The Archaean of Brazil

Earth-Science Reviews, 17 (1981) 31--48

31

Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

The Archaean of Brazil EBERHARD WERNICK Dept. Mineralogia e Recursos Minerais, Instituto Geoci$ncias e Ci$ncias Exatas, Rio Claro, SP (Brazil)

ABSTRACT Wernick, E., 1981. The Archaean of Brazil. Earth-Sci. Rev., 17: 31--48. The principal litho-structural units of the Brazilian Archaean terranes are characterized by mafic--ultramafic complexes, granulitic belts, greenstone belts and gneissic--granitic areas. The lithological, geosynclinal and structural aspects of every unit have been presented as well as their mutual relations. Data are still insufficient for a sure analysis of the current hypotheses about the evolution of Archaean terranes. Neither the genetic and chronological relations between the Atlantic and Goi~s granulitic belts and the associated gneissic--granitic areas have been explained, nor whether the greenstone belts implanted in them belong to one, two or even more generations. Important resources of gold, iron and manganese occur in association with these rocks. The large mafic--ultramafic complexes of the State of Goi~s may represent fragments of the primitive crust of our planet.

INTRODUCTION

After its consolidation at the end of the Middle Precambrian, the South American Platform suffered (during the Late Precambrian) various reactivation and regeneration processes, even within its cratonic nuclei. The identification of areas and nuclei of proven Archaean origin is a difficult task requiring great patience, because these areas and nuclei are restricted to geotectonic units that were to some extent spared from the tectono-magnetic processes of the Late Precambrian, such as the cratons and median massifs as well as those areas in which the basement of the Late Precambrian folded areas is exposed. Archaean complexes may be grouped into four lithological--structural units: (1) mafic--ultramafic complexes; (2) granulitic belts; (3) greenstone belts; (4) gneissic--granitic complexes. Due to the fact that only n o w major attention is being paid to the Archaean terranes and that geological data on Central Brazil and the Amazon region are still scarce, the present paper aims at a short presentation of the principal characteristics of the Archaean units, especially those of the eastern part of the c o u n t r y (Fig. 1; Table I). 0012-8252/81/0000--0000/$ 04.50 © 1981 Elsevier Scientific Publishing Company

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I

38 ° I

Fig. 1. Principal A r c h a e a n areas c e n t r a l - e a s t a n d n o r t h - e a s t S o u t h A m e r i c a : I = areas chiefly o f A r c h a e a n age ( G u r i e n s e a n d J e q u i ~ Cycles), l a t e r r e j u v e n a t e d or n o t ; I I = areas chiefly o f Early P r o t e r o z o i c age ( T r a n s a m a z o n i a n Cycle); I I I = areas chiefly o f Early P r o t e r o z o i c age, locally w i t h A r c h a e a n nuclei or r e j u v e n a t e d d u r i n g t h e late P r o t e r o z o i c ; I V = s e d i m e n t a r y a n d v o l c a n i c - - s e d i m e n t a r y covers related t o t h e r e a c t i v a t i o n a n d r e g e n e r a t i o n o f t h e S o u t h A m e r i c a n P l a t f o r m d u r i n g t h e Late P r o t e r o z o i c ( B r a z i l i a n o Cycle); V -- E s p i n h a ~ e b e l t a n d covers associated w i t h t h e C h a p a d a D i a m a n t i n a , possibly r e l a t e d t o t h e U r u a ~ u a n o Cycle; V I = L a t e P r o t e r o z o i c m o b i l e b e l t s a n d regions (Brazillano Cycle); V I I = old b a s e m e n t p r o f o u n d l y r e m o b i l i z e d d u r i n g t h e L a t e P r o t e r o z o i c ; V I I I = P h a n e r o z o i c s e d i m e n t a r y a n d v o l c a n i c - - s e d i m e n t a r y covers; I X = a p p r o x i m a t e c r a t o n b o r d e r s ; X = faults.

33 TABLE I Subdivision of Brazilian Precambrian, adapted to its principal cycles Late Precambrian (1800--570 m.y.)

Middle Precambrian (2600--1800 m.y.) Early Precambrian (>2700 m.y.)

Late Proterozoic

Early Proterozoic Late Archaean (3000--2600 m.y.) Early Archaean (>3000 m.y.)

Brasiliano Cycle (1100--570 m.y.) Uruaquano (Espinha~o) Cycle (>1500--1100 m.y.) Transamazonian Cycle (2000 ± 200 m.y.) Jequi~ Cycle (2700 ± 100 m.y.) Guriense (pre-Jequi~) Cycle

(>3000 m.y.)

