U–Pb laser ablation ICP-MS zircon dating across the Ediacaran–Cambrian transition of the Montagne Noire, southern France

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Petrology,

Geochemistry

(Geochronology)

U–Pb

laser

ablation

ICP-MS

zircon

dating

across

the

Ediacaran–Cambrian

transition

of

the

Montagne

Noire,

southern

France

Maxime

Padel

a,

*

,

J. Javier

A´lvaro

b

,

Se´bastien

Clausen

a

,

Franc¸ois

Guillot

c

,

Marc

Poujol

d

,

Martim

Chichorro

e

,

E´ric

Monceret

f

,

M. Francisco

Pereira

g

,

Daniel

Vizcaı¨no

h

a_UMR₈₁₉₈_EEP_CNRS,_{universite´ de}_Lille-1,_baˆtiment_SN5,_avenue_{Paul-Langevin,}₅₉₆₅₅_{Villeneuve-d’Ascq}_cedex,_France b

InstitutodeGeociencias(CSIC-UCM),Novais12,28040Madrid,Spain

c

UMR8187LOGCNRS,universite´ deLille–universite´ duLittoralCoˆted’Opale,SN5SciencesdelaTerre,59655Villeneuve-d’Ascqcedex, France

d

Ge´osciencesRennes,UMR6118,universite´ deRennes-1,campusdeBeaulieu,35042Rennes,France

e

GEOBITEC/DepartamentodeCieˆnciasdaTerra,UniversidadeNovadeLisboa,Portugal

f_18,_rue_des_Pins,₁₁₅₇₀_Cazilhac,_France

g_{IDL/Departamento}_de_{Geocieˆncias,}_ECT,_Universidade_de_E´vora,_Portugal h

7c/oJean-BaptiseChardin,Maquens,11090Carcassonne,France

C.R.Geoscience349(2017)380–390

ARTICLE INFO Articlehistory: Received28July2016

Acceptedafterrevision25November2016 Availableonline1August2017

HandledbyMarcChaussidon Keywords: Ediacaran Cambrian U–Pbdating MontagneNoire ABSTRACT

U–Pblaserablationinductivelycoupledplasmamassspectrometrywasusedfordating

zircongrainsextractedfromfour sedimentaryandvolcanosedimentaryrocks ofthe

Montagne Noireencompassing thepresumedEdiacaran–Cambrianboundaryinterval.

MagmaticzirconfromtwosamplesfromthebasalandmiddlepartsoftheRivernous

Formation(arhyolitictuff)weredepositedat542.51Maand537.12.5Ma,bracketing

the541MaagepresentlyadmittedasbeingattheEdiacaran–Cambrianboundary.Inaddition,

apieceofsandstonefromtheunderlyingRivernousFormationcontainingmostlyeuhedral

zircongrains,suggestingproximalmagmaticsources,yieldsNeoproterozoicdatesranging

from574Mato1Ga,and subsidiaryolderdates from1.25to2.75Ga.Anotherpieceof

sandstone fromthe overlyingMarcoryFormation yielded mostly roundedzircon grains

probablyissuedfrommoreremoteareas,withalargespectrumdominatedbyNeoproterozoic

datesaswellasolderagesupto3.2Ga.Acomparisonofbothkindsofsandstonesuggestsa

signiﬁcantchangeinprovenance,changingfromarestrictedsourceareaduringtheEdiacaran

toamuchlargersourcedomainduringtheCambrianEpoch2thatrecordedcontributions

fromdifferentcratonsofGondwana.

C ₂₀₁₇_Acade´mie_des_sciences._Published_by_Elsevier_Masson_SAS._This_is_an_open_access

articleundertheCCBY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/

4.0/).

* Correspondingauthor.

E-mailaddresses:maxime.padel@etudiant.univ-lille1.fr(M.Padel),jj.alvaro@csic.es(J.J.A´lvaro),sebastien.clausen@univ-lille1.fr(S.Clausen),

Francois.Guillot@univ-lille1.fr(F.Guillot),marc.poujol@univ-rennes1.fr(M.Poujol),ma.chichorro@fct.unl.pt(M.Chichorro),eric.monceret@orange.fr

(E´.Monceret),mpereira@uevora.pt(M.F.Pereira),daniel.vizcaino@wanadoo.fr(D.Vizcaı¨no).

