Trading partners: Tectonic ancestry of southern Africa and western Australia, in Archean

12 Pages·2012·0.84 MB·English

by  Aleksey V. Smirnov

PrecambrianResearch224 (2013) 11–22

ContentslistsavailableatSciVerseScienceDirect

Precambrian Research

journal homepage: www.elsevier.com/locate/precamres

Trading partners: Tectonic ancestry of southern Africa and western Australia, in

Archean supercratons Vaalbara and Zimgarn

AlekseyV.Smirnova,b,∗,DavidA.D.Evansc,RichardE.Ernstd,e,UlfSöderlundf,Zheng-XiangLig

aDepartmentofGeologicalandMiningEngineeringandSciences,MichiganTechnologicalUniversity,Houghton,MI49931,USA
bDepartment of Physics,M ichig anTech nologicalUn iver sity,Houg hton,MI4 9931,USA
cDepartment of Geology andGeop hysics,YaleUn iversity,Ne wHaven,C T0 6520,U SA
dErnstGeosci en ces,Otta waK 1T3Y2,Can ada
eCarlet onUniversity ,Ottaw aK1 S5B6 ,Canada
fDepartm entofEarth andEco syst emS ciences,DivisionofGeology,LundUniversity,SE22362Lund,Sweden
gCenterofEx cel lence forC oretoCrus tFluidSy stems,De pa rtmento fApp liedGeology ,C urtin U nivers ity,Perth,WA6845,Australia

a r t i c l e i n f o a b s t r a c t

Articlehistory: Originalconnectionsamongtheworld’sextantArcheancratonsarebecomingtractablebytheuseof
Receiv ed26April2012 integrate dpaleomagn etican dge ochrono logicst udieson Paleopro tero zoicmafic dykeswa rm s.H erew e
R19ecSeeivpetedm inb er erv2i0se1 d2 form report new high-quality p aleo magnetic data f rom the ∼2 .41 Ga Widgiemo oltha dyke swarm o f the Yil-
garncratoninwesternAustralia,confirmingearlierresultsfromthatunit,inwhichtheprimaryorigin
Accepted21September2012
ofcharacteristicremanentmagnetizationisnowconfirmedbybaked-contacttests.Thecorrespond-
Available online xxx in g paleomagnet ic pole (10 .2◦S, 159.2◦E, A 95= 7.5◦) , in combin ati on with newly availab le a ges on dykes
fromZimbabwe,allowforadirectconnectionbetweentheZimbabweandYilgarncratonsat2.41Ga,
Keywords:
withimpliedconnectionsasearlyastheircratonizationintervalsat2.7–2.6Ga.Theproposed“Zimgarn”
Paleomagnetism
Paleoproterozoic supercraton was likely distinct from Vaalbara (Kaapvaal plus Pilbara) at 2.4 Ga, but both of those entities
Yilgarn independentlyfragmentedatca.2.1–2.0Ga,reassemblingintotheKalahariandWestAustraliancratons

Pilbara by1.95–1.8Ga.

Zimbabvecraton © 2012 Elsevier B.V. All rights reserved.

Kaapvaalcraton

1. Introduction The Widgiemooltha mafic dykes of the Yilgarn craton (West-

ernAustralia)constituteoneofthemostprominentdykeswarms

Fundamental changes across the entire Earth system during of the world’s Precambrian shields (e.g., Sofoulis, 1966; Parker

the Archean–Paleoproterozoic transition (ca. 3000–2000Ma) are et al., 1987). The dykes generally trend east-west and outcrop

reflectedinglobaldynamicsthatbecamemoresimilartomodern- over the entire craton (Fig. 1). Most of the dykes are of picrite,

style plate tectonics through that interval (Condie and Kröner, olivine–doleriteorquartz–doleritecompositionandhavenotbeen

2008;ReddyandEvans,2009),althoughthekinematicrecordfrom affectedbysignificantmetamorphiceventssincetheirformation

thateraiscurrentlylimitedbyadearthofhigh-qualitypaleomag- (e.g.,Hallberg,1987).

netic data (Evans and Pisarevsky, 2008). However, recent work The age of the Widgiemooltha dyke swarm is constrained by

usingacombinedpaleomagneticandgeochronologicapproachon isotopic dating of its three largest dykes. Nemchin and Pidgeon
mafic d yke swarm s within sever al A rchean cratons, for examp le (1998) r eported a ba ddeley ite U–P b age of 2418± 3Ma for the
Slave (Buch anetal., 2009,2 012),Su perior(E vansand Ha lls,2010; wester nmost ex te nsion of the ∼600 -km -lon g Bin n er ing ie D yke

definingmemberoftheSuperiasupercratonofBleeker,2003),and (Fig.1).ElectronmicroprobeU–Pbanalysesofbaddeleyitesfrom

Vaalbara(deKocketal.,2009),hasstartedtochangethissituation, an eastern location of the Binneringie Dyke yielded a slightly
enabling ust otest an dq uantif yth eorigin al connect ions thatmay you ngerag eof2410 .3± 2.1 Ma(Frenche tal.,2 002).Ab ad deleyite
haveexis ted b etwe ent hem. U–Pbag eof 24 10.6+2 .1 /−1 .6M awasa ls oo btained f ortheCele-

brationDyke(DoehlerandHeaman,1998).Fletcheretal.(1987)
reporte dRb– SrandSm –Nd isochron ageof 2411±3 8M a forthe
ultramafi ccore ofth e∼180- km-longJ imb erl anain tr usi on, loca ted
∗Correspondingauthorat:DepartmentofGeologicalandMiningEngineeringand south of th e Bi nn erin gie Dyke (Fig. 1). These d ata broad ly con-
Sciences,Michigan Techno log icalUniversi ty, 630DowE CEB uilding, 1400Towns end
firm the earliest geochronological study of the Widgiemooltha
Drive, Houghton, MI 49931, USA. Tel.: [phone]. dyke swa rm (Tur ek, 1966), which yielded a 242 0±30Ma Rb–Sr
E-mailaddress:[email protected](A.V.Smirnov).

0301-9268/$–seefrontmatter© 2012 Elsevier B.V. All rights reserved.

http://dx.doi.org/10.1016/j.precamres.2012.09.020
12 A.V.Smirnovetal./PrecambrianResearch224 (2013) 11–22

-20o processing techniques, and it lacked a rigorous field test to confirm
theprimaryoriginofdykemagnetization.Herewereportnewhigh-
Pilbara
qualitypaleomagneticdatafromtheWidgiemoolthadykeswarm,

with implications for tectonic style of craton amalgamation and

separationduringtheArchean–Paleoproterozoictransition.

