Geology and petrology of the Austroalpine Châtillon slice, Aosta valley, western Alps

Geodinamica Acta 17 (2004) 91 105 Geology and petrology of the Austroalpine Châtillon slice, Aosta valley, western Alps Franco Rolfo a, b *, Roberto Compagnoni a, b, Dario Tosoni a a Dipartimento di Scienze Mineralogiche e Petrologiche, Via Valperga Caluso 35, I-10125 Torino, Italy b C.N.R. Istituto di Geoscienze e Georisorse, c/o DSMP, Via Valperga Caluso 35, I-10125 Torino, Italy Abstract Slices of continental crust pertinent to the lower Austroalpine domain of the western Alps, crop out within the ophiolitic Piemonte Zone. Among them, the Châtillon slice was studied in detail. The slice consists of orthogneiss with subordinate metabasics and very minor paraschist. The garnet-phengite-epidote-albite orthogneiss is characterised by polyphase garnet porphyroclasts. Metabasics consist of prasinite lenses and eclogite relics. Phengite-clinozoisite eclogite is characterised by small garnet idioblasts with prograde zoning; jadeite content in omphacite increases towards the rim; Si content in phengite decreases towards the rim. Garnet-glaucophane-phengiteparagonite micaschist is characterised by polymetamorphic garnet porphyroclasts, and small Alpine garnet idioblasts. A pre-alpine amphibolite-facies metamorphism is inferred for the polymetamorphic rocks of the Châtillon slice. Paragneiss and micaschist probably derive from pre-alpine kinzigites ; the orthogneiss protolith was a late-variscan porphyritic granitoid. Thermobarometry in the eclogite constrains the metamorphic peak at T 560 C and P = 16 kbar. The HP minerals were partly retrogressed to greenschist-facies assemblages during the late Alpine tectono-metamorphic recrystallisation. The inferred Alpine P-T conditions are consistent with those for other Penninic and Austro-Alpine nappes of the northwestern internal Alps. The Châtillon slice is very similar to the Eclogitic Micaschists Complex of the Sesia-Lanzo Zone and to the other eclogite-facies Austroalpine slices of the Dent Blanche Nappe, but it could also represent a portion of the Sesia-Lanzo Zone basement, which experienced a somewhat different subduction depth. The tectonic position of the Châtillon slice within the Piemonte Zone is essential to reconstruct the geometric relationships in the Austroalpine-Piemonte nappe stack of the northwestern internal Alps. 2004 Lavoisier SAS. All rights reserved. Keywords: High-pressure metamorphism; Eclogite; Austroalpine; Western Alps; Châtillon 1. Introduction The Austroalpine is the uppermost tectonic domain of the western Alps. It consists of continental crust lithologies and includes the Sesia-Lanzo Zone (in the following referred to as Sesia Zone) and the Dent Blanche Nappe system (Fig. 1). The Sesia Zone comprises the Eclogitic Micaschists Complex and the Gneiss Minuti Complex [1] and an upper element, the Second dioritic-kinzigitic Zone (II DK) [2, 3]; the Dent Blanche Nappe system comprises two independent, lower and upper, units [4, 5]. The Austroalpine units overly ocean-derived units of the Piemonte Zone. The Piemonte Zone consists of a complex pile of meta-ophiolites and calcschists, which may be subdivided into two main nappe systems: the overlying epidote-blueschist-facies Combin Zone and the underlying eclogite-facies Zermatt-Saas Zone [6, 7]. At the main tectonic contact between these two nappe systems, named Combin Fault by Ballèvre & Merle [8], * Corresponding author. Telephone: 00-39-011-6705106 Fax: 00-39-011-6705128 E-mail address: franco.rolfo@unito.it 2004 Lavoisier SAS. All rights reserved.

92 F. Rolfo et al. / Geodinamica Acta 17 (2004) 91 105 Fig. 1 Tectonic sketch-map of the northwestern Alps, modified from the 1:500.000 Structural Model of Italy [51]. 1: Southalpine system (SA), undifferentiated. 2: Canavese Zone (CA). 3: Austroalpine system; 3a: Roisan (R) and Mt. Dolin (D) Mesozoic cover and undifferentiated mylonites; 3b: Units with prevailing amphibolite- to granulite-facies pre-alpine overprint: Valpelline Series (VP), Second dioritic-kinzigitic Zone (DK), Vasario (VA) klippe; 3c: Units with Alpine eclogite-facies overprint: Eclogitic Micaschists Complex (EMC) of the Sesia Zone, and tectonic slices of Mt. Emilius (E), Glacier-Rafray (GR), Tour Ponton (TP), Santanel (S), Châtillon (CH), Etirol-Levaz (EL), Perrière (P), Grun (G) and Eaux Rousses (ER); 3d: Units lacking Alpine eclogite-facies overprint: Gneiss Minuti Complex (GMC) of the Sesia Zone, and thrust sheets of Dent Blanche (DB), M. Mary (MM), Pillonet (PI), and Verres (V); 3e: thrust sheets of the Rocca Canavese Unit. 4-6: Penninic Zone. 4: Piemonte Zone: 4a1: units related to the epidote blueschist-facies Combin Zone (CO); 4a2: units related to the eclogite-facies Zermatt-Saas Zone (ZS), Antrona (A) meta-ophiolites and corresponding units south of Dora Baltea river (SH); 4b: Ultramafic Lanzo massif (LA); 5: Internal Crystalline Massifs ; 5a: Upper Penninic units with Alpine eclogite-facies overprint: Gran Paradiso (GP), Monte Rosa (MR) and Arcesa-Brusson (AB). 5b: Intermediate Penninic units with Alpine epidote blueschist-facies overprint: Gran San Bernardo nappe (SB) undifferentiated and Camughera-Moncucco Zone (CM). 6: Outer Penninic Valais units and Sion-Courmayeur Zone, undifferentiated. 7: Helvetic-Ultrahelvetic system; 7a: Ultrahelvetic cover nappes and M. Chetif massif, undifferentiated; 7b: Helvetic "External Crystalline Massifs" with low grade Alpine overprint: Mont-Blanc massif (MB). 8: Post-orogenic Oligocene magmatic rocks: Brosso-Traversella (BT) and Valle del Cervo (C) intrusions. Tectonic lineaments: FP: Penninic front, SC: Sempione line, CL: Canavese line, AJR: Aosta-Colle di Joux-Colle della Ranzola fault system.

