Petro-structural map of the Money Unit (Gran Paradiso Massif, Valnontey valley, Western Alps)

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1 Journal of Maps ISSN: (Print) (Online) Journal homepage: Petro-structural map of the Money Unit (Gran Paradiso Massif, Valnontey valley, Western Alps) Paola Manzotti, Christian Le Carlier De Veslud, Benjamin Le Bayon & Michel Ballèvre To cite this article: Paola Manzotti, Christian Le Carlier De Veslud, Benjamin Le Bayon & Michel Ballèvre (2014) Petro-structural map of the Money Unit (Gran Paradiso Massif, Valnontey valley, Western Alps), Journal of Maps, 10:2, , DOI: / To link to this article: Paola Manzotti View supplementary material Published online: 10 Dec Submit your article to this journal Article views: 112 View related articles View Crossmark data Citing articles: 3 View citing articles Full Terms & Conditions of access and use can be found at Download by: [ ] Date: 24 November 2017, At: 00:05

2 Journal of Maps, 2014 Vol. 10, No. 2, , SCIENCE Petro-structural map of the Money Unit (Gran Paradiso Massif, Valnontey valley, Western Alps) Paola Manzotti a, Christian Le Carlier De Veslud a, Benjamin Le Bayon b and Michel Ballèvre a a Université de Rennes, Géosciences Rennes, UMR-CNRS 6118, University of Rennes1, Rennes Cedex, France; b BRGM, Direction des Géoressources, Orléans Cedex 2, France (Received 25 March 2013; resubmitted 25 October 2013; accepted 14 November 2013) The Gran Paradiso Massif is one of the Internal Crystalline Massifs (Penninic Zone) in the Alps. This Massif comprises the widely investigated Gran Paradiso Unit and the underlying, much less-explored Money Unit, which is a well-exposed tectonic window below the Gran Paradiso Unit. This contribution provides a new detailed petro-structural map of the Money Unit. An integrated structural and metamorphic study can relate different structures to the evolving metamorphic stages. There is also recognition of evidence of Permo-Carboniferous sedimentation, contact metamorphism and polyphase Alpine evolution, mainly developed under blueschist to greenschist facies conditions. The 1:5000 scale map (and the cross sections) display evidence for the Alpine evolution of the Money Unit and for the structural and geological setting of this area. From these data, a 3D model of the Money Unit and its folded contact with the Gran Paradiso Units is constructed. Keywords: Money Unit; Gran Paradiso Massif; structural and metamorphic evolution; folded contact; Alps 1. Introduction The Valnontey valley deeply cuts the Western Italian Alps south-west of Aosta. In this area, the Gran Paradiso Massif crops out as a tectonic window within the Piemont-Ligurian ophiolites. Previous work (Compagnoni, Elter, & Lombardo, 1974; Le Bayon & Ballèvre, 2006) has identified a monometamorphic complex (Money Unit) exposed as two tectonic windows below the Gran Paradiso Unit, in the Valnontey and Valeille valleys, respectively. Nevertheless, analytical and interpretative mapping of the Money Unit was still lacking. The present study describes the geological, structural and metamorphic history of the Money Unit, exposed in the Valnontey valley; the new petro-structural map summarises information on lithology, metamorphic mineral associations, and tectonic and sedimentary structures that characterise this unit. Moreover, our work has allowed us to distinguish two detrital sedimentary formations that were strongly metamorphosed during the Alpine orogeny. These two formations occur at different structural positions and present contrasting sedimentological and mineralogical characteristics. Corresponding author. paola.manzotti@univ-rennes1.fr # 2013 Paola Manzotti

