exhumation of the schistes lustrés complex: in situ laser probe 40ar/39ar constraints and...

Exhumation of the Schistes Lustres complex: in situ laser probe40Ar ⁄ 39Ar constraints and implications for the Western Alps

P. AGARD1, P. MONIE2 , L . JOLIVET1 AND B. GOFFE3

1Laboratoire de Tectonique, Universite PM Curie, ESA 7072, case 129, 4 place Jussieu, 75252 Paris Cedex 5, France([email protected])2Laboratoire de Geophysique, Tectonique et Sedimentologie. UMR-CNRS 5573, UM2. Pl. E.Bataillon, 34095 Montpellier, France3Laboratoire de Geologie de l’Ecole Normale Superieure, UMR-CNRS 8532, 24 rue Lhomond, 75231 Paris Cedex 5, France

ABSTRACT The Schistes Lustres (SL) suture zone occupies a key position in the Alpine chain between the high-pressure (HP) Brianconnais domain and the ultrahigh-pressure (UHP) Dora Maira massif, and reachedsubduction depths ranging from c. 40–65 km (Cottian Alps). In order to constrain the timing of HPmetamorphism and subsequent exhumation, several phengite generations were differentiated, on thebasis of habit, texture, paragenesis and chemistry, as belonging to the first or second exhumationepisode, respectively, D2 or D3, or to earlier stages of the tectono-metamorphic evolution.Ten carefully selected samples showing D2, D3 (D2 + D3), or earlier (mostly peak temperature)

phengite population(s) were subjected to laser probe 40Ar ⁄ 39Ar analysis. The data support the results ofthe petrostructural study with two distinct age groups (crystallization ages) for D2 and D3 phengite, at51–45 and 38–35 Ma, respectively. The data also reveal a coherent age cluster, at 62–55 Ma, for peaktemperature phengite associated with chloritoid which were preserved in low strain domains.The age of the D3 event in the SL complex appears very similar to ages recently obtained for

greenschist facies deformation on the border of most internal crystalline massifs. Exhumation rates ofthe order of 1–2 mm yr)1 are obtained for the SL complex, which are compatible with velocitiesdocumented for accretionary wedge settings. Similarly, cooling velocities are only moderate(c. 5 �C Myr)1), which is at variance with recent estimates in the nearby UHP massifs.

Key words: exhumation velocities; HP–LT metamorphism; laser-probe 40Ar ⁄ 39Ar dating; phengite;Western Alps.

INTRODUCTION

Understanding the dynamics of orogenic wedgesrequires quantitative constraints on key parameterssuch as exhumation trajectories and velocities, tem-perature and on the rheology of the wedge material(Platt, 1993; Burg & Ford, 1997; Beaumont et al.,1999; Ring et al., 1999b; Burov et al., 2001). Radio-chronological methods, in conjunction with structuralstudies linked to precise P–T paths, have increasinglybeen used to constrain cooling rates, exhumation velo-cities and mechanisms, in particular for high-pressure(HP) and ultrahigh-pressure (UHP) metamorphicrocks (e.g. Duchene et al., 1998; Perchuk & Philippot,1997; Hacker et al., 2000; Ring et al., 1999a). Con-ceptual models for exhumation (Platt, 1993; Ducheneet al., 1997b; Ring et al., 1999b) now rely on numerousreports of deeply buried continental (Chopin & Monie,1984; Dobretsov et al., 1995; Wain, 1997; Hackeret al., 1998; Ernst & Liou, 2000) and oceanic crustalrocks (Reinecke, 1991; Reinecke, 1998; Messiga et al.,1999), and on studies dealing with erosion processes(Burbank et al., 1996; Allen, 1997) or geospeedometry(Perchuk & Philippot, 2000 for a review).

In the Alpine chain, one of the best documentedorogens world-wide (Coward & Dietrich, 1989; Fig. 1),many geochronological studies have attempted toconstrain the timing of UHP to HP metamorphism,mostly on crystalline rocks of internal massifs andeclogite facies ophiolite rocks (Gebauer, 1999 for arecent review, and below). The most recent studies(Gebauer et al., 1997; Amato et al., 1999) have iden-tified high exhumation velocities. However, precisemodels are hindered by uncertainty about the assign-ment of dated minerals to the UHP paragenesis (e.g.Mont Rose: Rubatto & Gebauer, 1999; Dabie Shan:Hacker et al., 1998), as well as the validity of the clo-sure temperature concept (Dodson, 1973; Jager, 1973)being challenged by the recognition that such param-eters as grain size, composition, deformation, pressureand fluids are likely to affect isotopic closure too (e.g.Villa, 1998; Dahl, 1996a; Scaillet, 1998).Western Alpine studies over the past decade have

recognized (see Cliff et al., 1998; Von Blanckenburg &Davies, 1995; Michard et al., 1996; Rubatto &Gebauer, 1999; Amato et al., 1999; Duchene et al.,1997a; and below for a review) that: (i) ages for UHPmetamorphism mainly cluster around 70–35 Ma

J. metamorphic Geol., 2002, 20, 599–618

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(ii) UHP to HP metamorphism is probably diachro-nous, with older ages reported for the Sesia zone thanfor the internal crystalline massifs (ICM, e.g. DoraMaira) (iii) �eoalpine� phengite ages must be treatedwith caution owing to repeated reports of excess argoncontamination (e.g. Arnaud & Kelley, 1995; Scaillet,1996; Giorgis et al., 2000; Jahn et al., 2001), eventhough comparable U ⁄Pb ages have been reported(Paquette et al., 1989).Yet, several factors still hamper the understanding

of the exhumation history of Alpine HP and UHProcks: (1) few studies have combined metamorphic,tectonic and geochronological data from the regionalto the microscopic scale to enable a critical assess-ment of which metamorphic stage is being dated, (2)there are few constraints on the tectono-metamor-phic history of oceanic metapelite units, whichnevertheless represent the vast majority of the deeply

buried Pennine rocks accessible today (amongwhich the HP Schistes Lustres complex, WesternAlps).Our study aims to fill this gap by unravelling the

exhumation chronology of the oceanic Schistes Lustrescomplex (Cottian Alps; Fig. 1), for which only K ⁄Arphengite ages and fission track data exist. We presentresults of in situ laser probe 40Ar ⁄ 39Ar dating of suc-cessive phengite generations whose composition andP–T-deformation history were recently studied (Agardet al., 2001a,b). Implications for the exhumation of theWestern Alps as a whole and for the geodynamicevolution are then discussed.

GEOLOGICAL SETTING

The Schistes Lustres (SL) complex occupies an internalposition in the Alpine orogenic wedge (Fig. 1). It ismade of intensely folded Liguro-Piemontese upperJurassic (Malm; De Wever & Caby, 1981) to lateCretaceous calcschists (Coniacian-Santonian; Lemoineet al., 1984), with a few mantle slivers (mainly ser-pentinites) representing the floor of this Alpine realmmostly devoid of oceanic crust (Lagabrielle &Lemoine, 1997). These rocks followed a progradeHP–LT evolution between 100 and 60 Ma duringsubduction (e.g. Le Pichon et al., 1988; Lemoine et al.,2000). By the time collision developed (c. 35–30 Ma;Sinclair & Allen, 1992), the SL complex was probablylargely exhumed since it escaped the MP–MT meta-morphic imprint documented further north (Lepontinedome, e.g. Todd & Engi, 1997).The study of metamorphic occurrences shows that

carpholite-bearing assemblages are present in thewestern part of the study area (zones I–II; Fig. 2a),while chloritoid-bearing assemblages occur in theeastern part (zones III–V; Fig. 2a; there, carpholite isonly preserved as quartz-hosted relics). On the basis ofmetapelite mineralogy, P–T estimates at maximumdepth increase from west to east across the studyarea (Fig. 2a,b), from c. 12–13 kbar ⁄ 300–350 �C to20–21 kbar ⁄ 450–500 �C (Fig. 2b) (Agard et al., 2001a).These estimates are comparable to those from meta-basites located further south in the SL complex ofbetween 10 kbar ⁄ 400 �C and 25 kbar ⁄ 600 �C (Messigaet al., 1999; Schwartz et al., 2000).Very little is known about early deformation sta-

ges coeval with HP conditions. Yet, two successive,opposite-vergence exhumation stages, termed D2 andD3, respectively, were recently recognized (Agardet al., 2001a). On Fig. 2(b), P–T conditions pre-vailing during D2 and D3 are indicated. D2 is aductile, fairly coaxial, E-vergent deformation eventassociated with a regional S2 schistosity andoriented phengite-chlorite assemblages, that essen-tially postdates HP deformation stages coeval withthe crystallization of carpholite. In contrast, D3 is ahighly noncoaxial W-vergent deformation event(Fig. 2a), which is of a brittle-to-ductile character in

Fig. 1. Location of the study area (framed area) within theFranco–Italian Western Alps. Abbreviations used for the label-ling of the different domains (from the external to the internaldomains): ECM: external crystalline massifs cropping out on theEuropean plate; flysch: large flysch overthrust nappes; Brianc:Brianconnais domain; BS: noneclogitic Schistes lustres unit;Eclog: eclogitic Schistes lustres unit; ICM: internal crystallinemassifs, from south to north: Dora Maira (DM), Grand Paradis(GP), Mont Rose (MR); A.Alpine: austroalpine and south-alpine continental units (DB: Dent Blanche). Thick line:location of the cross-section of Fig. 2.

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the west of the study area and ductile in the east.D3 is responsible for the undulation of the S2schistosity, localized phengite recrystallization, andis similar to W-vergent deformation documentedon the western flanks of the oceanic UHP Monviso(Ballevre et al., 1990; Philippot, 1990) and the ICM(Henry et al., 1993). These results are summarizedin Fig. 2 (for further details, see Agard et al.,2001a,b).The selected phengite generations dealt with in

this study are all referenced with respect to the meta-morphic P–T conditions and the D2 or D3 exhumationstages.

TIME CONSTRAINTS

Stratigraphic constraints are scarce in the SL complex (De Wever &Caby, 1981; Lemoine et al., 1984). The youngest reported meta-sediments to delimit the timing of metamorphism in the SL complexare Maastrichtian (72–65 Ma; Deville, 1986), and metamorphismmust predate the Chattian (28–23.5 Ma), since metasedimentaryrocks from external units of the Western Alps contain HP minerals(synclinal de Barreme; Bonhomme et al. 1980), including carpholite(Goffe, unpublished data).Only a limited number of geochronological constraints are

available for the SL complex (as already noted by Cliff et al., 1998).Three phengite fractions from the SL complex immediately north(Bardonnechia) and south (Aigue valley) of the study area gave62 ± 0.5 and 43 ± 0.3 ⁄ 41 ± 0.4 Ma, respectively (Delaloye &Desmons, 1976). Two phengitic fractions (<2 lm) from Sestrieres

Fig. 2. (a) Position of the samples on a diagrammaticcross-section summarizing the main deformationpatterns and P–T conditions across the transectlocated in Fig. 1 (sample numbers as given in Agardet al. (2001b); except for new samples marked bystars). The samples are grouped into five differentzones (I through V; see also Fig. 4). Grey shear planesunderline the existence of discrete, non coaxial, mostlywest-vergent, ductile (in the east) to brittle (in thewest)D3 shear planes. D2 deformation is distributed morepervasively: places where unambiguous, noncoaxial,east-vergent D2 kinematic indicators were found areindicated by dark shear planes.(b) Summary of the metamorphic P–T constraints forthe samples considered in this study (zones I to V as inFig. 2a). Peak P–T conditions for zones I–V afterAgard et al. (2001b). Specific P–T estimates are givenfor each phengite generation of a given sample (seeTable 1), when available, with error bars give an ideaof the 2r uncertainties involved. The ranges of P–Testimates for the D2 and D3 retrograde stages basedon phengite-bearing assemblages, is indicated (hat-ched and empty areas, respectively). The stronglytemperature-dependent reaction Car ¼ Qtz +Cld + w (Cld +) separates a low-temperaturedomain, marked by carpholite-bearing rocks (seeFig. 2a) from a chloritoid-bearing domain wherecarpholite relics in quartz can often be found (see textfor details and Fig. 6d–f). Other reactions: Gln +Lws ¼ Zo + Chl + Pg + Qtz + w (Gln Lws-);Lws + Phg3.3 ¼ Zo + Ms + Chl + Qtz (Lws-);Jd + Qtz ¼ Ab (Jd +); Cal ¼ Arg (Arg +).

