Revista Brasileira de Geociências 12(1-3): 60'64, Mar .-Sel., 1982 - Silo Paulo

CRUSTAL EVOLUTION OF THE KAAPVAAL CRATON - TIMING AND NATURE OF TECTONIC EVENTS

J. M. BARTüN JR.·

ABSTRACT The component parts ofthe basement complex ofthe Kaapvaal Craton ofsouthern África formed at different times over a span of 800 Ma starting prior to 3,800 Ma ago. Exactly how each of these parts formed is a matter for speculation but they were brought ínto their present relative positions by tectonic processes such as diapiric intrusion, thrusting and shearing. As a result, adjacent rock units within the basement complex may have little genetic relationship to one another. Until the sense and magnitude of the displacements accompanying tectonic activity can be evaluated, it is difficult if not impossible to reconstruct the paleoenvironments of crustal for~ mation.

INTRODUCTION The granite-greenstone and high­ with those of the Swaziland Sequence of the Barberton -grade gneiss terrains of the Kaapvaal Craton of southern Granite-Greenstone Terrain and the late-stage granite Africa are exposed as a series of basement windows through plutons were thought to have been of a single generation. cover rocks of diverse ages ranging from Archaean to Karoo The leucocratic gneisses surrounding the greenstones were (Fig. I). TraditionalIy, it was believed that the rocks compri­ envisioned to form a fundamental unit and the high-grade sing these windows are correlative and formed in essentialIy gneisses of the Limpopo Mobile Belt and the Vredefort their present relative positions. For example, alI of the Dome were interpreted to be merely exposures of lower low-grade volcanic and sedímentary rocks were equated leveis of a single crustal block. More recently, however, it has become apparent that this simple paradigm is invalid. .Results of geochronologic studies have revealed that em­ placement ages vary and are geographicalIy controlIed. ln addition, results of isotopic and chemical studies suggest that different units formed by different mechanisms. Jt follows, therefore, that the basement complex of the Kaap­ vaal Craton represents a number of accreted crustal frag­ ments each with a distinct history. ln this paper, the evidence for this hypothesis is reviewed and its possible significance is discussed.

EMPLACEMENT AGE RELATIONSHIPS lN VA­ RIDUS AREAS DF THE KAAPVAAL CRATDN The Berberton Grenite-Greenstone Terrein The largest number of isotopic data pertaining to the Kaapvaal Craton is derived from rocks making up the Barberton Granite­ -Greenstone Terrain (Fig. I; see data compilation in Barton, 1982a). ln this area, nearly ali contacts among units are tectonically deformed so that interpretation of stratigraphic relalionships is ambiguous, Nevertheless, the Rb-Sr isotopic data (Figs, 2 and 3) are interpreted to best refiect condi­ o 500 tions of emplacement of the units involved (Barton, 1982a). kllometre& The Rb-Sr isotopic data fo~ most units are characterized by smalI inilial "Sr/S6Sr ratios and may be divided into four stalislically separable age groups (Fig. 2; Barton, Figure J - Geologíc map ofa portion ofsouthern Afríca showíng the 1981 l. The largest ages cluster around 3,500 Ma (lO· distribution ofexposures of the basement complex of the Kaapvaal years). These are derived from the Komati Formalion ofthe Craton (blank) through Archaean, Proterozoíc and Phanerozoic cover Barberton Greenstone Belt, the leucocratic orthogneiss rocks (diagonal pattern). 7he basement complex of lhe Rhodestan component of the Bimodal Suite of the Ancient Gneiss Craton is shown tn the vertical panem. L = Limpopo Mobile Belt : Complex and three tonalitic and trondhjemitic diapirs in­ Ln = Nonhern Marginal Zone; Lc = Central Zone; and Ls = Sou­ 8 6Sr them Marginal Zone. M = Murchison Granite-Greenstone Terrain ; trusive into the Greenstone Belt (Fig. 3).The ínitíal "Sr/ B = BarbertonGrantte-Greenstone Terraín;J = Johannesburg Dome; ratios of these units are indislinguishable and plot on a V = Vredefort Dome; and MA = Marydale Comp~ex reasonableSr-isotopic evolulion curve for depleted sub-

