New 40 Ar/ 39 Ar constraints for the “Grande Nappe”: The largest rhyolitic eruption from the Mont-Dore Massif (French Massif Central)

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New 40 Ar/ 39 Ar constraints for the “Grande Nappe”:

The largest rhyolitic eruption from the Mont-Dore Massif (French Massif Central)

Sébastien Nomade, Jean-François Pastre, Alison Pereira, Alexandra Courtin-Nomade, Vincent Scao

To cite this version:

Sébastien Nomade, Jean-François Pastre, Alison Pereira, Alexandra Courtin-Nomade, Vincent Scao.

New 40 Ar/ 39 Ar constraints for the “Grande Nappe”: The largest rhyolitic eruption from the Mont- Dore Massif (French Massif Central). Comptes Rendus Géoscience, Elsevier, 2017, 349 (2), pp.71-80.

�10.1016/j.crte.2017.02.003�. �hal-01515754�

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Petrology, Geochemistry (Geochronology)

New

⁴⁰

Ar/

³⁹

Ar constraints for the ‘‘Grande Nappe’’:

The largest rhyolitic eruption from the Mont-Dore Massif (French Massif Central)

Se´bastien Nomade

^a,

*, Jean-Franc ¸ois Pastre

^b

, Alison Pereira

^a,c,d,e

, Alexandra Courtin-Nomade

^f

, Vincent Scao

^a

aLaboratoiredessciencesduclimatetdel’environnement(IPSL-CEA-CNRS-UVSQ)etUniversite´ Paris-Saclay,domaineduCNRS,baˆt.12, avenuedelaTerrasse,91198Gif-sur-Yvette,France

bLaboratoiredege´ographiephysique,Environnementsquaternairesetactuels,UMR8591CNRS,Universite´sParis-1etParis-12,1,place Aristide-Briand,92195Meudoncedex,France

cDe´partementdepre´histoireduMuse´umnationald’histoirenaturelle,UMR7194duCNRS,1,rueRene´-Panhard,75013Paris,France

dSezionedisciencepreistoricheeantropologiche,DipartimentodiStudiUmanistici,Universita` degliStudidiFerrara,C.soErcoled’EsteI, 32Ferrara,Italy

eE´colefranc¸aisedeRome,PiazzaFarnese,00186Roma,Italy

fUniversite´ deLimoges,GRESE,EA4330,FST,123,avenueAlbert-Thomas,87060Limoges,France

1. Introduction

Two large stratovolcanoes are known in the French Massif Central: the Cantal and the Mont-Dore Massif (Fig.1).Theyoungestoftheseediﬁces(Mont-Dore)covers

anarea of500km²and rests on theVariscan basement (CantagrelandBaubron,1983).Thetotalvolumeerupted fromtheMont-DoreMassifhasbeenestimatedbetween 220km³ and 70km³ (Brousse, 1971; Cantagrel and Baubron, 1983; Cantagrel and Briot, 1990; Vincent, 1980;Fig.1).Thismassifiscomposedoftwocoalescent stratovolcanoes:theGue´ryandtheSancy(Cantagreland Baubron,1983;Nomadeetal.,2014a;PastreandCantagrel, 2001). The diversity and the wide dispersion of its C.R.Geoscience349(2017)71–80

ARTICLE INFO

Articlehistory:

Received19January2017

Acceptedafterrevision9February2017 Availableonline18March2017 HandledbyMarcChaussidon

Keywords:

Mont-DoreMassif GrandeNappe

40Ar/³⁹Ar

Xenocrystscontamination Haute-Dordognecaldera France

ABSTRACT

Sincethe1960s,anearlyexplosiveactivityintheMont-DoreMassifisassociatedwitha majorpyroclasticrhyoliticeruption(5–7km³)knownasthe‘‘GrandeNappe’’(GN).This event, linked to the formation of a 6-km-diameter cryptic caldera named ‘‘Haute Dordogne’’,wasbeforeourinvestigationdatedby⁴⁰Ar/³⁹Arat3.070.04Ma.Ournew single-crystallaserfusion⁴⁰Ar/³⁹ArdatesobtainedontwooutcropsoftheGN(Rochefort- MontagneandLudie`res)questionedseveralhypothesesmadeconcerningthis‘‘landmark’’

eventoftheMont-DoreMassifhistory.Wedemonstratethat:(1)theGNrhyoliticeruptionhas occurred much later than previously estimated (i.e. 2.770.02–0.07Ma full external uncertainties);(2)thecorrelationmadebetweentheVendeixrhyoliticcomplexes(intra- caldera position) dated back to 2.740.04Ma and the GN is proposed; (3) xenocryst contaminationcouldbeveryhigh(i.e.70%fortheRochefort-MontagneGNoutcrop)and explainsthenoticeableolderageobtainedpreviously;(4)alinkbetweentheGNeruptionand theformationofacalderaisquestionable;thehypothesisofanorthward-orientedblast channeledeastwardtowardthepaleo-AllierRiveristhusproposed.

C 2017Acade´miedessciences.PublishedbyElsevierMassonSAS.Thisisanopenaccess articleundertheCCBY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/

4.0/).

* Correspondingauthor.

E-mailaddress:sebastien.nomade@lsce.ipsl.fr(S.Nomade).

ContentslistsavailableatScienceDirect

Comptes Rendus Geoscience

w ww . sc i e nce d i re ct . co m

http://dx.doi.org/10.1016/j.crte.2017.02.003

1631-0713/^C 2017Acade´miedessciences.PublishedbyElsevierMassonSAS.ThisisanopenaccessarticleundertheCCBY-NC-NDlicense(http://

creativecommons.org/licenses/by-nc-nd/4.0/).

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pyroclastic products have been used for stratigraphic correlationanddatingofthePlio-Pleistoceneformationsin theFrench MassifCentral and elsewhere (Degeai etal., 2013;Nomade etal., 2010;Pastre and Cantagrel,2001;

Pastreetal.,2015).Severalrecent⁴⁰Ar/³⁹Arinvestigations haveallowedreﬁningthehistoryoftheMont-DoreMassif explosiveactivity(Nomadeet al.,2012,2014a,b).These newchronologicalconstraintshavehighlightedthat the pyroclasticactivityofthemassifwasmorecomplexthat previouslythoughtandcomprisedofeightcycles(3.1Ma to300ka)lastingabout100kaeach(Nomadeetal.,2014a).

Despite these recent efforts, several questions remain open,inparticularamongtheearlyexplosiveactivityof theGue´rystratovolcano(cycleG.IandG.IIinNomadeetal., 2014a).Theaccurateage,thevolumeandtheoriginofthe iconicrhyolitic eruption known as the ‘‘GrandeNappe’’

(GN hereafter; Vincent, 1979; 1980) remain elusive, mainly because of the lack of single crystal ⁴⁰Ar/³⁹Ar investigation.

