Dating low-temperature deformation by 40Ar/ 39Ar on white mica, insights from the...

Lithos xxx (2011) xxx–xxx

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Dating low-temperature deformation by 40Ar/39Ar on white mica, insights from theArgentera-Mercantour Massif (SW Alps)

Guillaume Sanchez a,⁎, Yann Rolland a, Julie Schneider a, Michel Corsini a, Emilien Oliot b,Philippe Goncalves b, Chrystèle Verati a, Jean-Marc Lardeaux a, Didier Marquer b

a GeoAzur, UMR 6526, Université de Nice-Sophia Antipolis, CNRS, IRD, 28 Av de Valrose, BP 2135, 06103 Nice, Franceb UMR6249 Chrono-environnement, 16 route de Gray, 25030 Besançon, France

⁎ Corresponding author. Tel.: +33 492 07 68 05; fax:E-mail addresses: [email protected] (G. Sa

(Y. Rolland), [email protected] (J. Schneider), [email protected] (E. Oliot), philippe.goncalv(P. Goncalves), [email protected] (C. Verati), [email protected]@univ-fcomte.fr (D. Marquer).

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Please cite this article as: Sanchez, G., et al.,Mercantour Massif (SW Alps), Lithos (2011

a b s t r a c t
a r t i c l e i n f o
Article history:Received 26 September 2010Accepted 9 March 2011Available online xxxx

Keywords:40Ar/39Ar datingPhengite–chlorite geothermobarometryShear zonesSouth-Western AlpsArgentera-Mercantour External Crystallinemassif

In order to date low-temperature deformation, intensely strainedmuscovite porphyroclasts and neocrystallizedshear band phengite from greenschist-facies shear zones have been dated by 40Ar/39Armethod in the Argentera-Mercantourmassif. Shear zones are featured by gradual mylonitization of a Variscan granite, gneiss and Permianpelite protolith (300–315 Ma) during theAlpine orogenic event.Mineralogical and textural observations indicatethat phengites and chlorites developed frombiotite and plagioclase influid systemduring deformation followingdissolution–transport–precipitation reactions of the type biotite + plagioclase + aqueous fluid = chlorite +albite + phengite + quartz + titanite + K-bearing fluid in the granite-gneiss mylonite. Contrariwise, phengitedeveloped at the expense of clays following substitution reaction in pelite mylonite. Based on conventionalthermobarometry on phengite and chlorite and Pressure–Temperature-aqueous fluid (P–T-MH2O) pseudosec-tions calculated with shear zone bulk compositions, the conditions during shear deformation were estimated at375±30 °C and 4.8–7±1 kbar in an H2O-satured system. In this low temperature environment, 40Ar/39Aranalysis of the Variscanmuscovite for various grades of ductile strain intensity shows a limited 40Ar/39Ar isotopicresetting, all ages scatteringbetween296 and315 Ma.Under conditions of intenseductile deformation and large-scalefluid circulation,muscovite grains formedduring theVariscan retain theirmucholder ages. 40Ar/39Ar datingof very fine grained synkinematic phengite grains, neoformed during the Alpine history, give consistent plateauages (34–20Ma) for each shear zone. In detail, 40Ar excess can be detected in the pelite mylonitic sample wherephengites crystallized by substitution process while the other mylonitic samples where phengites grow fromfluid-induced reactions do not evidence any 40Ar excess. These results demonstrate that the 40Ar/39Ar dating ofneocrystallized synkinematic white mica allows the determination of precise ages of deformation and fluidactivity. Together with precise thermobarometry undertaken on the basis of mineral chemistry and whole-rockcomposition, 40Ar/39Ar dating of white mica leads to the reconstitution of precise depth-deformation history oflow-grade (b400 °C)metamorphic units. At the Argentera-Mercantour massif scale, several stages of shear zonedevelopment at 15–21 kmdepth are dated between33 and 20Ma. In the SEpart of themassif shear zone ages arewell constrained to be either (1) 33.6±0.6 Maor in the range (2) 26.8±0.7 Ma–26.3±0.7 Ma. In theWest of themassif, younger shear zone ages range between (3) 22.2±0.3 Ma and (4) 20.5±0.3 Ma.

+33 492 07 68 16.nchez), [email protected]@unice.fr (M. Corsini),[email protected] (J.-M. Lardeaux),

l rights reserved.

Dating low-temperature deformation by 40A), doi:10.1016/j.lithos.2011.03.009

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

The determination of geological process rates and the elaborationof consistent tectonic models is critically dependant on accuratedetermination of the timing of deformation Müller et al., 2000. Giventhat K-rich mineral phases (micas) are common in highly deformedzones (Mitra, 1992), the application of the 40Ar/39Ar dating technique

on micas has frequently been attempted to constrain mylonitizationages (Dunlap, 1997; Goodwin and Renne, 1991; Kelley, 1988; Kelleyet al., 1994; Kirschner et al., 1996; Kliegfield et al., 1986; Reddy et al.,1996;West and Lux, 1993; Wijbrans and McDougall, 1986). However,in mylonitic shear zones characterized by intense localized ductileshear, fluid circulation and associated metamorphic reactions, argonisotopes are usually prone to strong mobility still making theinterpretation of 40Ar/39Ar ages spectra difficult.

Inherited deformed white mica porphyroclasts are most oftenused in low-temperature deformation 40Ar/39Ar dating studies (e.g.Mulch and Cosca, 2004; Mulch et al., 2002). Analytical (Kramar et al.,2001) and experimental (Dunlap and Kronenberg, 2001; Mares andKronenberg, 1993) works undertaken to quantify the influence ofshear deformation on the opening of Ar isotopic system in these pre-

r/39Ar on white mica, insights from the Argentera-

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2 G. Sanchez et al. / Lithos xxx (2011) xxx–xxx

existing minerals (porphyroclasts) have clearly shown that internaldeformation induces or enhances Ar loss. Isotopic argon exchangeduring deformation is controlled by the nature of deformationmechanisms (kink-band development, dynamic recrystallization,intra-grain microfractures) or by discrete intra-grain neocrystalliza-tion (Dunlap, 1997; Hames and Bowring, 1994; Villa, 1998; Villa et al.,1997). So that, it has been suspected that extremely deformedporphyroclast dating allows providing a mean of deformation age.However, some authors reported unreset Ar ages even in cases ofextrememylonitic deformation (e.g., Kramar et al., 2001). In this case,40Ar/39Ar ages could reflect mixing between partially preserved andre- or neocrystallized mineral ages (Dunlap et al., 1991; Mulch andCosca, 2004; Villa et al., 1997).

Neocrystallized white mica during shear deformation mayrepresent an alternative to constrain low-temperature deformationages (e.g., Challandes et al., 2003; Dunlap, 1997; Kirschner et al., 1996;Reddy and Potts, 1999). However, mylonite shear zones arecommonly characterized by intense localized deformation, fluidcirculation and mineral reaction. Thus, even for deformation-relatedneocrysts, it is unclear if ages represent the time of crystallization orthe cessation of fluid circulation (Challandes et al., 2003, 2008).

In the Argentera-Mercantourmassif (SWAlps, France), amuscovite-bearing Late Variscan and Permian basement is deformed by severalshear zone events during the Alpine compressive phase (~22–23 Ma;Corsini et al., 2004). Both pre-existing Variscan white mica which areextremely deformed during the alpine shearing and neocrystallizedsynkinematic white mica are present in the same greenschist-faciesmylonitic shear zones providing a good opportunity to resolve theissue of low-temperature deformation dating. Given that Ar isotopicexchange is controlled by deformation mechanisms and metamor-phism, petrological and thermobarometrical investigations arefundamental and have been performed defining mineral reaction,type of deformation within white mica and P–T conditions duringthe alpine deformation events. Further, 40Ar/39Ar dating is appliedon pre-existing Variscan muscovite porphyroclasts to investigatethe robustness of the 40Ar/39Ar system during fluid-assisteddeformation. Finally, 40Ar/39Ar dating is performed on the neo-crystallized syntectonic Alpine white micas in the granitic, gneissicand pelite shear zone to test the potential effects of fluid flow,deformation and type of mineral reactions on intragrain 40Ar/39Arpatterns. In particular, the reliable significance of neo-crystallizedwhite mica 40Ar/39Ar ages will be discussed.

2. Geological overview of the Argentera-Mercantour massif

The Argentera-Mercantour massif belongs to the ExternalCrystalline Massifs (ECM) of South-Western Alps, which is part ofthe European continental crust (Fig. 1). It is mainly composed ofmigmatitic gneisses interlayered with amphibolites, marbles, quart-zites and late anatectic granitoids. Permian and Early Triassic siltsand sandstones are frequently attached to the basement, whereas theoverlying Triassic to Early Cretaceous carbonates are detached aboveLate Triassic evaporitic rocks (Faure-Muret, 1955). Within the Variscanbasement the overall structure andparticularly the schistosity and shearzones result from the superposition of Variscan and Alpine deformationhistories (Bogdanoff, 1980; Faure-Muret, 1955; Malaroda et al., 1970;Vernet, 1964). During the Variscan orogeny, the Argentera-Mercantourmassif experienced a polyphase deformation history culminating in ahigh-grade metamorphism. Geochronological studies have providedevidence for an early high pressure–high temperature metamorphismduring the LateOrdovician–EarlyDevonian (462–332 Ma-U/Pbonzircon;Paquette et al., 1989; Rubatto et al., 2001) followed by medium to lowpressure metamorphism and anatexis during the Upper Carboniferous(330–315±7Ma-40Ar/39Ar on biotite; Monié and Maluski, 1983 and323±12 Ma-U/Pb on zircon; Rubatto et al., 2001). The emplacementof the Argentera granite marks the end of the high temperature

Please cite this article as: Sanchez, G., et al., Dating low-temperature defMercantour Massif (SW Alps), Lithos (2011), doi:10.1016/j.lithos.2011.

metamorphism and associated melting during the Upper Stephanian/Lower Permian (285–293 Ma, Rb/Sr; Ferrara and Malaroda, 1969).