3 Journal of Maps Methods Lithological associations, superimposed fabrics and the metamorphic environments to which they relate are presented on the map. The relative chronology of the superimposed structures is deduced using structural correlation criteria (Pollard & Fletcher, 2005; Spalla, Siletto, Di Paola, & Gosso, 2000; Spalla, Zucali, Di Paola, & Gosso, 2005; Turner & Weiss, 1963). Analysis of the superimposed fabric elements and their supporting mineral assemblages allows us to establish the relative timing of structures and the related metamorphic conditions in each lithology. A sequence of geological events (stages) is reconstructed from the cross-cutting (or overprinting) relationships between sedimentary (Sed), magmatic (Mag), metamorphic (Met), and deformational (D) signatures (Gosso & Spalla, 2009), each of which is labelled according to their relative order by subscript numerals (e.g., D 1 older than D 2 etc). The foliation orientation at outcrops is used to interpolate or construct foliation trace (dotted lines) (Connors & Lister, 1995; Johnson & Duncan, 1992; Zucali, 2002). Different colours of these traces refer to successive fabric generations and associated metamorphic conditions (Baletti, Zanoni, Spalla, & Gosso, 2012; Manzotti, 2011; Spalla, Di Paola, Gosso, Siletto, & Bistacchi, 2002; Spalla, Zucali, Salvi, Gosso, & Gazzola, 2004; Spalla et al., 2005). The geological mapping was undertaken at 1:5000 scale. Cross-sections are drawn perpendicular to the main structures (i.e., fold axes and fold axial surfaces). The structural features are represented in lower hemisphere Schmidt (equal-area) projections. Symbols used for structural elements are: S for syn-metamorphic surfaces (with the exception of S0 for layering), P for axial surfaces of folds, L for stretching lineations and A for fold axes. Numbering indicates successive deformation phases. Mineral abbreviations are after Whitney and Evans (2010) plus Wm for white-mica. The topographic base map is the Carta Tecnica Regionale, provided by the Regione Valle d Aosta, scale 1: 5000, UTM ED 1950 coordinate system: T7046, T7042. A preliminary 3D model has been built using the gocad 3D modeller (Mallet, 2002). A 3D model appears as a first order tool for integrating data and ensuring their consistency (see Appendix for details). In addition it allows the development of a 3D structural interpretation with the aim of showing the 3D geometry of the contact between the Money and the Gran Paradiso Units. 3. Geological outline The Gran Paradiso Massif belongs to the Briançonnais terrane and, together with Monte Rosa and Dora Maira, forms the Internal Crystalline Massifs of the European Alps. It represents a large tectonic window overthrusted by eclogite-facies oceanic units, derived from the Piemont-Ligurian Ocean (Figure 1). Two main units, the Money Unit and the overlying Gran Paradiso Unit, constitute the northern part of the Gran Paradiso Massif (Compagnoni et al., 1974; Le Bayon & Ballèvre, 2006) Money Unit The Money Unit mainly comprises a Late Palaeozoic leucocratic metagranite (Erfaulet metagranite) and a thick sequence of metasediments with different origins (Money Complex) (Compagnoni et al., 1974). The latter consists of Gr-rich metapelites, meta-conglomeratic layers (Compagnoni et al., 1974; Le Bayon & Ballèvre, 2006), Bt-Amp orthogneisses (Ballèvre, 1988) and paragneisses (metagreywacke) with interlayered amphibolites (Le Bayon & Ballèvre, 2006). The protolith age of the Money Complex is considered to be Permo-Carboniferous, on the basis of the lack of pre-alpine high-temperature relics (Compagnoni et al., 1974; Le Bayon &

4 326 P. Manzotti et al. Figure 1. The Gran Paradiso nappe in the Western Alps (in evidence the Money unit): simplified structural and geological map (modified after Le Bayon and Ballèvre, 2006). Ballèvre, 2006). Pre-Alpine garnet in metapelites at the boundary between the Erfaulet metagranite and the Money Complex has been attributed to contact metamorphism (Le Bayon & Ballèvre, 2004).

5 Journal of Maps 327 Although some micaschists from the Money Unit display garnet-chloritoid-rutile assemblages, which may suggest a relatively high-pressure imprint, the peak pressure-temperature conditions of the Alpine metamorphism are not quantitatively established. It is generally assumed that peak pressure was much lower than in the Gran Paradiso Unit (Beltrando, Compagnoni, & Lombardo, 2010; Bousquet et al., 2012; Le Bayon & Ballèvre, 2006), but this remains to be substantiated or refuted. Moreover, there are no radiometric data that constrain specific stages of the kinematics and tectonometamorphic evolution of the Money Unit Gran Paradiso unit The Gran Paradiso Unit consists of abundant augen-gneisses derived from porphyritic granitoids of Permian age (Bertrand, Paquette, & Guillot, 2005; Ring, Collins, & Kassem, 2005) intruded into metasedimentary rocks, that mainly comprise polymetamorphic paragneisses and micaschists. Lenses of mafic rocks, derived from pre-alpine amphibolites (Ballèvre, 1988; Battiston et al., 1984; Benciolini, Martin, & Tartarotti, 1984; Brouwer, Vissers, & Lamb, 2002; Compagnoni & Lombardo, 1974; Dal Piaz & Lombardo, 1986) or from Late Palaeozoic gabbros (Gasco, Borghi, & Gattiglio, 2010; Pognante, Talarico, Rastelli, & Ferrati, 1987) occur in the paragneisses. Remnants of a thin Mesozoic metasedimentary cover lay over this pre-triassic basement (Elter, 1960, 1972; Polino & Dal Piaz, 1978). The Gran Paradiso Unit records a metamorphic history that comprises: (i) regional Variscan amphibolite facies metamorphism (Le Bayon & Ballèvre, 2006); (ii) Permian contact metamorphism (Compagnoni & Prato, 1969); (iii) Early Alpine eclogite facies metamorphism (Compagnoni & Lombardo, 1974; Dal Piaz & Lombardo, 1986; Le Bayon, Pitra, Ballèvre, & Bohn, 2006); and (iv) Late Alpine epidote amphibolite/greenschist facies metamorphism (Le Bayon & Ballèvre, 2006). Evidence of pre-alpine regional metamorphism has been described in metasediments, whereas contact metamorphism (i.e., hornfelses) has been reported along intrusive contacts (Compagnoni et al., 1974). Pressure and temperature conditions have been estimated at 4 6 kbar C (Le Bayon et al., 2006) for the pre-alpine regional metamorphism, and at kbar, C (Gabudianu Radulescu, Compagnoni, & Lombardo, 2011) for the contact metamorphism. Alpine eclogite facies assemblages have been described in a variety of lithologies in the Gran Paradiso Unit. A pressure of kbar at C represents the minimum P-T conditions estimated for the eclogite facies event (Ballèvre, 1988; Benciolini et al., 1984; Brouwer et al., 2002). Moreover the Al- and Mg-rich micaschists (called whiteschists or silvery micaschists ) record eclogite facies conditions at kbar and C (Vidal, Parra, & Trotet, 2001; Wei, Powell, & Zhang, 2003; Gabudianu Radulescu et al., 2011). Few relics of the high-pressure history are found in the porphyritic orthogneisses: epidote, Ca-rich garnet, and perthitic K-feldpsar and biotite replaced by phengite, rutile and titanite are the sole evidence of eclogite facies event in the porphyritic orthogneisses (Le Goff & Ballèvre, 1990). The ages of the Alpine structural and metamorphic evolution are still discussed. Rb-Sr geochronology on whiteschists indicates an age of Ma and of Ma (Freeman, Inger, Butler, & Cliff, 1997; Inger & Ramsbotham, 1997; Meffan-Main, Cliff, Barnicoat, Lombardo, & Compagnoni, 2004). The first age is interpreted as the eclogite facies event (Meffan- Main et al., 2004), the second age as the Late Alpine greenschist facies re-equilibration (Freeman et al., 1997; Inger & Ramsbotham, 1997; Meffan-Main et al., 2004). In contrast, U- Pb SHRIMP dating on whiteschists yielded monazite ages at Ma and allanite ages at Ma (Gabudianu Radulescu, Rubatto, Gregory, & Compagnoni, 2009). The monazite is interpreted as a prograde phase, in equilibrium with an early HP paragenesis (chlorite-talc-