-24o
2. Methods
23
A 24 Yilgarn Wesampled27sitesrepresenting14separateWidgiemooltha
10 11,12 19,20 dykess panning mo stof theYilgarnc rato n(Fig.1 ).Fivetothirty-
9 twofi eld-drilled orien te dco resamp leswe reco llec tedf rom each
-28o 26 C,I 22 21 18 site. Orientationw asdone with bothsola randm agneticc ompa sses.
16 17 Low -fieldtherm oma gneti cana lyses ,rem ane nceunblo ckingtem-
5 7DH,GE 1415 perature and magnetic h ysteresis measureme nts indicate the
F 13 presenceofpseudosingle-domainmagnetiteorlow-Tititanomag-
CD netiteinm o stdykes(Fig.2).
RD
-32o Perth 4 JBDD UniMveargsniteytaicn dreMmiachniegnacneT mecehansoulroegmiceanltUsn wiveerresi tcyounsdiuncgtaeudt oamt aYtaelde
271 23 three-axis DC SQUID 2G rock magneto meters ho used in magnet-
ically shielded environments. After measurement of the natural

remanentmagnetization(NRM),thesampleswerecycledthrough
the Verwe y transition a t ∼120 K (V erwey, 1 939) by im mersing

themintoliquidnitrogenforabouttwohoursinordertoreduce

-36o aviscouscomponentcarriedbylargermagnetitegrains(Schmidt,
116o 120o 124o 1 993).Ne xt,alowalte rnating fiel d(AF) pre-treatm ent(to 100mTin

20mTsteps)wasappliedtoremoveanyremaininglow-coercivity
Fig. 1. Locations of sampling sites of this study (circles) and of Evans’ (1968) (dia- vis cou soriso ther malrem an ence.Fin ally, 15–20ther maldemagne-
monds)samplingsites.Open(orgrey)symbolsshowthesitesused(ornotused)
tizationstepswereperformedinaninert(nitrogen)atmosphere.
tPoil cbaalrcauclarateto tnhse. mTheianng praayleloinmeasginndeticica tdeirtehcetiWonids.g Siehmadoeodl tahraeadsy skheosww athrme Y;isloglaidrnl ianneds Progress ive d emagn etization w as ca rried out until t he magnetic

showtheBinneringie(BD),Jimberlana(JD),Celebration(CD),andRandalls(RD) intensityofthesamplesfellbelownoiseleveloruntilthemeasured
dykes .Ins etmapshow sthe areastudied byE vans(1968). direction sb eca meerrat ica nduns table (typic all yat5 80– 590◦C).

Thecharacteristicremanentmagnetization(ChRM)forsamples

age(usingadecayconstantof1.39×10−11yr−1);andtheconsis- displayingnearlylineardemagnetizationtrajectorieswasisolated
tenc yofis ot opica gesattest st othe e xcellentpre serva tion ofthe usingprinc ipal-co mpon entanalysis(PCA )(Kirschvink ,19 80).The
dykes . best-fi tlinewasusedifdefin edbyatl eastth reeconsecut ivedem ag-

Magnetic surveys show both positive and negative anoma- netizationstepsthattrendedtowardtheoriginandhadamaximum
lies associate d with the W idgiem ooltha d ykes (e.g., Tuc ker and angleofde viatio n(M AD)les sthan2 0◦. Forth em ajori ty ofdykes,
Boy d, 1987), su gges ting the presence of dual-p olarit y rema nent the C hR M was iso lated w ithi n a n arrow (1 0–2 5◦C) tem pe rature
magn etizatio n. The only pr evious pa leo magnetic stu dy of the ran geabov e520 ◦C(Figs .3–5).T he meand irectionswe recalculated
Widgiemooltha dyke swar m(Evans, 1968)foundrem anenc edi rec- using Fisher statist ics(Fi sher, 195 3).As itemeanw asac ceptedfor
tions of both p olarit ies from four indep enden tly cooled d ykes. furthe rcalcu lationsif itwaso btained f rom three orm oresamp les,
Howe ve r,that studywas confin edto alimitedareain thesou theast- thecon fidencecircle ( ˛ 95)w assmalle rthan 15◦, an dthep recision
ernYilgar n(Fig .1),it did notempl oy m odern dem ag neti zationand par ameterkwa sgrea terth an5 0.

a b

900 ) 15
u
m M /M = 0.143
rs s
e
m H = 159 Oe
c
(
nits) 600 ent Hc = 408 Oe
u m 0
SI Mo
κ ( 300 c
eti
n
g
a
0 M -15
-200 0 200 400 600 -0.5 0.0 0.5

Temperature (oC) Magnetic field (kOe)
Fig.2. (a)Atypicallow-fieldthermomagneticcurve((cid:2)(T))measuredinargonfrom−192◦Cto700◦CusinganAGICOKLY-4SmagneticsusceptibilitymeteratYaleUniversity.
The (cid:2)( T)cu r veindic atesthep resenceofasingl emag neticp hasewith aC uriet empe ratureb etw een57 0–58 0◦ C(low- Tititano magnetit eormagnetite ).The pr esenc eofnearly
stoic hiom etric magnetit eis furthers up p orted byachar acteris ticp ea kobs ervedatabo ut−153◦ C,associate dwith theVerwey(193 9) transition. (b)A typicalm a gnetic

hysteresis loop indicating pseudo-single domain behavior of the sample. Abbreviations are Hc, coercivity; Hcr, coercivity of remanence; Mrs, saturation remanence; Ms,
saturationmagnetization.
A.V.Smirnovetal./PrecambrianResearch224 (2013) 11–22 13

a b

510
540
590
585
c d

525
545
575
585
e
Fig.3. PaleomagneticresultsfromtheBinneringiedyke.(a–d)Typicalorthogonalvectorplotsofthermaldemagnetization(vertical/horizontalprojectionsshownby
oinpdein ca/fit elletedm sypmerbaotulsr)e. Es qteupasl-(ainre ◦aC p).lo(e ts) sThh oewsi tthe-em acecaenp dteirde pcta iloenosmfra ogmnetthicis d sirtuecdtyio(ncisr (colep se)na snqduf arroems) Ea vnadn tsh(e1ir9 m68e) a(ntr (isaonlgidle csi)rculsee)d wt iothc athlceu 9la5t%e cthonefimdeeannc ed ciriercclteio (n˛9(5s )t.a Nr)uamnbd eirtss
˛95circl e (solid line) for the Binner ing ie d yke (Table 1). Open sq uare show s the directio n fro m Sit e K (Ev ans, 19 68) collect ed fro m baked gr anit e abou t five me ters fr om t he
Binneringiedyke.Althoughbasedononlytwoindependentlyorientedsamples(Table1),thesiteKdirectionhintsattheprimaryoriginofthedykemagnetization.

3. Results geomagneticfieldattheArchean–Proterozoicboundary(Smirnov

etal.,2011).