LITHO-STRUCTURAL UNITS OF THE BRAZILIAN ARCHAEAN

Mafic--ultramafic complexes Mafic--ultramafic complexes constitute marked features of the Brazilian Archaean terranes. Among the best-known complexes, those of Barro Alto, Cana Brava, Tocantins (Niquel~ndia) and Caraibas are the most important. The first three, with elongated or boomerang-like forms, are situated in the State of Goi~s, associated with the granulitic belt of this state, east of the Central Massif of Goi~s. The Caraibas Complex, rich in copper (Barbosa, 1970), and situated in the region of the Curaqa river (State of Bahia) belongs to the S~o Francisco Craton and is composed of gneisses, granulites, schists, basic rocks, granites and migmatites with paleosoms of granulitic and mafic-ultramafic rocks. Among the complexes situated in the median massifs of the folded regions are the complex of Barra Velha in the State of Santa Catarina (Minioli, 1972) and that of Pien in the State of Paran~ (Girardi, 1974), both located in the median massif of Joinville of the southeastern fold belt. These complexes are bodies of c o m m o n occurrence in the oldest parts of the South American Platform. In the State of G o i ~ alone more than 150 are known; they have various sizes, ranging from very small units to large complexes hundreds of square kilometers across such as those of Niquel~mdia, Barro Alto and Cana Brava. The arrangement of the complexes is also variable; they occur as strings of bodies that are more or less aligned, or as irregularly distributed occurrences, apparently without marked structural control. The large complexes of Goi~s belong to the former type. The lithology is diversified and includes dunites, serpentinites, pyroxenites, websterites, hornblendites, gabbros, hyperites, chromitites, peridotites, lherzolites, anorthosites, amphibolites, norites and others. In general, the larger bodies exhibit heterogeneous and differentiated lithological characters (Mello Jr. et al., 1963; Berbert, 1970; Minioli, 1972; Souza, 1973; Girardi, 1974; Figueiredo et al., 1975).

34 The structural and textural features are of magmatic as well as of a metamorphic nature, with one or the other being dominant, this makes it rather difficult to incorporate them in the current litho-structural classifications of these rock types (Berbert, 1970). The complexes are associated either with gneisses, anatexites, migmatites and granites; with granulitic intercalations, or with chiefly granulitic areas. The metamorphic rocks of the complexes frequently show the same facies as the enclosing ones. There is also often a geochronological conformity. Where metamorphic and geochronological discordances occur, the contacts are tectonic (Souza, 1973); otherwise, the contacts are either abrupt or diffuse, graded or irregular. In the rocks surrounding such ultramafic bodies is evidence of lens-shaped mafic and ultramafic intercalations, even constituting the paleosome of migmatites. On the other hand, subordinate pieces of the surrounding rocks are also found enclosed in the larger mafic to ultramafic complexes. The mafic--ultramafic complexes, as well as the surrounding rocks frequently exhibit signs of remobilization and rejuvenation b y later events. These features are rendered more evident in the surrounding rocks by migmatization, remobilization, feldspathization, intrusion of granitoid bodies and lithologically varied dykes, recrystallization and isotopic rejuvenation. In the larger complexes, due to their lithological characteristics, such modifications are less intense and less well marked. Even so, one may observe polymetamorphic features, recrystallization, partial remobilization, metamorphic differentiation, intrusion of dykes and isotopic rejuvenation. The result of these processes is an obliteration, or a more or less intense partial deformation of the original features of the massifs. Important resources of chromium, nickel, cobalt and copper, as well as talc and asbestos occur in the mafic--ultramafic complexes. Due to the superposition of numerous geological events, it must be suspected that part of these mineralizations underwent concentration, dispersion or more or less complex modification. The Archaean age of part of the complexes has been determined through geochronological analyses by various authors. In Goi~s, the data reveal a large dispersion, between ages greater than 3000 m.y. and ages of the Late Precambrian Brasiliano Cycle (1000--500 m.y.). In Bahia, part of the Caraibas complexes showed an Archaean age, with remobilization in the Middle Precambrian. A similar situation may hold for the Pien Complex and some other areas of the same type. The origin of the complexes is still a subject of discussion. Almeida (1967) interpreted the complexes of Goi~s as part of a serpentine belt. Also Thayer (1970) saw, in their association with small peridotitic bodies, characteristics of mafic--ultramafic bodies of the mobile geosynclinal areas, similar to the Alleghenies and Dinarides. Fleisher and Routhier (1970) consider them to be differentiated stratiform massifs, comparable with the Stillwater Complex; Berbert (1970) and Figueiredo et al. (1975) emphasize that they exhibit

