Paleomagnetic and geochronological studies of the mafic dyke swarms of Bundelkhand craton, central India: Implications for the tectonic

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Precambrian Research

j o u r n al ho m e p age :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

Paleomagnetic and geochronological studies of the mafic dyke swarms of Bundelkhand craton, central India: Implications for the tectonic

evolution and paleogeographic reconstructions

Vimal R. Pradhan

, Joseph G. Meert

, Manoj K. Pandit

, George Kamenov

, Md. Erfan Ali Mondal

aDepartmentofGeologicalSciences,UniversityofFlorida,241WilliamsonHall,Gainesville,FL32611,USA

bDepartmentofGeology,UniversityofRajasthan,Jaipur302004,Rajasthan,India

cDepartmentofGeology,AligarhMuslimUniversity,Aligarh202002,India

a r t i c l e i n f o

Articlehistory:

Received12July2011

Receivedinrevisedform6November2011 Accepted18November2011

Available online xxx

Keywords:

Bundelkhandcraton,Maficdykes,Central India,Paleomagnetism,Geochronology, Paleogeography

a b s t r a c t

Thepaleogeographic position ofIndia within thePaleoproterozoic Columbia andMesoproterozoic Rodiniasupercontinentsisshroudedinuncertaintyduetothepaucityofhighqualitypaleomagnetic datawithstrongagecontrol.NewpaleomagneticandgeochronologicaldatafromthePrecambrianmafic dykesintrudinggranitoidsandsupracrustalsoftheArcheanBundelkhandcraton(BC)innorthernPenin- sularIndiaissignificantinconstrainingthepositionofIndiaat2.0and1.1Ga.Thedykesareubiquitous withinthecratonandhavevariableorientations(NW–SE,NE–SW,ENE–WSWandE–W).Threedis- tinctepisodesofdykeintrusionareinferredfromthepaleomagneticanalysisofthesedykes.Theolder NW–SEtrendingdykesyieldameanpaleomagneticdirectionwithadeclination=155.3^◦andanincli- nation=−7.8^◦(=21;˛⁹⁵=9.6^◦).Theoverallpaleomagneticpolecalculatedfromthese12dykesfalls at58.5^◦Nand312.5^◦E(dp/dm=6.6^◦/7.9^◦).TheoverallmeandirectioncalculatedfromfourENE–WSW Mahobadykeshasadeclination=24.7^◦andinclination=−37.9^◦(=36;˛95=15.5^◦).Thevirtualgeomag- neticpole(VGP)forthesefourdykesfallsat38.7^◦Sand49.5^◦E(dp/dm=9.5^◦/16.3^◦).Athird,anddistinctly steeper,paleomagneticdirectionwasobtainedfromtwooftheNE–SWtrendingdykeswithadeclina- tion=189.3^◦andinclination=64.5^◦.U–PbgeochronologygeneratedinthisstudyyieldsaU–PbConcordia ageof1979±8MafortheNW–SEtrendingdykesandamean²⁰⁷Pb/²⁰⁶Pbageof1113±7Maforthe MahobasuiteofENE–WSWtrendingdykes,confirmingatleasttwodykeemplacementeventswithin theBC.WepresentglobalpaleogeographicmapsforIndiaat1.1and2.0Gausingthesepaleomagnetic poles.Thesenewpaleomagneticresultsfromthe∼2.0GaNW–SEtrendingBundelkhanddykesandthe paleomagneticdatafromtheBastar/CuddapahsuggestthattheNorthandSouthIndianblocksofthe PeninsularIndiawereincloseproximitybyatleast2.5Ga.

ThepaleomagneticandgeochronologicaldatafromtheMahobadykeissignificantinthatithelpscon- straintheageoftheUpperVindhyanstrata.ThepolecoincidesintimeandspacewiththeMajhgawan kimberlite(1073Ma)andtheBhander–RewapolesfromtheUpperVindhyanstrata.Themostparsimo- niousexplanationforthiscoincidenceisthattheageoftheUpperVindhyansedimentarysequenceis

>1000Ma.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

TheIndiansubcontinentholdsakey positionwhenattempt- ing to unravel the intricacies of Precambrian paleogeography, suchas the PaleoproterozoicColumbia supercontinent (Rogers, 1996; Rogers and Santosh, 2002; Meert, 2002; Santosh et al., 2003; Zhao et al., 2004a,b), the Meso-Neoproterozoic Rodinia supercontinent(McMenaminandMcMenamin,1990;Meertand Torsvik,2003;Meertand Powell,2001; Lietal.,2008),andthe

∗Correspondingauthor.Fax:+13523929294.

E-mailaddress:vimalroy@ufl.edu(V.R.Pradhan).

Ediacaran–Cambrian Gondwana supercontinent (Meert, 2003;

MeertandVanderVoo,1996;BurkeandDewey,1972;Pisarevsky etal.,2008).Indiaofferstargetrocksthatarebothaccessibleand ofappropriateageforconductingpaleomagneticandgeochrono- logicalinvestigations(Meert,2003;PowellandPisarevsky,2002;

MeertandPowell,2001;Pesonenetal.,2003).Thesetargetrocks includePrecambrianmaficdykesanddykeswarmsthatintrude theArchean–Paleoproterozoiccratonicnuclei,aswellasvolcanic andsedimentarysuccessionsexposedintheDharwarandAravalli protocontinentsoftheIndianpeninsularshield(Fig.1).

Thecorrelationofmaficdykeswarmsintermsoftheirdistri- bution,isotopicage,geochemistryandpaleomagnetismiscritical inordertofullyevaluatePrecambrianplatereconstructionsand 0301-9268/$–seefrontmatter© 2011 Elsevier B.V. All rights reserved.

doi:10.1016/j.precamres.2011.11.011

Fig.1.GeneralizedgeologicalandtectonicmapoftheIndiansub-continentwiththePrecambrianmaficdykeswarmscross-cuttingvariousArcheancratonicblocks.C:

Cuddapahbasin;Ch:ChattisgarthBasin;CIS:CentralIndianShearZone;GR:GodavariRift;M:MadrasBlock;Mk:Malanjkhand;MR:MahanadiRift;N:NilgiriBlock;NS:

Narmada–SonFaultZone;PC:Palghat–CauveryShearZone;R:RengaliProvinceandKerajangShearZone;S:SinghbhumShearZone;V:VindhyanBasin.

