the johannesburg dome, south africa: new single zircon u ... · stages; hence it became important...

Precambrian Research 108 (2001) 139–157

The Johannesburg Dome, South Africa: new single zirconU–Pb isotopic evidence for early Archaean

granite–greenstone development within the central KaapvaalCraton

M. Poujol a,b,*, C.R. Anhaeusser b

a Hugh Allsopp Laboratory, Uni6ersity of the Witwatersrand, Pri6ate Bag 3, Johannesburg WITS 2050, South Africab Economic Geology Research Institute, Uni6ersity of the Witwatersrand, Pri6ate Bag 3, Johannesburg WITS 2050, South Africa

Accepted 30 November 2000

Abstract

The Johannesburg Dome, located in the central part of the Kaapvaal Craton, constitutes one of the key areas tobetter understand the Archaean crustal evolution of this part of the craton. The dome comprises a variety ofArchaean granitic rocks intruded into mafic–ultramafic greenstone remnants. This study presents new precise U–Pbsingle zircon dating for seven different granitoid samples and an amphibolite dyke collected from the JohannesburgDome. A trondhjemitic gneiss sampled on the northwestern part of the dome yielded an age of 334093 Ma andrepresents the oldest granitoid phase recognized so far. This result has important implications with regard to the ageof the mafic and ultramafic greenstone remnants scattered throughout the dome as it implies that the greenstoneremnants are older than c.3.34 Ga. This initial magmatic episode, involving early greenstone development and theintrusion of trondhjemitic and tonalitic granitoids on the northern half of the dome, was followed by the emplacementof a 320195 Ma hornblende–biotite–tonalite gneiss in the south. Following the trondhjemite–tonalite gneissemplacement a further period of magmatism took place on the dome, which resulted in the intrusion of mafic dykesthat are manifest as hornblende amphibolites. The age of these dykes has yet to be determined quantitatively, but theyfall within the time constraints imposed by the age of the trondhjemitic gneisses (3340–3200 Ma) and later,crosscutting, potassic granitoids. These rocks, consisting dominantly of granodiorites constitute the third magmaticevent and occupy an area of batholithic dimensions extending across most of the southern portion of the dome. Thesouthern and southeastern parts of the batholith consist mainly of medium-grained, homogeneous, grey granodioritesdated at 312195 Ma. Their porphyritic granodiorite equivalents in the southwestern part of the dome yielded an ageof 311492.3 Ma. An age of 3117912 Ma, from zircons extracted from one of the mafic dykes possessing graniticmicroveins, provided confirmation of the timing of this third magmatic event. Lastly, pegmatites that crosscut allthese earlier granitoid events are younger than 3114 Ma and might be at least 3.0 billion-years old. These new dataprovide confirmation of the conclusion that the Witwatersrand Basin was deposited after c.3074 Ma on an Archaean

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* Corresponding author. Fax: +27-11-3393026.E-mail address: [email protected] (M. Poujol).

0301-9268/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.

PII: S 0301 -9268 (00 )00161 -3

M. Poujol, C.R. Anhaeusser / Precambrian Research 108 (2001) 139–157140

basement as young as c.3120 Ma. The data, combined with that from other parts of the Kaapvaal Craton, furthersupports the view that the evolution of the Craton was long-lived and episodic, and that it grew by accretionaryprocesses, becoming generally younger to the north and west of the c.3.5 Ga Barberton-Swaziland granite–greenstoneterrane situated in the southeastern part of the Craton. © 2001 Elsevier Science B.V. All rights reserved.

Keywords: U–Pb; Zircon; Archaean; Greenstone belt; Kaapvaal Craton

1. Introduction

The Johannesburg Dome, which is uncon-formably overlain by the sedimentary successionsof the Witwatersrand Supergroup, is one of thefew mid-Archaean granite–greenstone inliers ex-posed in the central part of the Kaapvaal Craton.With an areal extent of approximately 700 km2, itprovides a unique window through which theArchaean basement rocks in this part of the cra-ton can be examined.

Anhaeusser (1973) provided the first compre-hensive geological map of the dome. This workdescribed the Johannesburg Dome as a mosaic ofdifferent granitic rocks that had intruded an olderArchaean mafic–ultramafic ‘greenstone’ crust.The granitic rocks display distinctive field charac-teristics and variable geochemical, mineralogicaland textural properties. The oldest granitic rockscomprise a suite of tonalitic and trondhjemiticgneisses and migmatites that occupy most of thenorthern half of the dome (Fig. 1). The south-cen-tral portion consists mainly of a variety of homo-geneous, medium-grained granodioritic rockswhich, in the west, are somewhat coarser grainedand are commonly porphyritic in texture. Peg-matitic dykes and veins are also common.

Despite the fact that the Johannesburg Domeprovides an opportunity to better understand theArchaean history of this part of the KaapvaalCraton, very few geochronological data arepresently available (see later). The purpose of thiswork is to present new U–Pb single zircon agesfor the Johannesburg Dome. Seven samples repre-sentative of the different granitic rock types werecollected. These include three trondhjemiticgneisses from the northern part of the dome, onetonalitic gneiss from the south and three homoge-neous granodioritic rocks from other more centrallocalities shown in Fig. 1. In addition, a mafic

dyke cropping out on the northern part of thedome was also sampled (Fig. 2).

