The Dom Feliciano Orogen extends for about 1,200 km from Punta del Este in Uruguay to the northeast of Santa Catarina State in Brazil, whereas the São Gabriel Orogen occupies a region of about 5,000 km2 in the extreme southwest of Brazil (Figure 21). The orogens are situated marginal to the Rio de La Plate Craton to the SW, the Paranapanema Craton to the west (covered by the Paraná Basin) and the Luis Alves cratonic massif that separates the Dom Feliciano and Ribeira Orogens.
Figure 21. Geological map of the southern segment
Geological map of the southern segment of the Mantiqueira Province, simplified from "Mapa Geológico da Província Mantiqueira, 1:250.000" by Silva et al., 2002a. The Uruguay segment is after Hartmann and Fernandes (2000). Legend: 1- Phanerozoic cover and magmatic rocks. Dom Feliciano Orogen: 2- late to post-tectonic foreland basins (Itajaí and Camaquã); 3- post-collisional G3 granitoids (I- and A-types, locally alkaline and Rapakivi-types); 4- late-collisional high-K calc-alkaline G2 granitoids; 5- pre to syn-collisional G1 granitoids with tonalitic xenoliths (*) (Pinheiro Machado Suite); 6- passive margin basins (Brusque, Porongos and Lavalleja groups with slices of ocean floor rocks. Reworked basement: 7- Kibarian orthogneisses (Punta del Este Terrane); 8- Rhyacian orthogneisses (Encantadas and Arroio dos Ratos Complex/Microcontinent); 9-Paragneisses (Várzea do Capivarita Complex); 10- Capivarita Meta-anorthosite; 11- Rhyacian tonalitic gneiss; 12-Paleoproterozoic gneisses with ca. 610 Ma metamorphic overprint. Eastern continental margin of the Rio de La Plata Craton: 13- Santa Maria Chico Complex. Luis Alves Craton: 14- A/Psg = Archaean TTG gneisses with granulite facies Paleoproterozoic overprint (Santa Catarina Complex). Caçapava do Sul Transpressional Event (560 Ma): 15- I-type granite. São Gabriel Orogen: 15- calc-alkaline gneisses of the oceanic arc (TTG-Cambaí Group); 17- metavolcano-sedimentary sequence (Vacacaí Supergroup); 18- back-arc metavolcano-sedimentary sequence (Passo Feio Formation); 19- metamafic and ultramafic rocks (ophiolitic remnants?) of the Cerro Mantiqueira Complex; 20- Tonian metadiorite.
The São Gabriel and Dom Feliciano Orogens have distinct tectonic identities, with metamorphic peaks separated in time by about ca. 70-90 Ma, providing another example of the diachronic tectonic evolution during the Neoproterozoic.
The early tectonic evolution (Tonian-Criogenian) is only registered in the São Gabriel Orogen and is characterised by a prolonged accretionary stage (from ca. 880 Ma to ca. 700 Ma) with a metamorphic peak at ca. 700 Ma. The later tectonic evolution, well represented in the Dom Feliciano Orogen, includes a magmatic evolution dated at ca. 650-560 Ma with a metamorphic peak between ca. 630 Ma and ca. 610 Ma.
This synthesis is based on the papers of Babinski et al. (1996, 1997), Fernandes et al. (1992, 1993), Holz and De Ros (2002), Basei (2000), Silva et al. (2002a,c, 2005), Basei et al. (2000, 2003), Hartmann and Fernandes (2000), and Hartmann et al. (1979, 1999, 2000).
The small part of the Rio de La Plata Craton that is exposed, is composed of orthogneisses of a typical TTG sequence and basic tholeiitic rocks. These gneisses have crystallization ages of ca. 2510 Ma and were metamorphosed to granulite facies around ca. 2022 Ma (Figure 21).
