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Dec 3, 2010 - Keywords Palaeozoic 4 Cape Supergroup 4 Rıo de la. Plata and Kalahari Craton 4 Detrital zircon age dating 4. Provenance 4 Gondwana.
Int J Earth Sci (Geol Rundsch) (2011) 100:527–541 DOI 10.1007/s00531-010-0619-x

ORIGINAL PAPER

Provenance and reconnaissance study of detrital zircons of the Palaeozoic Cape Supergroup in South Africa: revealing the interaction of the Kalahari and Rı´o de la Plata cratons Pieter H. Fourie • Udo Zimmermann • Nicolas J. Beukes • Thanusha Naidoo • Katsuro Kobayashi • Jan Kosler • Eizo Nakamura • Jenny Tait • Johannes N. Theron

Received: 15 December 2009 / Accepted: 30 October 2010 / Published online: 3 December 2010 Ó Springer-Verlag 2010

Abstract In order to facilitate the understanding of the geological evolution of the Kalahari Craton and its relation to South America, the provenance of the first large-scale cratonic cover sequence of the craton, namely the Ordovician to Carboniferous Cape Supergroup was studied through geochemical analyses of the siliciclastics, and age determinations of detrital zircon. The Cape Supergroup comprises mainly quartz-arenites and a Hirnantian tillite in the basal Table Mountain Group, subgreywackes and mudrocks in the overlying Bokkeveld Group, while siltstones, interbedded shales and quartz-arenites are typical Electronic supplementary material The online version of this article (doi:10.1007/s00531-010-0619-x) contains supplementary material, which is available to authorized users. P. H. Fourie  U. Zimmermann  N. J. Beukes  T. Naidoo Palaeoproterozoic Research Group, Department of Geology, University of Johannesburg, Johannesburg, South Africa U. Zimmermann (&)  T. Naidoo Department of Petroleum Engineering, University of Stavanger, 4036 Stavanger, Norway e-mail: [email protected] K. Kobayashi  E. Nakamura Pheasent Memorial Laboratory for Geochemistry and Cosmochemistry, ISEI, Okayama University, Misasa, Japan J. Kosler Centre for Geobiology and Department of Earth Science, University of Bergen, Bergen, Norway J. Tait School of Geosciences, The University of Edinburgh, Edinburgh EH9 3JW, Scotland, UK J. N. Theron Department of Earth Sciences, University of Stellenbosch, Stellenbosch, South Africa

for the Witteberg Group at the top of the Cape Supergroup. Palaeocurrent analyses indicate transport of sediment mainly from northerly directions, off the interior of the Kalahari Craton with subordinate transport from a westerly source in the southwestern part of the basin near Cape Town. Geochemical provenance data suggest mainly sources from passive to active continental margin settings. The reconnaissance study of detrital zircons reveals a major contribution of Mesoproterozoic sources throughout the basin, reflecting the dominance of the Namaqua-Natal Metamorphic Belt, situated immediately north of the preserved strata of Cape Supergroup, as a source with Archaean-aged zircons being extremely rare. We interpret the Namaqua-Natal Metamorphic Belt to have been a large morphological divide at the time of deposition of the Cape Supergroup that prevented input of detrital zircons from the interior early Archaean Kaapvaal cratonic block of the Kalahari Craton. Neoproterozoic and Cambrian zircons are abundant and reflect the basement geology of the outcrops of Cape strata. Exposures close to Cape Town must have received sediment from a cratonic fragment that was situated off the Kalahari Craton to the west and that has subsequently drifted away. This cratonic fragment predominantly supplied Meso- to Neoproterozoic, and Cambrian-aged zircon grains in addition to minor Silurian to Lower Devonian zircons and very rare Archaean (2.5 Ga) and late Palaeoproterozoic (1.8-2.0 Ga) ones. No SiluroDevonian source has yet been identified on the Kalahari Craton, but there are indications for such a source in southern Patagonia. Palaeozoic successions in eastern Argentina carry a similar detrital zircon population to that found here, including evidence of a Silurian to Lower Devonian magmatic event. The Kalahari and Rı´o de la Plata Cratons were thus in all likelihood in close proximity until at least the Carboniferous.

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Keywords Palaeozoic  Cape Supergroup  Rı´o de la Plata and Kalahari Craton  Detrital zircon age dating  Provenance  Gondwana

Introduction The siliciclastic Palaeozoic Cape Supergroup is developed along the western, southern and southeastern margins of the Kalahari Craton along the west, south and southeast coasts of South Africa (Fig. 1). At the time of deposition, the Cape basin was probably linked to similar aged basins in other parts of southwestern and southeastern Gondwana (Fig. 2). Most significant for the scope of this paper is to note the link with the Rı´o de la Plata Craton and Patagonia in Argentina (Fig. 2). It is generally assumed that the Kalahari Craton, including its interior Archaean Kaapvaal and Zimbabwe cratonic blocks, provided sediment to the Cape basin with perhaps some sources from South America (Fig. 2). However, apart from palaeocurrent directions (Fig. 1) and proximal to distal sedimentary facies transitions, there are no solid provenance data available to support this notion and define the nature and age of the source terranes. In particular, nothing is known about the composition and age of rocks in the subordinate western source terrane (Fig. 2). Recently, provenance studies in Neoproterozoic to Lower Palaeozoic successions of the Kalahari Craton were carried out, but did not provide a comprehensive view of the evolution of the craton (Naidoo 2008; Zimmermann et al. 2008a, b, 2010; Van Staden et al. 2006, 2010a, b). This paper addresses these questions through analyses of detrital zircon ages of samples from the Cape Supergroup. One profile in the eastern and western part of the basin was sampled throughout the succession in order to reveal the

Fig. 1 Map of southern Africa (representing the southern part of the Kalahari Craton) with the geology, palaeocurrents and the main sampling areas for the Cape Supergroup (after Thamm and Johnson 2006)

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signature of exposed rock material. The basin is divided, on the basis of sedimentological and stratigraphical differences, into western and eastern sub-basins, with the dividing line between them being roughly at 21°W. The quartz-rich nature of the rocks and their position around an extensive part of the craton qualify the Cape Supergroup as ideal succession to reveal more about the composition of the source areas of the Kalahari Craton and adjacent then rifted source areas.

