Eclogitic metamorphism in the Alpine far-west: petrological constraints on the Banchetta-Rognosa tectonic unit (Val Troncea, Western Alps)

The Banchetta-Rognosa tectonic unit (BRU), covering an area of 10 km2 in the upper Chisone valley, consists of two successions referred to a continental margin (Monte Banchetta succession) and a proximal oceanic domain (Punta Rognosa succession) respectively. In both successions, Mesozoic meta-sedimentary covers discordantly lie on their basement. This paper presents new data on the lithostratigraphy and the metamorphic evolution of the continental basement of the Monte Banchetta succession. It comprises two meta-sedimentary sequences with minor meta-intrusive bodies preserving their original lithostratigraphic configuration, despite the intense Alpine deformation and metamorphic re-equilibration. Phase equilibrium modeling points to a metamorphic eclogitic peak (D1 event) of 20–23 kbar and 440–500 °C, consistent among three different samples, analyzed from suitable lithologies. The exhumation P–T path is characterized by a first decompression of at least 10 kbar, leading to the development of the main regional foliation (i.e. tectono-metamorphic event D2). The subsequent exhumation stage (D3 event) is marked by a further decompression of almost 7–8 kbar associated with a significant temperature decrease (cooling down to 350–400 °C), implying a geothermal gradient compatible with a continental collision regime. These data infer for this unit higher peak P–T conditions than previously estimated with conventional thermobarometry. The comparison of our results with the peak P–T conditions registered by other neighboring tectonic units allows to interpret the BRU as one of the westernmost eclogite-facies unit in the Alps.

The Western Alps, consisting of a stack of European and Adriatic continental units and Liguria-Piemonte oceanic units (Fig. 1a), show metamorphic peak conditions decreasing from east to west. Alpine peak P-T conditions range from eclogite-facies conditions in the inner part of the belt (Internal Crystalline Massifs, Sesia zone, Zermatt-Saas-like units), with locally coesiteeclogite-facies conditions (Lago di Cignana unit, Internal Piedmont Zone on the left side of the Aosta valley, and Brossasco-Isasca unit, southern Dora Maira massif ) to lower greenschist-facies conditions in the more external part (Briançonnais-Houillere zone). Based on the Fig. 1 a Simplified tectonic map of the Western Alps (redrawn from Bigi et al., 1990); b Schematic geological map of the Susa and Chisone valleys current state of knowledge, the continental Acceglio-Col Longet nappe (Michard et al., 2003;Schwartz et al., 2000) and the Ambin-Vanoise massifs (Strzerzynski et al., 2012) are the westernmost units ( Fig. 1a) presently exhumed within the Liguria-Piemonte domain registering a metamorphic peak at the blueschist to eclogite-facies transition.
This study focuses on the Banchetta-Rognosa tectonic unit (BRU; Corno et al., 2019Corno et al., , 2021, which is a composite unit including continental and oceanic derived rocks, exposed between Chisonetto and Troncea valleys (tributary valleys of the upper Chisone valley, Italian Western Alps; Figs. 1 and 2). According to the available, large-scale, metamorphic maps of the Western Alps (e.g. Ballèvre et al., 2020;Beltrando et al., 2010;Bousquet et al., 2008), the BRU is located in the external part of the belt, characterized by low temperature, high pressure (LT/HP) metamorphism with peak conditions at the blueschist-eclogite-facies transition. However, at present, quantitative studies aimed at estimating the metamorphic peak conditions of the BRU are lacking. This contribution aims at filling this gap: the metamorphic peak conditions registered by the BRU continental basement are quantified using a petrologic approach based on phase equilibria modeling, and the metamorphic evolution of this poorly investigated sector of the Western Alps is constrained for the first time. Finally, the peak P-T conditions recorded in the BRU are compared with peak P-T estimates published for other HP oceanic-and continental-derived units exposed in the proximity of the BRU.

