U-Pb zircon evidence for an extensive early Archean craton in Zimbabwe: A reassessment of the timing of craton formation, stabilization, and growth Matthew S. A. Horstwood* Department of Geology, Southampton Oceanography Centre, Southampton SO14 3ZH, UK Robert W. Nesbitt Stephen R. Noble NERC Isotope Geoscience Laboratory, Kingsley Dunham Centre, Keyworth, Nottingham NG12 5GG, UK James F. Wilson Department of Geology, University of Zimbabwe, P.O. Box MP167, Harare, Zimbabwe ABSTRACT U-Pb single-zircon analyses provide direct evidence for an enlarged early Archean craton forming the core to the present Zimbabwe craton. Virtually identical dates from the south-central Tokwe segment (3455 ± 2 Ma) and Midlands (3456 ± 6 Ma) parts of the craton strongly suggest their synchronous formation, during an event that formed a single early cratonic nucleus which we propose to call the “Sebakwe protocraton.” This is considered to underlie most of the current Zimbabwe craton. Parts of the craton are at least 3565 ± 21 Ma, a rock age reported here that represents the oldest rock dated from Zimbabwe. A ca. 3350 Ma relatively undeformed and unmetamorphosed intrusive granitic phase constrains the timing of the high-grade metamorphism and the stabilization of the protocraton. Comparison with published Re-Os data for the Zimbabwe craton strongly indicates a depleted subcontinental lithospheric mantle underlying the entire Sebakwe protocraton. Subsequent intrusive and volcanic activity from 3.0 to 2.6 Ga represents a second major period of magma genesis and crustal formation within which the predominant rocks of the exposed Zimbabwe craton were generated. INTRODUCTION The development of the Zimbabwe craton of southern Africa occurred predominantly in two major events in the early and late Archean, each of which involved volcanism, crustal growth, and tectonism. Macgregor (1951) defined the oldest of the resulting volcanic rocks, the Sebakwian Group, from exposures in the Sebakwe River in the Midlands region of the country. These greenstones were invaded by various early Archean granitoids, deformed to produce a complex arrangement of gneisses and infolded Sebakwian lithologies, and then intruded by additional early Archean granitoids. Subsequent workers proposed that these ancient granitoid-gneiss complexes were confined to an area farther south around the Tokwe River. This area of high-grade rocks became known as the Tokwe segment (Wilson, 1990) and formed the focus of early geochronological work. Macgregor (1951) described a widespread, mafic-felsic (Bulawayan) greenstone sequence succeeded by clastic deposits (the Shamvaian) overlying the Sebakwian Group; sedimentation of the clastic deposits marks the end of Archean greenstone belt formation. Subsequent mapping, however, revealed a more complex late Archean picture, in which older (Belingwean) greenstone sequences underlie the Bulawayan formations and both greenstone sequences con*Present address: NERC Isotope Geoscience Laboratory, Kingsley Dunham Centre, Keyworth, Nottingham, NG12 5GG, UK.
tain tonalite-granodiorite plutons. A craton-wide influx of monzogranitic sills (the Chilimanzi Suite) succeeded the greenstone belt formation and is widely considered to herald the stabilization of the craton ca. 2.6 Ga (see Bickle and Nisbet, 1993; Wilson, 1979; Wilson et al., 1995). Geochronological work (Rb-Sr, Hickman, 1974; Pb-Pb, Moorbath et al., 1976) suggested that the oldest gneisses of the Tokwe segment were ca. 3.5 Ga rocks, the last deformation of which was fixed at pre-3.35 Ga by the presence of the crosscutting Mont d’Or granitoid in the north of the segment (see Taylor et al., 1984, for review). Detrital zircons in ca. 3.0 Ga quartzites indicated rocks in this region with concordant U-Pb ages ranging from 3.8 to 3.2 Ga (Dodson et al., 1988), but the 3.8 Ga source has yet to be identified. Here we present new U-Pb singlezircon data from the Midlands region and the Tokwe segment (Fig. 1). Our data demonstrate the presence of older (>3.5 Ga) early Archean continental crust far beyond the limits of the Tokwe segment and confirm previous estimates of the age of the Tokwe segment. SAMPLES AND RESULTS Conventional ID-TIMS (isotope dilution–thermal ionization mass spectrometry) U-Pb singlezircon data have been obtained on four samples from the Archean of Zimbabwe: two from the Tokwe segment and two from the Midlands region ~200 km to the northwest (Fig. 1). One Tokwe sample represents gneiss (Zimb.95/14)
