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RevistaBrasileira de Geociências 12(1-3): 15-31, Mer.câet., 1982 - Silo Paulo

ARCHAEAN TO EARLY TECTONICS AND CRUSTAL EVOLUTION: A REVIEW

ALFRED KRONER'

ABSTRACT Hypothesis on the formation and evolution of the Earth's continental crust in the Early Precambrian have to account for the independent and directionally consistent motion of crustal segments of considerable dimensions since at least 3,5 Ga ago, and at minimum veJocities comparable to those of today. It seems established, therefore, that has governed lithospheric evolution since the generation of rigid crustal segmenta, and it is speculated that a change in style of pia te interaction, 'rather than a change in fundamental mechanisms, has deter­ mined non-uniformitarian crustal evolution towards the present Wilson cycle. Uncertainties in the reliable reconstruction of Archaean tectonic settings are caused by low­ -temperature rock alteration that may obliterate primary magma tic trends, by the variable possible sources and melting processes for the generation of bimodal and calc-alkaline associations, and by disagreement on the interpretation of rocks with "primitive" isotopic characteristics. It is sug~ gested that magmatic underplating played a major role in lithospheric growth and. that reworking of continental crust during large-scale crustal differentiation was more important in the Archaean than recognized by most currently popular models. The generation and survival of first continental crust is seen in analogy with the evolution of Iceland, and later greenstone belt development was probably largely .intracontinental, following a -and-sag mede] in response to crustal rearrangements. Most high-grade terrains are considered to be older than neighbouring or overlying greenstone associations and were brought to the surface through low-angle thrusting. Archaean lithospheric growth built stable cratons that experienced little internal deformation in ear1y Proterozoic times. Less stabilized crust reacts to large-scale distortions predominantly by stretching, elongate basin-formation and "ensialic" through mechanisms ranging from crust restacking to transform shearing, finally gcnerating mobile belts. Locally, modern-type con­ vergent plate margins become recognizable at about 2.2 Ga ago while the full Wilson cycle is finally established in the late Proterozoic. '

INTRODUCTION Some 20 years of plate teetonies, crustal sutures merely by tiny occurrences of mafic-ultra­ eombined with new insights into the fine strueture of the mafic rocks, nappe tectonics and the presence of'calc-alkaline lithosphere as derived from seismie data, the application rock assemblages (e.g. Burke et al., 1977) and by inferring of multi-element geoehemieal and isotopie studies, and teetonie processes almost exclusively from geochemical data paleomagnetism have profoundly influeneed present think­ (e.g. Taylor, 1967; Glikson, 1972; Tarney, 1976) and thus ing on the origin and evolution of the Earth's continental postulating uniformitarian crustal evolution back to the crust. Early Arehaean (Burke et al., 1976; Burke, 1981), is now Although there is now general agreernent on how the succeeded by more realistic models that recognize funda­ Earth worked for the last 200 Ma beeause of observable mentai changes in the Earth's internal processes following evidenee in the oeeans and eontinents (e.g. Bird, 1980; Con­ global cooling and concomitant changes in lithospheric die, 1982a), it has proved diffieult to extend this history behaviour through time (Baer, 1981; Goodwin, 1981a; into more ancient times in view of the lost oceanic record Króner, 1981 a). It is also satisfying to see that modern and the ambiguity and eomplexity of the pre- rock workers place increasing importance on field relationships relationships in the (Dewey, 1982). However, pre­ rather than on conceptual interpretations, particularly as served eharaeteristie rock assemblages uniquely identifying regards such controversial problems as the evolution of modern Wilson eyclc processes have now been abundantly Arehaean greenstone-granitoid-gneiss terrains and Pro­ recognized in continental terrains as old as ea. 800-900 Ma terozoic mobile belts. and provide strong evidence for the conclusion that the The plate tectonie eoneept is now flexible enough to present global teetonic regime has governed the evolution aecomodate such previously rejected models. as "ensialic" of the lithosphere at least sinee the Late Precambrian (Baer, orogeny (Bally, 1981; Dewey, 1982; Króner, in press), while 1977; Krõner, 1977). adversaries of Precambrian piate motion have toaccept the Profound disagreement on the older erustal history, how­ dominance of horizontal tectonics in continental crust for­ ever, prevails to the present day, but it seems clear that mation sinee the earliest Arehaean (Myers, 1976) and the the earlier period of dogmatism that followed the introdue­ growing evidence for independent motion of rigid litho­ tion of the plate teetonic concept is now giving way to a . spheric plates ofconsiderable dimensions since at least 3.5 Ga more balaneed approaeh in attempts to decipher the Pre­ ago (Krõner and McWilliams, in press). The conclusion cambrian evolution of the continents. Thus, the early is inescapable, therefore, that has been recy­ enthusiasm of postulating vanished oeeans and defining cled back into the mantle since plates move around, although

'" Institut für Geowissenschaften, Johannes-Gutenberg~Universitat,Saarstr. 21, 6500 Mainz, West Germany 16 Revisto Brasileiro de Geociêncías, Volume 12 (1-3), 1982

the detailed process of aneient as well as the is seen in the widespread occurrence of MgO-rich komatiitic mechanism that triggered it remain speculative (Hargraves, volcanic rocks in Archaean terrains thatare commonly ascri­ 1981; Krôner, 1981a; Lambert, 1981). bed to high degrees ef partial melting in the mantle (Green, Uniformitarianists acknowledge significant difTerences 1975,1981). Since there is no evidence that signiticant heat between Archaean and younger terrains (e.g. Sleep and was lost by conduction through the Archaean continental Windley, 1982), yet they appeallargely to crustal-evolution crust (Tarney and Windley, 1977), the greater heat flux must models that are based on processes occurring along modern havebeen removed via theoceaniccrust, either byan increa­ plate boundaries in explaining crustal growth through time sed rate ofplate production or by an increased totallength of (e.g. Windley, 1979; Moorbath, 1978), in line with the oceanic ridges (Biekle, 1978). About six times the current glo­ dogma: comparable rocks imply cornparable origins (Dic­ bal oceanic crust formation rate of 3km'!a would thus be kinson, 1981). required by 2.8Ga ago or an average age ofocean crust at the The crucial questions not answered by these models are time of subduction of only ca. 20Ma (Sleep and Windley, why crusta I growth took place episodically rather than 1982). continuously (DePaolo, 1981) and why 70%-85% of the 1t is also argued that plate motion would have been faster present continental crust apparently formed in Archaean due to a lower viscosity in the mantle. Hager and O'Connell times despite the lack of evidence for signiticantly faster (1980), for example, calculated that a temperature increase plate motion in the early Precambrian as compared to later of 100'C in the mantle results in a viscosity decrease by a periods. The present Wilson cycleand associated subduction­ factor of 10, Ieading to a 3·fold increase in platevelocities. -related continental margin and island-arc magmatísm, there­ The high calc-alkaline magma generation rate of the Ar­ fore, appear to be less efficient -building mecha­ chaean (Moorbath, 1978) is seen as a consequence of this nisms than required by the Archaean rate of crustal growth. postulated greater plate motion (Dewey and Windley, 1981). This paper reviews the situation and speculates on alterna­ However, presently available palaeomagnetie data for tive processes that seem compatible with the geological, Achaean rocks do not support this concept in that they geochemical and palaeomagnetic data as well as the general fail to reveal signiticantly higher minimum continental ve­ concept of piate tectonics. locities with respect to the poles in the early Precambrian. These data come from the Superior Province in Canada (Dunlop, 1981), the Pilbara Block of western Australia PRECAMBRIAN PLATE MOTION It has been a wide­ (McElhinny and Senanayake, 1980) and the Kaapvaal Iy held opinion that higher heat flow in the Precambrian craton of southern Africa (Krõner and McWilliams, in resulted in faster plate motion and that this explains the press) (Fig. I). The reader should be aware of the Iimitations decreasing rate of crust-forrnation through time (e.g. Bickle, of palaeomagnetie data in that they only record longitudinal 1978; Dewey and Windley, 1981; Park, 1981; Smith, 1981). motion with respect to the poles and that motion is also The argument goes Iike this: Archaean terrestrial heat flow difficult to detect in cases where palaeomagnetic and rota­ was two-three times present rates (Lambert, 1981)and requi­ tion poles are in close proximity (McWilliams, 1981). red upper mantle temperatures to have been up to 150'C The Canadian results imply a minimum average veloeity higher than now (Davies, 1979; Cook and Turcotte, 1981; of about 2cm!a for the period 2.8 to 2.5 Ga ago (Ullrich Smith, 1981). Direct evidence of these higher temperatures and Van Der Voo, 1981), comparable to the Western Aus-

