton, parallel to the axes of isoclinal folds, and related to Acque Rosse) the Zermatt-Saas meta-ophiolite. a D2 deformation phase that occurred under eclogite There, only the Santanel slice seems to lie between facies (Tartarotti, 1988; Martin et al., 2004). Relics the Zermatt-Saas and Combin units. of an earlier prograde deformation (D1) have been These intermediate or lower Austroalpine continental recognised only in the core of garnet crystals. The slivers are mainly made up of pre-Alpine high-grade D2 foliation is further folded by a D3 deformation paragneiss, marbles, granitoids and continental PR01 to B15 n° 1 - from Volume phase with axes still oriented N-S. An E-W-trending gabbro bodies (Mt. Emilius, Etirol-Levaz), which D4 regional tectonic phase developed under the have undergone an eclogite-facies metamorphism greenschist facies (e.g., Elter, 1960; Ballèvre, 1988). and greenschist-facies retrogression (see Dal Piaz, South dipping fault planes belonging to the Aosta- 1999; Dal Piaz et al., 2001). Kienast (1983) and Ranzola normal fault system (Bistacchi & Massironi, Ballèvre et al. (1986) obtained an estimate of 550°C 2000), locally reactivated as N-vergent thrusts, and 1.6-1.7 GPa for the peak metamorphism of the represent the last deformation episode, D5 (Martin & Etirol-Levaz slice. Although the metamorphic history

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Volume n° 1 - from PR01 to B15 B02 - B02 Leader: S.Martin zona Dioritico-Kinzigitica and (iii)some facies Franchi, 1900,1902),(ii)theexternal greenschist- marble, quartziticschistsandmafi The Combinunitconsistsofcalcschists,impure The Combinmeta-ophioliteunit Saas meta-ophiolite. Lanzo domain,but roughly thesameasZermatt- al. of theirhigh-Pmetamorphism(40-49Ma:DalPiaz to thatoftheSesia-LanzoZone(seebelow), theage of thesebasementslivers ismore-or-less comparable oxides, sulphidesandtourmaline)(DalPiaz metacherts anddisseminatedoredeposits(Cu-Fe- rock geochemistryandthepresenceofMn-rich oceanic hydrothermalismisdocumentedbywhole common intheunderlyingZermatt-Saasunit. An eclogite-facies assemblagesthat,incontrast,are blueschist-facies relics,withouttracesofthe facies overprint andpreserves someepidote- meta-ophiolites. Itdisplaysapervasive greenschist- (Delaloye &Desmons,1976; Ayrton 400°C arevery scattered,rangingfrom49to30Ma (Martin &Cortiana,2001).Coolingagesbelow internal The Sesia-Lanzozonemainlyconsistsof(i)the promontory. interpreted asslicesofthe Adria microplateor African of platetectonics,theseunitshave been widely et al. (Argand, 1916;Stutz&Masson,1938;Compagnoni traditionally groupedastheDentBlanchenappe (ii) numerousexternal slicesofcontinentalcrust belt boundedtotheeastbyCanavese fault, and Lanzo internalzone,a90-km-longand25-kmwide Valley, bytwo fi The Austroalpine domainisrepresented,inthe Aosta (Sesia-Lanzo andDentBlanchenappes) The Adria continentalcrust Saas unit. These agesarequitesimilartothoseoftheZermatt- Tournalin bythe obtained foraCombinsodicamphibolefromGrand A crystallisationorcoolingageof43.0±0.3Mawas 2001). and 350-400°C(Sperlich,1988;Martin&Cortiana, facies paragenesishave beenestimatedat0.5-0.7GPa P-T conditionsforthedevelopment oftheblueschist- 1979a; Castello,1981;Martin&Cortiana,2001). The , 2001)is20-25MayoungerthanthatintheSesia- , 1977).Sincethedevelopment ofthetheory Gneiss minuti Micascisti eclogitici klippen rst-rank units,namely(i)theSesia- 40 complex (Gastaldi, 1871-1874) Ar/ ofthe Adria lower crust( 39 Ar totalfusiontechnique ). complex (Stella, 1894; c toultramafi et al. , 1982). e.g., et al.

et c II , basement (Ayrton Dolin) andhasbeenmetamorphosedtogetherwiththe Blanche-Mont Marysystem(Roisanzone,Mont conglomerate, etc.islocallypreserved intheDent quartzite, marble,dolostone,sedimentarybreccia, A Mesozoicsedimentarycover, composedof of theCervino/Matterhorn:DalPiaz Diehl Lardeaux &Spalla,1991),kinzigiticgneisses( granitoids ( et al. of serpentinizedmantleperidotite( of felsicandmafi The pre-Alpine Adriatic crustwas mainlycomposed Merle, 1993;DalPiaz,1999). The DentBlanchenappe( & Martin,1988;Canepa units (Diehl Dent Blanche( different structuralposition:(i)theblueschist-facies by acontrastingsubductionmetamorphismand two maingroupsofunitswhicharecharacterized or insidethelatter(Ballèvre located betweentheCombinandZermatt-Saasunits, Rafray, Etirol-Levaz andothersimilarunits, outliers, representedbytheMt.Emilius,Glacier- 1999); (ii)theeclogite-facies lower Austroalpine be reportedasupper Austroalpine outliers(DalPiaz, entire ophioliticPiedmontZoneandthereforemay 2001), which,like theSesia-Lanzozone,override the amphibole mafi and thenphengite-jadeiteorthogneissNa- micaschists (DentBlanche:Kienast&Nicot,1971), paragenesis. The they have preserved ratherwelltheeclogite-facies Lanzo zoneareofparticularinterest,because The metagranitoidandmetapelitesoftheSesia- Gneiss Arolla transformed into Adria basementduringtheEarlyPermian,they were As fortheleucocraticgranitoidsthatintruded to Mary, Pillonet)conditions,giving rise,respectively, (the upper Austroalpine outliers:DentBlanche,Mont and lower Austroalpine outliers)orblueschist-facies metamorphosed ineclogite-facies (theSesia-Lanzo metamorphic andigneousbasementwas During the Alpine orogeny, thehigh-grade 1995). been describedintheSesia-Lanzozone(Venturini, Similarly, ametamorphosed sedimentary cover has and studiedbyStella(1894)Franchi(1900, micascisti eclogitici , 1989),marbles,abundant UpperPaleozoic et al. , 1952;Canepa e.g., et al. (DentBlanche c boudins(Pillonet:DalPiaz,1976). s.s. Arolla) andgabbros( , 1952; Ayrton micascisti eclogitici Gneiss minuti et al. ), MontMaryandPillonetthrust c granulites( ( , 1982;Canepa auct et al. et al. .) andchloritoid-bearing s.l. et al. s.l. , 1990;DalPiaz ) issubdivided into ). et al. , 1990),afew slices (Sesia-Lanzo)and , 1986:Ballèvre & e.g., , 1982;DalPiaz Nicot,1977; , fi et al. e.g., et al. e.g. rt defi rst thebase , Cesare , 1977). 20-05-2004, 15:31:47 , 1990). et al. ned e.g., ,

THE SUBDUCTED TETHYS IN THE AOSTA VALLEY (ITALIAN WESTERN ALPS) B02

1902), are coarse-grained micaschists, with quartz, Field trip itinerary phengite, paragonite, large garnet crystals, omphacite, glaucophane, chloritoid and rutile (e.g., Lillianes, see DAY 1 Stop 2.7). Some granites gave rise to the famous “Contact between Europe (the Monte Rosa unit) eclogite-facies rocks, with jadeite, quartz, phengite, and the Tethys ocean (the Zermatt-Saas meta- and garnet (e.g., Monte Mucrone: Compagnoni & ophiolites)” Maffeo, 1973; Compagnoni et al., 1977; Oberhänsli et al., 1982, 1985; Rubbo et al., 1999). From the meeting point at the Novara railway station, In the Dent Blanche nappe, the blueschist-facies P-T we leave at 13:00 for Val di Gressoney, where we will conditions were estimated at 400-550°C and 0.7-0.8 take a cableway up to Passo dei Salati (N 45° 52.617’; GPa (Kienast & Nicot, 1971; Cortiana et al., 1998), E 7° 52.069’; alt. 2936 m). If we arrive early, we whereas higher P-T-values have been obtained for the shall go uphill from the top of the cableway towards Sesia-Lanzo eclogite-facies rocks (e.g., 550±50°C Stolemberg Peak. and 1.4-2.1 GPa: Oberhänsli et al., 1985; Inger et al., 1996; Tropper et al., 1999). Stop 1.1: The peculiarity of the Sesia-Lanzo nappe and of the Tectonic contact between Monte Rosa gneiss and upper Austroalpine outliers is the relatively old age Zermatt-Saas ophiolite, at the klippe of Punta of their high-P metamorphism, which is dated as Late Stolemberg (N 45° 52.762’; E 7° 51.936’; alt. Cretaceous (110±13 Ma, Jäger et al., 1990; 69.2±2.7 3068 m) Ma, Duchêne et al., 1997; 65±5 Ma, Rubatto et al., The Stolemberg peak is a beautiful example of a klippe 1998). A similar age was obtained for the Pillonet of Zermatt-Saas ophiolitic metabasalts resting on the klippe (75-73 Ma: Cortiana et al., 1998). These ages Monte Rosa basement. On the slopes of the peak, we have been interpreted as the crystallization age of the fi rst observe typical Monte Rosa micaschists, with eclogite-facies paragenesis at a depth of 50-100 km. Alpine imprint and some white aplitic gneiss. At a In the Sesia-Lanzo zone, Gosso (1977) and Gosso platform (N 45° 52.698’; E 7° 51.983’; alt. 3043 m), et al. (1979) identifi ed four Alpine deformation some metabasic lenses occur within the Monte Rosa phases (D1-D4), on the basis of fi eld interference gneisses, which display an almost horizontal fabric structures and microstructures. The D1 phase was (foliation 012/11). A little further on, going uphill, we coeval to the eclogite-facies event and produced an observe the contact between the Monte Rosa gneiss almost complete transposition, whereas D2-D4 are and the Zermatt-Saas meta-ophiolites (N 45° 52.762’; post-nappe deformation phases, which occurred at E 7° 51.936’; alt. 3068 m). Large amphibole, talc different steps of the retrogression or during the late and chlorite reaction rims develop between gneiss mesoalpine greenschist-facies overprint. and serpentinite (the pathway can be slippery). The tectonic contact between the Austroalpine domain The Stolemberg meta-ophiolites mainly consist of (i.e., the Sesia-Lanzo and Dent Blanche nappes) over banded albite-bearing amphibolite, with garnet relics, the Piedmont meta-ophiolites (i.e., the Zermatt-Saas deriving from former eclogite. Some eclogite relics, and Combin units) is locally outlined by Permian which escaped subsequent retrogression, consist of gabbro bodies (e.g., Dal Piaz et al., 1977; Zanella, omphacite, garnet and amphibole. 1992) that were strongly deformed under blueschist We walk along the contact. As this one is folded, we or greenschist-facies conditions during accretion come across different lenses of Monte Rosa gneiss or exhumation (e.g., Monte Pinter, Monte Tantané, and meta-ophiolite. We thus fi nd several repetitions Cervino/Matterhorn, and Moussallion). Therefore, of the two units. it seems that the thrust plane reworked pre-existing normal shear zones of the Adria crust, along which Stop 1.2: PR01 to B15 n° 1 - from Volume the gabbro bodies had been exhumed and laterally Panoramic viewpoint (N 45° 52.934; E 7° 51.874) juxtaposed to coeval shallower intrusions (Dal Piaz, We reach a point (N 45° 52.934; E 7° 51.874), from 1993). In contrast with the Monte Rosa/Zermatt-Saas where we have a sweeping view, but the path becomes contact, which is folded, the Dent Blanche/Combin diffi cult (very steep and with ropes), so we stop here. contact is generally straight and regular. Looking towards the south, we see closest to us Corno Rosso and Lago Gabiet, which belong to the Combin ophiolitic unit, and, in the distance, a ridge with the

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Volume n° 1 - from PR01 to B15 B02 - B02 Leader: S.Martin Figure 3-Itineraryofdays1and2.Notethatthepaths for different groupsaretracedwithdifferent symbols. 20-05-2004, 15:31:50 THE SUBDUCTED TETHYS IN THE AOSTA VALLEY (ITALIAN WESTERN ALPS) B02

Figure 4 - Folded contact between the Monte Rosa Basement and Zermatt Saas ophiolites at Mt. Stolemberg.

Straling and Corno Bianco peaks which belong to in the Monte Rosa massif (e.g., Curti, 1987; Lattanzi, the overlying Sesia-Lanzo zone (Gneiss minuti). The 1990; Pettke et al., 1999), noticeably near Alagna (Val folded contact between the Sesia-Lanzo gneiss and Sesia), where they have been mined for centuries. The the meta-ophiolites is visible on the western side of ore is related to the mesoalpine hydrothermal activity the Gressoney valley, from Monte Pinter (to the south) (Stella, 1943). to Testa Grigia and Monte Rothorn. Further north, the Combin unit occurs between Passo del Rothorn We return downhill. After passing the top of the and Passo Bettaforca, where there is the tectonic cableway, we descend a few metres and leave the contact between the Combin and Zermatt-Saas meta- path, moving some tens of metres to our right, below ophiolites. The latter unit crops out on the southern Corno Camoscio. slopes of the Monte Rosa massif. North of a line going from Monte Rosso to Lago Bleu, all the snowy peaks Stop 1.3: of Monte Rosa consist of Penninic gneiss. The contact between Monte Rosa gneisses and At the base of the cliff, at about 300 m in the direction Zermatt-Saas ophiolites at Corno Camoscio (N 45° of N240, one can see the ruins of an old gold mine 52.481’; E 7° 52.136’; alt. 2952 m) (19th-20th centuries; Lorenzini, 1998, p. 157-158). At the base of Corno Camoscio, which consists Quartz sulphide gold-bearing veins are quite abundant mainly of serpentinite, we can observe again the Volume n° 1 - from PR01 to B15 n° 1 - from Volume

Figure 5 - Stolemberg-Col d’Olen section. The south-dipping tectonic contact between the Monte Rosa basement and the Zermatt- Saas meta-ophiolites is represented. The Monte Rosa-Zermatt Saas infolding is not evidenced. Legend: MR= Monte Rosa, ZS= Zermatt-Saas, Co=Combin, e=eclogite, g= metgabbro, c=calcschists, s= serpentinite.

