LETTERSPUBLISHED ONLINE: 23 JANUARY 2011 | DOI: 10.1038/NGEO1060

Tracing two orogenic cycles in one eclogite sampleby Lu–Hf garnet chronometryDaniel Herwartz1*, Thorsten J. Nagel1, Carsten Münker2, Erik E. Scherer3 and Nikolaus Froitzheim1

Subduction of rocks into the mantle results in high-pressuremetamorphism and the formation of eclogites from basalticprecursor rocks. At the Earth’s surface, eclogites often occuras isolated fragments embedded in crustal rocks that lackevidence for high-pressure conditions1–3. The high-pressurerocks are therefore often viewed as dismembered fragmentsthat have been assembled and intercalatedwith rocks devoid ofany high-pressure history at shallow crustal levels4–8, forminga tectonic mélange. Such mélange models were supportedby age discrepancies among high-pressure rocks from theAdula nappe (Central Alps)9–12, which was thought to representa classic example of such a situation4,5. Here we presentLu–Hf age data from two populations of the high-pressuremineral garnet, found within a single eclogite sample takenfrom Trescolmen, in the Central Adula nappe. We report aminimum Variscan age of 332.7Myr and a maximum Alpineage of 38Myr for the two populations. We suggest thatthe Trescolmen eclogite was subducted to mantle depth andsubsequently exhumed, becoming part of a continental crustduring the Variscan orogeny. Later, during the Alpine orogeny,the Adula nappe must have been subducted to—and exhumedfrom—mantle depth a second time, as one coherent unit. Weconclude that the Adula nappe is not a mélange, and therefore,the crustal rocks that envelope the eclogites have also beensubjected to high-pressure conditions through deep subductionduring the Alpine event13,14.

Subduction to mantle depths of >40 km and exhumation backto the surface are documented by the occurrence of eclogite-faciesrocks formed at high-pressure in Alpine-type orogens. Such rocksoccur either in units derived from subducted oceanic crust or,alternatively, in tectonic nappes dominantly made of felsic crustfrom a subducted continental margin. Rocks that have reachedultrahigh-pressure conditions (that is, the stability field of coesite;>70 km) typically derive from the second type of high-pressureunits and are associated with abundant continental basement1–3.The high-pressure mineral assemblages are usually preservedwithin isolated blocks of eclogite or garnet peridotite, whereas theenveloping crustal rocks typically lack evidence of high-pressureconditions1–3. Thus, a paramount question in reconstructing thegeologic history of high-pressure domains is whether these nappesrepresent (1) units having different metamorphic histories thatwere mechanically juxtaposed within a subduction channel4–8,11or (2) coherent terranes of pre-existing continental basement thatunderwent high-pressure metamorphism and exhumation as awhole2,3,14–16 (Fig. 1).

The Adula nappe in the Lepontine Alps (Switzerland) has servedas a natural laboratory for many studies addressing subduction-zone processes4,5,11,13,14,17–20. It comprises crustal gneisses, meta-sediments and mica schists, as well as mafic and ultramafic

1Steinmann-Institut, Universität Bonn, Bonn 53115, Germany, 2Institut für Geologie und Mineralogie, Universität zu Köln, Köln 50674, Germany, 3Institutfür Mineralogie, Universität Münster, Münster 48149, Germany. *e-mail: d.herwartz@uni-bonn.de.

units including eclogites, amphibolites and peridotites13,14,17,19,20. Sofar, peak-pressure conditions have been recognized in eclogites,associated garnet mica schists, and scarce garnet peridotites13,14,19,20.These rocks show a gradient of southward-increasing pressure andtemperature (Fig. 2), interpreted to reflect burial in a southward-dipping subduction zone during the Tertiary Alpine orogeny19,20.After the high-pressure event, the entire unit was subjected toamphibolite-facies (low-pressure, high-temperature) conditions,with the intensity of the overprint also increasing to the south.However, previous geochronology yielded inconsistent results, sothe timing of high-pressure metamorphism in the Adula napperemains ambiguous. Eclogites and garnet peridotites from thesouthwestern part of the nappe yield ages between 42 and 35Myrusing Lu–Hf and Sm–Nd dating of garnet9,11 and U–Pb dating ofzircon10. In marked contrast, zircon grains from eclogites in thecentral and northeastern part of the nappe yield only Palaeozoicages for the high-pressure stage12 (Fig. 2). These observationsspurred models suggesting that the Adula nappe was not alwaysa single coherent unit but rather comprises different tectonicslices that were metamorphosed at different times before finalassembly5,11 (Fig. 1a).