4 0Ar ⁄ 3 9A r D AT I N G O F T H E S C HI S TE S L U S T R E S , W . A L P S 60 1

(Italy) gave 38.2 ± 1.2 and 48.8 ± 1.3 Ma (Bonhomme et al.,1980). K ⁄Ar isochrons from two samples immediately south of thestudy area (Fig. 1) gave 58–50 Ma and 54–49 Ma, respectively, andwere interpreted as two slightly diachronous steps of the samedeformation event (Liewig et al., 1981). Takeshita et al. (1994)reported ages scattered between 115 (likely resulting from thepresence of detrital components) and 40 Ma, and suggested an ageof 50 ± 10 Ma for the cooling of blueschist facies rocks in thestudy area. Finally, Caby & Bonhomme (1982) reported evidence ofexcess argon from two blueschist facies radiolarite samples from StVeran (Aigue valley) and concluded that the HP event should beplaced either at 85 or 60 Ma. Overall, these studies give ages scat-tered about 50 Ma, which is similar to the age of the adjacent UHPMonviso area (Monie & Philippot, 1989; Duchene et al., 1997a;Cliff et al., 1998).However, these studies do not provide strong constraints on the

age of metamorphism because: (i) white mica separates generallyconsist of mixtures of distinct phengite populations (ii) the authorsprovide little or no constraints on the tectono-metamorphic history,and (iii) K-Ar geochronology is poorly adapted to the dating ofsuccessive metamorphic steps as recorded by the SL complex.The late stages of exhumation were recently studied by fission

track methods on apatite and zircon, in the south of the studyarea (Schwartz, 2001). Zircon cooling ages (at c. 290 �C; Tagami& Shimada, 1996) and apatite cooling ages (at 110–60 �C; Hur-ford, 1990; Gunnell, 2000) decrease eastward from 28 to 22 Maand from 22 to 9 Ma, respectively (Schwartz, 2001). In the studyarea, the Chenaillet unit, which escaped HP metamorphism, con-sistently yields Zr and Ap–FT ages of 118 and 65 Ma, respectively,which suggests that the two chronometers have escaped completeresetting.All the above results are compared in Fig. 3 to the recent (i.e.

essentially more recent than the compilation by Hunziker et al.,1992) available data on HP and UHP metamorphism for the adja-cent internal domains of continental (Brianconnais, ICM and Sesia)and oceanic origin (Monviso, Zermatt–Saas; see also Rubatto &Gebauer, 1999; and Gebauer, 1999 for a review). Whether ages relateto greenschist facies or HP facies metamorphism, and which methodwas used, are indicated.This compilation emphasizes the considerable dispersion in the

data set (which in part derives from the use of different techniques)and the need for more constraining data—only a small number ofstudies provide petro-structural constraints at a comparable scale(e.g. Freeman et al., 1997; Fig. 3). In most cases, the assignment ofages to specific metamorphic events remains uncertain. For example,factors controlling zircon overgrowths or the isotopic behaviour ofthe different radiometric systems are still poorly known, speciallyunder HP to UHP conditions, not to mention the contaminationsuch as caused by Nd-rich inclusions in garnet (see the discussion byAmato et al., 1999), or repeated reports of contemination of40Ar ⁄ 39Ar ages by excess argon (Scaillet, 1998 and references therein).Note that some very young ages (c. 35–30 Ma for UHP metamor-phism in DMM) are difficult to reconcile with the geological con-straints (e.g. Stampfli & Marchant, 1997), and may reflect laterre-equilibration rather than the age of UHP–HP metamorphismitself. This point will be discussed later.

SAMPLE SELECTION

Metapelite samples containing phengite generations referenced withrespect to the tectono-metamorphic evolution were selected fromacross the SL complex (locations on Figs 2 & 4). The relationshipsbetween different phengite generations and mineral assemblagesthroughout the metamorphic evolution are documented in Fig. 4(boxes on the upper left and lower right part of figure): phengite canoccur as a precursor for carpholite, or associated with carpholite orchloritoid at HP conditions, or can be associated with chlorite as partof the retrograde assemblage (Agard et al., 2001b). The size,microtextural site, and time of crystallization of phengite with respectto deformation (deformation events, in Table 1) and metamorphicreactions throughout (parageneses, in Table 1), composition and

estimated P–T conditions of formation are listed in Table 1 (Agardet al., 2001a,b).Several points need to be highlighted:(1) the crucial observations in assessing the time of phengite cry-

stallization with respect to D2 and D3 are briefly listed under thedeformation event heading in Table 1.(2) analyses of phengite 1–19 are from Agard et al. (2001b), and

only representative variations in Si, Mg and interlayer content (whichmay influence argon retentivity, e.g. Chopin & Maluski, 1980;Monie, 1985; Scaillet et al., 1992; Scaillet, 1998; Giorgis et al., 2000)are listed.(3) most phengite crystallized during the metamorphic evolution

and cannot be mistaken with detrital white mica. The coexistence ofdifferent phengite generations with contrasting compositions (seealso: Frey et al., 1983; Chopin et al., 1991) suggests that successivephengite neocrystallization events took place, rather than a completeresetting of pre-existing phengite (which would have led to a homo-geneous phengite composition).(4) P–T estimates for the phengite generations (Fig. 2b) show that

it crystallized T <380 �C in the western carpholite zone, and at400 < T < 500 �C in the eastern chloritoid zone.

ANALYTICAL TECHNIQUES

The in situ laser ablation Ar ⁄Ar technique is described elsewhere indetail (by, e.g. Maluski & Monie, 1988). Rock sections of c. 500 lmthickness were double polished to 1 lm. Photographs of the polishedsurface and of the corresponding thin section (Agard et al., 2001b)were taken in order to have an accurate reference frame during laserprobe experiments. All samples were rinsed in acetone, methanol anddistilled water, then packed in aluminium foils and irradiated for 60h in the McMaster nuclear reactor (Canada) with several aliquots ofthe MMHb-1 (520.4 ± 1.7 Ma; Samson & Alexander, 1987) andHD-B1 (24.21 ± 0.32 Ma; Hess & Lippolt, 1994) flux monitors.After irradiation, the monitors and rock sections were placed on aCu-holder inside an UHV gas extraction system and baked for 48 hat 200 �C. The analytical device for laser probe experiments consistsof: (a) a multiline continuous 6 W argon-ion laser; (b) a beam shutterfor selection of exposure times, typically 30 milliseconds for eachpulse; (c) divergent and convergent lenses for beam focusing; (d) asmall inlet line for the extraction and purification of gases; (e) aMAP 215–50 noble gas mass spectrometer. Each analysis involves5 min for gas extraction and cleaning and 15 min for data acquisi-tion by peak jumping from mass 40 to mass 36. For each analysis,argon was extracted from a surface of 200 · 250 lm2, which con-cerned 1–6)7 crystals depending on the size of the (often tiny)phengite grains (c. 100–350 lm in length; Figs 5–7). Incision of thesample did not exceed 10–20 lm deep, and was limited by slowlymoving the laser about the rock surface. For most analyses, mineralsadjacent to phengite were not affected by lasering. However, forsmall phengite located in the vicinity of chlorite and calcium-bearingminerals such as calcite and epidote (e.g. sample 19*), the release ofthe 38Ar and 37Ar interfering isotopes with chlorine and calcium(Brereton, 1970; McDougall & Harrison, 1988) reveals that argonwas extracted both from the phengite and adjacent K-poor mineralor inclusions. In general, such analyses have been discarded, exceptwhen the corresponding ages remain consistent with those from purephengite.System blanks were evaluated every three experiments and ranged

from 3 · 10)12 cc for 40Ar to 4 · 10)14 cc for 36Ar. Ages anderrors were calculated according to McDougall & Harrison (1988;eq. 3.41–44, p. 84–85), and were corrected for the presence ofatmospheric argon assuming that all 40Ar is radiogenic except for acomponent associated with 36Ar in the same proportion as in thepresent atmosphere (% atm; Table 2). These are the so-calledapparent ages reported in Table 2 and Fig. 8. The quoted errorsrepresent one sigma deviation and do not include uncertainty onthe monitor. This apparent age can be compared to the interceptage of each phengite generation (black dotted lines; Fig. 8) calcula-ted without an a priori estimate of the initial 36Ar ⁄ 40Ar ratio,using 36Ar ⁄ 40Ar vs. 39Ar ⁄ 40Ar correlation diagrams.

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RESULTS

Results (Table 2; Fig. 8) are presented below fromwest (carpholite zone) to east (chloritoid zone).

Carpholite zone

Samples 1 & 3* (D3)

In sample 1, phengite associated with a D3 veinletpseudomorphs the edges of carpholite crystals(Fig. 5a–d) and in sample 3* a D3 phengite from a D3

shear band (Fig. 5e–h) gives apparent ages betweenc. 43 (± 0.4; 1r) and 36 Ma (± 1.5). Analyses 91–93from sample 1 are contaminated by Ca-rich phases(high 37Ar ⁄ 39Ar; Table 2) but ages fall within the agerange of uncontaminated phengite. In sample 1 theages are more scattered (42.5–36.6 Ma) than for sam-ple 3* (37.7–36.0 Ma plus one value at 39.8 Ma).Sample 3* has an intercept age of 35.5 ± 1.1 Ma in the36Ar ⁄ 40Ar vs. 39Ar ⁄ 40Ar correlation plot (40Ar ⁄36Ar ¼ 366 ± 76). For sample 1, the intercept agewith all spots is 38.2 ± 1.2 Ma with an initial argonratio of 337 ± 27.

Fig. 3. Compilation of recently published radiochronological data on the different domains of the Western Alps (i.e. essentiallymore recent than the compilations by Hunziker et al., 1992 or von Blanckenburg & Davies, 1995). For each study, the methodand minerals used (see legend box on the left hand side) and the overall domain of P–T conditions investigated (HP & UHP:high-pressure and ultrahigh-pressure conditions; GS: greenschist facies conditions) are given. Studies are listed as follows:(1) Liewig et al. (1981); (2) Bonhomme et al. (1980); (3) Takeshita et al. (1994); (4) Delaloye & Desmons (1976) (5) Caby &Bonhomme (1982); (6) Schwartz (2001); (7) Cliff et al. (1998); (8) Monie & Philippot (1989); (9) Duchene et al. (1997); (10)Reddy et al. (1999); (11) Rubatto et al. (1998); (12) Bowtell et al. (1994); (13) Barnicoat et al. (1995); (14) Amato et al. (1999);(15) Arnaud & Kelley (1995); (16) Scaillet et al. (1990); (17) Scaillet et al. (1992) (18) Monie & Chopin (1991); (19) Paquette et al.(1989); (20) Scharer et al. (1999); (21) Gebauer et al. (1997); (22) Tilton et al. (1989, 1991); (23) Chopin & Maluski (1980);(24) Freeman et al. (1997); (25) Hurford et al. (1992) (26) Monie (1985); (27) Rubatto & Gebauer (1999); (28) Chopin & Monie(1984) (29) Ruffet et al. (1995); (30) Hurford et al. (1989); (31) Cortiana et al. (1998); (32) Reddy et al. (1996) (33) Inger et al.1996; (34) Rubatto et al. (1999); (35) Ramsbotham et al. (1994).