* Bernard Price lnsütute of Geophysical Research, University ofThe Witwatersrand. 1 Jan Smuts Avenue , Johannesburg 2001, South Africa Revista Brasileira de Geociéncias. Volume 12 (1~3). 1982 61

0.710 , ,, ,, r ~i7J J' .; ~.; ~ 0.705 / i'ii ,i 1./'; «--' .:r~ ~ I.;" J..-- ...... r ..:' ~ • _ .... 7. ,F-...... ~-----;-l -,,~::::' 0.700r---- .' ., TO0.7ÕZ5 2 3 4 - 4 3.5 3 2.5 AGE 1109 yeorsl

Figure 2 - Diagram of intüal 87 Sr 86Sr versus age fqr the Rb~Sr.· isotopíc data from rock units comprising the Barberton Granite­ -Greenstone Terrain. 'Ihe data may be divíded ínto four age groups. The solid line is the Sr-isotopic evolution curve for the posndated depleted sub-oceanic mantíe (Rb Sr = 0.017). 1he dotted line is LEGEND the Sr-isotopíc evolutton curve for the Komati Formation of'the Onverwacht Group (Rb Sr = 0'()48). 1he dashed line corresponds E:3 to a RbtSr ratio ofO.53. The triangles are lhe data from the Lochiel _ :~~~2~ro~R~IUP 4 Batholith, Diagram modtfted from Fig. 1 of Barton (l982a) [[[JJ :~~~~~0~:f~~03mY.1 O ~:~~g~'~~~~3~~ÕlmYI !SSS.1 :g~KM:o~~~~~mYI ~~f~~~:o~~Ea"&~BERTON -oceanic rnantle (Fig. 2; Barton, 1981, 1982a). This rela­ _1 f7777lj ROCKS cr GAOUP I tionships suggests that these units are genetically and tem- ULLJ AGE(3560Io3430my) porally related to this type of mantle. . c=J ~~~:mcR~~~t~ED Ali of the other units within the Barberton Gramte­ o 'o 2,0 ~,O-.:.O kilom.lr•• -Greenstone Terrain have emplacement ages smaller than about 3,350 Ma and the corresponding initial 87Sr/'6Sr ratios plot above the postulated depleted sub-oceanic man­ Figure 3 - Geographic distributíon emplacement ages for units tle Sr-isotopic evolution curve (Fig. 2). The data for these comprísing the Barbenon Grantte-Greenstone Terraín (Modified units plot in a region bounded by the Sr-isotopic evolution from Fig. 2 of Barton (1982a)]. Note how the Lochiel Batholith curve ofthe Komati Formation and a steeper sloping curve separates a northern are containíng abundam greenstone remmants of uncertain significance. These younger units may be in­ and early díapirs from a southem area composed mostlv ofgneisses terpreted to be derived primarily from crustal rocks (Barton, 1982a). The units with compositions plotting on the Sr-iso­ topic -evolutíon curve of the Komati Formation are likely candidates for having been derived episodically from ma­ northern and southern regions contain diapirs and late­ terial of a composition similar to the mafic volcanic rocks -stage granite plutons of GroupB emplacement ages but of the Barberton Greenstone Belt. Those units with initial in addition, the southern region contains large accumula­ 87Sr/'6Sr ratios plotting above the Sr-isotopic evolution tions of sediments and voleanic rocks of the Pongola Se­ curve of the Komati Formation would be derived episodi­ quence and the igneous Usushwana Cornplex, both em­ cally from sialic crustal material. placed about 2,900 Ma ago. The rocks or the Pongola Se­ One unit with a rather exceptional emplacement age is quence and Usushwana Complex were emplaced in gra­ the approximately 3,030 Ma old areally significant but bens. The trends of some of these grabens continue through volumetrically minor sheet-like granitic Lochiel Batholith the Lochiel Batholith and are marked by intrusive rocks of (Fig. 3).This unit was emplaced with a small initial 87Sr/86Sr Group 3 emplacement ages in the northem region. Units ratio between the second and third periods of regional em­ with emplacement ages less than about 2,600 Ma are con­ placement activity (Fig. 2). centrated primarily in the southern region. The pattern of emplacement ages reveal a biased geo­ graphic distribution with respect to the Lochiel Batholith lhe Limpopo Mobile Belt The Limpopo Mobile Belt is a (Fig. 3). The region north of this batholith contains the zone of high-grade gneisses situated on the northern mar­ majority of the greenstone remnants and the Barberton gin of the Kaapvaal Craton (Fig. I). It may be divided by Greenstone Belt proper is intruded by diapirs ofboth Group major shear zones into a Central Zone and two marginal I and Group 2 emplacement ages. The region south of this zones. The marginal zones contain the highly metamor­ batholith consists primarily of a gneiss terrain with few phosed remnants of granite-greenstone terrains. The Cen­ greenstone remnants. The gneiss terrain comprises the