2. The‘‘GrandeNappe’’:geologicalbackgroundand currentageconstraints

Theterm‘‘GrandeNappe’’,created byVincent(1979, 1980),designatesthemainpyroclasticformationofMont- Dorewitharhyoliticcomposition.This‘‘eruption’’isalso known under different names: ‘‘nappe supe´rieure’’

(Brousse,1963),‘‘napperhyolitique’’,‘‘nappedeRochefort’’

and ‘‘nappe of Sailles’’ (Besson, 1978), ‘‘nappe externe’’

(Brousse, 1984), or Mont-Dore ignimbrite (Pastre and Cantagrel, 2001). According to the original suggestion

ofVincent(1979),theGNincludesunderthisgeneralname all rhyolitic products found under ‘‘the intermediate complex’’deﬁnedbythis authorandthe‘‘upperpumice stones’’foundintheexternalﬂanksoftheMont-Dore.The formation of this GN is suggested to be linked to the formation of a cryptic caldera named C1 or ‘‘Haute- Dordogne’’(Vincent,1979,1980).Thelimitofthiscaldera remainsunclearastracesofthisdepressionareprobably buriedbelowmorerecentproducts.Fromachronological pointofview,wenowknowthatintheinternalpartofthe Gue´ry stratovolcano the ‘‘upper pumice stones’’ corre- spond in reality to several trachytic pyroclastic units emplaced in two phases (G.II: 2.73–2.60Ma and G.III:

2.40–1.95Ma)aswellastotheyoungerSancystratovol- canostarting toeruptmuchlater,atabout1.1Ma (C.I.) (Nomade et al., 2012, 2014a). Therefore, the GN is by deﬁnitionolderthan2.730.02Ma,whichistheageofthe oldest trachyticpyroclasticrocklocatedabovetheGN(La BourbouleHaut,Nomadeetal.,2014a).Thecurrentageofthe GN(i.e.3.070.04Ma(2

s

uncertainty),Fe´raudetal.,1990;

Fig.1)correspondstotheweightedmeanofthreeindepen- dentages(i.e.sanidinepopulationsincluding5–7crystals) coming fromasingle outcrop(i.e.Sailles).Because ofthe ubiquistxenocrysticcontaminationoftheGue´rypyroclastic productsshowninNomadeetal.(2014a),theaccuracyofthis agecouldlegitimatelybequestioned.

Historically, the GN outcrops weredivided into two groups: proximal/internal or distal/external deposits corresponding to outcrops found inside (e.g., Brousse, 1963; BrousseandLefe`vre, 1966;Me´nard, 1979;Pastre, 1987; Pastre and Cantagrel, 2001; Vincent,1979, 1980) Fig.1.Mont-DoreMassifsyntheticgeologicalmapandsamplinglocation(modiﬁedfromCantagrelandBaubron,1983).TheMont-DoreMassifis constitutedbytheSancyandGue´rystratovolcanoes(PastreandCantagrel,2001).ThelowerandupperextensionlimitsoftheGNcalderaarefromMossand etal.(1982)andVaretetal.(1980),respectively.

S.Nomadeetal./C.R.Geoscience349(2017)71–80 72

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andoutside(Besson,1978;Me´nardetal.,1980;Ly,1982;

Pastre,1987; PastreandCantagrel, 2001)ofthelimit of what is considered to be the Haute Dordogne caldera limits. The GN volume was estimated between 7km³ (Brousseand Lefe`vre, 1966)and 5km³ (Vincent, 1980), makingitthelargesteruptiveformationoftheMont-Dore Massif (Equiv. 5<VEI<6; Newhall and Self, 1982).

However, the GN eruption remains small compared to themajoron-landignimbrites,whichcanrangebetween 100and3000km³(CasandWright,1988).

Intheinternalzone(i.e.intra-caldera),theGNincludes the zeolithized facies found in la ‘‘Bugette’’ and the rhyolitic pyroclastite pumiceousflow of Fenestre–Bour- boule(‘‘nappeinfe´rieure’’ofBrousse(1963),Brousseand Lefe`vre(1966),Meńard(1979)(Fig.1)aswellasaccording to Pastre (1987) and Pastre and Cantagrel (2001) all pumiceous surges and flows visible between 1000 and 1040ma.s.l.intheVendeixsmallvalleyalongroadD.88 (‘‘intermediate complex’’of Vincent,1979). These pyro- clasticrocksarealsofoundthroughouttheperipheryofthe LaBourboulecityandexposedalongtheEausaleéravine andtheVendeixstream(i.e.theVendeix-Sie`geformation inNomadeetal.,2014a).BetweenLaBourbouleandthe southernpartoftheVendeixvalley,theseformationswere dropped by120mdue totheactivityof a normalfault visiblenorthofthecityofLaBourboule(Fig.1).Inthe innerzone,theGNliesontopofthevolcano-sedimentary sequence madeoftuffites,which followedthetrachytic pyroclastic flowof LaBourboule (Pastre,1987;Pastre and Cantagrel, 2001) or ‘‘infrabasale’’ for Meńard (1979). However, the presence of the GN inside the Bourbouledepression,interpretedasthenorthernedge of theHauteDordognecaldera,wasquestionedbythe

40Ar/³⁹Ar ages obtained by Nomade et al., 2014a (see below), whichsuggestan ageof 2.740.02Mafor the

‘‘Vendeix-Sie`ge’’formation,whichismuchyoungerthan thesuggestedagefortheGN(c.a.3.070.04Ma;Fe´raudetal., 1990).

Intheexternalzone(i.e.extra-caldera),theGNoutcrops in the northern and eastern borders of the Mont-Dore Massif(Fig.1),whereithasthecharacteristicfaciesofa coherentrhyolitictuffincludedvariousamountsofﬁbrous pumicestones(12–15mthickintheRochefort-Montagne oldquarry,forexample)(Fig.2)aswellaslithicfragments fromoldervolcaniceventsandHercynianbasementsrocks (Fe´raudetal.,1990). Theﬁbrouspumicestoneshavean alkalinerhyolitecomposition(SiO2=73–74%and33–36%

of normative quartz) similar to that of the rhyolite of LuscladefoundintheCantalstratovolcano(Besson,1978).

Theyrepresenttheultimateproductofthedifferentiation processbyfractionalcrystallization(Me´nardetal.,1980;

Villemantetal.,1980).Theparagenesisofthesesubaphyric pumicestonesincludesalkalifeldspars(sanidine),green amphibole (edenite) and magnetite. Quartz,brown amphibole,biotiteandtitaniteareonlyfoundinthematrix.

