Geology of the Saint-Marcel valley metaophiolites (Northwestern Alps, Italy)

ABSTRACT The geological map of the Saint-Marcel valley at the scale of 1:20,000 illustrates the tectonic setting of metaophiolites from the southern Aosta Valley, in the Italian side of the Western Alpine belt. The map highlights the sharp contact between the metaophiolitic basement and its metasedimentary cover, which mainly consists of quartzites, marbles, and calcschists. In spite of the Alpine tectonics, this contact is regarded as deriving from the original oceanic crust/sediments interface. Metaophiolites mostly consist of metabasalts hosting Fe–Cu sulphide mineralisations, characterised by high-pressure metamorphic imprint. These rocks likely represent the shallowest portion of the Tethyan oceanic lithosphere created near the axis of the slow-spreading ridge where hydrothermal fluid circulation was active. Selected key-sections through metasediments reveal a consistent internal lithostratigraphy, in spite of the pervasive metamorphic and tectonic reworking acting during the Alpine evolution. Metasediments reflect various sedimentation episodes starting from pelagic and proximal settings to the onset of the orogenic stage. The Saint-Marcel valley metasediments thus reflect a changing in the sedimentation environments through time and space during the overall geologic evolution.

The geological map at 1:20,000 scale presented here derives from a compilation of maps originally realised at the 1:10,000 and 1:5000 scales, including PhD's and Master's Degree theses (Campana, 1988;Martin-Vernizzi, 1982;Tartarotti, 1988), implemented by a field mapping recently carried out by the authors of the present paper. This map represents the first geological detailed map produced in the Saint-Marcel valley. The aim of the present study is twofold: (i) to illustrate the tectonic setting of metaophiolites and overlying Austroalpine units, and, foremost, (ii) to describe the lithostratigraphy of metasediments associated with metaophiolites. The map is integrated with five lithostratigraphic columns and two geological sections.
In the northern Saint-Marcel valley, the ductile early-alpine structures are still recognisable and represented by dm-to-m-sized isoclinal folds with N-S trending sub-horizontal axes and steeply dipping axial planes. These represent the main D 2 structures, which fold earlier foliations. The main foliation (S 2 ) strikes N-S and dips 60°-80°to W. It has been interpreted as an eclogite-facies foliation defined by glaucophane, garnet, and omphacite (Martin & Tartarotti, 1989). The eclogite boudins within glaucophanite preserve D 1 eclogite structures (see Figure 2(b)).
Relics of an earlier deformation pre-dating the eclogite peak have been recognised only in the core of zoned garnets of the glaucophanites, chlorite-schists, and quartzites (Tartarotti & Caucia, 1993), whereas the D 1 structures are commonly absent in the matrix due to the dominant mylonitic character of the D 2 deformation phase. The D 2 phase was superimposed by greenschists-facies D 3 phase characterised by folds with 20°-60°steep axial planes and N-S trending axes (Martin & Tartarotti, 1989).

Key-sections lithostratigraphy
From north to south along the Saint-Marcel valley, five key-sections were selected to illustrate the lithostratigraphy of the metasedimentary cover sequence, and its relation with the ophiolitic basement: the Bois de Fontillon-Servette, Mont-Roux, Mont-de-Corquet, and the Pointe-de-Plan-Rue sections (located on the eastern side of the valley); the Rouallaz section located on the western side of the valley. These sections are characterised by a well defined, often sharp, contact between metaophiolite and metasediments, with scarce repetition of metaophiolitic layers within metasediments. The present thickness of the metasediments increases from a few metres up to 800 m going from the north to the south along the valley.