F. Rolfo et al. / Geodinamica Acta 17 (2004) 91 105 93 slices of continental crust with lithologic and metamorphic features similar to those of the Austroalpine units are discontinuously exposed [5]. Some of these slices, usually a few hundred meters long and a few tens of meters thick, are shown in the 1:100.000 Geologic Map of Italy [9], in the 1:200.000 Geologic Map of the northwestern Alps of Hermann [10], and in the 1:100.000 Carte Géologique de la Vallée d Aoste of Elter [11]. In the last decades, new slices of continental crust petrographically similar to the Austroalpine units have been reported from the middle Val d Aosta [12-16]; petrologic studies aimed at better constraining the metamorphic evolution of these slices are, however, relatively rare. These Austroalpine slices crop out both to the north and to the south of the Aosta valley (Fig. 1), their structural trend being possibly related to the occurrence of the Aosta - Col di Joux - Colle della Ranzola fault system (AJR fault), which according to Bistacchi et al. [17] may be considered as a conjugate group of Oligocene faults forming a E-W trending halfgraben. Since the study of such slices is important in order to trace the boundary between the two nappe systems of the Piemonte Zone, a new 1:10.000 geologic survey was carried out in the middle Aosta valley. The aim of this study was to study in detail the Châtillon slice, and to understand its relationship with the other Austroalpine slices and the surrounding units of the Piemonte Zone. In this area, three Austroalpine tectonic slices were preliminary described by Conte et al. [14] and named after the villages of Châtillon, Perrière and Grun, respectively (Fig. 1). These Austroalpine slices are shown in the new 1:50.000 Geologic Map of Italy, Sheet 91 (Châtillon) [16]. The Grun and Perrière slices crop out just at the tectonic contact between the Combin and the Zermatt-Saas Zones. The Châtillon slice, studied in detail in this paper, is exposed within the Piemonte Zone between the Dora Baltea river and the village of Châtillon (Fig. 2). The slice dips 20 to 40 towards NW, and is overlain by prasinite, calcschist, and Ti-clinohumite-bearing serpentinite, which may be referred to the Zermatt-Saas nappe system of the Piemonte Zone. The main foliation of the slice is consistent with the regional foliation of the surrounding Piemonte Zone. The lower tectonic contact with the meta-ophiolites of the Piemonte Zone is hidden to the east by the fluvial-lacustrine deposits of Saint-Vincent, and to the south and the west by the alluvial deposits of the Dora Baltea river. Fig. 2 Geological map of the outcrops between the villages of Breil, Chameran and Châtillon. Modified after Tosoni [15].

94 F. Rolfo et al. / Geodinamica Acta 17 (2004) 91 105 Though briefly described in the Regional Geologic Guide of the Aosta Valley edited by the Italian Geological Society [18] (Stop 1.13, pag. 114), the Châtillon slice was never studied in detail. The most recent original contribution is that of Dal Piaz & Martin [19], which gives a very brief description of the slice and four microprobe analyses of garnet, amphibole and phengite from a fine-grained gneiss. 2. Geological setting The Châtillon slice is about 400 m wide and 1,200 m long (Fig. 2). The two main outcrops occur along the gorge of the Marmore river and from the village of Chameran to the Dora Baltea River south of Breil (Fig. 2). The Châtillon slice consists of three lithologies: orthogneiss, minor prasinite with relics of eclogite, and micaschist. The regional foliation dips 20 to 40 towards NW-NNW. The light grey-greenish medium-grained orthogneiss locally contains K-feldspar porphyroclasts up to 1 cm across and shows in places a banded structure for the presence of dm-thick alternating horizons variably rich in epidote + biotite and albite + white mica, respectively. Garnet is rare in the eastern side of the slice, whereas it is quite common in other sectors where its size may reach 2-3 mm. Locally, garnet is totally replaced by dark green chlorite. Quite often, less than 50 cm thick leucocratic layers occur, which are usually parallel to the main regional foliation, but are locally oblique at low angle to the foliation. Decimetres to centimetre-thick metamorphic quartz veins are locally found. Prasinite (general name for albite actinolite epidote chlorite greenschist) occurs within the orthogneiss as layers or lens-like intercalations up to a few metres thick, which are widespread in the structurally higher portion of the slice, and are lacking in the lower one (Fig. 2). Prasinite is fine-grained and shows a well developed metamorphic foliation, mainly due to the preferred orientation of white mica. Locally, prasinite contains relics of eclogite. Lens-like albite + Fecarbonate pegmatoid pods also occur in the prasinites. Glaucophane-garnet-phengite micaschist is exposed at the base of the orthogneiss, close to the Dora Baltea river (Fig. 2). The micaschist crops out as a layer, half a metre thick, in which garnet is several