Lithotypes
Internal Complex
Micaschists range from fine to coarse grained; the former characteristically show intense rusty orange to dark brown weathering, the latter weather to pale orange or greyish colours.
Typical of (eclogitic) micaschists in the Sesia Zone is their compositional banding, with phyllosilicate-rich, felsic and mafic layers. Minerals in light layers comprise Wm + Qz + Ep ± Gln ± Rt, rarely also Cld; minerals in the basic layers typically include Gln ± Omp ± Wm ± Grt, Ep ± Qz.
Leucogneiss bands occur from a few centimetres to several meters in thickness, mostly parallel to the main foliation in micaschist; they consist of Wm, Qz, Zo, and sulphides.
Qz bands occur mostly parallel or at small angles to the main foliation with a mean thickness of 5-15 cm.
The main structural characteristic is a composite foliation marked by the eclogite facies mineral association described above.
Close to the contact with the External Complex (e.g. N of Albard, N of Cime Bianche, W of Weiss Weib) a marked retrograde greenschist facies overprint is observed, such that “augen” Ab, green Amp, Ep or Czo, Chl, Ttn (rimming Rt and as single crystals) almost completely replace the HP assemblage. Greenschist retrogression is particularly strong close to the contact with the External Complex (details in chapter 4).
Gneisses are from completely massive to foliated in texture, grain size varies from 2-30 mm; the weathering colour is mostly light grey or pale orange. Gneisses are dominantly felsic; they range from compositionally homogeneous to banded, with light Qz-feldspars bands and bluish green mafic ones. They contain Qz, Wm, blue Amp ± Omp, Grt, Zo, and Rt.
The main eclogitic foliation can be retrogressed to greenschist facies, with overgrowths of Ab, Chl, Ep-Czo, green Amp, and Ttn (details in chapter 4).
East of Prial, unusual bands are observed with dark red Grt (ø 2-4 cm); they probably are pre-Alpine.
Some metagranite - metagranodiorite bodies occur (e.g. Mt Las d’ Arnes, Pont de Trenta), massive in texture, weathering light grey. They consist of Wm, Zo, magmatic Aln, Gln, Grt, Qz, and Rt; where overprinted at greenschist facies Ab, Ep, green Amp, Chl, and Ttn occur.
Eclogites are dark green to bluish in outcrop and appear as massive or slightly foliated rocks. They are composed of Omp, Grt, Ep, Qz, Rt ± blue Amp ±Wm.
Some eclogites contain veins, in which Omp and Gln can be several centimetres in size. Banded eclogites occur at Col Fenêtre, showing alternating pink (Grt-rich) and green (Omp-rich) layers, both mm to cm thick.
Metagabbros appear as massive rocks with white cm-size patches of pseudmorphic after magmatic plagioclase and dark ones after magmatic pyroxene. They now contain variable proportions of Zo, blue Amp ± Omp, ± Wm, and Rt. Local greenschist facies overprint produced green Amp, Ab, Czo, Chl, and Ttn rimming Rt.
Eclogitized amphibolites crop out close to the top of Croix Courma as a massive lithotype of dark green-bluish colour. They are composed of coarse (cm), dark blue to black Amp with high lustre, Grt, plagioclase, and Qz; the partially eclogitized types also contain Wm, Omp, and Rt, and Grt coronae around the pre-Alpine Amp.
Silicate marbles range from light yellow to white massive marbles, to yellow cornieules, to grey phyllosilicate marbles. The massive marbles are partially dolomitized, show sugary texture and mm-cm bands of silvery silicates, notably Tlc and Tr (E and S of Mont de Prial). Cornieules are blocky and contain Qz veins (E and S of Mont de Prial). The greyish marbles are rich in Wm and Tlc with breccia fragments, millimetres to metres in size, of the same lithotype (E and S of Mont de Prial). Rarely, the silicate marbles contain also Grt, Ep, plagioclase, Amp and Ttn (e.g. E of Croix Courma).
Marbles with HT relics outcrop near Fontainemore. They contain green, pre-Alpine Cpx, phyllosilicates, Zo and sulphides; they host basic boudins (dm to m in size; Castelli, 1991).
Quartzitic gneisses are grey in colour; they show a cm-spaced foliation. They consist of Qz, Wm, Zo, Grt, and Rt; where overprinted in greenschist facies, Chl, Czo, Ab, and green Amp are present.