The Bois de Fontillon-Servette section is located in the northern valley ( Figure A in the map). The ophiolitic basement is well exposed near the Fontillon Damon farm, along a N-S-trending ridge (roche moutonnée), about 800-m long and 70-m high. Rocks are garnet-bearing glaucophanites including cm-to dm-sized eclogite boudins (Figures 2(a-c). Layers of glaucophanite alternate with layers of garnet-chloritoid-glaucophane-bearing chlorite-schists and minor talcschists (see Figure 2 (a)), as a result of tectonic transposition of basalt and basalt-derived rocks (Alpine D 2 phase; Martin & Tartarotti, 1989) that occurred under eclogite-facies metamorphic conditions. The Bois de Fontillon ophiolitic basement continues southwards to the Servette mining district, where it hosts Cu-Fe sulphide ores (i.e. the Servette and Chuc deposits; Castello, 1981;Dal Piaz & Omenetto, 1978;Martin et al., 2008). Metaophiolites are here associated with a metasedimentary cover sequence, about 10 m-thick, made of (from bottom to top) fine-grained metarodingite (probably original basalt), garnet-bearing micaceous quartzites including clinopyroxene, blue-amphibole, and relict cummingtonite included in garnet (Tartarotti & Caucia, 1993). Actinolite-schists occur at the contact between metasediments and metaophiolites (see Figure A in the map).
The Mount-Roux section ( Figure B in the map) is characterised by a ca. 100 m-thick metasedimentary sequence lying on top of Mg-metagabbros and prasinites (i.e. fine-grained albite-actinolite-chlorite-epidote-bearing rocks deriving from metabasalt or metagabbro) enclosing eclogite boudins. The contact between mafic rocks and metasediments is marked by actinolite-schists (bearing Cr-rich mica). Metasediments consist of (from bottom to top; see Figure B in the map) micaceous quartzites, impure marbles (Figure 3(a)), manganiferous quartzites (Figure 3(b)), carbonate-rich micaceous quartzites (Figures 3(c,d) alternating with fine-grained pale-grey marbles ( Figure  3(e); see also Figure 2(d)). This sequence is followed by calcschists with a typical flysch-like appearance consisting of siliceous marly limestone alternating with shaly beds (Figure 3(f); see also Figure BIII in the map), and minor quartz-carbonate-rich micaschists ('terrigenous metasediments' in the legend of the map) and micaceous quartzites. Calcschists may also include mafic layers or boudins.
The Mont-de-Corquet section ( Figure C in the map) is characterised by a 150 m-thick metasedimentary sequence making a cliff along the eastern side (right) of the Saint-Marcel valley. The metaophiolitic basement is not exposed, although amphibolites are interlayered within metasediments. The cover sequence is here overturned and consists (from top to bottom) of manganiferous metacherts, green micaceous quartzites, micaceous marbles, and flysch-type calcschists.
The Pointe-de-Plan-Rue section ( Figure D in the map) includes a metasedimentary sequence cropping out on the overturned limb of a hectometric recumbent fold exposed on the eastern side of the valley (Figure 2 (e)). The core of the fold is made of glaucophanite rich  Siivola and Schmid (2007). in pseudomorphs after lawsonite. Metasediments are represented by micaceous quartzites (Figure 3(f)), manganiferous quartzites, impure marbles, carbonaterich micaceous quartzites, quartz-chloritoid-rich micaschists covered by marbles and flysch-type calcschists. On the Plan-de-Rue peak, calcschists reach a thickness of about 50 m (see Figures DI, DII in the map and Figure 2(e)).
The Rouallaz section ( Figure E in the map) crops out on the western side of the Saint-Marcel valley, below the tectonic contact separating the Austroalpine Mount-Emilius klippe from the metaophiolites, here consisting of garnet-bearing glaucophanites with relict pillow structures. The metasedimentary sequence is overturned (see Figure EIII) and is made of (from top to bottom) micaceous quartzites, amphibole-chlorite-rich quartzites, flysch-type calcschists alternating with graphitic micaschists and marbles.
Summing up, the Saint-Marcel valley metasedimentary sequence starts, from bottom to top, with manganiferous quartzites followed by fine-grained, impure marbles. These two lithologies are associated with several types of quartzites and micaschists. This rock assemblage is then covered by flysch-type (like the Palombini shales) calcschists with variable thickness, from tens to hundreds metres, as observed along the Mount-Roux-Grand Crète ridge (see the map). The contact between the quartzites/micaschists series (the thickness of which never exceeds 10-15 m) and the flysch-type calcschists is often marked by terrigenous calcschists or by graphitic calcschists, as observed for example in the Rouallaz section.