ContentslistsavailableatScienceDirect

Comptes

Rendus

Geoscience

ww w . sci e nc e di r e ct . com

http://dx.doi.org/10.1016/j.crte.2016.11.002

1631-0713/ C ₂₀₁₇_Acade´mie_des_sciences._Published_by_Elsevier_Masson_SAS._This_is_an_open_access_article_under_the_CC_BY-NC-ND_license₍_http://

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1. Introduction

The pre-Variscan succession of the Montagne Noire

cropsoutasafold-and-thrustcomplexdividedintotwo

sedimentary-dominated, northern and southern ﬂanks

fringinganessentiallymetamorphic,AxialZone(Arthaud,

1970; Ge`ze, 1949). Several tectonic models have been

proposedfortheMontagneNoireandarestillsubjectof

debate(BrunandVanDenDriessche,1994;Charlesetal.,

2009; Faure and Cottereau, 1988; Fre´ville et al., 2016; Mattaueretal.,1996;Poujoletal.,inpress;Soulaetal., 2001;VanDenDriesscheandBrun,1992).The

Precambri-an–Cambrianboundaryhastraditionallybeententatively

located in the lowermost formation exposed in the

southernMontagneNoire,namelytheMarcoryFormation

(A´lvaro et al., 1998). However, recent reviews of the

northernsuccessionschallengedtheformerstratigraphic

chart (A´lvaro et al., 2014b; Devaere et al., 2013) and

establishedalowerstratigraphicpositionforthe

volcano-sedimentarysuccessionsoftheGrandmont,Rivernousand

Layrac formations, exclusively exposed in the northern

MontagneNoire.Theonlypreviousradiometricdatafrom

theRivernousFormation(Fm.)yieldedanOrdovicianage,

ranging from 47319Ma to 44340 (Rb/Sr method;

Demange, 1982), which was subsequently ruled out by

biostratigraphicagesyieldedbyacritarch-bearing,laterally

equivalent deposits (the so-called ‘‘Schistes X’’) from the

Axial Zone (Fournier-Vinas and Debat, 1970). There, the

‘‘SchistesX’’arecappedbytheSe´rie`sTuff(‘‘Se´rie`s’’isavillage

name) tentatively correlated with the Rivernous Fm. The

Se´rie`sTuffwasdatedat54515MabyPb-evaporationon

zircon from a metadacite (Lescuyer and Cocherie, 1992).

Somescarcezirconcrystalsweresampledinagarnet-grade

Cambrianmeta-siltstonefromthesouthernMontagneNoire,

givingamaximumdepositionalageof556Mabasedonlyon

asinglezircon(Gebaueretal.,1989).Thisageattributionmay

lookdisputable,giventhemorerecentstatisticalguidelines

for provenance studies (see below) and also taking into

accountthemarkedmetamorphiccharacterofthisrock.In

fact,inapreviouspaper,GebauerandGru¨nenfelder(1977)

admittedthatabout80%oftheprimitiveradiogenicPbmight

havebeenlostduetoPhanerozoicthermalevents.Inorderto

improvethestratigraphicframeworkoftheNeoproterozoic–

Cambrian boundary interval, and the lithostratigraphic

nomenclaturallinksbetweentheAxialZoneandthenorthern

andsouthern ﬂanksof theMontagneNoire, zircongrains

weresampledfromtheRivernous,GrandmontandMarcory

formationsanddatedbyinsituLA-ICP-MSU–Pbanalysis.Our

results place new constraints upon the palaeogeographic

afﬁnitiesofthedifferenttectonostratigraphicunitsthatform

theMontagneNoire,aswellasonthedetritalprovenanceof

theEdiacaran–Cambriansedimentspreservedin

neighbour-ingtectonostratigraphicareas.

2. Geologicalandstratigraphicsetting

LocatedinthesouthernpartoftheFrenchMassifCentral

(Fig.1A),theMontagneNoirerepresentsasegmentofthe

external,southwesterncomponentoftheVariscanBeltin

Europe(Demange,1998;Poujoletal.,inpress;Rogeretal.,

2004).Assummarizedabove,thisENE–WSW-strikingrange

isdividedintothreetectonicunits:acentralmetamorphic

dome,theso-calledAxialZone,fringedbyitsnorthernand

southernﬂanks(Demange,1985;Ge`ze,1949).

TheAxialZoneisessentiallycomposedofmicaschist,

minor marble, paragneiss and migmatized orthogneiss

(Ge`ze,1949).Theprotolithageoftheorthogneissandits

relationshipwiththemetasedimentaryrocks havebeen

disputed.Someauthorsinterpretedtheorthogneissasa

granitic Precambrian basement (Demange,1975, 1998),

whereas others considered it as Palaeozoic intrusions

(BardandLoueyit,1978).Recentconventional(ID-TIMS),

SHRIMP and LA-ICP-MS U–Pb datings of various

ortho-gneisssamples(Cocherieet al.,2005;Pitra etal.,2012;

Rogeretal.,2004)suggestthatthegraniticprotolithwas

emplacedduringtheOrdovician.