2. Geological and chronological settings

2.1. General geology

A variety of mafic and ultramafic rocks, manydisplaying affinities with komatiites, high-magne-sian basalts and tholeiites, were described as theearliest recognized greenstone rocks exposed onthe Johannesburg Dome (Anhaeusser, 1977, 1978,1992). Field mapping undertaken by Anhaeusser(1973) did not reveal the presence of any ancientgneissic crust predating these greenstones, theremnants of which have also been equated withsimilar rocks elsewhere on the Kaapvaal Craton(e.g. the c.3500 Ma rocks of the Barberton green-stone belt), that have been intruded, metamor-phosed and migmatized by successive granitoidevents as described by Anhaeusser and Robb(1981).

Regional mapping, coupled with selected de-tailed studies of key outcrops, such as the Nooit-gedacht migmatite platform (Fig. 2) seen in a riverexposure in the northwestern sector of the dome,led to the establishment of a field-based relativechronology of granitic emplacement events (An-haeusser, 1973, 1999). The earliest granitoid rocksinclude a suite of trondhjemitic and tonaliticgneisses (TTG’s), most of which occupy thenorthern half of the Johannesburg Dome (e.g.samples JHBD 98-8, 98-9, 98-10, Fig. 1). Expo-sures of similar rocks also occur on the southernedge of the dome (represented by hornblende–tonalite gneiss sample JHBD 98-1, Fig. 1) andunconformably underlie the Witwatersrand Su-pergroup sediments that dip to the south. Fieldrelations (Anhaeusser, 1973) suggested that the

M. Poujol, C.R. Anhaeusser / Precambrian Research 108 (2001) 139–157 141

Fig

.1.


Fig

.2.

Geo

logi

cal

map

ofth

eN

ooit

geda

cht

mig

mat

ite

plat

form

(aft

erA

nhae

usse

r,19

99).


TTG granitoid suite, which includes dioritic,tonalitic and trondhjemitic gneisses andmigmatites, may have been emplaced at differentstages; hence it became important to establish theisotopic ages of the various granitic phases distin-guishable on the basis of their mineralogical, geo-chemical and textural differences.

Following the emplacement of the TTG suite,an early mafic dyke event (sample JHBD 98-11)can be recognized on the dome (e.g. on the Nooit-gedacht platform, Fig. 2). These mafic dykes, nowrepresented by hornblende amphibolites, precededthe intrusion of the potassic granite suite thatoccupies most of the southern half of the dome(Fig. 1).

The potassic granitoids consist of a variety ofhomogeneous granodiorites that differ texturallyacross the dome. Homogeneous, grey, medium-grained granodiorites occur in the south-centraland southeastern sectors (sample JHBD 98-3, Fig.1) whereas coarser-grained, homogeneous, por-phyritic granodiorites occupy the southwesternsector (sample JHBD 98-5, Fig. 1). A furthertextural variation of the homogeneous granodior-ite suite is developed along the southern contactof the main potassic massif or batholith, adjacentto the hornblende–tonalite gneisses. Thesemedium-to-coarse-grained, pinkish granodioritesare represented in this study by sample JHBD98-2 (Fig. 1).

Fine-grained, homogeneous granodioriticdykes, considered to be genetically related to thepotassic granitoids described above, transgress thetrondhjemite gneiss–migmatite terrane on thenorthern half of the dome. Also transgressingthese gneiss–migmatite exposures are coarse-tex-tured pinkish pegmatite dykes that probably rep-resent the final stages of granitoid emplacementon the dome. The pegmatites, which are alsoencountered in the homogeneous granodiorites,were sampled for isotopic dating, but the fewzircons found in these rocks proved to be unsuit-able for this purpose.

The Johannesburg Dome has also participatedin various episodes of tectonism and epeirogenicuplift beginning in the early Archaean and extend-ing to post-Transvaal Supergroup times (�2250Ma), or even to post-Bushveld Complex times

(�2000 Ma). Shear zones, like the prominentnorth–northeast-trending structure shown in Fig.1, and many others like it recorded by Anhaeusser(1973), were reactivated by successive periods ofuplift and tectonic disturbance on the KaapvaalCraton (Anhaeusser, 1973; Hilliard, 1994). Noevidence could be found in support of a claim byRoering et al. (1990) that a series of northward-verging thrust faults were responsible for wide-spread ramping of granite sheets on the domeitself. Hence, in this paper, all the granitic rela-tionships discussed are regarded as in situ mag-matic and not structural in origin. What thrustingexists in the vicinity of the dome was probably ofa thin-skinned variety involving the supracrustalcover rocks, but not the granitic basement. Thedome was also intruded in Ventersdorp andTransvaal times (�2700–2224 Ma, Walraven etal., 1990) by numerous dykes. These include post-Transvaal mafic dykes, which preceded and ac-companied the emplacement of the �2060 MaBushveld Complex. Later intrusive events in-cluded the subalkaline and mafic dykes associatedwith the c.1300 Ma Pilanesberg Alkaline Com-plex, and mafic dykes linked to the early Meso-zoic Karoo igneous activity that occurred between190–170 Ma ago.

