Integration of new Sm-Nd isotopic data (Table 2) with previously published regional Sm-Nd data (Table 3) allows the re-interpretation of the architecture and tectonic framework of this part of the Ribeira Belt during the Neoproterozoic (ca 600 Ma).
The data presented above suggest that:
The basement rocks to the supracrustal sequences are dominated by Archaean to Paleoproterozoic units. The bimodal distribution of TDM model ages of migmatites and orthogneiss (2.65 to 2.7 and 3.39 Ga; Figure 17A) from the basement rocks indicate that source rocks in this region are isotopically comparable to basement rocks elsewhere in the Ribeira Belt (Fischel et al., 1998; Ragatky et al. 2000; Fetter et al. 2000, Figure 19). The consistency in the isotopic characteristics of basement rocks over these regions (> 200 km lateral extent) may indicate that they share a common Archean crustal basement, except for Costeiro Domain, where no old basement rocks have been identified.
The Sm-Nd data of supracrustal rocks demonstrates that each domain has a distinct Nd isotopic signature. Despite the likely existence of an Archean basement, most of the supracrustal sedimentary rocks in these domains have much younger TDM model ages (1.4 - 2.2 Ga) reflecting significant addition of juvenile components at or after 2.2 Ga (Figure 18A). Along an E-W transect across the Ribeira Belt, TDM model ages and Nd(600) values decrease to the Costeiro Domain in the east, as represented in Figure 20.
Another potentially important result is that the supracrustal rocks of the Embu Domain may define a bimodal distribution of TDM model ages. Biotite gneiss with TDM model ages of around 1.5 to 1.6 Ga and small negative Nd(600) values (-7 to -11) suggest the important participation of a Neoproterozoic juvenile source, whereas other rock types (sillimanite-gneiss) yielded predominant Paleoproterozoic model ages (1.86 to 1.96 Ga);
The gneisses and migmatites of the Costeiro Domain yielded TDM model ages between 1.27 and 1.42 Ga, with Nd(600) values around -4 and -5. The isotopic signature of these rocks is very homogeneous compared with those of the other domains in the Ribeira Belt. Also in the Costeiro Domain, the source rocks are more radiogenic than other domains, suggesting the participation of a significant (but certainly not dominant) Neoproterozoic juvenile source (Figure 18, 19).
In contrast to the Costeiro Domain, the granitoids of the Andrelândia, Embu and São Roque Domains display similar and large variations of TDM model age values between 2.2 - 1.6 Ga, with dominant values around 1.7 Ga. The observed heterogeneity may reflect derivation from both Paleoproterozoic and younger lithosphere/crust.
Integrated isotopic signature of basement, supracrustal sequences and granitoids in the Ribeira Belt in this area suggest that the Andrelândia, Embu and São Roque Domains constitute a single lithospheric block. A similar situation is described by Dantas et al. (2000) in the southern portion of the Ribeira Belt. However, this interpretation is in disagreement with the proposal that the different domains represent distinct tectono-stratigraphic terranes separated by suture zones (Campos Neto, 2000; Almeida, 2002).
The isotopic signature contrast between the Costeiro and Embu Domains reinforce the interpretation that the Cubatão shear zone represents a suture zone, based on the gravimetry data by Almeida (2002). Also, the Costeiro Domain have similar TDM model ages to those observed in rocks of the Rio Negro Magmatic Arc, about 150 km to the east of the area studied (Tupinambá et al., 2000). We suggest, therefore, that the arc extends to the Paraíba do Sul Valley region, as proposed by Trouw et al. (2000) and Campos Neto (2000).
The Ribeira Belt resulted from the collision between the São Francisco-Congo and Rio de La Plata Cratons during the assembly of west Gondwana during the Brasiliano orogeny. Fragments of these cratons may be present as slices in the crystalline basement of the Ribeira Belt, as indicated by the isotopic results on basement rocks (Figure 20). Zircon inheritance work could help define the absolute ages of these buried slices. The lack of TDM model ages between 2.6 and 2.3 Ga in the metasedimentary sequences indicate minor participation of Archean rocks as sediment sources to form the Neoproterozoic sequences.
Figure 19. Tectonic subdivisions of the Mantiqueira Province
Tectonic subdivisions of the Mantiqueira Province, compiled and modified after Heilbron, 1995, Morais et al, 1999; Bizzi et al., 2001.
Figure 20. Schematic geologic cross-sections
A: Schematic geologic cross-section of the Ribeira Belt.
B and C: TDM model age profiles for basement and supracrustal rocks and granitoids.
The profile shows that the basement rocks (purple) yield Archean TDM model ages, and that the supracrustal rocks in the Embu Domain are consistently younger than those in the Andrelândia Domain, and Costeiro Domain is younger than the other domains.