doi: 10.1111/j.1365-3121.2009.00870.x

The Zermatt-Saas ophiolite: the largest (60-km wide) and deepest (c. 70–80 km) continuous slice of oceanic lithosphere detached from a subduction zone? S. Angiboust,1 P. Agard,1 L. Jolivet1 and O. Beyssac2 1 Laboratoire de Tectonique, UMR CNRS 7072, Universite´ Pierre et Marie Curie, Tour 46-00, 2e`me e´tage, Case 129, 4 Place Jussieu, 75252 Paris, France; 2Laboratoire de Ge´ologie, UMR CNRS 8538, Ecole Normale Supe´rieure, 24 rue Lhomond, 75005 Paris, France

ABSTRACT The Western Alps are a classic subduction-related collisional orogen with well-preserved, deeply subducted ophiolitic remnants of oceanic lithosphere. Some (e.g. Monviso, Voltri) were recognized as a palaeo-subduction channel, with tectonic blocks showing a wide range of pressure–temperature conditions. We herein evaluate for the first time the metamorphic homogeneity of the extensive Zermatt-Saas ophiolite. ZermattSaas peak eclogitic assemblages are represented by omphacite– garnet ± phengite ± epidote ± lawsonite ± glaucophane in MORB-derived metabasalts and garnet–chloritoid–talc ± lawsonite ± phengite in hydrothermalized metabasalts. Thermobarometric estimates with THERMOCALC and Raman Spectroscopy

of carbonaceous material reveal homogeneous peak burial conditions at around 540 ± 20 °C and 23 ± 1 kbar. P–T paths indicate that the whole of the ophiolite, at least 60 km across, strikingly underwent similar burial and exhumation patterns and detached from the slab at depths around 80 km. The Zermatt-Saas ophiolite thus appears to be the world’s largest oceanic lithosphere fragment exhumed from such depths, which provides important constraints on interplate coupling mechanisms.

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Introduction Subduction ÔfactoriesÕ are crucial areas in terms of seismic hazard, element recycling or arc-magmatism, and the distinctive locus of high-pressure lowtemperature metamorphism (HP-LT; Ernst, 1970; Maekawa et al., 1993; Peacock et al., 1994). P–T-time paths for these HP-LT rocks provide constraints on both exhumation mechanisms and on deep structures and processes at work along the subduction plane (Shreve and Cloos, 1986; Platt, 1993; Ernst and Liou, 2000; Chopin, 2003; Hacker et al., 2003; Jolivet et al., 2003, 2005). Surprisingly, the fate of continental pieces transiently dragged at depth during continental subduction attracted much more research attention (Chopin, 1984; Hacker et al., 2000) than the exhumation of fragments of oceanic lithosphere during protracted subduction. A recent, worldwide review showed that the detachment and exhumation Correspondence: S. Angiboust, Laboratoire de Tectonique, UMR CNRS 7072, Universite´ Pierre et Marie Curie, Tour 4600, 2e`me e´tage, Case 129, 4 Place Jussieu, 75252 Paris, France. Tel.: +33 144275943; fax: +33 144275085; e-mail: samuel. [email protected]

 2009 Blackwell Publishing Ltd

Fig. 1 Spatial distribution of mainly sedimentary (light grey) and crustal ⁄ mantellic (dark grey) oceanic units in the internal units of the Western Alps (Zermatt-Saas and Monviso are indicated). Note the internal position of the mafic ⁄ ultramafic bodies and their close association with the Internal crystalline massifs (Dora Maira, Gran Paradiso, Monte Rosa). Recent P–T estimates on the Zermatt-Saas ophiolite are as follows: [1] Bucher and Grapes (2008); [2] Bucher et al. (2005); [3] Barnicoat and Fry (1986); [4] Reinecke (1991, 1998); [5] Martin et al. (2008). 171