Source:(modifiedafterFrenchetal.,2008).

possibleconfigurationofsupercontinents(Halls,1987;VanderVoo andMeert,1991;HallsandZhang,1995;Mertanenetal.,1996;Park etal.,1995;BleekerandErnst,2006;ErnstandSrivastava,2008;

Piispaetal.,2011).TheIndianpeninsularshield istraversedby numerousPrecambrianmaficdykeswarms(Drury,1984;Murthy etal.,1987;Murthy,1995;Ramchandraetal.,1995;Frenchetal., 2008;FrenchandHeaman,2010;Meertetal.,2010;Pati,1999;

Patietal.,2008;Pradhanetal.,2008;Ernstetal.,2008;Srivastava andGautam,2008;Srivastavaetal.,2008).Thesedykes intrude thegranite-greenstoneterranesofthemajorIndiancratonicnuclei,

namelyDharwarinthesouth,Bastarintheeastcentral,Singhbhum intheeastandtheAravalliandBundelkhandinthenorth–west andthenorth(Halls,1982;Fig.1).Thedykesarethefocusofcon- siderablescientificattentionaimedatdefiningtheirgeochemical andgeophysicalcharacteristics,geochronology,paleomagnetism andtectoniccontrolsontheiremplacement(Devarajuetal.,1995, RadhakrishnaandPiper,1999;Srivastavaetal.,2008;Hallsetal., 2007; French et al., 2008; French and Heaman, 2010; Pradhan etal.,2008,2010;Pati,1999;Patietal.,2008;Piispaetal.,2011;

Ratre et al., 2010; Ernst et al., 2008; Ernst and Bleeker, 2010;

Meertet al.,2011).Paleomagneticandgeochronologicalstudies onthesePrecambrianmaficdykeswarmsaswellasthevolcanic andsedimentarysuccessionsoftheGwalior,CuddapahandVind- hyanbasins,provideinsightintoIndia’schangingpaleogeographic positionduringthecriticalPaleo-toMesoproterozoicintervaland may,insomecircumstances,allowforfurtherageconstraintstobe placedonthepoorlydatedsedimentarybasinsofIndia(Pradhan etal.,2008,2010;Gregoryetal.,2006;Maloneetal.,2008;Meert etal.,2010,2011;Frenchetal.,2008;French,2007;Hallsetal., 2007;Ratreetal.,2010;FrenchandHeaman,2010;Piispaetal., 2011).

This study focuses on the Precambrian mafic dyke swarms intruding the Bundelkhand craton in north-central India (Pati, 1999). The most prominent of the Paleo–Mesoproterozoic (2.5–1.0Ga)magmaticeventsarerepresentedbythreemaficdyke swarmsthatintrudetheArcheangranitic-gneissicbasementofthe Bundelkhandcraton.Thedykesfollowtwomajortrends,suggest- ingatleasttwomajorpulsesofmagmaticemplacementwithinthe craton.Basedonfieldcross-cuttingrelationships,theENE–WSW trendingdykesaretheyoungestinthecraton,andarerepresented bythe“GreatDykeofMahoba”(Fig.3candd).Previouspaleomag- neticstudiesonthesedykeswerehinderedbypoorageconstraints (Poitouetal.,2008).Existingisotopicagesallowanagebracket between2150and1500Mafortheoldersuiteofmaficdykes(Rao, 2004;Raoetal.,2005;Basu,1986;Sarkaretal.,1997;Sharmaand Rahman,1996).

Wereportnewpaleomagneticandgeochronologicaldataonthe BundelkhandmaficdykesfromtheMahoba,Banda,Khajuraoand nearbyareasoftheBundelkhandprovinceinthecentralIndia(Fig.

2).Thisstudyoffersthefirstrobustageconstraintsonthepaleo- magneticpolescalculatedfromthesemaficdykes.Thesedatawill ultimatelyaidinimprovingourmodelsforthePrecambriantec- tonicevolutionoftheIndiancratons,clarifytheroleofIndiain theColumbiaandRodiniasupercontinents,andgeneratedatafor developinganapparentpolarwanderpath(APWP)forIndiainthis poorlyresolvedperiodofEarthhistory.

2. Geologicalsettingandpreviouswork

The Archaean–early Proterozoic Bundelkhand craton (BC), commonlyknownasBundelkhandGraniteMassif(BGM)isasemi- circulartotriangularprovincethatformsthenorthernmostpartof theIndianpeninsula(Fig.2).Itcoversanareaof∼29,000km²and liesbetweenlatitudes24^◦30and26^◦00Nandlongitudes77^◦30 Eand81^◦00E(Sharma,1998).TheBCisdelimitedtothewestby theGreatBoundaryFault,tothenortheastbytheIndo-Gangetic alluvialplainsandtothesouthandsoutheastbytheNarmada–Son lineament(Fig.2).Thesouthwesternmarginismarkedbyrelatively small outcrops of Deccan Basalt; additionally, Paleoproterozoic rocksoftheBijawarandGwaliorGroupsareexposedintheSW andNEpartsoftheBC.ThearcuateVindhyanbasinoverliesthe BCinthesouthandsoutheasternsections(Goodwin,1991;Naqvi andRogers,1987;Patietal.,2008;Meertetal.,2010).Sharmaand Rahman(2000)dividetheBundelkhandcratonintothreelitho- tectonicunits:(a)the∼3.5Gahighlydeformedgranite-greenstone basement; (b) ∼2.5 old multiphasegranitoid plutons andasso- ciatedquartz reefs; and (c) themaficdykes and dykeswarms.

TheBGMrepresentsasignificantphaseoffelsicmagmatismasso- ciatedwith a complex of pre-granitesedimentary rocks (Basu, 1986).Thebasementisrepresentedbyahighlydeformedgranite- greenstoneterranethat consistsof variousenclavesof Archean gneisses,amphibolites,ultramafics,BIF’s,tonalite–trondhjhemite- gneiss,marble,calc–silicaterocks, fuchsitequartzitesandother metasediments(Royetal.,1988;Basu,1986;Sharma,1998;Sinha Royetal.,1998;MondalandZainuddin,1996;Mondaletal.,2002).

Three differentphases of granitoid emplacement are identified intheBGM.Inorder ofdecreasing²⁰⁷Pb/²⁰⁶Pbage,thesegran- itesinclude2521±7Mahornblendegranite,2515±5Mabiotite granite,and2492±10leucogranitesandconstitute80–90%ofits exposed area(Gopalanet al., 1990;Wiedenbeck and Goswami, 1994;Wiedenbecketal.,1996;Mondaletal.,1997,1998,2002;

Malviyaetal.,2004,2006).GranitoidemplacementintheBCwas followedbyanumberofminorintrusionsincludingwidespread pegmatitic veins, porphyry dykes and dykeswarms, and felsic units(rhyolites,dykesofrhyoliticbrecciaswithangularenclaves ofporphyries).Numerousquartzveinsofvariedsizewithmainly NNE–SSWandNE–SWtrendsareobservedinpartsoftheBCrepre- sentingepisodictectonicallycontrolledhydrothermalactivity(Pati etal.,1997,2007).

TheyoungestphaseofmagmatismintheBGMisrepresentedby maficdykesanddykeswarmsthattraversealltheabovelitholo- gies.Morethan700maficdykesareknowntointrudethegranitoid rocksoftheBC(Patietal.,2008).Themajorityofthesedykestrend ina NW–SEdirection,withsubordinateENE–WSWand NE–SW trending dykesincludingtheENE–WSW Great dykeofMahoba (Basu,1986;MondalandZainuddin,1996;Mondaletal.,2002;Fig.