Quartzandsanidinecrystalsarepresentinthematrixwith scarcepyroxene,biotiteandtitanite(PastreandCantagrel, 2001).TheGNissuggestedtoreachtheAllierRiveronthe east(Fig.1)(Ly,1982;Pastre,1987).BetweenthePerrier plateau and Rochefort-Montagne, several outcrops are reported north of Saint-Nectaire – i.e. Ludie`res; Sailles

(3.070.04Ma;Fe´raudetal.,1990);FargesandLesArnats (Fig.1).TheeasternextensionoftheGNwasinterpretedas thechannelingofthiseruptioninanorth-andeast–west- oriented system of paleo-valleys (Pastre and Cantagrel, 2001).Thesameeastwardchannelingprocesswasproposed forthefourdebrisavalanchesduetowestwardcollapsesof the Gue´ry stratovolcanodated backto between 2.61and 2.58Ma(Nomadeetal.,2014a).Theseeventsfossilizedthe paleo-AllierriverandarenowexposedinthePerrierplateau sequence(Bernardetal.,2009;Nomadeetal.,2014b;Pastre, 2004).

ThecurrentvolumeandtheextentoftheGNarebased on the assumption that virtually all rhyolitic products found north and east of the Mont-Dore belong to this event.Asa matteroffact, the‘‘Vendeix-Siege’’rhyolitic surgesandﬂowsfoundintheleftbankofVendeixstream in intra-caldera position, considered as an internal equivalentoftheGN(Pastre,1987;PastreandCantagrel, 2001), were dated at 2.740.02 Ma by Nomade et al.

(2014a) (Fig. 1) suggesting that despite their chemical, lithologicalandmineralogicalsimilaritiesthesetwoevents cannotbecorrelatedanymore.Finally,Nomadeetal.(2014b) alsodatedarhyoliticpumicefallstratigraphicallybelowthe GN inthePerrier Plateau(i.e.PER128at3.090.01Ma), suggesting that at least one older event pre-dates the proposedcalderaformationandGNeruption(Fig.1).

Inordertostartunravelsomeofthequestionsraisedby therecentgeochronologicalinvestigationsandtoimprove theprecisionandaccuracyoftheGNeruption,wedecided toconductadetailed⁴⁰Ar/³⁹Arinvestigationusingsingle sanidinecrystallaserdatingontwowell-knownoutcrops bothlocatednorthandeastoftheMont-DoreMassif(i.e.

Rochefort-MontagneandLudie`res).Thestudiedoutcrops arepresentedindetailbelow.

3. Detailsampling

Twooutcropswerechosenforour⁴⁰Ar/³⁹Arinvestiga- tion. They represent two different facies of the GN according to previous works (Pastre and Cantagrel, 2001;Vincent,1979).Oneofthesamplesislocatednorth of theproposed calderalimit (i.e. Rochefort-Montagne).

The last one (i.e. Ludie`res) belongs to the numerous outcropsknownnorthofSaint-Nectaireandbelongingto theeasternextensionoftheGN(Fig.1).

3.1. Rochefort-Montagne

We sampled in an old quarry northeast of the - Montagne town (entrance along D2089 road, 45841⁰30⁰⁰N; 2848⁰30⁰⁰E) (Fig. 2a). The GN outcrop is comprised 12–15-m-high white cliffs visible from the road(Fig.2b).Thefaciesfoundinthisquarryconsistofa whitecoherentrhyolitewithonlysmall(mainly<1cm) and dispersed ﬁbrous pumice stones. We collected the samples(i.e.1kgofhomogeneousﬁnerhyolitewithfew pumice visible)on the cliff located eastof the quarry, about 1mabove the current ground.Only centimetric darker rock xenoliths were visible to the naked eye.

Becauseofthelackoflargeﬁbrouspumicestonesandof the presence of surge deposits of similar composition

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below the Rochefort-Montagne, this outcrop is inter- pretedasrepresentingtheearlyphaseoftheGN(Pastre andCantagrel,2001).

3.2. Ludie`res

TheGNoutcropsintwosmallquarrieslocated300m northeast of the hamlet of Ludie`res (town of Vernet- Sainte-Marguerite) (45837⁰12⁰⁰N; 2857⁰38⁰⁰E), 12km out- sideoftheproposed calderalimits(Fig.1). Aminimum thicknessof8mcouldbeestimatedfortheGNinthese quarries.ThefaciesoftheGNisdistinctfromtheprevious sampleandconsistsmainlyoflargeﬁbrouspumicestones (i.e.4–6cm,Fig.2d)withinawhitecoherentcindymatrix.

Thisfaciesisinterpretedas‘‘themaineruptivephase’’of theGN(Pastre,1987).Asimilarfaciesisrecognizedinthe nearbyoutcropofLesArnats(Fig.1).Wecollectedabout 30 largeﬁbrous pumicestonesin several zonesfor our

investigation. Sanidine as wellas edenite crystals were visibletothenakedeyesinseveralpumicestones.

4. ⁴⁰Ar/³⁹Armethod

Because of the lack of large sanidine in the ﬁbrous pumicestonesinRochefort-Montagnesamples,wedecid- edtoseparatesanidinefromthecoherentrhyoliticmatrix, whereas we worked exclusively on the large ﬁbrous pumicestonesfromLudie`res.Aftercrushingandsieving, sanidines ranging from 400 to 500

m

m in size were handpicked under a binocular microscope and then slightlyleachedfor5minin7%HFacid.Foreachsample, about thirty crystals were handpicked and separately loadedinaluminumdisksandirradiatedinthreedifferent irradiationsof90min(IRR37forRochefort-Montagne)and 120min(IRR68forLudie`res).Irradiationsweremadein the

b

1tubeoftheOSIRISreactor(FrenchAtomicEnergy Fig.2.DetailedmapsofthetwoinvestigatedGNoutcropswith(aandb)Rochefort-Montagnequarrylocationandpicture;(candd)Ludie`resquarry locationandphotographsofthelargewhiterhyoliticpumicestonesfoundinthislocality.

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Commission, Saclay, France). After irradiation, samples weretransferredintoacoppersampleholderandloaded individually into a differential vacuumCleartran^C win- dow.CrystalswereindividuallyfusedusingaSynradCO₂ laserat10–15%ofnominalpower(ca.25W).Arisotopes were analyzed using a VG5400 mass spectrometer equipped with a single-ion counter (Balzers SEV 217 SEN) following the procedure outlined in Nomade et al.(2010).EachArisotopemeasurementconsistedof 20cyclesofpeakswitchingoftheargonisotopes.Neutron ﬂuence(J)wasmonitoredbyco-irradiationofAlderCreek sanidines (Nomade et al., 2005). These standards were placedinthesamepitasthesamplesduringirradiation.

