Dft 01
Pre-Congress Field Trip -
Precambrian gneisses in Rio: from the Sugar Loaf to the Arpoador outcrops
Claudio M. Valeriano Universidade do Estado do Rio de Janeiro - UERJ Faculdade de Geologia - Depto. Geologia Regional e Geotectonica Rua São Francisco Xavier, 524/4006-A - Maracanã 20559-900 - Rio de Janeiro - RJ - Brazil
[email protected] Fax: +55-21-2546675 Tel: +55-21-5877102 Julio Cesar H. de Almeida Universidade do Estado do Rio de Janeiro - UERJ Faculdade de Geologia Rua São Francisco Xavier, 524/4006-A - Maracanã 20559-900 - Rio de Janeiro - RJ - Brazil
[email protected] Monica Heilbron Universidade do Estado do Rio de Janeiro - UERJ Faculdade de Geologia Rua São Francisco Xavier, 524/4006-A - Maracanã 20559-900 - Rio de Janeiro - RJ - Brazil
[email protected] Recommended citation: Valeriano, C.M.; Almeida, J.C.H. de; and Heilbron, M., 2000. Precambrian Gneisses in Rio: From the Sugar Loaf to the Arpoador Outcrops. During-Congress Field Trip. 31 st International Geological Congress, Rio de Janeiro, Brazil, August 6 - 17, 2000, Field Trip Dft 01, 18p.
Acknowledgements: The authors wish to acknowledge all the persons involved with the production of basic geological data, specially geologistAntonio Carlos Magalhãesand geology studentsElisa Souza Bento, Renata Seibel and Thamy Cristine Sales Domingos da Silva(UERJ) for their support during preparation of this field trip guide.
INTRODUCTION This one day Field Trip Guide is intended to give the visitor a representative synthesis of the main lithologic and structural features of the Sugar Loaf Arpoador area, through general description of the most interesting and easily accessible outcrops. Emphasis is given to the structure and deformational fabrics associated with the typical megacrystic orthogneiss that occurs in the area (“Gnaisse Facoidal”) and is responsible for most of Rio’s striking topographical features.
GEOLOGICAL SETTING OF RIO DE JANEIRO Geotectonics of Ribeira Belt During Gondwana Supercontinent Amalgamation and its Break Up The city of Rio de Janeiro is located within the Neoproterozoic/Cambrian Ribeira belt, which integrates the network of orogenic belts[1] that resulted from the amalgamation of the former Gondwana supercontinent (Figure 1). Break up of this continent during upper Paleozoic through Mesozoic times led to the individualization of the present South American, African, Antarctic, and Australian continents and India, the latter colliding later to Eurasia. Although diachronic in nature, the succession of geological processes associated with the long-lasting continental amalgamation leading to the formation of Gondwana are collectively referred in Brazilian and African literature as the Panafrican-Brasiliano orogeny. Main development took place during the Neoproterozoic III-Cambrian times and the latest tectonic stages extended up to the Ordovician. The Ribeira belt[2, 3, 4, 5] extends for 1400 km along the Atlantic coast (Figures 1 and 2), its name given for the Ribeira do Iguape River, in São Paulo and Paraná State. The counterpart of the belt is presently found along the matching west African coast, as the belt was split apart when the south Atlantic Ocean started to open during the upper Cretaceous. It is a complex orogenic belt developed along the eastern and southeastern borders of the São Francisco craton [6] in response to the convergence between the São Francisco and Congo paleocontinents and other intervening microcontinental fragments presently located at the coastal region of Brazil [7, 8, 9, 10, 11]. Recent interpretations of the tectonic evolution of the Ribeira belt envisage an early southeastward subduction of the oceanic lithosphere belonging to the former São Francisco paleoplate beneath the Congo paleoplate, followed by the collision of these two paleocontinental masses. A pre-collisional magmatic arc developed on the overriding Congo plate between 640 Ma and 600 Ma[12]. The deeply eroded