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............................................................................................................................................................. of oceanic fragments in the subduction channel are discontinuous and generally take place at depths shallower than c. 80 km, probably because of buoyancy constrasts (Agard et al., 2009). Petrological and numerical models generally envision the circulation of independent subducted blocks in a weak serpentinite melange between the plates (Guillot et al., 2001, 2008; Schwartz et al., 2001; Gerya et al., 2002; Federico et al., 2007; Yamato et al., 2007). Natural constraints are still few, however, and the following questions remain unanswered: how do these rocks detach from the sinking slab, at which depth(s), do they circulate in a loose channel or as large-scale imbricated slices? In the Western Alps, oceanic rocks were exhumed from somewhat deeper than average (24–28 kbar; Reinecke, 1998; Messiga et al., 1999) towards the end of oceanic subduction, and two strikingly different situations are found: (1) interleaved tectonic slices with diachronous and contrasting metamorphic evolutions in the Monviso and Voltri ophiolites (c. 450– 620 C, 12–25 kbar; Messiga et al., 1995, 1999; Schwartz et al., 2000; Guillot et al., 2004; Federico et al., 2004, 2007) and (2) a presumably coherent, 30-km-long tectonic body for the Zermatt-Saas (ZS) ophiolite (550–600 C, 25–30 kbar; Bucher et al., 2005). The latter finding, if confirmed, has severe implications on geodynamic and mechanical models of subduction zones. However, P–T determinations on the ZS ophiolite have given contrasting estimates over the past 30 years, and no one has yet evaluated the homogeneity of peak conditions across the entire ZS ophiolitic fragments. The aim of this paper was therefore to reinvestigate the P–T conditions of metamorphism of the whole ZS ophiolite in order to shed light on subduction channel processes.

Geological setting The eclogite facies ZS ophiolites (Bearth, 1967) extend across Switzerland and Italy (Figs 1 and 2) and represent (together with the noneclogitic Combin unit) a relict of the Mesozoic Liguro-piemontese oceanic 172

lithosphere subducted below the Apulian margin between c. 50 and 40 Ma (Lemoine et al., 1986; Dercourt et al., 1993; Bowtell et al., 1994; Rubatto et al., 1998; Amato et al., 1999; Agard et al., 2002). The ZS ophiolite complex is now part of a collisional nappe stack and is closely associated in space with the continental internal crystalline massifs (Monte Rosa, Gran

Paradiso, Fig. 2) and shows similar exhumation-related tectonic patterns (e.g. Chopin and Monie, 1984; Van der Klauw et al., 1997; see references therein for further details). The ZS ophiolite consists of serpentinites (Li et al., 2004), gabbros (e.g. Allalin gabbro, Meyer, 1983), and various types of metabasalts including pillow lavas (Bearth, 1967) and hydro-

Fig. 2 Simplified geological map after Elter (1987) and Bearth (1953) showing the sample localities and the results of our THERMOCALC and RSCM estimates. SRTM topographical map is used as background. Inset: simplified geological map in the vicinity of Lago di Cignana, modified after Van der Klauw et al. (1997). Idealized cross-section along AB, showing how the Zermatt-Saas and Combin units are sandwiched between the Dent Blanche unit and the Monte Rosa internal crystalline basement.  2009 Blackwell Publishing Ltd

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(a)

(b)

(d) (c)

(e)

(f)

Fig. 3 Representative optical microscope sketches (a,c,e,f), photograph (b) and microprobe view (d) of high-pressure assemblages in the Zermatt-Saas ophiolite. Numbers on the upper left refer to samples shown in Fig. 2. (a) Garnet porphyroblasts from a classical metabasite containing mainly epidote inclusions in the cores and omphacite and lawsonite pseudomorphs in the rims. Lawsonite pseudomorphs are also wrapped by the amphibole-rich (mainly glaucophane and barroisite) foliation. (b) Lozengeshaped lawsonite pseudomorphed by epidote and paragonite in a calcschist. Plane polarized light. (c) Lawsonite pseudomorph included within omphacite in a classical metabasite. Silica released by lawsonite destabilization likely reacted with jadeite to form a thin albite rim. (d) Chemical zonation of a porphyroblastic garnet from a ÔmagnesianÕ metabasite showing a strong Mn and Ca enrichment in the core and an Fe and Mg increase towards the rim. Abundances are given in structural formula units. (e) Porphyroblast garnet from a magnesian metabasalt wrapped by elongated talc crystals, showing quartz and chloritoid inclusions. Glaucophane and talc coexist in the schistosity overprinting the HP matrix. (f) Chloritoid, garnet, talc and omphacite in the Allalin metagabbro. Mineral abbreviations after Kretz (1983), except barroisite (brs).

