Comment on “Ultrahigh temperature granulites and magnesian charnockites: Evidence for the Neoarchean accretion along the northern margin of the Kaapvaal craton” by Rajesh et al.

PrecambrianResearch255(2014)455–458

ContentslistsavailableatScienceDirect

Precambrian

Research

j ou rn a l h o m e pa ge :w w w . e l s e v i e r . c o m / l o c a t e / p r e c a m r e s

Commentary

Comment

on

“Ultrahigh

temperature

granulites

and

magnesian

charnockites:

Evidence

for

the

Neoarchean

accretion

along

the

northern

margin

of

the

Kaapvaal

craton”

by

Rajesh

et

al.

O.

Laurent

_,

_G.

_Nicoli

_,

_A.

_Zeh

_,

_G.

_Stevens

_,

_J.-F.

_Moyen

_,

_A.

_Vezinet

a_{InstitutfürGeowissenschaften,FacheinheitMineralogie,JohannWolfgangGoetheUniversität,Altenhöferallee1,D-60438FrankfurtamMain,Germany} b_{CentreforCrustalPetrology,DepartmentofEarthSciences,StellenboschUniversity,PrivateBagX1,Matieland7602,SouthAfrica}

c_{UMR6524-CNRS-IRD,UniversitéJean-Monnet,23rueduDr.Michelon,42023Saint-Étienne,France}

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received15May2014 Accepted14July2014 Availableonline25July2014

1. Introduction

Over the years, the Southern Marginal Zone (SMZ) of the Limpopo mobilebelt hasbeenintensively studied,as it poten-tiallyresultsfromoneoftheoldestcontinent-continentcollision onEarth.Indeed,thegeneralconsensusabouttheevolutionofthis terraneisthatitunderwentasingletectono-metamorphiceventat ∼2.7Ga,ascribedtocollisionbetweentheKaapvaalandZimbabwe Cratons(Bartonetal.,2006;EglingtonandArmstrong,2004;Kröner etal.,2000;Kreissigetal.,2001;Roeringetal.,1992;Rigbyetal., 2008;StevensandvanReenen,1992a,b;Zehetal.,2009).Thestudy oftheLimpopobelt,andinparticulartheSMZ,isthereforeof pri-maryinteresttounravelthetectonicregimesthatprevailedback inthelate-ArcheanandtheirevolutionthroughoutEarthhistory.

Inarecentcontribution,Rajeshetal.(2014)proposedanew “out-of-the-box”interpretationofthegeologicalrecordintheSMZ, whichwouldrepresentamicro-continentaccretedtothenorthern marginoftheKaapvaalCratonduringtheNeoarcheanat2.72Ga. Thekeyelementonwhichthisinterpretationisbasedisthespatial andtemporalassociationofultra-hightemperature(UHT) gran-ulitesand magnesian charnockites,which would be a criterion toidentifysubductionandcollisionsettingsinPrecambrian ter-ranes.Todemonstratetheoccurrenceofboth rocktypesinthe SMZ, Rajeshet al.(2014) provide (1) newzirconU–Pb ages of 2.72Ga,obtainedfrommetamorphiczirconovergrowthsin pro-posedUHTmetamorphic rocks, and(2) anewinterpretationof major-elementdatafromthespatially associatedMatokpluton,

∗ Correspondingauthor.Tel.:+496979840133.

E-mailaddresses:laurent@em.uni-frankfurt.de,oscarlaurent86@gmail.com

(O.Laurent).

originallyproducedbyBohlender(1992),toreflectamagmaticarc settingatthetimeofintrusion.

Thisnewinterpretation,however,isinconflictwithprevious modelsandotherpublisheddata,whichstronglysupportthat(1) theSMZunderwentmetamorphismat“normal”granulitefacies temperaturesof850–875◦C,withthepreservationofasubstantial volumeofpeakmetamorphicbiotiteinmostrocks;(2)granitoids oftheMatokplutonwerenotgeneratedwithinanarcsettingbut ratherduringthepost-collisionalstageoftheca.2.72Gaorogeny,as testifiedbyboththeirage(∼2.68Ga)andchemicalcompositions; (3)theSMZrepresentreworkedmaterialoftheadjacentand over-thrustedPieterburgBlock,andthuscannothavebeenanisolated terranepriortoamalgamationwiththeKaapvaalCraton.