Figure 1 - Preliminory apporent polar wander paths for the -Kaapvaal craton, southem Africa, the Pílbata B/ock, Australía, and the Superior Provínce, Canada,for vatious períods ofear/y Precambrían time. Note the divergem but consistem motíon ofthe three cratons. Data f1'0;1I Duntop , (/981), Mclilhinnv and Senanayake (1980) and Krõner and McWl/Iiams (unpub/.). Ctrctes = Koapvoal craton; squares = Superior Provínce .. diamonds = Pilhara Block Revista Brasileira de Geocténctas, Volume 12 (1-3), 1982 17

tralian data that suggest a drift rate of ca. 1.5 cm/a for ln summary, present data favour the existence, since the the period 3.5-2.4 Ga ago. The Kaapvaal craton experienc­ Early Archaean, of comparatively large plates that drifted ed an appreciably higher minimum velocity of about 4 cm/a independently at minimum velocities comparable to those during the sarne period, but still cornparable to present rates. of today. Since there is no evidence to suggest large-scale The limited data do not yet permit to recognize strongly andesite-dominated crust formation in the Archaean (see fluctuating drift rates in the Archaean as has been postulated below), crustal growth rates greater than about 6 km'/a, for later times (Ullrich and Van Der Voo, 1981), but an or approximately ten times present rates, as deduced from important conclusion to emerge is that although each geochemical modelling (Mcl.ennan and Taylor, 1982) re­ Archaean craton has its own velocity signature, the overall main unexplained by uniformitarian models. mean minirnum velocity of 2.8 cm/a for the continents in post-Archaean times (UlIrich and Van Der Voo, 1981) does TECTONIC SETTINGS AND ALTERATION OF, not appear to have been significantly higher in earlier periods VOLCANIC ROCKS The application of geochemical ' as is suggested as one possibility by the above models. discrimination diagrarns 10 infer tectonic settings (e.g. This is also borne out by the geological data. Dewey (1982) Pearce, 1976, 1982; Pearce and Cann, 1973; Pearce et ai., has argued that increased rates ofplate motim, are signified 1977; Floyd and Winchester, 1975; Winchester and Floyd, by marine transgressions, a higher incidence Df 1977) has resulted in a flood of papers over the last ten obduction and the generation of blueschists and granitoid years in which major and trace element data of volcanic rocks. Except for the granitoids, whose origin and role in rocks regardless of age and state of preservation were used Archaean crustal evolution will be discussed below, none to construct evolutionary models in line with the particular of these processes are characteristic of the early earth. ln setting predicted by the diagrarns applied. fact, no ancient or blueschists have been reported Although the authors ofthese discrimination methods ex­ despite excellent preservation of Archaean upper crustal plicitly state that their diagrams are derived from data of segments, and it remains ODe of the enigmatic problems for modem, carefully selected, fresh volcanic rocks and do not most uniformitarian models to explain this phenomenon necessarily apply to Precambrian environments, this warn- i if early Precambrian plates moved indeed much faster and ing is frequently ignored since relative immobility of the i subduction-complexes would thus have risco correspond­ criticai elements during post-depositional alteration and ingly fast to preserve primary high-pressure metamorphic recrystallization is claimed, and there is widespread belief assemblages. Also, if Archaean oceanic crust was non­ that geochemical patterns resulting frorn present magmatic -subductible as is sometirnes claimed (Green, 1975; Baer, processes can confidently portray tectonic settings as far 1977, 1981), because its turnover was fast and thus did not back as the Archaean (Glikson, 1971; Naqvi, 1976; Dickin­ allow for substantial cooling (Bickle, 1978), why did this son, 1981; Hamilton, 1981; Weaver and Tarney, 1982). crust not preferentially obduct, particularly if there was Thus, Archaean and Proterozoic metabasalts of tholeiitic less elevation contrast between surfaces of continents and affinity are frequently interpreted as remnants of ancient the mean depth ofthe ocean floor (Dickinson, 1981; Dewey oceanic crust, aríd rocks classified as calc-alkaline andesites and Windley, 1981)? N.T. Arndt (pers. comm., 1982) invariably denote ancient ares related to subduction (e.g. suggests that Archaean ocean crustwas komatiite-dominated Glikson, 1972). and was therefore dense enough some 20 Ma after generation Àlthough it is not denied that, in many cases, the geo­ to be subducted. Survival ofsuch crust is therefore unlikely. chemical discrimination method is supported by Pre­ A second, equally important, consequence of the palaeo­ cambrian rock associations and geological field rela­ magnetic data is that ancient continental nuclei must have tionships (e.g. Roobol et al., 1982; Condie, 1981a) there is belonged to piates that drifted independently of each other growing evidence that many so-called "distinctive .rock at different velocities and in different directions (Fig. I) types" rnay occur in more than one envíronment. For exam­ since theearly Archaean, thusseriously questioning models pie, Archaean komatiites and basalts have been interpreted that infer one-plate scenarios for the early Earth (e.g. Fyfe, as primitive ocean crust (Glikson, 1976; Naqvi, 1976; 1974; Shaw, 1976) or the aggregation of early sialic nuclei Anhaeusser, 1981), yet such rocks are often found in se­ into a (Goodwin, 1974; Condíe ; 1980). quences that demonstrably rest on granitoid basement The apparent "smoothness" of Archaean polar wander (Nisbet et al., 1977; Krõner, 198Ic), are interfingering with path for the 3 era tons cited above (Fig. I) also seems to calc-alkaline volcanics in shallow-water environments (Bar­ contradict the scenario of "jostling platelets on a turbulent jey, 1981) and/or are interbedded with continental-type asthenosphere (Krõner, 1981a; Goodwin, 198Ib), at least for sediments (Archibald el ai., 1981; Schau, 1977). Likewise, post-3.5 Ga times! unless such motion was at scales not Archaean and early Proterozoic andesites that classify as detectable by palaeomagnetism. To the contrary, the direc­ calc-alkaline lavasand thus would demand a plate-margin tional consistency in continental mo tion over severa1 hun­ setting have been shown to belong to intracratonic rift-re­ dreds million years as shown by the apparent polar wander lated sequences (e.g. Hallberg et ai. 1976; Hegner et aI., paths (APWP) of Fig. I implies the existence of relatively 1981; Schweitzer, 1982). large plates and the operation of mantle convection cells Even if great care is taken in the selection ofsamples and comparable to those of the present Earth, alteration is at a minimum, discrimination diagrams often It is, however, seen as significant that both the Australian do not give conclusive results in that analysis plot in ali and Kaapvaal Archaean APWPs display distinct bends at possible fields, as demonstrated for the early Archaean those intervals of time that are considered as major crust­ Pilbara greenstones of Western Australia (Glikson and -forrning episodes due to the emplacement of voluminous Hickman, 1981). granitoid complexes. ln analogy with loops in younger More serious for the tectonic interpretation of geochem­ APWPs these changes in plate motion may signify major ical data, however, is the effect of low-temperature altera­ rearrangements in Archaean lithospheric geometry that tion in volcanic and volcaniclastic rocks and the effect may have had their cause in changing mantle dynamics. of crustal contamination. Superheated aqueous solutions 18 Revista Brasileira de Geoctênctas, Volume 12 (1·3), 1982

have profoundly altered modem oceaníc crust to depth of MacOeehan and McLean (1980), forexample, have shown more than 5 km in places (East Pacific Rise Study Group, that many "andesites" of Canadian greenstone belts are, 1981),and it iseasily visualized that hydrothermal convection in fact, altered tholeiitic basalts, and Krõner and Schweitzer in marine volcanic sequences was even more important in (unpubl. data) also demonstrate that superficially fresh Archaean times when mantle and ocean temperatures were but profoundly altered Early Proterozoic volcanics that significanlly higher than today (Fyfe, 1978). plot in the shoshonite and calc-alkaline fields in some dis­ Dimroth and Lichtblau (1979) demonstrated alteration­ crimination diagrams are. in reality within-plate tholeiites -induced element variations greally exeeeding the apparent (Fig. 3). magmatic trends from Archaean greenstone volcanics at Noranda, Canada, thus confirming earlier studies (sum­ Q2 marized in Condie, 19820) that many elements, including Q1 F.- ai some of those commonly regarded as immobile, are affected 1 to varying degrees by ehloritization, epidotization and ear­ -u WPB bonization. Such alteration as well as post-depositional -1.4 silicifieation (Fig. 2)tend to impose an apparent calc-alkaline feO' element distribution pattern on originally tholeiitic volcanic -IS cl rocks and can therefore lead to completely erroneous in­ terpretations. -1.6 F, -1.3 -l> -,S -lh é-o ! l • -1.3 . .SHO

-1.4

-1.5

-1.6 OfB Til100

bl dI

10 Zrl Y

50 100 Zr soo Ippml z-

Figure J- Dtscríminationdiagrams to determine tectoníc seníngs of mafic volcanic rocks of the /ower Proterozoíc Venlersdorp Super­ group, South Africa (Schweítzer, 1982). Diagramsa and d suggest plate margin ca/c-a/ka/ine associations wtth shoshonites, while dia­ grams b and c clearly support thc correcr wíthin-pkue origin of these rocks