We have investigated an eclogite from the classic Trescolmenlocality in the central Adula nappe (Fig. 2). Pressures andtemperatures associated with the main eclogite-facies assemblagesare about 2.1–2.2GPa and about 680 ◦C (ref. 20; Fig. 2). Thesample investigated is composed of garnet, omphacite, minoramounts of quartz, euhedral amphibole, and rutile (Fig. 3).There is no sign of amphibolite-facies retrogression and thesample is feldspar free. Omphacite crystals are equigranular andchemically homogeneous, whereas garnet porphyroblasts exhibitstrong chemical zonation and a bimodal grain-size distribution.A few large garnet grains up to 4mm in diameter comprisedark cores and pale rims. A second population of smaller (0.1–0.3mm), euhedral garnet porphyroblasts is distributed throughoutthe sample. High-resolution X-ray maps confirm the presenceof two generations (Fig. 3; Supplementary Fig. S1). Cores ofthe large porphyroblasts (grt1) are corroded, have oscillatorygrossular zoning, and irregular, almandine+ spessartine-rich andpyrope-poor patches. These relict cores are cross-cut by jaggedfissures filled with grossular-rich garnet (grt2a), which alsomantles grt1 cores. The outermost rim (grt2b) is again poorerin grossular component. The small, euhedral garnet populationconsists solely of grt2a (in cores) and grt2b (at rims). Bothgarnet generations contain rare inclusions of quartz, rutile andapatite. In addition, grt1 contains inclusions of Al-rich amphiboleand grt2 contains omphacite. We interpret the grt1 populationto have been partially resorbed at high temperature, therebyproducing the fissures. We also attribute the patchy distribution ofalmandine, pyrope and spessartine components to this event, with

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Unit comprising a large, coherent ultra (high) pressure body

0(k

m)

20

40

60

80

100

0

(km

)

20

40

60

80

100

Unit containing separate ultra (high) pressure a rocksbefore they are welded togetherat shallow crustal levels

a b

Figure 1 | Two contrasting models for the exhumation mechanism of geological units comprising (ultra) high-pressure rocks. a, Small parts of thesubducting slab are plucked off at various depths and then exhumed within a ‘subduction channel’ and welded into a megascale mélange at shallow crustallevels. b, Nappes are subducted and exhumed as coherent bodies. Hence, in model a only small parts of the nappe have reached (ultra) high-pressureconditions, whereas in model b the entire nappe is subjected to high-pressure conditions.

Bergell

Leventina

Simano

Bündnerschiefer

Tambo

Suretta

Southern Alps

Adula

Chiavenna

Du

CaGo

Ar

GaGa: Gagnone740 °C/3 GPaAbout 40 Myr (Sm¬Nd)

Ar: Alpe Arami840 °C/3.2 GPa1120 °C/5 GPa1180 °C/5.9 GPaAbout 35 Myr (U¬Pb)About 39 Myr (Sm¬Nd)About 37 Myr (Lu¬Hf)

Ca: Caurit720 °C/ 2.4 GPa

Du: Duria830 °C/3 GPa830 °C/2.8 GPaAbout 35 Myr (U¬Pb)

Tr: Trescolmen660 °C/2.2 GPa700 °C/2.1 GPaAbout 370 Myr (U¬Pb)

Cf: Confin570 °C/2.2 GPa700 °C/1.9 GPaAbout 340 Myr (U¬Pb)

Va: Vals520 °C/ 1.3 GPa580 °C/ 1.7 GPaAbout 330 Myr (U¬Pb)

VL: Val LargeAbout 370 Myr (U¬Pb)

10 kmPeriadriatic (Insubric) line

Go: Gorduno750 °C/2.3 GPaAbout 38 Myr(Lu¬Hf)