2).Thesemaficdykesaresubalkalinetotholeiiticincomposition anddisplaycontinentalaffinity(Patietal.,2008).Thedykesare commonlyexposedasaseriesofdiscontinuousandboulderyout- cropsextendinginlengthfromfewtensofmeterstomorethan 17km. The‘Great Dyke’of Mahobahasmaximumstrike length of≥50km(Figs.2 and3c andd).Thedykes aregenerallynon- foliated, relativelyunaltered and exhibit sharp chilled contacts withthehost granitoids(Fig.3b).Basu (1986)distinguishedat leastthreegenerationsofdykesbasedontheircross-cuttingrela- tionships.Theoldest,coarsegrainedNW–SEtrendingsuiteiscut byENE–WSWandNE–SWtrending medium-graineddykesthat includesmallbodiesandlenticlesofanaphaniticdolerite.Numer- ousintermediatetofelsicdykesareexposedinthewest-central partofthemassifbetweenJhansiand Jamalpur,includingdior- iteporphyry,syeniteporphyryandfine-grainedsyeniteporphyry (Basu,1986).

Theagesfor theArchaeanrocksintheBGMare poorlycon- straineddue tolimited isotopicstudies.Theoldestrocksin the massif are associated with the TTG magmatism intruding the basementrocksandareassignedanageof3503±99Ma(Rb–Sr isochron;Sarkaretal.,1996).Zirconsfromthebasementgneiss yieldanionmicroprobe²⁰⁷Pb/²⁰⁶Pbageof3270±3Ma(Mondal etal.,2002).TheBastar,DharwarandAravallicratons(Sarkaretal., 1993;Wiedenbeck etal.,1996)in peninsularIndiaalsocontain coevalArcheanbasementrockssuggestingthattheseprotoconti- nentsplayedanimportantroleintheearlieststagesofnucleation oftheIndianshield.Theoverallstabilizationageforthemassifhas beeninterpretedtobe∼2.5Gabasedonthe²⁰⁷Pb/²⁰⁶Pbagesof thegranitoids(Meertetal.,2010;Crawford,1975;Mondaletal., 1997,1998,2002;RoyandKröner,1996).Thelargescalegranitic magmatismintheBCoverlapswithsimilareventsofgranitemag- matismandmineralizationinadjacentBastar(2490±10Ma;Stein etal.,2004)andDharwarcratons(2510Ma;Jayanandaetal.,2000) indicatingwidespreadgraniticplutonismthroughoutmuchofthe IndianshieldattheArchean–Proterozoicboundary.Agecontrolon theBundelkhandmaficdykesisproblematic andislooselycon- strainedtobetweenca.2.1and1.5Ga(Crawford,1975;Crawford and Compston, 1970; Sarkar et al., 1997). More recent in situ

40Ar/³⁹Arlaserablationdatayielded2150and2000Maagesfor maficdykes(Rao,2004;Raoetal.,2005)indicatingtwopulsesof dykeemplacement,however,nodetailsonthelocationsofthesam- pleswereprovidedinthosepublications.Themajorityofthelaser spotsfortheinsitu⁴⁰Ar–³⁹Aranalysiswereinthe2000Marange, suggestingthatsomeoftheolderspotagesmaysufferfromexcess argonwithinthesamples.

Fig.2. SketchmapofthemajorunitsintheBundelkhandcraton,NWIndia(modifiedafterMalviyaetal.,2006).Theasterisksonthemapshowthesitessampledforboth paleomagneticandgeochronologicalanalysis(NW–SEtrendingdykeI9GS13andENE–WSWtrendingGreatDykeofMahoba).

Fig.3. (a)NW–SEtrendingdyke(I925)exposedinanoutcrop∼50kmawayfromMauranipur,M.P.(b)GranitichostrockatthesamesiteI925trellisedbynumerousmafic dykelets.(candd)ENE–WSWtrendingdykeI923exposedinaquarryinMahobadistrict,MadhyaPradesh,CentralIndia.

3. Samplingandmethodology

3.1. Paleomagneticmethods

We sampled 27 sites (dykes) and collected a total of ∼380 coresamples fromthe maficdykes intruding theBundelkhand cratonfor paleomagneticanalysis(Fig.2).Outofthese,only 18 sites(dykes)yieldedconsistentresultsandarediscussedinthis paper(Table1).Allsamplesweredrilledinthefieldusingawater cooledportabledrill.Thesampleswereorientedusingbothsunand magneticcompassandreadingswerecorrectedforlocalmagnetic declinationanderrors.SampleswerereturnedtotheUniversityof Floridawheretheywerecutintostandardsizedcylindricalspec- imens.These specimens were stepwisedemagnetized by using boththermalandalternatingfield(AF)methodsinordertoiden- tifythebesttreatmentforisolatingvectorcomponentswithinthe samples.Alternatingfielddemagnetizationwasconductedusinga home-builtAFdemagnetizerandwithfieldsupto100mT.Ther- maldemagnetizationwasconducteduptotemperaturesof600^◦C withanASC-ScientificTD-48thermaldemagnetizer.Basedupon themagneticstrengthofthesamplesandinstrumentsensitivity themeasurementsweremadeoneitheraMolspin^®spinnermag- netometerora2G77RCryogenicmagnetometerattheUniversity ofFlorida.InsamplesthatshowedaveryhighinitialNRM,sam- plesweretreatedinliquidNitrogenbathspriortothermalorAF treatmenttoremoveanyviscousmulti-domainmagnetism.Lin- earsegmentsofthedemagnetizationtrajectorieswereanalyzed viaprincipalcomponentanalysisusingtheIAPDsoftware(Torsvik etal.,2000).

3.1.1. Rockmagnetictests

Curie temperature experiments were run on representative samplefragmentsfromeachsiteonaKLY-3SsusceptibilityBridge withaCS-3heatingunit.Isothermalremanenceacquisition(IRM) studieswereperformedonsamplesusinganASC-IM30impulse magnetizertofurthercharacterizethemagneticbehavior.

3.2. Geochronologicalmethods

TwosamplesfromtheNE–SWtrending“GreatDykeofMahoba”

and five samples from the NW–SE trending older suite of mafic dykes from the Bundelkhand craton were processed for geochronology(Fig.2).Toascertainthepresenceofuraniumbear- ingphases,selectedsampleswerethinsectionedandthenimaged via scanning electron microscopy (SEM) for zircon/baddeleyite usingthebackscattereddiffraction(BSD)method.Outofthedozen samplesimaged,onlytwofromtheNE–SWtrendingyoungersuite (GDMandGDM-1)andfour(includinganalyzedsampleI9GS-13) fromtheNW–SEtrendingoldersuitedisplayedbrightspotsrep- resentativeofzirconorbaddeleyite,ranginginsizebetween2and 100␮m(Fig.4aandb).WepulverizedsampleI9GS13fromthe oldersuiteandMahobadykesamples(GDMandGDM-1)forcon- ventionalmethodsofmineralextraction.Using standardgravity andmagneticseparationtechniques,thezircongrainswerecon- centratedfrompulverizedsamplesatUniversity ofFlorida.The sampleswerecrushed,thendiskmilledandsievedto<80-mesh grainsizefraction.ThefractionswerethenrinsedusingCalgon(an anionicsurfactant),followedbywatertabletreatmentwithslow samplefeedrates.Thiswasfollowedbyheavyliquidmineralsep- arationwithmultipleagitationperiodstoreducethenumberof entrappedgrainsinthelowerdensityfraction.Finally,thesamples wererepeatedlypassedthroughaFrantzIsodynamicmagneticsep- aratoruptoacurrentof1.0 ˚A(2–4^◦tilt).Approximately15–20clear, euhedraltonearlyanhedralzircongrainsand zirconfragments werehandpickedfromthetwo samplesoftheNE–SWtrending youngerMahobasuite(GDMandGDM-1) and25–35subhedral

