Evidence for protracted prograde metamorphism followed by...

Evidence for protracted prograde metamorphism followed byrapid exhumation of the Zermatt-Saas Fee ophiolite

S. SKORA,1 , 2 N. J . MAHLEN,3 C. M. JOHNSON,3 L. P. BAUMGARTNER,1 T. J . LAPEN,4 B. L . BEARD3

AND E. T. SZILVAGYI31Institute of Earth Sciences, University of Lausanne, Geopolis, 1015 Lausanne, Switzerland2Institute of Geochemistry and Petrology, ETH Zurich, Clausiusstrasse, 25NW 8092 Zurich, Switzerland([email protected])3Department of Geoscience, University of Wisconsin-Madison, 1215 W Dayton St, Madison, WI 53706, USA4Department of Earth and Atmospheric Sciences, University of Houston, 4800 Calhoun Road, Houston, TX 77004, USA

ABSTRACT Major and trace-element zoning in garnet, in combination with Rb–Sr, Sm–Nd and Lu–Hfgeochronology, provide evidence for a protracted garnet growth history for the Zermatt-Saas Fee(ZSF) ophiolite, western Alps. Four new Lu–Hf ages from Pfulwe (c. 52–46 Ma) and one from Cha-mois (c. 52 Ma) are very similar to a previously published Lu–Hf age from Lago di Cignana. Overall,the similarity of geochronological and garnet zoning patterns suggests that these three localities had asimilar prograde tectonic history, commensurate with their similar structural position near the top ofthe ZSF. Samples from the lower part of the ZSF at Saas Fee and St. Jacques, however, producedmuch younger Lu–Hf ages (c. 41–38 Ma). Neither differences in whole-rock geochemistry, whichmight produce distinct garnet growth histories, nor rare-earth-element zoning in garnet, can accountfor the age differences in the two suites. This suggests a much later prograde history for the lowerpart of the ZSF, supporting the idea that it was subducted diachronously. Such a model is consistentwith changes in subduction vectors based on plate tectonic reconstructions, where early oblique sub-duction, which produced long prograde garnet growth, changed to more orthogonal subduction,which corresponds to shorter prograde garnet growth. Six new Rb–Sr phengite ages range from c. 42to 39 Ma and, in combination with previously published Rb–Sr ages, constrain the timing of thetransition from eclogite to upper greenschist facies P–T conditions. The proximity of the ZSF in theSaas Fee region to the underlying continental Monte Rosa unit and the similarity of peak-metamor-phic ages suggest these two units were linked for part of their tectonic history. This in turn indicatesthat the Monte Rosa may have been partly responsible for rapid exhumation of the ZSF unit.

Key words: geochronology; Lu–Hf; metamorphism; Rb–Sr; Zermatt-Saas Fee.

INTRODUCTION

High-pressure (HP) and ultrahigh-pressure (UHP)metamorphic terranes that are associated with conti-nent–continent collision record subduction of crust todeep levels, followed by rapid exhumation, asrequired to preserve evidence for (U)HP conditions(e.g. Coleman & Wang, 1995; Hacker & Liou, 1998;Chopin, 2003). Mechanisms for rapid exhumation ofhigh-density mafic rocks are debated, and possibilitiesinclude exhumation through attachment to low-density serpentinites (e.g. Hermann et al., 2000;Schwartz et al., 2001; Pilchin, 2005) or attachment tocontinental fragments (e.g. Cloos, 1993; Lapen et al.,2007). Indeed, many workers argue that buoyancymechanisms are the most likely means by which rapid(>20 mm yr�1) exhumation of (U)HP terranes occurs(e.g. Platt, 1987; Wheeler, 1991; Duchêne et al.,1997b).

The Alpine chain is a classical continent–continentcollisional orogen, where such theories might betested. Despite intensive petrological and geochrono-logical studies of the (U)HP terranes of the westernAlps, there still exists great uncertainty in the timingof Alpine metamorphism. Part of this uncertainty liesin the relations between ages and metamorphism –some ages may indeed reflect peak metamorphism,but mineral growth (of garnet and zircon, for exam-ple) along the prograde path may produce a range ofages (e.g. Lapen et al., 2003; Anczkiewicz et al.,2007; Kylander-Clark et al., 2007; Schmidt et al.,2008; Smit et al., 2010; Zirakparvar et al., 2011;Kirchenbaur et al., 2012). The aim of this study is toplace better constraints on the metamorphic historyacross the Zermatt-Saas Fee (ZSF) unit (Fig. 1) byapplying the Lu–Hf garnet geochronometer to a rela-tively large number of samples (n = 10) from differ-ent structural positions in the alpine stack. The new

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J. metamorphic Geol., 2015, 33, 711–734 doi:10.1111/jmg.12148

ages are linked to trace-element zonation in garnet,which together place tight constraints on the timingof prograde and peak metamorphism. New Rb–Srisochron ages for phengite provide additional con-straints on the transition between eclogite to uppergreenschist facies P–T conditions (see also de Meyeret al., 2014). Last, in the light of the new data set, aswell as previously published geochronological dataon the underlying continental Monte Rosa unit, weexamine the timing of metamorphism across the ZSFsheet to better understand the relations between con-tinental and oceanic units during subduction-relatedmetamorphism (e.g. Lapen et al., 2007).

GEOLOGICAL SETTING

Alpine rocks (before collision) can be generally subdi-vided into: (i) pre-Triassic basement rocks and (ii)Triassic to early Cenozoic cover sedimentary rocksthat were deposited in a predominantly shallow watershelf environment (e.g. Tr€umpy, 1980). During theEarly Jurassic, this carbonate platform broke up inconjunction with establishment of the Tethyan-

Atlantic junction, which gave rise to the developmentof the Liguro-Piemont Ocean basin (c. 165–160 Ma,Rubatto et al., 1998; Schaltegger et al., 2002), andseparation of Europe from Apulia/Africa. The open-ing of the Atlantic Ocean towards the north in theLate Jurassic to Early Cretaceous led to shorteningbetween Europe and Apulia/Africa (e.g. Stampfliet al., 1998). Southeast-directed subduction andnorthwest-directed thrusting during the Late Creta-ceous through early Cenozoic eventually resulted inclosure of the Liguro-Piemont Ocean basin and cre-ation of the Alpine tectonic nappe system (e.g. Hun-ziker, 1974; Dal Piaz & Ernst, 1978). The ZSFophiolite represents the (U)HP metamorphic remnantof that oceanic material. It is comprised of peri-dotites, serpentinites, eclogitized metagabbros andmetabasalts that contain local examples of deformedsheeted dyke systems and well preserved pillow struc-tures, and a cover series of calcareous and siliceousmetasedimentary rocks (e.g. Bearth, 1967; Dal Piaz &Ernst, 1978; Barnicoat & Fry, 1986).The paleogeographic organization used throughout

this paper is from north to south (Fig. 2a): (i) Euro-pean continental margin (Helvetic: Late Paleozoiccrystalline basement rocks, covered by various sedi-mentary Mesozoic/Cenozoic nappes), (ii) Valais Basin(Lower Cretaceous ‘B€undnerschiefer/Schiste Lustr�e’),(iii) Brianconnais domain (Late Paleozoic crystallinebasement rocks, covered by various Mesozoic sedi-mentary rocks), (iv) remnants of the Jurassic Liguro-Piemont oceanic crust (ophiolitic HP zone, overlain bythe sediment-dominated, low-pressure Combin zone),(v) northernmost Apulian/African continental marginor a distal micro-continent (Austroalpine: pre-Triassiccrystalline rocks), (vi) Apulian/African continentalmargin (non-metamorphosed Southalpine). Note thatthe low-P (upper greenschist/lower blueschist facies)Combin zone has been further subdivided into the (i)Mont Fort nappe (Triassic/Mesozoic cover sedimen-tary rocks of the Saint Bernard nappe, which is part ofthe Brianconnais) and (ii) Tsat�e nappe (ophioliticm�elange zone with overlying Upper Cretaceous calc-schists, Sartori, 1987). The Tsat�e nappe represents theaccretionary wedge below which the ZSF was sub-ducted (Sartori, 1987; Marthaler & Stampfli, 1989).Because subduction was southeast directed (Fig. 2b),

the oldest peak-metamorphic ages are recorded in fel-sic eclogites from the Sesia-Lanzo zone (Austroalpine)at c. 75–65 Ma (Inger et al., 1996; Duchêne et al.,1997a; Rubatto et al., 1999, 2011). Subsequent sub-duction of the ZSF unit to eclogite facies conditionsproduced a suite of metamorphic ages rangingbetween c. 50 and 40 Ma (Duchêne et al., 1997a;Rubatto et al., 1998; Amato et al., 1999; Dal Piazet al., 2001; Lapen et al., 2003; Rubatto & Hermann,2003; Gouzu et al., 2006; Herwartz et al., 2008).Focusing on ages from north of the Aosta fault, theSm–Nd garnet isochron age of 40.6 � 2.6 Ma mostclosely dates the peak of metamorphism (Amato et al.,

Zermatt

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St. Jacques

Pfulwe path:P-02,80bP-80c,96

Lago di Cignana:96JA-32, 01NM-45

Chamois:CH-48

Sample location

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Saas Fee:05NM-214,215

Saas Fee:05NM-212

St. Jacques:SJ-87

Lago di Cignana:08ES-03

Pfulwe pass:P-98,100

SaasFee

Fig. 1. Geological map showing the Liguro-Piemont oceanicremnants at the western Swiss/Italian border (simplified afterSteck et al., 1999). The studied samples from Zermatt-Saas Feeophiolite are from the Pfulwe, Chamois, Lago di Cignana, St.Jacques and Saas Fee areas, and sample names are listed foreach locality.

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712 S . SKORA ET AL .

