Thermal structure and metamorphic evolution of the Piemont-Ligurian metasediments in the northern Western Alps

Abstract

In the Western Alps, the Piemont-Ligurian oceanic domain records blueschist to eclogite metamorphic conditions during the Alpine orogeny. This domain is classically divided into two “zones” (Combin and Zermatt-Saas), with contrasting metamorphic evolution, and separated tectonically by the Combin fault. This study presents new metamorphic and temperature (RSCM thermometry) data obtained in Piemont-Ligurian metasediments and proposes a reevaluation of the P–T evolution of this domain. In the upper unit (or “Combin zone”) temperatures are in the range of 420–530 °C, with an increase of temperature from upper to lower structural levels. Petrological evidences show that these temperatures are related to the retrograde path and to deformation at greenschist metamorphic conditions. This highlights heating during exhumation of HP metamorphic rocks. In the lower unit (or “Zermatt-Saas zone”), temperatures are very homogeneous in the range of 500–540 °C. This shows almost continuous downward temperature increase in the Piemont-Ligurian domain. The observed thermal structure is interpreted as the result of the upper and lower unit juxtaposition along shear zones at a temperature of ~500 °C during the Middle Eocene. This juxtaposition probably occurred at shallow crustal levels (~15–20 km) within a subduction channel. We finally propose that the Piemont-Ligurian Domain should not be viewed as two distinct “zones”, but rather as a stack of several tectonic slices.

1 Introduction

The northern part of the Western Alps, located between the Rhône-Simplon and the Aosta-Ranzola faults, represent a “transition” zone where many paleogeographic domains were continuously accreted within the alpine orogenic wedge. Within this orogenic wedge, the Piemont-Ligurian oceanic domain recorded blueschist to eclogite facies metamorphic conditions during the Alpine orogeny. The Piemont-Ligurian zone is classically divided, according to their metamorphic evolution, into a greenschist to blueschist facies unit (Combin zone), and an eclogite to UHP facies unit (Zermatt-Saas zone) (e.g. Bearth 1976; Dal Piaz 1988; Ballèvre and Merle 1993). Much attention has been paid to the metamorphic evolution of the HP to UHP Zermatt-Saas zone because of well-preserved eclogite facies assemblages in metabasites (e.g. Bearth 1967; Ernst and Dal Piaz 1978; Oberhänsli 1980; Barnicoat and Fry 1986; Bucher et al. 2005), coesite inclusions within garnets in metaradiolarites (Reinecke 1991) and micro-diamonds in metamorphosed seafloor Mn nodules (Frezzotti et al. 2011). The Combin zone contains only few relics of blueschist facies assemblages in both metabasites (Ayrton et al. 1982; Sperlich 1988; Bucher et al. 2004) and metapelites (Pfeifer et al. 1991; Bousquet et al. 2004), which makes its metamorphic evolution difficult to assess. The contact between both zones is interpreted as a major contact, known as the Combin fault (Ballèvre and Merle 1993), that played an important role in the exhumation of HP metamorphic rocks within the orogenic wedge (e.g. Reddy et al. 1999; Froitzheim et al. 2006). However, the limit between both zones is sometimes difficult to define in the field (e.g. Cartwright and Barnicoat 2002; Bucher et al. 2004).

In order to better constrain the metamorphic evolution of the Piemont-Ligurian domain, we investigated the temperature record in the oceanic metasediments of the different units, using Raman spectroscopy of carbonaceous material (RSCM, Beyssac et al. 2002). This method allows quantifying the maximum temperature reached during the metamorphic evolution. As carbonaceous material is ubiquitous in metasediments in the area of investigation, this method is particularly suitable for quantifying the metamorphic evolution. We also analysed the metamorphic assemblages in metasediments, in order to correlate the estimated temperatures with P–T evolution of the different units. Samples were collected in the northern Western Alps at different structural levels of the Combin zone, across the Combin fault, and in the Zermatt-Saas zone.