TheJvalueforeachsamplewascalculatedfromanalysesof twoACs-2singlecrystalmeasurementswithanominalage of1.193Ma(0.5%)andusingthetotaldecayconstantof SteigerandJa¨ger(1977).Massdiscriminationwasassessed byanalysisofairpipettesthroughouttheanalyticalperiod, andwascalculatedrelativetoa⁴⁰Ar/³⁶Arratioof298.56(Lee etal.,2006).Severalproposedcalibrationsofthe⁴⁰Ar/³⁹Ar chronometerarecurrentlyinuse,yieldingagesthatvaryby 1%(Kuiperetal.,2008;Renneetal.,2011).Thisimplieda differenceinthecalibratedageforoursampleswithinthe reportedtotaluncertainty.Asaconsequence,weusedthe valuesofNomadeetal.(2005)andofSteigerandJa¨ger(1977) foralltheagesreportedhereafter.Proceduralblankswere measuredeverytwoorthreeunknowns.Foratypical10-min static blank, the backgrounds were generally about 3.0–

4.010¹⁷ and 6.0–7.010¹⁹ moles for ⁴⁰Ar and ³⁶Ar, respectively. The nucleogenic production ratios used to correctforreactor-producedArisotopesfromKandCaare reportedinthesupplementarydataset(S1andS2).

Because of the lack of large sanidine in the ﬁbrous pumicestonesinRochefort-Montagnesamples,wedecid- edtoseparatesanidinefromthecoherentrhyoliticmatrix, whereas we worked exclusively on the large ﬁbrous pumicestonesfromLudie`res.Aftercrushingandsieving, sanidines ranging from 400 to 500

m

m in size were handpicked under a binocular microscope and then slightlyleachedfor5minin7%HFacid.Foreachsample, about thirty crystals were handpicked and separately loadedinaluminumdisksandirradiatedinthreedifferent irradiationsof90min(IRR37forRochefort-Montagne)and 120min(IRR68forLudie`res).Irradiationsweremadein the

b

1tubeoftheOSIRISreactor(FrenchAtomicEnergy Commission, Saclay, France). After irradiation, samples weretransferredintoacoppersampleholderandloaded individually into a differential vacuumCleartran^C win- dow.CrystalswereindividuallyfusedusingaSynradCO₂ laserat10–15%ofnominalpower(ca.25W).Arisotopes were analyzed using a VG5400 mass spectrometer equipped with a single-ion counter (Balzers SEV 217 SEN) following the procedure outlined in Nomade et al.(2010).EachArisotopemeasurementconsistedof 20cyclesofpeakswitchingoftheargonisotopes.Neutron ﬂuence(J)wasmonitoredbyco-irradiationofAlderCreek sanidines (Nomade et al., 2005). These standards were placedinthesamepitasthesamplesduringirradiation.

TheJvalueforeachsamplewascalculatedfromanalysesof twoACs-2singlecrystalmeasurementswithanominalage of1.193Ma(0.5%)andusingthetotaldecayconstantof

SteigerandJa¨ger(1977).Massdiscriminationwasassessed byanalysisofairpipettesthroughouttheanalyticalperiod, andwascalculatedrelativetoa⁴⁰Ar/³⁶Arratioof298.56(Lee etal.,2006).Severalproposedcalibrations ofthe⁴⁰Ar/³⁹Ar chronometerarecurrentlyinuse,yieldingagesthatvaryby 1%(Kuiperetal.,2008;Renneetal.,2011).Thisimplieda differenceinthecalibratedageforoursampleswithinthe reportedtotaluncertainty.Asaconsequence, weusedthe valuesofNomadeetal.(2005)andofSteigerandJa¨ger(1977) foralltheagesreportedhereafter.Proceduralblankswere measuredeverytwoorthreeunknowns.Foratypical10-min static blank, the backgrounds were generally about 3.0–

4.010¹⁷ and 6.0–7.010¹⁹ moles for ⁴⁰Ar and ³⁶Ar, respectively. The nucleogenic production ratios used to correctforreactor-producedArisotopesfromKandCaare reportedinthesupplementarydataset(S1andS2).

5. ⁴⁰Ar/³⁹Arresults

Analytical details for each measured crystal are reported in the supplementary dataset (Tables S1 and S2).Resultsarepresentedasprobabilitydiagrams(Deino andPotts,1990)andinverseisochronesinFig.3.Weighted meanagesanduncertaintiesarecalculatedusingIsoPlot 3.0 (Ludwig, 2001) and given at 95% of probability, includingtheJﬂuxuncertainty.Wealsoaddedthetotal uncertainty, therefore including the decay constant between brackets for each sample. A homogeneous juvenile population is considered relevant when the weighted mean of these crystals has a probability ﬁt 0.1.Theweightedaverageagesarecalculatedincluding theJuncertaintythroughoutthetext.

5.1. Rochefort-Montagne

Atotalof14sanidinesweredatedforthissample.The probability diagram is multimodal with four modes (Fig.3a). Theyoungest population (juvenilepopulation) is comprisedofonly fourcrystalsand givesa weighted meanageof2.760.03Ma(0.07Ma;MSWD=0.7,P=0.5), whichsuggeststhepresenceofmainlyxenocrysts(70%ofthe crystals).Becauseofthelowspread(94.7–96.5%of⁴⁰Ar*)and ofthelimitednumberofjuvenilecrystals,wewerenotableto calculateavalidinverseisochrone(Fig.3bandTableS1).

Atotalof14sanidinesweredatedforthissample.The probability diagram is multimodal with four modes (Fig.3a). Theyoungest population (juvenilepopulation) is comprisedofonly fourcrystalsand givesa weighted

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meanageof2.760.03Ma(0.07Ma;MSWD=0.7,P=0.5), whichsuggeststhepresenceofmainlyxenocrysts(70%ofthe crystals).Becauseofthelowspread(94.7–96.5%of⁴⁰Ar*)and ofthelimitednumberofjuvenilecrystals,wewerenotableto calculateavalidinverseisochrone(Fig.3bandTableS1).

5.2. Ludie`res

We measured a total of 16 crystals. Excluding one obviouslyolderxenocrystandthreecrystalsthatcreateda small tailon theprobability diagram, all other crystals (n=12) belongto a juvenile population (Fig.3c). These crystals allow calculating a weighted mean age of 2.780.03Ma(0.07Ma;MSWD=0.9, P=0.5).The corres- pondinginverseisochroneagecalculatedusingthisjuvenile populationisself-coherent(i.e.2.780.03Ma;MSWD=0.9).