1999), because of strong enrichment of Sm at thegarnet rim (Skora et al., 2009). Rb–Sr metamorphicages of the Tsat�e nappe range from c. 44 to 37 Ma(Reddy et al., 1999; Cartwright & Barnicoat, 2002).Peak-metamorphic ages from subducted continentalslices of the southernmost Brianconnais continentalmargin such as Monte Rosa, the Gran Paradiso andthe Dora Maira nappes are c. 42–35 Ma (Tilton et al.,1991; Duchêne et al., 1997a; Gebauer et al., 1997;Rubatto & Gebauer, 1999; Engi et al., 2001; Meffan-Main et al., 2004; Lapen et al., 2007). Subsequentexhumation through upper greenschist facies is datedat c. 40–37 Ma in the Sesia-Lanzo zone (Inger et al.,1996), c. 40–35 Ma in the ZSF and adjacent Tsat�enappe (M€uller, 1989; Barnicoat et al., 1995; Reddyet al., 1999; Cartwright & Barnicoat, 2002) andc. 35–32 Ma in the units from the southernmost Brian-connais domain (Freeman et al., 1997; Engi et al.,2001; Meffan-Main et al., 2004).

SAMPLES AND PETROLOGY

Samples were taken to reflect a broad geographic dis-tribution as well as different structural levels (Fig. 1).The Lago di Cignana unit is from the structurallyhighest position, just beneath the Tsat�e nappe. TheChamois locality is only 5 km from Lago di Cignana,and likely at a similar structural position within theZSF unit. The Pfulwe samples are also from a struc-turally high position, although field relations suggestthat it is slightly lower than Lago di Cignana. The St.Jacques locality lies in the interior of the ZSF, but itsstructural position is unclear. In contrast, the Sass-Feesamples clearly reflect the structurally lowest positionin the ZSF package, directly lying above the MonteRosa nappe. The petrology of the eclogite samples isdiscussed first, followed by the metasedimentary sam-ples collected at Lago di Cignana and near Saas Fee.Thin section images for representative samples areshown in Figs 3a–f, 4a–f and 5a–d. Sample locationsare given in Table S1 (eclogites only) and Fig. 1.

Eclogites

All eclogites contain abundant omphacitic clinopy-roxene and garnet porphyroblasts. Other minerals,such as white mica (mostly paragonite, some phen-gite), glaucophane, epidote–clinozoisite solid solu-tions (collectively called epidote hereafter ineclogites), lawsonite (pseudomorphs), carbonate,quartz and rutile also occur in the ZSF eclogites.These minerals may or may not be part of the peak-metamorphic assemblage, co-existing with garnet.Minor retrogression is present in all samples. It ischaracterized by transformation of omphacite intoNa–Ca hornblende � albite; garnet rims into Na–Cahornblende � albite � chlorite � epidote; transfor-mation of glaucophane into Na–Ca hornblende (Na–Ca hornblende is either barrosite, taramite or kato-phorite); as well as rutile that is retrograded intotitanite and occasionally ilmenite at the rims.

Pfulwe

The samples P-80b, P-80c, P-02, P-96 come from asmall outcrop just below Pfulwe pass (~0.5 kmENE), whereas samples P-98 and P-100 come fromthe actual Pfulwe pass, which is located ~7.5 km eastof Zermatt. The geology and petrology of this area isoutlined in, for example, Bearth (1967), Ernst & DalPiaz (1978), Oberh€ansli (1982), Barnicoat & Fry(1986) and Barnicoat (1988). Both outcrops are dis-cussed together as the Pfulwe area hereafter. Ompha-cite often occurs as small grains that have undulatoryextinction in these samples, preserving a radial,flower-like growth structure (Fig. 3a,b). This appear-ance suggests that these rocks record little or nodeformation. Nearly idiomorphic garnet is coarse(Fig. 3c,d) reaching up to 1 cm. Omphacite, rutile,glaucophane and quartz inclusions occur in garnet,whereas epidote and ilmenite inclusions are moreabundant in the cores. Matrix paragonite and glauco-phane likely belong to the prograde assemblage for

Paleogeographic setting in Jurassic times

Today

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Europe

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1: Zone Houillere/Pontis nappe2: Grand Saint Bernard nappe3: Mont Fort nappe 4. Monte Rosa nappe5: Zermatt-Saas Fee zone6: Tsaté nappe 7: Dent Blanche nappe

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sub-continental mantle

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(b)Fig. 2. (a) Schematic diagram showing thepaleogeographic setting of the Zermatt-SaasFee and related units in a profile from NWto SW during Jurassic oceanic rifting(modified after Labhart, 1992). (b)Geological profile approximating thecurrent tectonic situation of the WesternAlps (modified after Labhart, 1992).


GEOCHRONOLOGY OF THE ZERMATT -SAAS OPHIOL I TE 713

omp

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96JA-32

1 mm 1 mm

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(c) (d)

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Fig. 3. Photomicrographs illustrating the metamorphic textures of dated samples (left columns are in PPL, and right columns aresame views in XP). All ZSF eclogites contain omphacitic clinopyroxene and garnet porphyroblasts that vary considerably in size.Other minerals, such as white mica (phengite or paragonite), glaucophane, epidote, lawsonite, carbonate, quartz and rutile alsooccur. (a, b) Eclogites from the Pfulwe area are relatively undeformed. In places, omphacite even preserve a radial, flower-likegrowth structure. (c, d) Garnet can grow exceptionally large at Pfulwe, occasionally reaching up to 1 cm in diameter. (e, f)Lawsonite pseudomorphs are commonly observed at Lago di Cignana, as both inclusions in garnet and in the matrix. grt, garnet;omp, omphacite; Na–Ca hbl, Na–Ca hornblende; gln, glaucophane; rt, rutile; lws pseudom., lawsonite pseudomorphs.



reasons discussed in Skora et al. (2008). Epidote,Fe-rich dolomite, rutile, apatite and quartz often occurin various but small amounts. The peak-metamorphic

assemblage is: omphacite + garnet + paragonite +glaucophane + epidote + rutile (+ carbonate +quartz).

SF-25b

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Fig. 4. Photomicrographs illustrating the metamorphic textures of dated samples (left columns are in PPL, and right columns aresame views in XP). The degree of deformation in these samples is strongly increased when compared with textures of samples fromPfulwe and Lago di Cignana (Fig. 3a–d), although the mineral paragenesis is comparable. (a, b) The sample from Chamoisexhibits a strong metamorphic foliation defined by oriented omphacite and rutile. (c, d) Garnet at Saas Fee is sometimes ruptured,suggesting that deformation was very strong. (e, f) The sample from St. Jacques differs from all other samples in that garnet ismuch smaller on average. Abbreviations as in Fig. 3; pg, paragonite; ttn, titanite.



Lago di Cignana

The petrology of this sample is described in detail inAmato et al. (1999), Lapen et al. (2003) and Skoraet al. (2009). Briefly, the matrix is composed of rela-tively coarse-grained omphacite, glaucophane, epi-dote, paragonite and phengite, which define a weakmetamorphic foliation (data not shown). Garnet por-phyroblasts are ~0.4–4 mm in diameter. Lawsonitepseudomorphs are relatively common in this sample(Fig. 3e,f). The peak-metamorphic assemblage of96JA-32 is garnet + omphacite + glaucophane +epidote + rutile + carbonate (+ phengite).

Chamois

One sample (CH-48) taken from the Nuarsax/Cha-mois area in the Valtournenche is layered and well

foliated. The layering is marked by the presence orabsence of lawsonite pseudomorphs (data not shown),and foliation is defined by oriented omphacite(Fig. 4a,b). In lawsonite-free layers, the foliationwraps around ~1–3 mm sized garnet. Garnet inclusionpatterns differ from Pfulwe samples in that titanite is avery frequent inclusion (besides omphacite), especiallyin garnet cores. Rutile supplants titanite as the Ti-richmineral inclusion in garnet rims. The few paragonitegrains that occur in the matrix often crosscut the folia-tion (Fig. 4a,b), hence, they are likely to have grownpost-deformational. Given that paragonite is stable atblueschist facies conditions, it is likely that it wouldhave been deformed alongside omphacite if paragonitebelonged to the prograde assemblage. Rutile grainsare also present in the matrix. The peak-metamorphicassemblage of the omphacite-rich layers is omphacite+ garnet + rutile. The other layers contain additional

08ES-03

1 mm 1 mm

0.5 mm 0.5 mm

08ES-03

ph

qtz

ph

qtz

pmt pmt

cbcbphph

05NM-21905NM-219

qtzqtz

bt bt

(a) (b)

(c) (d)

Fig. 5. Photomicrographs illustrating the metamorphic textures of dated samples (left columns are in PPL, and right columns aresame views in XP). The appearance of metasedimentary rocks varies from quartz-rich schists to calcsilicates. All samples containrelatively coarse-grained phengite that was used for Rb–Sr dating. (a, b) Sample 08ES-03 from Lago di Cignana is unusuallymanganiferous, as suggested by the presence of piemontite (=manganiferous epidote). (c, d) All samples from the Saas Fee areaare relatively rich in carbonate in addition to phengite and quartz. ph, phengite; cb, carbonate; qtz, quartz; pmt, piemontite; bt,biotite.



coarse-grained paragonite + epidote � albite assem-blages, in which minerals are not oriented. Theseassemblages are similar to the lawsonite pseudo-morphs described by Skora et al. (2009) for Lago diCignana (Fig. 3e,f), except that the lawsonite habit isoften not clearly preserved. The matrix lawsonite isnevertheless interpreted to have formed during earlyprograde metamorphism followed by transformationon the retrograde path, given the undeformed andunoriented appearance of mica.

Saas Fee

Two samples were collected close to the BritanniaHut (Saas Fee). The matrix of the eclogites consistsof fine-grained omphacite, epidote and rare glauco-phane, all of which define a very strong foliation.Garnet in these samples is up to 3 mm in diameter,and extremely inclusion rich. Grains are partly bro-ken, suggesting that some deformation occurred aftergarnet growth ceased. Titanite is more abundant thanrutile in the inclusion population, especially in garnetcores. In addition, titanite (�ilmenite) surrounds orreplaces rutile in the matrix. Lawsonite pseudo-morphs, marked by epidote + paragonite � albite,occur in both samples as garnet inclusions (Fig. 4c,d).Matrix lawsonite pseudomorphs appear to berestricted to sample SF-26. Saas Fee samples arecharacterized by the peak-metamorphic assemblageomphacite + garnet + rutile + glaucophane + epidote.