Although this orogen is only exposed in a limited area, it exhibits the most complete register of Brasiliano juvenile crust in the whole region described in this paper. It is characterized as an accretionary orogen or a "subduction-related orogen" (Sengör, 1990). The southern limit of the orogen is the NW-SE transpressive sinistral Ibaré Shear Zone, that separates the orogen from the Archean high grade terranes (Santa Maria Chico Granulitic Complex) of the Rio de La Plata Craton. The eastern limit of the orogen is marked by the NE-SW Caçapava Shear Zone (Figure 21). It is partially covered by the Camaquã Basin of the Dom Feliciano Orogen and it separates the orogen from an eastern block composed of reworked Paleoproterozoic basement (Encantadas Complex) and the fold-and-thrust belt (Porongos Complex) of the Dom Feliciano Orogen.
This basin is represented by mafic to ultramafic associations, magnesian schists, serpentinites, metabasalts and tholeiitic amphibolites, of possible ophiolitic nature (Cerro Mantiqueira Complex). These rocks constitute tectonic remnants now surrounded by syn-orogenic units that represent a magmatic arc. There are no reliable radiometric ages for these rocks, but a minimum age for the crystallization of the protoliths of about 750 Ma, obtained in the arc rocks, seems reasonable. The Passinho Metadiorite, geographically associated with the mafic-ultramafic units, has a crystallization age of ca. 880 Ma but its tectonic significance is at present not clear.
The pre-collisional or accretionary stage of this orogen is characterized by the construction of an island arc with several calc-alkaline volcanic-plutonic units (Figure 21). The plutonic part is composed of orthogneisses with TTG character (Cambaí Group), associated with the tholeiitic mafic-ultramafic rocks referred to above. Its crystallization age is about 740-730 Ma and the metamorphic age is ca. 730 Ma. The volcanic part (Vacacaí Group) is composed of meta-andesites, metabasalts, intermediate metatuffs intercalated with pelitic schists, marbles and amphibolites, apart from mafic to ultramafic intrusions. The crystallization age of the protoliths is ca. 750 Ma and the age of the metamorphism is ca. 700 Ma.
The Sm-Nd isotopic signature with positive eNd(t = 750 Ma) and model ages around 1.0 Ga indicate a juvenile origin for the magmatic arc that is therefore interpreted as an intra-oceanic magmatic arc.
The syn-collisional stage developed in response to the tectonic transport of the arc towards WNW until collision and amalgamation with the Rio de la Plata Craton at the time of the metamorphic peak, at ca. 700 Ma.
This orogen is constituted by Neoproterozoic passive margin sequences (Brusque Group in Santa Catarina, Porongos Group in Rio Grande do Sul and Lavalleja Group in Uruguay), extensive segments of magmatic arcs (Florianópolis, Pelotas and Aygua, the last one in Uruguay), and remnants of Paleoproterozoic to Archean basement (Figure 21). Its present NE-SW elongated structure is related to oblique continental collision between the Rio de la Plata, Paranapanema and Kalahari paleocontinents. As a result, the evolution of the orogen is associated with transpressive tectonics related to lateral escape that originated deep transcurrent shear zones, now the limits of distinct segments of the orogen. These shear zones control also the shape and formation of volcanic-sedimentary basins and the generation of syntectonic and post-tectonic plutons.
The most important transpressive system of shear zones (Figure 21) is composed of the Major Gercino, Dorsal do Canguçu and Sierra Ballena shear zones. In Rio Grande do Sul State and adjacent areas to the south these shear zones are sinistral. They separate the Porongos metavolcanic-sedimentary units to the west from the Pelotas Batholith to the east. The continuation in Uruguay is known as the Sierra Ballena Shear Zone, separating the Lavalleja Complex from the Aygua arc. On the other hand, the dextral Major Gercino Shear Zone, at Santa Catarina State, separates the metavolcanic-sedimentary Brusque Domain to the west from the magmatic arc to the east, The significance of this extensive shear zone system is still the subject of debate. Although the shear zones contain deep crustal mylonites and locally reach 5 km in thickness they were interpreted as intracontinental (Fernandes et al., 1992), separating passive margin deposits of the fold and thrust belt to the west from the magmatic arc to the east. Other authors interpret the shear system as the limit of paleoplates. According to this interpretation the metavolcanic-sedimentary basins of the fold and thrust belt would not represent the remnants of a Neoproterozoic passive margin, but rather accreted Mesoproterozoic terranes ("Schist Belt"), separated by the lineament from the "Granite Belt" (Basei et al., 2000).
