Journal of the Virtual Explorer

The Neoproterozoic framework of the eastern part of the South American Platform comprises mostly older cratons surrounded by Brasiliano-Pan-African mobile belts (Fig. 1). Convergence between the São Francisco – Congo, Amazonian and Rio de La Plata Cratons during the assembly of West Gondwana generated two major Neoproterozoic belts: the Brasília and Ribeira belts (see Cordani et al., 2000; and Heilbron et al., this volume, for a full description of the geology of the belts). The Brasília belt resulted from the closure of the Goianides Ocean (e.g., Pimentel et al., 2000) along the western-southwestern margin of the São Francisco Craton, and merges with the Borborema Province to the north (Fonseca et al., this volume), whereas the Ribeira belt is associated with the closure of the Adamastor Ocean to the east-southeast of the craton (e.g., Campos Neto, 2000). During oceanic closure, subduction gave rise to magmatic arc suites that preceded arc-continent and continent-continent collisions. A sequence of rocks were involved in the amalgamation of West Gondwana including: Paleo- and Mesoproterozoic basement rocks and intracratonic successions; Neoproterozoic passive margin successions; magmatic arcs of intraoceanic and active continental margin settings with pre- and syn-collisional granitoids; late-orogenic sedimentary succession and post tectonic granites and pegmatites (e.g., Heilbron et al., 2004).

The Brasília belt in the region is mainly comprised of a Neoproterozoic passive margin metasedimentary succession (Araxá and Andrelândia Groups), (Table 1). The Araxá Group comprises quartzite, schist and mafic rocks (Valeriano et al., 2004), whereas the Andrelândia Group contains paragneiss with amphibolite, quartzite, schist, gondite and calc-silicate rocks (Ribeiro et al., 1995; Paciullo et al., 2000). Pimentel et al. (2004a, b) summarized the evolution of the Brasília belt and its relation to the Goiás Magmatic Arc (Table 1), as follows: a) formation of intraoceanic island arcs, Goiás Magmatic Arc, characterized by calc-alkaline volcanic rocks and tonalitic to dioritic plutons, between 890-800 Ma; b) intrusion of mafic- ultramafic complexes in high metamorphism grade, probably linked to a continental rift environment at ca. 800 Ma; c) high-grade metamorphism related to collision between the northern Goiás Magmatic Arc and the western border of the São Francisco proto-craton, between 770-760 Ma (Ferreira Filho et al., 1994); d) period of igneous quiescence between 760-680 Ma; e) intrusion of tonalitic, granodioritic, granitic and mafic-ultramafic bodies between 670 and 600 Ma, considered by Junges et al. (2002) as the second period of generation of tonalites and crustal accretion of the arc; f) peak of metamorphism in the Goías Magmatic Arc associated with the final closure of the Goianides Ocean between ca. 630 and 600 Ma (Fischel et al., 1998; Pimentel et al., 2000; Piuzana et al., 2002); and g) cooling and regional uplift between 580-560 Ma (K-Ar, Pimentel et al., 2004b).

In the southern Brasília belt, closer to the Ribeira belt, the final oceanic closure (point f above) was associated with the development of significant eastward thrust of nappes, towards the São Francisco Craton or tangential to it (Fig. 1, Table 1; Valeriano et. al, 2004). This Nappe System (subdivided into the Guaxupé and Socorro Domains) represents a thick Neoproterozoic sheet composed of a pile of high grade metamorphic crustal rocks (Campos Neto and Caby, 1999; Campos Neto, 2000). Ages between 611 to 604 Ma in the Socorro and Guaxupé Domains were interpreted by Trouw and Pankhurst (1993) as a thermal peak associated with the earlier thrusting and stacking of the Nappe System. The Socorro-Guaxupé Domains were affected by a late- to post-orogenic magmatism between 630 – 620 Ma (Töpfner, 1996) and 610 – 590 Ma (Janasi et al., 1993; Ebert et al., 1996; Töpfner, 1996; Wernick, 1998), respectively Numerous NE-trending, dextral, transcurrent-transpressive shear zones of the Ribeira belt (Ebert and Hasui, 1998) overprint the Brasília belt thrust sheets, particularly in the southern reworked margin of the São Francisco Craton (Fig. 2, Table 1). This has also led to relative vertical displacement of the main Precambrian units of the Ribeira along the Atlantic coast (Hackspacher and Godoy, 1999; Heilbron et al., 2004).

Figure 2 and Table 1 show the studied area, with division of the regional geology into tectonic units: a) the São Francisco Craton, composed of Archean to Paleoproterozoic orthogneisses and migmatitic complexes; b) the Reworked Cratonic Border, composed mainly of Archean to Paleoproterozoic tonalitic and granodioritic orthogneisses, strongly reworked during the Neoproterozoic; c) the Brasília belt and associated Metasedimentary Succession Domain (Araxá and Andrelândia Groups), the Guaxupé and the Socorro Domains; and d) the Central Ribeira belt (Heilbron et al., 2004).

Like the Brasília belt, the Central Ribeira belt (Figure 1), across the states of Rio de Janeiro and São Paulo, is a result of convergence between the São Francisco Craton and some Neoproterozoic fragments, and the Congo Craton. Collision processes resulted in the docking of distinct tectonostratigraphic terranes, such as (see Heilbron et al., this volume):

Geochronological studies in the Central Ribeira belt have shown that the closure of the Adamastor Ocean was a protracted process. The evolution of the Rio Negro Magmatic Arc started as early as 790 Ma ago (Heilbron and Machado, 2003). The earliest collisional stages followed, between 640 to 620 Ma, with west-verging thrusting towards the São Francisco Craton. Sediments of the São Roque Group were deposited 610 Ma in a back-arc basin (Hackspacher et al., 2000). Major collision of the Ribeira belt with the Sao Francisco Craton was accompanied by an important tectono-thermal event dated between 590 and 565 Ma (Machado et al., 1996), interpreted to be associated with thrusting and development of dextral shear zones. In addition, the tectonic history of the Ribeira belt includes an important escape tectonic event, with transpressional character, juxtaposing different crustal levels (amphibolite and greenschist facies), along NE-trending shear zones. This event took place between 600 and 580 Ma ago, as constrained by U-Pb age of post-kinematic syenitic rocks (Töpfner, 1996; Hackspacher and Godoy, 1999). Vertical movement during this event led to the exhumation of the Brasília and Ribeira belt rocks, with erosion and continental sedimentation in small NE-trending basins developed between 570-540 Ma (Teixeira et al., 1999; Zanardo and Oliveira, 1990).

Subsequent thrusting and development of shear zones occurred in the Central Ribeira belt between 535 and 520 Ma, during the postulated docking of the Cabo Frio terrane (Schmitt et al., 1999) to the Oriental terrane in the eastern part of the belt. The late-stage magmatism related to this docking took place between 503 and 492 Ma (Wiedemann 1993, Machado et al., 1996, Schmitt et al., 1999).

Figure 1. Precambrian framework of Western Gondwana

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Precambrian framework of Western Gondwana with the location of the studied area; the southern Brasília belt. (modified from Trompette, 1994): 1- Phanerozoic basins; 2- Neoproterozoic Cratons: SF- São Francisco, LA- Luis Alves, RP- Rio de La Plata, C- Congo, K- Kalahari; 3- Neoproterozoic mobile belts: B- Brasília, A- Araçuaí, R- Ribeira. Locality: CF: Cabo Frio.

Figure 2. Geologic map of SE Brazil

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Geologic map of SE Brazil (modified after CPRM, 2002) with U-Pb and ⁴⁰Ar/³⁹Ar sample locations and numbers.