Geological setting The Cape Supergroup was deposited in a craton-fringing basin (here referred to as the Cape basin) around the southern perimeter of the Kalahari Craton (Figs. 1, 2). It is divided into three major lithostratigraphic units, namely, from the base upwards, the Table Mountain, Bokkeveld and Witteberg Groups (Tankard et al. 1982). The Table Mountain Group is considered to be Ordovician to Silurian and lowermost Devonian in age (Fig. 3). The Bokkeveld Group is essentially lower to middle Devonian and the Witteberg Group upper Devonian to Carboniferous in age (Fig. 3). The mineralogical composition of the siliciclastic sedimentary rocks are described in detail by Theron (1972), Rust (1973), Tankard et al. (1982) and Thamm and Johnson (2006) and will not be repeated here. Suffice to say is that the Table Mountain Group are dominated by quartzarenites in contrast to the feldspathic subgreywackes and shales of the overlying Bokkeveld Group. The Witteberg sandstones are feldspar poor, but the finer-grained lithologies contain alkali-feldspar and are very micaceous. The Table Mountain Group is characterised by thick packages of medium- to coarse-grained quartz-arenites

Int J Earth Sci (Geol Rundsch) (2011) 100:527–541

529

Fig. 2 Sketch of the location of different cratons of southern Gondwana during the Palaeozoic (revised from Rapela et al. 2003; Zimmermann et al. 2009). ANT Antarctica, E Ellsworth Mountains, KA Kalahari, KV Kaapvaal Craton as interior part of the Kalahari Craton, MFP Malvinas-Falkland Plateau, PAT Patagonia, RP Rı´o de la Plata. The Palaeozoic deposits in Argentina combine the Sierras Australes and Tandilia area on the Rı´o de la Plata Craton and the Sierra Grande and the Deseado Massif on Patagonia

(Figs 3, 4a, and b), conglomerates and subordinate finegrained lithologies (Rust 1973). Deposition was initiated during the Lower Ordovician and lasted until the Lower Devonian with facies ranging from shallow marine to fluvial, with a Hirnantian glacial deposit, comprised of shales and diamictites (Young et al. 2000; Thamm and Johnson 2006), in the middle of the succession (Fig. 3). Palaeocurrent data indicate the main transport direction to have been from north to south (Fig. 1). The boundary of the Bokkeveld Group with the Table Mountain Group is represented by a marine-flooding surface with the quartz-arenites of the former rapidly, but conformably grading into shelf and pro-delta shales and mudstones of the latter (Theron 1972). Up to five upwardcoarsening megacycles grading from shales and mudstones to siltstones (Figs. 3, 4c, e), and finally capped by immature sandstones or wackes (Fig. 4d), are present in the Bokkeveld Group. The group as a whole is thought to represent a sequence of progradational, laterally coalescing deltaic successions, vertically stacked due to tectonically controlled transgressions in an underfilled epeiric basin (Theron 1972; Tankard et al. 2009). The basin-fill thickens rapidly from proximal deltaic facies in the north and west to distal facies in the south and east. Palaeocurrents indicate sediment transport to have been dominantly from northwest to southeast. However, in the southwestern part of the basin, palaeocurrent directions indicate sediment dispersal from the west towards the east (Fig. 1). The Witteberg Group, which conformably overlies the Bokkeveld Group, comprises quartz-rich sandstones and

Fig. 3 Stratigraphic table for the Cape Supergroup (after Thamm and Johnson 2006). The red stars indicating the sampling horizons for detrital zircons. SG subgroup, m relative thickness; star indicates the stratigraphic position of the samples for detrital zircon dating

mica-rich mudstones with intercalated siltstones. The thickness of the group increases rapidly from north to south and west to east (Thamm and Johnson 2006). The age ranges from Middle Devonian (Givetian) to Early Carboniferous (Fig 1). A glaciogenic unit, the Miller Formation, is developed in the upper part of the stratigraphy. Several Carboniferous diamictites are reported from Argentina (Desjardins et al. 2009) and may be coeval with the Miller Formation (Fig. 4). The Witteberg basin-fill records several transgressive and regressive events, which produced thinly, laminated mudrocks and fluviodeltaic deposits, respectively (Figs. 4f, g). Palaeocurrent directions indicate sediment transport mainly from northern and eastern sources (Thamm and Johnson 2006; Fig. 1). A number of siliciclastic successions in the south and east of the Kalahari Craton are possibly coeval with the Cape Supergroup. However, their age constraints are weak and correlation is not beyond doubt. Current understanding would correlate the Kansa Group and the Schoemanspoort Formation in the Oudtshoorn area (Fig. 2 central area) with

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Int J Earth Sci (Geol Rundsch) (2011) 100:527–541

Fig. 4 Photos of exposures of the Cape Supergroup. a Impression of the massive quartz-rich successions of the Table Mountain Group near Oudtshoorn (central to eastern sampling area). b Mature quartz-arenites from the Table Mountain Group. c Bokkeveld Group at Ceres arc with the lower Witteberg Group on top of the mountains. The association of weatheringresistant quartz-rich rocks and softer shale units is very well documented (from Theron 1972). d Base of the Bokkeveld Group at Worcester (southern sampling area). e Black shales from the top of the Bokkeveld Group (eastern sampling area). f Base of the Witteberg Group in the western sampling area. g Typical siltstones of the Witteberg Group at Ceres (eastern sampling area)

the lower part of the Table Mountain Group (Naidoo 2008). The Natal Group in the east of the Kalahari Craton (Fig. 2 close to Durban) seems to be of similar age (Marshall 2006), but the Msikaba Formation, deposited north of

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Durban, would be an equivalent of the Witteberg Group (Late Devonian; Marshall 2006). Based on these uncertainties, the existing data for these successions are not included in the study.