6 328 P. Manzotti et al. carpholite?), whereas the allanite records the high-pressure metamorphic peak, being in equilibrium with chloritoid, talc, chlorite, phengite, rutile and apatite. Rosenbaum et al. (2012) have described a sequence of ages, at 39, 36 and 33 Ma, interpreted to record decompression. The difficulty in interpreting Ar-Ar ages in polycylic rocks from the Gran Paradiso Unit, a classic problem (Chopin & Maluski, 1980), has been recently illustrated by Beltrando, Di Vincenzo, and Ferraris (2013). Low temperature exhumation is recorded by fission-track data on zircon ( C at Ma) and apatite ( C at Ma) (Hurford & Hunziker, 1989; Malusà et al., 2005). 4. Textural and compositional features of lithologic types 4.1. Money Unit (Permian (?) Erfaulet metagranite and Permian-Upper Carboniferous Money Complex) Erfaulet metagranite (up to 100 m-thick) is a medium to fine-grained, homogeneous, leucocratic body. It mainly consists of quartz, albite, microcline, biotite, white mica and minor garnet (Figure 2(a)). A strain gradient is locally observed at the boundary with the Money Complex: the low-strain igneous domains that preserve magmatic textures (twinned plagioclase, subhedral mm-scale K-feldspar, quartz, subhedral allanite, biotite, and white mica), show a transition (metres in thickness) to S-tectonitic orthogneisses in which white mica defines a weak foliation. Bt-Amp-bearing orthogneisses display a grey or brown weathering colour (Ballèvre, 1988). These rocks contain quartz, black biotite, alkali feldspar, white mica, garnet and a blue-green amphibole (Figure 2(b)). Alkali feldspar occurs as porphyroclasts ( mm in size). The main foliation is defined by mm-thick layers of quartz + garnet and by the shape-preferred orientation of biotite and amphibole. These rocks commonly contain deformed quartz-feldspar coarsegrained veins, that could represent former pegmatites. Ab-paragneisses are banded in texture. The strong regional foliation is expressed by the alternation of dark green (white mica and locally chlorite) and lighter (quartz, albite) bands and by the alignment of white mica (Figure 2(c)). The paragneisses contain white mica, albite, quartz, garnet, graphite, biotite, chlorite, zoisite and local tourmaline, rutile, and a blue-green amphibole. Albite occurs as porphyroblasts, locally flattened parallel to the main regional foliation: idioblastic crystals of garnet are frequently found in the albite porphyroblasts. Amphibolites occur as dark green fine-grained layers in albitic paragneisses (Figure 2(c)). They are frequently boudinaged in metre- to centimetre- lenses. These rocks consist of colourless, blue-green, and green amphiboles, albite, biotite, ilmenite, titanite, quartz and local garnet, rutile, and epidote. The main foliation is marked by amphibole and biotite, which are parallel to albiterich layers. Detailed mapping of the Money window has revealed two detrital metasedimentary formations displaying conglomeratic layers. The most distinctive characteristics of the two metasedimentary formations are the diversity of the clasts and the amount of graphite. For simplicity, they are hereafter referred as MMF (monogenic meta-sedimentary formation, almost devoid of graphite) and as PMF (polygenic graphite-rich meta-sedimentary formation) respectively. On the map they are depicted in light blue (MMF) and dark blue (PMF). The monogenic metasedimentary formation (MMF), strongly metamorphosed and deformed during the Alpine evolution, mainly comprises meta-conglomeratic levels interlayered with Qzmicaschist bands. Meta-conglomeratic layers are 20 cm to 5 m thick. They essentially consist of quartz nodules (mm- to cm-size), representing former pebbles of quartz veins (Figure 2(d)). Locally the quartz nodules contain new metamorphic crystals of garnet and sparse lamellae of white mica in a