trai Zone, by contrast, is composed of an epicontinental Ancient Gneiss Complex largely of Group I emplacement sequence of metasedimentary 'rocks and granodioritic age and 'the Granodiorite Suite of Group 2 age. Both the gneisses. It has been postulated that the granodioritic gneis- 62 Revista Brasileira de Geocténcías, Volume 12 (1·3), 1982 ses could constitute a basement ontowhich lheepicontinental The Vredefort Dome The Vredeforl Dome (Fig. I) com­ sedimentary rocks were deposited (see e.g, Barlon and Key, prises primarily highly metamorphosed granitoid gneisses, 1981). However, lhe contacts between these are sheared and some of which contain xenoliths of metamorphosed greens­ hence the basement-cover relationship can not be demons­ lone material. Hart et ai. (198Ia) divided lhe granitoid trated unequivocally in the field. gneisses into an outer granite gneiss surrounding the In­ . The emplacement age oflhe epicontinental metasedimen­ landsee Leucogranofels conlaining high-grade melaigneous tary rocks is unknown but these units are in excess of 3,270 and melasedimentary xenoliths called lhe Steynskraal Ma old and rnay be less than about 3,570 Ma old (Barton Metamorphic Zone. The age data of these authors plus and Key, 1981). The emplacement ages of the granite­ those of Slawson (1976) suggest that emplacement of lhe -greenstone components of the marginal zones are also granitoid gneisses occurred about 3,OSO Ma ago and em­ unknown but both lhe Central Zone and the marginal zones placemenl of the Steynskraal Melamorphic Zone occurred contain late-stage granite plutons emplaced between about between about 3,600 Ma and 3,SOO Ma ago. However, 2,700 Ma and about 2,SOO Ma ago (Barton and Key, 1981). Welke and Nicolaysen (1981) have interpreted the Pb-iso­ The granodioritic gneisses include both the Sand River topic data frorn lhe Inlandsee Leucogranofels and lhe Gneisses and the Zanzibar Gneiss, The latter unit is at least Steynskraal Metamorphic Zone lo indicate that these units 3,200 Ma old (Barton and Key, 1982) and lhe Sand River actually were emplaced just over 3,800 Ma ago and have Gneisses were metamorphosed about 3,800 Ma ago (Barton subsequently undergone major metasomatic alteration in et al., 1982). When compared to data for compositionally chemical character, Regardless of whether this older age similar units in the Barberton Granite-Greenstone Terrain of emplacement is correct or not, a major break in isotopic (Fig. 4), the Rb-Sr isotopic data for the Zanzibar Gneiss characteristics exists between the outer granite gneiss and pIot in the field ofunits with Group 2 emplacement ages but lhe units in lhe centre of lhe dome (Hart et ai., 1981a and the data for lhe Sand River Gneisses plot uniquely and above 198Ib). This relationship could indicare that lhe contact lhe postulated Sr-isotopic evolution curve for depleted between these units, rather than being a primary igneous sub-oceanic mantle. The Sr-isolopic evolution vectors of feature, is a tectonic dislocation such as mark the contacts these two gneissic units intersect lhe postulated depleted sub­ between most units in lhe basernent complex of lhe Kaap­ -oceanic mantle Sr-isotopic evolution curve at a point cor­ vaal Craton. Unfortunately, proper structural mapping has responding lo an age in excess of 4,000 Ma (Fig. 4), sugges­ never been carried out in the centre of the Vredefort Dome ting that these units may have been emplaced into the crust so that lhe data do not exist with which to test this possibility. at a very early period in lhe earth's history. Compared with those data characterizing lhe Barberton Granite-Greenstone Terrain, the Rb-Sr isolopie data for lhe granitoid gneisses of lhe Vredeforl Dome plot in the field of units with Goup 3 emplacement ages while lhe data 0.710 for lhe Steynskraal Metamorphic Zone plót in the field of ,, units with Group 1 emplacemenl ages (Fig. 4). The steep , Sr-isolopie evolution vectors ofthe granitoid gneisses appear , to preclude lhe possibility for an appreciable pre-3,OSO Ma I ,{ I I,I I crustal hislory for these rocks. The vector for lhe rocks of II I I the Sleynskraal Metamorphic Zone could allow for only a I I , I , II I I short pre-3,Soo Ma crustal history forthis unit. Conceivably, 'I I I I II ~: I however, ali of lhe rocks comprising lhe Vredefort Dome ,I II ,I could have formed from older crustal rocks.