Int J Earth Sci (Geol Rundsch) (2011) 100:527–541

The dramatic thickening of the Cape basin-fill from west to east has been ascribed to basin architecture, and the availability of accommodation space to complex local and regional tectonic control (Tankard et al. 2009). The basement structure is highly complex as reflected by several horst-and-graben fault blocks that formed immediately prior to sedimentation of the Cape Supergroup (Tankard et al. 2009). At present, the northern margin of the Cape Supergroup is largely obscured by a cover of late Palaeozoic and Mesozoic Karoo strata. However, along the west coast of South Africa, the northern erosional margin of the succession is exposed and it onlaps onto Meso- and Neoproterozoic metamorphic complexes of the Namaqua and Gariep provinces, respectively (Fig. 1). The Cape Basin possibly extended to the Malvinas Islands (Tankard et al. 2009) and towards the Rı´o de la Plata Craton in eastern Argentina (Sierras Australes, 400 km south of Buenos Aires; 38°S) and the Sierra Grande in Patagonia (418S) (Fig. 2). The basin is partly floored by Mesoproterozoic aged basement, the NamaquaNatal Metamorphic Belt, stretching from southern Namibia to southeast South Africa. However, it is controversial what caused that large magmato-metamorphic belt to form between c. 1,300 and 1,000 Ma (e.g. Dalziel 2000; Cornell et al. 2006). Other underlying rocks are Neoproterozoic to Cambrian metasedimentary and igneous rocks exposed in the southwestern part of the Kalahari Craton. Age equivalent rocks are described in the southern part of the Rı´o de la Plata Craton (Oyhantc¸abal et al. 2010) and Patagonia (Pankhurst et al. 2003) including Cambrian igneous rocks (e.g. Rozendaal et al. 1999; Scheepers and Armstrong 2002; Rapela et al. 2003, 2007; Pankhurst et al. 2006).

Sampling Two complete stratigraphic profiles of the Cape Supergroup were sampled for geochemical analysis. One profile is situated in the western part of the basin between Nieuwoudtville and Ceres in the Western Cape Province (Fig. 1). The other is situated in the eastern part of the basin north of Port Elizabeth in the Eastern Cape Province (Fig. 1). A third section in the southwestern part of the outcrop area close to Worcester was sampled through the Bokkeveld Group exclusively, as palaeocurrents around the area revealed input from the west (Theron 1970, 1972), and it was hoped that provenance information in the area might reveal source areas to the west of the Kalahari Craton. A set of 3–10 samples was collected for each of the formations in the three areas sampled. Samples for detrital zircon analyses were taken from arenites of the Wuppertal and correlative Karies formations in the middle of the Bokkeveld Group and from the

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immediately underlying Nardouw Subgroup of the Table Mountain Group and overlying Weltevrede Subgroup of the Witteberg Groups (Fig. 3). The samples were collected in the western and eastern area of the basin (Fig. 1). In addition, one sample was collected from the Wuppertal Formation of the Bokkeveld Group in the southwestern part of the basin where palaeocurrent directions indicate transport from a westerly direction (Fig. 1). Several samples per formation were combined to gain as much information as possible about the age distribution of detrital zircon populations in the different rock units and areas of the basin.

Geochemistry Table 1 presents a summary of the most important geochemical proxies averaged for the various formations. We shall focus on the main trends in the different sedimentary successions and the differences between the western and eastern basin. Figure 5a demonstrates the trends for the main units regarding their general chemical composition (after Winchester and Floyd 1977; Fralick 2003; Lacassie et al. 2006; van Staden et al. 2006; Bertolino et al. 2007). The quartz-rich Table Mountain Group displays predominantly higher Zr/Ti ratios than typical unrecycled Upper Continental Crust (UCC after McLennan et al. 2006) with only a few samples from the glaciogenic Pakhuis Formation (diamictite) in the western basin having lower ratios (Fig. 5a, Table 1). The dominant trend is towards a rhyolitic composition in both, the west and east, with some samples from the eastern basin plotting towards more alkaline compositions (Fig. 5a). The geochemistry of rocks of the Bokkeveld Group displays bimodal chemical compositions related to sampling of arenitic (above UCC composition) and argillaceous (below UCC composition) units in the succession. Thus, provenance data seems to have been separated into more mafic and felsic components most probably due to grain-size sorting. The argillaceous samples trend towards a detrital mix dominated by less fractionated material than felsic rocks and are geochemically more homogeneous than the arenitic units (Table 1). The western arenitic units display a strong alkaline trend, whereas the southwestern and eastern units trend towards rhyolitic fields. In contrast to the Bokkeveld Group, Zr/Ti ratios in rocks of the Witteberg Group displays a definite alkaline trend in both the eastern and western arenitic units, but the trend is even more pronounced than in the Bokkeveld Group in the western basin (Fig. 5a, Table 1). Most of the samples from the eastern basin plot around, but mostly higher than UCC (Fig. 5a). Figure 5b displays trends in recycling and fractionation of the bulk sediment for all the units. The Th/Sc (Y-axis) ratio measures the degree of fractionation of the mixed

123

123

Witteberg

East

Table Mountain

Bokkeveld

Witteberg

West

Group

Shale

Wagen Drift

Sand Shale

Gamka

Gydo

Sand Shale Sand-silt Sand-shale Sand-cong

Pakhuis

Cederberg

Cederberg

Skurweberg

Goudini Rietvlei

Shale-silt Shale-silt Shale-sand Silt-sand

Witpoort

Lake Mentz/Kommadagga

Floriskraal

Dirkskraal

Sand

Til-shale

Peninsula

Weltevrede

Sand Sand-cong

Pikinierskloof

Silt-sand-cong

Silt

Voorstehoek

Graafwater

Shale Sand

Hex Rivier

Boplaas

Tra-Tra

Shale Sand

Waboomberg

Shale Sand

Silt-sand

Swartruggens

Sand

Sand

Blinkberg

Klipbokkop Wuppertal

Sand-silt

Witpoort

Osberg

Shale-silt

Lake Mentz (undiff.)

