An isotopic and geochemical study of the northern Kaapvaal Craton and the Southern Marginal Zone of the Limpopo Belt: are they juxtaposed terranes?

Lithos 50 Ž2000. 1–25 www.elsevier.nlrlocaterlithos

An isotopic and geochemical study of the northern Kaapvaal Craton and the Southern Marginal Zone of the Limpopo Belt: are they juxtaposed terranes? Katharina Kreissig a

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a , Thomas F. Nagler , Jan D. Kramers a , Dirk D. van Reenen b, ¨ C. Andre´ Smit b

Gruppe Isotopengeologie, Mineralogisch Petrographisches Institut, UniÕersitat ¨ Bern, Erlachstrasse 9a, 3012 Bern, Switzerland b Department of Geology, Rand Afrikaans UniÕersity, Auckland Park, Johannesburg, South Africa Received 12 January 1999; accepted 23 April 1999

Abstract To test existing lateral accretion models for the Archean Kaapvaal Craton in South Africa, metapelites and leucocratic granitoids from two different provinces, the granulite facies Southern Marginal Zone of the Limpopo Belt and the northern part of the adjacent greenschist to amphibolite facies Kaapvaal Craton, have been studied. Both provinces resemble each other in terms of isotope and trace element characteristics. They show a general depletion of incompatible elements independent from the grade of metamorphism. This observation conflicts with theories involving mobilisation during granulite facies metamorphism. A further common fingerprint is the unusually high Cr and Ni concentrations in the metapelites. In addition, they define a very narrow range of Nd model ages Ž2.9–3.05 Ga. constraining the average crustal age of their cratonic source region. The granitoid gneisses show somewhat larger variations of Nd model ages indicating some age heterogenity within the crust. However, the data from both provinces completely overlap. The Southern Marginal Zone and the Kaapvaal Craton also show uniform low 207 Pbr204 Pb relative to their 206 Pbr204 Pb ratios that differ clearly from other Southern African provinces. These results support the view that the Southern Marginal Zone represents a part of the Kaapvaal Craton exposed at a deeper crustal level; it rules out a hypothesis that both provinces are separate amalgamated terranes. Furthermore, there is very little difference between granitoids and metapelites which indicates that the latter largely derived from the former. The exposure of the granulite facies metapelites of the Southern Marginal Zone at the same level as the lower grade metamorphic rocks of the Kaapvaal Craton indicates uplift along a sharp tectonic boundary. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Archean; Geochemistry; Isotopes; Plate tectonics; Limpopo Belt; South Africa

1. Introduction

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Corresponding author. Fax: q41-31-631-49-88; E-mail: [email protected]

Archean provinces are investigated to obtain information about crust formation processes and the role of plate tectonics in the Archean, both of which are not yet well understood. Mechanisms that lead to

0024-4937r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 4 - 4 9 3 7 Ž 9 9 . 0 0 0 3 7 - 7

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K. Kreissig et al.r Lithos 50 (2000) 1–25

formation or destruction of the continental crust may have changed with time. A major process to form continental crust today is partial melting above subduction zones in active continental margins. According to Reymer and Schubert Ž1984., 1.65 km3 continental crust per year is produced in such a scenario including 36% of crustal recycling. The resulting crustal growth rate of about 1 km3 ay1 can account for only about 60% of the existing continental crust over the history of the Earth, and therefore net crustal growth must have been considerably greater at times in the past, presumably in the Archean ŽTaylor and McLennan, 1995.. Further, there are arguments that in the Archean, crust recycling into the mantle was much less than today Že.g., isotopic evidence: Patchett et al., 1984; Kramers and Tolstikhin, 1997; structural evidence: Choukroune et al., 1993.. Despite these indications for changes of the tectonic style through time, there exist also several arguments that modern style plate tectonics played a major role in Archean time. The Kaapvaal Craton in South Africa ŽKC. is a classical and extensively studied example of Archean continental crust. An influential model of its history proposed by de Wit et al. Ž1992. postulates two formation stages within 1 Ga by processes similar to those operating at present. In this model, formation of a shield-like continental crust framework by intra-oceanic obduction and amalgamation of oceanic terranes was followed by melting and chemical differentiation in the upper lithosphere. After extensional tectonics, Cordilleratype accretion of the crustal fragments and Alpinestyle continent–continent-collision would have led to the stabilisation of a continent consisting the KC and the Zimbabwe Craton ŽZC.. Since it has been shown that individual Archean cratons have individual Pb isotopic characteristics indicating highly variable time integrated UrPb ratios, as well as other characteristics such as HREE depletion ŽMoorbath et al., 1969; Black et al., 1973; O’Nions and Pankhurst, 1974; Barton et al., 1979, 1992; Wooden and Mueller, 1988; Taylor et al., 1991; Berger and Rollinson, 1997. it is likely that the same applies to allochthonous accreted terranes, and therefore such isotope and trace element chemistry can be used as a test for terrane accretion models.

This is done in this study for the case of the Southern Marginal Zone ŽSMZ. of the Limpopo Belt and the northern part of the KC. These two provinces consist of granitoid gneisses and greenstone belt material and are separated by a sharp tectonic boundary. The SMZ has undergone high grade metamorphism whereas in the northern KC the supracrustals are at greenschist to amphibolite facies. To test whether the SMZ is a separate amalgamated terrane or is in fact a high grade equivalent of the KC, and if so, whether it represents lower crust, we have carried out a comparative geochemical and isotope geochemical study with metapelites and leucocratic gneisses from both tectonic units. Metapelites were included to provide constraints for tectonic models of supracrustal Žgreenstone. belts and to address the question of their exotic Žallochthonous ophiolitic terranes. or regional origin.

2. Geological setting and sampling In the model of de Wit et al. Ž1992. the Limpopo Belt, in the north of the KC, has been regarded as the key example of an Archean orogeny in the modern style and thus as a proof for plate tectonics operating early in Earth history. The Limpopo Belt is a granulite facies province which occupies the border area between Zimbabwe, Botswana and South Africa ŽFig. 1.. It is wedged between the Zimbabwe Craton in the north and the Kaapvaal Craton in the south and is commonly divided into a Northern Marginal, a Central and a Southern Marginal Zone. These are separated from each other by shear belts. Following earlier suggestions ŽMason, 1973; Robertson and Du Toit, 1981; van Reenen and Hollister, 1988., both marginal zones would represent high grade equivalents of the adjacent Kaapvaal and Zimbabwe Cratons which were thrust onto these cratons along shear belts. In the model of de Wit et al. Ž1992. north-ward movement of the KC and southward subduction between 2.9 and 2.7 Ga were followed by a collision with the ZC 2.68 Ga ago resulting in the formation of the Limpopo Belt as a granulite facies root of an orogen Že.g., McCourt and Vearncombe, 1987, 1992; de Wit et al., 1992; Roering et al.,

K. Kreissig et al.r Lithos 50 (2000) 1–25

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Fig. 1. Geological map showing the SMZ of the Limpopo Belt and the northern part of the Kaapvaal Craton ŽKC.. NMZs Northern Marginal Zone; CZ s Central Zone; SMZ s Southern Marginal Zone of the Limpopo Belt; ZC s Zimbabwe Craton.