A great part of the Alpine metamorphism in the Argentera-Mercantour massif is localized in a network of ductile shear zonesthat mostly reactivated late-Variscan shear zones punctuated by lateCarboniferous metasediments (Fig. 2). Deformed Triassic rocks arealso involved within the shear zones. Moreover, a pervasive cleavageformedduring theAlpine orogeny is defined bymicron-scale phengite,chlorite and thin green biotite all over the massif outside the mainshear zones. TwomainNW–SE trending shear zones, the Valletta shearzone (VSZ) and the Bersezio shear zone (BSZ), crosscut the massif anddisplay dextral transpressive shearing (Baietto et al., 2009; Corsiniet al., 2004; Fig. 2). The southward reverse E–W trending Fremamorteshear zone (FSZ) crosscuts the Argentera granite and connectsnorthward to the VSZ (Fig. 2). To the East, the FSZ merges into theAutier (ASZ), Neiglier (NSZ) and Casterino (CSZ) shear zones.Preliminary 40Ar/39Ar dating of Alpine synkinematic phengites withinthe FSZ provided an age of ~22–23 Ma (Corsini et al., 2004). ZirconFission Track thermochronology provided ages in the range 29–20 Ma(Bigot-Cormier et al., 2006), which suggests that the earlier coolingstages of the massif happened within the same time scale as the shearzone activity and that subsequently basement uplift can be related tothe activity on the Fremamorte Shear Zone. Field analysis indicatesthat the Alpine Argentera-Mercantour shear zone network is mainlyright-lateral. The shear zones formed atmid-crustal depths (PN4 kbar)during the Alpine orogeny, at an average temperature of 350 °C(Corsini, et al., 2004). Right-lateral strike-slip displacements occurredat ~22–23 Ma in response to a sub-meridian shortening in the South-Western Alps. As for the rest of the ECM, burial was achieved byunderthrusting below the Alps internal units following the activationof the Penninic Frontal Thrust at 34–31 Ma (Simon-Labric et al., 2009).Part of the shear zone history is prograde, such as in the PelvouxMassif, while in other ECMs the preserved shear zone ages are mostlyretrograde. Exhumation and cooling are ascribed to a major phase ofshortening and dextral strike-slip reactivation of the Penninic Frontcoeval with greenschist facies metamorphism at 20–14 Ma along theSimplon (Campani et al., 2010), at 20–16 Ma along the Mont BlancMassif (Leloup et al., 2005; Rolland et al., 2007, 2008) and at 14–12 Maalong the Grimsel Massif (Rolland et al., 2009b). In the Argentera-Mercantour these right-lateral strike-slip mylonitic shear zones werereactivated in brittle regime during exhumation in the last 15 Ma(Bigot-Cormier et al., 2006; Bogdanoff et al., 2000; Sanchez et al.,2011). The active character of these dextral faults has been identifiedby recent studies which report meter-scale offsets of glacial polishedsurfaces (Sanchez et al., 2010a,b) and current seismicity, predomi-nantly with a dextral component (Jenatton et al., 2007).

3. Analytical procedures

Following field mapping and sample collection, selected repre-sentative specimens were prepared for laboratory analyses. Sampleselection in the field focused on high-strain zones (mylonites andultra-mylonites), in which large grains of Variscan muscovite(N500 μm) were sampled to investigate the extent of K-Ar reset inpre-existing white mica (Corsini et al., 2004). These results arecompared to dating of neo-grains of white mica (phengite) thatcrystallized during Alpine deformation. Samples were examined inthin section and selected samples were also analyzed for electronmicroprobe and bulk rock major element analysis. Seventeen sampleswere selected for 40Ar/39Ar dating. These samples were analyzed bysingle grain step-heating techniques. The separation of Variscanmuscovite and Alpine phengite is made by hand-picking underbinocular as the two populations have clearly distinct mineralogicalmorphologies: large flakes of transparent mica and thin-grainedcloudy aggregates, respectively.

ormation by 40Ar/39Ar on white mica, insights from the Argentera-03.009


Fig. 1. Geological map of the Western Alps modified after Bigi et al. (1990) and Polino et al. (1990). The External zones comprise: 1, the Dauphinois zone, which is limited by thePenninic Frontal Thrust (PFT), by the Subalpine Frontal Thrust (Digne-Castellane, Nice) and by the Jura Frontal Thrust. It comprises (1a) External Crystalline Massifs formed duringthe Variscan period and (1b) their Mesozoic (Trias-Cretaceous) sedimentary cover. 2, the Upper Cretaceous Helminthoid flysch. Internal zones comprise: 3, Briançonnais andPiemontais zones, which are made of metamorphic rocks from (3a) the European passive continental margin and (3b) the Tethyan oceanic domain; 4, the Austro-Alpine unitsincluding mainly the Dent Blanche nappe (DB) and the Sesia Lanzo zone (SL), which represent the Adriatic continental margin; 5, Oligocene to Quaternary molasse basins in the Alpsperiphery. Black and white dashed lines represent main active faults system, whereas in the SW edge of the western Alpine arc they represent principal faults in the Argentera-Mercantour ECM. External Crystalline Massifs: Mt Bl., Mont Blanc; Arg., Argentera-Mercantour. Internal Crystalline Massifs: DM, Dora-Maira; GP, Grand Paradis; MR: Monte Rosa;PFT: Penninic Frontal Thrust; Ca: Canavese line. The black rectangle is the investigated area located in Fig. 2. Projection latitude/longitude coordinates according the Lambert IIextended system.

3G. Sanchez et al. / Lithos xxx (2011) xxx–xxx

3.1. Chemistry and petrology

The chemical compositions of white mica and chlorite wereobtained by Electron Microprobe Analysis (EPMA) in order to checkthat the homogeneity of mineral compositions from core to rim. Theanalyses were carried out at 15 kV and 1 nA using Cameca CamebaxSX100 electron microprobe of the Blaise Pascal University inClermont-Ferrand. Natural samples were used as standards.

Bulk rock major element concentrations were performed at the‘Service des Roches et Minéraux' (SARM laboratory-CRPG-CNRS,Nancy, France). Powders of bulk rocks were digested with LiBO2,dissolved with HNO3, analyzed and calibrated with internationalgeostandards. Major elements were analyzed using inductivelycoupled plasma atomic emission spectroscopy (ICP-AES). The LossOn Ignition (LOI) is obtained by fusion of a sample aliquot at 1000 °Cfor over 11 h. The amounts of Fe for bulk rocks were arbitrarilycalculated as Fe2O3%.


3.2. 39Ar/40Ar method

Pure white mica monograins and aggregates less than 1 mm wereseparated by careful hand-picking under a binocular microscope toavoid altered grains or inclusions. All the samples were irradiated foraround 70 h (J1) and 10 h (J2, J3) in the nuclear reactor at McMasterUniversity in Hamilton (Canada), in position 5c along with Hb3grhornblende fluence monitor (1073.6 Ma±5.30 Ma; Jourdan et al.,2006) and Fish Canyon sanidine monitor (28.03±0.08 Ma; Jourdanand Renne, 2007) for J1 and J2, J3 respectively. The estimated errors of40Ar*/39ArK ratio range between ±0.1% (2σ), and ±0.6% (2σ) in thevolume where the samples were included.

All the samples were analyzed by single-grain CO2 laser fusionanalysis. Isotopic ratios were measured using a VG3600 massspectrometer, working with a Daly detector system, in the Universityof Nice (GeoAzur laboratory). The typical blank values measured atevery three heating steps for extraction and purification of the laser



Fig. 2. Structural map of the External Crystalline Argentera-Mercantour massif (SW Alps). Main structural features of the massif are shown. These make up the trend and width ofmajor shear zones, and the pattern of Alpine deformation markers (stretching/mineral lineations, foliations). BSZ: Bersezio shear zone; FSZ: Fremamorte shear zone which crosscutthe Argentera granite; ASZ: Autier shear zone CSZ: Casterino shear zone; NSZ: Neiglier shear zone; VSZ: Valletta shear zone. Stars indicate location of samples collected for this study:Valletta shear zone (MC24, MC26, MC27: Variscan muscovite; Me.05.08: phengite neocrystallized); Fremamorte shear zone (MC6a: phengite neocrystallized); Autier shear zone(Mer-Autier, Mer-Autier 2: phengite neocrystallized); Neiglier shear zone (GOR.06.05, Merc.07.08: phengite neocrystallized); Casterino shear zone (Me.06.11, Me.04.01). Thelocation of samples analyzed in previous study (Corsini et al., 2004) is also shown.


system are in the range 4.2–8.75, 1.2–3.9 cc STP for masses 40 and 39,respectively. Mass discrimination for the mass spectrometer wasmonitored by regularly analyzing one air pipette volume. Decayconstants are those of Steiger and Jäger (1977). The criteria used fordefining a plateau age are those described in McDougall and Harrison(1999). We also used the term “pseudo-plateau age” for plateausmaller than 70% of total 39Ar following discussions in Rolland et al.(2008). Uncertainties on apparent ages, plateau and pseudo-plateauages are given at 2σ level, which includes the error on the 40Ar*/39Arkratio of the monitor.

