Journal of South American Earth Sciences, Vol. 6, No. 1/2, pp. 49-57, 1992 Printed in Great Britain

0895-9811/92 $5.00+ .00 1992 PergamonPress Ltd & Earth Sciences & ResourcesInstitute

The Brasiliano Sao Francisco craton revisited (central Brazil)R. TROMPE'ITE1'3 , A. UI-n.F.IN , M. E. DA SILVA and I. KARMANN 2 3, 3 ICNRS/ORSTOM, France; 2Instituto de Geoei~ncias,UniversidadeFederal de Minas G-erais,Campus da Pampulha, 31270 Belo Horizonte, MG, Brazil; 3Instituto de Geoeieneias, Universidadede S~o Paulo (USP), CP 20899, 01498 S~toPaulo, SP, Brazil(Received September 1991; Revision Accepted July 1992) Abstract---The Brasiliano (~600 Ma) her of Pararnirim divides the SAo Francisco eraton (as defined by Almeida, 1967, 1969) into two parts: to the west is the Silo Francisco craton proper, to the east is the Salvador eraton, forming an extension of the Congo craton. The basement of the Paramirim belt, of Archean and Early Proterozoic age, is intruded by subalkalioe granitoids dated around 1700 Ma. The cover, known as the Espinhaqo Supergroup, consists of felsic magmatic rocks, overlain by clastic sediments depoited in a continental dR during the time interval ~1700-1100 Ma. The Espinhaqo Supergroup was deformed during the Brasiliano orogeny, at about 600 Ma. It is characterized by shaUow-dipplng, submeridional, large-scale ductile shear zones with western vergence and mineral and/or extension lineation indicating tectonic transport from east to west. These ductile shear zones extend south into the outer units of the Brasiliano Araquaf mobile belt and the Lower Proterozoic of the Quadrilatero Ferrifero. There are no structural data supporting the hypothesis of a Middle Proterozoic deformation of the Paramirim belt that should have been obliterated during the Brasilieno orogeny. However, a possible pre-Brasiliano deformation does not affect the question of the division of the Brasiliano-Sao Francisco craton. Resumo--A faixa brasiliana do Paramirim divide o CrOton do Silo Francisco (como definido por Almeida, 1967, 1969) em duas partes: a oeste est~i o C~ton do Silo Francisco propriarnente dito, a leste tem-se o Cr~ten do Salvador, formando urea extensilo do Cr~iton do Congo. O embasamento da faixa Paramirim, de idade arqueana e proteroz6ico inferior, est~ intrudido por granit6ides subalcalinos datados em cerca de 1700 Ma. Os metassedimentos, conhecidos como o Supergrupo Espinhaqo, consiste de rochas magrn~ticas f61sicas, sebrepostas por sedimentos cl~ticos depositados em urn "rift" continental durante o intervalo de 1700-1100 Ma. O Supergrupo Espinhaqo foi deformado somente no Brasiliano, a cerca de 600 Ma. H a est~ caracterizada pot zonas de cisalhamento dl~cteis de grande escala, submeridionais de mergulhos suaves, corn vergencia para oeste e lineae,~o mineral e/ou de extensao indicando transpurte tect&tico de leste para oeste. Para o su! estas zonas de cisalhamento dt~eteis cortarn em seguida as unidades externas da faixa m6vel brasiliana Arauqaf, e as rochas do Proteroz6ico Inferior do Quadril~tero Ferrffero. Nilo h~l dados estruturais que sustentam a hip6tese de urea deforma~ilo da falxa do Paramirim no Proteroz6ico M~dio que poderia ter sido obliterada durante a orog~nia do brasUiano. Emretanto, urea tal deformaqilo pr6-brasiliana nilo afeta a validade da divisilo do cr~iton brasUiano do Silo Francisco.

INTRODUCTION The SAo Francisco craton delimited by the Brasiliano orogeny (-600 Ma), first defined as the Sao Francisco platform by Almeida (1967, 1969). was described by Cordani et al. (1968) and Almeida et al. (1973). According to the original definition, the craton included most of the rectasediments currently assigned to the Ara~uat belt of Brasiliano age (Fig. 1). Later, Almeida (1977) established the presently accepted limits of the craton. The S~o Francisco craton is the western extremity of the much larger Congo (Za~re)-S[lo Francisco craton, which occupied the center of Western Gondwana and was split in two by the opening of the South Atlantic (Fig. 1). Pflug et al. (1969) proposed subdividing the Sao Francisco craton into a southwestern block and an eastern block. referring to the latter as the Lenqois craton. The dividing zone, passing by the Paramirim Valley (Fig. 2), was regarded as a fold belt deformed during the TransamazonJan orogeny (~2000 Ma). The present limits of the Sao

