THE RIBEIRA OROGEN: NEOPROTEROZOIC/CAMBRIAN TECTONIC EVOLUTION AND OPEN QUESTIONS
 

Heilbron, M; Almeida, J.; Tupinambá, M.; Duarte, B.P.; Valladares, C.; Schmitt, R.; Nogueira, J.R.; Valeriano, C.; Ragatky, D.; Eirado, L.G.; Palermo, N.; Geraldes, M.

 

TEKTOS/Faculdade de Geologia-UERJ. Rua São Francisco Xavier 524, 4-andar, Bloco A. Rio de Janeiro - RJ, Brasil. heilbron@uerj

 

ABSTRACT

The Neoproterozoic orogenic evolution at SE-Brazil started with the closure of the Goianides and Adamastor oceans, located respectively at the western and southeastern sides of the São Francisco paleocontinent. At the southeastern side (Ribeira belt), E-vergent subduction generated magmatic arc suites  (ca. 790 and 635-610 Ma) and was followed by arc-continent and continent-continent collisions at ca. 580 Ma and ca. 520 Ma.

Key-words: Ribeira belt, SE-Brazil, Western Gondwana, Brasiliano Collage

 

TECTONIC SUBDIVISION

The southern tip of the São Francisco craton (SFC) is bordered by the Brasília belt at the western side and by the Ribeira belt at the eastern side. The southern part of the Brasília Orogen has an overall NNW-SSE trend and is subdivided in nappes with tectonic movement to E or ESE, towards the SFC or tangent along its southern border. These nappes result from an early collisional stage (collision I, ca. 630-625 Ma, Trouw et al., 2000, Campos Neto, 2000)

On the eastern side, the NE-SW trending Ribeira Orogen results from the interaction between the SFC with microplates to the SE and with the southwestern part of the Congo Craton. Two collisional stages (Collision II, ca. 580 Ma and Collision III, ca. 520 Ma) result on the docking of four northwest vergent terranes (Occidental, Paraíba do Sul, Oriental and Cabo Frio, Heilbron et al. 2000).

 

NEOPROTEROZOIC SUCCESSIONS: PRECURSOR BASINS OF THE RIBEIRA OROGEN         

Andrelândia Megasequence of the Occidental terrane:  AMS

The AMS represent the Neoproterozoic passive margin sucession of the São Francisco paleocontinent. The sucession is subdivided in two sequences (Carrancas, Serra do Turvo), that grade laterally to distal pelagic facies that is virtually indistinguishable (Ribeiro et al., 2003).

From bottom to top, the Carrancas Sequence is composed of banded paragneisses with amphibolites, quartzite and grey phyllite intercalations; greenish muscovite-quartzite with minor schist intercalations; and grey graphitic phyllites and schists with quartzite intercalations. The Serra do Turvo Sequence overlaps both the Carrancas Sequence and the basement. The lower part is composed of biotite phyllites and plagioclase-biotite-garnet schists/ gneisses with dropstones of basement fragments. The upper part contains similar schists and gneisses, with a stratified appearance due to intercalations of amphibolite, quartzite, garnet-rich metachert and calcsilicate 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 (Paciullo et al., 2000).   The chemistry of the AMS amphibolites indicates a transition from continental to transitional oceanic environment, with TDM model ages of 1.20 and 1.05 Ga.

U-Pb data from detrital zircons reveal essentially Paleoproterozoic sources, with subordinate Archean and Mesoproterozoic contribution (Valladares et al., 2001). The age of the youngest detrital zircon (ca. 900 Ma, Valeriano et al., 2004) and the metamorphic ages of the Brasília (ca. 630-580 Ma) and Ribeira (ca. 605-560 Ma) orogens constrain the depositional age of the MAS

 

Metasedimentary Units of the Paraíba do Sul and Oriental Terranes

The metasediments that outcrop in the Paraíba do Sul and Oriental terranes comprise a succession of pelitic and psammitic rocks, with carbonatic and calc-silicate intercalations. The metasediments from the Paraíba do Sul Terrane are essentially composed of two main lithotypes: psammitic biotite gneisses and pelitic sillimanite-biotite gneisses. Garnet and tourmaline are common especially in the pelitic gneisses.   Centimetric to metric lenses of calc-silicate rock, sillimanite quartz schist, garnet rich metachert and impure marble are frequent.