The⁴⁰Ar/³⁶Arinitialintercept(i.e.29510)isequivalentto theatmosphericvalue(Fig.3dandTableS2).

We measured a total of 16 crystals. Excluding one obviouslyolderxenocrystandthreecrystalsthatcreateda small tailon theprobability diagram, all other crystals (n=12) belongto a juvenile population (Fig.3c). These crystals allow calculating a weighted mean age of

2.780.03Ma (0.07Ma; MSWD=0.9, P=0.5).The corres- pondinginverseisochroneagecalculatedusingthisjuvenile populationisself-coherent(i.e.2.780.03Ma;MSWD=0.9).

The⁴⁰Ar/³⁶Arinitialintercept(i.e.29510)isequivalentto theatmosphericvalue(Fig.3dandTableS2).

6. Discussion

Thenew⁴⁰Ar/³⁹Arageconstraintswepresentbringa newandimportantcontributiontotheunderstandingof theGNandquestionedhypothesesconcerningitsageand origin. First we will discuss the chronostratigraphic position of this event within the Gue´ry stratovolcano historyofNomadeetal.(2014a).Inasecondphase,wewill discuss thegenetic link between this eruption and the cryptic‘‘HauteDordogne’’caldera.

6.1. AbouttheageoftheGN

Sofar,theageofthismajorrhyoliticeruptionreliedona multi-crystals ⁴⁰Ar/³⁹Ar investigation, made more than 25yearsago(Fe´raudetal.,1990).Sincethiswork,theage ofthiseruptionaswellasthatoftheco-geneticcaldera Fig.3.⁴⁰Ar/³⁹Arresultspresentedasprobabilitydiagramsandinverseisochrones.

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wasﬁxedat3.070.04Ma,earlyinthehistoryoftheMont- Dore Massif (Nomade etal., 2014a; Pastre andCantagrel, 2001).Theagesof2.780.03Maand2.760.03Mathatwe obtainedfortheRochefort-MontagneandLudie`resoutcrops, respectively, are 300ka younger thanexpected. We com- binedinFig.4bothresultsandobtainedaweightedageof 2.770.02Ma (0.07Ma, n=16, MSWD=1.1; P=0.3). We consideredthisagetobethemoreaccurateonefortheGN eruption.Thedifferenceinagebetweenourworkandtheage obtainedbyFe´raudetal.(1990)iseasilyexplainedbythe

presence of xenocrysts within the sanidine populations analyzedatthetime(Fig.4).Wefoundmorecontamination intheRochefort-Montagnesample(i.e.70%ofthecrystals analyzed),probablybecauseweworkedonthecrystalsfrom thematrixratherthanonesextractedfromﬁbrouspumice stones.Axenocrysticcontaminationwasalsofoundinthe

‘‘juvenile’’ Ludie`res large ﬁbrous pumice stones that we investigated(Fig.3).Suchdiscretexenocrysticpopulations arelikelytohave beenincorporatedintothemagmavery nearoratthesurfaceinarapidlycooledpyroclasticﬂow;

otherwise,itwouldhaveresultedinatotalresettingoftheK/

Arisotopicsystem(e.g.,Spelletal.,2001).Ourresultsexplain whypreviousattemptsataccuratelydatingthiseventwere unsuccessful.TheagesweobtainedforRochefort-Montagne andLudie`resareindeedidenticalwithinuncertaintylimits with that of the rhyolitic ‘‘Vendeix-Sie`ge’’ formation (i.e.

2.740.02Ma;Nomadeetal.,2014a)(Fig.4).Basedonthis newgeochronologicaldataaswellasongeochemicaland mineralogicalsimilarities,thisprovesthattheseformations belongtothesameeventastheoneﬁrstsuggestedbyPastre (1987).Accordingto these newpiecesofinformation, we couldwithconﬁdencesaythatthepositionoftheGNwithin the history of the Mont-Dore Massif should now be reconsidered.

Followingthetephro-stratigraphichistoryproposedby Nomadeetal.(2014a),theGNignimbritecouldnowbe incorporatedintotheG.IIphaseoftheGue´ryandcannotbe considered anymoreas partoftheearlyactivity ofthis stratovolcano(G.IofNomadeetal.,2014a)(Fig.5).Wenow proposedthattheGue´rystratovolcanoexplosivehistory started with rather small rhyolitic eruptions scattered between 3.10and3.00Ma (G.Iphase, Fig.5). Thisearly Fig.4.Ludie`resandRochefort-Montagneoutcrops:combined⁴⁰Ar/³⁹Ar

probabilitydiagramcomparedtotheageofthe‘‘Vendeix-Siege’’rhyolitic pumicestones(Nomadeetal.,2014a).The⁴⁰Ar/³⁹Armulti-crystalagesof Fe´raudetal.(1990)arealsoreportedintheﬁgure.Theblueboxesarefor juvenilecrystals,theredonesforxenocrysts.

Fig.5.ChronologyofthepyroclasticunitsoftheGue´rystratovolcanomodifiedfromNomadeetal.(2014a).Thenew⁴⁰Ar/³⁹Arsingle-crystalanalyses(blue forjuvenilecrystals,redforxenocrysts)andthecorrespondingprobabilitydiagramaresuperposedonthepreviousresultsofNomadeetal.(2014a,b).GN andPAcorrespondtotheGrandeNappeandthePerrierdebrisavalanches(flankcollapsesoftheGue´ry),respectively.ThePAmarkstheendofthemain explosiveactivityofcycle‘‘G.II’’oftheGue´rystratovolcanoasdefinedbyNomadeetal.(2014a).ThebluestripcorrespondstothenewagefortheGN, whereastheredonecorrespondstothepreviouslysuggestedage(Fe´raudetal.,1990).

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phaseis nowfoundat thebaseofthePerriersequence (Nomadeetal.,2014b;Pastre,2004).Thisinitialexplosive phasewasfollowedbyatrachyticexplosiveactivityatthe beginningoftheG.IIphase (Fig.5).Thisactivityis well preservedin theVendeix area, suchas in theso-called

‘‘Bourboulebas’’pyroclasticunitwepreviouslydatedback to2.860.02Ma(seeﬁg.9inNomadeetal.,2014a).TheGN eruptionhasoccurred2.770.02Maago,inthemiddleof theG.IIphase(GNinFig.5).Inthisnewtephrostratigraphic history,theGNprecededbyonly200ka(ca.2.60–2.58Ma) the four lateral ﬂank collapses that affected the Gue´ry stratovolcano(Bernardetal.,2009;Nomadeetal.,2014a;PA inFig.5). Thenewtephrostratigraphichistory summaries aboveinvolvedthatmostofthepyroclasticproductsofthe Gue´rywereemplacedinarelativelyshortperiodoftime(i.e.

