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ZircongrainsinA‐typegraniteandtheirinclusionsasrecorderofuppermantleconditionsintheCroatiansegmentoftheLateCretaceouscollisionalzonebetweenEuropeand
Adria
Balen,D.1,Schneider,P.1,*,Massonne,H.‐J.2,Opitz,J.2,Putiš,M.3,Luptáková,J.4,Petrinec,Z.1
1DepartmentofGeology,FacultyofScience,UniversityofZagreb,Croatia2InstitutfürMineralogieundKristallchemie,UniversitätStuttgart,Germany
3FacultyofNaturalSciences,ComeniusUniversity,Bratislava,Slovakia4EarthScienceInstitute,SlovakAcademyofSciences,BanskáBystrica,Slovakia
*E‐Mail:[email protected]
Zircon, one of the ubiquitous accessoryminerals in granites is a valuable indicator ofvarious geological processes. In addition, it includes and preserves various types ofinclusionsandservesasanaturalpreservationcapsuleallowinginsideviewsintoearlierstage(s)ofthe(geodynamic)evolution.Theso‐calledSava(‐Vardar)ZoneisoneofthestillgeologicallyintriguingareasintheSEEuropewithasuturezoneafterclosingthewesternNeotethysbranchfollowingcollisionofthesouthernmarginoftheEuropeanplatewiththeAdriaplate(Pamić1993;Schmidetal.2008;Ustaszewskietal.2010).Thiszoneshowsrareoutcropsofvarioustypesofgraniticrocks.Amongthemisthepeculiarred‐colouredCretaceous granite from Mt. Požeška Gora (N Croatia). This granite, associated withrhyolites,cropsoutasseveralsmallintrusivebodiescoveringatotalareaofapprox.6‐7km2(Pamić1987)andcaughtourattentionduetoitsgeotectonicposition,appearance,colour, accessory mineral content and still disputable age (Pamić et al 1988; Jamičić2007).
Fig.1.BSEimagesofaMt.Požeškagoragranitetexture;Fsp=alkalifeldspar,Hem=hematite,Qtz=quartz.bhematitewithcrystallographicallyorientedilmeniteexsolutions
TheMt.PožeškaGoragraniteismassivewithafine‐tocoarse‐grainedgranitictexture.
It is mainly composed of alkali feldspar and quartz (Fig. 1a), with small amounts ofplagioclase, white mica and hematite with crystallographically oriented ilmeniteexsolutions(Fig.1b).Alkalifeldsparisdominantlyperthitewhereasplagioclaseisalmostpurealbite.Quartz‐alkalifeldsparintergrowthsandmicrographictexturesarecommon.
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Accessorymineralsfoundbypolarizingmicroscopeincludezircon,apatiteandFe‐(Ti)‐oxides.Theanalyzedzirconpopulationconsistsof184grainsrangingfrom60‐75μminthe
longand25‐35μm in the short axis.Aspect ratios range from1.7:1 to2.9:1,with themedianvalueof2.2:1.Theexternalmorphologyisgovernedbypredominantlydeveloped{100}prismsand{101}>>{211}bipyramids(Fig.2a),amongthemprevailD(50%ofthetotalpopulation),J5(30%)andJ4(11%)typesafterPupin’s(1980)zircontypology(Fig.2b). Zircon crystals are colourless to slightly yellowish with high transparency andbirefringence. Cathodoluminescence (CL) and back‐scattered electron (BSE) imagesreveal clear signs of growth oscillatory zoning typical ofmagmatic growth conditionswithoutanysignsofdissolutionofthesurface,whichindicatessaturationofZrinthemelt.The morphology of the zircon grains separated out of the Mt. Požeška Gora granitecombinedwithwholerockgranitegeochemistrypointstoahigh‐temperature,dryA‐typegranite(Balenetal.2017).
Fig.2.aTypicalmorphologyofMt.PožeškaGorazircon.bModifiedzircontypologydiagramZircongrainsarefoundincludedinthelatelycrystallizedminerals,alkalifeldsparand
quartz,andsomeofthemcompriseinclusionsofhematite,apatiteandearlyzircon.TheRamanspectraof inclusions(Fig.3) inzircon(preliminaryfindings)alsorevealedtiny(<10μm)mineralsofkokchetaviteandkumdykolite(?) (polymorphsofK‐feldsparandalbite,respectively)representingmetastablephasesinmeltinclusionsthatrequirerapidcooling(Ferreroetal.2016).Thefindingssetmagmagenesistothedepth(mostlikely)correspondingtotheuppermantlesource,aspredictedbyzircontypologyandrevealtheearlypathofA‐typegranitemagmaevolution.