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Volume n° 1 - from PR01 to B15 B02 - B02 Leader: S.Martin cm indiameter)thatarewrappedthepre-Alpine melanosome). Notethelarge garnetcrystals(upto2 rocks (leucosome)andmica-rich(former thick alternatinglayersofquartz+feldspar-enriched Monte Rosagneisses,consistingof3-to-20-cm- alt. 2902m). A 2-m-highsectionshows typical few metrestotheleft(N45°52.469’; E7°52.308’; Mosso, leave thepathtoreachan outcropthatisa Where thepathway passesnearIstituto 52.469’; E7°52.308’;alt.2902m) and gneiss Monte Rosafolded garnet micaschists Stop 1.4: Vigevano. pathway thatleadstoRifugioCittàdi Go downhill andreturnalongthe Corno Camoscio. the myloniticgabbroandserpentiniteof serpentinite. Furtheruphill,onecansee between micaschists(orgneiss)and of thereactionrimsthatdeveloped tremolite +talcepidote±garnetrocks, is typical,togetherwithchlorititeand and abundant tremolitite. This latter rock observe garnet+chloritoidmicaschists units (N45°52.481’;E7°52.136’;alt.2952 m), we contact betweentheMonteRosaandZermatt-Saas shows pseudotachylites. A few metresup,atthe and variously folded.Somedebrisofthesegneisses with mafi leucocratic quartz+feldspar-rich layersalternating Rosa gneissesconsisthereofbandedrocks,with belonging totheZermatt-Saasophiolites. The Monte folded contactofMonteRosagneissandmetabasites = Zermatt-Saas. eclogite–facies Zermatt-Saasmeta-ophiolites.For legend,seethepreviousfi North ofMt.Bettaforca, atColleBettaforca, theCombinmeta-ophiolitesareintectoniccontactwithunderlying by theinfolding ofcalcschists andprasinite(Combinmeta-ophiolites)byGneissMinuti(theSesia-Lanzounit). Figure 6-M.Pinter-Testa Grigia-M.Rothorn-ridgeontherightsideofGressoneyvalley. Theridgeischaracterized c layers(5cmthick)thatareboudinaged nearIstitutoMosso(N45° and geology. Itwas recentlycompletelydestroyed by physiology athighaltitudes,meteorology, glaciology For dinner, reachRifugioCittàdi Vigevano (N45° again someMonteRosagneissandophioliticslices. Return tothepathway andgodownhill, crossing a fi is devoted toscientifi managed bythe Torino University, theIstitutoMosso Inaugurated byQueenMargherita in1907and gneisses. right, aboudin(1x3m)ofmetabasiteisfoldedwithin contrasted behaviour of thevarious layers.Onthe foliation andthedisharmony offolds,duetothe Figure 7- Alternating layers ofquartz+feldspar-enriched re, but isnow underreconstruction. rocks (leucosome) andmica-rich rock Axes oftheasymmetric folds areoriented280/12. gures. CO=Combin,SLSesia-Lanzo,ZS c researchinthefi s (melanosome). ed of elds 20-05-2004, 15:31:55 THE SUBDUCTED TETHYS IN THE AOSTA VALLEY (ITALIAN WESTERN ALPS) B02

52.251’; E 7° 52.413’; alt. 2865 m), built in 1914, visible near a small lake. Looking towards Corno where we will illustrate the next day’s excursions. Camoscio, a 3-m-high ridge is visible (N 45° 52.321’; E 7° 52.371’; alt. 2861 m). It is mainly made of DAY 2 foliated serpentinite and shows a 50-cm-thick dyke “Crossing the Tethys ocean, from Europe (Monte of metabasite, slightly rodingitised, and consisting Rosa) to Adria (Sesia-Lanzo zone)” of dark-green amphibole, Ca-rich garnet and epidote. The transposed dyke, is foliated, almost horizontal The second day is devoted to a section across various and slightly boudinaged. A 2-cm-thick rim of Alpine tectonic slices, traversing the Europe plate chloritite occurs between the dyke and the enclosing (Monte Rosa unit), the Tethys ocean (Zermatt-Saas serpentinite. The latter displays a number of brownish ophiolites), up to the Adria microplate (the Sesia- rounded spots that were preferentially dissolved by Lanzo zone). Two itineraries are proposed, depending weathering and are made of carbonates. on the physical capabilities of the participants. A fi rst group (Group A) of fi t people with mountain experience can go uphill to Punta Indren and then to Rifugio Città di Mantova to observe the pre-Alpine Monte Rosa migmatitic basement that was subsequently eclogitized during Alpine metamorphism. This is recommended for people with a good knowledge of the mountain, as it demands severe physical efforts, having to cross a glacier and to climb a hill (via ferrata). Alpine equipment is mandatory (strong boots with creepers, alpine clothes). A second group (Group B) will follow an itinerary that starts from the Monte Rosa unit and crosses the Zermatt-Saas ophiolite unit up to Lake Gabiet, where the upper contact of the meta-ophiolites with the Sesia-Lanzo unit can be seen. The two groups will join in the valley, at Gressoney, and then visit continental eclogitized rocks of the Adria microplate (Lillianes; the Sesia-Lanzo unit). Figure 8 - Foliated serpentinites and a 50-thick Groups A and B transposed dyke of a slightly rodingitised metabasite (Zermatt-Saas meta-ophiolites). Stop 2.1: Metabasic dyke in serpentinite, north of Rifugio Group A Città di Vigevano (N 45° 52.321’; E 7° 52.371’; alt. 2861 m) Stop 2.2: The mountains that can be seen to the east of the the Monte Rosa basement at Punta Indren (N 45° refuge, on the other side of the Sesia valley (Monte 53.01’; E 7° 51.89’; alt. 3109) Tagliaferro, Cima Carnera, and Cima delle Croci), From Rifugio Città di Vigevano, we turn back show a jagged profi le due to regional foliation towards Punta Stolemberg. Passing through Stop and faults. They belong to the Combin ophiolite

1.2 from yesterday, we descend the roped slope of PR01 to B15 n° 1 - from Volume (Tagliaferro) and overlying Sesia-Lanzo unit (Gneiss Punta Stolemberg and, walking along the northern minuti and II Dioritico-kinzigitico klippen). contact of the ophiolitic klippe with Monte Rosa Around the refuge we can again observe the main gneiss, we reach Col delle Pisse (N 45° 53.01’; E 7° lithologies of the Zermatt-Saas and Monte Rosa units. 51.89’; alt. 3109), where numerous lenses (boudins) The Monte Rosa garnet-bearing micaschists, with of metabasite from pre-Variscan protoliths are found chloritoid ± kyanite, are folded by rounded folds. in the Monte Rosa basement. A few tens of metres north of the Rifugio Città di From that point we will cross typical Monte Rosa Vigevano, serpentinite of the Zermatt-Saas unit is

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Volume n° 1 - from PR01 to B15 B02 - B02 Leader: S.Martin Rifugio CittàdiMantova. made ofMonteRosagneissandmigmatitetoreach cross theIndrenglacierandclimbuprocky ridge garnet-bearing micaschists.FromPuntaIndren,we Climbing toPuntaIndren,notethebeautiful,typical normal MORB( from eclogite-facies metamorphismofophiolitic nearby Stolemberg eclogite(Stop1.2)whichresults subduction. Their originisdifferent fromthatofthe and eclogitizedwiththelatterduring Tertiary Rosa continentalcrust,andsubsequentlyboudinaged et al. mafi Rosa eclogitesarethoughttorepresentpre-Variscan their Variscan equivalent, thesepeculiarMonte reaction (omphacite+garnetquartzH glaucophane) hasgrown lately, accordingtoasliding are observed. The blueamphibole(crossiteand/or crystals, whosecoreisfi numerous microcrystallineatoll-shapedgarnet section, phengite,rutile,quartz,omphaciteand blasts overgrowing thefi of avery fi gneiss. These rockscontaincm-to-m-sizedboudins gneiss, withgarnet-richmicaschistsandleucocratic base ofthephotocorrespondsto1mm. and enclosingsomegarnetcrystals.Plane-polarizedlight, quartz), withblue-amphiboleblastovergrowing thematrix Figure 9-Fine-grained eclogite(garnet+omphacite belt ( Malpica-Tuy eclogitesthatareknown inthe Variscan these rocksareidenticaltotheChamptoceauxand crystals intheamphiboleblasts.Surprisinglyenough, but preserved somegarnet,occurringascorroded glaucophane) thathasconsumedquartzandomphacite c sillsofcontinentaltholeiiticaffi , 2002;DalPiaz,2001)intrudedintheMonte e.g. , Godard ne-grained eclogitewithblue-amphibole e.g. et al. , Beccaluva lled withquartzorphengite, , 1981;Godard,1988). As ne-grained matrix.Inthin et al. , 1984). nity (Ferrando nity 2 O = metamorphism. Duringsuchametamorphism, plagioclase, aretypicalofahigh-Peclogite-facies phengite attheexpense ofbiotite,ilmeniteand These reactions,whichproducedgarnet, rutile,and Cordierite =garnet+micaskyanite ±quartz Ilmenite +plagioclase=garnetrutilequartz Biotite =garnet+phengiterutile[biotite Biotite +plagioclase=garnetphengitequartz metamorphic reactions: high-P metamorphismthatisindicatedbyseveral During asecondstage,therocksunderwent ilmenite. plagioclase +garnet±cordieriteK-feldspar with migmatisation,consistsofbiotite+quartz typical ofhigh-Tlow-P conditionsandcoincident distinct metamorphicstages. An earlyparagenesis, These rocksshow petrologicalevidence fortwo after cordierite. encloses cm-sizeddarkspotsthatarepseudomorphs nebulitic structures.Insomeplaces,migmatite occurrence ofleucosomes,melanosomesand Migmatisation isobvious duetothecommon paragneiss withtransposedleucocraticdykes. NW oftheRifugio,outcropsshow migmatitic Mantova Eclogitized migmatites Stop 2.3: melanosomes ineclogitizedmigmatitenearRifugioCittà di Mantova. Theblack patches arepseudomorphafter [cordierite pseudomorph;Figure.15]; [coronas aroundilmenite;Figure.14]. pseudomorph; Figure.13]; magnifying lens]; + rutile[coronasoccasionallyvisiblewitha Figure 10-Nebulitic structures,leucosomesand atRifugioCittàdi cordierite. Lenscap=3cm. 20-05-2004, 15:31:59 THE SUBDUCTED TETHYS IN THE AOSTA VALLEY (ITALIAN WESTERN ALPS) B02

Figure 11 - Eclogitized migmatite at Rifugio Città Figure 13 - Eclogitized migmatite at Rifugio Città di Mantova: Pseudomorph after biotite. Biotite (Bi: di Mantova: Pseudomorph after cordierite. Plg: remnants) isolated in a matrix of quartz (Q) has been pseudomorph after plagioclase, mainly made of albite, replaced by an assemblage of garnet (Ga), phengite (Phg) possibly after jadeite (dark grey); Cd: Cordierite and rutile (white inclusions in phengite). Back-scattered pseudomorph made of garnet (light grey), micas (grey), electron image. kyanite and minor quartz (dark grey). Back-scattered electron image. plagioclase in metapelitic rocks is ordinarily replaced by cryptocrystalline aggregates of jadeite + quartz (+ kyanite + zoisite), which in turn are retrogressed into polycrystalline albite or oligoclase during decompression (e.g., Tropper et al., 1999; Bruno et al., 2001). As for the Monte Rosa migmatites, the initial plagioclase was replaced by cryptocrystalline albite that contains rodlets of kyanite, zoisite and phengitic muscovite (Figure. 15). Although jadeite has not been observed yet, it is likely that this plagioclase recrystallisation was due to the albite- to-jadeite transition followed by the reverse reaction during retrogression. The two metamorphic stages belong to two distinct orogenic cycles, namely pre-Permian (pre-granitic high-T cordierite-bearing migmatites) and Alpine (eclogite-facies coronas and pseudomorphs after cordierite). This metamorphic history is characteristic of the whole Monte Rosa basement, which represents

a slice of the Variscan European continental crust that PR01 to B15 n° 1 - from Volume underwent subduction and subsequent eclogite-facies Figure 12 - Eclogitized migmatite of Rifugio Città di metamorphism during Alpine convergence. Mantova: high-pressure corona between ilmenite and Group A returns to Passo dei Salati to take the plagioclase. Garnet and quartz grew at the expense of cableway to Gressoney. plagioclase, whereas a thin rutile corona occurs on the ilmenite side. Note that the corona is interrupted at contacts with quartz. Plane-polarized light.

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Volume n° 1 - from PR01 to B15 B02 - B02 Leader: S.Martin pathway towards Gabiet(e.g., N45°52.25’;E7° observable inthecalcschists. listvenite (altered carbonate-bearingserpentinite)are chloritoid. A littlefurtheralongtheridge,lensesof garnet porphyroclasts(upto2cmindiameter)and a lookatthecalcschists.Somelayerscontainlarge Following theridgeafew metressouthwards, give at anangleofabout25°. Although irregular, thecontactdipstowards theSSE calcschists withprasinitebandsandthinquartzites. south). The contact isoutlinedbycataclasticorfolded the Combinunitonleft( Corno Camosciotothenorth),andserpentinitesof serpentinite oftheZermatt-Saasunitonright( the retrogradedgreenschist-facies metabasitesand 2885 m) islocatedatthetectoniccontactbetween Col d’Olen(N45°52.229’;E7°52.129’;alt. goes uptoCold’Olen. m), wherewetake thepathway that 52.353’; alt.2869 Guglielmina, built in1878(N45°52.235’;E7° Group Breachestheneighbouring Albergo 52.129’; alt.2885m) meta-ophiolites The contactbetweenZermatt-SaasandCombin Stop 2.4: Group B Lens capdiameteris3cm. chloritoid bearing“calcschists” atCold’Olen. Figure 14-Largegarnetporphyroclastsingarnet- Zermatt-Saas andCombinmeta-ophiolites Stop 2.5: atCold’Olen(N45°52.229’;E7° i.e., CornoRossotothe onthe i.e.,

51.76’; alt.2800m) calcschist level observed atCold’Olen. On theway down toGabiet,wecrossagainthesame Col d’Olen. deformed bythesamepost-nappefoldsobserved near and theMonteRosaunitisagainvery irregular, being The contactbetweentheZermatt-Saasmeta-ophiolite Cima Indren-Gabietsection(Gosso geologic setting”section),whichoccursalongthe Furgg zone(see“MonteRosaunit”inthe“Regional 57.272’; alt.2798 m). Itmaybeattributed tothe albite-bearing amphibolite(N45°52.272’;E7° bearing micaschistswithboudinsofeclogiteand ophiolites. Here,thebasementismadeofgarnet- Monte Rosapre-Triassic basementinsidemeta- cableway, reachesaslicedrecumbentfoldofthe The pathway crossesaskirun,and,closetothe Zermatt-Saas unit. tremolitite, calcschists,androdingitefromthe rather complex associationsofserpentinite, Descending thepathfromCold’Olen,weobserve prasinite, allbelongingtotheCombinunit. capped bythinalternancesofcalcschistsandtabular To theeastCornoRossoismadeupofserpentinite (below). (above) andthelight-brownish MonteRosabasement Zermatt-Saas greenishmeta-basaltsandserpentinites of CornoCamoscio,thefoldedcontactbetween Looking backtothenorthwecansee,atbase dell’Uomo StortoandCornoBianco(theSesiaLanzo zone) intheback. Theridgebetween thelake andthe ophiolites) withPuntaStraling,CornoGrosso,P.ta Figure 15- View ofLagoGabiet(restingonmeta- peaks ismadeofcalcschists andserpentinites. et al. , 1979). 20-05-2004, 15:32:04 THE SUBDUCTED TETHYS IN THE AOSTA VALLEY (ITALIAN WESTERN ALPS) B02