1992.. The SMZ of the Limpopo Belt was considered to be the backthrust part, which thrust southward over the middle to low grade KC along a set of

shear zones ŽSmit et al., 1992.. Besides the general conflicting isotope geochemical considerations on Archean crustal evolution Žsee above., this model is

K. Kreissig et al.r Lithos 50 (2000) 1–25

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Table 1 Major and trace element data of the TTG-gneisses from the four greenstone belts of the Kaapvaal Craton ŽKC. Elements measured by XRF, U measured by INAA, 8 measured by ICP-MS. bdl s below detection limit. MB s Murchison greenstone belt; PB s Pietersburg greenstone belt; RK s Rhenosterkoppies; SB s Sutherland greenstone belt. Sample Locality Lithology

96r201 KC-MB TTG

96r202 KC-MB TTG

96r233 KC-MB TTG

96r246 KC-PB TTG

96r247 KC-PB TTG

96r204B KC-RK TTG

96r210 KC-RK TTG

96r211 KC-RK TTG

96r239 KC-SB TTG

96r228 KC-SB TTG

Major elements in wt.% SiO 2 72.74 TiO 2 0.28 Al 2 O 3 14.43 Fe 2 O 3 1.20 MnO 0.03 MgO 0.32 CaO 0.91 Na 2 O 5.10 K 2O 3.10 P2 O5 0.09 Sum 98.94 ArNK 1.23 ArCNK 1.08

72.95 0.21 14.88 1.52 0.01 0.40 2.19 5.79 1.08 0.06 99.55 1.39 1.01

74.84 0.09 13.95 0.70 0.01 0.23 1.15 5.61 1.68 0.02 98.84 1.26 1.06

69.50 0.31 16.71 2.11 0.03 0.70 3.03 6.03 0.98 0.10 100.02 1.52 1.01

68.55 0.42 16.12 2.95 0.04 1.12 2.93 5.06 1.69 0.14 100.09 1.59 1.04

74.76 0.04 14.72 0.50 0.01 0.10 2.09 4.48 3.11 0.03 100.60 1.37 1.01

67.40 0.51 16.38 3.83 0.04 1.33 4.22 4.60 1.28 0.19 100.27 1.83 0.99

73.83 0.15 14.83 1.21 0.03 0.47 2.22 4.65 2.06 0.07 100.03 1.50 1.07

70.11 0.36 15.98 1.91 0.02 0.75 2.11 5.90 1.96 0.14 99.68 1.35 1.02

72.66 0.21 15.32 1.30 0.02 0.39 2.00 5.38 2.18 0.08 99.86 1.37 1.03

Trace elements in ppm Ba 714 Cr 6 Cu 4 Nb 6 Ni 5 Pb 15 Rb 202 Sr 517 V 18 Y 7 Zn 34 Zr 189 ScU 1.8 ThU 5.9 UU 1.1

293 bdl bdl 5 3 16 43 488 9 8 35 153 2.0 4.9 0.6

519 bdl bdl bdl bdl 15 59 376 7 bdl 10 123 1.0 2.0 0.4

267 7 10 bdl bdl 11 71 1014 25 6 52 147 3.1 1.5 0.4

475 5 bdl bdl 7 10 126 808 56 10 54 118 4.7 2.4 1.0

969 bdl 15 bdl bdl 13 58 382 8 10 3 45 1.2 2.4 1.2

421 15 9 5 13 9 52 537 71 15 52 196 6.2 3.5 0.9

434 bdl 7 9 5 15 69 258 15 17 31 94 3.1 4.2 1.1

521 14 bdl 7 9 21 74 1021 28 11 50 240 2.9 4.8 0.8

623 bdl bdl 5 bdl 17 63 914 12 11 41 169 1.6 5.8 1.9

REEU in ppm La Ce Nd Sm Eu Gd8 Tb Yb Lu EurEuU ŽLarLu.N ŽLarGd.N ŽGdrLu.N Qtz

23.3 4 17 2.48 0.79 2.1 0.2 0.40 0.07 1.06 34.57 9.27 3.73 29.71

10.5 17 6 0.86 0.60 0.7 0.1 0.20 0.03 2.36 36.35 12.53 2.90 32.89

15.0 37 15 1.96 0.59 1.5 0.2 0.21 0.06 1.05 25.97 8.35 3.11 22.98

19.8 48 20 3.51 1.17 2.9 0.5 0.81 0.08 1.12 25.71 5.70 4.51 24.45

8.4 17 8 1.67 0.55 1.4 0.2 0.62 0.09 1.10 9.69 5.01 1.93 32.14

19.0 40 18 2.81 0.75 2.5 0.5 2.00 0.26 0.87 7.59 6.35 1.20 33.35

38.7 77 30 4.10 1.13 3.2 0.4 0.77 0.08 0.95 50.25 10.10 4.97 22.77

47.2 96 34 5.26 1.37 3.9 0.4 0.73 0.10 0.92 49.03 10.11 4.85 28.15

30.8 64 25 3.06 0.83 2.5 0.3 0.44 0.06 0.92 53.32 10.29 5.18 28.91

36.8 78 28 4.51 1.32 3.9 0.5 1.30 0.19 0.96 20.12 7.88 2.55 24.29

K. Kreissig et al.r Lithos 50 (2000) 1–25

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Table 1 Žcontinued. Sample Locality Lithology

96r201 KC-MB TTG

CIPW norm C 1.25 Or 18.32 Ab 43.15 An 3.93 Di – Hy 1.09 Mag 0.75 Ilm 0.53 Ap 0.21

96r202 KC-MB TTG

96r233 KC-MB TTG

96r246 KC-PB TTG

96r247 KC-PB TTG

96r204B KC-RK TTG

96r210 KC-RK TTG

96r211 KC-RK TTG

96r239 KC-SB TTG

96r228 KC-SB TTG

0.35 6.38 48.99 10.47 – 1.69 0.87 0.40 0.14

0.86 9.93 47.47 5.57 – 0.89 0.42 0.17 0.05

0.46 5.79 51.02 14.38 – 2.71 1.21 0.59 0.23

0.97 9.99 42.82 13.62 – 4.22 1.65 0.80 0.32

0.26 18.38 37.91 10.17 – 0.51 0.30 0.08 0.07

0.21 7.56 38.92 19.69 – 5.44 2.01 0.97 0.44

1.08 12.17 39.35 10.56 – 1.80 0.69 0.28 0.16

0.65 11.58 49.92 9.55 – 2.49 1.16 0.68 0.32

0.67 12.88 45.52 9.40 – 1.47 0.78 0.40 0.19

challenged by recent structural, petrological and geochronological studies in the Limpopo Belt Že.g., Barton et al., 1994; Holzer et al., 1997, 1998; Jackel ¨ et al., 1997; Schaller et al., 1997.. The Northern Marginal Zone ŽNMZ. and the Central Zone ŽCZ. are characterised by anticlockwise p–T-evolution and peak granulite facies metamorphism at 2.52 and 2.55 Ga, respectively ŽKamber and Biino, 1995; Holzer et al., 1998. together with important magmatic activitiy. A second granulite facies metamorphism for the CZ Žclockwise p–Tevolution. was reached at 2.0 Ga as a result of a dextral transpressive orogeny ŽKamber et al., 1995; Schaller et al., 1997, 1999; Holzer et al., 1998.. This Proterozoic major tectono-metamorphic event did not affect the SMZ which underwent an Archean clockwise p–T-evolution Žgranulite facies metamorphism at 2.69 Ga; Kreissig and Holzer, 1997.. According to van Reenen Ž1983. and van Reenen et al. Ž1987., the SMZ was affected by a high pressure granulite facies event, followed by decompression and near isobaric cooling. Decompression was initiated by the thrusting of the SMZ onto the greenschist facies KC along the Hout River Shear Zone. This led to a ‘hot iron effect’ with rapid pressure and temperature increase corresponding to amphibolite facies conditions in the footwall and rehydration in the hanging wall ŽPerchuk et al., 1996.. The resulting Orthoamphibole zone is mapped within the Granulite Zone of the SMZ by an Orthoamphibole isograd Žvan Reenen, 1983, 1986.. An alternative explanation for this rehydration zone is given by Stevens Ž1997.. Accord-

ing to his model the Orthoamphibole isograd reflects a fault boundary between two different high grade blocks with different H 2 O contents resulting from different degrees of melt loss during granulite facies metamorphism. The SMZ mostly consists of pyroxene-bearing leucocratic tonalitic–trondhjemitic–granodioritic ŽTTG. gneisses, the Baviaanskloof Gneisses, and metapelites forming discontinuous units which are summarised under the name Banderlierkop Formation. Ultramafic and mafic granulites, banded iron formation as well as charnoenderbites and granites of the Matok pluton are of minor importance. The northern part of the Kaapvaal Craton is characterised by four elongated greenstone belts ŽSutherland belt, Pietersburg belt, Rhenosterkoppies and Murchison belt. which are associated with voluminous granitoid bodies. There is a sharp tectonic boundary between the SMZ and the KC but no indications for important post Archean relative vertical movement between both provinces. The tectonic break and metamorphic contrast could thus result from a tectonic event affecting an existing whole crustal province, or a process during terrane amalgamation. For this study, TTGs and metapelites from greenstone belts in the northern Kaapvaal Craton and from the two zones of the Limpopo-SMZ ŽOrthoamphibole zone and Granulite zone. were sampled. In the case of migmatites, representative portions of leucosome, melanosome and mesosome were included, in order to avoid systematic errors due to SmrNd

