Synthesis of the Proterozoic tectonic evolution of southern Brazil

The Neoproterozoic to Cambrian tectonic evolution recorded in southeast Brazil and in southwest Africa (Figure 2) provides important information to understand the progressive agglomeration of Western Gondwana.

The principal paleoplates with their respective paleocontinents, now preserved as cratons, involved in this evolution are: São Francisco-Congo, Paranapanema-Paraná, Rio de la Plata and Kalahari, apart from smaller continental blocks like Luís Alves, Apiaí (?), Oriental Terrane-Serra do Mar Microplate and Cabo Frio Terrane.

The pre-Brasiliano geological evolution is long and complex, with the main rock forming events concentrated in the Archean and in the Paleoproterozoic. The interval between ca. 2.2 and ca. 1.9 Ga often referred to as Transamazonian Thermo-Tectonic Event is especially important for the generation of numerous granitoids, related to both partial reworking (melting) of Archean crust and to accretion of juvenile island arcs.

The ca. 1.75-1.6 Ga taphrogenesis originated intracontinental basins in the São Francisco-Congo Craton, preserved as the depositional sequences of the Espinhaço Supergroup and the São João del Rei Megasequence. These sequences were affected by the Brasiliano deformation and metamorphism related to the Araçuaí and Ribeira Orogens. Some terranes contain remains of ca. 1.4 Ga sedimentary basins, for example the Serra do Itaberaba Group and some formations of the Açungui Group located in the Apiaí Terrane.

The beginning of the Neoproterozoic is characterized by the Tonian taphrogenesis that resulted in the generation of rift basins (ca. 930-880 Ma) that later evolved to continental passive margin basins. The continental rift phase is characterized in the West Congo Belt (ca. 930-910 Ma; Tack et al., 2001) by extensive continental felsic to mafic volcanism and in the Araçuaí Orogen by the anorogenic Salto da Divisa intrusion (ca. 880 Ma; Silva et al., 2000b2) and by mafic dykes (ca. 906 Ma) that cut the Espinhaço Supergroup.

The stage of ocean opening (ca. 840-800 Ma) is well documented in most orogens treated here that show clear evidence for passive margin sequences and remnants of ophiolitic associations. Examples are the Ribeirão da Folha Formation (distal Macaúbas Group) in the Araçuaí Orogen, the Andrelândia Megasequence of the southern Brasília Orogen and probably the Brusque, Porongos and Lavalleja groups in the Dom Feliciano Orogen. These thick passive margin deposits are contemporaneous with the formation of juvenile magmatic arcs (island arcs) in the northern Brasília Orogen (Mara Rosa Arc) and in the São Gabriel Orogen. Hence, the paleogeographic scenario in the early Neoproterozoic suggests the presence of considerable oceans (Goainides and the Adamastor Oceans) separating the São Francisco paleocontinent from the Paranapanema, Rio de La Plata, Kalahari and other microcontinents.. If this is correct these continental masses were located far away from each other in the early Neoproterozoic and the São Francisco-Congo paleocontinent would not be part of the Supercontinent Rodinia as suggested by Pisarevsky et al. (2003) and Krönner and Cordani (2003).

In contrast, the Araçuaí Orogen would represent a confined orogen that formed largely by closure of a large gulf (an inland-sea basin). This is inferred from the available paleogeographic reconstructions (references) that show that the São Francisco and Congo cratons remained fixed to each other from about 2 Ga until the opening of the South Atlantic Ocean in the Mesozoic. Around 800 Ma the Araçuaí-West Congo Basin was situated in a wide gulf, linked in the north with the Paramirim and Sangha aulacogens. Although limited oceanic opening occurred in this basin, it was open to a larger oceanic region only in the south. The closure of this latter ocean, the Adamastor Ocean, is now represented by the Ribeira Orogen and other ramifications of the Brasiliano-Pan-African system.