from the type area of the Tokwe River in the center of the segment, and the other represents a leucosome component (Zimb.95/09) crosscutting the gneisses ~40 km to the northeast. The Midlands samples are from the Kwekwe Gneiss (Zimb.226) and from a small gneissic body here referred to as the Sebakwe River Gneiss (Zimb.141). The Kwekwe Gneiss, the western marginal unit of the granitoid-gneiss complex named the Rhodesdale batholith by Macgregor (1951), abuts the eastern margin of the late Archean Midlands greenstone belt; sample Zimb.226 was collected ~2.6 km east of this contact in the Sebakwe River. The Sebakwe River Gneiss is a much smaller gneissic body occurring entirely within the Midlands greenstone belt (Harrison, 1970; Campbell and Pitfield, 1994) and is exposed for ~800 m in the bed of the Sebakwe River. It crops out about 2.1 km west of the granite-greenstone contact and is dominated by amphibole-bearing quartz diorites (from which the sample was taken) and amphibolites. Their metamorphic grade of lower to middle amphibolite facies suggests that they are older than the adjacent, lower greenschist facies, greenstone belt volcanic rocks. Figure 2 displays the plots for the samples analyzed in this study1. All zircon fractions used in this study were selected on the basis of clarity, morphology, size, and lack of any visible inherited component. As such, and as a result of backscattered electron imaging on other members of the populations, all grains are considered to be primarily magmatic with no xenocrystic sectors. This is supported by the data; for any of the following results to reflect ages older than the true age of the rock, all the zircon fractions in each result would have to be xenocrystic in origin. Following Mezger and Krogstad (1997), no attempt is made to interpret the lower intercepts of the defined discordia. Four zircons from the Tokwe River Gneiss (Zimb.95/14) define a discordia line with an upper intercept at 3455 ± 2 1GSA Data Repository item 9958, U-Pb zircon data for Zimbabwe granitoids, is available on request from Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301,
[email protected], or at www.geosociety.org/pubs/drpint.htm.
Data Repository item 9958 contains additional material related to this article. Geology; August 1999; v. 27; no. 8; p. 707–710; 2 figures.
707
lization age of the rock, although the high MSWD of 27 suggests the involvement of more complex geological factors than those exhibited by the other samples in this study. Even so, the Sebakwe River Gneiss indicates the exposure of continental crust in the Zimbabwe craton at least 100 m.y. older than any previously measured.
N
Africa
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Shamva HARARE
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Figure 1. Geology of Zimbabwe and sample localities.
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Late Archean greenstone belt rocks and related ultramafic intrusions North Marginal Zone of the Limpopo belt Granitoids of Great Dyke Proterozoic cover various ages 1 - Tokwe River Gneiss (Zimb.95/14) 3 - Sebakwe River Gneiss (Zimb.141) Phanerozoic cover
2 - Leucosome sample (Zimb.95/09)
Ma and a mean squared weighted deviation (MSWD) of 1.7. This collinear data array essentially rules out any possible effects resulting from factors other than analysis, and indicates that the zircons have all reacted to the various Pb loss events in exactly the same manner, although to different extents. A fifth point, not included in the regression, is off this array and reflects the result of Pb loss events not affecting the other grains. Seven zircons from the leucosome component (Zimb.95/09) define a discordia line with an upper intercept of 3368 ± 9 Ma. When compared to the other samples, the relatively high MSWD of 15 is considered an artifact of the large grain size, abundant Pb, and consequently high 206Pb/ 204Pb ratio, and precise analyses, resulting in extremely small error ellipses and apparent scatter (Kalsbeek, 1992). As such, 708
4 - Kwekwe Gneiss (Zimb.226)
this result is interpreted as the age of crystallization of the leucosome. The samples from the Midlands area also display early Archean ages. At 3456 ± 6 Ma, the Kwekwe Gneiss (Zimb.226) is virtually identical in age to the Tokwe River Gneiss. The low MSWD of 1.4 also correlates, exhibiting remarkable collinearity of the regressed zircon fractions (especially in view of their large discordancy), suggesting that they all reacted in a similar fashion to the Pb loss. A single point omitted from the regression changes the result little but increases the MSWD such that factors not affecting the other fractions are inferred. The Sebakwe River Gneiss (Zimb.141) records the oldest magmatic event measured from the Zimbabwe craton. At 3565 ± 21 Ma, the discordia line is considered to reflect the crystal-
