Isotopic and geochemical evidence for crust-mantle interaction during late Archaean crustal growth

Geochimicaet CosmochimicaActa, Vol. 61, No. 22, pp. 4809-4829, 1997 Copyright© 1997 ElsevierScienceLtd Printed in the USA.All rights reserved 0016-7037/97 $17.00 + .00

Pergamon

PII S0016-7037(97) 00271-8

Isotopic and geochemical evidence for crust-mantle interaction during late Archaean crustal growth MICHAEL BERGER1 and HUGH ROLLINSON2'* JMineralogische-Petrographisches Institut, Gruppe Isotopengeologie, Universita't Bern, Erlachstrasse 9a, CH-3012 Bern, Switzerland 2Department of Geography and Geology, Cheltenham & Gloucester College of Higher Education, Francis Close Hall, Swindon Road, Cheltenham GL50 4AZ, UK (Received February 17, 1997; accepted in revised form July 15, 1997)

Abstract--The charnockites and enderbites of the North Marginal Zone of the Limpopo Belt in southern Zimbabwe represent a suite of dry magmas of tonalite-trondhjemite-granodiorite (TTG) affinity which crystallised directly in the lower crust. They were emplaced between 2.6 and 2.7 Ga and were derived from a source which showed remarkable lead isotope homogeneity. Unlike many other lower crustal rocks they have low K/Rb and low K / U ratios and have not experienced depletion of their heat producing elements Th, U, K, and Rb and thus represent an unusual suite of granulites. In this study we have combined the constraints of trace element and isotope geochemistry to characterise the nature of the protolith to the NMZ enderbites and charnockites. We use a combination of high /z-values, calculated lead and strontium isotope initial ratios and old Nd model ages to show that the enderbites and charnockites were derived from a protolith which was, in part, formed more than 500 Ma before magma genesis. We argue from the very high/z-values that this protolith constituted two separate components, an old high-/z, low Sm/Nd crustal component and a younger low-/z high Sm/Nd mantle-derived component, both of which were homogenised by extensive mixing prior to 2.7 Ga. The presence of an older crustal component contrasts with other Archaean T r G suites worldwide which show little evidence for a crustal prehistory. The process of mantle-crust interaction during late Archaean crustal growth in southern Zimbabwe invites comparison with modern convergent margins such as the Andes. Copyright © 1997 Elsevier Science Ltd 1. INTRODUCTION

by Kilpatrick and Ellis (1992). We demonstrate that the protolith to the NMZ charnockites and enderbites has a significant crustal prehistory. This contrasts with observations on Archaean TTG suites from other areas which show little evidence for crustal prehistories. Our observations demonstrate that the standard model of TTG generation through the simple melting of oceanic crust does not work for these rocks. The Limpopo Belt is located between the Zimbabwe and Kaapvaal Archaean Cratons of southern Africa and has a long geological history from 2.0 Ga to at least 3.2 Ga (Barton et al., 1994; Kamber et al., 1995). Conventionally the Limpopo Belt is divided into three zones: the Northern Marginal, Central, and South Marginal zones (Cox et al., 1965; Mason, 1973; Rollinson, 1993a). They show contrasting lithologies and structural trends and are separated by major shear zones. The rocks described here are located in the Northern Marginal Zone, which is situated in southern Zimbabwe (Fig. 1 ). The geology of the NMZ was first described in detail by Robertson (1973) who showed that it comprises an Archaean granulite terrain dominated by quartzo-feldspathic gneisses of tonalitic composition. Rocks of probable supracrustal origin, principally metabasalts and associated ultramafic rocks and banded iron formation, form less than 10% of the present-day outcrop. Intrusive into the quartzofeldspathic granulites is a suite of porphyritic granites, known locally as the Razi granite suite. Razi granite intrusions are found throughout the NMZ but are most common at the northern boundary of the NMZ, adjacent to the Zimbabwe Craton. Following Robertson and Du Toit (1981)

There is now a well established link between the process of crust generation in the Archaean and the genesis of magmas of the Tonalite-Trondhjemite-Granodiorite (TTG) suite (Martin, 1994). A wealth of geochemical, geophysical, and petrological studies have converged in recent years to suggest that TTG magmas are most commonly derived from a basaltic parent (Martin, 1994). What is not clear, however, and currently the subject of much debate, is the precise mechanism whereby TFG magmas are produced from their basaltic precursors and the tectonic setting or settings in which such a process took place on the early earth. Martin ( 1986, 1993) advocated a subduction related setting for TTG genesis. More recently a crustal underplating model has been proposed (Atherton and Petford, 1993; Muir et al., 1995; Rudnick, 1995), a view which has some affinity with an earlier model proposed by Kramers (1988) and Ridley and Kramers (1990) who argued that Archaean T r G s originated in a subcrustal magma layer. In this paper we attempt to constrain the process of Archaean crust formation for the Northern Marginal Zone (NMZ) of the Limpopo belt in Zimbabwe. We use trace elements for petrogenetic modelling and radiogenic isotopes to trace the protolith history. We show below that the NMZ comprises charnockites and enderbites which are igneous in origin and compositionally belong to the q"FG magma-suite and conform in composition to the C-type granites described *Author to whom correspondence should be addressed. 4809

4810

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many authors have considered that the NMZ of the Limpopo Belt represents a lower structural level of the Zimbabwe Craton which was subsequently uplifted and thrust onto the Zimbabwe Craton (Mkweli et al., 1995). Ridley (1992) drew attention to the quartzo-feldspathic gneisses of the NMZ and argued that they were charnockites and enderbites, granulites which crystallised directly from a melt in the lower crust. More detailed studies have confirmed these ideas, and there is now a significant body of field, petrographic, mineral, chemical, and geochemical evidence to show that the quartzo-feldspathic gneisses of the NMZ are primary magmatic granulites (Berger et al., 1995; Kamber and Biino, 1995; Rollinson and Blenkinsop, 1995; Rollinson and Huizenga, 1997.). In some areas of low strain we have been able to identify individual plutons and recent geochronology by Berger et al. ( 1995 ) has shown that they were emplaced at between 2.7 and 2.6 Ga (see Table 1 ). Their emplacement into the lower crust is confirmed by a thermobarometric study of a mafic inclusion assimilated by an enderbitic melt at Renco Mine (Rollinson, 1989; sample

42) which indicates crystallisation at 8.4 _+ 1 kb, 850 _+ 50°C. These rocks differ from the better-known charnockites of south India (e.g., Hansen et al., 1987) inasmuch as the NMZ charnockites are clearly igneous in origin. In this study we describe the geochemistry of the NMZ charnockites and enderbites. These rocks are the plutonic assemblage of Rollinson and Blenkinsop (1995). Samples were collected from about twenty separate plutons over an area of 4500 sq km throughout the northern part of the NMZ (Fig. 1), with the highest sampling densities along the Mundi and Mwenezi rivers. One pluton, the Sarahuru Pluton, was sampled in particular detail. A recent study by Kamber et al. (1995) has shown the importance of a 2.0 Ga event in the Central area of the Limpopo Belt. This metamorphic event postdates the emplacement of the charnockites and enderbites and is well developed in the Triangle Shear Zone to the south of the area of interest and has to some extent overprinted rocks in the southern part of the NMZ, the Transition Zone (NMZ s.1.) of Fig. 1. In this study we have selected our samples from the northern part of the

Late Archaean crustal growth in Zimbabwe

4811

Table 1. Compilation of age data from the Northern Marginal Zone of the Limpopo Belt and the Zimbabwe Craton. U-Pb Limpopo Belt NMZ Charnoenderbites

Charnoenderbite suite Razi-type granites and charnockites

2710 ± 38 Ma: 11 2718 _ 2637 _ 19 Ma: 11 2603 _+ 64 Ma: 11

Sm-Nd (TDM)

61 Ma: 12

2690 +_ 55 Ma: 12 2627 ___ 7 Ma: 10 2699 ± 67 Ma: 10

Zimbabwe Craton Tokwe Segment Tokwe gneiss Shabani gneiss Mushandike granod. Mont d'Or tonalite Mont d'Or granite Mid- to Late Archean Chingezi tonalite