The southern and northern sedimentary-dominated

ﬂanksoftheAxialZoneareafold-and-thrustcomplexof

nappes (Fig. 1). The Precambrian–Cambrian boundary

interval, only reported in thenorthern MontagneNoire

(A´lvaro et al., 2014b), comprises four formations, from

bottom to top, the Grandmont, Rivernous, Layrac and

Marcou formations(Fig. 2). The Grandmont Fm., about

700m thick, consists of grey to black shales with

subsidiary sandstone interbeds (Fig. 2). The Rivernous

Fm.,upto200mthick,comprisesslightlymetamorphosed

rhyolitictuffsthatincluderarebrecciaandshaleinterbeds

(Fig.2). Bothformationscrop out intheAve`ne–Mendic

parautochthon,whichincludestheLode´voisinlierandthe

Lacaunethrustslice(Murat,Fig.1B).IntheLacauneunit,a

lateral equivalent of the Rivernous Fm. (locally named

Murat Fm.),withbaseand toptruncatedby faults,was

datedat53212Ma(U–Pbonzircon;Demangeetal.,1995;

Ducrotet al., 1979). In the Ave`ne–Mendicparautochthon

(Fig.1),therhyoliticpalaeorelief formedbytheRivernous

Fm.(uptoabout300mhigh,afterA´lvaroetal.,2014b)is

unconformably onlapped by the volcano sedimentary

conglomerates andsandstones of the LayracFm. (Fig. 2).

TheLayracFm.isitselfoverlainbythecarbonate-dominated

MarcouFm.,about400mthickandassignedtotheCambrian

Stage2byrecentbiostratigraphic studies(Fig.2,Devaere

etal.,2013).

Theabove-reportedformationsarenotexposedinthe

southern Montagne Noire, where the oldest outcrop is

represented by the up to 1000m-thick Marcory Fm.

(Fig. 2), a monotonous alternation of sandstones and

shaleswithsubordinatecarbonatenodulesandlayers.The

upperpartoftheMarcoryFm.hasbeenassigned tothe

CambrianStage2–3transitionduetotheoccurrenceofthe

ichnogenera Psammichnitesand Taphrelmintopsis (A´lvaro

andVizcaı¨no,1999),andtheoldesttrilobitesfoundinthe

Montagne Noire, i.e. Blayacina miqueli (Cobbold, 1935;

Geyer, 1992). The MarcoryFm., although absent in the

Ave`ne–Mendicparautochthon,isexposedinotherthrust

slicesandnappesofthenorthernMontagneNoire.

3. Materialandmethods

3.1. Material

TheGrandmontandRivernousFormationshavebeen

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(about4kmtotheeastofLode`ve):asandstonefromthe

baseoftheexposedGrandmontFm.(sampleMN1),anda

rhyolitictufffromthebaseoftheRivernousFm.(sample

MN2).TheRivernousFm.wasalsosampledatitsmiddle

partneartheColduLayrac(sampleMN3).MN1toMN3

samplesbelongtotheso-calledAve`ne–Mendic

parautoch-thon.InordertoinvestigatetheprovenanceoftheMarcory

Fm.,asandstone(sampleMN4)wasalsoselectedfromthe

Psammichnitesgigas-bearinglevelinthesouthern

Monta-gneNoire (A´lvaroand Vizcaı¨no,1999),along theOrbiel

riversectionoftheMinervoisnappe(Fig.1).

3.2. Samplespreparation

Zirconseparationfromfreshsamplesstartedwithrock

grindingusingasteelcrusher.Theresultingpowderswere

sievedintherangeof50–250

m

m.Grainswereseparated

ﬁrstusinga heavyliquid (sodium heteropolytungstates,

density 2.85gcm–3_), _then _using _a _Frantz _magnetic

separator.FollowingSla´maandKosˇler(2012),theselected

grainswereobtainedfromrandomhandpickingundera

binocularmicroscopewhatevertheirsize,shape,orcolor,

inordertoavoidanyoperatorbias.Theywereﬁnallysetin

anepoxyresinpuckandpolishedtoexposetheircore.