DATA REVIEW Previous data from the Tokwe River Gneiss (Pb-Pb, Sm-Nd, and Rb-Sr whole-rock analyses; see Taylor et al., 1991, for review) indicate ages from ca. 3.6 to ca. 3.2 Ga. Our Tokwe River Gneiss result (3455 ± 2 Ma) confirms this consensus view and demonstrates the potential of the single-grain U-Pb zircon technique in elucidating the history of the area. Equally important, our result from the leucosome phase crosscutting the Tokwe gneisses (3368 ± 9 Ma) is coincident with the melt phase recorded by the intrusion of the Mont d’Or granodiorite (3345 ± 55 Ma, Pb-Pb whole-rock analyses, Taylor et al., 1984). The discovery of pre-3.4 Ga dates from the Midlands region, however, was unexpected. Previous geochronological work on samples from the Rhodesdale granitoid complex gave an age of 2976 +121/–132 Ma (Pb-Pb whole-rock analyses), which was supported by an Sm-Nd depletedmantle model age (TDM) of 2.99 Ga (Taylor et al., 1991). The sample on which these ages were obtained was considered by Stowe (1979) to represent the oldest phase of the Rhodesdale granitoid complex. Unless the Pb-Pb and Sm-Nd systematics have been disturbed, our U-Pb zircon age of 3456 ± 6 Ma from close to the margin argues otherwise. This result suggests that the granitoid is more complex than a single 2.99 Ga separation event and that, in part, it contains relicts of a basement of the same age as that of the Tokwe segment to the south. The 3565 ± 21 Ma date obtained on the Sebakwe River Gneiss represents the oldest date recorded from the Archean of Zimbabwe. Its location within the outcrop of the greenstone belt suggests an unconformable relationship similar to that observed in the National Monument section of the Mberengwa (Belingwe) greenstone belt, where 2.7 Ga greenstone belt volcanic rocks directly overlie a ca. 3.5 Ga granitoid basement (see Blenkinsop et al., 1993, and Bickle and Nisbet, 1993, for review). DISCUSSION OF CRATON DEVELOPMENT Current models for the formation of the Zimbabwe craton focus on the crustal-forming events of the late Archean to the virtual exclusion of those of the early Archean. These models regard the extensive monzogranitic sills of the ca. 2.6 Ga Chilimanzi Suite, derived from anatexis of a thickened sialic crust, as marking the culmination of the cratonic stabilization process (e.g., Wilson, 1979; Wilson et al., 1995; Kusky, 1998; Dirks and Jelsma, 1998). The Great Dyke (2461 ± 16 GEOLOGY, August 1999
0.74
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ZIMB.95/14 Tokwe River Gneiss
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ZIMB.95/09 Leucosome of Tokwe Gneiss
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Intercepts at 3565 ± 21 and 942 ± 315 Ma MSWD = 27
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Ma, Hamilton, 1977) thus becomes an excellent postcratonization strain marker heralding the Archean-Proterozoic transition. In our view, these models ignore the 600 m.y. of pre-3.2 Ga crustal history recorded by the Sebakwian volcanic rocks and granitoid-gneiss complexes. Geochronological and field data provide ample evidence of an extensive sialic basement to the late Archean greenstone assemblages (Bickle and Nisbet, 1993; Dougherty-Page, 1994; Jelsma et al., 1996; Hunter et al., 1998). The important questions, therefore, are whether any of this basement achieved cratonic stability in the early Archean and, if so, to what areal extent and when? In the south of the craton an important part of this basement is the Tokwe segment (Fig. 1; Wilson, 1979; Wilson et al., 1995). Detrital zircons extracted from late Archean clastic sedimentary rocks exposed in the extreme northwest and south of the segment, and derived from its erosion, yield concordant ages ranging from 3.8 to 3.2 Ga (Dodson et al., 1988). Abundance peaks at 3.8, 3.6, 3.46, 3.35, and 3.2 Ga emphasize the prolonged and consistent early Archean history of crustal formation in this area. The 3.8 Ga source has yet to be identified, and the 3.2 Ga source is not entirely clear. A ca. 3.6 Ga event is represented by Sm-Nd TDM model ages (Moorbath GEOLOGY, August 1999
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Figure 2. U-Pb concordia plots for Zimbabwe granitoid samples. MSWD— Mean squared weighted deviations.