35 characteristics of differentiated stratiform massifs as well as of alpine intrusions. Girardi (1974) considered the Pien Complex to be an alpine-type intrusion of synorogenetic penetration. The emplacement of complexes partially referrable to the Early Archaean must have occurred in a thin sialic crust (Armstrong, 1968; Condie, 1973). Such a situation favoured the abundant infiltration, as intrusions, after the model of Glikson (1971), as well as the tectonic linking of mantle material. The intrusions became subject to a more or less penetrating metamorphism in conditions of amphibolitic to granulitic facies, either on the occasion of their emplacement, or by immediately subsequent metamorphic cycles. On the other hand, taking into account the great ages obtained for some complexes of Goi~s, comparable to those of lunar rocks, Cordani and Hasui (1975) suggested that the complexes could be genuine representatives of the primitive crust of our planet and formed by mantle differentiation some 4000 m.y. ago. Based on the analysis of the bodies of Barro Alto and Tocantins, the organization of these complexes is as follows (Figueiredo et al., 1975): (1) Mafic zone, essentially of gabbroic composition, with norites, hyperites, and smaller quantities of pyroxenites, chiefly bronzitites, with a thickness of 3 km in Barro Alto and 4.5 km in the Tocantins Complex. (2) Ultramafic zone, composed of peridotites (harzburgites and lherzolites}, dunites, pyroxenites (bronzitites, enstatolites, websterites) and smaller intercalations of gabbroic rock and chromites, the latter being absent in Barro Alto; thicknesses are 2.7 km in Tocantins and 1.1 km in Barro Alto. (3) Anorthositic and central zone, the first occurring in the Barro Alto Complex, with a thickness of 6.5 km and almost entirely composed of anorthosites; and the second characterizing the Tocantins Complex, about 8 km thick, including norites, hyperites, gabbros and anorthositic gabbros. (4) Top zone, 5 and 3.7 km thick in Tocantins and Barro Alto, respectively, composed of various proportions of norites, gabbros, pyroxenites, anorthosites and anorthositic gabbros, with, in Tocantins, small and discontinuous lenses of ilmo-magnetites and vanadiferous magneto-ilmenite. Figueiredo et al. (1975) concluded that the two complexes of Goi~s resulted from the differentiation of a saturated basaltic magma, with a high alumina content and a tholeiitic affinity. Granulitic rocks

Granulitic rocks are generally present in Brazilian crystalline areas as inte:calations of varying size in chiefly gneissic--migmatitic--granitic rocks, or contributing to large areas or belts in which they form the dominating rock type. The firstcase includes a great part of the Amazonian Craton, where the formation of the Transamazonian units was accompanied by intense migmatization, granitization and remobilization of the older rocks; the median massifs of the northeastern and southeastern folded regions, besides the areas

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KM. Fig. 2. Principal o c c u r r e n c e s o f granulitic r o c k s and basic--ultrabasic c o m p l e x e s i n c e n t r e and east o f S o u t h A m e r i c a : I = c r a t o n s : 1, 2, 3 = A m a z o n i a n C r a t o n (1 = G u y a n a Shield, 2 = G u a p o r 6 Shield, 3 = east Paraguay massifs) 4 = SEo Luiz C r a t o n , 5 = SEo F r a n c i s c o C r a t o n , 6 = Rio d e La Plata C r a t o n ; II = principal massifs: 7 = Central Massif o f Goi~s, 8 = P e r n a m b u c o - - A l a g o a s Massif; I I I = m o s t i m p o r t a n t o c c u r r e n c e s o f granulitic and basic-ultrabasic c o m p l e x e s : A = A t l a n t i c Granulite Belt, B = Goi~s Granulite Belt, w i t h Barro A l t o , Niquel~ndia and Canabrava c o m p l e x e s ; I V - - U r u a ~ u Belt; V = m o b i l e belts and r e g i o n s related t o t h e Brazilian Cycle: 9 = Paraguay--Araguaia Belt, 10 = Brasflia Belt, 11 = Ara~uai Belt, 12 = Sergipano Belt, 13 = n o r t h e a s t m o b i l e region, 14 = s o u t h e a s t m o b i l e region; VI = old b a s e m e n t p r o f o u n d l y r e m o b i l i z e d d u r i n g t h e Brazilian Cycle; VII = Sierras P a m p e a n a s Massifs; VIII = s e d i m e n t a r y covers related t o t h e Brazilian Cycle; I X = P h a n e r o z o i c s e d i m e n t a r y a n d volcanic---sedimentary covers; X = a p p r o x i m a t e c r a t o n borders; XI = Subandine foredeep.

