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Ducea, M.N. and Giosan, L. and Carter, Andrew and Stoica, A.M. and Relu,D.R. and Balica, C. and Balintoni, I. and Filip, F. and Petrescu, L. (2018)U-Pb detrital zircon geochronology of the lower Danube and its tributaries;implications for the geology of the Carpathians. Geochemistry, Geophysics,Geosystems 19 , pp. 3208-3223. ISSN 1525-2027.

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U-PBDETRITALZIRCONGEOCHRONOLOGYOFTHELOWERDANUBEAND2

ITSTRIBUTARIES;IMPLICATIONSFORTHEGEOLOGYOFTHE3

CARPATHIANS4

5

MihaiN.Ducea1,2,LiviuGiosan3,AndrewCarter4,AdrianaM.Stoica1,2,ReluD.Roban2,6

ConstantinBalica5,IonBalintoni5,FlorinFilip6,LucianPetrescu278

1UniversityofArizona,Tucson,AZ85721,USA92FacultyofGeologyandGeophysics,UniversityofBucharest,010041,Bucharest,Romania10

3WoodsHoleOceanographicInstitution,USA114-UniversityofLondon,UK12

5-Babes-BolyaiUniversity,ClujNapoca,Romania136InstituteforFluvialandMarineSystems,Romania14

15

16

Correspondingauthor:MihaiDuceaducea@email.arizona.edu17

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August19,201819

KeyPoints:20

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∗ A detrital zircon U-Pb study ofmodern sands from the lower Danube and its tributaries22

documents themainmagmatic events that led to the continental crustal formation of the23

nearbyCarpathians;24

∗ThegreatmajorityofbasementwasformedinlatestProterozoic–Ordovicianislandarcs,a25

findingthatisconsistentwithpreviousstudies;26

∗AnunexpectedandprominentCarboniferousmagmaticpeak in thedetrital recordhasno27

knownsourceinthenearbyCarpathians.28

2

29

30

ABSTRACT31

32

We performed a detrital zircon (DZ) U-Pb geochronologic survey of the lower parts of the33

DanubeRiverapproachingitsDanubeDelta-BlackSeasink,andafewlargetributaries(Tisza,34

Jiu, Olt and Siret) originating in the nearby Carpathian Mountains. Samples are modern35

sediments.DZagespectrareflectthegeologyandspecificallythecrustalageformationofthe36

sourcearea,whichinthiscaseisprimarilytheRomanianCarpathiansandtheirforelandwith37

contributions from the Balkan Mountains to the south of Danube and the East European38

Craton.39

40

The zircon cargo of these rivers suggests a source area that formed during the latest41

ProterozoicandmostlyintotheCambrianandOrdovicianasislandarcsandbackarcbasinsin42

a Peri-Gondwanan subduction setting (~600 -440 Ma). The Inner Carpathian units are43

dominatedbyaU-PbDZpeakintheOrdovician(460-470Ma)andlittleinheritancefromthe44

nearby continental masses, whereas the Outer Carpathian units and the foreland has two45

main peaks, one Ediacaran (570-610 Ma) and one in the earliest Permian (290-300 Ma),46

corresponding to granitic rocks known regionally. A prominent igneousVariscanpeak (320-47

350Ma)intheDanube’sandtributariesDZzirconrecordisdifficulttoexplainandpointsout48

to either an extra Carpathian source or major unknown gaps in our understanding of49

Carpathiangeology.YoungerpeakscorrespondingtoarcmagmatismduringtheAlpineperiod50

makeupasmuchasabout10%oftheDZarchive,consistentwiththemagnitudeandsurface51

exposureofMesozoicandCenozoicarcs.52

53

KEYWORDS:Danube,Carpathians,detritalzircon,U-Pbgeochronology,continentalcrust54

55

1. INTRODUCTION56

57

3

AlthoughitiswellestablishedthattheSouthandEastCarpathiansaswellastheApuseniand58

theBalkanmountains(comprisingaZ-shapeddoubleoroclineoftheeasternmostpartofthe59

CarpathiansandBalkans)wereassembledduringtheAlpineorogenyasignificantcomponent60

ofpre-Jurassicbasement (Schmidetal.,2008,Matencoetal.,2010,2017) recordsanolder61

history that, inmost places, is poorly known and sometimes controversial (Balintoni et al.,62

2014).Amajorobstacle is thatmorethan70%oftheorogen isdenselyvegetatedandthus63

poorly exposed. Some progress has been made over the past decade helped by modern64

geochronologydata (seeBalintonietal.,2014 fora reviewofbasementgeochronologyand65

thegeologicalbackgroundbelow).Thishasledtomostoftheolderinterpretationsregarding66

the origin and evolution of the Carpathians basement (Krautner, 1994, for a review) being67

revisedorabandoned.68

69

TherelativelyfewzirconU-Pbgeochronologicalstudiesofbasement(preJurassicigneousand70

metamorphic)rocks(Balintonietal.,2009,2010,2011,2014,BalintoniandBalica,2016)from71

eachofthemajorgeologicdomainsortheirsyn-tectoniccoverrocks,(Stoicaetal.,2016)have72

shownthat themajorityofbasement rocks in theRomanianCarpathiansandtheir foreland73

regions are Ediacaran to early Paleozoic island arcs. Confirmation and dating of Variscan74

magmaticandmetamorphicrockshasalsohelpedplacetheRomanianCarpathianswithinthe75

regionalgeologicframeworkofnearbyEuropeanbasementterrains(vonRaumeretal.,2013).76

But, despite these advances large areas, such as the Fagaras Mountains of the South77

Carpathians(thehighestmountainrangeintheCarpathians),havenotbeenvisitedbyrecent78

studies. As a consequence fundamental questions remain, such as whether there are79

PrecambrianbasementrockstotheCambro-Ordovicianarcsandiftherewasasuccessionof80

magmaticeventsassociatedwiththeVariscancollision.Futureadvancesrequirenewgeologic81

andgeochronologicaldataasdemonstratedforexamplebyarecentstudyofaductileshear82

zone in theSouthCarpathiansbasement (Duceaet al., 2016).Results in that study showed83

thatterraneassemblytookplaceduringthelatestPermian,muchlaterintheevolutionofthe84

Paleotethys thanpreviousmodelsallowedand this changedunderstandingof the timingof85

metamorphismandterraneassemblyoftheSouthCarpathians. It iswithinthiscontextthat86

4

weconductedaDZU-PbstudyofmodernriversedimentsfromtheDanubeanditstributaries;87

ourapproach isa reverseengineeringattemptat fillinggeochronologic/tectonicgaps in the88

scarcelyknownhistoryoftheregionalbasementthroughthelensofthesedimentaryrecord89

ofmodernrivers.Bycomparingtheknownincompletegeologicrecordofthebasementinthe90

Carpathianswiththe limitedbutspatiallysignificantcollectionofzirconages fromthemost91

important rivers draining the Carpathian mountains (Radoane et al.,2003) and the lower92

Danube itself,we aim to detectwhat ismissing from the regional geologic knowledge and93

wheretotargetfuturelocalizedstudiesofthebasement.94

95

Detrital zircon U-Pb geochronology is routinely used to investigate continental regions96