The interval between ca. 790 and ca. 585 Ma (Criogenian/Neoproterozoic III) is characterized by the generation of magmatic arcs, both oceanic, like the São Gabriel Orogen (ca. 750-730 Ma) and the Rio Negro I Magmatic Arc (ca. 790 Ma, Ribeira Orogen), and on active continental margins, like the Rio Negro II-Suite G1 (ca. 635-585 Ma) of the Ribeira and Araçuaí Orogens, and the Pelotas Arc (ca. 650 to 610 Ma) in the Dom Feliciano Orogen. Fore- and back-arc basins were also formed in this period, as testified by their remains found in several orogens. They were recognized by the occurrence of volcanic and volcanoclastic rocks intercalated with metasedimentary successions of arc provenance. Part of the São Roque and Açungui groups in the Apiaí Terrane, part of the metasedimentary successions of the Ocidental and Oriental Terranes of the Ribeira Orogen (Paraíba do Sul, Cambuci, Costeiro and Búzios) and part of the paragneissic complex of the Araçuaí Orogen are probable representatives of these pre- to syn-collisional basins.

The construction of the Brasiliano Orogens in the region considered was markedly diachronous and will be briefly summarised below:

  1. Metamorphic ages of about 790 Ma were reported for the Embú Terrane. The tectonic significance of this event is at present not clear. This metamorphic episode is not known from adjacent terranes. Next is a collisional episode at about 700 Ma, is registered in the São Gabriel Orogen that probably represents the docking of the magmatic arc to the west of the Rio de La Plata Craton.

  2. The period between 630 and 600 Ma is characterized by important tectonism and metamorphism in the southern extreme of the Brasília Orogen. The collision of Paranapanema and São Francisco paleocontinents result on nappe formation accompanied by relatively high-pressure metamorphism in the lower nappes and high T low P metamorphism at the upper nappes (Guaxupé and Socorro nappes).. Similar activity occurs in the Dom Feliciano Orogen resulting from the collision between the Rio de La Plata and Kalahari paleocontinents. Similar ages in the Apiaí Terrane are probably related to the same setting and metamorphism as in the Southern Brasília Orogen.

  3. The interval between 605 and 550 Ma is marked by intense tectono-metamorphic activity in the Ribeira and Araçuaí Orogens. This event, with its climax around 580 Ma, resulted from the collision between the Oriental Terrane (or Serra do Mar Microplate, including the Rio Negro Magmatic Arc and Suite G1 of the Araçuaí Orogen) with the southeastern border of the São Francisco paleocontinent. Expressive magmatism was generated in this interval, including leucogranites, porphyritic granites, hornblende-biotite granites and charnockites. Most of the metamorphic and magmatic ages from the Embú Terrane (600 to 590 Ma) fall also in this interval. The collisional events led also to a system of steep dipping dextral transpressive shear zones that cut earlier consolidated orogens (southern Brasília, Dom Feliciano and Apiaí Terrane).

  4. Finally, the last collisional event registered in the region happened at about 520 Ma. This event was originally only recognised in the Cabo Frio Terrane (Búzios Orogeny, Schmitt et al., 1999), but its records in other terranes of the Ribeira Orogen have now been recognized (Heilbron and Machado, 2003). The tectonic context of this event is still in discussion, but it may well represent the closure of a back-arc basin during the final amalgamation of Western Gondwana.

Late-syntectonic basins are associated to these collisional events. They are filled with volcanic-sedimentary successions with provenance from the recently constructed orogens. The formation and deformation of these basins is also diachronous. Examples are the Eleutério, Pouso Alto, Castro e Camarinha Units in the central part of the region, the Camaquã unit in the south and the Salinas Formation in the north.

After the amalgamation of Western Gondwana was finished a transition occurred to an extensional regime, probably related to orogenic collapse. This stage was recognised in the Araçuaí and Ribeira Orogens through inclined shear zones with down dip movement, gravity folds with subhorizontal axial planes and steep transtensional shear zones at high angles to the orogens. Important bimodal magmatism with ages between 510 and 480 Ma is associated with this stage.