4. Petrology

4.1. Sample description

Undeformed protolith to ultramylonites have been sampled inVariscan granite (FSZ), gneiss (VSZ, ASZ, NSZ) and Permian pelite(CSZ) shear zones. Granite and gneiss shear zones vary gradationallyfrom underformed, protomylonite (P) and mylonite (M) and ultra-mylonite (U) rocks (Fig. 3a). Pelite shear zone is narrower (meter-scale) and does not display deformation gradient.

Undeformed granite and gneissmainly contain biotite+muscovite+plagioclase + K-feldspar + quartz. The major mineralogical changeobserved across granite and gneiss shear zones is the breakdown ofplagioclase and biotite in phengite and chlorite. Additionally, there is a


progressive increase in titanite, Ti-oxide (rutile) and decrease in primarymuscovite, plagioclase, K-feldspar, biotite and quartz. The changes inmineral mode across the deformation gradient are consistent with theprogress of metamorphic reactions (described in Section 4.3 ). Despitemajor changes in matrix mineralogy, porphyroclasts of K-feldspar andmuscovite are preserved but vary in size and proportion in the bulk rock.These porphyroclasts present evidence of intra-crystalline deformationand pressure-dissolution processes.

Undeformed pelites are clay rich and also contain detrital quartz,K-feldspar, plagioclase and Fe-oxide. During mylonitization the mainreaction is the replacement of clay minerals and plagioclases byphengite that defined the foliation. This reaction implies substitutionprocess as phengite growth occurred in the same textural site.

Since granitic and gneissic shear zones evidence clear deformationgradient, a detailed study across this latter has been performed for eachstage in order to precise mineral reaction and deformation mechanism.

4.2. Mineralogy and microstructures across deformation gradient

4.2.1. ProtomyloniteIn protomylonite, the original igneous assemblage is heteroge-

neously deformed and shows partial reaction textures. The coarsegrain structure of both the Variscan granite and gneiss is preserved,although fracturing of K-feldspar and minor ductile deformation inbiotite and muscovite is observed (Fig. 3b).



Fig. 3. Microphotographs of studied shear zone samples showing textural and mineralogical evolution from (a) undeformed zones to (h) ultamylonites. a, deformation gradient atoutcrop scale within the Fremamorte shear zone. P: protomylonite, M: mylonite, U: Ultramylonite. b–c, protomylonites showing textural evidence of brittle deformation of K-feldspar megacrysts with aggregates of fine-grained neocrystallized phengite and chlorite along fractures or in pressure-shadows, some grain boundary migration and dynamicrecrystallization of quartz. d–g, mylonites showing widespread crystallization of the phengite–chlorite assemblage, which defines a continuous foliation and mantles inherited K-feldspar and muscovite porphyroclasts. The latter display internal deformations. Deflection of phengite–chlorite associated to shear bands indicates intense strain with a dextraldirection of shear (S–C structures, mica fishes, K-feldspar fracturation). Inherited biotite shows signs of sagenitisation as rutile becomes more abundant. h, ultramylonites showingjuxtaposition of felsic bands enriched in quartz and layers formed by the phengite–chlorite assemblage surrounding rare and extremely sheared inherited muscovite and K-Feldsparporphyroclasts.


K-feldspar megacrysts behave in a brittle–ductile manner showingsome porphyroclast features, undulose extinction and micro-perthite(Fig. 3c). Generally, plagioclase is fully altered, and recrystallized intoalbite and patchy muscovite. In addition, grain boundary migration anddynamic recrystallization of quartz are common, especially aroundporphyroclasts (Fig. 3b,c). Themain observed deformationmechanisms


for micas appear to be: rigid rotation, grain boundary sliding, and intra-granular sliding along basal (001) planes (Fig. 3b). Biotite is partiallyreplaced by phengites, chlorite and rutile (Fig. 3b). Aggregates of fine-grained phengite and chlorite intergrown with quartz can be observedalong grain contacts, fractures or in pressure-shadow fibers nucleatedon K-feldspar and muscovite megacryst rims (Fig. 3b,c).




4.2.2. MyloniteThe mylonite deformation stage is characterized by a mineralogy

and microstructure slightly different compared to previous stage ofmylonitization. Albite, quartz, biotite are extensively replaced byphengite–chlorite–titanite–rutile assemblages andnumerous opaques(Fig. 3d–g).

The foliation formed by ductile strain is defined by elongatedphengite and chlorite grains which are aligned between feldspar andmuscovite porphyroclasts (Fig. 3f,g). Phengite and chlorite rich shear-bands show deflections that are typical of S–C and C′ structures (Berthéet al., 1979) and provide consistent dextral sense of shear in all samples(Fig. 3f,g). Thin phengite ribbons are observed mantling individualmuscovite and K-feldspar porphyroclasts indicating that frictionalsliding of the grains is an important deformation process (Fig. 3e–g).Inherited muscovite grains show widespread evidence for intraand inter-folial deformation, featured by sliding on basal (001)planes with or without fractures, subgrain development and kinkinitiation (Fig. 3e–g). Plagioclase exhibits some recrystallization intophengite+quartzon the strain fringes that crystallizedduring shearing.Pressure shadows formed by polycrystalline phengite, chlorite, albite,quartz ribbons are common around porphyroclasts (Fig. 3f,g). Miner-alized veins which contain chlorite–quartz–titanite assemblage can beobserved. These veins show deflection in the foliation which suggeststhat they crystallized during deformation.

4.2.3. UltramyloniteUltramylonite samples show a very intense foliation defined by

well-aligned and interconnected phengite (N80%) and chloritegrains, elongate lenses of fine-grained quartz, rare muscovite fishand pod-shaped K-feldspar and plagioclase (Fig. 3h). The latter areoften very small, truncated, rounded and totally mantled byneocrystallized phengite and quartz indicating complete breakdownof plagioclase (Fig. 3h). Pressure shadows are strongly transposedand sheared into the matrix. Biotite has almost totally disappearedat the expense of chlorite, phengite, quartz and titanite. Variscanmuscovite grains are rare and when present are extremely shearedand entirely surrounded or replaced by the phengitic matrix(Fig. 3h). Intra-granular deformation of muscovite is featured bysliding on basal (001) planes, subgrain development and kinkingresulting in a marked undulose extinction. Quartz ribbons displayundulose extinction, intracrystalline subgrains and evidence fordynamic recrystallization (Fig. 3h).

4.3. Mineral reactions across deformation gradient

The textural relationships and changes in mineral mode describedin the previous section show that phengite and chlorite are mainlyformed at the expense of biotite and plagioclase by intense fluid–mineral reactions coeval with shearing by pressure–dissolution–recrystallization (Fig. 3). In details, distinct mineral reactions occurredat different textural sites along the deformation gradient.

In protomylonite samples, the biotite breakdown produces chlorite,quartz and rutile and is represented by the following equation assumingmagnesian end-members:

2½KðMgÞ3ðSi3ðAl;TiÞO10ÞðOHÞ2� þ 2Hþ ¼ ðMgÞ6ðSi3AlO10ÞðOHÞ8

þ3SiO2 þ TiO2 þ 2Kþ

ð1Þ

2 Biotite þ acid fluid ¼ 1 Chloriteþ 3 Quartz þ 1 Rutile þ K�bearing fluid

In the same stage of deformation, breakdown of plagioclase locatedin another textural site occurred and lead to albite and phengitecrystallization. The K component required for phengite crystallization iscarried by fluid as reaction (1) predicts. This suggests strongmobility ofK promoted by fluid at sample scale. Thus, a second reaction can be


written following the equation where muscovite end-member is usedfor phengite:

NaCaAl3ðSi5O16Þ þ SiO2 þ Kþ þ Al

3þ þ 2OH� ¼ NaAlðSi3O8Þ

þKðAlÞ2ðSi3Al1O10ÞðOHÞ2 þ Ca2þ

ð2Þ

Plagioclase þ quartz þ K�Al�fluid ¼ Albiteþ Phengite

þCa�bearing fluid

Mineralogical observations show that ultramylonite samples arestrongly homogenized. Phengite aggregate iswidespread throughoutthe rock and no longer confined around plagioclase textural site. Inaddition, chlorite forms well defined continuous layers in associa-tion with titanite instead of rutile as observed in protomylonitestage. This substitution of rutile to titanite demonstrates enhance-ment of Ca mobility at sample scale which is in agreement withreaction (2)predictionasCa is releasedduringplagioclasebreakdown.Allof these observations argue for strong element mobility at ultramylonitestage, promoted by deformation mechanism and fluid circulation.Therefore, a singlemineralogical reaction can bewritten in ultramylonitewhereproducts of biotite andplagioclase breakdownare carriedbyfluidsallowing chlorite, phengite and titanite crystallization through precipi-tationprocess. This reaction iswrittenusingmagnesianend-members forbiotite and chlorite and muscovite end-member for phengite:

2½KðMgÞ3ðSi3ðAl;TiÞO10ÞðOHÞ2� þ NaCaAl3ðSi5O16Þ þ 2H2O

¼ ðMgÞ6ðSi3AlO10ÞðOHÞ8 þ NaAlðSi3O8Þ þ KðAlÞ2ðSi3Al1O10ÞðOHÞ2þSiO2 þ CaTiSiO5 þ K

þ þ Hþ

ð3Þ

2 Biotite þ 1 Plagioclase þ aqueous fluid ¼ 1 Chlorite þ 1 Albite þ 1 Phengite

þ1 Quartz þ 1 Titanite þ K�bearing acid zfluid

The above reactions predict the release of Si and in situ precipitationof quartz,which is confirmedbymineralogical observationsespecially inthe synkinematic veins. However, the modal decrease in quartz duringprogressive evolution from protomylonite to ultramylonite suggeststhat shear zones are mostly depleted in SiO2 (Fig. 3). Thus, as silica ishighly soluble inHTfluids, this confirms that such lowgrade shear zonescan be considered as ‘open systems’ (e.g. Streit and Cox, 1998).