Stop 2.6: Panoramic viewpoint on the upper contact between the meta-ophiolites and the Sesia-Lanzo zone, at Lago Gabiet (alt. 2367 m) We arrive at Lago Gabiet, which was created by a gravity dam (2367 m). From here, beautiful panoramic views allow us to summarise what we have seen and to add some regional perspective. From bottom to top, we notice three elements : (i) the Combin meta- ophiolites; (ii) the contact between the Combin meta- ophiolite (below) and the Sesia-Lanzo unit (above); and, further up (iii) the albitic banded orthogneiss (Gneiss minuti auct.) of the Sesia-Lanzo zone (Adria continental crust, Austroalpine domain). The Combin Figure 16 - Eclogite-facies Sesia-Lanzo rocks at Lillianes. meta-ophiolite is made up of bands of calcschists, Note the dark lamprophyre dikes that crosscut all prasinite, metagabbro and serpentinite, which crop structures. all around Lago Gabiet. Also notice that the contact between the Combin meta-ophiolite and the Sesia- pressure unit in the Lys River. Lanzo unit passes along the slopes of Punta Straling Here, we leave the bus and cross the arched medieval and Corno Grosso; it is marked by its morphology and stone bridge over the Lys River. We descend to the a contrast in colours. Near Punta Starling, this contact western river bank to observe rocks of the Sesia- is deformed by a large-scale south-verging fold and, Lanzo unit (note the large erratic block consisting at Corno Rosso, it dips gently southwards. of beautiful eclogitic micaschists with cm-sized We take the cableway at Gabiet towards Gressoney. garnet crystals). Some metres down from the bridge This cableway follows the small Mos valley, where an outcrop displays a whole range of eclogite-facies serpentinite and retrogressed eclogite (near the Mos rocks (micaschists, gneiss, leucocratic dykes and house) from the Zermatt-Saas unit crop out. eclogite) boudinaged and deformed together. These rocks show a polyphase metamorphic evolution, with a pre-Alpine high-grade evolution typical of a Groups A and B continental crust, followed by the Alpine eclogite- facies imprint. Several late-Alpine undeformed dykes We all meet at the parking lot, and should be on the of a dark ultrapotassic lamprophyre (Venturelli et al., bus at 4:30 pm (remember that the cableway takes 20 min from Passo dei Salati, and 10 min from Gabiet). We descend the Gressoney valley, passing from the meta-ophiolites to the Sesia-Lanzo zone. Stop 2.7: The Sesia-Lanzo unit (i.e., Adria microplate) at Lillianes (N 45° 37.985’; E 7° 50.634’; alt. 646 m) From Gressoney the bus goes southwards along the Val di Gressoney for about 30 km. After the fi rst 6 km, near Chemonal (Gressoney-Saint-Jean), we cross the tectonic contact between the Piedmont nappe (the Combin meta-ophiolite with dominant calcschists) PR01 to B15 n° 1 - from Volume and the Sesia-Lanzo zone (Gneiss minuti unit, orthogneiss without eclogite-facies relics). At Issime, 14 km south of Gressoney-Saint-Jean, we enter into Fig. 17 - Microphotograph of the contact between the inner part of the Sesia-Lanzo zone, which is Eclogitic Micaschist (on the left, note garnet) and characterized by a well-preserved eclogite-facies lamprophyric dyke (note pyroxene and biotite phenocrysts). Plane-polarized light, width of the imprint (Micascisti eclogitici unit). We stop a few km photograph = 3 mm. further to the south, at Lillianes, to observe this high-

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Volume n° 1 - from PR01 to B15 B02 - B02 Leader: S.Martin paths canbefollowed toreturnthebus: readtextfor details. Figure 18-Itineraryofday3.Notethatpathsfor different groupsaretracedwithdifferent lines,andthat two different 20-05-2004, 15:32:09 THE SUBDUCTED TETHYS IN THE AOSTA VALLEY (ITALIAN WESTERN ALPS) B02

Figure 19 - Panorama from the road to Druges looking towards the North. Mountain and localities names are in white. In black the different units: DB = Dent Blanche; CO = Combin greenschist ophiolites; ZS = Zermatt-Saas eclogitized ophiolites; MRU = the Monte Rosa Unit.

1984) crosscut the high-pressure fabric. In the Western times (Servette), archaeological and historical aspects Alps, such post-metamorphic intrusions crosscut will also be considered (ruins, old foundry and slags). the Alpine nappe pile (Dal Piaz et al., 1979b). They Furthermore, the trip reproduces the one followed by display Late Oligocene ages (Venturelli et al., 1984; Horace-Bénedict de Saussure on 20th August 1792 Pettke et al., 1999), and were probably generated by (Voyages dans les Alpes, t. 4, pp. 454-460), on which partial melting of a metasomatised mantle, with some occasion he described blueschists (“schorl bleuâtre”) contamination by the Alpine crust (Dal Piaz et al., for the fi rst time. 1979b; Venturelli et al., 1984). Moreover, we can see in the river bed some huge loose blocks of eclogite Stop 3.1: that come from the Zermatt-Saas meta-ophiolite. Panoramic viewpoint on the road between Druges Basses and Druges Hautes (N 45° 42.469’; E 7° DAY 3 28.608’; 1585 m) “Eclogitized hydrothermal deposits in ophiolites: We leave Collegio Gervasone by minibus at 8:00 am the Servette and Praborna mines” after breakfast and head to Druges (Saint-Marcel). After a 1h15 drive, on the way up, we stop at a During this day we visit the eclogitized oceanic panoramic viewpoint. deposits of Saint-Marcel, namely the Fe-Cu Spectacular view of (from E to W): Monte Rosa hydrothermal sulphide deposit of Servette and the (Penninic nappe), Cervino/Matterhorn (Dent Blanche famous Praborna Mn mine, which allow observation Austroalpine nappe), Grandes Jorasses and Mont of peculiar high-pressure lithologies and minerals. Blanc (Helvetic nappe). Lower down, on the northern As these mines, now abandoned, had been exploited side of the Aosta valley, we observe the eclogite- and since the 17th century (Praborna) and even in Roman blueschist-facies ophiolites of the Piedmont nappe Volume n° 1 - from PR01 to B15 n° 1 - from Volume

Figure 20 - Panorama from the road to Druges (continued from Fig. 19).

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Volume n° 1 - from PR01 to B15 B02 - B02 Leader: S.Martin of charcoal (usedfor dating). Figure 22-Hugeslagheaponthepathbetween theTreves foundry andServette mine.Intheinset,slagwithfragments furnace structure. Figure 21-Remainsoftheoldfoundry atTreves: a chalcopyrite (CuFeS In thisfoundry, coppermetalwas extracted from (N 45°42.444’;E7°27.432’;alt.1672m) Two mainkindsofslagsareobserved (Casartelli,in from theprocessingofServette ore. road, wecanobserve hugeamountsofslagsderiving Near thefoundry, onthesouthernsideof themain (N 45°42.449’,E7°27.304’,alt.1659m) Slag heap Stop 3.3: Old foundry of Treves Stop 3.2: of theoldfoundry Treves. leave thebus, andhave a30-mnwalk uptotheruins We reachDrugesHautesinfi margin (Châtillon,PillonetandMontMary). and slicesofcontinentalcrustattributed tothe Adria since the15 slags andcopper. Suchblastfurnaceshave beenused furnace. The processproducedgas,iron-richsilicate stones andcoal,usedasfuel,wereinterlayeredinthe that supportedthecatwalk). Layersofmineralised through apathcomingfromthemine(seepillars mine. The blastfurnacewas loadedbythetop,

th century. 2 ) fromthenearbyServette

ve minutes.Herewe 20-05-2004, 15:32:13 THE SUBDUCTED TETHYS IN THE AOSTA VALLEY (ITALIAN WESTERN ALPS) B02

preparation). Some are porous and contain fragments Stop 3.4: of gangue and pieces of carbonised wood. The more Eclogitized oceanic hydrothermal ore of the abundant slags, however, look like lava. Servette mine They display an upper face with fl uidal structures, Introduction: The Saint-Marcel eclogite-facies internal fl ow channels, and a rough lower face (soil meta-ophiolites, making part of the Zermatt-Saas imprint). unit, are overthrust by the Monte Emilius klippe The slags are mainly composed of fayalite, wustite, (see “the intermediate continental slices” above). spinel, relict sulphides and interstitial glass that The uppermost section of these meta-ophiolites, concentrated such residual elements as Ca, K, and well exposed at Servette, consists of interlayered Na.Weathering has produced green crusts of Cu chloriteschists, talcschists, glaucophanite, quartzite, sulphates (Casartelli, in preparation). slices of eclogite-facies metagabbro and serpentinite. A preliminary dating of a piece of carbonised wood At Servette, these rocks include a Cu-Fe sulphide (Picea excelsa) enclosed in a piece of slag provided ore, consisting mainly of pyrite and chalcopyrite. The an age of 1120±40 BP (GX-29281; Mambretti, 2003). deposit occurs between 1717 and 1890 m in altitude. This indicates that the mine was active during the It is concentrated in two major ENE-dipping layers of Middle Ages, far before the activity of the Treves 3-4 m in thickness, and some minor levels (less than foundry. 1 m) located at the boundary between chloriteschists From the slag heap, leave the main road and take a and glaucophanite, but it is also disseminated pathway that climbs in the woods towards the ancient throughout the surrounding rocks. mine of Servette. The Servette mine was probably exploited in the Along the path, some outcrops of magnetite-bearing middle ages, as testifi ed by the slag datation (1120±40 serpentinite can be observed. BP: see stop 3.3) and the existence of old excavations At a fork, take the left branch of the way that leads to th the uppermost level of the Servette mine, which was (see stop 3.4.2 below). During the 18 century, dug in sulphide-bearing meta-ophiolites. Servette was exploited by the Challant family, as recorded by Nicolis de Robilant (1786-87). However, most of the remnants actually date from the last period of exploitation (1854-1950; see Lorenzini, 1998). The lithologies have been described in detail by Krutow-Mozgawa (1988), Martin & Tartarotti (1989) and Martin et al. (2004), whose work can be summarised as follows: (a) Chloriteschists have chlorite, garnet, quartz, ±talc, ±chloritoid, ±crossite, ±paragonite and accessory sulphides, rutile, epidote, ilmenite, all aligned along the main foliation. These rocks display S1 planes (schistosity) and C-planes (shear planes), outlined by large fl akes of primary chlorite, and which intersect each other at about 35°. The C-planes coincide with the main foliation of the associated glaucophanite. Garnet occurs as zoned euhedral porphyroclasts up to 1 cm in diameter, with inclusion trails (ilmenite, apatite and epidote) occurring in the core, whereas rare inclusions of rutile, crossite and chloritoid are found in the rim. Chloritoid forms cm-sized porphyroclasts PR01 to B15 n° 1 - from Volume elongated along the schistosity and stretched grains along the C-planes. It is often replaced by secondary Figure 23 - Piece of slag from the ancient Servette mine. chlorite. Rare blue amphibole (crossite) crystals show Spinel (Spn) and skeletal fayalite (Fa) grew in a liquid matrix, now made of glass (black) and skeletal wustite rims of blue-green secondary hornblende (barroisite). (grey). The small white grains are sulphides of Cu and/or (b) In talcschists, centimetric, zoned garnet crystals, Fe, whose presence indicates an incomplete oxidation. characterized by a pink core and red rim, and large, Back-scattered electron image. dark, chloritoid porphyroclasts up to 5 cm long are

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Volume n° 1 - from PR01 to B15 B02 - B02 Leader: S.Martin (CuFeS sulphides andoxides:pyrite (FeS chloriteschists, talcschistsandquartzite,consistof (e) The ore assemblages ore (e) The sulphides, cummingtoniteandcrossite. may includesilicates,but garnetinturnincludes found alignedalongthemainfoliation.Sulphides silicate ((Fe,Mn) & Caucia,1993).Deerite,ararehigh-pressureiron besides quartz(Martin& Tartarotti, 1989; Tartarotti and blueamphibole(crossite)arethemajorminerals, Pyrite, chalcopyrite, magnetite,garnet,cummingtonite (c) Glaucophanite retrograded. = chlorite+paragonite.Glaucophaneisrareandnot retrograde reactionsuchaschloritoid+glaucophane shows chlorite+paragoniterimsthatareduetoa irregularly disseminatedinthematrix.Chloritoid glaucophane, chloritoid,rutileandsulphidesare whereas chloritoidandtalcalsooccurinrims.Garnet, and quartzoccurasinclusionsingarnetcores, immersed inafi (Cu (Cu (d) Sulphide-bearingquartzite generally inclusion-free. rocks, chloritoidmayreach3-4cminlength,andis abundant talcandlarge garnets. Inthesetransitional crystals, whereastheassociatedtalcschistshave an abundance ofglaucophaneandsmallgarnet characterized bytheabsenceoflawsonite, and Glaucophanite incontactwithtalcschistsisgenerally interpreted aspseudomorphsafterlawsonite. paragonite, ±calcite,albiteandchlorite,are rock. They consistofzoisite,and/orclinozoisite, a lozenge-shapedbasalsectionarefrequentinthis unzoned garnetcrystalsofthematrix.Prismswith foliation, inclusionsarerare,asinthesmaller, the outermostrims,whichgrew aftertheexternal accordingly withtheoutsidemainfoliation.In after lawsonite, ilmeniteandglaucophaneelongated include rutile,quartz,chloritoid,pseudomorphs a coreenclosingquartzandtitanite. The innerrims The biggest,upto5mmindiameter, arezonedwith hornblende. Garnetcrystalsshow different sizes. glaucophane isrimmedbyblue-greensecondary aggregates fi lineation. Talc +magnetite±albiteandparagonite microboudinaged paralleltotheN-Strending is stronglyalignedinthemainfoliationand phengite, rutile,magnetiteandilmenite.Glaucophane garnet, chloritoid,epidote,paragoniteandaccessory 9 5 S FeS 5 ), pyrrotine, marcassite,mackinawite ((Fe,Ni) 2 4 ) withminorsphalerite(ZnS),bornite ), othersecondarysulphidessuchusdigenite ll thestretching-relatedfracturesand ne-grained talcmatrix.Onlyrutile 6 iscomposedofglaucophane, (Fe, Al) , hostedbyglaucophanite, 3 Si formsthinlayers. 6 O 20 (OH) 2 ), chalcopyrite 5 ), isalso 9

replaced bychloriteandparagonite. chlorite andquartz;chloritoidglaucophaneare several observations: garnetispartiallysubstitutedby greenschist-facies partialretrogressionistestifi talcschists), as550±60°Cand2.00.3GPa. Finally, 3 different rocks(glaucophanite, chloritschistsand the peakmetamorphicP-Tconditionsformain paragonite +talc.Martin assemblage garnet+chloriteglaucophane main paragenesisischaracterisedbytheequilibrium paragonite andchloritoid.Inglaucophanite,the of chlorite,lawsonite pseudomorphs,glaucophane, assemblage preserved ingarnetcoresasinclusions after lawsonite andsomerelicsofanearlymineral In glaucophanite,itisrevealed bypseudomorphs of cummingtoniteandcrossiteincludedingarnet. is evidenced bythepresenceofdeeriteandthat 2004). Inquartzite,theearlyprogradeevolution can besummarisedasfollows (Martin The metamorphicevolution oftheServette rocks (Martin & Tartarotti, 1989;Martin quartz-rich sediments,mafi under high-pressuremetamorphicconditionsof hydrothermal oceanicalterationanddeformation and talcschistshasbeeninterpretedasresultingfrom The interlayeringofglaucophanite,chloriteschists after lawsonite. the lattercontainglaucophaneandpseudomorphs The formermaycontaingarnetandchloritoid,whereas rich light-colouredfl (f) Inthewalls ofthehouse,abeautifulCr- andMg- hematite rims. the eclogitepeak.Inmatrix,ilmenitemayshow intergrowth structures,whichweregeneratedafter includes rutile,exsolved ilmeno-hematiteandFe-Ti and incoresrimsofzonedgarnetcrystals.It pyrite. Ilmeniteisobserved bothinthemainfoliation Bornite hasbeenobserved onlyassmallcrystalsinside later asinterstitialphasesandfi ilmenite. Chalcopyrite andsphaleritemaycrystallize Occasionally, earlierpyrite hasbeen observed inside or itbordersearlierpyrite, suggestingexsolution. Chalcopyrite generallydefi 1873), rutile,ilmenite,hematiteandmagnetite. 1979; Castello S (g) Micaschists green Cr-rich omphacite. shows centimetricgarnetporphyroclastsandlarge 8 ) (Natale,1969;Gruppoofi andcalcschists et al. aser eclogite-facies metagabbro eclogite-facies aser , 1980),native copper(Jervis, et al. candultramafi ns “fl nes (2004)have estimated areinterlayeredrocks. oliti, 1977;Castello, ll fracturesofgarnet. et al. ames” inpyrite, , 2004). cmaterials 20-05-2004, 15:32:17 et al. ed by ed ,