37 with strong signs of migmatization and remobilization, as is the case with the coastal strip of the States of Espirito Santo and Rio de Janeiro; and the marginal areas southeast, south and southwest of the S~o Francisco Craton. The second case corresponds to the granulitic belt of Goi~s and that of the Atlantic (Fig. 2). A tlantic granulitic belt This belt has a length of about 700 km and a width of up to 250 km, and consists of various units extending from the north of the State of Bahia in the S~o Francisco Craton to the State of Rio de Janeiro, more or less following the coast. The northern part is formed by the Caraibas Complex (Barbosa, 1970) beginning north of parallel 10°S in the extreme north of the State of Bahia. It consists of gneisses, anatexites, migmatites, leptites, schists, quartzites, dolomitic marbles and numerous bodies of basic--ultrabasic rocks, part of which bear copper of chromium, besides granitic rocks. The association results from the metamorphism of a magmatic--sedimentary sequence formed by pure and impure sandstones, arkoses, greywackes, pelites, marls, dolomites and mafic--ultramafic rocks. The metamorphism which affected the granulitic facies generated large areas with rocks belonging to the garnet--sillimanite subfacies. It was accompanied by anatexis and intrusion of granitoid rocks, dominantly quartz
38 oxygen pressure {Allard and Fujimori, 1966; Stormer, 1973; Sighinolfi and Fujimori, 1972). The relations between the Jequi~ Complex and the Salvador area are varied, occurring as fault contacts as well as through a gradual transition or through an intermediate migmatitic--granulitic zone. The granulitic area of Salvador seems to bend around the Jequi~ Complex, forming an arc with its concavity toward the west and a branch with a dominant NE direction. Both units, as well as the Caraibas Complex, show a general N--S direction and a sinuous configuration with inflections toward the northeast and northwest. More to the south, beyond the border of the S~o Francisco Craton, the granulites of the Jequi~ Complex and the Salvador area decrease, occurring as nuclei in a zone of dominantly kinzigitic gneisses with a structural direction parallel to the coast (Silva Filho, 1974). The age obtained for the gneisses is about 550 + 80 m.y., but Cordani (1973) considered them to be of Early Proterozoic age and completely rejuvenated during the Brasiliano Cycle. There are some indications that part of these granulitic rocks may even represent still older material. The rocks exhibit evident signs of superposition of high- and low-pressure metamorphism besides granitization, feldspathization and recrystallization, with two or three mineral generations. Basic granulites dominate, constituting as a whole a differentiated calc-alkalic sequence with norites, gabbros, diorites and granodiorites (Guimar~es, 1956). A similar case occurs in the whole border area southeast, south and southwest of the S~o Francisco Craton, as for instance the northeast of the State of S~o Paulo (Oliveira, 1973; Oliveira and Alves, 1974; Wernick and Penalva, 1974). These granulitic nuclei of areas strongly remobilized during the Late Precambrian represent the destruction of the Transamazonian and older granulitic belts, which occur in the S~o Francisco Craton in the States of Minas Gerais, Espirito Santo and Rio de Janeiro, which are represented, among others, by the Paraiba Group {Barbosa, 1959a; Rosier, 1957, 1965; Ebert, 1968; Delhal et al., 1969; Cordani et al., 1973). Goia's granulitic belt The Goi~s granulitic belt occurs at the eastern side of the Central Massif of this state. It has a length of about 500 km, an orientation N30E, and it is covered on the east by the Uruquano fold belt. It has been only sparsely described, by Barbosa (1959b), Nilson and Motta (1968, 1969) and Bittencourt and Correa (1970). The belt is of sedimentary--magmatic origin, including gneisses, amphibolites, pure feldspathic quartzites containing alumina-silicates and iron, calco~silicatic rocks, and dolomitic marbles. An important part is represented by gneisses of gabbroic to quartz-dioritic composition, pyroxenites, garnet--pyroxenites and hypersthene--plagioclase gneisses. Part of the banded gneisses with layers of variably concentrated andesine, hornblende and pyroxene, with graded to rapid passages may represent metavolcanics and metapyroclastics of basic to intermediate composition. The Goi~s belt is richer than the Atlantic belt in mafic--ultra-

39 mafic material, with the occurrence of the already mentioned and dated complexes of Tocantins, Barro Alto and Cana Brava. Common aspects of the granulitic belts In both the Atlantic and Goi~s belts the aluminous rocks associated with the granulites display either sillimanite, kyanite plus sillimanite, cordierite plus sillimanite, or complex assemblages among these minerals, similarly to the other Archaean areas (Anh~/user et al., 1969; Mehnert, 1969). The granulitic belts are bordered by areas with a monotonous sequence of biotite or hornblende-gneisses, frequently anatexitic locally with more or less developed ovoid structures, and profusely cut through by granitoid rocks. In these areas of the S~o Francisco Craton, greenstone belts are implanted, but not yet localized west of the Goi~s granulitic belt. The contacts between these belts and the gneissic--granitic areas are either tectonic, by faults or progressively intense fault zones, or are diffuse and graded. The first case is apparently more frequent in Bahia and the second in Goi~s. The dip of the foliation of the granulitic belts is accentuated, often subvertical. Frequently one may observe a structural conformity between the granulitic belts and the younger geological units of lower-grade metamorphism, such as a few Transamazonian units of the S~o Francisco Craton. Such evidence suggests that the granulitic belts had a great influence upon the emplacement of the younger superposed metamorphic units, appearing to support the contention of Dolginov et al. (1973) that the Archaean granulitic belts present greater mobility during following geological periods than the associated gneissic--granitic areas. Other occurrences o f granulitic rocks Granulitic rocks of various types occur in the different median massifs of the folded region of the northeast and the southeast (Franco and Coutinho, 1957; Wernick, 1967; Sial, 1968; Basumallick et al., 1969; Minioli, 1972; Oliveira, 1973; Wernick and Penalva, 1974; Oliveira and Alves, 1974; Girardi, 1974; Brito Neves, 1975). They constitute either intercalations in gneisses and associated rocks, or relict structures in migmatitic surroundings. They are of Archaean to Early Proterozoic age, the oldest rocks being frequently associated with ultrabasic ones; they probably correspond in part to former extensions of the Atlantic granulitic belt. Their original mineralogy was more or less differentiated by thermic or tectonic events, accompanied by migmatization, granitization and recrystallization. Polyphase granulites with two or three mineral generations are common. These modifications are particularly intense in the median massifs, strongly remobilized during the Late Precambrian; they were selective, with the almost total destruction of the acid granulitic rocks and consequent apparent increase of the rate of intermediate and basic granulites. Occurrences similar to those observed in the median massifs of the folded regions of the northeast and southeast are present in many areas of the Ama-