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............................................................................................................................................................. thermalized mafic rocks (Martin and Tartarotti, 1989). Jurassic and Cretaceous oceanic metasediments are locally preserved, (Bearth and Schwander, 1981; Reinecke, 1991). During the past 30 years, contrasting peak pressure conditions were deduced for the ZS ophiolite (e.g. 10– 28 kbar, Fig. 1; Ernst and Dal Piaz, 1978; Oberhansli, 1980; Barnicoat and Fry, 1986; Reinecke, 1991, 1998). The rare UHP coesite-bearing sedimentary rocks at Lago di Cignana (Fig. 2) suggest that part of the ophiolite underwent pressures between 26 and 29 kbar at temperatures between 590 and 630 C (Reinecke, 1991, 1998). A more recent study on the Allalin gabbro (Saas-Fee area, Fig. 2) suggests P–T conditions of 25–30 kbar and 550–600 C (Bucher et al., 2005), whereas metabasites from the southern part of the ZS ophiolite yielded somewhat lower P–T conditions of 21 ± 3 kbar and 550 ± 60 C (Martin et al., 2008).

Petrographical description To better constrain the ZS P–T estimates, we conducted an extensive

sampling covering the whole ZS unit (Fig. 2). Our petrographical study focuses on metabasites, which show a wide range of mineral parageneses and textures (Fig. 3), and on the Allalin gabbro. Representative electron microprobe analyses are given in Table 1. ÔClassicalÕ metabasites paragenesis Zermatt-Saas ÔclassicalÕ metabasites (showing N-MORB geochemical signatures; Dal Piaz et al., 1979) contain relict eclogite facies assemblages with mm- to cm-large garnet porphyroblasts hosting inclusions of other high-pressure phases (mainly epidote, omphacite, paragonite and quartz, Fig. 3a). Omphacitic pyroxene occurs as subhedral crystals both within garnet and locally in the matrix, where it is commonly replaced by low-grade amphibole. Peak omphacite is locally rimmed by a more Ca-rich omphacite overgrowth. Garnet and omphacite, together with rutile and epidote, define the standard eclogitic peak burial assemblage typical of classical metabasites. Glaucophane modal

amounts are highly variable (0–70 vol.%) and increase towards pillow rims. Glaucophane is rarely included within garnet and overgrows eclogitic assemblages in most of the samples. Epidote is stable early within the HP assemblages, as shown by its inclusion within garnet and omphacite cores. Epidote is also found associated with paragonite and quartz in mm-sized lozengeshaped areas, which are interpreted as pseudomorphs after lawsonite (e.g. Bearth, 1973; Fig. 3a). Lawsonite pseudomorphs, which are ubiquitous in ZS metabasites and adjacent calcschists (Fig. 3b), are found within garnet and omphacite cores and rims in some samples (Fig. 3a,c), and coexist with peak burial assemblages. In contrast, lawsonite breakdown and glaucophane crystallization are characteristic of post-eclogitic assemblages. In the most retrogressed samples, relict HP minerals are replaced by patchy low grade amphiboles such as barroisite, magnesio-hornblende, and actinolite together with epidote, paragonite, albite ± titanite. In few samples, garnet is rimmed by biotite.

Table 1 Representative electron microprobe analyses of garnet, omphacite, chloritoid, talc, phengite, glaucophane, barroisite and Mg-hornblende from classical metabasites (CM) and magnesian metabasites (MM) of the ZS ophiolite. Mineral

Grt

Grt

Grt

Grt

Omp

Omp

Cld

Tlc

Phg

Gln

Brs

Mg-Hbl

Sample

CM-7

CM-17

MM-23

MM-23

CM-7

CM-11

MM-23

MM-24

MM-24

MM-24

CM-16

CM-17

Location

Core

Rim

Core

Rim

Core

Rim

Core

Core

Core

Core

Core

Core

SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O Sum Si Ti Al Fe Mn Mg Ca Na K Sum XJd XPrp XMg

36.73 0.19 20.74 30.42 1.29 1.34 9.4 0.04 0 100.15 2.96 0 1.97 2.05 0.09 0.16 0.81 0 0 8.04