Cv

VL

Tr

Cf

Va

European marginApulian margin

South Penninic oceanTertiary intrusions

Zürich

Milano

10 km IL

Bergell

Trescolmen

Simano

A A

A

Cv: CalvarescAbout 370 Myr (U¬Pb)

N

′′A′

A′

A′′

a

46° 20′

46° 10′46° 10′

46° 20′

46° 30′

9° 30′9°

9°

100 km

Latit

ude

(° N

)

Longitude (° E)

Figure 2 | Location of the Adula nappe and the Alp Trescolmen. a, Tectonic sketch map of the Adula nappe. Stars denote prominent eclogite or garnetperidotite localities. Rectangles illustrate a compilation of peak pressure–temperature conditions20, as well as known high-pressure ages9–12. AmbiguousAr–Ar (ref. 22), K–Ar and Rb–Sr (ref. 23) data are not shown (Va, Vals; Cf, Confin; VL, Val Large; Tr, Trescolmen; Cv, Calvaresc; Ga, Gagnone; Ar, AlpeArami; Go, Gorduno; Ca, Caurit; Du, Duria). Insets show an overview of the Alpine realm (top) and a tectonic cross-section through the Adula nappe20

(bottom; IL, Insubric line).

the almandine+spessartine-rich areas representing the original grt1composition. Other grt1 domains have apparently been subjectedto diffusive re-equilibration. This view is corroborated by theobservation that the spessartine content within the patches stillreflects original growth zoning; that is, it consistently increasestowards the grain centres as defined by the concentric, oscillatory

grossular zoning. Garnet generations grt2a and grt2b clearly grewafter the resorption event, with grt2a healing the fissures andforming new nuclei within the rock matrix. The presence of sharpchemical gradients between grt1 domains and the surroundinggrt2 indicates that the aforementioned diffusive re-equilibrationmust have occurred before grt2 growth. Enclosed relics of grt1 in

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grt1

grt1 relics

grt2a

grt2b

am

ompgrt

qtz

AlmandinePyrope

GrossularSpessartine

Oscillatory zoning

B

B

A

B

A A

B

B

A

1 mm

A

0.1

0.3

0.5

0.7

0.1

0.3

0.5

0.7

a b

c d

e f

h

j

B

g

i

A A

B

BB

A A

A′ B ′

Fe

Ca Mn

Mg

X in

gar

net

X in

gar

net

Figure 3 | Illustration of the different garnet generations in Trescolmen sample TRC1. a–d, Element maps of Fe (a), Mg (b), Ca (c) and Mn (d). In b, bluishcolours are garnet (grt), green is omphacite (omp), yellow is amphibole (am) and black denotes quartz (qtz). e,f, Compositional cross-section throughlarge (e) and small (f) garnet crystals. Location of cross-sections is indicated in a–d. g–j, Illustration of garnet generations. grt1 represents corroded garnetrelics from a first (Variscan) orogenic cycle (g,h) that have been partially replaced and overgrown by two successive (Alpine) generations, grt2a (i) andgrt2b (j).

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Table 1 | Lu–Hf data obtained from the Trescolmen (Adula nappe) eclogite sample TRC1.

Sample ID Fraction Lu (ppm) Hf (ppm) 176Lu/177Hf 176Hf/177Hf

DH55 Whole rock 1(selective tabletop digestion)

0.662 0.221 0.4240 (13) 0.283987 (21)

DH78 Whole rock 2(PARR bomb digestion)

1.27 6.04 0.02990 (9) 0.283074 (9)

DH61 Omphacite 0.0205 0.249 0.01167 (11) 0.283171 (14)DH53 Variscan garnet 2.47 0.0496 7.114 (23) 0.32333 (14)DH62 Variscan garnet 2.68 0.0752 5.092 (18) 0.312218 (30)DH63 Variscan garnet 3.29 0.112 4.187 (16) 0.30722 (11)DH134 Variscan garnet 4.38 0.135 4.638 (4) 0.311691 (7)DH156 Variscan garnet 2.09 0.0392 7.647 (24) 0.32567 (16)DH157 Variscan garnet 1.91 0.0746 3.637 (11) 0.301684 (30)DH158 Variscan garnet