(Cawoodetal,2012;Gehrels,2014).Theabilitytomeasurelargenumbersofzirconsbyin-situ97

massspectrometry,mostlybylaserablationICP-MS(Gehrelsetal.,2008)hasturneddetrital98

zircon chronology into one of the most widely used quantitative provenance tools. Most99

studies aim to identify source area(s) of a sedimentary package by comparing zircon age100

distributions with bedrock ages from potential source areas (e.g. Barbeau et al., 2005;101

Thomas, 2011, Robinson et al., 2012; Gehrels and Pecha, 2014). Source regions are often102

distinguishable because each plausible source area has a specific geologic/tectonic history103

that includesdifferent times,durationsand fluxesof zirconproducingmagmatismand toa104

lesser extent, metamorphism. The goal of this study is the opposite in that we want to105

expand regional geochronological datasets by usingmodern river sediments to capture the106

zirconU-Pbagestructureofrocksinrivercatchmentareas.Here,wefocusontheDanubeand107

its tributaries thatdrain theeasternmost segmentof theCarpathianMountains inRomania108

(Matenco et al., 2016). We found an unexpected abundance of Carboniferous (Variscan)109

zircons,a relativelyyoungprovenanceageof theEastCarpathian foreland,aswellas some110

unexpected Eocene ages that amongother datahelp to clarify existinghypotheses. Results111

alsohelpguidefutureregionalwork.112

113

2. GEOLOGICBACKGROUNDANDZIRCONAGES114

115

5

The Romanian Carpathians comprise a series of Alpine units each comprising several116

individualthrustsheets,stackedupduringcompressionaltectonics.Themainunitsareshown117

inthesimplifiedmap(Fig.1afterMatencoetal.,2010,andpreviousworkcitedtherein).From118

top to bottom, the major units are: (a) Tisza, which makes up the northern part of the119

ApuseniMountainsandpartsoftheTransylvanianBasin(CiulavuandBertotti,1994),(b)east120

Vardar, a sequence of several thrusts that includes some primitive island arc rocks and121

pseudo-ophiolites of Jurassic age, (c) the Supragetic,making up the northern,western and122

easternpartsofthesouthCarpathians,(d)theGeticintheSouthCarpathiansandequivalent123

Bucovinic thrust sheets in the East Carpathians (the Supragetic and Getic are sometimes124

collectively referred to as Dacia), (e) the Ceahlau-Severin thrust sheets represented in the125

South Carpathians by a narrow belt of serpentinites and attenuated flysch and in the East126

Carpathians by a larger flysch belt, (f) theDanubian, the lowest trust sheet package in the127

SouthCarpathiansand(g)thethinskinnedthrustsoftheEastCarpathians,whichoverridethe128

forelandtotheeast.Indetail,thesearecomplicatedstructuresandhavenumerousalternate129

namesandinterpretationsintheliterature.InthisstudywefollowtheschemeofMatencoet130

al. (2010), which at the large scale is not fundamentally different from earlier syntheses131

(Burchfiel,1976,1980).132

133

The compressional assembly of these distinct blocks took place between mid- and late134

Cretaceousduringat leasttwopoorlydateddistincttectonicevents(confusinglyreferredto135

as“Austrian”and“Laramide”orogenicphasesintheRomanianliterature,Sandulescu,1984),136

followedbyalatersequenceofthrustingintheEastandSouthCarpathians,whichstartedin137

theMioceneandextendedintothePlioceneandQuaternary.TheApuseniMountains,which138

contain internal evidence of Cretaceous thrusting, may have been translated from more139

southerly latitudes and has almost certainly been rotated clockwise during aMid-Miocene140

(Ballaetal.,1987;Patrascuetal.,1994;DupontNivetetal.,2005)episodeoftectonicescape141

attributedtotheTiszabloc (Ratschbacheretal.,1993).Thus, thepostulatedpositionof the142

ApuseniMountains on top of the other Carpathian units may be the result of a relatively143

youngtectonicevent.Clearly, theoverallassemblyof these thrustsheets ismultiphaseand144

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their structural position today is complicated by translation along strike slip faults145

(Ratschbacher et al., 1993; Tischler et al., 2007; Ducea and Roban, 2016) and by the146

reactivationofsomethrustfaultsasextensionalstructures(Schmidetal.,1998;Fügenschuh147

andSchmid,2005).148

149

FormoredetailonthetectonicelementsandAlpineevolutionoftheRomanianCarpathians,150

whichremainhighlydebatedintheregionalliterature,wereferthereadertoseminalpapers151

bySchmidetal(2008),Matencoetal(2010;2017),CsontosandVörös(2004)andtheearlier152

reviewbyBurchfiel (1976),whosemainpointsweremadepopular in the local literatureby153

Sandulescu (1984). What is of importance to this paper is that the major units appear to154

containthinnedcontinentalbasement(igneousandmetamorphicrocksofpre-Mesozoicage)155

andareseparatedbysomerelativelynarrowbasinsinwhichsedimentationwasmarine(East156

Vardar,Ceahlau-Severin,andits latervariantfoundintheEastCarpathians,theParatethys).157

None of these appear to have been part of themajor Tethys ocean,whosemain suture is158

located to the southof theCarpathiansand theBalkans (Schmidetal., 2008). Instead they159

were basins possibly linked to the greater Tethys at times, formed on thinned continental160

crustandpossiblycontainingsmallfragmentsofoceaniccrust.Thisthinningpresumablytook161

place at the end of the Variscan orogeny, when a collisional belt collapsed in a Basin and162

Range-like fashion (Menard and Molnar, 1988), thus priming the Eastern European163

continental crust for the later development of the Tethys and related basins. Few of the164

Carpathian units described previously as ophiolites (Sandulescu, 1984, 1988) are165

geochemically or even geologically in the larger sense true ophiolites (Ianovici et al., 1976;166

Ionescuetal.,2009;Gallhoferetal.,2017).Forexample,theonesintheSouthApuseniarea,167

whichwereemphatically labeledas the “MainTethysian Suture”by Sandulescu (1984), are168

actually rocks found in association with a predominantly calc-alkaline suite ranging from169

basalt to rhyolites (Gallhoferet al., 2017). In thebroader sense thesebasinswerebackarc170

domainstothegreaterTethysocean,similartobasinsoftheCaucasus(Cowgilletal.,2016).171

ClosureofthesebasinsintheSouthCarpathiansledtoAlpinemetamorphism(Ciulavuetal.,172

2008). Tisza, part of the east Vardar thrust sheet, the Supragetic, Getic, Bucovinic and the173

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Danubian units, all contain pre-Alpine continental basement, as does the foreland to the174

southandeast.ThethinskinnednappesoftheEastCarpathiansdonothavemuchexposed175

basementperse,butpetrographyoftheseunitsshowclearlythattheyweresourcedbyrocks176

fromthenearbyforelandtotheeast.177

178

179180Figure1.ModernconfigurationoftheCarpathianorocline,withmajorgeologicalandstructuralunits,magmatic181arcs (Ca=Neogene calc-alkaline, A-Ca= late Cretaceous calc-alkaline, and K-Na =Jurassic) andmajor faults. The182mapiscompiledonthebasisoftheGeologicalMapsofRomaniaexecutedbytheGeologicalInstituteofRomania183at various scales (1∶1,000,000, 1∶200,000, and 1∶50,000) and subsequent work by the Free University of184Amsterdam/UniversityofUtrechtgroupsledbyProf.LiviuMatenco(e.g.Mațencoetal.2010).185186