4.4. Mineral chemistry

Typical biotite, K-feldspar, muscovite and plagioclase composi-tions are given in Supplementary data. Representative analyses andstructural formulas of phengite and chlorite are given in Table 1.

4.4.1. PhengiteNo significant variations in phengite compositions were observed

between microstructural sites for a given sample, and overall mineral-ogical variations within individual samples are moderate (Table 1).Thus, it appears that there is only one phengite population in eachshear zone, with Si contents of 3.24±0.05 p.f.u. (per formula unit)for the Autier shear zone; 3.20±0.06 p.f.u. for the Valletta shear zone;3.36±0.05 p.f.u. for the Casterino shear zone and 3.32±0.05 p.f.u. forthe Neiglier shear zone.

4.4.2. ChloriteChlorite shows very consistent compositions in all shear zones,

with an average XMg value of 0.41±0.08 and Si content in the range2.76±0.1 p.f.u (Table 1).



Table 1Representative phengite and chlorite compositions from shear zone samples analyzed by EPMA. Standard deviations (2σ) are expressed within parentheses.

Shear zones Autier Valetta Casterino Neiglier

Samples Autier 06 CF.06.29 Me.06.11 Gor.06.05

Mineral Phg Chl Phg Chl Phg Phg

SiO2 48.12 (1.09) 24.97 (0.06) 45.82 (1.13) 24.47 (0.33) 48.57 (1.39) 48.03 (1.47)Al2O3 30.37 (1.28) 19.50 (0.02) 28.60 (1.99) 18.75 (0.54) 26.02 (0.63) 27.77 (0.92)FeO 3.09 (0.60) 29.03 (1.00) 4.77 (1.06) 32.56 (0.54) 4.01 (0.45) 4.04 (0.89)MgO 2.40 (0.36) 11.68 (0.44) 2.07 (0.38) 9.33 (0.20) 3.13 (0.27) 2.94 (0.36)MnO 0.03 (0.02) 0.16 (0.04) 0.04 (0.03) 0.40 (0.05) 0.01 (0.00) 0.06 (0.05)K2O 10.78 (0.35) 0.06 (0.00) 10.98 (0.41) 0.07 (0.01) 10.64 (0.55) 9.13 (0.52)Na2O 0.11 (0.04) 0.07 (0.02) 0.12 (0.04) 0.09 (0.08) 0.41 (0.71) 0.18 (0.30)CaO 0.07 (0.04) 0.25 (0.19) 0.14 (0.27) 0.12 (0.04) 0.05 (0.04) 0.04 (0.03)TiO2 0.33 (0.16) 0.18 (0.09) 0.76 (0.67) 0.02 (0.03) 0.28 (0.06) 0.48 (0.11)Cr2O3 0.05 (0.04) 0.10 (0.08) 0.05 (0.03) 0.01 (0.01) 0.03 (0.01) –

Total 95.30 (0.98) 85.97 (0.08) 93.27 (1.65) 85.80 (0.51) 93.10 (1.15) 92.69 (1.43)

O basis 12 18 12 18 12 18Si 3.24 (0.05) 2.75 (0.001) 3.20 (0.06) 2.77 (0.01) 3.36 (0.05) 3.32 (0.05)Al 2.41 (0.09) 2.53 (0.01) 2.35 (0.14) 2.50 (0.04) 2.12 (0.05) 2.26 (0.06)Fe 0.17 (0.04) 2.68 (0.10) 0.28 (0.06) 3.08 (0.09) 0.23 (0.03) 0.21 (0.1)Mg 0.24 (0.04) 1.92 (0.07) 0.22 (0.04) 1.57 (0.01) 0.32 (0.03) 0.61 (0.08)Mn 0.00 (0.00) 0.02 (0.00) 0.00 (0.00) 0.04 (0.00) 0.00 (0.00) 0.01 (0.01)K 0.92 (0.04) 0.01 (0.00) 0.98 (0.03) 0.01 (0.00) 0.94 (0.06) 1.61 (0.01)Na 0.02 (0.01) 0.01 (0.00) 0.02 (0.01) 0.02 (0.02) 0.05 (0.09) 0.05 (0.08)Ca 0.00 (0.00) 0.03 (0.02) 0.01 (0.02) 0.01 (0.00) 0.00 (0.00) 0.65 (0.46)Ti 0.02 (0.01) 0.01 (0.01) 0.04 (0.04) 0.00 (0.00) 0.01 (0.00) 0.05 (0.01)Cr 0.00 (0.00) 0.01 (0.01) 0.00 (0.00) 0.00 (0.00) 0.00 (0.00) –


4.5. Pressure–Temperature-fluid estimates

The pressure, temperature and H2O-saturation conditions duringmetamorphism were investigated in three shear zone samples(Autier Shear Zone: Mer-Autier; Valletta Shear Zone: CF.06.29; andCasterino Shear Zone: Me.06.11), using conventional thermobaro-metry and pseudosection modeling. Phase relation modeling has beenperformed in Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O system andcalculated with PerpleX_07 (Connolly, 2005), using the internallyconsistent thermodynamic database of Holland and Powell (1998,and revised in 2002). Solution models used are biotite [Bio(HP)],chlorite [Chl(HP)],whitemica [Mica(CHA)], garnet [Gt(HP)], plagioclase[Pl(h)], K-feldspar [San], chloritoid [Ctd(HP)], and carpholite [Carp](solution model references available at http://www.Perplex.ethz.ch).Average bulk compositions used as model input are given in Table 3.NaCaKFMASH P-MH2O for Autier shear zone sample and P–T pseudo-sections for these threemylonitic shear zones samples are presented forthis bulk composition in Figs. 4 and 5. In the following section andfigures, mineral abbreviations used are those of Kretz (1983).

4.5.1. Temperature estimatesThe homogeneous EPMA composition of synkinematic chlorite and

phengite in each sample suggests that thermodynamic equilibriumwas reached during deformation. The temperature of shear zonesparageneses can be estimated from the location of the followingequilibrium (e.g., Cathelineau and Nieva, 1985):

ðclinochloreþ sudoiteÞchl ¼ ðMg� amesiteÞchl þ quartz þ H2O ð2Þ

Table 2Temperature estimates based on the chemistry of chlorites (Cathelineau and Nieva,1985; Vidal et al., 2001, 2005). Temperatures are provided in °C.

Samples Autier-06 ±(2σ) CF-06-29 ±(2σ) Me 05-08 ±(2σ)

TFeMg 416 65 399 12 405 69TChl-Qtz 329 22 367 41 386 47TCathlineau 354 15 373 10 337 3


Temperature calculationswere doneusing the chlorite solid solutionmodel and thermodynamic data from Vidal et al. (2001, 2005),assuming activity of water equal to unity. Conventional chloritethermometry based on the Fe–Mg ratio of chlorite (Cathelineau andNieva, 1985) was also done for comparison. Results are shown inTable 2. Temperatures obtained for analyzed shear zones are all within375±30 °C, using both independent methods. The temperature ofCasterino shear zone samples could not be estimated because chloritecrystals are too small andhardly analyzable.However, its temperature isdiscussed below in the light of P–T pseudosection modeling.

4.5.2. Effect of H2O saturation on the Si content in phengite, and bearingon phengite barometry

To test the effect of H2O in the NaCaKFMASH system for mediumpressure–low temperaturemetapelite rocks, a representative P-MH2Opseudosection was drawn at T=375 °C for the Autier shear zone(Fig. 4).

Most of the P-MH2O field (at MH2ON0.12 and Pb7 kbar) is featuredby the main assemblage biotite–chlorite–phengite–epidote–albite–quartz, which is in agreement with the shear zone petrography. Thecalculation shows that the Si content in phengite is stabilized in theH2O-saturated domain. Indeed, in H2O-saturated domains the Si substitutionvalues in phengite are much lower than in water-undersaturateddomains. For instance, the Si value in phengite measured in the Autiershear zone is 3.24 p.f.u., which is concordantwith a pressure estimate of5.3 kbar in the water-saturated domain. In contrast, the same Si valuewould feature a pressure value ranging from 5.3 to at least 8 kbar in thewater-undersaturated field.

In the case of ECM shear zones, textural observations and mineralreactions 1–3 imply that H2O was in excess in the shear zone systemthroughout the phengite crystallization process, thus it is necessaryto use the phengite Si-values calculated in the water-saturateddomain. Indeed, in this domain, the phengite Si-values appear to behighly pressure-dependent. Further, even if the water-saturatedcondition evolution is complex and may evolve towards a morewater-undersaturated system during deformation, the estimatedP value will be a minimum estimate as shown by Si isopleths inthe under-saturated domain. Consequently, our results from the


http://www.Perplex.ethz.ch


Table 3Average bulk geochemical composition of shear zone samples.