mm across. Close to the micaschist-orthogneiss contact, the orthogneiss is rich in mm-thick quartz veins. North of the Châtillon slice, calcschist, serpentinite, and medium grained massive prasinite of the Piemonte Zone are exposed, which may be referred to the Zermatt-Saas Zone (Fig. 2). Massive calcschists crop out NE of the Breil village and along the Marmore gorge, where the contact with serpentinite is marked by a decimetre-thick horizon rich in black garnet. Serpentinite is typically strongly foliated and characterised by the local occurrence of metamorphic veins rich in olivine, Mg-chlorite and Ti-clinohumite, the latter mineral suggesting eclogite-facies conditions. On the southern side of the Aosta valley, south of the Châtillon slice and the AJR fault system, relics of eclogite minerals within greenschist also suggest eclogite-facies conditions for the Piemonte Zone as already recognized by Dal Piaz & Nervo [20]. In the Châtillon slice, two main sets of joints are pervasively developed: a first set dipping about 80 towards ESE, and a second one almost vertical with E-W direction. These late joints, mostly sealed by quartz ± chlorite ± albite ± calcite ± Mn-oxides ± pyrite, locally show a vertical offset of a few dm. The E-W vertical set of joints is probably related to the AJR fault system: one of these faults is evident along the road between Châtillon and Pontey. 3. Petrography and mineral chemistry In order to constrain the metamorphic evolution of the Châtillon slice, the three major lithologies were studied in detail. Minerals from representative samples of orthogneiss (sample DB 356), eclogite (sample DB 365) and micaschist (sample DB 358) were analysed with a Cambridge SEM- EDS, operating at 15 kv accelerating potential and 50 s counting time. Natural and synthetic minerals and oxides were employed as standards. Contents of Fe 3 + and structural formula of amphiboles were calculated according to Holland & Blundy [21]. Analyses of other minerals were processed with the software of Ulmer [22]. Representative analyses of minerals used for thermobarometric calculations are reported in Table 1. 3.1. Orthogneiss The garnet-phengite-epidote-albite orthogneiss is mediumgrained and consists of albite, quartz, locally porphyroclastic K- feldspar, white mica, epidote, garnet, chlorite, biotite, clinozoisite, carbonate, and accessory titanite, sulphide, apatite, rutile, allanite. The orthogneiss is crossed by mm-thick fractures filled with chlorite and green-brown biotite, the latter one frequently overgrowing the former one, and by late calcite veins. The main regional metamorphic foliation is defined by the elongation of epidote aggregates and quartz ribbons, and by the preferred orientation of phengite flakes (Si max = 3.37 a.p.f.u.; Fig. 8). The rock matrix consists of a fine-grained aggregate of quartz + albite (An 1-3 ) (Fig. 3a). The mm-spaced foliation is defined by thin layers enriched in epidote, phengite + green biotite, and albite + quartz + phengite, respectively. The occurrence of domains preserving fold hinges, where the original phengite is recrystallised to lower-celadonite phengite (Si = 3.19-3.24 a.p.f.u.), indicates that the main foliation is a transposition foliation. Very rare fine-grained rounded aggregates of white mica + epidote also occur, which are most likely pseudomorphs after a former plagioclase. Albite porphyroblasts are locally abundant: they host inclusions of epidote, white mica, quartz, titanite and amphibole needles ± garnet, which define an internal S i discordant to the main S e foliation. K-feldspar is microcline, partially replaced by a symplectite of white

F. Rolfo et al. / Geodinamica Acta 17 (2004) 91 105 95 Table 1 Representative microprobe analyses of garnet, pyroxene, amphibole, and phengite from the main lithologies of the Châtillon slice. Structural formulae are calculated on the basis of 8 cations and 12 oxygens for garnet, 4 cations and 6 oxygens for pyroxene, 23 oxygens and 13 cations (+ Na + K + Ca) for amphibole and 11 oxygens and all Fe = Fe 2+ for phengite. Eclogite (DB 365) Micaschist (DB 358) Orthogneiss (DB 356) Grt (core) 1Grt 79c Grt (rim) 2Grt 105r Omp 2Px 103r Phe 1Wm 92c Grt (core) 11Grt 7c Grt (rim) 11Grt12r Glph 9Amph 67r Phe 8Wm 50c Grt (core) 16Grt7c Grt (rim) 16Grt 1r Phe 16Wm 36 SiO 2 38.12 38.33 56.81 50.96 37.40 38.09 58.10 51.01 37.93 37.64 50.38 TiO 2 0.03 0.30 0.00 0.20 0.38 Al 2 O 3 21.17 21.31 10.60 26.61 21.17 21.13 12.06 27.35 21.20 21.34 27.40 Fe 2 O 3 1.11 0.22 0.78 0.00 0.11 0.69 0.31 0.00 0.60 0.13 0.00 FeO 27.39 26.88 3.19 1.89 36.16 27.39 10.32 1.68 18.53 28.41 3.27 MnO 0.88 0.22 0.00 0.00 0.94 0.36 0.00 0.10 8.73 0.50 0.06 MgO 4.07 1.89 8.10 3.50 3.62 1.83 9.49 3.14 0.41 0.99 2.57 CaO 7.84 11.99 14.63 0.19 0.90 11.34 0.76 0.04 13.47 11.18 0.00 Na 2 O 6.80 0.31 7.37 0.42 0.63 K 2 O 0.02 11.06 0.07 10.56 10.45 SUM 100.58 100.84 100.96 94.82 100.30 100.83 98.48 94.50 100.87 100.19 95.14 Oxygens 12 12 6 11 12 12 23 11 12 12 11 Si 2.989 3.008 2.002 3.425 2.997 2.999 7.924 3.421 2.996 2.995 3.386 Ti 0.001 0.015 0.000 0.010 0.019 Al 1.956 1.971 0.440 2.108 1.999 1.961 1.938 2.162 1.973 2.002 2.170 Fe 3+ 0.065 0.013 0.021 0.000 0.006 0.041 0.031 0.000 0.036 0.008 0.000 Fe 2+ 1.796 1.764 0.094 0.102 2.424 1.804 1.178 0.094 1.224 1.891 0.184 Mn 0.058 0.015 0.000 0.000 0.064 0.024 0.000 0.006 0.584 0.034 0.004 Mg 0.476 0.221 0.425 0.351 0.432 0.215 1.929 0.314 0.048 0.118 0.258 Ca 0.659 1.008 0.552 0.014 0.077 0.957 0.111 0.003 1.139 0.953 0.000 Na 0.465 0.040 1.949 0.055 0.082 K 0.001 0.948 0.012 0.903 0.896 SUM 7.999 8.000 4.000 7.003 8.000 8.000 15.072 6.967 8.000 8.001 6.999