toeuhedralzircongrainsfromtheNW–SE trendingoldersuite (I9GS13)underanopticalmicroscopetoensuretheselectionof onlytheclearest grains and fragmentsof grains.Furtherhand- pickingofthegrainsreducedthenumbertoonly7–8goodgrains fromtheMahobadoleritesand10–15grainsfromtheI9GS-13dyke sample.Thegrainswerethenmountedinresinandthenpolished toexposemedialsections.Furthersonicationandcleaningofthe plugsin5%nitricacid(HNO3)helpedtoremoveanycommon-Pb surfacecontamination.

U/PbisotopicanalyseswereconductedattheDepartmentof GeologicalSciences(UniversityofFlorida)onaNuPlasmamulti- collectorplasmasourcemassspectrometerequippedwiththree ion counters and 12 Faraday detectors. The LA–ICPMC–MS is equippedwithaspeciallydesignedcollectorblockforsimultane- ousacquisitionof²⁰⁴Pb (²⁰⁴Hg),²⁰⁶Pband²⁰⁷Pb signalsonthe ion-countingdetectorsand²³⁵Uand²³⁸UontheFaradaydetectors (Muelleretal.,2008).Mountedzircongrainswerelaserablated usinga New-Wave 213nmultraviolet laser beam.During U/Pb analyses,thesamplewasdecrepitatedin aHestreamandthen mixedwithAr-gasforinductionintothemassspectrometer.Back- groundmeasurementswere performedbeforeeach analysisfor blankcorrectionandcontributionsfrom²⁰⁴Hg.Eachsamplewas ablatedfor∼30sinanefforttominimizepitdepthandfraction- ation.Datacalibrationanddriftcorrectionswereconductedusing theFC-1DuluthGabbrozirconstandard,longtermreproducibil- itywere2%for²⁰⁶Pb/²³⁸U(2)and1%for²⁰⁷Pb/²⁰⁶Pb(2)ages (Muelleretal.,2008).Therewasnosignificant²⁰⁴Pbdetectedin thesamplesandtherefore,nocommonPbcorrectionwasappliedto theU–Pbdata.Datareductionandcorrectionwereconductedusing acombination ofin-housesoftware andIsoplot(Ludwig,1999).

AdditionaldetailscanbefoundinMuelleretal.(2008).

4. Results

4.1. Geochronologicalresults

TheU/Pb agesfromthezircon/zirconfragmentsweredeter- minedfor theolderNW–SEtrending dykesampleI9GS13 and theGreat dyke of Mahobasamples GDM and GDM1. The dyke sampleI9GS13yieldedanumberofwellfacetedzirconsandzir- con fragments suitable for U/Pb isotopic analysis. Eleven laser analyseson five differenteuhedral zircons yielded a concordia ageof1979±8Ma(2;MSWD(Conc.+Equival.)=0.63;probabil- ity(Conc.+Equival.)=0.90;Fig.5a;Table2a).Thisageisinrough agreementwithreportedinsitu⁴⁰Ar/³⁹Arlaserablationdatacited above(Rao,2004;Raoetal.,2005).Inaddition,nocrustalagesof 2.0GaareknownfromtheBundelkhandcratonandnodetritalcom- ponentofthisagehasbeenidentifiedintheadjacentVindhyanand Marwarbasins(Maloneetal.,2008;Meertetal.,2010).Therefore, itisourinterpretationthatthe1978Maagereflectsthecrystalliza- tionageofthedyke.Tenanalysesonfourotherfragmentaryzircons fromsampleI9GS13yieldedmoderatelydiscordant²⁰⁷Pb/²⁰⁶Pb datesbetween2777and2686Ma(Fig.5b;Table2a).Theseresults arebroadlyconsistentwiththeagesofthebasementrocksinthe region(Mondaletal.,2002).Threeanalysesofasinglezirconare slightly tomoderatelydiscordant,and the twoleast-discordant analysesagreetowithinuncertaintyandyieldaweightedmean

207Pb/²⁰⁶Pbdateof3254±3Ma(MSWD=0.88)thatisalsoacom- monageforbasementrocksintheBundelkhandcraton(seeMeert etal.,2010).

ThetwoMahobadykesamplesyieldedonlytwowellfaceted grainsandseveralfragments/tipsofzircons(Fig.4a).Theregression derivedfrom²⁰⁷Pb/²⁰⁶Pbratiosfromfivelaseranalysesonthree differentzircongrainsandzirconfragments/tipsyieldedamin- imumconcordantageof1096±19Ma(2;MSWD=0.3;Fig.5c;

Table1

Bundelkhandpaleomagneticresults.

Site/study Latitude/longitude N/n Dec Inc Kappa() ˛95 VGPlatitude VGPlongitude dp/dm

OlderSuite(NW–SE)

SiteI442 25.383^◦N,78.536^◦E 5 141.0^◦ −09^◦ 11.3 7.2^◦ 47.2^◦N 325.7^◦E 3.7/7.3

SiteI443^a

DykeA 25.425^◦N,78.672^◦E 2 139.0^◦ −12.6^◦ 70.0 11.1^◦ 46.5^◦N 329.8^◦E 5.8/11.3

DykeB 25.425^◦N,78.672^◦E 4 141.4^◦ 5.8^◦ 70.0 11.1^◦ 43.2^◦S 137.1^◦E 5.6/11.1

DykeC^b 25.425^◦N,78.672^◦E 2 339.0^◦ −21^◦ 70.0 11.1^◦ 48.5^◦S 110.5^◦E 6.1/11.7

SiteI444 25.408^◦N,78.669^◦E 7 174.0^◦ −14.8^◦ 16.0 15.5^◦ 71.4^◦N 277.3^◦E 8.1/15.9

SiteI454 25.147^◦N,80.050^◦E 7 156.5^◦ −17.3^◦ 50.0 8.6^◦ 62.4^◦N 318.2^◦E 4.6/8.9

SiteI455 24.935^◦N,79.902^◦E 18 149.0^◦ −21^◦ 47.3 5.1^◦ 57.6^◦N 330.6^◦E 2.8/5.5

SiteI925 24.946^◦N,79.911^◦E 34 151.3^◦ −19.5^◦ 34.0 4.3^◦ 59.1^◦N 326.5^◦E 2.3/4.5

SiteI927 25.017^◦N,80.475^◦E 27 167.5^◦ −15.9^◦ 42.0 4.3^◦ 69.3^◦N 297.6^◦E 2.3/4.4

SiteI929 25.192^◦N,80.464^◦E 9 168.2^◦ −10.3^◦ 107 5.0^◦ 67.1^◦N 291.8^◦E 2.6/5.1

