The geologic framework of the central portion of the Mantiqueira Province is characterized by the diachronous evolution of the Brasília and Ribeira Belts from the Early Neoproterozoic to the beginning of the Paleozoic (750-480 Ma; Unrug, 1997). These belts were formed in association with the collisional events between the São Francisco, Congo and Paraná-Rio de La Plata Cratons (Brito Neves et al., 1999; Trouw et al., 2000; Cordani et al., 2000), which resulted in the formation of a series of superposed structures between the two belts (Interference Zone, Figure 1) that hinder the understanding of their evolutionary history (see also Hackspacher et al, this volume).
Figure 1. Tectonic framework of the central segment of the Mantiqueira Province
Tectonic framework of the central segment of the Mantiqueira Province in southwest Brazil (modified and compiled from Heilbron, 1995, Morais et al, 1999; Bizzi et al., 2001). Simplified geotectonic outline of western Gondwana. A complex structural interference zone is formed due to the superposition between the Brasilia and Ribeira orogenesis.
There are major complications in the stratigraphic correlations within the different units of the Ribeira and Brasilia Belts, which become apparent in comparisons between the classical geological papers on the region (Hasui and Oliveira, 1984; Machado Filho, et al., 1983; Machado and Endo, 1993; Heilbron et al., 1995 and 2000; Machado and Demange, 1992; Ebert and Hasui, 1998; Morais et al., 1999; Trouw et al., 2000; Campos Neto, 2000; Ribeiro et al., 2003). There is no consensus as to nomenclature, rock components or names of different stratigraphic units (Table 1). The major controversy refers to a unit of meta-sedimentary rocks, including gneissic and migmatitic rocks with calc-silicate, marble and metabasic rock intercalations. This unit is interpreted to be part of the Paraíba do Sul Complex (Hasui and Oliveira, 1984) or the Embu Complex (Morais et al., 1999; Campos Neto, 2000) or even the Andrelândia Group (Heilbron, et al., 2000 and 2003, Trouw et al., 2000 and 2003) and the Juiz de Fora Complex (Campos Neto and Figueiredo, 1995). A good example of this divergence is shown in Figure 2.
Figure 2. Comparative geological maps of the studied area
Comparative geological maps of the studied area showing the contrasting interpretations between A: Campos Neto (2000) and B: Heibron et al. (2000).
More recently, Sm-Nd isotopes are being used as tracers of tectonic environments to help understanding the complex evolutionary history of the Neoproterozoic fold belts, enabling the identification of different juxtaposed terranes involving similar structural and lithologic characteristics, and showing distinct isotopic signatures (Dantas et al. 2000; Pimentel et al. 2001; Van Schmus et al. 2003). This paper presents new isotopic results related to the rocks exposed along the border between São Paulo and Rio de Janeiro states, in SE Brazil, where the tectonic-stratigraphic correlation is uncertain. The aim is to assess the tectonic framework of this central part of the Ribeira Belt, based on a systematic Sm-Nd isotopic study combined with remote sensing and digital terrain model data, in order to constrain the nature and location of the crustal boundaries between different Precambrian terranes.