The overview presented below is mainly based on the following references: Hasui et al. (1975), Heilbron et al. (1995, 1998a, 2000, 2003a,b,c), Figueiredo and Campos Neto (1993), Campos Neto and Figueiredo (1995), Ribeiro et al. (1995), Schmitt et al. (2004), Valeriano (1999), Campos Neto et al. (2005), Machado and Demange (1992, 1994), Valeriano et al. (1993, 1995, 2000, 2004), Machado et al. (1996a,b), Tassinari and Campos Neto (1998), Campanha and Sadowsky (1999), Campos Neto (2000,), Ribeiro et al. (1995), Trouw et al. (1986, 2000), Campanha (2002), Heilbron and Machado (2003), Pedrosa-Soares et al. (2003b), Silva et al. (2005). For new geochronological results for the Ribeira and Brasília Orogens see Hackspacher et al. (this volume).
The southern part of the Brasília Orogen has an overall NNW-SSE trend (Figure 6, 7, 8 and 9) and is subdivided in nappes with tectonic movement to E or ESE, towards the São Francisco Craton or tangent along its southern border. These nappes result from an early collisional stage (ca. 630-625 Ma) between the São Francisco paleocontinent and another paleocontinent to the west, called Paranapanema, now hidden under the Phanerozoic Paraná Basin. The nappes can be subdivided into a group of lower nappes and a group of upper nappes. The lower nappes are characterized by inverted metamorphism reaching relatively high pressure granulite facies in their upper portion (Table 1; Figure 8, 9 and 16) and contain basement slices related to the São Francisco Craton. The upper nappes are also partially in the granulite facies but with lower pressures; they contain numerous calc-alkaline granitic intrusions interpreted to represent remnants of a magmatic arc from the upper plate. Recent syntheses on this region are given in Valeriano (1999), Ribeiro et al. (1995, 2003), Ebert et al. (1991, 1996, 1998), Paciullo (1997), Paciullo et al. (2000), Campos Neto (2000), Campos Neto and Caby (1999, 2000), Trouw et al. (2000) and Janasi (2002).
Figure 6. Tectonic map of the central segment
Tectonic map of the central segment of the region discussed in this paper. Legend: 1- Cenozoic rifts; 2- Late Cretaceous to Paleogene alkaline rocks; Brasília Orogen (3-4): 3- Lower Nappes; 4- Upper Nappes; 5- Basement of CSF and autochthonous domain of CSF; 6- São Francisco Supergroup; 7- Metasedimentary rocks of autochthonous domain; Ribeira Orogen (8-13): 8- Andrelândia Domain and 9- Juiz de Fora Domain of the Occidental Terrane; 10- Paraíba do Sul Klippe; 11- Oriental Terrane including 12- Rio Negro Arc; 13- Cabo Frio Terrane; Apiaí Terrane (14-15): 14- São Roque and Açungui Terranes; 15- Embú Terrane. CTB- Central Tectonic Boundary. AB-section on Fig. 7; rectangles indicate the insert of Figs 8 and 10.
Figure 7. Structural section of the Ribeira Orogen
Legend: Occidental Terrane (1-6): 1-3 Andrelândia Megasequence divided into Autochthonous, Andrelândia and Juiz de Fora Domains, respectively; 4 to 6- basement associations (Barbacena, Mantiqueira and Juiz de Fora Complexes); Paraíba do Sul Terrane (7-8): 7- Paraíba do Sul Group; 8- Quirino Complex; Oriental Terrane (9-13): 9- Cambuci Sequence; 10- Italva Sequence; 11- Costeiro Sequence; 12- Rio Negro Arc; 13- collisional granitoids; Cabo Frio Terrane (14-15): 14- Búzios and Palmital Sequences; 15- Região dos Lagos Complex. CTB- Central Tectonic Boundary.
Figure 8. Geologic map of the southernmost Brasília Orogen
Geologic map of the southernmost Brasília Orogen (modified from Trouw et al., 2000; Ribeiro et al., 2003; Heilbron et al., 2004). Legend: Andrelândia Mega sequence (1-6): Carrancas Sequence (1-4): 1+2- paragneiss with amphibolite, quartzite and schist; 3- greenish muscovite-bearing quartzite; 4- gray phyllite/schist with minor quartzite; Rio do Turvo Sequence (5-6): 5- plagioclase biotite schist/gneiss; 6- biotite schist/gneiss with amphibolite, quartzite, gondite and calc-silicate rocks; GA- anatectic granites; Gn- Guaxupé Nappe; MC- Carandaí Megasequence: Barroso and Prados sequences; DL- Lenheiro delta: Lenheiro Sequence; PT- Tiradentes Megasequence (Tiradentes, São José and Tejuco Sequences). Basement associations: I- Greenstone Belts; II - Gneissic Complexes, Gr- Granitoids, Mg- Metagabbro, Ms- Minas Supergroup, Dc- Capivarí Diorite, Gp- Piedade gneiss, Gm- Matola Syenitic Gneiss.
Figure 9. Structural sections of southern Brasília Orogen
Structural sections of southern Brasília Orogen. For legend see Figure 8.
Table 1. Tectonic subdivision of the southern Brasília Orogen
| Orogen | Type | Terranes | Structural Units | Collisional Events |
|---|---|---|---|---|
| Brasília Orogen | Allochthonous | Lower nappes | Carrancas Klippe/Nappe Luminárias Nappe São Tomé das Letras Nappe Carmo da Cachoeira Nappe Varginha Nappe | Collision I ca. 630 Ma |
| Upper nappe | Guaxupé Nappe | |||
| Zone of interference with Ribeira Orogen | Allochthonous | Lower nappes | Andrelândia Nappe, Liberdade Nappe (within the Andrelândia Domain of the Ribeira Orogen) | Collision I ca. 630 Ma (remnants) Collision II ca. 580 Ma |
| Upper nappe | Socorro Nappe | Collision I ca. 630 Ma Collision II ca. 580 |
The NE-SW trending Ribeira Orogen results from the interaction (collision) between the São Francisco Craton with microplates to the SE and with the southwestern part of the Congo Craton (Figure 2). This collisional stage (Collision II, ca. 580 Ma) produced several northwest vergent imbricated terranes. Since this collision was oblique the deformation was partitioned in frontal compression and dextral transpressive components. In consequence, the contacts between tectonic units are usually steep and with oblique movement in this orogen, in contrast to the southern Brasília Orogen where they are flat.