Although present dogma holds that ealc-alkaline magmas are predominantly produeed in the mantle-wedge overlying subduction zones (Gill, 1980; Thorpe, 1982), there are examples of anorogenie basalt-tholeiitie andesite-daeite­ -rhyolite assoeiations from the Tertiary (Wilkinson and Binns, 1977)to thc Archaean (Hegner et 01., 1981),and these assemblages apparently oceur in extensional environments where crustal melting is required to aceount for the observed chemical variations. Several reeent investigations have shown that andesitic to rhyolitie rock types in sueh settings may be produced by varying degrees ofmixing oflower erust and manlle material (for summaries see EOS, 61, 67-68, 1980) and that under­ plating of dense picritic to gabbroie magmas at the base of the crust played an important role in this process (Betton and Cox, 1979; Ewart et -al., 1980). ln aecounting for the origin of the c1early ensialic meta­ Figure 2 - Low-temperature alteratíon ln basaltíc andesite of the voleanies of the Arehaean Yellowknife Supergroup in the lower Proterozoic í/entersdorpSupergroup, SmithAfrica,as reveaíed Canadian Slave Province, Baragar (1966) eoncluded that by (ovstttctftcouonand(b)carbonatization.CrossedNicols, 20 x mag­ mafic magmas generated at the base of the erust were con­ nijication taminated by crustal melts before they could be extruded, Revista Brasileira de Geoct'ncias. Volume 12 (1-3). 1982 19

resultíng in strong calc-alkaline trends superimposed on isotopic characteristics indicating derivation from older originally basaltic magmas. Significantly, the setting as sialic crust of tonalitic or more mafic composition (e.g. revealed by excellent exposures and detailed fieldwork, Hunter et ai., 1978, Collerson et ai., 1982; Hanson, 1981; was one of stretching and rifting in the > 3 Ga old pre­ Muhling, 1981; Condie, 1981b). On the other hand, lhe -Yellowknife granitic crust (Padgham. 1981). postulated mantle-source for a considerable number ofsuch Taylor and Hallberg (1977) also invoke a rift-setting for complexes as deduced from low 8'Sr/86Sr initial ratios the generation ofcalc-alkaline andesite-bearing volcanics in has led to the proposal of major crustal accretion events, the Archaean Marda Complex of Western Australia and particularly at lhe end of lhe Archaean 2.8-2.5 Ga ago concIude that crustal anatexis played an important role. (Moorbath, 1978). However, such speculations lose some Giles (1981), in addition, showed that calc-alkaline rocks of their significance in the light of recent demonslrations in greenstones of the Yilgarn Block do not occur in ares that simpie one-stage Sr-evolution diagrams may be inap­ but in rather discrete, isolated centres. He advocates sub­ propriate in portraying the crustal history of granitoid duction-unrelated wet melting in the upper mantie and rocks whose Rb/Sr ratio is significantly higher than that subsequent fractional crystallization from a mafic source of their source (e.g. Hart et al., 1981; Davies and Allsopp, at the base of the crust, perhaps related to a hot spot, The 1976; Cooper et al., 1982). The implications of this conclu­ conditions ofmagma generation created in the upper mantle sion for crustal evolution models are further discussed would essentially duplicate those produced in the mantie below. wedge above a descending oceanic crustal slab in a modern Condie (l98Ib) arguedfrom geochemical model studies 'subduction zone (Giles, 1981). that tonalite-trondhjemite and high-grade gneiss cornposi­ To summarize, many c1aims for modern-type tectonic tions define acceptable source rocks for Archaean granites se.ttings as deduced from the geochemistry of aneient vol­ and that most of the ancient lower crust must have been canic rocks are suspect and are not supported by geological involved in the production ofthese rocks, leaving a depleted field evidence. Low-temperature alteration and contamina­ residue. This model, however, is in sharp contrast lo the con­ tion of basaltic rocks can produce secondary calc-alkaline c1usion of Weaver and Tarney (1982) who suggesled that trends that are easily misinterpreted, particularly in com­ most granulites represent liquid compositions rather than re­ plexly deformed terrains, and that may suggest wrong tec­ fractory residues, while the crustal layering and chemical tonic environments, Thegreatmajority ofArchaean volcanic differentiation revealed by seismic (Drummond et ai., 1981) 'sequences belongs to bimodal basalt-dacite/rhyolitesuites and isotopic data (Hart et ai., 1981) as well as pelrological while true andesites are rare except for the Canadian shield considerations (O'Hara, 1977) seem to support the partial (Condie, 1981 a; Goodwin, 1981b). This bimodal distribution melting módel. pattern is also retlected in the composition ofmost Archaean The volumetrically most important granitoid rocks of grey gneiss complexes (Barker and Arth, 1976)- a situation most Archaean terrains are tonalites and cogenetic trondh­ that is in conflict with crustal evolution models advocating jemites as well as their tectonized gneissicequivalents. Much Archaean continental growth predominantly through pro­ controversy exists as to the origin of these rocks that typi­ cesses at destructive plate margins. cally form large composite "gregarious batholiths" (Mac­ The assessment of geochemical data for most Early Pro- Gregor, 11951) and that often contain xenoliths of green­ . terozoic metavolcanic successions is more difficult due to stone belt material as well as banded gneisses and/or rnig­ few published reliable high-quality analysis (Condie, 1982b). matites that may either represent older sialic basement or However, the sarne limitations as detailed above apply, and products of interaction between greenstones and granitoid the generally high degree of metamorphism in the mobile melt. zones renders deduction of the tectonic setting from the Anhaeusser (1973), Anhaeusser and Robb (1981) and geochemistry of'such recrystallized volcanicrocksevenmore Glikson (1979) have suggested that the tonalite-trondhje­ speculative than in the low-grade Archaean terrains. mite complexes are derived from anatexis of a primitive Magma mixing and crustal underplating may be processes mafic-ultramafic crust as found in lower greenstone sue­ that enable lhe generation of calc-alkaline rock assemblages cessions and that the evolving magmas ascend to higher in extensional environrnents, and this concept is further crustal leveis where they intrude the higher greenstone units explored below. and form multilobate diapirs, This results in extensive assimi­ lation of greenstone material and inthe formation of mar­ SOURCE Of ARCHAEAN BIMODAL SUITES All ginai migmatite zones that develop into banded gneisses in presently known Archaean terrains consist of varying pro­ high strain zones. portions ofgreenstone belts, upper crustal granitoid intrusi­ Petrogenetic considerations do not favour this model ves and a variety of banded medi um- to high-grade "grey (Condieãnd Hunter,.1976; Hanson, 1981), and it is parti­ gneisses" that frequently contain metasedimentary remnants cularly difficult to explain the derivation of the widespread and that are either older or formed broadly contemporaneo­ interbanded bimodal basalt-trondhjemite gneisses from such usly with the greenstone belt rocks (forsummaries see Wind­ a processo Bettenay et ai. (1981) have summarized the field ley, 1977; Glover and Groves, 1981). Taken together, an Ar­ evidence against greenstone anatexis, and most of their chaean crustal column composed of these units would have arguments are confirmed by my own observations in the broadly andesitic composition (Taylor, 1967) while lhe in­ Barberton region and Swaziland, southern Africa. dividual rock units themselves are rernarkably andesite­ An alternative origin is proposed by most uniformitarian free and almost universally reveal a pronounced "Daly gap" models that see similarities between Archaean tonalites (Barker and Arth, 1976). and modern calc-alkaline batholiths and their gneissic roots The origin ofthe potassic granitoid complexes that appear that characterize Andean and Cordilleran belts (Windley, late in the evolutionary history of rnost Archaean terrains 1979; Hamilton, 1981; Weaver and Tarney, 1982). Tonalites (Anhaeusser and Robb, 1981) is least controversial since and trondhjemites are generated from upper mantle-derived many of these batholithic bodies have geochemical and melts that originate above subduction zones and either in- 20 Revis~a Brasileira de Geocilncias. Volume 12 (I~3). 1982 trude into the evolving are or continental margin or accrete Hanson (1981) pointed out that mantle-derived mafic at or near the base of the crust (Weaver and Tarney, 1982) melts high in MgO and FeOsuch as komatiites have densities where they solidify and develop into granulite-gneiss belts higher than most crustal rocks and that many ofsuch melts (Windley, 1979). These processes require no or only short would therefore not reach upper crustal leveis. Trapped at crustal residence times for the granitoid precursors - a the base of the crust, they would form a possible source for feature supported by many isotope data (e.g. Moorbath, tonalite, diorite and granodiorite. Mafic underplating, as 1978). advocated here, has previously been discussed as a Iikely The great volumes of tonalite-trondhjemite produced phenomenon in extensional environments and may also in the Archaean are conveniently ascribed to rapid ocean play a major role in greenstone volcanism as discussed crust recycling, and the frequent collision of fast-moving, below. numerous small plates (Dewey and Windley, 1981), but this The tonalite-trondhjemite suites with "primitive" isotope scenario neither accounts for the spatial and temporal dis­ characteristics satisfy the above model but do not preclude tribution ofthese rocks nor for their generation during world­ the possibility that their source terrains had very low Rb/Sr wide well-defined crustal accretion events (Moorbath, so that considerable crustal residence times up to several 1978). The conflict of this hypothesis with the available hundred million years are possible for the granitoid pre­ palaeomagnetic .data has already been discussed. cursors (e.g. Barton, 1981). The equation of ancient high-grade bimodal gneisses and Barker et ai. (1981) distinguish between relatively homo­ their associated metasediments with modero accretionary geneous tonalites that are often compositionally similar to thrust stacks that were transferred to deep crustal leveis dacitic volcanics in neighbouring greenstone belts and that during the accretion event (Dewey and Windley, 1981) may originate through the process as described above, from completely ignores the fact that the Archaean granulites well foliated, commonly migmatitic and heterogeneous frequently contain metasedimentary assemblages derived varieties that often form part of extensive gneiss terranés from shallow-water sandstone-shale-limestone sequences and may include metasedimentary remnants. These latter (Katz, 1977). The extensive granulite terrains of southern rocks are ascribed to remobilization of older tonalite or India, Enderby Land and Southern Africa are also contrary "grey gneiss", and examples of this type include the Vikan to the narrow, short, linear belts that would be expected gneisses of North Norway (Moorbath and Taylor, 1981), from subduction-accretion along small continental plates. the Kiyuktok gneisses of Labrador (Collerson et al., 1982), An alternative origin for the Archaean high-grade gneisses as well as several granitoid complexes in southern Africa has been suggested by Myers (1976) and Krõner (l98Id), (Davies and Allsopp, 1976) and in the Yilgarn Block of and this is more fully explored below. Western Australia (Archibald et aí., 1981). A third origin for the tonalite-trondhjemite suite is ln sumrnary, Archaean tonalite-trondhjemite suites and proposed by Barker and Arth (1976), Barker et ai. (1981), compositionally similar grey gneiss terrains are unlikely to Condie and Hunter (1976), Condie (198Ia), and Hanson represent the products of greenstone anatexis, From the (1981) on the basis of petrogenetic and geochemical model­ many dissimilarities of these rocks to Cordilleran batholiths Iing. These models show that K-poor granitoid magmas may and their root zones, their spatial distribution and their be derived from rnafic-ultrarnafic sources, either as products structural evolution it is considered unlikely that they of partial melting of amphibolite or eclogite, or through formed aiong modern-type convergent zones. Models that fractional crystallization of basaltic liquid. The scarcity of consider underplating ofdense mafic magmas in extensional intermediate magmas and melting experiments that yielded environments, generation of tonalite-diorite-trondhjemite trondhjemite from an aniphibolite source (Helz, 1976) fa­ plutons from these sources and from remelting of older vour the partial melting mechanism. sialic gneissic crust are considered more realistic and satisfy Partial melting of 10:;{,-20% amphibolite of basaltic to constraints from field relationships, petrology and geo­ komatiitic composition at or near the amphibolite-granulite chemistry. transition, i.e. near the base of the crust, produced tonalite­ Recent seismicand isotopic studies lend support to large­ -trondhjemite melts (Barker et al., 1981). ln the lower crust -scale crustal difTerentiation due to metamorphic segregation these magmas intrude as sheets and, together with their in the Archaean crust, a process that almost certainly accom­ host rocks, they are involved in plastic flow by lateral ex­ panied the evolution of granite-greenstone terrains and tension and form high-grade gneisses with generally flat probably caused early cratonization of some shield areas. foliations (Gastil, 1979). The 3.8 Ga old Sand River Gneisses The Yilgarn Block of western Australia has a 3-layer ca. of the Limpopo belt (Fripp, 1981) may be an example of 40-50 km thick crust with the lowest part consisting of high­ this mode!. At higher leveIs the melts may still intrude as -velocityj7.0-7.4km. s" ') material considered to be residual sheets and form complex structural relationships with the mafic granulite (Drummond et ai., 1981; Archibald et al., tectonized roots of greenstone belts, including marginal 1981). The deep crustal profile through the Vredefort Dome, zones of migmatites (own observations in Southern Africa, South Africa, exhibits a similarly layered crust that formed see also Bettenay et al., 1981; McOregor, 1979). But the at least 2.80a ago (Hart et ai., 1981), and the deposition majority of the magma rises diapirically to form large com­ of cratonic sedimentary rocks on parts of the Kaapvaal posite batholiths that often engulf the previously deformed craton as early as 3Ga ago (Krõner, 1981a) suggests comple­ greenstones and obliterate their relationships with older tion ofcrustal difTerentiation before that time. The specula­ basement. Structural work (e.g. Archibald et ai., 1981; tion that little differentiation, uplift and erosion occurred Platt, 1980; Williams and Furnell, 1979; De Wit, 1982) in the Archaean and should therefore explain the lack ofanc­ demonstrates that the batholitic granitoids are not respon­ ient subduction-accretion prisms (Dewey and Windley, 1981) sible for the first deformation in most greenstone belts but is in conflict with the above data and with the evidence for rather modify existing structures by steepening originally crustal thicknesses, at least locally, comparable to those flat foliations and thrusts. of today since 3.8Ga ago. RevistaBr06i1eira de Geocíências, Volume 12.(I~j). 1982 21