Intheforelandthereareseveraldistinctblocks,someofwhich(Moesia,ScythiaandtheEast187

EuropeanPlatform)were viewedhistorically as platformsor even cratonic blocks basedon188

theapparentlackofdeformationoftheircoverrocks.Inbetweenthem,liesNorthandSouth-189

CentralDobrogea,whichareexposed intheprovinceofDobrogea(butcontinueunexposed190

undertheRomanianplains)andclearlyhaveacomplicatedPaleozoicand,inthecaseofNorth191

Dobrogea, even a Mesozoic history. The North Dobrogea does not represent a platform,192

whereasSouth-CentralDobrogea,whichcontinuesundereastMoesiaisconsideredbysome193

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asplatformal.Asmoreindustrydata(drillingandseismic)becomeavailablefromtheforeland194

regionsoftheCarpathians,itisclearerthatnoneoftheseareasactedasrigidlyaspreviously195

thought during the Alpine orogeny (Krezsek et al., 2017). In addition, they have a complex196

(and at the moment poorly resolved) Paleozoic history including magmatism as young as197

Carboniferous(e.g.inWesternMoesia,Paraschiv,1979).198

199

WhileMoesia,Dobrogea, Scythia andeven thewestern-most reachesof theEast European200

platformclosetotheCarpathiansremainhighlydebatedinthegeologicliteratureandpoorly201

knownduetolackofexposure,oneaspectrelevanttothisstudyhasbecomeclear:noneof202

these areas are legitimate cratonic areas free of deformation and magmatic activity for a203

sizablefractionoftheEarth’shistory.Instead,aswewillshowbelow,theyaredominatedby204

Neoproterozoic U-Pb ages (560-610 Ma) and a distinctive array of less abundant 1-2 Ga205

zirconsthatarerathersimilartotheDanubianunitintheSouthCarpathiansbutverydifferent206

fromthehigherAlpinestructuralunitsfoundtowardstheCarpathiansinterior.207

208

Figure 2 is a compilation Kernel Density Estimate (KDE) type plot of igneous and (mostly)209

detritalzirconsmeasuredinvariousCarpathianandforelandunitsoverthepastdecade.The210

great majority of these data were acquired at the Arizona Laserchron facility (and its211

precursors)at theUniversityofArizonaandpublishedbyvariousgroupscited in the figure.212

Recently,Gallhoferetal.(2015,2017)havecontributedtotheU-Pbgeochronologyknowledge213

of theJurassicandCretaceousCarpathianmagmaticarcrocksofSouthApuseniandBanat,214

respectively. Older (pre-2008) zircon data from this segment of the Carpathians and its215

forelandarefew:someTIMSworkfromtheEdmontonlaboratoryintheearly2000sonafew216

Jurassic plutons of the east Vardar region (Pana et al., 2002), earlier TIMS work on217

Neoproterozoic Danubian granitic plutons (Liégeois et al., 1996) and limited 1960s-1980s218

geochronology performed in Soviet laboratories and published in local journals, without219

analyticaldetails.220

221

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Zircon U-Pb ages from the interior of the Carpathians orocline (basement and cover units222

derived from them) are dominated by latest Precambrian (~600Ma) to Cambro-Ordovician223

ages,insomeareascontinuingintotheSilurian(Balintonietal.,2009,2010,2011,2013,2014)224

(Figure 2). The following structurally higher units share this history: Tisza, East Vardar,225

Supragetic,andDacia(GeticandBucovinic).WerefertotheseastheInnerCarpathianUnits.226

These rocks represent the products of relatively long-lived peri-Gondwana island arcs over227

thattimeperiod(600–420Ma)withisadistinctdominantpeakat466±10Marepresenting228

theculminationofahighfluxmagmaticevent(Stoicaetal.,2016).Olderzirconsarefewand229

theymakeupasmallGrenvillepeakandfewagesolderthan2.0Ga-theyarethoughttobe230

derived from a landmass closest to the island arcs. A less pronounced peak at 330±20Ma231

represents the Variscan orogen and is believed to record a period of high grade232

metamorphism (Dallmeyeretal., 1998;DragusanuandTanaka,1999;Medarisetal., 2003),233

which marks a continent-continent collisional event. Variscan zircons are mostly234

metamorphic. The end of high-grade metamorphism is tentatively constrained by the235

extension-relatedtectonicemplacementofsomemantleperidotiteintothecrust(Medariset236

al.,2003)at316±4Ma.237

238

Incontrastwiththeinteriorunitsdescribedabove,theDanubianoftheSouthCarpathians,all239

threeDobrogea blocks, the sedimentary units of the thin skinned thrust sheets of the East240

Carpathians (presumably derived from eastern sources) and the limited data on Moesia’s241

basement all show a differentU-Pb signature (Figure 2). They are referred to as theOuter242

CarpathianUnitsbelowandarefoundstructurally in lowerpositions intheAlpinestackand243

towardorwithintheforeland.TheyhavealateVariscan(285-300Ma)signature,whichinthe244

exposedDanubianisrepresentedbypost-collisionaloftenS-typegranitoidscommonlyfound245

elsewhereinthePeriGondwananbasementofEurope(StampfliandBorel,2002;Stampfliet246

al.,2011;vonRaumeretal.,2013)butwhichisnotpresentintheinnerunitsoftheRomanian247

Carpathians(wheretheVariscanpeakisat320-350Maandispredominantlymetamorphic).248

The dominant age peak of the Outer Units is Neoproterozoic one, 580±30 Ma with249

progressivelyfeweragestoward~800Ma.IntheexposedDanubian,Neoproterozoicplutons250

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are intruded into a metamorphosed sequence of 800 Ma island arc rocks (Liégeois et al.,251

1996);presumablyasimilarscenarioappliestolesserexposedunitsoftheforeland.Cambro-252

Silurianpeaks(withamaximumat460Ma)whicharecommonintheInnerUnitsdonotexist253

here. Theother distinctive featureof theouter units is the relatively abundant Proterozoic254

peaksat1.2Ga,1.5Ga,1.75Ga,2.0GaandthelateArcheanpeakat2.7Ga(Figure2).While255

thereisnoevidenceforbasementofthatagetohaveexistedintheDanubianandprobably256

the other outer units, they may have been located near to such a source (and the exact257

continental fragment representing that source remains debated, see Balintoni et al., 2014,258

BalintoniandBalica,2016)whereastheinnerunitswerenot.259

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260Figure2.KDE(KernelDensityEstimate)ageprobabilitydiagramsofCarpathiansandforelandzircons261(compiledfromvariouspublishedsources).Thepinklinerepresentingsedimentarycover(thinskinned262nappes)ofAlbian rocks combine zirconsderived from InnerandOuterCarpathianunits. TheOuter263CarpathiansincludetheforelanddomainstotheeastandsouthoftheCarpathians264265

266

Overall, despite cursory knowledge of the geologic history of the Romanian Carpathian267

basement,allbasementunitsabovetheCeahlau-Severinsuture(theInnerCarpathianUnits)268

have a distinctive U-Pb zircon age distribution from the one found below it, including the269

foreland(theOuterCarpathiansUnits).270

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271

Alpinemagmatismisrelativelyscarce;theRomanianCarpathiansarecoveredbyabout12%272

volcanic and intrusive rocks of post Permian rocks. A few early extension-related rocks in273