2. UHTmetamorphism

AdetailedcommentontheoccurrenceofUHTmetamorphism proposedbyBelyaninetal.(2012)andRajeshetal.(2014)was providedbyNicolietal.(2014);thereaderisreferredtothiswork forfurtherdetails,onlythekeypointsbeingrepeatedhere.The P–TestimatessuggestedbyBelyaninetal.(2012)andRajeshetal. (2014)(i.e.∼1000◦_{Cand∼12kbar)areinconsistentwithbothfield}

observationsandexperimentalstudies.Thedetailedmetamorphic analysisprovidedbyTayloretal.(2014)clearlydemonstratesthat the presence of peak metamorphic biotite in theBandelierkop formationmetapelitesisinconsistentwithpeakmetamorphic tem-peraturesin excessof 900◦C. Thisis inagreementwithearlier metamorphicstudies(e.g.StevensandvanReenen,1992a,b),more recent conclusions from phase equilibrium modelling (Koizumi et al.,2014),as wellasa verylargebody of experimentaldata onpartialmeltingofbiotite-bearingmetasediments(e.g.Montel http://dx.doi.org/10.1016/j.precamres.2014.07.010

456 O.Laurentetal./PrecambrianResearch255(2014)455–458

andVielzeuf,1997;Pati ˜noDouceandHarris,1998;Pati ˜noDouce andJohnston,1991;PickeringandJohnston,1998;Stevensetal., 1997;VielzeufandHolloway,1988)arguingforatemperaturepeak aroundfluid-absentbiotitebreakdownpartialmeltingconditions, i.e.820–870◦C.

Inadditiontotheseconsiderationsaboutthepeak tempera-tureattainedbytheSMZ,itisimportanttonotethatRajeshetal. (2014)providenometamorphicanalysisforthetwodated sam-ples(i.e.DR-19CandDR-20).Therefore,itisnotclearwhetherthe obtainedmetamorphicagesdocumentedbyRajeshetal.(2014)

actuallyreflectpeakmetamorphism,andifso,ifthedated sam-plesunderwentthesameP–Tconditionsasthesampleusedforthe metamorphicstudy(i.e.DR-19A).

3. GeochemicalsignatureoftheMatokpluton

3.1. Terminologyissues

Rajeshet al. (2014) suggest that the ∼2.68Ga granitoids of theMatokplutonare“magnesiancharnockites”,afamilyofrocks supposedlyformedinsubductionsettings(FrostandFrost,2008; Rajesh,2012),beingcompositionallysimilartoarcmagmas.Inthis context,itmustbenotedthatthe“charnockite”appellationusedby

Rajeshetal.(2014)isincorrect,becauseitshould(strictly speak-ing)onlybeusedfororthopyroxene-bearinggranites(Frostand Frost,2008).However,theMatokplutonisacompositeintrusion, madeupofdifferentmagmaticrockshavingawiderangeof sil-icacontents(SiO2=55–70wt.%;Laurentetal.,2014andFig.5f–g

ofRajeshetal.,2014),i.e.notonlygranites.Thismisuseis confus-ing,asBohlenderetal.(1992)clearlydemonstratedthatthereare atleasttwodifferentgenerationsoforthopyroxene-bearingrocks intheSMZ,namely(1)metamorphiccharnockites(sensustricto), inwhichorthopyroxeneisthedominantmaficphaseandformed attheexpenseofbiotite,presumablyduringdehydrationmelting, forexampleintheBaviaanskloofTTGgneisses;and(2)asuiteof so-called“charnockiticrocks”(jotunites, enderbitesand charno-enderbites),restrictedtotheMatokplutonandigneousinorigin. Theselattercontainorthopyroxeneinplaces,butalwaysmuchless thanclinopyroxene,andbothphasesarecompletelyabsentinthe morefelsicrocks(Bohlender,1992;Laurentetal.,2014;Rapopo,

2010).Therefore,itislikelythatseveral,ifnotmost,ofthe sam-plesplottedbyRajeshetal.(2014)intheirdiscriminationdiagrams arenotcharnockitesatall,andprobablynoteven orthopyroxene-bearing.