RElATIONSHIPS lN GREENSTONE BELT STRATI­ Another matter of continued considerable dispute is the GRAPHIES AND ORIGIN OF VOlCANIC ROCKS basement-coverrelationship in Archaeangranite-greenstone Much early speculation on greenstone belt development terrains. Field evidence is often equivocai due to faulted has been based on "model" stratigraphies that were contacts, insufficient exposure or obliteration of criticaI erected in the belief that there is a cyclieal evolution in relationships by later intrusive rocks. ali belts from mafic-ultrarnafic rocks at the base through Discovery of several well preserved original contacts of andesites to highly differentiated volcanics and overlying, greenstone or metasedimentarysequences as old as 3Ga predominantly sedimentary successions íe.g, Anhaeusser, with clearly underlying older sialic basement has led to 1971). The basal units were regarded as primeval mafic rejection of ensimatic models for most late Archaean green­ crust while the stratigraphically higher and more evolved stone belts in favour of rift or marginal basin evolution (for sequences, together with the neighbouring granitoids, were summaries see Windley, 1977; Platt, 1980; Krõner, 19810), ascribed to ensimatic island-arc evolution (Anhaeusser, but "hard-core" defenders of intraoceanic crustal evolution 1973; Glikson, 1976). Apparent geochemical similarities still envisage the Early Arcbaean ea. 3.5 Ga greenstone ofthe mafic-ultrarnafic volcanics and the more differentiated generation to have formed 00 primeval oceanic crust des­ rocks with modem MüRB and intraoceanic are complexes pite the observational fact that such crust has never been (Condie, 1981a) led to ensimatic subduction-related models identified and that even the oldest preserved greenstone that were the main reason for speculations that the present assemblages contain evidence for shallow-water deposition Wilson-cycle operated since the earliest Archaean (Burke and the presence ofsialic components (e.g. Hickrnan, 1981; et al., 1976; Burke, 1981). Archibald et al., 1981). J.F. Wilson (pers. comm.) has now Much fieldwork and more refined geochemical and also documented first direct evidence for sialic basement geophysical data have resulted in a drastic revision of the under the >3.4Ga old Selukwe greenstone belt in Zim­ above simplistic models (for summaries see Krôner, 1981h; babwe. Condie, 1981a; Glover and Greves, 1981) since it was recog­ ln some cases sialic gneisses show structural complexities nized that each greenstone belt has its own distinctive strat­ not recorded in neighbouring greenstones, and, for this igraphy, with the proportion ofindividual rock types varying reason, have been interpreted as basement. Rb-Sr isotopic widely between belts, that there is evidence for older granitoid data, however, revealed very low 87Srj86Sr initial ratios for crust inmost cases ar thatsialicmaterial wasat least nearby, such rocks and ages that are virtually indistinguishable from and that original greenstone basins had dimensions often those of tbe presumed overlying greenstones (e.g. Barberton greatly exceeding their present outcrop size (Schwerdtner, greenstones 3.51 ± 0.06 Ga, Ancient Gneiss Complex 3.56 ± 1980; Krõner, 1981("). ± 0.11 Ga; Barton, 1981; Pilbara lower greenstones 3.56~ Of considerable importance are the discoveries that felsic - 3.45 Ga, Shaw batbolith 3.42 ±0.03 Ga; McCulloch, volcanic associations may appear low in the greenstone quoted in Hickman, 1981). ln view of the long-standing stratigraphy and may be overlain by, or intercalated with, dogma that rocks with "prirnitive" Sr isotopic ratios must komatiitic lavas (Schulz, 1980; Bariey. 1981; Hickman, be regarded as juvenile additions to the crust and thus 1981), that there is abundant evidence for shallow-water cannot have appreciably older crustal parents (Moorbath conditions during initial greenstone deposition (Dunlop. 1975, 1978) these granitoid ages were interpreted as primary, 1978; Lowe and Knauth, 1977; Eriksson, 1981) and that thus overruling the above structural evidence. terrigenous sediments appear throughout greenstone se­ As is demonstrated below the isotopic criteria are open quences (Archibald et al., 1981). Barley (1981) found that to reinterpretation in the light of recent work, and many calc-alkaline volcanics and volcaniclastic sediments are so-called "juvenile gneisses" may have been in existence interlayered with tholeiitic basalts in the early Archaean prior to greenstone generation. Warrawoona Group of tbe Pilbara Block, and that there Modern geochernical data also contradict the earlier view is neither a genetic relationship between the tholeiitic and of ensimatio greenstone evolution since neither the tholeiitic calc-alkalinerocksnor a consistent trendtowardmoreacidic nor the calc-alkaline suites are strictly identical to modern composítion in the evolution of the entire sequerice. Such ocean floor and island are volcanics (e.g. Hawkesworth and results cast serious doubt on island-arc and marginal basin O'Nions, 1977; Grachev and Fedorowsky, 1981; Glikson, greenstone models (Tarney and Windley, 1981), particularly 1979; Condie, 198Ia). The pronounced bimodality ofgreen­ ir seen in conjunction with the occurrence of calc-alkaline stone successions strongly favours a rift-related setting for rocks in discrete, isolated centres rather than in ares (see tbe initial stage of greenstone belt formation. p. 19) and the generally broad rather than linear form of Oceanie such as mid-ocean ridge systems are not a many early greenstone basins {e.g. Hickman, 1981; Gee er al., plausible setting since they do not explain ·the appreciable 1981; Nisbet et ai., 1981I. sialic detritusfound in some greenstonebelt sediments. Also, The stratigraphic continuity of even thin units such as modero ocean crust experiences amphibolite-grade rneta­ cherts over distances up to 430km (Hickman 1981I, the rnorphism at crustal depths of about 5 km (East Pacific occurrence of evaporites and stromatolitic carbonates Rise Study Group, 1981) while greenstone belt successions (Dunlop, 1978), platform (alluvial) sediments (Eriksson. with apparent thicknesses of more tban 10 km display only 1981I and subaerial acid volcanics (Thurston, 1980; D.I. greenschist grade, thereby suggesting some thermal "shield- . Groves, pers. comm., 1982) as well as the considerable size ing" mechanism below their bases, believed to be conti- of some greenstone basins ali argue for intracontinental nental crust. . evolution of most greenstone belts on older sialic crust Early models assumed tbat greenstone belts follow a of remarkable stability and of dimensions exceeding several pattern from ultramafic-to-mafic-to-siliceous volcanics, sue­ hundred kilometres (Krôner, 198Ie). This is also in accord ceeded by a suite of granitoids evolving from Na-rich to with the palaeomagnetic data and casts doubt onthe are­ K-rich types, and that this process should reflect "matur­ -microcontinent collision model for the Archaean. ing" of intraoceanic ares to microcontinents (e.g. Glikson, 22 Revisto Brasileira de Geocíêncías, Volume 12 (1~3). 1982