THE SUBDUCTED TETHYS IN THE AOSTA VALLEY (ITALIAN WESTERN ALPS) B02

Stop 3.4.1: The uppermost level of the Servette mine (N 45° 42.111’, E 7° 27.327’, alt. 1820 m) All the rocks and ore described above are visible along the path, occurring mainly as loose blocs in the mine debris. Advance along a path across the debris, up to a platform where the path is interrupted (N 45° 42.054’; E 7° 27.319’, alt. 1828 m). Then, go a few steps eastwards to the entrance of the uppermost gallery (N 45° 42.059’; E 7° 27.339’, alt. 1829 m). The gallery was opened in glaucophanite and Mn-rich quartzite cm-to-dm-thick layers that grade into carbonate- rich micaschists. This Mn-bearing rock is rich in Figure 26 - Servette: glaucophanite with pseudomorphs alurgite (pink Mn-muscovite), yellow Mn-garnet, red after lawsonite, centimetric red garnet and prismatic black piemontite or/and Mn-epidotes. The main schistosity chloritoid. Lens cap: 3 cm dips 20-30° towards ENE. Above the entrance (N 45°

Figure 24 - Servette: mineralised quartzite. Note Figure 27 - Servette: talcschist with large zoned garnet centimetric garnet crystals and abundant sulphides (pyrite crystals and prismatic chloritoid. Note that small and chalcopyrite). glaucophane prisms are found with talc in the matrix. Lens cap is 3 cm. Volume n° 1 - from PR01 to B15 n° 1 - from Volume

Figure 25 - Servette: rodingite consisting of red grossular-rich garnets + green diopside + epidote, in the Figure 28 - Praborna: boudinaged bands of piemontite- surroundings of stop 3.4.1. Lens cap: 3 cm. rich (reddish) and braunite-rich (black) quartzites.

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Volume n° 1 - from PR01 to B15 B02 - B02 Leader: S.Martin Cr+Fe Figure 30-Praborna: quartz (Q),withminorreddishpiemontite(P). Cpx+albite symplectite(S),inamatrixofbraunite(B)and Figure 29-Praborna:violan(V)corrodedbya Plane-polarized light. Garnet rimisspessartine-rich. Fig. 31-Praborna:garnet-bearingMn-rich quartzite. band inbraunite-rich quartzite. 3+ -clinopyroxene+garnet-rich quartzite The zonedpiemontitedisplaysmechanical twinsthathave Figure 33-Praborna:piemontitecrystalsinquartzite. a peculiarrelationshipswiththesubgrainsofhost quartz(Q)andviolan(V)partlytransformed intoa Cpx+albite symplectite(S)duringretrogression. Figure 32-Praborna;rock withpiemontite(P), quartz. Cross-polarizedlight. Plane-polarized light. 20-05-2004, 15:30:58 THE SUBDUCTED TETHYS IN THE AOSTA VALLEY (ITALIAN WESTERN ALPS) B02

Figure 34 - Green fountain (Acqua Verde): blue gel covering the stones of the river.

Figure 35 - Crepin: completely eclogitized gabbro. Pyroxene is green, plagioclase white, olivine (transformed to talc) is brownish. Note coronitic rims developed between the different microdomains.

42.061’; E 7° 27.338’), a rodingite (grossular-rich garnet + diopside + epidote) formed by metasomatism at the expense of a metagabbro, at the contact with an overlying serpentinite slice. This one separates the Servette sequence from the overlying Mont Roux rock complex, which is made up of retrogressed metagabbro, prasinite and minor serpentinite. At the northern extremity of the uppermost level of the mine (N 45° 42.112’; E 7° 27.361’; 1820 m), one can see what used to be the upper terminal of the cableway towards the Chuc mine village and factory (see stop 3.7). Volume n° 1 - from PR01 to B15 n° 1 - from Volume Stop 3.4.2: The old mine (N 45° 42.066’; E 7° 27.280’, alt. 1792 m) Return to the fork (Figure. 37) and take the right branch of the path heading downhill, which leads to Figure 36 - Map of the Servette mine a 7-8m long cleft opened in the wall but partly hidden after Castello (1979). by debris. This cavity results from the excavation of

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Volume n° 1 - from PR01 to B15 B02 - B02 Leader: S.Martin from 1854to1950(Lorenzini,1998). exploited during the18 authors afterhim. Actually, theminewas mainly Nicolis deRobilant(1786-87),aswellbyseveral to Roman,oreven pre-Roman( pillars, thiswork was attributed, withoutmuchproof, technique used,whichhaspreserved partoftheoreas a sulphide-richlayer. Becauseoftheold-fashioned “Fig. 4-Profi copper mineofancientRomansinSt.Marcel”; the work ofRobilant(1786-87). “Fig. 3-Mapplanofthe Figure 37-MapandsectionoftheServette mineafter mine house,whichdatesfromthelastperiodof Continue afew metressouthwards, uptotheruined (N 45°42.053’;E7°27.297’,alt.1800m) The modern mine Stop 3.4.3: le ofthecave withthebedsofstone”. th centuryandintheperiod i.e., Celtic),timesby going backtostop3.3. recommend youreachthemainroaddownhill by this partofthemineisquitedangerous,andwe tailings (Lorenzini,1998;Zinetti,2002).However, still visible:powder magazine,galleryentrances,and mine house,several remnantsoftheexploitation are lithologies (see“Stops3.4,Introduction”).Below the we canseeagoodsummaryofalltheServette exploitation (1854-1950).Onthewalls ofthehouse, period ofexploitation oftheServette mine. GX-29282: Mambretti,2003). They datefromthelast wood whichhave given a recentage(<100B.P.; porous, irregular, containingpiecesofcarbonised 41.660’; E7°27.134’;alt.1734m). These slagsare heap isvisibleonthewesternsideofpath(N45° to thenorth. A few metressouthwards, alarge slag 1738 m),anoverview oftheServette mineisvisible “Strada Cavour” (N45°41.811’;E7°27.252’;alt. heading toPraborna. At theintersectionwith From stop3.3,take themainroadtowards theSouth, (N 45°41.660’;E7°27.134’;alt.1734m) Slag deposit Stop 3.4.4: by theChallantandDavise families duringthe17 1415 (Pelloux,1913),andwas intensively exploited Mn occurrencesworldwide. Itwas alreadyknown by The oldPrabornamineisoneofthemostfamous cliff atabout50mabove themeadow. thrown fromtheoldmine,whichwas excavated inthe metagabbro occuratthefootofhill. They were wide looseblocksofmanganiferous quartziteand turn totherightheadingtowards thecliff. Hugeand Where thepathway crossestheSaint-MarcelRiver, (N 45°40.774’;E7°26.968’,alt.1894m) Praborna Mnmine Stop 3.5: mineralogy. Itisthetypelocalityforseveral rare The Prabornaoreiswellknown forits peculiar underlie it. bearing glaucophanite,chloriteschistsandmicaschists metagabbros overlie theMnore,whereassulphide- metamorphosed intoeclogitefacies. Serpentiniteand Mn thanks totherelatively highelectronegativity ofMn: glassmakers ofMurano,near Venice, tofade glass, Aosta; “fondsChallant”).Braunitewas usedbythe (Mn a 4-8m-thickboudinagedlayerrichinbraunite The oreconsistsofmanganicquartzite,including and 18 3+ 2+ +Fe Mn th centuries(archives ofthe Aosta province, 3+ 2+ 6 SiO (coloured)=Mn 12 ). Itisassociatedwithophiolites

2+ +Fe 3+ (transparent). 20-05-2004, 15:33:25 th

THE SUBDUCTED TETHYS IN THE AOSTA VALLEY (ITALIAN WESTERN ALPS) B02 Volume n° 1 - from PR01 to B15 n° 1 - from Volume

Figure 38 - Lithological map of the lower St. Marcel valley with a schematic geological section (s) and lithostratigraphy of the Servette deposit (after Martin et al., 2004).

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Volume n° 1 - from PR01 to B15 B02 - B02 Leader: S.Martin Q = Ab]. Back-scattered electronimage. Cpx, withminorK-feldspar (K-F)[i.e.Jds.s.inviolan+ quartz (Q)toasymplectiteofalbite(Ab)andjadeite-free Mn-rich omphacite) was transformed atcontactswith Figure 40-Praborna:duringretrogressionviolan(i.e. Descloizeaux, 1862-74;Bondi Mn-bearing clinopyroxene (Breithaupt,1838; Mn minerals:violan,asemipreciousviolet-blue Figure 39-EntranceofthePrabornamine. 1997); strontiomelane(Meisser Fe (Damour, 1841;Pelloux,1913;Brugger 1893); romeite,acomplex oxideofSb,Mnand the Mn-bearingvariety ofmuscovite (Penfi (Napione, 1788-89;Kenngott, 1853);alurgite, et al. , 1978);piemontite,themanganicepidote et al. et al. , 1978;Brown , 1999).Many et al. eld, , accumulation ofoceanicMn-bearingnodulesand The oreisthoughttobeametamorphosed (Na, Ca, Al…). from coretorim(Mn,Fe,andO) 1987). Itcanbeinterpretedintermsofdiffusion fronts silicate-rich levels oftheboundary(Martin&Kienast, from thebraunite-richlayerofcoretowards the quartzite thatwas exploited; anirregular dm- aegyrine-jadeitite; thebraunite+piemontite-rich micaschists; alurgite+braunite+piemontite-bearing & Kienast,1987),namely(frombottomtotop): Vernizzi, 1982;Kienast&Martin,1983;Martin scales, atypesequencehasbeendefi numerous andvarious layersthatalternateatdifferent Although theoreisstronglybandedanddisplays thick level ofCr+Fe braunite (Mn other manganesemineralshave beenobserved: decrease inMnvalency ( chloriteschists; prasinite. The sequencerefl bearing quartzite;garnet-bearingclinopyroxenite; quartzite, wheregoldwas found;garnet+hematite- richterite (Martin-Vernizzi, 1982);thulite(anMn and chloromelanite;rhodonite(Mn Wurmbach blythite, calderite,etc.:Martin-Vernizzi, 1982; Abs- epidote); hollandite;rhodochrosite(MnCO Mn-rich phengite)was partlytransformed toasymplectite Figure 41-Praborna:duringretrogression,alurgite(i.e., Albite occursinalatesmallvein. Pi=Piemontite. et al. 2+ Mn , 1983);Mn-bearingaugite,jadeite of manganophylliteandhyalophane. 3+ 6 SiO 3+ i.e., Back-scattered electronimage. 12 -clinopyroxene+garnet-rich ); garnets(spessartine, adecreaseofOactivity) 2+ 2 Si 2 nd (Martin- ned O 6 ); K-F-Mn- 3 ); etc. 20-05-2004, 15:33:31 et a ects 2+

THE SUBDUCTED TETHYS IN THE AOSTA VALLEY (ITALIAN WESTERN ALPS) B02

umbers (Martin-Vernizzi, 1982; Mozgawa, 1988; the meta-ophiolites (greenish) and the Austroalpine Tumiati, personal communication). This is supported gneisses (brownish) of the Monte Emilius klippe by the abundance of certain elements (Ba, Sb, and Sr) (intermediate continental slice). Mylonitic that are typical of such an environment. The Mn-rich serpentinites occur along this contact. body was highly deformed and boudinaged during the eclogite-facies metamorphism, and transposed parallel Group B to the foliation of the surrounding glaucophanite. The most competent levels (pyroxene-rich and braunite- Stop 3.7: rich layers) were boudinaged and fractured. These The abandoned mining village of Chuc fractures were fi lled with high-pressure minerals (N 45° 42.044’; E 7° 26.863’; 1422 m) such as violan, Mn jadeite, alurgite, piemontite, On the way back from Praborna, after a 30-mn walk, braunite, greenovite, K-Cl-richterite and quartz turn to the left onto the old “strada Cavour” that leads (Martin-Vernizzi, 1982). Other fractures and veins to Plout (fork at N 45° 41.811’, E 7° 27.252’, alt. developed during retrogression, as they were fi lled by 1738 m). The pathway zigzags down to the valley. low-pressure minerals (Mn-tremolite, rhodochrosite, In a few points, slags similar to those of Stop 3.4.4 Mn-phlogopite, Mn-chlorite), accompanied by are visible. recrystallised braunite and piemontite. Whereas the The abandoned village of Chuc was active from host rocks were strongly retrogressed, the high-P 1854 to 1950 (Cesti, 1978; Lorenzini, 1998). It parageneses were preserved in the Mn ore, which consists mainly of 3 ruined houses (an offi ce, behaved as a gigantic clast in a highly deformed a guard house, and a workers’ dormitory). The matrix. Nevertheless, some static retrogression is site was linked to the Servette and Chuc mines observed, such as low-pressure symplectites around through cableways whose terminal is still alurgite and clinopyroxenes. visible. On the way back from Praborna, two alternative paths The copper mine of Chuc, whose geological setting are proposed. A short path, with a stop at Fontillon is quite similar to that of Servette, was located on the (Stop 3.6; glaucophanite, eclogite and view on other side of the river at altitudes between 1283 and the Emilius klippe), allows us to come back to the 1443 m (very diffi cult access). It consists of fi ve major departure point at Druges Hautes (Group A), whereas mineralised levels, WNW-trending and SSW-dipping, a longer and more diffi cult way (“Strada Cavour”) that are located at the glaucophanite/chlorite-schist passes through the abandoned mining village of Chuc boundary, as observed in the galleries of the mine. and descends along the Saint-Marcel river down to The ore was sent via another cableway to a laver Plout (Stops 3.7 and 3.8) (Group B). located at an altitude of 1211 m, on the western bank of the river, but visible from “Strada Cavour”. There, Group A the ore was crushed and enriched in sulphide, and then transported to Saint-Marcel by a 4-km-long pipe Stop 3.6: or, in the last years, by a cableway. Fontillon (N 45° 42.111’; E 7° 27.327’, alt. 1819 m) On the way back to Druges, in front of stop 3.3’s slag Stop 3.8: deposit, turn to the left (i.e., to the north) into the “Green fountain” of Acqua verde woods. After a 5-minute walk, we’ll climb onto the (N 45° 42.094’; E 7° 26.940’; 1371 m) Fontillon ridge. The rocks consist of banded garnet A few hundred metres below Chuc, the pathway glaucophanite, with lawsonite pseudomorphs, cm- crosses a small river that descends from Servette. The thick green veins of clinopyroxene, N-S-trending water, being saturated in Cu, has deposited a blue gel decimetre-thick bands of glaucophane-bearing that covers the rocks and pebbles of the river bed. This eclogite and chloriteschists. This latter rock has been spectacular “fontaine colorée” was described by Saint- PR01 to B15 n° 1 - from Volume exploited, likely during the middle ages, for millstone Martin de La Motte (1784-85), Horace-Bénedict de manufacturing. Traces of this activity are visible at Saussure (1796, t. 4, p. 459), Prosio (1903), Noussan the northern extremity of the eastern wall of the ridge (1972) and Zinetti (2002). The gel is made of an (very diffi cult access). amorphous Cu hydroxide, which likely precipitates A view towards the Saint-Marcel river and Monte during a change of pH. A few metres uphill it can be Emilius (N 45° 42.480’; E 7° 27.239’, alt. 1642 m) seen that the deposit actually results from the mixing allows us to observe the N-dipping contact between of two streams, at the confl uence of two small rivers,

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Volume n° 1 - from PR01 to B15 B02 - B02 Leader: S.Martin m (right). Antique photographsinLorenzini (1995). Figure 42- Abandoned village andmineofChuc. Workshop (top);terminalofthecableway (left);wagon atlevel 1331 20-05-2004, 15:33:38 THE SUBDUCTED TETHYS IN THE AOSTA VALLEY (ITALIAN WESTERN ALPS) B02 Volume n° 1 - from PR01 to B15 n° 1 - from Volume

Figure 43 - Itinerary of day 4.