96 F. Rolfo et al. / Geodinamica Acta 17 (2004) 91 105 mica + quartz. The slightly zoned epidote, crowded with fluid inclusions, has an iron content increasing from core to rim (Ps 67-77 ) and usually overgrows relics of a magmatic REE-rich allanite. Titanite often includes rutile relics. Garnet porphyroclasts, light pinkish in colour, are crowded with inclusions of a zoned clinozoisite (Ps 62-42 from core to rim) and of phengite, apatite, titanite, zircon, which define a rough S i. Garnet is clearly zoned with Fe increasing and Ca decreasing, from core (Alm 39 Grs 39 ) to rim (Alm 63 Grs 28 ), respectively (Fig. 4). Mn shows a typical bell-shaped pattern (Sps 22-1 ). Pyrope content is very low, but a gradual increase from core to rim (Prp 1-5 ) is observed (Fig. 5). The irregular variation of some elements, especially calcium, observed in the compositional profiles (Fig. 5) is considered to be either an analytical artefact due to the contribution of both the hosting garnet and the included clinozoisite, or a retrograde zoning acquired at the garnet- clinozoisite interface. 3.2. Micaschist Fig. 3 Microphotographs of representative lithologies of the Austroalpine Châtillon slice. A) Garnet phengite epidote albite orthogneiss (sample DB 356). A relict porphyroclastic garnet (Grt) shows a clear S i defined by alignment of clinozoisite (Czo). Note the replacement of garnet by an earlier chlorite (Chl) and a later biotite (Bt). The foliated matrix mainly consists of quartz (Qtz), albite (Ab), phengite (Phe), clinozoisite, and minor titanite (Ttn). Plane polarised light. B) Glaucophane phengite garnet micaschist (sample DB 358). Note the two garnet generations: pre-alpine porphyroclasts (Grt1) wrapped by the main rock foliation, and Alpine eclogite-facies small idioblasts (Grt2). Phengite (Phe), glaucophane (Gln), and quartz (Qtz) ribbons define the foliation. Plane polarised light. C) Phengite clinozoisite eclogite (sample DB365). Minerals shown in the picture are garnet (Grt), omphacite (Omp) and phengite (Phe). Note the presence of tiny rutile inclusions within omphacite, which suggest its derivation from a former Ti-bearing mineral, most likely brown hornblende and/or biotite. Plane polarised light. Mineral symbols after Kretz [52] and Bucher & Frey [53]. The garnet-glaucophane micaschist mainly consists of quartz, white mica, glaucophane, garnet, and of minor chlorite, clinozoisite, calcite, and accessory titanite, sulphide, apatite, graphite, and zircon. The main metamorphic foliation is defined by the preferred orientation of Alpine white mica and amphibole (Fig. 3b). Frequently, hinges of rootless isoclinal folds also occur, which are marked by thin layers alternatively enriched in white mica and quartz. Garnet occurs as both pinkish porphyroclasts surrounded by a colourless corona and small colourless idioblasts (Fig. 3b). Porphyroclastic garnet (Fig. 4), up to 6 mm across, consists of a homogeneous almandine-rich core lacking grossular (Alm 78 Prp 18 Sps 4 on average), and a Ca-rich rim with the same composition as the small idioblasts (Alm 59 Grs 35 Prp 5 Sps 1 on average). Fractures in porphyroclastic garnet are healed by white mica and glaucophane, whereas pressure shadows consist of chlorite. White mica is both phengite and paragonite (Fig. 8). Phengite, which defines the main foliation, is slightly zoned with Si ranging from 3.42 to 3.30 a.p.f.u. from core to rim, respectively. On the contrary, phengite included in garnet is homogeneous (Si = 3.34-3.36 a.p.f.u.). Blue amphibole is a homogeneous glaucophane (Fig. 7), occasionally mantled by Mg-hornblende. The Al IV content is 0.01-0.26 in glaucophane and 0.61-0.95 in Mg-hornblende; Al VI is 1.83-1.99 in glaucophane and 0.49-0.88 in Mg-hornblende, respectively. Glaucophane hosts inclusions of garnet, white mica, titanite, graphite and quartz, and is replaced by a very fine-grained symplectite of albite + chlorite. Accessory Al-rich titanite (Al up to 0.11 a.p.f.u.), slightly zoned clinozoisite (Ps 35-41 from core to rim), and magnetite also occur. 3.3. Metabasics The eclogite is fine-grained and consists of garnet, omphacite, amphiboles, clinozoisite, white mica, quartz, green biotite, calcite, chlorite, and accessory titanite, rutile, sulphides, allanite, and apatite (Fig. 3c). The high-p peak minerals are only preserved as microporphyroclasts wrapped