" : B~I I I, I iíB I •••• III I e;-- I ' •• , ••••••••••• j' I A I II B ...... ;. II The Johannesburg Dome Like lhe Vredeforl Dome, lhe I .-i '....'...., I -'.-'----c----;:::;;:;;:n• Johannesburg Dome (Fig. I) consists of prirnarily granitoid 0,700, ---'~.~."-'..;_"~.,;_. r~ "A ' ITo0:702.5 I- III II , gneisses. However, lhe gneisses of the Johannesburg Dome 2 3 4 .. appear to be less highly metamorphosed and a significanl greenstone remnant occurs within them. The contacts among 4 3.5 3 2.5 AGE 1109 yecrsl the various gneiss units and between the gneisses and the greenstone remnant are tectonically deformed so ·that stratigraphic relalionships are irnpossible lo determine in lhe field. No age data exist for lhe rocks comprising lhe Figure 4 - Díagram of ínítíal 87Sr 86Sr versus age for the Rb-Sr greenstone remnant but lhe granitoid gneisses appear lo isotopíc data from selected rock units from elsewhere in the base­ have been emplaced between aboul 3,ISO Ma and aboul ment comp/ex of the Kaapvaal Craton. 'Ihe age groupings and 3,000 Ma ago and were metarnorphosed between about límí ung Sr-isotopie evolutíon curves lrom lhe data from lhe 2,SOO Ma and 2,400 Ma age (Anhaeusser and Burger, 1982; Barberton Granite-Greenstone Terrain are shown for comparison. Barton and Callow, 1982). Rb-Sr isotopic data for the The hexagons are from lhe Central Zone of lhe Limpopo Mobile graniloid gneisses oe lhe Johannesburg Dome indica te that Beh : A = the Sand River Gneisses (Barton' et al., 1982) and B = lhe these rocks were emplaced belween lhe second and lhird Zanzibar Gneiss (Barton and Key, 1982)~ 1he squares are from lhe periods of regional emplacemenl aClivity in lhe Barberlotl Vredefort Dome: A = the Steynskraal Metamorphic Zone; B = the oUler granile gneiss; C = the In/andsee Leucogranofels (ali fro.m Oranile-Oreenslone Terrain (Fig. 4). ln addilion, lhe gneis­ Hart et aI., 1981a); and D = lhe dolo from Slawson (1976). 7he ses ofgranodiorilic composition in lhe Johannesburg Dome circle is {mm lhe Murchisol1 Granite~Greens(one Terra/fi Barrol1, have inilial ·'Sr ··Sr ratios lhal plot on lhe poslulaled (l982b) ànd lhe inverted triangles are from lhe Johanneshurg Dome Sr-isolopic evolulion curve for depleled sub-oceanic mantle. (Barton and Callow, 1982) This relalion.hip precludes lhe possibilily of an appreciable Revista Brasileira de Geociênctoe, Volume 12 (1~3). 1982 63