The sulphides and manganese mines of the Saint-Marcel valley and their anthropic significance
The Fe-Cu hydrothermal sulphide deposits of Bois de Fontillon-Servette and Chuc-Eve Verda spring and the famous Praz-Bornaz (or Praborna) Mn mine, now abandoned, have been exploited since the seventeenth century or since the Roman time. The ruins of the old foundry of Treves (lat. 45°42.444 ′ N, long. 7°2 7.432 ′ E, alt. 1672 m) and huge amounts of slags derived from the Servette ore processing (long. 7°2 7.304 ′ E, lat. 45°42.449 ′ N, alt. 1659 m) can be still observed in the Druges-Damon area, in the Northern Saint-Marcel valley. The slags are composed of fayalite, wustite, spinel, sulphides, and interstitial glass and include carbonised wood. The 14 C datation of a piece of carbonised wood (Picea excelsa) enclosed in a slag of Servette suggests exploitation in the Middle Age (1120 ± 40 BP) (GX-29281; Tumiati et al., 2005). Near the intersection with the old 'Strada Cavour' a large heap of slags (lat. 45°41.660 ′ N, long. 7°27.134 ′ E, alt. 1734 m) occurs. These slags have given a more recent age (<100 B.P.; GX-29282; Tumiati et al., 2005), probably referred to the period when Servette was exploited by the Challant family, as recorded by Nicolis de Robilant (1786-87; see also Cesti, 1978;Lorenzini, 1998;Martin, Godard, & Rebay, 2004).
The Cu-Fe Chuc mine is located on the left side of the river at altitudes between 1283 and 1443 m (see the map); it has been in activity from 1854to 1950(Lorenzini, 1998. The village of Chuc is located on the right side of the river (long. 7°26.863 ′ E, lat. 45°42.044 ′ N, altitude 1422 m; not shown in the map) and now consists of ruined houses. A few hundreds of metres below Chuc, the pathway crosses a small river that descends from Servette to the Eve Verda spring (long. 7°26.940 ′ E, lat. 45°42.094 ′ N, altitude 1371 m). The water, when saturated in Cu, pours out a blue gel that covers the pebbles on the river bed. This blue gel was described by Saint- Martin de La Motte (1784-85), de Saussure (1796 and Prosio (1903). The gel is made of almost amorphous Cu hydroxide (woodwardite), which likely precipitates during a change of pH (Tumiati, Godard, Masciocchi, Martin, & Monticelli, 2008).
The ancient Praz-Bornaz (or Praborna) mine (long. 7°26.968 ′ E, lat. 45°40.774 ′ N, alt. 1894 m) was intensively exploited by the Challant and Davise families during the seventeenth and eighteenth centuries (archives of the Aosta Province bureau). Braunite was used by the glassmakers of Murano, near Venice to fade glass, thanks to the relatively high electronegativity of Mn. The ore consists of manganiferous quartzites, including a 4-8 m-thick boudinaged layer rich in braunite (Mn 2 Mn 3 6 SiO 2 ) associated with the glaucophanites and eclogites.

Methods
The map is at 1:20,000 scale and encompasses an area of about 110 km 2 . The geological map derives from original fieldwork including (i) a compilation of maps originally realised at the 1:10,000 and 1:5000 scales during PhD's and Master's Degree theses by S. Martin, R. Campana, and P. Tartarotti; (ii) additional maps recently performed by the authors at the 1:10,000 scale. Cross-sections are oriented perpendicular to the overall trend of the main structures.
Geological mapping was performed using topographic maps (Carta Tecnica Regionale) released by the Regione Autonoma Valle d'Aosta -Assessorato territorio e Ambiente (Aut. n. 1611 del 17/07/2015) and represented on a vector topographic map derived from the CTRN (Carta Tecnica Regionale Numerica 1:25,000; Coordinate System represented in meters ED50-European Datum 1950, UTM Zone 32 Nord). Quaternary deposits have been mapped by original fieldwork supported by photointerpretation of aerial images of the Regione Valle d'Aosta.