7 Journal of Maps 329 Figure 2. Main rock types and structures in the Money Unit. Letters in white at the bottom of each picture indicate the cardinal points. (a) Erfaulet metagranite: preserved igneous texture with subhedral Kfs crystals; (b) Bt-Amp orthogneiss: L4 lineation is marked by Amp; (c) Ab-paragneiss with a banded structure defined by Wm-Chl layers alternating to Qz-Ab-rich bands. A deformed dark green layer of amphibolite is enclosed in Ab-paragneiss; (d) Quartz meta-conglomerates (MMF), folded by D5 folds and strongly stretched parallel to L4 lineation; (e) Quartz-micaschist (MMF) with mm-size garnet crystals; (f) Stratigraphic contact between meta-conglomeratic layers (PMF) and a cm-thick band of graphite micaschist. The meta-conglomeratic layer contains pebbles and lenses of quartz and fine-grained leucocratic aplitic gneisses. matrix of re-crystallised quartz. Other types of pebbles (e.g., aplites) are rare. Nodules are frequently flattened and stretched parallel to the dominant Alpine structures (i.e., foliation and lineation). The quartz-rich matrix of the meta-conglomeratic layers is devoid of many minerals and mainly contains white mica, garnet, biotite and local chlorite and chloritoid. Locally, thin layers ( m-thick) of fine-grained meta-sediments (former sandstones or siltstones) define the original stratification.

8 330 P. Manzotti et al. Qz-micaschists layers (cm- to m-size) display a grey or brown weathering colour, with reddish brown domains and bands. They consist of white mica, quartz, garnet, albite, graphite ( 5%), chlorite and local biotite and ankerite (Figure 2(e)). Chloritoid occurs as subhedral crystals, generally included in garnet porphyroblasts. Shape-preferred orientation of white mica, local chloritoid, and rarely graphite, as well as quartz-rich layers mark the main foliation. The polygenic metasedimentary formation (PMF), equally strongly deformed and metamorphosed during the Alpine evolution, is well exposed between the right-side moraine of the Money Glacier and the rocky cliffs immediately below the actual front of the glacier (moraines are indicated on the topographic map as lines defined by triangles). It consists of cm- to m-thick grey to black layers of Gr-micaschists, alternating with light grey layers of Qz-micaschists and of grey meta-conglomeratic layers (Figure 2(f)). Differently from the first meta-sedimentary formation, meta-conglomeratic layers contain abundant elongated lenses (i.e., former pebbles) of quartz, fine-grained, leucocratic aplitic gneisses, coarse-grained granitic gneisses and dark, fine-grained, graphite-rich lenses (i.e., former mud clasts). The sedimentary layering is only locally identifiable from the alternation of meta-conglomeratic levels (0.2 1 m-thick) and quartz and graphite-micaschists layers (0.2 1 m- thick). Some graphite-micaschist layers are also irregularly distributed within the meta-sedimentary sequence and may represent former organic-rich mudstones. Matrix of both types of micaschists and meta-conglomeratic layers contain white mica, graphite (.20%), chlorite, titanite, quartz and garnet, which is randomly distributed Gran Paradiso Unit (Permian granitoids transformed to gneisses, pre-permian basement) A brief description of lithologies, belonging to the Gran Paradiso Unit and cropping out close to the boundary with the Money Unit (i.e., up to 100 m above the contact), is presented. Higher up, the lithologies of the Gran Paradiso Unit are left undifferentiated on the map, because they are out of the scope of this paper. Valmiana orthogneisses (Le Bayon, 2006) crop out as lenses (10 15 m-thick) of fine-grained, leucocratic gneisses along both slopes of the Valnontey valley (Figure 3(a)). They define most of the contact between the Gran Paradiso and the Money Units. They contain quartz, feldspar, white mica, biotite, titanite, chlorite and local garnet. They are frequently banded with alternate layers (mm-thick) rich in quartz + feldspar or in white mica. Mylonitic orthogneisses crop out as continuous lenses (5 10 m-thick) observable on both flanks of the Valnontey valley. They are characterised by quartz, albite, muscovite, biotite, titanite and by centimetre-sized porphyroclasts of potassic feldspar (Figure 3(b)). Shape preferred orientation of biotite defines the dominant mylonitic foliation. Grt + Cld-bearing micaschists display a brown-reddish weathering colour. They are finegrained rocks with a pervasive, strongly folded foliation, with submillimetre spacing, marked by alternating white mica + chloritoid- and quartz-rich layers (Figure 3(c)). Garnet, ilmenite, rutile are also frequently observed, whilst retrograde chlorite also locally occurs. Grt-Ab-bearing paragneisses are banded in texture. The strong foliation is expressed by alignment of white mica and by the alternation of darker (garnet-biotite) and lighter (quartzalbite) bands. The gneisses consist of garnet (Figure 3(d)), quartz, albite, white mica, rutile, biotite, chlorite, titanite, ilmenite, and local epidote, tourmaline, and graphite. The common feature of these rocks is the presence of black and round (mm-size) blasts of albite that are often flattened parallel to the main foliation.