Eighteen sites from thirteen separate dykes yielded well- Although Evans (1968) found consistent directions between
defined (˛ <15◦ ) site mean dir ections, i ncludin g twelve sites dykes and b aked g ranite h ost ro cks at two sites, thos e are not
95

(from eight different dykes) that yielded directions consistent strictlypositivebaked-contacttestsbecausetheylackstable,dif-

with those reported by Evans (1968) (Fig. 6a, Table 1), and the ferentresultsfromunbakedhostrocks.Weaugmenttheaggregate
comb inedd atasetyie lds apale omagn eticp ole at10. 2◦S, 159 .2◦E Widgi emoolth a da taset wi th po sitive bak ed-conta ct t ests con-
(A =7.5◦ ). The an gular di spersion of vir tual g eo magnet ic poles firmingtheprim aryageo fmag netizatio ninthesedykes .First isthe
95
(VGPs ) S=( 11.7 ±1.9)◦ f rom the W idg iemool tha dykes is lower inverse bak edconta ctt est fromNarrogin q uarry (Site1 )nea r the
thanth at o f15– 17 ◦observed att hecomparablel atitude ba ndfor western endo ftheBin neri ngieD yke,whe rethe dyke isc ross- cut

the last 5 million years (e.g., Johnson et al., 2008). However, the byathin,younger,east-westdykebearingaremanencedirection

lowSvalueisconsistentwiththesystematicallylowVGPdisper- identical to those of the Marnda Moorn large igneous province

sionvaluesmeasuredfromotherreliablepaleomagneticdatasetsof (LIP), also known in the Narrogin region as Wheatbelt or Fraser

theNeoarcheanandPaleoproterozoicage,suggestingmorestable dykes(Pisarevskyetal.,2003;WingateandPidgeon,2005)(Table2;
1
4
Table1
anS uubmmetmbw eaerre oyn fo- ssfa iptmea plmeloeesma nua sgdenidree ttcoict i coraenlsc.u u(cid:3)llptas,t ˚efr otPhmaer etph atehle eWo lmaidtaigtgiunedmeetoi aco ndltdeh claol indngyaikttieuosdn. e S( Diotfe) taIhDned: v niinurctmulibanlea grtsieo odnme (naIo)g;t ne˛e 9tt5hicea pnoodril gek i(anVraeGl tsPhi)t.ee A s99 5c5o%al lnceodcn tKefi dad refeon rtc hethe c i9isr 5cs%lteu c daonyn,d fil etdhteteen rccsoe dncecirencnolettre aa tnthidoe nt sh pietae crsao fmnroceemtne trEr vfaoatrino psn a( 1lpe9aor6ma8m)a.g e(cid:3)ntsee, rt˚ ifcos dra iVrreeG ctPth ideo insssitt.r eiBb l uaistt iittohuned. en uamndb elorn ogfi tsuitdees . uNs eisd tfhoer
Site ID (cid:3)s(◦S) ˚s(◦E) N B D (◦) I (◦) k ˛95(◦) (cid:3)p(◦N) ˚p(◦E) K A95(◦)
Binneringie 27 33.0 116.95 4 258.9 −64.3 95 9.5 16.3 342.1
1 32.9 4 117.11 11 248.1 −62.3 731 1.7 08.6 339.9
3 32.49 118.54 8 272.3 −59.8 1212 1.6 22.1 353.3
26 31.80 121.42 9 236.2 −64.1 1607 1.3 2.8 336.8
9,25 31.53 122.32 19 225.1 −57.6 71 4.0 –8.2 336.3
C 31.7 121.7 4 246.0 −75.5 26 –e 17.9 327.9
I 31.7 121.7 4 233.5 −68.0 160 –e 5.2 332.2
Mean–BinnerinKg ai,edDykeb 31.6 122.1 2 7 224445..58 −−6650..40 2755 –7e.0 9.3 338.3 38 9.9 A.V
Mean – Binneringie Dyke(thisstudyonly)b 5 247.8 −62.7 75 8.9 8.3 341.6 35 13.1 .Sm
Jimbe rlana 57d 3322..1168 112211..7341 77 211370..43 −−6472..18 12377 151..29 −1.3 324.8 irnov
FED 333222...211 111222121...807 533 222746830...505 −−−666916...505 1161969 −––eee 22860...607 333534780...307 etal./P
Mean–JimberlaGnaDyke 32.1 121.8 3 5 225521..56 −−6687..08 17616 –9e.2 1143..79 333490..81 28 14.8 recam
Other dikesc 11 31.32 121.59 6 262.7 −65.2 562 2.8 17.9 346.6 brian
14 30.18 121.97 16 242.5 −56.4 76 4.3 −1.0 347.1 Re
15 29.46 121.93 8 264.8 −68.8 146 4.6 19.9 342.4 sea
17 dKingoftheHillDyke 28.63 121.33 5 354.6 −81.4 51 10.9 rch
121809ddd 222866...033686 111222000...555299 1770 323395592...504 −−654640...214 15240450 1242...705 224 (20
2221 d 2278..8043 111199..9425 86 232586..39 −4681..52 178997 42..04 −7.0 333.7 13) 11
23CelebrationDyke 30.90 121.97 5 238.1 −67.6 172 5.8 6.2 335.3 –2
24 RandallsDy ke 30.90 122.38 7 266.1 −68.1 866 2.5 21.3 344.1 2
Ad, f 30.9 122.0 3 239.0 −67 25 –e 6.1 335.9
H 32.1 121.8 7 51.0 73 .5 86 –e 10.7 325.6
TotalWidgiemoolthamean–Thisstudyonly 8 245.3 −65.5 92 5.8 8.2 336.0 27 10.9
Thiss tudyplusEvans (1968 ), 2po larities g 9 247.5 −66.6 117 4.8 10.2 339.2 48 7.5

a Samplefrombakedgranite5mawayfromtheBinneringieDyke(Evans,1968).Notusedforthemean.

b NositemeandirectionwascalculatedfortheBinneringiesites2,4,and8,whichyieldedscattereddirections.

c NositemeandirectionwascalculatedforSites12,13,and16,whichyieldedscattereddirections.

d Sitenotusedforcalculationofthemeandirections.
e ˛95 is no t pro vid ed in Evans (1 968 )
f SiteA(Evans,1968)isequivalenttoourSite23andwasnotusedforcalculationofthetotalmeandirection.

g TheSiteHdirectionwasinvertedforthetotalmeancalculation.
A.V.Smirnovetal./PrecambrianResearch224 (2013) 11–22 15

Site 7
a c
N
0
500
5
4
-
E
0
9 D
W 575 E - FG
7
S
-45
b
-45 0
180
0
Fig.4. PaleomagneticresultsfromtheJimberlanadyke.(a)Atypicalorthogonalvectorplotofthermaldemagnetization(vertical/horizontalprojectionsshownbyopen/filled
sym bo ls)fromSite7.N umber sindi cate temperatu reste ps( in ◦C).(b) Acceptedp aleom agne tic directio ns(opensquares) andtheirmean(sol idcircle)wit hthe9 5% confidence
circle (˛9 5) for Site 7 (equal-ar ea projec tion). (c) The mean direct ion for the Jim berlana dyke (open squa re) an d its ˛95 area (soli d circl e) calc ulated from Sit e 7 ( this study)
andSitesD,E,F,andG(Evans,1968)(Table1).