Pb 235 U
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et al., 1986; Taylor et al., 1991), but ca. 3.35 and ca. 3.46 Ga abundance peaks are well represented, both in the early Rb-Sr and Pb-Pb wholerock studies (see Taylor et al., 1991, for review) and in the zircon data presented here. The ca. 3.35 Ga Mont d’Or granitoid (3348 ± 120 Ma, Rb-Sr whole-rock analyses, Moorbath et al., 1976; 3345 ± 55 Ma, Pb-Pb whole-rock analyses, Taylor et al., 1991) intrudes deformed Sebakwian Group rocks in the northwest part of the Tokwe segment. Its anatectic origin is supported by the high initial Sr isotopic ratio (Sri = 0.711 ± 0.002) and the Sm-Nd TDM model age of 3.64–3.67 Ga (Taylor et al., 1991), demonstrating that processes commonly associated with welldeveloped continental crust were operating at this time. The Mont d’Or data and field relationships, in conjunction with our data from the crosscutting leucosome ~80 km east-southeast of the Mont d’Or granitoid and the abundance peak of Dodson et al. (1988), indicate a widespread crustal melting event ca. 3.35 Ga. We interpret the resultant undeformed ca. 3.35 Ga granitoids, which intrude the ca. 3.46 Ga greenstone-granitoid gneisses, as marking the crustal stabilization of the Tokwe segment ca. 3.35 Ga. The granitic event indicated by the 3.2 Ga ages would thus represent a later episode of early Archean crustal addition.
26
30
Our ca. 3.5 Ga dates from the Midlands area and the synchroneity of our results for the Tokwe and Kwekwe gneisses strongly suggest that the two areas are contiguous. The true areal extent of ca. 3.5 Ga rocks in the Rhodesdale granitoid complex is unknown. Banded gneisses in the complex, dated as ca. 2.9 Ga (Taylor et al., 1991), contain infolded greenstones that also occur in the Kwekwe Gneiss. Although more recently assigned to the late Archean (Wilson et al., 1995), previous workers (Stowe, 1971; Wilson, 1979) regarded these greenstone remnants as Sebakwian and implied correlation with the Tokwe segment. The conclusions of the earlier workers are favored here and clearly, although much more work is needed to define it, a considerable area of ca. 3.5 Ga rocks could be present in the Rhodesdale granitoid complex. No ca. 3.35 or ca. 3.2 Ga dates have been recorded from the Midlands region. However, a ca. 3.2 Ga zircon age (Pb-Pb Kober technique, Dougherty-Page, 1994) from a granitic pebble in Shamvaian conglomerates of the Harare-Shamva greenstone belt ~300 km northeast of our Kwekwe Gneiss sample also suggests an early Archean component to this northern region. Gneissic basement with infolded greenstones, which flanks the southeastern part of this greenstone belt, may 709
therefore be older than the ca. 2.9 Ga suggested by Wilson et al. (1995). Thus, instead of the relatively small area in the south of the craton represented by the Tokwe segment, the distribution of pre-3.2 Ga crustal rocks in Zimbabwe may have been much greater, with a north-south dimension of more than 450 km. Support for an extensive pre-3.2 Ga basement also comes from the work of Nägler et al. (1997), who analyzed Os isotopes from a series of chromites taken from ultramafic rocks in and around the Tokwe segment. Their data demonstrate consistently older model ages for the chromites than ages deduced from geologic field relationships. They suggested that these older model ages require a substantial period of Os isolation from Re to allow the time-integrated evolution of a retarded 187Os/188Os isotope ratio. To achieve this they postulated a major depletion event within the subcontinental lithospheric mantle beneath the Zimbabwe craton beginning ca. 3.8 Ga. Such a depletion event is compatible with stabilization of the Tokwe segment ca. 3.35 Ga. If this interpretation is correct, it is highly unlikely that such a major event, and in turn stabilization, would have been confined to the relatively small area of the Tokwe segment; rather, the event would have involved the entire pre-3.2 Ga basement of Zimbabwe. It is for this postulated larger area of stabilized early Archean continental crust that we propose the name “Sebakwe protocraton.” This reflects the “protocratonic” terminology of Stowe (1971) but does not imply any genetic mechanism of generation such as indicated by the “Tokwe terrane” terminology of Kusky (1998). CONCLUSIONS Our U-Pb