9 Journal of Maps 331 Figure 3. Main rock types in the Gran Paradiso Unit. Letters in white at the bottom of each picture indicate the cardinal points. (a) Valmiana orthogneiss occurs as a ten-metre-long scale layer; (b) Mylonitic orthogneiss showing centimetre-sized porphyroclasts of potassic feldspar. The pervasive foliation (S4) is marked by Qz-Fd lithons and by Bt films; (c) Grt-Cld micaschist showing a well-developed foliation (S4); (d) Grt-paragneiss characterised by mm-scale garnet crystals. 5. Geological history of the Money Unit 5.1. Evolutionary stages Meso- and micro-structural analysis, based on overprinting criteria, allowed reconstruction of a polyphase evolution (6 stages) in the Money Unit. The first stage comprises sedimentary and volcanic events probably of Carboniferous age. The second stage is a magmatic event probably of Permian age. It is followed by a poly-deformational and metamorphic Alpine evolution (see Table 2 for details). Previous work (Compagnoni et al., 1974; Le Bayon & Ballèvre, 2006) identified three major events; one pre-alpine and two Alpine stages (Table 1). This work presents a more detailed account of both the pre-alpine and the Alpine history. Table 2 summarises the Table 1. Deformation, metamorphic and magmatic history of the Money Unit, according to Compagnoni et al. (1974) and Le Bayon and Ballèvre (2006). Stage Event Type 1 Pre-Alpine history Upper Deposition of a volcano-sedimentary sequence (Money Complex) Carboniferous-Permian Granitic intrusion (Erfaulet) 2 Alpine collisional history Transposition of the sedimentary layering (Money Complex) Epidote-amphibolite-facies metamorphism 3 Alpine collisional history Open folds with subhorizontal axial planes E-W trending stretching lineation with top-to-the-west shear sense Epidote amphibolite-facies metamorphism

10 Table 2. Relative chronology of meso-structures and their mineral support and schematic relationships between deformation, magmatism, metamorphism and sedimentary history in the Money Unit. Stage Rock Structures Mineralogical Assemblages Metamorphic Conditions 1 Carboniferous? SED1 MAG1 MMF, PMF Sedimentary layering \ \ (S 0 ) Bt-Amp orthogneiss Phenocrysts of Afs Afs \ 2 Upper Carboniferous- MAG2 Erfaulet granite Igneous texture Pl + K-Fs + Qz + Bt +Wm + Aln +Op +Zrn \ Permian? + Ap + Ttn MMF Contact Grt Amphibolite metamorphism 3 Alpine MET3 D3 MMF S3 Grt core + Cld + Rt + Qz + Op Blueschist PMF Grt + Gr + Wm + Rt + Qz Ab-paragneiss Grt + Wm + Rt + Zo + Ttn + Gr Amphibolites Colourless Amp + Grt + Rt 4 Alpine MET4 D4 MMF S4, L4, C type shear bands Wm + Qz + Grt + Cld + Rt + Ap + Op + Zrn + Mnz + Aln + Gr + Ank Blueschist to Albite- Epidote-Amphibolite PMF Wm + Qz + Grt + Gr + Ab + Tur Ab-paragneiss Wm + Grt + Chl + Qz + Zo + Bt +Tur Amphibolites Amp + Bt + Qz + Grt + Ab + Czo/Zo + Ilm + Ep + Chl Bt-Amp orthogneiss Qz + Grt + Bt + Amp + Aln + Ep/Zo + Op + Zrn + Ap + Ab + Wm Erfaulet metagranite Pl + Kfs + Qz + Bt + Grt + Wm + Op + Ttn + Ep/Zo 5 Alpine MET5 D5 MMF S5, F5 P5, A5 Wm + Qz + Grt + Ap + Zrn + Mnz + Aln + Albite-Epidote-Amphibolite Ab + Chl + Gr + Ank PMF Wm + Qz + Grt + Gr + Ab + Ttn Bt-Amp orthogneiss Amp + Bt (Continued) 332 P. Manzotti et al.

11 Table 2. Continued. Stage Rock Structures Mineralogical Assemblages Metamorphic Conditions 6 Alpine MET6 D6 MMF F6, P6, A6 Qz + Chl + Bt + Gr + Zo + Ap Greenschist PMF Wm + Gr + Chl + Qz + Bt + Ep/Zo + Ank Ab-paragneiss Wm + Chl + Qz + Bt + Tur + Ap + Gr Amphibolites Qz + Ep + Chl + Ilm + green Amp + Bt Bt-Amp Qz + Wm + Ep/Zo + Cal + Chl + Op + Ap orthogneiss Erfaulet metagranite Chl + Wm + Zo/Ep + Op (MET metamorphic; D deformational; MAG magmatic; SED sedimentary). Abbreviations are as follows: S foliation; L stretching lineration; F fold; P axial plane; A fold axis. Amphibolite metamorphic conditions associated with contact metamorphism (stage 2) as constrained by Le Bayon and Ballèvre (2004). Journal of Maps 333