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Volume n° 1 - from PR01 to B15 B02 - B02 Leader: S.Martin by H.-B.deSaussurre,whoundertookthefi The Valtournanche valley was visitedin1792 ophiolites andtheoverlying DentBlanchenappe. the tectonicrelationshipsbetweenPiedmontmeta- The excursion also offers theopportunitytoobserve Crepin, whichstillpreserves itsmagmaticstructure. di Cignanaandtheeclogite-facies metagabbroof the famous coesite-bearingoccurrenceofLago ophiolite duringthe Alpine orogeny. We shallvisit P metamorphismthatdeveloped intheZermatt-Saas The fourthdayismainlydevoted tothe(ultra-)high- ophiolites: Cignanaandcoronitic metagabbros” “Ultrahigh- andhigh-pressure metamorphism in one comingfromthe Cime Bianche unit)thatseparatestheCombinandZermatt-Saasunits,which generallyappearsaswhitecliffs. ZS=Zermatt-Saas meta-ophiolites.Thewhitearrows pointtothethinsliceofPermo-Mesozoic sediments(Pancherot- Figure 44- View northwards fromGillaray(stop4.1).DB=DentBlanche nappe;CO=Combinmeta-ophiolites; can bevisited. so-called This highlyretrogressedeclogite-facies gneiss(the continental unit)thatoverlies the meta-ophiolites. comes fromtheMonteEmiliusklippe(intermediate of marble,eclogiteandeclogite-facies gneissthat meta-ophiolite oftheZermatt-Saasunit,large blocks River. Intheriver bed,onecansee,togetherwiththe Continue thepathnorthwards, alongtheSaint-Marcel size, canbeobserved. and chloriteschists,withchloritoiduptoafew cmin 2002). At thesameplacelooseblocsofglaucophanite (1789 m)andtheotherfromChucvillage(Zinetti, down toPlout,whereasplendid17 rare omphacite,glaucophaneorcrossite.Continue layers andlensesrichinchloriteactinolite,with + phengitemicroclineepidote)alternatingwith banded rockwithleucocraticbands(quartz+albite Gneiss pipernoidi DAY 4 Galleria Ribasso of Amstutz, 1951)isa th centurychurch ofServette rst 4505 m(insteadof4478m). At theendof19 Cervino (theMatterhorn),obtaininganestimateof trigonometric calculationsoftheheightMonte overlying DentBlanche-MontMarynappesystem, the Combinmeta-ophiolites(MonteMeabè)and To the West, wecanseethetectoniccontactbetween the smallhillbehindittoobserve thepanorama. leave theminibus neartheGillarayoratory, andclimb Valtournanche valley andCervinoappeartous. We us toGillaray. As weclimbup,grandviews ofthe calcschists oftheCombinunitaredominant,leads a private road(permissionrequired),alongwhich We headup Torgnon andMangnod,and,fromthere, up thehillsonbothsidesofvalley. assemblages), whereastheCombinophiolitesmake metasediments, locallywithwellpreserved eclogitic ophicalcite, metagabbro,metabasaltandminor meta-ophiolites oftheZermatt-Saasunit(serpentinite, follow theMarmoreRiver which cutsacrossthe Valtournanche valley. NorthofChamplong,we am andheadtowards Cignana,enteringintothe We leave Collegio Gervasone byminibus at8:00 Gillaray (alt.2186m.)-Panoramic viewpoints Stop 4.1: Valtournanche areaandsurroundings. subduction metamorphismandplatetectonicstothe and Jean-RobertKienasthave appliedtheconceptsof Piaz, 2001b).Morerecently, Giorgio Vittorio DalPiaz Alps intermsofmultiplerecumbentfolding(seeDal tectonic structureofthePenniniczonein Western studied theDentBlanchenappeandinterpreted Cervino (DalPiaz,1996). Afterwards, Émile Argand geological survey andbarometricmeasurements of century, in1869,FeliceGiordanomadethefi 20-05-2004, 15:33:43 rst th

THE SUBDUCTED TETHYS IN THE AOSTA VALLEY (ITALIAN WESTERN ALPS) B02

granitoids and overlying Valpelline kinzigite. The Combin ophiolite makes up the tectonic substratum of the Cervino basement rocks (Dent Blanche nappe) as well as the ridge between the Cervino and the Theodul Pass and the Plateau Rosa (Testa Grigia) klippe, whereas the Breithorn, on the right, already belongs to the Zermatt-Saas unit. The contact between these meta-ophiolites and the underlying Monte Rosa basement nappe occurs further east, between the snowy Polluce and Castore peaks. East of the Valtournanche valley, the contact between the ophiolitic Zermatt-Saas (Plan Maison) and Combin (Grand Tournalin) units is outlined by a light- coloured horizon of Permian-Mesozoic rocks, mainly Triassic (Pancherot-Cime Bianche decollement unit). Further south, the Combin meta-ophiolites are overthrust by the Pillonet klippe (Dent Blanche s.l.) which is characterized by a blueschist-facies imprint of Late Cretaceous age (Cortiana et al., 1998). To the south, on the opposite side of the Aosta valley, one can see the Zermatt-Saas Mt. Avic ultramafi c massif, the Austroalpine Monte Emilius continental Figure 45 - Simplifi ed geologic map of Cignana Lake slice, and the Penninic Gran Paradiso nappe. surroundings, redrawn from Reinecke (1998). 1 = serpentinized ultramafi cs; 2 = coesite-eclogites and Stop 4.2: derived greenschists; 3 = UHPM metasediments; Panorama of the Cignana valley, 4 = greenschists interlayered with minor calcschists from Rifugio Barmasse (Combin Zone); 5 = calcschists with minor greenschists (N 45° 52,590’; N 7° 35,385’; alt. 2182 m) and marbles (Combin Zone); 6 = dolomite – calcite marbles; 7 = Austroalpine undifferentiated; 8 = scree and We continue our way on the minibuses, until we reach morain deposits. Solid thick lines = tectonic contacts. the parking lot of Rifugio Barmasse (2169 m), near the Cignana lake (2162 m), which is a famous coesite with the Arolla Gneiss basement unit (Monte occurrence. Before looking at the rocks in detail, take Miracolo) and its Mesozoic cover (Cima Bianca). a look at the panorama to the north-west, in front of To the North, the Pancherot hill is made up of a huge you. ophiolitic serpentinite capped by Triassic quartzite and Your feet are resting on the high-P metabasites and platform carbonates of the Pancherot-Cime Bianche metasediments of the Zermatt-Saas unit (micaschists decollement unit. Behind that we can observe Testa and quartzite with large garnet crystals). To the del Leone and Cervino (southern face), made, from north-east (i.e., on your right looking at the lake), bottom to top, of Permian metagabbro, Arolla gneissic Mt. Pancherot (2614 m) is composed of a slice of Volume n° 1 - from PR01 to B15 n° 1 - from Volume

Figure 46 - View of Cignana Lake and surrounding peaks from Rifugio Barmasse.

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Volume n° 1 - from PR01 to B15 B02 - B02 Leader: S.Martin to thenorthwest. area ( again ontheslopesofBeccadiSalèandCortina Saas-Combin andCombin-DentBlanche,canbeseen Salè). Furthersouth-west,thetwo contacts,Zermatt- cover (Roisanzone:ChâteaudesDamesandBeccadi basement rocks(mainly Arolla Gneiss)andMesozoic occurs fromthewaterfall tothehorizonline,with Dent Blanche-MontMarycompositenappesystem exposed. Above theCombinunit, Austroalpine serpentinite bands,istectonicallyfl of calcschistswithminorprasinite,metagabbroand the lake, theCombinunit,heremainlyconsisting the lake shorenearasmallchurch(2178 m). Behind quartzite, marbleanddolomite),whichcropoutalong Mesozoic continentalmetasediments(mainly Triassic unit). Itisoverlain, tothenorth,byasliceofPermian- folded serpentinite(Zermatt-Saasorlower Combin Cignana Ultrahigh-pressure meta-ophiolitesofLake Stop 4.3: i.e. , ontheleftsideoflake), wherethey dip attened andpoorly Figure 47-Eclogitizedgabbro boudinagedinmetasediments. similar tothatofmodernoceanicrocks(N-MORB). been identifi and retrogressedeclogites.Pillow structureshave Reinecke studies byReinecke andcoworkers (Reinecke, 1991; following descriptionissummarisedfromthedetailed relationships amongvarious lithologies.Mostofthe observing thesuperpositionofdifferent fabrics andthe The lake shoreoffers acontinuousoutcropthatallows predominate tothewestandnorthwestoflake. rocks prevail over metasediments,whichgradually overlying Combinunit.Southofthedam,metabasic Saas unit,immediatelybelow thecontactwith belong totheupperpartofophioliticZermatt- and southeasternshoresofLake Cignana. They crop outasathinlevel thatextends onthesouthwestern Introduction Reinecke, 1998;Reinecke Metabasites similar metamorphicultrahigh-Pevolution. section ofoceaniccrust,seemtohave undergone a rocks, whicharebelieved torepresentaformer et al. arerepresentedbyvariously deformed ed, andthegeochemicalsignatureis : The CignanaMesozoicmetasediments , 1994;van derKlauw et al. , 2000). The various et al. 20-05-2004, 15:33:48 , 1997; THE SUBDUCTED TETHYS IN THE AOSTA VALLEY (ITALIAN WESTERN ALPS) B02

Samples that are devoid of retrogression and post- others rich in glaucophane. Phengite and epidote are eclogite deformation have an ultrahigh-P assemblage also visible. The foliation and lineation are outlined consisting of garnet, clinopyroxene, glaucophane, by prismatic minerals of the high-P assemblage, clinozoisite, zoisite, rutile, apatite and paragonite. especially glaucophane. The foliation wraps garnet Occasionally, phengite and dolomite are found. crystals, and pressure shadows consist of coarse- However, the presence of paragonite texturally in grained omphacite. The eclogites are variously equilibrium with the high-P assemblage is not clearly retrogressed to greenschist facies, and retrogression understandable, as it should not be stable at the P-T is complete towards the rims. conditions determined for these rocks (2.6-3.0 GPa, 600°C). Various degrees of retrogression affect Stop 4.3.3: these rocks, from greenschist-facies assemblages N 45° 52.673’; E 7° 35.495’; alt. 2159 m. rimming high-P minerals to completely retrogressed Continue on towards the southwest, along the lake rocks, especially at the high-strain rims of boudins, shore, where a few dm-to-m-sized lenses of coarse- consisting of actinolite, albite, chlorite and epidote. grained metagabbro occur in the metasediments Numerous shear bands, variously oriented veins and (micaschists; quartzite containing garnet crystals up folds help to constrain the exhumation path (see van to 5 cm in diameter). The undeformed metagabbro, der Klauw et al., 1997, for details). Metasediments are described in detail by Reinecke (1998). The ultrahigh-P assemblage consists of garnet, dolomite, aragonite, lawsonite, coesite, phengite in calcschists and micaschists made of garnet, phengite, coesite ± epidote, talc, dolomite, Na-pyroxene and Na-amphibole. Boudinaged levels of Mn-rich quartzite are also observed. Contrasting ages of the high-P metamorphism have been recently obtained by two works (Rubatto et al., 1998, U-Pb=44.5±2.3 Ma and 43.9±0.9 Ma; Amato et al., 1999, Sm-Nd=40.6±2.6 Ma). Moreover, Amato et al. (1999) infer a rapid exhumation based on the Rb- Sr cooling ages. Note that coesite-free eclogites from the same unit in the nearby upper Ayas valley display Sm-Nd (garnet-pyroxene) and Rb-Sr (phengite) ages Figure 48 - Deformed reaction rim between a basic of 49 and 46 Ma, respectively (Mayer et al., 1999). boudin (to the top) and metasediments (to the bottom). Note dark chlorite layers and light epidote layer towards Stop 4.3.1: the metasediment. Lens cap is 3 cm. N 45° 52.668’; E 7° 35.574’; alt. 2164 m. Go to the small mound, situated in the parking lot, near the southwestern extremity of the dam. There, you can see a 2-m-high outcrop consisting of banded garnet quartzite, with late quartz veins. You can distinguish two 2-cm-thick pink layers containing alurgite (Mn-mica), spessartine, piemontite and rare lenses of braunite (Dal Piaz et al., 1979a). A green level consists of phengite, epidote, hematite ± garnet. Volume n° 1 - from PR01 to B15 n° 1 - from Volume Stop 4.3.2: N 45° 52.703’; E 7° 35.551’; alt. 2159 m. Descend to the lake shore, near the southern extremity of the dam. Eclogite bands and lenses, deformed together with metasediments, occur on the shore. They are often Figure 49 - Decimetric garnet in quartz-rich layered, with bands rich in garnet+omphacite and metasediments. Lens cap diameter 3 cm.