The Ribeira Orogen is subdivided into five tectono-stratigraphic terranes (in the sense of Howell, 1989), separated by thrust faults or by transpressive shear zones with oblique movement. These terranes are: Ocidental, Paraíba do Sul, Embú, Oriental (Serra do Mar Microplate) and Cabo Frio Terranes (Table 2, Heilbron et al., 1998b). The docking of these terranes resulted in crustal scale thrust sheets with NW vergence, towards the São Francisco Craton (Figure 6, 7, 10 and 11). The first four terranes were docked around ca. 580 Ma and the last, the Cabo Frio Terrane, at ca. 520 Ma.
Figure 10. Geological map of the Ribeira Orogen
Geological map of the Ribeira Orogen modified after Heilbron et al. (2004). Legend: 1- Quaternary cover; 2- Tertiary rift basins; 3- Late Cretaceous to Paleogene alkaline rocks. (4-9): Syn- to post-collisional granitoids; 4- post-collisional biotite granite (510-480 Ma, G5); 5- syn-D3 granitoids (535-520 Ma, G4); 6- late-collisional granites and charnockites (ca. 560 Ma, G3); 7- Porphyritic syn-collisional granites (590-560 Ma); 8- syn-collisional leucogranites and S-type to hybrid charnockites (ca. 580 Ma, G2). Granitoids of undetermined ages (9-10): 9- Hornblende granite; 10- Anta and São Primo suites; 11- Rio Negro Magmatic Arc (790-620 Ma, G1-pre-collisional). Occidental Terrane (12-17): Andrelândia Mega sequence (12-14): 12- Rio do Turvo Sequence (high-P granulite facies); 13- Rio do Turvo Sequence; 14- Carrancas Sequence; 15- Mantiqueira Complex; 16- distal facies of the Andrelândia Megasequence in the Juiz de Fora Domain; 17- Juiz de Fora Complex; 18- Embú Complex; Paraíba do Sul Terrane (19-20): 19- Paraíba do Sul Group; 20- Quirino Complex. Oriental Terrane (21-22): 21- Italva succession; 22- Costeiro succession; Cabo Frio Terrane (23-24): 23- Búzios and Palmital succession; 24- Região dos Lagos Complex.
Figure 11. Structural sections of Ribeira Orogen
Structural sections of Ribeira Orogen after Heilbron et al. (2004). Legend as in Fig. 10.
Table 2. Tectonic subdivision of the central part of the Ribeira Orogen
| Terranes | Structural domains | Collisional period | ||||
|---|---|---|---|---|---|---|
| Ocidental Terrane |
|
Collision II ca. 580 Ma (605-560 Ma) | ||||
| Paraíba do Sul Klippe | ||||||
|
|
|||||
| Cabo Frio Terrane | Collision III ca. 520 Ma (535-510 Ma) |
An important feature of the southern Brasília Orogen and of the Ribeira Orogen is the relatively high proportion of basement rocks present in most terranes, with the exception of the Socorro-Guaxupé Nappe and the Oriental Terrane. From a structural viewpoint the basement is exposed in antiforms in the autochthonous areas and in tectonic slices within the nappes or allochthonous domains. Apart from recent unpublished geochronological data this section is based on the following publications: Pires (1978), Oliveira (1980), Barbosa and Grossi Sad (1983a, b), Pires et al. (1990), Heilbron (1993, 1995), Machado and Gauthier (1996), Machado and Noce (1993), Figueiredo and Teixeira (1996), Machado et al. (1996a,b), Duarte et al. (1997), Valladares et al. (1997, 2003), Duarte (1998), Heilbron et al. (1998a), Fischel et al. (1998), Ragatky et al. (1999); Nogueira and Choudhuri (2000), Schmitt (2000), Valença et al. (2000), Noce et al. (2000), Ávila et al. (2000), Quéméneur et al. (2003); Heilbron et al. (2003b,c, 2004), Ribeiro et al. (2003), Silva et al. (2003a), Duarte et al. (2003, 2004).
From a lithologic viewpoint the basement may be subdivided in the following units (Figure 10):
Greenstone belt-like sequences of probable Archean age, striking NE-SW, crop out in the Autochthonous Domain (Figure 8). They contain mafic and ultramafic rocks, of volcanic and subvolcanic origin, and metasedimentary rocks, including Mn-rich metachert). They are usually strongly deformed and metamorphosed to upper greenschist or lower amphibolite facies.
Bimodal igneous rocks with ages between ca. 2.22 and 2.12 Ga, intrusive in the greenstone belts form a continuous belt in the Autochthonous Domain called Mineiro Belt (Figure 8; Teixeira et al., 2000). This association includes gabbros, diorites, granitoids and subvolcanic rocks of mafic, intermediate or felsic composition, transformed to schists and gneisses. The metamorphism is similar to the greenstone belt sequence.
Migmatitic orthogneisses, granitoids and metabasic rocks either with Archean (ca. 2.8-2.7 Ga) or Paleoproterozoic (ca. 2.2-2.0 Ga) ages. Some authors report a metamorphic episode at ca. 2.06-2.05 Ga. The orthogneisses are tonalitic to granitic and locally trondhjemitic in composition. Geochemical and isotopic data suggest a continental magmatic arc environment at least for the Paleoproterozoic suite (Figure 8, 9 and 10). According to their outcrop area they are grouped in the following complexes: Mantiqueira, Piedade, Campos Gerais and Amparo.
Paleoproterozoic orthogranulites comprise a very heterogeneous unit called the Juiz de Fora Complex. The protoliths of these granulites include calc-alkaline granitoids of both continental and oceanic magmatic arcs and also collisional granitoids. The ages fall in the range of ca. 2.14-2.07 Ga. The metabasites may be subdivided in two suites, an alkaline one with intraplate affinity of about 1.7 Ga, and another more heterogeneous one of about ca. 2.4 Ga, composed of tholeiitic rocks with chemical signature of a convergent tectonic environment, varying from E-MORB to arc tholeiite. Sm-Nd isotope data (TDM: 2.22-2.13 Ga) suggest juvenile contribution for the calc-alkaline rocks. Silva et al. (2000b, 2005) obtained an Archean age (Shrimp, U-Pb) for felsic orthogranulites of this complex from an outcrop close to Juiz de Fora city (Figure 10 and Figure 11).