Fig.6b).AlthoughtheyoungerNarrogindykeisundated,itsorien- extensionoftheMuggamurraswarm,apartoftheMardnaMoorn

tationandpaleomagneticdirectionssuggestaMarndaMoornage LIP(Myers,1990;WingateandPidgeon,2005)intothenorthern

thatwouldimplytheBinneringieremanencepre-dates1.2Ga. Yilgarn. Alternatively, these sites may represent Widgiemooltha

An additional positive baked contact test that was conducted dykesthatrecordedtransitionalfielddirections.OneofJimberlana

atBooylgooHills(Site21)whereaWidgiemoolthadykeintrudes dykesites(Site5)resultedinameandirectionthatissignificantly

an Archean lava sequence provides more direct evidence of a different from the consistent directions obtained from the other

primary magnetization in the Widgiemooltha swarm (Table 2; Jimberlana sites (Table 1; Fig. 4c). We speculate that this direc-

Fig.6c).ThesedataindicatethattheBooylgoodykecarriesapri- tionmayrepresentalocalremagnetizationevent.Finally,Sites19

mary magnetic remanence, whose direction is identical to other and22yieldedwell-definednortherlydirectionswithintermediate

WidgiemoolthadykesandthusconfirmsthattheWidgiemooltha upwardinclinationpossiblyreflectingarecentfieldoverprint.

grandmeanpoleisrepresentativeoftheYilgarncratonat2.41Ga.

ThecombinedresultfromourstudyandthatofEvans(1968)mer- 4. Discussionandconclusions
itsa perfect7 pointc lassifi cat ionon Van der Vo o’s(1 990)Q- scale

ofpaleomagneticreliability. A moderate to high paleolatitude for Yilgarn is directly com-
Six additional sites yielded site-mean directions significantly parab le to that of the Zimbabwe cra ton , for wh ich recent U–Pb
differe ntfromthe Widg iemoolt hagrandm ean(Table 1).Themean datinga llo wsre con stru ctionat2.4 1Gaacc ord ingtot hepub lished
direction sfrom th reedykes(Sites 17,18 ,and 20)are cl ose tothe Sebang a Poor t Dyke virtual ge oma gne tic pole ( Mu sha yandebvu
Fraser dyk e me an (Pi sarevs ky et a l., 2 00 3) an d to th e dire cti ons etal.,19 95;see Appe ndixA) .Themoderat eunc ertaintiesinboth
yielded byth eintru dingthindy ke atN arrogin (Ta ble 2;F ig.6b)thus po les permit ava rietyofal low able juxtaposit ionsbetween the two
offering a poss ibilitytha tth esed yk esmayre presen ta ne astw ard craton s,whic h wepro po sewerej oinedtogether inasupe rcra ton

Table2

SummaryofpaleomagneticdirectionsusedforthebakedcontacttestsshowninFigure6b,c.Nisthenumberofsamplesusedtocalculatethepaleomagneticdeclination(D)
and inclin ati on (I); ˛95and k are the 95 % co nfid enc e circle and th e con centrat ion param eter fo r p aleo magnet ic direction s.
Site ID N D (◦) I (◦) k ˛95(◦)
TotalWidgiemoolthamean(Table1) 9 247.5 −66.6 117 4.8

Narrogininversebakedcontacttest
Site1(>5 mfrom thein truding dyke) 11 248.1 −62.3 731 1.7
Site 1 (<5 cm from the intruding dyke ) 1 322.8 −72.5 – –
1 325.7 −73.2 – –
Sampleswithintheintrudingdyke 1 326.3 −61.6 – –
1 331.1 −63.4 – –
1 355.7 −58.9 – –
FraserDykemean(Pisarevskyetal.,2003) 23 332.6 −74.1 110 2.9

BooylgooHillsbakedcontacttest
Widgiemo olth adyke 3 265.4 −60.0 44 18.7
Archeanlavasat 3mformthecontact 7 266.1 −64.8 54 8.3
Archean lavas at >9 m from th econtac t 15 333.8 −40.0 25 7.2
16 A.V.Smirnovetal./PrecambrianResearch224 (2013) 11–22

Site 11 Site 14
a b
90
0 -
9
-
N
N
555 -45
-45
535
575
W E

0
W 600E 0 180
180
S S
Site 15 Site 21
c d
90 -90
-
N 500 N
535 W E -45
-45 585
0
565 0 180
W E 180
S S
e Site 23 f Site 24

0 0
9 9
- -
N N
475
525 -45 -45
0 585 0
W 565 E 180 W E 180
S S
Fig.5. PaleomagneticresultsfromtheotherdykesusedtocalculatethegrandWidgiemoolthadirection(Table1).Typicalorthogonalvectorplotsofthermaldemagnetization
(ver tic al/horizontalpr ojectio nssh own byop en/fil ledsy m bols).Nu mbe rsind icatetemperatu resteps( in◦C). Equ al-area plotsshow theac cepte d paleoma gneticdirections
(open squares) and their mean ( solid ci rcle ) with the 9 5% confid ence circl e (˛95) fo r each site.
“Zimgarn”(Fig.7).Weemphasizethatuntiladditionalpaleomag- thesweepingpatternofyoungercratonstabilizationfromwestto

neticdataareobtainedfromthetwocratons,aprecisemodelfor eastacrossZimbabwe(JelsmaandDirks,2002).Also,sinistraloff-

Zimgarnwillremainelusive;nonetheless,wetentativelychoose setonmajorterrane-boundingshearzonesintheeasternYilgarn

apreferredjuxtapositionguidedbyseveraladditionalgeometric (Czarnotaetal.,2010)canberestoredtoaproto-Zimgarnconfigu-

constraints.First,wealigntheSebangaPoortDykeapproximately ration(Fig.7b)thatnearlyalignstheproposedKurnalpimagmatic

paralleltotheWidgiemoolthadykes.Suchparallelismisallowed arcatca.2700MawithanotherproposedarccrossingZimbabwe

paleomagneticallybecausebothdykesuiteshaveremanencedecli- atthesametimeinterval(JelsmaandDirks,2002).

nationsroughlyinlinewiththedykestrikes.However,evenafter PostulationofaZimgarnsupercratonpresentsanoveldilemma

satisfyingthisconstraint,withintheuncertaintiesofthepoles,any for Neoarchean reconstructions: are dyke swarms or basement

relative position of the two cratons is permitted (i.e., they could provincesmorediagnosticoforiginalconnectionsbetweenextant

betranslatedaroundeachother).Second,werestrictsuchtrans- cratons?BleekerandErnst(2006)presentedthesimplegeomet-

lationsothatthe2.57GaGreatDykeofZimbabweprojectsoffthe ric argument that as vertical features, dykes provide piercing

marginofYilgarn,becauseithasnotyetbeenidentifiedinWestern pointswhosepositionsarenotsensitivetocurrentexposurelevel;

Australia. This constraint restricts the cratons to be either north in addition, analogy with more completely preserved radiating

orsouthofeachotherinpaleogeographiccoordinatesat2.41Ga dyke swarms (e.g., Central Atlantic Magmatic Province) shows