2.86–2.58Ma)during theG.IIphase.Thishastobelinked withhighmagmaproductionratesduringthisphaseofthe Gue´rystratovolcanoaswellaswithparticularconditionsin the magmatic chamber(s) that would require further investigations. The comparison between the new single- crystalanalysesfromtheGNoutcrops andthepreviously publishedagessuggestthatamajorityofthecontamination found within the GN belongs to the trachytic episode occurringatthebeginningoftheG.IIphase(Fig.5).

6.2. Aboutthelinkwiththecryptic‘‘Haute-Dordogne’’

caldera

As said above, Nomade et al. (2014b) also dated a rhyoliticpumiceousfallbelowwhatisconsideredtobethe GNinthePerrierplateau,whichismucholderthantheGN.

Asaconsequence,theproposedvolumeof5–6km³should betakenwithgreatcaution,assomerhyoliticoutcropsare not linkedto the GNeruption. However, as we do not systematically investigate all rhyolitic outcrops, it will remain hazardous to propose a more precise volume withouta morewidespreadstudy.Theexistenceoftwo calderas(C.IfortheGue´ryandC.IIfortheSancy)withinthe Mont-DoreMassifwasheavilydebated,especiallybecause theirexistenceismainlybasedongeophysicalinvestiga- tion,thereforeonlyon indirectevidences(e.g.,Cantagrel and Briot, 1990; Lavina, 1985; Me´nard et al., 1980;

Mossand, 1983; Nercessian et al., 1984; Varet et al., 1980; Vincent, 1980). The location and size of the C.I

‘‘Haute Dordogne’’ caldera is still an open question:

centered on the La Bourboule area for some authors (Me´nard etal., 1980; Varetet al., 1980; Vincent,1980;

Fig.1),itwasalsoproposedtobeclosertotheGue´ryLake areawheresomedifferentiateddomesmaymarktheedge of the caldera (Cantagrel and Briot, 1990). This last hypothesis could be rejected as we know that some domes are much younger than the GN (e.g., 2.0950.038Ma(U–Pbage)forSanadoire;Cocherieetal., 2009)relatedtocycleG.IIIoftheGue´ryarea(Nomadeetal., 2014a;Fig.5).Someotherdomeswereemplacedfollowing severaleasternﬂankcollapsesoftheGue´rystratovolcanoes that markthe endofcycle G.II(Perrier Avalanche;PA in Fig.5),andthereforethereisprobablynolinkwithacaldera formation occurring 200–300ka before. One of the argu- ments put forward to prove the existence of theHaute–

DordognecalderaisthewidespreadignimbriteGNsupposed

tobeemplacedearlyintheGue´rystratovolcanohistory.The positionoftheGNwithinthevolcano-sedimentaryinﬁlling of the supposed caldera does not support a genetic link between the depression and/or low speed anomalies (i.e.

Nercessianetal.,1984;Varetetal.,1980)observedaround theLaBourbouleareaandtheGNeruptionitself.Thecircular structure within the basementhighlighted by Nercessian etal.(1984)arguesinfavoroftheexistenceofadepression interpretedasa‘‘caldera’’bytheauthors.Wewouldlikehere tonoticethattheageofsuchastructureisunknown and cannotbedirectlylinkedtotheGNat2.77Ma.Furthermore, thiscirculardepressionisbasedonextremelylow-resolution 3Dseismicdatacomparedtothecurrentstateoftheartof technology.Anotherobservationwhichcouldbemadeisthat thecalderalimitssuggestedbythiscircularstructureinclude the so-called C.2 caldera (Lavina, 1985), which is much younger,datedbackto71920ka(fullexternaluncertainty;

Nomade et al., 2012). This depression also includes geo- graphicallytheyounger(250-m-thick)pyroclasticrockspile outcroppingintheMont-DoreValleydatedbacktobetween 640and390ka(Nomadeetal.,2012).Thissuggeststhatthe low-speed zone highlightedbyNercessianetal.(1984)is mainlyfoundbelowunconsolidatedpyroclasticmaterialsof various ages, covering 3.0Ma of volcanic history, two superposed stratovolcanoes, and eight pyroclastic cycles (Nomadeetal.,2014a).Inconclusion,itisimpossibletolink thiscircularstructuretoanyspecificeruptioncycleorperiod ofactivityoftheMont-DoreMassif.Finally,accordingtothe proposedvolumesandvarioussizesofthecaldera,theGN wouldhaveleftadepressionofabout200–250mindepth (CantagrelandBriot,1990),whichisnotcompatiblewiththe geophysicalandgeologicalevidencesputforwardbyMeńard etal.(1980),Nercessianetal.(1984),Varetetal.(1980),and Vincent(1980),whoallsuggestedamuchlargeranddeeper caldera.Itremainsthatwecannotrejectthehypothesisthat the GN islink toa caldera formationbutthishypothesis currentlylacksgeological,geophysical,andgeochronological evidence. If the circular structure within the basement knownas‘‘HauteDordogne’’ calderareally exists,hasthis structurebeenlikelyformedbeforetheGNeruption?Thisisa hypothesis that needs to be considered; however, the question is: where are the co-eruptive deposits resulting fromits formation? Onethingis certain:it isneitherthe small-volumerhyoliticeruptionfoundreworkedatthebase ofthePerrier plateau,northetuffites andtrachytic flows belowtheGNaroundLaBourboule,whichcouldbelinkedto theformationofalargecaldera.

To ﬁnalize this discussion, we would like here to exploreanotherhypothesis:hastheGNbeengeneratedby anorthward-trendinglateralblast,orisitratherlinkedto theformationofacaldera?Withsuchmechanism,theco- eruptiveproductsaredepositednorthwardﬁrst,following theblast,andthenalonganeast–westdirectionchanneled bythePaleo-Alliervalleysystem.Thishypothesisexplai- ned the position of the GN within the Vendeix–La Bourboule volcano-sedimentary succession as well as thecurrentoutcropsoftheGNfoundalongthenorthern andsouthernedgesoftheVernetstream(Fig.1).Thesame channelingprocesseshaveoccurredafterthefourcollap- sesoftheeastwardpartoftheGue´rystratovolcanodated backtobetween2.60and2.58Ma(Nomadeetal.,2014a) S.Nomadeetal./C.R.Geoscience349(2017)71–80

78

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(PAinFig.5).Wecurrentlycannotfavoranyhypothesis, but our work clearly shows that many things are still unresolvedandthatworksmainlydonemorethan30years agoneedtobereevaluatedinthelightofthestateoftheart ofanalyticaltechniques.Despitetherecentinvestigations that improved ourknowledgeoftheMont-Dore Massif, moregeochronology,geochemistryandgeophysicsstudies arecriticallyneededinthefuturetodeciphertheoriginof the GN, but also reconstruct the eruptive history and geochemicalevolutionofthesecondlargestdifferentiated volcanicmassifofEurope.

