The Ribeira Belt (Hasui et al., 1975) extends over approximately 1,400 km along the southeastern Brazilian coast, where its central segment - to the south of the São Francisco Craton - is formed by different juxtaposed crustal slices trending NE-SW, metamorphosed at medium to high-grade conditions (Heilbron, 1995; Trouw et al., 2003). Tectonothermal and magmatic activities occurred between 700 - 450 Ma (Trouw et al., 2000). The metamorphic peak took place around 595-565 Ma (Machado et al., 1996), contemporaneously with a thrusting tectonic event with NE-vergence, and was associated with syn-collisional granite magmatism. A system of dextral strike slip shear zones crosscuts older fabrics of the Ribeira Belt and controls late-collisional (540-520 Ma) to post-tectonic (520 and 480 Ma) granite intrusions (Heilbron, 1995; Trouw et al., 2000).
The tectonostratigraphic framework discussed here is based on the integration of published maps, radar images and the digital terrain model (DTM) of the Shuttle Radar Topography Mission (SRTM) project, description of 276 outcrops, and whole-rock Sm-Nd isotopic data. The main geologic maps used in the integration were: the tectonic framework map from the central Ribeira Belt in Rio de Janeiro state (Heilbron et al., 2000), Santos and Guaratinguetá sheets (Morais et al., 1999), at 1:250.000 scale; geologic maps from Fernandes (1991), Pereira (2001) and Almeida (2002) at the 1:100.000 scale; and the semi-detailed work of Cordani (1976) and Riccomini (1989). The nomenclature used does not follow a specific author but uses the tectonostratigraphic denomination most suitable to the local geologic context, and the more accepted terms found in literature.
In order to simplify the understanding of regional geology, the results are described according to three major units: basement rocks (Mantiqueira Complex); meta volcano-sedimentary and sedimentary sequences defining the São Roque, Andrelândia, Embu and Costeiro Domains; and Brasiliano granite plutons (Figure 3).
Figure 3. Geological map of the Paraíba do Sul Valley
Geological map of the Paraíba do Sul Valley in the States of São Paulo and Rio de Janeiro.
The rocks of the basement (Mantiqueira Complex) form an elongated slice trending NE-SW, tectonically interlayered with rocks from the Embu Domain, or along the contact between the Andrelândia and the Embu Domain (Figure 3). Basement rocks are commonly migmatitic, porphyritic hornblende-biotite gneiss, with composition varying from tonalite to granodiorite (Figure 4), with a variety of migmatite structures - stromatic, agmatic, schöllen, schlieren - with prevalence of stromatic structure showing layers and lenses of hornblendite (Figure 5). Leucosomes are layered, centimetric to decimetric, and may present hornblende porphyroblasts. Its composition varies from granite to monzogranite, while the mesosome is of tonalite composition. Locally homogeneous metagranitoids are associated with the migmatites (e.g. south of the town of São Luiz do Paraitinga, Figure 3).
Figure 4. Porphyritic hornblende-biotite gneiss of the Mantiqueira Complex
Porphyritic hornblende-biotite gneiss of the Mantiqueira Complex, showing a sub-horizontal foliation. (UTM23S - 420113/7423675)
Figure 5. Migmatitic gneiss of the Mantiqueira Complex
Migmatitic gneiss of the Mantiqueira Complex showing layers and lenses of hornblendites. (UTM23S - 508924/7504977)
Andrelândia Domain. The main components of this unit are migmatitic and mylonitic (sillimanite - garnet)-biotite banded gneiss (Figure 6), and hornblende tonalitic to granodioritic gneiss (Figure 7). There are centimetric to metric intercalations of quartzite, quartz-schist, thin layers of marble and sperssatite quartzite, amphibolite and common calc-silicate rocks associated with the garnet-biotite gneiss. This domain occurs to the north of the Rio Preto shear zone (Figure 3), and is considered to be part of the Andrelândia Group of Trouw et al. (1980, 2000), although some authors believe that they belong to the Piracaia Complex - Socorro Terrain (Morais et al., 1999; Campos Neto, 2000; Pereira et al., 2001).
Figure 6. Sillimanite-biotite gneiss
Sillimanite-biotite gneiss of the Andrelândia Group showing regular banding. (UTM23S-521141/7509504)
Figure 7. Banded hornblende migmatitic gneiss
Banded hornblende migmatitic gneiss of the Andrelândia Domain. (UTM23S-421327/7501520)
Embu Domain. The Embu domain crops out between the NE-SW trending Rio Preto and Cubatão shear zones, and is covered by the Tertiary Taubaté and Resende Basins (Figure 3). This unit is composed of a variety of rock types, including hornblende-biotite gneiss, biotite gneiss, garnet-biotite gneiss, sillimanite-garnet gneiss (kinzigite), quartz-biotite-feldspar migmatite, quartz-feldspar schist, quartzite, calc-silicate rock, amphibolite and orthogneiss (Hasui et al., 1975; Fernandes, 1991; Morais et al., 1999 and Pereira et al., 2001). Orthopyroxene-bearing granulites were described by Cordani (1976) and Almeida (2002) as part of this unit.