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Fig. 3. P r o v o n a n d possible g r e e n s t o n e b e l t s o f t h e S~o F r a n c i s c o C r a t o n : I = Transamaz o n i a n o r o l d e r b a s e m e n t ; I I = L a t e P r e c a m b r i a n m o b i l e belts; I I I = Espinba~o belt and r e l a t e d covers o n t h e C h a p a d a D i a m a n t i n a ; I V = o l d e r b a s e m e n t p r o f o u n d l y r e m o b i l i z e d during t h e Brazilian Cycle; V = s e d i m e n t a r y covers o f t h e Brazilian Cycle. V I = P h a n e r o z o i c s e d i m e n t a r y a n d s e d i m e n t a r y - - v o l c a n i c covers: V I I = g r e e n s t o n e belts: 1 = Serrinha, 2 = C o n t e n d a s - - M i r a n t e , 3 = B r u m a d o , 4 = J a e o b i n a , 5 = Capim, 6 = Urandi, 7 = Bequira, 8 = C o l o m i , 9 = Rio das V e l h a s - - L a f a i e t e ; V I I I = A t l a n t i c G r a n u l i t e Belt; A = Caraibas C o m p l e x , B = Jequi~ C o m p l e x , C = Salvador C o m p l e x , D = Paraiba C o m p l e x ; I X = a p p r o x i m a t e b o r d e r s o f t h e S~o F r a n c i s c o Craton.

41 zonian Craton where more or less extensive areas of granulitic rocks are associated with gneisses, anatexites, migmatites, and granitoid rocks as in the Guyana and Xingu Complexes (Issler et al., 1974; Silva et al., 1974); in the Juiz de Fora Group (Ebert, 1968) southeast of the S~o Francisco Craton (Minas Gerais) with an age of 2780 m.y. (Cordani et al., 1973); and in the basal complex of Goi~s (Almeida, 1967) in the Central Massif of this state. These intercalations possibly form part of ancient and extensive granulitic areas, strongly modified and obliterated by later migmatization, remobilization and granitization during one of the chelogenic periods of Sutton (1971). Greenstone belts

The identification and study of the greenstone belts is fairly recent in Brazil, including work in the S~o Francisco Craton in the State of Bahia (Mascarenhas, 1973, 1976) and in the area of the 'Quadril~tero Ferrffero' in the State of Minas Gerais (Fig. 3). Greenstone belt structures in Bahia

In the central northern part of the S~o Francisco Craton some types of structures similar to greenstone belts have been identified by Mascarenhas (1973, 1976). These are volcanic--sedimentary sequences situated on top of the Archaean gneissic-granitic substratum; they occur at Serrinha, Contendas--Mirante, Brumado, Capim, Boquira, Urandi and Colomi. They are of small size (2000--5000 km2), situated upon a gneissic-granitic basement partly proven to be of Archaean age (Travora et al., 1967; Jardim de S~ et al., 1967a, b). They are generally oriented in a N--S direction, approximately parallel to the trend of the Jequi~ and Caraibas Complexes, but with numerous irregular expansions. Grandiorites and biotite (hornblende) granites as well as diatexitic granitoid bodies of intrusive character, border or invade the sedimentary, volcanic and ultramafic sequences of the greenstone belts. T h e y are medium- to coarse-grained, equigranular and porphyritic, isotropic or gneissic. Around various bodies the sequences of greenschist facies cut through exhibit a higher-rank metamorphism, including the formation of hornfelses. Generally, the same sequences occur in other Archaean greenstone belts and ultramafic sequence, such as volcanic--pyroclastic sequence with associated hypoabyssal bodies, chiefly mafic but including also intermediate and silicic volcanites, or a sedimentary sequence with psefitic, coarse and fine psammitic, pelitic and chemical rocks including some containing iron and manganese, generally intercalated within the metavolcanics. The basal parts of the sedimentary sequence are chiefly pelitic and chemical grading upwards into beds of psammites. Important economic resources such as chromite, gold, uranium, magnesite and manganese are sometime found in the same lithologic association.

42

The greenstone belts exhibit a structural pattern characterized by isoclinal folds, refolding, inversions, and normal to asymmetric folds. Although the axes of the primary folding are oriented after the principal alignment of the greenstone belt, minor and secondary folds may occur in any direction, strongly influenced by bordering and intrusive granitic and diatexitic bodies. Secondary foliation, intense fracturing, shearing and milonitization are common; locally one may observe fault contacts with the surrounding gneissic-granitic complex.