plutonic roots of the Rio Negro arc is well preserved at Rio de Janeiro city and the adjacent Serra dos Órgãos Range. The collisional stage of convergence resulted in intense deformation of both passive margins. At the São Francisco side expressive reworking of basement rocks took place. Due to the Cretaceous Gondwana break up, part of the former Congo paleoplate probably stayed attached to the South American continent[11]. In southeast Brazil (Figure 3) the break up and dispersion of Gondwana was recorded by the development of the offshore Santos and Campos basins and, onshore, by reactivation of the Precambrian crust through faulting and anorogenic magmatism. Main expressions of this tectonic episode are tholeiitic basalt dike swarms of Cretaceous age and the Eo-Cretaceous/Tertiary Southeast Brazil Rift System with associated alkaline plutonic and volcanic magmatism[13, 14, 15, 16, 17]. LOCAL GEOLOGY OF THE RIO DE JANEIRO CITY Historical background The geological investigation of Rio de Janeiro county (former Guanabara State) started in late nineteenth and early twentieth centuries with the pioneering works of Pissis[18], Laboriau[19], Paes Leme[20, 21], Lima e Silva[22], and Backheuser[23]. The first systematic work on the local structure and lithology was accomplished by Lamego[24, 25, 26], who first brought comprehensive maps and structural cross sections of the south and central zones of the county. A geological map of the county in the scale of 1:50.000 published in 1965[27] represents a landmark for the local cartography, when the city completed 400 years of existence. From 1980 on, renewed efforts in the study of the geology of Rio de Janeiro county started, with growing detailed mapping and petrologic investigations from important contributions from the Rio de Janeiro Federal and State universities[28, 29, 30, 31, 32, 33, 34, 35]. Geological overview Steep relief, subhorizontal lithologic contacts and vertical faults concur to the resulting complicated outcrop pattern of the southeastern portion of Rio de Janeiro, as seen in Figure 3. In simple terms, the geologic Precambrian substratum may be described as a high-metamorphic grade terrain comprising metapelitic gneisses and quartzite intruded by a broad variety of sin- to post-tectonic granitoid rocks. Cretaceous basaltic and Tertiary alkaline dikes with associated vertical block faulting controlled subsequent erosion that sculpted the present rocky pinnacles and deep valleys. Largely controlled by sea-level variations, Quaternary alluvial and coastal sedimentary plains are presently the preferential sites for the development of the urban environment. Many generations of man-made landfills substantially modified the original outline of lakes, drainage channels and seashore. Granite-gneiss lithology Metasedimentary sequence: The oldest lithological unit of Rio de Janeiro county
is composed by supracrustal rocks. This metasedimentary pile is represented by: a) cordierite-sillimanite-garnet-biotite gneisses (kinzigite) with lenses of calcsilicate rocks, amphibolite and feldspatic quartzites; b) biotite gneisses rich in quartzitic layers; and c) leucogneisses (leptinite). Preliminary Rb/Sr isochrons[7] give an indication of at least ca. 489 Ma for the metamorphism of the metasedimentary sequence. Archer granodioritic to tonalitic gneisses: This unit crops out at the northern and western zone of the county (Figure 3) and has been regarded as basement to the Metasedimentary sequence. More recently, this unit has been interpreted as the local representative of the the Neoproterozoic Rio Negro pre-collisional magmatic arc[12], which has been documented at the Serra dos Órgãos Range. The unit comprises hornblende and biotite bearing metaluminous orthogneisses of granodioritic to tonalitic compositions. Granitic compositions are locally found. More basic rocks such as diorite, quartz-diorite and gabbros occur as metric to centimetric lenses within the gneisses. In more deformed zones, these rocks display a black and white banded pattern, due to stretching of basic lenses. Augen Gneiss (Facoidal Gneiss): The Augen Gneiss is the most representative lithology of the southern portion of the city. Most of the proeminent relief of this area, such as the Pão de Açucar, Corcovado and Dois Irmãos hills, were built by this rock. Abundance, durability and texture beauty are the main reasons for the widespread use of this rock as construction and ornamental material