DobrogeaandtheEastCarpathiansareknowntobeofTriassicage;theDitraualkalinemassif274

oftheEastCarpathians(Ar-Arageof230Ma,Dallmeyeretal.,1997)withadiameterofabout275

20kmissizable,butunlikelytobeamajorsourceofdetritalzircons..TheEastVardarregion276

compriseslatestJurassicophiolitesandislandarcrocksrangingincompositionfrombasaltto277

rhyolites and syn- to late- granitoid plutons (Ianovici et al., 1976, Pana et al., 2002). These278

rocksoccupya sizableportionof the southernApuseniMountainsbut areprobably zircon-279

poor due to their relativelymafic average composition. A belt of intermediate calc-alkaline280

rocksextendingintotheSouthCarpathianstotheApuseniMountains,sometimesreferredto281

as banatites (Berza et al., 1998), formed in response to the Sava ocean subduction to the282

southoverashortperiodbetween75and82MaintheRomaniansegment(Zimmermanet283

al.,2008;Gallhoferetal.,2015).Althoughwellstudiedfortheirnumerousoredepositsand284

richmineraldiversityassociatedwith them,outcropscoveronlya small area.MidMiocene285

volcanicandhypabyssal intermediate to silicic intrusivesare found in the southernApuseni286

Mountains (Seghedi et al., 2004, 2011; Rosu et al., 2004) mainly along an extensional287

lineament, but they toooccupy a small fractionof theoverall Carpathian regionmaparea.288

Finally, volcanic and some hypabyssal intrusions of Miocene (~15 Ma) to Quaternary are289

foundimmediatelytothewestoftheEastCarpathians;theyrepresentasizablevolcanicarc290

associatedwiththeclosureoftheParatethysocean;thearcrangesincompositionfrombasalt291

torhyolite(Seghedietal.,2011),andisonaverageanandesite.Giventhecatchmentareasof292

the river sands sampled for this study (only the Tisza draws somewhatmore heavily from293

riversthatcrosscutthisarc),Mioceneoryoungerdetritalzirconsareunlikelytobecommon.294

Overall,thesevariousplutonicandvolcanicrocksareexpectedtobepresentintheriversands295

butonlyinlownumbers.296

297

One of the critical assumptions in the interpretations below is that the lower Danube in298

Romania and Bulgaria was closed in recent times to sediment supply from west of the299

13

Carpathianbarrier(theGoldenGategorge)(Matencoetal.,2016)andthusthegreatmajority300

ofzirconsmeasuredinthisstudyaresourcedfromthemoderncatchmentareasinthenearby301

mountainrange.ThisassumptionmaybreakdownindetailastheDacianBasin,forexample302

(theforelandoftheSouthCarpathians),mayhavebeenconnectedinthepast(e.g.Miocene)303

toother segmentsof Paratethys andas such,mayhave recycled zircons frommostdistant304

sources contained in basin sediments. If zircons did come from outside of the Carpathian305

orocline,thisassumptionstatesitisunlikelytheywouldbeabundant.306

307

3. SAMPLES308

309

Finetomediumgrainedsandsampleswerecollectedfromtherecentalluvialdepositsofthe310

activebarsandbanksalongtheDanubeRiverand fourof itsmain tributaries (Siret,Olt, Jiu311

andTisza)(Figure3andTable1).Samplesontributarieswerecollectedascloseaspossibleto312

theirconfluencewiththeDanube.313

314

Zircon concentrates were prepared using standard heavy liquids and a Franz Isodynamic315

magneticseparatorset<0.5amps.Zirconrichfractionswerethenmounted inepoxyresin316

and polished to expose internal surfaces. At no pointwere zircons hand picked as this can317

introduce bias. Analyses weremade on every zircon like grain intersected during scanning318

along transects across polished grainmounts using a NewWave 193 nm aperture-imaged,319

frequency-quintupledlaserablationsystemcoupledtoanAgilent7700quadrupole-basedICP-320

MS.Atypicallaseroperatingconditionforzirconusesanenergydensityofca2.5J/cm2anda321

repetitionrateof10Hz.RepeatedmeasurementsofexternalzirconstandardPlesovice(TIMS322

referenceage337.13±0.37Ma;Slamaetal.,2008)wasusedtocorrectforinstrumentalmass323

biasanddepth-dependentinter-elementfractionationofPb,ThandU.Temora(Blacketal.,324

2003) and 91500 (Wiedenbeck et al., 2004) zircon were used as secondary age standards.325

Ageswerebasedon the 206Pb/238U ratio for grains <1000Maand the 207Pb/206Pb ratio for326

oldergrains.DatawereprocessedusingGLITTER4.4datareductionsoftwareandgrainswith327

14

a complex growth history or disturbed isotopic ratios, with > +5/-15% discordance, were328

rejected.329

DatatablesareprovidedintheSupplementaryMaterial.330

331

Table1.SamplinglocationsalongDanubeandtributaries.332

333334

335336

Samplelocation Latitude Longitude Distancetomouth[km]

DanubesamplesTulcea 45°13'15.99"N 28°42'46.94"E 115Brăila 45°19'22.59"N 28°0'8.84"E 164Turnu 43°42'46.30"N 24°53'28.30"E 596

TributarysamplesSiret 45°23'56.80"N 28°0'41.90"E 155Olt 43°44'50.97"N 24°46'35.68"E 604Jiu 44°34'09.80''N 23°27'19.80''E 694Tisa 46°08'48.76"N 20°03'52.47"E 1214

15

Figure 3. Sample locations along the Danube (blue symbols) and tributaries (red circles) from337downstreamtoupstream:1.Tulcea;2.SiretRiver;3.Braila;4.Turnu;5.OltRiver;6.JiuRiver.7.Tisza338River. Catchment areas are show for each tributary sample. The map is color coded based on339elevation- blue-green colors correspond to low elevations, whereas brown and red are higher340elevations.Thehighestpeaksareshowninthebrightestoftherednuance.341342

4. RESULTS343

344

Figure 4 shows KDE age distributions of Danube tributary samples and figure 5 shows KDE345

plotsofDanubesamplesatdifferentlocations.TheKernelDensityEstimateplots(Vermeesch,346

2012)usedanadaptivebandwidth.Belowwepresenttheseresultsindetailandhighlightthe347

important age groups found in each sample. First, we discuss the results on the tributary348

samplesfollowedbythepresentationoftheDanubesamples.Allsamplesaredominatedby349

igneouszirconsbasedonU/Thratios(seesupplementarydatatables).U/Thratiosisexcessof350

10 are taken to reflect ametamorphic origin,whereas lower ratios (most commonly lower351

than2-3)are typicalof igneous zircons.Despite the fact thatadominantproportionof the352

Carpathianbasementareaismetamorphic,veryfew(<2%ofthetotalanalyzedpopulationof353

zircons from theDanubeand tributaries) zirconsaremetamorphic; they reflect the igneous354

crystallization of the protoliths. In previous studies (e.g. Balintoni et al., 2009, 2010), we355

observed that when zooming in at zircon rim scales (10 microns or less), metamorphic356

overgrowthsarenotuncommonamongCarpathianbasement zircons. In thisdetrital study,357

wemeasuredonlycoresofzircongrainsanddidnotfocusonintragraincomplexities.358

359

360

4.1.Danubetributarysamples361

362

TiszaRiver363

Tisza has a broad and complex provenance in that it mixes major tectonic units from the364