single-zircon data show, for the first time, the presence of sialic rocks of ca. 3.5 Ga age in areas of Zimbabwe other than the Tokwe segment and reveal the oldest dated rocks in the craton. The 3.46 and 3.37 Ga results for the Tokwe segment confirm earlier published wholerock Rb-Sr, Pb-Pb, and Sm-Nd model age data and support existing arguments for a complex magmatic and metamorphic history for the early Archean sialic basement of Zimbabwe. Published Os isotope data (Nägler et al., 1997) and detrital zircon studies (Dodson et al., 1988) require that the early Archean crust of Zimbabwe began developing ca. 3.8 Ga with a culminating event ca. 3.2 Ga. We propose that a pre-3.35 Ga craton extended from the Tokwe segment in the south, northward to the Midlands area and probably as far north as Shamva, a much larger area than proposed by Wilson et al. (1995). The late Archean 3.0–2.6 Ga events, including eruption of the Belingwean and Bulawayan greenstones that now dominate exposures in the Zimbabwe craton, therefore represent a second round of intracontinental, mafic-felsic volcanism with associated granitoid intrusion and crustal growth. The primary stabilization of the Zimbabwe craton is seen 710
as occurring ca. 3.35 Ga, marked by the intrusion of granitoids. This was followed by a final stabilization event at 2.6 Ga, marked by the intrusion of the Chilimanzi granite suite and occurring ~750 m.y. after the principal event. As such, a 300– 400 m.y. gap is apparently present in the geochronological record between the stabilization of the Sebakwe protocraton and the onset of late Archean volcanism. Although Kusky (1998) defined the Tokwe terrane as a stable coherent block by ca. 2.9 Ga, stabilization and coherency of the real ancient core of the Zimbabwe craton actually occurred more than 400 m.y. earlier during the formation of the Sebakwe protocraton. ACKNOWLEDGMENTS Supported by the Natural Environment Research Council through a postgraduate scholarship (NERC grant GT 4/94/410/G to Horstwood) and analyses conducted at the NERC Isotope Geoscience Laboratory (NIGL project no. 20175). Horstwood thanks the British Council for funding a link visit and attendance at the Harare ’97 conference and the staff and technicians at the Department of Geology, University of Zimbabwe, for logistical help and support during time spent in Zimbabwe. We thank M. E. Bickford and R. L. Gibson for their reviews. This is NIGL publication 325. REFERENCES CITED Bickle, M. J., and Nisbet, E. G., 1993, The geology of the Belingwe greenstone belt, Zimbabwe: A study of Archean continental crust: Geological Society of Zimbabwe Special Publications 2: Rotterdam, Balkema, 239 p. Blenkinsop, T. G., Fedo, C. M., Bickle, M. J., Eriksson, K. A., Martin, A., Nisbet, E. G., and Wilson, J. F., 1993, Ensialic origin for the Ngezi Group, Belingwe greenstone belt, Zimbabwe: Geology, v. 21, p. 1135–1138. Campbell, S. D. G., and Pitfield, P. E. J., 1994, Structural controls of gold mineralisation in the Zimbabwe craton—Exploration guidelines: Zimbabwe Geological Survey Bulletin 101, 270 p. Dirks, P. H. G. M., and Jelsma, H. A., 1998, Horizontal accretion and stabilization of the Archean Zimbabwe craton: Geology, v. 26, p. 11–14. Dodson, M. H., Compston, W., Williams, I. S., and Wilson, J. F., 1988, A search for ancient detrital zircons in Zimbabwean sediments: Geological Society of London Journal, v. 145, p. 977–983. Dougherty-Page, J. S., 1994, The evolution of the Archean continental crust of Northern Zimbabwe [Ph.D. thesis]: Milton Keynes, Open University, 244 p. Hamilton, P. J., 1977, Sr isotope and trace element studies of the Great Dyke and Bushveld mafic phase and their relation to early Proterozoic magma genesis in southern Africa: Journal of Petrology, v. 18, p. 24–52. Harrison, N. M., 1970, The geology of the country around Que Que: Rhodesia Geological Survey Bulletin 67, 125 p. Hickman, M. H., 1974, 3500 m.y. old granite in southern Africa: Nature, v. 251, p. 295–296. Hunter, M. A., Bickle, M. J., Nisbet, E. G., Martin, A., and Chapman, H. J., 1998, Continental extensional setting for the Archean Belingwe greenstone belt, Zimbabwe: Geology, v. 26, p. 883–886. Jelsma, H. A., Vinju, M. L., Valbracht, P. J., Davies, G. R., Wijbrans, J. R., and Verdurmen, E. A. T., 1996, Constraints on Archean crustal evolution or
Printed in U.S.A.