Rio das Velhas--Lafaiete greenstone belt (Minas Gerais) Another greenstone belt-type structure has been described from the southern part of the S~o Francisco Craton, represented by the Rio das Velhas Supergroup (Dorr et al., 1957; Dorr, 1969) and by the Lafaiete Formation (Ebert, 1956, 1962, 1963) in the region of the Quadril~tero Ferrifero of the State of Minas Gerais. This greenstone belt, with ages greater than 2800 m.y. (Herz, 1970), is implanted upon a gneissic--migmatitic basement, rich in agmatitic migmatites and 'schollen' with amphibolitic paleosom, partly represented by the Late Archaean Barbacena Group {Delhal et al., 1969; Cordani et al., 1973). Some portions of the Barbacena Group are intrusive in the Rio das Velhas Supergroup. This is, therefore, an older unit, intensively remobilized during the Jequi4 Cycle. The Rio das Velhas Supergroup and the Lafaiete Formation occur over an area of about 7000 km 2, and present many characteristics of a greenstone belt; the former is composed of schists, metasedimentary and metavolcanic phyllites, ferruginous formations (alternating beds of quartz, siderite and magnetite), metagreywackes, ankeritic quartz rocks, sericitic quartzites and schistose conglomerates. Mafic--ultramafic rocks, representing a conformable or unconformale dunitic--gabbroic sequence, are present as soapstone, serpentinites, talc, chlorite, tremolite or actinolite schists and amphibolites. The schists and metasedimentary phyllites contain abundant chlorite amphibole, quartz, Na-plagioclase and epidote minerals; biotite occurs locally. This sequence, with an estimated thickness of about 4200 m, constitutes the Nova Lima Group, the basal part of the Rio das Velhas Supergroup. Its upper part (Maquin4 Group) is represented by quartz--sericite schists and phyllites in its lower section and by conglomerates and massif sericite or chlorite quartzites in its upper section. Thicknesses have been estimated at between 600 and 1400, and between 250 and 400 m, respectively. In the Lafaiete Formation part of the section of the Rio das Velhas Supergroup is associated with more or less thick intercalations of manganese-bearing rocks. The Rio das Velhas--Lafaiete greenstone belt provides important gold resources (Morro Velho, Raposos, Morro Vermelho, Bela Fama, Bicalho, Urubu) situated preferentially in quartz-ankeritic and manganese-bearing rocks (districts of S~o Jo~o del Rei, Lafaiete and Saude). The iron deposits of the Nova Lima Group are still unexploited. The Rio das Velhas Supergroup locally exhibits a complex isoclinal

43

folding, sinuous and influenced by the intrusive granitoid bodies, often with inverted synclines and anticlines. The structural pattern is obliterated by the deformation and metamorphism of the overlying Minas Supergroup, of Early Proterozoic age. Gneissic--granitic areas

Gneissic--granitic complexes of an original Archaean age occur in the cratons, in the central massif of Goi~s and in some of the median massifs of the Brasiliano folding regions. Their identification represents a recent advance due to the improvement of geochronological determinations. The lithology of these areas is often m o n o t o n o u s , involving chiefly biotite (hornblende) gneisses, anatexites, amphibolites and migmatitites; numerous a u t o c h t h o n o u s (diatexites) and allochthonous granitoid bodies of a quartzdioritic to granitic composition occur in association. In minor quantities pure and impure, locally ferruginous quartzites, calco-silicate rocks and dolomitic marbles are found, besides schists, cordierite, sillimanite and kyanite gneisses with the most diverse associations among the alumino-silicates. Acidic to mafic granulites and ultramafic rocks occur more or less frequently as intercalations. Among the migmatites those of agmatitic structure dominate, 'schollen', 'schlieren', bedded and folded, having as their paleosom gneisses, amphibolites, granulites and ultrabasic rocks and as neosom granitoid, dioritic to granitic material. The rocks are of amphibolitic and granulitic facies, c o m m o n l y with polymetamorphic and metasomatic features. A m e b o i d or boomerang-like structures as well as tongue-like domes have been found only locally. Given the observed polycylical evolution in many areas, it was n o t always possible to distinguish which structures are really Archaean and which were superimposed by later deformations. In many places it is supposed that the structures of younger units reflect those of an older basement; in other cases unconformable structural patterns are evident. The Archaean age of these complexes has heen proven by radiometric methods, chiefly during the last few years. In many areas, however, age determinations are still lacking, with the supposed Archaean age based only on various kinds of correlation. Many Archaean gneissic--granitic areas have possibly been remobilized, migmatized and isotopically rejuvenated by later events with the result that in the central and western parts of the Amazonian Craton, and in the western part of the central massif of Goi~s, basement rocks referrable to the Transamazonian Cycle are dominant (Hasui and Almeida, 1970; Amaral, 1974). DISCUSSION AND CONCLUSION

In Brazil the same Archaean units occur as in other parts of the world, represented by large mafic--ultramafic massifs, granulitic belts, greenstone