pre 3,100 Ma crustal history for these granodioritic gneisses are correct, they were active at different times to forrn rocks and suggests that they may be related genetically in some at different locations in what is now the basement complex manner to depleted sub-oceanic mantle. of the Kaapvaal Cratori.

The Murchlson Granita-Greenstone Terraln The Mur­ chíson Granite-Greenstone Terrain contains tonalitic and trondhjemitic gneisses surrounding a highly deforrned CONTACT RELATIONSHIPS The geographic distri­ greenstone belt intruded by the Iayered Rooiwater Complex bution of the different ages of emplacement of the various (Fig. I). Ali three of these units are intruded by diapirs of rock units comprising the basement complex of the Kaap­ late stage granite. Contacls between the gneisses and the vaal Craton led Barton (1981) to suggest that the craton greenstone belt and Rooiwater Complex are deforrned formed by the episodic accretion of crustal rocks onto so that their stratigraphic relationship is uncertain. The a crustaI nueleus containing the what is now the Cen­ available isotopic data for these rocks are summarized by trai Zone of the Limpopo Mobile Belt. He envisioned Rarton (I 982b). The emplacement age ofthe greenstone belt this accretion to be concentric to the nueleus and he fai­ rocks is unknown but the Rooiwater Complex was emplaced led to take into consideration the significance of the facts prior to about 2,960 Ma ago and the late-stage granite that nearly ali contacts between rock units are sheared diapirs were emplaced about 2,820 Ma ago and about and that major shear zones occur throughout the craton. 2,660 Ma ago. Some of the trondhjemitic gneisses surroun­ Among these shear zones , lhe one containing the Palala ding the greenstone belt were emplaced about 3,270 Ma Shear Zone and separating the Central Zone of the Lim­ ago with an initial s'Sr/'·Sr ratio lha Iplots on the Sr-isotopic popo Mobile Belt from the rocks to the south is over 8 km evolution curve postulated for depleted sub-oceanic rnantle wide and extends completely across the Kaapvaal Craton (Fig. 4). As a result, these gneisses could be genetically (Barton and Key, 1981). Episodic movement along this related to mafic igneous rocks ofan approximately 3,270 Ma shear zone has occurred until the presento The Koedoes age derived from this type of mantle, River Lineament (du Toit and van Reenen, 1977) is a shear zone several kilometres wide that truncates the trends of the isograds ofthe Southern Marginal Zone ofthe Limpopo Mobile Belt. Major shear zones contain the rocks compri­ The Marydale Group The Marydale Group comprises a sing the Pietersburg, Sutherland and Murchison Greenstone. greenstone sequence on the western edge of the Kaapvaal Belts and movement aiong these has occurred inlermittently Craton (Fig. I). Isotopic data suggest lha I these rocks were until at least about 1,000 Ma ago (Barton, 1982b). The emplaced about 3,000 Ma ago (Cornell and Barton, 1979; northern boundary of the Barberton Greenstone Belt is Reid, 1981)and, as such, they are not correlative with either marked by a shear zone at least a kilometre wide that may the rocks of the Barberton Greenstone Belt or the volcanic be part of an imbricate thrust (Fripp et 