(purest sample)4.37 0.0889 7.037 (25) 0.327306 (36)

DH159 Variscan garnet 4.27 0.0813 7.508 (23) 0.327553 (30)DH160 Variscan garnet 3.72 0.0759 7.013 (23) 0.326723 (46)DH161 Alpine garnets≤150 µm

(purest sample)0.945 0.0259 5.174 (18) 0.286748 (44)

DH64 Alpine garnet≤150 µm 0.960 0.0259 5.272 (17) 0.287067 (54)DH133 Alpine garnet≤150 µm 1.02 0.0266 5.442 (3) 0.287197 (26)DH54 Alpine garnet≥250 µm 0.965 0.0294 4.665 (14) 0.287089 (15)DH65 Alpine garnet≥250 µm 0.951 0.0274 4.939 (16) 0.287077 (56)DH66 Alpine garnet≥250 µm

(impure garnet separate)0.974 0.0558 2.478 (8) 0.285292 (63)

DH55-R Residue of whole rock 1 – – 0.0001875 (7) 0.283022 (9)DH53-R Residue of Variscan garnet – – 0.001832 (6) 0.283038 (14)DH156-R Residue of Variscan garnet – – 0.0009642 (71) 0.282959 (22)DH157-R Residue of Variscan garnet – – 0.001369 (8) 0.283067 (25)DH159-R Residue of Variscan garnet – – 0.002141 (9) 0.283063 (29)

Reported uncertainties on the last decimal places (in parentheses) are the estimated 2σ external reproducibility for 176Lu/177Hf and 2σm internal run statistics for 176Hf/177Hf. For the isochron regressions,the empirical relationship between external and internal uncertainties was employed, where the 2σ external uncertainties of 176Hf/177Hf are about twice the 2σm internal analysis statistics27 . Externalreproducibility for 176Lu/177Hf of ±0.25% (2σ ) includes average error magnification for over- or under-spiked samples. ‘Residues’ are the undigested fraction left after the selective dissolution of wholerock and garnet by the tabletop procedure.

grt2-filled fissures are as small as 10 µm and still preserve theirdistinct chemical composition.

We separated several garnet fractions by hand picking darkred fragments of grt1 and light red, euhedral crystals of grt2in two different grain sizes. Two different digestion methodswere applied, one of which, in steel-jacketed Teflon bombs(PARR bombs), ensures full sample digestion21, whereas a tabletopdigestion method efficiently dissolves garnet while leaving behindrefractory Hf-bearing phases such as zircon and rutile21. Theundigested refractory minerals (residues) from the tabletopdigestion were rinsed several times and then digested in PARRbombs. The Lu–Hf data from the different garnet separates definetwo well-constrained arrays in 176Hf/177Hf versus 176Lu/177Hfspace, documenting two distinct metamorphic events (Table 1;Fig. 4). Eight out of nine grt1 fractions yield Carboniferousages (336–299Myr), whereas <150 µm grt2 fractions consistentlydefine Eocene ages (40–37Myr). We interpret the Eocene age todate prograde growth of grt2 as Alpine peak-pressure conditionswere approached, whereas the Carboniferous age may representgrowth or re-equilibration of grt1. As our physical separation ofthe two main garnet generations was not perfect, the Variscanand Alpine isochrons strictly represent minimum and maximumages, respectively. There is clear evidence for contamination of>250 µm grt2 fractions by grt1 inclusions, as the former clearlyplot above the isochron defined by the smaller grain size fractionsof grt2 (<150 µm). Although not visible in thin section, there isprobably a minor grt1 component in the <150 µm grt2 fractionsas well, as evident from a slight scatter in 176Hf/177Hf that is