Francisco craton, inherited from the Brasiliano orogeny, were incorrectly extended here to pre-Brasiliano time. The geological province of Paramlrim was established by Cordani (1973a,b, 1978), mainly on the basis of geochronological data (see, e.g., Tavora et al., 1967). This province has been defined by K/Ar ages ranging from 620 to 450 Ma, both from basement rocks and from quartz schists of the Espinha~o Supergroup metasedimentary cover. The Paramirim Province was interpreted as a Brasiliano mobile zone with low-grade metamorphism. It separated the SAo Francisco craton (sensu stricto) in the southwest from the Salvador craton in the east. The limits of the latter were similar to those of the Lenfois craton of Pflug et al. (1969). The Salvador craton was the South American extremity of the Congo craton of Africa (Fig.l). This division in two of the S~io Francisco craton had already been outlined by Hurley et al. (1969), on the predrift reccmstrnction of western Gondwana. According to Almeida (1977, and cited in Cordani. 1973b), the province of Paramirim, which was defined using K/Ar ages, could not be regarded as a Brasiliano

Address all correspondence and reprint requests to Dr. Roland Trompette at Insfituto de Gex~ienias, Universidade de S~ Paulo, Brazil: telephone [55] (11) 813-8777; fax [55] (11) 210-4958; telex 113695 OUSPO.49

50

R. TROMFETrE,A. UHLEIN,M. E. DA SILVA,and I. KARMANN

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Fig. 1. Proposed division of the Congo (Za~e)-Slo Francisco craton in two parts: to the east, the Congo-Salvadorcraton; to the southwear, the S ~ Francisco (s.s.) craton. The fit of Africa and South America was taken from De Wit et al. (1988): A) the Slo Francisco craton sensu A h n ~ d a (1967, 1969); B) the S ~ Franciso) (s.s.) and Salvador cratons sensu Cordani (1973a,b). Legend: 1, Pan-AfricanBrasitiano fold belts, mainly metasedimentary, with indication of thrust direction; 2, puzzle of Pan-Mrican-Brasiliano mini-cratons and mini-fold belts; 3, ctatons, areas stable during the Pan-African-Bra~liuno orogeny.

province or mobile belt because its metamorphism is dated at 1300-1100 Ma (S~l e t al.. 1976) - - i.e. pre-Brasiliano. This was substantiated by the subhorizontal attitude of Brasiliano cycle sediments (Una Supergron p. equivalent to the SAD Francisco or Bambul s e n s u lato Supergroup) on craton margins adjacent to the Paramirim Province. Theref(ge, in the absence of slz~ctural studies demonsl~ating evidence of Brasiliano deformation, the SADFrancisco craton has been described in recent Brazi-

llan publications (Teixeira and Figueiredo. 1991: Brito Neves and Cordgni. 1991) as a Middle Proterozoic unit. following the definition of Almeida (1977). Torquato (cited in Torquato and Cordani. 1981) suggested that the Brasiliano mobile belts of Sergipe and Arafuaf (Figs. 1 and 2) joined along the Atlantic coast of Brazil. separating the Slo Francisco craton from the Congo craton, but no structural or geochronological data support this hypothesis (Trompette. 1984).

The Brasiliano S~o Francisco craton revisited (central Brazil)

51

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Fig. 2. The Brasiliano Sao Francisco (s.s.) and Salvador cratons in their structuralframework (from Almeida, 1967, 1977; Inda and Barbosa, 1978): C, Concei~Io do Mat Dentro; D, Diamantina; P, Paramirim; S, Salinas; SB, Serra do Boquer(lo; SE, Serra do Estreito; SSO, Serra de Santo Onofre. Legend: 1, Meso-Cenozoic cover; 2, Late Proterozoic cover; 3, Middle Proterozoic cover; 4, basement with individualization of the Minas Supergroup (Quadril~iteroFerrifero); 5, Brasiliano Paramirim-Quadril~itero Ferrifero shear zone; 6, medasediments of Late Proterozoic age; 7, metasedim~ts of Middle Proterozoic age; 8, polycyclic basement reworked by the Brasiliano orogeny.

THE PARAMIRIM PROVINCE: A MIDDLE PROTEROZOIC (1300-1100 Ma) OR BRASILIANO (-600 Ma) FOLD BELT? Until about 1985, the Paramirim Province was considexed to be a continental rift filled with Middle Protero-

zoic rocks and folded at 1300-1100 Ma (S~i et al., 1976; Bdto Neves et al., 1979, 1981). Recent work suggests a predominantly Brasiliano age of the deformation (Caby and Arthaud, 1987; Uhlein. 1991). This reopens the question of the coherence of the S~lo Francisco craton sensu Almeida (1977).

52

R. TROMPETIE,A. UHLEIN,M. E. DA SILVA,and I. KARMANN

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Fig. 3. Schematicgeological map of the Brasiliano ParamirimFold Belt or Province(from Inda and Barbosa, 1978; Caby and Arthaud, 1987; Uhlein, 1991). Legend: 1, Cenozoic and Quaternary cover; 2, Late Proterozoic cover (Una SGr.); 3, Middle Proterozoic cover (EspinhaetoSGr.); 4, basement;5, Middle Proterozoicmetasedimentsand metavolcanics;6, LagoaReal orthogneissesand S ~ "Iim6teo granitoids -1725 Ma; 7, Polycyclic basement; arrowsindicate tectonic vergence. The ParamirimProvincecan be subdividedinto three structural units: the western subautochthonousmetasedlmentarySanto Onofre Hills unit (Pt); the median polycyclic basementunit intruded by Middle Proterozoicgranitoids (P2); and the eastern m e ~ e n t a r y and metavolcanic unit (P3) which grades eastward to the Middle Proterozoic cover of the Salvadorcraton. Key: B, Boquira; C, Caetitd;I, Ibiajara;LB, Livramentodo Brumado;LR, Lagoa Real; OB, Oliveirados Brejinhos;P, Paramirim.