3.3. LA-ICP-MSinsituU–Pbdating

Toidentifyinternalgrowthtexturesandmorphologies,

zircongrainswereimagedbyscanningelectronmicroscopy

(SEM) to get cathodoluminescence and back-scattered

electron images (atthe ‘‘Laboratoire d’oce´anologie etde

ge´osciences’’,UniversityofLille,France).TheU–Pbagesof

zirconsweredeterminedinsituattheGe´osciencesRennes

laboratoryby LA-ICP-MSusingan ICP-MSAgilent 7700x

coupled with an ESI laser Excimer system producing a

radiationwithawavelengthof193nm(NWR193UC),with

ablationspotdiametersof25

m

m,energypulsesof7Jcm–2_,

and repetitionratesof5Hz.Ablations wereoperated on

both grain rims and cores. Where necessary, distinct

domainsofazircongrainwereanalyzedtocomparetheir

ages.TheresultingablatedmaterialwasmixedinaHe,N

andArgasmixturebeforebeingtransferredintotheplasma

sourceoftheICP-MSdevice.Eachanalysislasted80sand

consisted of a ﬁrst 20-s background measurement

Fig.1.SimpliﬁedgeologicalmapoftheMontagneNoire;modiﬁedfromDevaereetal.(2014).A,LocationoftheFrenchMassifCentral(grey)andMontagne Noire(rectangle)inFrance.B,Structuralunitsandpreviousradiometricages:(a)Demange,1982(Rb/Sr)discardedbyourresults(seetext);(b)Ducrotetal., 1979(U–Pb)inDemangeetal.,1995;(c)LescuyerandCocherie,1992(U–Pb);(d)Rogeretal.,2004.

M.Padeletal./C.R.Geoscience349(2017)380–390 382

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followed by 60-s ablation with measurements of

204

(Hg+Pb), 206Pb, 207Pb, 208Pb, 232Th, and 238U, and a

15swash-outdelaybeforethenextacquisition.Thedata

werecollectedinbatchof43analysesdividedinthreesetsof

10unknowns,bracketedbytwomeasurementsoftheGJ-1

primaryzirconstandard(Jacksonetal.,2004)tocorrectfor

U–Pb and Th–Pb laser-induced fractionation and for

instrumentalmassdiscrimination,followedbyoneanalysis

of thePlesovice secondaryzircon standard(Sla´ma etal.,

2008)inordertochecktheprecisionandaccuracyofthe

measurements. During the course of this study, the

Plesovice zircon standard yielded a Concordia age of

336.80.67Ma (N=32). The operating conditions forthe

LA-ICP-MSequipmentcanbefoundinSupplementaryTable

1. For more information on the acquisition protocol, see

Manzottietal.(2015).Datatreatmentwasperformedwiththe

GLITTERsoftware(VanAchterberghetal.,2001)andplotted

using the Isoplot 3.75 software (Ludwig, 2012) in both

Wetherill and Tera-Wasserburg Concordia diagrams. For

rhyolitic tuffs, theages werecalculated using theTuffZirc

Agealgorithm(LudwigandMundil,2002)togetherwiththe

SambridgeandCompston(1994)algorithm.Forthe

sandsto-nes,agedistributioncurveswithprobabilitydensityplotwere

obtained using the density plotter freeware proposed by

Vermeesch(2004).Fordates>1Ga,wereportedthe207Pb/

206_Pb_dates_and_for_ages_<₁_Ga,_we_used_the206_Pb/238_U_dates.

The analyses out of the [90–110%] concordance interval,

calculated with 100(207Pb/235Uage)/(207Pb/206Pbage) for

ages>1Ga(Meinholdetal.,2011)and100(206_Pb/238_U_age)/

(207_Pb/235_U_age) _for_ages _<₁_Ga, _were_rejected ₍_Faure _and

Mensing, 2005 and Talavera et al., 2012). The age ofthe

youngestzirconpopulationisderivedfromaclusterofatleast

threeanalysesfromthreedifferentgrainsoverlappinginageat

2

s

(standarddeviation),asproposedbyDickinsonandGehrels

(2009) to ensure a statistically robust estimate of the

maximum depositional ages. Percentages of concordance,

isotopicratiosandageswith1

s

errors,aswellasUandPb

concentrations are providedin Supplementary Table 2. In

sedimentaryrocksamples(MN1andMN4),about110grains

wereanalyzedinordertogetthebestrepresentationofthe

detritalzirconpopulations.Fortuffs(MN2andMN3),about

50grainswereanalyzed,followingthesuggestionsofBowring

etal.(2006),togivearobustestimateofthebestageforthe

relatedvolcanicevent(s).

4. Results

4.1. GrandmontFormation(MN1)

ZirconsfromthesampleMN1,medium-grained

sand-stone, are mostly in the 100–250

m

m range, euhedral,

facetted,rarelysub-rounded,colourlessandgenerallywell

zoned(Supplementarydata,Fig.S1).107ofthe114MN1

analyseswereconcordant [90–110%],amongwhich94%

are Neoproterozoic (101 grains), 3% Mesoproterozoic

(3 grains), 2% Paleoproterozoic (2 grains) and 1 grain

(1%)isArcheaninage(Fig.4B).Intheintervalrangingfrom

500to1100Ma,theprobabilitycurveshowsadominant

Ediacaran group within the cluster 550–850Ma, which

displaysamainpeakaround605Maandasecondaryone

around 635Ma (Fig. 4B). In this same cluster, four

subsidiary peaks are identiﬁed around 690Ma, 760Ma,

805Ma and 835Ma. The Tonian-aged zircon grains are

characterized by one peak around 906Ma, in an 890–

920Maclusterandanotherpeakaround1Gainthe950–

1050Macluster.Thefouryoungestandconcordantzircon

grains from this group rangingfrom 567.26.12Ma to

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579.56.28Ma,yieldanaveragedateof5746Ma (95% conf.,Fig.3E).