in many of the city’s buildings and monuments. The Augen Gneiss is a deformed coarse megacrystic granitoid that intrudes both units described above. Rb-Sr isochrons[7, 8] of 714 and 644 Ma were reported from outcrops located at the western zone of the city, while other isochrons of 564 and 533 were reported[36] at the Niteroi town localized at the opposite side of Guanabara bay. Therefore, the age of the Augen Gneiss batholith is still a matter of debate. U/Pb dating of this rock are ongoing efforts in order to solve the question. Late and post-tectonic granites: The Pedra Branca batholith[35, 37] crops out at the western zone of the county and constitutes a huge sill like intrusion of granitic to granodioritic porphyritic body. The Favela Granite [28] comprises post-tectonic stocks and metric dikes of a gray porphyritic metaluminous granite, locally associated with basic rocks. The granitic sill which caps the flat top of the Pedra da Gávea Hill is a visible example of this rock. Small pink and white aplitic dikes crosscut all the above-mentioned rocks and problably mark the end of the Neoproterozoic-Cambrian Brasiliano orogeny. Preliminary Rb/Sr isochron data[7, 8] indicate ca. 482 Ma for the emplacement of the post-tectonic granites. Metamorphism and deformation The peak conditions of regional metamorphism generated cordierite bearing mineral parageneses in the kingizitc gneisses of Rio de Janeiro, characteristic of high-temperature low-pressure granulite facies[30]. These rocks display a typical mineralogical assemblage of biotite, sillimanite, garnet, cordierite and K-feldspar. The regional metamorphism was also associated with the widespread anatexis of both the ortho-gneisses (including the Augen Gneiss) and the metasedimentary
sequence. The leucossomatic anatectic veins are concordant to the main (S1) foliation, resulting in the typical stromatic migmatitic structure displayed by the gneisses in Rio. The main foliation was subsequently folded by large recumbent (D2) folds that define the general geological architecture (e.g., section A-A’ in Figure 6). An expressive example of this folding can easily be seen at the top of the northern face of the Sugar Loaf hill (Figure 7), best viewed from Flamengo beach and other points along the shore of downtown area. A new phase of local anatexis generated coarse granitoid pods and veins (Utinga Granite[28]), specially common within the orthogneisses. Discordant pegmatites and leucogranites intrude all the previously described units. They are spatially and genetically related to the late deformations represented by two sets of open to tight upright folds and shear zones. An important mega-antiformal structure (Rio de Janeiro Antiform[10] deflects the foliation from subhorizontal attitudes at the Serra da Carioca hills, to steep south-dipping at the costal region of the city (see section A-A’, in Figure 6).
FIELD TRIP SUMMARY Meeting point Central monument obelisk located at the Praça General Tiburcio (Square), Praia Vermelha.
STOP 1 Top of the Sugar Loaf Objectives General view of the landscape and morpho-structural outline of southeastern Rio de Janeiro. STOP 2 Top of Urca Hill Objectives View of the western face of the Sugar Loaf and geomorphological contrast between the biotite (kinzigitic) metasedimentary gneiss and the Augen Gneiss. STOP 3 Cláudio Coutinho trail, starting at the northern end of Praia Vermelha (beach), along the southern face of the Urca Hill. Objectives Observation of deformational gradients with the development of mylonite zones in the Augen Gneiss related to ductile shear zones. Lunch break
STOP 4 São Teodosio Point, Cara de Cão Hill (Ponta de São Teodosio, Morro Cara de Cão) Objectives Observation of the lithology of the metassedimentary unit; sheared contact between the Augen Gneiss and the metasedimentary unit; ductile shear zones along pegmatitic dikes; observation of superposition of mesoscopic structures that represent regional deformation phases. STOP 5 Ponta do Arpoador Objectives Observation of strain gradients and resulting textures in the Augen Gneiss related to inhomogeneous ductile shear; discussion of regional tectonic implications of local deformation patterns.