Geological overview
The Alpine belt is the result of progressive Late Cretaceous to Eocene subduction of the Alpine Tethys and its adjacent European continental distal margin beneath the Adria plate, leading to continental collision during Late Eocene-Early Oligocene (Coward & Dietrich, 1989;Michard et al., 1996;Stampfli & Marchant, 1997;Lemoine et al., 2000;Dal Piaz, 2010). During the collisional event, deeply subducted continental and oceanic lithospheric segments were exhumed and stacked in the growing mountain belt Beltrando et al., 2010;Manzotti et al., 2018;Rubatto & Hermann, 2001).
The Liguria-Piemonte domain comprises ophiolitebearing units with Upper Jurassic to Cretaceous metasedimentary covers (Bearth, 1967;Beltrando et al., 2010 andBalestro et al., 2019 and references therein for a detailed review). At a regional scale, a distinction can be made between eclogitic units (Zermatt-Saas type or Internal Piedmont Zone) and overlying blueschist units (Combin type or External Piedmont Zone). UHP conditions are recorded in the Lago di Cignana unit (Groppo et al., 2009;Reinecke, 1998) in the upper Valtournenche valley. On the other hand, at the uppermost levels, the Chenaillet ophiolite lacks an Alpine metamorphic overprint (e.g. Lewis & Snewing, 1980;Manatschal et al., 2011;Mével et al., 1978).