The small age discordance between these two sam-ples is the consequence either of a different initial40Ar ⁄ 36Ar ratio in the veinlet phengite (sample 1) and

in the metapelite (sample 3*), or of some slightly dia-chronous growth of phengite during D3. Moreover,minor variations in the initial argon ratio at the samplescale could be partially responsible for the intrasampleage scattering, as well as deficiencies in the interlayeredcontent revealed by microprobe analyses (Table 1;Agard et al., 2001b).

Sample 5 (D2-D3)

Phengite growing after carpholite is oriented alongthe S2 foliation together with chlorite, or occurs indisoriented patches within the matrix, and so itcannot be attributed with confidence to either the D2or D3 phase. Seven analyses of the phengite give anage range between 48.7 and 38.0 Ma (1r < 1 Ma).Despite contamination by Ca-rich phases for someanalyses (Table 2), there is no systematic correlationbetween the 37Ar ⁄ 39Ar ratios and the ages obtained.In the correlation diagram, five analyses plot linearlyand give an intercept age of 36.2 ± 1.1 Ma with aninitial ratio of 385 ± 14, which is above the present-day atmospheric ratio of 295.5.

Sample 8 (D2)

Oriented phengite associated with chlorite replaced astretched carpholite-bearing quartz segregation bou-dinaged by D2 deformation and realigned on S2 foli-ation (Fig. 6). This phengite generation (with the highestSi4+ content in the carpholite zone; Table 1) is a goodreference forD2 deformation since the outcropwas littleaffected byD3. The determined ages range between 46.9

Fig. 4. Location of the samples (1–19; same numbers as inAgard et al., 2001b; except for new samples marked by stars)in the study area (enlargement from Fig. 1). Numbers I throughV refer to the zones defined in Fig. 2. Boxes show the generalshape of P–T paths in the West and in the East of the study area,and the successive phengite generations (labelled Phg) thatformed during the P–T evolution. Phg: stands for abundantphengite; (Phg): underlines the fact that only a few phengitecrystals of this type could be studied (sample 12); crystals whichwere too tiny or rare to be studied. Jd (+), Cld (+) and Lws (–):crystallization isograd for jadeite, chloritoid, and breakdownisograd for lawsonite in metapelites, respectively. Shaded areason the map: places where carpholite is most pervasivelypreserved.

Table 1. Main characteristics of the phengite sampled in the Schistes Lustres metapelites (location in Figs 2a & 4; same numbers asin Agard et al., 2001b; except for new samples marked by stars): size, microtextural site, timing of crystallization with respect todeformation events (the criteria leading to the attribution to a specific tectonic event, and the tectonic event itself, are listed in separatecolumns), timing of crystallization with respect to metamorphic reactions (parageneses), composition and estimated P–T conditionsof formation (Agard et al., 2001a,b; see also Fig. 2). In order to ease reading, apparent ages from this study (Fig. 8a) are alsogiven. **: the precision on P–T estimates is about 1–2 kbar, 50 �C.

6 04 P . A G AR D E T A L .

and 44.5 Ma (plus one value at 42.2 Ma; 1r c. 1–2 Ma).The larger error bars (c. 2–3 Ma) in analyses 12, 13 and17 are due to both a weak argon signal and a highatmospheric contribution (Table 2). An intercept age of44.9 ± 1.5 Ma was calculated for all ablation data,whose initial argon ratio (40Ar ⁄ 36Ar ¼ 288 ± 16)suggests that no excess argon was trapped after thephengite became closed to argon diffusion.

Chloritoid zone

Samples 12* (D2) and 12 (D2 + peak pressure)

Phengite from sample 12* is part of the syn-kinematicchloritoid–chlorite–phengite assemblage which marks

the S2 foliation (Fig. 6c) and includes quartz por-phyroclasts hosting carpholite. Ages range between51.3 and 43.3 Ma (1r c. 1.5 Ma), except one value at40.1 Ma (Fig. 8). As for sample 8, the larger error barsare caused by weak argon signals. The data cluster onthe correlation diagram, precluding a precise evalua-tion of the intercept age and initial 40Ar ⁄ 36Ar ratio.Sample 12 occurs in a less deformed outcrop a few

metres away from sample 12* and has identical min-eralogy, except that relics of the prograde carpholite +phengite assemblage are found in quartz porphyro-clasts (Fig. 6d–f). Phengite from the foliation yieldages between 53.6 and 45.9 Ma (1r c. 1 Ma) whereasthree tiny, quartz-hosted phengite associated withcarpholite (analyses 136–138; Table 2) give slightly

Fig. 5. Two samples representative of D3 deformation (samples 1 & 3*). Mineral abbreviations after Kretz 1983. PPL: plane polarizedlight; XPL: cross-polar light; S2: schistosity associated with D2 deformation. Sample 1: (a) Photograph of the sample stub which wasirradiated and dated. Numbers refer to analyses given in Table 2, and white overlays show surface areas from which argon wasextracted. (b) Close-up view of the microprobe thin section made from the same stub as Fig. 5 (a) (see text for detail; its locationis given by the frame in Fig. 5a): carpholite crystals realigned along S2 (some fresh, tiny carpholite needles hosted in quartz arestill visible on the lower left portion of the photograph) were partially replaced by a mixture of chlorite and phengite. (c) Enlargementfrom Fig. 5 (b): ovoid-shaped phengite crystals are recognizable among chlorite. Analysis 41 corresponds to the crystal at the headof arrow. (d) Cross-polar light view of Fig. 5 (c). Sample 3*: (e,f) Microphotograph and interpretative sketch, respectively, of thethin section that was dated: oblique D3 shear zones are readily distinguishable.(g,h) Enlargement (PPL and XPL) from Fig. 5 (e):newly formed phengite and chlorite crystals crosscut the D2 schistosity.


older ages (c. 60–56 Ma; Fig. 8). The 1r error of3–5 Ma for the latter phengite is due to the weak argonsignal obtained with only one crystal. Phengite fromthe foliation yields an intercept age of 51.9 ± 1.6 Ma(Fig. 8) with an initial 40Ar ⁄ 36Ar ratio of 291 ± 9.

Samples 14–16 (pre-D2)

These two little-deformed samples come from the coreof a metre-scale boudin of calcschists limited by twoD2 shear zones. Hence they predate D2, but are littleaffected by D2. Both samples contain the character-istic static peak-temperature chloritoid-phengite-chlorite assemblage in the P–T trajectory (c. 450 �C;Fig. 2, Table 1), and contain carpholite only as relicswithin quartz grains devoid of prograde phengite. Theonly difference between the samples is that no retro-gression can be detected in sample 16 (Fig. 7c;constant phengite composition with Si ¼ 3.5 pfu;Table 1), whereas different phengite compositions arefound in sample 14 (Fig. 7a,b; 3.35 < Si < 3.5;Table 1).

There is a large dispersion in ages for samples 14 and16, from 66.2 to 46.9 Ma (8 ages in the range 58–51 Ma), and from 71.0 to 59.3 Ma (plus one valueat 55.0 Ma; 1r c. 1 Ma), respectively. These data(Table 2) do not show any important and significantvariations in the 37Ar ⁄ 39Ar and 38Ar ⁄ 39Ar ratios, or inatmospheric contribution, that could be correlatedwith the age dispersion. In both cases the data do notfit linearly in the correlation plot. Sample 16 has atentative intercept age of 58.9 ± 4.7 Ma based on thesix least discordant ages which range from 71 to64 Ma, with an initial argon ratio of 663 ± 170.However, the large errors attached to these valuespreclude any definite conclusion as to the meaning ofboth ages and 40Ar ⁄ 36Ar intercepts, i.e. on the presenceor not of excess argon in the dated mica. The youngerand more scattered ages for sample 14, togetherwith scattered phengite compositions, point to laterpartial phengite recrystallization (likely during D2),in contrast with sample 16 that preserves olderages and more clustered, celadonite-richer phengitecompositions.

Fig. 5. (Cont’d).

6 06 P . A G AR D E T A L .

Fig. 6. Two samples representative of D2 deformation (samples 8 & 12*), and one sample representative of D2 including relicsformed prior to D2 (sample 12; phengite associated with carpholite from the peak of pressure). Same abbreviations as in Fig. 5. Sample8: (a,b) close-up view of the section showing oriented, parallel crystals of phengite and chlorite formed at the expense of carpholitealong the S2 foliation. Sample 12*: (c) chloritoid-bearing phengite ⁄ chlorite matrix oriented along S2. Large, neighbouring quartzcrystals host relic carpholite needles. Sample 12: (d–e) chloritoid-phengite-chlorite matrix showing a large, inclusion-rich quartzporphyroclast. S2 foliation is visible, although less clearly than in Fig. 6 (a–c). (f) enlargement from Fig. 6 (e), showing inclusionsin quartz: carpholite is associated with early phengite which were dated (analyses 136–138, Table 2).


Sample 19 (D2-D3)

In this sample, phengite is aligned with chlorite in thegarnet–epidote ± calcite-bearing S3 foliation. Relicsof chloritoid and earlier higher pressure (and Si4+)phengite are found in adjacent domains. It was difficultto identify which phengite population was dated (cir-cles on Fig. 8, as for sample 5), so this sample a prioriyields data pertaining to D3 and D2. The ages rangebetween 48.5 and 36.6 Ma, with two groupings at48.5–45.8 Ma and 39.2–36.6 Ma (plus one inter-mediate value at 42.2 Ma). High 37Ar ⁄ 39Ar ratios(Table 2) are probably due to the degassing of epidoteand ⁄ or calcite in the matrix, but again no systematiccorrelation between the variation of these ratios and

the age dispersion was observed. The intercept agefor the younger group is 37.6 ± 1.2 Ma (40Ar ⁄36Ar ¼ 309 ± 28; Fig. 8b), while the data clusters forthe older group, precluding any linear extrapolation.

Sample 19* (D2 + pre-D2)

This sample is a few metres distant from sample 19,where D2 is little affected by D3. Some of the phengiteoriented along the S2 foliation contain cores that havea distinct phengite composition (Si4+c. 3.53; Fig. 7d;Table 1) and therefore likely predate D2. Laser probeinvestigations on these two populations yield agesbetween 46.1 and 42.2 Ma for matrix D2 phengite(analyses 32–34; 1r < 1 Ma), and 56.0–53.7 Ma for

Fig. 7. Two samples showing metamorphic assemblages formed prior to D2 (samples 14 & 16), and one sample representative of D2deformation yet preserving early phengite relics (sample 19*). Same abbreviations as in Fig. 5. Sample 14: (a) characteristic disoriented,almost equant chloritoid–phengite–chlorite assemblage not reworked by D2. (b) chloritoid–phengite–chlorite assemblage littlereworked by D2: a preferred left–right orientation is however, noticeable. Sample 16: (c) characteristic disoriented, almost equantchloritoid-phengite-chlorite assemblage not reworked by D2. Sample 19*: (d) rough sketch (the sample was unfortunately destroyed)showing the S2 foliation underlined by phengite flakes, some of which show inclusions ⁄ cores with a distinctive chemical compositioninterpreted as phengite crystals predating D2.