St. Jacques

One sample was taken northeast of St. Jacques in theVal d0Ayas (SJ-87). Matrix omphacite is aligned anddefines a foliation (Fig. 4e,f). Garnet in this samplediffers from all other samples in that it is much smal-ler (≤1 mm, Fig. 4e,f) but very abundant. In terms ofinclusion patterns, it has abundant titanite in thecores, and rutile in the rims. Other minerals that arealigned in the matrix include epidote, omphacite,glaucophane and paragonite. Glaucophane com-monly has rutile inclusions, in contrast to garnetcores, suggesting that it started to grow close to peakconditions. This sample also contains lawsonite pseu-domorph textures (paragonite + epidote � albite),very similar to the Saas Fee samples. The peak-meta-morphic assemblage is omphacite + garnet + glauco-phane + epidote + rutile + paragonite.

Metasedimentary rocks

Metasedimentary rocks in the ZSF were sampled forRb–Sr geochronology at Lago di Cignana, and nearSaas Fee, representing the lower and upper parts ofthe ZSF respectively. The mineralogy and composi-tions of metasedimentary rocks in the ZSF varygreatly, as primarily reflected in variable proportionsof quartz, garnet, carbonate and mica.

Lago di Cignana

Samples are garnet-bearing, quartz-rich schists(quartz: 40–65%, phengite: 15–25%, carbonate:

Petrological studies conducted on eclogites fromthe Western Alps all indicate that Tpeak and Ppeakwere reached at roughly the same point in time (e.g.Reinecke, 1998). This is in contrast to studies oneclogites from the Central Alps (e.g. Brouwer et al.,2005) that appear to have reached their Tpeak duringthe collisional stage. Eclogites from the Central Alpsare heavily overprinted. Their overprint assemblage isconsistent with exhumation through amphibolitefacies, in contrast to eclogites from the Western Alpsthat only show a greenschist facies overprint. The lat-ter is indicative of cooling during exhumation.Because of the shape of garnet isopleths in P–T spacein eclogites (e.g. Hoschek, 2001), garnet cannot havegrown during exhumation unless it was subjected toheating upon exhumation. The term peak metamor-phism is thus used to describe the point of deepestburial, which equates to Tpeak and Ppeak in the Wes-tern Alps, and it is assumed that garnet growth musthave ceased upon reaching this point.

ANALYTICAL METHODS

Whole-rock (WR) powders were prepared from slabsof rock, which were trimmed to remove weatheredportions, crushed in a steel jaw crusher and reduced tosand-sized particles using an alumina-lined disc mill(University of Wisconsin). The samples were thensplit, where a portion was saved for mineral separates,a portion was powdered using an alumina-lined shat-terbox and a portion was powdered using a tungsten-carbide shatterbox. WR data given in Table 1 areXRF data (University of Lausanne), obtained on thesame WR powder that was used for geochronology.

Mineral chemistry

Garnet

Central cuts of garnet in thick sections of eclogiteswere prepared for major and trace-element analyses,using X-ray tomography at the University of Lau-sanne. Major-element X-ray maps of all measuredgrains were obtained prior to acquiring wavelength-dispersive quantitative analyses using a Cameca SX-

50 (5 spectrometers) electron microprobe (Lausanne).The same thick sections and garnet profiles werethen used to obtain trace-element profiles using laserablation inductively coupled plasma mass spectrome-try (LA-ICP-MS). Trace-element data were acquiredon a Perkin-Elmer ELAN 6100 DRC ICP-MS atLausanne. Samarium data for one Pfulwe garnetwere obtained by secondary ion mass spectrometry(SIMS) at the Max Planck Institute of Chemistry(Mainz). A detailed description on garnet sectioning,EMPA, LA-ICP-MS and SIMS is published inSkora et al. (2006). Garnet chemical data are plottedin Fig. 6a–c.

Phengite

Chemical compositions of phengite in the metasedi-mentary rocks were measured using the CamecaSX51 electron microprobe at 15 keV and 20 nA atthe University of Wisconsin-Madison. Data are givenin Table S2.

Rb–Sr, Sm–Nd and Lu–Hf isotopes

Mineral separates and WR samples of eclogites wereprocessed for Sm–Nd and Lu–Hf geochronology fol-lowing the methods in Lapen et al. (2004) at theUniversity of Wisconsin-Madison. Analytical detailsare only briefly summarized here, whereas a fulldescription is given in Appendix S1. Eclogite samplesand minerals were dissolved in Parr bombs, followedby several dry-down steps. Complete dissolution wasconfirmed after centrifuging each sample. The Amatoet al. (1999) leaching procedure was used on garnetfractions spiked only with the mixed 149Sm–150Ndtracer. Garnet-rich fractions spiked with both mixed176Lu–178Hf and 149Sm–150Nd tracers were not sub-jected to sequential dissolution procedures as out-lined in Amato et al. (1999), because Mahlen et al.(2008) found that this method could potentially frac-tionate Lu and Hf. There was further concern thatleaching or partial dissolution may preferentially dis-solve certain garnet populations, and our geochrono-logic modelling is intended to model total garnetevolution.

wt% P-80b P-80c P-96 P-02 P-98 P-100 96JA-32a CH-48 SF-25b SF-26 SJ-87

SiO2 48.4 50.0 50.4 48.7 49.7 51.4 50.0 50.3 46.2 47.7 48.5

TiO2 1.8 1.6 2.5 1.7 2.2 2.4 1.8 1.9 1.9 2.1 2.2

Al2O3 14.6 15.4 15.3 15.5 15.9 16.0 14.8 15.5 15.7 15.7 14.6

Fe2O3 9.4 8.3 10.0 9.9 11.1 10.8 9.5 8.6 10.4 9.8 11.4

MnO 0.2 0.1 0.2 0.1 0.1 0.1 0.2 0.2 0.2 0.1 0.2

MgO 5.9 4.9 4.2 5.4 4.3 4.4 6.4 4.9 4.4 4.9 5.7

CaO 9.2 10.5 7.8 8.8 8.1 7.0 10.5 11.7 15.6 13.5 11.3

Na2O 6.2 6.9 6.7 6.4 5.8 5.8 3.9 5.4 3.8 4.2 3.9

K2O 0.1 0.0 0.1 0.0 0.2 0.3 0.2 0.0 0.0 0.1 0.1

P2O5 0.2 0.3 0.4 0.2 0.3 0.4 0.2 0.2 0.2 0.2 0.2

LOI 3.0 1.0 1.7 2.3 1.2 0.5 1.7 0.3 0.7 0.8 0.8

Sum 99.0 99.0 99.3 99.0 98.9 99.1 99.2 99.0 99.1 99.1 98.9

Major elements determined by XRF; LOI, loss of ignition.aSample from Amato et al. (1999) and Lapen et al. (2003).

Table 1. Whole-rock data for eclogites (wt%).



Lutetium and Hf isotopes were measured on aMicromass Isoprobe. The 176Lu decay constant ofS€oderlund et al. (2004) was used, at1.865 9 10�11 yr�1, which is essentially identical tothat of Scherer et al. (2001). Samarium and Nd iso-topes were measured on a Micromass Sector 54 ther-

mal ionization mass spectrometer (TIMS). The decayconstant for 147Sm used was 6.54 9 10�12 yr�1. Iso-tope data for both Lu–Hf and Sm–Nd are given inTables 2 and 3, and are plotted in Fig. 7a–s.Mineral separates and WR samples of metasedi-

mentary rocks were analysed for Rb–Sr isotopes at

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160

0.1

0.2

0.3

LuSm (ICP)Sm (SIMS)

Lu [p

pm]

Rim-to-rim zoning [mm]

MnFeMgCa

Cat

ions

[pfu

] C

atio

ns [p

fu]

Cat

ions

[pfu

]

0.5

1.0

1.5

2.0

Pfulwe (sample P-80b)

Chamois (sample CH-48)

10

20

30

40

50Lu

[ppm

]

1.0

1.5

2.0

Sm

[ppm

]S

m [p

pm]

Saas Fee (sample SF-25b)

0.5

1.0

1.5

2.0

0.5 1.0 1.5 2.0 2.5 3.0 3.5 0.5 1.0 1.5 2.0 2.5 3.0 3.5

0.5 1.0 1.5 2.0 2.5 0.5 1.0 1.5 2.0 2.5

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

2

4

6

8

10

12

14

16

18

Lu [p

pm]

0.5

0.6

0.7

0.8

0.9

0.1

0.2

0.3

0.4 Sm

[ppm

]average quantificationlimit of Sm

average quant.limit of Sm

(a)

(b)

(c)

Fig. 6. Dual plots of major elements (EMPA) and Lu and Sm (LA-ICP-MS, SIMS) zoning pattern for representative garnetsamples, one from each investigated location. All data were collected on garnet central sections, as determined by X-raytomography. Most Sm concentrations of garnet cores are below the limit of quantification for LA-ICP-MS. The data are stillplotted, but this only serves to illustrate that the garnet core is depleted in Sm when compared with the rim compositions.



the University of Wisconsin. A 87Rb–84Sr mixed iso-tope tracer was added to the samples; two mixedspikes were used, one for high Rb/Sr ratio phasessuch as mica (spike molar Rb/Sr = 191) and one sui-ted for materials with lower Rb/Sr ratios (spikemolar Rb/Sr = 3.8). Rubidium and Sr were separatedusing cation-exchange chromatography and analysedusing a VG Instruments Sector 54 TIMS. The 87Rbdecay constant of Rotenburg et al. (2012) was used,1.3971 9 10�11 yr�1, which is close to other recentdeterminations by Kossert (2003) and Nebel et al.(2011). For samples in the age range of 40 Ma, thedecay constant of Rotenburg et al. (2012) producesages c. 0.6 Ma older than those calculated using theold decay constant of Steiger & J€ager (1977). Rb–Srdata for the metasedimentary rocks are given inTable 4 and are plotted in Fig. 8a–f.

Errors used in isochron calculations reflect mea-sured uncertainties of individual analyses (internal 2standard error, SE) for 143Nd/144Nd, 176Hf/177Hf and87Sr/86Sr. The uncertainties in all parent/daughter

Table 2. Sm–Nd isotope data for Zermatt-Saas Fee eclogites.