General features and lithostratigraphy
The BRU crops out within an area of 10 km 2 on the mountain ridge between Troncea and Chisonetto valleys, where it is tectonically juxtaposed to several oceanic units (Servizio Geologico d'Italia, 2020;Fig. 2a, b). This unit consists of two successions respectively recording the Mesozoic tectono-depositional evolution of (i) a continental margin, i.e. Monte Banchetta succession, and (ii) a neighboring oceanic sector, i.e. Punta Rognosa succession. The continental and oceanic successions of the BRU are both covered by the same post-rift sediments consisting of Upper Jurassic?-Cretaceous carbonate micaschist. This peculiar architecture suggests pre-orogenic proximity (juxtaposition) of continental-and oceanic-derived rocks at the hyperextended European distal margin (Corno et al., 2021). Due to the occurrence of jadeite in the pre-Triassic basement and to the lithostratigraphic features of its overlying Mesozoic sedimentary cover, the continental part of the BRU has been correlated to the Acceglio-Col Longet nappe system (Caron, 1971;Caron & Saliot, 1969).
This study focuses on the Monte Banchetta continental succession (Fig. 2c), whose main features can be observed between the Banchetta gorge and the Vallonetto stream (hereafter named North La Grangia section; Fig. 3a) and to the south of the Vallonetto stream (hereafter named Vallonetto section; Fig. 3c).
In the North La Grangia section, the continental basement mainly consists of a white-greyish strongly foliated micaschist sequence (CBm in Fig. 3a) hosting layers and bodies of different lithologies. The medium-to fine-grained micaschist consists of quartz, white mica, chlorite, albite, epidote and graphite. The main lithological bodies embedded within the micaschist include: (i) chloritoid + phengite-bearing glaucophanic schist (b in Figs. 3a and 4), up to 2 m in thickness, mostly located in the central portion of the tectonic slice (see detailed petrographic description in Sect. 5.1); (ii) white quartzite layers (a in Fig. 3a), usually occurring in the upper portion of the micaschist; (iii) fine-grained gneiss (c in Fig. 3a), consisting of quartz, white mica, chlorite, albite widely occurring along the whole Banchetta eastern side, in decametric-thick levels; (iv) a pluri-decametric body of metabasite is exposed in the southernmost part of the North La Grangia section (d in Fig. 3a and b), including discontinuous white meta-aplites (1-2 m in length and up to 30 cm in thickness). This metabasite is a mediumgrained massive rock consisting of glaucophane, garnet, chlorite, epidote, albite, titanite and minor paragonite, rutile, K-feldspar and quartz (Fig. 5a). The main foliation is defined by the alignment of glaucophane and epidote. Garnet porphyroblasts (up to 1 mm in diameter and pale yellow in color) occur in discontinuous domains wrapped by the main foliation. A sharp discontinuity in their composition suggests the existence of two different generations of garnet (Fig. 5b): a locally embayed Ca + Fe-rich core (Grt1), likely pre-Alpine in age, is surrounded by a Mn-rich Alpine rim (Grt2; see Additional file 1 for compositional diagrams). K-feldspar, interpreted as a relict phase, occurs in small crystals (up to 15 µm) in submm patches with chlorite, epidote and rare muscovite (Fig. 5c).The whole micaschist sequence is unconformably covered by Upper Permian-Lower Triassic siliciclastic deposits (CBq in Fig. 3a), represented by white-greenish massive quartzite, locally micro-conglomeratic in the lower part (with detrital pink quartz clasts) and with phengite-bearing quartzite schists in the upper part. Upsection, Triassic meta-dolostone (up to 20-30 m-thick) and monomictic, clast-supported, meta-breccia occur (CBd in Fig. 2c). The dolomitic clasts of the meta-breccia are polycrystalline, up to few decimeters in size, and are set within a dolomitic matrix with sporadic decametric levels of black shale, carbonatic micaschist and phyllite. Then, the overlying syn-rift cover (up to 200 m (See figure on next page.) Fig. 3 a Lithostratigraphic succession of the North La Grangia section. Red polygon shows the location of the analyzed sample, used for P-T pseudosection modeling. Acronyms are: CBm, Ab + Chl micaschist; a, quartzite levels; b, Cld + Ph-bearing glaucophanic schist; c, fine-grained gneiss; d, metabasite body with meta-aplites (white bodies); CBq, Ph-bearing quartzite; b Field photograph of the metabasite body (d) within the Ab + Chl micaschist of the North La Grangia section. The main foliation (S1) is highlighted by whitish meta-aplites layers and is sub-parallel to primary lithological surface, S2 grows in axial plane of D2 folds; c Lithostratigraphic succession of the Vallonetto section. Red polygons show the location of analyzed samples, used for P-T pseudosection modeling. Acronyms are: CVpm, Jd-bearing gneissic micaschist; CVbm, Gr-bearing micaschist; m, Tlc + Aeg-bearing impure marble; CVm, Cld-bearing micaschist; g, Cld-bearing glaucophanite bodies; fg, fine-grained gneiss; dq, dark quartzites; d Field photograph of Jd-bearing gneissic micaschist of the Vallonetto section ( modified from Corno et al., 2021); D2 folds transpose S1 compositional banding and develop S2 axial plane schistosity, D3 tectono-metamorphic event is expressed by S3 crenulation cleavage thick, see also Corno et al., 2021) consists of polymictic meta-breccia, black micaschist, and carbonate-bearing quartzite. The polymictic meta-breccia is composed of meta-dolostone and quartzite clasts in a carbonate matrix containing minor Cr-bearing white mica, talc and detrital K-feldspar. In the uppermost part of the The Vallonetto section identifies a tectonic slice (only a few hundred meters long and 30 m thick) of pre-Triassic basement rocks whose lower terms are made of dark grey jadeite-bearing gneissic micaschist (CVpm in Figs. 3c, d and 4). Locally, bodies (2-3 m in size) of medium-to coarse-grained impure marble occur above the gneissic micaschist (m in Fig. 2c). The marble consists of calcite, dolomite, talc, amphibole, epidote, aegirine, chlorite, hematite, and relict spinel. Due to the high oxygen fugacity in these marble bodies, all iron is ferric, stabilizing a Garnet porphyroblasts partially retrogressed to chlorite and wrapped by S1 foliation, defined by glaucophane + epidote + quartz, partially retrogressed to poikiloblastic albite (Plane Polarized Light, PPL); b Garnet porphyroblasts displaying an outer Mn-rich Alpine rim and an inner Ca + Fe-rich, likely pre-Alpine core (Back Scattered Electron image, BSE); c Detail on a sub-mm patch made of relict K-feldspar + epidote + chlorite + white mica (BSE). (d, e, f ) Tlc + Aeg-bearing impure marble; d S1 hematite foliation transposed by S2 schistosity (BSE); e Calcitic matrix with relict crystals of dolomite + hematite and talc flakes oriented along the S1 foliation (BSE); f Large chromite crystals wrapped by chlorite, growing also along micro-fractures. Note aegirine partially retrogressed by Ca-amphiboles and talc flakes dispersed in the calcitic matrix (BSE) hematite and aegirine (see Additional file 1). Due to the absence of ferrous iron, Mg-rich minerals like talc become stable. The matrix is mostly made of calcite while the main foliation is defined by discontinuous mm-thick layers of hematite + chlorite + talc (Fig. 5d). A second, poorly developed, foliation is defined by the oriented growth of large crystals of zoned Mg-riebeckite amphibole (up to 250 µm) + chlorite + epidote. Aegirine porphyroblasts, up to half a mm long, are wrapped by the main foliation and are concentrated in discontinuous domains, often in association with talc. Dolomite crystals, up to 300-400 µm in diameter, are dispersed in the matrix and are partially replaced by calcite. They often include hematite flakes and are partially enveloped by talc (Fig. 5e). Widespread through the rock, large relict crystals (up to 2 mm) of Cr-bearing spinel occur, wrapped by the main foliation (Fig. 5f ). Late Ca-amphiboles overgrow talc and aegirine. Up section, a graphitic micaschist follows (CVbm in Fig. 3c), consisting of white micas (both phengite and muscovite), chlorite, albite, quartz, graphite and rare calcite. The tectonic slice ends upward with a ~ 10 m -thick reddish chloritoid-bearing micaschist (CVm in Fig. 3c), with local levels of fine-grained gneiss (consisting of white micas, quartz, chloritoid, chlorite, albite and allanitic epidote; fg in Fig. 3c) and dark quartzites (dq in Fig. 3c). Especially in the upper part, this chloritoid-bearing micaschist embeds minor metric bodies of chloritoid-bearing glaucophanites (g in Figs. 3c and 4).