Samples SiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O TiO2 P2O5 PF Total% % % % % % % % % % % %

Autier-06 (ASZ) 64.77 16.59 5.20 0.04 2.22 0.42 2.02 4.63 0.79 0.31 2.77 99.75CF-06-29 (VSZ) 64.40 16.21 6.35 0.07 1.83 0.68 1.65 4.48 0.83 0.21 2.63 99.33ME-06-11 (CSZ) 64.84 16.32 4.22 0.03 1.90 0.90 4.04 3.72 0.59 0.08 2.79 99.44


Argentera-Mercantour shear zones show that the phengite barometer isan efficient tool to estimate pressure in water-saturated conditions andprovides consistent pressure values considering the Alpine geothermalgradient at 375 °C (25 °C·km−1; Crouzet et al., 1999).

4.5.3. Pressure estimatesFollowing previous sections, all the P–T pseudosections were

calculated in H2O saturated conditions.Autier and Valletta shear zone samples (Mer-Autier and CF.06.29)

show relatively similar pseudosection results. These predict a largestability field for the shear zone paragenesis (300≤T≤475 °C;3≤P≤10 kbar) with a biotite + chlorite + phengite + epidote +albite + quartz mineral assemblage (Fig. 5a). This is consistent withobserved parageneses as described in previous sections, except forepidote, the content of which is predicted below 2% in the calculation.Phengite is present in theentire P–T range. Biotite+albite is replacedbychlorite+ jadeite+ lawsonite ± glaucophane in the higher part of P–Tspace, but these parageneses are unlikely from petrographic observa-tions. In addition, the chlorite-out reaction is predicted at ~425–475 °C.Contours of phengite Si isopleths were calculated using Mica(CHA)solution model (Auzanneau et al., 2010; Coggon and Holland, 2002).

H2O

sat

ur

biot kfphg gt ky ep

biot phg gt ky ep

biot pl phg

biot pl

phg ep

biot phg gt ep

biot phgep

biot chl phg ep

biot phgpar ep

biot

biot pl kf phg ky

3.153.10

3.0512

3 45

67

8

9 chl i

n

phg par

3

4

5

6

7

8

P (k

bar)

0.11 0.120.10

MH2O

NaCaKFMASH T°

chl

ep

Fig. 4. P-MH2O pseudosections in the NaCaKFMASH system calculated at 375 °C for the


They predict a Si content range of 3.10–3.35 p.f.u. across most of thestability field of the observed mineral assemblage. This compositionalprediction closely agrees with measured EPMA Si contents in phengite,which is 3.24 and 3.20 p.f.u. on average for the Autier and Valletta shearzones, respectively.

The Casterino shear zone sample (Me.06.11) displays a slightlydifferent pseudosection result (Fig. 5c). The chlorite-out reaction inthe considered P range is estimated at ~325–425 °C. The stability fieldof the observed natural paragenesis (biotite + chlorite + phengite +epidote+albite+quartz) assemblage is the following: 310bTb425 °C,3bPb10 kbar. This field coincides with phengite Si isopleths rangingbetween 3.30 and 3.45 p.f.u. This Si content range fits verywell with theanalyzed EPMA Si contents of 3.36 p.f.u. on average.

The above P–T pseudosections yield relatively similar within errorpressure estimates at T=375 °C of 5.4±1 kbar and 4.8±1 kbar for theAutier and Valletta shear zones, respectively (Fig. 5). In contrast,the Casterino Shear Zonepseudosection provided a pressure estimate of7±1 kbar at the same temperature. This difference in phengite Sicontents, and resulting barometry, is ascribed to a difference of protolithshear zone composition as Casterino shear zone derived to a pelite andphengites crystallized from clay mineral by substitution process.

ated

biot chl phg ep H2O

chl phg ep H2O

3.35

3.30

3.25

3.20Si pfu in phengite

1. biot kf phg gt ky

9. biot pl kf phg

2. biot phg gt ky 8. biot pl phg ky

5. biot pl phg gt ky 4. biot pl phg gt ky ep

6. biot pl phg gt

10. chl phg ep

7. biot pl phg gt ep 3. biot pl kf phg gt ky

10

0.14 0.150.13

(mol)

Autier SZ

ab + qz= 375°C

Autier shear zone sample. The thick black line marks the limit of H2O saturation.



3.20

3.25

3.30

3.153.10

3.35

3.40

3.45

3.50

3.55

Si pfu in

phengitebiotchl phg preab

biot chl phg

pump ab

biot

chl phg law gl ab

biot chl phg ep ab

biot phg ep ab

biot Kf phg ep ab

biot pl Kf phg ab

phglaw

ab

chl

6. biot chl phg pump pre ab

4. biot chl phg law pump ab 3. chl phg law ab

1. chl phg law jd gl

5. biot chl phg ep pump ab

7. biot phg ep pre ab

2. biot chl phg law gl ab

9. biot pl phg ep ab 8. biot pl Kf phg ep ab

1 2

3 4

5

6

7

8

9

3

4

5

6

7

8

9

10

P (k

bar)

T (°C)350 400 450 500300

NaCaKFMASH qz

15°C/km

20°C/km

30°C/km

Casterino SZ

chl i

n

3.30

3.35

biot chlphg ep ab phg ep ab

3.40

3.30

34

chl phg law ab

chl phg par law ab

chl phgep pump

ab

chl phg ep ab

chl phg ep law ab

biot chl pl phg ab

biot

biot pl phg

biot phg par ep ab

biot pl phg ab

biot chl phg ep ab

chl pl

phg

1

2

3

4

5

6

7

8

9

10

chl i

n

biot

in

3

4

5

6

7

8

9

10

P (k

bar)

T (°C)350 400 450 500300

NaCaKFMASH qz

15°C/km

20°C/km

30°C/km

3.20

3.25

3.30

3.15

3.10

3.35

3.403.45

Si p

fu in

phe

ngite

6. biot chl phg pre ab 5. chl phg pre ab

3. chl phg pump ab 2. chl phg law pump ab

1. chl phg par law jd

4. chl phg ep pre ab

7. biot phg ep ab 8. biot pl phg ep ab

9. biot chl phg par ep ab 10. biot chl pl phg ep ab

Valletta SZ

chl phg law ab

chl phg par

law ab

chl phg par law gl ab

biot chl phg ep ab

chl phg ep ab

biot chl pl phg ab

biot pl phg ab

biot phgpar ep

ab

biot pl phg ep ab

biot phg ep ab

biot chl pl phg ep ab

chl phg pump ab

5. biot chl phg pre ab 4. chl phg law pump ab

1. phg par chl law jd gl 2. chl phg par ep law ab 3. biot chl phg par ep ab

3.20

3.25

3.30

3.15

3.10

3.35

3.40

3.45

Si pfu

in ph

engi

te

chl i

n

biot

in

1

2

3

4

5

3

4

5

6

7

8

9

10

P (k

bar)

T (°C) 350 400 450 500300

Autier SZ

NaCaKFMASH qz

15°C/km

20°C/km

30°C/km

Fig. 5. NaCaKFMASH P–T pseudosections calculated for shear zone samples formed in the gneiss lithology (Autier and Valletta shear zones) and in the Permian pelite lithology(Casterino shear zone). Dotted lines indicate phase assemblage boundaries that correspond to biotite-in and chlorite-in reactions. Dashed lines represent phengite Si compositionalisopleths. Solid lines represent geothermal gradients. The gray-filled ellipses correspond to the P–T range estimates based on phengite Si isopleths and temperature calculated withthe chlorite thermometer. Note that the water content used in the calculation is 1.0 wt.% and that silica saturation is indicated by quartz present in all fields of each pseudosection.


5. 40Ar/39Ar dating

5.1. Dating of synkinematic Alpine white mica

Seven laser step-heating experiments were performed on phen-gite neoblasts from four shear zones (Autier, Casterino, Neiglier andValletta shear zone) (Table 4). Three duplicates have been done tocheck the robustness and reproducibility of Ar ages. Age spectra arepresented in Fig. 6. All samples yielded plateau ages, except sample


Mer-Autier and its duplicate Mer-Autier 2. Autier shear zone samples(Mer-Autier and Mer-Autier 2, Fig. 6a) display disturbed age spectra.Old apparent ages are observed (10–40% of 39Ar released) in the LTsteps of each experiment. In their mid to high temperature parts, thespectra steps down to a relatively flat portion (40–90% of 39Arreleased) with younger apparent ages. Sample Mer-Autier yielded a“pseudo-plateau age” of 32.7±0.5 Mawith 49.1% of 39Ar released. Thesample Mer-Autier 2 yielded no plateau age but an apparent age of34.5±0.1 Ma calculated for seven steps excluding the lower and



Table 4Summary of phengite 40Ar/39Ar dating results from the Argentera-Mercantour shear zones. Two duplicates have been performed for each sample except the Me.05.08 sample. Threedistinct J values correspond to three distinct irradiations.