Shale

Sand

Karoopoort

Silt-sand

Floriskraal

Rock type

Waaipoort

Formation/subgroup

Patterson

Uitenhage

Steytlerville

Patterson/Steytlerville

Patterson

Franschhoek Ceres

Doringriver

Doringriver/Franschhoek

Doringriver

Pakhuis pass

Klawer

Doringbaai

Doringbaai

Nieuwoudtville

Nieuwoudtville

Nieuwoudtville

Ceres

Ceres

Ceres

Ceres

Ceres Citrusdal

Ceres

Ceres

Ladismith

Ladismith

Buffelsriver

Beervleidam/N Ceres

Beervleidam

Ceres

N Ceres

Locality

74.9

69.1

68.8

69.7

86.4

78.9 87.3

69.7

62.2

91.2

65.0

89.1

87.6

77.2

66.0

91.2

67.8

91.2

64.7

93.2

59.0

66.1 89.4

91.6

58.93

67.1

78.5

84.8

83.8

71.2

82.7

72.5

SiO2

0.54

0.76

0.67

0.53

0.28

0.60 0.08

0.77

1.14

0.19

0.54

0.12

0.16

0.39

0.83

0.28

1.02

0.50

0.75

0.42

0.92

0.91 0.52

0.78

1.06

0.85

0.53

0.51

0.34

0.80

0.31

0.58

TiO2

11.9

17.5

18.6

10.1

8.3

14.8 5.1

20.2

26.3

7.1

13.9

7.4

8.7

14.3

17.5

7.6

20.8

11.2

17.5

9.9

18.6

19.0 12.6

14.7

21.8

22.1

15.3

12.8

9.5

18.7

10.6

6.6

Nb

310

211

271

354

211

528 71

334

225

115

229

80

219

333

259

250

871

417

127

319

162

353 326

647

248

351

552

361

203

363

180

96

Zr

7.0

7.9

11.0

5.9

1.6

4.4 1.4

9.7

23.3

0.9

10.2

0.8

1.6

6.4

15.4

1.7

11.8

2.7

17.4

2.1

20.3

16.2 2.8

3.3

21.0

16.0

9.7

2.9

2.6

9.9

2.2

9.0

Sc

13.5

11.9

18.7

26.0

12.8

17.6 4.5

13.6

21.8

7.2

13.2

1.9

5.6

11.4

15.2

6.4

29.2

9.7

13.1

15.6

18.4

20.8 12.8

25.1

20.3

16.2

16.0

11.0

10.4

13.4

5.8

8.5

Th

23

24

58

31

15

37 9

38

52

12

41

16

15

32

41

15

64

24

33

24

41

62 22

44

52

52

43

28

20

42

15

25

La

0.710

0.570

0.460

1.060

2.400

0.920 0.988

0.618

0.594

0.734

0.490

0.682

0.759

0.490

0.495

1.356

0.372

1.809

0.541

1.423

0.412

0.523 1.313

1.689

0.543

0.600

0.610

3.280

3.010

0.690

2.230

2.130

Nb/Y

0.096

0.046

0.067

0.111

0.126

0.142 0.163

0.076

0.033

0.108

0.070

0.124

0.268

0.175

0.052

0.149

0.143

0.140

0.028

0.128

0.029

0.065 0.104

0.138

0.039

0.069

0.174

0.118

0.100

0.075

0.097

0.028

Zr/Ti

3.29

3.04

5.27

5.25

9.38

8.32 6.38

4.25

2.25

14.33

4.01

21.14

10.22

5.45

2.69

8.82

5.40

9.00

1.89

11.59

2.03

3.80 7.83

13.32

2.49

3.25

4.43

9.66

7.69

4.24

6.82

2.78

La/Sc

44.29

26.71

24.64

60.00

131.88

122.81 59.58

39.23

9.37

133.56

22.45

96.80

131.00

59.40

16.81

146.98

74.02

156.34

7.28

152.00

7.98

21.73 117.93

196.05

11.80

21.94

56.91

124.48

78.08

36.67

81.82

10.67

Zr/Sc

Table 1 Averages of geochemical data for the different sampled formations and members of the three groups of the Cape Supergroup in the western and eastern basins

1.93

1.51

1.70

4.41

8.00

3.79 2.38

1.73

1.03

5.77

1.29

5.96

3.70

2.33

0.99

3.76

2.48

3.63

0.75

7.41

0.91

1.28 4.61

7.60

0.97

1.01

1.65

3.79

4.00

1.35

2.61

0.94

Th/Sc

10.44

21.59

14.82

8.98

7.96

7.11 6.78

14.39

32.29

10.32

14.29

7.94

3.78

8.61

19.27

6.72

7.00

7.14

35.25

7.83

34.01

15.41 9.62

7.23

25.63

14.53

5.76

8.47

10.04

13.26

10.32

36.22

Ti/Zr

532 Int J Earth Sci (Geol Rundsch) (2011) 100:527–541

5.63 11.67 4.81 1.40 142.30 39.93 12.99 4.14 0.180 0.134 0.744 0.550 33 38 11.8 13.0 3.0 10.2 331 284 Sand Silt-sand Goudini/Tchando Baviaanskloof

cong conglomerates, sand sandstone; silt siltstone, til tillite (from Fourie 2009)

0.31 0.53 80.7 75.8

Sand Skurweberg/Kouga

Meiringspoort Meiringspoort

0.24 88.5

Silt-sand Cedarberg

Meiringspoort

detritus, while the Zr/Sc (X-axis) ratio measures the degree of recycling (McLennan et al. 1990). The plots clearly reveal an increase in Zr/Sc ratios with increasing sediment maturity of the formations (Table 1). Samples with Th/Sc ratios below 0.89 (UCC, after McLennan et al. 2006) are present in all the argillaceous units, and as such are more common in the Bokkeveld Group (Table 1). Recycling decreases from the Table Mountain Group to the Bokkeveld Group and slightly from the Bokkeveld Group to the Witteberg Group. The western part of the basin seems to have undergone more intense recycling than the southern and eastern parts of the basin, especially during Bokkeveld Group sedimentation (Fig. 5b). Figure 5c displays a common plot of trace geochemical data to differentiate palaeotectonic setting of siliciclastic sedimentary rocks using the trace elements Ti, La, Sc and Th (Floyd and Leveridge 1987). Also, on these diagrams it is clear that the lithological composition of samples determines in which tectonic setting or field they plot (see Table 1). In general, arenaceous rock units trend to the ‘‘passive margin’’ field and the argillaceous units to the ‘‘active margin’’ fields. Exceptions are represented by arenaceous units in the lower part of the Bokkeveld Group from the southwestern part of the basin that plot within the ‘‘active continental margin’’ field (Table 1). Units from the western Witteberg Group do not plot in ‘active continental margin’ fields, but the argillacous rocks in the eastern basin show such a trend.