K. Kreissig et al.r Lithos 50 (2000) 1–25

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Table 2 Major and trace element data of the TTG-gneisses and a leucosome from the two subzones of the Limpopo Southern Marginal Zone ŽSMZ. Elements measured by XRF, U measured by INAA, 8 measured by ICP-MS. bdl s below detection limit. OAZ s Orthoamphibole Zone; GZ s Granulite Zone. Sample Locality

96r225 OAZ of SMZ TTG

96r227 OAZ of SMZ TTG

96r226-G OAZ of SMZ TTG

96r226-L OAZ of SMZ Leucosome

Major elements in wt.% SiO 2 70.25 TiO 2 0.29 Al 2 O 3 15.39 Fe 2 O 3 2.22 MnO 0.03 MgO 1.64 CaO 2.69 Na 2 O 5.50 K 2O 1.39 P2 O5 0.16 Sum 100.10 ArNK 1.46 ArCNK 1.00

68.75 0.46 16.33 2.51 0.02 1.16 2.84 5.55 1.38 0.15 99.82 1.54 1.03

67.83 0.39 16.10 2.75 0.04 1.58 3.51 4.98 1.72 0.15 99.70 1.60 0.98

62.28 0.80 16.63 5.60 0.08 2.64 5.46 4.51 1.30 0.22 100.19 1.88 0.89

74.40 0.17 14.60 1.25 0.01 0.41 1.99 5.65 1.05 0.10 99.93 1.40 1.04

Trace elements in ppm Ba 442 Cr 64 Cu 17 Nb 5 Ni 31 Pb 13 Rb 50 Sr 721 V 42 Y 10 Zn 44 Zr 132 ScU 5.0 ThU 3.6 UU bdl

672 12 5 5 6 16 63 774 50 11 48 169 6.5 2.9 bdl

376 45 6 5 23 15 60 346 62 14 37 138 5.2 1.8 bdl

299 41 27 9 45 12 44 472 122 28 67 194 11.4 1.6 0.5

457 bdl 8 bdl bdl 18 43 628 8 11 25 259 1.9 12.8 0.4

REEU in ppm La Ce Nd Sm Eu Gd8 Tb Yb Lu EurEuU ŽLarLu.N ŽLarGd.N ŽGdrLu.N

27.5 52 19 3.13 0.87 3.0 0.4 0.50 0.08 0.87 35.71 7.66 4.66

18.7 40 18 3.42 0.98 3.2 0.4 0.88 0.13 0.91 14.94 4.88 3.06

Lithology

96r203 OAZ of SMZ TTG

34.5 64 22 3.52 1.01 3.8 0.4 0.50 0.10 0.84 35.84 7.59 4.72

26.0 68 31 6.06 1.87 4.8 0.8 3.00 0.45 1.06 6.00 4.53 1.33

76.7 170 50 6.36 1.27 5.1 0.5 0.79 0.10 0.68 79.67 12.57 6.34

96r232 OAZ of SMZ TTG 69.11 0.44 15.73 3.21 0.04 0.82 2.18 5.65 1.82 0.19 99.75 1.40 1.03

427 13 18 11 7 15 87 562 25 12 73 142 4.4 1.8 0.5

9.9 24 13 2.68 0.73 2.3 0.4 0.64 0.11 0.90 9.35 3.60 2.60

96r217 GZ of SMZ TTG

96r248 GZ of SMZ TTG

96r230 GZ of SMZ TTG

68.98 0.36 15.72 3.08 0.05 1.10 3.63 5.11 0.99 0.16 99.49 1.66 0.98

68.83 0.57 15.56 3.51 0.04 1.52 4.13 4.83 0.52 0.24 99.95 1.83 0.97

62.89 0.60 14.05 5.55 0.07 4.69 4.70 4.15 1.66 0.16 99.42 1.63 0.82

184 14 3 4 8 5 17 597 35 8 58 161 3.5 1.0 bdl

246 13 31 6 17 5 10 658 55 15 26 284 5.4 bdl bdl

381 252 26 bdl 162 17 68 492 82 11 65 171 10.9 2.6 0.8

24.4 53 21 2.84 1.14 3.1 0.3 0.37 0.05 1.17 50.69 6.58 7.71

15.2 31 15 2.62 1.01 2.7 0.4 1.17 0.17 1.16 9.29 4.70 1.97

39.1 75 29 3.48 1.00 2.9 0.3 0.81 0.11 0.96 36.92 11.26 3.28

K. Kreissig et al.r Lithos 50 (2000) 1–25

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Table 2 Žcontinued. Sample Locality Lithology

96r203 OAZ of SMZ TTG

96r225 OAZ of SMZ TTG

96r227 OAZ of SMZ TTG

96r226-G OAZ of SMZ TTG

96r226-L OAZ of SMZ Leucosome

96r232 OAZ of SMZ TTG

96r217 GZ of SMZ TTG

96r248 GZ of SMZ TTG

96r230 GZ of SMZ TTG

CIPW norm Qtz C Or Ab An Di Hy Mag Ilm Ap

24.69 0.33 8.21 46.54 12.30 – 5.17 1.26 0.55 0.37

23.34 0.90 8.15 46.96 13.11 – 3.88 1.43 0.87 0.35

22.21 0.02 10.16 42.14 16.43 – 5.29 1.53 0.74 0.35

15.54 – 7.68 38.16 21.29 3.52 8.06 2.86 1.52 0.51

32.66 0.79 6.20 47.81 9.22 – 1.60 0.71 0.32 0.23

23.28 0.96 10.75 47.81 9.57 – 3.46 1.89 0.84 0.44

25.70 0.03 5.85 43.24 16.96 – 4.50 1.65 0.68 0.37

27.45 0.12 3.07 40.87 18.92 – 5.67 1.78 1.08 0.56

15.83 – 9.81 35.12 14.81 5.97 12.29 2.84 1.14 0.37

fractionation during migmatisation ŽChavagnac et al., 1997, 1999.. No metapelitic rocks were sampled in the Pietersburg greenstone belt. 19 TTG gneisses, 13 metapelites, one leucosome-band within metapelites, as well as one amphibolite were chosen for isotope and geochemical analysis. Sample localities are shown in Fig. 1.

3. Petrography

the SMZ, reflecting the different grades of metamorphism. 3.2.1. Of the KaapÕaal Craton The KC metapelites are fine grained, greenish with a well defined schistosity. They consist mainly of micas and quartz with porphyroblastic or poikiloblastic garnet, staurolite or kyanite. Minor and accessory phases are opaques, rutile, monazite and tourmaline. Zircons, if present, occur as inclusions in biotite.

3.1. TTGs The sampled TTGs are light grey to grey, medium grained and banded leucocratic gneisses. The most frequent mineral association is plagioclaseq quartz q biotiteq microcline. Quartz and feldspar form a seriate–interlobate texture with an average grain size of 0.8 mm. Accessory minerals are allanite, titanite, zircon, apatite, ilmenite and magnetite in varying amounts. Chlorite, epidoterclinozoisite, carbonates and muscovite are frequent as secondary minerals, but the latter was found only in KC samples. On the other hand, the leucocratic gneisses of the SMZ contain orthopyroxene or hornblende.

3.2.2. Of the Southern Marginal Zone The metapelitic gneisses of the SMZ are medium grained, dark grey gneisses. They possess a heterogranular texture and a major assemblage of biotite q orthopyroxene q feldspar q quartz " garnet " sillimanite" cordierite. Accessory minerals are almost the same as in the KC-schists, except for tourmaline, which was not observed in the SMZmetapelites.