1976). However, the cyclicity concept has already been lines" may be erroneous or at least imprecise. For the Rb-Sr shown to be of only local significance, and careful modem systern such calculations may give ages too low or too high, dating has demonstrated that both extrusive and intrusive while for the Sm-Nd system the model ages are always too rocks are broadly contemporaneous (e.g. Barton, 1981; low (DePaolo, 1981). DeLaeter et al., 1981h thereby again implying that the latter Second, most evolutionary diagrams tacitly assume that . are unlikely to represent melting products of the former. the isotopic ratios measured on a few samples of a given Large-scale melting of lower greenstone units to form gran­ dated rock unit retlect the average ratio for the parent and, itoids and in order to explain the missing primeval oceanic therefore, isotopic growth lines with corresponding slopes crust is an implausible mechanism and Bettenay et ai. (1981) are drawn in calculating maximum model ages. Hart et at, succintly demonstrate that greenstone belts are probably (1981) have recently demonstrated that this assumption may shallow structures restricted to upper crustal layers and that be invalid, and that significant amounts of Rb and U move the average crustal composition underneath granitoid­ upwards in the crust during high-grade dehydration events, -greenstone terrains probablycorresponds to siliceous (band­ thus profoundly changing the isotopic systematics. Recent ed) gneisses with intercalated cornponents derived from studies show that CO 2 may be a major vapour phase during older supracrusta) sequences including minor greenstones high-grade metamorphism (Newton et ai., 1980) and may and, characteristically, shallow-water metasediments as be able to cause significant depletion of heavy REE in the well as early granitoid intrusives. Hart et ai. (1981) have dehydrated lower crust with complementary enrichment of given a superb demonstration of precisely this association Iight REE in higher leveis (Collerson and Fryer, 1978). from the centre of the Vredefort Dome in South Africa, Such REE mobility may therefore also affect model age cal­ believed to be a uniquely preserved "window" of the Early culations using the Sm-Nd system. Archaean lower continental crust. Hart et ai. (1981) demonstrated by combining several Given the scenario of fracturing of continental crust dur­ dating techniques, that upper crustaI granitic gneisses in ing continental breakup or for the formation of pull-apart the Vredefort Structure of the Archaean Kaapvaal Craton, basins, and assuming that mafic underplating is an important South Africa, derived their present Rb/Sr and U/Pb ratios process in this setting, the origin of Archaean bimodal vol­ from a major lower crusta) high-grade "reworking" event canicsuites may now be broadly compared with the evolution about 3 Ga ago. ln spite of the "primitive" 87Sr/"'Sr ratio ofmagmas in modem rift environments wherc both juvenile ofO.7019 ± 2 and an average Rb/Sr ratio ofO.35 established mantle-derived melts and anatectic melts derived from par­ in these rocks at that time (and therefore implying a juvenile tial melting of previously solidified mafic crust play a origin in terrns of conventional interpretation) they can in . crucial role in the petrogenesis ofobserved bimodal associa- fact be shown to originate from ca. 3.5 Ga old sialic crust tions (e.g. Oskarsson et ai., 1979). with primary Rb/Sr of ca. 0.1. 1t is contended that such Hanson (1981) argued that large volumes of dense, iron­ cases are the rule and not the exception in Archaean terrains -rich melts representing low degrees of mantle melting and that reliable crustaI evolution models must therefore could have existed near the Archaean crust-mantle boun­ be based on combined studies of several isotopic systems dary, since there is evidence for such more recent mande (e.g. Moorbath and Taylor, 1981). Many greenstone-gran­ melting from the study of mantle xenoliths. -Sucb melts. itoid-gneiss relationships and models will have to be re­ too dense to rise to the surface, would tend to form imrnis­ -evaluated if the concept of world wide massive juvenile cible liquids, separate gravitationally, andthen ascend into growth throughout the Archaean requires substantial re­ the crust to form either basalt and/or mafic tonalite, or to vision. solidify as dacite/rhyolite and/or granite. The models of Cox (1980) and Ewart et ai. (1980) likewise see the primary ORIGIN OF HIGH·GRAOE TERRAINS Archaean source for bimodal ritt-generated volcanics in underplated high-grade metamorphic assemblages support the concept rnafic magmas, and there is also a strong Iikelihood ofmelt­ of average gradients and average thickness of continental ing and mixing with cornponents of mafic lower continental crust between 30 km and 40 km as early as 3 Ga ago although crust during this processo A conceptual model has been there seems to be a tendency for average metamorphic suggested by Krõner (198Ia) and is further developed pressures to have increased through time (Grambling, 19s'1). below. Generation of isobaric or prograde pressure regimes toge­ ln summary, basement-cover relationships and internal ther with prograde temperatures have probably been the greenstone belt stratigraphies favour evolution of green­ cause for the generation of most granulite complexes in stone basins in rift environments on thick, relatively stable the lower crust and have been ascribed either to magmatic continental crust. crustal accretion mechanisms (Wells, 1980; 1981) or to tectonic thickening through crustaI interstacking (Bridg­ CRUSTAL REWORKING ANO JUVENILE CRUS­ water et ai.. 1974) or (Dewey and Burke, TAL GROWTH The demonstration that low Sr-initial 1973). ratios for a rock body do not automatically imply a The first model is based on the concept of massive crustal juvenile mantle-rclated origin but may also be found in accretion through juvenile mantle-derived magmas (e.g. suites derived from parents with extended crustal prehistory Moorbath, 1978) at Cordilleran-type continental margins must be regarded as one of the major recent achievements and is discussed in detail by Tarney and Windley (1977) in isotope geochemistry (e.g. Collerson et ai.; 1982; Hart and Wells (1981). The composition and age of Archaean et al., 1981) and may significantly change our understanding high-grade terrains with their ubiquitous metasedimentary of crustal evolution processes. assemblages as well as their size and dimension, however, First, it is now demonstrated without reasonable doubt render this model unlikely for the early Earth since it implies that the mantle was inhomogeneous with respect to most a predominance of[uvenile sialic rocks in granulite complex­ isotopic systems since at least 3.6 Ga ago and that simple es, a feature not seen in presently exposed Archaean terrains model age calculations based on so-called "mantle growth (Katz, 1977; Hart et al., 1981). Revista Brasileirade Oeocténctas, Volume 12 (1·3), 1982 23