Gwenoro dam migt. gneiss*** Somabula tonalite*** Sesombi tonalite*** Chilimanzi Suite

Pb-Pb

Rb-Sr

3.08-3.13 Ga: 11 2669 ± 230 Ma: 12" 3.05 Ga: 11 2.96 Ga: 11 3.06 Ga: 11 3.00 Ga: 11 2.99 Ga: 11 2644 +__ 39 Ma: 12" 2.87-3.09 Ga: 11 2583 ±

# value** 9.1

9.1

7 Ma: 10

3475 3088 2917 2946 3345

+ 97/-93 Ma: 8 3.64-3.68 Ga: 7 + 44/-46 Ma: 8 3.36-3.55 Ga: 8 +_ 171 Ma: 8 3.62 Ga: 8 + 125/-135 Ma: 8 ± 55Ma:6 3.71-3.74 Ga: 8

3500 +_ 400 Ma: 1 3495 ___ 120 Ma: 3 3350 ± 120Ma:2

9.4

2874 2825 2800 2686 2720 2752 2579

+_ 32 Ma: 8 3.09-3.18 Ga: 8 + 94/-100 Ma: 8 + 72-76 Ma: 8 + 88/-94 Ma: 8 ± 60 Ma: 8 + 50/-52 Ma: 8 2.86 Ga: 8 + 154/-173 Ma: 8 2.81 Ga: 8

2810 _ 70 Ma: 5 2818 _ 91 Ma: 8 2684 +__ 102 Ma: 8

8.5 8.2 8.3 8.2 8.5 8.1 8.0

2601 ± 14 Ma: 9

2705 2594 2633 2570

+ 60/-63 Ma: 8 + 80 Ma: 3 ± 140 Ma: 1 ± 25Ma:4

9.1 9.8

1 = Hawkesworth et al. (1975), 2 = Moorbath et al. (1976), 3 = Moorbath et al. (1977), 4 = Hickman (1978), recalculated from h = 1.42 × 10-". yr -~ , 5 = Hawkesworth et al. (1979), 6 = Taylor et al. (1984), 7 = Moorbath et al. (1986), 8 = Taylor et al. (1991), 9 = Jelsma (1993), 10 = Mkweli et al. (1995), 11 = Berger et al. (1995), 12: present study. All Nd-model ages are (re-)calculated for the model of Goldstein et al. (1984); * = regression of enderbites only; ** = recalculated for an age of the earth of 4.55 Ga; *** = Pb-Pb data of these intrusives are compiled in Figure 6.

NMZ, the NMZ(s.s.) of Fig. 1, in order to avoid the possible influence of the 2.0 Ga recrystallisation on the geochemistry of the 2.7 Ga magmas.

1.1. Petrography Following Berger et al. (1995), we have subdivided the rocks of the N M Z into enderbites, charnockites, retrogressed charnockites, and Razi granites. Here the term enderbite includes rocks of tonalitic and trondhjemitic composition (petrographicaUy they lack K-feldsp), whereas charnockites are of granodioritic and granitic composition and contain Kfeldspar (Fig. 2a). Enderbites are even-grained and comprise quartz ( 2 0 30%), plagioclase ( 6 0 - 6 5 % ) , orthopyroxene ( 5 - 1 0 % ) , and biotite ( 3 - 5 % ) with accessory Fe-Ti oxides, apatite, and zircon. Some samples contain clinopyroxene, hornblende, and/or garnet. On occasions a relict cumulate texture is evident with cumulus plagioclase (sometimes as phenocrysts), intercumulus pyroxene, and patches of late crystallised finer grained granular quartz. Plagioclase is antiperthitic; some grains are zoned with more calcic cores. More sodic samples contain mesoperthite in addition to plagioclase. Pyroxenes contain lamellar and dusty exsolution of iron oxides and more rarely contain abundant quartz inclusions. Red-brown biotite overgrows orthopyroxene to form

biotite-quartz symplectites. This partial hydration of orthopyroxene is thought to indicate the back reaction of orthopyroxene with late-magmatic water (Ridley, 1992). Charnockites contain quartz ( 3 0 - 4 0 % ) , plagioclase ( 2 0 30%), perthitic microcline or orthoclase ( 2 5 - 3 5 % ) , and biotite ( 5 - 1 0 % ) . Orthopyroxene, where present, is frequently extensively altered to biotite or biotite + carbonate. The main accessory minerals are Fe-Ti oxides, apatite, and zircon. Antiperthitic plagioclase and perthitic alkali feldspar both occur as phenocrysts. Retrogressed charnockites are charnockites which appear in outcrop as diffuse zones which show a distinct colourchange to white or pink. They are best seen in the southern part of the N M Z s.s. of Fig. 1, at Sarahuru, and are thought to represent fluid pathways through the charnockites and enderbites. They are characterised by the growth of microcline or orthoclase at the expense of plagioclase (Berger et al., 1995) and are of granitic or granodioritic composition (Fig. 2a) Many features of the slightly later Razi granite suite, notably their dry, granulite facies mineralogy suggest that there is a close association between their emplacement and that of the magmatic granulites of the N M Z (Berger et al., 1995 ). They range in composition from granite to granodiorite (Fig. 2a). In thin section they contain quartz ( 3 0 - 5 0 % ) , plagioclase ( 2 0 - 5 0 % ) , orthoclase, a n d / o r microcline ( 0 - 4 0 % )

4812

M. Berger and H. Rollinson an

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Fig. 2. (a) NMZ enberbites (filled squares), charnockites(open squares), Razi Granites (triangles). and retrogressed charnockite and enderbites (diamonds) plotted on the normative An-Ab-Or classificationdiagram of O'Connor ( 1965) modified by Barker (1979). (b) Compositional variation within individual plutons from the NMZ enderbites and charnockites; the plutons are Naude's Quarry pluton (light stipple), Manjirenji Dam (diagonal shading), Renco (dark stipple), and Sarahuru (cross hatching). Plagioclase compositions plot in the solid black field. Data sources are from this study, Berger et al. (1995), and Tabeart (1989).

and up to about 20% ferromagnesian minerals: biotite, hornblende, and orthopyroxene. Accessory minerals include apatite, ilmenite-magnetite intergrowths, zircon, allanite, sphene, and calcite. Orthopyroxene is the earliest formed ferromagnesian mineral, but it is invariably altered either to hornblende or to biotite, although some rocks show no evidence for the former presence of orthopyroxene indicating that these magmas show a wide range of H20 activities. 2. ANALYTICAL METHODS

2.1. REE, Thorium, Uranium, and Major Elements For the 85/series sample set REE elements were analysed using inductively coupled plasma spectrometry (ICP) at Royal Holloway College, Egham, UK using the method of Walsh et al. ( 1981 ). REE elements of other samples were measured by instrumental neutron activation analysis (INAA) at Activation Laboratories Ltd/Canada. Concentrations of Th and U were determined either by INAA at Activation Laboratories LTD/Canada or by X-ray fluorescence analysis on pressed powder pellets at the Department of Mineralogy and Petrology, University of Bern. Average uncertainties on XRF analyses are 6% (2 s.e.) for Th, and 10% (2 s.e.) for U, while average errors (2 s.e.) on INAA data are 5% for both Th and U. Major elements were determined by X-ray fluorescence analysis on fusion beads at the University of Leicester, UK. Results are documented in Table 2 for major elements, Table 3 for REEs, and in Table 4 for Th and U.