3.2. Matokgranitoidsarenotsimilartoarcmagmas

Apartfromtheterminologyissue,thegeochemicalarguments usedbyRajeshetal.(2014)todiscriminatethegeodynamic set-tingofthe“Matokcharnokites”arealsoproblematic.Indeed,the interpretationofmagmaticarcaffinitysolelyreliesontheuseof afewmajor-elementdata.Infact,manypreviousstudiesclearly showed that “subduction signatures” are commonly equivocal, evenwhentraceelementsystematicsareconsidered(e.g.Bédard, 2006;Rollinson,2009;Willboldetal.,2009).The“magnesian” sig-natureusedbyRajesh(2012)andRajeshetal.(2014)asasmoking guntoreflectasubductionoriginisthereforenotunambiguous,and couldalsocharacterizemagmasformedinverydifferent geody-namicenvironments.Tosupportthemagmaticarcsetting,Rajesh etal.(2014)arguethatthe“Matokcharnockites”formedby dehy-drationmeltingofamphibolites,inaroughlysimilarprocesstothat proposedfortheformationofArcheanTTGs(e.g.MoyenandMartin, 2012andreferencestherein).However,ArcheanTTGs,although mostlymagnesianincharacter,canbegeneratedinawiderange ofgeodynamicenvironments(Fig.1),notnecessarilyrequiringa convergentplatemargin(Moyen,2011;MoyenandMartin,2012). Inaddition,themagnesiansignatureoftheMatok “charnock-ites” is not that obvious. It is clear in Fig. 5f of Rajesh et al. (2014)thattheirsamplesarericherinFethantypical charnock-ites from the Northern Marginal Zone (NMZ), several of them straddlingtheboundarybetweenmagnesianandferroangroups definedbyFrostetal.(2001).Moreover,thediagrampresentedin Fig.5hofRajeshetal.(2014)alsoshowsthattheMatok granit-oidsfollowaverydistinctivetrendwithrespecttotheNMZones, characterizedbylowerAl2O3/CaOratios.Thesetwo

characteris-tics(highFeandlowAlcontents)havebeenrecentlyhighlighted byLaurentetal.(2014),whoconductedadetailed petrogenetic studyoftheMatokgranitoidsonthebasisof major-,trace ele-ment andSm–Nd isotopegeochemistryas wellasgeochemical modelling. In contrast tothe suggestionmade by Rajeshet al.

50 55 60 65 70 75

FeO

/ (FeO

+ MgO)

SiO

(wt.%)

SiO

(wt.%)

SiO

(wt.%)

High-Pressure TTG

«Hot» subduction zone

Delaminated restites beneath oceanic plateau

Medium-Pressure TTG

Melting a the base of a thick oceanic plateau Delamination at the base of orogenic crust

Low-Pressure TTG

«Intra-oceanic» differentiation Melting at the base of a thin oceanic plateau

Ferroan Magnesian Ferroan Magnesian Ferroan Magnesian

Fig.1. FeOt/(FeOt+MgO)vs.SiO2diagramofFrostetal.(2001)presentingthecompositionofrepresentativesamplesofthethreegroupsofsodicTTGsdefinedbyMoyen (2011).Asspecifiedonthetopofeachplot,thesedifferentgroupscanformincontrastedgeodynamicenvironments,implyingconvergentplatesettings,butalsointraplate orevendivergentones(e.g.intra-oceanicdifferentiationatamid-oceanridge–seeMoyen,2011fordetailsanddiscussion).Morethan90%ofTTGsaremagnesian,regardless thepetrogeneticgrouptheybelongtoandthusthegeodynamicsettinginwhichtheyformed.Thelightgreyfieldrepresentsthefieldof“magnesiancharnockites”defined byRajesh(2012).