Mat. Sm (ppm) Nd (ppm) 147Sm/144Nd 143Nd/144Nd 2SE eNd

Pfulwe sample: P-80b

grt l 0.432 0.181 0.2482 0.513076 �15 8.5grt la – – – 0.513077 �17 8.6WR 17.0 4.51 0.1618 0.513069 �7 8.4gln 8.93 2.25 0.1536 0.513061 �8 8.3Pfulwe sample: P-80c

grt l 0.433 0.282 0.3887 0.513131 �64 9.6WR 13.1 3.50 0.1627 0.513067 �8 8.4Pfulwe sample: P-02

grt l 0.804 0.281 0.2120 0.513099 �92 9.0grt la – – – 0.513081 �38 8.6grt la – – – 0.513088 �15 8.8grt c. 3.87 1.07 0.1684 0.513052 �12 8.1WR 15.3 4.24 0.1687 0.513078 �8 8.6omp 9.38 2.53 0.1637 0.513056 �9 8.2Pfulwe sample: P-98

grt 1.50 0.54 0.2190 0.513053 �14 8.1grta – – – 0.513058 �31 8.2WR 21.38 5.76 0.1638 0.513059 �8 8.2Pfulwe sample: P-100

grt 0.84 0.54 0.3912 0.513119 �24 9.4WR 19.95 6.03 0.1608 0.513049 �7 8.0omp 26.74 6.94 0.1577 0.513055 �8 8.1Chamois sample: CH-48

grt 2.95 1.20 0.2469 0.513084 �8 8.7WR 16.0 4.36 0.1656 0.513053 �5 8.1Saas Fee sample: SF-25b

grt 1.44 3.42 0.2558 0.513120 �7 9.4WR 5.30 16.36 0.1970 0.513090 �7 8.8Saas Fee sample: SF-26

grt l 0.702 1.14 0.3722 0.513100 �23 9.0grt la – – – 0.513103 �34 9.1grt 1.96 4.25 0.2799 0.513090 �13 8.8WR 6.30 20.3 0.1889 0.513086 �7 8.7omp 3.06 9.76 0.1906 0.513073 �9 8.5gln 4.03 13.5 0.1821 0.513074 �7 8.5St. Jacques sample: SJ-87

grt 7.69 2.92 0.2305 0.513094 �6 8.9WR 19.4 6.13 0.1927 0.513078 �7 8.6

Mat., material; grt, garnet; omp, omphacite; gln, glaucophane; WR, whole rock; l, lea-

ched; c, core.

2SE are from in-run statistics.

Where replicate analyses occur, isochrons are calculated from averages.aAnalyses that are repeat measurements of the same dissolution.

Table 3. Lu–Hf isotope data for Zermatt-Saas Fee eclogites.

Mat. Lu (ppm) Hf (ppm) 176Lu/177Hf 176Hf/177Hf 2SE eHf

Pfulwe sample: P-80b

grt 3.00 3.12 0.1363 0.283184 �5 14.1grta – – – 0.283189 �8 14.3grta – – – 0.283187 �7 14.2WR 0.492 3.90 0.0180 0.283081 �7 10.5WRa – – – 0.283079 �11 10.4omp 0.131 3.44 0.0054 0.283062 �9 9.8gln 0.052 3.41 0.0021 0.283062 �8 9.8Pfulwe sample: P-80c

grt 3.93 3.17 0.1764 0.283235 �5 15.9grta – – – 0.283233 �12 15.8grta – – – 0.283231 �8 15.8WR 0.352 3.77 0.0130 0.283076 �7 10.3WR 0.381 3.90 0.0139 0.283079 �6 10.4omp 0.100 3.85 0.0037 0.283062 �8 9.8Pfulwe sample: P-96

grt 4.66 4.88 0.1356 0.283184 �6 14.1grta – – – 0.283198 �6 14.6grta – – – 0.283195 �7 14.5grta – – – 0.283197 �6 14.6grta – – – 0.283191 �6 14.4WR 0.710 5.83 0.0174 0.283093 �5 10.9omp 0.152 5.49 0.0040 0.283076 �8 10.3Pfulwe sample: P-02

grt 2.85 3.41 0.1190 0.283175 �6 13.8grta – – – 0.283173 �8 13.7grt 2.80 3.53 0.1132 0.283176 �6 13.8grta – – – 0.283176 �6 13.8grta – – – 0.283176 �6 13.8grt c 4.24 3.01 0.2006 0.283249 �10 16.4WR 0.442 3.74 0.0167 0.283088 �9 10.7WRa – – – 0.283083 �13 10.5omp 0.051 1.29 0.0055 0.283078 �7 10.4ompa – – – 0.283083 �6 10.5Pfulwe sample: P-98

grt 2.47 4.20 0.0837 0.283164 � 7 13.4grta – – – 0.283160 � 5 13.3grta – – – 0.283163 � 8 13.4WR 0.660 5.21 0.0179 0.283096 � 7 11.0Pfulwe sample: P-100

grt 1.95 4.44 0.0624 0.283122 �8 11.9grta – – – 0.283126 �7 12.1grta – – – 0.283122 �8 11.9WR 0.661 5.62 0.0166 0.283088 �5 10.7omp 0.162 5.76 0.0038 0.283080 �6 10.4Chamois sample: CH-48

grt 3.13 3.05 0.1458 0.283221 �7 15.4grta – – – 0.283222 �6 15.6grt 3.05 2.95 0.1466 0.283224 �6 15.5grta – – – 0.283220 �8 15.4WR 0.524 4.31 0.0173 0.283097 �7 11.0WRa – – – 0.283100 �5 11.1WRa – – – 0.283096 �6 11.0omp 0.0621 3.37 0.0024 0.283077 �5 10.3ompa – – – 0.283080 �6 10.4Saas Fee sample: SF-25b

grt 3.87 3.89 0.1412 0.283195 �8 14.5grta – – – 0.283196 �6 14.5WR 0.661 4.00 0.0236 0.283111 �5 11.5WRa – – – 0.283112 �7 11.6WRa – – – 0.283113 �5 11.6Saas Fee sample: SF-26

grt 6.47 3.71 0.2482 0.283299 �11 18.2grta – – – 0.283289 �7 17.8grta – – – 0.283286 �8 17.7WR 0.720 4.69 0.0218 0.283120 �10 11.6WRa – – – 0.283119 �5 11.8omp – 1.62 – 0.283112 �10 11.6gln – 3.70 – 0.283117 �10 11.7St. Jacques sample: SJ-87

grt 4.53 2.92 0.2208 0.283284 �6 17.7grta – – – 0.283286 �7 17.7WR 0.771 4.91 0.0224 0.283139 �5 12.5omp 0.049 3.54 0.0020 0.283126 �7 12.1

Mat., material; grt, garnet; omp, omphacite; gln, glaucophane; WR, whole rock; c, core.

2SE are from in-run statistics.

Where replicate analyses occur, isochrons are calculated from averages.aAnalyses that are repeat measurements of the same dissolution.



ratios were set equal to 0.5%, but this has little effecton the propagated errors for the isochrones due tothe relatively young age of the samples.

RESULTS

Results are presented first for garnet and phengitechemistries, which were used for Sm–Nd and Lu–Hfand Rb–Sr geochronology respectively. Sm–Nd andLu–Hf ages for eclogites and Rb–Sr ages formetasedimentary rocks are summarized in Tables 5and 6 respectively.

Garnet chemistry

Major-element profiles determined by electron micro-probe exhibit concentric prograde growth zoning thatis characterized by spessartine and grossular contents

that are highest in the cores, and pyrope and alman-dine contents that are highest at the rims. The exactzoning pattern, as well as the magnitude, variesslightly by location. Rare-earth-element profiles for allgarnet grains vary systematically from Lu to Sm, verysimilar to that described in Skora et al. (2006). OnlyLu (enriched in early grown cores) and Sm (enrichedin late-grown rims) data are shown; these elements arethe most important for understanding the geochrono-logical results. Selected examples of characteristic zon-ing patterns of each locality are given in Fig. 6a–c.Note that no central garnet section of the St. Jacquessamples was obtained because they are too small.

Phengite chemistry

Phengite from Lago di Cignana exhibits Si that variesfrom 3.26 to 3.48 atoms per formula unit (apfu; aver-

97 ± 320 MaMSWD = 8.2

0.51322

0.51298

0.51314

0.51306grt

leachedgrt

WRomp

–10 ± 51 Ma

0.51322

0.51298

0.51314

0.51306WR grt

P-96 Pfulwe

143 N

d/14

4 Nd

20 ± 21 MaMSWD = 1.7

43 ± 43 Ma

0.51322

0.51298

0.51314

0.51306

0.51298

0.51314

0.51306

147Sm/144Nd

0.1 0.3 0.5 0.7

leachedgrt

WR

leachedgrt

WR

glc

0.51322

Sm-Nd

46.7 ± 2.1 MaMSWD = 2.2

grtcore

grt

WRomp

0.28335

0.28305

0.28325

0.28315

54.0 ± 6.4 Ma

0.28335

0.28305

0.28325

0.28315 grt

WR

grt

46.1 ± 2.2 MaMSWD = 1.3

WR

0.28335

0.28305

0.28325

0.28315

omp17

6 Hf/1

77H

f

grt

49.6 ± 2.5 MaMSWD = 0.68

51.7 ± 2.1 MaMSWD = 1.04

WRomp

glc

WRomp

grt

0.28335

0.28305

0.28325

0.28315

0.28335

0.28305

0.28325

0.28315

176Lu/177Hf

0.04 0.12 0.20 0.28

Lu-Hf

20 ± 14 MaMSWD = 2.4

grt

omp

WR

glc

leachedgrt

78 ± 25 Ma

grtWR

58 ± 17 Ma

grt

WR

44 ± 16 MaMSWD = 1.7

grt

WRomp

147Sm/144Nd

P-02 Pfulwe

P-98 Pfulwe

P-80b Pfulwe

P-80c Pfulwe

SF-26 Saas Fee

SF-25bSaas Fee

CH-48 Chamois

SJ-87 St. Jacques

P-100 Pfulwe

65 ± 37 Ma

0.1 0.3 0.5 0.7

grtWR

40.7 ± 1.8 Ma

grt

WR

38.1 ± 2.7 Ma

grt

WR

52.6 ± 1.7 MaMSDW = 1.5

grt

WRomp

40.3 ± 5.9 MaMSWD = 0.18

grtWR

omp

176Lu/177Hf

0.04 0.12 0.20 0.28

39.2 ± 1.6 MaMSWD = 0.20

grt

WRomp

P-02 Pfulwe

P-98 Pfulwe

P-80b Pfulwe

P-80c Pfulwe

SF-26 Saas Fee

SF-25b Saas Fee

CH-48 Chamois

SJ-87 St. Jacques

P-100 Pfulwe

no Sm-Nd data availablefor sample P-96 (Pfulwe)

(l)

(c)

(d)

(a)

(b)

(h)

(g)

(f)

(i)

(e)

(m)

(n)

(j)

(k)

(r)

(q)

(p)

(s)

(o)

Fig. 7. (a–i) Sm–Nd and (j–s) Lu–Hf isochrons for Alpine eclogites. Data are from Tables 2 and 3. Note that all isochrons are atthe same scale for Sm-Nd and Lu-Hf respectively. grt, garnet; WR, whole rock; omp, omphacite; glc, glaucophane.



age = 3.36 � 0.05; calculations are based on 11 oxy-gen), indicating moderately high celadonite contents(Table S2). Saas Fee samples display generallylower average phengite contents and larger variation(Si range = 3.05–3.34 apfu; average = 3.24 � 0.09).Paragonite contents in all samples are low, varyingbetween ~0.04 and 0.08 apfu. Phengite in the piemon-tite-bearing sample 08ES-03 has measurable Mn con-tents (0.3 � 0.1 wt%). Calcium contents are alwayslow, often below detection. All phengite in themetasedimentary rocks at Lago di Cignana and SaasFee is interpreted to reflect eclogite to upper green-schist facies P–T conditions.