7. Conclusions

Thenewsinglecrystals⁴⁰Ar/³⁹Arinvestigationledusto thefollowingconclusions:

(1)the GN eruption is now precisely dated back to 2.770.02Ma (0.07Ma full external uncertainty) and belongstotheG.IIphaseoftheGue´rystratovolcano;

(2)thepreviouslyproposed⁴⁰Ar/³⁹Aragewascorrupted byunrecognizedxenocrysticcontamination;

(3)thecorrelation madebetween theVendeix rhyolitic complexes (intra-caldera position) dated back to 2.740.04MaandtheGNisnowﬁrmlyestablished;

(4)weproposeanalternatehypothesisfortheformation oftheGN:anorthwardblasteruptionchanneledlater eastwardtowardthepaleo-AllierRiver.

Acknowledgments

TheauthorswouldliketothankProf.H. Bril(GRESE, UniversityofLimoges,France)andDr.G.Delpech(GEOBS, Universite´ Paris-Sud, Orsay) for fruitfuldiscussions.The sampleswereirradiatedintheOsirisreactor(CEASaclay) thankstoDr.J.-L.Joron.ThisisLSCEcontributionNo.6032.

AppendixA. Supplementarydata

Supplementarydataassociatedwiththisarticlecanbe found, in the online version, at http://dx.doi.org/10.1016/

j.crte.2017.02.003.

References

Bernard,B.,VanWyk deVries,B.m.,Leyrit,H., 2009.Distinguishing volcanicdebrisavalanchedepositsfromtheirreworkedproducts:

the Perrier sequence(FrenchMassif Central).Bull. Volcanol. 71, 1056–1071.

Besson,J.-C.,1978.Lesformationsvolcaniquesduversantorientaldu massif du Mont-Dore (Massif central français),feuille 1/25000ê Veyre-Monton5-6.The`se3êcycle,Clermont-Ferrand,France.

Brousse,R.,1963.Identiﬁcationdedeuxcoule´esdeponcesdanslemassif volcaniqueduMont-Dore(Auvergne).C.R.Acad.Sci.Paris,Ser.D257, 2869–2871.

Brousse,R.,1971.Magmatologieduvolcanismeneóge`neetquaternaire duMassifcentral.In:SymposiumGeólogie,geómorphologieetstruc- tureprofondeduMassifcentralfrançais,PleinAirService,Clermont- Ferrand, pp.377–478.

Brousse,R.,1984.Comblementdelafossevolcaniqueetsocledumassif duMont-DoreinProgrammege´ologieprofondedelaFrance.The`me 9,Volcanismere´centduMassifcentral(Mont-Dore).Doc.BRGM81–

9,9–19.

Brousse,R.,Lefe`vre,C.,1966.NappesdeponcesduCantaletduMont- Dore.Leursaspectsvolcanologiques,pe´trographiquesetmine´ralogi- ques.Bull.Soc.geol.France7,223–245.

Cantagrel,J.-M.,Baubron,J.-C.,1983.ChronologieK-Ardese´ruptionsdans lemassifvolcaniquedesMonts-Dore:implicationsvolcanologiques.

Geol.France2,123–142.

Cantagrel,J.-M.,Briot,D.,1990.Avalanchesetcoule´esdede´bris:leVolcan duGue´ry–Ou` estlacalderad’effondrementdanslemassifdesMonts Dore? C.R.Acad.Sci.Paris,Ser.D311,219–225.

Cocherie,A.,Fanning,C.M.,Jezequel,P.,Robert,M.,2009.LA-MC-ICPMS andSHRIMPU–PbdatingofcomplexzirconsfromQuaternarytephras fromtheFrenchMassifCentral:magmaresidencetimeandgeochem- icalimplications.Geochim.Cosmochim.Acta73,1095–1108.

Cas,R.A.F.,Wright,J.V.,1988.VolcanicSuccessions.ModernandAncient.

UnwinHyman,528p.

Degeai, J.-P.,Pastre, J.-F., Gauthier, A., Robert,V., Nomade,S.,Bout- Roumazeilles,Guillou,H.,2013.Lase´quencelacustredumaard’Alle- ret(Massifcentral,France):Te´phrochronologieete´volutionpale´oen- vironnementale en Europe occidentale au de´but du Ple´istoce`ne moyen.Quaternaire24,443–449.

Deino,A.,Potts,R.,1990.Single-crystal⁴⁰Ar/³⁹ArdatingoftheOlor- gesailie Formation, Southern Kenya Rift. J. Geophys. Res. 95, 8453–8470.

Fe´raud,G.,LoBello,P.,Hall,C.M.,Cantagrel,J.M.,York,D.,Bernat,M.,1990.

DirectdatingofPlio-Quaternarypumicesby⁴⁰Ar/³⁹Arstep-heating andsingle-grainlaserfusionmethods:theexampleoftheMonts- Doremassif(MassifCentral,France).J.Volcanol.Geotherm.Res.40, 39–53.

Kuiper,K.F.,Deino,A.,Hilgen,F.J.,Krijgsman,W.,Renne,P.R.,Wijbrans, J.R.,2008.SynchronizingrockclocksofEarthhistory.Science320, 500–504.

Lavina,P.,1985.LevolcanduSancyetle«Massifadventif».E´tudes volcanologiquesetstructurales. (Ph.D.thesis).Universite´ deCler- mont-Ferrand,France,211p.

Lee,J.Y.,Marti,K.,Severinghaus,J.P.,Kawamura,K.,Yoo,H.-S.,Lee,J.B., Kim,J.S., 2006.Aredeterminationof theisotopicabundances of atmosphericAr.Geochim.Cosmochim.Acta70,4507–4512.

Ludwig,K.R.,2001.Isoplot3.0ageochronologicaltoolkitforMicrosoft Excel.In:SpecialPublicationNo.4.BerkeleyGeochronologyCenter, Berkeley,CA,USA.