(Fig.7a).Third,wechoosethenorthernoptionforZimbabwerel- thattherift-relateddykescanshownearest-neighborconnections

ativetoYilgarn,becausethemorejuvenileNeoarcheancharacter evenamongsuture-boundedprovincesofdifferentage.Thisrea-

oftheeasternYilgarn(Czarnotaetal.,2010)isabettermatchfor soning provides a rationale for juxtaposing cratons with distinct
A.V.Smirnovetal./PrecambrianResearch224 (2013) 11–22 17

0o 0o
a b

Narrogin inverse
baked contact test
intruding dyke
< 5 cm from
intruding dyke
o o û Fraser dyke mean o
0 0 0 0
9 9 9 9
- -
> 0.5 m from
0o intruding dyke
Booylgoo Hills
baked contact test
Archean lava
180û 180û
c > 9 meters
from contact
Dyke
o o
0 0
9 3 meters 9
-
from contact
180o
Fig.6. (a)Stereonetofacceptedsite-meanpaleomagneticdirections(diamonds–thisstudy;triangles–Evans,1968).Thickandthincircles(alsoin(bandc))showthemean
sbheotww es e tnh-es imtee dainr eCchtiR oMn adnirde cittsi o˛n 9 5focro Sniet ec a1l c (uthlaet ˛ed95 friso mno at lsl haoccwenp)t.e Sdq d uiarreecst isohnosw (T tah bel eC 1 h)R. M(b )d Ti rheec tiinovnesr sf oe r b tahkee dsa cmonptleasc tt ateksetn a ftr tohm e Nthaer rBoigninne q ruinagrirey D(S yikt ee 1w; i stehein t e5x ctm). Dfrioa mmo tnhde
contactwiththeintrudingdyke.TrianglesshowtheChRMdirectionsyieldedbytheintrudingdyke.ThemeanFraserdykedirection(star)iscalculatedforSite1basedon

thepaleomagneticpolefromPisarevskyetal.(2003).(c)TheBooylgoobakedcontacttest(seetext).SquareandtriangleshowthemeandirectionfromtheArcheanlavas

sampledat3mand>9mfromthecontact,respectively.

cratonizationhistories,suchasYilgarnandZimbabwe,whichwere cratonic stabilization (Dirks and Jelsma, 1998; Frei et al., 1999).

previously grouped into separate supercratons (Bleeker, 2003). Second, Zimgarn and Vaalbara magmatic “barcodes” share no

The distinctive 2.41-Ga age match between Widgiemooltha and knownmatchesthroughtheinterval2.6–2.0Gainterval(e.g.,Ernst

Sebanga Poort dykes (Söderlund et al., 2010) may be diagnostic andBleeker,2010).Third,paleomagneticlatitudesaresomewhat

oftheiroriginalconnectiondespitethemismatchofcratonization distinctbetweenthetwosupercratonsat2.69Ga(althoughuncer-

agesforYilgarnandZimbabwe. taintiesarelarge;Table3).

TheZimgarn-likeassemblagemighthavebeenjoinedtoSuperia Zimgarn probably fragmented into separately drifting

fromNeoarcheancratonizationtoearlyPaleoproterozoicsepara- Zimbabwe and Yilgarn cratons sometime between 2.2 and

tion,asproposedbySöderlundetal.(2010),butpreciselycoeval 2.0Ga (Figs. 7c and 8a). Two rift- to passive-margin successions

paleomagnetic poles at 2.41Ga are not yet available from Supe- areevident:theMagondiSupergroupinZimbabwe,whichisolder

rior or adjoining cratons to test that hypothesis. In the Scotland than 2.0Ga (McCourt et al., 2001) and may indicate rifting as

sector of the North Atlantic craton, the older Scourie dykes are earlyas2.26Ga(Manyerukeetal.,2004);andtheYerridaBasinin
nearly th esam eage (2418+ 7/−4Ma ;He aman andTar ney,19 89) north ern Yilg arn ,thatextend s from asea rlya sca .2.2–2.1 Gain a

as the Widgiemooltha and Sebanga dykes, but Scourie paleo- sag phase (Windplain Group) to a rift stage (Mooloogool Group)

magnetic remanences have been reset by Laxfordian (ca. 1.7Ga) thatisonlyconstrainedtobeolderthan1.84Ga(Rasmussenand

metamorphism(Beckmann,1976;Piper,1979).Superialayatlow Fletcher, 2002; Pirajno et al., 2004). Despite the uncertainty of

paleolatitudesat2.45Ga,aswelldocumentedbydatafromtheMat- associatingpreservedepicratonicstratawithanorthernYilgarnrift

achewandykes(summarizedbyEvansandHalls,2010),butboth margin,thatmarginmusthavebeenestablishedpriortoaccretion

YilgarnandZimbabwelackpaleomagneticresultsfromthatolder oftheNarracoota/Dalgaringaarc(perhapsalsothePilbaracraton)

age. intheearlystagesofthe2.0–1.95GaGlenburghorogeny(Pirajno

TheVaalbarasupercratonhasrecentlybeenquantifiedforthe etal.,2004;Johnsonetal.,2011).BoththeYerridaandMagondi

2.8–2.7Ga interval (de Kock et al., 2009). It is likely that Vaal- successionscontaincarbonate–evaporiteplatformscharacterized

barawaspaleogeographicallyseparatedfromZimgarn,forseveral by the Lomagundi carbon-isotopic excursion; according to our

reasons. First, basement ages and cover successions of Vaalbara model,theyarecorrelativesuccessionsalongacommoncratonic

substantially differ from those of Zimgarn, including extensive margin (Fig. 7c), representing rifting of Zimgarn from other,

carbonate platform development at 2.6Ga (reviewed by Knoll originallyconjoinedcratonsyettobeidentified(possiblySuperia,

and Beukes, 2009), when Zimgarn was still undergoing final according to Söderlund et al., 2010). By 1.88Ga, Zimbabwe and
18 A.V.Smirnovetal./PrecambrianResearch224 (2013) 11–22

a
Sebanga
VGP
Widgie-
m ooltha
Zimbabwe
6
Z imbabwe
Yilgarn
Yilgarn

3
P aleo-Equator
2410Ma 2410 Ma
b c
Zimbabwe
v
v
v
v Zimbabwe
v
v
Yilgarn
v
v 2690Ma
v
v
2100Ma
Yilgarn
paleolatitude and orientationunknown
Fig.7. (a)Zimgarnsupercratonat2410Ma,achievedbyrotationofYilgarntoZimbabweaccordingtoEulerparameters(–47◦,077◦,+157◦),andpaleogeographicrestoration

accordingtotheWidgiemoolthadykepaleomagneticpoledeterminedherein.Greenstonebeltsareshadedlightgray.GD=GreatDyke(paralleledbytheUmvimeelaand

EastDykes),SPD=SebangaPoortDyke,BD=BinneringieDyke,JD=JimberlanaDyke.Youngermarginalbasinsandorogensarelabeledforreference,despiteanachronism.