not correlated with 176Lu/177Hf. Assuming that the grt2 fractionhaving the lowest 176Hf/177Hf at a given 176Lu/177Hf is the leastcontaminated, we obtain a maximum garnet–omphacite isochronage of 37.10± 0.94Myr for the Alpine event. Alternatively, anage of 38.5 ± 2.5Myr is calculated using the residues of grt1and the purest grt2 fraction. As residual components of grt1cores preserve unradiogenic Hf that was not available when newgarnet formed during Alpine eclogite-facies metamorphism, theuse of this low Variscan initial 176Hf/177Hf is probably unrealistic.Nevertheless, it constrains a firm upper age limit for Alpinegarnet growth. Analogous to our reasoning for grt2 fractions,we regard grt1 separates that tend towards lower 176Hf/177Hfat a given 176Lu/177Hf as being affected by the younger grt2domains. A minimum age of 336.0 ± 3.3Myr is obtained forthe older event by an isochron defined by grt1 separates withthe comparatively highest 176Hf/177Hf and the solid residuesthat remained after garnet dissolution. The residues consist of>99% rutile and should have preserved a Variscan initial Hfisotope composition. Using the fully digested whole rock insteadof the residues for isochron regression yields an identical ageof 337.1± 1.3Myr.

Eclogites from Trescolmen can now be shown to have beensubjected to the high-pressure phases of two orogenic cycles17, ahistory that may also apply to the entire basement of the Adulanappe, as we will argue below. The Variscan ‘minimum’ age of332.7Myr is consistent with more precise U–Pb zircon ages ofabout 370Myr typically found at Trescolmen12. These 370Myr agesprobably reflect the true timing of garnet growth because they

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176 H

f/17

7 Hf

176 H

f/17

7 Hf

176Lu/177Hf

176Lu/177Hf

0.2829

0.2830

0.2831

0.2832

0.2833

0.2834

0.2835

0 0.01 0.02 0.03

WR1 (tabletop)WR2 (PARR bomb)Omphacitegrt1

grt2 ≤ 150 μm

Residue of WR1Residues of grt1

grt2 ≥ 250 μm

b

Maximum age

Minim

um ag

e

0.28

0.29

0.30

0.31

0.32

0.33

Age = 336.0 ± 3.3 Myr(176Hf/177Hf)

i = 0.283020 ± 0.000034

MSWD = 3.3

Age = 37.1 ± 0.94 Myr(176Hf/177Hf)

i = 0.283163 ± 0.000027

10

Age = 336.0 ± 3.3 Myr(176Hf/177Hf)

i = 0.283020 ± 0.000034

MSWD = 3.3

Age = 37.1 ± 0.94 Myr(176Hf/177Hf)

i = 0.283163 ± 0.000027

a

b

0 2 4 6 8

Figure 4 | Isochron plots illustrating the different ages obtained for twogarnet generations present in eclogite sample TRC1 from Trescolmen.a, A six-point isochron defined by the purest (with respect to differentgarnet generations) grt1 separate, four residues of grt1 fractions and onewhole-rock residue (WR1) yields a Variscan age of 336.0±3.3 Myr. Atwo-point isochron defining an Alpine age of 37.10±0.94 Myr is obtainedfrom the purest grt2 fraction and the omphacite separate. MSWD=meansquare of weighted deviates. b, Close-up showing the low-Lu/Hf analyses.

were interpreted to reflect high-pressure conditions12. The Eocene‘maximum’ age of 38Myr is consistent with high-pressure agesdetermined by Lu–Hf (ref. 11), Sm–Nd (ref. 9) and U–Pb (ref. 10)chronometry in the southwestern Adula nappe (Fig. 2). It alsoconcurs with Ar–Ar phengite ages (about 40.5Myr; ref. 22) fromthe central Adula nappe as well as a prominent Rb–Sr phengite-agepopulation (about 38–35Myr; ref. 23) that has been interpreted todate the Alpine high-pressure event20,23. As the intensity of Alpinemetamorphic overprint increases towards the south, evidence of theVariscan metamorphism is best preserved in the northeastern partof the nappe. Fortunately, our sample from the central part of thenappe was only partially overprinted such that two distinct garnetgenerations are present.