Sample no. Laser power/step temperature Atmospheric cont (%) 39Ar (%) 37Arca/39Ark 38Arcl/39Ark 40Ar*/39Ark Age (Ma)

Mer Autier Phengite J1=0.017079325 15.94 7.426 0.004±0.0002 0.015±0.0013 1.037±0.019 31.69±0.61335 21.68 5.877 0.002±0.0003 0.014±0.0017 1.143±0.019 34.87±0.61345 18.51 13.35 0.002±0.0001 0.014±0.0013 1.206±0.009 36.77±0.36349 14.81 8.082 0.002±0.0002 0.014±0.0018 1.162±0.020 35.44±0.65356 13.23 16.153 0.002±0.0001 0.014±0.0010 1.143±0.010 34.89±0.38360 11.74 11.289 0.002±0.0001 0.014±0.0012 1.098±0.013 33.52±0.44366 12.48 8.582 0.002±0.0002 0.014±0.0087 1.065±0.018 32.53±0.57376 12.99 6.333 0.002±0.0002 0.014±0.0018 1.036±0.025 31.65±0.79381 13.17 11.903 0.002±0.0001 0.014±0.0010 1.071±0.012 32.69±0.40750 13.16 11.005 0.002±0.0001 0.014±0.0011 1.065±0.014 32.53±0.46

Total gas age 33.86±1.66Mer Autier 2 Phengite J1=0.017079

650 95.72 0.699 0.016±0.1221 0.022±0.0044 0.070±0.137 2.165±4.2750 20.03 7.005 0.002±0.0161 0.016±0.0075 0.899±0.012 27.49±0.40850 6.27 12.532 0.001±0.0111 0.014±0.0081 1.136±0.009 34.67±0.3950 6.20 9.232 0.001±0.0134 0.014±0.0057 1.086±0.011 33.15±0.41000 4.52 12.775 0.001±0.0133 0.014±0.0077 1.178±0.007 35.92±0.31050 4.10 23.679 0.000±0.0068 0.014±0.0042 1.142±0.005 34.85±0.31100 3.45 15.89 0.001±0.0097 0.014±0.0048 1.097±0.006 33.48±0.31150 2.24 6.138 0.002±0.0273 0.013±0.0070 1.131±0.015 34.52±0.51200 5.17 6.266 0.002±0.0244 0.014±0.0088 1.130±0.015 34.49±0.51300 7.93 3.873 0.003±0.0308 0.014±0.0012 1.200±0.025 36.59±0.81450 26.99 1.131 1.085±0.1309 0.016±0.0027 1.794±0.077 54.44±2.31550 71.76 0.781 0.014±0.2190 0.019±0.0051 1.145±0.094 34.94±2.9

Total gas age 34.09±4Me.04.01 Phengite J1=0.017068

318 10.23 19.154 0.014±0.0002 0.015±0.0011 0.942±0.011 28.79±0.4330 4.01 27.416 0.006±0.0001 0.013±0.0062 0.849±0.010 25.96±0.3344 2.84 14.011 0.002±0.0002 0.012±0.0013 0.859±0.022 26.25±0.7359 2.95 32.069 0.002±0.0001 0.013±0.0090 0.869±0.009 26.55±0.3370 15.12 1.685 0.011±0.0015 0.016±0.0091 1.208±0.157 36.83±4.8800 8.57 5.666 0.047±0.0006 0.018±0.0023 2.423±0.051 73.11±1.6

Total gas age 29.59±11Me.06.11 Phengite J3=0.003422

376 29.57 0.805 0.003±0.0600 0.028±0.0016 9.870±0.537 59.91±3.2406 7.70 4.655 0.268±0.0112 0.016±0.0049 5.628±0.138 34.40±0.8440 2.18 10.884 0.469±0.0071 0.013±0.0025 4.095±0.047 25.09±0.3480 0.60 22.778 0.240±0.0021 0.013±0.0017 4.437±0.028 27.17±0.2513 0.00 22.858 0.035±0.0019 0.012±0.0016 4.476±0.029 27.41±0.2535 0.30 16.142 0.000±0.0026 0.012±0.0012 4.434±0.039 27.16±0.2562 0.12 8.172 0.000±0.0064 0.012±0.0038 4.537±0.073 27.78±0.42000 2.05 13.705 0.009±0.0029 0.017±0.0015 10.78±0.049 65.31±0.3

Total gas age 32.88±13Gor.06.05 Phengite J3=0.003440

385 16.98 4.23 0.013±0.0070 0.020±0.0021 3.653±0.1 22.52±0.619405 0.31 2.248 0.001±0.0119 0.014±0.0054 4.242±0.110 26.12±0.671436 2.91 3.5 0.000±0.0080 0.014±0.0045 4.063±0.06 25.03±0.388465 0.09 7.149 0.003±0.0053 0.013±0.0035 4.237±0.09 26.10±0.574487 0.16 4.153 0.005±0.0053 0.013±0.0033 4.214±0.07 25.96±0.444522 0.04 13.61 0.005±0.0023 0.013±0.0010 4.246±0.04 26.15±0.22545 0.04 17.219 0.001±0.0017 0.013±0.0011 4.289±0.02 26.42±0.14567 0.04 15.638 0.004±0.0020 0.012±0.0012 4.315±0.03 26.57±0.176600 0.02 19.582 0.009±0.0016 0.013±0.0017 4.375±0.02 26.94±0.138640 0.14 3.124 0.042±0.0126 0.011±0.0030 4.447±0.09 27.38±0.5432000 0.00 9.549 0.381±0.0041 0.012±0.0011 4.534±0.040 27.91±0.245

Total gas age 26.42±1.018Merc.07.08 Phengite J1=0.017071

334 9.990 5.135 0.008±0.0004 0.018±0.0028 0.888±0.05 27.14±1.475345 79.037 6.763 0.008±0.0003 0.018±0.0025 0.799±0.03 24.43±1.016353 0.670 6.846 0.007±0.0003 0.014±0.0010 0.841±0.02 25.72±0.675360 2.745 6.901 0.007±0.0003 0.014±0.0016 0.831±0.02 25.43±0.522369 0.916 20.757 0.005±0.0001 0.013±0.0057 0.860±0.01 26.3±0.289379 1.331 13.209 0.004±0.0001 0.013±0.0068 0.858±0.01 26.24±0.42395 1.964 25.017 0.003±0.0001 0.013±0.0058 0.870±0.01 26.61±0.317415 2.664 8.916 0.011±0.0002 0.013±0.0011 0.906±0.06 27.69±1.748465 9.994 0.827 0.149±0.0019 0.019±0.0012 1.261±0.14 38.42±4.293750 1.873 5.627 0.188±0.0019 0.014±0.0022 0.936±0.03 28.59±0.96

Total gas age 26.54±1.413Me.05.08 Phengite J2=0.003839

320 52.26 3.00 u.b.l±u.b.l 0.01±0.0031 3.45±0.36 23.71±2.5370 32.72 17.57 u.b.l±u.b.l 0.01±0.0052 3.02±0.05 20.78±0.4400 22.02 25.59 u.b.l±u.b.l 0.01±0.0037 2.95±0.03 20.29±0.2430 8.22 33.97 u.b.l±u.b.l 0.01±0.0024 2.90±0.03 19.93±0.2


Please cite this article as: Sanchez, G., et al., Dating low-temperature deformation by 40Ar/39Ar on white mica, insights from the Argentera-Mercantour Massif (SW Alps), Lithos (2011), doi:10.1016/j.lithos.2011.03.009


Table 4 (continued)

Sample no. Laser power/step temperature Atmospheric cont (%) 39Ar (%) 37Arca/39Ark 38Arcl/39Ark 40Ar*/39Ark Age (Ma)

Me.05.08 Phengite J2=0.003839460 0.00 6.09 u.b.l±u.b.l 0.01±0.0011 3.19±0.13 21.92±0.91111 16.60 13.78 u.b.l±u.b.l 0.01±0.0038 3.00±0.08 20.64±0.5

Total gas age 20.50±0.8


highest step (1, 2, 10, 11, and 12) which corresponds to 86.5% of 39Arreleased. A concordant isochron age of 34.0±1.1 Ma can be defined.The 37ArCa/39Ark spectra for the two samples display flat pattern with0.01–0.001 value which is similar with the Ca/K obtained on phengiteby EPMA. 38ArCl/39Ark values are homogeneous reflecting a singlephengite population.

Casterino shear zone samples (Me.04.01 and Me.06.11; Fig. 6b)exhibit similar saddle-shaped age spectrum with an apparent stepages ranging from 73.1±1.6 to 26.0±0.3 and from 65.3±0.3 to25.1±0.3 from high to low temperature steps, respectively. Thesepatterns reflect some very slight Ar excess, which likely influencedthe lower and higher temperature steps (Arnaud and Kelley, 1995).However, the spectra central portions preserve flat and concordant agesteps. The two experiments have weighted average ages of 26.3±0.2(with steps 2–4) and 27.3±0.1 (with steps 3–7), which include 73.5%and 80.8% of 39Ar released, respectively. So, we interpret these ages asmaximum ages. The 37ArCa/39Ark spectra for both samples display asimilar saddle shape with lower values identical to Ca/K obtained onphengite by EPMA.

Neiglier shear zone samples (GOR.06.05 and Merc.07.08; Fig. 6c)show flat Ar spectra, which allow computation of precise plateau agesat 26.3±0.1 Ma (steps 4–9) and 26.3±0.5 Ma (steps 1–7) using77.3% and 84.6% of released 39Ar, respectively. The Ca/K ratio is lowand consistent with EPMA values. Ca/K and Cl/K ratios are homoge-neous and unrelated to any age variation, which is concordant with amonogenic phengite population.