F. Rolfo et al. / Geodinamica Acta 17 (2004) 91 105 97 Fig. 4 Chemical composition of zoned garnet porphyroclasts and small garnet idioblasts from the phengite - clinozoisite eclogite (sample DB365), the garnet glaucophane micaschist (sample DB358) and the garnet phengite epidote albite orthogneiss (sample DB356). Arrows point to porphyroclast rims and idioblast compositions. around by an isoclinally-folded greenschist-facies foliation. This phengite-clinozoisite eclogite contains poorly zoned garnets with cores richer in mineral inclusions than rims. The cores, which include glaucophane, titanite and omphacite, are slightly Ca-poorer and Mg-richer (Alm 65 Grs 17 Prp 16 Sps 2 on average) than the rims (Alm 59 Grs 33 Prp 7 Sps 1 on average) (Fig. 4). Na-pyroxene is slightly zoned, from ferrian omphacite in the core to omphacite in the rim according to the definitions of Rock [23]; its jadeite content increases (Jd 39-44 ) and the aegirine content decreases (Acm 9-0 ) from core to rim (Fig. 6). Omphacite is locally replaced by a very fine-grained symplectite of amphibole or of diopside + albite-oligoclase (An 1-16 ) or, more rarely, by a felt of plagioclase + biotite + chlorite ± calcite. Phengite shows a weak compositional zoning with the core richer in Si (Si = 3.43 a.p.f.u.) than the rim (Si = 3.38 a.p.f.u) (Fig. 8). In addition to relict glaucophane included in garnet, blue-green to green amphiboles also occur, which mainly define the greenschist-facies metamorphic foliation: they are zoned from actinolite to Mghornblende and to Fe-pargasite in the rim, with a strong increase in Al IV from core (0.06 a.p.f.u.) to rim (1.22 a.p.f.u.) (Fig. 7). The clinozoisite xenoblasts are compositionally homogeneous (Ps 41-43 ); they locally show thin rims with slightly higher birefringence. Titanite, still preserving rutile relics in the core, often forms small elongated aggregates. The prasinite, derived from eclogite retrogression, consists of amphiboles, albite, relict garnet, clinozoisite, chlorite, minor white mica, green biotite and carbonate, and accessory titanite, rutile, sulphide and apatite. The amphibole nematoblasts have zoning and composition similar to those found in eclogites. Large amphiboles also occur, which are crowded with small oriented rutile inclusions: they are interpreted as structural relics of a pre-alpine hightemperature Ti-bearing brown hornblende, from which Ti was exsolved as rutile during the Alpine eclogite-facies event. An oligoclase rim surrounds the albite poikiloblasts, which include green amphibole and clinozoisite. Locally, albite + amphibole symplectites occur, which derive from omphacite retrogression. The small garnet relics are almost totally replaced by chlorite and green biotite. Medium to fine-grained clinozoisite often shows a complex zoning, with slightly Fe-enriched rims. White mica, which includes sagenitic rutile, most likely derives from pre-alpine brown biotite. Titanite often includes rutile relics.

98 F. Rolfo et al. / Geodinamica Acta 17 (2004) 91 105 4. P-T conditions and metamorphic evolution On the ground of geological, petrological and minerochemical data, a complex tectono-metamorphic evolution may be envisaged for the rocks of the Châtillon slice. This evolution includes both pre-alpine and Alpine events (Table 2). 4.1. Pre-Alpine history The garnet-glaucophane micaschist shows an evident polymetamorphic evolution, with relics of a pre-alpine amphibolite-facies metamorphism, and a polyphase Alpine evolution. The pre-alpine history is documented by the core composition of the zoned garnet, an almandine with minor pyrope and very low grossular (Fig. 4), which is consistent with an amphibolite-facies metamorphic imprint [24]. On the contrary, the garnet rim has a composition enriched in the grossular component, which is consistent with growth under Alpine eclogite-facies conditions. In metabasics, the presence of amphibole porphyroclasts crowded with very fine-grained rutile droplets or needles, and the presence of high-si phengite including sagenitic rutile suggests the former occurrence of a pre-alpine Tibearing hornblende of high T, and of Ti-bearing biotite, respectively. These two minerals point to a relatively high-t assemblage, which is compatible with two possible protoliths: an intrusive mafic rock (i.e. a hornblende gabbro) or a metamorphic amphibolite. In the case of an igneous protolith, the metabasic lenses would derive from a former gabbro dyke cutting across the late-variscan granitoids, boudinaged and recrystallised during the polyphase Alpine evolution. In the case of a metamorphic protolith, the metabasic lenses would derive from former xenoliths of Variscan amphibolite-facies basement, engulfed by late-variscan granitoids during their emplacement. On the ground of the petrologic data available from the amphibole-bearing eclogites of both the Eclogitic Micaschists Complex of the Sesia Zone [25] and the Austroalpine outliers [26], both interpretations are possible. As to the orthogneiss, its size, homogeneity, mineralogy and mineral mode suggest an igneous protolith with granodioritic composition. This interpretation is further supported by the common occurrence of microstructural relics, such as fine-grained monomineralic aggregates derived from Alpine recrystallisation of a former single igneous mineral grain like quartz, plagioclase and biotite, and of mineralogical relics, such as K-feldspar and allanite. Chemical composition and zoning of garnet porphyroclasts and mineral inclusions, especially epidote, seem to rule out a pre-alpine metamorphic origin. Therefore, it may be concluded that the Châtillon orthogneiss derives from the Alpine tectonic and metamorphic reworking of a late-variscan porphyritic biotite granodiorite. 