Calcschists display greyish-brown weathering and a mm-spaced foliation marked by aligned Wm, Ep, Gr ± Omp in addition to Cal. The schists contain centimetric bands of gneiss and eclogite boudins. Locally an increase in phyllosilicate contents produces calc-micaschists.
Close to Pesse calc-micaschists are associated with eclogites, impure marbles and impure Quartzites.
Andesitic dykes are massive and fine grained, weathering green to light grey. Ab and green Amp occur as phenocrysts in a dark grey matrix, which locally contains lithic fragments with Ep and Wm defining a foliation. The dykes show sharp contacts to their host rocks.
East of Faretta the percentage of phenocrysts is higher, and a melanocratic enclave is found. Close to Rechantier possibly two generations of andesites occur, with crosscutting relations; feldspar, Bt and Amp are phenocrysts.
External Complex
2DK lithotypes
Amphibolites and granulites preserve high metamorphic grade assemblages of pre-Alpine age with a weak to almost no Alpine overprint. Outcrop colours vary from dark to light grey. These rocks are composed of Qz, Grt, plagioclase ± hornblende and minor Wm + Chl; pyroxene may be present locally (e.g. Forka Valley, also in Dal Piaz et al., 1971). Bt is replaced by Chl and opaque minerals along the cleavage. Centimetric Grt bands and pods are abundant (e.g. ponds East of Corno del Lago).
Migmatites showing variable leucosome structures occur north of Trentostag. Pegmatites up to some decimetres wide are found as whitish to light blue bands; they show cm-grain size and are mainly composed of blue Qz, plagioclase ± light pink Grt (e.g. Mt. Aquila).
Banded gneisses appear with their typical rusty weathering colour; bands are from <1 mm to dm, light and dark-bluish when fresh, with sharp contacts. Light bands are composed mainly of Qz and minor Wm, dark bands are composed of Wm, Ep, Chl and Gr. Large Ttn crystals and earthy pseudomorphs of Aln and Ap after pre-Alpine monazite are characteristic. The main foliation is in greenschist facies, pervasive to mylonitic, with very narrow spacing (mm and less).
Pre-Alpine structures are visible in some outcrops (e.g. migmatite folds at Münu); leuco- and melanosome portions are distinguishable in thin sections.
This lithotype show sharp contacts against orthogneisses of the External Complex.
Amphibolites outcrop as dark green to brown bodies near Machaby village and east of Col Chasten (also in Dal Piaz et al., 1971). They consist of a pre-Alpine green-brown pleochroic hornblende overgrown by green-light blue Amp (Machaby), Ab, Czo, ± Wm, ± Chl, Ttn, sulphides and oxides. East of Col Chasten the grain size can increase up to several mm.
Orthogneisses range from massive and homogeneous, grey types to banded ones with light and dark layers. Qz, Ab, Wm, Ep/Czo, Chl ± green Amp ± sulphides occur, often with magmatic relics such as Kfs, brown Aln, and Ap.
The main greenschist facies foliation is tight, with a spacing of several millimetres. Locally the foliation is mylonitic (e.g. close to Chavanisse), with <1 mm spacing; grain size reduction and a strong stretching lineation marked by Qz and Wm occur (e.g. north of Arnad); Kfs, where present, shows “augen”.
Leucocratic layers are frequent, from a few cm to several dm, oriented parallel to the main foliation. They contain Ab, Qz, Ep and Wm ± Kfs. In a few localities (e.g. Prouvy-Arnad) basic layers and/or boudins are present, parallel to the main foliation. They appear green and contain Ep, green-blue Amp, Chl, Ab, Ep, and Wm; Rt is seldom preserved in Ttn cores.
Layers of orthogneiss with pink-reddish Grt (mm to cm in size) occur in few localities (i.e. E of Pelluas, S and E of Mont Crabun, NW Albard de Bard).
Transitions from orthogneiss to micaschist are usually gradual, with an increase in phyllosilicate contents.
Paragneiss / micaschists show bright bands rich in Ab + Qz alternating with dark ones rich in Wm, Chl, and Ep. Layers are mm to cm thick, fine grained, showing rusty colours on weathered surfaces.
Marbles outcrop South of Verdoyen near the boundary with the Piemonte-Liguria Ocean derived units. They are grey and <1m thick and in sharp contact with overlaying micaschists. Apart from Cal, they contain accessory Wm and green Amp.