2.2. Isotope Geochemistry Measurements of U, Pb, Rb, and Sr were performed at the Institute of Mineralogy and Petrology, Isotope-Geology,

University of Bern. Samples of 100-500 mg whole rock powder were digested in HF in Savilex® screw top vessels followed by cation exchange procedure. Samples for Pb and U were aliquoted, and a 2°spb-235U mixed spike was used for concentration measurements, whereas for those samples where only Pb was determined a 2°7pb spike was used. This was followed by Pb and U extraction in 5 mL quartz-columns and standard HBr-HCI chemistry. Lead was loaded on single Re filaments with silica gel and H3PO4 and was measured statically on a VG Sectora ® 5 collector mass spectrometer. The values obtained on standard NBS 981 through the course of this work are, for the 2°vpb/2°rpb ratio, 0.91443 _+0.00045, for 2°spb/2°6Pb, 2.1645 _+ 0.0019, and for 2°6pb/2°4pb, 16.9322 _+ 0.0131 (errors quoted are 1 s.d. of population of 108 measurements), and fractionation corrections were made accordingly. Lead blanks of the total procedure were generally about 0.6 ng. Because all sample preparations involved the separation of several #g of Pb, no correction was made for the blank. Uranium was loaded on triple Ta-Re-Ta filaments and was measured on an Avco 90 °, 350 mm radius single collector instrument. The errors (2 s.e.) on U concentrations are better than 2%. Model #, values were calculated for a single stage model with an age of the earth of 4.55 Ga and primordial 2°6pb/2°4Pb and 2°Tpb/z°4pbratios of 9.307 and 10,294. Samples for Rb-Sr isotopic analysis were prepared using standard cation exchange. Strontium was loaded on single Ta filaments and analyzed on the VG Sector~ instrument. 875r/86Sr ratios were corrected for mass fractionation by norrealizing to a value of 8.37521 for 88Sr/86Sr. Twenty-five measurements of the NBS Srm 987 standard on this instrument during this work gave a S7Sr/S6Sr ratio of 0.710253 -40.000042 (error quoted is 2 s.d. of this population). No other

Late Archaean crustal growth in Zimbabwe

4813

Table 2. Major element and selected trace element XRF analyses of selected samples from different plutons in the NMZ, Limpopo Belt, Zimbabwe. Sample No Pluton Lithology

85/4 Ngundu e

SiO2 (wt%) TiO2 (wt%) A1203 (wt%) Fe203 (wt%) MnO (wt%) MgO (wt%) CaO (wt%) Na20 (wt%) K20 (wt%)

62.58 71.47 68.55 0.56 0.25 0.47 18.10 14.82 16.20 5.34 3.04 3.73 0.09 0.04 0.03 1.71 0.63 1.13 4.38 3.22 4.24 6.01 5.16 4.47 1.30 1.04 1.31 0.20 0.04 0.13 nd nd nd 1 0 0 . 2 7 99.71 100.26 0.964 0.964 1.002 145 159 354 32 9 57 24 14 11 21 33 26 185 178 285 19 10 10

P205 (Wt%) H20 (wt%) Total (wt%) ASI Sr (ppm) Rb (ppm) Y (ppm) Th (ppm) Ba (ppm) Nb (ppm)

85/11 Lundi e

85/134 Lundi e

85/40 Renco e

90/89H Manjirenji e

72.81 0.39 14.15 3.16 0.06 0.68 3.41 3.90 1.75 0.10 nd 100.41 0.992 96 74 22 22 283 7

65.38 0.47 16.27 4.75 0.06 2.46 5.35 2.92 1.92 0.04 1.14 100.76 1.027 310 18 10 0 660 nd

85/123 Lundi e

85/105 85/110 Neshuro Sarahuru c c

85/135 Lundi c

85/158 Tods Q c

85/90 Manjirenji rc

85/104 Neshuro rc

67.31 67.89 74.60 76.99 74.98 0.31 0.37 0.10 0.05 0.17 18.37 15.72 14.44 13.08 13.57 2.77 2.45 1.04 0.24 1.33 0.03 0.03 0.01 0.01 0.03 0.93 0.64 0.13 0.01 0.16 4.23 2.66 1.76 1.11 1.31 5.75 4.86 3.96 2.70 3.22 1.21 3.18 4.52 5.95 5.53 0.10 0.10 0.04 0.02 0.03 nd nd nd nd nd 1 0 1 . 0 1 97.90 100.60 1 0 0 . 1 6 100.33 1.008 0.981 0.995 1.108 0.998 735 226 161 386 99 42 111 104 162 174 5 8 7 5 10 1 24 6 1 32 576 733 1034 2421 975 2 6 bd bd 3

74.83 0.16 14.61 0.73 0.01 0.19 2.41 3.33 3.36 0.03 nd 99.93 1.065 216 67 2 3 1532 1

73.58 0.23 13.93 1.47 0.02 0.34 1.58 3.90 4.28 0.05 nd 99.38 1.009 179 181 10 16 943 7

nd = not determined; bd = below the detection level; e = enderbite, c = charnockite, rc = retrogressed sample.

correction has been applied to the Sr data reported in this work. Rubidium was measured on the single collector instrument on triple Re-Ta filaments. 87Rb/s6sr ratios were determined to better than 1.2% (2 s.e.) except for a few higher errors which are indicated in Table 5. Errors given for the Pb-Pb and Rb-Sr ages are 2 cr errors.

3. GEOCHEMISTRY 3.1. Major Element Geochemistry On the triangular normative A n - A b - O r classification diagram o f O ' C o n n o r (1965) modified by Barker (1979), enderbites plot, by definition, in the fields o f tonalite and trondh-

Table 3. REE analyses of rocks from the NMZ, Limpopo Belt, Zimbabwe.

Sample Primitive enderbites 85/4 85/11 85/134 Enderbites 85/40 90/89H 92/113 85/123 90/78f Charnockites 85/105 85/110 85/135 85/158 90/79a Retrogressed samples 85/90 85/104 90/78a 92/115 Razi type 92/066L

Pluton

Analytical La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu method (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm)

Ngundu Lundi Lundi

ICP ICP ICP

43.5 41.7 70.1

Renco Manjirenji Manyuche Lundi Sarahuru

ICP INAA INAA ICP INAA

41 35.4 54.1 9.77 53.3

Neshuro Sarahuru Lundi Tods Q Lundi

ICP ICP ICP ICP INAA

37.7 18 11 59.1 106

61.4 24.9 14.4 106 169

Manjirenji Neshuro Sarahuru Manyuchi

ICP ICP INAA INAA

19.8 25.7 17 34.1

Musumhe

INAA

21.9

nd = not determined.

78.8 72.8 115

7.54 6.9 10.2

30.3 26.3 37.9

5.89 5.1 5.2

0.67 0.92 1.16

5.12 4.55 3.93

nd nd nd

3.71 2.45 2.07

0.69 0.4 0.39

1.8 1 1.26

1.06 0.44 0.47

0.14 0.06 0.06

7.04 nd nd nd nd

26.9 19.2 34 6.53 31

4.85 3.1 4.68 1.07 4.18

0.87 1.11 1.1 0.68 0.72

4.54 nd nd 1.1 nd

nd 0.36 0.6 nd 0.6

3.01 nd nd 0.66 nd

0.68 nd nd 0.15 nd

1.91 nd nd 0.74 nd

1.68 0.83 1.15 0.31 0.88

0.25 0.12 0.15 0.03 0.11

5.5 18.7 1.99 7.29 1.17 4 . 0 1 9.33 34.9 nd 54

2.77 1.12 0.54 5.08 8.43

0.65 0.69 1.09 0.67 1.55

2.17 1.05 0.42 3.61 nd

nd nd nd nd 0.9

1.18 0.58 0.26 1.9 nd

0.23 0.11 0.05 0.33 nd

0.68 0.44 0.15 1 nd

0.33 0.23 0.14 0.32 1.17

0.04 0.04 0.02 0.04 0.12

27.7 40.7 24 56

2.37 3.56 nd nd

8.07 12 8 17

0.99 1.89 1.09 2.42

0.99 0.52 0.7 0.9

0.78 1.5 nd nd

nd nd 0.3 0.2

0.23 1.1 nd nd

0.05 0.21 nd nd

0.23 0.56 nd nd

0.06 0.28 0.41 0.47

0.01 0.03 0.05 0.04

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1.43

nd

0.2

nd

nd

nd

0.88

0.09

72.7 59.2 90 17.9 88

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Table 5. Rb-Sr isotope data. Sample Charnockites 90/76 92/013 92/014 92/023 92/103 92/110 Granites 90/80 92/112 93/508 Retrogressed samples 90/78A 90/78B 90/78C 90/78H 90/78K 92/020 92/115 92/116 92/118 Enderbites 90/75B 90/78D 90/78E 90/78F 90/78G 92/002 92/003 92/025 92/029 92/038 92/043 92/050 92/053 92/060 92/067 92/074 92/109 92/111 92/113 92/114 92/117