44 belts and gneissic-granitic areas. The mutual relations between these units are identical to those c o m m o n l y reported elsewhere. When the diverse Brazilian mafic--ultramafic complexes are dated, it appears that they are virtually absent in areas formed during the Brasiliano Cycle and rare in those referred to the Uru~uano Cycle. Their frequency increases in the Middle Precambrian (Transamazonian Cycle) and their number and extent reach a maximum in the Archaean. The petrographical studies carried o u t on Brazilian Archaean rocks suggest that the genetic mechanism of basaltic magmas proposed by Green and Ringw o o d (1967) was already effective during the Archaean and that the mafic rocks present in terranes of that age have been derived from different depths. The relations between the Brazilian granulitic belts and the gneissic--granitic bordering areas are still poorly known. Two phases of granulitic rock formation can be referred to the Early Precambrian and another one to the Early Proterozoic (Transamazonian Cycle). Rocks of this t y p e of certain Late Proterozoic age are n o t known. The Transamazonian granulitic areas can be distinguished from the Archaean ones b y a lesser quantity of mafic-ultramafic rocks; also, they are n o t associated with or bordered by gneissic-granitic areas with ameboid structures. An association between granulitic strips and those of para-metamorphites derived from metamorphism of platformed sedimentary rocks is more clearly seen. However, in a few cases field relationships resemble those observed b y Arriens and Lambert (1969), Windley and Bridgwater (1971) and Dolginov et al. (1973), suggesting older sequences, isotopically rejuvenated during the Early Proterozoic. The s t u d y of the Brazilian greenstone belts is still in an early stage. According to Mascarenhas (1976), the gneissic--granitic basement should have developed from a protocraton by continental growth, in the sense of the models proposed by Anh~user et al. (1969), Glikson (1971), and Katz {1974). Taking into account a minimum age 2700 m.y. for the Jequid Complex, the formation of the greenstone belts in Bahia must have occurred between 3000 and 2700 m.y. ago. This compares with ages generally varying between 3000 and 2400 m.y. for the formation of the greenstone belts in various other parts of the world. However, the different occurrences are n o t sufficiently known to assure their penecontemporaneous origin or to be sure that they belong to two distinct generations as postulated by Coward et al. (1976) in South Africa. It is n o t known if they also contain greenstone belts of Early Proterozoic age such as in Canada and Guyana (Bell et al., 1975; Choudhuri, 1976). The radiometric data indicate the widespread occurrence of Brazilian Archaean terranes with ages compatible with the Jequid Cycle. This involves, in many cases, intense episodes of metamorphism, migmatization, granitization, remobilization, intrusion and isotopic rejuvenation. All this had a great importance for the formation of the South American Platform basement, similarly to other parts of the world (Sutton, 1963, 1971), obliterating the Early Archaean structures more or less completely. Conversely, this cycle,

45

instead of representing the growth of new material around the oldest Archaean protocratons in a process of 'onion-skin' type, may shape the destruction of extensive Archaean cratonic areas, dominantly of sialic composition, thus confirming the conclusions of Wynne-Edwards and Hasan (1970) and Bridgwater and Fyfe (1974). The chiefly gneissic--granitic structure comprise the most stable areas of the Early Precambrian. They sustain the structures of the greenstone belt type and integrate cratons upon which the covers associated to the tectoorogenetic cycles of the Late Precambrian developed. Upon these, the metamorphism of the Late Precambrian folding zones generally do not exceed the greenschist facies. There was neither migmatization nor synkinematic granitization. The granulitic belts constitute areas of prolonged tectonic instability, serving as a basement for the internal zones of the Late Precambrian geosynclines in which metamorphism frequently attained the amphibolite facies associated with abundant granitization and migmatization (Almeida, 1976). At the present level of knowledge, any hypothesis about the evolution of :Brazilian Archaean formation is highly speculative. The theory of continuous growth of Archaean nuclei (Engel and Kelm, 1972) and that of superimposition of mobile intrusions on large sialic masses (Cahen and Snelling, 1966; Clifford, 1968, 1970; KrSner, 1977) have received some support among Brazilian researchers. ACKNOWLEDGEMENTS

The author thanks J. Mascarenhas of CPRM and Prof. F.F.M. de Almeida of Instituto de Geoci~ncias, Sao Paulo University, for permission to use unpublished material (final texts presented at the 29th Brazilian Geological Congress); Prof. B.B. Brito Neves of the Geology Department, Federal University of Pernamuco, for personal communications; Profs. J.S. Bittencourt and A. Choudhuri of the Instituto de Geoci~ncias e Ci~ncias Exatas of the Paulista State University, for discussions, suggestions and revision of part of the manuscript. REFERENCES Allard, G.O. and Fuijmori, S., 1976. A new occurrence of sapphirine in Salvador, Bahia, Brazil. Can. Mineral., 8: 660. Almeida, F.F.M., 1967. Origem e evoluq~o da plataforma brasileira. Dep. Nac. Prod. Min., Div. Geol. Mineral., Bol., 241 : 36 pp. Alrneida, F.F.M., 1976. Estruturas do Pr~-Cambriano Inferior brasileiro. XXIX Cong. Bras. Geol., Ouro Proto - - Belo Horizonte, resumos trabalhos, pp. 201--202. Amaral, G., 1974. Geologia precambrian da regi~o Amaz6nica. Instituto Geoci~ncias, Univ. S~o Paulo, unpublished thesis. Anh~/user, C.R., Mason, R., Viljoen, M.J. and Viljoen, R.P., 1969. A reappraisal of some aspects of Precambrian shield geology. Geol. Soc. Am. Bull., 80: 2175--2200. Armstrong, R.L., 1968. A model for the evolution of strontium and lead isotopes in a dynamic earth. Rev. Geophys., 6: 175--199. Arriens, P. and Lambert, J.B., 1969. On the age and strontium isotopic geochemistry of granulite-facies rocks from the Fraser Range, Western Australia and Musgrave Range, Central Australia. Geol. Soc. Aust. Spec. Publ., 2: 13--21.