Continental collision is an elegant and plausible mecha­ a plate thickness of at least 200 km (Davies and Strebeck, nism for the generation of high-grade sialic rocks and pre­ 1982). That such thick Iithosphere was already present in sently converts much of the old Indian upper crust that the Archaean is substantiated by the occurrence of kim­ has disappeared underneath Asia over the last 30 Ma to berlite-derived diamonds in the ca. 2.60a old Witwaters­ high-grade assemblages that may reappear at the surface rand conglomerares of South Africa (Hallbauer et ai., 1980). after substantial erosion following crustal thickening. The Jordan has argued repeatedly (1979,1981) that subcrustal process, in the present Wilson-cycle regime, is restricted to Iithospheric growth results from basalt extraction in fertile, regions commonly classified as "orogenic" and is the final undepleted mantle, leaving a peridotitic residue that is less product of mountain-building. Since Archaean greenstone dense than the starting material. This depleted, 1ight residue belts are no longer considered to be ancient analogues of would therefore tend to float on undepleted mantle and modern orogens (Anhaeusser et ai., 1969) it is unlikely that would tend to be incorporated in the overlying lithospheric ocean closure and continental collision in the present sense piate. Repeated continental basaltic volcanism would cons­ were responsible for the large areas of high-grade gneisses titute primary episodes ofbasalt extraction from the mantle in aneient cratons. and would thus thicken the continental lithosphére from Models proposing plane-strain thrust restacking of thin­ below, resulting in a relatively buoyant, thick subcontinental ned or mechanically weakened continental crust as present­ root that is not likely to be the site offurther large-scale melt­ Iy observed in the Alps (Hsü, 1979; Dewey, 1982) do not ing. An important implication of this persuasive model is require the previously discussed conditions and, in my opi­ that it explains cratonization of continental crust as a direct nion, account for the rock assemblages and structural fea­ consequence of early formation of a thick lithosphere (Jor­ tures observed in ancient granulite complexes. The Limpopo dan,1981). belt of Southern África, with several granulite-forming The above vertical lithospheric growth model does not events, the oldest dated at 3.80a, has been cited as an necessarily depend on the operation of continental margin example ofthis type (Coward, 1976; Barton, 1981) and much subduction processes in the Archaean but accounts for of the Archaean development of the North Atlantic craton crustal stabilization through widespread and cxtensive basalt is ascribed to this process (Park, 1981). extraction from the mantle. Sueh extraction may be seen in Crustal thinning through faulting and development of the form of massive crustal underplating as discussed in shallow continental basins is shown by many Archaean previous sections and the generation of greenstone belt supracrustal sequences found in high-grade gneisses and volcanics as well as granitoid intrusives from this underplated may have preceded thrust- or wrench-faulting that took reservoir and from the mantle below. The areally extensivo place along detachment surfaces sometimes reaching to the worldwide crustal accretion event at the end ofthe Archaean base of the crusl. It is speculated that such motion is per­ has almost certainly resulted in substantial cratonization as haps reflecting major rearrangements of crustal segments revealed by thick, late Archaean to early Proterozoic intra­ within relatively large continental piates. The thrusting me­ continental volcano-sedimentary successions some ofwhich chanism, coupled with intense deformation and granitoid may constitute "failed" greenstone belts (Hegner et ai., 1981). intrusions, transferred the once shallow-level assemblages Continentallithosphere that was not sufficiently thickened in to deep crustal regions where they recrystallized and even­ the Archaean through mantle-derived magmatism remained tually returned to the surface after isostatic equilibration tectonically mobile and was therefore substantially modi­ of the thickened block. fied, in some cases beyond recognition,during Proterozoic Katz (1976) ascribes the generation ofArchaean granulite and later deformation. terrains essentially to large-scale intracratonic transform The "subcontinenta1 lithospheric root" concept may also motion along zones of weakness demarkated by sediment­ be able to explain the observation that, in some cases, iso­ -filled aulacogens, but other scenarios are equally cornpa­ topic heterogeneities have persisted in local closed systems tible with the available data as suggested above. in the upper mantle under old cratons since Archaean iimes Common to ali these suggestions is that horizontal tec­ (Kramers, 1979) since this material has obviously not par­ tonics probably played a decisive role in the generation of ticipated in later re-equilibration with undepleted mantle Archaean granulites but that Cordilleran or continental through convection or significant melting. lf this interpret­ collision settings were not required. ation is correct, model age calculations for rocks derived from old, depleted and heterogeneous subcontinenta1 man­ ARCHAEAN L1THOSPHERIC THICKNESS AND tle should be treated with caution. CI'lATONIZATION The preservation of Archaean lower There are several ways is which Archaean lithospheric crustal assemblages with mineral parageneses suggesting growth by underplating can be envisaged. The Jordan (1979, normal average geothermal gradients despite an appreciably 1981) model requires a heat source in the mantle to cause higher global heat Ilow at that time has led to the suggestion melting and basalt extraction. Under continental (and that the ancient oceanic lithosphere was thin (20-30km) modem oceanic) arcas such sources are generally seen in whereas continental lithosphere under the cratons must hot-spots or mantle-plumes (Hofmann and White, 1982) have been fairly thick, thus insulating the continental crust and, in some cases. these thermal anomaliesmust have been against heat Ilux from the convecting mantle (Davies, 1979). of considerable dimensions and duration as judged by the By analogy, Wells (1981) has suggested that the survival voluminous continental basalt suites such as the Early of 30-40km thick Archaean crust implies the presence of a Proterozoic Ventersdorp and Fortescue Supergroups of stable subcrustal root zone to continents, not less than South Africa and Western Australia or by the Mesozoic 200km thick , that probably grew during crustaI formation Karoo volcanism of Gondwanaland, and thickening. These concepts are supported by heat Ilow Since mantle plumes are major agents for transferring heat data from Archaean cratons (Oxburgh, 1981), by seismic ScS from the deeper mantlc, they may have been more common wave travei-time variations (Jordan, 1979, 19811 and by in the Archaean than today, perhaps forming planar arrays geotherms deduced from kimberlite pyroxenes that require (Hanson, 1981). They may ultimately have been responsible 24 Revista Brasileira de Oeoctenctos, Volume 12 (1-3), 1982 for greenstone belt formation and granitoid magmatism in stabilizes the overlying crust and contributes to itscontinued the crust while the subcrustal Iithosphere gradually (or survival, episodically) thickened and thus contributed to continental Regions ofthe early crust that do not experience extensive stabilization. and continued volcanism are likely to maintain relatively An alternative mechanisrn to cause lithospheric under­ thin lithospheres and are easily dragged down back into the plating may be seen in the pro posai by Ringwood (in press) mantle, while continental segments over plumes grow rapid­ whereby subducted oceanic Iithosphere sinks into the mantle Iy and tend to survive longer, to depth of about 600 km where the depleted basaltic com­ The evolution of Iceland may be an appropriate scenario ponent is denser than the surrounding mantle and thus for the next stage of continental difTerentiation (Krõner, keeps sinking while the former harzburgite part of the 1981d) since the present heat tlow there is about 200'mW/m2 slab becomes buoyant and rises diapirically. This material, (Smith, 1981). Continued melting in the mantle produces being less dense than undepleted mantle just as in the Jordan further tholeiitic magmas but a large portion does no longer model, underplates the lithosphere and causes thickening. reach the surface of the growing crust but accumulates at ln this model old era tons have thick Iithosphere sirnply its base and forms the reservoir for bimodal volcanic suites because they have been subjected to this type of underplat­ and the first rising tonalite-trondhjemite diapirs (Fig. 4). ing for long times. As the volcanic pile thickens its lower parts and the under­ Ringwood (op. cit.) further suggests that the process from lying ultramafic protocrust melt and form a vast reservoir subduction ta thermal equilibration with the mantle and for further differentiated magmas. It is speculated that the subsequent diapirism and underplating takes place on a dense residue remaining from these processes may become timescale of 1-2Ga. For the Archaean this may mean that decoupled from the crust through viscous drag to allow the extensive global rnagrnatism and continental stabiliza­ recycling into the convecting mantle. Continued growth of tion at 2.7-2.5 Ga ago may be due to massive underplating this kind gradually reduces the overall densíty of the evolv­ as a resuit of substantial ocean crust recycling 1-2Ga ing crust which is now less than about 3 cni'li! and, through earlier, i.e. at a time when most models predict that con­ partial melting at its base, establishes a two-layered structure tinental crust formation began. with the lower part transformed into high-grade assemblages ln'summary, thick continental roots under old shields and the upper part consisting of both primitive and diffe­ rnay have been responsible for the preservation of ancient rentiated volcanics and granitoid intrusives. lower crust and may be the prime reason for cratonization. Eventually a situation may be reached where a mixture Current models in support of this concept see the cause of of granitoid intrusives and differentiated volcanics consti­ thick subcontinental Iithosphere in underplating and the tutes the surface of this now predominantly sialic early operation of mantle-plumes, upper crust. If it emerges above sealevel, erosion ensues and early sediments with strong calc-alkaline affinities that retlect A MODEL FOR CONTINENT-FORMATION Fol­ thecomposition oftheir parents are deposited. The ea. 3.8 Ga lowing rapid cooling of the originally melted surface of the old Sand River tonalites ofthe Limpopo belt could be exam­ Earth after core formation (Ringwood, 1979), a very thin pies of this type, and I visualize that the crust had evolved crust composed chiefly of uitramafic rocks (Condie, 1980; to this stage by about 4Ga ago. However, no extensive sedi­ Nisbet and Walker, 1982) forms small but coherent rigid mentary basins formed at that time since only small parts segments Iike the crust on lava lakes of present active vol­ of the microcontinents emerged significantly above sea levei canoes. These segments frequently break up again and are because vertical continental accretion was compensated by recycled back into the underlying melted mantle because of fusion at depth and, perhaps, sideways tlow by ductile their high density and viscous drag at their bases resulting spreading of the lower crust (cf. Hess, 1962). from convection below. The breakup and recycling is further The scenario postulated above guarantees survival of enhanced through heavy meteorite bombardment. these early continental nuclei which are now too Iight to As cooling progresses the early crustaI segments thicken be recycled and whose subcrustal roots gradually thicken and komatiitic to basaltic volcanism becomes widespread and thus "shield" the overlying crust against extensive dís• as outlined by Condie (1980) and Nisbet and Walker (1982). ruption. Neighbouring Iithospheric segrnents, where mantle An Early Archaean heat tlow of 200-250mW/m2 implies melting and volcanism were insufficient to form sialic volcanic activity over virtually the entire surface ofthe early nuclei, remained comparatively thin and dense, i.e, oceanic Earth (Smith, 1981). It is unlikely that this volcanism took in character, and were sites of drag-induced "sag-subduc­ place along linear zones comparable to present oceanic tion" (Goodwin, 1981b; Krõner, 198Ia). Such subduction ridge systems. Rather, the closely spaced convection cells may have been caused by meteorite impacts or drag above in a vigorously convecting mantle would suggest a system downgoing mantle currents (Fig. 4). Whatever the mecha­ of thermal plumes (Krõner, 1981a; Lambert, 1981; Smith, nism, none of this early "oceanic" crust has survived while 1981 ). the contemporaneous sialic differentiates are considered to The early ultramafic crust with overlying komatiitic vol­ form the oldest preserved crustal remnants. canics was still too dense to survive and was completely recycled back into the hot mantle at comparatively short time GREENSTONE BELT EVOLUTION Accepting the rates. At about 4 Ga ago mantle temperatures and heat above highly speculative model for early crustaI differentia­ tlow had decreased sufficiently to permit komatiite-domi­ tion, and given the constraints of field relationships as mated crust that now had a longer survival rate. Continued detailed in previous sections, the following development is volcanism then had the chance to build volcanic centres that envisaged for greenstone-granite-gneiss terrains (Krõner. grew rapidly in height. Growth of the volcanic pile through 1981a, 198Ic): partial melting of the subcrustal mantle immediately below By about 3.6 Ga ago continental crustal segments of di­ results in basalt depletion and cooling ofthe depleted region mensions exceeding several hundred km in size and of so as to form a minitectosphere (Jordan, 1979) that further sufficient thickness and rigidity to support large basins were Revista Brasileira de Geócíêncies, volume 12 (I~3). 1982 25