The Valletta shear zone sample (Me.05.08; Fig. 6d) displays a well-defined plateau age (steps 1–6) of 20.5±0.3 Ma with 100% of released39Ar. The Ca/K ratios were below detection limit due to the low contentof Ca in phengite and to the time spent between irradiation and analysis.However, the Cl/K ratios are low and homogeneous, which reflects, asfor previous samples, a monogenic phengite population.

Three previously published ages of neocrystallized phengites fromthe Fremamorte shear zone (MC6a; Fig. 6e) undertaken by Corsini et al.(2004) are included in this paper for comparison. The three experimentsdisplay a concordant age spectra defining an average plateau age of22.2±0.3 Ma for N75% of 39Ar released in each analysis.

5.2. Dating of Variscan white mica porphyroclasts

The seven analyses performed by Corsini et al. (2004) on strainedVariscan white micas have been included in the paper for discussion,and briefly described below. Analyzed samples are (1) strained gneissesof the Valletta shear zone and (2) mylonitized Argentera granite withinthe Fremamorte shear zone (Fig. 2; Fig. 7).

Valletta shear zone samples (MC24,MC26 andMC27; Fig. 7a) displaysimilar staircase shaped age spectrum with apparent ages rangingfrom 147.8±6.0 to 316.3±0.7 and 104.0±9.7 to 316.2±0.8 and151.2±4.0 to 321.6±0.7, respectively. Each age spectra convergestowards an age range of 305–315 Ma (for N88% of 39Ar released) inhigh temperature steps, whereas apparent ages of low temperaturesteps are younger and more variable (~104 to ~151 Ma). This patternclearly indicates some Ar loss or mixing with a younger, probablyAlpine, component that affect the lower temperature step ages.Additionally, in intermediate temperature steps the age spectradisplay a saddle-shape which is interpreted as some partialrecrystallization of the muscovites into phengites during the Alpinemetamorphic event.


Fremamorte shear zone samples (MC6a, MC6b and MC7; Fig. 7b)exhibit staircase shaped spectra similar to samples from the previousgroup of the Valletta shear zone. The high temperature part of the agespectrum converges to an age at around 296–299 Ma for N60% 39Arreleased. It can be noticed that the least deformed sample (MC8)displays a more disturbed age spectrum than the ultramyloniticsample (MC6a).

6. Discussion

The main question addressed in this paper is “how can we datedeformation events”?

In the following we discuss this issue in the light of 40Ar/39Ar datingof preserved inherited Variscan white mica porphyroclasts andneocrystallized synkinematic phengite within distinct low temperatureshear zones of the Argentera-Mercantour massif that are characterizedby intense fluid circulations, and are sites of mineral reactions involvingbreakdown of Variscan minerals.

6.1. Robustness of the 40Ar/39Ar system in white mica porphyroclastsduring low temperature (b400 °C) intra-granular ductile deformation

Studies applying the 40Ar/39Ar laser microprobe technique haveshown that single mineral grains can be partially reset duringdeformation (e.g., Kramar et al., 2001; Mulch et al., 2002). Severalgrain-scale deformation mechanisms may be considered to accountfor the Ar loss, such as grain boundary sliding, cleavage sliding andfracturing, kinking, pressure-dissolution and internal grain boundarymigration (Hames and Cheney, 1997; Kramar et al., 2001; Lee, 1995;Mulch et al., 2002; Reddy and Potts, 1999; Reddy et al., 1999, 2001).For instance, based on observed lattice microstructures such as kinkbands, zones of intense dislocation density and fractures, Mulch et al.(2002) proposed that argon loss diffusion occurred microstructurallywithin segments over length scales defined by the orientation andspacing of the internal structures. These processes are thought toaffect less than 20% of the age spectra, at low and intermediatetemperature steps while the high temperature steps preserve an ageclose to the initial crystallization age, or a minimum age. In this case,Scheuber et al. (1995) suggested that the younger age obtained on thelower part of perturbed 40Ar/39Ar age spectra would represent adeformation age.

Ar age spectra of Variscanmuscovite porphyroclasts presented in thisstudy show similar patterns as those described by Mulch et al. (2002).Step 40Ar/39Ar age frequency gives a peak in the range 305–315 Ma forthe Valletta shear zone and 296–299 Ma for the Fremamorte shear zone,corresponding to the HT steps. Similar ages were also obtained in theVariscan basement that was not affected by any Alpine metamorphicoverprint in the Maures-Tanneron massif located 30 km SW of theArgentera-Mercantour massif (Corsini et al., 2010). These 300–310 Maages on white mica in Argentera-Mercantour massif should beconsidered as crystallization ages during the late high temperatureVariscan thermal event (Corsini andRolland, 2009; Rolland et al., 2009a).In the LT spectra part, 20% of 40Ar/39Ar spectrum is still perturbed,witnessing an Ar loss process (Fig. 6). Several mechanisms (volumediffusion, deformation, fluid circulation, neocrystallization) can beresponsible for such loss and are discussed below.

At these low metamorphic temperatures (375±30 °C), thermallyactivated volume diffusion of radiogenic argon in micas cannot be



Age : 32.7 +/- 0.5 Ma

Age : 34.5 +/- 0.1 Ma

Autier Shear Zone

0

10

20

30

40

50

App

aren

t Age

(M

a)

10020 40 60 80

Cumulative % 39Ar released

Mer-Autier Mer-Autier 2

Phengite

Maximum age : 26.3 +/- 0.2 Ma

Maximum age : 27.3 +/- 0.1 Ma

Casterino Shear Zone

0

16

32

48

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80

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10020 40 60 80

Me.06.11Me.04.01

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Average Ca/K obtained on phengite by EPMA

Neiglier Shear Zone


21

15

27

33

39

45

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t Age

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a)

20 40 60 80 100

Age : 26.3 +/- 0.1 Ma

Age : 26.3 +/- 0.5 Ma

Gor.06.05 Merc.07.08

Phengite

Average Ca/K obtained on phengite by EPMA

10

14

18

22

26

30

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aren

t Age

(M

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10020 40 60 80



Age : 20.5 +/- 0.3 Ma

Valetta Shear Zone Me.05.08

Phengite

10^-4

10^-3

10^-2

10^-1

10^0

10^-4

10^-3

10^-2

10^-1

10^0

10^-4

10^-3

10^-2

10^-1

10^037

Ar c

a/39

Ar k

37A

r ca/

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r k

37A

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39A

r kAverage Ca/K obtained on phengite by EPMA

ba

c

d

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a)

Cumulative % 39Ar released0 20 40 60 80 100

0

6

12

18

24

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22-23 Ma

MC6a : Neocrystallized

phengite duplicates Fremamorte Shear Zone

eFrom Corsini et al., 2004

Fig. 6. 40Ar/39Ar spectra of Alpine neo-crystallized phengites in the Alpine shear zone samples. a–d. 40Ar/39Ar spectra obtained in this study in Autier (ASZ), Casterino (CSZ), Neiglier(NSZ), and Valletta (VSZ) shear zones. e. 40Ar/39Ar spectra obtained in the Fremamorte (FSZ) shear zones by Corsini et al., 2004.


Please cite this article as: Sanchez, G., et al., Dating low-temperature deformation by 40Ar/39Ar on white mica, insights from the Argentera-Mercantour Massif (SW Alps), Lithos (2011), doi:10.1016/j.lithos.2011.03.009


20 40 60 80 1000

350

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MC26

MC27 Valetta Shear Zone

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Cumulative % 39Ar released0

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Strained Muscovitesporphyroclasts

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MC6b

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296-299 Ma


Fremamorte Shear Zone

a

b

Fig. 7. 40Ar/39Ar spectra of deformed Variscan gneiss muscovites in (a) Valletta and(b) Fremamorte shear zones from Corsini et al., 2004.


significant (Harrison et al., 2009). Above all, the samples were takenon deformation gradients showing increasing and heterogeneousstrain from undeformed to ultramylonite gneiss, from rim to core ofshear zones. A noticeable fact considering the 40Ar/39Ar age spectraon muscovite porphyroclasts obtained on protomylonites, mylonitesand ultramylonites is that patterns are globally identical. However,the ‘bias’ consisting in hand-picking well preserved porphyroclastswhatever the ductile strain intensity does not lead to a well establishedlink between bulk strain and the amount of argon loss. In any cases,along the deformation gradient, differentmechanisms ofmica porphyr-oclasts deformation can be observed such as grain-boundary slidingespecially in less deformed samples and intra-granular sliding, kink-band, dislocation network and (micro)-fracture. Thus, the presence ofabove-mentioned deformationmechanisms can control argon diffusionby promoting lattice migration and enhancing interconnectivity(Hames and Cheney, 1997; Kramar et al., 2001; Mulch and Cosca,2004; Mulch et al., 2002). Isotopic and compositional heterogeneity indeformed white mica due to recrystallization and neocrystallizationduring deformation and metamorphism are also suspected to controlargon diffusion and thus influence 40Ar/39Ar age spectra of muscoviteporphyroclasts (e.g. Scaillet et al., 1990). However, we are unable todiscriminate the possible impact of inclusions of neocrystallizedphengites of very small size (b10 μm) from the effect of strain-inducedrecrystallization of muscovite. Last factor often invoked to explain Arloss is the presence of fluids. Diffusion can be enhanced by grainalteration due to fluid advection at grain boundaries. In experimentalstudies Harrison et al. (2009) observed similar stair-cased patterns inlow-temperature portions of the argon age spectra of samples thatexperienced intense hydrothermal alteration. Thus, it is possible that Arloss could be ascribed to some alteration of grain rimsdue tofluid influx.