Clearly, vestiges of Variscan metamorphism are preserved onlyas relict cores of garnet, whereas the rest of our eclogite hasequilibrated to Alpine high-pressure conditions. Hence, the pres-sure and temperature estimates for the Trescolmen locality prob-ably reflect Eocene conditions13,14,19,20. This assumption is evenmore likely to apply for the southwestern Adula nappe wherethe Alpine overprint was even more pronounced. Although itcannot be resolved here whether the petrological features in thenorthern Adula nappe record Eocene or Palaeozoic conditions, itis simpler to attribute a coherent pressure–temperature gradientto a single event rather than to two. Collectively, we infer thatthe north-to-south pressure gradient in the Adula nappe is anentirely Alpine feature and may well reflect the position of theAdula nappe as a coherent unit in an Alpine subduction zone15,20.Subduction as a coherent unit is also supported by our new age databecause (1) Alpine high-pressure metamorphism is consistentlydated as upper Eocene within a variety of rocks throughout theAdula nappe and (2) our Variscan high-pressure age in sampleTRC1 clearly shows that this sample was part of the continentalbasement before the Alpine orogeny. These features would beexpected for basement nappes that are subducted and exhumed ascoherent slabs, whereas they must be viewed as rather coincidentalin a mélange model.

Although the continental host rocks in theAdula nappe generallylack evidence for deep subduction, ‘coherent unit type’ subductionmodels require that they must have been subjected to peakpressures similar to those of the associated eclogites2. Regardlessof whether these rocks had actually recrystallized to high-pressureassemblages, they would have had large buoyancies, contributingto fast exhumation1–3,24. The complete absence of any diffusivere-equilibration between Variscan grt1 and Alpine grt2 populationsduring the entire Alpine cycle indicates that exhumation indeedoccurred rapidly. As evident from the available age data, deepsubduction of the Adula nappe did not commence until about37Myr ago and the unit reached peak-pressure conditions about35Myr ago10, before it was exhumed to shallow crustal levels(0.55GPa) about 33–32Myr ago10,12. Hence, it seems unlikely thatthe Trescolmen eclogites have resided in a deep subduction channelfor several million years as proposed for other high-pressure rocksin the Adula nappe5,11 and elsewhere in the Alps6,7.

Our data support models where the Adula nappe represents abasement nappe that was subducted and rapidly exhumed as onecoherent unit3,14–16,20. The methods applied here can now be usedto test for similar scenarios in other continental high-pressure andultrahigh-pressure units in the Alps and elsewhere.

MethodsThe eclogite sample was crushed and divided into two splits, one of which waspowdered in an agate mill, whereas the other was processed for mineral separation.Two different digestion procedures were applied. (1) Selective dissolution of garnet,omphacite, and one whole-rock fraction (TRC1-WR1) efficiently dissolves majorsilicate phases, while leaving behind refractory, Hf-bearing accessory phases suchas rutile and zircon21. (2) One whole rock (TRC1-WR2) and the refractory mineralinclusions remaining after selective dissolution (‘residues’) were digested in Savillexvials placed inside steel-jacketed Teflon PARR bombs for 3–5 days at 180 ◦C,ensuring full sample digestion21. A mixed 176Lu–180Hf tracer was added before alldigestions. The refractory mineral inclusions remaining from the selective tabletopdigestion were rinsed repeatedly with 3N HCl and deionized water before respikingand digestion in PARR bombs. Separation of Lu and Hf from the rock matrix wasachieved by Ln-Spec resin column chemistry25. Lutetium and Hf measurementswere carried out on a Finnigan Neptune multi-collector inductively coupledplasma mass spectrometer (MC-ICP-MS) at the Steinmann Institut, Bonn21,26.Measured 176Hf/177Hf values are reported relative to 176Hf/177Hf= 0.282160 forthe Münster Ames Hf standard, which is isotopically identical to the JMC-475standard. For isochron calculations, the external reproducibility was estimatedby the empirical relationship 2σ external reproducibility ≈4σm, where σm is thestandard error of an individual analysis27. An external reproducibility of ±0.2%(2σ ) for the 176Lu/177Hf is typical for ideally spiked sample solutions if naturallyoccurring Yb in the Lu cuts is used for mass bias and interference corrections26.Isochron regressions and ages were calculated using ISOPLOT v.2.49 (ref. 28) and

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NATURE GEOSCIENCE DOI: 10.1038/NGEO1060 LETTERSλ176Lu= 1.867×10−11 yr−1 (refs 29,30). Blanks for Lu and Hf ranged between3–15 pg and 6–25 pg, respectively. Total amounts of Hf analysed ranged from 1.2 to584 ng. Sample-to-blank ratios were all>80. The blank uncertainty results in slightvariations of 176Hf/177Hf composition, but the effect on isochron ages is negligible.Electron microprobe analyses were carried out using the JEOL-JXA-8900 at theSteinmann Institut, Bonn.