4. Analytical techniques

3.2. Metapelites

4.1. Whole rock for major, trace and rare earth element analyses

In contrast to the TTGs, the metapelites differ markedly in their mineralogy between the KC and

Major and trace element analyses were performed by XRF at the University of Fribourg. Major ele-

Trace elements in ppm Ba 343 381 Cr 1593 1310 Cu bdl 55 Nb 11 11 Ni 552 679 Pb 13 12 Rb 114 429 Sr 26 26 V 203 148 Y 19 17 Zn 58 128 Zr 138 110 ScU 28.2 24.5 U Th 4.9 4.3 U U 1.3 1.2

Major elements in wt.% SiO 2 64.76 55.53 TiO 2 0.76 0.55 Al 2 O 3 22.06 16.47 Fe 2 O 3 10.3 7.66 MnO 0.05 0.05 MgO 8.56 9.08 CaO 0.08 0.2 Na 2 O 0.33 0.46 K2O 3.76 6.07 P2 O 5 0.14 0.07 Sum 114.6 98.54 ArNK 4.78 2.25 ArCNK 4.64 2.14

399 1228 15 10 416 19 145 14 167 22 235 140 23.6 4.0 1.5

60.08 0.59 17.6 8.26 0.04 6.68 0.09 0.29 3.82 0.07 99.96 3.82 3.69

179 1102 16 6 20 6 19 40 47 11 12 153 4.0 5.3 1.0

84.20 0.29 12.99 0.31 0.01 0.09 0.14 0.50 0.48 0.02 99.81 9.75 8.20

350 441 20 14 81 8 63 156 144 36 40 272 13.7 4.0 1.7

70.88 1.13 13.82 5.98 0.12 0.64 0.91 0.38 3.75 0.67 99.7 2.95 2.18

477 5189 45 bdl 1129 2 10 15 365 12 94 34 36.4 0.6 0.1

63.03 0.45 6.43 20.37 0.34 5.65 1.90 0.24 0.04 0.02 99.08 14.68 1.65

209 1790 5 bdl 504 13 35 115 88 11 20 81 14.1 1.3 0.6

81.08 0.31 12.5 1.26 0.01 1.04 0.65 1.97 1.31 0.02 101.29 2.68 2.14

211 1771 18 5 670 20 30 281 95 11 34 143 15.3 2.4 0.9

72.08 0.58 14.44 2.22 0.02 2.18 1.98 3.29 0.51 0.02 99.67 2.42 1.51

367 559 38 8 211 10 80 168 181 22 72 125 25.3 3.5 1.1

63.18 0.62 15.49 8.9 0.13 5.31 1.79 1.93 1.84 0.14 100.23 3.00 1.84

362 1320 54 6 494 7 59 133 217 19 122 105 28.8 2.9 1.1

59.06 0.67 14.58 10.78 0.13 9.27 1.29 1.78 1.3 0.08 99.98 3.36 2.18

500 2837 44 5 759 5 107 60 281 22 137 166 35.6 2.5 1.0

50.64 0.95 16.88 12.03 0.14 12.37 1.07 1.11 2.66 0.08 99.34 3.59 2.54

585 1384 43 9 526 8 112 133 304 20 133 137 31.5 4.8 0.9

54.78 0.89 18.61 10.01 0.09 9.08 1.31 1.72 2.57 0.06 99.59 3.32 2.33

141 98 7 bdl 41 10 19 389 84 13 46 106 10.4 1.2 0.6

63.23 0.46 17.41 6.2 0.1 3.23 4.69 3.32 0.59 0.1 99.82 2.85 1.19

1227 7 25 bdl 32 37 16 796 bdl 7 bdl 44 3.2 1.3 0.1

73.21 0.02 15.57 0.82 0.01 0.31 2.3 5.38 1.14 0.06 99.41 1.54 1.09

262 1722 93 6 586 15 67 155 262 19 132 120 34.9 2.1 0.8

57.57 0.84 17.19 8.87 0.08 8.66 1.60 2.08 1.53 0.06 100.01 3.39 2.15

Sample LF3 MDR 1 41-106 E 96r538 96r234 95r085 95r086 95r087 96r235-grt 96r235-ath 96r236 96r224 96r075 95r079 96r238 Locality KC-MB KC-MB KC-MB KC-RK KC-SB KC-SB KC-SB KC-SB OAZ of SMZ OAZ of SMZ OAZ of SMZ GZ of SMZ GZ of SMZ GZ of SMZ GZ of SMZ Lithology Metashale Metashale Metashale Metaarkose Metagreywacke Amphibolite Metaarkose Meta-fe-shale Meta-fe-shale Meta-fe-shale Meta-fe-shale Meta-fe-shale Metagreywacke Leucosome Meta-fe-shale

Table 3 Major and trace element data of the metasediments from the four greenstone belts of the Kaapvaal Craton ŽKC . and the two subzones of the Limpopo Southern Marginal Zone ŽSMZ . Elements measured by XRF, U measured by INAA, 8 measured by ICP-MS. bdl s lower detection limit. MBs Murchison greenstone belt; RK s Rhenosterkoppies; SB s Sutherland greenstone belt; OAZ s Orthoamphibole Zone; GZ s Granuline Zone.

8 K. Kreissig et al.r Lithos 50 (2000) 1–25

50.4 75 46 7.77 1.20 5.2 0.8 2.01 0.28 0.58 18.70 8.10 2.31

13.58 8.95 35.87 3.89 0.53 – 27.61 – 4 1.04 0.16

REEU in ppm La 31.9 Ce 70 Nd 27 Sm 5.47 Eu 1.20 Gd8 4.2 Tb 0.7 Yb 2.14 Lu 0.31 EurEuU 0.77 ŽLarLu .N 10.69 ŽLarGd .N 6.35 ŽGdrLu .N 1.68

CIPW norm Qtz 32.08 C 17.45 Or 22.22 Ab 2.79 An – Di – Hy 29.24 Hem Mag 4.64 Ilm 1.44 Ap 0.32 30.85 12.99 22.57 2.45 – – 23.16 – 3.64 1.12 0.16

30.2 68 25 5.32 0.89 4.9 0.9 2.70 0.40 0.53 7.84 5.15 1.52

79.08 11.44 2.84 4.23 0.56 – 0.22 0.08 – 0.46 0.05

28.4 53 19 2.99 0.56 2.3 0.3 0.82 0.13 0.65 22.69 10.32 2.20

51.72 9.09 22.16 3.22 0.14 – 5.04 – 2.16 1.69 1.5

60.4 160 86 16.40 3.86 11.9 1.6 2.56 0.31 0.84 20.24 4.24 4.77

38.6 2.59 0.24 2.03 9.3 – 37 – 6.21 0.85 0.05

0.8 2 2 0.61 0.51 1.0 0.3 1.19 0.16 2.00 0.52 0.67 0.78

61.45 6.71 7.74 16.67 3.09 – 3.2 – 0.58 0.59 0.05

11.2 24 10 1.98 0.56 1.7 0.3 0.84 0.12 0.93 9.69 5.50 1.76

43.09 4.92 3.01 27.84 9.69 – 6.45 – 1.01 1.1 0.05

19.0 39 15 2.87 1.12 2.6 0.4 0.80 0.13 1.25 15.18 6.11 2.49

30.23 7.4 10.87 16.33 7.97 – 20.53 – 3.87 1.18 0.32

22.1 47 20 4.41 1.29 4.1 0.8 3.00 0.45 0.93 5.10 4.50 1.13

23.01 8.09 7.68 15.06 5.88 – 32.66 – 4.31 1.27 0.19

16.6 42 16 3.87 0.90 3.4 0.6 2.30 0.34 0.76 5.07 4.08 1.24

8.85 10.42 15.72 9.39 4.79 – 41 – 4.9 1.8 0.19

13.5 38 19 3.04 0.50 2.9 0.6 2.73 0.39 0.51 3.60 3.89 0.92

15.23 10.76 15.19 14.55 6.11 – 30.39 – 4.37 1.69 0.14

28.5 65 28 5.34 0.98 4.4 0.7 0.95 0.14 0.62 21.15 5.41 3.91

24.8 3.02 3.49 28.09 22.61 – 13.05 – 2.73 0.87 0.23

17.2 41 16 3.33 1.06 2.8 0.5 0.95 0.14 1.06 12.76 5.13 2.49

32.08 1.45 6.74 45.52 11.02 – 1.32 – 0.46 0.04 0.14

24.8 47 15 1.76 1.94 1.4 0.2 0.30 0.06 3.78 42.93 14.80 2.90

20.21 9.35 9.04 17.60 7.55 – 28.67 – 3.70 1.60 0.14

16.7 36 15 3.57 1.01 3.1 0.6 2.31 0.30 0.93 5.78 4.50 1.28

K. Kreissig et al.r Lithos 50 (2000) 1–25 9

10

K. Kreissig et al.r Lithos 50 (2000) 1–25

2-ml silica gel and was measured on a VG Sector w five collector mass spectrometer. Pb-fractionation correction was based on standard NBS 981 and set to 0.0010 " 0.0001 per atomic mass unit. A total Pb blank of 53 " 30 pg was obtained during time of analysis. 4.3. Sm–Nd on whole rock