O.Laurentetal./PrecambrianResearch255(2014)455–458 457 0.0 0.1 0.2 0.3 0.4 0.5 1.0 0.8 0.6 0.4 0.2

FeO

/ (FeO

+ MgO)

Al

O

/ SiO

(Both with SiO2 < 65 wt.%)

Matok pluton :

Modern arc magmas :

0.9 Ga-old HBG Fe-K suite (Southern Norway)

Fig.2. FeOt/(FeOt+MgO) vs. Al2O3/SiO2 diagramwhere thewhole-rock

com-positions of granitoids from the Matok pluton (data from Laurent et al., 2014) are reported, together with that of the Proterozoic “ferro-potassic” Hornblende–Biotite–GranitoidsuiteofsouthernNorway(datafromBogaertsetal., 2003andVanderAuweraetal.,2007)aswellastypicalA-type,Fe-richgranites andmodernsubduction-relatedmagmasfrombothcontinentalandoceanicarcs (>5000samplesfromtheGEOROCdatabase).InaverageandatsimilarSiO2contents

(<65wt.%),rocksfromtheMatokplutonclearlyshowhigherFeOt/(FeOt+MgO)and

lowerAl2O3/SiO2thanarcmagmas.

(2014),Laurentetal.(2014)concludedthattheMatokgranitoids areakin to“ferro-potassic”suites, very commonin Proterozoic terranes (e.g. Duchesne et al., 2010; Ferré et al., 1998; Peucat et al., 2005; Vander Auwera et al., 2011) and intermediate in composition between (1) Al-, Mg-rich sanukitoids and (2) Al-poor,Fe-richanorthosite–mangerite–charnockite–granite(AMCG) suites. This intermediate composition is well illustrated in the FeOt/(FeOt+MgO)vs.Al2O3/SiO2 diagramofFig.2.Thisdiagram

alsoshowsunequivocally thattheMatokgranitoidsareglobally moreferroanandlessaluminousthanmodern,classicalarc mag-masfrombothoceanicandcontinentalconvergentmargins(Fig.2).

4. Geodynamicmodel

ThegeodynamicmodelproposedbyRajeshetal.(2014)isthat theSMZwasamicrocontinent,whichwasaccretedtothenorthern marginoftheKaapvaalCratonby∼2.72Gaatthelatest,aftera periodofnorth-vergingsubduction.Thisinterpretation,however, sufferstwomajorproblems:

(1)This model is in conflictwith thefact that the Matok plu-ton(suggestedtobesubductionrelated)intrudedat∼2.68Ga (U–Pb agedata of Barton etal., 1992; Laurent etal., 2013; Zeh et al., 2009), whereas terrane collision happened at 2.71–2.72Ga,asindicatedbyU–Pbagesofmetamorphic zir-consobtainedfromgranulite-faciesrocksoftheSMZ(Rajesh etal.,2014; Tayloretal.,2014).Furthermore,field observa-tionsandadditionalagedatafromtheHoutRiverShearZone indicatethatupliftandsouthwardthrustingoftheSMZ gran-ulitesoverthePietersburgblockstartedpriortotheintrusion oftheMatokpluton,i.e.around2.70Ga(Kreissigetal.,2001; Laurentetal.,2013).Insummary,theMatokplutonisclearly 10–30Mayoungerthantheinferredageofcollision,suchthatit cannotberelatedtosubductionandratherrepresentsatypical post-collisionalintrusionasrecentlyproposedbyLaurentetal. (2014).