Ly,M.H.,1982.Le PlateaudePerrier etlaLimagne duSud:E´tudes volcanologiquesetchronologiquesdesproduitsmontdoriens(Massif centralfranc¸ais).The`sede3^ecycle.,Clermont-Ferrand,France,180p.

Me´nard,J.-J.,1979.Contributiona` l’e´tudepe´trographiquedesnappesde poncesdumassifvolcaniqueduMont-Dore(Massifcentralfranc¸ais).

The`sede3^ecycle.Universite´ Paris-Sud,Orsay,105p.

Me´nard,J.-J.,Clocchiatti,R.,Maury,R.C.,Brousse,R.,1980.Originedes poncesrhyolitiques duMont-Dore(Massif central,France):Argu- mentspe´trologiques.C.R.Acad.Sci.Paris,Ser.D290,559–562.

Mossand,P.,1983.Levolcanismeante´-etsyn-caldeiradesMonts-Dore (Massifcentralfranc¸ais).Implicationsge´othermiques. (Ph.D.thesis).

Universite´ deClermont-Ferrand,France,197p.

Mossand,P., Cantagrel,J.-M., Vincent,P., 1982.Lacaldera deHaute Dordogne: aˆge etlimites (massif desMonts-Dore, France). Bull.

Soc.geol.France7,727–738.

Nercessian,A.,Hirn,A.,Tarantola,A.,1984.Three-dimensionalseismic transmissionprospectingoftheMont-Dorevolcano,France.Geophys.

J.R.Astron.Soc.76,307–315.

Newhall,C.G.,Self,S.,1982.Thevolcanicexplosivityindex(VEI):an estimateofexplosivemagnitudeforhistoricalvolcanism.J.Geophys.

Res.87,1231–1238.

Nomade,S.,Renne,P.R.,Vogel,N.,Deino,A.L.,Sharp,W.D.,Becker,T.A., Jaouni,A.R.,Mundil,R.,2005.AlderCreeksanidine(ACs-2):aQua- ternary⁴⁰Ar/³⁹ArdatingstandardtiedtotheCobbMountaingeo- magneticevent.Chem.Geol.218,315–338.

Nomade,S.,Gauthier,A.,Guillou,H.,Pastre,J.-F.,2010.⁴⁰Ar/³⁹Artemporal frameworkfortheAlleretmaarlacustrinesequence(FrenchMassif Central):volcanologicalandpaleoclimaticimplications.Quat.Geo- chem.5,20–27.

Nomade,S.,Scaillet,S.,Pastre,J.-F.,Nehlig,P.,2012.Pyroclasticchronol- ogyoftheSancystratovolcano(Mont-Dore,FrenchMassifCentral):

newhigh-precision⁴⁰Ar/³⁹Arconstraints.J.Volcanol.Geotherm.Res.

225–226,1–12.

Nomade,S.,Pastre,J.-F.,Nehlig,P.,Guillou,H.,Scao,V.,Scaillet,S.,2014a.

TephrochronologyoftheMont-DorevolcanicMassif(MassifCentral, France).New⁴⁰Ar/³⁹ArconstraintsontheLatePlioceneandEarly Pleistoceneactivity.Bull.Volcanol.76,798.

Nomade,S.,Pastre,J.-F.,Guillou,H.,Faure,M.,Gue´rin,G.,Delson,E., Debard,E.,Voinchet,P.,Messager,E.,2014b.⁴⁰Ar/³⁹Arconstraintson someFrenchlandmark LatePliocenetoEarlyPleistocenelarge

(11)

mammalianpaleofauna:paleoenvironmentalandpaleoecological implications.Quat.Geochronol.21,2–15.

Pastre,J.-F.,1987.Lesformationsplio-quaternairesdubassindel’Allieret levolcanismere´gional (Massifcentral, France).The`se.Universite´

Pierre-et-Marie-Curie(ParisVI).Me´m.Sci.Terre87-32,733p.

Pastre,J.-F.,2004. ThePerrierPlateau: aPlio-pleistocenelongﬂuvial recordintheriverAllierBasin,MassifCentral,France.Quaternaire 15,87–102.

Pastre,J.-F.,Cantagrel,J.-M.,2001.Te´phrostratigraphieduMont-Dore (Massifcentral,France).Quaternaire12(4),249–267.

Pastre,J.-F.,Debard,E.,Nomade,S.,Guillou,H.,Faure,M.,Gue´rin,C., Delson,E.,2015.Nouvellesdonneésgeólogiquesette´phrochronolo- giquessurlegisementpaleóntologiquedumaardeSene`ze(Pleísto- ce`neinfe´rieur,Massifcentral,France).Quaternaire26(3),225–244.

Renne,P.R.,Balco,G.,Ludwig,K.,Mundil,R.,Min,K.,2011.Responsetothe CommentbyW.H.Schwarzetal.,on‘‘Jointdeterminationof⁴⁰K decayconstantsand⁴⁰Ar/⁴⁰KfortheFishCanyonsanidinestandard, andimprovedaccuracyfor⁴⁰Ar/³⁹Argeochronology’’Geochim.Cos- mochim.Acta75,5097–5100.

Spell,T.L.,Smith, E.I.,Sanford,A., Zanetti,K.A.,2001. Systematicsof xenocrysticcontamination:preservationofdiscretefeldsparpopu- lationsatMcCullough PassCaldera revealedby⁴⁰Ar/³⁹Ardating.

EarthPlanet.Sci.Lett.190,153–165.

Steiger,R.H.,Ja¨ger,E.,1977.Subcommissionongeochronology:conven- tionontheuseofdacayconstantsingeo-andcosmochronology.

EarthPlanet.Sci.Lett.36,359–362.

Varet,J.,Stieltjes,L.,Gerard,A.,Fouillac,C.,1980.Prospectionge´other- miqueinte´gre´edanslemassifduMont-Dore.In:AdvancesinEuropean GeothermalResearch.ReitelPublishing,Strasbourg,France155–201.

Villemant,B.,Joron,J.-L,Treuil,M.,1980.Originedesquartzdesnappesde poncesduMont-Dore(Massifcentral,France):argumentsge´ochimi- ques.C.R.Acad.Sci.Paris,Ser.D290,687–690.

Vincent,P.M.,1979.Unrepe`rechronologiquedanslacalde´radesMonts Dore(Massifcentralfranc¸ais):lespyroclastitesdudoˆmedelaGache- rie.C.R.Acad.Sci.Paris,Ser.D289,1009–1012.

Vincent,P.M.,1980.Volcanismeetchambresmagmatiques:l’exemple desMont-Dore.LivrejubilairedelaSocie´te´ ge´ologiquedeFrance.

Me´m.hors-se´rie10,71–85.