All the above mentioned mechanisms may explain the average 20%argon loss observed in the 40Ar/39Ar age spectra of deformed Variscanmuscovite. However, younger ages obtained in the low temperatureperturbed part of 40Ar/39Ar age spectra are not consistent from sampleto sample, ranging between 65 Ma to 195 Ma depending on the shearzone considered. Therefore, the younger ages cannot be considered aseven a minimum deformation age. They always fall significantly above(between 30 and 160 Ma older) the ‘true’ Alpine ages given byphengites. Consequently, our results demonstrate that intenselydeformed Variscan muscovites within the Argentera-Mercantourgreenschist facies shear zones preserve most of their initial 40Ar/39Arages. Deformation mechanisms and fluid circulation were not efficientenough to totally reset the 40Ar/39Ar isotopic system in deformed whitemicas. Thus, the dating of deformation events by the Ar–Ar techniqueapplied to extremely deformed K-bearing minerals that experiencedductile strain and metamorphism does not appear to be a suitableapproach.

6.2. Significance of 40Ar/39Ar ages on synkinematic phengitesneocrystallization

Mylonitization within Argentera-Mercantour shear zones duringAlpine deformation occurred under greenschist-facies conditions. Thisleads notably to the synkinematic growth of phengite, a new potassicmineral phase, at the expense of biotite and plagioclase or clays. Theseneocrystallized white micas are well-developed in the myloniticfoliation and in shear bands. Their presence and proportions increasealong the deformation gradient from protomylonite to ultramylonite.All thesemicrostructural and petrographical observations evidence thatthese phengites were neocrystallized synkinematically during theAlpine deformation events. Thus, 40Ar/39Ar ages obtained on thesesynkinematic neocrystallized white micas are most likely to representdeformation ages. Nevertheless, several processes such as (i) addition ofexcess 40Ar or (ii) 40Ar loss can potentially lead to aging or rejuvenationof 40Ar/39Ar ages respectively and affect their geological significance.

(i) Generally, incorporation of an excess of 40Ar in crystalline latticeof analyzed mineral is discussed in terms of solubility of argon in thedifferent mineral and fluid phases (e.g. Kelley, 2002). Indeed, Argon is ahighly incompatible trace element, which strongly favors partition fromminerals into grain boundary fluids. Thus, this highly incompatiblenature of Argon in fluid/mineral systems makes the fluid an infinitereservoir for this gas once it is released in the fluid. The mylonites fromthe Argentera-Mercantour massif have been formed at mid-crustaldepth during intense fluid–rock reaction, as evidenced by the synkine-matic phengite–chlorite assemblage (see ‘Petrology’ Section 4). Only40Ar/39Ar age spectra of Casterino shear zone show typical U-shapereflecting possible 40Ar excess. Textural and mineralogical observationsin this shear zone evidence that phengite crystallized by substitutionprocess of clay minerals. Thus, in this case, phengite can partly inheriteradiogenic argon derived fromK-bearing clayminerals. Contrariwise,in the other shear zones, 40Ar/39Ar age spectra are not affected by40Ar excess. However, in this case, neocrystallization of phengiteoccurred from dissolution–transport–recrystallization (or precipita-tion) in different textural site than the K-rich Variscan minerals,unlike the Casterino shear zone. During the Alpine greenschist faciesmetamorphism, the argon located in these minerals (white mica, K-feldspar, and biotite) was progressively released in the rock duringtheir breakdown, which was accelerated by intense shearing andfluid–rock interaction. Thus, argon was easily transferred into thehydrous fluid phase (mainly H2O) as its solubility in the fluids islargely higher than in the neocrystallizing white mica (Kelley, 2002).Commonly, the mid-crustal shear zones are considered as “opensystems” where the time for fluid residence is particularly short(Dipple and Ferry, 1992; Fourcade et al., 1989). Consequently, theexcess argon that concentrated into the fluid phasewas removed andevacuated from the system. Such a process is necessary to account for




the homogeneity of age spectra obtained on Alpine white micas andtheir reproducibility. One limitation to this model will be to considerthat the system is argon-saturated (Schneider et al., 2008). In thiscase, the phengites that crystallized from Variscan mineral break-down during Ar-saturated fluid flow should incorporate argon intothe new mineral phase and lead to 40Ar excess. Nevertheless, thegood reproducibility of the 40Ar/39Ar age spectra strongly suggeststhat 40Ar excess is insignificant.

(ii) Radiogenic argon loss may be the result of either diffusionprocess or leaching of phengites due to circulating fluids. Thermallyactivated diffusion is probably limited given that the estimatedmetamorphic temperature (375±30 °C) is low compare to minimalargon diffusion temperature in white mica (e.g. 425 °C; Harrison et al.,2009). Furthermore, intracrystalline diffusion in water-saturateddomain is greatly enhanced (Faquhar et al., 1996; Freer, 1981).Thus, as shear zones are characterized by intense fluid flow, the argonloss effect by thermally activated diffusion will be insignificant inregard to the leaching of argon by fluids. The 40Ar/39Ar age spectra ofneocrystallized phengites from Fremamorte, Neiglier and Autier shearzones present slight perturbations in their lower temperature part(Fig. 7). These perturbations reflect small amounts with a maximumof 10–15% of 40Ar loss into the Argentera-Mercantour neocrystallizedphengites. However, the duplicate 40Ar/39Ar age spectra have similarpatterns and give identical plateau ages. Thus, even if phengites weresubject to 40Ar exchange probably due to the ongoing fluid–mineralinteraction after mineral crystallization, the 40Ar loss does not seem toaffect the inner part of the grains that contribute to the Ar spectra hightemperature part.

In the light of the different above-discussed processes which canpotentially influence 40Ar/39Ar age, the 40Ar/39Ar plateau ages obtainedon synkinematically neocrystallized phengites represent the age ofdeformation and associated fluid activity.

6.3. Timing and condition of deformation in the Argentera-Mercantourmassif

As discussed above, the dating of synkinematic phengite that can belinked directly to P–T-x (Pressure–Temperature-fluid) estimates andtectonic analysis allows quantification of the timing and conditions ofdeformation in crustal shear zones. This is essential for the reconstruc-tion of precise P–T-t-ε (Pressure–Temperature-time-deformation)paths. In the case of the Argentera-Mercantour massif, phengite whichcrystallized in strain shadows and foliation domains indicates consis-tently inverse or dextral shear sense in the five shear zones analyzed.These shear zones were formed at mid-crustal depths (4.8bPb7 kbar)and temperature conditions (375±30 °C) suggesting shear zoneactivation at depths between 15 and 21 km. Several stages of shearzone development have been constrained between 34 and 20 Ma. Thefirst age group (34 Ma) is within the range of ages obtained for thePenninic Frontal Thrust activity in the Pelvoux ECM closer to the NWpart of themassif and are likely related tounderthrusting below internalzones (Simon-Labric et al., 2009). The other age groups (26–20 Ma) arecompatible with a transpressional zone in a constant N–S shorteningcontext (Baietto et al., 2009; Corsini et al., 2004). Based on FT data(Bigot-Cormier et al., 2006; Sanchez et al., 2011), we propose that theorientation of the main shortening direction in the Argentera-Mercantour massif has not changed significantly since 26 Ma. ThisOligo-Miocene tectonic evolution is in strongagreementwith themodelof continental deformation of the Periadriatic system related to theanticlockwise Adriatic plate indentation during the Alpine collision (seeFig. 10 in Ciancaleoni and Marquer, 2008).

7. Conclusions: implications for the construction of P–T-t-σ paths

This study emphasizes the main processes controlling the 40Ar/39Arsystem in mylonitic ductile shear zones during low temperature


(b400 °C) metamorphic events. To summarize, the main contributionsof this paper are that:

1. Fluid circulation and ductile deformation have limited effects onthe resetting of the 40Ar/39Ar system in inherited white mica. Thus,even extremely deformed minerals cannot provide the time ofdeformation, not even ‘a minimum age close to the realdeformation age’ as previously suggested.

2. To obtain an age of deformation, it is necessary to separate amineral that crystallized during the shearing event in an opensystem with respect to the fluid phase. Dated mineral has tocrystallize from a different mineral species in other textural sites ordirectly by a precipitation reaction from a fluid phase. Indeed,substitution reactions during the shearing event could lead tosignificant 40Ar inheritance. Furthermore, fluids in open system canact as a vector for radiogenic argon evacuation and thus limit thepotentiality of 40Ar excess. As deformation and fluid flow in shearzone are major controls on the neocrystallization, the argon ageprovides a reliable age of shear deformation and fluid activity.

3. The Si substitution of phengite in NaCaKFMASH system is highlydependent on fluid saturation. In the water-saturated domain, theSi-phengite can be used as a good geobarometer and providespressure over-estimates in water-undersaturated conditions.

Supplementarymaterials related to this article can be found onlineat doi:10.1016/j.lithos.2011.03.009.

Acknowledgments

We are grateful to the deep-insight and constructive reviews of S.Reddy and S. McLaren and to the editorial work of I. Buick, whichsignificantly improved the previous version of this paper. We wish tothank the efforts of Marc Hassig who significantly improved theEnglish language of this paper.

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