Discussion and conclusions
Field work data allowed us to elaborate the geological map provided at the scale of 1:20,000 with two geological sections, and to characterise some significant lithostratigraphic series. Petrographic observations also furnish further data on the nature and composition of metasedimentary rocks covering the metaophiolites. We summarise in the following the main results and highlight some topic of research to be developed in the future.
The metaophiolitic basement mostly consists of metabasalts showing Alpine high-pressure metamorphic imprint, and hosting Fe-Cu sulphides ores. These rocks likely represent the shallowest portion of the Tethyan oceanic lithosphere characterised by lava flows, pillow basalts, and derived rocks. The abundant Fe-Cu and Mn-rich ore deposits characterising the Saint-Marcel valley metaophiolites indicate that the corresponding portion of oceanic lithosphere was probably created near the axial part of the ocean ridge where hydrothermal fluid circulation was active. The close association of metabasalts and Fe-Cu-Mn mineralisations make the Saint Marcel valley metaophiolites almost unique among the eclogite-facies Alpine metaophiolites. A similar association has been described only in the eclogite-facies Monviso metaophiolites, which preserve the signature of mid-ocean ridge hydrothermal alteration (e.g. Nadeau, Philippot, & Pineau, 1993) and mineralisations (e.g. Rolfo et al., 2015 and refs. therein). By contrast, the Saint-Marcel valley metaophiolites strongly differ from other Alpine metaophiolites (e.g. the Platta and Tasna ophiolites) which are located in a palaeogeographic position consistent with ocean-continent transition zones (i.e. 'OCT' settings; e.g. Manatschal & Müntener, 2009).The sharp contact between the Saint-Marcel valley metaophiolites and metasediments, starting with manganiferous quartzites, indicates the partial preservation of the original oceanic stratigraphy in spite of the Alpine deformation. This contact may be regarded as a marker for reconstructing the lithostratigraphy within the metasedimentary cover sequence. The basal manganiferous quartzites may represent the metamorphic analogoues of pelagic sediments rich in Mn-oxides, like those found in hydrothermal Mndeposits distributed in modern oceans (e.g. Usui & Someya, 1997; see also discussion in Tumiati et al., 2010). The Praborna Mn-rich deposit in the Saint-Marcel valley has been interpreted as the high-P metamorphosed equivalent of the Jurassic Mn-rich cherts (e.g. Tumiati et al., 2010).
The Saint-Marcel valley metaophiolites also differ from many sequences of the Piemonte Nappe (e.g. Lagabrielle et al., 2015 and refs. therein;Tartarotti, Festa, Benciolini, & Balestro, 2015;Tricart & Lemoine, 1991) for not showing the direct juxtaposition of oceanic sediments upon serpentinised peridotites and associated gabbroic intrusions. Comparisons with modern slow-spreading oceans have lead many authors to interpret the primary association of serpentinites and sediments as a signature of tectonic mantle denudation in the Jurassic Tethys (e.g. Lagabrielle & Cannat, 1990;Lemoine, Boillot, & Tricart, 1987;see review in Lagabrielle, 2009). Such model is supported by the occurrence in the Alpine metaophiolites of thick shear zones recently interpreted as a record of oceanic detachment faults (e.g. Balestro, Festa, & Tartarotti, 2014;Manatschal et al., 2011).
The Saint-Marcel valley metasedimentary sequence is reconstructed from the key-sections described in this paper, and consists, from bottom to top of: (i) a basal series characterised by manganiferous quartzites commonly grading to fine-grained, impure marbles; (ii) an intermediate series consisting of micaceous quartzites and micaschists; (iii) a top part made of three different rock types: quartz-rich calcschists ('terrigenous metasediments' in the map legend), graphitic calcschists, and flysch-type calcschists ('undifferentiated calcschist' in the map legend). In the upper Saint-Marcel valley, as in the nearby Pointe Tersiva and Rosa dei Banchi peaks, these upper calcschists reach a thickness of a few hundred metres.