Stratigraphic DataThe craton (or cratons, depending on the assumed hypothesis) flanking the Paramirim Province consist of an Aw,hean and Transamazonian (-2000 ]via) basement and a subhorizontal metasedimentary cover. The Espinhafo Supergroup, forming the lower part of the latter, is well

developed in the east of the province. The upper part of the cover, the Una Supergroup. is the equivalent of the SaD Francisco or Bambui (sensu law) Supergroup defined north of Belo Horizonte by Costa and Branco (1961). The Espinhafo Supergroup, of Middle Proterozoic age. ccmsists predominAndy of elastic metasedimentary rocks. The Late Proterozoic Una Supergroup begins with diamictites.

The Brasillano SAoFrancisco cratou revisited (central Brazil) which are tillites sensu stricto north of Belo Hodzonte (Isotta et al., 1%9; Rocha Campos and Hasui, 1981), followed by alternating fine-grained clastic sediments and carbonates with rare stromatolites. In the Paramirim Province, the basement consists of Archaean and Transamazonian (Early Proterozoic) rocks intruded by numerous Middle and Late Protexozoic graultoids. The cxatonic cover is restricted to the Espinl~o Supergroup, subdivided into three groups (Inda and Barbosa, 1978; Costa and Inda, 1982). At the base, the Rio dos Remedios Group consists of lenticular bodies of acid metavolcanics: rhyolites, rhyodacites, dacites, breccias, and tufts, intruded by quartz porphyries and accompanied by coarse-grained clastic sediments. The middle Parag u r u Group censists of metaconglomerates, metaarkoses, quartzites, metasiltstones, and schists. The upper Chapada Diamantina Group consists of two quartzite units (the Tombador and Morro do Chapeti Fc~'mations), with shales, siltstones, and carbonates, sometimes stromatolitic, of the Cabcclo Formation sandwiched between them. In the Paramirim Province, the fully developed Espinha~o Supergroup is represented by the above three groups. On the adjacent Salvador craton, the Espinhac,o Supergroup is represented by all three groups with reduced thickness (S~iet al., 1976), or, according to Costa and Inda (1982), only by the Chapada Diamantina Group. The latter interpretation is generally accepted. In the eastern part of the Paramirim Province (Fig. 3, P3), acid magmatism of the Rio dos Remedios Group is tholeiitic to subalkaline (Sishinolfi and Concei~ao, 1974; Sa et al., 1976; McReath et al., 1981). Intense secondary alteration caused by tectonism and hydrothermal action (Frtes, 1986) makes a geochemical characterization of this magmatism difficult. According to the above-mentioned authors, this volcanism of uncertain age resulted from partial melting of continental crust. The large dispersal of isotopic ratios on Rb/Sr isochron diagrams was interpreted in two different ways. Brito Neves et al. (1980) calculated an isochron giving an age of 1090+ 57 Ma (Ri = 0.715 + 0.004), whereas S~i et al. (1976) and McReath et al. (1981) obtained an errochron with an age of 1200 :!: 120 Ma. These ages are indicative of isotopic re,homogenization, generally considered as resulting from a metamorphic event. In the western part of the Paramirim Province, a second volcanic complex is present along the western slopes of the Santo Onofre Hills, between Boquira and Macat~bas (Fig. 3, P1). It consists of rhyolites, trachytes, and quartz trachytes with peralkaline affinities. These axe little deformed and not affected by hydrothermal alteration (Sa et al., 1976; McReath et al., 1981). This complex, of unknown age, is younger than the Rio dos Remedios Group but forms part of the Espinha~o Supergroup. The basement of the Paramirim Province and the metasediments of the Espinha~ Supergroup are intruded by basic dikes and sills locally deformed and yielding K/ Ar ages ranging from 1200 to 500 Ma (Tavora et a l . 1%7; Brito Neves et al., 1980). A gabbro dike cutting the Espinhafo Supergroup quartzites was dated by K/Ar on feldspars at 1111 + 56 Ma (S~tet al., 1976).