Paleoproterozoic hornblende orthogneisses called the Quirino and Região dos Lagos complexes occur respectively in the Paraíba do Sul and Cabo Frio Terranes (Figure 10 and Figure 11). The Quirino Complex (ca. 2.19-2.17 Ga) contains tonalitic to granodioritic granitoids with enclaves of ultramafic, mafic and calc-silicate rocks. The Região dos Lagos Complex (ca. 1.9 Ga) is composed of tonalitic to granitic orthogneisses, with dioritic enclaves and abundant amphibolite lenses (up to about 10 m) interpreted as boudinaged mafic dykes. Isotope Sr and |Nd data suggest both melting of Archean crust and juvenile Paleoproterozoic sources.
Towards the end of the Paleoproterozoic and during the Mesoproterozoic two intracontinental basins developed on the southern part of the São Francisco paleocontinent, called São João del Rei and Carandaí Basins (Ribeiro et al., 1995). The weak deformation and low metamorphic grade allow recognition of most primary sedimentary features so that a reasonable interpretation of the sedimentary evolution of these basins is possible.
The São João del Rei Basin was filled by the São João del Rei Megasequence, redefined after the São João del Rei Group (Ebert, 1957, 1958, 1968). It comprises a quartzitic succession of about 1000 m thick that crops out at the São José, Tiradentes and Lenheiro Ranges, in the vicinity of São João del Rei (Figure 8, 9 and Figure 12). Internal unconformities allow the separation, from bottom to top of four depositional sequences: the Tiradentes, São José, Tejuco and Lenheiro sequences. The first three amount to a thickness of about 150 m whereas the last one is about 500 m thick. Andreis et al. (1989) and Ribeiro et al. (1995, 2003) studied all the depositional sequences in detail. Based on the lithofacies associations and successions these authors suggested the following evolution of the sedimentary paleoenvironment: a shallow shelf environment evolved to lagoon systems and tidal flats, with a braided river delta developed on top.
Figure 12. Stratigraphic organization
Stratigraphic organization of the São João del Rei and Carandaí Megasequences, according to Ribeiro et al. (1995, 2003).
U-Pb ages of detrital zircons from the Lenheiro Sequence indicate predominance of Paleoproterozoic sources (ca. 2.2 to 1.8 Ga) with secondary contributions from Archean rocks (ca. 2.9 to 2.5 Ga). The age of the youngest zircon (ca. 1.8 Ga) is taken as the maximum depositional age of this sequence (Valladares et al. 2001, 2004). The megasequence is correlated with the Espinhaço Supergroup of similar age? that crops out along the eastern border of the São Francisco Craton.
The São João del Rei Megasequence is cut by mafic dykes with alkaline tendency, related to extension that led to the development of the overlying Carandaí Basin (Ribeiro et al., 1995, 2003). This basin is filled with the Carandaí Megasequence that rests unconformably on the São João del Rei Megasequence and adjacent basement. It contains two sequences, separated by a paleokarst surface, the Barroso Sequence, rich in calcareous rocks, covered by the Prados Sequence, of pelitic composition. Metadiamictites at the base of the Barroso Sequence were interpreted as debris flow deposits along the border of the basin, during a transgressive system tract? My ignorance but I don't know this term, is this correct. The overlying phyllites probably represent a condensed section of pelites during the maximum inundation and the calcareous unit may represent a carbonatic shelf or ramp, generated during a high sea level system tract??. The Prados Sequence would correspond to the pelitic cover of this subsiding carbonate shelf (Figure 8 and Figure 12). No direct geochronological data are available for the Carandaí Basin, but Sm-Nd TDM model ages of 1.7 and 1.3 Ga from the mafic dykes suggest a maximum age for sedimentation of ca. 1.3 Ga.
Most of the Neoproterozoic metasedimentary successions, related to the southern Brasília and Ribeira Orogens are passive margin successions of the Andrelândia Megasequence. However, in the internal part of the Ribeira Orogen and in the upper nappes of the Brasília Orogen successions occur that were probably deposited in fore and back-arc basins, related to the orogens. Since these rocks are allochthonous, intensely deformed and strongly metamorphosed they are described separately for the different terranes in which they occur (Figure 10 and Figure 11).
The Andrelândia Megasequence was redefined after the Andrelândia Group (Ebert, 1958) and includes also the Itapira Group. Apart from metasedimentary rocks the megasequence includes also mafic and ultramafic intercalations of igneous origin. It occurs in the Autochthonous Domain, in the lower nappes of the southern Brasília Orogen and in the Andrelândia and Juiz de Fora Domains. The main references for the megasequence are: Paciullo (1997), Söllner and Trouw (1997), Paciullo et al. (2000), Ribeiro et al., (1995, 2003), Campos Neto (2000) and Heilbron et al. (2000). Machado et al. (1996b), Heilbron et al. (1989), Gonçalves and Figuereido (1992), Valladares et al. (2001, 2004) and Valeriano et al. (2004) presented new isotopic and geochemical data for the sequence.
The Andrelândia Megasequence is subdivided in the Carrancas and the Serra do Turvo Sequences. These sequences grade laterally to distal pelagic facies that are virtually indistinguishable (Figure 8, 9 and 13).
Figure 13. Interpretation of the stratigraphy
Interpretation of the stratigraphy of the Andrelândia Mega sequence (Paciullo, 1977).
From bottom to top, the Carrancas Sequence is composed of banded paragneisses with amphibolite intercalations, similar gneisses with quartzite and grey phyllite intercalations, quartzite with minor schist intercalations, containing greenish muscovite and grey, graphitic phyllites and schists with quartzite intercalations.
The Serra do Turvo Sequence covers the Carrancas Sequence, the Carandaí Megasequence and the basement. The lower part, composed of biotite phyllites and plagioclase-biotite-garnet schists grading to gneisses, is typically homogeneous, apart from local occurrences of basement granitoid fragments, interpreted as dropstones. The upper part contains similar schists and gneisses, now with a stratified appearance due to intercalations of amphibolite, quartzite, garnet-rich metachert and calc-silicate rocks.