STOP DESCRIPTIONS Meeting Point (7:00 a.m.): Central monument (Retirada da Laguna) obelisk located at the center of the Praça General Tiburcio (Square), Praia Vermelha (beach), at the end of Avenida Pasteur (Pasteur Avenue). Displacement: Short walk to the Pão de Açúcar cable car station; cable car to the Morro da Urca (Hill), then the other one directly to the top of the Sugar Loaf. STOP 1 Top of the Sugar Loaf (Pão de Açúcar), 392.5 m above sea level. The Sugar Loaf is a conspicuous pyramidal rock monolith that stands at the entrance of the Guanabara Bay. It owes its name for the resemblance of the peculiar shape of the mountain with the sugar cones that were produced from sugar cane originally in the Azores and later massively in Brazil. Special Features: Panoramic view of southern Rio de Janeiro and Niteroi; General landscape and morpho-structural outline: the Rio de Janeiro Antiform; faults related to Cretaceous-Tertiary reactivation; Quaternary coastal sedimentation. Displacement: Cable car down to the Urca Hill. STOP 2 Top of Urca Hill, 222 m above sea level. Special Features: View of the western face of Sugar Loaf and geomorphological contrast between the
biotite (kinzigitic) metasedimentary gneiss and the Augen Gneiss (Figures 5 and 6). Displacement: Cable car down to Praia Vermelha, then short walk to the Claudio Coutinho track, at the northern end of the Praia Vermelha beach.
STOP 3 Cláudio Coutinho trail. Location: The trail starts between the ECEME Military School and the Gabriela Mistral Elementary School at the northern end of Praia Vermelha (beach); walk to east along the southern face of the Urca Hill. Special Features: Close view of textures of the Augen Gneiss. The Augen Gneiss is derived from metamorphism and deformation of a coarse porphyroid granite, sin-tectonic to the Brasiliano orogeny in the Ribeira belt, and shows intrusive relationships with the regional metasedimentary gneisses. The trail intersects one of several mylonite zones related to ductile shear zones displaying continuous deformational gradients in the Augen Gneiss. Lunch break Displacement: 15 min bus ride to the Fortaleza de São João Fortress (Fortaleza de Sao João) at Urca, with short stop at the Urca Square (Quadrado da Urca). STOP 4 Ponta de São Teodosio (São Teodosio Point), Morro Cara de Cão (Hill). Location: Eastern tip of the Cara de Cão Hill, inside de Fortaleza de São João, Urca. The main gate to the Fortaleza de São João is located at the Avenida João Luis Alves, Urca. As this is military area, allowance is restricted and previous permission is required. Rio de Janeiro was originally settled in mid-16th century between the Sugar Loaf and the Cara de Cão Hill. Its location was strategy-motivated, guarding the entrance to the Guanabara Bay. The city was later moved to a site located more to the interior of the Guanabara Bay, presently at the downtown area. The new site offered a far better harbor and defense conditions, with more abundant water supply. Nevertheless, the old fortress still remaining at the São Teodósio Point is one of the relics attesting the importance of the place for defense against French efforts to establish a new colony to be called “la France Antartique”. Special Features: Lithology of the metassedimentary association: garnet-cordierite-biotite gneiss with calc-silicate lenses and layer of graphitic quartzite; sheared contact between the
Augen Gneiss and the Metasedimentary unit; ductile shear zones developed along pegmatitic dikes, associated with both main and late deformation phases; superposition of mesoscopic structures that represent regional deformation phases: folds and ductile shear zones; large isoclinal folds visible at the north wall of the Sugar Loaf (Figure 7). Displacement: Approximately 30 min. bus ride to the Arpoador outcrop. STOP 5 (3:30 p.m.) Ponta do Arpoador. Location: Rocky point that separates Copacabana from Ipanema beaches. It may be reached by foot from the eastern end of the Avenida Vieira Souto (Avenue), which runs along the Ipanema beach, and walk along the Avenida Francisco Bhering to the outcrop. When coming from Copacabana beach (Posto 6), take the Francisco Otaviano Street at the southern end of the Avenida Atlântica and cross through the Praça do Arpoador (Square) to the Avenida Francisco Bhering, then walk to the outcrop. The Arpoador (i.e., harpooner) outcrop owes its name to