2.2. Pre6ious geochronological studies

Allsopp (1961) carried out the first geochrono-logical investigations using the Rb–Sr system onthe granitic rocks. He examined both whole-rockand separated mineral fractions from samples col-lected exclusively from the granodioritic phasesdeveloped in the central portion of the dome. Awhole-rock Rb–Sr age (recalculated with l=1.42×10−11 per year) was found to be 3132965Ma (Allsopp, 1961) with an initial 87Sr/86Sr ratioof 0.706090.0030 (Allsopp, 1964). Widely differ-ing apparent ages were obtained for the separatedmineral fractions and Allsopp concluded that thediscordance of the mineral ages was the result ofthe diffusion of radiogenic strontium from min-eral to mineral. Contrasting with this whole-rockRb–Sr age is a 207Pb/206Pb zircon age of 2585965 Ma (Burger and Walraven, 1979) obtainedfrom one of the granodiorite samples analyzed by


Allsopp. More recently, Barton et al. (1999) con-ducted a Rb–Sr, Pb–Pb and Sm–Nd study ongranitoid rocks from the Johannesburg Dome.The whole-rock Rb–Sr data on the granodioritesdefine ages at 31589179 and 3081933 Ma, re-spectively. The Pb-isotope data for the same unitsdefine ages at 3062926 and 3112914 Ma, whilezircon evaporation data define an age of 309393.2 Ma. These authors concluded that the gran-odiorites were emplaced �3090 Ma ago and werederived from a source between �3300 and 3500Ma old.

Very few data are available for the tonalitic–trondhjemitic gneisses. An U–Pb age of 3170934 Ma (Anhaeusser and Burger, 1982) wasdetermined from multiple zircons obtained from atonalite cropping out on the southern edge of theJohannesburg Dome. The least discordant iso-topic data were found to closely conform to a3200 Ma Wasserburg-type diffusion curve. Morerecently, Barton et al. (1999) obtained a whole-rock Pb age of 3001+132/−146 Ma for tonalitefrom the same sample locality. In addition, thistonalite yielded a whole-rock Rb–Sr age of23859127 Ma and a biotite Rb–Sr age of2321923 Ma.

3. Sampling

Seven samples of the different granitic phasesand one sample of the early mafic dykes werecollected from various localities on the Johannes-burg Dome (Fig. 1). The tonalitic–trondhjemiticgneisses (TTG) were sampled at three differentlocations; JHBD 98-1 is a hornblende–biotite–tonalitic gneiss cropping out along the southernmargin of the dome, whereas JHBD 98-9 repre-sents a sample of leuco-biotite trondhjemiticgneiss from the northwestern sector of the dome(Fig. 1). Samples JHBD 98-8 and 98-10 consist ofleuco-biotite trondhjemitic gneisses from theNooitgedacht migmatite platform (Fig. 2) de-scribed recently by Anhaeusser (1999). The sam-ple JHBD 98-11 corresponds to a mafic dykecropping out on the same platform. This exposurealso occurs on the northwestern side of the domeand is situated approximately 2 km east of local-ity JHBD 98-9.

The homogeneous potassium-rich granitoidsuite was sampled at four separate localities. Sam-ple JHBD 98-2 is a relatively coarse-grained, ho-mogeneous granodiorite from the southern half ofthe dome; sample JHBD 98-3 is a medium-fine-grained granodiorite from the south-central partof the dome, and sample JHBD 98-5 is a coarse-grained porphyritic granodiorite from the west-central part (Fig. 1).

4. Methodology

All the samples were prepared and analyzed atthe Hugh Allsopp Laboratory, University of theWitwatersrand, Johannesburg. Rock sampleswere pulverized using a heavy-duty hydraulic rocksplitter, jaw crusher and swing mill. Mineral sepa-ration involved the use of a Wilfley Table, heavyliquids (bromoform and methylene iodide) and aFrantz Isodynamic Separator. Zircons were exam-ined with a binocular microscope in order toassess grain quality, degree of fracturing and thepossible existence of inherited cores. Handpickedzircons were abraded using the techniques ofKrogh (1982) and washed in ultra-pure acetone,diluted nitric acid and hydrochloric acid. Singlegrains or small populations of zircons were thenplaced into 0.35 ml Teflon vials together with30-ml HF and a mixed 205Pb–235U spike. Eight ofthese Teflon vials were then placed in a ParrContainer for 2 days at 220°C. The samples werechemically processed without separating U andPb (Lancelot et al., 1976) and loaded on a rhe-nium filament together with a 0.25 N phosphoricacid–silica-gel mixture. The analyses were per-formed on an automated VG54E mass spectrome-ter using a Daly collector and corrected by 0.002(90.05%) for mass fractionation. Total Pbblanks over the period of the analyses range from15 to 30 pg and a value of 30 pg was assigned asthe laboratory blank (206Pb/204Pb=18.9791,207Pb/204Pb=15.7390.5 and 208Pb/204Pb=39.1991.5). The calculation of common Pb wasmade by subtracting blanks and then assumingthat the remaining common Pb has been incorpo-rated into the crystal and has a composition de-termined from the model of Stacey and Kramer


(1975). Data were reduced using PbDat (Ludwig,1993a). Analytical uncertainties are listed at 2s

and age determinations were processed using Iso-plot (Ludwig, 1993b).