Presentnorthisindicatedforeachcraton.Intheright-sidepanel,polesareshownwiththeir95%uncertaintyregions.VGP=virtualgeomagneticpole.(b)Tectonicelements

ofZimgarnat2690Ma.PaleolatitudesandorientationofZimbabweconformtotheReliancepole(Table3),withpolaritychosentominimizemotionbetween2690,2575,and

2410Ma.Yilgarnisdisplacedtorestorepost-2690-Masinistraloffsetonmajorterrane-boundingshearzones(simplifiedinthisillustrationbythegeneralizedshearcouple),

thusbringingproposed2690-Mavolcanicarcs(JelsmaandDirks,2002;Czarnotaetal.,2010)nearertoalignment.(c)Proposedcontinuouspassivemarginextendingfrom

theYerridamargin(Yilgarn)totheMagondimargin(Zimbabwe)atca.2100Ma.PaleolatitudeandorientationofZimgarnarenotconstrainedatthatage.

Yilgarn were widely separated from each other, as indicated by is so widespread across Kaapvaal at 2.06Ga (Reischmann, 1995;

the moderate versus low paleolatitudes, respectively implied by Cawthorn et al., 2006; Mapeo and Wingate, 2009; Prendergast,

theMashonalandandFrerepoles(Table3). 2012), has not yet been recognized in Pilbara, suggesting that

KaapvaalandPilbarasharesimilartectonostratigraphicrecords Vaalbara could have split prior to 2.06Ga. If so, then the 2.2Ga

through much of the early Paleoproterozoic Era, such simi- maficeventsinKaapvaal(Cornelletal.,1996)andPilbara(Müller

larity providing the original basis for the Vaalbara hypothesis et al., 2005) could document their initial separation. Unfortu-

(Cheney, 1996). However, the Bushveld-related magmatism that nately,amongthefourcratonsdiscussedherein,onlyKaapvaalhas

a b
Vaalbara SPD
Kalahari
breakup
craton Zimbabwe
50°
Zimbabwe
assembly
SPD Kaapvaal
Pilbara
40°
Kaapvaal Yilgarn
BD
JD 30°
WestAustralian craton

20° BDJD assembly
Zimgarn
Yilgarn
breakup 10°
2050 Ma 1800 Ma
Pilbara

Fig.8. Kinematicsequenceofthetradingcratonicpartners.(a)VaalbaraandZimgarnintheearlystagesofbreakup,2050Ma.OnlyKaapvaalisconstrainedpaleomagnetically

atthisage(Table3),butothercratonsarepositionedforconsistencyinthekinematicsequence.(b)KalahariandWestAustraliancratonsafterfinalassembly,1800Ma;

paleolatitudesandorientationsofbothcratonsareconsistentwiththeca.1800MapoleslistedinTable3.
A.V.Smirnovetal./PrecambrianResearch224 (2013) 11–22 19

Table3

PaleomagneticpolesbearingonZimgarn,Vaalbara,andtheirtransitiontowardunitedKalahariandWestAustraliancratons.

Craton/Paleomagnetic pole Age (Ma) (cid:3)p(◦N), p (◦E) (cid:3)r(◦N), r (◦E) A95(◦) 1234567 Q Test (cid:3)(cid:5)(◦) Reference
Zimgarn: ZimbabweRef.

Reliancemiddle-temp. 2690 44,128 44,128 17 10111015 f 21 YoshiharaandHamano(2004)

GreatDykegrandmean 2575 24,057 24,057 9 11111005 C,c 37 SeeAppendixA

SebangaPoortDyke(VGP) 2410 17,006 17,006 14 10011003 C 45 SeeAppendixA
Widgiem oolth adike s 2410 −10, 159 06, 013 8 1111111 7 C ,c 50 This study

Vaalbara: KaapvaalRef.
Allanridge basaltcombined 2685 −70,346 −70,346 6 11111016 G 36 deKocketal.(2009)
Rykoppies E-Wd ikesonly 2685 −63, 339 −63, 339 7 1111101 6 C 37 Lub nina et al. (2010)
Woongarra /We eliWo lli 2450 −50, 220 −78, 235 5 1111100 5 g 11 Evans(2 00 7) (abstractonly)
Ongeluklava 2220 −01, 101 −01, 101 5 1111101 6 G 17 Evans etal.(1 997)

WaterbergUBS-I 2055 37,051 37,051 11 11111005 F,G 28 deKocketal.(2006)

Bushveldcomplex 2050? 19,031 19,031 6 11111106 f 49 Lettsetal.(2009)

Transitionalpoles: KaapvaalRef.
WaterbergU BS-II ∼1950? −11,330 −11,330 10 01111105 c,g 33 deKocketal.(2006)
BlackHills dikes ∼1900? 09, 352 09,3 52 5 0111110 5 C 42 Lub nina et al. (2010)

Mashonaland+Mazowe 1880 07,338 18,016 7 11111106 C 48 Hansonetal.(2011)

Waterbergsills&Soutp.lavas 1875 16,017 16,017 9 11111106 C 50 Hansonetal.(2004)
Frereironf orm ati on(Yil garn) 1890 −45, 220 2 1110111 6 (f ) 10 William se ta l.(2004)

Kalaharicraton:
Soutpan sbergUBS-II ∼1800? 25,008 25,008 8 01111105 M 39 deKock(2007)

WestAustraliancraton:
Hame rsleyover printHP3 ∼1800? −35,212 −35,212 3 01101014 – 12 Lietal.(2000)

+(cid:3)N1ro5(t◦e7N:◦ (cid:3))),,p rP((i◦◦lEbN)a):r, papa t(lo◦eEoK)ma: apapagvlneaeoatlmicina pgVonlaeeat rliboc atparoatel(edd iienn Kttoho ecthk reee frteearfele.n,re2cn0e c0fer9a ;fmr−aem5 9oe◦f ,itn2hd5ei2 cs◦aa,tm+e9dp3 lae◦b)d,o Zuvienm ietbasa.cbhw seecttoioKna. aRpovtaatailoinn ptahreatmraentesirtsi:o Ynialglaprenr itood Z(iHmabnasbownee tina lY.,il2g0a1rn1 ;(t−h6is9 s◦,tu0d5y1;◦ ,−−4379◦◦, )0.77◦,
A95(◦): F isher’s (19 53) confi de nce cone radi us.
1–7andQ:reliabilitycriteriafromVanderVoo(1990).Thefourthcriterionofafieldstabilitytest,ifpositive,isabbreviatedasfollows:C=baked-contacttest,c=inverse

baked-contacttest,F=soft-sedimentfoldtest,f=tectonicfoldtest,G=intraformationalconglomeratetest,g=conglomeratetest,M=magnetostratigraphytestofstratabound

reversalsinsequence.Intheseabbreviations,uppercasesymbolsindicateprimarypaleomagneticremanence;lowercasesymbolsindicateancientremanencethatmightbe

primarybutisnotyetdemonstratedassuch.
(cid:3)(cid:5)(◦)=p aleo la titud ec omputedforre fer encelocalitiesintheLimpopobeltbetweenZimbabweandKaapvaal(−22◦N,029◦E)andintheCapricornorogenbetweenYilgarn
andP il bara(−24◦N, 120◦E).

reliablepaleomagneticpolesfromthe2.2to1.9Gainterval(Table3; AppendixA.