9.4 15.1

5.79

7.46 5.73 166.83 17.37 0.141 1.459 17 6.7 1.0 169

12.15 Sand

Shale

Peninsula Table Mountain

Gydo

Meiringspoort

0.48

8.2

2.04 98.45 6.19 0.175 0.756 30 11.4 4.9 495

33.26

78.0

12.2

3.20

0.74 9.38

79.18 8.16

2.61 0.030

0.110 0.800

0.822 49

20 7.8

13.9 18.9

2.7 189

177

0.36 91.0 E of Oudtshoorn

11.1

1.0 60.6 Port Elizabeth

21.9

34.42

7.31 2.78

0.73 8.71

73.51 5.21

2.10 0.029

0.137 0.439

0.609 41

52 27.8

14.4 19.7

10.0 735

171 21.6 1.0

0.9

60.1

17.0

533

77.4

Port Elizabeth

Port Elizabeth

Shale

Sand Gamka

0.7 Port Elizabeth Sand Hex Rivier

Voorstehoek

28.63

12.58 1.48 30.52 3.72 0.080 0.555 41 16.2 10.9 334

12.62

72.0

16.4

0.85

1.53 35.01

11.69 2.08

4.59 0.079

0.035 0.678

0.545 42

32 13.1

14.0 9.1

15.4 180

320 16.1

19.4

0.7

0.9

72.1

56.4 Shale Tra-Tra

Port Elizabeth Sand Boplaas

Port Elizabeth

27.02 0.97 12.16 2.81 0.037 0.643 49 16.8 17.3 211 1.0 Shale Karies

Port Elizabeth

61.6

22.0

19.00

6.49 5.07

1.38 19.19

120.45 8.64

3.86 0.053

0.154 0.559

0.473 46

39 22.8

16.5 12.0

4.5 542

230 16.9 0.7 70.1

12.0 0.6 87.2

Sand-shale

Port Elizabeth

Sand

Adolphspoort

Bokkeveld

Sandpoort

Steytlerville

TiO2 Group

Table 1 continued

Formation/subgroup

Rock type

Locality

SiO2

Nb

Zr

Sc

Th

La

Nb/Y

Zr/Ti

La/Sc

Zr/Sc

Th/Sc

Ti/Zr

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Detrital zircon age populations The detrital zircon age populations of the samples of the Bokkeveld Group from the western and southwestern areas (see sample locations in Fig. 1) are very similar (Table 1 supplementary material) and are therefore grouped together as part of the western sample area in Fig. 6a. The Bokkeveld Group, sampled in the southern area, contained the youngest detrital zircon in the Cape Supergroup, which was dated as 388 ± 36 Ma (BV-SW3; Fig. 6a; Table 1 supplementary material). A larger group of similar aged detrital zircons are dated in the western and southwestern Bokkeveld samples, which dates from the Lower Palaeozoic ranging from Upper Ordovician to Devonian (Table 1 supplementary material). This age is close to the proposed depositional age of the Bokkeveld Group (Boucot and Theron 2004) (Fig. 3). The youngest detrital zircon from the underlying Table Mountain Group in the western area was dated as Late Neoproterozoic (526 ± 30 Ma, TMG-W1, Table 1 supplementary material), whilst the youngest detrital zircon grain from overlying Witteberg Group in that area falls into the Ordovician (424 ± 54 Ma, WB-W1, Table 1 supplementary material). In the eastern

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Fig. 5 a Geochemical trends after Winchester and Floyd (1977). b Main geochemical compositional characteristics for clastic sandstones using Th/ Sc versus Zr/Sc ratios (McLennan et al. 1990). c La/Sc versus Ti/Zr ratios (after Floyd and Leveridge 1987) pinpoint the probable tectonic setting of clastic rocks

area, the best yield of concordant detrital zircon age data came from the Bokkeveld Group, as almost 80% of the zircon grains from the Table Mountain and Witteberg Groups were strongly discordant. However, the youngest grain was discovered in the Table Mountain Group sample with a Lower Palaeozoic age (TMG-E1 with an age of 469 ± 26 Ma, Table 1 supplementary material and Fig. 6b). The oldest detrital zircon grain found in the western area comes from the Table Mountain Group and has a Neoarchaean age of 2556 ± 20 Ma (Fig. 6a and

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Table 1 supplementary material). In the eastern area, the oldest detrital zircon dated has an age of 2682 ± 17 Ma and comes from the sample of the Bokkeveld Group (Table 1 supplementary material and Fig. 6b). All three groups contain detrital zircon grains from the Cape Granites, an intrusive suite with an age between 550 and 505 Ma (Scheepers and Armstrong 2002) and/or their equivalents in eastern Argentina (Rapela et al. 2003; Gregori et al. 2004). Neoproterozoic detrital zircons are also abundant in all samples. Detrital zircon grains derived

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Fig. 6 a Schematic diagram for detrital zircons of the western basin of the Cape Supergroup according to data from Table 1 supplementary material. b Schematic diagram for detrital zircons of the eastern basin of the Cape Supergroup. The oldest and youngest zircons are shown. Vertical green lines indicate the ion-probe data. Examples of selected zircons in CL-light can be found in Fig. 2 data repository. All data shown here are with 2-sigma error

from the Mesoproterozoic metamorphic rocks of the Namaqua-Natal Metamorphic Belt are widespread in the Cape Supergroup, while those from Palaeoproterozoic source are meagre (Table 1 supplementary material) and identified in the sample of the Witteberg Group (WB-W48, WB-W49) and in the Bokkeveld Group with palaeocurrents from the west BV-SW46, BV-SW47). The oldest detrital zircon grain found in the western basin area transported from the north (Fig. 2) is of Archaean age and found in the Table Mountain Group sample (Table 1 supplementary material, TMG-W24). The sedimentary rocks deposited in the eastern basin are characterised by palaeocurrents mainly directed to the south (Fig. 2). The best zircon yield was from the

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Bokkeveld Group sample (Table 1 supplementary material), as most of the grains found in the Table Mountain and Witteberg Groups were strongly discordant (c. 70% of the measured detrital zircon grain). However, the youngest grain was discovered in the Table Mountain Group sample with a Lower Palaeozoic age (Table 1 supplementary material, TMG-E1; Fig. 6b). Constraints for the depositional age of the Bokkeveld Group failed using detrital zircons, as the youngest detrital zircon grain dated was Cambrian (Table 1 supplementary material) and fall in the same time frame as the Cape Granites (550–505 Ma) although no exposure of granite is known to the north of the sample region (Scheepers and Armstrong 2002; Thamm and Johnson 2006). Detrital zircons of Neoproterozoic ages are amply abundant in the Bokkeveld Group sedimentary rocks (Table 1 supplementary material). Similar to the samples from the western part of the basin, the influence of the Mesoproterozoic Namaqua-Natal Metamorphic belt is strong and accounts for a large number of zircons. One grain (BV-E44), from the Bokkeveld Group, has an early Mesoproterozoic age, which is relatively rare for the Kalahari Craton. One Archaean detrital zircon could be identified (Table 1 supplementary material) in the Bokkeveld Group (BV-E46) together with one of Palaeoproterozoic age (BV-E45). Overall, the detrital zircon age populations of the western and eastern areas are similar with only minor differences (Fig. 6). In both areas, there are peak populations of zircons with late Neoproterozoic (600–540 Ma) to very early Palaeozoic (540–450 Ma i.e. Cambrian to Ordovician) ages and a second peak population of detrital zircon ages spans the late Mesoproterozoic between *1.0 Ga and *1.2 Ga. In both the western and eastern areas, a few of the detrital zircons are also of late Paleoproterozoic (*1.8–2.0 Ga) and early Paleoproterozoic to Late Neoarchaean age. However, no zircons older than Neoarchaean were found in the samples (Fig. 6).