38.21 0.08 21.09 28.35 0.53 4.11 8.45 0.04 0 100.86 2.99 0 1.95 1.86 0.04 0.48 0.71 0 0 8.03

36.76 0.11 21.44 28.81 5.31 2.16 5.45 0.07 0 100.11 2.95 0 2.03 1.94 0.36 0.26 0.47 0 0 8.01

37.76 0 21.22 30.41 1.07 4.32 4.57 0.02 0 99.39 3.01 0 1.99 2.02 0.07 0.51 0.39 0 0 7.99

55.57 0.03 12.16 8.43 0.00 4.87 8.78 9.5 0 99.34 2.01 0 0.52 0.26 0 0.26 0.34 0.67 0 4.06 0.56

54.16 0.15 4.65 6.31 0.11 11.56 19.54 3.25 0 99.73 1.99 0 0.2 0.19 0 0.63 0.77 0.23 0 4.01 0.23

24.6 0 41.67 19.75 0.17 5.48 0.00 0.02 0 91.69 1.00 0 1.99 0.67 0 0.33 0 0 0 3.99

60.47 0.07 0.32 6.42 0 25.91 0.16 0.04 0.1 93.49 8.02 0 0.05 0.71 0 5.12 0.02 0 0.02 13.94

51.82 0.13 25.48 2.44 0 3.9 0.00 0.21 9.37 93.35 3.5 0 2.03 0.14 0 0.39 0.00 0.03 0.81 6.90

59.97 0.01 11.18 6.81 0.01 12.46 0.43 7.43 0.03 98.34 8.03 0 1.77 0.76 0.00 2.49 0.06 1.93 0.01 15.05

45.16 0.04 10.58 16.76 0.13 10.29 8.75 3.9 0.26 95.87 6.84 0 1.89 2.12 0.02 2.32 1.42 1.15 0.05 15.81

50.5 0.07 9.31 11.65 0.14 13.58 9.96 2.79 0.2 98.2 7.38 0 1.21 1.22 0.02 3.23 1.58 0.7 0.02 15.36

0.05 0.07

0.16 0.2

0.08 0.1

0.14 0.2

0.33

0.89

0.7

0.77

0.52

0.67

Mineral abbreviations after Kretz (1983), except barroisite (brs).

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............................................................................................................................................................. ÔMagnesianÕ metabasites paragenesis ÔMagnesian metabasitesÕ, mainly found in the southern Aosta valley, show peculiar high-pressure lithologies (garnet glaucophanites, talcschists and chlorite-schists), Fe–Cu sulphides and Mn ore deposits related to oceanic hydrothermalism (Martin and Tartarotti, 1989). We focused our study on these chloritoid–talc-rich layers found within garnet glaucophanites. Garnet porphyroblasts show a stronger zonation than in classical metabasites, with Mn- and Ca-enriched cores and Mgenriched rims (Fig. 3d; Table 1). These garnets host numerous inclusions (chloritoid, talc, quartz and locally glaucophane and chlorite). The most common HP matrix assemblage is characterized by garnet, chloritoid, talc and more rarely lawsonite pseudomorphs. Chloritoid occurs locally as cm-sized porphyroblasts wrapped by elongated talc crystals marking the main foliation (Fig. 3e). As for classical metabasites, lozengeshaped lawsonite pseudomorphs are locally disseminated in the matrix (especially within glaucophanites) and post-eclogitic assemblages are characterized by lawsonite breakdown and glaucophane crystallization. A barroisitic blue–green amphibole often rims glaucophane crystals because of later equilibration. Fractures in glaucophane are filled by the lowgrade assemblage talc + albite (Martin and Tartarotti, 1989; Corona and Jenkins, 2007). Chloritoid is rimmed by chlorite and paragonite. Allalin metagabbro paragenesis This peculiar metagabbro (Bearth, 1967) is characterized by widespread saussuritized plagioclase phenocrysts in an eclogitic matrix where omphacite and garnet coexist with chloritoid and talc (Fig. 3f). Chloritoid occurs either as inclusions in the pyroxene or as rims surrounding saussuritized plagioclase. Glaucophane is found in pressure shadows around porphyroblasts. Post-eclogitic garnet and chloritoid crystallize within shear bands underlined by talc and epidote. Later recrystallizations are marked by actinolite rims on glaucophane and paragonite–chlorite intergrowths between glaucophane and chloritoid.  2009 Blackwell Publishing Ltd

In both types of metabasalt, up to four successive mineral assemblages (prograde, peak pressure, post-eclogitic and low-grade respectively) can be recognized texturally and used to estimate burial and exhumation P–T conditions.