6 08 P . A G AR D E T A L .

Table 2. Full results of argon data; %atm: proportion of atmospheric argon.

Thin sect. N� Apparent Age 1r 40* ⁄ 39 1r 36 ⁄ 40 1r 39 ⁄ 40 1r 37 ⁄ 39 1r 38 ⁄ 39 1r %atm

*1000 *100 *1000 *1000

1B 40 41.7 0.6 1.627 0.024 0.715 0.022 48.478 0.514 1.855 2.646 0.274 0.006 21.12

41 37.7 0.4 1.470 0.018 0.765 0.027 52.657 0.339 14.027 1.792 0.270 0.009 22.60

42 42.1 0.7 1.643 0.027 0.939 0.028 43.974 0.415 13.905 1.664 0.398 0.010 27.74

43 41.4 0.9 1.615 0.037 0.743 0.053 48.337 0.443 14.002 1.565 0.285 0.021 21.95

74 38.9 1.1 1.517 0.045 0.709 0.067 52.104 0.727 0.000 3.624 0.253 0.027 20.94

75 36.6 1.5 1.426 0.061 1.399 0.070 41.110 0.674 0.000 5.184 0.636 0.030 41.36

91 41.8 0.6 1.628 0.025 1.190 0.032 39.828 0.155 1320.118 11.817 0.558 0.016 35.16

92 42.5 1 1.656 0.041 1.638 0.031 31.150 0.387 863.952 14.057 0.985 0.020 48.41

93 39.1 0.8 1.522 0.033 0.830 0.021 49.606 0.853 611.325 9.372 0.312 0.006 24.52

3*A 53 39.8 0.4 1.552 0.018 0.778 0.020 49.626 0.371 184.919 2.062 0.293 0.007 23.00

56 37.4 0.4 1.455 0.015 0.475 0.009 59.071 0.516 7.536 0.319 0.150 0.002 14.05

57 37.3 0.7 1.452 0.029 0.465 0.016 59.389 1.057 7.366 0.287 0.146 0.005 13.74

58 36.5 0.5 1.420 0.019 0.410 0.020 61.870 0.646 2.899 0.919 0.124 0.005 12.13

95 37.7 0.6 1.466 0.024 0.623 0.028 55.649 0.679 21.409 1.911 0.208 0.011 18.41

96 37.5 0.6 1.460 0.023 0.552 0.040 57.329 0.374 5.286 1.675 0.179 0.014 16.30

97 36 0.4 1.400 0.017 0.540 0.032 60.022 0.278 7.765 1.265 0.167 0.010 15.96

98 36.2 0.9 1.410 0.038 0.677 0.047 56.744 1.012 13.730 1.114 0.223 0.016 20.01

5A 45 42.4 0.6 1.654 0.023 1.125 0.026 40.364 0.256 2675.098 19.731 0.520 0.134 33.24

47 37.7 0.6 1.468 0.025 0.424 0.042 59.581 0.487 5.808 1.334 0.133 0.014 12.53

48 45.3 1 1.766 0.039 0.642 0.052 45.886 0.469 1590.627 14.078 0.262 0.020 18.97

49 38.5 0.7 1.500 0.030 0.324 0.055 60.301 0.504 255.447 4.732 0.100 0.016 9.57

116 40.2 0.3 1.567 0.011 0.830 0.008 48.177 0.253 0.000 0.002 0.321 0.004 24.52

117 48.7 0.6 1.904 0.025 0.903 0.027 38.511 0.256 9.567 1.747 0.439 0.012 26.69

126 38 0.4 1.479 0.016 0.532 0.014 56.972 0.512 3.094 0.663 0.175 0.005 15.71

8A 12 46.9 2.3 1.833 0.089 0.664 0.126 43.852 0.567 0.000 6.841 0.283 0.047 19.63

13 45.2 3.2 1.765 0.126 1.332 0.134 34.350 0.703 11.754 9.876 0.731 0.063 39.37

17 45.6 2.6 1.778 0.103 2.057 0.064 22.064 0.599 27.999 2.995 1.739 0.068 60.78

50 45.3 1 1.767 0.038 0.470 0.057 48.714 0.433 4.939 2.432 0.180 0.023 13.90

51 45.7 0.6 1.784 0.024 0.332 0.027 50.551 0.493 97.406 1.858 0.123 0.011 9.81

77 45.8 1.2 1.787 0.049 0.369 0.063 49.837 0.838 39.346 4.002 0.139 0.028 10.92

79 44.5 2 1.738 0.078 1.143 0.073 38.103 0.894 19.384 6.305 0.561 0.030 33.78

81 42.2 1.1 1.646 0.042 1.293 0.048 37.554 0.305 24.450 5.496 0.644 0.026 38.19

12*A 2 40.1 0.6 1.564 0.025 0.469 0.037 55.082 0.494 0.000 2.044 0.161 0.015 13.86

4 46.7 2.6 1.824 0.103 0.234 0.170 51.027 0.788 0.000 15.799 0.087 0.065 6.93

5 43.3 3.1 1.689 0.124 0.443 0.211 51.457 0.661 0.000 14.801 0.162 0.081 13.09

6 48.8 1.7 1.906 0.069 0.414 0.086 46.055 0.932 32.515 6.975 0.169 0.036 12.21

7 46.4 2.4 1.811 0.094 0.692 0.135 43.926 0.526 1.147 9.129 0.293 0.061 20.45

87 49.3 0.7 1.926 0.027 0.366 0.015 46.300 0.570 0.000 1.523 0.149 0.007 10.83

88 51.3 0.6 2.007 0.026 0.287 0.018 45.603 0.505 3.139 1.038 0.117 0.007 8.49

89 44.5 0.9 1.737 0.035 0.442 0.050 50.062 0.505 0.854 1.739 0.166 0.010 13.05

90 45.5 0.7 1.776 0.027 0.451 0.038 48.799 0.371 3.170 1.448 0.173 0.015 13.31

12A 131 49.2 0.7 1.921 0.028 0.467 0.036 44.867 0.324 0.000 0.000 0.197 0.013 13.81

132 51.8 0.5 2.025 0.021 0.173 0.017 46.870 0.407 1.459 1.205 0.068 0.007 5.11

133 45.9 1.1 1.792 0.045 0.643 0.062 42.439 0.274 8.380 4.952 0.361 0.030 23.95

134 51.8 1.2 2.025 0.047 1.272 0.044 30.820 0.281 0.000 0.000 0.771 0.025 37.58

135 53.6 0.6 2.098 0.026 0.617 0.016 38.978 0.380 0.000 0.002 0.296 0.008 18.22

136 56.9 2.8 2.226 0.110 1.103 0.101 30.283 0.536 0.000 0.000 0.688 0.063 32.58

137 59.9 5.4 2.346 0.214 0.182 0.274 40.339 1.223 0.000 0.000 0.094 0.141 5.36

138 55.9 2.8 2.186 0.111 2.285 0.048 14.862 0.308 17.905 8.178 2.868 0.582 67.51

139 46.8 1.5 1.828 0.061 2.398 0.030 15.935 0.147 14.324 2.388 2.813 0.037 70.87

140 51.7 0.8 2.022 0.031 1.040 0.020 34.270 0.343 6.203 1.611 0.567 0.009 30.72

14A 20 49.3 1 1.927 0.040 0.596 0.042 42.761 0.560 3.430 3.418 0.259 0.019 17.60

21 54.3 1.1 2.123 0.043 0.523 0.039 39.836 0.582 0.000 3.656 0.248 0.022 15.44

22 53.4 0.6 2.088 0.023 0.307 0.029 43.554 0.221 0.000 1.757 0.131 0.011 9.07

25 46.9 0.6 1.833 0.024 0.500 0.018 46.513 0.529 5.436 1.962 0.202 0.007 14.76

26 64.5 1.1 2.531 0.043 0.657 0.021 31.831 0.447 14.222 3.650 0.389 0.014 19.43

27 57.3 0.5 2.242 0.020 0.448 0.008 38.702 0.287 3.460 0.313 0.217 0.003 13.24

28 58.1 2.5 2.271 0.099 0.949 0.074 31.680 0.821 14.403 4.818 0.555 0.051 28.04

99 51.4 1 2.009 0.039 0.382 0.055 44.161 0.250 9.682 2.862 0.160 0.022 11.29

100 52.7 1 2.060 0.041 0.426 0.057 42.445 0.172 2.972 1.965 0.187 0.028 12.58

101 66.2 0.8 2.599 0.033 0.311 0.015 34.928 0.406 0.000 0.879 0.168 0.009 9.20

102 62.7 0.7 2.460 0.030 0.406 0.024 35.766 0.312 0.933 2.564 0.211 0.015 12.00

103 50.9 0.8 1.990 0.031 0.357 0.039 44.945 0.386 0.000 2.353 0.149 0.018 10.56

104 55.9 0.7 2.188 0.027 0.363 0.026 40.807 0.332 2.013 1.315 0.165 0.011 10.73

16A 64 66.6 0.4 2.615 0.015 0.349 0.006 34.291 0.176 0.000 0.537 0.190 0.004 10.32

65 55 1.1 2.154 0.044 0.510 0.047 39.431 0.438 0.000 0.754 0.243 0.021 15.07

66 66.9 0.7 2.628 0.028 0.295 0.023 34.735 0.251 1.201 1.193 0.160 0.011 8.72

68 71 0.4 2.792 0.015 0.394 0.008 31.651 0.144 0.000 0.628 0.232 0.005 11.63

69 61.4 0.7 2.406 0.029 0.524 0.020 35.128 0.316 0.000 1.257 0.278 0.010 15.49

70 68.2 0.7 2.676 0.030 0.303 0.018 34.019 0.317 0.000 0.974 0.166 0.010 8.95

71 68.1 0.4 2.674 0.017 0.275 0.007 34.358 0.190 0.000 0.566 0.149 0.004 8.12


core-rich phengite (plus one value at 61.2 Ma; 1rc. 1.5 Ma). The 37Ar ⁄ 39Ar and 38Ar ⁄ 39Ar ratios arelow compared to sample 19, suggesting minor contri-bution from chlorite and Ca-bearing phases. Anintercept age of 53.7 ± 1.7 Ma was calculated withthe older phengite group, corresponding to an initialargon ratio of 337 ± 53, statistically indistinguishablefrom the atmospheric value of 295.5.

DISCUSSION

Interpretation of the results

Isotopic closure

Geochronological data are classically interpreted torecord closure of the isotopic system at a specific

temperature which can vary from about 800–100 �Cor less, depending on the isotopic diffusion proper-ties of the mineral (e.g. Dodson, 1973; Cliff et al.,1998; Villa, 1998). Many previous studies assumethat argon closure in phengite occurs at a tempera-ture close to that of muscovite argon retention, i.e.350–430 �C depending on cooling rate, grain size andother effects controlling closure (e.g. Wijbrans &McDougall, 1986; Hames & Bowring, 1994; Reddyet al., 1996).A number of Alpine studies have shown that the

closure temperature for phengite is still uncertain,largely due to chemical and crystallographic variationsbetween phengite end-members (e.g. Chopin&Maluski,1980; Scaillet et al., 1992; Scaillet, 1998; and the dis-cussion by Dahl, 1996a). These studies have empha-sized the complexity of controlling factors on argon

Fig. 8. Apparent age results for each sampleare presented by zone, from west to east,differentiating samples where chloritoid ispresent (squares) or absent (diamonds).Symbols follow the chronological con-straints based on structural criteria, whichare shown at the bottom of the figure: grey,black and white symbols correspond to agesobtained on phengite formed before D2,during D2 or during D3, respectively.Intercept ages are given as a thick dottedline, for samples for which they could becalculated. Transparent overlays super-imposed on the data (light grey, grey, andwhite overlays relate to phengite formedprior to D2, during D2 or during D3,respectively) underline the consistency of theresults, namely: (i) phengite formed earlier(when several generations were clearlyrecognized, e.g. samples 12 & 19*) effectivelyyield older apparent ages (ii) D2 ages aredistinct from D3 ages (iii) ages across strikeare broadly similar for each tectonic event.See text for detailed comments.