5. Results

5.1. Tonalitic and trondhjemitic gneisses

The hornblende–biotite–tonalitic gneiss(JHBD 98-1; equivalent to sample RP7 of An-haeusser, 1971, 1973) consists mainly of quartz,sodic-plagioclase, hornblende and biotite. Acces-sory minerals include sphene, apatite, magnetite,zircon and microcline. The plagioclase (albite-oligoclase) is generally saussuritized (to epidote)or sericitized, whereas the hornblende is partly or

totally altered to chlorite. The tonalitic gneisseshave a distinctive chemical composition character-ized by high Na2O (4.23 wt.%) and low K2O (2.24wt.%) contents. As mentioned earlier this tonaliticgneiss was found to be approximately 3170 Ma byAnhaeusser and Burger (1982) using multiple zir-con populations. In the present study, all thezircons extracted from sample JHBD 98-1 werefound to be translucent and pink in colour. Sixindividual grains were analyzed (Table 1) and theresults were plotted in a concordia diagram (Fig.3). Four sub-concordant points define an upperintercept age of 3200.995.2 Ma (MSWD=3.5)with a weighted mean 207Pb/206Pb age of 3199.992 Ma. These new data, together with that fromAnhaeusser and Burger (Fig. 3 inset) define asimilar age of 3201.795.3 Ma. Consequently, weconsider that the emplacement age of this tonalite

Fig. 3. Concordia diagram for hornblende–biotite–tonalite sample JHBD 98-1. Inset diagram shows these data together with thatfrom Anhaeusser and Burger (1982).


Tab

le1

U–P

bis

otop

icda

tafo

rth

esa

mpl

esfr

omth

eJo

hann

esbu

rgD

omea

Rad

ioge

nic

Rat

ios

Wei

ght

(mg)

App

aren

tA

ges

(Ma)

U(p

pm)

Pb

(ppm

)20

6 Pb/

204 P

bG

rain

9(%

)20

7 Pb/

235 U

9(%

)20

7 Pb/

206 P

b9

(%)

206 P

b/23

8 U20

7 Pb/

235 U

207 P

b/20

6 Pb9

Cor

.C

oef

206 P

b/23

8 U

JHB

D98

-11.

021

.257

1.0

0.24

500.

230

9810

131

5080

3184

30.

995

Zr

1-1,

p,t

810

0.61

700.

521

.727

0.5

0.25

260.

131

2631

7232

01Z

r1-

2,p,

t2

40.

9767

5458

20.

6240

0.6

22.1

080.

70.

2522

0.1

3127

3171

0.63

5831

9968

20.

9911

02Z

r1-

3,p,

t5

820.

6511

40.

522

.693

0.5

0.25

280.

231

8631

9632

023

0.95

8668

985

Zr

1-4,

p,t

Zr

1-5,

p,t

0.7

420

.421

0.8

0.24

420.

430

5631

1231

476

0.89

4634

272

0.60

660.

621

.882

0.6

0.25

230.

131

4631

7932

002

0.62

900.

97Z

r1-

6,p,

t5

8064

597

JHB

D98

-26

0.20

801.

46.

066

1.9

0.21

151.

212

1819

8529

1720

0.77

309

80Z

r2-

1,p,

t33

10.

96.

021

0.9

0.21

160.

412

1019

790.

2064

2918

56

0.91

Zr

2-2,

y,t

271

4720

00.

0603

88.

51.

665

8.5

0.20

040.

537

799

528

309

0.99

444

3222

4Z

r2-

3,p,

dZ

r2-

4,p,

d3.

68

2.61

43.

70.

2004

0.7

583

1305

2829

110.

9850

359

255

0.09

46

JHB

D98

-34.

014

.872

4.0

0.25

681.

322

6161

2807

3532

2721

0.95

5Z

r3-

1,p

234

0.42

0135

50.

5735

0.6

18.7

470.

70.

2371

0.3

2922

3029

3101

50.

8965

Zr

3-2,

p47

61.

912

.253

2.1

0.22

080.

821

8026

240.

4024

2987

365

130.

93Z

r3-

3,p,

t5

110

560.

69.

967

0.7

0.21

470.

318

7024

32Z

r3-

4,p,

d29

425

50.

9040

515

917

670.

3366

0.9

17.0

450.

90.

2337

0.2

2736

2937

0.52

8830

78Z

r3-

5,p,

d3

0.98

379

7611

85

0.8

13.8

291.

00.

2264

0.5

2364

2738

Zr

3-6,

p,d

3027

57

0.88

222

117

590

0.44

300.

79.

274

0.8

0.20

400.

418

3623

650.

3296

2859

Zr

3-7,

p,d

60.

8836

690

242

6Z

r3-

8,p,

d0.

76

12.4

780.

70.

2213

0.1

2210

2641

2991

20.

9813

263

2151

0.40

89

JHB

D98

-564

80.

6198

0.7

20.4

100.

80.

2388

0.5

3109

3111

3112

70.

8058

Zr

5-1,

p,t

523

0.9

18.2

900.

90.

2372

0.2

2864

3005

0.55

9331

01Z

r5-

2,p,

d3

0.99

2122

9614

64

4.1

20.7

014.

40.

2377

1.7

3156

3125

Zr

5-3,

p,t

3105

327

0.93

5650

694

0.63

160.

917

.037

0.9

0.23

560.

227

1829

370.

5245

3091

Zr

5-4,

p,d

30.

9712

0076

125

7Z

r5-

5,p,

d1.

66

6.38

61.

60.

2060

0.3

1308

2030

2874

50.

9825

063

438

0.22

49

JHB

D98

-835

40.