53

Model for a Middle Proterozoic Aulaeogen Deformed Between 1300 and 1100 MaAccording to the model described by S~i et al. (1976), S~ (1981), Inda and Barbosa (1978), Costa and Inda (1982). And Brito Neves et al. (1979, 1980), a N-NW/SSEtrending anlacogen opened at about 1800-1700 Ma. This age of rifting is based on correlation of the acid magmatic rocks of the Rio dos Remedios Group with the distant rhyolitic intrusions in the Concei~Ao do Mato Dentro rogkm (Fig. 2). The latter, which, in Minas Gerais, are thought to mark the beginning of Espinhaf,o Supergroup accumulation, are dated at 1770 Ma by the U/Pb method on two zircon samples (Brito Neves et al., 1979). Conventional Rb/Sr isotopic analyses of acid metavolcanic rocks of the Rio dos Remedios Group in the Paramirim Province yielded model ages of 1500 and 1600 Ma, using minimal initial ratios of Ri of 0.706 and 0.710, respectively (S~ et al., 1976; Brito Neves et al., 1980). The metamorphism of the Paramirim aulacogen is dated at 1200 to 1100 Ma by S~ et al. (1976) and around 1300 Ma by Brito Neves et al. (1979, 1981). The range 1200-1100 Ma is based on the dating of metavolcanic rocks in the eastern part of the Paramirim Province (Fi8. 3, P3). The 1300 Ma age was obtained for undeformed and unmetamorphosed shales of the cratonic Caboclo Formation (1290 + 52 Ma Rb/Sr, Ri = 0.7360 + 0.002) on five whole-rock samples and on the fine fraction of five other samples. This is interpreted as the age of diagenesis, indicating the effect of the Paramirim Province orogeny in the cratonic domain. This Middle Proterozoic deformation was attributed to vertical movements of basement blocks, unit P2 in Fig. 3, forming a fan structure (S~iet al., 1976; S~, 1981; McReath et al., 1981). In the west, in the Santo Onofre Hills (Fig. 3, PI), a western vergence towards the Sao Francisco craton sensu stricto is present, whereas in the east, in the P2 unit (Fig. 3), the vergence is to the east. The deposition of the Espinha~o Supergroup would therefore be bracketed between 1770 and 1300 or 12001100 Ma, depending on the accepted age for orogeny. In this model, the influence of the Brasillano (-600 Ma) event is minimized to granite emplacement and minor deformation non-commensurate with the principal Middle Proterozoic tectonism. The Brasiliano event is poorly dated by a three-point isochron on schists of the Espinha~ Supergroup of the Santo Onof~e Hillsgiving an age of 600 + 63 Ma, Ri = 0.733 + 0.06 (Brito Neves et al., 1980). The dispersion of Rb/sr ages representing the Rio dos Remedios Group magmatism and the 620-450 Ma K/Ar ages obtained from various lithologies of the Espinha~ Supergroup are attributed to the Brasiliano event (Tavora et al., 1%7).

Model of a Middle Proterozoic Aulacogen Deformed During the Brasiliano Event (-600 Ma)New ideas on the structure of the Paramirim Province, stemrnin~ from detailed works on uranium minaralization in the Lagoa Real region (Fig. 3), have led us to propose a

54

R. TROMPETIE,A. UHLEIN,M. E. DA SILVA,and I. KARMANN

model that differs frownthe previous one in the style and age of deformation. The mineralization is associated with lenses of albitite with pyroxene and garnet occurring in the Lagoa Real orthogneisses (P2 unit, Fig. 3). The orthogneisses are products of deformation of the subalkaliue, iron-rich, Sao T'unoteo granitoids. The study of the mineralization and its structural context resulted in recognition of west-vergent ductile shear zones (Lobato et al., 1983, Souza et al., 1986) with a geometry iucompatible with the vertical model of S~i (1981). According to Caby and Arthaud (1987), only one polyphase deformation affected the orthogneisses of Lagoa Real and meta-sediments of the Espinhaq~ Supergroup in the Santo Onofre Hills (Fig. 3, PI). The D 1 phase formed submeridional ductile shear zones and metamorphic foliation in gneisses associated with mineral (biotite, amphibole) and/or stretching lineations. This foliation cuts at a low angle across the magmatic layering of the orthogneisses and albitites. In the schists and quartzites of the Santo Onofre Hills (Espinhafo Supergroup), D1 is associated with greenschistfacies metamorphism. L1 mineral or stretchin~ liueation is identical with that described above. The Lagoa Real orthogneisses and the metasediments were transported to the west. D2 is represented by subvertical ductile shear zones also oriented north-south and associated with subhorizontal L2 lineation. D3 is represented by large-scale folding along N/S-trending axes. Farther south in the Araguaf fold belt around Diamantina (Fig. 2), polyphase deformation of the same nature has been identified in the prolongation of the Espinhaf, o Supergroup (Uhlein et al.. 1986; Uhiein. 1991; Marshak and Aikmi,, 1989). The principal deformation Dp or D1 is ductile shearing with tectonic transport to the west. toward the Slo Francisco (sensu stricto) craton. Lineations L1 or Lp, oriented N90 and plunging generally to the east. are marked by elongation of metamorphic minerals and stretching of pebbles. West of Diamantina, Dp or D1 deforms the sediments of the BambuI Supergroup and is therefore of Brasiliano age (Uhlein et al., 1986; Uldein and Pedreira, 1989). By comparison with the Diamantina area, the deformation affecting the Paramirim Province is Brasiliano in age. The Lagoa Real orthogueisses were dated by the U/Pb method on zircons at 1724 + 5 and by the Rb/Sr wholerock method at 1629 30 Ma, Ri ffi 0.7104 (Turpin et al., 1988). The S ~ Timoteo granitoids are dated at 1710 45 by Rb/Sr, Ri = 0.715 (Cordaul et al., 1989). A mixture of seven samples of orthogneisses and five samples of granitoids yielded an age of 1710 100 Ma by the Pb/Pb whole-rock method (Cordani et al., 1989). The albitites and uranium mineralization are associated with a hydrothermal event postdating the emplacement of the Sao Tunotoo granitoids. The uranium mineralization was dated by the U/Pb method at 1397 9 Ma (Turpin et aL, 1988). In addition, U/Pb isotopic data for zircons obtained for the Lagoa Real orthogneisses and the uranium mineralization suggest probable Pb loss around 525 and 480 Ma. Turpin et al. (1988) interpreted these to be ages of metamorphism speci:fically that of the metamorphic event associated with the Brasiliano D1 deformation.