Table 2. (Cont’d).

Thin sect. N� Apparent Age 1r 40* ⁄ 39 1r 36 ⁄ 40 1r 39 ⁄ 40 1r 37 ⁄ 39 1r 38 ⁄ 39 1r %atm

72 63.7 1.4 2.500 0.055 0.258 0.045 36.959 0.590 6.837 1.041 0.130 0.023 7.62

73 59.3 1 2.321 0.040 0.496 0.046 36.766 0.212 9.724 3.331 0.250 0.026 14.66

19A 59 45.8 0.8 1.786 0.033 1.305 0.034 34.403 0.235 3611.694 41.414 0.709 0.018 38.57

60 38.1 0.5 1.485 0.019 0.718 0.018 53.040 0.512 293.509 4.731 0.254 0.007 21.22

63 48.5 0.4 1.892 0.016 1.255 0.016 33.253 0.099 1784.370 14.867 0.705 0.009 37.08

82 36.6 2.5 1.424 0.100 0.557 0.194 58.674 0.787 465.009 18.204 0.181 0.060 16.46

83 38.2 3.9 1.486 0.154 1.125 0.226 44.925 0.823 273.633 15.783 0.456 0.098 33.24

84 39.2 2.6 1.527 0.102 1.673 0.091 33.120 0.802 822.438 16.708 0.943 0.050 49.43

85 42.2 2.4 1.646 0.094 1.209 0.117 39.049 0.601 509.677 14.516 0.575 0.056 35.73

86 45.9 1.1 1.793 0.043 0.786 0.030 42.809 0.792 264.470 6.291 0.345 0.012 23.23

19*A 29 54.7 2.6 2.142 0.104 1.128 0.089 31.122 0.683 8.693 8.573 0.676 0.049 33.34

30 61.2 1.9 2.397 0.078 0.625 0.040 34.014 0.920 9.357 4.181 0.345 0.021 18.47

31 55.5 2.3 2.171 0.091 0.599 0.109 37.919 0.502 0.000 9.277 0.296 0.052 17.70

32 46.1 0.6 1.801 0.025 0.568 0.032 46.206 0.327 28.541 1.998 0.230 0.012 16.80

33 42.2 0.4 1.646 0.018 0.338 0.016 54.676 0.511 17.161 0.919 0.115 0.005 10.00

34 45.7 1 1.785 0.041 0.350 0.041 50.230 0.858 0.000 1.008 0.131 0.015 10.35

35 53.7 0.6 2.099 0.025 0.291 0.021 43.542 0.388 0.000 0.896 0.126 0.008 8.61

## 56 0.8 2.192 0.033 0.436 0.012 39.744 0.509 9.182 2.100 0.206 0.004 12.88

6 10 P . A G AR D E T A L .

diffusion, both intrinsic (composition of the host min-eral and grain size, crystal polytypes,…) and extrinsic(temperature, time, deformation, pressure, fluids), anddeveloped a new approach through modelling (e.g.ionic porosity models, Dahl, 1996b). Clearly, a closuretemperature value around c. 350–430 �C for phengiteis only valid as a first approximation, and furtherappropriate diffusion experiments are needed. Thecomplexity of alpine radiochronological data probablystems from such uncertainties, and also from possibleincorporation of excess argon in UHP phengite underappropriate conditions (e.g. Arnaud & Kelley, 1995;Ruffet et al., 1995; Scaillet, 1996; Giorgis et al., 2000and references therein).

Temperature constraints

The temperature constraints (± c. 30 �C) for thecarpholite zone are <380 �C, and <450–500 �C forthe chloritoid zone in the east (Fig. 2). Thus thephengite ages from the carpholite zone (samples 1, 5 &8) likely represent crystallization ages. Accordingly,referring to the intercept age of each sample, we sug-gest that D3 phengite crystallized mainly in the interval38–35 Ma and D2 phengite at c. 45 ± 2 Ma.In contrast, phengite ages from the chloritoid zone

(samples 12, 12*, 14, 16, 19 & 19*) may either representcooling ages or crystallization ages, depending on theexact closure temperature assumed for this phengite.According to the diffusion equations, and taking intoaccount the pressure effect on closure temperature(Harrison & McDougall, 1985; Monie, 1990), it islikely that phengite from the chloritoid zone, thatreached pressures about 5 kbar higher than thecarpholite zone (Fig. 2), began to retain argon at amore elevated temperature. Assuming a theoreticalpressure dependence estimate of 6–7 �C ⁄kbar (Dahl,1996b), the closure temperature would then increase byabout 30–40 �C.Figure 9(a) shows the range of temperatures at

which closure may have taken place, given uncertain-ties on the exact closure temperature assumed formuscovite (e.g. Hames & Bowring, 1994; Kirschneret al., 1996; Villa, 1998) and on the pressuredependence of phengite. It shows that it is difficult totell whether HP phengite from zone IV and HP and D2phengite from zone V formed below or above theclosure temperature, hence whether or not theyremained closed to argon diffusion throughoutexhumation. Overall, the dispersion of the data shownby samples 14, 16 and 19* rather suggests that some ofthe ages may represent cooling ages rather than cry-stallization ages, but this cannot be strictly demon-strated. The interpretation of 40Ar ⁄ 39Ar ages from theeastern phengite in terms of crystallization ages istherefore convenient, though probably only relevant asa first approximation. These ages range from37.6 ± 1.2 Ma for D3 phengite (sample 19) to 65–54 Ma for pre-D2 phengite (samples 12, 14, 16 & 19*).

Main conclusions

Several conclusions can be drawn from Fig. 8:(1) Phengite assigned to D3, D2 or before D2,

respectively, yield progressively older ages. Absolutechronology therefore faithfully reproduces the relativechronology based on tectono-metamorphic observa-tions: D3 < D2 < pre-D2.(2) For each tectonic event, comparable ages are

obtained, particularly for D3 (samples 1, 3* & 5:38.2 ± 1.2, 35.5 ± 1.1, and 36.2 ± 1.1 Ma, interceptages, respectively) and D2 (sample 8: 44.9 ± 1.5 Ma,intercept age; sample 12*: 51–43 Ma, apparent ages).This conclusion also pertains to phengite formedprior to D2, although the scatter of ages is somewhatlarger (sample 14: mainly 58–51 Ma, apparent ages;sample 16: 71–55 Ma, apparent ages; sample 12early:51.9 ± 1.6 Ma, intercept age; sample 19*early: 53.7 ±1.7 Ma, intercept age).(3) These data further confirm the existence of two

distinct deformation stages (Agard et al., 2001a), sincethere is virtually no overlap between ages obtained forD3 (samples 1 & 3*: 38–35 Ma) and D2 (samples 8 &12*: 51–45 Ma).(4) In samples with two phengite generations as

based on textural and chemical grounds and datedseparately (12, 19 & 19*), older ages are systematicallyobtained for early phengite (Fig. 8).We therefore propose that D2 took place during

the period 51–45 Ma, and D3 between 38 and 35 Ma.Events predating D2 occurred during the period71–51 Ma (samples 14, 16, 12early & 19*early). Morespecifically, the early phengite of sample 12 suggeststhat the HP event in the centre of the study area tookplace at 60–56 Ma. For samples 1, 14 and 16 con-taining mainly a single generation of HP or LPphengite, there are several causes that might con-tribute to the scatter in ages: (i) variations of theinitial argon ratio (ii) duration of the metamorphicevent (for samples 14 & 16 whose phengite predateD2) and (iii) impact of cooling vs. crystallization(samples 14 & 16) (iv) interlayer deficiency inpotassium (for sample 1) (v) contamination due tothe ablation of adjacent K-poor minerals (e.g.chlorite) or (vi) minor amounts of excess argon.The latter (and most debated) source of scatter isdiscussed in the following section.

Is excess argon present?

Incorporation of excess 40Ar (extraneous or inheritedargon) at the time of growth (at least above the closuretemperature) and ⁄ or during one of the recrystallizingstages, or from diffusion through grain boundaries(Reddy et al., 1996) can give rise to meaningless ages.While the amount of atmospheric argon can be eval-uated with some precision, it is not the case for theradiogenic excess component (e.g. Ruffet et al., 1995)which is potentially sourced from degassing of


pre-existing minerals or from fluids circulating duringmetamorphism.For the SL complex, it is first worth stressing that

the following systematics would not be observed if

excess argon was important (for similar conclusions,see Bosse et al., 2000; de Sigoyer et al., 2000):(1) The results of absolute chronology are remark-

ably consistent with the relative tectono-metamorphic

Fig. 9. (a) Proposed P–T–t paths: results are plotted as a function of pressure and temperature, on the P–T paths of Fig. 2 (b). Valuesin square symbols are apparent ages; these circles are isochron ages. Thin solid line: evolution of the closure temperature assuming aclosure temperature of 350 �C at atmospheric pressure and a pressure dependence of 6–7� kbar)1 (Dahl, 1996b) for phengite. Givenuncertainties commonly reported on such estimates (± 50 �C), the exact closure temperature probably falls within the dashedrange. This suggests that HP phengite from zone IV and HP and D2 phengite from zone V may have formed above the closuretemperature, thus partly yielding cooling ages rather than crystallization ages. See text for discussion. (b) Comparison betweenpressure(depth)-time paths obtained for the Schistes Lustres samples and for UHP rocks from Dora Maira (after Duchene et al.,1997a). Exhumation velocities are one order of magnitude smaller for the former. Fission-track data on zircon (FT ⁄Zr) are fromSchwartz (2001). The thin lines labelled with sample numbers from this study connect data obtained for the different phengite in eachof those samples. (c) Implications of the results of this study with respect to the timing of HP–UHP and greenschist facies meta-morphism in the Western Alps (data as in Fig. 3). Constraints for the oceanic domains are greatly improved. See text for details.

6 12 P . A G AR D E T A L .

chronology established independently on the basis ofmacro- and microtextures: D2 phengite are indeedsystematically older than D3 phengite. Further, agesobtained for the SL complex (c. 60–35 Ma) fit in thesame range as previous radiometric (K ⁄Ar) data on SLphengite (see Time constraints and Table 1), or asestimates for the adjacent ophiolitic Monviso unit (60–40 Ma). Therefore, if excess argon exists, it would havea very homogeneous composition at the scale of thevarious oceanic units, which seems very unlikely.(2) Ages deduced for D3 and D2 are broadly similar

within the same zone (e.g. samples 12 & 12* from zoneIII) and across strike (e.g. timing of D2 for zones II etIII; Fig. 9a).Reported evidence of excess argon is in fact mostly

restricted to little deformed, polymetamorphic con-tinental crustal rocks that were subducted to HPconditions (DMM: Arnaud & Kelley, 1995; Scaillet,1996; Scaillet, 1998 and references therein; Sesia:Ruffet et al., 1995; Dabie Shan: Li et al., 1994; Giorgiset al., 2000), where the prograde destabilization ofancient minerals results in excess argon being releasedand potentially trapped in HP phengite. In such envi-ronments, the circulation of fluids is limited (Giorgiset al., 2000; Philippot & Rumble, 2000 for a review),and the existence of argon pods was hypothetized toaccount for reports of excess argon (e.g. Scaillet,1996). The existence of metre-scale fluid and mineraltransfer in the SL complex (Agard et al., 2000) arguesagainst the preservation of possible chemically con-trasted argon pods. On the other hand, the absence ofsignificant large scale fluid circulation in the SL com-plex (Henry et al., 1996; Agard et al., 2000) arguesagainst a systematic resetting (or shift) of the phengiteradiometric data. Though minor amounts of excessargon cannot be ruled out for some samples (e.g.samples 14, 16), our results suggest that an overallcontamination by excess argon in the SL samples isunlikely.