Bucovinic in the East Carpathians (mostly the ApuseniMountains),with source areas of its365

tributaryMures,whichdraws fromboth theGetic-Suprageticandeven theDanubianunits.366

Therefore,therangeofdetritalzirconagesisexpectedtobediversealthoughdominatedby367

16

zirconsfromtheInnerCarpathianunits.TheTiszaageplot(Figure4A)hasamixedagesignal368

which is dominated by Inner Carpathians (Cambro-Ordovician 550-440 Ma with Variscan369

zircons320-350Ma)asexpected,butitalsoshowssignificantinputfromtheDanubian(290370

Ma,whichonFigure4ccanbeseenasasecondarypeaktolarger320Maand600Mapeaks).371

AfewPrecambrianzirconsarealsopresent.Themostnoteworthyfeature,asisthecasewith372

the Olt River, is the predominance (50% of measured ages) of Variscanmagmatic zircons,373

whichissignificantlydifferentthanonewouldpredictbasedonexistingdatafromanyofthe374

InnerCarpathiansunits.ThepresenceofafewJurassiczirconsisattributedtothesouthern375

Apuseni island arc-MORB corridor whereas the late Cretaceous grains are part of the376

banatititcarc.Lessexpectedarea few latestPermiantoTriassicages,whicharedifficult to377

correlatetoknownmagmaticrocksintheCarpathians.378

379

JiuRiver380

TheriverJiudrainsmostlyDanubianrocks,withonlyaminorsetoftributariesbeingsourced381

intheGeticunit.Inthisrespect,theJiuzirconpopulationshouldbethesimplestofallsamples382

inthisstudyandreflecttheDanubianbasement,whichitdoesratheraccurately(Figure4C).383

The main age peak (575 Ma) corresponds to the dominant Danubian Neoproterozoic384

magmatic event found in the Danubian (Liégeois et al., 1996) as well as in other outer385

Carpathianunits(Figure2).Neoproterozoicagesdecreaseinnumbertowardsthe800Maage386

of the Dragsan Series, which is considered the oldest basement of the lower Danubian387

(Balintonietal.,2011).AlsopresentaresomeCambro-SilurianagesfromtheGeticunitabove388

whichisdrainedbyafewoftheeasterntributariesoftheJiu.Thereisnoevidencethatsuch389

agesexistintheDanubianthrustsheets.ThereisalsonotarealVariscansensustrictopeakat390

330MaintheDanubian,apartfromacoupleofagesthatmayagainbederivedfromtheGetic391

unit. A second peak (290-300 Ma) corresponds to abundant late Variscan post-tectonic392

granitoids known from theDanubian.Older inherited age peaks at 1.0Ga, 1.5Ga, 1.75Ga,393

1.95Ga, 2.4Ga and 2.7Ga are also typical forOuter Carpathians units. The dominance of394

NeoproterozoicversusPermianagesinthedetritalrecordisexpectedbecausetheJiuandits395

tributariesdrainprimarilythesouthernslopesoftheSouthCarpathians,wheretheolderages396

17

prevailatoutcrop.RiverswashingthenorthernslopesoftheDanubiandomain(e.g.Retezat397

Mountains)aremore likely tobedominatedbyc.300Maagesbecauseof thehighsurface398

areaoccupiedbyagraniticbatholithofthatage.399

400

Also present are a few late Permian and Triassic age (263 to 219Ma) that do not fitwith401

known Carpathian rocks. A few Mesozoic and Cenozoic ages are also present (see402

SupplementaryDataTableastheydonotproperlyshowonFigure4duetothebandwidthof403

the age spectrum) although these are attributable to the banatiticmagmatism orMiocene404

tuffsthatareknowntobepresentintheforelandoftheSouthCarpathians.405

406

407Figure4.Danubetributariesriversamples(A.Tisza,B.Olt,C.JiuandD.Siret;)KDE(n=numberofzirconsages).408Seetextforfurtherexplanations.409

410

411

412

OltRiver413

18

TheOltRiverdrainsmostlyGeticandSuprageticunitsandsinceitoriginatesintheSouthEast414

Carpathians,itmaycontainsomeBucovinic(Geticequivalent)init.Sometributaries(Lotruor415

Cibin)maybringintotheprovenancemixmaterialfromtheDanubianandsouthernApuseni416

butitissubordinatecomparedtoGetic-Suprageticsources.Thegreatestamountofsediment417

isdriven from theglaciatedFagarasMountains aswell as theelevatedareasof the central418

South Carpathians, all of which are Getic and Supragetic units and are overwhelmingly419

metamorphic(basement)rocks.420

421

TheOltsample(Figure4B)containsalargenumberoflatestPrecambriantoCambro-Silurian422

ages typical for the Inner Carpathian units previously seen in the basement proper and in423

various sediments derived from it (Figure 2). However, the peak distribution is unlike the424

averageoftheGeticunits.ThesurprisehereisthatmorethanVariscan(320-350Ma)zircons425

dominate(≥50%)whichisunlikeanypreviousstudyoftheGeticbasement.426

427

TheOltsamplehasthecharacteristicallylowabundanceofolderPrecambrianpeaksespecially428

between1.2and2Ga(Balintonietal.,2014),althoughthereisasizableGrenvillepeak(1-1.2429

Ga) which is not common to Inner Carpathian units. A single late Eocene age is similar to430

Eocene zircons detected in some of the other samples but puzzling because such ages are431

unknown in anyof theCarpathianunits to thenorthof theDanube,or in the south in the432

BalkanMountains.EocenemagmatismisprominentintheRhodopeMountainssouthofthe433

BalkansbutthisareadoesnotdrainintotheDanube.434

435

436

SiretRiver437

TheSiretRiveristhelargesttributaryoftheDanubeoriginatingontheeasternslopesofthe438

EastCarpathians.Throughitstributaries,theSiret’ssourceareasincludetheInnerCarpathian439

UnitsaswellastheouterUnitsandtheEastCarpathiansforeland.ThezirconKDEplot(135440

zircons, Figure 4D) is complex and dominated by magmatic zircons, only two Precambrian441

grains showmetamorphicTh/U ratios (>20).The InnerCarpathianUnitsdiagnosticCambro-442

19

Ordovician-Silurian age pattern with a lesser group of Neoproterozoic ages is clearly443

distinguishable.SomezirconagesbelongingtotheNeoproterozoicpeakmaycomefromthe444

Outer Carpathian Units. The range of Variscan ages suggest mixing of Getic metamorphic445

rocks(~330Ma)withlateVariscanS-typegranitoidsoftheDanubian(290-300Ma).446

447

Inaddition,amorepronounced1-2Gaspectrumofagesisfoundinthissamplecomparedto448

allotherssamplesandrelevantsourceareas(Figure2).Theseagesmakeupmorethanhalfof449

thezirconagepopulation.More than5%areArchean (2.4and2.7Ga),more than inother450

samples and is indicative of the Sarmatian craton as a source area. Taken together, these451

olderthan1.0GaagepeaksmakeupthelargestgroupingofpreGrenvillezirconsfoundinany452

basement or cover rock from the Romanian Carpathians and nearby foreland. Harder to453

explainarethepresenceofMesozoiczircons.Eightzircons(makingup6%ofthepopulation)454

are Triassic, Jurassic or Cretaceous. Although there is a large Triassic alkaline massif455