Distinct metasedimentary successions occur in the three structural domains of the Oriental Terrane. It is possible that the metasedimentary successions of the Oriental Terrane represent a carbonate passive margin of a continent opposed to the SFP. 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 & Machado, 2003).

Migmatitic garnet-biotite gneisses with lenses of dolomitic olivine marble and calcsilicate rocks occur in the Cambuci Domain. Lenses of basic 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 & 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, calcsilicate rocks and amphibolites. Detrital zircon ages from the quartzites indicate sources 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). 

Búzios-Palmital Sucession of the Cabo Frio terrane

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. Sillimanite-garnet-biotite gneisses with intercalations of calcsilicate rocks and garnet quartzites predominantly compose the Palmital Succession. 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., 2003).

The lithological composition, available geochronological data and geographic position of the successions, all point to deposition in a Neoproterozoic back-arc basin related to the arc (Heilbron & Machado, 2003). 

NEOPROTEROZOIC-CAMBRIAN CONVERGENCE

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. It results from the progressive collision of the following continents: Paranapanema, SF-Congo, Oriental and Cabo Frio. In both orogens, subduction of oceanic lithosphere leads 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. Finally, a third collision (Collision III) took place between the Cabo Frio and Oriental terranes at ca. 520-510 Ma.

 

Subduction of the Adamastor Ocean

Magmatic arc rocks that testify pre-collisional subduction in the Ribeira Orogen are located in the Oriental Terrane. The subduction was probably towards E-SE producing the Rio Negro Magmatic Arc. 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?), demonstrating that the sedimentation was coeval with the subduction process. 

 

Collision II: ca. 590-560 Ma
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. This 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, characterized by relatively steep juxtaposed terranes and abundant strike slip 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 strike slip shear zones.

Collision II led to the docking of the Paraiba do Sul and Oriental terranes (including the Rio Negro Magmatic Arc) with the Occidental Terrane. The intense deformation related to this collision generated tight to isoclinal folds, strong mylonitic schistosity and stretching lineations. The metamorphism of the Occidental Terrane varies from greenschist facies on the cratonic border to medium pressure granulite facies at the contact with the Occidental and Paraíba do Sul terranes. The Juiz de Fora Domain, in the upper part of the Occidental Terrane, is characterized by a tectonic mélange-like structure, defined by a crustal scale duplex with alternating basement and cover interleaved in numerous tectonic lenses.

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 Costeiro 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), a late suite of high-K calc-alkaline bodies (ca. 575-560 Ma; e.g. the coarse augen gneiss 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 Occidental Terrane (Juiz de Fora Domain) and in the Oriental Terrane.

 

Collision III (ca. 535-510 Ma)

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 SF-Congo Craton were consolidated, leaving little space for moving micro-continents. Some authors suggest that this collision resulted from the closure of an oceanic basin 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. The metamorphism is of relatively high pressure and temperature, characterized by the kyanite-K-feldspar association (with late sillimanite) in meta-pelitic 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 strike slip 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., 1996) and the igneous activity produced granites of the same age. The Além Paraíba Shear Zone, which is the largest one, extends from the state of São Paulo until the NE of Rio de Janeiro State, and 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. It seems prossible that these thermal and deformational events are related to the collision of the Cabo Frio Terrane (Collision III).