Brasiliano folding of the Paramirim Province takes into account the we,stem vergence present in the Santo Onofre Hills and in the basement immodiately adjacent to the east (Fig. 4). The eastern vergence, known from the eastern part of the Paramirim Province (P3 unit, Fig. 3). rem,dns poorly understood. Perhaps it is associated with subverticad ductile D2 shear zones. The Brasiliano deformation is intense in the Parsmirim Province but disappears rapidly to the east and west of it. The sediments of the Una Supergroup are only slightly folded along the adjacent craton margins.

DISCUSSION AND CONCLUSIONS: DIVISION OF THE S~O FRANCISCO CRATON The Paramirim Province is characterized by the presence of the complete and thick Espinhago Supergroup be~innlng with a magmatic event that is chemically difficult to characterize because of secondary alteration but is probably alkaline. The opening of the paramirim ccmtinental rift is poorly dated. Leaving aside uncertain conelations over large distances, two hypotheses are possible. In the first, rifting is coeval with the intrusion of the SAo Timoteo granitoids (Caby and Arthaud, 1987). beginning around 1725 Ma. Alternatively. rifting is synchronous with the hydrotherreal event responsible for the uranium mineralization around 1400 Ma. Broad geochemical affinities between the magmatism of the Rio dos Remedios Group and the Sao Tunoteo granitoids favor the first hypothesis. The Middle Proterozoic age of the metamorphism and tectonism affecting the Paramirim Province is based on Rb/Sr dating of sedimentary rocks and acid metavolcanic rocks strongly affected by hydrothermal alteration. These radiometric data have low credibility and are difficult to interpret because of the large scatter of points on isochron diagrams, resulting in errochrons or poor isochrons. Ri values are always high, indicating that these rocks are of crustal origin. A Brasiliano age of the main tectonic event affecting the Paramirim Province is based entirely on structural data. The various deformation phases of the Paramirim Province and of the Brasiliano Araguat belt can be correlated. Specifically, the first deformation phase is represented in both areas by ductile shearing with tectonic transport towards the S ~ Francisco craton sensu stricto. This shearing, well defiued in outer units of the Araguat fold belt. affects the Paramirim Province in the north, whereas in the south it reworks the Early Proterozoic basement of the Quadrilatero Ferrifero. Around Salinas in the Araguat fold belt, the main metamorphism, roughly synchronous with the first deformation characterized by low dipping shearing, is dated at 630 + 30 Ma by Rb/Sr on gneisses of the outer structural units, and at 590 + 28 Ma by U/Pb on zircons from gueisses of the inner units (Siga et al., 1988). U/Pb ages of 525 and 480 Ma for the Lagoa Real complex, interpreted by Turpin et al. (1986) as dating the Brasiliano deformation, are considered here to be lateto post-Brasiliano resettings.

The Brasiliano Slo Francisco craton revisited (central Brazil)

55

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Fig. 4. Schematicgeological section of part of the Santo OnofreHills unit (PI) and of the polycyclic basementunit (P2); see Fig. 3 for location (slightly modifiedfrom Caby and Arthaud, 1987).

Refoldins of the northern extremities of the Serra do Estreito and Serra do Boqueir~lo, clearly visible on the 1/ 1.000.000 geological map of Bahia State (Inda and Barbosa, 1978), was often interpreted as proof that a preBrasiliano orogeny had occurred in the Paramirim Province. The meridional structures of both ranges, formed during the Middle Proterozoic, were thought to be refolded by Brasiliano movements of adjacent North-East Brazil or Borborema Province. However, refolding is more likely a result of interference of two Brasillano folding events: an earfier, submefidional one of the Par~mirim Province, and a later E/W-trending event affecting the North-East Brazil Province. This interpretation is supported by observations made on the Late Proterozoic Una Supergroup in the ~ basin of the Salvador craton, where two phases of folding are superposed. The older phase produced large folds with N/ S- to NNW/SSE-trending axes (Inda and Barbosa, 1978). that are parallel to, and synchronous with, the folds of Serra do Estreito and Serra do Boqueirao in the Parsmirim Province. The younger fold phA~ caused the general d~collement of the Una Supergroup. The metric to hcctomettic-sized folds with E/W-trending axes (Brito Neves, 1967) are associated with thrust faults along which the carbonates of the Una Supergroup were transported to the south (Lagoeiro, 1990). This second phase represents the effect of ddormation of the Borborema or North-East Brazil Province in the Salvador cratonic domain (Brim Neves, 1983). In the Paramirim Province, them are no structural data supporting the existence of an older, Middle Proterozoic, deformation obliterated by the Brasiliano orogeny. However, such a deformation would not affect the question of fragmentation of the Slo Francisco craton, a geological province of Brasiliano age. It must be concluded, therefore, that the Paramirim Province of Brasiliano age cuts the Sao Francisco craton (sensu Almeida, 1967, 1969) into two parts. To the west and southwest is the Sao Francisco craton senau stricto; to the east is the Salvador eraton, a western extension of the Congo craton. This subdivision, suggested since the 1970s (Cordani, 1973a,