The Perimbó Shear Zone is another important component of the NE-SW transpressive system in Santa Catarina. It separates the passive margin deposits thrust onto Archean basement from the Luís Alves Craton.
The most extensive segment of Archean basement preserved in the region described in this paper is the Granulitic Complex of Santa Catarina that constitutes the basement of the Dom Feliciano Orogen. It extends for 250 km in the states of Santa Catarina and Paraná with a maximum width of 100 km (Figure 21). This complex is composed of an association of TTG granitoids with basic tholeiitic rocks, metamorphosed to granulite facies. It contains tonalitic and trondhjemitic gneisses related to the evolution of an oceanic arc, with common intercalations of pyroxenitic ultramafic rocks and supracrustals that include principally iron formation and locally garnet biotite gneisses. The crystallisation age of the protoliths of the trondhjemitic gneisses is ca. 2.7 Ga and the age of the metamorphism is ca. 2.17 Ga. There is no geochronological evidence for metamorphic heating during the Neoproterozoic, although numerous granitic plutons intruded this segment of Archean basement and several volcanic-sedimentary basins (e.g. Itajaí) were installed on top. This Archean-Paleoproterozoic domain was named Luís Alves Craton and is interpreted to represent a microcontinent.
Other important remnants of basement (Encantadas, Arroio dos Ratos and Várzea do Capivarita complexes) are Paleoproterozoic terranes submitted to metamorphic recrystallization during the collisional stage. An example is the Encantadas Gneiss with magmatic crystallization age of ca. 2.2 Ga and metamorphic ages of ca. 630 Ma (this gneiss could also represent an ancient microcontinent). Other minor components of Paleoproterozoic basement were submitted to migmatisation by anatexis around 610 Ma (Camboriú and Águas Mornas complexes).
These basins contain quartzites, marbles, locally with stromatolites, graphitic phyllites, pelitic schists and metaturbidites, deposited in shallow to deep environments of a passive margin. Local occurrences of mafic metavolcanics with remnants of variolitic pillow lavas and banded iron formation suggest an ocean floor environment (Brusque Basin). Mafic to ultramafic associations, possibly related to a stage of ocean opening, are also described in the Porongos Basin.
The passive margin successions were deformed and metamorphosed under lower greenschist facies conditions (Brusque) and lower amphibolite facies conditions (Porongos and Lavalleja), building a fold belt around 630-610 Ma. The pre-, syn- and post-collisional stages of the Dom Feliciano Orogen are best registered in the magmatic arc and are characterized by the generation of numerous granites during the Neoproterozoic to Cambrian.
The pre- to syn-collisional plutonism was dated in the interval of ca. 650-610 Ma. Most of these plutons have chemical and isotopic signatures of a mature magmatic arc of an active continental margin (Pelotas Arc). The granitoids are calc-alkaline, medium to high K, with a dominant component derived from a metaluminous continental source (orthogneisses), and rare S-type granites. The Sm-Nd model ages are predominantly Paleoproterozoic and secondarily Archean and Mesoproterozoic. The Nd (t = 600 Ma) values are moderately to strongly negative.
There is still much controversy about the subduction polarity of the Dom Feliciano Orogen: some authors defend subduction to the west, but most authors agree on subduction to the east based on geochemical zoning in post-collisional granitoids and on structural vergence (Basei et al., 2000, 2003).
The post-collisional stage extends from ca. 610 to ca. 590 Ma and is characterized by extensive I-type subalkaline to alkaline magmatism (Caledonian type, Pitcher, 1983). Local shoshonitic bodies are related to post-orogenic collapse. The granitoids of this stage are characterized by dominantly crustal isotopic signatures with a strong influence of Archean to Mesoproterozoic crust.
Apart from the granitoids that intrude the Pelotas Arc, the post-collisional stage is also characterized by extensive volcanic-sedimentary back-arc basins, with abundant alkaline and more rarely shoshonitic volcanic and volcanoclastic filling. The age of these volcanics is confined between ca. 600 and ca. 560 Ma. The evolution of these basins is from continental to marine, starting with anastomosing fluvial systems (red beds) with conglomerates and arcosic sandstones, and evolving to deltaic marine systems with proximal and distal turbidites and finally deposits of the continental slope.