4.2. Alpine history 4.3. The Alpine eclogite-facies event The Alpine eclogite-facies event is mainly testified by the relics of eclogite discovered in prasinites. Garnet and omphacite zoning is consistent with the available data from other eclogites from both the Austroalpine and the Piemonte Zones [27]. The moderate garnet zoning from almandine-richer cores to grossular-richer rims (Fig. 4) suggests a two-stage eclogite-facies recrystallisation, pointing to a prograde metamorphism. This garnet zoning is frequently found in many eclogites from both the western Alps and other HP/UHP mountain belts [27-29]. This early high-pressure metamorphic event is also evident in the orthogneiss and micaschist, which contain highceladonite phengite, almandine- and grossular-rich garnet, and rutile. This metamorphic event was accompanied by the development of a pervasive foliation, now preserved only as relics of intrafolial fold hinges within the main regional greenschist-facies foliation. In the phengite-bearing eclogites, the peak metamorphic conditions of the eclogite-facies recrystallisation were estimated by means of conventional thermobarometry. Temperatures were estimated by applying to core and rim compositions the Mg-Fe exchange in garnet - clinopyroxene [30, 31] and garnet phengite [32] geothermometers. Pressures were estimated from the garnet omphacite phengite geobarometer [33, 34]. For core and rim compositions, T ~ 430 C [30] and P ~ 16.5 kbar, and T ~ 490 C and P ~ 15.5 kbar, respectively, were obtained, which point to a T increase of about 60 C and a slight P decrease of about 1 kbar during the Alpine evolution (Fig. 9). Equilibration temperatures about 25 C lower result from the Powell s [31] calibration. From the garnet-phengite thermometer, experimentally calibrated by Green & Hellman [32] for basaltic rock compositions, maximum temperatures of about 560 C were obtained for a nominal pressure of about 16.0 kbar (Fig. 9) and considering total iron as Fe 2 + in phengite. The garnet-phengite thermometer was also applied to rim compositions in the garnet and phengite bearing micaschist and orthogneiss, considering total iron as Fe 2 + in phengite; maximum temperatures of about 530 C and 550 C respectively were obtained for a nominal pressure of about 16.0 kbar (Fig. 9). It is difficult to make a choice on which is the most reliable temperature for the peak metamorphic conditions; however, the values of 530-560 C obtained for all lithologies from the garnet-phengite thermometer [32] at 16 kbar matches the best T (and P) estimates given for peak conditions of the Eclogitic Micaschists Complex of the Sesia Zone (T = 500-600 C, P 15-17 kbar [25]; T = 490-570 C at 15 kbar [47]; T = 550-600 C, P 15-18 kbar [48]) and of the Mt. Emilius klippe [35, 36]. Moreover, the Fe 3 + content of the phengites both in the eclogite and in the micaschist is supposed to be very small, since they plot near the muscovite-celadonite joint (Fig. 8), and consequently the

F. Rolfo et al. / Geodinamica Acta 17 (2004) 91 105 99 Table 2 Metamorphic evolution of the orthogneiss, micaschist and metabasics of the Châtillon slice, Aosta valley. Grey lines refer to minerals whose occurrence was inferred from their pseudomorphs Orthogneiss Micaschist pre-alpine history Alpine history Magmatic assemblages Metamorphic assemblages Eclogite-facies event Decompressional evolution Greenschist-facies event allanite apatite biotite Ca-carbonate chlorite clinozoisite epidote glaucophane Na-Ca amphibole graphite garnet I garnet II ilmenite K-feldspar monazite plagioclase white mica quartz rutile sulphides titanite Metabasite allanite apatite biotite Ca-carbonate chlorite clinozoisite epidote Ti-hornblende glaucophane Na-Ca amphibole garnet I garnet II pyroxene plagioclase white mica quartz rutile sulphides titanite pre-alpine history Alpine history Eclogite-facies event Decompressional evolution Greenschist-facies event

> Apogee FrameMaker Noir G17_1_ Page 100 Mardi, 15. juin 2004 10:04 10 100 F. Rolfo et al. / Geodinamica Acta 17 (2004) 91 105 Fig. 5 Qualitative X-ray maps for Fe, Mg, Ca and Mn of a garnet porphyroclast from the garnet phengite epidote albite orthogneiss (sample DB356). The images are 512x512 pixels. White patches in the Ca map are inclusions of clinozoisite. The compositional profile along the double arrow in the Fe X-ray map refers to the same garnet porphyroclast, which is about 3 mm across.