12 334 P. Manzotti et al. relative chronology of mesostructures and their time relationships with mineral assemblages. Orientation of structural elements (fold axial planes, fold axes, foliations, lineations) related to the different stages are shown in stereoplots on the geological map. Stage 1 (Carboniferous?): In the PMF the sedimentary layering (S 0 ) is locally identifiable by the alternation of meta-conglomeratic layers (0.2 1 m-thick) and of finer-grained rocks. In the MMF, thin layers ( m-thick) of fine-grained sediments (former sandstones or siltstones, now Qz-bearing micaschists) define the bedding (S 0 ) (Figure 4(a)). Despite rather good outcrop conditions, the shape of the conglomeratic layers is difficult to ascertain. However, the lenticular character of some layers is consistent with deposition in a fluvial environment. In the Bt-Amp-bearing orthogneiss, porphyroclasts ( mm in size) of alkali feldspar are here interpreted as former phenocrysts. For this reason, this orthogneiss is considered as deriving from volcanic rocks. These probably represent a volcanic episode that took place between the deposition of the two sedimentary formations. Stage 2 (Upper Carboniferous-Permian?): This corresponds to the intrusion of the Erfaulet granite. Undeformed hundred- to metre-scale domains of granitoids preserve igneous texture (plagioclase, K-feldspar, quartz, allanite, biotite, and white mica). The boundary between the Erfaulet metagranite and the metasediments was an intrusive contact (Le Bayon & Ballèvre, 2004). Evidence of contact metamorphism is found at the microscale (i.e pre-alpine relics of garnet, Figure 4(b)) in samples collected close to the contact with the Erfaulet metagranite (Le Bayon & Ballèvre, 2004). The lack of mylonites facilitates the preservation of the primary magmatic contact. In addition, the observed meso-structures (i.e., presence of aplitic and pegmatitic dykes (linked to the Erfaulet intrusion) in the Money Complex, and crosscutting relationships with the sedimentary layering) also support this interpretation (left side of the Valnontey valley, close to the Erfaulet bridge, see Figure 3 in Le Bayon & Ballèvre 2004). Stage 3 (Alpine): This stage is only seen on the microscale. An internal S3 foliation is visible in garnet (meta-conglomeratic layers of the MMF) and in albite porphyroblasts (Qz-micaschist of the PMF, Ab-paragneiss, and amphibolite). S3 is expressed by (i) chloritoid + rutile + quartz + opaque (meta-conglomeratic layers of the MMF) (Figure 4(c)); (ii) graphite + white mica + rutile + garnet (quartz-micaschist of the PMF); (iii) garnet + white mica + titanite + zoisite + rutile (Ab-paragneiss); (iv) colourless amphibole + garnet + rutile (amphibolite). Numerical modelling in a large variety of bulk-rock chemistries of metapelites shows that the mineral assemblage garnet + chloritoid + rutile + quartz is stable at pressures higher than 12 kbar. For this reason, we consider that stage 3 developed at blueschist facies conditions (Le Bayon et al., 2006; López-Carmona, Abati, & Reche, 2007, 2010; López-Carmona, Pitra, & Abati, 2013; Žáčková, Konopásek, Jeřábek, Finger, & Košler, 2010). Stage 4 (Alpine): D4 deformation corresponds to S4 foliation, generally parallel to the main lithological boundaries and to the sedimentary layering (S 0 ). Depending on lithology, S4 is marked by quartz + albite lithons (Qz-micaschist and meta-conglomerate of the MMF, Bt- Amp-bearing orthogneiss, Ab-paragneiss, Erfaulet metagranite) and by white mica (Qz-micaschist and meta-conglomerate of the MMF, Erfaulet metagranite, Ab-paragneiss) (Figure 2(c)) + chloritoid + graphite films (Qz-micaschist and meta-conglomerate of the MMF) or it is expressed by the shape preferred orientation of biotite and/or amphibole (Bt-Amp-bearing orthogneiss, amphibolite, Erfaulet metagranite). During this stage a stretching lineation (L4) also developed. Depending on the lithology, L4 is expressed by the shape preferred orientation of white mica (Qz-micaschists of the MMF and of the PMF and Ab-paragneiss), of biotite and blue-green amphibole (Bt-Amp-bearing orthogneiss) (Figure 2(b)) and by the elongation of quartz pebbles in meta-conglomeratic layers of the MMF (Figure 2(d)).

13 Journal of Maps 335 Figure 4. Main structures in the Money and Gran Paradiso Units. Letters in white at the bottom of each picture indicate the cardinal points. (a) 20 cm-thick layers alternate to Qz-micaschist bands (MMF); (b) Petrological evidence of contact metamorphism (Le Bayon and Ballèvre, 2004): Pre-Alpine garnet core in a Qzmicaschist (MMF) close to the contact with the Erfaulet orthogneiss (plane-polarised light); (c) Qz-micaschist (MMF): Garnet porphyroblast with chloritoid and quartz inclusions (stage 3) (plane-polarised light). (d) Metaconglomeratic layer (PMF): centimetre-scale D5 fold; (e) Ab-paragneiss: metre-scale isoclinal D5 fold; (f) Grmicaschist associated with meta-conglomeratic layers (PMF): S4 foliation is folded by D5 folds; (g) Contact between the Money and the Gran Paradiso Units continuously outcroppoing over hundred metres distance; (h) Qz-micaschist associated to meta-conglomeratic layers (MMF): centimetre-scale D6 folds.