The paleoenvironment interpretation points to deposition in a passive margin basin with shelf deposits grading to more distal deep marine turbidites with transitions to ocean floor sediments and igneous rocks. The presence of dropstones and regional correlations suggest the influence of glaciation (Paciullo et al., 2000).
Isotopic Sr and Nd data from the plagioclase gneisses, such as very low initial Sr87/Sr86 ratios (Heilbron et al., 1989; Campos Neto, 2000) and Mesoproterozoic mixture TDM ages, indicate the presence of a juvenile source, apart from the basement. This may either be explained by intrabasinal magmatism or by contribution from a Neoproterozoic magmatic arc (Campos Neto, 2000). If the latter hypothesis is correct the upper part of the Andrelandia Megasequence would be contemporaneous to the initiation of the Brasiliano Orogeny.
Geochemical data from the amphibolites of the Andrelândia Megasequence indicate a progressive transition from continental to transitional oceanic environment. Sm-Nd model ages between 1.20 and 1.05 Ga constrain the maximum depositional age of the megasequence at ca. 1 Ga.
207Pb/206Pb (LA-ICPMS) ages from detrital zircons of the Carrancas Sequence from the Autochthonous Domain reveal essentially Paleoproterozoic sources, with subordinate Archean and Mesoproterozoic contribution. The age of the youngest zircon (ca. 900 Ma, Valeriano et al., 2004) is considered the best estimate for the maximum depositional age of the Andreândia Megasequence. Metamorphic ages of the Brasília (ca. 630-580 Ma) and Ribeira (ca. 605-560 Ma) Orogens constrain the minimum depositional age.
The metasedimentary rocks that crop out in the Paraíba do Sul Klippe (as in Table 2) and Oriental Terrane comprise a succession of pelitic and psammitic rocks, with abundant carbonatic and calc-silicate intercalations (Figure 10 and Figure 11). The metasedimentary rocks from the Paraíba do Sul Klippe are essentially composed of two main lithotypes: psammitic biotite gneisses and pelitic sillimanite-biotite gneisses (Figure 10 and 11). Garnet and tourmaline are common especially in the pelitic gneiss. The lithotypes occur intercalated on all scales of observation, defining a conspicuous gneissic layering. Centimetric to metric lenses of calc-silicate rock, sillimanite quartz schist, garnet-rich metachert and impure marble are abundant, especially in the pelitic gneisses.
Distinct metasedimentary successions occur in the three structural domains of the Oriental Terrane. These successions have received several local denominations and have also been attributed also to the Paraíba do Sul Group. Originally they were included in Paraíba-Desengano Group by Rosier (1957, 1965) or in the Paraíba Series by Ebert (, 1957). These units will be described separately below.
Migmatitic garnet-biotite gneisses with lenses of dolomitic olivine marble and calc-silicate rocks occur in the Cambuci Domain. Lenses of mafic rocks, metamorphosed to garnet-diopside granulites are also abundant. The leucosomes of the migmatites are locally charnockitic in composition. U-Pb data from detrital zircons indicate sources from the basement and from a Neoproterozoic magmatic arc (Heilbron and Machado, 2003).
In the Costeiro Domain peraluminous gneisses, rich in garnet, sillimanite and locally containing cordierite, are predominant. This succession contains many intercalations of up to about 10 m thick of impure quartzite, banded biotite gneisses, calc-silicate rocks and amphibolites. Pb207/206 detrital zircon ages from the quartzites indicate sources ranging from Archean to Neoproterozoic (Valladares et al., 2001).
The metasedimentary succession of the Italva Domain includes banded biotite gneisses, calcitic marbles, amphibolites and amphibole schists. The succession is interpreted as representing a shelf environment with basaltic volcanism, now metamorphosed to amphibolite facies. The U-Pb zircon age of ca. 840 Ma, from amphibolite, is the best estimate for the depositional age of this succession (Heilbron and Machado, 2003).
Some authors suggest that the metasedimentary successions of the Oriental Terrane may represent a carbonate passive margin basin of a (micro)continent (Oriental Terrane or Serra do Mar Microplate). The successions were intruded by Neoproterozoic magmatic arc rocks showing the transition from passive to active margin as a consequence of the initiation of subduction (Heilbron and Machado, 2003).
The Búzios Succession is composed of sillimanite-kyanite-garnet-biotite gneisses with K-feldspar and abundant intercalations of calc-silicate layers and amphibolite lenses and layers (Figure 10 and 11). Minor intercalations of garnet-quartz gneisses and feldspathic quartzites occur sporadically. The amphibolites may contain garnet, diopside and sphene; some grade to meta-hornblendites. The calc-silicate rocks are mainly garnet-diopside and diopside gneisses that occur in boudinaged layers of variable thickness (few cm to 10 m).
The Palmital Succession is predominantly composed by sillimanite-garnet-biotite gneisses with intercalations of calc-silicate rocks and garnet quartzites. U-Pb (SHRIMP) ages of detrital zircon from this succession showed the following sources: Archean (ca. 2.5 Ga); Paleoproterozoic (ca. 2.0 Ga); and Neoproterozoic (ca. 1.0 Ga and ca. 800-600 Ma; Schmitt et al. 1999,2003). The geographic position of these successions, their lithological composition (pelites, carbonates and basalts) and their geochronological data, all point to deposition in a Neoproterozoic back-arc basin related to the Rio Negro Magmatic Arc (Heilbron and Machado, 2003).
In this section we will use the rock sequences and their ages to describe the tectonic evolution of the southern Brasilia and Ribeira Orogens The region described here shows clearly the diachronous nature of the different stages of the Brasiliano Orogeny, both in the Ribeira and the southern Brasília Orogens (Campos-Neto and Figueiredo, 1995; Campos-Neto, 2000; Trouw et al., 2000). This diachronism results from the progressive collision of the following (micro)continents: Paranapanema (or Paraná), São Francisco-Congo, Oriental (or Serra do Mar) and Cabo Frio. In both orogens, subduction of oceanic lithosphere led to the formation of continental volcanic arcs. The collisional stage in the southern Brasília Orogen took place around ca. 630 Ma (Collision I), whereas in the Ribeira Orogen it happened at ca. 580-560 Ma (Collision II). Collision II affected also the southern extreme of the recently structured Brasília Orogen, leading to the complex Zone of Interference between the two orogens (Figure 6). Finally, a third collision (Collision III) took place between the Cabo Frio and Oriental Terranes at ca. 520-510 Ma.