long gone whale hunters that gathered at the excellent lookout point to watch the movements of their prey. It is an approximately 40 x 100 m rocky cape between the Ipanema and Copacabana beaches. Special Features: The outcrop (Figure 8) is part of the southern limb of the Rio de Janeiro Mega-Antiform, with foliation dipping 65 degrees to SSE. The Augen Gneiss predominates, with lenses of leucogranite and one lens of amphibolite. Presence of strain gradients in the Augen Gneiss are related to inhomogeneous ductile shear (Figure 9): a lens-shaped less-deformed domain shows the predominance of flattening of feldspar megacrysts; predominant high-strain zones with sinistral sense of shear[38] display a variety of shear-related deformation textures such as S-C, S-C-C’ and C-C’ foliation patterns[39, 40]; a stretching lineation is locally present, with low plunges to azimuths between 240 and 260. Pegmatite dikes cross cut the foliation, associated to late- phase ductile shear zones. Topic for Discussion: When the foliation is restored to horizontal position, a tectonic transport verging to ENE is suggested, associated to the generation of the main foliation of the Augen Gneiss. Tectonic implications for the local deformation in opposition to the predominance of dextral oblique convergence postulated for the late deformation stages of Ribeira belt. End of Field Trip
REFERENCES 1- Unrug, R. 1997. Rodinia to Gondwana: the geodynamic map of Gondwana supercontinent assembly. GSA Today 7(1):1-6. 2- Cordani, U.G.; Melcher. G.C.; Almeida F.F.M. 1967. Outline of Precambrian Geochronology of South America. Canadian Journal of Earth Sciences, 5, 629-632. 3- Cordani, U.G.; Amaral, G.C.; Kawashita, K. 1973b. The Precambrian Evolution of South America. Geologische Rundschau 62 (2), 309-317. 4- ALMEIDA, F.F.M., 1967. Origem e Evolução da Plataforma Brasileira. Boletim da Divisão de Geologia e. Mineralogia, DNPM, Rio de Janeiro, 241, 1-36. 5- ALMEIDA, F.F.M. de; AMARAL, G.; CORDANI, U.; KAWASHITA, K. 1973. The Precambrian evolution of the South American Cratonic Margin,South of Amazon River. In: The ocean basin and margins (Nairn & Stelli, eds.),1:411-416, Plenum, Nova York. 6- Almeida, F. F. M. de -1977- O Cráton do São Francisco.Revista Brasileira de Geociências. 7(4):349-364. SBG, São Paulo. 7- Fonseca, A. C.; Cordani, U.G.; Kawashita. 1984. Dados preliminares sobre a geocronologia e suas encaixantes na cidade do Rio de Janeiro. Método Rb/Sr. Anais, XXXIII Congresso Brasileiro de. Geologia., Rio de Janeiro, SBG, V.6, 2333-2345. 8- FONSECA, A.C. 1994. Esboço Geocronológico da Região de Cabo Frio, Estado do Rio de Janeiro, Unpublished Doctoral Thesis, IG/USP, São Paulo, 186 p. 9- Campos Neto, M.C. & Figueiredo, M.C.H. 1995. The Rio Doce Orogeny, Southeastern Brazil. Journal of South American Earth Sciences, 8(2): 143-162 10- Heilbron, M.; Tupinambá, M.; Almeida, J.C.H.; Valeriano, C.M.; Valladares, C.; Duarte, B. 1998. New constraints on the tectonic organization and structural styles related to the Brasiliano collage of the central segment of the Ribeira belt, SE Brazil. Basement Tectonics Symposium, Ouro Preto, Minas Gerais, Brasil, Ext. Abstr. p: 15-17. 11- 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 Margins, Talwani & Mohriak (eds.), 354p. AGU-Geophysical Monograph Series, V 115. 12- Tupinambá, M.; Teixeira, W.; Heilbron, M. 1998.The Pan-African/Brasiliano Arc-related Magmatism at the Costeiro Domain of the Ribeira belt, Southeastern Brazil. Basement Tectonics Symposium, Ouro Preto, Minas Gerais, Brasil, Junho de 1998, Ext. Abstr. p: 12-14. 13- VALENÇA, J. G., 1976. Geologia dos maciços alcalinos do Estado do Rio de Janeiro. Parte II Correlações geológicas. in: I, II e III Semana de Estudos Geológicos da Universidade Federal Rural do Rio de Janeiro, Col. Trab., Itaguaí (1976), p. 247 - 259. 14- ALMEIDA, F.F.M, 1976. The system of continental rifts bordering the Santos Basin. An. Acad. Bras. Cienc. 48, supl, 15-26. 15- Mello, M.S., C. Riccomini, Y. Hasui, F.F.M. Almeida, A.M. Coimbra, 1985. Geologia e evolução do sistema de bacias tafrogênicas continentais do sudeste do Brasil, Revista Brasileira de Geociências, 15, 3, 193-201. 16- Mohriak, W.; Mello, M.R., Dewey, J.F., Maxwell, J.R. 1990. Petroleum Geology of the Campos Basin, offshore Brazil. In: Classic Petroleum Provinces (edited by J. Brooks), pp. 119-141, Geological Society, London.