Locality

Sr (ppm)

Rb (ppm)

Bonde Mataga Mataga Mundi N Mundi S Mwenezi

333.11 336.59 309.47 474.60 302.93 281.48

31.90 13.14 106.57 107.43 34.73 78.78

0.277 1.149 0.999 0.656 0.332 0.812

Maraehuru Musumhe Samba

225.58 151.88 396.25

137.77 170.52 75.49

1.778 3.285 0.552

Sarahuru Sarahuru Sarahuru Sarahuru Sarahuru Mundi N Manyuche Manyuche Mwenezi

170.42 164.45 110.01 145.00 146.03 181.40 192.80 174.64 146.41

134.22 127.12 129.14 131.60 140.79 62.90 93.35 136.17 116.00

2.297 2.254 3.437 2.650 2.817 1.006 1.407 2.273 2.310

Matibi Sarahuru Sarahuru Sarahuru Sarahuru Tokwe R Tokwe R Mundi N Mundi N Mundi N Tshaba R Bangala Mbire Manjirenji Musumhe Munde S Mwenezi Mwenezi Manyuche Manyuche Mwenezi

558.34 174.32 210.02 157.42 202.17 328.60 270.66 296.98 154.19 291.07 271.32 166.13 121.79 301.17 709.27 248.35 196.91 181.71 204.35 221.01 150.82

58.06 15.57 32.78 63.68 24.09 6.88 41.87 7.72 20.56 109.06 24.73 31.01 104.15 14.33 36.42 12.46 64.35 35.99 53.23 13.17 71.50

0.301 0.259 0.452 1.175 0.345 0.061 0.448 0.075 0.386 1.088 0.264 0.541 2.497 0.138 0.149 0.145 0.178 0.574 0.755 0.173 1.378

jemite whereas charnockites lie in the fields of granodiorite and granite (Fig. 2a). Retrogressed samples plot principally as granites (Fig. 2), as expected from their observed Kenrichment. Of particular note is the rarity of rocks of trondhjemitic composition, indicating that the N M Z crust is less sodic than many Archean cratons (Martin, 1993). In addition to their Na-poor character the enderbites and charnockites of the N M Z are peraluminous. On a plot of the alumina-silica index ( A S I - - Z e n , 1986) vs. SiO2 (not shown), there is a trend of increasing ASI with increasing silica and a large proportion of the samples plot in either the mildly per-aluminous field ( A S I > 1.0) or the per-aluminous field (ASI > 1.1), see Table 2.

3.1.1. Harker variation diagrams The enderbites and charnockites of the N M Z show well defined trends on Harker diagrams. These are illustrated in

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Berger et al. (1995) who described negative correlations with SiO2 for the oxides MgO, CaO, Fe203, TiO2, and MnO. Negative trends for CaO and total Fe as FeO and A1203 are shown in Fig. 3 for the larger dataset used in this study. A plot of measured mineral compositions plotted onto the Harker variation diagrams (Fig. 3) indicate that the observed chemical variation can be interpreted in terms of crystal fractionation of the assemblage plagioclase + orthopyroxene + clinopyroxene. This is in broad agreement with the findings of Berger et al. (1995) who interpreted the variation in terms of plagioclase and clinopyroxene fractionation.

3.1.2. Compositional variation within plutons In Fig. 2b we plot the compositional variation within four plutons on the normative An-Ab-Or diagram. Two important features are apparent. First, each pluton shows a common trend from the An-Ab boundary towards the Or apex. This

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is most easily explained by plagioclase and pyroxene fractionation as suggested by the trends on the Harker diagrams. Second, the plutons show a range of initial compositions

each with similar Or contents but with variable Ab/An ratios. The broad trend in the chemistry of the complete suite of enderbites and charnockites (Fig. 2a) is in reality com-

Late Archaean crustal growth in Zimbabwe posed of a number of better defined lines of magmatic evolution (for individual plutons) each showing fractionation from enderbite to charnockite.

3.2. Trace Element Chemistry 3.2.1. Multi-element diagrams Average trace element concentrations in the enderbites, charnockites, retrogressed charnockites, and Razi Granites are presented on a mantle normalised multi-element diagram in Fig. 4a. The four groups show small differences in absolute concentrations of trace elements but the pattern of high Rb, Ba, Th, U, and K and low Nb, Ti, Y, and Tb is apparent in all lithologies. When compared with charnockites and granulite facies TTG gneisses from other terrains, it is apparent that charnockites and granulites in general vary in their concentrations of the heat producing elements K, Rb, U, and Th. The NMZ charnockites and enderbites are similar to Archaean charnockites from south India and Canada and Proterozoic charnockites from Antarctica (Fig. 4b) inasmuch as they carry high concentrations of Rb, Th, U, and K. In contrast, however, the NMZ charnockites and enderbites are enriched in heat producing elements and depleted in Sr relative to the Archaean Lewisian granulites, the Proterozoic Lapland granulites, the Cretaceous Western Fjordland Orthogneiss granulites of New Zealand (Fig. 4c), and the average lower continental crust of Rudnick and Fountain (1995). When the NMZ samples are compared with the database for Archaean granulites, post-Archaean granulites, and granulite xenoliths of Rudnick and Presper (1990), their distinctive chemistry is further emphasised. Average K/Rb ratios in the NMZ granulites are between 328 (enderbite) and 340 (retrogressed granulites). Even at K concentrations of less than 1 wt% K / R b ratios rarely exceed 1000, in marked contrast to the dataset of Rudnick and Presper (1990). Similarly K / U ratios are low. Values range from 1060 to 7300 for enderbites and from 6000 to 31000 for charnockites ( K data from Berger et al. (1995), U data from Table 4). In contrast granulites in the Rudnick and Presper dataset frequently have K / U ratios greater than 100,000. These observations demonstrate that not all granulite facies rocks are depleted in the elements Rb, U, and Th and cast doubt on the widely accepted concept of element depletion during granulite facies metamorphism (Rollinson, 1996). The low K/Rb ratios and high levels of U and Th in the NMZ charnockites and enderbites suggest that granulite facies elemental depletion has not occurred in these rocks. The arguments developed below indicate that this is a function of their source rather than of process.

4817

within this suite we have identified three samples which, on the basis of their major element chemistry, are thought to represent the least fractionated members of the suite for they plot close to the An-Ab boundary in the normative An-AbOr plot in Fig. 2. These samples, our primitive enderbites, are thought to approximate in composition to unevolved igneous liquid compositions. These primitive enderbites (Fig. 5b) are enriched in total REEs relative to the majority of the suite, have small negative Eu anomalies, and have strongly fractionated REE patterns. Retrogressed samples (Fig. 5c) plot within the same compositional range as the charnockites and enderbites and appear to have an undisturbed, magmatic REE chemistry. The similarity between the REE profile of the single Razi granite sample and that of the field of charnockites and enderbites (Fig. 5c) supports the view advanced earlier that there is a close geochemical association between the two. The high degree of REE fractionation (normalised La/Yb ratios are between 27 and 100) in the NMZ charnockites and enderbites is typical of Archean tonalites (Martin, 1986, 1993) and indicative of either the melting of a garnet bearing source rock or garnet fractionation in the source. The small negative Eu anomaly in the primitive enderbites is unusual for Archean tonalites (Rudnick and Taylor, 1986) and suggests either the presence of plagioclase in the source rock or that the samples are not entirely primitive and have experienced some plagioclase fractionation. Both possibilities are consistent with the relatively low Sr levels in these melts. The variability in REE chemistry is consistent with plagioclase and pyroxene fractionation as indicated by the major element chemistry. In Fig. 5f we have modelled the effects of 20% and 30% fractional crystallisation of the assemblage plagioclase + clinopyroxene + orthopyroxene from a primitive enderbite. The calculated liquid and residue compositions conform well to the range of compositions present in the charnockites and enderbites and suggest that samples with the most extreme positive Eu anomalies represent liquids which are enriched in cumulus plagioclase and pyroxene. A similar pattern of REE fractionation was described by Seifert et al. (1977) from the Tupper-Saranac Mangerites in the Adirondacks, rocks which are also interpreted as magmatic granulites. The gabbroic nature of the fractionating phases constrains this stage of the evolution of these melts to crustal levels.