SiteI930 25.189^◦N,80.469^◦E 16 164.0^◦ −13.2^◦ 71.3 4.4^◦ 66.1^◦N 302.7^◦E 2.3/4.5

SiteI931 25.286^◦N,79.919^◦E 10 152.9^◦ −8.8^◦ 46.2 7.2^◦ 56.8^◦N 315.3^◦E 3.7/7.3

Mahobadykes(ENE–WSW)

SiteI451 25.284^◦N,79.851^◦E 23 14.6^◦ −31.7^◦ 25.0 6.1^◦ 45.3^◦S 59.5^◦E 3.8/6.9

SiteI452 25.284^◦N,79.851^◦E 13 27.6^◦ −48.7^◦ 232.0 2.7^◦ 29.1^◦S 52.1^◦E 2.3/3.6

SiteI923 25.184^◦N,79.401^◦E 20 14.2^◦ −37.0^◦ 125.0 3.3^◦ 42.2^◦S 61.2^◦E 2.3/3.9

SiteI924(greatcircles) 25.301^◦N,79.934^◦E 10 40.7^◦ −29.2^◦ – 2.8^◦ 33.1^◦S 31.0^◦E 1.7/3.1 NE–SW(thirddykesuite)

SiteI448 25.134^◦N,79.750^◦E 20 200.0^◦ 60.0^◦ 42.4 14.3^◦ 21.5^◦S 63.3^◦E 16.3/21.6

SiteI928 25.052^◦N,80.486^◦E 20 175.0^◦ 68.0^◦ 48.6 4.7^◦ 13.8^◦S 83.5^◦E 6.6/7.9

Overallmean–older(NW–SE) 25.00^◦N,80^◦E 141/12 155.3^◦ −7.8^◦ 21.4 9.6^◦ 58.5^◦N 312.5^◦E 4.9/9.7 Overallmean–Mahoba(ENE–WSW) 25.17^◦N,79.5^◦E 76/4 24.7^◦ −37.9^◦ 35.9 15.5^◦ 38.7^◦S 49.5^◦E 9.5/16.3 N:numberofsamplesused;n:numberofdykes/sites;Dec:declination;Inc:inclination;:kappaprecisionparameter;˛95:coneof95%confidenceaboutthemeandirection;

VGP:virtualgeomagneticpole.

aDykeA,BandCare3smalldykesfromSiteI443withDykeC.

bShowingreversepolarity.

Table2a

GeochronologicalResultsfromtheBundelkhandoldersuiteofdykes(I9GS13).

Ratios Ages

Grain ²⁰⁷Pb/²⁰⁶Pb ±2␴ ²⁰⁶Pb/²³⁸U ±2␴ ²⁰⁷Pb/²³⁵U ±2␴ ²⁰⁶Pb/²³⁸U (Ma)

±2␴ ²⁰⁷Pb/²³⁵U (Ma)

±2␴ ²⁰⁷Pb/²⁰⁶Pb (Ma)

±2␴ %Disc RHO

I9GS132 0.1222 0.0012 0.34907 0.0179 5.88 0.30 1930 43 1959 22 1989 8 3 0.98

I9GS 136 0.1230 0.0016 0.35774 0.0239 6.07 0.42 1971 57 1985 30 2000 12 1 0.98

I9GS137 0.1218 0.0026 0.36297 0.0223 6.10 0.40 1996 53 1990 28 1983 19 −1 0.95

I9GS138 0.1210 0.0014 0.36787 0.0208 6.14 0.36 2019 49 1995 25 1970 10 −3 0.98

I9GS 13 9 0.1212 0.0012 0.37220 0.0221 6.22 0.38 2040 52 2007 26 1975 9 −3 0.98

I9GS1310 0.1220 0.0016 0.36876 0.0188 6.20 0.32 2023 44 2004 23 1985 11 −2 0.97

I9GS 1311 0.1215 0.0012 0.35601 0.0164 5.96 0.28 1963 39 1970 20 1978 9 1 0.98

I9GS1314 0.1218 0.0012 0.35741 0.0198 6.00 0.34 1970 47 1976 24 1983 9 1 0.98

I9GS1315 0.1214 0.0012 0.35357 0.0157 5.92 0.26 1952 37 1964 20 1977 9 1 0.98

I9GS1316 0.1211 0.0014 0.35077 0.0178 5.86 0.28 1938 39 1955 21 1972 10 2 0.97

I9GS1317 0.1213 0.0012 0.35355 0.0163 5.91 0.30 1951 42 1963 22 1975 9 1 0.98

I9GS1321 0.1883 0.0008 0.45398 0.0221 11.79 0.58 2413 98 2588 45 2728 7 12 0.99

I9GS1323 0.1928 0.0007 0.47373 0.0190 12.59 0.51 2500 83 2649 38 2766 6 10 0.99

I9GS1324 0.1939 0.0008 0.46927 0.0187 12.55 0.50 2480 82 2646 37 2776 6 11 0.99

I9GS1326 0.2572 0.0020 0.47423 0.0696 16.82 2.47 2502 301 2924 136 3230 12 23 1.00

I9GS1327 0.2611 0.0007 0.63422 0.0162 22.83 0.59 3166 64 3220 25 3253 4 3 0.99

I9GS1328 0.2615 0.0008 0.56674 0.0566 20.44 2.04 2894 231 3112 95 3256 5 11 1.00

I9GS1330 0.1941 0.0007 0.50044 0.0099 13.39 0.27 2616 43 2708 19 2777 6 6 0.98

I9GS1331 0.1900 0.0007 0.48971 0.0130 12.83 0.34 2569 56 2667 25 2743 6 6 0.99

I9GS1333 0.1864 0.0008 0.48857 0.0105 12.56 0.28 2564 45 2647 21 2711 7 5 0.98

I9GS1334 0.1896 0.0012 0.45310 0.0204 11.85 0.54 2409 90 2592 42 2739 10 12 0.99

I9GS 1335 0.1837 0.0006 0.46585 0.0121 11.80 0.31 2465 53 2588 24 2686 6 8 0.99

I9GS1337 0.1852 0.0009 0.47595 0.0122 12.16 0.32 2510 53 2616 24 2700 8 7 0.98

I9GS1338 0.1888 0.0016 0.49585 0.0112 12.91 0.31 2596 48 2673 23 2732 14 5 0.98

Table2b

GeochronologicalresultsfromtheMahobadykes(GDMandGDM1).

Ratios Ages

Grain ²⁰⁷Pb/²⁰⁶Pb ±2␴ ²⁰⁶Pb/²³⁸U ±2␴ ²⁰⁷Pb/²³⁵U ±2␴ ²⁰⁶Pb/²³⁸U(Ma) ±2␴ ²⁰⁷Pb/²³⁵U(Ma) ±2␴ ²⁰⁷Pb/²⁰⁶Pb(Ma) ±2␴ %Disc RHO

GDM5a 0.07697 0.0006 0.17605 0.0051 1.868 0.06 1046 28 1070 20 1120 17 7 0.96

GDM6a 0.07697 0.0007 0.17509 0.0040 1.858 0.05 1041 22 1066 16 1120 18 7 0.93

GDM 7a 0.07671 0.0006 0.17551 0.0041 1.856 0.05 1043 23 1066 16 1114 16 6 0.95

GDM8a 0.07654 0.0006 0.17623 0.0041 1.860 0.05 1047 23 1067 16 1109 17 6 0.94

GDM 18a 0.07631 0.0006 0.18242 0.0041 1.919 0.05 1081 22 1088 16 1103 16 2 0.94

Table2b).Theweightedmean²⁰⁷Pb*/²⁰⁶Pb*ageforthesefiveanal- ysisyieldedanageof1113±7(2;MSWD=0.75;probabilityof fit=0.56;Fig.5d;Table2b).