We present in this work a combined petrological and RSCM study. Results of 62 RSCM temperatures obtained in the Combin and Zermatt-Saas zones in the northern part of the Western Alps are compared with new data on the metamorphic evolution of the Piemont-Ligurian units. The distribution of temperatures is discussed in the frame of the nappe structure and the P–T evolution of the Piemont-Ligurian units in the northern Western Alps.

2 Geological setting

2.1 Regional geology

The study area is located in the northern part of the Western Alps. This region is characterised by a nappe-stack of different protolithic and paleogeographic origin (Fig. 1):

Fig. 1

Tectonic map of the northern Western Alps, modified after Bousquet et al. (2012b)

• oceanic derived units: the Sion-Courmayeur zone from the Valaisan ocean, and the Tsaté and Zermatt-Saas units from the Piemont-Ligurian ocean.

• continent derived units: the Grand St. Bernard nappe system and the Monte Rosa nappe from the Briançonnais microcontinent, and the Dent Blanche nappe and the Sesia zone from the Apulian margin.

The Piemont-Ligurian domain, is a structurally composite domain which is classically divided in two “zones” based on stratigraphic, tectonic and metamorphic criteria: an upper unit, also called “Combin zone”, and a lower unit, the “Zermatt-Saas zone” (e.g. Dal Piaz 1965; Bearth 1967, 1976; Dal Piaz and Ernst 1978; Sartori 1987; Dal Piaz 1988). In the following we will describe the main characteristics of both units, mainly on the stratigraphic, tectonic and metamorphic descriptions available in the Western Swiss Alps, underlining the correlation of units with the Italian Alps (Aosta valley).

2.2 The upper unit or “Combin zone”

Since the work of Argand (1909), who introduced the term of “Combin zone”, several studies related to the stratigraphy and the internal structure of this zone have been published (e.g. Bearth 1976; Marthaler 1984; Sartori 1987; Dal Piaz 1988; Escher et al. 1993; Sartori and Marthaler 1994). However, there are still different interpretations regarding the internal structure and the tectonic evolution of this complex zone. The upper unit (UU) is composed of several tectonic nappes of different paleogeographic origin.

The Tsaté nappe (Sartori 1987) mainly consists of oceanic rocks: metagabbros, serpentinites, metabasites, calcschists and marbles of Jurassic-Cretaceous age. The metabasites and metasediments of the Tsaté nappe show few occurrences of mineral assemblages indicating HP metamorphism. Sodic amphibole (glaucophane or crossite) and lawsonite relics, sometimes associated with garnet, have been described locally in metagabbros, metabasalts and prasinites (Dal Piaz and Ernst 1978; Ayrton et al. 1982; Sperlich 1988; Vannay and Alleman 1990; Cartwright and Barnicoat 2002; Bucher et al. 2004). Carpholite pseudomorphs have also been described in the calcschists located NW of the Dent Blanche nappe (Pfeiffer et al. 1991), as well as relics of garnet, Mg-rich chloritoid and phengite in the Zermatt area (Bousquet et al. 2004, 2008). Maximum P–T conditions are however still poorly documented. The available estimates are in the range of 9–12 kbar and 300–475 °C for metabasites (Reddy et al. 1999; Cartwright and Barnicoat 2002) up to 15 kbar and 450 °C in metasediments (Bousquet et al. 2008).

At the base of the Tsaté nappe, or sometimes intermingled inside the Tsaté nappe, occur discontinuous exotic sheets: the Cimes Blanches (Pancherot-Cime Bianche unit in the northern Aosta valley) and Frilihorn nappes (Dal Piaz 1988; Escher et al. 1993). They mainly consist of rocks of continental origin: quartzites, schists, marbles, dolomites and conglomerates of Permian to Cretaceous age. These nappes are interpreted as exotic decollement sheets derived from Mesozoic continental cover sequences of the Briançonnais domain (Marthaler 1984; Sartori 1987; Dal Piaz 1988; Vannay and Alleman 1990; Escher et al. 1993) that were incorporated during early stages of formation of the Piemont-Ligurian accretionary prism (Marthaler and Stampfli 1989; Sartori and Marthaler 1994). Other studies also propose that the Frilihorn and Cimes Blanches nappes are derived from the Mesozoic sedimentary cover of the Sesia-Dent Blanche basement nappes, incorporated within the accretionary prism during early stages of deformation (Froitzheim et al. 2006; Pleuger et al. 2007). According to Escher et al. (1993) and Steck et al. (1999), the Cimes Blanches nappe is considered to represent the base of the UU, which overlies the Grand St. Bernard nappe system to the north and the Zermatt-Saas zone to the south. These rocks locally display jadeite-quartz-phengite mineral assemblages (Bousquet et al. 2004).