Provenance constraints Geochemical signatures reveal a decrease in the amount of reworking of the sediments in the Cape basin from the Table Mountain Group to the Bokkeveld Group, with reworking increasing again in the Witteberg Group. This is indicated by Zr/Sc ratios in argillaceous and arenaceous samples (Fig. 5b) and confirmed by petrographic data (Fourie 2009). A general decrease in the degree of reworking from west to east is also noticeable geochemically (Fig. 5b), and petrographically (Fourie 2009). This reflects the existence of independent basin cycles rather than one large basin evolution for the Cape Supergroup and supports the evolutionary model of Tankard et al. (2009).

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Fig. 7 The Cape Supergroup in southern Africa deposited on the Saldania belt and Mesoproterozoic basement rocks and the possible source areas. The most abundant source is the underlying rocks and the Namaqua-Natal belt. Possibly, an exotic block to the south (Patagonia) existed during deposition of the Cape Supergroup and supplied Lower Palaeozoic zircons. The source for the Ordovician to Silurian igneous zircons is still not clear. Magmatic rocks of such an age are known from southern Patagonia (Fig. 2) and Buenos Aires province (marked by a red star). The euhedral nature of these young detrital zircons (Fig. 2 supplementary material) favours a source close to the depositional area like a volcanic centre positioned in the Palaeozoic basins (volcano symbol). Brown boxes mark Palaeozoic sedimentary successions in Argentina with post-Cambrian Palaeozoic detrital zircons. Blue circles indicate the main sources for the detritus and silver circles together with stippled arrows important but scarce source rocks. The red circle in the east represents the unknown Cryogenian source for the eastern basin deposits. The green stippled line marks the extension of the Cape basin inferred from detrital zircon data (this study) and earlier studies (Tankard et al. 1982; Rapela et al. 2003; Pankhurst et al. 2006; Van Staden et al. 2010b). BA Buenos Aires, CT Cape Town; the Saldania belt underlies the Cape basin

In the western basin, a significant alkaline trend in the arenitic beds of the two upper units, the Bokkeveld and Witteberg Groups (Fig. 5a), indicates the exposure of a new alkaline source(s) during Bokkeveld sedimentation. This trend is not evident in the Table Mountain Group (Fig. 5a). The southwestern part of the basin reflects input of arc-related material in the lower Bokkeveld Group (Fig. 5b, c; Table 1), whereas all other units show a typical ‘‘passive margin’’ signature across the whole basin. The schematic detrital zircon age populations in both the western and eastern parts of the basin clearly display the dominance of late (*1.0–1.2 Ga) Mesoproterozoic sources. The late Mesoproterozoic Namaqua-Natal Metamorphic Belt (Cornell et al. 2006; Eglington 2006) forms the basement immediately north of the Cape Supergroup on the Kalahari Craton and is thus, considering palaeocurrent data (Theron 1970, 1972), the most likely source for this age fraction (Fig. 7). The metasedimentary Bushmanland Group that forms part of the Namaqua-Natal Metamorphic

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Belt contains detrital zircons of early to middle Mesoproterozoic age and could have served as source for the few detrital zircons with ages around 1.4 Ga (Figs. 6, 7). However, perhaps more importantly is to note that the Richtersveld Subprovince along the border between South Africa and Namibia represents an ancient *2.0–1.7 Ga volcanic arc complex (Reid 1982; Reid et al. 1987; Cornell et al. 2006) and could also have contributed zircons of that age to especially the western part of the Cape Basin (Fig. 7). The second largest population of detrital zircons is of late Neoproterozoic age in both the western and eastern parts of the Cape Basin (Fig. 6). These grains most obviously must have been sourced from the so-called PanAfrican orogeny that includes the Gariep and Damara metamorphic belts along the west coast of South Africa and Namibia (Tankard et al. 1982), the Dom Feliciano belt in South America (Basei et al. 2008), and the Mozambique Belt along the far northeastern coast of South Africa and east coast of Mozambique (Jacobs et al. 2003; Fig. 7). The source of relatively large population of detrital zircons with early to middle Neoproterozoic (*0.9–0.7 Ga) ages in both the western and eastern parts of the Cape Basin (Fig. 6) is more difficult to account for in the eastern basin. The only magmatic event recorded so far in southern Africa with that age relates to the Richtersveld Igneous Complex situated at the frontier between Namibia and South Africa (Frimmel et al. 2001). In South America comprises the Punta del Este Terrane, in eastern Uruguay (Fig. 7), a magmatic event of such an age (Oyhantc¸abal et al. 2009) and might have functioned as the source for the western basin part. Late Neoproterozoic to Cambrian-aged zircons were most probably originated from granitoid plutons and felsic volcanic rocks that mark the extensional tectonic event that preceded deposition of the Cape Supergroup (Rozendaal et al. 1999; Rapela et al. 2003; Gregori et al. 2004). The Terra Australis Orogen that was situated along the southern coast of Gondwana is also of this age (Cawood 2005) and could also have sourced some sediment into the Cape Basin (Fig. 7). The presence of a significant population of detrital zircons with lower Palaeozoic (Ordovician–Silurian) ages (*500–400 Ma) in the Cape Supergroup is perhaps most problematic but at the same time most interesting regarding tectonic setting and palaeogeographic reconstructions of the basin and its source areas. The Ross Orogeny in the adjacent Antarctica continental fragments was active at that time (Foden et al. 2006) and in addition the Balcarce Formation in eastern Argentina (Fig. 7), an equivalent of the upper part of the Table Mountain Group, contains massive tuff beds of possible Silurian (Zimmermann and Spalletti 2009; Van Staden et al. 2010b) age. These tuffs