The timing of HP metamorphism and exhumationin the Schistes Lustres complex

The above data have discriminated between twoexhumation stages, D2 and D3 (Agard et al., 2001a). Itis proposed that D2 took place during 51–45 Ma, andD3 between 38 and 35 Ma. The HP event, marked bythe phengite-carpholite assemblage hosted in quartz(sample 12), probably occurred at 60–56 ± 4 Ma, atleast in the central part of the study area. At the otherend, recent fission track data for the SL complex yieldages on zircon in the range 28–20 Ma (Schwartz,2001).These results are plotted on Fig. 9(a,b) to estimate

cooling rates and exhumation velocities for the SLcomplex. The figure shows that both for the carpholiteand chloritoid zones, cooling rates did not exceeded10 �C yr)1 and were most probably in the range of5 ± 2 �C yr)1 throughout the studied area. These

estimates are not particularly high, in contrast withcooling rates up to 80 �C yr)1 recently proposed for thenearby Dora Maira crystalline massif (Gebauer, 1999),which suggests that different cooling mechanisms pre-vailed in the two areas. Exhumation rate estimates, onthe order of 1–2 mm yr)1, fall between velocitiesattributed to erosion processes (c. 0.05–0.5 mm yr)1,with contrasted rates up to 1 cm yr)1; Ring et al.,1999b) and velocities deduced for the (early stages ofthe) exhumation of UHP rocks (several cm ⁄ year;Duchene et al., 1997b; Perchuk & Philippot, 1997;Amato et al., 1999; Perchuk & Philippot, 2000 for areview). Despite uncertainties on pressure estimates(c. ± 1–2 kbar; Agard et al., 2001b), and on theprecise age of D2 and D3 (± 2–3 Ma; Figs 9a,b),these first order constraints preclude fast exhumationvelocities for the SL complex.Since D2 and D3 are opposite vergence exhumation

stages (top-to-the East and top-to-the West, respect-ively), somewhat diachronous ages would be expectedfor each stage along the transect (progressivelyyounger D2 ages in the east and younger D3 ages inthe west, respectively). Unfortunately, the scatter ofages for D3 or D2 (cumulated error bars c. 2–3 Ma;Fig. 9a) precludes any precise evaluation of the extentof spatial diachronism for these events.Exhumation and cooling velocities documented here

are comparable to exhumation velocities found inpresent day accretionary wedges (c. 0.8 mm yr)1, asdeduced from FT data for the Olympic mountains;Brandon et al., 1998). Given the nature of the materialand the continuity of P–T estimates, the SL complexshould indeed probably be seen as of palaeoaccre-tionary complex (Platt, 1993 and references therein;Ring et al., 1999a). Yet, details of the palaeogeo-dynamic setting for the exhumation of the SL complexremain somewhat unclear, in part because the geo-metry during HP deformation was obscured by laterdeformation. Further research should focus on deter-mining the extent of underplating and ⁄ or extensionalmovements, the respective influence of erosion, normalfaulting and ductile thinning (Ring et al., 1999b),and whether the maximum pressure reached (Agardet al., 2001a; Schwartz, 2001) compares to maximumpressure conditions attainable at the bottom of anaccretionary prism.

Implications for the Western Alps and for the ageof the HP–UHP event

These new radiochronological data better constrainthe timing of the tectono-metamorphic events in theSL complex, when compared to previous results forthe Cottian Alps or for other parts of the SL complex(Figs 3 & 9c). In particular, they constrain the mini-mum age of HP metamorphism to c. 55 Ma, andthe greenschist stage to c. 35–40 Ma (Fig. 3). Theseresults also demonstrate that the scatter of agesdocumented by previous studies for the SL complex


was a result of dating S2 + S3 phengite from therocks (Fig. 3).These data on the age of HP metamorphism in the

SL complex (c. 55–60 Ma) can be compared withconstraints for the adjacent domains in the WesternAlps (Figs 3 & 9d). There are three main age groupingsfor the ICM (Fig. 3; Hunziker et al., 1992; VonBlanckenburg & Davies, 1995): (i) c.100 Ma (studiesnumbered 17, 18, 19 & 28, Fig. 3) (ii) c. 70 Ma (studies23 & 28, Fig. 3) (iii) c. 30–40 Ma (studies 9, 21 & 22,Fig. 3). The timing of the west-vergent greenschistfacies metamorphic stage (D3) in the SL complex at40–35 Ma gives data identical to that for the ICM(studies 16, 18 & 28, Fig. 3). This conclusion empha-sizes the importance of the top-to-the West extensionaldeformation taking place at 40–35 Ma all along theWestern arc, that predated both the onset of collisionand generalized extensional movements documented inthe Mediterranean area (Jolivet & Faccenna, 2000). Italso strengthens the view that ages for the ICM at40–35 Ma for the greenschist facies and <35 Ma(study 9) for the UHP stages may not be entirelycompatible, unless exhumation of the ICM proceededat an extremely fast rate (c. 10 cm yr)1; Fig. 9c), incontrast with the exhumation rates inferred for theSchistes Lustres unit.Rapid exhumation (Amato et al., 1999) along a

lithospheric scale east-vergent extensional shear zoneoperating during the period 45–36 Ma has recentlybeen proposed (Wheeler & Butler, 1993; Reddy et al.,1999; Wheeler et al., 2001) for the coesite bearingUHP metapelites from the Zermat–Saas ⁄Cignana area(Reinecke, 1991). This east-vergent extensional defor-mation is similar to the D2 phase here and the tectonicepisode documented by Ballevre & Merle, 1993) in thenorth-western Alps, but appears to have lasted longerand developed at a much faster rate. These differencesalong the Alpine chain suggest that either palaeogeo-dynamic settings and exhumation mechanisms differradically for HP and UHP metapelites, that importantlateral variations along this arcuate chain cannot beignored, and ⁄or that exhumation stages rather thanUHP metamorphism were dated in some previousstudies. Intermediate retrograde stages such as D2were not dated in the ICM (except by Freeman et al.,1997), so that the comparison with the SL complexunfortunately cannot be made.These SL data are used in a geodynamic model for

the Western Alps (Fig. 10), that follows a classicalscheme. South-eastward subduction of the Alpineocean, early subduction of Sesia (ages for both HP andgreenschist facies metamorphism in Sesia, c. 70 Maand 50–45 Ma in Fig. 3, respectively, are older thanfor the SL complex, e.g. Scharer et al., 1999), isfollowed by later underplating of the European margin(e.g. Coward & Dietrich, 1989; Michard et al., 1996;Stampfli & Marchant, 1997; Gebauer, 1999). The SLcomplex is considered as a palaeoaccretionary wedge,and the position of the ICM is problematic (Fig. 10a).

For the latter, our results do not directly discriminatebetween the two end-member models proposed forthe much debated UHP metamorphism in theDMM: early subduction (i.e. sometime between 110and 65 Ma) vs. late, very fast subduction (i.e. between40 and 35 Ma). In the latter case, very different sub-duction and exhumation dynamics are required for theSL complex and the DMM in order to account for thestriking contrast in the duration of their respectivemetamorphic evolution (Fig. 9b). The tectonic settingproposed for the exhumation of the SL complex duringD2 (Fig. 10b) is an east-vergent extensional removal ofthe overburden (with the obducted Chenaillet unitlying on the hanging wall of this detachment), associ-ated with underplating at depths. As supported by ageconstraints for the D3 stage in the SL complex(38–35 Ma) and by structural similarities betweengreenschist facies deformation in the SL complex and

Fig. 10. Tentative geodynamic evolution based on the radio-chronological data from the Schistes Lustres complex. Dottedlines: main detachment faults; black dot: approximate positionof the rocks from Assietta. The position of the Dora Mairamassif throughout orogeny remains problematic: the twoextreme, conflicting positions are given for each tectonic stage.See text for details.

6 14 P . A G AR D E T A L .

in the ICM (Philippot, 1990; Henry et al., 1993; Agardet al., 2001a), this D3 event is interpreted to becontemporaneous (and genetically associated) with thewest-vergent extensional deformation participating inthe exhumation of the ICM at 40–35 Ma (Fig. 10c;studies 16,18, Fig. 3).

CONCLUSIONS

Age constraints for the Schistes Lustres complex con-firm that HP metamorphism took place at c. 55 Mafollowed by two distinct exhumation stages, D2 andD3, at 51–45 Ma and 38–35 Ma, respectively.Most importantly, absolute chronology results

directly support the relative chronology establishedon textural and metamorphic grounds. There is inparticular a strong correlation between phengitechemistry (e.g. Si content), textural location and age,which demonstrates the usefulness of the 40Ar ⁄ 39Armethod, despite several reports of excess argon andgeneralized suspicion cast on 40Ar ⁄ 39Ar dating ofphengite in HP terranes (Scaillet et al., 1992; Scaillet,1998; Giorgis et al., 2000). The coherence of theresults presented here show that an overall contam-ination by excess argon can be ruled out for the SLmetapelites. Our data also suggest that 40Ar ⁄ 39Arages of high-pressure phengite formed at temperatureof 450 �C or slightly above are likely interpretable interms of crystallization ages rather than cooling ages,even when cooling and exhumation rates werenot specifically high. This is due to the preservation ofa relatively cold P–T gradient during the entiremetamorphic evolution, coeval with the permanentunderthrusting of cold rocks at depth preventingthermal reequilibration in the middle crust.Exhumation rates on the order of 1–2 mm yr)1 are

obtained for the SL complex, which are compatiblewith velocities documented for accretionary wedgesettings (Brandon et al., 1998). Exhumation mecha-nisms for the SL complex appear to be very differentfrom those involving the HP–UHP internal crystal-line massifs (among which the Dora Maira massif),but also from those involving UHP oceanic metap-elites (Amato et al., 1999; Wheeler et al., 2001).Although our study does not directly provide con-straints on the internal crystalline massifs, age con-straints for the D3 stage emphasize the importanceof the top-to-the West extensional deformationtaking place in the internal units at 38–35 Ma allalong the Western arc, which indicates that a 35-Maage for UHP metamorphism has to be consideredwith some caution.

ACKNOWLEDGEMENTS

The authors thank G. Ruffet and J. Wheeler forconstructive reviews. Special thanks are due toJ. Cassareuil for technical assistance.

REFERENCES

Agard, P., Goffe, B., Touret, J. & Vidal, O., 2000. Fluidevolution in HP metamorphic settings. A case study inblueschist and greenschist metapelites from the Western Alps.Contributions to Mineralogy and Petrology, 140, 296–315.

Agard, P., Jolivet, L. & Goffe, B., 2001a. Tectonometamorphicevolution of the Schistes Lustres complex: implications for theexhumation of HP and UHP rocks in the Western Alps.Bulletin de la Societe Geologique de France, 172(5), 617–636.