Conditions of metamorphism THERMOCALC

calculations

Temperatures and pressures were estimated with the computer program THERMOCALC v.3.25 using an updated (2003) version of the Holland and Powell (1998) thermodynamic dataset. Activities of mineral end-members were calculated following Holland and Powell (1998). We followed the calculation method described in Agard et al. (2006). P–T estimates for each paragenesis (Table 2) have been carried out on seven metabasites and one metagabbro sampled across the studied area. THERMOCALC estimates are given in Table 3 and plotted in Fig. 4 (see also Fig. 2 for a map view; analyses considered are available on request). Average P–T

results (Table 2) for the four successive assemblages considered are the following: 1 prograde P–T estimates range between 17–21 kbar and 486– 524 C; 2 peak burial conditions are P = 22.8–24.7 kbar and T = 530– 555 C; 3 post-eclogitic assemblages yield P–T conditions of 14.5–17 kbar and 506–552 C; 4 lower grade reequilibration gives more scattered P–T estimates, between 381–528 C and 3.9– 9.4 kbar.

Raman spectroscopy on carbonaceous matter We used the geothermometer of Beyssac et al. (2002) based on the irreversible transformation of carbonaceous matter into graphite during metamorphism with increasing temperature. This allows determining maximum temperatures in the range 330–650 C, with absolute temperature uncertainties on the order of 50 C but much smaller internal reproducibility, on the

Table 2 Assemblages selected for the different P–T calculations and average results for each equilibrium step (T range, P range).

Classical metabasites

Mg-rich metabasites

Prograde

Peak burial

Post-eclogitic

Low grade

Omph inc. Grt core ±Lws ±Phg inc. ±Ep inc. ±Pg inc. ±Gln inc. (4) Grt core Ctd inc. Tlc inc. Lws

Omph mid. Grt mid. ±Lws ±Phg inc. ±Ep inc. ±Gln inc.

Omph rim Grt rim Gln mx Ep mx Phg mx ±Pg mx

b ⁄ g. amph Ep Ab ±Pg ±Chl ±Bt

(6) Grt mid. Ctd inc. Tlc inc. ±Lws ±Phg inc.

(1) Grt rim Ctd mx Tlc mx Gln mx Ep mx Pg mx (2) Grt rim Omph rim Ctd rim Tlc mx Gln mx (1) 506–552 14.5–17

(6) b ⁄ g. amph Ep Ab Pg Chl

(1) Allalin metagabbro

T range P range

482–524 17–21.5

(2) Grt core ⁄ mid Omph core ⁄ mid Ctd core Tlc (1) 530–555 22.8–24.7

(2) g. amph Ep Ab Chl ±Pg (1) 381–528 3.9–9.4

Italicized numbers in the lower right of each box refer to the number of thin sections for which calculations were made. Mineral abbreviations after Kretz (1983). Other abbreviations: b ⁄ g, blue ⁄ green; inc., inclusion; mx: matrix.