6289

1.0

20.3

711.

60.

2349

1.0

3145

3109

3086

160.

8014

1Z

r8-

1,p,

t10

35

0.5

16.8

230.

50.

2376

0.1

2671

2925

0.51

3431

05Z

r8-

2,p,

t2

0.97

511

149

248

560

10.

4614

0.6

15.1

840.

70.

2386

0.2

2446

2827

3111

40.

94Z

r8-

3,p,

t25

35

134

0.6

18.4

730.

70.

2397

0.2

2862

3015

0.55

8931

184

30.

96Z

r8-

4,p,

t10

5188

138

0.37

974

7.7

12.5

707.

70.

2401

0.2

2075

2648

3121

41.

026

211

470

0Z

r8-

5,p,

t0.

5173

50.

617

.304

0.6

0.24

260.

226

8829

5231

374

0.92

144

8644

8Z

r8-

6,p,

d4.

414

.432

4.5

0.24

550.

435

5433

040.

4612

3156

Zr

8-7,

p,t

71.

010

6611

813

83

0.59

465

9.0

18.2

229.

00.

2222

0.4

3008

3002

2997

71.

011

275

1565

Zr

8-8,

p,d

1.8

20.4

382.

00.

2487

0.7

3014

3112

0.59

6131

7640

911

0.94

Zr

8-9

(4,

t)3

105

701.

617

.050

1.8

0.24

060.

626

7429

3831

24Z

r8-

10,

p,d

95

0.95

141

8661

20.

5140

0.7

14.3

780.

90.

2284

0.6

2475

2775

3041

0.46

819

Zr

8-11

,p,

d0.

804

272

6011

0


Tab

le1

(Con

tinu

ed)

Rad

ioge

nic

Rat

ios

Wei

ght

(mg)

App

aren

tA

ges

(Ma)

U(p

pm)

Pb

(ppm

)20

6 Pb/

204 P

bG

rain

9(%

)20

7 Pb/

235 U

9(%

)20

7 Pb/

206 P

b9

(%)

206 P

b/23

8 U20

7 Pb/

235 U

207 P

b/20

6 Pb9

Cor

.C

oef

206 P

b/23

8 U

JHB

D98

-94.

421

.198

4.5

0.27

820.

928

3627

231

4818

433

5315

0.98

7Z

r9-

1,p,

t68

30.

5525

3.6

24.6

133.

90.

2761

1.4

3215

3293

3341

Zr

9-2,

p,t

56

0.94

7213

696

90.

6466

1.5

16.2

801.

60.

2757

0.3

2298

2893

0.42

8333

3917

43

0.99

644

Zr

9-3,

p,d

731

40.

2123

81.

48.

070

1.5

0.27

560.

212

4122

3933

387

0.99

233

6290

1Z

r9-

4,p,

tZ

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Fig. 4. Concordia diagram for the trondhjemitic gneiss sample JHBD 98-9 from the northwestern part of the Johannesburg Dome(Fig. 1).

is �3200 Ma. Zircons 1 and 5 (Table 1), al-though identical in shape, are slightly discordantand plot well to the left of the discordia definedby the other four grains. It is suggested that theposition of these grains may be the consequenceof a multiple discordancy with some Pb loss at anearly stage in addition to some more recent Pbloss. The presence of such grains within the zirconpopulation of the tonalite gneiss may also explainthe slightly younger age (�3170 Ma) recorded byAnhaeusser and Burger (1982).

Sample JHBD 98-9 is a trondhjemitic gneiss(equivalent to sample SK7 of Anhaeusser, 1971,with 6.25 wt.% Na2O and 0.97 wt.% K2O) fromthe northwestern part of the dome (Fig. 1). Zir-cons extracted from this sample were typicallypink in colour and most often translucent.

However, some of the zircons were darker. Sixgrains in total were analyzed from this rock(Table 1). Plotted in a concordia diagram (Fig. 4)they are slightly to highly discordant. All thetranslucent grains define a discordia pointing to awell-defined upper intercept age of 334093.3 Ma(MSWD=1.7) with a lower intercept age of 5913 Ma. This �3340 Ma age is considered to bethe best estimate for the emplacement of thetrondhjemite. Two of the darker zircons are dis-cordant (Fig. 4) and, relative to the others, arecharacterized by younger 207Pb/206Pb ages as wellas very low 208Pb/206Pb ratios (Table 1). Thesegrains can, therefore, be interpreted as a reflectionof a post-crystallization (migmatization or gneiss-forming?) event leading to a complex lead lossand/or partial recrystallization.