4.2. RivernousFormation(MN2andMN3)

Samples MN2 and MN3 are both rhyolitic tuffs.

Accordingly,zircon grains fromsamplesMN2 andMN3

areeuhedral,clearandcolourless.Rarelycored,internal

structures highlighted by the CL-imaging show clear

magmaticzoning(Supplementarydata,Fig.S1).

InsampleMN2,59zircongrainswereanalysed,among

which 50 gave concordant dates [90–110% as deﬁned

above]. Seven of the 50 analyses wererejected due to

possibleleadloss.Thustheyoungestandmainpopulation

fromthissamplerepresent65%ofthese43zircongrains

with individual 206_Pb/238_U _dates _ranging _from

534.86.01Mato5466.02Ma.Thesecondgroup

repre-sents23%ofthetotalpopulation,andgives206_Pb/238_U_dates

ranging from 576.96.3Ma to 600.86.7Ma (Fig. 3A).

Finally, ﬁve inherited core grains were dated at

Fig.3. LA-ICP-MSresultsforallsamplesusing206

Pb/238

Uages.DiagramA:mixtureanalysisforsampleMN2usingSambridgeandCompston’s(1994) approach;theage579.84.3Maisconsideredasinheritedage.DiagramB:mixtureanalysisforsampleMN3usingSambridgeandCompston(1994)approach; theage5247.2Maisconsideredasgeologicallymeaningless,duetoleadloss.DiagramsCandD:datapointagedistributionforsamplesMN2andMN3,resp., usingTuffZircAge(LudwigandMundil,2002);chosenemplacementagefortherhyolitictuffsoftheRivernousFm.are:5423Ma(2s)forsampleMN2and 5373Ma(2s)forsampleMN3.DiagramsEandF:averageagecalculatedfortheyoungestconcordantzirconpopulationofGrandmontandMarcoryFm.(sample MN1andMN4),derivedfromaclusterofatleastthreeanalysesfromthreedifferentgrainsoverlappinginageat2s(standarddeviation)asproposedbyDickinson andGehrels(2009).

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651.67.2Ma,661.87.1Ma,66237.1Ma,685.87.6Ma

and704.77.6Ma(SupplementaryTable2).

Looking attheyoungestpopulation, theTuffZircAge

algorithmreturnedadateof542.5+0.7/–0.6Ma,whilethe

SambridgeandCompstonalgorithmyieldedacomparable

dateof541.92.3Ma(Fig.3AandC).Therefore,choosing

between those two within error identical results, we

concludethatthisrhyolitictuffwasemplaced542.5+0.7/–

0.6Ma.

InsampleMN3,58zircongrainswereanalysed,among

which 52 yielded concordant results [90–110%]: two

compositegrainswithcores(Zr1andZr27)yieldingU–

Pb datesof1837.719.1Maand583.16.37Ma

respec-tively. The youngest zircon population suggested by the

SambridgeandCompstonapproachforsampleMN3returned

adatearound524Ma(Fig.3B),whichispoorlyconstrained,

by only one concordant zircon among seven somewhat

discordantdatapoints(onaConcordiaplot).Therefore,this

dateisinterpretedasgeologicallymeaninglessasitcouldbe

linkedtoapossibleleadloss.

Keepingonlyagroupcomprisingthreeconcordantdata

points(greybars in Fig.3B),theTuffZircAge algorithm

yielded adate of537.35+2.35/–1.25Mawhile the

Sam-bridge and Compston approach returned a comparable

date of 537.12.5Ma. This second rhyolitic samplewas

thereforeemplaced537.12.5Maago(2

s

)(Fig.3BandD).

4.3. MarcoryFormation(MN4)

SampleMN4isaﬁne-grainedand maturesandstone.

Accordingly, its zircon crystals are mostly anhedral,

rounded to subrounded, often broken and rarely

bi-pyramidal, largely in the 50–100-

m

m size range. They

are colourless to yellowish, though the biggest zircon

grainsarereddishincolour.Intheanalysedfraction,104of

the112MN4analyseswereconcordant[90–110%],among

which 87%are Neoproterozoic(90grains), 4%

Mesopro-terozoic (4 grains), 5% Paleoproterozoic (6 grains), 2%

Neoarchean(2grains),onegrainisMesoarcheanandthe

oldestoneisPaleoarcheaninageat3.2Ga(Fig.4A).