F. Rolfo et al. / Geodinamica Acta 17 (2004) 91 105 101 maximum temperatures obtained are likely to be close to the real ones. Pressure for the peak metamorphic phase can be also estimated in the orthogneiss using the equilibrium 3 Cel = Phl + 2 Kfs + 3 Qtz + 2 H 2 O [37]. Considering K-feldspar as a relict phase and Si max = 3.37 a.p.f.u. in the phengite (Fig. 8), the minimum pressure inferred in the KMASH system is on the order of 11-12 kbar (Fig. 9). However, because the experimental work of Massonne & Schreyer [37] considers the pure magnesian system, and since the X Mg is lower in the biotite than in the phengite of the orthogneiss, the equilibrium curve moves towards higher pressures [38]. Moreover, this reaction is not water conserving and, as a consequence, the position of the Si-isopleths will displace to higher pressures in case of decreased ah 2 O [39]. The decompressional evolution between the Alpine eclogite- and the greenschist-facies events is rarely preserved. A relatively early step in the decompression history is the omphacite breakdown, which may be constrained by the pyroxene composition of the symplectite, which contains about 10 Mole% of jadeite molecule. Since the clinopyroxene is in equilibrium with albite, a pressure of about 10 kbar is estimated (Fig. 9) for a temperature of about 450 C as found in the Eclogitic Micaschists Complex [25, 60]. 4.3.1. The Alpine greenschist-facies event After the eclogite-facies peak and the following decompression, the high-p minerals were extensively retrogressed Fig. 7 Al VI vs. Al IV diagram of amphiboles from the garnet - glaucophane micaschist (sample DB358) and phengite clinozoisite eclogite (sample DB365). The bold line separates low Al VI (low pressure) from high Al VI (high pressure) amphibole compositions [55]. Compositional fields of amphiboles for the greenschist- (a), epidoteamphibolite- (b), amphibolite- (c), and granulite-facies (d) are from [56]. Fig. 6 Compositions of peak and retrograde clinopyroxenes from phengite clinozoisite eclogite (sample DB365) of the Châtillon slice, in the jadeite aegirine (enstatite + ferrosilite + wollastonite) diagram of Morimoto et al. [54], modified by Rock [23]. Na-pyroxene core compositions plot in the field of ferrian omphacite, while rims plot in the field of omphacite. to greenschist-facies conditions. In metabasics, omphacite was replaced by earlier amphibole + albite symplectite and later chlorite + albite ± biotite symplectite. In the micaschist, glaucophane was replaced by Na-Ca amphibole ± albite aggregates. In the orthogneiss, porphyroblastic albite grew at the expenses of phengite, and in both orthogneiss and metabasics chlorite developed at the expense of garnet, together with epidote. The microstructural evidence shows that during retrograde evolution, phengite first re-equilibrated to phengite with lower celadonite contents, then albite grew mainly at the expense of phengite contemporaneously to chlorite, and finally albite was overgrown by an oligoclase rim, whose formation was synchronous with the biotite development. This sequence of minerals and mineral assemblages suggests that the post-eclogitic evolution started with significant decompression coupled with a moderate cooling, and was followed by a small, but detectable, T increase within greenschist-facies conditions. This part of the P-T path is consistent with those estimated for other eclogite-facies tectonic units of the western Alps [40].

102 F. Rolfo et al. / Geodinamica Acta 17 (2004) 91 105 Fig. 8 Compositions of di-octahedral micas in the Na vs. Si, and total Al vs. Si diagrams from the garnet phengite epidote albite orthogneiss (sample DB356), garnet glaucophane micaschist (sample DB358), and phengite clinozoisite eclogite (sample DB365) of the Châtillon slice. Cel = celadonite; Ms = muscovite. 5. Discussion: the tectonic position of the Châtillon slice It is difficult to define the exact tectonic position of the Châtillon slice within the Piemonte Zone nappe pile for two main reasons: the lack of outcrops, due to the presence of an extensive and thick Quaternary cover, and the complexity of the local geology. The geologic setting is furthermore complicated by the occurrence of the Combin fault which marks a significant metamorphic gap [4, 5, 41] and by a late feature, the AJR fault [17, 42], which mostly produced vertical displacements under brittle conditions at 32-30 Ma [43, 44]. According to a simple geometric westward extrapolation from the Val d Ayas (Fig. 1), the AJR fault should be traced to the north of the Châtillon slice. However, the outcrops of the Piemonte Zone north of the village of Châtillon do not show clear evidence of such an important discontinuity. Furthermore, within the Châtillon slice the mean attitude of foliation gradually rotates clockwise moving southwards, and close to the Dora Baltea river it is almost E-W, i.e. parallel to the trend of the AJR fault (Fig. 2). These observations, supplemented by geologic data from a wider area including the nearby Austroalpine slices of Grun [13] and Perrière [14], suggest that the Châtillon slice is located to the north of the AJR fault (Fig. 1). Therefore, the AJR Fig. 9 Thermobarometric estimates for peak and post-peak metamorphic conditions of the phengite-bearing eclogites, the garnetglaucophane micaschists and the orthogneiss of the Châtillon slice. Average metamorphic temperatures are estimated with the garnetclinopyroxene geothermometer [30] and the garnet-phengite geothermometer [32]. Average metamorphic pressures are estimated with the garnet-omphacite-phengite geobarometer [33]. Results suggest a T increase and slight P decrease during the Alpine evolution (arrow). Isopleths modelling the jadeite content in omphacite (Ab Qtz Jd) follow the diopside-jadeite solid solution model of Gasparik [57], and are calculated according to Connolly [58]. Isopleths of Si-content in phengite of the orthogneiss (KMASH system for the assemblage Phe + Kfs + Phl + Qtz, after Massonne & Schreyer [37]) are labelled with Si cations per formula unit. Phase relations for Al 2 SiO 5 after Holdaway & Mukhopadhyay [59]. Mineral abbreviations after Kretz [52]. fault runs somewhere to the south of the Châtillon slice, most likely by the course of the Dora Baltea river. Bistacchi et al. [17] consider the AJR fault as a conjugate group of faults forming an E-W trending half-graben all along the middle Aosta valley with the normal master fault located along the southern side of the Aosta valley. In any case, the position of the Châtillon slice cannot be compared with that of most Austroalpine slices, such as Etirol-Levaz, Tour Ponton, Eaux Rousses, Santanel or Grun, which occur right at the tectonic contact between the Combin and the Zermatt-Saas Zones, i.e. along the Combin fault. In fact, due to the occurrence of HP relics both north and south, the Châtillon slice may be considered as tectonically belonging to the Zermatt-Saas Zone, as are the Glacier Rafray and other southern Austroalpine outliers located far from the Combin fault [17]. 6. Conclusions A new 1:10.000 geologic survey, supported by a petrographic and petrologic study, has shown that the Châtillon