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48 Nilson, A.A. and Motta, J., 1968. Piroxenitos e gnaisses g~bricos de f~tcies granulito no complexo basal de Goianira--Trindade, Goias. Soc. Brasil. Geol., An. XXII Congr., Belo Horizonte, pp. 241--242. Nilson, A.A. and Motta, J., 1969. Geologia da ~rea Goianita--Trindade, Goias. Dep. Nac. Prod. Mineral, Div. Fore. Prod. Mineral, Bol., 133. Oliveira, M.A.F., 1973. Petrologia das rochas metam6rficas da regigo de S~o Jos4 do Rio Pardo, SP. Rev. Brasil. Geoci~nc., 3 : 257--278. Oliveira, M.A.F. and Alves, F.R., 1974. Geologia e petrologia da regigo de Caconde, SP. Soc. Brasil. Geol., An. XXVIII Congr., Porto Alegre, 5: 133--143. Rosier, G.F., 1957. A geologia da Serra do Mar, entre os Picos de Maria Comprida e do Desengano. Dep. Nac. Prod. Min., Div. Geol. Mineral., Bol., 1 6 6 : 5 8 pp. Rosier, G.F., 1965. Pesquisas geol6gicas na parte oriental do Rio de Janeiro e na parte vizinha do Estado de Minas Gerals. Dep. Nac. Prod. Min., Div. Geol. Mineral., Bol., 2 2 2 : 4 1 pp. Sial, A.M., 1968. Petrologia dos charnockitos de duas localidades em Pernambuco. J. Mineral., 6 : 41--60. Sighinolfi, G.P., 1971. Investigations into deep levels of the continental petrology and chemistry of the granulite facies terrains of Bahia (Brazil). Atti Soc. Toscan. Sci. Nat., Mem. Ser. A, pp. 237--341. Sighinolfi, G.P. and Fujimori, S., 1972. Granadas nos granulitos de Salvador, Bahia: discuss~o sobre relaq~es entre variaq~o qufmica e reaq~es metam6rficas. Rev. Brasil. Geoci~nc., 2 : 141--150. Silva, G.G., Bima, M.I.C., Andrade, A.B.F., Issler, R.G. and Guimar~'es, G., 1974. Geologia da folha SB-22 -- Araguaia e parte da folha SC-22 -- Tocantins. Dep. Nac. Prod. Min., Projeto RADAM, 4: 1--143. Silva Filho, M.A., 1974. Os kinzigitos do extremo sul da Bahia, sua origem e p o s i ~ o estratigr~fica relativa. Soc. Brasil. Geol., An. XXVIII Congr., Porto Alegre, 4: 159-164. Souza, A., 1973. Aspectos geolSgicos e geocronolSgicos do complexo de Barro Alto, Goias. Instituto Geoci~ncias e Ci~ncias Exatas, Univ. Est. Paulista, Rio Claro, S~o Paulo, unpublished thesis. Stormer, Jr, J.C., 1973. The composition of sapphirine from Salvador, Bahia, and condition o f its formation. Rev. Brasil. Geoci~nc., 3 : 192--200. Sutton, J., 1963. Long-term cycles in the evolution of the continents. Nature, 198: 731--735, Sutton, J., 1971. Some development in the crust. Geol. Soc. Austr., Spec. Publ., 3 : 1--10. Travora, F.J., Cordani, U.G. and Kawashita, K., 1967. Determinaq~es geocronolSgicas na regia~o central da Bahia pelo m~todo pot~ssio--argSnio. Soc. Brasil. Geol., An. XXI Congr., Curitiba, pp. 234--244. Thayer, T.P., 1970. Notes on the geology and resources of some peridotites and related rocks in Brazil. In: Departamento Nacional P r o d u ~ o Mineral; Carta geolSgica do Brasil ao milion~simo - - folha Goias SD-22 (unpublished report BR-32, U.S. Geol. Survey). Wernick, E., 1967. A geologia da regi~o de Amparo, leste do Estado de S~'o Paulo. Institute Geoci~ncias e Ci~ncias Exatas, Univ. Est. Paulista, Rio Claro, S~o Paulo, unpublished thesis. Wernick, E. and Penalva, F., 1974. M i g m a t i z a ~ o e f e l d s p a t i z a ~ o de charnockitos a granulitos no leste paulista e sul de Minas Gerais. Soc. Brasil. Geol., An. XXVIII Congr., Porto Alegre, 5 : 155--160. Windley, B.R. and Bridgwater, D., 1971. The evolution of Archaean low and high-grade terrains. Geol. Soc. Austr., Spec. Publ., 3: 33--36. Wynne-Edwards, H.R. and Hasan, O., 1970. Intersecting orogenic belts across the North Atlantic. Am. J. Sci., 268: 189--308. [Received September 4, 1980 ]