Samples JHBD 98-8 and 98-10 are additionaltrondhjemitic gneisses (equivalent to samples N14and N2 of Anhaeusser, 1999, and which average6.08 wt.% Na2O and 0.95 wt.% K2O) that cropout on the Nooitgedacht migmatite platform (Fig.2). Zircons from these samples are euhedral, gen-erally pink in colour and vary from translucent todark. Eleven grains from sample JHBD 98-8 andtwo grains from sample JHBD 98-10 were ana-lyzed (Table 1) and plotted in a concordia dia-gram (Fig. 5). They are concordant to discordantand occur scattered in the diagram. The youngestpoint (JHBD 98-8, Zr 8, Fig. 5) is concordantwith a 207Pb/206Pb age of 299797 Ma, whereasthe oldest (JHBD 98-10, Zr 2, Fig. 5) is 4.6%discordant with a 207Pb/206Pb age of 3213910Ma. As shown in Fig. 2, the Nooitgedacht plat-form is a very complex exposure showing most ofthe igneous phases recognized on the dome. Thescatter of the data could, therefore, be a conse-

quence of the complex history of this platform,which has been influenced by a succession ofdifferent fluid injections. This could have led topartial melting of the zircons followed by recrys-tallization. As these trondhjemitic gneisses areintruded by a �3.12 Ga (see later) granodioriticphase (Fig. 2) they cannot be 3 billion-years old.Therefore, the positions of the data can best beexplained in terms of a crisis polygon (Fig. 5),defined by three apices at 3340, 3000 Ma andzero, respectively. The first apex at c.3340 Ma(defined by sample JHBD 98-9) represents the ageof the trondhjemite emplacement; the second atc.3000 Ma (defined by the youngest concordantpoint) could represent the youngest significantevent to have influenced the rocks exposed on theplatform (crystallization of new zircons associatedwith the emplacement of the pegmatitic dykes?)and the third, at zero, representing recent leadloss.

Fig. 5. Concordia diagram for trondhjemite gneiss samples JHBD 98-8 and 98-10 from the Nooitgedacht river platform shown inFig. 2.


Fig. 6. Concordia diagram for the medium-to-coarse-grained pinkish granodiorite sample JHBD 98-2 located approximately 5 kmnorth of central Johannesburg (Fig. 1).

5.2. Potassic granitoids

Sample JHBD 98-2 (equivalent to sample VP2of Anhaeusser, 1973, with 4.12 wt.% Na2O and3.97 wt.% K2O) is a coarse-grained, pinkish, ho-mogeneous granodiorite (Fig. 1). The zirconsfrom this rock are generally pink in colour (someare yellowish) and translucent to dark. Four zir-cons were analyzed (Table 1) and, when plotted ina concordia diagram (Fig. 6), display high degreesof discordance. They point to a relatively poorlydefined upper intercept age of 2947957 Ma(MSWD=22) and a lower intercept age of 49956 Ma. This age of �2950 Ma is, therefore,

considered to be a minimum age for the emplace-ment of this potassic granitoid.

Sample JHBD 98-3 (similar to sample FD2 ofAnhaeusser, 1973, with 3.98 wt.% Na2O and 4.30wt.% K2O) is a medium-grained, grey granodior-ite cropping out in the central part of the dome(Fig. 1). All the zircons extracted from this samplewere translucent to dark-pink in colour. Eightzircons were analyzed (Table 1) and have beenplotted in a concordia diagram (Fig. 7). They aresub-concordant to very discordant. The five mostconcordant zircons analyzed define a relativelywell-constrained upper intercept age of 3121.295Ma (MSWD=0.8) with a lower intercept age of


636925 Ma that does not correspond to anyrelevant geological event. The most concordantgrain, zircon 2, defines the absolute minimum ageof this sample at 310195 Ma. This age is re-garded as the best estimate for the emplacementof this granodiorite in the south-central part ofthe dome. The eighth zircon (Zr 1, Table 1) isslightly discordant and is defined by a 207Pb/206Pbage of 3227 Ma. This zircon is interpreted as axenocryst, probably extracted from the earlierTTG granitoid suite.

Sample JHBD 98-5 (similar to sample HD30 ofAnhaeusser, 1971, with 4.14 wt.% Na2O and 4.45wt.% K2O) is representative of the porphyritic

granodiorites that crop out in the southwesternpart of the dome. Most of the zircons from thissample are pink and translucent and occur to-gether with some darker-pink grains. Five zirconswere analyzed (Table 1), two of them being con-cordant and the remaining three presenting differ-ent degrees of discordance (Fig. 8). They define awell-constrained upper intercept age of 3114.292.3 Ma (MSWD=0.47) with a lower interceptage of 358911 Ma, the latter without any appar-ent geological meaning. The age of 311492 Ma isonce again considered to be the age of emplace-ment of the porphyritic granodiorite in this partof the dome.

Fig. 7. Concordia diagram for the medium-grained granodiorite sample JHBD 98-3 from the central part of the Johannesburg Dome(Fig. 1).


Fig. 8. Concordia diagram for the porphyritic granodiorite sample JHBD 98-5 from the west-central part of the Johannesburg Dome(Fig. 1).

5.3. Mafic dyke

Sample JHBD 98-11 (similar to sample N12 ofAnhaeusser, 1999) is representative of the maficdykes cropping out in the Nooitgedachtmigmatite platform. These mafic dykes intrudedthe trondhjemitic gneisses and the amphiboliticgreenstones prior to the late intrusive granodi-oritic event (Fig. 2) and, in their turn, are in-truded by granitic veins (Anhaeusser, 1999).Extreme care was taken to separate the appar-ently vein-free mafic dyke from its vein-rich equiv-alent. Zircons were found in both vein-free andvein-rich samples, but were more abundant withinthe vein-rich sample material. All the zircons were

very small in size (�30–50 mm) and pink incolour. Three grains (Zr 11-1 to Zr 11-3, Table 1)from the vein-free sample and five grains (Zr 11-4to Zr 11-8, Table 1) from the vein-rich samplewere analyzed. Plotted in a concordia diagram(Fig. 9), they are discordant to very discordant,but define a very well-constrained upper interceptage of 3117912 Ma (MSWD=1.7) with a lowerintercept age of 707933 Ma. The upper interceptage is undistinguishable from the ages found forthe granodiorite and, therefore, it is assumed thatall the zircons found in this sample were probablylinked to the emplacement of the �3120 Magranitic veins within the mafic dykes. It was as-sumed, furthermore, that the granitic veins, which


intruded the mafic dykes, were linked to the em-placement of the granodioritic phase within thedome.