Sm–Nd and Lu–Hf geochronology – eclogites

The Sm–Nd and Lu–Hf results obtained on the exactsame samples and dissolutions are compared inFig. 7a–s, and it is immediately apparent that virtu-ally none of the Sm–Nd ages (Fig. 7a–i) produceduseful isochron ages due to very low measured147Sm/144Nd ratios for garnet, which in turn pro-duced ages of very low precision. Errors in Sm–Ndages range from 14 to 320 Ma. In contrast to thegenerally unsuccessful Sm–Nd geochronologicalresults from the ZSF, most samples produced Lu–Hfisochrons that are geologically meaningful (Fig. 7j–s).

We note that although some samples produced ageswith relatively high uncertainties, sequential dissolu-tion methods were not used for Lu–Hf garnetanalyses for reasons detailed in the methods section.Lu–Hf isochron precision is proportional to the Lu/Hf ratios that were obtained on garnet, as expected.At Pfulwe, four samples produced Lu–Hf ages thathad uncertainties of

ages, a second age group of ‘40 Ma’ is defined. Abroadly similar age range (c. 45–42 Ma) wasobtained by Herwartz et al. (2008) for the upperValle di Gressonay.

Rb–Sr geochronology – metasedimentary rocks

Rb–Sr isochrons for the metasedimentary rocks arelargely controlled by phengite, the highest Rb/Sr min-eral in the samples (Table 4). The intercepts are con-trolled by carbonate and an epidote-group mineral.WR analyses provide an assessment of isochronintegrity and may identify open-system behaviour.Isochron ages that include the WR for Lago di Cig-nana samples have large errors. Phengite–clinozoisite(01NM-45) and phengite–piemontite (sample 08ES-03) ages at Lago di Cignana, however, have relativelysmall errors of 41.6 � 0.2 Ma (Fig. 8a) and39.8 � 0.2 Ma (Fig. 8b) respectively. Phengite–(clino)zoisite ages for the Saas Fee samples are39.1 � 0.3 Ma (05NM-212; Fig. 8c), 38.7 � 0.2 Ma(05NM-214; Fig. 8d), 39.4 � 0.2 Ma (05NM-215;Fig. 8e) and 40.6 � 0.2 Ma (05NM-219; Fig. 8f).The weighted average of these samples is39.7 � 0.2 Ma. All these ages are indistinguishablewith isochron ages constructed using all components,including whole rocks (Table 6). Although very smallage uncertainties are reported here (0.3 Ma or less,

based on analytical 87Rb/86Sr and 87Sr/86Sr errors),the uncertainty in the geological age interpretation iscertainly much larger; this will be important to con-sider in the discussion section. Collectively, all of theRb–Sr phengite ages are close to the 40 Ma agegroup, as defined in the previous section for eclogites.The initial 87Sr/86Sr ratios are all high, >0.710(Table 4), consistent with significant sourcing of themetasedimentary rocks within the ZSF from conti-nental basement, as reflected by the local graniticnappes (Mahlen et al., 2005).

DISCUSSION

Below, the discussion commences with some impor-tant issues related to Rb–Sr, Sm–Nd and Lu–Hf iso-chron geochronology, such as isotopic closure(Dodson, 1973) and equilibrium assemblages, and thevery high errors obtained for the Sm–Nd techniqueare addressed in the context of REE abundances ingarnet. This is followed by a short explanation onhow Lu–Hf and Rb–Sr ages need to be interpretedbased on REE zoning pattern and other petrologicalconsiderations. Individual Lu–Hf and Rb–Sr ages arethen placed in the context of previously publisheddata, as well as their structural position in the alpinestack. Lastly, the implications of the new data for thesubduction history of the ZSF unit are discussed.

Assessment of Rb–Sr, Sm–Nd and Lu–Hf geochronology

Geochronological investigations aimed at determiningthe P–T path of (U)HP terranes require rocks thatdid not exceed the closure temperatures of appropri-ate geochronometers for significant periods of time.In addition, meaningful geochronology requireslithologies that do not contain inherited components(e.g. Scherer et al., 2000), as well as samples thatexhibit minimal retrograde overprints. Retrogradegarnet resorption is especially problematic for Lu–Hfgeochronology (Kelly et al., 2011). The eclogites ofthe ZSF unit, however, satisfy all of the above crite-

Table 5. Sm–Nd and Lu–Hf ages (Ma) for Alpine eclogites.

Sample Location Sm–Nd Age Initial 143Nd/144Nd 2SE eNd Lu–Hf Age Initial176Hf/177Hf 2SE eHf

P-80b Pfulwe 20 � 21 0.513045 �24 7.9 49.6 � 2.5 0.283061 �5 10.8P-80c Pfulwe 43 � 43 0.513021 �47 7.5 51.7 � 2.1 0.283063 �4 11.0P-96 Pfulwe – – – – 46.1 � 2.2 0.283076 �5 11.3P-02 Pfulwe 97 � 320 0.51296 �36 6.2 46.7 � 2.1 0.283075 �4 11.2P-98 Pfulwe �10 � 51 0.513069 �58 8.4 54.0 � 6.4 0.283078 �9 11.6P-100 Pfulwe 44 � 16 0.513006 �18 7.2 40.3 � 5.9 0.283076 �5 11.296JA-32 Lago di Cignana 40.6 � 2.6a 0.513046 �11 8.0 48.8 � 2.1b 0.283101 �5 12.3CH-48 Chamois 58 � 17 0.512990 �22 6.9 52.6 � 1.7 0.283079 �3 11.5SF-25b Saas Fee 78 � 25 0.512989 �38 8.1 38.1 � 2.7 0.283095 �4 12.2SF-26 Saas Fee 20 � 14 0.513054 �19 6.9 40.7 � 1.8 0.283103 �6 11.8SJ-87 St. Jacques 65 � 37 0.512996 �52 6.9 39.2 � 1.6 0.283123 �4 12.9

Sm–Nd and Lu–Hf isochrons are calculated with relative uncertainties of 0.5% for 147Sm/144Nd and 176Lu/177Hf, and the analytical uncertainties (internal 2SE) for 143Nd/144Nd and176Hf/177Hf as given in Tables 2 and 3.aAmato et al. (1999).bLapen et al. (2003).

Table 6. Rb–Sr ages (Ma) for Alpine metasedimentary rocks.

Sample Location Rb–Sr agea Rb–Sr ageb Initial 87Sr/86Src 2SE

01NM-45 Lago di Cignana 41 � 19 41.6 � 0.2 0.711079 �1508ES-03 Lago di Cignana 39.6 � 8.4 39.8 � 0.2 0.710033 �1005NM-212 Saas Fee 40.3 � 3.8 39.1 � 0.3 0.711512 �1305NM-214 Saas Fee 38.5 � 1.2 38.7 � 0.2 0.710166 �905NM-215 Saas Fee 39.2 � 0.9 39.4 � 0.2 0.710002 �1305NM-219 Saas Fee 40.8 � 0.7 40.6 � 0.2 0.709470 �13

Rb–Sr isochrons are calculated with relative uncertainties of 0.5% for 87Rb/86Sr, and theanalytical uncertainties (internal 2SE) for 87Sr/86Sr as given in Table 4.aAge calculated using the WR and all mineral fractions.bAge calculated using phengite–(clino)zoisite only.cGiven are initials and 2SE that are calculated from phengite–(clino)zoisite isochrons;they overlap within error with calculated intercepts using multiple minerals.



ria for robust geochronology. For example, all sam-ples show only minor indications of retrogression. Inaddition, the peak temperatures of the ZSF unit didnot exceed ~600 °C, which is below the minimumblocking temperatures of both Sm–Nd and Lu–Hfgarnet geochronometers (e.g. Th€oni & Miller, 1996;Duchêne et al., 1997a; Ganguly et al., 1998; VanOrman et al., 2002; Tirone et al., 2005; Skora et al.,2008), and this is confirmed by the preservation ofprograde REE zoning in garnet (Fig. 6a–c). Hence,Lu–Hf and Sm–Nd ages should represent timeswithin the prograde-metamorphic cycle.

The precursors of the eclogites were young, oceanicmaterial that should contain minimal or no inheritedcomponents nor relict minerals that would complicatethe isotope systematics. The high eNd (+6 to +8) andeHf (+10 to +13) values (Table 5) indicate that theZSF eclogites were derived from depleted mantle andtherefore should contain no inherited components.This is important because eclogitic garnet is inclusionrich, which could potentially supply non-radiogenicHf through, for example, sub-microscopic zirconinclusions, or inherited Nd from epidote inclusions.Although zircon was not observed in the samples,LA-ICP-MS data suggest its presence at sub-micronscale. Such zircon most likely grew during progrademetamorphism when formerly Zr-rich host mineralsbroke down. This is corroborated by a publishedaverage U–Pb zircon age from the Lago di Cignanaunit, which is relatively young (c. 44 Ma, Rubattoet al., 1998), and only slightly older than the Sm–Ndage determined from the same area (c. 41 Ma; Amatoet al., 1999). Baxter & Scherer (2013) show thatalthough inclusions of the same age as the dated min-eral may decrease the isochron precision, they wouldnot alter the accuracy of the age. Hence, we concludethat the small amount of zircon dissolved alongsidegarnet cannot significantly alter the Lu–Hf agesexcept to decrease precision.