Structural evolution
A polyphasic evolution is recorded in the BRU, characterized by the overprinting of HP (D1 and D2 phases) and LP-LT deformation events (D3 phase), followed by a late folding (D4 phase) (Corno et al., 2019(Corno et al., , 2021.
The oldest D1 event is responsible for the development of the S1 schistosity sub-parallel to primary compositional surfaces (S0) (Fig. 3b). S1 schistosity is deformed and transposed during D2 event into tight to isoclinal folds, whose axial plane schistosity S2 is usually the most penetrative planar fabric in the BRU (Fig. 3d), defined by epidote + phengite/paragonite ± glaucophane ± chloritoid assemblage. S2 mainly dips to W-NW and contains a pervasive L2 stretching lineation. A non-cylindrical folding for the D2 event is suggested by NE-SW trending A2 fold axis, sub-parallel to the L2 stretching lineation. Major contacts between continental-and oceanic-derived successions, as well as their minor intra-succession tectonic contacts, were deformed since the earliest deformation events (D1-D2). D3 event is recorded by mesoscopic folds and crenulations ( Fig. 3d) with sub-horizontal ENE-WSW-trending axes and axial planes usually dipping at high angle to SSE. Some rocks record an incipient S3 crenulation cleavage, widely associated with retrogression and development of LT-LP assemblages (chlorite + muscovite + albite ± stilpnomelane ± pumpellyite; Corno et al., 2019Corno et al., , 2021. The late D4 event is responsible for the development of gentle km-scale folds, generally displaying sub-horizontal N-S trending axes and high-angle dipping axial planes. Tectonic contacts recorded a late top-to-S-SW extensional reactivation.

Petrography and mineral chemistry
A Scanning Electron Microscope (JEOL JSM-IT300LV) equipped with an energy-dispersive X-ray spectrometer (EDX), with a SDD (a silicon drift detector from Oxford Instruments), hosted at the Earth Science Department of the University of Turin, was used for the determination of major elements. The experimental conditions include: accelerating voltage 15 kV, 1 nA probe current, counting time 50 s, process time 5 μs and working distance of 10 mm. The measurements were conducted in high vacuum conditions. The EDX acquired spectra were corrected and calibrated both in energy and in intensity thanks to measurements performed on cobalt standard introduced in the vacuum chamber with the samples (see Reed, 2005 andGoldstein et al., 2017). The Microanalysis Suite Oxford INCA Energy 300, that enables spectra visualization and elements recognition, was employed. This technique, with adequate counting statistics (> 10 6 cnts), allows to reach sensitivity of the order of 0.1 wt% and accuracy around 1%. A ZAF data reduction program was used for spectra quantification. The resulting full quantitative analyses were performed, using natural oxides and silicates from Astimex Scientific Limited ® , as standards. All the analyses were recalculated using the MINSORT computer software (Petrakakis & Dietrich, 1985).

