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THE BRASILIANO OROGENY AT THE CENTRAL SEGMENT OF RIBEIRA BELT, BRAZIL The structure of the central segment of the Ribeira belt is defined by three large thrust sheets (Lower, Intermediate and Upper) which override the autochthonous domain, towards the southern portion of the São Francisco Craton. Four lithotectonic associations are individualized within the domains: basement rocks (pré-1,8 Ga rocks); orthogneisses of unknown age; Post-1,8 Ga metasedimentary cover and Brasiliano granitoid rocks. Continental tholeiitic deformed dykes and disrupted lenses within the cover indicate extensional periods. Investigation of the tectono-metamorphic and magmatic aspects, associated with new U/Pb geocronological data support the subdivision of Brasiliano Orogeny within three main tectonic periods: a) the syn-collisional period (590-563 Ma) is represented by the main deformation (D 1+2), coeval with the MI metamorphic stage and γl-γ3 granitoid rocks. Crustal shortening took place initially through ductile thrusts and tight recumbent folds. The late stage of this collisional event is characterized by oblique convergence, with pervasive deformation throughout the belt. MI had its thermal peak during D 2 , and is characterised by intermediate to high pressures. The spatial distribution of the metamorphic zones defines an inverted gradient. Deformed I and S type granitoid rocks occur during this period, b) the post-collisional period (535-520 Ma) is characterized by D 3 deformation, coeval with M 2 metamorphic stage. During DS phase, the oblique convergence continued, but was accomodated by late to post-metamorphic peak strike slip ductile shear zones. Between these shear zones, shortening was achieved by steep folding of S 2 foliation. The Ma stage with lower pressure regime is associated with intense melting of the basement and the cover, resulting in the generation of I-type magmatism and subordinated S-type; and c) the post-tectonic period is represented by the Post-D4 γ 5 magmatism registered between 503 to 492 Ma. This magmatism may be related to melting of crustal rocks associated with mantelic contribution related with high temperature conditions and uplift just after the Brasiliano Orogeny.

2l 30 30 5,1.3-Metabasitos do cDÀ 5.1.4-ortognaisse do conpLexo t'lantiqueira 5.1.5-Rochas granitóides intrusivas 5.1.5.1-Granito porfiróide xistoso 5.1.5.2-Granito porfíróide Serra do Lagarto 5.1.5.3-Leucogranitos 5.2-DOMÍNIO TECTÔNICO JUIZ DE FORÀ (DrJr) ciclo Depos ic iona I 5.2.3-Metabasitos no cDA 5.2.4conplexo Juiz de Fora (cJF) 5.2.5-Rochas granitóideg intrusivas 5.2.5.1-Granltóide Rio Turvo 5.2.5.2-Granito Porfiróide 5.3-DOMfNIO TECTÔNICO PÀRAÍBÀ DO SUL (DTPS) 5.3.1-Introdução ã-E*A-Grupo Parafba do su1 (GPs) 5.3.3-sufte Quirlno-Dorândla (SIQD) 5.3.4-Rochas granitóides intrusivas 5.3.4.1-Granitóide Serra da Concórdia 5.3.4.2-Leucogranito serra do lpíranga 5.4-,UNTDADES LITOLóGICAS FANEROZóICÀS 5.4.1-Diques de diaþásio e microgabro 5.4.2-Rochas alcalinas e brechas tectônicas CÀPÍÎULO 6-GEOLOGIÀ EATRUTTRATJ 6.1-TNTRODUçÃo 6.2-ÀNÃLISE GEOMÉÎRICA DÀ DEFORMÀçÃO PRECÀMBRTANA 6.2,L-Fases da Deformação Principal 6.2.L.1-Donfnio Àndrelândia Alóctone (DTAND) Dobras Dl Foliação S1 Dobras D2 Foliações s2 e s1+2 Lineação de-crenülãção e de intersecção Lineação rnineral 6.2.L.2-Donfnio Juiz de Fora (DTJF) Foliação s2 Dobras L,ineação ¡nineral-6.2.L.3-Do¡nfnio Rectônico Parafba do suL (DTPS) Xistosidade S1 Dobras D2 Foliação s2 5.2.L-Introdução 5,2.2-l,fetassedinentos do Andrelândia 49 58 lll lìffi:Eå: *i"ål:i""'ção Li2 6.2.2-Deformações tardias D3 e D4 6.2.2.1-Dobras D? 6.2.2.2-Zonas de-cisaLharnento Di 6.2.2.3-ctivagèrn disjuntiva e xÍstosidade de crenulação s? 6,2.2.4-Lineações de-intersecção e crenulação D3 6.2.2.5-Fraturas de extensão e peg'natitos associados à defornação D3 6.2.2.6zonas de cisalha¡nento e dobras D4 6.3-SÍNTESE DA DEFORMAçÃO pRecrulaRl¡r'le E ]NTERPRETAçÃO CINEMÁTTCA 6.3.1-Deformação Principal (D1+D2) 6.3.2-Deforrnaçóes tardias 6.4-DEFORMAçÃO RÍIPTIL MESO-CENOZóICA cApf TI¡1,o 7-Evor,¡tçÃo Dog PgrJSos t'fETÀt'rôRFrcoa PREcÀllBRrÀNoa i-sul i¡rlcÃo cott å onronueçÃo! coNsxDERAçõEs DE À¡TBTENTAçÃo GEOTECTôNICÀ