Three different sequences were identified in this domain. A paragneiss sequence is characterized by biotite gneiss, garnet-biotite gneiss, sillimanite-garnet gneiss, quartz-muscovite-sillimanite schist, with rare layers of quartzite and amphibolite and occasional layers of sillimanite schist (Figure 8). A biotite gneiss sequence is fine to medium-grained and gray and has migmatites of tonalitic to granodioritic composition. Banded migmatites are the dominant lithology and have white to pink, coarse-grained leucosome composed of quartz, biotite and K-feldspar, and melanosome composed of (hornblende)-biotite banded gneiss. Pegmatite dikes are also common (Figure 9). A schist sequence is composed of fine-grained muscovite-biotite schist, with subordinate quartz-biotite schist, sillimanite-biotite and quartz-feldspar schist (Figure 10), and is commonly deeply weathered, making their mapping difficult. Tourmalinite layers can be used as a regional layer guides in this unit, and laminated rhythmites alternated with biotite gneiss, quartzite, and amphibolite occur locally. The presence of low-grade schists (including biotite, muscovite and feldspar) was registered by Ebert (1968).
Figure 8. Sillimanite-garnet-biotite schist
Sillimanite-garnet-biotite schist of the Embu Domain showing a small centimeter lens rich in sillimanite. (UTM23S-481508/7471975)
Figure 9. Migmatitic biotite gneiss of the Embu Domain
Migmatitic biotite gneiss of the Embu Domain with pegmatite intrusions related to a sub-vertical shear zone. (UTM23S-533705/7496420)
Figure 10. Quartz-biotite schist
Quartz-biotite schist interlayered with quartzite showing tourmaline rich lenses. Embu Domain. (UTM23S-492424/7463760)
Paraíba do Sul Domain. Metasedimentary rocks and orthogneisses of the Paraíba do Sul Domain occur in the southeast portion of the studied area (Figure 3). This domain corresponds to the Paraíba do Sul klippe defined by Heilbron (1995) and is represented by the Quirino Complex (Valladares, 1996; Machado et al., 1996) and Paraíba do Sul Complex (Machado Filho et al., 1983). The Quirino Complex includes homogeneous granite to granodiorite orthogneiss, with enclaves of basic and calc-silicate rocks. The Paraíba do Sul Complex include a sequence of pelitic schists and biotite gneiss with marble, calc-silicate and a few quartzite layers (Almeida et al., 1993).
Costeiro Domain. This domain appears close to the coast of São Paulo and Rio de Janeiro States, separated from the Embu and Paraíba do Sul Domains by the Cubatão and Paraiba do Sul shear zones (Figure 3). Two different rock sets prevail in this domain: a) biotite granite gneiss and migmatite (Figure 11), and b) paragneiss intercalated with schists and quartzites (Figure 12). Lenticular hornblende-biotite tonalite gneiss bodies and xenoliths of amphibolite are commonly migmatized.
São Roque Domain. There are several meta volcano-sedimentary sequences, characterized by low-grade (greenschist facies) metamorphism, cropping out between the Jundiuvira and Buquirinha shear zones grouped into the São Roque Domain (Figure 3; Janasi, 1999; Juliani et al., 2000, Hackspacher et al., 2000; Campos Neto, 2000). This unit is composed of a prevailing psammite and pelite associations (phyllites, quartzite, (sillimanite-garnet)-biotite schist, garnet schist) with carbonate layers and basic to intermediate metavolcanic rocks.
There are several granitoid bodies intruding the domains described above and originated during the Brasiliano orogeny (Heilbron, 1995; Wernick, 2000). As a rule, plutons in the Ribeira Belt are elongated with a NE-SW trend, associated with strike slip shear zones (Figure 3), and present a well-developed tectonic fabric. These bodies are easily recognized in satellite and radar images. On the SRTM digital terrain model (DTM), plutons can be distinguished from the host rocks by high topography (Figure 13). Four magmatic episodes were identified in the Ribeira Belt in the Rio de Janeiro segment (Heilbron, 1995; Trouw et al. 2000): 1) pre-collisional magmatism (630-600 Ma); 2) early to syn-collisional (595-565 Ma) generated during the peak of metamorphism at around 580 Ma, and associated with the main deformational event; 3) syn-collisional magmatism (565-540 Ma), associated with the main transcurrent shear zones; and 4) late-collisional magmatism, granitoid plutons and dikes not foliated (520-480 Ma).
Figure 13. Santa Branca Granitoid associated with the Taxaquara shear zone
The granitoid shows sigmoidal form. The area of flat topography to the north is underlain by basin sediments. The NE-SW regional fabric of the area can be seen in this image. SRTM image with a directional filter. Insert on the upper left-hand side indicates the location of the image on the regional geological map of Fig. 3.
S-type intrusions are predominantly gray to white, porphyritic, two-mica granitic to granodioritic composition, represented by the Quebra Cangalha Granites in the Embu Domain (Morais et al., 1999; Trouw et al., 2003). I-type intrusions are characterized by (hornblende)-biotite granites, monzogranite, granodiorite and tonalite, represented by the Natividade pluton in the Costeiro Domain and the Lagoinha-Turvo pluton in the Embu Domain.