1978), is now supported by structural and geochronological data. The Paramirim Province has two peculiarities compared with other Brasiliano-Pan African mobile belts of western Gondwana. Late Proterozoic sediments, equivalent to the Una Supergroup of the Salvador craton, are entirely missing; either they were never deposited or they were removed by erosion. A similar situation exists in the Brasiliano mobile belts of southern BrasH: in the Tijncas belt, forming the northern part of the Dom Feliciano belt (Basel, 1990), and possibly also in parts of the Ribeira mobile belt. The other peculiarity is that deformation is predominantly by ductile shear. The Parsmlrim mobile belt of Brasiliano age is interpreted either as an intracratonic belt formed by deformation of a narrow rift overriding the eastern margin of the SAo Francisco craton, or as a Himalayan-type collision zone (Caby and Arthaud, 1987; Uhlein and Pedreira, 1989). The moderate width of the deformed zone (150 lan), the absence of a suture, and the similarity of the Upper Proterozoic cratonic covers on both sides of the mobile belt all favor the first model.A negative gravimetric anomaly of up to - 5 0 regals trends NNW-SSE in the southern part of the Paramirim Province between Caetit~ and Oliveira dos Brejinhos (Fig. 3). This anomaly was regarded by geologists favoring a Himalayan-type belt as a zone of crustal thickening marking a collisional suture. However, Ussami and Bott (1989) interpret the anomaly as due to a shallow granitic body, plunging steeply to the east and shallowly to the west, located at a depth of 8 to 13 kin, depending on the density contrast adopted. Almowl~dgraents--Thefieldwork of da Silva, Karmann, and Trompettein Bahia was supported by the Secretariat of Mines and Energy of the State of Bahia. Work on the Ars~uaf mobile belt in the northeastern part of Minas G-eraisby Uhlein and Trompene was partly supported by Petrobras, FAPESP (Fundaqlo de Amparo a Pesquisa do Estado de Sao Paulo), and ORSTOM (France). We aclmowledge B. Brito Neves who provided helpful comments on an early version of the manuscript, R. Unrug (USA) who kindly translated it into English, and A. KrOner for his constructive review.