Fig.8a).

Followingthesebreakupevents,thefourcratons“tradedpart- DiscussionofsomepaleomagneticpoleslistedinTable3.

ners”toreassembleintotheircurrentlypreservedaffinitieswithin GreatDykeandrelatedintrusions.TheGreatDyke(GD)wasone

West Australian and Kalahari cratons (Fig. 8b). Yilgarn and Pil- ofthefirsttargetsforpaleomagneticstudyofPrecambrianrocks.

baracollidedviasuturingintheCapricornorogen,mostlikelyat ThepioneeringworkofMcElhinnyandGough(1963)ontheGD
abou t1.95Ga (Joh nsoneta l.,2 011 ).Zimbabw eandK aapva aljoin ed gene ratedame anpa leo pole(20.7◦ N,62 .6◦E,K =27.5, A =11 .7◦;
95

togetheracrosstheLimpopobelt,possiblyaslateasca.1.8Ga,by sevensites)corroboratedindependentlybyNairn(1963).Amore

wayoflarge-scalesinistralshearingsuggestedbydiscordantpale- regional study by Jones et al. (1975) discovered partial remag-

omagneticdatabetweenthetwocratonsat1.87Ga(Hansonetal., netizationonitssouthernandsoutheasternextensions.Southof

2011). the GD, the Msiningira Dyke generated a mean from four site-
Ourhypothesizedtectonicmodel,albeitspeculativegiventhe gen erate dVG Psat20.4◦ N,52.1 ◦E,K=13.6 ,A =25 .8◦(Jo nes etal.,
95

modestnumberofavailabledata,demonstratesthepossibilitythat 1975). Parallel to the GD, the East Dyke has also yielded robust

supercratonshadrichtectonichistoriesofinteractionpriortotheir paleomagneticresults(McElhinnyandGough,1963;Jonesetal.,

amalgamationsintomorefamiliarcontinentalassociationsembed- 1975)withasimilar,assumed-primaryremanencedirection(mean

dedwithinyoungersupercontinents.Decipheringthosehistories of three low-dispersion VGPs from sites 9, A, and X), yielding a
will require continue defforttoobtain precisegeoc hrono logicand po le at 3 2.3◦N, 52.7◦E, K =182 .5, A =9.2 ◦. A more re cent pape r
95

reliablepaleomagneticdatafromthetectonicallystableinteriorsof byMushayandebvuetal.(1994)describedpaleomagneticresults
exposed Archeanterrai ns.In tegra ted U–Pbbadde leyite datingan d fro m many new sit es in the Um vimeela D yke, ∼15km w est of

paleomagneticstudyfromearliestProterozoicmaficdykesconsti- andparalleltotheGreatDykealongmostofitslength.Although

tuteapowerfulapproachtoward“fulfillingWegener’slegacy”and Mushayandebvuetal.(1994)computedameanoflocalpaleomag-

producingarobustglobalpaleogeographyfortheentirelatterhalf neticdirections(declinationandinclination),theca.550kmdyke

ofEarthhistory(ErnstandBleeker,2010). lengthindicatesthatamoreaccurateapproachwouldbetoaverage
theVG Pscalcula tedf ro meac hsite.Th isyieldsa positio n of (20.8◦N,
Acknowledgments 61.8 ◦E; K =60.3, A95 =4.8 ◦), ca lcul ated from 1 6 well c on strained

Umvimeelasites,includingthosefromearlierstudiesbutexclud-

This research was funded by the David and Lucile Packard ingsitesUS(alsoexcludedbyitsauthorsduetolocationwithinthe
Founda tion, and the N ational Scie nce Found ation . We th ank the def orme dn orthe rnendof the dy ke)and UM (e xcluded hereind ue
Geological S urve y of Western Austral ia, the Unive rsity of W est- toits˛95 valueof2 3.3◦ ).T his mean isre gard edasaVG Pinit self,
ernAustral ia,Mich ae lWingate ,MartinVa nKr anendonk, and Peter i.e ., n ot averagi ng out th e pal eosecu la r variatio n o f Earth ’s mag-
Caw oodforlo gisticals upport.M ichaelW ing ate,DanfordM oor e,Ian neti cfie ld,because of itslo wdispersion ofdirecti on samong sites
Rose,an dT aylorKili anprovid edassis tanceint hefield. Wetha nk separ atedb yasmuc ha s5 00k m.Resultsf ro mthePopo tekeGa bbro
twoa nony mous review ersforth eirusefulc om me nts. arecitedb yW il sonet al .(19 87) andMu shaya nde bvuetal. (1994),
20 A.V.Smirnovetal./PrecambrianResearch224 (2013) 11–22

butarenotfullyreportedinthepublishedliterature.Wetherefore thegeochronologystudy,U–Pbbaddeleyiteupper-interceptages
com put eth eme anpaleo ma gne ticpolefr omthefou rdy ke-mean ran ged from 2685. 5 ±9.3 Ma, to 2661.9 ±7. 7Ma (decay con stant
resultsca lcul ateda bove(23.6◦N,5 7.4◦E ,K=1 07.6 ,A =8.9◦,N=4 uncerta inties includ ed in the qu oted er rors) . Th e youn gest age,
95

dykemeanscomprisingdatafrom219samplesdistributedamong however,derivesfromadykethatwasnotsampledbyLubninaetal.

30 sites), as the best representation of the time-averaged paleo- (2010)forpaleomagnetism;thereforetheolderrangeofages,atca.

geography for Zimbabwe craton during emplacement of the GD 2685Ma,ispreferredforthepaleomagneticpole,whichincludes

swarm.Jonesetal.(1975)foundthatregionalmetamorphicover- threedykesintheRykoppiesarea.Thebaked-contacttestforoneof

printingfromtheLimpopobelt,tothesouthofthesitesdescribed thesedykesintograniticbasementappearspositive,thoughitisnot

above,producedadifferentpaleomagneticmeandirection,which fullydescribedwithstatisticsofmeandirectionsfromthebakedor

thusconstitutesaregionalinversebaked-contacttestfortheGD unbakedhostrocks.AfourthdykenearBarbertonhassimilarpale-

pole,whichisthusolderthanca.2000Ma(Morgan,1985).Inaddi- omagnetic remanence to the type-Rykoppies dykes, and a strike

tion, the remanence directions from sites in the Sebanga Poort similartothatofanESE-WNWdyke(BCD1-04)inthesamearea

Dyke,asdescribedbelow,aredistinctfromthoseoftheGDswarm, thatwasdated—althoughwithlargeregressionerrors—byOlsson

despite geographic proximity. This also argues against regional et al. (2010) of approximately Rykoppies age. Although Lubnina

remagnetizationsubsequenttoemplacementoftheSebangaPoort etal.(2010)combinedtheirRykoppiesresultwiththatfromafifth