3.3. Isotope Geochemistry The results of the lead and strontium isotope geochemistry are presented in Tables 4 and 5.

3.3.1. Lead isotopes 3.2.2. REE Diagrams The NMZ charnockites and enderbites are characterized by REE patterns which are highly fractionated, have a wide spread in total REE concentrations, and a variable Eu anomaly (see Fig. 5a). Samples with the highest total REE contents have negative Eu anomalies, samples with the lowest total REE contents have positive Eu anomalies, and samples in the middle of the range do not show a Eu anomaly. From

Lead isotope measurements on the charnockite and enderbite suite define a straight, narrow band on a 2°Tpb/z°apb2°spb/2°4pb plot (Fig. 6) well above the evolution curve of Stacey and Kramers (1975). A regression line through all the data points yields an errorchron with an apparent age of 2690 _+_ 55 Ma and an MSWD of 44.5. If the two most radiogenic points are excluded from the regression, the data yield an apparent age of 2788 ___ 150 Ma with an MSWD

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is obtained if a t w o stage lead i s o t o p e evolution m o d e l is used. A suite o f nine s a m p l e s was c o l l e c t e d f r o m Sarahuru

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Fig. 5. Chondrite normalised rare earth element plots. (a) enderbites (squares) and charnockites (circles); (b) primitive enderbites compared with the field of charnockites and enderbites (shaded); (c) retrogressed samples (circles) and Razi granite (triangles) compared with the field of charnockites and enderbites (shaded); (d) melt compositions (circles) calculated for 5%, 10%, 20%, and 30% partial melting of a basaltic source (squares) with chondritic REE ratios; melt compositions are in equilibrium with a garnet granulite residue (0.5 plagioclase, 0.45 clinopyroxene, 0.05 garnet); primitive enderbite compositions (unornamented) are shown for comparison. (e) melt compositions (circles) calculated for 5%, 10%, 20%, and 30% partial melting of a LREE-enriched basaltic source (squares); melt compositions are in equilibrium with a garnet granulite residue (0.5 plagioclase, 0.4 clinopyroxene, 0.1 garnet); primitive enderbite compositions (unornamented) are shown for comparison. (f) liquid compositions (filled circles) and residue compositions (open circles) produced by 20% and 30% fractional crystallisation of the assemblage (0.74 plagioclase + 0.13 clinopyroxene + 0.13 orthopyroxene) from enderbite 85/11 (triangles), compared with the field of charnockites and enderbites (shaded). Partition coefficients for andesites in (d) and (e) and for rhyolites in (f) taken from Rollinson (1993b). Normalizing values taken mainly from Nakamura (1974).

quarry in order to image the isotopic characteristics of a single NMZ pluton for which a U-Pb zircon age of 2710 Ma (interpreted as the intrusion age) and the Nd tDM age

(3,1 Ga) have already been obtained (Berger et al., 1995). Regression of the 2°Tpb/2°4pb- 2°tpb/2°4Pb data (Fig, 7) yields an isochron age of 2718 +_ 61 Ma with an MSWD of 0.74.

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206Pb/204Pb Fig. 6. 2°7pb/2°4pb-2°rpb/2°4pbplot of enderbites (open squares), chamockites and granites (open circles) and retrogressed charnoekite and enderbites (open pentagons). The grey shaded rectangle represents the main array of the age corrected 2°7pb/2°4Pb-2°rPb/2°4Pbratios. The iregular shaded fields are for six 2.6-2.8 Ga tonalitic plutons from the Zimbabwe Craton--the Somabula tonalite (So), the Sesombi tonalite (Se), the Chingezi tonalite (Ch), and the three plutons included in the Gwenoro Dam migmatitic gneisses field (GD) from Taylor et al. (1991). S.r. = standard reproducibility. Evolution curve of Stacey and Kramers (1975) plotted for reference.

This age coincides with the U-Pb zircon age of 2710 _+ 38 Ma obtained on one of the samples and lies within error of the 2690 +_ 55 Ma age obtained for the entire NMZ suite. The model #~ value of 9.1 for the Sarahuru suite is identical to that obtained for the whole NMZ charnockite and enderbite suite We regard the 2690 Ma age as the rough average intrusion age of the charnockite and enderbite suite of the whole NMZ. This age has been derived for a dataset that covers approximately twenty individual intrusive bodies over a large geographic area. Two lines of evidence support this reasoning. First, the age is in broad agreement with U-Pb zircon ages obtained on individual plutons which range between 2.6 and 2.71 Ga (Berger et al., 1995; Mkweli et al., 1995). Second, the average age does not contradict the apparent ages obtained on the individual lithologies such as Sarahuru. The scatter of sample points about the errorchron may be attributed to a range in the initial lead isotopic compositions in the protolith and/or to a change in the radiogenic 2°rpb/2°4Pb, 2°7pb/2°4pb ratios of the protolith during the time interval of the magmatic activity (2.6-2.71 Ga; Berger et al., 1995; Mkweli et al., 1995). There are a number of important differences between the lead isotope chemistry of the NMZ charnockite and enderbite

suite and that of tonalitic plutons of similar age on the adjacent Zimbabwe Craton (Taylor et al., 1991; Fig.7; Table 1 ). These have been identified as follows: (1) Although the NMZ data represent a far larger number of individual plutons, the 2°7pb/2°4pb-2°rpb/2°4pb band they define is much narrower than that outlined collectively by the plutons of the Zimbabwe Craton. The closest comparison is with the Gwenoro Dam plutons (Fig. 6) on the Zimbabwe Craton (2720 _ 60 Ma, /z = 8.5) and yet even here the twenty plutons of the NMZ suite have the same compositional range as three on the Zimbabwe Craton. These results suggest a particular homogeneity in the lead isotope chemistry of the NMZ plutons. (2) In contrast to the Zimbabwe Craton tonalites, which range in lead isotope signature from mantle derived to crustal, the NMZ suite consistently shows high 2°TPb/2°4Pb ratios for given 2°6pb/2°4pb ratios, indicating a high # time-integrated protolith. (3) The model #1 value of 9.1 calculated for the 2690 Ma errorchron is extremely high compared with the model #1 value of 8.5 (recalculated for an age of the earth of 4.55 Ga) for the Gwenoro Dam migmatitic gneisses, the most uranogenic of the tonalites from the Zimbabwe Craton in which the Pb/Pb age is realistic (Taylor et al., 1991; Table 1 ). (4) The high/zj value for the charnockite and enderbite suite precludes a mantle origin for the char-

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4821

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nockite and enderbite suite at 2.69 Ga. This observation is also indicated by the high Nd model (tDra) ages of around 3.0 Ga reported by Berger et al. (1995). On the other hand, it should be noted that while mantle derived plutons were intruded until 2.6 Ga in the Zimbabwe Craton, no similarly young, mantle derived plutons are found in the NMZ. In Fig. 8 we show age corrected 2°Tpb/2°4pb-2~pb/2°4pb ratios for the charnockite and enderbite suite. They were calculated using either measured intrusion ages or intrusion ages estimated from the field relationships and U concentrations measured by ID, XRF, or INAA (Table 4). 63% of the data points occupy a small area shown by the shaded field on Fig. 8. This field is defined by the 2°7pb/2°4pb-2°~Pb/ 2°4pb coordinates: 15.060/13.854, 15.226/13.531, 15.261/ 14.879, and 15.410/14.556 and is thought to represent the average lead isotopic composition of the protolith in the time span 2.71 to 2.6 Ga. Samples with initial 206/207 ratios falling to the fight of the shaded field in Fig. 8 probably result from either recent U-loss or sample inhomogeneity, whereas initial ratios to the left of the field, assuming that they do not reflect originally lower initial ratios, can only result from sample inhomogeneity (e.g., higher concentrations of zircon in the analyzed quantity compared to the average sample). Some evidence for this sample inhomogeneity is evident from the data presented in Table 4 where multiple aliquots of the same sample (90/78 series) show differing 2°TPb/2°4pb-2°6pb/2°4Pb ratios. The average 2°6Pb/ 2°4Pb and 2°7pb/2°4pbratios of this cluster are 14.2 and 15.24,