(2)AccretionoftheSMZasanindividualmicroterranetothe north-ernedgeoftheKaapvaalCratonisalsoatoddswithprevious studiesbasedonHf–Sr–Nd–Pbisotopes(e.g.Bartonetal.,1992, 2006;Kreissigetal.,2000;ZehandGerdes,2012;Zehetal., 2009).ThesedataunequivocallyshowthattheSMZandthe adjacentrocksofthePietersburgblockactuallybelongtothe samecrustaldomain.Specifically,theSMZresultsfrominternal reworkingofthePietersburgblockcrustowingtocontinental collisionat2.72Ga(Laurentetal.,2014;Zehetal.,2009,2013), withanorthward-locatedterranethatcouldberepresentedby theCentralZoneoftheLimpopobelt.Internalreworkingisfor instancesupportedbyPbisotopicdataindicatingsimilarlylow ␮-values(238_U/204_{Pb)of≤10forallrocksoftheNorthern}

Kaap-vaalCraton(includingtheSMZ),whichareverydifferenttothe high-␮rocksexposedintheCentralZoneoftheLimpopobelt andtheZimbabweCraton,bothhavingvalues≥11.5(Barton etal.,2006).CombinedageandHfisotopedataalsosupport thatallgranitoidsofthenorthernKaapvaalCraton,including theSMZ,formedbyreworkingofasinglecrustalcomponent, whichderivedfromadepletedmantlesourcebetween3.3and 3.0Ga(e.g.ZehandGerdes,2012;Zehetal.,2009,2013). Ortho-andparagneissesfromboththeSMZandthePietersburgblock also showundistinguishableNdmodel ages (2.9–3.2Ga),as wellassimilarmajor-andtrace-elementsystematics(Kreissig etal.,2000).Itisworthwhilenoting,inaddition,thatdetrital zircongrainsinthemetasedimentarysamplesinvestigatedby

Rajeshetal.(2014)yieldagesof∼3.40,∼3.33,3.00–2.95and 2.85–2.75Ga.Theseagesactuallycorrespondtothemain mag-maticepisodesinthePietersburgblock(Laurentetal.,2013; Zehet al.,2009).Theyoungestages(2.85–2.75Ga)are typi-calforgraniteemplacement(Turfloopbatholithandassociated intrusions;Hendersonetal.,2000;Kröneretal.,2000;Laurent etal.,2013;Zehetal.,2009),whereasallages≥2.95Gaarethat ofthePietersburgTTGs(Kröneretal.,2000;Laurentetal.,2013; Zehetal.,2009)andarealsorecordedbydetritalzirconsfrom low-tomediumgrademetasedimentaryrocksofthe Murchi-sonandPietersburggreenstonebelts(ZehandGerdes,2012; Zehetal.,2013).Alllinesofevidencethereforesupportthat rocksoftheSMZandthePietersburgblockbelongtothesame crustaldomain,butunderwentcontrastedP–Tevolutionduring the∼2.72Gacollisionevent.

5. Conclusion

TherecentcontributionbyRajeshetal.(2014)proposesanew geodynamicmodelfortheevolutionoftheSMZoftheLimpopo belt and theKaapvaalCraton in theNeoarchean,implying that bothterraneswereamalgamatedat∼2.72Gaasaresultof north-vergingsubductionandsubsequentcollision.Themainsupporting evidenceforsuchamodelisthespatialandtemporalassociation ofUHTgranulitesinonehand,andsubduction-relatedmagmatic rocksreferredtoas“magnesiancharnockites”ontheotherhand. However,theirargumentstosupportboth(1)UHTmetamorphic conditionsintheSMZ;and(2)asubduction-relatedoriginforthe Matokgranitoids areunreasonable and/orequivocal. Moreover, thisnewgeodynamicmodeldoesnotfitatallwiththeresultsof agreatdealofpreviousstudiesinthearea,andmustthereforebe regardedwithsomecriticism.

Nevertheless,webelievethatthedatapresentedbyRajeshetal. (2014),especially theirnewU–Pb agesondetritalzircons from theSMZ,providevaluable information regarding(1) thetiming ofgranulite-faciesmetamorphismandcontinentalcollisioninthis terrane; and (2) theprovenanceof thestudiedmetasediments. Thesenewdatamustbereconsideredinthescopeofongoingwork

458 O.Laurentetal./PrecambrianResearch255(2014)455–458

abouttheevolutionofthePietersburgblockandtheSMZduring theNeoarchean.

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