Fig. 2. Modal Quartz–Alkali feldspar-Plagioclase triangle of Streckeisen Ž1975.. Open squares correspond to sampled leucocratic TTG-gneisses from the KC and filled squares to those from the SMZ. 1 s granodiorite; 2 s tonalite; 3 s quartz–monzodioritergabbro; arrow s low-K calc-alkaline trend as defined by Lameyre and Bowden Ž1982..

ments were analysed from glass discs whereas the XRF analyses of trace elements were obtained on pressed powder pellets. REE, Sc, Th and U were analysed at ACTLABS, ŽOntario, Canada. by INAA, except Gd, which was measured by ICP-MS.

100 mg rock powder were digested in Savillex w screw-top beakers. A 149 Smr150 Nd-tracer was previously added. A cold HF–HNO 3 step for 1 day was followed by a hot HF–HNO 3 step for 6 days using the ultrasonic bath every day. After evaporation the samples were re-dissolved in 6 M HCl. REE separation followed standard chromatographic procedures. Sm and Nd were isolated from the other REE and from each other by reversed phase extraction chromatography ŽRichard et al., 1976.. Sm measurements were made on a single collector AVCO w mass spectrometer. Nd was measured as NdOq from single Re filaments on a VG Sector w mass spectrometer. The oxygen composition used for isotopic corrections was determined by analysing pure 150 Nd spike as described by Wasserburg et al. Ž1981. and the average composition gave 18 Or16 O s 2.024 = 10y3 and 17Or16 O s 3.83 = 10y4 " 0.03 = 10y4

4.2. U–Pb on whole rock About 150 mg rock powder were digested in a cold HF–HNO 3-mixture in Savillex w screw-top beakers for one day and subsequently evaporated. The samples were attacked again in hot HF and HNO 3 and evaporated after 4 days on a hot plate. Some samples were digested following the procedure described by Nagler and Kamber Ž1996.. All samples ¨ were aliquoted and a 235 Ur208 Pb-spike was used for concentration measurements. U and Pb extraction was performed in 5-ml quartz glass columns and 300-ml teflon w micro columns using standard HBr– HCl–HNO 3 chemistry. U was loaded with HNO 3 on Ta side filaments in a triple filament setup with a Re center filament. U-measurements were made on a single collector AVCO w mass spectrometer. Pb was loaded as phosphate on Re single filaments with

Fig. 3. K–Na–Ca-triangle for the TTGs from the SMZ and the KC using the same symbols as in Fig. 2. CA s classical calc-alkaline trend according to Barker and Arth Ž1976.; grey Tdh-field s field of trondhjemitic affinity proposed by Martin Ž1994..

K. Kreissig et al.r Lithos 50 (2000) 1–25

11

Fig. 4. Debon–Le Fort diagram ŽDebon and Le Fort, 1988. for the TTG-gneisses of the SMZ and the KC using the same symbols as in Fig. 2. The A- and B-factors are calculated in millications. A reflects the ‘‘alumina index’’ and B could be considered as a ‘‘mafic index’’, as it is directly proportional to the weight content of dark minerals in the rock. The metaluminous and peraluminous fields correspond to the CAFEM and ALUM-suites according to Debon and Le Fort Ž1988..

Fig. 5. Metapelites from the SMZ Žfilled circles. and from the KC Žopen circles. in a classification diagram for terrigenous sandstones and shales using ŽFe 2 O 3 rAl 2 O 3 . vs. ŽSiO 2 rK 2 O. after Herron Ž1988..

12

K. Kreissig et al.r Lithos 50 (2000) 1–25

ŽChavagnac et al., 1999.. Mass interference, oxygen, fractionation and spike correction are done online

during measurements. The Nd ratios were normalized to 146 Ndr144 Nd s 0.7219. Values obtained on

Fig. 6. Correlation diagrams Žspidergrams. for the TTGs Ža. and metapelites Žb. from the SMZ Žfilled symbols. and the KC Žopen symbols. normalised to primitive mantle ŽWanke et al., 1984; Th, Nb, Zr, Y: Jagoutz et al., 1979.. ¨

K. Kreissig et al.r Lithos 50 (2000) 1–25

the La Jolla standard were 0.511839 " 0.000029 Ž2 s S.D., n s 5. during the first series and 0.511875 " 0.000026 Ž n s 15. during the second one. All data were corrected for a 143 Ndr144 Nd standard value of the La Jolla of 0.511860.

13

5. Results 5.1. Major and trace Element Geochemistry The geochemical data are given in Tables 1–3. The leucocratic gneisses are plotted in a QAP-di-

Fig. 7. LILE distribution for the TTGs Ža. and metapelites Žb. from the SMZ Žfilled symbols. and the KC Žopen symbols. as well as the average Middle Proterozoic upper crust Žcrossed squares. normalised to average Early Archean upper crust ŽCondie, 1993..

14

K. Kreissig et al.r Lithos 50 (2000) 1–25

agram ŽFig. 2. according to Streckeisen Ž1975.. All samples fall in the granodiorite–tonalite field, except sample 96r226-G, which is on the border of the quartz–monzo-diorite or –gabbro field due to the low amount of normative Qtz Ž15.54.. They follow

the typical low K-calc-alkaline trend of Archean TTGs ŽLameyre and Bowden, 1982.. As can be seen in Fig. 3, most of the samples plot within the trondhjemitic field in a K–Na–Ca-triangle ŽMartin, 1994; Barker and Arth, 1976., with a tendency towards

Fig. 8. Trace metal distribution for the TTGs Ža. and metapelites Žb. from the SMZ Žfilled symbols. and the KC Žopen symbols. as well as the average Middle Proterozoic upper crust Žcrossed squares. normalised to average Early Archean upper crust ŽCondie, 1993..

K. Kreissig et al.r Lithos 50 (2000) 1–25

higher K andror Ca concentrations reminiscent of the calc-alkaline trend. Additionally, the Debon–Le Fort diagram ŽFig. 4. was applied to discriminate between peraluminous and metaluminous suites and to obtain information on mineral fractionation processes. With the exception of the three samples 96r226-L, 96r232 and 96r225, all TTG-gneisses from the SMZ are metaluminous, indicating a derivation from a mafic magma by hornblende fractionation. The KC-TTGs have peraluminous character, sample 96r210 being the only exception. The fields for the two populations overlap. The sampled metapelites are classified using the Fe 2 O 3rAl 2 O 3 vs. SiO 2rK 2 O plot of Herron, 1988 ŽFig. 5.. The metasediments from the SMZ are clearly enriched in Fe compared to the KC samples, which are more scattered. According to this graph, sample 96r234 is a metagreywacke and samples 95r086 and 96r538 are both meta-arkoses. All others plot in the shale or Fe-shale fields. To compare the rocks from the different provinces the geochemical data of the TTGs and metapelites ŽTables 1–3. are plotted in typical spidergrams ŽFig. 6.. The rocks from the KC and those from the SMZ show very similar patterns. The TTGs are characterised by positive Rb, negative Nb and Ti anomalies and a depletion of compatible elements like Mg, Cr and Ni. As can be seen from Fig. 6a, the samples have wide ranges of concentrations, except for Na and Si. As illustrated in Fig. 6b, the metapelites behave similarly to the TTGs. They show less depletion of compatible elements and no significant Ti-anomaly. There is much greater scatter in Na, Ca and Fe than for the TTGs, with some differences between the KC and the SMZ. Only a slight distinction can be recognised for the elements Nb, Sr and Zr. But still both provinces overlap in general. By normalising to the average value of the KC Žcf. Haskin, 1990. the SMZ rocks plot within the 2 s-range of the KC, indicating no significant differences between both provinces in terms of their trace element characteristics. Comparing the trace element values of the rocks from the SMZ and the KC to average upper crust values published by Taylor and McLennan Ž1981. or Condie Ž1993., it is obvious that both the metapelites and the TTGs are generally depleted in incompatible

15

elements. To illustrate this aspect, certain trace element plots of Condie Ž1993. were adapted.