2.3 The lower unit or “Zermatt-Saas zone”

The lower unit (LU), or Zermatt-Saas zone (ZSZ), consists of an ophiolite sequence derived from the Piemont-Liguria Ocean and composed of metaperidotites, metagabbros, metabasalts and serpentinites (Bearth 1967) of middle Jurassic age (Rubatto et al. 1998). An oceanic sedimentary cover mainly composed of metapelites, marbles and metacherts of Jurassic-Cretaceous age (Bearth 1967, 1976; Bearth and Schwander 1981; Dal Piaz 1965) completes the sequence. This zone is considered either as a homogeneous unit (Angiboust et al. 2009) or, based on the different metamorphic evolutions, as an assemblage of several tectonic slices (Bousquet et al. 2008; Groppo et al. 2009).

The typical HP-metamorphic assemblages in metabasalts and metagabbros of the ZSZ are composed of garnet-omphacite-glaucophane ± chloritoid ± talc, first described in the Zermatt area (Bearth 1959), documenting maximum P–T conditions in the range of 16–20 kbar and 550–600 °C (e.g. Ernst and Dal Piaz 1978; Oberhänsli 1980; Barnicoat and Fry 1986). Despite no new petrological findings, these P–T conditions were recently revaluated to upper values ~23–27 kbar and ~550–600 °C on the basis of new thermobarometric calculations (Bucher et al. 2005; Angiboust et al. 2009). Furthermore, the discovery of coesite inclusions within garnet in metasediments at Lago di Cignana, indicate locally UHP conditions of 27–32 kbar and 590–630 °C (Reinecke 1991, 1998; Groppo et al. 2009). More recently, microdiamonds were discovered in some garnets of the Lago di Cignana UHP unit, indicating pressures over 32 kbar (Frezzotti et al. 2011). The UHP Cignana unit, composed of large boudins of metabasalts and metagabbros embedded in metasediments, is located in the upper part of the LU (Forster et al. 2004; Groppo et al. 2009).

2.4 Structure of the Piemont-Ligurian nappe stack

The difference in metamorphic record between the UU and the LU led some authors to propose the existence of a major tectonic contact in between, known as the Combin fault (Ballèvre and Merle 1993) or Gressoney Shear Zone in the Aosta valley (Reddy et al. 1999). The nature of this contact (i.e. extensional and/or compressional), the amount of displacement and its role in the exhumation of HP Piemont-Ligurian units is still widely debated (Reddy et al. 1999, 2003; Froitzheim et al. 2006; Pleuger et al. 2007). Studies realised in the vicinity of this contact underline that a distinction of both units is difficult to assess due to pervasive retrogression and deformation (Cartwright and Barnicoat 2002; Groppo et al. 2009). Most of the authors locate the limit between the UU and the LU (i.e. the Combin fault) at the base of the Cimes Blanches nappes (Escher et al. 1993; Ballèvre and Merle 1993; Reddy et al. 2003; Pleuger et al. 2007). However other studies locate the contact lower than the base the Cimes blanches nappe (or Pancherot-Cime Bianche unit) in the Zermatt region and the northern Aosta valley (Compagnoni and Rolfo 2003; Bucher et al. 2004; Forster et al. 2004). In addition to lithological differences, the distinction between the UU and the LU is frequently based on the estimated metamorphic record in metabasites. Most of the studies distinct the two units by the presence or absence of eclogite facies mineral assemblages (Ballèvre and Merle 1993; Ring 1995; Reddy et al. 1999, 2003; Pleuger et al. 2007; Beltrando et al. 2008). The UU is considered to be greenschist (Ballèvre and Merle 1993; Reddy et al. 1999) to blueschist facies (Bousquet 2008; Bousquet et al. 2012a).