These successive stages are described below, based on the following literature: Heilbron et al., 1982, 2000; Machado et al., 1996b; Trouw et al., 2000; Campos Neto and Caby, 2000; Campos Neto, 2000, 2001, 2002; Janasi et al., 2001; Cordani et al., 2002; Heilbron and Machado, 2003; Valeriano et al., 1993, 1995, 2000, 2004; Silva et al., 2002b, 2005; and Schmitt et al., 2004). The tectonic evolution of the region will be described starting by the closure of the Goianides Ocean which separated the Paranapanema and São Francisco paleoplates, which started with the evolution of a magmatic arc (granitoids of the Guaxupé and Socorro Nappes) and eventually gave rise to the Brasilia Orogen (collision I). We will then move on to closure of the Adamastor Ocean, at the eastern side of São Francisco paleoplate, that started with the development of the Rio Negro Arc (at Rio de Janeiro State) and the Pelotas Arc (at Santa Catarina and RioGrande do Sul states), followed by their collision with São Francisco, Luis Alves and Rio de La Plata paleoplates (collision II). Finally, the collision (collision III) of the Cabo Frio terrane represents the final stage of the amalgamation of Western Gondwana at E-SE Brazil
The early stages of subduction of the Goianides Ocean at the western side of the São Francisco paleoplates is characterized by the development of a magmatic arc represented by orthogneisses of the Guaxupé and Socorro terranes...
Remnants of this magmatic arc are preserved both as deep-seated batholiths in the Socorro-Guaxupé Nappe and as shallow intrusions in a carbonatic shelf in the Apiaí-São Roque Terrane (Campos Neto, 2000; Figure 14). However, since this terrane is separated from the Socorro-Guaxupé Nappe by transcurrent shear zones the regional correlations are still speculative. For this reason the Apiaí-São Roque Terrane and associated units are described in a separate section below.
Figure 14. Geological map of the Socorro-Guaxupé Nappe
Geological map of the Socorro-Guaxupé Nappe and São Roque Domain according to Campos Neto (2000). Socorro Guaxupé Nappe (1- 8): 1- Pico do Itapeva Formation (pit) and related formations in Andrelândia Domain (e- Eleutério Formation, pa- Pouso Alegre Formation); 2- aluminous A-type granites; 3- Itu Province (calc-alkaline/A-aluminous) jotunite-mangerite series (m) and ca. 585-590 Ma granites; 4- Capituva and Pedra Branca syenites (ca. 610 Ma); 5- porphyritic, calc-alkaline, Hbl-Bt orthogneisses, Grt-Bt granites (np) and mangerite-granite orthogneisses (620-630 Ma); 6- gneisses and stromatic migmatites (with metasedimentary mesosome) of the Piracaia (Pi) and Caconde (Ca) Complexes, tonalite-granodiorite-granite orthogneisses (640-655 Ma); 7- foliated granitic metaluminous diatexites of the Pinhal (Ph) and Paraisópolis (Pa) complexes; 8- basic garnet granulites and enderbites; Brasília Orogen and SFC (9- 13): 9- metasedimentary successions in the nappe system, (gl) Ky-granulitic nappes; 10- São João del Rei Megasequence (t); 11- tonalite and granodiorite orthogneisses, São Gonçalo do Sapucaí Complex (sg); 12- Serra Negra tonalites (Neoarchaean), Amparo Complex (ap - Mesoarchaean); 13- orthogneisses and migmatites, including the southern extreme of SFC. São Roque Terrane (14-17): 14- Granites; 15- potassic calc-alkaline porphyritic Hbl-Bt granites; 16- São Roque Group; 17- Serra do Itaberaba Group; Apiaí Terrane: (18-21): 18- A-type syenogranites; 19-Ms-Bt granites and granodiorites; 20- potassic calc-alkaline porphyritic Hbl-Bt granites; 21-Votuverava Formation (Vt) and schists that grade to gneisses and migmatites to NE; 22- Rio Jaguari mylonites.
As stated above, the Paranapanema Craton is covered by the Paraná Basin (Brito Neves et al., 1999; Campos Neto, 2000). K-Ar ages of minerals of the basement of this basin (obtained from drill cores) allowed the delineation of a Paleoproterozoic core, defining a craton, surrounded by Neoproterozoic (Brasiliano) rims (Cordani et al., 1984, Milani, 1997; Milani et al., 2000; Quintas, 1995). The shape and constitution of this craton are mainly derived from gravity data (Marangoni, 1994) and from variations in P and S wave velocities (Van Decar et al., 1995). The Apiaí-São Roque Terrane and the Socorro-Guaxupé Nappe correspond to the eastern margin of this craton/paleocontinent with calc-alkaline arc plutonism.
The pre-collisional rocks of the southern tip of the Brasília belt are related to subduction of the Goianides Ocean, located between São Francisco and Paranapanema paleoplates. Rocks related to the magmatic arc are orthogneisses and metasedimentary rocks derived from the arc that crop out at the Socorro and Guaxupé nappes (Figure 6)These nappes have about 15 km thick and are composed of three superposed units corresponding to different crustal levels: the Lower Granulite Unit, the Intermediate Diatexite Unit and the Upper Migmatite Unit (Campos Neto and Caby, 2000). The nappes forms two big lobes separated by high angle lateral ramps, the northern one being called Guaxupé and the southern one Socorro (Figure 6 and 14).
The Lower Granulite Unit (Figure 14 and 15) consists of banded garnet-biotite-orthopyroxene granulites of enderbitic to charno-enderbitic composition with local intercalations of noritic to gabbroic gneisses. The granulites of intermediate composition are calc-alkaline, poor in K and Rb, but the mafic granulites have a tholeiitic signature. Migmatised tonalitic to granodioritic orthogneisses occur as discontinuous lenses in the top of this unit. The basal orthogranulites are not clearly distinct in their chemical signature from the younger potassic calc-alkaline batholiths. This unit is considered to be the product of convergent pre-collisional tectonics. U-Pb ages ca. 655-640 Ma; Basei et al., 1995; Ebert et al., 1996; Hackspacher et al., 2004) indicate that this magmatism took place shortly before the main metamorphism, possibly as related processes in the deeper part of the arc.