F. Rolfo et al. / Geodinamica Acta 17 (2004) 91 105 103 slice largely consists of orthogneiss, leucocratic layers, and of a thin horizon of glaucophane-bearing micaschist. The orthogneiss hosts small inclusions of prasinite with eclogite relics. The intrusive origin of the orthogneiss is suggested by its size and homogeneity, as well as by the preservation of relict igneous K-feldspar phenocrysts and of igneous allanite. The leucocratic layers, which are still locally discordant to the main orthogneiss foliation, have been interpreted as partially transposed aplitic dykes of pre-alpine, most likely late-variscan, age. The petrographic study has shown that the orthogneiss is a monometamorphic rock, which derives from the Alpine tectonometamorphic reworking of a former granitoid of probable late-variscan age. The prasinite boudins with eclogite relics, which are hosted within the orthogneiss, may derive either from former amphibolite xenoliths from the Variscan basement incorporated by the granitoid magma during its emplacement, or from late-variscan mafic dykes, originally cutting across the granitoid. The glaucophanebearing micaschist is a polymetamorphic rock, since it preserves mineral relics of a pre-alpine, most likely Variscan, metamorphism of amphibolite-facies. Therefore, the micaschist may be interpreted as a relic of the metamorphic Variscan basement into which the granitoids intruded in late- Variscan times. In all lithologies of the Châtillon slice the main regional foliation is an Alpine foliation, which developed under greenschist-facies conditions. However, this foliation is the result of transposition of an earlier eclogite-facies foliation, as suggested by the preservation of isoclinal fold hinges defined by higher-celadonite (higher-p) phengites, and by the occurrence within eclogite-facies garnet porphyroclasts of a S i discordant to the main greenschist-facies S e foliation. It is impossible to estimate the P-T conditions of the Variscan metamorphism; however, the composition of the garnet relics from the glaucophane micaschist is compatible with the amphibolite-facies conditions, recognised in the Eclogitic Micaschists Complex of the Austroalpine Sesia Zone [24, 45] and Mt. Emilius klippe [26, 35, 36, 46]. The poorly zoned garnet from eclogites, hosting glaucophane inclusions in the core and showing compositions and zoning similar to those of other Alpine eclogites, on the other hand, entirely grew under Alpine eclogite-facies conditions. The peak conditions of the Alpine eclogite-facies metamorphism have been estimated at T 560 C and P 16 kbar, which are in good agreement with the peak conditions of the Eclogitic Micaschists Complex [25, 47, 48], though lower than the recent estimates by Tropper & Essene [60] (T = 550-650 C, P = 17-21 kbar). However, though the P estimates appear to be more accurate than the previous ones, temperatures seem to be too high since chloritoid instead of staurolite is the stable phase all over the Eclogitic Micaschists Complex in metapelites of suitable composition [25]. In the Châtillon slice, zoning of the peak eclogite-facies minerals records a T increase and slight P decrease during growth (Fig. 9). The post-eclogitic evolution is characterised by decompression mainly recorded by the breakdown of omphacite to produce a clinopyroxene + albite symplectite, in which the clinopyroxene in equilibrium with albite has a jadeite content of 10 mole% suggesting metamorphic P of about 10-12 kbar. Decompression is followed by the Alpine greenschist-facies overprint, which is especially evident in the orthogneiss, where porphyroblastic albite extensively grows at the expense of phengitic white mica. This reaction, which is responsible for the conversion of phengite micaschist into albite gneiss, has been reported from most tectonic units of the western Alps, including the Gneiss Minuti Complex of the Sesia Zone [25]. The last thermal event affecting the Châtillon slice is a small, but detectable, T increase within greenschist-facies conditions recorded by oligoclase rims overgrowing albite and by the development of biotite. Summing up, the Alpine P-T conditions of the Châtillon slice are consistent with those recorded in other Penninic and Austroalpine nappes of northwestern internal Alps [49]. On the ground of its lithologic association and tectonometamorphic evolution, the Châtillon slice is genetically related to the Austroalpine domain. In particular, the Châtillon slice is similar from a lithologic and metamorphic point of view, to the eclogite-facies Mt. Emilius klippe [35, 36, 50] and to Glacier Refray slice [20] whose eclogite-facies mineral assemblages have been strongly retrogressed to greenschist facies. However, it cannot be excluded that the eclogitefacies peak conditions of the Châtillon slice could be slightly lower than those of the Eclogitic Micaschists Complex, thus representing portions of the Sesia-Lanzo Zone basement which experienced a somewhat different subduction depth. Acknowledgements Careful and constructive reviews by G.V. Dal Piaz and B. Lombardo greatly improved the manuscript. This work was carried out with financial support of Italian C.N.R, C.S. Geodinamica delle Catene Collisionali, Turin (now Istituto di Geoscienze e Georisorse) and M.U.R.S.T., Grant ex-40%. 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