01., 1980). The rocks of the Pongola Sequence. Lochiel Batholith within the Barberton Granite-Greenstone Terrain was probably emplaced along a major crustal break. The shearéd contacts separating rock units vary' in width MECHANISMS Of EMPLACEMENT Many mecha­ but are generally only a few tens to a few hundreds ofmetres nisms have been poslulated for the emplacement of the wide and may reflect fairly small displacements of the various units comprising the basement complex of the order of hundreds of metres. The senses of these displace­ Kaapvaal Craton. Crudely, these may be divided into tho­ ments are not always certam but many involve diapiric se that require ao original mafic crust (e.g. Anhaeusser, emplaeement ofone unit into another and others the tectonic 1973; Harl et 01., 19810), those that require an original interleaving of tabular units. The shear zones, in contrast, sialic crust (e.g. Hunter, 1974; Tarney et 01., 1976; Bar­ are several kilometres wide and reflect rela tive displace­ ton and Key, 1981) and those that require some of each ments ofhundreds ofkilometres. Thesenses ofdisplacements (e.g. Barton, 1982b). These mechanisms also may be divided in shear zones need not be constant but such large displa­ into those that are philosophically uniformitarian (e.g. cements can more easily be achieved aiong strike-slip faults Hunter, 1974; Tarney et 01., 1976; Barton and Key, 1981; than along thrust faults. The orientation of the shear zones Barton, 1981) and those that are not (e.g. Anhaeusser, is notconcentric ar radial to a specific area within theKaap­ 1973; Harl et 01.,19810). Consensus has not been reached, vaal Craton so that the order of accretion of crustal units is however, as to which one if any of these models is mosI not obvious. applicable. The lowgrade rnafíc volcanic rocks of the greenstone belts may be remnants of oceanic crust (e.g. Anhaeusser, 1973; de Wit and Stearn, 1980) or may have been intruded into rift zones in sialic crust (e.g. Hunter, CONCLUSION Clearly, most crustal elements of the 1974; Tarney et 01.,1976; Hart et 01.,19810). The granitoid basement complex of the Kaapvaal Craton formed over a gneisses, felsic volcanic rocks and late-stage granite plutons substantial period of time of over 800 Ma by processes not rnay have formed by partial rnelting of mafic and sialic yet understood, These elements were brought into their crustal rocks (e.g. Tarney et 01., 1976; Barton, 1981) present configuration by tectonic processes. As a result, or by the metasomatic addition of specific elements from adjacent rock units may have Iittle or no genetic relationship the mantle to a mafic crust, i.e. crudely granitization (Anha­ to one another. Until the sense and magnitude of the dis­ eusser, 1973; Harl et 01.,19810). The epicontinental rocks placements related to the tectonic processes can be evaluated of lhe Central Zone of the Limpopo Mobile Belt may have properly, reconstruction ofthe paleoenvironments ofcrustal been deposited in a graben (Barton and Key, 1981). Never­ formation can only be achieved for fairly small volumes of theless, no matter which mechanisms of crustal formation the basement complex of the Kaapvaal Craton, 6-4 Revista Brasileira de Geocténctas, Volume 12 (1·3), 1982