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............................................................................................................................................................. Table 3 THERMOCALC P–T estimates (mode two calculations; Holland and Powell, 1998) for each sample. Uncertainties on T and P (DT and DP, respectively). Selected assemblage Prograde assemblage 6 Grt ⁄ Omp ⁄ Phg 7 Grt ⁄ Omp ⁄ Gl ⁄ Ep ⁄ Pg ⁄ Lws 14 Grt ⁄ Omp ⁄ Phg ⁄ Ep ⁄ Lws 16 Grt ⁄ Fe-Brs ⁄ Ep ⁄ Pg ⁄ Lws 22 Grt ⁄ Ctd ⁄ Tlc ⁄ Lws Peak burial assemblage 3 Grt ⁄ Omp ⁄ Ctd ⁄ Tlc 6 Grt ⁄ Omp ⁄ Phg 7 Grt ⁄ Omp ⁄ Gl ⁄ Ep ⁄ Lws 11 Grt ⁄ Omp ⁄ Phg ⁄ Ep 14 Grt ⁄ Omp ⁄ Phg ⁄ Lws 16 Grt ⁄ Omp ⁄ Phg ⁄ Ep 17 Grt ⁄ Omp ⁄ Phg ⁄ Ep 23 Grt ⁄ Ctd ⁄ Tlc ⁄ Lws 24 Grt ⁄ Ctd ⁄ Tlc ⁄ Phg Post-eclogitic 3 Grt ⁄ Omp ⁄ Gl ⁄ Ctd ⁄ Tlc 11 Grt ⁄ Omp ⁄ Phg ⁄ Gl ⁄ Ep 23 Grt ⁄ Gl ⁄ Ctd ⁄ Tlc ⁄ Ep ⁄ Pg 24 Grt ⁄ Gl ⁄ Ctd ⁄ Tlc ⁄ Ep ⁄ Pg Low-grade assemblage 3 Act ⁄ Ep ⁄ Pg ⁄ Chl ⁄ Ab Act ⁄ Ep ⁄ Chl ⁄ Ab 6 Prg ⁄ Bt ⁄ Ep ⁄ Pg ⁄ Ab 7 Fe-Brs ⁄ Ep ⁄ Pg ⁄ Ab 11 Mg-Hbl ⁄ Ep ⁄ Pg ⁄ Ab ⁄ Sph 14 Ed ⁄ Ep ⁄ Pg ⁄ Ab 16 Brs ⁄ Ep ⁄ Pg ⁄ Ab Mg-Hbl ⁄ Ep ⁄ Pg ⁄ Ab 17 Mg-Hbl ⁄ Ep ⁄ Pg ⁄ Ab Mg-Hbl ⁄ Ep ⁄ Pg ⁄ Ab 23 Mg-Hbl ⁄ Ep ⁄ Pg ⁄ Chl ⁄ Ab 24 Brs ⁄ Ep ⁄ Pg ⁄ Chl ⁄ Ab Brs ⁄ Ep ⁄ Pg ⁄ Chl ⁄ Ab

Temp. (C)

Press. (kbar)

DT

DP

Sigfit

482 493 486 524 503

21.5 17.4 17 20 20.7

39 17 10 16 11

1.5 1 0.6 1.2 1.3

0.44 1.98 0.49 0.14 1.44

542 552 540 540 555 554 539 530 542

23.8 23 23.9 22.8 24.3 23.3 24.7 23.4 23.2

27 42 22 27 17 24 25 8 10

7.4 1.5 1.9 1.1 1 1 1.1 1 1.6

1.6 0.02 2 1.23 1.46 0.58 1.16 0.6 1.18

552 506 547 548

17 14.5 15.4 15.3

12 26 11 11

2.1 1.5 1.7 1.3

1.22 0.78 1.33 0.45

497 512 496 392 433 483 456 381 528 473 465 436 477

6.1 6.7 7.5 5 3.9 7.5 9.4 4.4 8.4 5.9 7 7.6 7

17 18 47 73 34 37 40 34 35 31 17 19 20

0.5 0.6 1.7 2.3 1.2 1.3 1.5 1.1 1.3 1.1 0.7 0.4 0.4

1.13 0.54 1.35 0.28 1.14 0.13 0.04 0.28 0.06 0.07 0.38 0.11 0.74

Grey shaded lines correspond to magnesian metabasites and Allalin metagabbro (sample 3). Mineral abbreviations after Kretz (1983) except ferro-barroisite (Fe-Brs), barroisite (Brs) and magnesio-hornblende (Mg-Hbl).

order of 10 C (Beyssac et al., 2004). To smooth out the within-sample structural heterogeneity, between 11 and 26 spectra were recorded for each sample, following the approach of Beyssac et al. (2002). Raman spectroscopy on carbonaceous matter (RSCM) bulk analyses are shown in Table 4. The structural heterogeneity is systematically low for all samples, and RSCM temperatures are remarkably clustered between 523 and 545 C for all ZS calcschists (Fig. 2).