Phase diagram modeling
Isochemical phase diagrams (i.e. P-T pseudosections) were calculated for a Cld + Ph-bearing glaucophanic schist (sample AC44 in Fig. 3a) from the North La Grangia section, a Cld-bearing glaucophanite (sample VT8 in Fig. 3b) and a Jd-bearing gneissic micaschist (sample AC74 in Fig. 3c)  High-resolution multispectral maps of the thin sections used for deriving the effective bulk compositions of the investigated samples were obtained using the same SEM instrument, described in Sect. 4.1. Operative conditions used for mapping the entire thin sections were: 15 kV accelerating voltage, 5nA probe current, 1 μs EDS process time, 10 5 cnts/s, 2.5 µm point step, 1 ms dwell time. The row data were processed using the MultiSpec© software in order to obtain the modal compositions (vol% of all the minerals).
For each sample, the processed X-ray maps are reported in Fig. 3.
Bulk rock compositions of these samples (Table 1) were calculated by combining the mineral proportions obtained from the quantitative modal estimate of SEM-EDS multispectral maps with mineral chemistry acquired at SEM-EDS.
The isochemical phase diagrams were calculated in the system MnNKFMASOH (MnO- for samples AC44 and AC74 and in the system NKFMASOH (Na 2 O-K 2 O-FeO-MgO-Al 2 O 3 -SiO 2 -O 2 -H 2 O) for sample VT8,using Perple_X 6.9.0 (Connolly, 1990(Connolly, , 2005(Connolly, , 2009, the internally consistent thermodynamic database of Holland and Powell (2011) (ds62) and the equation of state for H 2 O of Holland and Powell (1998). Fluid saturated conditions were assumed, and the fluid was considered as pure H 2 O (aH 2 O = 1). This last assumption is realistic for the studied samples, because of the large occurrence of hydrous phases and the absence of primary carbonates and sulphides.
CaO was neglected in all pseudosections, because Cabearing phases are lacking. TiO 2 was not included in the calculation because rutile is the only Ti-bearing phase stable at HP conditions in all the samples.

Petrography and mineral chemistry of selected samples
Three samples have been selected from the pre-Triassic basement rocks of the Monte Banchetta succession out of a total of about 30 samples, based on their mineral assemblages, which are considered as the most suitable for constraining the HP tectono-metamorphic evolution of the BRU. Petrographic features and mineral chemical data are briefly summarized here for the three samples that have been selected for further petrological investigations: AC44 (Cld + Ph-bearing glaucophanic schist), VT8 (Cld-bearing glaucophanite) and AC74 (Jd-bearing gneissic micaschist). The blastesis-deformation relationships of the selected samples, as well as their mineral chemical data, are summarized in Tables 2 and 3, respectively.

Sample AC74: Jd-bearing gneissic micaschist
This sample is a medium-grained gneissic micaschist consisting of quartz (21%), potassic white mica (49%), jadeite (3%), chloritoid (12%), chlorite (4%), albite (4%), paragonite (3%), and accessory allanite (3%) and rutile (1%). The main foliation (S2) is defined by the alignment of white micas, chlorite and chloritoid, concentrated in pluri-mm thick layers, alternated with discontinuous quartz-rich layers (Figs. 4 and 6e). A relic S1 foliation is defined by quartz + white micas + chloritoid + jadeite + rutile and is highlighted by polygonal arcs and intrafolial folds preserved within microlithons ( Fig. 6e and f ). Chloritoid occurs in two generations: an earlier syn-D1 generation, oriented at high angle with respect to the S2, and a syn-D2 generation ( Fig. 6e and f ). Jadeite porphyroblasts, up to 1.3 mm in size, are enveloped by the main foliation and are partially and variably retrogressed (Fig. 6f ). The preserved portions of jadeite display intergrowth relationships with quartz and pre-kinematic white mica, while retrogressed portions are completely replaced by a fine-grained aggregate of quartz and albite. Late albite, chlorite and muscovite grow statically on the main foliation. Among accessory phases, mm-sized relicts of allanite are wrapped by the main foliation and include zircon and monazite. A schematic metamorphic evolution through the three main tectono-metamorphic event is reported in Fig. 6g.
White mica occurs as potassic white mica and paragonite. Phengite and paragonite are related to D1 and D2 tectono-metamorphic events, and are partially replaced by static growth of syn-D3 muscovite. Phengite flakes have the highest Si contents (Si from 3.30 up to 3.56 a.p.f.u; Fig. 7a). X Mg in phengite ranges between 0.48 and 0.75. Na-pyroxene is almost a pure jadeite according to Morimoto (1988), with Acmite < 15% (see Additional file 1). Both chloritoid generations (i.e. syn-D1 and syn-D2) have low X Mg values of 0.13-0.14, and low MnO contents, always lower than 1 wt%. Chlorite plots in the clinochlore field and has X Mg ranging between 0.32 and 0.35.