VERTICAL SAG~SUBOUCTION THROUGH NEGATIVE BÜOYANCY ANO CONVECTIVE SHEAR-STRESS , . ~··"'''N ...... - ...... , ~ -, , , -, \ " -, \ \ \ MAFIC TO ULTRAMAFIC ~/. ; / / '" CRUS! ,- ~ , ' / V V -~'V 25 V V V V '\2)lV V V V~ (V~í:V V VV V V' V VVVV V COMPLETELY" v=)rMELTED . \/ V • MANTLE V V \, VVVV j VV V V VV 100 s := ~ ~ PARTLY MELTED c, w o i 1 i MANTLE 1

500 ------SOL!O MANTLE

FiKIII'l' 4 - Símplifled, schematic and highly specukuíve model for the origin and growth oflthe earliest continental crusl tmodijied after Condie, 1980). For expianatíon see text

in existence as demonstrated by the evolution of the Pilbara and lead to the production of calc-alkaline suites as well Block in western Australia (Hickrnan, 1981). For the late as crustal-derived granitoids (Fig. Se). Archaean greenstone basins similar or even larger dimen­ If the crust becomes sufficiently thin during stretching sions can be inferred. These crustal segments are subjected and sagging it may eventually break apart with the formation to crustaI attenuation or fissuring above mantle plumes or ofsmall ocean basins where the continental input is minimal "hot lines" (Hanson, 1981 I, thereby generating oval or (Fig. 5D). The mechanism of rupture proposed here may be elongate basins or sharply bounded, fault-controlled graben analogous to the formation of modem rift systems and is systems (Fig. 5A). These basins collect shallow-water sedi­ described in detail by Keen (1980). It is also possible that ments as well as mafie to ultramafic lavas produced by a strike-slip induced pull-apart basins as a result of trans­ high degree of melting in the subcrustal mantle (Fig. 5B). current movements following crustal distortion may develop OnJy a small proportion ofthese melts can rise to the surface, into greenstone depositories. Platt (1980) suggests large­ however, and cxtrude as Mg-rich lava while a much larger -scale ductile wrench-faulting as a possible mechanism. volume remains near the base of the crust to become the "Oceanic" greenstones as in Fig. 5D would have to' be source for bimodal volcanic assemblages and early tonal­ founded on ancient layered oceanic crust. Such crust has­ itic-trondhjemitic intrusives. Further stretching in the lower not been convincingly demonstrated from any Archaean crust causes continued block rotation along listric faults greenstone belt, but there are many belts whose bases are during basin widening, perhaps anologous to the Basin-and­ not exposed or whose basal contacts are concealed by later -Range Province of the western USA. ln this way exceptio­ tectonism or granitoid intrusives. nally thick sequences ofvolcanic rocks may be accumulated, Closure of the greenstone basin by horizontal shortening accompanied by subsidence in the proto-greenstone basis causes early recumbent folding and shallow thrusts (Fig. SE) and subsequent rise of the crust-mantle boundary. Even­ that may lead to stratigraphic repetition and obliteration tually the spreading and sagging process may lead to of original depositional relationships (DeWit, 1982; Stowe, the decoupling of lower crustal blocks together with dense 1974; Platt, 1980). Many of these early flat structures were residues of earlier underplated material, with further pro­ later modified and frequently steepened during the emplace­ duction of large volumes of basaltic rocks. At this stage ment of granitoid batholiths that caused further horizontal crustal fussion and crust-mantle mixing become important shortening in the greenstone pile. 26 Revista Brasileira de Geocíências, Volume 12 (1-3), 1982

The marginal basin model for lhe evolution of Archaean continuous stratigraphy in some regions, Furthermore, greenstones as proposed by Tarney et ai, (1976) fails lo available age data suggest that most greenstone belts and account for many of the observed greenslone belt relation­ associated granitoids had evolutionary histories extending ships (e,g, Groves et ai" 1978; Platt, 1980; Hickman, 1981; over ta, 200 Ma or more while, on the average. Recent Gee et ai" 1981; Archibald et ai" 1981), in particular lhe marginal basins have relatively short lifetimes of 40 Ma or apparently large greenstone basins and the remarkably less (Talwani and Langseth, 1981),

A FRACTURE- ANO GRABEN SYSTEMS, EARLY INTRACONTINENTAL BASINS ./' DEVELOPING INTO GREENSTONE BELTS ~ '"

~:---_----:x TH lN ARC",'" LITI

SHORT WAVELENGTH PATTERN OF ARCHAEAN CONVECTION SYSTEM lN THE ASTHENOSPHERE

B c PRIMITIVE I«X'Vl.TI ITIC TO TI-OLEJ me VOLCANISM ANO TERRIGENOUS SEDIMENTS EARLY GRAN!TO IDS BII"lJDAL VOLCAN1SM + 1+ UPPER " + + CRUST" + CRUST/Wi.NTLE MIXING ~ "*" + + +- _C~~S!/MtlNTLE R + 2:. __ --- --::,-:,---:,:::--::t...------MIXII\\S... ~ ~ I --HIGH-GRADE------_;--:;;"'~.i~,;,::~ DUCTi LE SPREAD ING ~ LO\'iER CRUST I- \ 5:! UNDERPLATED DENSE CRUSTAL THICKENING I PERIOOTITIC ~ THROUGH UNOERPLATING S'

HOT AND THIN SUB- CRUSTAL LITHOSPHERE \/

D E CLOSURE OF GREENSTONE BASIN ANO DEFORMA.TION

+ + + + LATE K-GRANlTES, lARGELY CRUSTAI...- .... DERIVED "Ç" ::...---~--~-_.._-+- -~ ((+_..--'-I'f:;:. BASEMENT ROORKIf'{; LOIrI1:R - --- ~ = DURII\\S DEFORMo\TIOO CRUST ~GRAN!TOID- SHEETS -...... • ~ ULTfW>\I\FIC INTRUSIOOS, SHEARlm, L(M-ANGLE THRUSTING &\BBRO-ANORTHJSITE ANO CRUSTAL THIC!<.ENING LAYERED C<»1PLEXES

COOLI NG OF SUBCRUSTAL LITHJSPHERE AFTER DECAY OF f/AATLE PLLME

Figure 5 - Schematíc cross-sections showtng suggested evolutíon ojearly Archaean Iithosphere in response to smatí-scate convection panem ín the mant!e (A) anil vartous stages in the development ola greenstone belt ,(B~D). For explanatíons see text. Modíjíed after Kroner (l98/a) Rtl'isla Brasileira de Geocilncias, Volume 12 (1-3), 1982 27