3 Methodology

During diagenesis and metamorphism, carbonaceous material (CM) present in the initial sedimentary rock is progressively transformed into graphite (graphitization). Because of the irreversible character of graphitization, CM structure primarily depends on the maximum temperature reached during the metamorphic cycle, and is therefore not sensible to retrograde or subsequent evolution at lower temperature (Wopenka and Pasteris 1993; Beyssac et al. 2002). A linear correlation between the structural organisation of CM, measured by Raman spectroscopy, and the metamorphic temperature has been calibrated (RSCM method—Beyssac et al. 2002). The calibration was made using samples from different regional metamorphic belts with well-known P–T conditions, and allows the determination of temperatures in the range of 330–650 °C with a calibration accuracy estimated to ±50 °C. The relative uncertainties on temperature are much smaller, allowing high resolution field mapping of temperatures (Negro et al. 2006; Wiederkehr et al. 2011). Other calibrations have also been published to extend this temperature range below 330 °C (Rahl et al. 2005; Lahfid et al. 2010) or for rocks affected by regional metamorphism (Aoya et al. 2010). These other calibrations were done on different Raman spectrometers with different laser wavelength. This may significantly affect the estimated temperatures (Aoya et al. 2010). We only used the original calibration and analytical setup of Beyssac et al. (2002) in this study.

Raman spectra were performed on a Renishaw InVIA reflex microspectrometer (Laboratoire de Géologie, ENS Paris, and IMPMC Paris, France) equipped with a 514 nm Spectra Physics (20 mW) argon laser. Laser beam was focused on the sample using a DMLM Leica microscope with a 100 × objective (NA = 0.90), with laser power at sample surface of ~1 mW. The signal was filtered by edge filters and dispersed with a 1,800g/mm grating to be analysed by a Peltier cooled RENCAM CCD detector. Before each session the spectrometer was calibrated with a silicon standard. Because Raman spectroscopy of CM can be affected by several analytical mismatches, we followed closely the analytical and fitting procedures described by Beyssac et al. (2002, 2003). Measurements were only performed below transparent minerals, generally quartz, to avoid any effect of polishing on the structure of CM (Beyssac et al. 2003). For each sample, 10–24 spectra were recorded in order to smooth out the within-sample structural heterogeneity. Spectra were processed using the Peakfit software following the procedure described in Beyssac et al. (2003).

4 RSCM temperatures in the Piemont-Ligurian units

The 62 samples were collected in the Piemont-Ligurian domain in 3 different areas (Fig. 2): North of the Dent Blanche nappe, in the Zermatt area and in the Lago di Cignana area. Among these samples, 50 were collected in the UU, and 12 in the LU, in order to highlight thermal structure of the Piemont-Ligurian nappe stack. For each sample, we also described the observed mineral assemblage in order to give a comparison with our estimated temperature (Figs. 2, 3, 6; Table 1).

Fig. 2

Tectonic map of the Piemont-Ligurian units around the Dent Blanche massif (modified after Steck et al. 1999) showing RSCM temperatures and mineral assemblages in the upper unit of the Piemont-Ligurian domain. The absolute uncertainty on RSCM temperatures is ±50 °C. The map is located in Fig. 1

Fig. 3

Tectonic map of the Zermatt area (modified after Bearth 1953 and Bucher et al. 2003) with RSCM temperatures and mineral assemblages in the upper and lower units of the Piemont-Ligurian domain. The absolute uncertainty on RSCM temperatures is ±50 °C. The map is located in Fig. 2

Table 1 Samples from the Piemont-Ligurian units with coordinates (CH1903) and elevation, observed mineral assemblages, number of Raman spectra (Sp.), R2 ratio (mean value and standard deviation) and RSCM temperature (mean value and 1-σ uncertainty)

4.1 Zermatt area

Samples were collected along different profiles in the Zermatt area across the two units (Fig. 3). A subset of representative spectra is depicted in Fig. 4.