6. Discussion

The first part of this study focused on thetrondhjemitic and tonalitic gneisses occurringmainly on the northern half of the JohannesburgDome, but which are also represented on thesouthern margin of the dome. In both localities

these gneisses intrude mafic and ultramafic igneousand volcanic rocks. The most interesting result wasderived from sample JHBD 98-9, which gave anage of 334093.3 Ma. The two trondhjemiticgneiss samples studied from the nearby Nooit-gedacht migmatite platform yielded very scattereddata (Fig. 5) that did not provide any directgeochronological constraints. This was interpretedas reflecting the complex multi-stage history thatthis granitoid platform had undergone, including alate-stage event, possibly linked to the emplace-ment of pegmatites at approximately 3000 Ma.

Fig. 9. Concordia diagram for the zircons extracted from the mafic dyke sample JHBD 98-11 from the Nooitgedacht migmatiteplatform.


The Johannesburg Dome granite–greenstoneterrane, which was previously considered to beapproximately 3170 Ma old (Anhaeusser andBurger, 1982) has now been shown to contain anolder granitoid phase. The �3340 Ma age deter-mined for the trondhjemitic gneisses representsthe oldest magmatic phase described from thegranitoid rocks of the dome. This result may alsohave important implications with regard to theage of the mafic and ultramafic greenstone rem-nants scattered throughout the dome and whichhave not yet been dated because of the absence ofmaterial suitable for this purpose. Consequently,it implied that the greenstone remnants, whichwere considered by Anhaeusser (1999) to haveformed in an Archaean oceanic or volcanic arc-like geotectonic setting, were older than 3.34 bil-lion years.

This initial magmatic episode, involving earlygreenstone and TTG granitoid development onthe northern half of the dome, was followed bythe emplacement of the hornblende–biotite–tonalite in the south at �3200 Ma, as has beendemonstrated by the data from sample JHBD98-1 (Fig. 3).

Following the trondhjemite–tonalite eventthere appears to have been a further period ofmafic plutonism manifest in the form of the am-phibolite dykes displayed on the Nooitgedachtmigmatite platform and shown in Fig. 2. Geo-chemical evidence, in the form of distinctly differ-ing REE abundances, led Anhaeusser (1999) tosuggest that more than one dyke event may haveoccurred. It was argued that if only a single stageof dyke emplacement had been involved then thetwo magma types would probably have formedfrom different mantle sources. The age of thesedykes has yet to be determined quantitatively, butthey fall within the time constraints imposed bythe age of the trondhjemitic gneisses (3340–3200Ma) and the crosscutting homogeneous granodi-orites discussed below (3114–3121 Ma).

The final stages of Archaean crustal evolutionevident on the Johannesburg Dome coincidedwith the emplacement of an extensive homoge-neous granodiorite–porphyritic granodioritebatholith or massif, the latter seen occupyingmost of the southern half of the dome (Fig. 1).

Manifestations of this event are also seen on thenorthern half of the dome in the form of granodi-orite and pegmatite dykes that intrude the earlier-formed greenstones, gneisses and migmatites (Fig.2).

Two samples representative of the medium-grained and porphyritic potassic granitoids havebeen dated in this study at 312195 (minimumage of 310195 Ma) and 311492 Ma, respec-tively. A third sample of coarse-grained granodi-orite yielded a poorly constrained minimum ageof 2947957 Ma and consequently did not confl-ict with the previous ages. Zircons extracted froma mafic dyke, but associated with granitic veinswithin the dyke, defined an upper intercept age of3117912 Ma. Consequently, we consider that thepotassic granitoid suite within the JohannesburgDome was emplaced 3114–3120 Ma ago. This ageis in good agreement with the 3132965 Ma agedetermined by Allsopp (1961), but contradicts thezircon evaporation age of 3090 Ma publishedrecently by Barton et al. (1999). One of the prob-lems of the zircon evaporation technique lies withthe difficulty in ascertaining the concordance ofthe zircons. The data presented in this study showthat very few zircons are concordant, some ofthem giving apparent 207Pb/206Pb ages at around3090 Ma (sample JHBD 98-3: Zr 2 3101Ma, Zr 53078Ma; sample JHBD 98-5: Zr 2 3101 Ma; Zr 43091 Ma). It is, therefore, possible to concludethat Barton et al. (1999) were dealing with sub-concordant zircons, which yielded younger 207Pb/206Pb ages. Another explanation might suggestthat the age of 3.09 Ga reflects a younger, discreetpulse of magmatism in this area, but this needs tobe confirmed.

7. Implications for the evolution of the KaapvaalCraton

Over the past decade much new geochronologi-cal data has been published relating to the Kaap-vaal Craton (Barton et al., 1999; Kroner et al.,1999, 2000; Nelson et al., 1999; Poujol and Robb,1999; Kreissig et al., 2000; McCourt et al., 2000).