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might have been related to the same volcanic chain or system as the one that sourced the Cape Basin on the Kalahari Craton (Fig. 6). Detrital zircons of Siluro-Devonian age were also identified in Devonian successions in Eastern Argentina (Uriz et al. 2010) and may point to a source in Patagonia. A Silurian magmatic event is known from the Deseado Massif in southern Patagonia (Fig. 2) (Pankhurst et al. 2003), and this event or lateral equivalents of it may have sourced these young detrital zircons. The reason is that some of the youngest detrital zircon grains from the Cape Supergroup are euhedral in shape and could not have been transported over the long distances needed if they had to have come from the Deseado Massif (Fig. 7). The similarity in detrital zircon age populations of the Cape Supergroup on the Kalahari Craton (this study) with those of early Palaeozoic successions in Eastern Argentina (Uriz et al. 2010) support the proposed initial extend of the Cape Basin from the Kalahari Craton into Eastern Argentina (Fig. 2) (Tankard et al. 1982; Rapela et al. 2003; Pankhurst et al. 2006; Van Staden et al. 2010b). Apart from the significance of these youngest Lower detrital zircons in the Cape Supergroup, one of the most remarkable features of the detrital zircon age populations is the absence of any Meso- to Palaeoarchaean zircon grains (Fig. 6). Such zircons are expected if for example the early Archaean Kaapvaal Craton, situated to the north of the Namaqua-Natal Metamorphic Belt, in the core of the Kalahari Craton would have sourced the Cape Supergroup from the north as indicated by palaeocurrent directions (Fig. 1). Perhaps the simplest explanation for this discrepancy is to assume that the Namaqua-Natal Metamorphic Belt still formed a major morphological boundary or divide between rivers that drained into the Cape Basin to the south and the Kaapvaal Craton to the north (Fig. 7).

Basin model During deposition of the Table Mountain Group, larger basins were developed on the Kalahari Craton, and the first cratonic cover sequence with several hundred kilometre extent existed (Tankard et al. 1982) and then changed slightly in its composition reflecting mainly a change of continental to marine sedimentation (Theron 1970, 1972; Tankard et al. 1982; Fourie 2009). It is widely accepted that the Table Mountain Group basin represents mostly shallow marine to fluvial coastal proximal deposits. However, rift-related sediments are absent (Theron 1970, 1972; Tankard et al. 1982; Naidoo 2008; Fourie 2009). Interpretation of the Cape granites and their equivalents in eastern Argentina as being related to extensional tectonics (Scheepers and Armstrong 2002; Rapela et al. 2003) would infer a rifted margin

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environment for the Table Mountain Group. However, apparently extensional granites can appear in other tectonic settings like, for example, Ordovician ‘‘extensional’’ granites associated with foreland basin successions of NW Argentina (Bahlburg 1990; Zimmermann and Bahlburg 2003; Zimmermann et al. 2009). This may be a comparable scenario for the Terra Australis Orogen of southern Gondwana (Cawood 2005), which was active until the Middle Cambrian, followed by extensional tectonics in its foreland basin. Rift-related magmatism is not reported from the southeast of the Kalahari Craton, which might point to extensional tectonics further east, as detrital zircon grains with ages, comparable to the Cape granite and their equivalents, have been found (Table 1 supplementary material; Fig. 7). The following sedimentation cycle, represented by the Bokkeveld Group, comprises immature rocks, partly wackes and a wide range of detrital zircon ages. The Bokkeveld basin was characterised by relatively short distances of sediment transport between source and sink areas, and only few detrital material have been derived from the underlying clean quartz-arenites. Tankard et al. (2009) mention fault-controlled subsidence as a major event during the deposition of the Bokkeveld Group. We observe magmatic activity definitely younger than the Cape granites in the sedimentary rocks in South Africa (this study) and eastern Argentina (Zimmermann and Spalletti 2009; Uriz et al. 2010; Van Staden et al. 2010b). Only few areas in eastern Argentina are so far identified comprising Silurian magmatism (Deseado Massif, southern Patagonia, Fig., 2; Pankhurst et al. 2003). Most of the detrital zircons found in the western basin of the Cape Supergroup are euhedral and cannot be sourced from that southern Patagonian area but related to other lateral extensions of this igneous event. The western Witteberg Group comprises the most recycled and most alkaline detritus and points to a new source, not exposed previously during the earlier sedimentation cycle of the Cape Supergroup (Fig. 5) but the detrital zircon grain population in the western basin part does not differ significantly from the older successions (Table 1 supplementary material). The occurrence of Ordovician to Silurian detrital zircons in the Cape Supergroup and Palaeozoic successions in eastern Argentina (Rapela et al. 2007; Uriz et al. 2010; Van Staden et al. 2010b) allows to define a larger basin covering the south of the Kalahari Craton and the eastern margin of Argentina, the Rı´o de la Plata Craton and Patagonia (Fig. 7) as previously proposed using evidence from metamorphic and magmatic rocks (Rapela et al. 2003; Gregori et al. 2004) or structural data (in Tankard et al. 2009). Recently, the identification of glacial deposits in eastern Argentina (Tankard et al. 1982; Zimmermann and

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Spalletti 2009; Van Staden et al. 2010b) allowed for the first time a lithostratigraphic correlation.

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Appendix: analytical methods X-ray fluorescence (XRF) analyses, INAA (Instrumental Neutron Activation Analysis), ICP-MS

Conclusions The detrital zircon age distribution of the Cape Supergroup reveals as the dominant source the Mesoproterozoic Namaqua-Natal Metamorphic Belt. Detrital input from the older (Palaeoarchaean to Palaeoproterozoic) craton interior (Kaapvaal Craton; Fig. 7) is almost completely absent throughout the deposition of the entire succession. We therefore suggest that the Namaqua-Natal Metamorphic Belt had been a large morphological divide, which have existed as such from the Neoproterozoic to the Lower Carboniferous. The Upper Neoproterozoic to Middle Cambrian detrital zircon age distribution throughout the Cape Supergroup might be related to several source areas on, and surrounding the Kalahari Craton. The western and southwestern sample areas might have received input of this age fraction from the Dom Feliciano belt that faced or represented the eastern margin of the Rı´o de la Plata Craton (Figs 2 and 7), as well as from the younger Cape Granite Suite that intruded the Kalahari Craton between 550 and 505 Ma. The latter can also be found in eastern Argentina on the Rı´o de la Plata Craton in the Sierras Australes, south of Buenos Aires. The proposed Terra Australis Orogen that bordered Gondwana to the south is also contemporaneous with this age fraction and might have provided input especially to the eastern part of the basin. Ordovician to Silurian detrital zircons in the western and especially the southwestern parts of the Cape basin have been derived from sources exotic to the Kalahari Craton. Coeval sedimentary successions from eastern Argentina also contain this age fraction, but the source of the zircons is still unknown. In southern Patagonia, Silurian igneous rocks are described and might be related to the same magmatic event. However, most of the detrital zircons of this specific age are euhedral and were thus not transported as far. This study, therefore, provides additional evidence for a similar basin evolution in eastern Argentina and the Kalahari Craton during the Palaeozoic and should assist in correlations of rocks between the two regions. Acknowledgments PHF thanks the National Research Foundation of South Africa and the Geological Society of South Africa for financial assistance. UZ and JT thank the Marie Curie FP6 EXT Action for financial support of the isotope laboratory studies. UZ acknowledge Marathon and Statoil for their financial support of geochemical and isotope geochemical analyses. We like to thank the reviewers Robert J. Pankhurst and Leo A. Hartmann for their thorough reviews, which improved our manuscript. We also like to thank S. Siegesmund and M. Basei for their kind invitation to this special volume and the editors for their effective handling.