Fig. 4

Representative Raman spectra obtained from samples collected in the upper and lower units of the Piemont-Ligurian domain. The R2 ratio value and corresponding temperature are given for each spectra. Location of the samples in Fig. 3 and Table 1

4.1.1 Upper unit

The Raman spectra in the metasediments of the UU show significant variation in the CM structural organisation (Fig. 4). The degree of organization increases from upper to lowermost levels, with a variation of the R2 ratio from 0.50 to 0.25 (Table 1).

Along the profile below the Matterhorn, from Hörnlihütte to Schwarzsee, temperature increases from ~450 °C at the contact with the Matterhorn Austro-Alpine units to 500–530 °C near the contact with the LU (Figs. 3, 5a). However, temperatures are mainly in the range 450–475 °C along this profile. The profile located from Trift to Zermatt shows very homogeneous temperature record in the range of 480–500 °C, except for one sample that shows a temperature of 420 °C (Figs. 3, 5b). Samples collected in the Oberrothorn area also show homogeneous temperature record ranging from 500 to 520 °C, similar to the Hörnlihütte-Schwarzsee and Trift-Zermatt profiles (Fig. 3).

Fig. 5

a Simplified geological cross-section between Schwarzee and Matterhorn showing the distribution of RSCM temperatures upper and lower units of the Piemont-Ligurian domain (modified after Bucher et al. 2004). b Simplified geological cross-section between Unter Gabelhorn and Gornergrat showing the distribution of RSCM temperatures in the upper and lower units of the Piemont-Ligurian domain (modified after Bearth 1953). The cross-sections are located in Fig. 3. The absolute uncertainty on RSCM temperatures is ±50 °C

4.1.2 Lower unit

The spectra obtained in the LU are very homogeneous, with an R2 ratio in the range 0.22–0.33 indicating that the CM is generally well-ordered in this unit (Fig. 4; Table 1). Estimated temperatures are always in the range of ~500–540 °C across the whole area, showing homogeneous peak T record in the metasediments of this unit (Fig. 3).

4.2 Lago di Cignana

Samples were collected along a NW–SE transect across the UU, and across the contact between the two units (Fig. 6). The Raman spectra are very similar to the Zermatt area, with relatively well ordered CM and R2 ratios in the range 0.25–0.42 (Table 1).

Fig. 6

Tectonic map of the Lago di Cignana area (modified after Compagnoni and Rolfo 2003) showing RSCM temperatures and metamorphic assemblages in the upper and lower units of the Piemont-Ligurian domain. The absolute uncertainty on RSCM temperatures is ±50 °C

Along the NNW-SSE profile passing through Mt. Pancherot, temperature increases from ~460–470 °C in uppermost sampled levels of the UU, to ~530 °C at the contact with the underlying LU (Fig. 6b). A single sample collected north of the Lago di Cignana shows temperature of ~475 °C similar to the ones observed along the profile (Fig. 6a). Samples collected near the contact, W of Lago di Cignana, show similar temperatures of ~510–520 °C in both the UU and the LU. Finally, one sample collected in the LU shows temperature of ~540 °C, very similar to the temperatures observed near the contact further north (Fig. 6a).

4.3 North of the Dent Blanche nappe

Samples were collected in the metasediments of the UU in different N–S valleys and around barriers lakes (Fig. 2). The Raman spectra are very similar to those observed within the UU in the Zermatt and Lago di Cignana areas, with relatively well ordered CM and R2 ratios in the range 0.33–0.50 (Table 1). Temperatures are slightly lower in the Evolène-Zinal area, in the range 420–450 °C, compared to the Mauvoisin lake area where they are in the range 460–500 °C. However these temperatures are in good agreement with those observed SE of the Dent Blanche massif, showing spatially coherent distribution of temperatures in the whole UU.