Discussion Implications for the subduction path Computed P–T paths for all the samples are shown in Fig. 4 and cluster 176

tightly. Although prograde assemblages are largely retrogressed, the burial path is constrained by (i) the widespread occurrence of lawsonite pseudomorphs in eclogites and calcschists across the area, and (ii) THERMOCALC P–T calculations on garnet inclusions. Both provide evidence for a low-temperature burial path, thus advocating for a cool geotherm and ⁄ or rapid subduction. Peak burial P–T conditions are strikingly homogeneous regardless of the kind of metabasalt, which demonstrates that the entire ophiolite, at least 60 km across, underwent similar eclogitic conditions (c. 23–25 kbar) and reached similar depths (Fig. 2). Post-eclogitic and low grade estimates suggest a near-isothermal decompres-

sion between 23 and c. 10 kbar, in agreement with earlier works (Reinecke, 1998; Bucher et al., 2005). Importantly, we note that the independent RSCM estimates of maximum temperatures are in very good agreement with the THERMOCALC calculations (Fig. 4). Our peak temperature estimates are, nevertheless, slightly lower than those of Bucher et al. (2005) or Reinecke (1998), and so are the peak pressures (by 3–5 kbar). This discrepancy can probably be attributed to the use of different thermobarometric programmes and thermodynamical databases and to the lack of coesite in our rocks. Indeed, the pressure estimates of Reinecke (1991, 1998) for the area of Lago di Cignana are tightly constrained by the presence of small coesite inclusions within garnet or tourmaline. Although our peak pressure estimates lie close to the coesite stability field (lower P limit at 25.5 kbar at 540 C; Fig. 4), coesite was never found in our samples (similarly to Bucher et al., 2005), despite thorough and extensive investigations. Note that coesite would shift P–T estimates towards higher P values but probably not affect the homogeneity of the results (as suggested by the consistency of our RSCM temperatures and the homogeneity of the P–T gradient in the Western Alps; Oberhansli and Goffe, 2004). The lack of ubiquitous coesite in the ZS ophiolite could be tentatively attributed to (i) the detachment of hm-scale bodies which went a few kilometres deeper than the rest of the ophiolite and were later juxtaposed at 23–25 kbar; (ii) the lack of quartz inclusions within garnet or omphacite; and (iii) small overpressures within the Lago di Cignana eclogitic unit, as those expected in the subduction channel from numerical experiments (c. 10%; Yamato et al., 2007; Raimbourg and Kimura, 2008). Implications for subduction zone mechanics Since the pioneering work of England and Holland (1979) and Shreve and Cloos (1986), most authors view the upper plate–slab interface as a subduction channel. Thermomechanical modelling of a subduction channel predicts either an intense mixing in a weak serpentinite matrix (Gerya et al.,  2009 Blackwell Publishing Ltd

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Fig. 4 P–T diagram showing the THERMOCALC estimates and ellipses associated with calculation uncertainties (see Holland and Powell, 1998) for eight samples taken across the Zermatt-Saas ophiolite (located on Fig. 2 for location). Range of peak metamorphic T estimated by RSCM thermometry (given in Table 4) is also depicted. Typical textures and parageneses (from top to bottom: eclogitic, post-eclogitic and lower grade) are represented for the classical metabasites. Background facies grid modified from Evans (1990). The experimentally determined phase equilibria are from Bohlen and Boettcher (1982) for the coesite ⁄ quartz reaction and from Holland (1980) for the albite = jadeite + quartz reaction. BS, blueschists; CEc, coesite eclogites; EA, epidote–amphibolites; EBs, epidote– blueschists; Ec, eclogites; GS, greenschists; LEc, lawsonite eclogites.

2002; Gorczyk et al., 2007) or the exhumation of individual slices (Yamato et al., 2007). Although Gerya et al. (2002) compare their lowviscosity model with the ZS ophiolite, our results reveal that the ZS area is not a deep serpentinite ÔmelangeÕ. Instead, the ZS ophiolite shows a striking homogeneity of peak condition estimates and appears to be a 60km-long continuous slice detached from the slab. The ophiolite detachment occurred at around 70–80 km depth along the subduction zone,  2009 Blackwell Publishing Ltd

slightly shallower than previous estimates and thus closer to most subducted oceanic lithosphere fragments returned to the surface (i.e. slightly outside of the negative buoyancy area; Fig. 5; Agard et al., 2009). Serpentinites, which represent about half of the ophiolite and are mostly slab-derived and equilibrated under similar P–T conditions (Li et al., 2004), therefore probably acted, during the exhumation of the ZS ophiolite, not so much as a weak matrix, in which slices are disintegrated, but as a posi-