(Dallmeyer et al., 1997) in the East Carpathians (Ditrau) the ages do not exactly match.456

YoungerMesozoicagesarealsopuzzling,andconceivablytheymayhavebeensourcedfrom457

someoftheMid-CretaceousflyschunitsoftheCeahlauunit(althoughthereisnomagmatic458

arcassociatedwiththeflyschunits).459

460

Tosummarize,theSiretriversandcontainsadiagnosticCarpathiansignalbutitalsocontains461

a significant number of Precambrian zircons and a group of Mesozoic ages that are not462

obviouslytiedtoknownsourcerockswithintherivercatchmentarea.463

464

4.2.Danubesamples465

DanubeatTurnu466

TheDanubeatTurnu(103zirconsmeasured,Figure5A)isamixofsedimentdownstreamof467

mostinputsfrommostofthemajorSouthCarpathianriversandthisprovidesagoodaverage468

of provenance prior to the arrival of rivers from the East Carpathians (e.g. Siret) and from469

moreEast European cratonal sources (Prut). In addition tobeing locateddownstream from470

themajor Carpathian rivers Turnu is also founddownstreamof the largest Bulgarian rivers471

20

drainingtheBalkanMountains.Variscan intrusionsareabundant intheBalkans (Carriganet472

al., 2005) but their age span, between 317- 297 Ma, is distinctively younger than our473

magmaticagesthatfallbetween350-320Ma.474

475

TheTurnusampleappearsamixturebetweenaJiu-like(Danubian)KDEandanOlt-like(Getic-476

Supragetic) age distribution. Both Neoproterozoic (at around 850 Ma) and late Variscan477

granitoidagesarepresentalthoughArguablyDanubian sourcesare slightlymore important478

than Getic-Supragetic ages supported by distinct Mesoproterozoic peaks. The unexpected479

Variscanpeak(igneouszirconsof320-360Ma)found intheOltRiversample isalsopresent480

and has the samemagnitude relative to the Cambro-Silurian peaks of the Getic-Supragetic481

units.AfewCenozoicages(45Ma,23Ma,6Ma)areunlikeanyigneousactivityknowninthe482

mountainous regions representing the source of the lower Danube at this location. It is483

possiblythatsometuffsorloessderivedfromthem,foundintheforelandoftheCarpathians484

andotherlesserstudiedvolcanicunitsintheBalkanstothesouthcouldprovetothesources485

of these zircons. At this point, however, these ages do not match the existing Cenozoic486

regionalgeologicrecordandaredifficulttouseforward.487

488

DanubeatBraila489

This sample (118 zircons measured, Figure 5B) location was chosen to give an integrated490

DanubesignalpriortothearrivalofthelasttworiversfromtheMoldovanforeland,someof491

which could carry amuchmore East European cratonal age signature (dominantlyArchean492

see Figure 2) compared to the ones derived from the Carpathian and Balkan orogens.493

OtherwisethesignalshouldnotbemuchdifferentfromthatatTurnu.494

495

ThebulkofofthedetritalzirconagedistributionisconsistentwithamixofInnerandOuter496

Carpathians plus foreland sources (including the now ubiquitous magmatic Variscan signal497

from the Getic-Supragetic). It is clear that overall no one event dominates, neither the498

NeoproterozoicoftheDanubian,northeCambro-SilurianarcsoftheGetic/Supragetic,northe499

VariscanagesoftheGetic(possiblyalsofromtheBalkansinthesouth),orthepostVariscan500

21

granitoidsoftheDanubian.Inall,theycontributedmoreorlesssimilarlytothezirconbudget501

andareconsistentwitherosionofthemodernCarpathians.Afewyoungoutlieragesexistat502

Braila,somederivedfromtheNeogeneVolcanicfield(<10Ma),otherrepresentingeitherthe503

Eoceneagesseeninothersamplesorthewell-establishedlatestCretaceousmagmatism.504

505

DanubeatTulcea506

Thissample(104individualzirconsmeasured,Figure5C)representstheintegratedDanubeDZ507

signal downstream from the arrival of the main cratonal tributary (Prut) and just before508

enteringtheDanubeDeltatodrain intotheBlackSea.Thebreakdownofages isabout40%509

InnerCarpathianUnits,40%OuterCarpathianUnits,10%Alpineages (JurassicEastVardar,510

late Cretaceous banatitic magmatism, Neogene magmatism including two unexplainable511

Oligoceneages),about5%inferredtobefromthenearbyNorthDobrogeaterrain(basedon512

thedominanceof250Maigneousagesthere,BalintoniandBalica,2016)andtheremainder513

probablybeingderived fromeither East European cratonormore likelyperi-cratonal areas514

similar to theOuter Carpathians but dominated by 1-2 Ga ages. Only two of these zircons515

(datedat420and322Ma)havehighTh/Urationsandarethereforemetamorphicinorigin.516

The integrated Danube signal shows that the Inner andOuter Carpathians each contribute517

abouthalfof thepresent-dayzirconcargodespitethegreaterexposureof InnerCarpathian518

units in the mountainous regions. The integrated signal undoubtedly contains zircons that519

were derived from sedimentary sources such as the thin-skinned nappes of the East520

Carpathians that primarily came fromOuterCarpathianUnits.Overall the greatmajority of521

the Carpathians and foreland were formed in the latest Proterozoic to the middle of the522

Paleozoic (600-420 Ma) followed by the enigmatic Variscan (320-350 Ma) episode of523

magmatism,whichstillmakesupabout17%oftheintegratedsignalatTulcea.524

22

525Figure5.DanubeRiversamples(A.Turnu,B.Braila,C.Tulcea) KDE(numberof individualzirconsshowninthe526diagram).Seetextforfurtherexplanations.527528

5. INTEPRETATIONSANDIMPLICATIONS529

530

Comparison of previously published data from the Romanian Carpathians basement and531

results fromthis studyallowus tomakeacrudeestimateof the igneousandmetamorphic532

A

B

C

0 300 600 900 1200 1500 1800 2100 2400 2700 3000

Turnu (n=118)

33

319

474

558

623

891

1008

1754

1222

1982

KDE

0 300 600 900 1200 1500 1800 2100 2400 2700 3000

Braila (n=103)

48

304

459

573

802

1004

704

1262

1620

1959

KDE

0 300 600 900 1200 1500 1800 2100 2400 2700 3000

Tulcea (n=104)

58

336

449

570

627

757 10

09

1154

1716

2004

2201

2700

2983

256

KDE

23

events responsible for themakingof thissegmentofcontinentalcrust (Figure6).Belowwe533

detailourfindingsanduncertaintiesassociatedwiththem.534

535

5.1.Natureofbasement536

ThemainresultsofthisstudyconfirmpreviousworkthatsuggestallCarpathianunitsandthe537

forelandformedduringtheNeoproterozoicandearlyPaleozoicinaseriesof islandarcsand538

marginal basins formed in a peri-Gondwanan setting (Balintoni et al., 2014). This scenario539

applies to much of the pre Alpine basement of mobile Europe (Stampfli et al., 2011; von540

Raumeretal.,2013).Resultsalsosupporttheviewthatagestructureandcompositionofthe541