4.2. Paleomagneticresults

The paleomagnetic directions/statistics for the individual NW–SE,ENE–WSWandNE–SWtrendingBundelkhandmaficdykes arelistedinTable1.Threedistinctpaleomagneticdirectionswere recordedforthedoleriticdykessuggestingthreeepisodesofdyke emplacementwithintheBCvaryinginspaceandtime.Astable uni-vectorialdemagnetizationtrendwasobservedforamajority ofthesesamplesduringthermal/AFtreatments.Thesamplesshow discreteunblockingtemperaturesbetween550and570^◦C.The‘B’

samplestreatedwithAFdemagnetizationshowgradualdecreasein theirmagneticintensityandbehaveconsistentlyoverawiderange ofcoercivityspectrum(Fig.6bandd).

TheNRMintensitiesfortheENE–WSWtrendingMahobadyke samplesrangebetween0.4and0.1A/m(Fig.6aandb).Theinten- sityplotsyieldedunblockingtemperaturesbetween550and560^◦C duringthermaldemagnetizationwhileAFdemagnetizationyielded widecoercivityspectrum,consistentwithlow-Timagnetiteasthe maincarrierofmagnetization(Fig.6a andb).Theoverallmean directioncalculated fromfour ENE–WSW Mahoba dykes has a declination=24.7^◦andinclination=−37.9^◦(=36;˛95=15.5^◦)and aresultantvirtualgeomagneticpole(VGP)at38.7^◦Sand49.5^◦E (dp/dm=9.5^◦/16.3^◦;(Fig.9a).

Fifteendykes weresampledfromtheolderNW–SEtrending suiteandtwelveyielded consistentstablepaleomagneticdirec- tions(Table1).TheNRMintensitiesforthesesamplesvaryfrom 0.44to0.11A/m.Fig.6canddshowsthedemagnetizationbehav- ioroftwopilotdykespecimenstothethermalandAFtreatments.

Themeanpaleomagneticdirectionobtainedfromthese12dykes hasadeclination=155.3^◦andinclination=−7.8^◦(=21;˛95=9.6) afterinvertingonereversepolaritydykeI443C(Fig.9b;Table1).

Theoverallpaleomagneticpolecalculatedfromthesetwelvedykes fallsat58.5^◦Nand312.5^◦E(dp/dm=6.6^◦/7.9^◦).

In addition to these two directions from the NW–SE and ESE–WNWdykes,a thirddistinctsteeppaleomagneticdirection wasobtainedfromtwooftheNE–SWtrendingdykes.Theoverall mean direction calculated for these two dykes has a declina- tion=189.3^◦ andinclination=+64.5^◦ (Fig.9c). Thisissuggestive, though not conclusive, evidence for a third episode of dyke emplacementwithintheBC.

TheBundelkhandgranitichostrocksampleswerealsocollected atsitesI925 and I927withtheintenttoperforma baked con- tacttest. Unfortunately, the vast majority of thegranites were dominatedbymulti-domainmagnetiteandnoconsistentdirec- tionswereobtainedwithoneexception.AtsiteI925 numerous maficdykeletswereobservedtrellisingthegranitichostrock(Fig.

3b). These graniticsamples in contact with themafic dykelets weretaken about 1.5 away from themain dyke (Fig. 3a) and stablepaleomagneticdirectionswereobtainedforthehostgran- itesthat weresimilartothemain dykeI925(Fig.7aand b).In addition,oneofthedykes(siteI443C)isofopposite polarityto thecharacteristicmagnetizationobservedinthemajorityofdykes (declination=339^◦andinclination=−21^◦;=70;˛95=11.1).While notconclusive,thepartialbakedcontacttest,thedualpolaritysig- nalandthefactthatthethreesuitesofdykes(NE–SW,ENE–WSW andNW–SE)yielddistinctpaleomagneticdirectionssupportapri- marymagnetizationintheNW–SEtrendingoldersuiteofdykes

and negates arguments for a younger remagnetization in the region.

4.2.1. Rockmagneticresults

Representativeresultsofthermomagnetic analysis(suscepti- bilityvs.temperature)ofENE–WSWand NW–SEtrending dyke samplesareshowninFig.8(a–d).Theheatingandcoolingcurvesof magneticsusceptibilityfortheENE–WSWtrendingMahobadykes asshowninFig.8aandcdisplaytwomagneticphases.Theheating curvesshowaprominentpeakcenteredcloseto250–300^◦Cand susceptibilitydropabove300^◦C,indicatingthelikelyexistenceof pyrrhotite.Arapiddecreaseinthemagneticsusceptibilityaround 550–575^◦Cindicatestheexistenceofmagnetite(Fig.8aand c).

Thecoolingcurveshowshighersusceptibilitywhichisprobably causedbytheex-solutionandconversionofTi-magnetitetopure magnetite(Fig.8aandc).

TherockmagneticstudiesonthedominantlyoccurringNW–SE trendingoldersuiteofdykesshownearlyreversibleCurietemper- aturerunscharacteristicofmagnetite(Fig.8bandd).Theheating CurietemperatureTcHofdykesampleI927-12is572.8^◦C,andthe coolingCurietemperatureT_cCis567.5^◦C(Fig.8d).IRMcurvesalong withbackfieldcoercivityofremanencefromboththeseENE–WSW andNW–SEtrendingmaficdykesalsoindicatemagnetiteasthe principalmagneticcarrierinthesamples.Arapidriseinintensity nearsaturationat∼0.25to0.3Twasobservedformajorityofthe dykesamplesandtheirmagnetizationremainsconstantathigher fields,uptothehighestappliedfieldof1.2T.Thevaluesfortheback fieldcoercivityofremanencerangesbetween0.07and0.1mT(Fig.

8eandf).