Theprobabilitydensitycurve(Vermeesch,2004)shows

a dominant Ediacaran group (clustered across 550–

850Ma),withamainpeakaround614Maandasecondary

peak around 575Ma (Fig. 4A). In this same cluster, six

otherpeaksareidentiﬁedaround651Ma,678Ma,700Ma,

737Ma, 800Ma and 850Ma. The Tonian-aged zircon

grains are characterized byone peak around900Ma in

an 890–920Ma cluster and another distinctive peak

around 1Ga in the 950–1050Ma cluster. The three

youngest dates obtained from this sample that are

concordantyieldanaveragedateof602.57.3Ma(Fig.3F).

5. Discussion

5.1. TheRivernousvolcanicactivitymarkingthe

Precambrian–Cambrianboundaryinterval

Thetwodates(537.12.5Maand542.5+0.7/–0.6Ma)

obtainedfromtheRivernousrhyolitictuffsaremucholder

than previouslyestimated(47319Ma and44340Ma;

Demange, 1982). These results allow us to conﬁdently

identify the Precambrian–Cambrian boundary (541Ma,

Gradstein et al., 2012) in the basal succession of the

MontagneNoire.Bycomparisonwiththepreviously

deter-mined age of the Se´rie`s Tuffs from the Axial Zone

(54515Ma;LescuyerandCocherie,1992)andthe

River-nous(formerMurat)Fm.intheLacaunethrustsliceofthe

northernﬂank (53212Ma, interceptsuperior; Demange

etal.,1995andreferencestherein;Figs.1–2),theseresults

supportthelateralequivalenceoftheSe´rie`svolcanicepisode

(AxialZone)andRivernousrhyolitictuffdeposition(northern

ﬂank),asalreadysuggested(Poucletetal.,inpress).

InotherVariscanunitsoftheIbero-ArmoricanArc,some

plutonicbodieshaverecentlybeendatedaround540Maby

LA-ICPMS(Supplementarydata,Fig.S2;seealsoCasasetal.,

2015;Castin˜eirasetal.,2008;Gutie´rrez-Alonsoetal.,2004; Melletonetal.,2010;Rubio-Ordo´n˜ezetal.,2015):the

Arc-de-FixandArde´choisaugengneissesintheMassifCentral,

with respective Concordia age of 541.83.1 and

542.53.1Ma (Couzinie´ et al., this issue), as well as the

LaparanorthogneissintheCentralPyrenees,withaConcordia

age of 5453Ma (Mezger and Gerdes, 2016). All these

plutonicandvolcanicevents,overlappinginagewithinerror,

shouldbelinkedtoacommonepisodeassociatedwiththe

voluminousmagmaticandanatecticCadomianevents

repor-tedforWestGondwana,amongothers,byLinnemannetal.

(2007, 2008). According to these authors, the numerous

plutonic and volcanic to volcano sedimentary complexes

identiﬁedintheOssa-Morena,Saxo-ThuringianandAnti-Atlas

zones(A´lvaroetal.,2014a;Bleinetal.,2014)canresultfroma

slabbreakoffofasouthwardsubductedoceanicplateending

withtheCadomiancycleatabout545–540Ma.Theendofthe

Pan-African/CadomiancycleledtotheonsetofaCambrian

magmaticcycle(Supplementarydata,Fig.S2),relatedtothe

riftingoftheNorthGondwanamargin(A´lvaroetal.,2014a,

2014b;Poucletetal.,inpress).

5.2. AgeandpotentialprovenancesoftheGrandmontand

Marcoryformations

Depositionalages.Theyoungestgroupofconcordant

zircongrainsfromtheGrandmontFm.(sampleMN1)yield

anaveragedateof5746Mathatisinterpretedhereasits

maximumdepositionalage(i.e.lateEdiacaran;Fig.3E).This

resultiscoherentwiththestratigraphicframeworkproposed

byA´lvaroetal.(2014b)andtheNeoproterozoicagebasedon

acritarchsreportedfromthe‘‘SchistesX’’ Fm.oftheAxial

Zone (Fournier-Vinas and Debat, 1970). As a result, this

maximum age of deposition supports their correlation

betweentheGrandmontand‘‘SchistesX’’formations(A´lvaro

etal.,2014b).

Theaveragedatecalculatedfromtheyoungestgroupof

concordant data for the Marcory Fm. at 602.57.3Ma

(Fig.3F)mightbeconsideredasamaximumdepositionalage.

However, it is far from the real depositional age, as

mentioned above. Indeed, this samplewas selected from

the Psammichnitesgigas-bearing level (Fig. 2), and

conse-quently must be assigned to the Cambrian Stage 2–3

transition,i.e.itislessthan521Maold.

Sourceofthepre-ca.1Gadetritalzircongrains.The

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(MN4)andGrandmont(MN1)formationsaresomewhat

similar, but display some differences. Although each

individualsingleton-dateshouldbetreatedwithcaution,

the distributions of pre-1 Ga ages are plainly distinct

(Figs.4and5).