Received 3 March 2010; accepted 7 December 2010;published online 23 January 2011

References1. Agard, P., Yamato, P., Jolivet, L. & Burov, E. Exhumation of oceanic blueschists

and eclogites in subduction zones: Timing and mechanisms. Earth Sci. Rev. 92,53–79 (2009).

2. Ernst, W. G., Marumyama, S. & Wallis, S. Buoyancy-driven, rapid exhumationof ultrahigh-pressure metamorphosed continental crust. Proc. Natl Acad. Sci.94, 9532–9537 (1997).

3. Brueckner, H. K. Dunk, dunkless and re-dunk tectonics: A model formetamorphism, lack of metamorphism, and repeated metamorphism ofHP/UHP terranes. Int. Geol. Rev. 48, 978–995 (2006).

4. Trommsdorff, V. Metamorphism and tectonics in the Central Alps: The Alpinelithospheric mélange of Cima Lunga and Adula.Mem. della Soc. Geol. Ital. 45,39–49 (1990).

5. Engi, M., Berger, A. & Roselle, G. T. Role of the tectonic accretion channel incollisional orogeny. Geology 29, 1143–1146 (2001).

6. Stöckhert, B. & Gerya, T. V. Pre-collisional high pressure metamorphismand nappe tectonics at active continental margins: A numerical simulation.Terra Nova 17, 102–110 (2005).

7. Bousquet, R. Metamorphic heterogeneities within a single HP unit: Overprinteffect or metamorphic mix? Lithos 103, 46–69 (2008).

8. Yamato, P., Burov, E., Agard, P., Le Pourhiet, L. & Jolivet, L. HP–UHPexhumation during slow continental subduction: Self-consistentthermodynamically and thermomechanically coupled model with applicationto the Western Alps. Earth Planet. Sci. Lett. 271, 63–74 (2008).

9. Becker, H. Garnet peridotite and eclogite Sm–Nd mineral ages from theLepontine dome (Swiss Alps): New evidence for Eocene high-pressuremetamorphism in the central Alps. Geology 21, 599–602 (1993).

10. Gebauer, D. in Earth Processes: Reading the Isotopic Code (eds Basu, A. &Hart, S.) (Geophys. Monogr. 95, 307–329, AGU, 1996).

11. Brouwer, F. M., Burri, T., Engi, M. & Berger, A. Eclogite relics in the CentralAlps: PT—evolution, Lu–Hf ages and implications for formation of tectonicmélange zones. Schweiz. Mineral. Petrogr. Mitt. 85, 147–174 (2005).

12. Liati, A., Gebauer, D. & Fanning, C. M. Geochronological evolution of HPmetamorphic rocks of the Adula nappe, Central Alps, in pre-Alpine and Alpinesubduction cycles. J. Geol. Soc. 166, 797–810 (2009).

13. Heinrich, C. A. Eclogite facies regional metamorphism of hydrous mafic rocksin the Central Alpine Adula Nappe. J. Petrol. 27, 123–154 (1986).

14. Meyre, C., De Capitani, C., Zack, T. & Frey, M. Petrology of high-pressuremetapelites from the Adula Nappe (Central Alps, Switzerland). J. Petrol. 40,199–213 (1999).

15. Froitzheim, N., Schmid, S. M. & Frey, M. Mesozoic paleogeography and thetiming of eclogite-facies metamorphism in the Alps: A working hypothesis.Eclogae Geol. Helv. 89, 81–110 (1996).

16. Beaumont, C., Jamieson, R. A., Butler, J. P. & Warren, C. J. Crustal structure:A key constraint on the mechanism of ultra-high-pressure rock exhumation.Earth Planet. Sci. Lett. 287, 116–129 (2009).