Figure 15. Structural section of the Socorro-Guaxupé
Structural section of the Socorro-Guaxupé and Lower Nappe System at Southern Brasília Belt (Modified from Campos Neto, 2000).
The Intermediate Diatexite Unit (Figure 14 and 15) is composed of meta-aluminous orthogneisses with discontinuous bodies of stromatic migmatites surrounded by grey to pink nebulitic gneisses. The mesosomes of the migmatites are composed of dark grey hornblende-biotite-clinopyroxene gneisses, of tonalitic to dioritic composition, cut by light grey tonalitic leucosome veins. The melanosomes are rich in biotite and hornblende. The grey to pink nebulitic gneisses contain variable migmatitic structures and have modal composition of granite with biotite and locally hornblende. They are leucocratic, equi- to inequigranular, medium to coarse grained rocks with transitional contacts to irregular bodies of porphyritic orthogneisses. Metasedimentary rocks appear as narrow bands of sillimanite-garnet-biotite gneisses with cordierite and spinel, or as xenoliths of calc-silicate gneisses containing garnet, diopside and scapolite. This unit has been referred to as the Pinhal Complex in the Guaxupé lobe (Wernick and Penalva, 1980) and as the Paraisópolis Complex in the Socorro lobe (Cavalcante et al., 1978).
The Upper Migmatite Unit (Figure 14 and 15; Piracaia Complex, Campos Neto and Basei, 1983) consists of a metasedimentary sequence in which the extent of melting decreases towards the upper part of the nappe. The dominant lithotype is banded cordierite-sillimanite-garnet-biotite gneiss with stromatic leucosomes composed of granite, locally with garnet. Subordinate intercalations of feldspathic quartzite and quartzitic gneisses with sillimanite and muscovite, calc-silicate gneisses, metabasic rocks, hornblende gneisses and rare thin marble lenses. Inside the metasedimentary successions several bodies of grey orthogneisses, of tonalitic to granitic composition, cut by stromatic leucosomes were mapped.
The pattern of Sm-Nd model ages shows values between 1.4 and 1.7 Ga in the eastern part of the nappe in contrast with values higher than 2.0 Ga in the western part (Janasi, 2001, 2002). These ages are interpreted to indicate mixture of juvenile NeoProterozoic material with old continental crust and suggest that the frontal part of the Socorro-Guaxupé Nappe represents the thinned rim of the Paranapanema paleocontinent.
The closing of the Goianides Ocean resulted in the collision between the Paranapanema (Paraná) and São Francisco Paleocontinents and built the southern part of the Brasília Orogen. This process led to the formation of an extensive system of subhorizontal east-verging nappes that cover the southwestern border of the São Francisco Craton (Figure 6, 8, 9 and 14). Intense deformation generated tight to isoclinal folds at micro, meso and macro scales, associated with a strong schistosity and stretching lineation. The nappes involve locally basement slices that were superposed on metasedimentary rocks through thrust faults. The frontal shortening related to the nappe movement has been estimated as at least 150 km (Trouw et al, 2000? Figure 9).
The metamorphism of the Lower Nappes exhibits inverse zoning with relatively high pressures, typical for exhumed subduction zones. The grade varies from lower greenschist facies in the Autochthonous Domain to granulite facies with kyanite-K-feldspar associations in the upper part (Figure 15 and 16). Local remnants of eclogitic rocks were also reported (Trouw, 1992; Trouw et al., 2000). Maximum pressures and temperatures were estimated at about 13.5-15 kbar and 800-900 °C. The metamorphic peak was probably attained at about ca. 630 Ma dating method? and younger ages at ca. 610-605 Ma were attributed to the exhumation of the nappes (Trouw and Pankhrust, 1993; Campos Neto and Caby, 2000; Trouw et al., 2000).
Figure 16. Simplified structural map of the southernmost Brasília Orogen
Simplified structural map of the southernmost Brasília Orogen, with major nappes and thrust systems (see also Fig. 8; modified from Trouw et al., 2000). Legend: High-T Upper Nappes: fold axes and stretching lineations. High- to medium-P Lower Nappes: NV- Varginha, NLB- Liberdade; NCC- Carmo da Cachoeira; NA- Andrelândia; NSTL- São Tomé das Letras; NL- Luminárias; KC - Carrancas klippe; DA - Autochthonous Domain. SEJF- Juiz de Fora Thrust System related to Ribeira Orogen. Black arrows: fold axes and stretching lineations related to Collision I (ca. 630 Ma); red arrows: fold axes and stretching lineations related to Collision II (ca. 580 Ma); green arrows: fold axes and stretching lineations in the Autochthonous Domain.
The metamorphism in the Upper Nappes (Socorro-Guaxupé Nappe; Figure 14) is of high temperature with lower pressure and is associated to the generation of abundant granitic rocks by anatexis. A subhorizontal metamorphic high-temperature LS fabric was developed under conditions of non-coaxial deformation accompanied by volume change. Most kinematic indicators show top to the E-NE thrust movements. Oblique shear zones with NW-SE mineral and stretching lineation are present in the Guaxupé Nappe, whereas subvertical NE-SW dextral shear zones are common in the Socorro Nappe.
In the Intermediate Diatexitic Unit (Figure 14) the metamorphism reached maximum temperatures and pressures of 900 ± 50°C and 6.5 ± 2 kbar, respectively (Negri, 2002; Campos Neto and Caby., 2000), with an almost isobaric heating path, also present in the upper unit. These very high temperatures reached at relatively shallow crustal levels, indicate a high thermal gradient characteristic for active margin environments with shallow asthenospheric mantle. High pressure conditions (14 kbar with 850°C; Del Lama et al., 2000; Campos Neto and Caby, 2000; Freitas, 2000; Garcia and Campos Neto, 2003) were attained in the Lower Granulitic Unit and indicate a tectonic environment compatible with the root zone of a magmatic arc, involved in a subduction zone.