It is commonly assumed that failure of Sm–Ndgarnet geochronology in terms of spread in147Sm/144Nd ratios reflects the impact of LREE-richinclusions such as metamorphic epidote, which is arefractory sink for all LREE, and this has prompteddevelopment of leaching techniques (e.g. Amatoet al., 1999; Baxter et al., 2002; Anczkiewicz & Thirl-wall, 2003; Pollington & Baxter, 2011). The methodof Amato et al. (1999) was adopted here for selectedsamples (Pfulwe, Saas Fee), given its previous successin removing the majority of LREE-rich inclusionsfrom the Lago di Cignana area. It is immediatelyapparent from Fig. 7a–i, however, that this leachingprocedure was unsuccessful. The measured range of147Sm/144Nd ratios of leached garnet fractions of~0.21–0.39, with absolute Nd concentrations as highas 0.18–1.14 ppm (Table 2), is indicative of contami-nation by LREE-rich inclusions (e.g. Th€oni, 2002;Baxter & Scherer, 2013). A key question is: why werethe Sm–Nd geochronology attempts at localities

other than at Lago di Cignana so unsuccessful? Oneexplanation is that the leaching methods are rathersensitive to garnet sizes, the sizes of inclusions, differ-ent inclusion populations, and the duration of acidleaching and leaching temperatures, raising the possi-bility that every sample potentially behaves differ-ently (e.g. Pollington & Baxter, 2011). Success orfailure in leaching methods applied to Sm–Nd garnetgeochronology may also be dependent on absoluteSm–Nd concentrations in garnet. Garnet enriched inSm and Nd is more resistant to the influence ofLREE-rich inclusions before 147Sm/144Nd ratios aredecreased to levels too low to provide useful iso-chrons. In contrast, garnet that contains very low Smand Nd will be much more difficult to use for Sm–Nd geochronology, and leaching methods may be lesseffective. This is illustrated for the different examplesin Fig. S1.Measured REE profiles across garnet indicate that

Sm concentrations in Pfulwe (Fig. 6a), Chamois(Fig. 6b) and Saas Fee (Fig. 6c) are enriched towardsthe rim, similar to that found in Lago di Cignana(Skora et al., 2009). Absolute concentrations of Sm,however, vary significantly among samples fromspecific localities, as well as between localities, reflect-ing differences in garnet growth histories. Rim-Smconcentrations are highest at Lago di Cignana(2.5 ppm, Skora et al., 2009) and Chamois (2 ppm,Fig. 6b), lower in Saas Fee samples (0.8 ppm,Fig. 6c) and lowest in Pfulwe samples (

amphibole), accessory phases (e.g. apatite, titanite)and whole rocks. Complexities in Rb–Sr isochronsmay reflect differential diffusional resetting of miner-als, or disturbance of the isotopic system due tointeractions with late, alkali-rich fluids or re-crystal-lization (e.g. Giletti, 1991; Jenkin et al., 1995; K€uhnet al., 2000; de Jong, 2003; di Vincenzo et al., 2006;Glodny et al., 2008). In this study, all Saas Fee sam-ples have well-equilibrated phengite–WR–(clino)-zoisite–carbonate isochrons, and moderate to smalluncertainties. In contrast, phengite–WR–clinozoisiteand phengite–WR–piemontite ages for Lago di Cig-nana samples have larger errors (8–19 Ma). This sug-gests that individual minerals were affected byadditional processes such as recrystallization, fluid–mineral interaction or partial re-equilibration at theLago di Cignana (U)HP locality. Similar observa-tions were also reported by Mahlen et al. (2005) forother ZSF metasedimentary rocks, where there wasclear evidence for Rb–Sr mobility during Alpinemetamorphism. Previous studies have found that pro-grade Sr zoning can be preserved in epidote in UHPterranes that have recorded significantly higher peak-metamorphic temperatures (>700 °C; Nagasaki &Enami, 1998). This suggests that Sr in epidote dif-fuses very slowly at eclogite facies conditions, andthat epidote-group minerals will likely remain closedat all times in the ZSF unit. We therefore use (clino)-zoisite–phengite and piemontite–phengite pairs forthe most accurate Rb–Sr isochron ages in all samplesin this study.

The isotopic closure temperature for Rb–Sr inphengite is not well constrained. J€ager et al. (1967)empirically determined a closure temperature of~500 � 50 °C for phengite-bearing alpine metamor-phic rocks. This is close to the peak-metamorphictemperatures of the ZSF unit of ~550–660 °C (e.g.Oberh€ansli, 1982; Barnicoat & Fry, 1986; Reinecke,1998; Bucher et al., 2005; Angiboust et al., 2009;Groppo et al., 2009), suggesting that phengite Rb–Srisochrons either date peak-metamorphic conditions(eclogite facies) or the onset of cooling (upper green-schist facies). We note that the availability of fluidsfor recrystallization, as well as the presence orabsence of an Rb and Sr exchange partner in thematrix, will also play a role in isotopic resetting ofphengite at peak temperatures (e.g. Giletti, 1991; Jen-kin et al., 1995; K€uhn et al., 2000; Glodny et al.,2008). The Rb–Sr ages in this study are thereforeinterpreted to date a period that encompassed eclog-ite to upper greenschist facies P–T conditions,depending on the individual rock sample and its P–T-fluid-deformation history.

Garnet geochronology constraints on prograde-metamorphic history

Lapen et al. (2003) first proposed that the very highLu/Hf distribution coefficient ratio for garnet should

result in Lu enrichment in early grown cores. Thiswould bias Lu–Hf garnet ages towards the onset ofgarnet growth, in contrast to Sm–Nd ages that wouldrepresent a volumetric mean age in the absence of anysignificant growth zoning. Assuming a Rayleigh modeland batch nucleation, the estimated duration of pro-grade garnet growth was c. 12 Ma based on the con-trast in Lu–Hf age (48.8 � 2.1 Ma) and Sm–Nd age(40.6 � 2.6 Ma) measured on the same sample. Thisled to the recognition that measured Sm–Nd and Lu–Hf bulk garnet ages reflect the integrated effects oftheir growth histories (see also Kohn, 2009).It was subsequently shown that REE profiles of

garnet from the ZSF unit, and some other areasworldwide, do not follow those predicted by Rayleighfractionation, but rather reflect diffusion-limitedtransport of REEs, which may produce REE deple-tion halos around growing garnet (e.g. Skora et al.,2006; Moore et al., 2013). The study of Skora et al.(2009) showed that although Lu is enriched in earlygrown cores due to diffusion-limited transport, Lu–Hf ages do not date the onset of garnet growth sim-ply because core concentrations are of minor abun-dance when weighted over a spherical geometry. TheLu–Hf ages may still be slightly skewed towards theonset of growth, but only when garnet grows overprolonged periods of time. Thus, interpretation ofLu–Hf ages are a complex matter as they can datenear-peak metamorphism, if garnet grew over a shortinterval (10 Ma). If thisis not the case, then Lu–Hf and Sm–Nd ages may be similardespite enrichment of Lu in the early grown cores.



reflect an earlier, prograde-metamorphic age if garnetgrowth was protracted.

Diffusion-limited transport also explains Smenrichments in the late-grown rims (Fig. 6a–c),reflecting relaxation of depletion halos in the matrixand faster diffusion rates at peak- or near-peak con-ditions. With the recognition that the Sm/Nd ratiosare highest in the rims of garnet in the Lago di Cig-nana sample dated by Sm–Nd (Amato et al., 1999),garnet growth modelling in Skora et al. (2009) pro-vides strong support for an age of c. 40 Ma for peakUHP metamorphism at Lago di Cignana. Note thatthis estimate is strongly dependent upon the confi-dence of the Sm–Nd age, which has a reported preci-sion of 2.6 Ma, and this uncertainty assumes thatany surviving LREE-rich inclusions that remainedafter leaching did not influence the age accuracy (e.g.prograde epidote is of roughly similar age as garnet).The age uncertainty noted by Amato et al. (1999)permits an integrated Sm–Nd age as old as 43.2 Ma,which would place the end of garnet growth andpeak metamorphism at c. 42 Ma. In contrast, if thetrue Sm–Nd age at Lago di Cignana lay towards theyounger end of the uncertainty, peak metamorphismcould be younger than 40–38 Ma.

Garnet crystal-size distributions (CSD) will alsoexert an important control on bulk garnet ages(Skora et al., 2009). Differently sized garnet grainsare typically interpreted to have grown during a pro-grade-metamorphic event, nucleating at differenttimes (continuous nucleation and growth). Large gar-net reflects early nucleation and the entire garnetgrowth history, whereas the smallest ones record latenucleation and growth towards the end of the pro-grade path (e.g. Jones & Galwey, 1966; Kretz, 1966;Cashman & Ferry, 1988). These studies also showthat intermediate to small (late-grown) garnet tend todominate over large (old) garnet in the population.

Comparison of Lu–Hf and Sm–Nd isochron agesto their respective model ages, taking into accountREE zoning in garnet and CSDs, suggests that garnetin the Lago di Cignana area grew over a longer per-iod (30 � 10 Ma, Skora et al., 2009). Although suchlong prolonged garnet growth times may sound sur-prising, other studies have subsequently found com-parable results (e.g. Cheng et al., 2011). Note that nonew age modelling was performed for this study,given the similarities of measured REE zoning pat-tern in all ZSF garnet. The essence of the quantita-tive modelling of Skora et al. (2009) is thusreproduced in Fig. 9, and our interpretation of themetamorphic history of the ZSF is based on theserelations.