5. Discussion

5.1. 1.1Gapaleomagnetism

TheVGPcalculatedfortheENE–WSWtrendingMahobadykes oftheBundelkhandcratonissignificantintermsofitsageand the direction.The U–Pb zirconage of 1113±7Ma for Mahoba dykesfallsinthesametimebracketastheMajhgawankimber- litethat intrudes both theLowerVindhyan sequence (∼1.6Ga) and the Baghain sandstone of the Kaimur Group (Upper Vin- dhyan). Gregory et al. (2006) dated Majhgawan kimberlite at 1073±13.7Mavia⁴⁰Ar–³⁹Aranalysisofphlogopitephenocrysts andobtainedavirtualgeomagneticpoleat36.8^◦S,32.5^◦E(dp=9^◦; dm=16.6^◦;alsoseeMillerandHargraves,1994)thatisstatistically indistinguishablefromthevirtualgeomagneticpolecalculatedfor theMahobadykeswarminthisstudy(Fig.10).Maloneetal.(2008) conductedapaleomagneticstudyontheBhander–RewaGroups (Upper Vindhyan) and obtaineda mean paleomagnetic poleat 44^◦S,34.0^◦E(A95=4.3). Wenotethat theVGP’s oftheMahoba dykesgeneratedinthisstudyandthepenecontemporaneousMajh- gawankimberlitearenearlyidenticaltothepaleomagneticpoles obtainedfromtheBhander–RewaGroupsintheUpperVindhyan sequence(Fig.10).

Azmietal.(2008)arguethatthepaleomagneticdatafromthe MajhgawankimberliteandtheBhander–Rewasequenceareindeed age-equivalentandlateNeoproterozoicinage,butthattheMajh- gawanphlogopitescrystallizedatdepthsome400millionyears earlier.Weacknowledgethatthepresenceofsimilarmagnetiza- tionsin three different unitsin close proximitymay alsoraise concernaboutthepossibilityofremagnetization.However,wenote thatthereisnoreported∼1.1Garemagnetizationeventwithinor inthevicinityoftheBundelkhandcraton.

Fig.4. (a)Backscatteredelectron(BSE)andcathodoluminescence(CL)imagesofselectedzircons/uraniumbearingmineralsfromENE–WSWtrendingMahobadykes.Scale barsin␮m.(b)Backscatteredelectron(BSE)andcathodoluminescence(CL)imagesofselectedzircons/uraniumbearingmineralsfromNW–SEtrendingdykes.Barlength correspondsto10–100␮m.

5.45.80.0735

0.0795

1100

1060

data-point error ellipses are 2

Sample GDM & GDM1 (5 analysis/3 grains) MSWD (of concordance) = 0.3 238 U 206 Pb [a] [c] Mean = 1 1 12.7 M a, 95% confidence MSWD = 0. 75, P robability = 0.56

± 7.4

207 Pb

Pb 206

00

1110

1120

1140 1130 1090

207Pb Pb 206

1096+/-19 Ma

Sample GDM & GDM1 [d]

5.25.66.0

0.0785 0.0775 0.0765 0.0755 0.0745 0.196 0.192 0.188 0.184 0.180 2.052.35

data-point error ellipses are 2

Sample I9GS_13 (10 analysis/ 4 g rains) MSWD (of concordance) = 4.5

2760

207 Pb 235 U [b]

206Pb U 238

2.152.251.951.85

2720

2680

1900

1980

2060

0.41 0.31 5.36.76.9

data-point error ellipses are 2

(1 1 analysis/ 5 grains) Concordia Age = 1979.1 MSWD (Conc. + Equiv .) = 0.63 Probability (Conc. + Equiv .) = 0.90 ± 7.9 Ma

206Pb U 238

207 U 235 Pb

0.350.39 0.37 0.33 5.56.35.9

Sample I9GS_13

Fig.5.(a)Concordiadiagramforthe11spotsfromfiveconcordantzircon/baddeleyitegrainsandtipsfromsampleI9GS13yieldinganageof1979.1±7.9Ma(2;

MSWD=0.63;probability=0.90).(b)Tera–Wasserburgplotobtainedfrom10analysesonapopulationoffourfragmentaryzircons/baddeleyitesfromNW–SEtrending dykesampleI9GS 13yieldingmoderatelydiscordant²⁰⁷Pb/²⁰⁶Pbdatesbetween2777and2686Ma(2;MSWD=4.5)(c)Tera–Wasserburgconcordiainterceptageof 1096±19Ma(2;MSWD=0.3)derivedfromtheregressionthroughtheuncorrecteddatafromthe²⁰⁷Pb/²⁰⁶Pbforthefiveanalysisonthreezircongrainsandtipsfrom sampleGDMandGDM1(d)Theweightedmean²⁰⁷Pb*/²⁰⁶Pb*ageforthefiveanalysisfromsampleGDMandGDM1yieldedanageof1113±7(2;MSWD=0.75;probability ofconcordance=0.56).

200 400 600 800 T emperature (°C) 0

0.5 0

J/J

1.0

N E 570 C 550 C 450 C

N

NRM

W, U p 0.2 A/m

W, U p NRM

10 mT 20 mT 80 mT 40 mT

E AF (mT)

1.0 0.5 0 0

J/J

N Sample I923-4B

Sample I923-1A

20 40 60 80 100

[a] [b]

AF (mT)

1.0 0.5 0

J/J

W,Up NRM 10 mT 20 mT

80 mT 40 mT

S

E

02 0 40 60 80 100 [d]

Sample I925-7B

200 400 600 800 T emperature (°C)

0

0.5 0

J/J

1.0

W, U p S

E Sample I925-22A NRM

300 450

550

N

V H [c]

0.1 A/m

Additionalsupportfortheprimarynatureofmagnetizationis foundinbothUpperVindhyansequenceandtheNW–SEtrending oldersuiteofBundelkhanddykes.UpperVindhyansedimentary unitsshowatleastelevengeomagneticreversalssupportingapri- marymagnetizationintheserocks.Similarly,thepresenceofpartial bakedcontacttestyieldedbythegranitichostrocksamplestra- versedbythemaficdykeletsatsiteI925(Fig.7eandf)andthedual polaritymagnetizationshownbyoneofthedykes(siteI443C)sup- portaprimarymagnetizationintheNW–SEtrendingoldersuiteof Bundelkhanddykes.

5.1.1. AgeimplicationsfortheBhander–Rewasequenceofthe UpperVindhyan

TheageofdepositioninVindhyanbasinlocatedtothesouth oftheBundelkhandCraton(Fig.1)inthenorthernIndianpenin- sularshield,hasbeendebatedforover100years(Oldham,1893;

Auden,1933;CrawfordandCompston,1970;Venkatachalaetal., 1996;Maloneetal.,2008;Azmietal.,2008).Theonsetofsed- imentationin thelowerVindhyanSupergroupis generallywell constrainedataround1.6–1.8Gabyradiometricdata(Rasmussen etal.,2002a,b;Rayetal.,2002,2003Sarangietal.,2004;Kumar, 2001);howevertheageoftheUpperVindhyanisstillenigmaticand highlycontentiousduetolackoftargetssuitableforgeochronol- ogy,controversialfossilfindsandpoorlyconstrainedglobalstable isotopic correlations (Vinogradov et al., 1964; Crawford and Compston, 1970;Paul et al.,1975; Srivastava and Rajagopalan, 1988;Chakrabarti,1990;Smith,1992;KumarandSrivastava,1997;

Kumaretal.,2002;De,2003,2006;Rayetal.,2002,2003;Raietal., 1997;Gregoryetal.,2006;Maloneetal.,2008;Azmietal.,2008).