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Powders were prepared by milling to a very fine mesh. Pressed powder pellets (for selected trace elements) contain 8.0 g of sample powder mixed with 4 g of binder, placed in an aluminium cup and pressed at a pressure of 20 tons. Glass beads (for major elements) were prepared by fusing sample powder with a flux containing 50% lithium metaborate and 50% lithium tetraborate with LiNO3 as oxidant. XRF precision and accuracy were controlled by international and internal rock standards to below 5% (1 s) for measured elements. INAA was performed by ACTLABS (Ontario, Canada). Sample powders were dissolved in lithium metaborate flux, and the resultant bead rapidly digested in dilute nitric acid. INAA precision and accuracy based on replicate analysis of international rock standards are 2-5% (1 s) for most elements and ± 10% for U, Sr, Nd and Ni. For ICP-MS, analyses were carried out by ICP-MS at ACME Laboratories (Vancouver, Canada). For more information about the analytical parameter, please visit www.acmelab.com. Ion-probe analysis Uranium and Pb isotopes were measured by high-resolution secondary ion mass spectrometry (HR-SIMS) using a Cameca ims-1270 system at ISEI, Okayama University, following the protocol of Usui et al. (2002). Zircons were analysed in situ in one-inch-round resin mounts. To reduce common Pb contamination on the sample surface, rounds were rinsed with 0.1 M HF and placed in a 0.1 M HNO3 ultrasonic bath for 3 min. The clean sample was coated with 30 nm thickness of gold. A focused O primary beam of 15 nA accelerated to 13 kV was used, resulting in a primary beam diameter of * 25 mm sampling diameter. Secondary acceleration power was 10 kV and over 5000 mass resolution power (M/DM) was applied to remove the mass interferences. Signals from 204Pb, 206Pb, 207Pb and 208 Pb were simultaneously collected using electron multipliers of the multi-collection system, and UO was collected by adjusting magnet power. Raw data were corrected for instrumental mass discrimination and mass fractionation of the U/Pb atomic ratio and Pb isotopic ratios using calibration curves obtained for the PML-Zr [Sri Lanka, Usui et al. (2002)] and 91500 zircon standard (Wiedenbeck et al. 1995, 2004) at the beginning of each analytical session. The Isoplot/EX program (after Ludwig 1999) was used to regress and calculate an U/Pb concordia age for each analysis.

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Laser ablation ICPMS dating of zircons The zircons were mounted in 1-inch epoxy-filled blocks and polished to obtain even surfaces suitable for laser ablation ICPMS analysis. Prior to analysis by laser ablation ICPMS, the sample surfaces were cleaned in DI water and ethanol. Isotopic analysis of zircons by laser ablation ICPMS followed the technique described in Kosler et al. (2002). A Thermo-Finnigan Element 2 sector field ICPMS coupled to a 213 NdYAG laser (New Wave UP-213) at Bergen University was used to measure Pb/U and Pb isotopic ratios in zircons. The sample introduction system was modified to enable simultaneous nebulisation of a tracer solution and laser ablation of the solid sample (Horn et al. 2000). Natural Tl (205Tl/203Tl = 2.3871—Dunstan et al. 1980), 209Bi and enriched 233U and 237Np ([99%) were used in the tracer solution, which was aspirated to the plasma in an argon–helium carrier gas mixture through an Apex dissolving nebuliser (Elemental Scientific) and a T-piece tube attached to the back end of the plasma torch. A helium gas line carrying the sample from the laser cell to the plasma was also attached to the T-piece tube. The laser was set up to produce energy density of ca 3 J/cm2 at a repetition rate of 10 Hz. The laser beam was imaged on the surface of the sample placed in the ablation cell, which was mounted on a computer-driven motorised stage of a microscope. During ablation, the stage was moved beneath the stationary laser beam to produce a linear raster (ca 10 9 40 microns) in the sample. Typical acquisitions consisted of a 35 s measurement of analyses in the gas blank and aspirated solution, particularly 203Tl–205Tl–209Bi–233U–237Np, followed by measurement of U and Pb signals from zircon, along with the continuous signal from the aspirated solution, for another 120 s. The data were acquired in time-resolved— peak jumping—pulse-counting mode with 1 point measured per peak for masses 202 (flyback), 203 and 205 (Tl), 206 and 207 (Pb), 209 (Bi), 233 (U), 237 (Np), 238 (U), 249 (233U oxide), 253 (237Np oxide) and 254 (238U oxide). Raw data were corrected for dead time of the electron multiplier and processed off line in a spreadsheet-based program (Lamdate—Kosler et al. 2002) and plotted on concordia diagrams using Isoplot (Ludwig 1999). Data reduction included correction for gas blank, laser-induced elemental fractionation of Pb and U and instrument mass bias. Minor formation of oxides of U and Np was corrected for by adding signal intensities at masses 249, 253 and 254 to the intensities at masses 233, 237 and 238, respectively. No common Pb correction was applied to the data. Details of data reduction and corrections are described in Kosler et al. (2002) and Kosler and Sylvester (2003). Zircon reference material 91500 (1,065 Ma—Wiedenbeck et al. 1995), GJ-1 (609 Ma—Jackson et al. 2004) and Plesˇovice

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(337 Ma—Slama et al. 2008) were periodically analysed during this study, and they yielded mean ages of 1065 ± 5.6, 591 ± 10 and 343 ± 4.8 Ma, respectively.

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