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Volume n° 1 - from PR01 to B15 B02 - B02 Leader: S.Martin the foldedcontactwithmetasediments. lenses withmylonites(interferenceFigureures)and allows ustoseetherelationshipsoflessdeformed along cracks. A few metresabove, apolishedsurface often retrogressedtogreenamphiboleatmargins and green clinopyroxene crystals,now mainlyomphacite, igneous texture demonstratedbyupto5cmemerald- wrapped bymyloniticmetagabbro,haspreserved its Zermatt-Saas meta-ophiolites Stop 4.4: with marblelevels. Figureures). Furthersouth,metasedimentsprevail, of different deformationphases(interference to themetasediment).Notealsosuperposition metabasite) andadm-thickbandrichinepidote(close thick discontinuousbandofchlorite(closetothe metasediments, reactionrimsconsistofacm- Between metabasitesandquartz-orcarbonate-rich metabasite boudinagedwithinmetasediments. Going furthersouthalongthelake, wecanobserve N 45°52.60’;E7°35.422’;alt.2160m. Stop 4.3.4: omphacite, rutileandglaucophane.Rimsofgreen Eclogites arefi opaques), andcouldrepresentformerlawsonite. of plagioclase,epidoteandwhitemica(plusminor main foliation.Roundedfl epidote andraretourmaline,allalignedalongthe Carbonate-rich rockscontaincalcite,whitemica, the northwest. and transposedlayeringdipofabout45-50°towards marble alternateandarefoldedtogether. The foliation glaucophanite, serpentinite,calcschistsandlayered 52.509’; 7°36.530’;alt.1891m.).Bandedeclogite, visible ontheleftsideofpathway (N45° At about350mafterthepipe,large outcropsare 36.295’, 1923m). power station(smalltunnelatN45°52.402’,E7° The pathpassesunderthepipeofhydroelectric left (forkatN45°52.302’,E7°36.218’,1908 m). After theruinsofvillageFalegnon, turntothe the path(give alookatthestairsteps). metagabbro, serpentinite)arefoundindebrisalong On theway down, various metabasites(eclogite, and metabasites(bandedeclogiteglaucophanite). micaschists, calcschistswithlawsonite pseudomorphs) dam, wecrossprevalent metasediments(quartzite, triangle), descendingsouthofthedam.Below the We leave Cignanafollowing pathway N1(yellow ( e.g. , N45°52.509’;E7°36.530’;alt.1891m) ned grained. They consistofgarnet, attened aggregates consist , below Cignanadam diopside andchlorite. mainly madeofCa-Fegarnetandepidote,withminor metagabbro arevisibleintheserpentinite. They are two boudinageddm-thickdykes ofrodingitized the upperbranchofpathway, near sign“17”, Along themainpath,20msouthofjunctionwith (N 45°52.878’;E7°36,954’;alt.1804m.) Rodingitized gabbro dykes inserpentinite Stop 4.5: same majorpaththatmustbefollowed northwards. divides intotwo branches,which,actually, leadtothe At N45°52.687’,E7°36,903’ (alt.1892m.),thepath see Stop4.1). coloured horizonof Triassic rocks(Grand Tournalin; Zermatt-Saas andCombinunitsoutlinedbyalight- Valtournanche valley. Notethecontactbetween the Grand Tournalin, ontheothersideof Gobba diRollin(Zermatt-Saasserpentinite)and From thepathway, wecansee theBreithornand barroisitic amphibolearepresent. Serpentinite lenswithinmetagabbro Stop 4.6: One oftheseserpentinitelensesshows a30-cm-thick size. serpentinite, whichoccursaslensesafew metresin displays afl metagabbro. Furthernorth,themetagabbro,which with afew transposedandboudinaged dykes of observe dominant strongly-foliatedserpentinite, Continue alongthepathtowards north. We fi (N 45°53.144’;E7°36,893’;1826m.) the pathtoLiortere Figure 50-Macroscopicreactionrimbetween gabbro (on aserstructurebyplaces,prevails over the the right)andserpentinite(onleft). See textfor details. , along

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reaction rim, with concentric bands: Stop 4.7: serpentinite | tremolitite | tremolite + talc | chlorite + Crepin eclogitic gabbro and troctolite (e.g., N 45° tremolite | transformed gabbro, with epidote + chlorite 52.619’; E 7° 37.095’; alt. 1630 m) | metagabbro. From Liortere, the pathway zigzags downhill towards Crepin. A huge mass of blocks, which resulted from an ancient rock fall, occurs at a few tens of metres

Figure 51 - Fine-grained metatroctolite with poecilitic Figure 52 - Coronitic troctolite from Crepin. Plagioclase clinopyroxene. Olivine: dark and rounded, plagioclase: is white, olivine dark grey and rounded, with lighter talc white, pyroxene: grey. and darker garnet concentric coronas. Volume n° 1 - from PR01 to B15 n° 1 - from Volume

Figure 53 - Partially eclogitized troctolite (Crepin). Figure 54 - Completely eclogitized troctolite. Texture Plagioclase is completely transformed to a fi ne-grained is distinguishable (rounded olivine and anhedral aggregate. Olivine is preserved. A thin, complex corona plagioclase). See text for assemblages. develops between the two. See text and following fi gure for Olivine grains are 2-4 mm. detailed assemblages. Olivine grains are 3-5mm, plane- polarized light.

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Volume n° 1 - from PR01 to B15 B02 - B02 Leader: S.Martin chlorite, kyanite andeven rarequartz.Cr-rich very fi large tremolitecrystalsarecommon,togetherwith Olivine ismainlytransformed to talc.Nonetheless, coronas attheformerplagioclase-olivine interface. paragonite), chloritoidandgarnetthatformsirregular clinozoisite, togetherwithmicas(phengiteand small kyanite crystals arefoundaroundlarge igneous plagioclase. Towards olivine microdomain, jadeitic clinopyroxene andclinozoisitereplaced In completelytransformedtroctolite,fi minerals bynew high-Passemblages. facies minerals,tocompletereplacementofigneous thin coroniticreactions,giving risetoeclogite- display awholerangeoftransformations,fromvery have undergone staticmetamorphicreactions.Rocks composed ofolivine, plagioclaseandclinopyroxene, transformations. Troctolite andgabbro,originally These rocksshow peculiartextures andmetamorphic Pancherot. They comefromaninaccessibleridgebelow Mt. and gabbro,withadominanteclogite-facies imprint. The blocksaremainlymadeofmetamorphictroctolite of Crepin(N45°52.619’;E7°37.095’;alt.1630 m). from thelastbendinpathway beforethevillage southwards ofthepathway. Itis easilyaccessible Back-scattered electronimage. and plagioclaseinincompletelyeclogitizedmetatroctolite. Figure 55-Crepin:multilayeredcoronadeveloping between olivine ne-grained omphacite,phengite,chloritoid, ne-grained charming oratoryofthevillage. at theparkinglotlocatedaround100mafter to thevillageofCrepin. The rendezvous pointisset We now return tothepathway, whichwefollow down green amphibole,chloriteandalbite. pressure metamorphism,withthedevelopment of greenschist-facies retrogressionfollowing thehigh- of metatroctoliteandmetagabbro thatunderwent Among theblocks,therearealsoseveral examples eclogite-facies reequilibration. P-T estimatesofP> of theserocks(Rebay&Powell, 2002)have provided of P-Tpseudosectionsforthevarious microdomains Geothermobarometric computationsandcalculation side, classicandhydrouseclogitizedmetagabbros. Tribuzio, 1991).Itisthuspossibletofi eclogite-facies metamorphicreactions(Messiga& igneous microdomainsandthekineticsofsubsequent which favoured thechemicalhomogenisation ofthe the development oflow-grade hydrousassemblages, 1999). The main effect ofoceanicmetamorphismwas through isotopestudies(Cartwright&Barnicoat, Mt. Viso:Messiga Valtournanche area:Ernst&DalPiaz,1978; Saas ophioliteshasalsobeenproven of oceanicmetamorphismintheZermatt- that tookplacebeforeeclogitization(Ayas- intensity oftheoceanichydrousalteration eclogite-facies reactionsarerelatedtothe The various degrees ofdevelopment of jadeite ±kyanite. clinozoisite +albiticplagioclasegarnet± Plagioclase pseudomorphismadeupof plagioclase pseudomorph. Augitic clinopyroxene |omphacite garnet |plagioclasepseudomorph. Olivine |talcclinopyroxene |chlorite coronas: are rimmedbyeclogite-facies complex minerals In incompletelyeclogitizedrocks,igneous talc. be overgrown byCr-rich chloritoidand overgrown byomphacite. This latter may clinopyroxene ispartiallytocompletely (olivine andclinopyroxene) 2GPa and T et al. , 1999). The effect ≈ 600°C forthe nd, sideby 20-05-2004, 15:33:59 THE SUBDUCTED TETHYS IN THE AOSTA VALLEY (ITALIAN WESTERN ALPS) B02

Figure 56 - Crepin: corona between augitic cinopyroxene and plagioclase in partially-transformed eclogitic gabbro. Back-scattered electron image.

DAY 5 those tectonic units we have been crossing on this “Spectacular views of the Western fi eld trip. Alps’ structure/nappes” Stop 5.1:

We leave Collegio Gervasone early in the morning, View from Plateau Rosà. PR01 to B15 n° 1 - from Volume for an optional excursion (only if the weather is good) The Testa Grigia summit is made up of a small to Rifugio Guide del Cervino (3480 m, Testa Grigia- klippe of the Pancherot-Cime Bianche unit, which Plateau Rosà), southeast of Cervino (Matterhorn). We separates the Zermatt-Saas meta-ophiolites (below arrive at Breuil (Cervinia) and take cableways, fi rst to Testa Grigia) from the Combin unit (above, present Plan Maison and Plan Tendre Lake, and then to Testa to the NW). It is a decollement cover sheet made up Grigia, from where we have a spectacular view of of tabular sediments deposited near a continent, like the collisional zone of the Western Alps, particularly white quartzites, marbles, sedimentary breccias and

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Volume n° 1 - from PR01 to B15 B02 - B02 Leader: S.Martin ophiolites; ZS–Zermatt-Saaseclogitizedophiolites. Piaz, 1992,vol. 1,Fig. 7.15).CO–Combingreenschist Sommette-Grand Tournalin ridge(SW)(fromDal Figure 58-Panorama fromTesta Grigiatowards the area, withtheeclogite-facies ophioliticrocksofthe At ourfeetwehave thePlan Tendre-Plan Maison Valtournanche. seen alongthewesternfl the CombinunitandEtirol-Levaz slice,canbe the DentBlanche-MontMarynappesystem,through whole nappestack,fromtheZermatt-Saasophioliteto Rous, throughtheGrandesetPetitesMurailles. The along thewholeridge,betweenCervinoandMt. (Africa-Tethyan ocean)tectoniccontactisvisible ophiolitic Combinunit. The DentBlanche-Combin thin basementmylonites;and(vi)theunderlying Lower Permianmetagabbrobody, underlainby(v) from metagranitoidsandmetagabbros;(iv) thehuge of the Arolla series;(iii)athickmylonitichorizon kinzigitic gneissand(ii)gneissicmetagranitoids observe several elements:theDentBlanche(i) From thetoptobaseofCervino,wecan 1992). calcschists, ofEo-Trias toJurassicage(DalPiaz, gabbros; CO–Combinophiolites;PCBPancherot-Cime Bianche-Bettaforca Unit. kinzigitic units, AR –GneissMinutiandmetagranitoidsof Arolla; Gb–Permian gabbros; m–milonitesaround Figure 57-Panoramic viewfromTesta Grigiatowards theSW(fromDalPiaz,1992,vol. 1,Fig. 7.14). VP – Valpelline ankoftheupperandmiddle Penninic ( between theophiolite( (Monte Rosaunit;4225m)pointoutthelimit peaks, Polluce(Zermatt-Saas;4090m)andCastore unit; 4139m)isvisibletotheeast. The two following The Breithornserpentiniticmassif(Zermatt-Saas Zermatt-Saas andCombinophioliticunits. exotic metasediments,presentlytrappedbetweenthe is againmadeupofthePancherot-Cime Bianche 57), aNNW-SSE-oriented light-colouredridge left ofthelake, fromSommettatoRoisetta(Figure Zermatt-Saas unit,allaroundtheGoilletLake. To the Helvetic basementintheBernOberland. along theMischabelbackfold;far tothenorthis Mid-Penninic GranSt.Bernard(Briançonnais)nappe nappe (Europeanmargin) and cappedinturnbythe are thrustover theUpper/InnerPenninic MonteRosa underlying CombinandZermatt-Saasunits,which of theCervino-Weisshorn ridge. We alsoseethe on the African (Austroalpine)continentalfragment From theChaletwehave aview towards Switzerland, Amato, J.M.,Johnson,C.M.,Baumgartner, L.P. Abs-Wurmbach, I.,Peters, T., Langer, K.&Schreyer, References cited provided someof theillustrations. Tumiati and Andrea Mambrettiarethanked forhaving for hiscarefulandconstructive review. Simone Giorgio Vittorio DalPiazisgratefully acknowledged Acknowledgements Jahrbuch Miner. Abh O: anexperimental andpetrologicalstudy. W. (1983).PhaserelationsinthesystemMn-Si- i.e. , European)domains. . 146 i.e. , 258-279. , Tethys ocean)andthe 20-05-2004, 15:34:04 Neues THE SUBDUCTED TETHYS IN THE AOSTA VALLEY (ITALIAN WESTERN ALPS) B02

Figure 59 - View from Testa Grigia (the Chalet) towards Switzerland (looking towards N). From Dal Piaz, 1992, vol. 1, Fig. 7.16. AR – Gneiss Minuti and metagranitoids of the Arolla Unit; CO – Combin greenschist ophiolites; ZS – Zermatt-Saas eclogitic ophiolites; MR – Monte Rosa; SB, C, BR and R – Gran San Bernardo Units. s – serpentinite; r – rodingite; gs – gabbro and serpentinite; mc – garnet-chloritoid micaschists with basic inclusions; cp – calcschists and prasinites; mo – morains.

& Beard, B.L. (1999). Rapid exhumation of Eclogae geol. Helv. 79, 57-74. the Zermatt-Saas ophiolite deduced from high- Barnicoat, A.C. & Botwell, S.A. (1995). Seafl oor precision Sm-Nd and Rb-Sr geochronology. Earth hydrothermal alteration in metabasites from high- Planet. Sci. Letters 171, 425-438. pressure ophiolites of the Zermatt-Aosta area of the Amstutz, A. (1951). Sur l’évolution des structures western Alps. Museo di Scienze Naturali di Torino, alpines. Archives des Sciences [Genève] 1951, Bollettino 13 (suppl.), 191-220. 323-329. Barnicoat, A.C. & Fry, N. (1986). High-pressure Argand, E. (1916). Sur l’arc des Alpes occidentales. metamorphism of the Zermatt-Saas ophiolite zone, Eclogae geol. Helv. 14, 145-191. Switzerland. J. Geol. Soc. of London 143, 607-618. Ayrton, S., Burgnon, C., Haarpainter, T., Weidmann, Barnicoat, A.C., Cliff, R.A., Inger, S. & Rex, M. & Frank, E. (1982). Géologie du front de la D.C. (1993). Assessing the age of high-pressure nappe de la Dent Blanche dans la région des Monts- metamorphism in the western Alps using Sm-Nd Dolins, Valais. Eclogae geol. Helv. 7, 269-286. and 40Ar/39Ar. Terra Nova 5, 380. Ballèvre, M. (1988). Collision continentale et chemins Bearth, P. (1952). Geologie und Petrographie des P-T. Mém. Doc. CAESS Rennes 19, 332 p. Monte Rosa. Matériaux pour la Carte géologique Ballèvre, M. & Kienast, J.- R. (1987). Découverte et signifi cation de paragenèses à grenat-amphibole bleue dans la couverture mésozoïque de la nappe de la Dent-Blanche (Alpes occidentales). C. R. Acad. Sci. Paris (série II) 305, 43-47. Ballèvre, M. & Merle, O. (1993). The Combin fault: compressional reactivation of a late Cretaceous-early Tertiary detachment fault in the western PR01 to B15 n° 1 - from Volume Alps. Schweitz. Mineral. Petrogr. Mitt. 73, 205-227. Ballèvre, M., Kienast, J.-R. & Vuichard, J.-P. (1986). La nappe de la Dent Blanche (Alpes occidentales): deux unités austroalpines indépendantes. Figure 60 - Authors ready to climb Piramide Vincent (in the back).