Agard, P., Vidal, O. & Goffe, B., 2001b. Interlayer and Sicontent of phengites in high-pressure carpholite-bearingmetapelites. Journal of Metamorphic Geology, 19, 1–20.

Allen, P., 1997. Earth Surface Processes. Blackwell Science,Oxford.

Amato, J. M., Johnson, C. L., Baumgartner, L. P. & Beard, B.L., 1999. Rapid exhumation of the Zermatt-Saas ophiolitededuced from high-precision Sm-Nd and Rb-Sr geochronol-ogy. Earth and Planetary Science Letters, 171, 425–438.

Arnaud, N. O. & Kelley, S. P., 1995. Evidence for excess argonduring high pressure metamorphism in the Dora MairaMassif (western Alps, Italy), using an ultra-violet laser ablat-ion microprobe 40Ar-39Ar technique. Contributions to Miner-alogy and Petrology, 121, 1–11.

Ballevre, M., Lagabrielle, Y. & Merle, O., 1990. Tertiary ductilenormal faulting as a consequence of lithospheric stacking inthe western Alps. Deep Structure of the Alps, 227–236, SocieteGeologique de France, Paris.

Ballevre, M. & Merle, O., 1993. The Combin fault: compres-sional reactivation of a late Cretaceous-early Tertiary detach-ment fault in the Western Alps. Schweizrische MineralogischePetrografische Mitteilung, 73, 205–227.

Beaumont, C., Ellis, S. & Pfiffner, A., 1999. Dynamics ofsediment subduction-accretion at convergent margins: short-term modes, long-term deformation, and tectonic implica-tions. Journal of Geophysical Research, 104, 17 573–17 601.

Bonhomme, M. G., Saliot, P. & Pinault, Y., 1980. Interpretationof potassium-argon isotopic data related to metamorphicevents in south-western Alps. Schweizrische MineralogischePetrografische Mitteilung, 60, 81–98.

Bosse, V., Feraud, G. & Ballevre, M., 2000. Geochronologie desroches eclogitiques du complexe de Champtoceaux (massifArmoricain, France): coherence des methodes U-Pb, Sm-Nd,Rb-Sr et 40Ar ⁄ 39Ar. 18e Reunion Des Sciences de la Terre,Societe Geologique de France, Paris.

Brandon, M. T., Roden-Tice, M. K. & Garver, J. I., 1998. LateCenozoic exhumation of the Cascadia accretionary wedge inthe Olympic mountains, northwest Washington state. Geolo-gical Society of America Bulletin, 110, 985–1009.

Brereton, N. R., 1970. Corrections for interfering isotopes inthe 40Ar ⁄ 39Ar dating method. Earth and Planetary ScienceLetters, 8, 427–433.

Burbank, D., Leland, J., Fielding, E., Anderson, R., Brozovic,N., Reid, M. & Duncan, C., 1996. Bedrock incision, rockuplift and threshold hillslopes in the northwestern Himalayas.Nature, 379, 505–510.

Burg, J. P. & Ford,M., 1997. Orogeny through time: an overview.In:Orogeny Through Time, Special Publication 121 (eds Burg, J.P. & F. M.), pp. 1–17. Geological Society, London.

Burov, E., Jolivet, L., Le Pourhiet, L. & Poliakov, A., 2001. Athermomechanical model of exhumation of HP and UHPmetamorphic rocks in Alpine mountain belts. Tectonophysics,342, 111–136.

Caby, R. & Bonhomme, G., 1982. Whole-rock and fine fractionK-Ar isotopic study of radiolarites affected by the alpinemetamorphism. 5th International Conference on Geochemistryand Isotope Geology, International Geological Congress,Tokyo.

Chopin, C., Henry, C. & Michard, A., 1991. Geology andpetrology of the coesite-bearing terrain, Dora Maira massif,Western Alps. European Journal of Mineralogy, 3, 263–291.


Chopin, C. & Maluski, H., 1980. 40Ar ⁄ 39Ar dating of high-pressure metamorphic micas from the Gran Paradiso area(Western Alps): evidence against the blocking tempera-ture concept. Contributions to Mineralogy and Petrology, 74,109–122.

Chopin, C. & Monie, P., 1984. A unique magnesiochloritoid-bearing, high-pressure assemblage from the Monte Rosa,Western Alps: petrologic and 40Ar-39Ar radiometric study.Contributions to Mineralogy and Petrology, 87, 388–398.

Cliff, R. A., Barnicoat, A. C. & Inger, S., 1998. Early Tertiaryeclogite facies metamorphism in the Monviso ophiolite.Journal of Metamorphic Geology, 16, 447–455.

Coward, M. & Dietrich, D., 1989. Alpine tectonics – anoverview. In: Alpine Tectonics, Special Publication, 45 (edsCoward, M., Dietrich, D. & Park, R. G.), pp. 1–29. GeologicalSociety, London .

Dahl, P. S., 1996a. The crystal-chemistry basis for Ar retentionin micas: inferences from interlayer partitioning and implica-tions for geochronology. Contributions to Mineralogy andPetrology, 123, 55–99.

Dahl, P. S., 1996b. The effects of composition on retentivity ofargon and oxygen in hornblende and related amphiboles: afield-tested empirical model. Geochimica et CosmochimicaActa, 60, 3687–3700.

De Wever, P. & Caby, R., 1981. Datation de la base des schisteslustres postophiolitiques par des radiolaires (Oxfordien-Kimmeridgien moyen) dans les Alpes Cottiennes (Saint Veran,France). Comptes Rendus de l’Academie Des Sciences, 292,467–472.

Delaloye, M. & Desmons, J., 1976. K-Ar radiometric agedeterminations of white micas from the Piemonte zone,french-italian western Alps. Contributions to Mineralogy andPetrology, 57, 297–303.

Deville, E., 1986. La klippe de la pointe du Grand Vallon(Vanoise-Alpes occidentales): un lambeau de metasediments aforaminiferes du Maastrichitien superieur couronnant lesnappes de �Schistes Lustres�. Comptes Rendus de l’AcademieDes Sciences, 303, 1211–1226.

Dobretsov, N. L., Sobolev, N. V., Shatsky, V. S., Coleman,R. G. & Ernst, W. G., 1995. Geotectonic evolution ofdiamondiferous paragneisses, Kokchetav complex, northernKazakhstan: the geologic enigma of ultrahigh-pressure crustalrocks within a Paleozoic foldbelt. Island Arc, 4, 267–279.

Dodson, M. H., 1973. Closure temperature in cooling geochron-ological and petrological systems. Contributions to Mineralogyand Petrology, 40, 259–274.

Duchene, S., Albarede, F. & Lardeaux, J. M., 1998. Mineralzoning and exhumation history in the Munchberg eclogites(Bohemia). American Journal of Science, 298, 30–59.

Duchene, S., Blichert-Toft, J., Luais, B., Telouk, P., Lardeaux,J. M. & Albarede, F., 1997a. The Lu-Hf dating of garnets andthe ages of the Alpine high-pressure metamorphism. Nature,387, 586–589.

Duchene, S., Lardeaux, J. M. & Albarede, F., 1997b. Exhuma-tion of eclogites: insights from depth-time path analysis.Tectonophysics, 280, 125–140.

Ernst, W. G. & Liou, J. G., 2000. Overview of UHPmetamorphism and tectonics in well-studied collisional oro-gens. In: Ultrahigh-Pressure Metamorphism and Geodynamicsin Collision-Type Orogenic Belts, International book series,Vol. 4 (eds Ernst, W. G. & Liou, J. G.), pp. 3–19, Final reportof the task group III-6 of the international lithosphere project,Geological Society of America, Boulder.

Freeman, S. R., Inger, S., Butler, R. W. H. & Cliff, R. A., 1997.Dating deformation using Rb-Sr in white mica: greenschistfacies deformation ages from the Entrelor shear zone, ItalianAlps. Tectonics, 16, 57–76.

Frey, M., Hunziker, J. C., Jager, E. & Stern, W. B., 1983.Regional distribution of white K-mica polymorphs and theirphengite content in the central Alps. Contributions toMineralogy and Petrology, 83, 185–197.

Gebauer, D., 1999. Alpine geochronology of the Central andWestern Alps: new constraints for a complex geodynamicevolution. Schweizrische Mineralogische Petrografische Mitt-eilung, 79, 191–208.

Gebauer, D., Schertl, H.-P., Brix,M.& Schreyer,W., 1997. 35Maold ultrahigh-pressure and evidence for very rapid exhumationin the Dora Maira Massif, Western Alps. Lithos, 41, 5–24.

Giorgis, D., Cosca, M. & Li, S., 2000. Distribution andsignificance of extraneous argon in UHP eclogite (Sulu terrain,China): insights from in situ 40Ar ⁄ 39Ar UV-laser ablationanalysis. Earth and Planetary Science Letters, 181, 605–615.

Gunnell, Y., 2000. Apatite fission track thermochronology: anoverview of its potential and limitations in geomorphology.Basin Research, 12, 115–132.

Hacker, B. R., Ratscbacher, L., Webb, L. et al., 2000. Exhuma-tion of UHP continental crust in east central China; lateTriassic-early Jurassic unroofing. Journal of GeophysicalResearch, 105, 13 339–13 364.

Hacker, B. R., Ratschbacher, L., Webb, L., Ireland, T., Walker,D. & Shuwen, D., 1998. U ⁄Pb zircon ages constrain thearchitecture of the ultrahigh-pressure Qinling-Dabie Orogen,China. Earth and Planetary Science Letters, 161, 215–230.

Hames, W. E. & Bowring, S. A., 1994. An empirical evaluationof the argon diffusion geometry in muscovite. Earth andPlanetary Science Letters, 124, 161–167.

Harrison, T. M. & McDougall, D. I., 1985. Diffusion of 40Ar inbiotite: Temperature, pressure and compositional effects.Geochimica et Cosmochimica Acta, 49, 2461–2468.

Henry, C., Burkhard, M. & Goffe, B., 1996. Evolution ofsynmetamorphic veins and their wallrocks through a WesternAlps transect: no evidence for large-scale fluid flow!. ChemicalGeology, 127, 81–109.

Henry, C., Michard, A. & Chopin, C., 1993. Geometry andstructural evolution of ultra-high-pressure and high-pressurerocks from the Dora-Maira massif, western Alps, Italy.Journal of Structural Geology, 15, 965–981.

Hess, J. C. & Lippolt, H. J., 1994. Compilation of K-Armeasurements on HD-B1 standard biotite, 1994 status report.In: Phanerozoic Time Scale (ed. Odin, G. S.), pp. 19–22,Bulletin de Liaison & Information, IUGS Subcom. Geochro-nologie, International Geological Correlation Programme,project 196.

Hunziker, J. C., Desmons, J. & Hurford, A. J., 1992. Thirty-twoyears of geochronological work in the Central and WesternAlps: a review on seven maps, pp. 1–59, Lausanne.

Hurford, A. J., 1990. Standardization of fission track datingcalibration: recommandation by the Fission Track WorkingGroup of the IUGS Subcommission on Geochronology.Chemical Geology, 80, 171–178.

Jager, E., 1973. Die Alpine orogenese im lichte der radiome-trischen Alterbestimmung. Eclogae Geologicae Helvetiae, 66,11–21.

Jahn, B., Caby, R. & Monie, P., 2001. The oldest UHP eclogitesof the world: Age of UHP metamorphism, nature of protolithsand tectonic implications. Chemical Geology, 178, 143–158.