Constraints on peak metamorphism and initial uplift

The relations between model ages and garnet growthduration shown in Fig. 9 highlight the importance ofknowing the age of peak metamorphism and hence

cessation of garnet growth when interpreting Sm–Ndand Lu–Hf garnet isochron ages. Rb–Sr phengiteages provide important constraints on the transitionfrom eclogite to upper greenschist facies P–T condi-tions. The average of four samples from Saas Fee is39.5 � 0.1 Ma, whereas the average Rb–Sr age forthe two samples from Lago di Cignana is40.7 Ma � 0.1 Ma. Another published Rb–Sr ageusing phengite–WR from Lago di Cignana is slightlyyounger at 38.5 � 0.1 Ma, using the decay constantof Rotenburg et al. (2012; original published age ofAmato et al., 1999; 37.9 � 0.1 Ma). The Rb–Sr agesfor other ZSF samples span a large range of c. 48–37 Ma (e.g. Barnicoat et al., 1995; Amato et al.,1999; Reddy et al., 1999; Cartwright & Barnicoat,2002). This large age range is in part related to thefact that varying mineral assemblages were used forgeochronology, and different studies selected theirrocks for different purposes, including samples thatare heavily overprinted. If, however, comparisons arerestricted to Rb–Sr ages of epidote–phengite pairs,the results converge to the same range observed inthis study. Recalculating the epidote–phengite Rb–Srisochron ages of Barnicoat et al. (1995) for theT€aschalp area, using Rotenburg et al. (2012), pro-duces a range of 43.0–40.1 Ma with a weighted aver-age of 41.4 � 0.5 (published ages using old decayconstant: 42.3–39.5 Ma). This area is spatially closeto Pfulwe (~1–2 km NNE of Pfulwe). Anotheryounger Rb–Sr age for T€aschalp is reported inAmato et al. (1999). They proposed an age of38.6 � 0.1 Ma (original ages using old decay con-stant: 38.0 � 0.1 Ma) based on a phengite–WR pair.Last, Reddy et al. (1999) published an epidote–phen-gite age of 41.2 � 0.6 Ma (original age using olddecay constant: 40.5 � 0.6 Ma) for the Val d’Ayas.The sample location is ~5 km north of St. Jacques.Hence, all previously published Rb–Sr ages are prin-cipally consistent with the results obtained in thisstudy.Although Rb–Sr ages can be difficult to relate with

the specific P–T conditions, a pattern emerges for thedata presented here. The age distributions at Lago diCignana (c. 41–38.5 Ma) and T€aschalp (c. 41–38.6 Ma) are consistent with the interpretation thatRb–Sr may effectively record a time period thatencompasses peak eclogite to post-peak upper green-schist facies P–T conditions. The oldest Rb–Sr ages(c. 43–40 Ma) fully overlap with what is interpreted aspeak metamorphism in these structurally highest units(e.g. with the Sm–Nd age of Lago di Cignana).Younger ages in the order of c. 38.5 Ma are just out-side the error bracket for peak ages, and are thereforeinterpreted to reflect a fluid event during exhumationthrough upper greenschist facies. Average Rb–Sr agesin Saas Fee samples closely overlap with average Lu–Hf ages at c. 39.4 Ma. This would suggest that SaasFee samples are slightly younger in terms of peakmetamorphism (c. 39 Ma) than samples from Pfulwe,



T€aschalp and Lago di Cignana where Rb–Sr (andSm–Nd) ages are closer to c. 41 Ma.

Collectively, our results suggest that peak meta-morphism occurred at c. 42–40 Ma, and uplift andgreenschist facies metamorphism occurred between c.39 and 38 Ma in the structurally highest units (e.g.Pfulwe, T€aschalp, Lago di Cignana). Peak metamor-phism might have been reached c. 1–2 Ma later inthe structurally lowest unit represented by Saas Fee

rocks (c. 40–39 Ma). The greenschist facies overprintis not constrained in these samples. Our resultspotentially indicate very rapid initial uplift fromeclogite to greenschist facies conditions within analmost irresolvable time interval, perhaps in the orderof only c. 1 Ma, although possibly as much as 4 Ma.

Evidence for diachronous prograde metamorphism in theZSF unit

The broad correlation between the two age groupsdefined by the Lu–Hf garnet geochronology at c.50 Ma (c. 52–46 Ma: Lago di Cignana, Pfulwe andChamois) and 40 Ma (c. 41–38 Ma: Saas Fee andSt. Jacques) suggest distinctly different progradegarnet-growth histories. The age differences could berelated to diachronous subduction of the ZSF zoneor they could be related to petrological differences.For example, garnet in different samples could havestarted nucleating at very different times yet follow-ing broadly similar P–T–t paths. The latter hypothe-sis might be tested through garnet inclusionassemblages and bulk chemistry effects on garnet-inphase relations. For example, the presence of titaniteinstead of rutile is often associated with lower P–Tconditions (blueschist instead of eclogite facies;e.g. John et al., 2011). Hence if garnet started togrow significantly earlier in one rock compared toanother, one could expect titanite inclusions to bemore abundant in garnet cores. Titanite-rich garnetcores are common in Lago di Cignana, Chamois andSaas Fee samples. Garnet cores that have very lowtitanite abundances are exclusively characteristic ofPfulwe samples. Inclusion populations therefore donot correlate with the distinct Lu–Hf age groups,suggesting that there are no systematic differences ingarnet growth timing relative to P–T paths. In termsof bulk-rock chemistry, it is well known that Mncontents, as well as Mg/(Fe + Mg) ratios, exert astrong influence on the garnet-in phase relationsduring prograde metamorphism (e.g. Spear, 1993). Itis shown in Fig. 10a,b that the distinct Lu–Hf agegroups do not correlate with either of these parame-ters. Although there are some differences in bulk-rock compositions in terms of CaO and Na2Oconcentrations, likely reflecting various degrees ofspilitization of the protolith basalts (Table 1), there isno clear correlation with Lu–Hf ages (Fig. 10c). Wetherefore conclude that there is no evidence frominclusion populations or bulk chemical compositionsfor distinctly different prograde garnet growth histo-ries relative to P–T conditions.Previous modelling of Lu–Hf garnet ages has

demonstrated that the core-rim Lu distribution pat-terns can have a strong influence on the Lu–Hf agemeasured for bulk garnet (Lapen et al., 2003; Kohn,2009; Skora et al., 2009). All Lu zoning profiles dis-play narrow central peaks that are consistent withdiffusion-limited Lu uptake (Fig. 6a–c). The minor

MnO

[wt%

]

0.11

0.13

0.15

0.17

0.19

MgO

(MgO

+FeO

)

0.25

0.30

0.35

0.40

Age [Ma]35 5040 45

CaO

/Na 2

O

1.0

2.0

3.0

4.0

(a)

(b)

(c)

Fig. 10. Whole rock v. age relations, showing that there is nosimple correlation between (a) Mn, (b) Mg/(Mg + Fe), whichare parameters that are known to influence when the garnet-inreaction is crossed during prograde metamorphism. (c) Thereis also no obvious correlation with Ca/Na, which characterizesthe different degrees of ocean–floor spilitization. These resultsshow that the different garnet growth intervals do not reflectsystematic changes in bulk chemical compositions.



differences between samples, such as the extent of Lushoulders towards the rim, cannot account for majorLu–Hf age differences. Moreover, all samples appearto contain a garnet population that is consistent witha single continuous nucleation and growth event dur-ing prograde metamorphism, as opposed to, forexample, bimodal garnet size distributions that arecommonly found in the Central Alps (e.g. Herwartzet al., 2011). The only difference is that Pfulwe sam-ples have, on average, significantly fewer but largergarnet crystals compared to all other samples, butthis distinction does not correlate with the differentLu–Hf age groups. We therefore rule out distinct dif-ferences in Lu profiles or garnet populations as anexplanation for the distinct Lu–Hf age groups.

Below we summarize the age groups and their link-ages to structural positions in the nappe stack. All theage data discussed below are summarized in Fig. 11.

Pfulwe, Lago di Cignana and Chamois samples

Lu–Hf ages from four samples from a single outcropjust below the Pfulwe pass are relatively well

constrained (uncertainties: 2.1–2.5 Ma), and rangefrom 51.7 to 46.1 Ma. This is very similar to the Lu–Hf age obtained by Lapen et al. (2003) for Lago diCignana (48.8 � 2.1 Ma). The Lu–Hf age of Cha-mois sample CH-48 is also well constrained to be52.6 � 1.7 Ma, again falling into the 50 Ma age cate-gory. Structurally, all these locations reflect the topof the exhumed ZSF sequence, beneath the Tsat�enappe, which is thought to represent the accretionaryprism below which the ZSF unit was subducted.Peak-metamorphic constraints are provided by Sm–Nd and Rb–Sr ages for Lago di Cignana (also closeto Chamois) and T€aschalp (close to Pfulwe). They allroughly fall within an age bracket of c. 42–40 Ma.The similarities in ages, zoning pattern and CSD sug-gest that these units have shared the same tectonichistory. It is important to stress that as in the Lu–Hfage at Lago di Cignana, the disparate Lu–Hf andpeak-metamorphic ages require a prolonged garnetgrowth interval of 30 � 10 Ma (Fig. 9). This hasbeen further discussed in Skora et al. (2009).

Saas Fee samples

Two samples have yielded considerably younger Lu–Hf ages (c. 41–38 Ma), overlapping with Rb–Sr agesobtained from four metasedimentary samples fromthe same area (average: 39.5 � 0.1 Ma). The samplescome from an area that lies at the structurally lowestposition in the obducted ZSF unit, directly above theMonte Rosa nappe, which reflects the distal part ofthe European continent. The age modelling (Fig. 9)suggests that this is best explained by a very shortgarnet growth period (

place them in the 40 Ma age group. Trace-elementzoning data and a CSD are not available, but petro-logical descriptions of the samples are close to thatobserved in other locations. If a peak-metamorphicage of 40 Ma is assumed for this area, similar to allother areas, then it is clear that garnet must havegrown for a relatively short period of time. This isstrikingly similar to that found for the Saas Fee sam-ples. Because the upper Valle di Gressonay area alsojust overlies the Monte Rosa nappe, this places thesesamples into a similarly low structural position as theSaas Fee samples.