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Volume n° 1 - from PR01 to B15 B02 - B02 Leader: S.Martin Botwell, S.A.,Cliff, R.A. & Barnicoat, A.C. Bigi Beccaluva, L.,DalPiaz,G.V. &Macciotta,G. Bearth, P. (1967).DieOphiolitederZonevon Bearth, P. (1958).Übereinen Wechsel der Bearth, P. (1956).GeologischeBeobachtungenim Bearth, P. (1954). Borghi, A., Compagnoni,R.&Sandrone,(1996). Bondi, M.,Mottana, A., Kurat, G.&Rossi, G.(1978). Bocchio, R.,Benciolini,L.,Martin,S.& Tartarotti, Bistacchi, A., DalPiaz,G.V., Massironi,M.,Zattin, Bistacchi, A. &Massironi,M.(2000).Post-nappe Bearth, P. (1952).Ueberdas Verhaeltnis von Helv basin. paleogeographic evolution ofthewestern Tethyan Antrona units,western Alps: implicationsforthe metabasites fromtheZermatt-Saas,Combinand (1984). Transitional tonormalMORBinophiolitic Schweitz. Zermatt-Saas Fee. Schweitz. Mineral. Petrogr. Mitt. Mineralfacies inder Walzerzone desPenninikums. Alpen. grenzgebiet derlepontischenundPenninischen Geol. Komm., Bern,1954. 25000: BlattZermatt,MonteMoro, Saas. 32 to theirthermo-mechanicalevolution. Massif ofthe Western Alps: Petrologicalconstraints Composite P-TpathsintheInternalPenninic St. Marcel. Cristallochimica delviolanoedellaschefferite di Mineralogia composition andeclogiticparagenesis. valley ophiolites,Italian Western Alps). Protolith gabbros fromdifferent lithologicalsetting(Aosta P. (2000).GeochemistryofeclogitizedFe-Ti Rundsch.) the northwestern Alps. evolution ofthe Austroalpine-Penninic wedgein extensional fault systemandOligocene-present M. &Balestrieri,M.L.(2001)- The Aosta-Ranzola Tectonophysics the northwestern Alps: anintegrated approach. brittle tectonicsandkinematicevolution of Firenze. Sheets 1and2.CNR,ProgettoGeodyn.,SELCA Zone der Alpen. Metamorphose und Tektonik inderpenninischen 94 p. de laSuisse. , 338-347. et al. . 89 Geologie enMijnbouw Eclogae geol. Helv , 1990.StructuralModelofItaly, 1:500.000, , 345-367.

132

90 Rend. Soc.It.Min.Petr. Kuemmerly &Frey. Bern,Switzerland,

69 : 654-667. , 130p.

, 217-237. 327 Geologische Atlas derSchweiz 1: Schweitz. Mineral. Petrogr. Mitt. , 267-292. Beitrag Geologische karte Int. J. EarthSciences(Geol. . 49

63 , 279-290.

, 165-177. 38

, 363-373. 34 Eclogae geol. , 15-25. Periodico di Schweiz.

Brown, Ph.,Essene,E.&Peacor, D.(1978). The Breithaupt, A. (1838).BestimmungneuerMineralien. Chopin, C.&M Cesti, G.(1978).Ilgiacimentopiritoso-cuprifero Cesare, B.,Martin,S.&Zaggia,L.(1989).Mantle Castello, P., DalPiaz,G.V., Gosso,G.,Kienast, Castello, P. (1981).Inventario dellemineralizzazioni Castello, P. (1979). Cartwright, I.&Barnicoat, A.C. (1999).Stable Canepa, M.,Castelletto,Cesare,B.,Martin,S. Brugger, J.H.,Gieré,R.,Graser, S.&Meisser, N. Compagnoni, R.&Maffeo, B.(1973). Jadeite- mineralogy andpetrologyofmanganese-richrocks pp. 329-330]. Journal fürPraktische Chemie Metam. Geol. eclogitization intheZermatt-Saasophiolite. (1994). Sm-Ndisotopicevidence ontheageof Valdôtaine d’Hist.naturelle di Chuc-Servette pressoSt.Marcel(Aosta). 69 (Western Alps). peridotites fromthe Austroalpine Mt.Marynappe Firenze, 1980,pp.171-192. VI ophiolitefi eld conference, Field excursion book In: GruppodiLavoro sulleOfi nappe inthe Aosta valley andrelatedoredeposits. & Venturelli, G.(1980). The Piedmontophiolite J.R., Martin,S.,Natale,P., Nervo, R.,Polino,R. d’Aosta. piemontese deicalcescisticonpietreverdi invalle a magnetite,ferro-rameemanganese delcomplesso Torino, unpublished,328p. nelle vallidiSt.Marcel eFenis Earth Sciences pressure metamorphism. constraints onfl window toocean-fl isotope geochemistryof Alpine ophiolites:a [Padova] nappe (Italian Western Alps). & Zaggia,L.(1990). The Austroalpine MontMary Mineral. Petrol. (1997). The crystalchemistryofroméite. Petrol. from St.Marcel,Piedmont,Italy. Mount Mucrone Area (Sesia-LanzoZone, Western bearing metagranites Mineral. Petrol. petrologic and assemblage fromtheMonteRosa,western Alps: mangesiochloritoid-bearing, highpressure , 91-97.

67 Ofi oliti 42 , 227-232. , 1-17.

12

40 88

uid-rock interactionduringhigh- 127 87 Ar- , 187-196. Studio geologico-giacimentologico 6 Schweitz. Mineral. Petrogr. Mitt. , 219-235. , 5-46. oor hydrothermalalterationand , 388-398. O 39 , 136-146. nié, P. (1984). A unique Ar radiometricstudy. l.s. andrelatedrocksinthe International Journal of

32

, 127-156. 15 oii Mediterranee, oliti , 321-338[Violan: Mem. Sci.Geol. Contrib. Mineral. . Thesis Univ.. Thesis Contrib. Contrib. 20-05-2004, 15:34:08 Rev. J. J. ,

THE SUBDUCTED TETHYS IN THE AOSTA VALLEY (ITALIAN WESTERN ALPS) B02

Italian Alps). Schweitz. Mineral. Petrogr. Mitt. 53, Sci. Geol. [Padova] 51, 1, 155-176. 355-378 + 2 pl. Dal Piaz, G.V. (2001a). Geology of the Monte Rosa Compagnoni, R., Dal Piaz, G.V., Hunziker, J.C., massif: historical review and personal comments. Gosso, G., Lombardo, B. & Williams, P.F. (1977). Schweiz. Mineral. Petrogr. Mitt. 81, 275-303. The Sesia-Lanzo Zone, a slice of continental Dal Piaz, G.V. (2001b). History of tectonic crust with alpine high-pressure low-temperature interpretations of the Alps. J. Geodynamics 32, assemblages in the Western Alps. Rend. Soc. It. 99-114. Min. Petr. 33, 281-334. Dal Piaz, G.V. & Lombardo, B. (1986). Early eclogite Cortiana, G., Dal Piaz, G.V., Del Moro, A., Hunziker, metamorphism in the Penninic Monte Rosa – Gran J.C. & Martin, S. (1998). 40Ar-39Ar and Rb-Sr dating Paradiso basement nappes in the northwestern Alps. of the Pillonet klippe and Sesia-Lanzo basal slice in Geol. Soc. Am. Memoir 64, 249-265. the Ayas valley and evolution of the Austroalpine- Dal Piaz, G.V., Martin, S. (1988). Dati microchimici Piedmont nappe stack. Mem. Sci. Geol. [Padova] sul metamorfi smo alpino nei lembi Austroalpini del 50, 177-194. Pillonet e di Chatillon (Valle d’Aosta). Rend. Soc. Curti, E. (1987). Lead and Oxygen isotope evidence geol. ital. 9, 15-16. for the origin of the Monte Rosa gold deposits Dal Piaz, G. V. & Omenetto, P. (1978). Brevi note su (Western Alps, Italy): A comparison with Archean alcune mineralizzazioni della Falda Piemontese in lode deposits. Economic geology 82, 2115-2140. Valle d’Aosta. Ofi oliti, 3, 161-176. Dal Piaz, G.V. (1965). La formazione mesozoica Dal Piaz, G.V., Hunziker, J.C. & Martinotti, G. dei calcescisti con pietre verdi fra la Valsesia e la (1972). La zona Sesia-Lanzo e l’evoluzione Valtournanche ed i suoi rapporti strutturali con il tettonico-metamorfi ca delle Alpi nord-occidentali ricoprimento del Monte Rosa e con la Zona Sesia- interne. Mem. Soc. Geol.It. 45, 433-460 Lanzo. Boll. Soc. Geol. 84, 67-104. Dal Piaz, G.V., De Vecchi, G. & Hunziker, J.C. (1977). Dal Piaz, G.V. (1966). Gneiss ghiandoni, marmi e The Austroalpine layered gabbros of the Matterhon anfi boliti antiche del ricoprimento Monte Rosa and Mt. Collon-Dents de Bertol. Schweiz. Mineral. nell’Alta Val d’Ayas. Boll. Soc. Geol. 85, 103-132. Petrogr. Mitt. 57, 59-88. Dal Piaz, G.V. (1971). Nuovi ritrovamenti di cianite Dal Piaz, G.V., Di Battistini, G., Kienast, J.R. & alpina nel cristallino antico del Monte Rosa. Rend. Venturelli, G. (1979a). Manganiferous quartzitic Soc. It. Min. Petr. 27, 437-477. schists of the Piedmont Ophiolite nappe in the Dal Piaz, G.V. (1974). Le métamorphisme alpin Valsesia-Valtournanche area. Mem. Sci. Geol. de haute pression et de haute température dans [Padova] 32, 24 p. l’évolution structurale du bassin ophiolitique Dal Piaz, G.V., Venturelli, G. & Scolari, A. (1979b). alpino-appenninique. 1e: Boll. Soc. Geol. It. 93, Calc-alkaline to ultrapotassic postcollisional 437-468; 2e: Schweiz. Mineral. Petrogr. Mitt. 54, volcanic activity in the internal northwestern Alps. 399-424. Mem. Sci. Geol. [Padova] 32, 16 p. Dal Piaz, G. V. (1976). Il lembo di ricoprimento Dal Piaz, G.V., Venturelli, G., Spadea, P. & Di del Pillonet, falda della Dent Blanche nelle Alpi Battistini, G. (1981). Geochemical features of occidentali. Mem. Sci. Geol. [Padova] 31, 61 pp. metabasalts and metagabbros from the Piemonte Dal Piaz, G.V. [ed.] (1992). Le Alpi dal M. Bianco al ophiolite nappe, Italian Western Alps. Neues lago Maggiore. Guide geologiche regionali, Soc. Jahrbuch Miner. Abh. 142, 248-269. geol. It., 1/3, 311 p.; 2/3, 211 p. Dal Piaz, G.V., Cortiana, G., Del Moro, A., Martin, Dal Piaz, G.V. (1993). Evolution of Austro-Alpine S., Pennacchioni, G. & Tartarotti, P. (2001). and Upper Penninic basement in the northwestern Tertiary age and paleostructural inferences of the Alps from Variscan convergence to post-Variscan eclogitic imprint in the Austroalpine outliers and extension. In: J. Von Raumer & F. Neubauer [eds.], Zermatt-Saas ophiolite, western Alps. Int. J. Earth PR01 to B15 n° 1 - from Volume Pre-Mesozoic geology in the Alps. Springer-Verlag, Sci. 90, 668-684. pp. 327-344. Damour, A. (1841). [Mémoire sur la Roméine (sic)]. Dal Piaz, G.V. (1996). Felice Giordano and the C. R. Acad. Sci. Paris 13, 476. geology of the Matterhorn. Acc. Sci. Torino, Atti Delaloye, M. & Desmons, J. (1976). K-Ar radiometric Sci. Fis. 130, 163-179. age determinations of white micas from the Piemont Dal Piaz, G.V. (1999). The Austroalpine-Piedmont zone, French-Italian Alps. Contrib. Mineral. Petrol. nappe stack and the puzzle of Alpine Tethys. Mem. 57, 297-303.

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Volume n° 1 - from PR01 to B15 B02 - B02 Leader: S.Martin Ernst, W.G. &DalPiaz,G.V. (1978).Mineral Engi, M.,Scherrer, N.C.&Burri, T. (2001). Elter, G.(1987). Elter, G.(1971).Schisteslustrésetophiolitesdela Elter, G.(1960).Lazonapenninicadell’alta Valle Diehl, E.A.,Masson,R.&Stutz, A.H. (1952). (1862-74). Descloizeaux, A. Franchi, S.,Novarese, V. &Stella, A. (1912). Franchi, S.(1902).UeberFeldspath-Uralitisirungder Franchi, S.(1900).Sopraalcunigiacimentidiroccie Ferrando, J.,ScambelluriM.,G.V., DalPiaz& Duchêne, S.,Blichert-Toft, J.,Luais,B., Téluk, P., Frey, M.,Hunziker, J.C.,O’Neil,J.R.&Schwander Jacques area. schists ofPiedmontophiolitenappe,Breuil-St. parageneses ofeclogiticrocksandrelatedmafi Petrogr. Mitt. single-grain monaziteagedata. Monte Rosanappe:Constraintsfrompetrologyand Metamorphic evolution ofpeliticrocksthe Florence, Italia. d’Aoste Géologie Alpine Graies). Hypothèsessurl’originedesophiolites. zone piémontaiseentreOrcoetDoireBaltée(Alpes Univ. Padova d’Aosta eleunitálimitrofe. Blanche. Contributo allaconoscenza delricoprimentoDent 248. I (1862),66-67,145-151,254-255;II(1874),245- und Palaeontologie Alpen). Glimmerschiefern derGebirge von Biella(Graiische Natron-Thonerde-Pyroxene ausdeneklogitischen 31 ligure. giadeitiche nelle Alpi Occidentalienell’Apennino 2002 Società geologica italiana, Torino 10-12settembre and retrogressedproducts. from tholeiiticcontinentalbasaltstoalpineeclogites southern Furgg-zone, MonteRosanappe,NWItaly: Piccardo, G.B.(2002). The mafi pressure metamorphism. dating ofgarnetsandtheages Alpine high- Lardeaux, J.-M.& Albarède F. (1997). The Lu-Hf 5-52. relations intheMonteRosagranite,western Alps: H.W. (1976).Equilibrium-disequilibrium Monte Rosa Geologica d’Italia,allascala1:100.000,Foglio 29, , 119-158. , p.254 Bollettino delr. Comitatogeologico d’Italia [1:100 000]. C.N.R.,Soc.Elab. Carte, Neues Jahrbuch für Mineralogie, Geologie Mem. Ist.Geol.Min.Univ. Padova . R.Uff. Geologico,Roma.

Am. Mineral. 81 22 , 305-328. , 1093p. 47 Carte géologique dela Vallée , 147-169.

1902 (II) Nature 81° Riunioneestivadella

Manuel deMinéralogie 64 , 112-126. Mem. Ist.Geol.Min. , 15-31.