tively buoyant ÔfloatÕ preventing the eclogitized crust from irreversibly sinking into the mantle. The apparent contrast between the heterogeneous tectonic slices of the Monviso and Voltri massifs and the homogeneous ZS body, all exhumed from the same subduction zone, could result from the earlier exhumation of smaller rock volumes in Monviso and Voltri (Fig. 5; e.g. Guillot et al., 2004; Federico et al., 2007). In any case, the exhumation of such a large piece of dense oceanic 177

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............................................................................................................................................................. Table 4 Peak metamorphic temperatures in the calcschists from the Zermatt-Saas ophiolite estimated by RSCM thermometry. See text and Beyssac et al. (2002) for details. Samp.

n

R2

T

SD

1 2 4 5 8 9 10 12 13 15 18 19 20 21 22 25 26 27 Average

12 17 11 14 26 11 14 11 18 23 14 12 12 12 26 19 12 12 15

0.22 0.23 0.24 0.24 0.22 0.25 0.22 0.25 0.24 0.23 0.24 0.27 0.23 0.22 0.22 0.23 0.21 0.26 0.23

545 537 536 535 542 531 542 531 535 541 533 523 537 544 544 541 545 527 537.2

12.4 19.9 15 16.7 29.7 17.3 14.9 18.6 17.3 19.8 24.4 21 25.1 16 29.5 18.4 12.4 15.8 19.1

R2 ratio with temperature in Celsius degrees (T) and 1r standard deviation (SD). n: number of spectra acquired for each sample.

lithosphere has critical implications for subduction zone mechanics (Fig. 5). The exhumation of the ZS ophiolite could have been triggered and enhanced by the entrance of the positively buoyant continental crust (e.g. Monte Rosa; Fig. 2), which is known to allow for the extrusion of gigantic crustal volumes (e.g. Norway: Andersen et al., 1991; Oman: Chemenda et al., 1996; Dabie Shan: Hacker et al., 1996). Singh et al. (2008) recently suggested that largescale oceanic crust detachment from the slab occurred on the plate–slab interface in the Sumatra subduction zone. Such a mechanism, in response to large earthquakes (Hacker et al., 2003; Andersen and Austrheim, 2006; Singh et al., 2008), could thus possibly be invoked for the detachment and exhumation of the ZS ophiolite. Although the downdip limit of the seismogenic zone is commonly around 40 ± 5 km (e.g. Peacock and Hyndman, 1999), it can reach down to c. 65 km (Seno, 2005; for T between 500 and 600 C), suggesting that deep slices like ZS can effectively detach from the slab at such depths. 178

(b) (a)

Fig. 5 (a) Tentative geodynamic scenario for the exhumation of the Zermatt-Saas ophiolite: (1) The thinned European continental margin is progressively dragged into subduction shortly after the last pieces of oceanic lithosphere (including ZermattSaas, according to available radiometric constraints). (2) Exhumation of the ZS ophiolite is likely triggered by the entrance of the positively buoyant continental crust. (3) Coeval exhumation of the internal crystalline massifs (GP, Gran Paradiso; MR, Monte Rosa) and of the Zermatt-Saas ophiolite (ZS). (b) P–T paths from this study (broad shaded arrow) compared with the P–T paths from Reinecke (1998) and Bucher et al. (2005). Boxes correspond to a compilation of the maximum P–T values reached by the oceanic lithosphere across the world (see Agard et al., 2009). Thick boxes represent P–T estimates from Western Alps in Voltri, Monviso and our estimates on ZS ophiolite. Grey field: domain where the oceanic crust is negatively buoyant with respect to the adjacent mantle.

Acknowledgements We thank S. Guillot and R. Oberhansli for constructive reviews and A. Nicolas for the handling of our manuscript. We gratefully acknowledge Christian Chopin and P. Monie´ for thoughtful remarks and Didier Devaux for promptly making our thin sections. We also thank Loic Labrousse and Benjamin Huet for insightful discussions. The project was partly funded by an IUF grant to Laurent Jolivet.

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