Fig. 9. REE diagrams for the TTGs Žsquares. and metapelites Žcircles. from the SMZ Žfilled symbols. and the KC Žopen symbols. normalised to chondrites ŽBoynton, 1984.. Ža. Mostly TTGs Žas well as metapelites: 95r086; 95r087 and 95r075.. Žb. Mostly metapelites Žas well as TTG’s: 96r211 and 96r226-G.. Žc. Samples which show neither typical TTG Ža. nor metapelite Žb. REE-pattern.

K. Kreissig et al.r Lithos 50 (2000) 1–25

16

Fig. 7a q b show the distribution of some highly incompatible elements in our samples normalised to Early Archean upper continental crust values of Condie Ž1993.. To display the trace element evolution through time, the Middle Proterozoic upper crust ŽCondie, 1993. is also plotted for comparison. It can be seen from Fig. 7a that in general the TTGs deviate in a sense opposite to these, being more depleted in Pb, Th and U especially than the average Early Archean upper crust model of Condie and more enriched in Sr. These specific characteristics are shown by both the SMZ and the KC. As can be recognised from Fig. 7b the metapelites are even more depleted in these incompatible elements than the TTGs. Similar observations can be made with respect to the measured high field strength elements ŽHFSE., Zr, Nb and Y.

In Fig. 8a q b, the trace metals Sc, V, Cr and Ni are plotted in the same way as the incompatible elements above. It can be seen that the sampled TTGs are depleted in all these, in contrast to the metapelites, which are generally enriched in these trace metals, especially in Cr and Ni. The two provinces resemble each other in this particular feature as well. Fig. 9a–c illustrates the chondrite-normalised REE concentrations of the TTGs and the metapelites. Nearly all samples are grouped into two types. The majority of TTGs define the first type. They are mostly strongly fractionated ŽLarLu. N s 20.1–79.7 and show either small negative ŽEurEuU ) 0.7. or small positive Eu-anomalies ŽEurEuU - 1.3.. The second type is almost exclusively defined by metapelites and as can be seen in Fig. 9b, they

Table 4 Sm–Nd isotopic data of TTGs and metapelites from the KC and the SMZ M s metapelite; G s TTG-gneiss; A s amphibolite. KC s Kaapvaal Craton; SMZs Southern Marginal Zone. MB s Murchison Belt; PB s Pietersburg Belt; RK s Rhenosterkoppies; SB s Sutherland Belt. Sample

Rock-type locality

Sm, ppm

Nd, ppm

147

96r201 96r233 MDRI 96r239 96r247 96r210 96r538 96r228 96r234 96r234a 95r085 96r203 96r232 96r232 a 96r226-G 96r226-G a 96r227 96r235-ath 96r236 96r230 96r217 96r248 96r238 95r075

G-KC-MB G-KC-MB M-KC-MB G-KC-PB G-KC-PB G-KC-RK M-KC-RK G-KC-SB M-KC-SB M-KC-SB A-KC-SB G-SMZ-OAZ G-SMZ-OAZ G-SMZ-OAZ G-SMZ-OAZ G-SMZ-OAZ G-SMZ-OAZ M-SMZ-OAZ M-SMZ-OAZ G-SMZ-GZ G-SMZ-GZ G-SMZ-GZ M-SMZ-GZ M-SMZ-GZ

2.76 0.764 6.76 3.60 3.44 3.95 2.67 4.71 15.45 15.68 0.743 3.71 2.55 2.58 5.39 5.40 3.49 2.68 2.66 2.95 3.02 2.64 3.27 3.11

19.0 5.25 37.6 23.2 19.6 24.9 17.6 29.9 80.4 81.6 1.75 23.5 11.7 11.8 27.3 27.3 18.5 12.4 12.7 21.0 21.0 14.5 15.5 15.7

0.08773 0.08793 0.1088 0.09410 0.1063 0.09575 0.09158 0.09533 0.1162 0.1160 0.2560 0.09548 0.1318 0.1321 0.1196 0.1194 0.1137 0.1302 0.1262 0.08480 0.08682 0.1100 0.1276 0.1196

U a

Smr

144

Nd

143

Ndr

144

0.510495 0.510475 0.511016 0.510662 0.510965 0.510709 0.510664 0.510596 0.510873 0.510859 0.513863 0.510663 0.511058 0.511081 0.510886 0.510925 0.510997 0.511374 0.511296 0.510549 0.510364 0.510961 0.511328 0.511213

Nd

"2 s mean

U TDM

´ Ž0.

26 26 26 29 29 29 26 26 29 26 26 29 26 26 26 29 29 26 26 29 29 29 26 26

3.10 3.10 2.90 3.00 2.95 3.00 2.95 3.15 3.40 3.40 2.95 3.05 3.75 3.70 3.50 3.45 3.10 3.05 3.05 2.95 3.20 3.05 3.00 2.95

y41.80 y42.20 y31.64 y38.54 y32.64 y37.63 y38.51 y39.83 y34.42 y34.70 23.89 y38.53 y30.83 y30.37 y34.18 y33.42 y32.01 y24.67 y26.18 y40.76 y44.36 y32.72 y25.55 y27.81

Model ages in Ga calculated after Nagler and Kramers Ž1998. and rounded to the smallest figures in multiples of five. ¨ Duplicate analyses.

K. Kreissig et al.r Lithos 50 (2000) 1–25

17

behave very similar to the average Archean shales pattern ŽCondie, 1993., which is plotted for comparison. This type is characterised by LREE fractionation, rather flat HREEs ŽŽLarLu. N s 3.6–10.7. and a distinct negative Eu-anomaly ŽEurEuU s 0.5–0.9. when the positive Eu-anomaly of TTG sample 96r226-G is disregarded. Two metapelites show stronger HREE-fractionation ŽŽLarLu. N s 21.2– 24.6.. In general it is impossible to distinguish between rocks of the KC and those of the SMZ, whereas there is a subtle systematic difference between metapelites and TTGs with five exceptions. Three metapelites behave like TTGs and two TTGs have metapelitetype REE-patterns. Four samples which do not fit into the two types, TTGs and metapelites from both provinces as well as the amphibolite sample 95r085 are shown in Fig. 9c. TTG samples 96r233 and 96r204B are characterised by low REE abundances and especially for 96r233 a extreme depletion in HREE and a positive Eu-anomaly is obvious. A similar pattern shifted to higher values is given by sample 95r079 which is taken from a leucosome band within a metapelite outcrop. Metapelite sample 96r234 shows the typical REE-pattern of allanite ŽMcLennan, 1989. which is intergrown with epidote. 5.2. Isotope geochemistry Nd results are shown in Table 4. All samples but the amphibolite Ž95r085. show 147 Smr144 Nd ratios below 0.132, typical for felsic crustal rocks. The SMZ-TTGs give both the highest and lowest ratios Ž0.085–0.132. whereas the KC-TTGs are somewhat less scattered Ž0.088–0.106.. TTG-gneiss data from both terranes completely overlap, whereas the narrow ranges of metapelites from KC and SMZ Ž0.092–0.116 and 0.120–0.130, respectively. show a small offset ŽFig. 10a.. Similarly, present day Nd isotopic compositions of the felsic KC samples and the SMZ-TTGs overlap and fall in the range of ´ Nd s y44 to y31, whereas the SMZ metapelites are somewhat higher Ž ´ Nd s y25 to y28, Table 4.. All but three of the analysed samples including the measured amphibolite give model ages very close to 3.0 Ga if the Nagler and Kramers Ž1998. model is ¨ applied. Therefore the Nd model ages do not allow a

Fig. 10. 147 Smr144 Nd Ža. and TDM Žb. distribution of the sampled TTGs ŽG. and metapelites ŽM. from both provinces KC and SMZ.

distinction between the rocks from the SMZ and those from the KC. Two TTG samples from the SMZ Ž96r232, 96r226-G. and one metapelite from within the Hout River Shear Zone Ž96r234. however have apparent Nd model ages which are significantly older Ž3.75, 3.50 and 3.40 Ga, respectively; Table 4 and Fig. 10b.. Whole rock Pb-isotope data are listed in Table 5 and plotted in standard uranogenic and thorogenic Pb-diagrams ŽFig. 11.. The majority of samples show a strong uniformity of the KC-SMZ provinces. Metapelites and TTGs are indistinguishable and plot well below the reference curve of Stacey and Kramers Ž1975., displaying a uniform low 207 Pbr204 Pb to 206 Pbr204 Pb.