 

Post-collisional stage (ca. 510-480 Ma)

A post-collisional deformation phase registered in the Oriental and Cabo Frio terranes marks the transition to an extensional tectonic regime. This phase is interpreted as the result of extensional collapse and is represented by two groups of structures: a) normal brittle-ductile shear zones, parallel to the orogen and 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 shear zones played an important role as channel ways for the ascending magmas, as shown by frequent magmatic flow structures. An important characteristic of these bodies is the common association with basic rocks, leading to structures of magma mixing along the contacts.

 

OPEN QUESTIONS

In spite of the great improvement of the knowledge of the Ribeira belt during the last decade, several open questions remain unanswered:

- The Paraíba do Sul terrane is a fragment of the 

SF margin or is related to another continent?

- What causes inversion of the dip of the major suture of the belt: indentation of the magmatic arc or changes of the subduction polarity during convergence.

- What is the polarity of subduction of the collision III? What is the relationship with the Panafrican belts at Angola and Namíbia?

- What are the relationship between central Ribeira belt and the São Roque-Apiaí terranes at the southern segment of the belt?

-How to constrain a very complex and long term evolution with the paleogeography of Western Gondwana at the Neoproterozoic III- Early Cambrian transition?

 

REFERENCES

Campos Neto, M.C., 2000. Orogenic Systems from Southwestern Gondwana, an approach to Brasiliano-Pan-African Cycle and Orogenic Collage in Southeastern Brazil. In: Tectonic Evolution of South America, Cordani e outros eds, pp. 335-365.

Heilbron, M.; Mohriak, W.; Valeriano, C.M.; Milani E.; Almeida, J.C.H.; Tupinambá, M. 2000. From Collision to Extension: The Roots of the Southeastern Continental Margin of Brazil. In:  Atlantic Rifts and Continental Margin, Talwani and Mohriak eds. AGU, 115, p. 1-32.Heilbron, M. & Machado, N. 2003. Timing of terrane accretion in the Neoproterozoic-Eopaleozoic Ribeira orogen (SE Brazil). Precambrian Research. 125: 87-112.

Ribeiro, A. et. al. 2003. Geologia da Folha São João del Rei. IN: Projeto Sul de Minas, Livro e 13 Mapas Geológicos na escala 1:1.000.000. Pedrosa Soares et al. (Editors). Capítulo 11.

Schmitt, R.S.; Pimentel, M.; Van Schmus, W.R.; Trouw, R.A.J.; Armstrong, R.A. 2003. Marine sedimentation related to the latest stages of Gondwana assembly in the Ribeira belt: new U/Pb data. IV South American Symposium on Isotope Geology. Salvador, Bahia, Brazil Special Volume of Short Papers: 294-297.

Schmitt, R.S.; Trouw, R.A.J.; Van Schmus, W.R.; Pimentel, M.M. 2004. Late amalgamation in the central part of Western Gondwana: new geochronological data and the characterization of a Cambrian collisional orogeny in the Ribeira belt (SE Brazil). Precambrian Research, 133: 29-61

Trouw, R.A.J.; Heilbron, M.; Ribeiro, A.; Paciullo, F.V.P.; Valeriano, C.M.; Almeida, J.C.H.; Tupinambá, M.; Andreis, R.R. 2000. The central segment of the Ribeira belt, in Tectonic Evolution of South America, Cordani e outros eds, p: 287-310.

Valeriano, C.M.; Machado, N.; Simonetti, A.; Valladares, C.S.; Seer, H.J.; Simões, L.S. 2004. U-Pb geochronology of the southern Brasília belt (SE Brazil): sedimentary provenance, Neoproterozoic orogeny and assembly of West- Gondwana. Precambrian Research, 130: 27-55

Valladares C.S.; Machado N.; Ribeiro A., Paciullo, F.V.P.; Heilbron,  M.; Gauthier G., 2001. Ages of detrital zircon from siliciclastic successions of the Brasilia belt, southern border of the São Francisco craton, Brazil: implications for the evolution of proterozoic basins. In: 3º South American Symposium on Isotope Geology, Pucon, Chile. CD-ROM Extended Abstracts, Servicio Nacional de Geologia y Mineria: 261-264