4.4 RSCM temperatures and Piemont-Ligurian nappe structure

We have reported the range of estimated RSCM temperatures on a simplified geological cross section of the studied area (Fig. 7) in order to assess the relation between the estimated temperatures and the Piemont-Ligurian nappe structure. These temperature ranges correspond to a synthesis of observations in the three studied areas, and have been projected on the cross section according to the structural position of the samples in the UU.

Fig. 7

Simplified geological cross section across the Penninic units of the northern Western Alps (modified after Escher et al. 1993) showing the range of estimated RSCM temperatures estimated in the upper and lower units of the Piemont-Ligurian domain. The cross section is located in Fig. 2

The RSCM temperatures in the UU show coherent distribution (Fig. 7). In the upper structural levels of the UU temperatures are in the range of 420–470 °C, observed below the contact with the Dent Blanche nappe. Intermediate levels, which have been intensely folded, as underlined by the position and the structure of the Frilihorn nappe, show temperatures in the range of 470–500 °C. In the lower structural levels of the UU, close to the contact with the LU, temperatures are in the range of 500–530 °C. These latter temperatures are quite similar to those observed in the underlying LU, in the range of 500–540 °C. The temperatures observed north of the Dent Blanche nappe, in contact with the underlying Grand St. Bernard nappe system, are slightly lower, in the range of 470–500 °C.

5 New metamorphic data in the Piemont-Ligurian metasediments

In addition to the estimation of peak T by the RSCM method, we studied systematically the mineral assemblage of each sample analysed. We also included some additional samples in order to complete the regional distribution of mineral assemblages (Figs. 2, 3 and 6). The mineral assemblages for all these samples are given in Table 1. Electron microprobe analysis of the most significant minerals assemblages were also performed using JEOL JXA8200 (University of Bern) and JEOL JXA8600 (University of Basel) using standard conditions (15 kV, 10 nA). Representative analyses of mineral assemblages in metasediments are given in Table 2.

Table 2 Representative electron microprobe analysis of mineral assemblages in the upper unit

5.1 Spatial distribution of mineral assemblages

Occurrence of mineral assemblages representative of HP conditions is scarce in metasediments of both units. Most of these metasediments are composed of quartz, calcite, phengite, with variable amounts of epidote, clinozoisite and plagioclase (Figs. 2, 3, 6; Table 1). In the area located north of the Dent Blanche nappe, relic garnet was only found in one sample from a metabasite south of Zinal (Fig. 2). In the Zermatt and Lago di Cignana areas, some samples depict a well preserved mineralogy, with garnet-chloritoid-phengite assemblage (Figs. 3, 6, 8a and b). This mineral assemblage is only observed locally, below the Hörnlihütte along the Matterhorn-Schwarwee profile (Fig. 3), in the Oberrothorn area (Fig. 3), and in the lower part of the Lago di Cignana profile (Fig. 6a-b). This mineral assemblage has been found both in the LU associated to eclogites and in the UU without eclogites (Figs. 3, 8a–c). The most common mineral assemblage in the UU is Ph-Chl-Cal-Qz ± Ep/Czo ± Plg (Figs. 2, 3, 6 and Table 1) typical of greenschist facies metamorphic conditions. At the regional scale, field occurrences of mineral assemblages evidencing HP metamorphism are decreasing toward the west, where the overprinting seems to be predominant (Figs. 2, 3). They can be only found as relic garnet or sometimes chloritoid inclusions in albite porphyroblasts (Fig. 8e).

Fig. 8

Photomicrographs of the metamorphic assemblages observed in the metasediments of the upper and lower units of the Piemont-Ligurian domain. Samples are located in Figs. 3 and 6 and Table 1. a Garnet relic with chloritoid and chlorite inclusion in the CIG0901 sample at Lago di Cignana (plane polarized light). b Garnet-chloritoid-phengite assemblage in the CIG0903 sample at Lago di Cignana (plane polarized light). c Garnet chloritoid-phengite assemblage in the GOL0812 sample at Gornergrat (plane polarized light). d Garnet partially replaced by chlorite and albite in the SZ0816 sample near Hörnlihütte (plane polarized light). e Relic garnet within albite porphyroblasts in the TRIF0805 sample near Trift (plane polarized light). f Sheared albite porphyroblast in the chlorite-phengite foliation in the CIG0915 sample at Lago di Cignana (crossed polars). Mineral abbreviations after Whitney and Evans (2010)