Fig.3.Ramanspectraofkokchetaviteandkumdykolite(?)frominclusionsinzircon
AllanalyzedzirconsfromtheMt.PožeškaGoragranitehavevariableuranium(398‐1988ppm)andthorium(263‐1371ppm)contents,withTh/Uratiosrangingfrom0.61to
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0.97,consistentwithamagmaticorigin(HoskinandSchaltegger2003).Accordingtotheirexternaltypology,internalstructure,andchemicalcomposition(highUmean=903ppmandThmean=666ppm,Th/U=0.74,Zr/Hf=54.96)itissuggestedthattheycrystallizedindeepmagma chambers at relatively high and constant temperature. The calculated Zrsaturationtemperaturesofthemagmaandtheonsetofthezirconcrystallization(Watsonand Harrison 1983) show values of up to 878 °C, indicating its formation in a high‐temperatureenvironment.Existenceofhematite(norm2.7wt.%),thatpaintsthegranitereddish, indicates an oxidised magma type. The positive Ce anomaly in zircon REEpatterns(Fig.4a)alsoprovidesevidenceofapositiveoxidationpotentialofthemagma.Exsolution lamellae of ilmenite in hematite with crystallographically oriented textureresultedfromrelativelyrapidcoolingand/orpressuredrop.
Fig.4.aREEpatternsnormalizedtochondrite(Boynton1984)forzirconsandhostgranite.bP‐TdiagramshowingthegraniticsolidusandisoplethsfortheSicontentinwhitemicaandmeltvolume.TheintersectionofthesolidusandaspecificSiisoplethforwhitemicaindicatesthe
intrusionlevelofthehostgraniteEmplacement P‐T conditions were estimated with PERPLE_X calculations of the
intersectionofthespecificSiisopleth(3.10Sip.f.u.),accordingtolate‐crystallizedwhitemica,withthesolidusforthehostgraniteinaP‐Tdiagram(Fig.4b).Theobtainedvaluesare5.2kbarand615°C,whichcorrespondtoacrustalintrusionlevelofca.20km,andfittheP‐TrangesresultingfromtheapplicationoftheOr‐Ab‐Qtz(300‐1000MPa)andAn‐Ab‐Or(600‐650°C)ternarysystems.The finding of red granite pebbles at the base of the Santonian‐Campanian‐
Maastrichtian geological column (e.g. Jamičić 2007) was in contradiction with thepreviouslyreportedMaastrichtianage(71.5±2.8Ma,Rb/Sr isochron) for threegraniteand twocogenitic rhyolitesamples (Pamićetal.1988).Agesobtained inourstudyonzircon using a LA‐ICP‐MS are 207Pb/235U=85.8±1.1 Ma, 206Pb/238U=86.1±1.5 Ma and208Pb/232Th=87.7±1.9Ma(RMSD),whichareinagreementwithstratigraphicconstraints.TheLateCretaceoussubductionofthewesternNeotethyanbranchinthestudyarea
was terminated and changed to a post‐subduction rift‐like tectonic process. Thesubsequent extension caused rhyolite volcanism along deep faults and youngerextension‐related basaltic volcanism (Belak et al. 1998). The A‐type granite with itsspecificzircontypologyoriginatedatgreatdepthsandhigh‐temperatures(uppermantleconditions)andintrudedrapidlymid‐toshallow‐crustallevels.Thepresentedevidencefits this scenario that invokes a geodynamic change from a compressional to anextensionalregimeduringtheLateCretaceous.
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Acknowledgments:ThisworkhasbeenfullysupportedbyCroatianScienceFoundationundertheprojectIP‐2014‐09‐9541.Theauthorsaregrateful toT.Theye (Stuttgart) forhelpduringmicroprobework; furtheranalyticalworkissupportedbyAPVV‐15‐0050andITMS26220120064.References:BalenD,SchneiderP,MassonneH‐J,OpitzJ,PetrinecZ(2017)ACretaceousA‐typegranitefromtheEurope‐
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