The geochronological data show evidence that the very high temperature metamorphism, responsible for the formation of the migmatites in the inner part of the Guaxupé and Socorro nappes, took place at about 625 5 Ma (Basei et al., 1995; Janasi, 1999; Vlach and Galda, 2000; Fetter et al., 2001). Migmatitic gneisses and granulites from the base of the nappe attained their metamorphic equilibrium ca. 615-612 Ma (Janasi, 1999; Vlach and Gualda, 2000; Negri, 2002). These ages are interpreted as the main transport period of the nappe and therefore also of the collision between the Paranapanema and São Francisco paleocontinents.
The Neoproterozoic syn-collisional magmatism involved extensive melting in the lower crust, resulting in a large volume of crustal granites in the whole nappe (Figure 14). Around 625 Ma temperatures close to 1000 ºC were attained in the deeper portions of the Socorro-Guaxupé Nappe (~14 kbar), leading to the melting of granulites and to the generation of charnockitic magmas (Janasi, 2002). A similar age was obtained for crustal granites generated at the temperature of the break down of biotite (~ 850 ºC) by melting of orthogneisses in the middle crust (anatectic biotite granites of Pinhal type) and at the temperature of the break down of muscovite (~ 750 ºC) by melting of paragneisses, possibly mixed with basement, in the upper crust (garnet-biotite granites of Nazaré Paulista type, southern part of Socorro lobe; Janasi, 1999). Janasi (2002) subdivided magmatism in the Socorro-Guaxupé Nappe as follows: mangeritic suite (ca. 630-625 Ma); calc-alkaline suite with high K (ca. 625-620 Ma); anatectic granites (ca. 625 Ma) and potassic syenites (ca. 610 Ma). The potassic calc-alkaline granites (ranging from monzodiorites to syenogranites) that make-up the principal volume of the Socorro and Pinhal-Ipuiúna batholiths have an age of ca. 625 Ma (Töpfner, 1996; Janasi, unpublished data). This age appears to represent a well-defined and relatively short thermal peak for the whole nappe. The batholiths are essentially granitic and of hybrid character, but the presence of several small bodies of mafic to intermediate composition demonstrates a component of magma contribution from the mantle (Wernick, 1984; Janasi and Ulbrich, 1991). The elevated thermal gradient, evidenced by high temperature charnockitic magmas, is probably related to extensive underplating of basaltic magma. In a compressive regime these magmas were trapped at the base of the continental crust with which they interacted, acquiring the "evolved" isotopic signatures, observed even in the mafic bodies (Janasi, 2002).
The post-tectonic syenites that appear as rounded plutons intrusive in the relatively shallow crust of the Intermediate Diatexitic Unit have ages of ca. 610 Ma (Töpfner, 1996) and register the initiation of the post-collisional evolution. Examples are the potassic syenites of the Pedra Branca and Capituva massifs, intrusive in the central part of the Guaxupé lobe (Töpfner, 1996; V.A. Janasi, unpublished data). The composition of these magmas requires source rocks of enriched subcontinental lithospheric mantle, demonstrating the influence of the mantle during this stage of the evolution of the Socorro-Guaxupé Nappe.
The youngest granites in the nappe (the Itu Granitic Province) were dated at ca. 590-580 Ma (Töpfner, 1996; Ebert et al., 1996). They intruded shallow crustal levels (< 3 kbar), after the emplacement of the nappe. These granites constitute a post-orogenic magmatic province that borders the Paranapanema plate (and also the present border of the Paraná Basin), that contains from potassic calc-alkaline granites to aluminous A-type granites, apparently with a continuous range of transitional compositions (Vlach et al., 1990, Wernick, 2000).
The subvertical transcurrent shear zones and the thermal structure of the Itu Granitic Province (Figure 14) played an important role in the exhumation of intermediate crustal levels of the nappe. This led to the formation of the Pico do Itapeva successor basin (correlated to the Pouso Alegre and Eleutério Basins in the Andrelândia Domain) with continental and shallow marine sediments. The fossils are of Neoproterozoic to Cambrian age (acritarcas Cloudina riemkeae and foraminifera Titanotheca coimbrae, 570-540 Ma) and rhyolitic pebbles were dated at about 600 Ma (Teixeira, 2000). The sedimentary rocks of these basins were deformed and metamorphosed to very low grade, resulting in NE-SW striking folds with vergence to NW and local development of axial planar slaty cleavage. The cooling of the regional metamorphism can be estimated from K-Ar ages in biotite of ca. 530 Ma (data from pebbles of the Eleutéria Basin; Teixeira, 2000).
Magmatic arc rocks that testify pre-collisional subduction of the Adamastor Ocean in the Ribeira Orogen are located in the Oriental Terrane (Figure 6 and 10). The subduction was probably towards E-SE producing the Rio Negro Magmatic Arc (Tupinambá, 1999; Tupinambá et al., 2000; Figure 10 and 11). Only the plutonic portion of this arc is preserved; it is composed of calc-alkaline tonalitic to granitic orthogneisses with associated gabbros. They intruded into paragneisses of the Costeiro Domain that probably represent the distal (turbiditic) part of the passive margin of the Oriental Terrane paleocontinent. Geochemical and isotopic data suggest at least two stages for the development of this arc: ca. 790 Ma and ca. 635-620 Ma. Important features of this arc are that the Pb isotopic data reveal the absence of a Paleoproterozoic or older inheritance and that the Nd data indicate two groups of rocks with contrasting levels of crustal contamination. U-Pb data suggest that the arc was a source area for the sediments of the Cambuci Domain (fore-arc basin?) and for the younger units of the Costeiro Domain (back-arc basin?), demonstrating that the sedimentation was contemporaneous with the subduction process.
The closure of the Adamastor Ocean led to the collision between the São Francisco paleocontinent with another continental fragment that now remains as the Oriental Terrane (or Serra do Mar Microplate). This second collisional episode (Collision II) took place between ca. 590 and 560 Ma, with its peak activity around 580 Ma, and resulted in the Ribeira Orogen. The structural style of large subhorizontal nappes, observed in the Brasília Orogen, resulting from frontal collision, is quite contrasting with the structural style in the Ribeira Orogen, characterised by relatively steep juxtaposed terranes and abundant transcurrent dextral shear zones. Collision II is therefore interpreted as oblique with partition of deformation in zones of frontal shortening with western to northwestern vergence and steep NE-SW transpressive dextral shear zones. For this reason, the limits of the tectonic units are reverse faults with dips between 30º and 60º or subvertical transcurrent shear zones (Almeida et al., 1998; Figure 10 and 11).