5.2 Mineral compositions and textural relationships

The main foliation in metapelites is defined by the alignment of phengite within a calcite-quartz matrix. Plagioclase frequently occurs as sheared porphyroblasts within the main foliation together with epidote and/or clinozoisite (Fig. 8f). Chloritoid is found as inclusion in big garnet pseudomorphs (Fig. 8a) together with phengite and chlorite, or within the main phengite foliation (Fig. 8b). The chloritoid compositions are homogeneous with an X_Mg in the range of 0.14–0.17 (Table 2). Different types of garnet have been observed in the UU samples, with regional consistency from Zermatt to Lago di Cignana (Figs. 3, 6; Table 2). Garnet porphyroblasts (type I) occur within the main foliation and show compositional zoning (Fig. 8b). The spessartine component is higher and the almandine component lower in the core of these garnets, compared to their rim (Table 2). Such kind of variation has been interpreted as a growth of garnet under increasing temperature conditions (Spear and Markussen 1997; Enami 1998; Tinkham and Ghent 2005). Smaller garnets (type II) occur within the main foliation (Fig. 8b) and their composition is similar to the rim of type I garnets (Table 2). Phengite associated with chloritoid and garnet within garnet pseudomorphs (Fig. 8a) or in garnet crystallization tail (Fig. 8b) show Si content in the range 3.38–3.46 (Table 2). Chlorite is found associated with chloritoid and phengite garnet within garnet pseudomorphs (Fig. 8a), in garnet crystallization tails (Fig. 8b, d–f), or within the main foliation, and show compositions with X_Mg in the range 0.43–0.46 (Table 2). Garnets partially (Fig. 8d) or almost totally (Fig. 8e) replaced by chlorite and plagioclase, are observed in some sample showing distinctive stages of retrograde evolution. Figure 8f depicts the most common assemblage and textural relationships observed in the UU samples.

6 Discussion

6.1 Metamorphic evolution of Piemont-Ligurian metasediments

The new petrological data on metasediments and their metamorphic record allow us to discuss the metamorphic evolution of the Piemont-Ligurian domain. The garnet-chloritoid-phengite assemblages, and their mineral compositions, are characteristic of blueschist facies metamorphic conditions in metasediments (Bousquet et al. 2008; Bucher and Grapes 2011). Peak P–T conditions of ~12–15 kbar and ~450 °C were estimated for the UU in the area of Zermatt in metabasites and metapelites (Cartwright and Barnicoat 2002; Bousquet et al. 2008). We therefore use these maximum P–T conditions (Fig. 9). Metasediments of the Piemont-Ligurian domain towards the south, north of the Gran Paradiso massif (Fig. 1), show similar petrological and metamorphic features. Indeed, the mineral assemblages, the mineral compositions and the textural relationships are very similar to our study (Bousquet 2008). Two distinct metamorphic stages, a LT-HP followed by heating during decompression have been identified. The P–T conditions are estimated for the first stage at 14 kbar and 450 °C for primary garnet core associated with chloritoid and primary phengite and for the second one at 7–8 kbar and 530 °C for secondary garnet associated with clinozoisite, chlorite and secondary phengite. On the basis of these data and our own investigations, we propose a P–T evolution for metasediments similar to the Zermatt area (Fig. 9). This will serve as a basis to discuss and interpret the observed thermal structure.

Fig. 9

Synthetic P–T grid for the Piemont-Ligurian units with proposed P–T evolution for the different tectono-metamorphic units. P–T conditions after: 1a,b Cartwright and Barnicoat (2002); 2 Bousquet et al. (2008); 3 Oberhänsli (1980), Barnicoat and Fry (1986); 4a,b Angiboust et al. (2009); 5 Bucher et al. (2005); 6 Reinecke (1991), Groppo et al. (2009)

6.2 RSCM temperatures and metamorphic evolution

In the lower levels of the UU, estimated temperatures are very consistent in the range of 500–530 °C. On the basis of the textural relationships and garnet compositional zoning, we relate these RSCM temperatures to the growth of secondary garnet within the main foliation. This interpretation is also in agreement with temperatures estimated by classical thermobarometry by Bousquet (2008). We therefore propose that the estimated RSCM temperatures in the range of 500–530 °C, observed at the base of the UU, likely correspond to heating during exhumation of metasediments (Fig. 9).