Asmentionedabove,zircongrains fromtheMarcory

Fm.(sampleMN4)aremuchsmalleranddeﬁnitelymore

rounded(Supplementarydata,Fig.S1)thanthosefromthe

GrandmontFm.(sampleMN1),suggestingalongdistance

oftransportfortheformergrains.Wehaveseparateddata

frominheritedcores(whiteboxesinFig.4),meaningless

regardingthesourceage,fromdatafromzonedmagmatic

rims(blackboxes,Fig.4)thatrepresentmostprobablythe

ageofthesource-rock.Fromthisrespect,theoldestzircon

rim from the Grandmont Fm. (MN1) is only Late

Paleoproterozoic(180619Ma),andonlytwograinrims

are older than 1 Ga.By contrast,the presence ofseveral

ArcheanzirconrimsinMN4pointstoamajorchangeinthe

sourceareas.MN1zircongrainscouldpossiblybederived,

ultimately,fromtheAmazoniancraton(Rhyacian,Orosirian

andStatherianevents),moreprobably fromthe Eburnean

WestAfricancraton,orfromtheSaharanmetacraton(Fig.5).

By contrast,eight pre-1Gazircon rims fromsampleMN4

form a Paleoproterozoic group (cluster ranging from

Fig.4.Frequencyandprobabilitydensityplotsofdetritalzircongrainsintherange500–3300MaforsamplesMN1(B)andMN4(A).Agegroupsofeach samplearepresentedinapiediagram.SectionCshowsacomparisonofagegroupswherewhiteboxesrepresentdatesfrominheritedcoreandblackboxes relatetodatesfromzonedrims.

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210919Mato178720Ma)andanArcheangroup.Those

groupsﬁtmuch betterwithan Africansourcecomprising

EburneanareasplusArcheanWestAfricancraton(Fig.5).

However,weshouldnotexcludethattheavailabilityof

zircon to erosion and transport from either primary

crystalline orrecycled sources requiresthat the

zircon-bearing rocks be exposed at the appropriate time, and

recyclingfromoldersedimentarydepositsmayconstitute

a more signiﬁcantsourcethan fromprimarycrystalline

rocks.

SourcesoftheStenian–Toniandetritalzircons.The

clusterpointingtotheStenian–Toniantransition(950to

1100Ma)insamplesMN1andMN4couldshareacommon

origin (Fig. 4). Oneshould expectanAmazoniancraton

afﬁnity (Linnemann et al., 2011) with detrital sources

related totheSan Ignacioand Sunsasevents andon its

eastern margin (Pereira et al., 2012). However, many

studies on sandstones from Lower Palaeozoic

peri-GondwananexposuresaroundtheMediterraneanregion

(e.g.,Israel,Jordan,Libya,Pyrenees,Sardinia,Greeceand

Sicily) rule out an Amazonian provenance and suggest

insteadaneasterntosoutheasternAfricanorigin(Altumi

et al., 2013; Avigad et al., 2003; Avigad et al., 2012; Kydonakisetal.,2014;Margalefetal.,inpress;Meinhold etal.,2011;Meinholdetal.,2013;Williamsetal.,2012).

Therefore,theArabian–Nubianshield,theSaharan

meta-craton, possibly the western edge of the Congo craton

(Tacketal.,2001),aswellasitseasternpartrecordingthe

‘‘Kibaran Event’’(Tacketal.,2010)andtheIrumidebelt

(Meinholdet al.,2011), ﬁt wellaspotentialsources for

MesoproterozoicandStenian–Tonianzircongrains.These

zircon-formingeventsoccurred simultaneouslywiththe

assembly of the supercontinent Rodinia (Grenvillian

orogeny). Different hypotheses have been advanced to

explainthisinputofMesoproterozoicandStenian–Tonian

zircon crystals (Altumi et al., 2013), including: (i) the

transportoflargeamountsofsedimentthrough

Neopro-terozoicglaciersfollowinganoriginalsouth–north

tran-sect,later reworkedanddeposited(Avigadetal.,2003);

and (ii) a sourceregion linked tothe Transgondwanan

supermountain range resulting from the East African–

AntarcticOrogenandformedduringtheprotractedLate

Neoproterozoic docking of East and West Gondwana

(Williamset al., 2012), involving the development of a

super-fan system (Kydonakis et al., 2014; Squire et al.,

2006).

SourcesofNeoproterozoicdetritalzircons.