Implications for the ZSF and related units

The discussion here, which builds on the modellingof Skora et al. (2009), that Lu–Hf ages date differentpoints in time, either near-peak metamorphism or atime that reflects integrated prograde growth, pro-vides an alternative interpretive context for Lu–Hfgeochronology studies of other units in the region.Herwartz et al. (2008) studied the ‘Balma unit’ in theupper Valle di Gressonay, and they could not recon-cile their c. 42 Ma Lu–Hf age with a peak-metamor-phic age of c. 40 Ma of the ZSF unit, which ledthem to suggest that the ‘Balma unit’ originated inthe Valais trough. Our new results document youngerLu–Hf ages (41–38 Ma: Saas Fee) in samples fromthe lower ZSF, a position that is structurally similarto rocks studied by Herwartz et al. (2008) in thatboth just overly the Monte Rosa unit. These relationssuggest that Lu–Hf age information must be inter-preted within the context of the structural and meta-morphic histories of the units as determined bymultiple geochronometers and P–T constraints. Wetherefore suggest that it is not appropriate to inferthat the upper Valle di Gressonay must have origi-nated in the Valais based on the Lu–Hf ages alone.In fact, the originally proposed paleoposition of theZSF unit (Bearth, 1967) is very well reconciled withour new interpretation that the lower ZSF units hadprograde-metamorphic histories that were distinctfrom those of the upper ZSF.

The results presented here suggest that the dis-parate ages for peak metamorphism reflect the pro-tracted nature of prograde metamorphism, as well asthe diachronous subduction that occurred across theorogen. The relatively young age of the Saas Fee(and upper Valle di Gressonay) samples in terms ofinitiation of garnet growth suggests that the lowersection of the ZSF unit may have been subducted lastwhen compared with the Pfulwe, Lago di Cignanaand Chamois areas, all of which require a prolongedprograde garnet growth history. This suggests twodifferent origins: the Pfulwe, Lago di Cignana andChamois areas must have subducted early, placingthem close to the African continental margin. In con-trast, the Saas Fee and the ‘Balma unit’ must haveoriginated much closer to the European continental

margin, near the Monte Rosa unit, and was sub-ducted significantly later (Fig. 12b). This is differentfrom current estimates for the paleopositions in theWestern Alps, where the entire ZSF unit is placed inthe northern realm (Fig. 12a, e.g. Labhart, 1992). Wenote that our interpretation requires a change in thepaleogeographic origin of the Tsat�e nappe, whichmust be placed towards the northern realm of theLiguro-Piemont Ocean, close to the European conti-nental margin (Fig. 12b). This is required to explainthe relatively young Rb–Sr ages (range: c. 44–37 Ma;without distinction in the different minerals used forisochrons, Reddy et al., 1999) in the light of thegreenschist to lower blueschist facies rocks of theTsat�e nappe that are unlikely to have exceeded theirblocking temperature of ~500 � 50 °C.Reconstruction of the relative movements of the

Apulian/African plate and the European plate sug-gests an initially strong sinistral movement, followedby a rotational movement and subsequently a moreperpendicular subduction (Fig. 13a,b, see also e.g.Dewey et al., 1989; Rosenbaum et al., 2002; Rosen-baum & Lister, 2005). Average subduction rates of0.3–1.0 cm yr�1 were estimated to explain the pro-grade history of the Lago di Cignana area (Skoraet al., 2009), which lies within the estimated range of0.4–1.5 cm yr�1 that is based on independent paleo-magnetic studies (e.g. Dewey et al., 1989; Schmidtet al., 1997). The initially oblique subduction shouldresult in slow subduction rates, reflecting a small per-pendicular subduction vector, and slow garnetgrowth. This is exactly what is proposed for the Lagodi Cignana, Pfulwe and Chamois areas, which weresubducted early based on the geochronological resultsobtained here. The protracted garnet growth periods,in turn, explain the disparate Lu–Hf, Sm–Nd andRb–Sr isochron ages. In contrast, oceanic crust thatwas originally located close to the European margin

Traditional position of the remnants of the Piemont-Ligurian ocean

NW SE

SesiaBriannconnais(e.g. Monte Rosa)

Zermatt-Saas Feehigh P remnants

Tsaté nappe low P remnants

Reinterpretation of the remnants of the Piemont-Ligurian ocean

NW SE

SesiaBriannconnais(e.g. Monte Rosa)

Zermatt-Saas Feehigh P remnants

Tsaté nappe low P remnants

(b)

(a)

Fig. 12. (a) Schematic illustration of the traditionalpaleoposition of the Zermatt-Saas Fee zone (white circles) andTsat�e nappe (white crosses). (b) Schematic illustration of there-interpretation that is based on geochronology in this study.



would have subducted last. Plate motion vectorstowards the end of subduction were almost perpen-dicular, which would have led to fast convergencerates and hence shorter garnet growth intervals forthe Saas Fee and the upper Valle di Gressonay areas.Finally, our model may be accommodated in the agerange and thermal structure inferred for the Liguro-Piemont basin, which initially formed c. 165–160 Ma(Rubatto et al., 1998; Schaltegger et al., 2002) andhad started to close at c. 110–90 Ma (e.g. Deweyet al., 1989). We speculate that the very young age ofthe oceanic crust that was subducted, combined withits small size, reflects alpine subduction that was dri-ven by compression from the different plate motions,rather than slab pull.

The ZSF peak age estimates are reasonably similarto a U–Pb rutile age, obtained from an eclogite faciestension crack in the Monte Rosa unit(42.6 � 0.6 Ma, Lapen et al., 2007). This suggeststhat the Monte Rosa and the ZSF zone were partlycoupled during prograde metamorphism and uplift(Lapen et al., 2007). In addition to the overlap inpeak-metamorphic ages of the Monte Rosa unit withthose determined for the ZSF unit, there is an addi-tional overlap of the greenschist to amphibolite faciesoverprint in both units, which has been estimated tohave occurred between c. 40 and 32 Ma. It is possiblethat both units closely shared paleogeographic posi-tions during basin closure, and subsequent exhuma-tion. Coupling of the granitic nappes of GranParadiso and Monte Rosa to the Western Alps ophi-olites during eclogite facies metamorphism could pro-vide an important buoyancy force, in addition to theserpentinite units, for rapid exhumation of the high-density eclogites.

CONCLUSIONS

New Lu–Hf geochronology on garnet, in concertwith Rb–Sr and Sm–Nd geochronology, indicates

that peak-metamorphic conditions from the WesternAlps define distinct age groups of 50 and 40 Ma. It isnot possible to correlate the disparate ages withpetrological differences such as REE zoning thatmight suggest different paragenesis. Instead, it isshown that the age record is a consequence of differ-ent origins within the Liguro-Piemont realm, com-bined with diachronous subduction and significantlydifferent prograde garnet growth intervals. The ZSFunit is suggested to reflect slivers of oceanic crust thatstarted subducting at different moments in time. Theindividual slivers nevertheless appear to have had abroadly common peak-metamorphic age of c. 42–39 Ma. Areas that have the oldest Lu–Hf ages(50 Ma age group: Pfulwe, Lago di Cignana andChamois) represent the structurally highest subunitsin the exhumed ZSF unit. They are adjacent to theoverlaying Cime Blanche and Tsat�e units, which inturn are overlain by remnants of the African conti-nental margin. Because their inferred prograde garnetgrowth interval is >20 Ma, close to that permitted byplate tectonic reconstructions, these areas must haveoriginated at the southernmost realm of the Liguro-Piemont Ocean, close to the African continental mar-gin. The early onset of garnet growth inferred for thisgroup indicates that these units were subducted first.The upper Valle di Gressonay and the Saas Fee unitthat have younger Lu–Hf ages (40 Ma age group)are structurally in similar positions in that they bothdirectly overly the Monte Rosa unit (Briannconais).Hence their paleoposition was likely the northern-most realm of the Liguro-Piemont Ocean, where itwas subducted as the last piece of the PiemontOcean. The distinctly different age groups andinferred prograde garnet growth intervals are wellexplained by rotation of plate motion of the Liguro-Piemont Ocean with time. An initially oblique sub-duction resulted in slow burial rates and long garnetgrowth durations. Subduction then gradually turnedtowards a near-perpendicular orientation, resulting in

Plate motion trajectories

42 Ma

81 MaSmith, 1971

80 Ma

63 Ma

Dewey et al., 1973

95 Ma

70 Ma

Livermore & Smith, 1985 Rosenbaum et al., 2002

60 Ma

110 Ma

Dewey et al., 1989

92 Ma

83 Ma

46 Ma

Europe

Africa/ Apulia

Alp-

Teth

ys

Vardar

ValaisBrianconnais

Paleogeographic reconstruction beforeinitiation of subduction

(a) (b)

Fig. 13. (a) Paleogeographic reconstruction of the Alpine area in the Early Cretaceous, before the initiation of subduction (afterStampfli et al., 2002). (b) Reconstruction of the relative movements of the Apulian/African against the European plate suggests aninitially strong sinistral movement, which should result in initially slow subduction rates (Smith, 1971; Dewey et al., 1973;Livermore & Smith, 1985; Dewey et al., 1989; Rosenbaum et al., 2002). This is followed by a rotational movement andsubsequently a more perpendicular subduction. This study does not differentiate between the different proposed models for platemotion, but finds that the geochronological results are fully consistent with rapid convergence rates near the end of subductionbased on ages from garnet and phengite in structurally lowest units (e.g. Saas Fee).



much faster burial, as well as rapid garnet growthrates in the subducted slab.

ACKNOWLEDGEMENTS

Financial support provided by the Swiss NationalScience Foundation grant SNF2100–066996(L. Baumgartner) and by the Soci�et�e Acad�emiqueVaudoise (S. Skora) is gratefully acknowledged. Sup-port was also provided by the U.S. National ScienceFoundation grant EAR-0309853 to C. Johnson andB. Beard. S. Skora acknowledges a SNF Ambizonegrant (PZ00P2_142575). The constructive reviews ofE. Baxter and two anonymous reviewers, as well asthe editorial handling of D. Robinson, were veryhelpful and are greatly acknowledged.

REFERENCES

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Anczkiewicz, R., Szczepanski, J., Mazur, S. et al., 2007. Lu-Hfgeochronology and trace element distribution in garnet:implications for uplift and exhumation of ultra-high pressuregranulites in the Sudetes, SW Poland. Lithos, 95, 363–380.

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