The Archaean high-grade terrains represent largely pre­ intense. The 3Ga old Pongola sequence of Southern Africa -greenstone sialic crust, together with predominantly shal­ and the 2.4-2.5 Ga old Dharwar suites are examples of this low-water sedimentary deposits and granitoids, produced kind and have been termed "failed greenstone belts" by during various stages of incomplete crustal fissuring and Hegner et ai. (1981) since they may reflect abortive attempts subsidence. Crustal interstacking moved much of this of the stabilized crust to break apart. material into lower crustal leveis with transformation of Elsewhere such: sequences were deposited on less stable granitoids and supracrustal rocks into high-grade metamor­ erust and formed successions that are somewhat inter­ phic assemblages. mediate between greenslones and deposits in Phanerozoic Periodic subcrustal mantle decoupling of dense, heavy "geosynclines" in thatthe proportion ofmore differentiated residues may provide the mechanism for ductile flow and volcanics including andesites as well as immature sediments horizontal tectonics in Archaean lower crust. Combined is appreciably higher than in most Archaean suites (e.g. with "thinned-crust collision" this may explain tectonic Condie, 1982b). The Birrimian of West Africa, the Jacobina mixing through interstacking (Park, 1981). It is suggested and Minas Groups of Brazil and the Huronian of the Cana­ thatthis type of lower crust underlies most, if not ali, gran­ dian shield may be examples of this type, ite-greenstone terrains in the ancient shields as suggested Although clearly associated with older continental crust by the crustal profile in the Vredefort Dome, South Africa the evolution of these sequences is still a matter ofconsider­ (Hart et al., 1981). able debate since they were deposited in elongate, well­ By the end of the Archaean large crustal segments of at -defined basins and experienced a tectonic history that least subcontinental proportions were in existence (Krõner, resembles modern collision belts of Alpine- or Himalaya­ 1981c) that moved around independently and with different -type or active continental margin evolution of Cordilleran- average minimum velocities comparable to those of today. -style. The whole discussion is now receiving an added The remaining questionsofwhy no Archaean ocean crust cornplexity in that it is likely that exotic terranes that played and no typical continental slope deposits are preserved a decisive role in the accretionary history of Phanerozoic 'remam enigmatic. As for the former it is speculated that Cordilleran belts (Coney et ai., 1980) have also existed in near-vertical subduction in the ancient oceans and the Proterozoic times as already suspected from palaeomagnetic comparatively high density of largely komatiitic ocean crust data of the Canadian shield (Irving and McGlynn, 1981) (N. Arndt, pers. comm., 1982) prohibited obduction. If and of the late Precambrian accretion complex of':Arabia "sag-subduction" caused complete recycling ofthe Archaean (Krõner, unpubl. data). oceanic lithosphere the viscous drag forces in the ancient Condie (I 982b) has reviewed the composition of Early upper mantle must indeed have been considerable (Hargra­ and IMiddle Proterozoic successions and concludes that ves, 1981). Vertical subduction involving no mantle wedge the period before ca. 1.7Ga was dominated by stable con­ would also explain the scarcity ofandesites in the Archaean tinental margin or intraeratonic basins while later assem­ (Barker et al., 1981) and would largely rule out the marginal blages suggest more mobile environments with frequent basin model for greenstone evolution. continental rifting and the local onset of buoyancy-driven The missing widespread turbidites that should be expected subduction. Suggestions for Middle Proterozoic convergent from the margins of Archaean continents may either be due plate boundary tectonics come from the Wopmay orogen to a considerably lower elevation contrast between conti­ and the Circum Superior belt of the Canadian shield, the nental surfaces and aneient ocean basins as expected from Svecofennides of the Baltic shield and the Mid-continent the relatively young average age of ocean crust atthat time region of North America (for revíewssee Medaris, in press), and/or such rocks were losr through processes of "subduc­ while contrary, but lessconvincing, data suggestthe existence tion erosion" (Dewey and Windley, 1981) where the down­ of a stable supercontinent throughout the Proterozoic (e.g. going slab drags down most of the oceanic and trench sedi­ Piper, 1982). ments as well as the underside of the forearc crust and thus The similarity ofsome early Proterozoic associations with inhibits the development ofsubduction-accretion complexes modern "geosynclinal" assemblages has resulted in strongly like at the Japan, Mariana and Middle America trenches divergent interpretations where one opinion holds that the (Talwani and Langseth, 1981). full modern Wilson-cycle was in operation while the other opinion favours "ensialic" tectonics with relatively little relative motion of crustal segments now bordering these EARLY PROTEROZOIC TECTONICS A period of mobile belts (see Medaris, in press, for full discussion). comparative crustal stability following Late Archaean era­ These contrasting points of view have now largely been tonization through voluminous granitoid magmatism eh a­ reconciled by the general acceptance of considerable sialic racterizes most old shields in the Early Proterozoic. ln underplating (A-subduction) during orogeny as presently southern Africa considerable crustal stability was already documented in the , and by the recognition of achieved some 3 Ga ago as documented by still little-deform­ plate tectonicians that several Phanerozoic orogenic beIts ed volcanics and sediments that were deposited in in­ such as the Alps and Pyrenees appear to have resulted from tracratonic basins (Krõner, 1981a,c) while elsewhere this largely intracontinental deformation whereby severely at­ stability is documented at about 2.6-2.4 Ga ago by such tenuated continentallithosphere was shortened and thicken­ sequences as the Fortescue in Western Australia and the ed by a variety of mechanisms ranging from thrust resta­ Dharwar in Peninsular India. Ali these units developed in cking to transform shearing (Dewey, 1982). continental rift environments on thick , older granitoid Dewey (1982) has discussed basin formation, lithospheric crust and their subsequent deformation, if any reflects stretching and crust restacking while Krõner (1977, 1980, the degree of stability of the underlying craton. ln some 1981b, 1982) applied identical concepts in models for the cases the basin evolution closely follows the greenstone belt evolution of African Proterozoic belts. It is clear, therefore, pattern of the Archaean but ultramafic volcanics and gran­ that plate tectonics does not require complete plate separa­ itoids are less frequent and deformation -is markedly less tion prior to orogeny, and a whole variety of different 28 Revista Brasileira de Oeociências, Volume 12 (1-3), 1982 settings can now be imagined that do not follow the Wilson of the crust anil by radioactive self-heating after crustal cycle pattern, interstacking. Following the speculation of Bird (1978) that subcrustal The model is entirely compatible with the concept of mantle lithosphere may delaminate during orogeny, Krõner horizontally moving plates but differs from the Wilson cycle (in press) has developed a model for the ensialic evolution in that no wet oceanic crust is generated during basin of Proterozoic mobile belts that includes the concepts dis­ formation and none is consumed during orogeny. Instead, cussed above and that infers subcrustal mantle rather than dry subcrustal lithosphere sinks down but does not cause ocean crust subduction during horizontal shortening of calc-alkaline magmatismo previously attenuated continental crusl. This model is pre­ It is suggested that the majority of Early Proterozoic sented in Fig. 6 in a simplified formo It invokes rifting, mobile belts, particularly those originating between 2.1 and heating and stretching of the crust as a result of lithospheric 1.8Ga ago, evolved along lines as suggested above, i.e. thinning over a . This mechanism eventually without significant relativc motion of cratonic blocks, and leads to a "geosynclinal" basin entirely 1I00red by conti­ this is in line with the palaeomagnetic record (for reriews see nental crust. The rise of asthenosphere enhances gravitatio­ Krõner, 198Ih).Towards the end ofthe Precambrian modem nal instabilities in the old and dense subcrustal lithosphere Wilson cycle evolution begins to dominate global tectonics and, on fracturing following crustal stretching, results in as documented by the frequent occurrence ofophiolites and spontaneous delamination. Hot asthenospheric material other collision and accretion signatures (Krõner, in press). rises to take the place of the detached and sinking litho­ spheric slab, thereby inducing A-subduction and intersta­ CONCLUDING REMARKS Crustal evolution and cking of continental crust. The much thickened crust is the style of global tectonics were dominated by horizontal partially melted at depth, intruded by syn- and post-orogenic motion since the formation of the first continental seg­ granites and finally uplifted and eroded to its present levei ments about 4 Ga ago. Since average minimum velocities of exposure. Episodic thermal anomalies during orogeny of the continents do not seem lo have changed significantly are caused by the rise ofasthenospheric magmas to the base over the last 3.5 Ga, it may have been the changing style of

( A ~'ií + ... + ., + ..... + + + +-- Continental crust + -- Subcrustal /( lithosphere .-, ~ ... \ / Asthenosphere I I I/

B + -- +"~+ + + /r .» ~ -- -:Mellmg ~ l~/

C ~ --:r;, .'L-.W?f __ + + + + + ------> -----;---- -" - »>: ,/ ,/ = ,/ ,/ ,/ ,/ ,/------,/ -"

,,'/ . /1//_/ " " , Figure 6 - Cartoon deptctíng threestages ín the evolution ofan "ensialic" foldbelt through ríftíng, lithospheric stretchíng, delamtnation ofmantle fithosphere and crustal interstacking or A-suhàuction causíng horizontal shortening and orogeny. (From Krôner. in press) Revista Brasileira de Geocilneias. Volume 12 (1M3), 1982 29

crustal interaction that determined non-uniforrnitarian de­ Acknowledgements The West German Research velopment towards modern plate tectonics. At present, many Council (DFG) has funded most ofthe research and traveI plate tectonicians still see models of intracrustal Archaean that 100 to theopiniorrs expressedin this paper, and Ialso ack­ evolution as advocated in this paper as idiosyncratic, non­ nowledge the stimulating discussions wlth many colleagues -uniforrnitarian mechanisms (Windley, 1981 I, but l feel con­ during the activities of fGCP Project 92 and at lhe Pretere­ fident that the final acceptance of ensialic orogeny as just zoic Symposium in Madison, Wisconsin, in 1981. The one form of the complex interplay of rigid piates (Dewey. comments of N.T. Arndt. A.l. Baer, D.1. Groves and I. 1982)demonstrates the tlexibility of the plate tectonic con­ Lambert are appreciated. This review is a contribution to cept whose expression in the rock record has left evidence IGCP Project N.' 92 and to the new Lithosphere Project. for a unidirectional development that we are still unable to decipher in detai!.

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