56

R. TROMPETIE, A. UHLEIN,M. E. DA SILVA, and I. KARMANN

REFERENCESAlmeida, E E M. de, 1967. Origem e Evolufao da Plataforma Brasileira. Departamento Nacional da Produ~lo Mineral, DivisAo Geologia Mineralogia (Rio de Janeiro), Boletim 216, 96 p. Almeida, F. F .M. de, 1969. Diferenciaf/[o tecttnica da plataforma Brasileira. Anais, XXII! Congresso Brasileiro de Geologia (Salvador), 29-46. Almeida, F. F. M. de, 1977. O craton do S/lo Francisco. Revista Brasileira de Geoci~ncias 7, 349-364. Almeida, E F. M. de, Amaral, G., Cordani, U. G., and Kawashita, K.,1973. The Precarnbrian evolution of the South America cratonie margin south of the Amazon river. In: Ocean Basins and Margins, Volume 1: The South Atlantic (edited by A. E. M. Nairn and E G. Stehli), pp. 411-446. Plenum Press, New York, NY, USA. Almeida, E E M. de, Hasui, Y., Brito Neves, B. B. de, and Fuck, R. A., 1981. Brazilian structural provinces: An introduction. Earth Science Reviews 17, 1-29. Basel, M. A. S., 1990. O grupo Brusque: Urea evoluqao monocfclica? Anais, XXXVI Congresso Brasileiro de Geologia (Natal) 6, 26492657. Brito Neves, B. B. de,1967. Geologia da Folhas de Upamirim e Morro do Chape~ [Ba. Relat. SUDENEICONESP 17]. Unpublished report, Companhia Nordestina Sondagens Perfuraq~es, Retire, Pernambuco, Brazil, 53 p. Bfito Neves, B. B. de, 1983. O Maps Geoltgico do Nordeste Oriental do Brasil. Escala 1 I 1.000.000. Tese Livre Do~ncia, Instituto de Geoci~cias, Universidade de Sac Panic, SAc Paulo, SP, Brazil, 177 p. Brito Neves, B. B. de, and Cordani, U. G., 1991. Tectonic evolution of South America during the Late Proterozoic. Precambrian Research 53, 23-40. Brito Neves, B. B. de, Cordani, U. G., and Torquato, J. R. E, 1980. Evoluqao geocronol6gica do Precambriano do Estado da Bahia. In: Geologia e Recursos Minerals do Estado da Bahia (edited by H. A. V. Inda and E B. Duarte). Secretsria de Minas e Energia, Coordenao da Produfao Mineral (Salvador), Textes B(tsicos 3, 1-101. Brito Neves, B. B. de, Kawashita, K., Cordani, U., and Delhal, J., 1979. A evolu~[lo geocronol6gica da corditheira do Espinhafo: Dados novos e integrsf~[o.Revista Brasileira de Geoci$ncias 9, 71-85. Caby, R., and Arthaud, M., 1987. Petrestructural evolution of the Lagea Real subalkaline metaplutonic complex (Bahia, Brasil). Revista Brasileira de Geoci~ncias 17, 636. Cordani, U. G., 1973a. Evoluftlo Geol6gica Pre-Cambriana da Faixa Costeira do Brasil Entre Salvador e Vitoria. Tese Livre Docencia, Instituto de Geociencias, Universidade de Silo Paulo, Sac Panlo, SP, Brazil, 98 p. Cordani, U. G., 1973b. DefiniqAo e caracterizaq[o do cr~lton do Sic Francisco. Anais, XXVI1 Congresso Brasileiro Geologia (Aracaju) 2, 142-145. Cordani, U. G., 1978. Comentarios filos6ficos sobre evoluct~ geol6gica prt-Cambriana e debates. Sociedade Brasileira Geologia, N~cleo Bahia (Salvador), Publica~tlo Especial_ 3, 33-65. Cordani, U. G., Kawashits, K., Sam, K., and Taylor, E N., 1989. Pb-Pb, Rb-Sr and K-Ar systematics of rock types in Lagoa Real Uranium Province, south-central Bahia, Brazil. Abstracts, XXVII1 lnterna. tional Geological Congress (Washington DC) 1, 327. Cordani, U. G., Melcher, G. C., and Almeida, E E M. de, 1968. Outline of the Precambrian geochronology of South America. Canadian Journal of Earth Sciences 5, 629-632. Costa, L. M. A. da, and Inda, H. A. V., 1982. O aulac6geno do Espinhaqo. Ci~ncias da Terra 2, 13-18. Costa, M.T. da, and Branco, J. J. R., 1961. Roteiro para a excursllo Belo Horizonte - Brssilia [ContribuiqAo ao XIV Congresso Brssileiro de Geologia]. Instituto de Pesquisas Radioactivas, Universidade de Minas Gerais, Publicafll Escola Engenharia 15, 9-25. De Wit, M., Jeffery, M., Bergh, H., and Nicolaysen, L, 1988. Geological Map of Sectors of Gondwona, Reconstructed to Their Disposition ~150 Ms, Scale 1:10.000.000. American Association of Petroleum

Geologists, Tulsa, OK, USA, and University of the Witwaterarand, Jobalmesburg, South Africa. Frdes, R. J. B., 1986. Efeitos cataclimticoe-hidrotermals nos vulc/mitos 6cidos da Chapada Diamantina - BA. Anais, XXXIV Congresso Brasileira de Geologia (Goit~nia) 4, 1440-1450. Hurley, P. M., Almeida, E E M. de, Melcher, G. C., Cordani, U. G., Rand, J. R., Kawashits, K., Vandoros, P., Pinson, W. H., and Fairbairn, Jr, H. W., 1967. Test of cominental ~il~ by comparison of radiometric ages: A pre-drifl reconstroction shows matching geologic age provinces in West Africa and northern Brazil. Science 157, 495-500. Inda, H. A. V., and Barbosa, J. F., 1978. Texto Explicativo Pars o Maps Geoldgico do Estado da Bahia, Escala 1:1.000.000. Seeretaria de Minas e Energia, Salvador, Bahia, Brazil, 137 p. hda, H. A. V., Shorscher, H. D., Dardenne, M. A., Schobbenhaus Hlho, C., Haralyi, N. L. E., Branco, P. C. A., and Ramatho, R., 1984. O crston do SAc Francisco e a faixa de dobramentes Araquaf. In: Geologia do Brasil (edited by C. Schobbenhaus, D. A. Campos, G. R. Derze, and H. E. Asmus), pp. 193-248. Departamento Nacional da Produqao Mineral, Brssflia, DF, Brazil. Isotta, C. A. L., and Rocha-Campos, A. C., 1969. Striated pavement of the Upper-Cambrian glaciation in Brazil. Nature 222, 466-468.