respectively, and these values are used below in our discussion of the history of the protolith of the NMZ charnockite and enderbite suite. Thorogenic lead vs. uranogenic lead ratios for the NMZ charnockite and enderbite suite are plotted in Fig. 9. The data array is well correlated except for three samples (90/ 78K, 92/050, and 92/117) which show increased 2°~Pb/ 2°4pb ratios relative to 2°spb/2°4pb ratios and are excluded from further discussion. The correlation of 208Pb/ 204Pb with 2°6pb/2°4Pb ratios for the NMZ charnockite and enderbites is in contrast to that observed for the 2.6-2.8 Ga tonalitic plutons of the adjacent Zimbabwe Craton (Taylor et al., 1991 ). In the Zimbabwe Craton 2°~pb/~Pb- 2°6pb/2°4pb isotope characteristics are thought to indicate U mobility either directly after or some hundred million years after intrusion (Taylor et al., 1991). The average time integrated Th/U ratio for the enderbites and charnockites (calculated using the average age of 2690 Ma) is 7.8, about twice the value of the average terrestrial ratio of ca. 4. The time integrated Th/U ratios also agree quite closely with the average present day Th/U ratio of 10.2, calculated from measured Th and U concentrations (Table 4). The samples analysed here were collected from localities where there is no macroscopic or petrographic evidence for the later 2.0 Ga event recently documented by Kamber et al. (1995). The only evidence we have found to indicate the possible resetting of isotopic compositions at 2.0 Ga comes from Pb leaching experiments on plagioclase in samples 90/

4822

M. Berger and H. Rollinson

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206Pb/204Pb Fig. 8. Age corrected 2°7pb/2°4pb-2°6Pb/2°4pbplot for whole rock data from the NMZ. The lead isotope data have been corrected for the decay of U using the U concentrations and the samples ages given in Table 4. The grey shaded field represents the main array of 2°7pb/2°4pb-2°6pb/2°4pbratios in which 63% of all data points plot. The star defines the average age corrected 2°7pb/2°4Pb-2°6pb/2°4pbratio pair. The evolution curve of Stacey and Kramers (1975) is plotted for reference. A tangent to the Stacey and Kramers curve from the average age-corrected 2°Tpb/2°4pb-2~pb/ 2°4pbratio constrains the minimum age of the protolith to 3.25 Ga. Symbols as for Fig. 6 with the addition of Sarahuru (solid symbols).

78G from Sarahuru and 92/111 from Mwenezi (Table 4). These experiments indicate that there has been a partial resetting of lead isotopic compositions in plagioclase (M. Berger, unpubl, data). However, since the lead isotope ratios of both samples sit on the 2.69 Ga whole-rock trend, there is no evidence to suggest that there has been 2.0 Ga isotopic reequilibration on the scale of our whole rock samples. Nevertheless, in the light of these experiments we thought it prudent not to use the 2°6pb/2°4Pb vs 2°7pb/2°4pb initials given by plagioclase residues to define an initial for the NMZ whole rock suite.

3.3.2. Strontium isotopes An 87Sr/86Sr-87Rb/86Sr errorchron regressed through all charnockite, enderbite, and retrogressed samples yields an apparent age of 2566 +_ 39 Ma with a high MSWD of 33.6 and an initial ratio of 0.70427 ± 79 (Fig. 10). This result is within error of the Rb-Sr errorchron of Mkweli et al. (1995), obtained on a smaller dataset. When regressed separately the enderbites yield an apparent age of 2644 _+ 58 Ma (intial ratio 0.70400 ± 97), within error of the Pb-Pb age for the NMZ suite. The charnockites give a younger apparent age of 2539 ± 94 Ma (initial ratio 0.70410 ± 156) as do

the retrogressed charnockites and enderbites 2598 ___ 70Ma (initial ratio 0.70377 ± 234, excluding samples 92/116 and 92/118). An Rb-Sr errorchron for the Sarahuru pluton (Fig. 11 ) yields an apparent age of 2538 _-. 54 Ma and a high 875r/86Sr initial ratio of 0.70616 ___ 130. If the four enderbite samples only are used then an age of 2669 ___ 230 Ma and an initial ratio of 0.70529 +_ 98 is obtained, whereas the five retrogressed samples give an age of 2635 ___340 Ma and an initial ratio of 0.70393 _+ 818. These ages have large errors and, therefore, cannot on their own form the basis of substantive arguments. These results suggest that the Rb-Sr data reflect two events. First, enderbite samples yield ages broadly comparable with the Pb-Pb ages for the charnockite enderbite suite. These ages are interpreted as magmatic crystallisation ages. The inital ratios of these samples are high and confirm the observation made above from lead and neodymium isotopes (Berger et al., 1995), that the protolith to the NMZ charnockite and enderbite suite contained a major crustal component at ca. 2.7 Ga. Furthermore, the strontium isotope data give no indication of any significant Rb depletion, consistent with the trace element results reported above. Younger ages are found in rocks which contain zones of retrogression, reflecting channellised fluid flow (Berger et al., 1995), and

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these young ages are thought to represent the time of retrogression of the gneisses.

used to provide a picture of the process of late Archaean crustal evolution in the NMZ.

4. DISCUSSION

4.2.1. The significance of the high # value and high Th/U ratios

4.1. The Timing of the NMZ Magmatism The geochronological results presented here indicate an intrusion age of ca. 2.7 Ga for the NMZ charnockites and enderbites. The apparent Pb-Pb age of 2690 _+ 55 Ma for the ca. 20 sampled plutons is interpreted as a broad average intrusion age for the suite. This is within error of the whole rock Pb-Pb age of 2718 _+ 61 Ma from Sarahuru and the Rb-Sr age of 2644 _+ 58 Ma for the enderbites. U-Pb zircon ages obtained on the same rocks are in the range 2710 _+ 38 to 2603 _+ 64 Ma confirm an intrusion age of ca. 2.7 Ga and suggest that the magmatism lasted for ca. 100 Ma (Berger et al., 1995). 4.2. Geochemical Constraints on the Nature of the Protolith Information about the source region of the NMZ magmas and about their magmatic evolution may be obtained from their trace element and isotope geochemistry. In the sections below the trace element chemistry of U, Th, and the REEs and the isotope geochemistry of the Pb and Nd systems are

The NMZ charnockites and enderbites are characterised by high concentrations of U and Th, have a high model/z~ value of 9.1, and high Th/U ratios. The high model #~ value is especially interesting since none of the late Archean granitoids in the Zimbabwe Craton have such high values (Taylor et al., 1991; Jelsma et al., 1996). Late Archean tonalites from the Zimbabwe Craton (Fig. 6, Table 1 ) have model/.t~ values between 8 and 8.5. Model #] values of 9.0 and above are only obtained on the granitoids and gneisses that are older than ca 3.25 Ga (Pb-Pb, Rb-Sr whole rock, Taylor et al., 1991; Table 1 ), rocks which have Nd model ages greater than 3.5 Ga (recalculated using the model of Goldstein et al., 1984). The high average time integrated Th/U ratio of 7.8 indicates that the NMZ enderbites and charnockites experienced either depletion of U or enrichment of Th relative to the average terrestrial Th/U ratio of between 3.5 and 4. The well defined linear array on the 2°spb/2°4pb-2°6pb/2°4pbplot for the Sarahuru data (Fig. 9) suggests that such fractionation occurred during the formation of the protolith rather than in the magma differentiation process that led to the

4824

M. Berger and H. Rollinson

0.84

~Wh0ierock Rb-Sr errorchron on J charnoenderbite suite samples

0.82 Q

0.80

878r

868r

0.78 0.76 0.74

0.72 ~ 0.70

-

~dr-

apparent age (all points): 2566+/-39 Ma, MSWD = 33.6 87Sr/868r init. , = 0.70427+/-79 ,, . . . . ,i i ; :~,

f I

0

I

I

1

2

I

I

I

3

4

87Rb/86Sr Fig. 10. ~TSr/86Sr-~TRb/8%r plot for whole rock data from the NMZ. Symbols as for Fig. 6.

present suite. Thus the high time integrated T h / U ratios were inherited from the protolith of the enderbites and charnockites. U / T h / P b fractionation is a crustal process, for U, Th, and

0.84

Pb are highly incompatible in silicate phases during mantle melting, such that U / P b , T h / P b , and T h / U ratios in the melt are the same as those of the source rock before melting. In contrast, in the continental crust U, Th, and Pb are partitioned

Whole rock RWSr errorchron on samples from the Sarahuru pluton, NMZ, Zimbabwe

0.82 0.80

878r 0.Te 868r 0.76 0.74 0.72

apparent age (all points): 2 5 3 8 + / - 5 4 Ma, MSWD = 7.8 8 7 8 r / 8 6 8 r init. = 0 . 7 0 6 1 6 + / - 1 3 0

'b)

0.70 0

1

2

3

4

87Rb/86Sr Fig. 11. STSr/S6Sr-87Rb/S6Srerrorchron for whole rock samples from the Sarahuru Pluton. (a) regression of enderbite samples yielding an age of 2669 _+ 230 Ma and an STSr/S6Sr initial of 0.70529 ± 0.00049 (MSWD = 7.6); (b) regression of retrogressed samples yielding an age of 2635 _+ 340 Ma and an 87Sr/S6Sr initial of 0.70393 _ 0.00409 (MSWD = 4.58).