Dykeatca.2410Ma(Söderlundetal.,2010).TheGDpoleisthus dykeinanisolatedexposuremorethan300kmtotheSouthofthe

considered to be “key” for the Zimbabwe craton at ca. 2575Ma. typeRykoppiesarea,wesuggestthatsuchcorrelationismerelyten-

Usingthe“Q”-scaleofreliability(VanderVoo,1990),theGDmean tative,especiallygiventhedyke’snearlyE–Wstrikethatishighly

paleopoleonlyfailstosatisfythecriteriaofdualremanencepolarity oblique to the fanning array defined by dated Rykoppies dykes

andsimilaritytoyoungerpaleopoles(1.2GaPremierkimberlites; (Klausenetal.,2010).Excludingthedual-polarityremanencedirec-

DoppelhammerandHargraves,1994). tionfromthatsite,wecalculateameanoffoursingle-polarityVGPs
SebangaPoor tDyk e.Thetype representativeoftheSebangaPoort at(– 63.0◦ N,33 8.7◦ E,K =162,A =7.2◦) .
95

Dyke (SPD) swarm has been studied paleomagnetically by both Ongeluklavas.Werejectthesinglesiteofoppositepolarityin

Jones et al. (1975) and Mushayandebvu et al. (1995), and dated thedatasetofEvansetal.(1997)becauseithasaslightlydiscordant
at 24 08 ±2 Ma, us ing U –Pb on baddele yite b y Söder lund et al. dire ctionan d anind et erm inate reversal st est. I talsode rivesfrom

(2010). At more northerly sites in this intrusion, the SPD gave a anintrusionthatisonlyspeculativelycorrelatedasafeederdyke

southeasterlyandupwardcharacteristicremanencedirectionthat forthelavas.

is distinct from that of neighboring sites in the Great Dyke and FrereFormation.Althoughapositivefoldtestforthe‘A’com-

EastDyke.Forthisreason,aswellastherecognizableoverprinting ponent is claimed by Williams et al. (2004) according to some

by a ca. 2000-Ma Limpopo overprint of nearly opposite polarity statisticaltests,inspectionofthedatatablesandfiguresshowsvery

inmoresoutherlysites(Mushayandebvuetal.,1995),wepropose similar levels of precision between the in situ and tilt-corrected

thatthenortherlySPDsitescarryaprimarypaleomagneticrema- means.TheFrereFormationhasmorerecentlybeendatedtoca.
nenc e.T hemeano ffou rVG Ps(17 .2 ◦N,6.1◦E ,K=42.5,A = 14.3◦) 1890M aby U–Pb oninterbe dde dvolc anicash units (Rasm us sen
95

is itself considered to be a VGP because it derives from a single etal.,2012).

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and B ubi D yk es, the fo rme r of whic h wa s dat ed at 2512 ±2Ma References

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ied p aleo magnet ically b y Ba tes and Jo nes (1996) . One, near t o but T2i.6m Ge aM. Ianr:k Herasn oskfiC, rEu.,s tMa leEr tvaonleunti,o Sn.., CR RämCöP,r eTs. ,s ,VTuaoyllloor, Ja. n(dEdFsr.a),n D ciysk,eL oSnw daornm,ps p–.
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nteusmt e(sriotue sSMDa1s)h. oTnhael aontdhesril, lslo,sctartiekde swNiEth(inno ttehteh iosudticsrtoinpc trieogniofnro mof Buchtearno,zKo.Lic.,. LCeaCn haedmi ainn aJonutr,nA a.Nl o.,fv Eaanr Bthre Secmi eennc,e Os .4,260, 31621.M –3a7ll9e.ydiab as edy kesofthe
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contin entalr econstruc tionsin thee arly Paleoproterozoic. Can adianJourna lof
(tdioynk eofs iotpepSoDsi2tei nptoruladriintyg rseillaltsiivtee tMo St1h5a)t. oAf tfuhlel ysipllo wsithiivceh bita kceudts- CawEtharotrhn S,Rci.Gen.,c Eeasl e4s9,,H 4.V35.,–W45al4r.a ve n,F. ,Uken ,R.,Watkeys,M.K. ,2006.The Bushve ld
contac ttes tatt hatlocality isi ndic atedby ac ross- overofm agnetic Compl ex.In :John son,M .R.,Anhae us ser,C .R., Thomas,R .J.(E ds.),T heG eologyof
SouthAfr ica. Geologic alSoc ietyofSouth Afr ica/Coun cilf orGeo scien ce,Joha n-
directions at a reasonable distance of conductive heat loss from nesbu rg/Pret oria,pp.26 1–281.
thedykeintoitscountryrock(BatesandJones,1996),indicating Cheney,E.S.,1996.S equ encestratigraphyandplatetectonicsignificanceofthe
no remag neti zat ion of th e sil l since the time o f dyke emplace- Tran svaa lsucce ssionofso uthernAfrica andi tsequ ivalentin WesternAu str alia.
me nt (positive inver se bake d-co ntact test for th e s ill) an d primary CornPerlelc,aDm.Hb.,ri Sacnh üRtetsee,aSr.cS h., 7E 9gl, i3n–g2to4n., B.L.,1 996. Th eOngeluk ba salticand esitefor-
remanenceinthedyke(positivebaked-contacttest).Theantipodal mat ionin Griqual and West,Sout hAf rica:s ubm arinealt eration ina2222 Ma
natureofth et wo direct ionsindic ateslikelypen econt emp oraneous Protero zoi csea.Precam brian Resea rch79, 101–123.
emplac em en t age s between the two intrus ions, separated in time CondGieeo, lKo.gCi.c, KRreö cnoerrd , ,Av.,o 2l.040480. W.Gh eeonlo Dgiidca Pll aStoec iTeetcytoonficAsm Beergicina?( ESvpiedceianlceP afrpoemr, tphpe.
byageomagneticfieldreversalbutnegligibleplatemotion. 281–294 ).

RykoppiesDykes.Lubninaetal.(2010)studiedseveralareasof Czarnota,K.,Champion,D.C.,Goscombe,B.,Blewett,R.S.,Cassidy,K.F.,Henson,P.A.,
pre-Transvaa l (pre-2 .65Ga) ma fic dykes in the e astern K aapva al Groen ew ald,P.B.,20 10.G eodynamics of theeaste rnY ilgarnCra ton. Precamb rian
Research183 ,175 –202 .
pcriaetso”nD. yInk ethseiisr tehaestsearmnmeoasst tshtuatdyd aatreeda, bayn EO–lsWso snweatrmal. o(f2 “0R1y0k)o.pIn- de Knoectki,s Mm.Oo.f , Ethvea nlos,w De.Ar.tDw., oDournlcaonndf, oHr.mC.i,t Bye-buokuesn,d Ne.dJ., sGeuqtuzemnecre,s J.o, 2f0t0h6e. WPaaleteormbeagrg-