Impure Quartzites are grey in colour and again <1m thick (south of Verdoyen, east of Arnad). These are composed of Qz, Wm, Czo, green Amp and Chl. West of Salé impure Quartzites display a purple colour (Mn), are of similar dimension and in contact with overlying gneiss. In thin section this Quartzite shows fine grained Chl and Grt-rich layers; Ab and opaque minerals are also present. These were interpreted as metacherts by Beltrando et al. (2014a).
Calc-micaschists are silvery to dark grey, with an earthy aspect when weathered. They are composed of Wm, Chl, Ep and Cal. They outcrop north and east of Arnad in bands up to several meters thick.
Barmet Shear Zone
Metadiorites-metagranites appear as homogeneous and massive rocks, light grey in colour, with grain sizes of a few mm. They are composed of Qz, Ab, Ep, Wm, Chl ± Kfs ± Bt ± Grt ± green Amp ± blue Amp ± Ttn. The main gneissic foliation is marked by alignment of the greenschist facies minerals.
Rare basic boudins (~ 3-10 cm) are found close to Simonetta. Their minerals include Omp, now preserved at the core of fine grained domains with Ab and green Amp, Ep, and Wm.
Banded gneisses occur with alternating layers, several cm to meters thick, of leucocratic and melanocratic types. The former show Qz, Ab, and Kfs, the latter are rich in Wm, Chl, and Ep.
Paragneisses show rusty weathering and are composed of Qz, Wm, Grt, Ab, Ep, Chl and Rt ± Gln ± Omp. The main foliation is in greenschist facies but HP minerals (Gln, Rt and Omp) commonly remain.
Marbles with yellow weathering colour mostly show an intense to mylonitic foliation. Typically they are silicate-rich with layers rich in Wm ± Tr ± Omp ± Tlc ± Ab ± Ep ± Ttn. However, bands of pure marbles with a sugary texture occur, e.g. at Pian Dzen. Marble bodies vary in thickness from metres to several decametres and show a variable degrees of dolomitization.
Metabasites commonly occur in bands of a few cm (e.g. Cime P. Blanches) and elongate bodies inside the marbles parallel to the main foliation. In thin section, Omp appears mottled and is overgrown by pale green to colourless Amp.
Gneiss and basic blocks from centimetric to decametric occur in a marble matrix and are wrapped by the main foliation (e.g. ridge E of Mont Crabun).
Eclogitic fragments with sharp edges occur sparsely. At Col D’Arche dm-size boudins containing long prisms of white carbonate were discovered, probably these are pseudomorphic intergrowths of Cal after Arg, which are commonly found in HP-LT terrains and are observed also near Quincinetto (lower Val d’Aosta; Compagnoni et al. 2014).
Carbonate breccia outcrop W of Mont the Prial with a thickness of few meters. It is a massive lithotype composed of a matrix rich in carbonate and Wm with minor Gln. It includes fragments (from mm to several cm) of Gln gneiss and micaschist. The HP foliation of the clasts shows random orientations; hence these fragments were incorporated in the carbonate matrix after they acquired their HP foliation.
Cornieules shows a thickness of circa 4 meters and outcrop E of Simonetta. Their appearance is massive with an intense yellow colour and numerous holes (cm to dm in size) with parallelepipedal to cubic shapes.
References
Beltrando, M., Manatschal, G., Mohn, G., Dal Piaz, G. V., Vitale Brovarone, A., & Masini, E. (2014a). Recognizing remnants of magma-poor rifted margins in high-pressure orogenic belts: The Alpine case study. Earth Science Reviews, 131, 88–115.
Compagnoni, R., Engi, M., & Regis, D. (2014): Val d’Aosta section of the Sesia Zone: Multi-stage HP metamorphism and assembly of a rifted continental margin. In 10th International Eclogite Conference, Syn-Conference Excursion, 5 September 2013, GFT-Geological Field Trips (Vol. 6 (1.2), pp. 1–44).
Castelli, D. (1991). Eclogitic metamorphism in carbonate rocks; the example of impure marbles from the Sesia-Lanzo Zone, Italian Western Alps. Journal of Metamorphic Geology, 9, 61-77.
Dal Piaz, G. V., Gosso, G., & Martinotti, G. (1971). La II Zona Diorito-Kinzigitica tra la Valsesia e la valle d’Ayas. Memorie della Società Geologica Italiana, 10, 257–276.
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