Similarities exist between the JohannesburgDome and the Barberton terrane where Kamo


and Davis (1994) reported a 335296 Ma agefrom zircons and badeleyites from gabbros in-truded into the Komati Formation. In addition, atuffaceous layer in the uppermost Kromberg For-mation gave a 207Pb/206Pb evaporation age of333493 Ma (Byerly et al., 1996). These two agesare identical, within error margin, to the 334093.3 Ma age found for the emplacement of sampleJHBD 98-9 in the northern part of the Johannes-burg Dome.

In the far northeastern part of the KaapvaalCraton recent data from Kroner et al. (2000)suggests that the Giyani (Sutherland) greenstonebelt was deposited at around 3.2 Ga on a base-ment as old as c.3.28 Ga. In addition, the oldestrocks in the vicinity of the Murchison greenstonebelt, to the south of Giyani, were dated at 3228912 Ma (Poujol et al., 1996). Similar ages havebeen reported in the Barberton terrane for the323691 Ma Nelshoogte pluton (De Ronde andKamo, 2000), the 322791 Ma Kaap Valley plu-ton (Kroner et al., 1991), and the 321692 MaDalmien pluton (Kamo and Davis, 1994). This,together with the new data from the Johannes-burg Dome, suggests that an important period ofmagmatism occurred between 3.34 and 3.2 billionyears ago on the Kaapvaal Craton. At this time itis possible that the Ancient Gneiss terrane ofSwaziland and that of the southern and northernBarberton terranes were welded together to forma stable nucleus in the manner described by DeWit et al. (1992).

Late potassic batholiths were then emplaced inboth the Barberton and central Kaapvaal (Johan-nesburg Dome) terranes. Kamo and Davis (1994)dated several granitoid batholiths and plutonsfrom the Barberton terrane, including the Stentor(310795 Ma), Mpuluzi (310794 Ma), Nelspruit(310693 Ma) and Salisbury Kop (3109910 Ma)bodies. These all have ages, within error, close tothe c.3114–3120 Ma emplacement ages of thepotassic granitoid suite found on the Johannes-burg Dome.

In contrast the Pietersburg and Murchisongranite–greenstone terranes, which are locatedadjacent to the northern and northeastern mar-gins of the Kaapvaal Craton, are generallyyounger (3.09–2.68 Ga) than the terranes found

farther south and in the central part of the Craton(Brandl et al., 1996; Poujol and Robb, 1999;Poujol et al., 1996; Kroner et al., 2000). Further-more, recent data (Poujol et al., 2000) havedemonstrated an episodic granitoid emplacementhistory in the western part of the Craton (Kraai-pan–Amalia terrane), which ranges between 3.01and 2.79 Ga. Thus, the western part of the Cratonappears to be significantly younger than the cen-tral and eastern portions and can, on the basis ofthe age relationships, be more suitably correlatedwith the rocks of the northern portion of theKaapvaal Craton.

These data demonstrate further that the amal-gamation of the Kaapvaal Craton was long-livedand episodic, and presumably involved the forma-tion of juvenile magmatic arcs that coalesced withexisting continental blocks in the period 3.65–2.65 Ga. The resulting scenario envisages that theKaapvaal Craton grew by accretionary processes,becoming generally younger towards the northand west with the central portion, represented bythe Johannesburg Dome, providing a region withages overlapping those of the surroundingterranes.

8. Conclusion

The Johannesburg Dome consists of a complexmosaic of granitoid rocks manifest by differencesin areal extent, composition, texture and age. Thisgeochronological study has demonstrated some ofthe difficulties that can be encountered in datingArchaean granite–gneiss–migmatite terranes.

Three main magmatic events have been defined.The first involved the emplacement of trond-hjemitic rocks at c.3340 Ma in the northern partof the dome, followed by a tonalitic phase atc.3200 Ma in the south. The greenstone remnantsoccurring widespread on the dome predate theearliest trondhjemitic gneisses and are, therefore,at least 3.34 billion-years old. The mafic dykesthat intruded the trondhjemite–tonalite gneisssuite were emplaced between c.3340 and c.3120Ma. The third event, namely the emplacement ofthe potassic granitoid suite, is shown to havetaken place at c.3120–3114 Ma, followed by a


pegmatite dyke episode possibly as young as 3.0Ga.

A similar age of 312095 Ma was obtainedfrom the granitoid basement that pre-dates the307496 Ma upper lava sequence of the Domin-ion Group, which underlies the WitwatersrandSupergroup successions southwest of Klerksdorpin the North West Province (Armstrong et al.,1991). Consequently, the new data from the Jo-hannesburg Dome, the latter situated approxi-mately 150 km to the northeast of Klerksdorp,provide confirmation that rocks of the Witwater-srand Basin were deposited unconformably on anArchaean basement as young as c.3120 Ma.

In conclusion, the Johannesburg Dome appearsto represent an intermediary, mid-Archaean ter-rane linking the eastern portion of the KaapvaalCraton with the northern and western parts of theCraton, with each of these terranes showing pro-gressively younger ages both to the north and tothe west.

Acknowledgements

We would like to acknowledge Sandra Kamo,Jan Kramers and Jay Barton who provided veryhelpful and insightful reviews of the originalmanuscript.

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the johannesburg dome, south africa: new single zircon u ... · stages; hence it became important...

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