Neopro-terozoic grains display similar age distributions in the

GrandmontandMarcoryformations:amain

Neoprotero-zoicpopulation of zircongrains (with onepredominant

Ediacarangroup)followedbyﬁvesimilarpeaksacrossthe

Tonian–Cryogenianinterval(Fig.4).Potentialsourcesfor

thesedetritalzirconsarelocatedintheeastern(Saharan

metacraton,Arabian-Nubianshield)andwestern

(Trans-Saharanbelt, Pan-Africansutureof theAnti-Atlas,early

and lateCadomianarcs, and AvalonianArc)area of the

North-Gondwanamargin(Fig.5).Theprobabilitydensity

curve of the Marcory Fm., as well as the shape and

roundnessofitszircongrains,impliesmoreremoteorigins

forthemthanfortheGrandmontFm.ones.Therefore,a

Fig.5.PotentialsourcesforsamplesMN1andMN4(modiﬁedafterDrostetal.,2011;Linnemannetal.,2011;Pereiraetal.,2011;Pereiraetal.,2012;Tack etal.,2001;Tacketal.,2010).AC1:SiderianeventoftheAmazoniancraton;AC2:Rhyacian,OrosirianandStatherianeventsoftheAmazoniancraton;AC3: SanIgnacioandSunsaseventsoftheAmazoniancraton;AC4:easternmarginoftheAmazoniancraton;Av:Avalonia;Cd:Cadomia;Bo:Bohemianmassif; AA:Anti-Atlas;WAC1:EburneaneventoftheWestAfricancraton;WAC2:LiberianeventoftheWestAfricancraton;WAC3:LeonaneventoftheWest Africancraton;TSB1:Trans-Saharanbelt,Benin-Nigerianshield;TSB2:Trans-Saharanbelt,Tuaregshield;SMC:Saharanmetacraton;ANS:Arabian–Nubian shield;CC:Congocraton.

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comparisonof themorphologicalandagedifferencesof

thesezircongrainssuggestsanevolution ofthe

deposi-tional basinand it sourcing froma narrow basinbeing

inﬁlledbytheGrandmontFm.,probablyaback-arcbasin

resulting from the Panafrican/Cadomian orogeny, to a

moreevolved andopened basininﬁlledby theMarcory

Formation, represents a more evolved and widespread

basinwithwiderpotentialsourceareas,developedduring

thebreak-upofWestGondwana.

AcompositezircongrainoftheGrandmontsandstone

(sampleMN1;ZR49)demonstrates thatcrustwithca.

586Maoldrocksbecamerecycledduringmagmatismat

ca.567Ma,and underlinestheexistenceoftwodistinct

Ediacaranmagmaticeventsinthesourcearea.

One should expect to identifythe Ediacaran

River-nous volcanic event as reworked zircon grains in the

overlying Cambrian Marcory Fm. However, this is not

the case. A rapid burial of the Rivernous volcano

sedimentary palaeorelief, related to high rates of

sedimentationandavailableaccommodationspace due

to active thermal subsidence, has been proposed for

FurongiantoEarlyOrdoviciantimesinWestGondwana

(Linnemannetal.,2011;Pereiraetal.,2012)andthelate

Neoproterozoic–earlyCambrianintheANS(Avigadand

Gvirtzman, 2009) or post-Cadomian rifting extension (Poucletetal.,inpress;VonRaumerandStampﬂi,2008),

whichshouldprecludereworkingoftheRivernousfrom

distal(northern)toproximalareas(southernMontagne

Noire).

6. Conclusions

TheEdiacaran–Cambrianboundaryhasbeen

conﬁdent-lyidentiﬁedwithinerror,basedonU–Pbzircondating,into

theRivernous Fm.ofthenorthernMontagneNoire.The

Rivernous volcanic event is indicated to be the lateral

equivalentoftheSe´rie`sTuffoftheAxialZone.Thisﬁtswell

with a latest Ediacaran depositional age (ca. 574Ma)

estimatedwithdetritalzirconU–Pbgeochronologyforthe

underlyingGrandmontFm.Thelatter shouldbe

consid-eredas a time-stratigraphicequivalentof the

acritarch-bearing‘‘SchistX’’Fm.oftheAxialZone(Supplementary

data,Fig.S3).

U–PbanalysisofthedetritalzirconsfromtheEdiacaran

GrandmontFm.and theCambrianSeries2 MarcoryFm.

suggestsachangeovertimeinthesourcing.TheEdiacaran

sedimentsweredepositedinanarrowback-arcbasinfar

from the inﬂuence of far cratonic sources, whereas

CambrianSeries2detritalsedimentswerederivedfrom

maturesourcerocksinvolvingthedenudationofdifferent

Gondwanancratons.

Acknowledgements

The authors thank constructive criticism made by

O. Bleinand B. Laumonier, and founding fromthe RGF

program of theFrench Geological Survey (BRGM). This

paperisacontributiontoprojectCGL2013-48877-Pfrom

SpanishMINECO.

AppendixA. Supplementarydata

Supplementarydataassociated withthisarticlecanbe

found,intheonlineversion, athttp://dx.doi.org/10.1016/j.

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