Thermodynamic modeling
The peak P-T conditions of the selected samples were constrained using the isochemical phase diagram approach, based on the predicted stability field of the observed assemblages, combined with the intersection of compositional isopleths modelled for chloritoid and glaucophane (samples AC44 and VT8). Phengite compositional isopleths have not been used, due to the difficulties in assigning each composition to a specific phengite generation (syn-D1 or syn-D2). The general topology of Fig. 7 Compositional diagrams for potassic white mica (a) and Na-amphibole (b) in the three selected samples the calculated phase diagram sections is similar for all the samples: chloritoid is predicted to be stable up to 530-560 °C and garnet appears in the temperature interval of 480-530 °C, depending on samples. Glaucophane and paragonite are predicted to be stable in almost the whole P-T region of interest; exceptions are for sample AC74, where glaucophane is limited to P < 21-23 kbar and for sample AC44, where paragonite is absent in the P-T range of 400-520 °C, 17.5-24 kbar.

Sample AC44
The modelled pseudosection is dominated mainly by quadri-and quini-variant fields (Fig. 8a). The observed peak assemblage (Gln + Cld + Ph + Pg + Qz) is modelled at T < 450 °C and P = 17-22 kbar; at T > 450 °C, garnet is predicted to occur in addition to these phases, whereas at P > 21-22 Kbar, jadeite appears at the expenses of paragonite. The modelled chloritoid and glaucophane compositional isopleths allow further constraining the peak P-T conditions for this sample. The X Mg measured in chloritoid (X Mg = 0.15-0.18) defines a T range of 420-480 °C, whereas the X Mg measured in the syn-D1 glaucophane (X Mg = 0.63-0.67) constrains pressure at 21-22 kbar, mostly in the narrow field where jadeite (< 0.15 vol%) coexists with paragonite. Very low modal amounts of garnet (< 1 vol%) are predicted to occur at these P-T conditions, although it has not been observed in the sample. Such low modal amount of garnet could have been likely replaced by retrograde chlorite, which is widespread in the sample. Similarly, quartz is not observed in the sample but the modelled pseudosection predicts its stability over a wide P-T range. In this case, the predicted modal amount of quartz is extremely low (< 0.2 vol%), and quartz could have been easily overlooked. Overall, peak P-T conditions of 21-22 kbar and 450 ± 25 °C are constrained for this sample.

Vallonetto section 6.2.1 Sample VT8
The modelled pseudosection consists of large quadrivariant fields and smaller tri-variant fields (Fig. 8b). The observed peak assemblage (Qz + Ph + Pg + Cld + Gln) is predicted to be stable in a large field ranging from 16 to 20-22 kbar and at T < 530 °C, limited by the garnet appearance at T > 530 °C. At pressures higher than 20-22 kbar (depending on temperature), jadeite becomes stable together with paragonite; this last disappears at P > 22-23 kbar. The modeled compositional isopleths of chloritoid corresponding to its measured composition (X Mg = 0.14-0.17) are nearly vertical in the jadeite-absent field, where they constrain temperatures in the range 460-520 °C; however, in the paragonite + jadeite field, the chloritoid isopleths become P-dependent and constrain P in the interval 21-22 kbar (for T = 420-520 °C). The X Mg isopleths modeled for glaucophane and corresponding to its measured composition (X Mg = 0.59-0.67) are concentrated in the paragonite + jadeite field and constrain P at 21-23 kbar. The intersection between chloritoid and glaucophane compositional isopleths further constrain peak P-T conditions at 450 ± 20 °C and 21-22.5 kbar, in the Qz + Ph + Pg + Cld + Gln + Jd field. The amount of jadeite predicted at peak P-T conditions is about 15-17 vol%; although jadeite is not preserved, its former occurrence in the HP assemblage is compatible with the high amounts of retrograde albite (38 vol%) observed in the sample.