17. Biino, G. G., Marquer, D. & Nussbaum, Ch. Alpine and pre-Alpine subductionevents in polycyclic basement of the Swiss Alps. Geology 25, 751–754 (1997).

18. Schmid, S. M., Pfiffner, O. A., Froitzheim, N., Schönborn, G. & Kissling, E.Geophysical–geological transect and tectonic evolution of the Swiss–ItalianAlps. Tectonics 15, 1036–1064 (1996).

19. Dale, J. & Holland, T. J. B. Geothermobarometry, P–T paths and metamorphicfield gradients of high-pressure rocks from the Adula Nappe, Central Alps.J. Metam. Geol. 21, 813–829 (2003).

20. Nagel, T. J. Tertiary subduction, collision and exhumation recorded in theAdula nappe, central Alps. Geol. Soc. Spec. Publ. Lond. 298, 365–392 (2008).

21. Lagos, M. et al. High precision Lu–Hf geochronology of Eocene eclogite-faciesrocks from Syros, Cyclades, Greece. Chem. Geol. 243, 16–35 (2007).

22. Santini, L. Geochemistry and Geochronology of the Basic Rocks of the PenninicNappes of East-Central Alps (Switzerland) PhD thesis, Univ. Lausanne (1992).

23. Jäger, E. Die alpine Orogenese im Lichte der radiometrischenAltersbestimmung. Eclogae Geol. Helv. 66, 11–21 (1973).

24. Duchêne, S. et al. The Lu–Hf dating of garnets and the ages of the Alpinehigh-pressure metamorphism. Nature 387, 586–589 (1997).

25. Münker, C., Weyer, S., Scherer, E. E. & Mezger, K. Separation of high fieldstrength elements (Nb, Ta, Zr, Hf) and Lu from rock samples for MCICPMSmeasurements. Geochem. Geophys. Geosystems 2, Nr. 2001GC000183 (2001).

26. Vervoort, J. D., Patchett, P. J., Söderlund, U. & Baker, M. Isotopic compositionof Yb and the determination of Lu concentrations and Lu/Hf ratios by isotopedilution using MC-ICPMS. Geochem. Geophys. Geosystems 5, Q11002 (2004).

27. Bizzarro, M., Baker, J. A., Haack, H., Ulfbeck, D. & Rosing, M. Early historyof Earth’s crust–mantle system inferred from hafnium isotopes in chondrites.Nature 421, 931–933 (2003).

28. Ludwig, K. R. Isoplot/Ex version 2.49, Geochronological Toolkit for MicrosoftExcel (Berkeley Geochronology Center Spec. Publ. 1a, 2001).

29. Scherer, E. E., Münker, C. & Mezger, K. Calibrating the Lu–Hf clock. Science293, 683–686 (2001).

30. Söderlund, U., Patchett, P. J., Vervoort, J. D. & Isachsen, C. E. The 176Lu decayconstant determined by Lu–Hf and U–Pb isotope systematics of Precambrianmafic intrusions. Earth Planet. Sci. Lett. 219, 311–324 (2004).

AcknowledgementsWe thank S. Weber for his help with preparing mineral separates, S. Oppel forprocessing thin sections, A. Luguet for technical assistance with the MC-ICP-MSinstrument, J.E. Hoffmann for critical discussion and S. Kramer for proofreading.We are grateful for the thorough and constructive reviews by J. Kramers, H.K.Brueckner and C. Beaumont.

Author contributionsD.H. acquired the Lu–Hf isotope data including MC-ICP-MS measurements and dataprocessing. T.J.N. collected, processed and refined electron microprobe analyses. C.M.,N.F. and E.E.S. substantially contributed to the study design, data evaluation andinterpretation. E.E.S. developed the selective digestion method and C.M. developed thecolumn chemistry and the analytical protocol. D.H. and T.J.N. wrote the manuscript andcontributed equally to the study.

Additional informationThe authors declare no competing financial interests. Supplementary informationaccompanies this paper on www.nature.com/naturegeoscience. Reprints and permissionsinformation is available online at http://npg.nature.com/reprintsandpermissions.Correspondence and requests formaterials should be addressed toD.H.

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