387 Schweiz. Mineral. c boudinsofthe , 586-589. Carta 17 c . ,

Gastaldi, B.(1871-1874).Studigeologicisulle Alpi Inger, S.,Ramsbotham, W., Cliff, R.A. &Rex, D.C. Godard, G.,Kienast,J.-R.&Lasnier, B.(1981). Godard, G.(1988).Petrologyofsomeeclogitesin Jervis, G.(1873). Jäger, E.,Schaltegger, U.,Stöckhert,B.,Hurford, Hurford, A.J., Hunziker, J.C.&Stöckhert,B.(1991). Hunziker, J.C.(1974).Rb-SrandK-Arage Gosso, G.,DalPiaz,G.V., Piovano, V. &Polino,R. Gosso, G.(1977).Metamorphicevolution andfold Hunziker, J.C.(1970).Polymetamorphisminthe Gruppo diLavoro sulleOfi Griffi Occidentali. Mineral. Petrol petrological, Rb-Srandstableisotopedata. system geochronology. Zone. Western Alps: timeconstraintsfrommulti- (1996). Metamorphicevolution oftheSesia-Lanzo France). eclogites from“Massif Armoricain” (eastofNantes, Retrogressive development ofglaucophaneinsome Developments inPetrology Eclogites andeclogite-facies rocks Armorican Massif,France.In:D.C.Smith[ed.], the Hercynides; theeclogitesfromsouthern 61. (2), 29-30. the Sesiazone, Western Alps. high pressuremetamorphisminametagraniteof A.J. &Hammer-Schmidt, K.(1990).Datingof 126 Tectonics the western Alps: evidence forepisodicrapiduplift. Constraints onthelatethermotectonicevolution of Padova the Western Alps. determination andthealpinetectonichistoryof 151-161. levels. nappes, postnappedeformationsandstructural (1979). High-pressureemplacementofearly-alpine Rend. Soc.It.Min.Petr. Gressoney valley (SesiaLanzozone, Western Alps). history intheeclogiticmicaschistsofupper Monte Rosa, Western Alps. 1977. falda piemontese, Alpi occidentali: 10-13ottobre Escursione adalcunigiacimentiaCu-FeeMndella 67 deposit, Val d’Aosta,Italy. of clinopyroxenes fromSt.Marcelmanganese , 568-586. n, W.L. &Mottana, A. (1982).Crystalchemistry , 152-168. Ofi oliti Mem. Sci.Geol

31 Contrib. Mineral. Petrol

10 , 5-55. , 758-769. Mem. r. Com.Geol.It.

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33 oliti Mediterranee(1977). Contrib. Mineral. Petrol. , 231-249. Eclogae geol. Helv. 12 American Mineralogist , pp.451-519. Terra Abstract . 32 78 , 16p.

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Volume n° 1 - from PR01 to B15 B02 - B02 Leader: S.Martin Mottana, A. &Griffi Napione, C.A.(1788-89).Manganepidotevon St. Mottana, A. (1986).Blueschist-facies metamorphism Moretti, A. (1948).Notiziesuigiacimenticupriferi Meyer, J.(1983).MineralogieundPetrologiedes Meisser, N.,Perseil,E.-A.,Brugger, J.,ChiapperoP.- Messiga, B.,Kienast,J.R.,Rebay, G.,Riccardi,M.P. Mozgawa, J.(1988). Mottana, A., Rossi,G.,Kracher, A., Kurat, G.(1979). Nicco, R.(1988).Lecomunitàvaldostane e Nicco, R.(1987).NotesuiMuttaelametallurgia del Natale, P. (1969).Recrystallizationandremobilisation of Minerals. Marcel, Val d’Aosta,Italy).In: chemistry ofpiemontitefromthetype-locality(St. Memoir Marcel. cycle, Univ. P. etM.Curie,Paris VI, 140p. occurrences. of manganiferouscherts:areview ofthealpine Italiana della Valled’Aosta. Allalingabbros. Ph.D. Thesis, BaselUniv., Basel. mineralogist Marcel-Praborna, AostaValley,Italy. mineral speciesofthecryptomelanegroupfromSt. J. (1999).StrontiomelaneSrMn of fl between olivine andplagioclaseasaconsequence Metam. Geol. the Monviso ophiolites(Western Alps, Italy). & Tribuzio, R.,(1999).Chloritoideclogitesfrom Petrogr. Mitt. Apennine ophiolites,Italy). metamorphism (Al-Mg-gabbros,Northern (mine dePraborna, Val d’Aoste). éclogitique d’unesérieocéaniquemanganésifère majeurs ettraces aucours dumétamorphisme diopside. Violan revisited: Mnbearingomphacite and Assoc. (Varna), 635-640. […] Leminieredella Valle d’Aostaindue l’industrializzazione ametàdelsec.XVIII. Resistenza in Valle d’Aosta”I–1987,89p. e documenti,“Quadernidell’IstitutoStoricodella XIX. In: tra lasecondametàdelsec.XVIIIel’inizio della metallurgia delferronellaBassa Valle d’Aosta Ferro in Valle d’Aosta(1650-1732). Alcune vicende metallici enon Convegno sullarimobilizzazionedeiminerali in somepyrite depositsofthe Western Alps. In: uid-rock interactionsduringsub-seafl

Mem. r. Acc. Sc. Torino 164 1948 L’industrializzazione in Val d’Aosta. Tsch. Min.Petr. Mitt. Reports13 , 267-299.

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71 17 Geological Societyof America , 637-678. , 405-414. , 287-299. n, W.L. (1986). The crystal Atti Congresso diMineralogia Comportement deséléments th , Cagliari,23p. GeneralMeetingInt.Min.

26

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Nicot, É.(1977). Oberhänsli, R.,Martinotti,G.M.,Hunziker, J.C.& Oberhänsli, R.,Hunziker, J.C.,Martinotti,G.& Noussan, É.(1972).Lesfontainescolorées. Pawlig, S.&Baumgartner, L.P. (2001).Geochemistry Pennacchioni, G.(1989). Penfi Pfeiffer, H.R.,Colombi, A., Ganguin,J.(1989). Perseil, E.-A.&Smith,D.C.(1995).Sb-richtitanite Perseil, E.-A.(1988).Laprésencedestrontiumdans Pettke, T., Diamond,L.W. & Villa, I.M.(1999). Pelloux, A. (1913).Nuove formedellaromeitadi St. italiennes). de la Valpelline (nappedelaDent-Blanche, Alpes 26 pl. Resistenza in Valle d’Aosta”I–1988,130 p.+ documenti “Quadernidell’IstitutoStoricodella Petrogr. Mitt. eklogitisierter permischerGranit. Stern, W.F. (1982). MonteMucrone;eineoalpin Geol. the Sesia-Lanzozone, Western Alps, Italy. Eo-Alpine eclogitizationofPermiangranitoidsin and petrologyofMonteMucrone:anexample of Stern, W.B. (1985).Geochemistry, geochronology Flore valdôtaine Paris VI. In: conservati pressol’Archivio diStato Torino. disegni sull’industrializzazionein Valle d’Aosta saggi diBenedettoSpiritoRobilant.Mappee Petrogr. Mitt. Rosa granite, Val d‘Ayas, Italy. of atalc-kyanite-chloritoid zonewithintheMonte Occidentali). metamorfi ca alpinadelM.Emilius(Alpi Am. J. ofSciences manganese minesofSt.Marcel,inPiedmont,Italy. ophiolites ofthewestern-central Alps. geochemical comparisonofpolyphasemetamorphic Zermatt-Saas and Antrona zone:apetrographicand crystal chemistry. Praborna, Aosta Valley, Italy:petrographyand in themanganeseconcentrationsatSt.Marcel- Deposita Marcel-Praborna (Val d’Aoste,Italie). les oxydesmanganésifèresdugisementdeSt. temporal link. devolatization inthenorthwestern Alps: The Mesothermal goldveins andmetamorphic Nat. Genova Marcel in Valle d’Aosta. eld, S.L.(1893).Onsomemineralsfromthe L’industrializzazione in Val d’Aosta.

52 , 165-184. 23 Thèse de3 , 306-308. (ser. 3) Tesi didottorato,Univ. Padova.

62 81 Geology , 486-487. , 329-346.

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27 cycle, Univ. P. etM.Curie, . Annali Museo CivicoSt. , 641-644. 46 Struttura edevoluzione , 288-295. Schweiz. Mineral. Schweiz. Mineral.

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Mineral. Petrogr. Mitt. 69, 217-223. Alps. Contrib. Mineral. Petrol. 132, 269-287. Prosio, P. (1903). L’eau verte de Saint-Marcel. Bull. Rubbo, M., Borghi, A. & Compagnoni, R. (1999). Soc. Flore valdôtaine 2, 76-78. Thermodynamic analysis of garnet growth zoning Rebay, G. & Powell, R. (2002). The formation in eclogite facies granodiorite from M. Mucrone, of eclogite facies metatroctolites and a general Sesia zone, Western Italian Alps. Contrib. Mineral.

petrogenetic grid in Na2O-CaO-FeO-MgO-Al2O3- Petrol. 137, 289-303.

SiO2-H2O (NCFMASH). J. Metam. Geol. 20, Saint-Martin de La Motte, comte de (1784-85). La 813-826. fontaine verte de St. Marcel dans la vallée d’Aoste. Reddy, S.M., Wheeler, J. & Cliff, R.A. (1999). The Mémoires de l’Acad. r. des Sciences [de Turin] II, geometry and timing of orogenic extension: an 1-12. example from the Western Italian Alps. J. Metam. Saussure, H.-B. de (1779-96). Voyages dans les Alpes, Geol. 17, 573-589. précédés d’un essai sur l’histoire naturelle des Reinecke, T. (1991). Very-high-pressure environs de Genève. S. Fauche, Neuchâtel, 4 vol. [t. metamorphism and uplift of coesite-bearing 4, pp. 454-460]. metasediments from the Zermatt-Saas zone, Scaini, F. (1971). La slavikite della miniera di Western Alps. Eur. J. Mineral. 3, 7-17. Servette a St. Marcel (Aosta). Natura 62, 135 p. Reinecke, T. (1998). Prograde high- to ultrahigh- Scherrer, N.C., Gnos, E. & Chopin, C. (2001). A pressure metamorphism and exhumation of oceanic retrograde monazite-forming reaction in bearthite sediments at Lago di Cignana, Zermatt-Saas zone, bearing high-pressure rocks. Schweiz. Mineral. western Alps. Lithos 42, 147-189. Petrogr. Mitt. 81, 369-378. Reinecke, T., Heinz-Jurgen, B. & Wirth, R. (2000). Sperlich, R. (1988). The transition from crossite Compositional zoning of calcite in a high-pressure to actinolite in metabasites of the Combin unit metamorphic calc-schist: clues to heterogeneous in Vallée St. Barthélemy (Aosta, Italy). Schweiz. grain-scale fl uid distribution during exhumation. Mineral. Petrogr. Mitt. 68, 215-224. Contrib. Mineral. Petrol. 139, 584-606. Stella, A. (1894). Relazione sul rilevamento eseguito Reinecke, T., van der Klauw, S.N.G.C. & Stöckhert, nell’anno 1893 nelle Alpi Occidentali (Valli B. (1994). UHP metamorphic oceanic crust of the dell’Orco e della Soana). Bollettino del r. Comitato Zermatt-Saas zone (Piemontese zone) at Lago geologico d’Italia 25, 343-371 + pl. iii. di Cignana, Valtournanche, Italy. High Pressure Stella, A. (1943). I giacimenti auriferi delle Alpi Metamorphism in the Western Alps. 16th IMA Italiana. Carta geologica d’Italia 27, 1-134. Meeting, Pisa, Italy. Sturani, C. (1973) Considerazioni sui rapporti tra Robilant, S.B. Nicolis de (1786-87). Description Appennino settentrionale e le Alpi occidentali. particulière du Duché d’Aoste, suivi d’un essai Quaderni Acc. Naz. Lincei 183, 117-142. sur deux minières des anciens romains, et d’un Stutz, A.H. & Masson, R. (1938). Zur tektonik der supplément à la théorie des montagnes et des mines. Dent-Blanche Decke. Schweiz. Mineral. Petrogr. Mémoires de l’Académie r. des Sciences [de Turin] Mitt. 18, 40-53. II, 245-274 + pl. vii-ix [Servette: pp. 257-264 + pl. Tartarotti, P. (1988). Le ofi oliti piemontesi nella ix]. media e bassa valle di St. Marcel (Aosta). Tesi di Robilant, S.B. Nicolis de (2001). Viaggi mineralogici. dottorato, Dipartimento di Geologia, Paleontologia A cura di V. Garuzzo [ed.]. Firenze, Olschki, 314 p. e Geofi sica, Università di Padova, 167 p. Rondolino, R. (1934). Sopra alcuni minerali di St. Tartarotti, P. & Caucia, F. (1993). Coexisting Marcel (Valle d’Aosta). Periodico di Mineralogia cummingtonite-sodic amphibole pair in 5, 123-139. metaquartzites from the ophiolite’s sedimentary Rubatto, D. & Gebauer, D. (1999). Eo/Oligocene (35 cover (St. Marcel Valley, Italian Western Alps): Ma) high-pressure metamorphism in the Gornergrat A X-ray structure refi nement and petrology study. PR01 to B15 n° 1 - from Volume Zone (Monte Rosa, Western Alps): implications for Neues Jahrbuch Miner. Abh. 165, 223-243. paleogeography. Schweiz. Mineral. Petrogr. Mitt. Tartarotti, P., Martin, S. & Polino, R. (1986). 79, 353-362. Geological data about the ophiolitic sequences in Rubatto, D., Gebauer, D. & Fanning, M. (1998). the St. Marcel valley (Aosta Valley). Ofi oliti 11, Jurassic formation and Eocene subduction of the 343-346. Zermatt-Saas-Fee ophiolites: implications for the Tropper, P., Essene, E.J., Sharp, Z.D. & Hunziker, geodynamic evolution of the Central and Western J.C. (1999). Application of K-feldspar-jadeite-

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Volume n° 1 - from PR01 to B15 B02 - B02 Leader: S.Martin Venturelli, G, Thorpe, R.S.,DalPiaz,G.V., Del Vannay, J.C.& Allemann, R.(1990).Lazone Van derKlauw, S.N.G.C.,Reinecke, T. &Stöckhert, Venturini, G.(1995) The geology, geochemistryad Von Raumer, J.F., Neubauer, F. (1993).Late Alps, Italy. Oligocene volcanic rocksfromthenorthwestern alkaline, shoshoniticandassociatedultrapotassic Moro, A. &Potts,P.J. (1984).Petrogenesisofcalc- d’Aoste, Italie). piemontaise dansleHaut-Valtournanche (Val record inmetabasites. Piemontese zone,western Alps: thestructural metamorphic oceaniccrustfromLagodiCignana, B. (1997).Exhumationofultrahigh-pressure Alps, Italy). and metapelitesintheSesiaLanzoZone(Western quartz barometrytoeclogitefacies metagranites (WesternItaly). Alps, geochronology oftheinnercentralSesiaZone Precambrian andPalaeozoic evolution ofthe Alpine [Lausanne] Contrib. Mineral. Petrol. 25 J. Metam.Geol. , 148p. Eclogae geol. Helv. Lithos Mémoires deGéologie

41, 17 , 195-209. 79-102.

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Zinetti, G.(2002). Zanella, P. (1992). Widmer, T., Ganguin,J.& Thompson, A.B. (2000). Wheeler, J.&Butler, R.W.H. (1993).Evidencefor (Como). Tesi diLaurea,Università degli Studidell’Insubria Marcel (Ao)eproposta direcupero ambientale. nel versante destro dellabassavallediSaint- e didegrado dellaminiera diCueFe diServette idrogeologico causatodallostatodiabbandono miloniti. Schweiz. Mineral. Petrogr. Mitt. rocks oftheZermatt-SaasZone,Switzerland. Ocean fl Letters continental Sesiaunits. between theoceanicPiedmontandoverlying extension inthewestern Alpine orogen:thecontact Alps. Neubauer [eds.], basement. An overview. In:J.von Raumer&F. Springer, Berlin,pp.625-639.

117 oor hydrothermalveins ineclogitefacies Tesi, Università diPadova, 167p. , 457-474. The Pre-Mesozoic Geology inthe Analisi ambientaledeldissesto I gabbridelCervinoelesue Earth PlanetaryScience

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