Late Archaean crustal growth in Zimbabwe differentially into mineral phases such as feldspar and the accessory minerals zircon and monazite. These elements are also fractionated during the interaction between crust and a hydrous fluid phase. For this reason the observed high /~ value and high Th/U ratio of the NMZ charnockites and enderbites must reflect processes which have taken place in a crustal protolith. This observation is supported by the high initial 87Sr/86Sr ratio of around 0.704, a value which is well above estimates for the late Archaean mantle, and is consistent with the old Nd-model ages of around 3.0 Ga recorded by Berger et al. (1995). An estimate of the age of the inferred crustal protolith can be made from the average 2°7pb/2°4pb and 2°tpb/2°4pb initial ratios of 15.24 and 14.2 calculated for the charnockite and enderbite suite. A minimum average protolith age is determined by laying a tangent on the Stacey and Kramers ( 1975 ) growth curve and forcing it through the average initial lead isotope ratio pair (Fig. 8). The minimum protolith age thus obtained is 3.25 Ga. The # value of this tangent approaches infinity, and this means that the protolith must be considerably older than 3.25 Ga. The older the protolith, the lower the average # value required to reproduce the initial of the chamockite and enderbite suite. The /z value ranges from about 11 at derivation age 4.0 Ga to infinite at 3.25 Ga. The observation that the high measured # value and the high Th/U ratios measured in the NMZ charnockites and enderbites were inherited from an older crustal source with an even higher # value is of profound importance. Two significant observations follow. First, the direct source of the NMZ charnockites and enderbites was produced by the mixing of two different sources at 2.7 Ga. Second, the mixing was between an old (crustal) source with a high # value and a younger lower-# source.

4.2.2. Neodymium isotope constraints on the nature of the protolith e-Nd values calculated from the data of Berger et al. (1995) for the NMZ charnockites and enderbites at 2.7 Ga define a narrow range between + 1.21 and - 1 . 9 6 (Table 4). Model ages recalculated using the new upper mantle growth curve of Th. F. Nagler and J. D. Kramers (pers. commun.) are between 2.74 and 2.97 Ga. However, given the constraints of the lead isotope system described above, these calculated ages must be now interpreted as mixing ages which are intermediate between the age of the old (>3.25 Ga), high # (crustal) source and a younger low # source (minimum age 2.7 Ga). The calculated e-Nd values allow further constraints to be placed upon the nature of these two sources. Old felsic crust typically has a subchondritic Srn/ Nd ratio and evolves through time to more negative e-Nd values. Thus to produce the observed range of e-Nd values at 2.7 Ga, there was mixing between low Srn/Nd crust, with negative e-Nd values, and a source with a high Sm/Nd ratio and with positive e-Nd values, values close to that of the late Archaean depleted mantle. Thus the low-/z, high Sm/ Nd component was a mantle-derived melt.

4.2.3. Lead isotope homogeneity A striking feature of the NMZ charnockite and enderbite suite is the homogeneity of the lead isotope compositions at

4825

ca. 2.7 Ga. We have shown that the 2°Tpb/2°4pb-2°tpb/2°apb band defined by ca. 20 plutons from the NMZ is more tightly constrained than the field for six plutons from the Zimbabwe Craton. The concordancy of ages between the Pb-Pb and Rb-Sr whole rock systems (apart from some minor Rb-Sr resetting) demonstrates that the U-Pb and Rb-Sr whole rock systems have remained undisturbed since emplacement. Thus the homogeneity of the lead isotope compositions at ca. 2.7 Ga is regarded as a primary feature of these magmas and not an artefact of later alteration. This isotopic homogeneity places additional contraints on the process of magma generation, for it requires that at 2.7 Ga the NMZ charnockites and enderbites were derived from a source which was thoroughly mixed. We showed above, however, that melts now forming an area of crust ca 4500 km 2' were extracted from a source which was the product of mixing of at least two separate components with very different #-values. Thus at, or prior to, 2.7 Ga, extensive mixing took place between an old crust and a younger mantle-derived component.

4.2.4. Constraints from the REE A small group of samples were identified on the basis of their major element chemistry as the least fractionated melts from the NMZ charnockite enderbite suite. These primitive enderbites have the highest concentrations of REEs within the charnockite enderbite suite and have strongly fractionated REE patterns with small negative Eu anomalies. These samples are here used to further constrain the REE chemistry of the source through the modelling of their REE chemistry. Following Martin (1986, 1994) and the results of recent experimental studies (e.g., Rapp and Watson, 1995), we have modelled the genesis of the NMZ charnockites and enderbites through the partial melting of a basaltic source. We used a typical Archaean tholeiite (sample 9, 8.85 wt% MgO; RoUinson and Lowry, 1992) with a flat 10 × chondrite REE pattern, as the starting material, the batch melting equation and low pressure partition coefficients for andesites from the compilation in Rollinson (1993b). Our calculations (Fig. 5d) showed that it is not possible to duplicate the measured REE concentrations for any reasonable fraction of partial melting (up to 30%) using either an eclogitic or garnet granulite residue. We repeated the calculations using a different starting material, in this case a basalt enriched in the LREEs but with fiat 10 x chondrite HREEs (sample 93: MgO = 4.37%, Rollinson and Lowry, 1992) and a garnet granulite residue and found a much closer correspondance between the calculated and measured REE compositions (Fig. 5e). These results suggest that the NMZ charnockites and enderbites were derived from a source enriched in the light REE. Given the evidence presented above for the protolith as a mixture, we do not believe that these results indicate an enriched mantle source for the basaltic protolith. Further, we do not insist on the batch melting of a basaltic source as the principal mechanism of magma generation in the NMZ. Rather, we believe that this result simply supports the view that the NMZ magmas were derived from a source produced from the mixing of a felsic component with a light REEenriched pattern, typical of Archaean continental crust and a mantle derived component with a flat REE pattern.

