226 THE FÉ GRANITIC GNEISS, IN THE CENTRAL PART OF THE PALEOPROTEROZOIC MINEIRO BELT, SW SÃO FRANCISCO CRATON: U-Pb GEOCHRONOLOGY, Sm-Nd ISOTOPIC GEOCHEMISTRY AND TECTONIC IMPLICATIONS Nunes, L.C. 1 , Teixeira, W. 2 and Ávila, C.A. 3 1 Geochronological Research Center of the University of São Paulo, SP, Brazil. Rua do Lago 562, Cidade Universitária, 05508-080. E-mail: luciana@igc.usp.br 2 Geochronological Research Center of the University of São Paulo, SP, Brazil. Rua do Lago 562, Cidade Universitária, 05508-080. E-mail: wteixeir@usp.br 3 Departament of the Geology and Paleontology, National Museum, Federal University of Rio de Janeiro, Rio de Janeiro, RJ, Brazil. Quinta da Boa Vista, São Cristóvão, 20940-040. E-mail: avila@mn.ufrj.br Key words: Fé granitic gneiss, U-Pb geochronology, isotopic geochemistry, Mineiro belt, São Francisco Craton. INTRODUCTION AND REGIONAL GEOLOGY The geological framework of the southern border of the São Francisco Craton comprises Archean and Proterozoic units of varied age and nature, which are now exposed side by side in response to multiple magmatic, tectonic and erosional processes (Fig. 1). Such a framework can be summarized as follows: i) an Archean crust composed mainly of granulites and partly migmatized gneisses (Campo Belo, Bonfim and Belo Horizonte complexes) and greenstone belt remnants (e.g., Rio das Velhas Supergroup in the Quadrilátero Ferrífero region) - further intruded by Archean felsic plutons; ii) a Paleoproterozoic domain occurring along the southern edge of the craton, containing greenstone belts, as well as voluminous mafic and felsic plutonic intrusions; and iii) Proterozoic supracrustal sequences of different ages (Minas Supergroup, and São João del Rei, Carandaí and Andrelândia megasequences) – e.g., Ávila et al., 2006. The Paleoproterozoic plutons constitute most of the Proterozoic domain and are genetically associated with the pre-, syn- and late-tectonic stages of the Mineiro magmatic belt (Ávila 2000, Teixeira et al. 2000). In addition, these plutons exhibit Nd-Sr isotopic characteristics and calc-alkaline affinities that are compatible with products evolved from accretionary arc settings (e.g., Noce et al. 2000, Quéméneur and Noce 2000; Teixeira et al. 2005). Recent geological mapping carried out in the central part of the Mineiro belt between Lavras and Conselheiro Lafaiete regions (Fig. 1) has shown that the mafic and ultramafic rocks can be assigned to two separated metavolcano-sedimentary units - the Nazareno and Rio das Mortes belts (e.g., Ávila et al. 2004). The mafic and ultramafic rocks of both belts were overprinted by two successive deformational and metamorphic events. The first one reached low- to medium grade amphibolite facies, whereas the second developed under greenschist- to low- amphibolite facies. The latter episode also overprinted the neighboring mafic and felsic plutons of the Mineiro belt (Ávila 2000). The present work deals with the magmatic evolution of the Fé granitic gneiss, one of the plutons that crop out in the central part of the Mineiro belt. New U-Pb data and isotopic and chemical studies are presented providing insights on the tectonic evolution of this belt. FÉ GRANITIC GNEISS – GEOLOGY AND PETROGRAPHY The Fé granitic gneiss occurs between Ritápolis and Coronel Xavier Chaves, underlying an area of 12 km 2 (Fig. 1), emplaced into banded gneisses end migmatites. This elongated body shows a NNE/SSW trending foliation, and is overprinted in the south by Lenheiros shear zone. The body, as well as the nearby coeval intrusions (Brumado and Rio Grande diorites), displays amphibolite and pyroxenite xenoliths that resemble, respectively, the typical lithologies of the Rio das Mortes greenstone belt and the pyroxenitic-gabbroic bodies that crop out in the vicinities (Ávila et al. 1999). In addition, pegmatitic injections cut randomly the Fé granitic gneiss as well as the adjacent bodies (e.g., Glória quartz-monzodiorite, Avila et al., 2006). These injections are coeval with the Ritápolis granitoid (2121 ± 7 Ma - Ávila 2000), that crops out extensively in the northwest part of the studied area. The Fé granitic gneiss specimens show usually felsic and mafic bands; the modal composition is characteristic of monzogranitic and sienogranitic terms. The main mineralogical assemblage is: Quartz, K-feldspar, plagioclase, biotite, epidote, titanite and allanite. The primary texture is dominantly equigranular and subordinated porphyritic with K-feldspar megacrysts. WHOLE ROCK GEOCHEMISTRY Whole rock major and trace elements in six selected samples of the Fé pluton were carried out at the Geosciences Institute of the University of São Paulo, Brazil. The resulting, main chemical characteristics indicate predominantly a peralumious and subordinately metaluminous composition (Fig. 2a), a sub-alkaline trend