Jolivet, L. & Faccenna, C., 2000. Mediterranean extension andthe Africa-Eurasia collision. Tectonics, 19, 1095–1106.

Kirschner, D. L., Cosca, M. A., Masson, H. & Hunziker, J. C.,1996. Staircase 40Ar ⁄ 39Ar spectra of fine-grained white mica:Timing and duration of deformation and empirical constraintson argon diffusion. Geology, 24, 747–750.

Lagabrielle, Y. & Lemoine, M., 1997. Alpine, Corsican andApennine ophiolites: the slow-spreading ridge model. ComptesRendus de l’Academie Des Sciences, 325, 909–920.

Le Pichon, X., Bergerat, F. & Roulet, M. J., 1988. Platekinematics and tectonics leading to the Alpine belt formation;A new analysis. Geological Society of America (special paper,218), 11–131.

Lemoine, M., de Graciansky, P. C. & Tricart, P., 2000. DeL’ocean A la Chaıne de Montagnes. Tectonique Des PlaquesDans les Alpes. Gordon & Breach, Paris.

6 16 P . A G AR D E T A L .

Lemoine, M., Marthaler, M., Caron, J. M. et al., 1984.Decouverte de foraminiferes planctoniques du Cretace super-ieur dans les schistes lustres du Queyras (Alpes occidentales).Consequences paleogeographiques et tectoniques. ComptesRendus de l’Academie Des Sciences, 229, 727–732.

Li, S., Wang, S., Chen, Y., Liu, D., Qiu, J., Zhou, H. & Zhang,Z., 1994. Excess argon in phengite from eclogite: evidencefrom dating of eclogite minerals by Sm-Nd, Rb-Sr and40Ar ⁄ 39Ar methods. Chemical Geology, 112, 343–350.

Liewig, N., Caron, J. M. & Clauer, R., 1981. Geochemical andK-Ar isotopic behaviour of alpine sheet silicates duringpolyphased deformation. Tectonophysics, 78, 273–290.

Maluski, H. & Monie, P., 1988. 40Ar ⁄ 39Ar laser-probe multi-dating inside single biotites of a Variscan orthogneiss (PinetMassif Central, France). Chemical Geology, 73, 245–263.

McDougall, I. & Harrison, T. M., 1988. Geochronology andThermochronology by the 40Ar ⁄ 39Ar Method. Oxford Univer-sity Press, New York.

Messiga, B., Kienast, J. R., Rebay, G., Riccardi, M. P. &Tribuzio, R., 1999. Cr-rich magnesio-chloritoid eclogites fromthe Monviso ophiolites (Western Alps, Italy). Journal ofMetamorphic Geology, 17, 287–299.

Michard, A., Goffe, B., Chopin, C. & Henry, C., 1996. Did theWestern Alps develop through an Oman-type stage? Thegeotectonic setting of high-pressure metamorphism in twocontrasting Tethyan transects. Eclogae Geologicae Helvetiae,89, 43–80.

Monie, P., 1985. La methode 40Ar ⁄ 39Ar appliquee au metmor-phisme alpin dans le massif du Mont-Rose (Alpes occiden-tales). Chronologie detaillee depuis 110 Ma. EclogaeGeologicae Helvetiae, 78, 487–516.

Monie, P., 1990. Preservation of Hercynian 40Ar ⁄ 39Ar agesthrough high-pressure low-temperature metamorphism in theWestern Alps. European Journal of Mineralogy, 2, 343–361.

Monie, P. & Philippot, P., 1989. Mise en evidence de l’age eocenemoyen du metamorphisme de haute-pression dans la nappeeclogitique du Monviso (Alpes occidentales) par la methode39Ar-40Ar. Comptes Rendus de l’Academie Des Sciences, 309,245–251.

Paquette, J. L., Chopin, C. & Pecaut, J. J., 1989. U-Pb zircon,Rb-Sr and Sm-Nd geochronology of high- to very-high-pressure meta-acidic rocks from the Western Alps. Contribu-tions to Mineralogy and Petrology, 101, 280–289.

Perchuk, A. L. & Philippot, P., 1997. Rapid cooling andexhumation of eclogitic rocks from the Great Caucasus,Russia. Journal of Metamorphic Geology, 15, 299–310.

Perchuk, A. L. & Philippot, P., 2000. Geospeedometry and timescales of high-pressure metamorphism. International GeologyReview, 42, 207–223.

Philippot, P., 1990. Opposite vergence of nappes and crustalextension in the French-Italian western Alps. Tectonics, 9,1143–1164.

Philippot, P. & Rumble, D., 2000. Fluid–rock interactionsduring high-pressure and ultrahigh-pressure metamorphism.International Geology Review, 42, 312–327.

Platt, J. P., 1993. Exhumation of high-pressure rocks: a review ofconcepts and process. Terra Nova, 5, 119–133.

Reddy, S. M., Kelley, S. P. & Wheeler, J., 1996. A 40Ar ⁄ 39Arlaser probe study of micas from the Sesia zone, ItalianAlps: implications for metamorphic and deformation histor-ies. Journal of Metamorphic Geology, 14, 493–508.

Reddy, S. M., Wheeler, J. & Cliff, R. A., 1999. The geometryand timing of orogenic extension: an example from theWestern Italian Alps. Journal of Metamorphic Geology, 17,573–589.

Reinecke, T., 1991. Very high-pressure metamorphism and upliftof coesite-bearing metasediments from the Zermatt-Saas zone,Western Alps. European Journal of Mineralogy, 3, 7–17.

Reinecke, T., 1998. Prograde high- to ultrahigh-pressure meta-morphism and exhumation of oceanic sediments at Lagodi Cignana, Zermatt-Saas Zone, western Alps. Lithos, 42,147–189.

Ring, U., Brandon, M. T., Willett, S. D. & Lister, G. S., 1999a.Ductile deformation and mass loss in the Franciscan subduc-tion complex: implications for exhumation processes inaccretionary wedges. In: Exhumation Processes: NormalFaulting, Ductile Flow and Erosion, Special Publication154, (eds Ring, U. B. M. T., Willett, S. D. & Lister, G. S.),pp. 55–86, Geological Society, London.

Ring, U., Brandon, M. T., Willett, S. D. & Lister, G. S., 1999b.Exhumation processes. In: Exhumation Processes: NormalFaulting, Ductile Flow and Erosion, Special Publication154, (eds Ring, U. B. M. T., Willett, S. D. & Lister, G. S.),pp. 1–27, Geological Society, London.

Rubatto, D. & Gebauer, D., 1999. Eo ⁄Oligocene (35 Ma) high-pressure metamorphism in the Gornergrat zone (MonteRosa, Western Alps): implications for paleogeography.Schweizrische Mineralogische Petrografische Mitteilung, 79,353–362.

Ruffet, G., Feraud, G., Ballevre, M. & Kienast, J. R., 1995.Plateau ages and excess argon in phengites: an 40Ar-39Ar laserprobe study of Alpine micas (Sesia zone, Western Alps,northern Italy). Chemical Geology, 121, 327–343.

Samson, S. C. & Alexander, E. C., 1987. Calibration of theinterlaboratory 40Ar ⁄ 39Ar dating standard MMHb-1. Chem-ical Geology, 66, 27–34.

Scaillet, S., 1996. Excess 40Ar transport scale and mechanism inhigh-pressure phengites: a case study from an eclogitizedmetabasite of the Dora Maira nappe, western Alps. Geochi-mica et Cosmochimica Acta, 60, 1075–1090.

Scaillet, S., 1998. K-Ar (40Ar-39Ar) geochronology of ultrahighpressure rocks. In:When Continents Collide: Geodynamics andGeochemistry of Ultrahigh-Pressure Rocks (ed. Hacker, B. R.& L. J. G.), pp. 161–201. Kluwer, Amsterdam.

Scaillet, S., Feraud, G., Ballevre, M. & Amouric, M., 1992.Mg ⁄Fe and (Mg, Fe) Si-Al2 compositional control on argonbehaviour in high-pressure white micas: a 40Ar ⁄ 39Ar continu-ous laser-probe study from the Dora Maira nappe of theinternal western Alps, Italy. Geochimica et CosmochimicaActa, 56, 2851–2872.

Scharer, U., Ballevre, M., Castelli, D., Feraud, G. & Ruffet, G.,1999. Evidence for a single short-lived phase of deepsubduction in the western Alps: new U-Pb and Rb-Sr data.EUG Meeting No. 10. J. Conf. abstr. 4, EUG, Strasbourg.

Schwartz, S., 2001. La zone piemontaise des Alpes occidentales:un paleo-complexe de subduction. Arguments metamorphi-ques, geochronologiques et structuraux. Documents DuB.R.G.M., no. 302. Bureau des Recherches Geologiques etMinieres, Orleans.

Schwartz, S., Lardeaux, J. M., Guillot, S. & T., 2000. Diversitedu metamorphisme eclogitique dans le massif ophiolitique duMonviso (Alpes occidentales, Italie). Geodinamica Acta, 13,169–188.

de Sigoyer, J., Villa, I. & Cosca, M., 2000. Informations39Ar ⁄ 40Ar sur des roches metamorphisees a HP-BT, a prendreou a laisser? 18e Reunion Des Sciences de la Terre, J. Conf.abstr., Paris.

Sinclair, H. D. & Allen, P. A., 1992. Vertical versus horizontalmotions in the Alpine orogenic wedge: stratigraphic responsein the foreland basin. Basin Research, 4, 215–232.

Stampfli, G. M. & Marchant, R. H., 1997. Geodynamicsevolution of the Tethyan margins of the Western Alps. In:Deep Structure of Switzerland. Results from NFP 20 (edsLehner, P. H., Frei, W., Horstmeyer, H., Mueller, S., Pfiffner,A. & Steck, A.), Binkhauser Verlag, Basel.

Tagami, T. & Shimada, C., 1996. Natural long-term annealing ofthe fission-track system around a granitic pluton. Journal ofGeophysical Research, 101, 8245–8255.

Takeshita, H., Shimoya, H. & Itaya, T., 1994. White mica K-Arages of blueschist-facies rocks from the Piemonte �calc-schists�of the Western Alps. Island Arc, 3, 151–162.

Todd, C. S. & Engi, M., 1997. Metamorphic field gradients inthe Central Alps. Journal of Metamorphic Geology, 15, 513–530.


Villa, I. M., 1998. Isotopic closure. Terra Nova, 10, 42–47.Von Blanckenburg, F., Davies, H. & J., 1995. Slab breakoff: amodel for syncollisional magmatism and tectonics in the Alps.Tectonics, 14, 120–131.

Wain, A., 1997. New evidence for coesite in eclogite and gneisses:Defining an ultra-high pressure province in the WesternGneiss region of Norway. Geology, 25, 927–930.

Wheeler, J. & Butler, R. W. H., 1993. Evidence for extension inthe western Alpine orogen: the contact between the oceanicPiemonte and overlying continental Sesia units. Earth andPlanetary Science Letters, 117, 457–474.

Wheeler, J., Reddy, S. M. & Cliff, R. A., 2001. Kinematiclinkage between internal zone extension and shortening inmore external units in the NW Alps. Journal of the GeologicalSociety of London, 158, 439–443.

Wijbrans, J. R. & McDougall, I., 1986. 40Ar ⁄ 39Ar dating ofwhite micas from an Alpine high-pressure metamorphic belton Naxos (Greece): the resetting of the argon isotope system.Contributions to Mineralogy and Petrology, 93, 187–194.

Received 11 June 2001; revision accepted 1 March 2002.

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