18

Table 5 Pb isotopic data of TTGs and metapelites from the KC and the SMZ MBs Murchison greenstone belt; RK sRhenosterkoppies. PBs Pietersburg greenstone belt; SBsSutherland greenstone belt. OAZ sOrthoamphibole Zone; GZ sGranulite Zone. Sample

Locality

Rock-type

206 204

KC-MB KC-MB KC-MB KC-MB KC-MB KC-MB KC-PB KC-PB KC-PB KC-RK KC-RK KC-RK KC-RK KC-SB KC-SB KC-SB KC-SB KC-SB OAZ of SMZ OAZ of SMZ OAZ of SMZ OAZ of SMZ

96r226-L 96r232 96r235-grt 96r235-ath 96r236 96r230 96r217 96r248 96r238 96r224 95r075 95r079

OAZ of SMZ OAZ of SMZ OAZ of SMZ OAZ of SMZ OAZ of SMZ GZ of SMZ GZ of SMZ GZ of SMZ GZ of SMZ GZ of SMZ GZ of SMZ GZ of SMZ

TTG TTG TTG metapelite metapelite metapelite TTG TTG TTG TTG TTG TTG metapelite TTG metapelite amphibolite metapelite metapelite TTG TTG TTG Goudplaas Gneiss leucosome TTG metapelite metapelite metapelite TTG TTG TTG metapelite metapelite metapelite leucosome

"2 sq

207 204

Pbr Pb

"2 sq

208 204

Pbr Pb

"2 sq

r1UU

r 2 ††

m

16.575 14.113 13.891 15.857 14.311 14.942 14.487 13.859 16.202 20.062 15.447 18.909 28.340 17.353 16.041 16.623 13.606 13.633 13.633 16.283 13.888 14.175

0.063 0.024 0.045 0.186 0.012 0.062 0.031 0.082 0.078 0.021 0.046 0.065 0.095 0.029 0.038 0.181 0.022 0.031 0.013 0.011 0.015 0.089

15.033 14.441 14.434 14.976 14.574 14.731 14.531 14.412 14.978 15.673 14.661 15.401 17.070 15.093 15.178 15.093 14.363 14.317 14.297 14.807 14.340 14.415

0.058 0.026 0.048 0.186 0.015 0.062 0.033 0.087 0.074 0.019 0.045 0.054 0.058 0.027 0.037 0.166 0.030 0.034 0.015 0.023 0.017 0.092

35.784 34.595 34.168 34.917 33.592 33.658 34.735 33.770 34.999 33.875 36.421 36.056 40.124 36.013 34.918 35.671 32.939 32.862 34.974 34.193 34.340 33.082

0.140 0.066 0.115 0.413 0.042 0.142 0.080 0.203 0.173 0.045 0.115 0.128 0.139 0.069 0.087 0.391 0.060 0.080 0.044 0.037 0.047 0.211

0.990 0.977 0.987 0.947 0.962 0.990 0.968 0.989 0.976 0.920 0.981 0.988 0.992 0.977 0.986 0.992 0.803 0.988 0.969 0.554 0.972 0.992

0.987 0.963 0.984 0.995 0.913 0.988 0.971 0.993 0.987 0.939 0.974 0.987 0.989 0.939 0.979 0.997 0.956 0.975 0.936 0.939 0.939 0.993

7.821 0.811 6.029 8.677 5.643 3.951 3.183 3.890 5.105 12.274 5.631 4.316 16.867 10.446 5.817 7.812 2.043 6.616 2.248 7.154 2.738 3.437

13.630 13.903 16.272 16.163 17.845 15.291 13.503 13.422 14.861 15.299 14.249 12.999

0.128 0.012 0.158 0.118 0.153 0.014 0.014 0.027 0.035 0.052 0.017 0.011

14.386 14.352 14.932 14.873 15.220 15.256 14.386 14.272 14.723 14.777 14.484 14.225

0.136 0.015 0.146 0.110 0.132 0.016 0.017 0.030 0.036 0.051 0.019 0.014

39.748 33.076 35.240 33.972 34.278 35.816 33.781 32.832 34.677 35.991 33.293 32.731

0.376 0.041 0.344 0.251 0.296 0.045 0.045 0.071 0.087 0.126 0.050 0.040

0.992 0.963 0.992 0.990 0.985 0.967 0.973 0.983 0.985 0.984 0.965 0.961

0.993 0.923 0.996 0.994 0.994 0.932 0.939 0.967 0.975 0.982 0.924 0.902

1.780 1.426 9.270 15.402 20.619 3.360 0.783 1.156 4.959 6.675 3.126 0.171

K. Kreissig et al.r Lithos 50 (2000) 1–25

96r201 96r202 96r233 LF3 MDR1 41-106E 96r239 96r246 96r247 96r204B 96r210 96r211 96r538 96r228 96r234 95r085 95r086 95r087 96r203 96r225 96r227 96r226-G

Pbr Pb

K. Kreissig et al.r Lithos 50 (2000) 1–25

Fig. 11. Uranogenic Ž207 Pbr204 Pb vs. 206 Pbr204 Pb; Ža. and thorogenic Ž208 Pbr204 Pb vs. KC; filled symbols: SMZ; squares: TTGs; circles: metapelites.

Only sample 96r230 lies nearly on the reference curve and metapelite sample 96r538 from the Rhenosterkoppies greenstone belt shows a very high uranogenic Pb. In Fig. 11b, the thorogenic Pb-values are plotted. The datapoints are grouped around the reference curve. This diagram also fails to discriminate KC from SMZ rocks on the one hand and TTGs from metapelites on the other. Sample 96r538 is enriched in both, 208 Pbr204 Pb and 207 Pbr204 Pb. The other high-208 Pb sample 96r226-L is the leucosome

206

Pbr204 Pb; Žb. Pb-diagrams. Open symbols:

part of the Goudplaas Gneiss, reflecting monazite inclusions.

6. Discussion Given the many parameters studied Žtrace element and isotope geochemistry, Pb and Nd isotope data., the overall similarities between the SMZ and the KC rocks is striking. Even metapelites and TTGs are

Notes to Table 5: r1 s206 Pbr204 Pb vs. 207 Pbr204 Pb error correlation after Ludwig Ž1988.. †† r 2 s206 Pbr204 Pb vs. 208 Pbr204 Pb error correlation after Ludwig Ž1988.. q Errors are two standard deviations absolute after Ludwig Ž1988..