REFERENCES

ANHAEUSSER, C.R. - 1973 - The evolution of the early Precambrian DeWIT, M,J. and STERN, C.R. - 1980 - A 3.5 b.y. ophlolhe complex crust of southern Africa. Phil. Trans. R. Soe. Lond. A273:359-388. from 'Barberton greenstone belt, South Africa: Archaean oceanic crust ANHAEUSSER, C.R. and BURGER, A.J. ~ 1982~ An Intcrprctatton of and its geotectonic sígnificance. Abstracts 2nd. In1. Archaean Symp. U-Pb zircon ages for Archacan tonalitic gneisses from lhe Johannesburg­ Perth. pp. 85-87. -Pretoria granite dome. Trans. Geai. Soe. S. AFim 85:(in prese]. Du TOIT, M.C. and VAN REENEN, D,D. - 1977 - The southem margin BARTON. J.M., Jr. - 1981 - The panem of Archaean crusta! evolution of lhe Limpopo Mobile Belt, northern Transvaal, with spccial rcfcren­ in _southern Africa as deduced from the evolution of lhe Limpopo ce to metamorphism and structure. Geai. Sur\'. Botswana Buli. 12:83-97. Mõbile BeÍtand the Barberton Granüe-Greenstone Terrane. Spec. Publ. FRIPP, R.E.P., VAN NIEROP, D.A., CALLOW, M,J., LILLY, P.A. and Geai. Soe. Australia 7:21-31. DU PLESSIS, L.U, - 1980 - Deformation in part of the Archeean BARTON, J.M., Jr. - 1982a - Isotopíc constraints on possible tectonic Kaapvaal Craton, South Africa. Precambríun Res. 13:241·251. models for crustal evolution in the Barberton Graníte-Greenstone Tér· HART, R.J., WELKE, H,J. and NICOLAYSEN, L.O. - 1981a ~ Geo­ raio, southern África. Spee. Pub/. Geai. Soe. S. Africa 9:(in presa). chronology ofthe deep profile through Archaean basement at vredefort. ~ BARTON, J.M., Jr. 1982b -r Timíng of ore emplacement and deforma­ with implications for early crustal evolution. J. Geonnvs. Res. 86: 10663· tion, Murchison and Sutherland Greenstone Belts. Kaapvaal Craton. ·10680. Spec. Publ. Geol. Soe. Zimbabwe, Goíd 8.2 (in press). HART, R.J., NICOLAYSEN, L.O. and GALE, N.H. ~ 1981b -' Radiocle­ BARTON, J.M .. Jr. and CALLOW, M. - 1982 - Rb and Sr elemental ment concentrations in thedeep pro file through Precambrían basement of and Sr-isotopie studies of certain rock units in the Johannesburg Dome. the Vredefort structure. J. Geoph)'s. Res. 86:10653·10661. Trans. Geot. Soe. S. Africa (submitted for publícationj. HUNTER, D.R. - 1974 - Crustal developmem in lhe Kaapvual craton. BARTON. J.M., Jr. and KEY, R.M. - 1981 ~ The tectonic development L lhe Archaean. Precambrian Res. t :259-294. of the Limpopo Mobile Belt and the evoluticn of the Archaean crarons REID, D.L. - 1981 - Sm-Nd ages from the Namaqua and Richtersvcld of southern Africa. ln: A. Krôner (ed.), Precambrian Plate Tectonics. provinces. Abstracts S. Afriean Geodynamie Symposium, Gcol. Soe. S. Elsevier, pp. 185·212. Afriea, pp. 173-174. BARTON, J.M., Jr. and KEY, R.M. - 1982 - Rb-Sr ages and geological SLAWSON, W.F. - 1976 - vredefon core: a cross-sectton af the upper setting ofcertam rock units from the Central Zoneofthe Limpopo Mobile crust. Geochim. Cosmochim, Acta 40:117-121. ~ Belt, near Zanzibar, castern Botswana. Speo. Pub/. GeaI. Soco S. Afríca TARNEY, J., DALZIEL, I.W.D. and De WIT, M.J. - 1976 Marginal 8:(in press). basin "roces verdes" eomplex of S. Chile: a model for Archaean greens­ tone bel! formation. ln: B.F. Windley (ed.], Barly History of lhe Earth, BARTON, J.M., Jr.. RYAN, B. and FRIPP, R.E.P. - 1982 ~ Rb-Sr and U-Th-Pb isotopic studies of the Sand 'River Gneisses, Central Zone, Wiley, pp. 131-146. Limpopo Mobile Belt. Spec. Publ. Geol. Soe. S. Africa 8:(in prese). Recebido em 31 de julho de 1982 CORNELL, D.H. and BARTON, E.S~ - 1979 ~ Age of lhe Marydale banded íron formation by the Pb-Pb rnethod. Extended Abstracts, 18th Congo Geol. Soe. S. África. pp. 111·114.