14 336 P. Manzotti et al. During stage 4 C -type shear bands also developed; they are marked by the shape preferred orientation of (i) chloritoid and phengite (Qz-micaschist and meta-conglomerate of the MMF); (ii) amphibole and biotite (amphibolite and Bt-Amp-bearing orthogneiss); (iii) white mica and graphite (meta-conglomeratic layers of the PMF); (iv) white mica, biotite and local chlorite (Erfaulet metagranite, Qz-micaschist of the PMF). Because the S3 foliation that is found in the core of garnet and albite porphyroblasts is defined by finer-grained minerals than outside the porphyroblasts, we consider that stage 3 probably represents the prograde evolution towards the development of the main regional foliation (stage 4). During this evolution, a slight change in P-T conditions is recorded by the change in mineral assemblages. For example, in the amphibolites, colourless actinolitic amphiboles, possibly associated to blue glaucophanitic amphiboles, were stable during stage 3 and are replaced by albite + blue-green amphibole (barroisitic) during stage 4. This would record the transition from the blueschist facies (stage 3) to the albite-epidote amphibolite facies (stage 4). Detailed microprobe work is needed to solve this issue properly. The first outcome of this work, now in progress, has been to discover in the MMF detrital pre-alpine garnet grains that were used as nuclei for the growth of Alpine garnet during stages 3 and 4 (Manzotti & Ballèvre, 2013). Stage 5 (Alpine): D5 structures consist of isoclinal, symmetric or asymmetric folds, ranging from centimetres to hundreds of metres in size (Figure 2(d), and 4(d f)). In the MMF and PMF the S4 foliation is transposed into a new S5 foliation, developed under albite-epidote-amphibolite facies conditions, as suggested by S5 mineral support. At the microscale, quartz and chlorite form strain shadows and strain caps around garnet porphyroblasts in Qz-micaschist and in meta-conglomerate of the MMF. The existence of a huge, recumbent fold at regional scale (i.e., at the scale of the Money window) developed during our stage 5 was already recognised by previous workers (Compagnoni et al., 1974; Le Bayon & Ballèvre, 2006). As discussed below, this folding event also affects the tectonic contact between the Money and the Gran Paradiso units, emphasising its regional significance. Stage 6 (Alpine): Epidote or very-fine grained aggregates of chlorite or of white mica sometimes replace garnet. White mica crystals, marking previous structures (i.e., S4, S5, and C -type shear bands), are replaced by chlorite and by green biotite. This stage, visible in all rock types, is locally represented by asymmetric folds, centimetres in size, which gently refold D5 folds (Figure 4(h)). No foliation was developed at this stage. Stage 6 mainly occurs during decompression at greenschist facies conditions, affecting all the lithologies and particularly observable close to the contact between the Money and the Gran Paradiso Units (Figure 4(g)) Tectonic contact between the Money and the Gran Paradiso Units The tectonic contact between the Money and the Gran Paradiso Units is visible on both flanks of the Valnontey valley. It is well exposed between the right-side moraine of the Money Glacier and the rocky cliffs lying immediately below the actual front of the Glacier (Figure 4(g)), whereas, on the left side of the Valnontey valley, it outcrops along the torrent de Les Cheseres, at about 2200 m asl. Field observations and structural data (see Schmidt equal area diagrams on the map) indicate that the contact between the Money and the Gran Paradiso Units occurred during stage 4 and was folded during stage 5, along with the entire Money Unit. This interpretation is in contrast with the previous one (Le Bayon & Ballèvre, 2006), which considered the contact as synchronous to our D5 and was not folded. In the Gran Paradiso Unit, close to the described contact, evidence of the D5 folding phase is also visible (e.g., isoclinal, symmetric or asymmetric folds, ranging from centimetres to hundreds of metres in size. See geological map: right-side moraine of the Money Glacier). In the micaschists from the Gran Paradiso Unit, S4 is marked by the shape