4826

M. Berger and H. Rollinson

4.3. The Process of Charnockite and Enderbite Formation The isotope and trace element chemistry of the NMZ charnockites and enderbites show that these magmas were derived by mixing between two different components. These may be characterised as follows: (1) An old (>3.25 Ga), high/z component (>9.1), with a high Th/U ratio, high U, Th, and Pb concentrations, a high 87Sr/86Sr ratio, an old model Nd age, a fractionated REE pattern and low Sm/Nd ratio and at 2.7 subchondritic e-Nd values. This component also has a high Pb/Nd ratio. It is identified as either old felsic crust or sediment derived from an old felsic crust, and it is in this early felsic crust that fractionation of U/Pb and Th/Pb took place. These observations are consistent with recent U/Th/Pb modelling by Kramers and Tostikhin (1997) who have demonstrated that old Archaean continental crust was enriched in U, Th, and Pb and had a high #value relative to modern continental crust, because it was generated from a mantle which was not depleted in these elements. (2) A younger (>2.7 Ga - 3.25 Ga) low # component (<9.1), with low U, Th, and Pb concentrations, a fiat or LREE-depleted REE pattern and high Sm/Nd ratio; at 2.7 Ga this component had an E-Nd value close to that of the depleted mantle. The Pb/Nd ratio was low. This component was a mantle derived melt. Our data require a model for late Archaean crust generation in the NMZ in which old felsic crust interacted with a mantle derived melt and the two components were sufficiently well mixed to produce isotopically uniform magmas throughout the 4500 km 2 of the NMZ at ca. 2.7 Ga. The two components made different contributions to the major element and trace element budget of the final melt. Of particular importance is the observation that the concentrations of the trace elements U, Th, and the LREEs were sufficiently high in the older crustal component to dominate the final mixture. Thus, at 2.7 Ga the protolith had a high/z, a high Th/U, and was enriched in light REE, with low e-Nd values. The presence of plagioclase _+ garnet in the residues suggest that the mixing took place close to the base of the crust. Modern analogues of large-scale crust-mantle interaction associated with the process of crust generation are found in convergent margins such as the Andes. Recent geochemical studies show that magmatism in such areas may include components from the subducting slab, the mantle wedge, ocean floor sediment, and the lower continental crust (Stern and Kilian, 1996). Central to such models is the process of crustal assimilation and fractional crystallisation (AFC). Here we model two possible scenarios of crust mantle interaction. First, we model the REE chemistry of a melt produced by the interaction of a basaltic melt underplating the lower continental crust. The AFC calculations were based upon Eqn. 6a of DePaulo ( 1981 ), the starting material was assumed to have a fiat REE pattern and a 10x chondritic REE concentration, and the old crust component was modelled on the composition of the 3.5 Ga Tokwe Gneiss (La/Yb~ estimated as 80, data from Hawkesworth et al., 1975; Taylor et al., 1991). Our calculations show that it is possible to obtain a close fit between calculated and measured values for melt fractions between 60 and 70% of the original mass

(Fig. 12a). Further calculations based upon the equations of Aitcheson and Forester (1994) using strontium and neodymium isotopes (data from this study and calculated from Hawkesworth et al., 1975; Taylor et al, 1991 ) suggest that the mass ratio of assimilated old crust to magma is between 0.15 and 0.25. Secondly, we model the interaction of a melt produced by the partial melting of the subducted slab and the continental crust. In this case a tonalitic melt is produced by the partial melting of a basalt with the same composition as that used in the calculations above. AFC modelling of REEs shows that the interaction between tonalite and old lower crust (based upon the composition of the Tokwe Gneiss, as above) also yields melt compositions which are close to those measured in the NMZ magmas (Fig. 12b). The solutions proposed are nonunique but strongly suggest that the parental magmas of the NMZ charnockites and enderbites were produced through AFC processes resulting from the interaction of a mantle-derived melt and continental crust. The modelling predicts that residues from the AFC of up to 30% of the mass of the NMZ magmas may be present in the lowermost part of this crustal section. It is possible that the AFC process can also explain the isotopic homogeneity observed in the initial 2°tpb/2°4pb and 2°7pb/2°4pb ratios of the NMZ charnockites and enderbites. Aitcheson and Forester (1994) show that AFC processes can give rise to the buffering of isotopic compositions, through rapidly rising elemental compositions as a consequence of crystal fractionation. 5. CONCLUSIONS The charnockite and enderbites of the NMZ are a suite of dry magmas ranging in composition from tonalite to granite which were emplaced at ca. 2.7 Ga (Berger et al., 1995). They are closely related in composition to TTG suites, typical of Archaean continental crust. Many models for Archean crust generation make assumptions about the nature of the protolith. Here we have used the combined constraints of trace elements, lead, strontium, and neodymium isotopes in order to characterise the nature of the protolith and understand the process of charnockite and enderbite generation. We show from REE modelling that the NMZ magmas cannot be generated simply through the partial melting of a basaltic source: the conventional approach to the genesis of Archaean TTG melts. Rather, from isotopic studies we have argued that the protolith contained a significant component of old crust. We propose that the protolith was produced by the mixing of old, high-# crust with a high concentration of U and Th and a younger, low-/z, mantle-derived melt. This contrasts with previous observations on Archaean TTG suites, which show little evidence for a crustal prehistory. Initial lead isotope ratios at 2.7 Ga are remarkably homogeneous suggesting that the process of mixing in the protolith was very thorough. Our observations demonstrate that it is not necessary to invoke an enriched mantle source in order to explain the existence of old high-# crustal provinces (see for example, Wooden and Mueller, 1988). Our data for the NMZ show that late-Archaean high-# crust can be formed through the interaction between mantle-derived melts and old continental crust, which by its very nature tends to have an enriched U/Pb ratio (Kramers and Tostikhin, 1997).

Late Archaean crustal growth in Zimbabwe

4827

1000 (a) A F C - Initial melt basaltic

100

10 o t~

La Ce

Nd

Sm Eu G d TbDy

Er

YbLu La Ce

Nd

Sm Eu Gd TbDy

Er

Yb Lu

Fig. 12. Chondrite normalised rare earth element plots showing the results of AFC calculations. In each case the starting material is shown as an heavy black line with symbols, the calculated compositions as grey lines with symbols, and the measured composition of the NMZ primitive enderbites as black lines without symbols. The ratio of the mass of crust assimilated to the mass of fractionated crystals (r) is set at 0.7. (a) the mixing of a basaltic melt with REE concentrations = 10x chondrite and old continental crust (based upon the 3.5 Ga Tokwe gneisses, composition not shown). Melt compositions are calculated for 90%, 70%, 50%, and 30% of the original mass. (b) the mixing of a tonalitic melt and old continental crust (based upon the 3.5 Ga Tokwe gneisses, composition not shown). Melt compositions are calculated for 90%, 70%, 50%, and 30% of the original mass. The composition of the tonalite is calculated from the 20% batch partial melting of a basalt with REE concentrations = 10x chondrite in equilibrium with an eclogitic residue (0.4grt 0.6 cpx), using the partition coefficients for andesites.

A contemporary analogue for crust-mantle interaction during crustal growth is to be found in modern convergent margins such as the Andes. In keeping with current models for Andean magmatism we demonstrate the importance of A F C processes as an integral part of crust mantle interaction. The present compositional range is in large part due to the effects of plagioclase and pyroxene fractionation indicating crystal fractionation in dry, high temperature melts at crustal levels. The A F C process and subsequent crystal fractionation may be better thought of as separate parts of a continuum. Our data do not, however, permit us to discriminate between competing models of crustal growth for we cannot distinguish between the magmatic underplating of old crust with a mantle derived melt and crustal growth through the mixing of a slab-derived partial melt and old lower crust. One of the important findings of this study is that, unlike many granulite facies terrains, the N M Z charnockite and enderbites have not experienced depletion of their heat producing elements Th, U, K, and Rb. Consequently, element ratios such as K / R b and K / U are low relative to some other granulite terrains, and heat production in this area is abnormally high for lower to middle crust. This may be an im-

portant control on the protracted magmatic history (up to 100 Ma) of the area. We believe that it is also significant that the major element compositions of the N M Z charnockite and enderbites are less sodic than typical Archean TTG compositions. Similarly, the negative Eu anomaly in some of the REE patterns is atypical of many Archaen TTGs. These geochemical features may be useful in other regions to signal a preexisting crustal contribution to Archaean TTGs.

Acknowledgments--MB was funded by the Swiss National Foundation grant No. 20.47157.96. This work forms part of a PhD study at the University of Bern supervised by Prof. J. D. Kramers. The British Council is thanked for a travel grant made jointly to HRR and J. D. Kramers. R. Frei and T. F. Nagler are thanked for helpful discussions and B.S. Kamber, T. Blenkinsop, and S. Mkweli for their company in the field. H. Oshidari of the University of Bern is thanked for XRF analyses of Pb, Th, and U. Other XRF and ICP REE analyses reported here were made by David Lowry whilst working as a research assistant at Cheltenham & Gloucester College. Leicester University and Royal Holloway College, University of London are thanked for access to their analytical facilities. This paper has benefited greatly from the thoughtful and constructive reviews of Catherine Chauvel, Jan Kramers, Beatrice Luais, Roberta Rudnick, and Klaus Mezger.

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