In the upper levels of the UU, at first sight, mineral assemblages are documenting greenschist facies conditions, and RSCM temperatures ranging between 420 and 500 °C are indicating similar T-conditions (Figs. 3, 5a, b). Along the Zermatt-Trift profile (Fig. 5b), the Tsaté and Frilihorn nappes are intensely deformed and folded (Bearth 1967; Mazurek 1986; Sartori 1987) and the temperature is very homogenous along this profile (480–500 °C). The temperature range of 480–500 °C also corresponds to temperature estimated in sheared calcschists in the Täschalp region for a pressure of ~5 kbar (Cartwright and Barnicoat 2002). Based on HP-relics in plagioclase, we therefore interpret RSCM temperatures as documenting the reheating during decompression. The more pervasive greenschist retrogression in this area (Figs. 3, 8e) was also favoured by more intensive deformation during exhumation.

6.3 Thermal structure and exhumation history of the Piemont-Ligurian units

The RSCM temperatures obtained for the LU are slightly lower but in good agreement with previous P–T estimates obtained with geothermobarometry (Bucher et al. 2005). According to Angiboust et al. (2009) the LU underwent nearly isothermal decompression from 24 to ~6–8 kbar at a temperature of ~550 °C. The RSCM temperatures therefore correspond to near peak P–T conditions. According to the P–T paths (Fig. 9), the different units were juxtaposed at a shallow crustal level within a subduction channel. This P–T evolution suggests that the UU and the LU were put together at a depth of ~15–20 km and a temperature of ~500 °C. This juxtaposition occurred along several shear zones (limits of the different units), that were active during the Middle Eocene (~40–35 Ma; Reddy et al. 1999; Cartwright and Barnicoat 2002). The difference in thermal overprint during exhumation may also evidence a change of the thermal gradient during nappe stacking in this period. Our study, associated with the diamond discovery in the Lago di Cignana UHP unit (Frezzotti et al. 2011), clearly shows that the different units of the Piemont-Ligurian domain in the northern Western Alps cannot longer be viewed as a juxtaposition of only two distinct “zones”. Finally, considering similar observation in the Graian Alps (Bousquet 2008) and in the Cottian Alps (Agard et al. 2001), we propose that the entire Piemont-Ligurian domain of the Western Alps is a stack of several tectonics slices with distinctive prograde and retrograde metamorphic evolution.

7 Conclusion

The results of this study based on a combined petrological and RCSM study provide new constraints on the metamorphic evolution of the Piemont-Ligurian domain in the northern Western Alps. The RSCM temperature distribution is coherent at a regional scale in the Piemont-Ligurian domain. The metasediments of the upper unit, or “Combin zone”, show variable peak temperatures in the range of 420–530 °C, with an increase in temperature from upper to lower structural levels. This temperature is related to greenschist metamorphic conditions and deformation during exhumation. Metamorphic data highlight an increase in temperature during retrograde evolution from blueschist to greenschist facies metamorphic conditions. The temperatures observed in the lower unit, or “Zermatt-Saas zone”, are in the range of 500–540 °C, showing almost continuous temperature record in the upper and lower units of the Piemont-Ligurian domain. We interpret the observed thermal structure as the result of juxtaposition of the upper and lower units along shear zones at a temperature of ~500 °C during the Middle Eocene. According to the proposed P–T paths for the different units, this juxtaposition occurred at shallow crustal levels within a subduction channel. At a larger scale, we propose that the Piemont-Ligurian Domain is not only composed of two units, separated by a major contact, but represents a stack of several tectonic slices with their own metamorphic history accreted together during the subduction.