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35- PORTO Jr. R. 1994. Petrologia das rochas graníticas das Serras da Pedra Branca e Misericórdia, município do Rio de Janeiro, RJ, Brasil. Unpubl. Thesis, IG-UFRJ, 222 p. 36- MACHADO, R.; DEMANGE, M. & VIALETE, Y. 1996. Idades geocronológicas Rb/Sr da granitogênese brasiliana no segmento setentrional da Faixa Ribeira, Estado do Rio de Janeiro. IN CONGRESSO BRASILEIRO DE GEOLOGIA, 39, Salvador. Anais.... Salvador, SBG, vol.1, pág. 38-40. 37- PORTO Jr. R. & FIGUEIREDO, M.C.H. 1996. Petrologia dos granitos da Serra da Pedra Branca, Rio de Janeiro, Brasil.Bol. IG/USP, Publ. Especial, 18: 89-90. 38- VALERIANO, CM.; HEILBRON, M.; ALMEIDA, J.C.H.; BENTO, E.; MELO, R.S.; SILVA, T.C. 1999. Particionamento da deformação em augen gnaisses na escala de afloramento: um estudo do Gnaisse Facoidal na Pedra do Arpoador, cidade do Rio de Janeiro. Boletim de Resumos do VI Simpósio de Geologia do Sudeste, Águas de São Pedro (SP). 39- PASSCHIER, C. W. & SIMPSON, C. 1986. Porphyroclast systems as kinematic indicators. J. Struct. Geol. 8(8):831-843. 40- BERTHÉ, D.; CHOUKROUNE, P. & JEGOUZO, P. 1979. Orthogneiss, mylonite and non-coaxial deformation of granites: the example of the South Armorican shear zone, J. Struct Geol 1:131-142
Figure 1: Location of the Ribeira belt, São Francisco and Congo cratons in relation to Gondwana during the Eo-Paleozoic (Unrug, 1997).
Figure 2: Major tectonic elements of southeastern Brazil (modified from Heilbron et al., 2000)
Figure 3: Geology of southeast Rio de Janeiro city (Heilbron et al. 1993). Section A-A' is displayed in Figure 5.
Figure 4: Map of the southeastern sector of Rio de Janeiro, with the location of the field trip stops (IBGE, Folha Baía da Guanabara, 1:50.000, contours in meters). Symbols: M Meeting Point; 5 Stop number.
Figure 6: Schematic structural sections indicated in Figures 3 (A - A') and 5 (B - B' to E - E')
Figure 7 - Main phase isoclinal style folds visible on the north face of Sugar Loaf, as traced
from an aereal photograph.
Figure 5: Geology of the Sugar Loaf area, Rio de Janeiro (Valeriano e Magalhães, 1984).
Figure 8 - geology of the Arpoador Outcrop, Rio de Janeiro (Valeriano, Almeida e Heilbron, 2000)
Figure 9 - Main deformational features associated with the Augen Gneiss at the Arpoador outcrop (Valeriano et al., 1999)