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This paper focuses the tectonic evolution of the southern Brasília belt, with emphasis on the Furnas segment, along the 21°S parallel. The uppermost structural unit (Passos Nappe -PN) comprises a highly deformed metasedimentary succession interpreted as a fragment of the Neoproterozoic passive margin of western São Francisco craton. An inverted metamorphic gradient ranging from greenschist to lower granulite facies of medium to high-pressure regime characterizes the PN as relict of a subduction zone. The External Domain displays a complex imbrication of basement rocks (Archean Piumhi greenstones, a turbiditic graywacke succession and a calc-alkaline granitoid suite) with undated siliciclastic low-grade metasedimentary rocks. The São Francisco Craton (SFC) comprises pre-1.8 Ga basement rocks covered by anchimetamorphic Neoproterozoic carbonatic shallow marine platform deposits of the Bambuí group. The Brasiliano thrust stacking generated a coarse clastic influx of molassic character on the foreland zone of São Francisco Craton, coeval with the exhumation of the External Domain thrust sheets. New K-Ar determinations on mineral separates are presented and interpreted among previous data. The SFC basement rocks display Paleo-to Mesoproterozoic cooling ages. The allochthonous units, in contrast, display K-Ar ages within the 560-675 Ma range. Brasiliano thrust stacking is therefore interpreted to have taken place onto a "cold" São Francisco craton foreland, in a thin-skinned style, as basement rocks were not heated enough to have their K-Ar systems reset during the allochthony.Keywords: Brasilia Belt, K-Ar, Neoproterozoic, thrust tectonics, Brasiliano orogeny 1 -UERJ -Rio de Janeiro State University (cmval@uerj.br); 2-USP -São Paulo University; 3-UNESP -São Paulo State University; 4-CNPq researcher. INTRODUCTIONThe Brasília belt is an important tectonic element of the Tocantins orogenic Province, developed during the Neoproterozoic Gondwana assembly, through convergence between the Amazonian, São Francisco and a third cratonic block presently covered by the Paraná Basin (Brito Neves et al. 1999). The southern portion of Brasilia belt (Fig. 1) has its eastern tectonic elements consistently thrusted eastwards towards the border of São Francisco craton (SFC). Southwards, the Ribeira belt, displaying NE-SW structural fabric along the Atlantic coast and a relatively younger tectonic development, with metamorphic peaks dating around 580 and 530 Ma (Machado et al. 1996) truncates it.This article presents an overview of the tectonic organization of southern Brasilia belt, with emphasis on the Furnas segment, along the 21°S parallel. In that region, the Passos nappe displays low to highgrade Brasiliano metamorphism and was exhumed and thrusted at least 250 km onto a relatively cold foreland zone (eastern border of the São Francisco craton) as thin-skinned tectonic slices. K-Ar data help to constrain the timing and thermal conditions of allochthony.

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RESUMO GEOQUÍMICA DOS ORTOGNAISSES GRANULÍTICOS DO COMPLEXO JUIZ DE FORA, SEGMENTO CENTRAL DA FAIXA RIBEIRA, BRAZIL O Complexo Juiz de Fora (CJF) foi investigado em sua área-tipo (região de Juiz de Fora, MG) e está inserido em um contexto de evolução geológica complexa, policíclica, com o envolvimento de eventos deformacionais e metamórficos de alto grau recorrentes, que se desenvolveram desde o Arqueano até o Neoproterozóico-Neocambriano. O CJF é constituído de ortognaisses, migmatitos e metabasitos na fácies granulito, tendo, subordinadamente, ortognaisses na fácies anfïbolito. O mapeamento geológico mostrou que o CJF ocorre como escamas tectônicas interdigitadas a rochas metassedimentares pós-1.8 Ga., com paragêneses da fácies anfïbolito, e a paragnaisses migmatíticos com ortopiroxênio. Eventos tectônicos recorrentes obliteraram generalizadamente as feições e a paleogeografïa original. O artigo visa investigar as características litogeoquímicas dos ortogranulitos do CJF e elucidar aspectos da sua evolução magmática original e ambiente geotectônico. Dados de campo e petrográficos permitem subdivir estes ortogranulitos em granulitos máficos, intermediários e félsicos. Os elementos imóveis e ETR sugerem que (a) os granulitos máficos são toleiitos de provável evolução a partir de um mesmo magma parental, (b) os intermediários e félsicos são calcioalcalinos e possuem evidências que sugerem relação petrogenética entre alguns de seus litotipos, e (c) nenhuma relação petrogenética parece existir entre os granulitos máficos e os demais.ABSTRACT The Juiz de Fora Complex (JFC) in the surroundings of Juiz de Fora (MG) town records a complex geological evolution, with recurring deformational and high-grade metamorphic events from the Archean to the Lower Cambrian. The JFC comprises pre-1.8 Ga. granulite facies orthogneisses, migmatites and metabasites, with subordinated amphibolite facies orthogneisses. The JFC occurs as thrust sheets within post-1.8 Ga amphibolite facies metasedimentary rocks and within orthopyroxene-bearing migmatitic paragneisses of undetermined age. Recurring tectonic events led to the general obliteration of original textures, structures and paleogeography. Field and petrographic data allow the subdivision of the orthogranulites into mafïc, intermediate, and felsic granulites. Imobile and Rare Earth elements suggest that (a) the mafic granulites are tholeiitic and that each sample represents different stages of differentiation of one parental magma, (b) the intermediate and felsic granulites are calc-alkaline, and (c) no petrogenetic link seems to exist between the tholeiitic and the calc-alkaline rocks, but some evidences point to possible link among the calc-alkaline rocks.