Lagoeiro, L. E., 1990. Estudo da Deformaf~lo nas Sequ~ncias Carbondticas do Grupo Una na Regido de Irec~, BA. Tese Mestrado, Universidade Federal de Ouro Preto, Ouro Preto, MG, Brazil, 102 p.Lobato, L M., Forman, J. M. A., Fyfe, W. S., Ketrich, R., and Barnett, R. L., 1983. Uranium enrichment in Arcbaean crustal basement associated with o v e r t ~ g . Nature 303, 235-237. Matshak, S., and Alkmin, E E, 1989. Proterozoic contraction/extension tectonics of the southern Sac Francisco region, Minas Gerais, Brazil. Tectonics 8, 555-571. Maruejol, P., Cuney, M., Fuzikawa, K., Netto, A. M., and Poty, B., 1987. The Lagoa Real subalkaline granitic complex (South Bahia, Brazil): A source for uranium mineralizations associated with Na-Ca metssomatism. Revista Brasileira de GeociOncias 17, 578-594. McReath, I., S~.,E. E J. de, and Fryer, B.J., 1981. As vulc~icas ~cidas proterozticas da regigtoda Bacia do Rio Paramirim, Bahia. In: Geologia e Recursos Minerals do Estado da Bahia (edited by H. A. V. Inda, M. M. Marinho, and E B. Duarte). Secretaria de Minas e Energia (Bahia) 4, 121-132. Pilug, R., Schobbenhans, C., and Renger, E, 1969. Contribuif~lo a GeotectOnica do Brasil Oriental [Serie Especial 9] [in Portuguese and English]. Ministerio Interior, SUDENE, Divisao C-eologia, Recif~, Pernambuco, Brazil, 55 p. Rocha-Campos, A. C., and Hasui, Y., 1981. Tillites of the MacatJbas Group (Proterozoic) in central Minas GerMs and southern Bahia, Brazil. In: The Earth's Pre-Pleistocene Glacial Record (edited by M. J. Hambrey and W. B. Harland), pp. 933-938. Cambridge Univ. Press, Cambridge, England, UK. S~, E. E J de,. 1981. A Chapada Diamantina e Faixa Santo Onofre: Un exemplo de tectonics intra-placa no Proteroz6ico medic do craton SFrancisco. In Geologia Recursos MineraIs do Estado da Bahia (edited by H. A. V. Inda, M. M. Marinho, and F. B. Duarte). Secretaria de Minas e Enargia (Bahia) 4, 111-120. S~l, E. E J. de, Barrels, R. L., Brito Neves, B. B. de, and McReath, I., 1976. Geocronologia e o modelo tectono-magrn~ltico da Chapada Diamantina e Espinhaqo setentrional, Bahia. Anais, XXIX Congresso Brasileiro de Geologia (Ouro Preto) 4, 205-227. Siga, Jr., O., Cordani, U. G., and Fonseca, A. C., 1988. The geotectonic setting of eastern Brazil and West Africa during the Brasiliano/Pan African orogeny. Abstracts, VII Gondwana Symposium (Sac Paulo), 6. Sighinolfi, G., and Conceiqito, T. M. L., 1974. Petrologia e geoquimica de riolitos alcalinos prt-carnbrianos do oeste da Bahia. Anais, XXVIII Congresso Brasileiro de Geologia (Porto Alegre) $, 89-100. Souza, S. L. de, Fr6es, R. J. B., and Moraes, A. M. V. de, 1986. Nova concepito sobre a evoluqao geotectdnica do Espinhao setentrional. Anais, XXXIV Congresso Brasileiro de Geologia (Goi,~nia) 3, 11761190.

The Brasiliano Sao Francisco cratcm revisited (central Brazil)Tavora, E J., Cordani,U. G., and Kaweshita, K.,1967. Determinaqees de idede potessio/arsonio em rochas da regiio central da Bahia. Anais, XX1 Congresso Brasileiro de Geologia (Curitiba), 234-244. Teixeira, W., and Figueiredo, M. C. H., 1991. An outline of Early Proterozoic crustal evolution in the Sao Francisco craton, Brazil: A review. Precambrian Research, $3, 1-22. Torquato, J. R., and Cordani, U. G., 1981. Brazil-Africa geological links. Earth Science Reviews 17, 155-176. Trompette, R., 1984. Geodynamic evolution of Western Gondwana in middle and upper Proterozoic time. Abstracts, XXVII International Geolologial Congress (Moscow) Z 404-405. Turpin, L., Maruejol, P., and Ctmey, M., 1988. U-Pb, Rb-Sr and Sm-Nd chronology of granitic basement, hydrothermal albitites and uranium mineralization (Lagoa Real, South-Bahia, Brazil). Contributions to Mineralogy and Petrology 98, 139-147.

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Uhlein, A.,1991. Transifilo Crdton-Faixa Dobrada: Exemplo do CrOton Silo Francisco e da Faixa Aragua{ (Ciclo Brasiliano) no Estado de Minas Gerais. Tese Doutoramento, Instituto de Geoci~ncias, Univexsidade de Sao Pauio, Slo Paulo, SP, Brazil, 295 p. Uhlein, A., and Pedreira, A. J., 1989. Considera~Cles sobre a 8eologia estrutural, tect6nica e facies sedimentares do Espinhaqo setentrienal e Chapada Diamantina (BA). Anais, V Simp6sio de Geologia de Minas Gerais (Belo Horizonte),180.183. Uhlein, A., Trompette, R.,and Silva, M. E. da, 1986. Estruturalo teotSniea do supergrupo Espinha.o na regilto de Diamantina (MG). Revista Brasileira de Geocidncias 16, 212-216. Ussami, N., and Bott, M. H. E, 1989. A gravity study of the Paramirim granite, Bahia State, Brazil. Revista Brasileira de Geocidncias 19, 170-176.