UU

19

20

K. Kreissig et al.r Lithos 50 (2000) 1–25

rather similar in terms of their trace element characteristics. These similarites lead to the assumption that the metapelites are formed by sedimentation of weathered and transported TTG and greenstone material. The contribution from mafic greenstone belts would explain the relative enrichment of compatible elements for the metapelites as well as the more or less flat HREE patterns ŽFig. 9b. shown by most of them. This is in agreement with many studies Že.g., Weaver and Tarney, 1981; McLennan and Taylor, 1984; Condie, 1993. which have demonstrated that the Archean crust could be regarded as a mixture of mafic and felsic igneous rocks. The variable relative amounts of both components result in a varying slope of the REE-pattern but change little in Eu ŽWeaver and Tarney, 1981.. The distinct negative Eu-anomalies which are common for the majority of metapelites conflict with the suggestion of McLennan Ž1989. that Eu-anomalies only occur in post Archean sediments. The observed wide variation for Rb, Sr and K could point to weathering effects and additionally these for Zr, Nb and Fe probably indicate transportation effects. Weaver and Tarney Ž1981. observed a depletion of K, Rb, Cs, Th and U in granulites when compared with amphibolite facies rocks. This conforms with other authors Že.g., Black et al., 1973. who explained the low abundances of certain highly incompatible elements in many Archean granulites by a removal with fluids during high grade metamorphism. The fact that five out of nine SMZ-TTGs have U-concentrations below the detection limit ŽTable 2. seems to confirm this hypothesis. Therefore the U and Th concentrations were calculated from the measured Pb isotopes assuming an initial composition according to Stacey and Kramers Ž1975. at 3.0 Ga and a closed system. An early disturbance Ž; 2.7 Ga metamorphism. would not significantly affect the results. The lower grade KC-TTGs contain 0.4–2.7 ppm model U and 1.0–7.1 ppm Th. This range is indeed greater than that of the granulite facies SMZ-TTGs ŽU: 0.1–1.1 ppm; Th: 0.5–4.7 ppm.. However, this is not the case for the metapelites. The concentration ranges overlap widely Ž0.2–0.7 ppm U in the KC and 0.2–1.0 ppm U in the SMZ as well as 0.5–1.5 ppm Th in the KC and 0.4–1.4 ppm Th in the SMZ.. Therefore, high grade metamorphism is not responsible for the depletion of incompatible elements. The

low U and Th concentrations are thus a special regional characteristic of both low grade KC and high grade SMZ. Another unusual feature is the extremely high Cr and Ni abundances of the metapelites ŽFig. 8b.. Dia et al. Ž1990. proposed a decrease of Cr and Ni with time of South African shales ŽFig. 12.. By interpolating the trend implicit in the data of Dia et al. Ž1990. to the Cr and Ni values of this study, the existence of a s 3.0 Ga old protholith for these sediments is indicated. This is in line with the Nd model ages. The fact that the sampled rocks Žmetapelites and TTGs. from both provinces are mostly more depleted in some incompatible and HFS elements than the Early Archean upper crust of Condie Ž1993. whereas younger samples are enriched, also supports this indication. However the enrichment of trace metals in the TTGs and the depletion in the metapelites does not conform to the postulated evolution trend ŽCondie, 1993. and shows the importance of the different formation processes as well. The high Cr and Ni concentrations in old shales cannot be explained by simple bulk erosion of the continental crust. McLennan et al. Ž1983. postulated that in some Archean areas the abundances of Cr and Ni in shales are too high for any realistic source composition and that therefore enrichment processes during formation must be assumed. Clearly, the consistent Nd results do not allow to be distinguished different sources or different crustal histories for the KC and SMZ. The metasediments are most likely to give a good estimate of the average crustal age of the respective craton, as they represent fine grained material, i.e., a long sedimentary transport. Excluding sample 96r234 which is also offset in major and trace elements, all metapelites from the KC and the SMZ define a very narrow Nd model age range of 2.9–3.05 Ga. This conformity is a strong indication that both provinces actually represent the same crustal material. Nd model ages of the TTG-gneisses from the low metamorphic KC indicate some age heterogeneity within the craton. The high grade ; 2.7 Ga event that exclusively affected the SMZ may have introduced some additional scatter on the SmrNd ratios of the SMZ gneisses. Migmatisation could cause changes in the SmrNd systematics Že.g., Chavagnac et al., 1999.. This might be the reason for both outliers 96r226-G

K. Kreissig et al.r Lithos 50 (2000) 1–25

21

Fig. 12. Cr and Ni abundances of South African shales with different deposition ages ŽDia et al., 1990.. The solid lines represent the average values of 400 and 1230 ppm for Ni and Cr, respectively of the examined SMZ and KC metapelites. The grey area are the two standard deviation concentration ranges.

and 96r232 which are characterised by anatexis and late dehydration Žsample 96r226-G is an outlier in major and trace elements as well.. Analytical problems can be excluded as the data for all three oultliers are reproduced within the errors ŽTable 4.. However, assuming reasonable changes in the SmrNd ratio during a later disturbance of the SmrNd system the

age of this event should be F 2 Ga which is not reported from this area. Therefore, the outliers probably reflect true crustal age heterogeneity in addition to possible SmrNd disturbance. The strong depletion in HREE of a majority of TTGs attests that garnet was one equilibrium phase during formation of the gneiss precursor. This is in

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good agreement with several authors who observed strongly HREE depleted Archean rocks Že.g., O’Nions and Pankhurst, 1974. and with Weaver and Tarney Ž1981. who showed that this feature is common in all Archean tonalites independent of crustal level. Additionally, the observed negative Ti-anomaly suggests that hornblende fractionation may also be important for the depleted HREE-pattern whereas the lack of significant Eu-anomalies indicates that plagioclase fractionation did not play a fundamental role. Pb-isotopes also show a conformity of the SMZKC provinces. On the other hand they differ clearly from many other Archean terranes, as Fig. 13 illustrates. The charnoenderbites from the NMZ of the Limpopo Belt ŽBerger and Rollinson, 1997. and

granitoids from the ZC plot well above the reference curve ŽStacey and Kramers, 1975.. Igneous rocks from the eastern Beartooth Mountains ŽUS. behave similar to the latter ŽWooden and Mueller, 1988.. Data from the CZ ŽChavagnac et al., 1999. lie in between, just on the SK-curve. Only the Amitsoq gneisses some of the oldest rocks of the world with the least radiogenic Pb-values known also plot below ŽKamber and Moorbath, 1998.. Pb isotopes appear to be quite suitable to distinguish between Archean terranes, a role mainly played by Nd model ages in the Proterozoic and Phanerozoic but probably not in the Archean. The similar Nd model ages in Southern Africa Že.g., Berger et al., 1995; Poujol et al., 1996. indicate major crust formation events at about 3.0 Ga.

Fig. 13. Pb-isotope data from the Zimbabwe Craton in a uranogenic Pb-diagram from the SMZ of the Limpopo Belt and the northern part of the KC compared with those from the NMZ ŽBerger and Rollinson, 1997., the CZ ŽChavagnac et al., 1999., the Beartooth Mountains, MT ŽWooden and Mueller, 1988. and the Amisoq gneisses, Greenland ŽKamber and Moorbath, 1998..

K. Kreissig et al.r Lithos 50 (2000) 1–25

The U depletion could result from a specific crust formation process or a U-depleted mantle. There are no obvious correlations between U and other elements ŽZr and Yb.. This does not favour a fractionation crystallisation process as a cause for the low U, but on the other hand does not rule it out completely. The amalgamation model of de Wit et al. Ž1992. proposed the tectonically juxtaposition of different granulite terrains with the low grade granite–greenstone terrains. However the high grade SMZ and the adjacent low grade northern part of the KC show exactly the same isotopic and geochemical characteristics. Even particularities such as the high concentrations of Cr and Ni in the metapelites and the general U-depletion are identical in the rocks from both provinces. Therefore, the assumption that the SMZ and the KC represent different separate amalgamated terranes seems unlikely.

7. Conclusions The geochemical and isotope geochemical comparison of the rocks from both examined South African provinces, the KC and the SMZ, provides evidence that these were derived from the same crustal material which was formed between 3.05 and 2.9 Ga. The strong depletion of some highly incompatible elements, especially U, Th and Pb, is not caused by a removal during high grade metamorphism but is considered to be a common feature. Another one is the enrichment of Cr and Ni in the metapelites from both provinces implying an old crustal precursor. The most striking common characteristic of the SMZ and the KC is their uniform Pb isotopic composition. It differs clearly from other Archean terranes including the Central and the Northern Marginal Zone of the Limpopo Belt. Therefore, the results do not support the terrane amalgamation model of de Wit et al. Ž1992. for the two provinces studied. Furthermore, the metapelites in both provinces are to an important extent derived from the TTGs. More metapelite studies and comparisons with their possible source rocks are needed in order to better characterise supracrustal provinces, as they provide indications of paleogeographicrdepositional relations between greenstone belts and cratons.

23

Acknowledgements J.M. Huizenga and J.M. Barton are thanked for discussion and support in the field. We also thank A. Schmidt-Mumm who provided samples from the Zimbabwe Craton. ACTLABS is acknowledged for the performance of INAA analyses. H. Rollinson and M. Whitehouse are thanked for their constructive reviews as well as G. Stevens for his suggestions. This project was funded by the Swiss National Foundation ŽGrant 20-47157.96..

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