15 Journal of Maps 337 preferred orientation of white mica, chloritoid and by Qz-rich layers, indicating similar P-T conditions than in the underlying Money Unit. The strong topographic relief of the study area allows the geological structures to be reconstructed in 3D. The proposed 3D model is constrained by structural field data. The D5 fold is non-cylindrical with its axis, varying in plunge direction from west to east of the mapped unit (i.e., N2508 and 108 in the east, at the front of the Money Glacier; N2708 and 158 in the axis of the Valnontey valley; N3008 and 258 in the west, close to the torrent de Les Cheseres). 6. Conclusions Petro-structural and lithostratigraphic data collected during mapping in the Valnontey valley led to a map at the 1: 5000 scale and improved the knowledge of the geological history of the Money Unit, better characterising the lithological, structural and petrological features of this unit (see Tables 1 and 2). Two detrital meta-sedimentary formations with diverse sedimentological and mineralogical characteristics are recognised, based on (i) the monogenic versus polygenic character of the metaconglomerates, implying different sources for the detrital material, and (ii) ubiquitous vs localised graphite, implying considerable differences in the rate of deposition of the detrital material with respect to the rate of destruction of the organic matter. Moreover, the two metasedimentary formations occupy different structural positions, being separated by a volcanic sequence. A polyphase history, with two pre-alpine and four Alpine stages of evolution has been recognised. The pre-alpine history comprises evidence of the original sedimentary features (i.e., layering recognised in both metasedimentary formations, and eruption of volcanic rocks) and of magmatism (i.e., intrusion of the Erfaulet metagranite). The Alpine history comprises four stages of deformation and metamorphism (from blueschist to albite-epidote amphibolite then greenschist facies conditions) that record the tectonic history of the whole sequence from its burial (D3) to its exhumation (D5 and D6). Detailed mapping and 3D modelling allowed tracing and delineating precisely the folded tectonic contact between the Money and the Gran Paradiso Units. This contact took place during stage 4 and was folded during stage 5, two stages of regional significance. Software The geological and structural map were drawn using ESRI ArcGIS. The structural data were plotted with GEOrient (version 9.5.0). The cross-sections presented in the geological map and the photographs were compiled using Adobe Illustrator. The 3D model was built using gocad software Acknowledgements This work has been financially supported by the Swiss National Science Foundation (project no. PBBEP2_142155). Careful reviews by J.P. Burg, A. Griffin, and D. Zanoni are greatly acknowledged. The Ente Parco Nazionale Gran Paradiso is thanked for allowing fieldwork and rock sampling in the Valnontey Valley. The Villaggio Alpino Don Bosco is thanked for the accommodation at the Alpe Money hut. References Baletti, L., Zanoni, D., Spalla, M. I., & Gosso, G. (2012). Structural and petrographic map of the Sassa gabbro complex (Dent Blanche nappe, Austroalpine tectonic system, Western Alps, Italy). Journal of Maps, 8(4),

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18 340 P. Manzotti et al. Spalla, M. I., Zucali, M., Di Paola, S., & Gosso, G. (2005). A critical assessment of the tectono-thermal memory of rocks and definition of tectono-metamorphic units: evidence from fabric and degree of metamorphic transformations. Geological Society Special Publications, 243, Spalla, M. I., Zucali, M., Salvi, F., Gosso, G., & Gazzola, D. (2004). Tectono-metamorphic map of the Languard-Campo, Serie del Tonale nappes between upper Val Camonica and Valtellina, Central Alps, Austroalpine domain. Memorie di Scienze Geologiche, 55, Turner, F. J., & Weiss, L. E. (1963). Structural analysis of metamorphic tectonites. McGraw-Hill, New York, pp Vidal, O., Parra, T., & Trotet, F. (2001). A thermodynamic model for Fe-Mg aluminous chlorite using data from the equilibrium experiments and natural pelitic assemblages in the 1008 to 6008C, 1 to 25 kb range. American Journal of Science, 301, Wei, C. J., Powell, R., & Zhang, L. F. (2003). Eclogites from the south Tianshan, NW China: petrological characteristic and calculated mineral equilibria in the Na2O-CaO-FeO-MgO-Al2O3-SiO2-H2O system. Journal of Metamorphic Geology, 21, Whitney, D. L., & Evans, B. W. (2010). Abbreviation for names of rock-forming minerals. American Mineralogist, 95, Žáčková, E., Konopásek, J., Jeřábek, P., Finger, F., & Košler, J. (2010). Early Carboniferous blueschist facies metamorphism in metapelites of the West Sudetes (Northern Saxothuringian Domain. Bohemian Massif). Journal of Metamorphic Geology, 28, Zucali, M. (2002). Foliation map of the Eclogitic Micaschist Complex (Monte Mucrone - Monte Mars - Mombarone, Sesia-Lanzo, Italy). Memorie di Scienze Geologiche, 54, Appendix A: 3D modelling approach with the gocad 3D modeller The gocad 3D modeler uses a discrete representation of geological objects through regular (grid) or irregular meshes (polygonal curves, triangulated surfaces, and tetrahedralised solids; Mallet, 2002). 3D reconstruction of geological objects (e.g., faults or stratigraphic contacts) from structural planar and linear data is based on the Discrete Smooth Interpolation (DSI, Mallet, 1997; Mallet, 2002). A wide range of constraints was used to take into account the different types of data, but also to ensure geometrica and geological consistency between objects (Mallet, 2002; Caumon et al., 2009). A geological database, including all available data has been integrated in to gocad and comprises:. a 20 m resolution Digital Elevation Model (DEM) obtained from the Geoportale Nazionale (the Italian National Geoportal; Polygonal features, representing geological boundaries, derived from this study and draped over the DEM,. field structural measurements (e.g., bedding, foliation, fold axes and planes) obtained from this study. A surface, representing the contact between the Money and the Gran Paradiso Units has been constructed. A purely 3D approach, based on geological field data, has been used for this purpose. This method avoids the use of cross sections as an intermediate step that can mask the real 3D geometry of geological structures (Fernández et al., 2004). The contact surface has been interpolated in 3D with the DSI methods, which it is based on the outcrop curve, foliation and axis orientation/plunge data, according to the procedure described in Le Carlier de Veslud et al. (2009), Salles et al. (2011), and Caumon et al. (2009).