InnerCarpathianunitsaredifferentfromtheOuterCarpathianunitsandforeland.TheInner542

unitshave little inheritance fromearlierPrecambrian zirconsandhave two stagesofmajor543

crustalgrowth,oneat560Maandtheotherat460Ma.Theseare followedbywell-known544

Variscanbarrovianmetamorphismandsomepoorlydocumentedassociatedmagmatism(320-545

350Ma).TheDanubianunitintheSouthCarpathiansandtheforeland(includingDobrogea),546

arebetterrepresentedbyinheritedagesinthe1-2Gainterval(Balintonietal.,2012;Balintoni547

and Balica, 2013), and record a distinct magmatic age between 570-620Ma, along with a548

lesserbutimportantagepeakataround800Ma(theoldestrocksoftheCarpathians),.There549

isalsoadistinctepisodeofpostcollisionalmagmatismat290-300Ma,somepostVariscanS-550

type granitoids known regionally in the Danubian but no obvious Variscan metamorphism551

(315-350 Ma) as seen elsewhere. The study detrital zircon age distributions of the main552

Danube tributaries match these established events and can therefore be considered553

representativeoftheregionalgeology.554

555

Each of these two major domains (separated by Alpine basins, now closed as sutures)556

contributes about 37% of the total DZ signal of the Danube as it enters its delta. The557

remainder ismadeupofsomeAlpinemagmaticages, limitedcraton inputfromtheEastern558

Europeanstableareatothenorthandanunexplainedbutsizable(17%ofthetotalDZbudget)559

group of Variscan magmatic ages not known in the Romanian Carpathians or Variscan560

magmatismintheBalkanMountainstothesouth(seebelow).Theseagesaside,thebulkof561

24

theCarpathiancontinentalcrustwasformedinislandandtransitionalarcsandothermarginal562

(e.g. backarc) basins close to Gondwana, between about 600 and 420 Ma, and with a563

dominantagepeakataround460Ma.Theoldestarc isfoundintheDanubianunitandisa564

maficislandarcremnantofabout800Ma.565

566

567Figure6.Majormagmatic (red) andmetamorphic (blue)events in theRomanianCarpathiansand foreland, as568evident from this andprevious studies. TheenigmaticVariscanmagmatic eventdescribed in this paper is not569shown here. DM – Dragsanmetamorphism (age unresolved but prior to Ediacaranmagmatic event). NDM –570North Dobrogea metamorphism, also age unresolved. Vertical length of the boxes shown in this figure is571proportionaltothezirconabundancebudgetintheriversstudiedhere,whereastheextentofmetamorphismis572basedonthesurfaceexposureofthesemetamorphicrocksrelativetomagmaticrocks.573574

575

5.2.AgesoftheEastCarpathianforeland576

577

Itisimpossibletoquantifyhowmuchoftheriversignalisforeland-derivedsincealmostallof578

the Carpathian foreland is covered by younger sediment and vegetation. The Dobrogea579

forebulge (Matencoetal.,2013) is located in the foreland to theCarpathianoroclinalbend580

anditszirconbudgetissomewhatsimilartotheOuterCarpathianunits(Balintonietal.,2014,581

25

BalintoniandBalica,2016).Ultimately,westillknowverylittleaboutthebasementofMoesia582

andtheEasternEuropeanforelandeastoftheCarpathiansandthisstudywasnotdesignedto583

addmuchtothat.However,anoteworthyfeatureisthattheSiretRiver,whichdrainstheEast584

CarpathiansandasignificantpartoftheEastEuropeanforeland,hasaDZpatternsimilarto585

OuterCarpathianunitsand isnotdominatedbycratonsources fromtheeast.Rivers to the586

east,suchastheDniesterinUkraineandMoldovaandcertainlytheVolgaandDoninRussia,587

may provide a more accurate craton signal. Recognizing that the Siret river DZ signature588

probablymostlyderivesfromthethinskinnednappesandnottheforeland,wearguethatthe589

immediateforelandtotheeastCarpathiansisnotpartofthecraton.TheScythianandother590

poorlyknownmobilebeltsborderingtheEastEuropeancraton(Kuznetzovetal.,2014)must591

makeup themajorityof thebasementof theMoldovan foreland,while legitimate cratonic592

Archeanblocksarenowherenear.593

594

5.3.Variscanages595

596

The Variscan (315-350Ma) igneous (low Th/U) ages so prevalent in the analyzed samples,597

represent themost surprising finding of this study, based on the known regional geology.598

Limitednumbersofmetamorphicagesaretobeexpectedbutnotmagmaticages.Thereare599

threepotentialexplanations:(1)magmatismofVariscanageismoreprevalentinpartsofthe600

Carpathians basement than previously recognized, (2) these grains were brought into the601

forelandbasins (e.g. theDacianbasin,orMoesia) froma southerlyorigin (theBalkans)and602

recycled inthemodernDanubeandtributaries,and(3)theseagesareextra-Carpathian, i.e.603

brought by the Danube or paleo-Danube from significantly upstream where Variscan604

magmatismismorewidespread(NeubauerandHandler,1999;vonRaumeretal.,2013).605

606

ThefirstexplanationishighlyunlikelydespiteourlimitedU-Pbgeochronologicknowledgeof607

largeareasoftheSouthCarpathians,inparticulartheFagarasMountains.Whiletheglaciated608

andelevatedFagarasMountainsmayrepresentamajorsourceofsedimentintheOltRiver,609

andonly one sample (detrital, Balintoni et al., 2009) has ever been analyzed forU-Pb ages610

26

fromthenorthernslopesoftherange,itisclearthattheyarenotmadeupofpredominantly611

igneousrocks,butinsteadtheyaredominatedbyvariousmetamorphicsequences(Panaand612

Erdmer, 1994). Garnet Sm-Nd geochronology data (Dragusanu and Tanaka, 1999) shows613

conclusively that metamorphism is Variscan (320-360 Ma). All areas from the Romanian614

Carpathians undergoing Variscan modifications are characterized by amphibolite grade615

metamorphismthatendedatabout315Ma(Medarisetal.,2003)withextensionalcollapse;616

neither the peak metamorphism nor the extension that followed it is associated with617

significantfelsicplutonism.ManyofthehighgraderocksofDaciacontainsomeleucogranite618

and pegmatites (Hann, 1995), but they represent less than 1% of the exposed area (Horst619

Hann,personalcommunication,2017),arePermian(255-280Ma)inageandhavedistinctively620

largeTh/Uashighas300(ourworkinprogress).Itisthusunlikelythatayettobeidentified621

Carpathianterraincanbethesourceofthesezircons.622

623

ThesecondandthirdpossibilitiespresupposethatVariscanzirconsweretransportedintothe624

peri Carpathian Paratethys basins at an earlier time either from the south (the Balkan625

Mountains) orwest of the Romanian Carpathians (various locations in central Europe) and626

laterincorporatedaslocalsourcesintothelowerDanubeandtributaries(seeFigures3and6627

invonRaumeretal.,2013).TheBalkanmountainsdocontain,incontrasttotheCarpathians,628

significant areas of Variscanmagmatism (Carrigan et al., 2005), but their age is somewhat629

younger(317-297Ma)thantheVariscanpeakidentifiedinthisstudy.Wedonotidentifythe630

Balkanageswiththoseinourdata.631

632

Anexo-Carpathiansource–tothewestandbeyondtheCarpathiandoublebend–isplausible633

for theDanube itself if it can carry coarsematerial through the IronGates gorge,which is634

debatable.Butforthetributaryrivers,suchastheglaringexampleofOlt,anOuterCarpathian635

source would presume that the Dacian basin fill (part of the Paratethys in recent times)636

contains relatively far traveled zircons from times when this basin was interconnected to637

othersoftheParatethys(Pannonian,etc.)(Matencoetal.,2013).ThoseVariscanzirconswere638

then transported further downstream by the Olt River from the Miocene-Pliocene639