Sample AC74
The modelled pseudosection is characterized mainly by quadri-and quini-variant fields (Fig. 8c). The observed peak assemblage (Qz + Jd + Ph + Pg + Cld) is predicted by a three-variant field at P > 22 kbar and T < 480-520 °C, limited toward lower pressures by the appearance of glaucophane, and toward higher temperatures by the appearance of garnet. The modelled compositional isopleths of chloritoid corresponding to its measured composition (X Mg = 0.09-0.14) plot both in this field and in the nearby glaucophane-bearing field (Jd + Gln + Pg + Cld + Qz + Ph), in which low modal amounts of glaucophane (< 8 vol%) are predicted to occur. The former occurrence of low amounts of glaucophane in the peak assemblage, now completely replaced by retrograde chlorite + albite, cannot be excluded. Therefore, peak P-T conditions have been estimated at 21-23 kbar, 470 ± 50 °C, at the boundary between glaucophane-absent and glaucophane-bearing fields.

What the basement rocks of the BRU tell us: from the protoliths to eclogite-facies metamorphism
The detailed lithostratigraphic, structural, petrographic and petrologic analysis of the poorly investigated basement rocks of the BRU allowed us to make some hypothesis about the nature of their protoliths and to reconstruct their metamorphic evolution.

Protoliths
The North La Grangia section can be interpreted as a heterogeneous and composite Paleozoic basement derived from an original sedimentary sequence, mostly consisting of intercalated pelites with different content and types of clay minerals (now transformed in Ab + Chl micaschist or Cld + Ph-bearing glaucophanic schist) intercalated with arenaceous pelites (now fine-grained gneiss), and of minor quartz-arenite (now quartzite), with the arenaceous fraction increasing upward. In this setting, the photolith of the metabasite body embedded in this sedimentary sequence can be interpreted as a pre-Alpine metamorphic rock derived from a mafic protolith, either of magmatic or of sedimentary origin. The occurrence of relicts of unzoned garnet cores and of few relict K-feldspars, suggests that relatively high-T conditions were reached during the pre-Alpine metamorphic event.
In this framework, the observed abrupt Mn-enrichment in garnet rims could be interpreted as the onset of high-P overgrowth on a pre-existent pre-Alpine HT garnet, as reported in other units of the Alps in similar tectonic positions (Bucher et al., 2019), The lower part of the Vallonetto section is made of Jdbearing gneissic micaschist, followed upsection by a dark grey graphitic micaschist. At the contact between these two lithologies, discontinuous metric bodies of Tlc + Aeg-bearing impure marble locally occur. We suppose a magmatic protolith for the Jd-bearing gneissic micaschist, whereas the dark grey graphitic micaschist likely derived from an original sedimentary sequence mostly consisting of pelite ± rich in organic matter. The uppermost part of the Vallonetto section is made of Cldbearing micaschist with detrital allanite and metric bodies of glaucophanite, suggesting a volcano-clastic origin for this sequence.
Both sequences containing some intrusive bodies were involved in the Alpine subduction and in the following exhumation, preserving their original lithostratigraphic configuration in spite of the intense deformation and metamorphic re-equilibration.