Collision II led to the docking of the Paraiba do Sul and Oriental Terranes (including the Rio Negro Magmatic Arc) onto the Ocidental Terrane that represents basement of the reworked margin of the São Francisco paleocontinent. The intense deformation related to this collision generated tight to isoclinal folds, strong mylonitic schistosity and stretching lineations. The metamorphism of the Ocidental Terrane varies from greenschist facies on the cratonic border to medium pressure granulite facies at the contact with the Ocidental and Paraíba do Sul Terranes. The Juiz de Fora Domain, in the upper part of the Ocidental Terrane, is characterised by a tectonic mélange-like structure, defined by a crustal scale duplex with alternating basement and cover interleaved in numerous tectonic lenses.
The metamorphic associations indicate an intermediate gradient with maximum T and P estimated at about 700 °C and 7 kbar. Some metabasic rocks contain remnants of granulitic associations of higher pressure. The metamorphic zones of the Ocidental Terrane are inverted, like in the Brasília Orogen. Radiometric ages of the metamorphism fall in the range 595-550 Ma. The metamorphism of the Paraiba do Sul Klippe is of amphibolite facies, but in the Oriental Terrane it ranges from upper amphibolite facies (Italva Domain) to granulite facies (Cambuci and Costeira Domains). The principal deformation in these terranes produced a penetrative schistosity associated with tight to isoclinal folds.
The crustal thickening that resulted from Collision II originated various suites of granitoids: an early suite of porphyritic high-K calc-alkaline granitoids (ca. 590-580 Ma), leucogranites and/or garnet charnockites (ca. 580 Ma, Duarte et al., 2000), a late suite of high-K calc-alkaline bodies (ca. 575-560 Ma; e.g. the coarse augengneiss of Rio de Janeiro) and finally biotite granites (ca. 560 Ma). These granitoids, related to Collision II, are more abundant in the upper part of the Ocidental Terrane (Juiz de Fora Domain) and in the Oriental Terrane (Figure 10 and 11).
The last collisional stage, between ca. 535 and 510 Ma, that led to the amalgamation of the Cabo Frio Terrane and the Ribeira Orogen, has also been referred to as the Búzios Orogeny (Schmitt et al., 2004). At this time (Cambrian) practically all orogens around the São Francisco-Congo Craton (Figure 2) were consolidated, leaving little space for moving microcontinents. Some authors suggest that this collision resulted from the closure of an oceanic basin (back-arc ?) located between the Rio Negro Magmatic Arc/Oriental Terrane and the southwestern border of the Congo paleocontinent (Heilbron et al., 2000; Heilbron and Machado, 2003).
Collision III generated important low angle structures in the Cabo Frio Terrane (Figure 10 and 11). The metamorphism is of relatively high pressure and temperature, characterised by the kyanite-K-feldspar association (with late sillimanite) in metapelitic rocks. P-T conditions for these rocks were estimated as at least 9 kbar and 780 °C (Schmitt et al., 2004).
In this same period renewed deformation, metamorphism and igneous activity occurred in the terranes previously amalgamated into the Ribeira Orogen. The deformation produced refolding and generated a number of transcurrent NE-SW oriented dextral shear zones. The metamorphism with age of ca. 535-520 Ma was of similar grade as the earlier one (M2 of Machado et al., 1996b) and the igneous activity produced granites of the same age. Examples of the dextral shear zones are the Além Paraíba, Três Corações and Caxambú shear zones, the last two with horizontal offset of about 15-18 km (Trouw et al., 2003). The Além Paraíba Shear Zone (Campanha, 1981), which is the largest one, extends from the state of São Paulo until the NE of Rio de Janeiro State. It is characterized by mylonitic and ultramylonitic anastomosing layers, with strong planar and linear fabric, that alternate with lenses and layers with many folds and protomylonitic structure. Kinematic studies and strain analyses point to a transpressive regime for the shear zone. Along the axis, in the region of Três Rios (Figure 10 and 11) granulitic rocks of the Juiz de Fora Complex and metasedimentary rocks of the Andrelândia Megasequence crop out in antiformal cores. The transpressive character of the shear zones was described by Chrispim and Tupinambá (1989) and Ebert et al. (1991). Some authors have interpreted these structures as "popups" related to transpression (Endo and Machado, 1993). Others advocate the reactivation of the mylonitic fabric in a second deformational stage, based on kinematic, microstructural and experimental studies (Almeida, 2000). It seems possible that these thermal and deformational events are related to the collision of the Cabo Frio Terrane (Collision III).
A post-collisional deformation phase registered in the Oriental and Cabo Frio Terranes, marks the transition to an extensional tectonic regime (Figure 10). This phase is interpreted as the result of extensional collapse of the orogenic edifice, in a similar way as in the Araçuaí Orogen (Heilbron et al., 2000; Heilbron and Machado 2003). This phase is represented by two groups of structures: a) brittle-ductile shear zones with down-dip movent, parallel to the orogen and associated with east vergent folds and b) subvertical NW-SE oriented shear zones, orthogonal to the orogen, with transtensional kinematics and with predominantly dextral and northeast block-down movement.
This tectonic regime is associated with post-collisional calc-alkaline granites that occur as circular stocks, sills or dykes. Radiometric dating of these bodies yielded ages between 510 and 480 Ma. The regional pattern of this magmatism shows an increase in alkalinity towards SW (Junho, 1993). The shear zones played an important role as channel ways for the ascending magmas, as shown by frequent magmatic flow structures (Figure 10). An important characteristic of these bodies is the common association with mafic portions, leading to structures of magma mixing along the contacts. Examples are the granites of Paratí, Ilha Grande, Pedra Branca (ca. 510 Ma), Suruí, Teresópolis, Nova Friburgo and Sana (Figure 10; Penha et al., 1980; Pires et al., 1982; Penha and Wiedemann, 1984; Junho, 1993; Heilbron et al., 1995; Machado and Demange, 1992, 1994; Porto Jr. and Valente, 1988; Tupinambá, 1999, Tupinambá et al., 2000).