27

sedimentaryfilloftheDacianbasin.Alternatively,intheso-called“spillandfill”model(Bartol,640

et al., 2012, Leever et al., 2010, 2011), a paleo-Danube that formed upstream in the641

Pannonianbasin infilledpartsof theDacianbasinandVariscanzirconsarenowerodedand642

carried by the Olt. The puzzling fact that the Jiu River, which also traverses the Moesian643

foreland(theDacianbasin)doesnothavesuchVariscanagesmaybeexplainedbyarapidinfill644

ofthewesternDacianbasinbylocalrivers(Fongngernetal.,2016).The“concurrentbasinfill”645

model(Olariuetal.,2018)callsforCarpathianriversinfillingmostoftheDacianBasin,which646

wouldhavelimitedtheimportofzirconsfrombeyondtheIronGates.647

648

Themorethan50%VariscanagesfoundatthemouthoftheOltRiver,thedominanceofthe649

samepeakintheTiszasampleandsmallerbutsignificantfractionsfounddownstreamalong650

theDanube(withabout17%ofthetotalzirconbeingVariscanatTulcea,theterminuspointof651

Danube before entering the Delta) remains unresolved and puzzling. Future studies should652

investigatethisinmoredetailusingdetritalrecords,especiallyfromalongtheOltRiverandits653

archiveimmediatelytothesouthoftheSouthCarpathiansandintotheMoesianplain.654

655

5.4.Youngerages656

657

Clearly,onlyasmall fractionoftheDZages intheDanube’sarchive ismadeofAlpineages.658

About10%ofthemountainousterrain intheCarpathiansconsistsofMesozoicandyounger659

magmatic rocks, and the Danube budget of zircons reflect roughly that (although the East660

CarpathianssocalledNeogenevolcanicbelt is located inanunusualpositionrelativetothe661

hydrographic network and is located far from the samples studied here). Of the Alpine662

magmaticrocksoftheCarpathiansandBalkans,theEastVardarislandarcanditsMORB-like663

basementarenotsignificantzirconproducers(becausethereareabundantgabbrosandmafic664

rocks),thelateCretaceousBanatiticarcispoorlyexposedinacoupleofnarrowbeltsthatare665

unlikelytoproducealargezirconcargo,andtheNeogenevolcanismintheEastCarpathians666

andApuseniMountainsaremostvolumetricallysignificant.Regardless,theNeogenearconly667

providesaminusculenumberofzirconsalongthelowercourseoftheDanube,asexpected.668

28

Theyarevolumetricallyoverpoweredbythemaincrustal-formingperiGondwananmagmatic669

arcsandmarginalbasinsofboththeinnerandouterCarpathiansanditsforeland.670

671

Twominorbutunusual groupsofages standoutamong theAlpineonesandgenerate two672

additionalproblemsinmatchingzirconssourcestosinks:TriassicandEoceneages.Theonly673

Triassic magmatic bodies known in the Romanian Carpathians and North Dobrogea,674

respectively,arethealkalinemassifofDitrauandasuiteofsmallalkalinebodies inwestern675

northDobrogeaassumedtobeofsimilarage.However,theexistenceofafewPermo-Triassic676

agesinalmostallanalyzedsamplessuggeststhatTriassicmagmatismmaybemoreprevalent677

intheCarpathians.WehavesomeevidencethatsuturingofPaleozoicterranes intheSouth678

CarpathianscontinuedintotheTriassic(Duceaetal.2016)andthattheGeticpegmatitesare679

ofthatageaswell.EitherotherearlyextensionplutonssimilartoDitrau(butvolumetrically680

smaller)existintheCarpathiannappesorpostcollisionalpegmatitesprovidethisagerangein681

thedetritalrecord.682

683

Eoceneages(40-30Ma)areevenmorepuzzlingbecausethereisnoCenozoicmagmatismof684

that age in the Romanian Carpathian realm (Seghedi et al., 2011). An Eocene arc is well685

developedintheRhodopemountainstothesouth,butthecurrenthydrographicnetworkor686

evenknownancientones(Matencoetal.,2013)donotlinkthatsourceareatotheDanubeor687

itslowertributaries.PossiblyearlierlinksbetweenvariousbasinsoftheParatethys(Matenco688

etal.,2016)mayhavebroughtzircons fromsucha farsourcearea intothe lowerDanube’s689

current basin. There is no obvious resolution to the question as to whether a previously690

continuousParatethyscouldhavetransportedlaterallyasignificantamountofmaterialfrom691

westoftheRomanianCarpathians;therearenosedimentologicaldatafromtheDacianbasin692

to support or refute that.However, a future study, perhaps aDZ studyof the sedimentary693

archive of the Dacian basin, could resolve this question. This and the other puzzling694

complexitiesfoundinourdataillustratehowtheDZrecordcanbecomplicatedbysecondor695

third order sedimentary processes that go beyond a simple (and direct) source to sink696

relationshipinafluvialsystem.697

29

698

6. CONCLUSIONS699

700

AdetritalzirconU-PbstudyofmodernsandsfromthelowerDanubeandthemostimportant701

four tributaries originating in the Carpathian Mountains documents the main magmatic702

events that led to the continental crustal formation of the nearby Carpathians. The main703

conclusionsofthisstudyare:704

705

1- Thegreatmajorityofbasementwas formed in latestProterozoic–Ordovician island706

arcs, a finding that is consistent with limited previous studies performed on the707

basementitself;708

709

2- A prominent Carboniferous (350-320 Ma, Variscan) magmatic peak in the detrital710

record has no known source in the nearby Carpathians, either because it was711

overlooked by previous basement studies or implying that lateral transport from712

outsideof the source area (and subsequent recycling) has takenplace in the recent713

geologicpast. SomeVariscan intrusionsdoexist in theCarpathiansbut according to714

thecurrentgeochronologicknowledge,aresmallvolumeplutons,andcannotaccount715

for such a large regional DZ peak. We cannot distinguish between these two716

explanationsatthispointduetolimitedexistingdata.717

718

3- A small proportion of unexplained igneous Eocene ages exist along theDanube and719

tributaries;theirclosestexposedplausiblesourcesareintheRhodopeMountainswell720

totheSouthwithoutaclearsedimentarypathwayfromsourcetosinkinthemodern721

configurationoftheriverdrainages.722

723

724

Acknowledgements, Samples and Data. M.N.D. acknowledges support from US725

National Science Foundation grant EAR 1725002 and the Romanian Executive Agency for726

30

Higher Education, Research, Development and Innovation Funding project PN-III-P4-ID-PCE-727

2016-0127. LG acknowledges support from the Ocean and Climate Change Institute of the728

Woods Hole Oceanographic Institution. There are no financial conflicts of interests for any729

author. The data supporting the conclusions are tabulated in the Supporting Information,730

availablewiththispaper.731

732

31

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