LITHOGEOCHEMISTRY OF PALEOPROTEROZOIC ORTHOGRANULITES FROM THE RIO PRETO (MG)-VASSOURAS (RJ) REGION, CENTRAL RIBEIRA BELT, SE BRAZIL MONICA HEILBRON*, RÔMULO MACHADO** & MARIO FIGUEIREDO** (in memorian) RESUMO LITOGEOQUÍMICA DOS ORTOGRANULITOS DO COMPLEXO JUIZ DE FORA NA SEÇÃO TRANSVERSAL BOM JARDIM DE MINAS (MG)-VASSOURAS (RJ), SEGMENTO CENTRAL DA FAIXA RIBEIRA As rochas granulíticas do Complexo Juiz de Fora integram o embasamento pré-1.8 Ga da escama tectônica Média do segmento Central da Faixa Ribeira. Este domínio é marcado pela interdigitação tectônica de granulitos com unidades metassedimentares meso a neoproterozóicas. O Complexo Juiz de Fora é representado, na região entre Rio Preto (MG) e Vassouras (RJ), por três conjuntos Iitológicos: ortogranulitos, ortognaisses e para- granulitos, que afloram em diversas escamas de empurrão relacionadas ao encurtamento da Orogênese Brasiliana. Os contatos tectônicos principais são caracterizados pela transformação dos granulitos em milonitos retrometamórficos para a fácies anfibolito. A investigação geoquímica dos ortogranulitos permitiu subdividi-los em duas associações tectônicas distintas: a) uma representada por duas séries cálcio-alcalinas geoquimicamente semelhantes às de arcos magmáticos modernos; b) outra com dois grupos de rochas básicas, urn toleítico e outro com basaltos transicionais a alcalinos, ambos indicando ambiente distensivo intracontinental. A relação entre estes ambientes tectônicos distintos deve ser investigada em termos geocronológicos, já que a tectônica Brasiliana obliterou a paleogeografia deste importante segmento paleoproterozóico da faixa Ribeira. Palavras chave: embasamento paleoproterozóico, margem continental ativa, magmatismo intraplaca, granulitos, Complexo Juiz de Fora ABSTRACT The granulites of the Juiz de Fora Complex (JFC) integrate the pre-1.8 Ga basement of the Intermediate Thrust Sheet of the Ribeira Belt's central segment. This tectonic domain is characterized by intense tectonic imbrication of the JFC granulites with Meso- to Neoproterozoic metasedimentary units. In the Rio Preto (MG)-Vassouras (RJ) region, the JFC comprises three lithological associations: orthogranulites, orthogneisses and paragranulites which crop out as several thrust sheets formed by intense shortening during the Brasiliano Orogeny. The main contacts are characterized by the transformation of the granulites into amphibolite facies mylonites. The geochemistry of the orthogranulites leads to the identification of two tectonic associations. One comprises two calc-alkaline series compositionally similar to rocks of modern magmatic arcs, while the other contains two groups of basic rocks, respectively the tholeiitic series and alkaline to transitional basalts, both of intraplate extensional tectonic setting. The relationships between these distinct tectonic environments need further geochronological background, as the Brasiliano shortening obliterated the original paleogeography of this important Paleoproterozoic segment of the Ribeira belt. Keywords: Paleoproterozoic basement, Continental active margin; Continental intraplate magmatism, Granulites, Juiz de Fora Complex INTRODUCTION AND TECTONIC SETTING The geochemical investigation of high grade metamorphic complexes, such as the Juiz de Fora Complex (JFC), provides key information on protoliths and their tectonic setting, as the primary textures and original relationships between different rock associations were destroyed by the intense deformation and high temperature metamorphism. Geologic investigation of granulite terranes also provides important inferences on the composition and processes of deeper segments of the conti- nental crust. This paper presents and discusses geological and geochemical data from granulites of the JFC, an important segment of the pre-1.8 Ga basement of the Central Segment of Ribeira Belt (Fig. 1) that was reworked during the Brasi- liano Orogeny. The term Juiz de Fora Series (Ebert 1957, 1968, Cordani et al 1973) was originally used to designate metasediment- derived granulites exposed in the central segment of Ribeira belt. Later, orthogranulites were also included in the JFC (Barbosa & Sad 1983a, b, Machado Filho et al. 1983, Oliveira 1980). U/Pb and Rb/Sr isotopic data indicate that the granul- ite facies metamorphism took place during the Paleoprotero- zoic Transamazonian Orogeny, further retrograded to amphi- bolite facies during the Neoproterozoic Brasiliano Orogeny (Cordani et al. 1973, Machado Filho et al 1983, Teixeira & Figueiredo 1991). The suggestion that the JFC protoliths may be Archaean (Fyfe & Leonardos 1974, Cordani et al, in Fonseca et al 1979, Oliveira 1980) completes the evolution scheme proposed for this complex. GEOLOGICAL CONTEXT OF JUIZ DE FORA COMPLEX IN THE INVESTIGATED TRANSECT Detailed geologic mapping in the Bom Jardim de Minas (MG)-Piraí (RJ) Rio Preto (MG)-Vassouras (RJ) section allowed the subdivision of the JFC into four lithologic units (Heilbron 1993): (a) orthogranulites, with amphibolite facies retrogressive mylonitic parageneses; (b) banded ortho- gneisses of amphibolite facies; (c) metasedimentary granuli- tes (kinzigites with minor orthopyroxene in leucossomes); and (d) amphibolite facies metasedimentary gneisses. In this paper and based on structural-metamorphic data combined with new U/Pb geochronological data (Machado et al 1996), we assume that the paragneisses represent Meso to Neopro- terozoic units, correlated to the Andrelândia Depositional Cycle (Andreis et al 1989). They were technically imbri- cated with the Paleoproterozoic orthogranulites and ortho- gneisses of the JFC during the Brasiliano orogeny. Thus, the JFC comprises the granulite facies basement (Pre-1,8 Ga) of the central segment of Ribeira belt. One U/Pb discordia (Machado et al. 1996) and Rb/Sr errorchrons (Heilbron 1993) support the Paleoproterozoic age of the granulite facies meta- morphism in the investigated region. * Departamento de Geologia Regional e Geotectônica, Faculdade de Geologia, Universidade do Estado do Rio de Janeiro. Rua São Francisco Xavier 524, sala A-4024, Maracana, Rio de Janeiro. CEP 20559-900. Fax- 021-2546675. Tel- 021-5877102 - e-mail: heilbron@uerj.br ** Institute de Geociências, Universidade de São Paulo, Caixa Postal 11348, São Paulo, SP, CEP 05422-970 27(l):83-98,mar9odel997 Revista Brasileira de Geociencias