The Upper Vindhyan sedimentary rocks were typically cor- related withtheMarwar Supergroup (Rajasthan);sequences in theSalt Range ofPakistan and with theKrol–Tal Groupof the LesserHimalayas(McElhinnyetal.,1978;Klootwijketal.,1986;

MazumdarandBanerjee,2001).Thesecorrelations,however,are basedonsomewhatsimilarlithologiesandtheproximityofthe undeformedMarwarandUpperVindhyanstrata.

Thelithologiccomparisonsbetweentheseunitsareratherprob- lematic.Forexample,theevaporitedepositsareprevalentwithin theMarwar andSaltRanges,but absentintheUpper Vindhyan sequence. Malone et al. (2008) examined detrital zirconsuites fromboth theUpper Vindhyan sedimentaryrocksin Rajasthan andthenearbyMarwarSupergroup.The²⁰⁷Pb/²⁰⁶Pbagedistribu- tionobservedinthedetritalzirconanalysisoftheUpperBhander sandstoneyieldedseveralnoteworthypeaksbetween1850and 1050Ma (Maloneet al.,2008).Incontrast, detritalzirconsana- lyzedfrom theSonia and Girbarkhar sandstone of the Marwar Supergroupyieldedagepeaksinthe840–920Marange,acom- ponentcompletelyabsentintheUpperBhandersandstoneofthe VindhyanSupergroup.Hence,theresultsfromthedetritalzircon geochronologysuggestthatpreviouscorrelationsbetweenthetwo depositionalsequencesareincorrect.Maloneetal.(2008)hypoth- esizedthattheclosureageoftheUpperVindhyansedimentation wasnoyoungerthan1.0Gabasedontheirpaleomagneticstudyon theBhander–RewaGroupsoftheUpperVindhyanandthedetrital zirconworkonBhander–RewaunitsandtheMarwarSupergroup.

AdditionalsupportforaLateMesoproterozoicclosureagecame fromapaleomagnetic/geochronologicalstudyoftheMajhgawan kimberlite(Gregoryetal.,2006).

ThepaleomagneticandgeochronologicaldatafromtheMahoba dykesgeneratedinthisstudyisgermanetothediscussionofthe

sedimentationagesintheUpperVindhyanbasin.TheVGPobtained forthe∼1113MaMahobadykes(37.8^◦S,49.5^◦E)isnearlyidentical tothemeanpaleomagneticpolefromtheBhander–RewaGroups andtheMajhgawankimberlite(Fig.10;seediscussionabove).Our interpretationoftheMahobapaleomagneticandgeochronologi- calresultstherefore lendsadditionalsupporttotheproposalof Maloneet al.(2008)and Gregory etal. (2006)that theclosure ofsedimentationintheUpper Vindhyansequenceisolderthan

∼1.0Ga.

5.1.2. IndiainRodiniasupercontinentat1100Ma

TheLateMesoproterozoic(ca.1100Ma)hasbeenpostulatedas thetimeintervalfortheformationofthesupercontinentRodinia (McMenaminandMcMenamin,1990;Dalziel,1991;Moores,1991;

Hoffman, 1991). The existence of Rodinia is supported by the presenceofanumberof1300–900Maoldorogenic/mobilebelts (Dewey andBurke,1973)andassociatedgeologiclinks between thevariouscratonicnuclei(Young,1995;Dalziel,1997;Rainbird etal.,1998;Karlstrometal.,1999;SearsandPrice,2000;Dalziel etal.,2000).However,thegeometry/paleogeographyandduration oftheRodiniasupercontinentremainsextremelyfluidandcontro- versialduetoapaucityofwelldated,highqualitypaleomagnetic polesfromvariouscontinentalblocksformingthesupercontinent (Weiletal.,1998;MeertandPowell,2001;Meert,2001;Meertand Torsvik,2003;Lietal.,2008).

Oneoftheoutcomesofthis studyisourabilitytoconstrain thepaleoposition ofIndian sub-continent in Rodiniaconfigura- tionat∼1.1Ga.Therecentpaleomagnetic andgeochronological studiesfromtheMajhgawankimberlite(Gregoryetal.,2006)and Bhander–RewaGroups ofUpperVindhyan(Maloneetal.,2008) providedhighqualitypolestoconstrainthepaleogeographicposi- tionofIndiaat1.1Ga.

Thereareanumberofpaleomagneticpolesavailablefromother elementsoftheRodiniasupercontinentthataremoreorlesscoeval withthosefromIndia.ConsideringtheEastGondwanaelements, thekeypoleforthisintervalinAustraliaisthe∼1070Madoleritic rocksofBangemallsills(Wingateetal.,2002).TheBangemallpole achievesascoreofQ=7inthereliabilityschemeofVanderVoo (1990).

ThepaleogeographicpositionforLaurentiaisbasedonthecom- binedmeanpaleomagneticdatafromthePortageLakevolcanics and Keweenawan dykes (Pesonenet al., 2003; Swanson-Hysell et al., 2009).Based on the extensivefield and laboratorytests these poles are inferred to be primary and have precise zir- con/baddeleyiteU–Pbdatesof1095Ma(PortageLakeVolcanics) and ∼1109Ma(Keweenawanvolcanics), respectively(Hallsand Pesonen,1982; Davis and Sutcliffe,1985; Goodgeet al., 2008).

Morerecently,Swanson-Hyselletal.(2009)providedhighreso- lutionpaleomagneticdatafromaseriesofwell-datedbasaltflows atMamainsePoint,Ontario,intheKeweenawanRiftandsuggested thatthepreviouslydocumentedreversalasymmetryforthesevol- canicrocksisanartefactofthefastmotionoftheLaurentianplate towardstheequatoratthisinterval (alsoseeMeert,2009).The combinedmeanofthePortageLakevolcanicsandtheKeweenawan doleritespoleplacesLaurentiaatintermediatepaleolatitudesinour

∼1.1Gareconstruction.

TherearetwopaleomagneticpolesavailablefromtheBaltica between ∼1100 and 1123Ma. Most of the Rodiniareconstruc- tionsat∼1100MautilizethepaleomagneticdatafromtheBamble intrusionsinsouthernNorwaytoconstrainthepositionofBaltica

Fig.6. OrthogonalvectorplotsfromtheNW–SEandENE–WSWtrendingdykesoftheBundelkhandcratonshowingtypicalcharacteristicremanentmagnetizationdirections.

(a)ThermaldemagnetizationbehaviorofENE–WSWtrendingdykesampleI923-1A.(b)AlternatingfielddemagnetizationbehaviorofENE–WSWtrendingdykespecimen I923-4B.(c)ThermaldemagnetizationbehaviorofNW–SEtrendingdykesampleI925-22A.(d)AlternatingfielddemagnetizationbehaviorofNW–SEtrendingdykesample I925-7B.Solidsquaresrepresentprojectionsonthehorizontalplaneindicatedby‘H’;opensquaresrepresentprojectionontoaverticalplaneindicatedby‘V’.

Fig.7.BakedcontacttestonNW–SEtrendingdykeI925.(a)ThermaldemagnetizationbehaviorofthedykespecimenI925-7A.(b)Thermaldemagnetizationbehaviorofthe granitichostrockspecimenI925-39Aabout1.5mawayfromthemaindykeshowingsimilardirectionsasthemaindyke.