the Zimbabwe craton: A U-Pb zircon, Sm-Nd and Pb-Pb whole rock isotope study: Contributions to Mineralogy and Petrology, v. 124, p. 55–70. Kalsbeek, F., 1992, The statistical distribution of the mean squared weighted deviation—Comment: Isochrons, errorchrons and the use of MSWD values: Chemical Geology (Isotope Geoscience Section), v. 94, p. 241–242. Kusky, T. M., 1998, Tectonic setting and terrane accretion of the Archean Zimbabwe craton: Geology, v. 26, p. 163–166. Macgregor, A. M., 1951, Some milestones in the Precambrian of Southern Rhodesia: Geological Society of South Africa Transactions, v. 54, p. xxvii–lxxi. Mezger, K., and Krogstad, E. J., 1997, Interpretation of discordant U-Pb zircon ages: An evaluation: Journal of Metamorphic Geology, v. 15, p. 127–140. Moorbath, S. M., Wilson, J. F., and Cotterill, P., 1976, Early Archean age for the Sebakwian group at Selukwe, Rhodesia: Nature, v. 264, p. 536–538. Moorbath, S. M., Taylor, P. N., and Jones, N. W., 1986, Dating the oldest terrestrial rocks—Fact and fiction: Chemical Geology, v. 57, p. 63–86. Nägler, T. F., Kramers, J. D., Kamber, B. S., Frei, R., and Prendergast, M. D. A., 1997, Growth of subcontinental lithospheric mantle beneath Zimbabwe started at or before 3.8 Ga: Re-Os study on chromites: Geology, v. 25, p. 983–986. Stowe, C. W., 1971, Summary of the tectonic development of the Rhodesian Archean craton, in Symposium on Archean Rocks: Geological Society of Australia Special Publication 3, p. 377–383. Stowe, C. W., 1979, A sequence of plutons in the central portion of the Rhodesdale Granitic terrane, Rhodesia: Geological Society of South Africa Transactions, v. 82, p. 277–285. Taylor, P. N., Jones, N. W., and Moorbath, S., 1984, Isotopic assessment of relative contributions from crust and mantle sources to the magma-genesis of Precambrian granitoid rocks: Royal Society of London Philosophical Transactions, ser. A v. 310, p. 605–625. Taylor, P. N., Kramers, J. D., Moorbath, S., Wilson, J. F., Orpen, J. L., and Martin, A., 1991, Pb/Pb, Nd-Sm and Rb-Sr geochronology in the Archean craton of Zimbabwe: Chemical Geology (Isotope Geosciences Section), v. 86, p. 175–196. Wilson, J. F., 1979, A preliminary appraisal of the Rhodesian Basement Complex: Geological Society of South Africa Special Publication 5, p. 1–23. Wilson, J. F., 1990, A craton and its cracks; some of the behaviour of the Zimbabwe block from the late Archean to the Mesozoic in response to horizontal movements, and the significance of its mafic dyke fracture patterns: Journal of African Earth Sciences, v. 10, p. 483–501. Wilson, J. F., Nesbitt, R. W., and Fanning, C. M., 1995, Zircon geochronology of Archean felsic sequences in the Zimbabwe craton: A revision of greenstone stratigraphy and a model for crustal growth, in Coward, M. P., and Ries, A. C., eds., Early Precambrian processes: Geological Society [London] Special Publication 95, p. 109–126. Manuscript received December 15, 1998 Revised manuscript received April 26, 1999 Manuscript accepted May 4, 1999
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