In spite of metamorphism and the absence of fossilifereous contents (the only age constraint is the Eocene age of the eclogite-facies metamorphism in the metaophiolites; Dal Piaz et al., 2001), we infer that the Saint-Marcel metasedimentary sequence records the tectonosedimentary events of the Tethyan basin, from oceanisation to subduction and collision in the Alpine belt. Based on correlations with other sections of the Western Alps (see the synthesis paper by Deville et al., 1992), and comparison with the unmetamorphosed stratigraphic sequences in the Northern Apennines (e.g. Abbate et al., 1980;Marroni et al., 1992), we suggest that the Saint-Marcel sediments were deposited in different environments during at least three main diachronous events. The basal series was deposited in the abyssal plain of the Tethys ocean during the Middle to Late Jurassic (e.g. Chiari et al., 2000;De Wever & Caby, 1981), when oceanic spreading was still active. The overlying series consisting of micaceous quartzites and micaschists may derive from detrital material delivered either by the proximal passive continental margin during the extensional stage or by the overriding continental plate after the onset of subduction. The age of this series cannot be easily constrained, since it differs from other sequences of the Western Alps (e.g. Valais, French-Italian Alps) or Apennines showing a similar stratigraphic position, which are commonly characterised by abundant detrital deposits and olistoliths of both continental and ophiolitic nature. The sequences from the Valais and French-Italian Alps have been attributed to early episodes of the Alpine compressive tectonics occurred during the early Late Cretaceous (e.g. Fudral, Deville, & Marthaler, 1987;Lagabrielle & Polino, 1988;Lemoine et al., 1984;Marthaler, 1984). The equivalent sedimentary units in the Apennines are represented by the 'Casanova' complex of Cenomanian-Turonian age (e.g. Abbate, Bortolotti, & Principi, 1984;Decandia & Elter, 1972).
The Saint-Marcel uppermost calcschists are interpreted as syn-orogenic flysch. These rocks although completely devoid of ophiolitic detrital material, likely correspond to the metaflysch series occurring on top of the Schistes Lustrés nappe of the French-Italian Alps (see Deville et al., 1992), and possibly to the 'Flysch a Helmintoides' formation in the Apennines dated Upper Cretaceous (e.g. Decandia & Elter, 1972).
In conclusion, the Saint-Marcel valley represents a meaningful case for the understanding of the origin and evolution of the Alpine metaophiolites. Among metaophiolites involved in the Alpine subduction, those of the Saint-Marcel valley are particularly rich in basaltic rocks which also host sulphides and manganese mineralisations. This assemblage can be interpreted as deriving from a mid-ocean ridge segment affected by hydrothermal fluid circulation. Moreover, the Saint-Marcel valley metasediments refer to sedimentation ranging from pelagic and proximal environments to the onset of the orogenic stage. Given the absence of any available age for these sequences, and taking into account the effect of pervasive tectonic transposition, we suggest that the different types of metasediments could reflect a changing in the sedimentation environments through time as well as through space during the overall geologic evolution. Indeed, the Alpine deformation could have contributed to juxtapose sedimentary sequences originated in different, even if adjacent, environments. Further structural and petrographic studies on the Saint-Marcel valley metasediments, integrated with data from present-day comparable settings are needed to better constrain the nature of the supraophiolitic cover sequence.

Software
The geological dataset has been implemented and validated in the ESRI ® GIS environment (ArcGis ® ). The geometric primitives and alphanumeric attributes of interest for the map production have been extracted in shp file format from the ESRI ® geodatabase. These features have been imported into the Adobe Illustrator ® environment through MAPublisher ® . This allowed keeping intact, readable, and editable both the geometric/spatial component and the descriptive part of these features. Finally, we have used Adobe Illustrator ® for the cartographic layout and letterpress, for all drawings and photo assemblage.
Piaz for constructive discussion in the field. The Regione Autonoma Valle d'Aosta, Assessorato Territorio e Ambiente bureau, is acknowledged for providing the necessary digital support (Authorization Nb. 1611 on July 17th 2015).

Disclosure statement
No potential conflict of interest was reported by the authors.