Alpine evolution
The Alpine tectono-metamorphic evolution of the Monte Banchetta succession of the BRU was reconstructed by applying the isochemical phase diagram modeling approach. The P-T path from peak conditions to the final exhumation was constrained, and related to the different deformation stages recognized in the study area.
The D1 tectono-metamorphic event − defined by phengite + paragonite + glaucophane I + chloritoid I + epidote I assemblage − occurred at the metamorphic peak, which has been constrained at 20-23 kbar and 440-500 °C. These peak P-T conditions have been constrained on the basis of the overlap of the P-T conditions (ellipses) inferred from the three modelled samples (Figs. 8 and  9). During the D1 event, jadeite developed in magmatic bodies with felsic composition of the Vallonetto Section. These peak P-T conditions are remarkably consistent among the investigated samples, which are representative of different chemical systems, and point to peak metamorphism within the eclogite-facies field (Fig. 9).
Based on minero-chemical composition of the metamorphic phases, similar P-T conditions (470-520 °C, at Fig. 9 a Compilation of P-T trajectories of different units of the Western Alps for comparison with the studied area. Whereas the D1 P-T estimates for the Banchetta-Rognosa unit were obtained by isochemical phase diagram modelling, the retrograde tectono-metamorphic events (i.e. D2 and D3) have been qualitatively inferred on the basis of mineral assemblages only. Different line patterns indicate different thermobarometric methods as reported in the legend. Reactions 1 to 8 are from: 1-2: Poli & Schmidt, 1995;3: Holland, 1980;4: Guiraud et al., 1990;5: Evans, 1990;6: Powell & Holland, 1990;7: Rao & Johannes, 1979;8: Nitsch, 1971; b Simplified tectonic map of the Western Alps (redrawn from the Structural Model of Italy, Bigi et al., 1990) and approximate location of units considered for comparison 17-19 Kbar) were inferred for the oceanic succession of the Monte Banchetta-Punta Rognosa unit (Corno et al., 2019). These data suggest a common metamorphic evolution for the oceanic and continental successions of the BRU, in agreement with the reconstructed tectono-stratigraphy and implying their pre-orogenic juxtaposition.
The D2 tectono-metamorphic event is testified by the development of the main foliation at the regional scale, defined by upper blueschist-facies assemblages: phengite ± paragonite + glaucophane II + chloritoid II + epidote II ± chlorite, and by the replacement of jadeite by albite + quartz in the gneissic micaschist of the Vallonetto section (sample AC74). Considering the reaction curves limiting the stability fields of the D2 assemblage (i.e. reactions 4 and 6 in Fig. 9), this event is qualitatively constrained at around 11-13 kbar and 450-500 °C. The D3 tectono-metamorphic event was responsible for the widespread retrogression of the HP mineral assemblages and the development of greenschist-facies assemblages (chlorite + albite + muscovite). Taking into account the fact that the oceanic succession of the BRU is tectonically juxtaposed with the continental succession since the first deformation stages, it is possible to suppose a common D3 event in the pumpellyite and stilpnomelane stability fields at about 350 °C and P < 5-7 kbar (Corno et al., 2019).
The exhumation path is characterized by an early decompression of at least 10 kbar, which was either isothermal or associated to a little T decrease, during which the Jd-out decompressional reaction was crossed, leading to the development of the main regional foliation (D2 event). This event developed in the glaucophane + paragonite stability field according to the reaction 4 in Fig. 9 and at lower T with respect to the amphibole + paragonite out/ garnet + albite in (reaction 6 in Fig. 9). Therefore, the S2 foliation was equilibrated at an apparent geothermal gradient of about 10-12 °C/km, corresponding to the upper blueschist-facies (UBS), and this suggests a subduction channel environment. The subsequent exhumation stage is marked by a further decompression of almost 7-8 kbar associated with a significant temperature decrease (cooling down to 350-400 °C), in the pumpellyite and stilpnomelane stability fields: this implies an increase in the geothermal gradient to ~ 25 °C/ km, compatible with continental collision regime (D3 event).

Comparison between the BRU and neighboring units
In the investigated sector of the Alpine chain, both eclogitic and blueschist-facies units are exposed (see Malusà et al., 2002, andAgard, 2021, for a detailed review). Six of these units have been selected for a comparison with the BRU (Fig. 9 for locations of each unit), due to their proximity to the BRU and because their P-T paths have been already constrained in the literature. The selected units include: (i) the eclogite-facies, oceanic derived, Susa-Lanzo-Orsiera unit (Zermatt-Saas type; Servizio Geologico d'Italia, 2002) exposed in the Susa Valley (location A in Fig. 9b), the blueschist-facies, oceanic units exposed in (ii) the Beth-Ghinivert area, (iii) the Albergian area, (iv) the Fraiteve area (locations B, C and D in Fig. 9b), and the blueschist-facies, continental units exposed in (v) the Ambin-Vanoise Massif, and (vi) the Acceglio-Col Longet nappe (locations E-F and G-H in Fig. 9b). It is important to note that the P-T evolution of these units were constrained with different methods, ranging from the pseudosection approach (i.e. for the Susa-Lanzo-Orsiera unit and the Ambin-Vanoise Massif ), to multi-equilibrium thermobarometry (average P-T method of THERMOCALC; Holland & Powell, 1998) (i.e. for the Beth-Ghinivert, Albergian and Fraiteve area), to conventional thermobarometry and/or the analysis of the position of relevant reactions in the P-T space (i.e. for the Albergian and Fraiteve zones and the Acceglio-Col Longet nappe).
For the Fraiteve area (C in Fig. 9a, 5-7 kilometers to the North-West with respect to the BRU), the same authors estimated peak P-T conditions of 16.5-18.5