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Pão de Açúcar, RJ Cartão postal geológico do Brasil
Abstract
O Pão de Açúcar com o seu bondinho é o cartão postal mais conhecido do Brasil e em nível mundial é um dos cartões de visita do Brasil. Além de seu interesse turístico, sua proposição como sítio geológico justifica-se pela importância que esse corpo granítico teve no desenvolvimento dos estudos geológicos e geomorfológicos no Brasil. O Granito do Pão de Açúcar está inserido na Suite Rio de Janeiro, que só muito recentemente teve sua natureza magmática comprovada. Essa suíte inclui diversos granitóides foliados, dominantemente peraluminosos (granitos tipo-S), assim designados: Granito Pão de Açúcar, Granito Corcovado e Granito Cosme Velho. Trabalhos geocronológicos recentes utilizando o método de datação U-Pb- SHRIMP (Sensitive High Resolution Ion Microprobe) determinaram a idade de cristalização de ca. 560 Ma representando um marco da amalgamação final do Supercontinente Gondwana ao final do Ciclo Orogênico Brasiliano/Pan- Africano.
... O espaço geográfico acima descrito assenta essencialmente sobre um conjunto geológico cuja composição e evolução são importantes de serem ressaltadas. As rochas mais antigas que estão presentes são os paragnaisses do Complexo Paraíba do Sul, de idade meso-neoproterozoica. Eles afloram entre os granitóides do Complexo Rio Negro, os ortognaisses da Suíte Rio de Janeiro, cujos principais representantes são o Pão de Açúcar, o Corcovado, e o Cosme Velho, identificado pela presença dos facóides de feldspato potássico, além de outros granitóides, de composição tonalítica e gabróica, conjunto esse de idade neoproterozoica a cambriana (SILVA & RAMOS, 2002). Todas essas rochas foram afetadas pelo evento Brasiliano, caracterizado nessa área por deformação compressional, cisalhamento transcorrente, metamorfismo de alto grau, fusão parcial de rochas crustais e granitogênese, o que acabou por alinhar a maior parte dessas litologias na direção WSW-ENE, durante o processo de formação do Gondwana (HEILBRON et al., 2004;MANSUR et al., 2008). ...
A Geoconservação é um ramo recente das Geociências que utiliza métodos para analisar quais sítios geológicos são representativos no contexto geológico de determinada região, avaliar suas características, valores e usos, bem como promover cuidados necessário para sua conservação e possibilidades de gestão. Após a seleção desses locais uma alternativa para a gestão dos sítios geológicos é por meio da interpretação ambiental, ferramenta que contribui para a conservação da natureza ao estabelecer relações com o que é observado. Dessa forma, este trabalho tem como objetivo apresentar os onze geossítios selecionados no inventário do município de Cananeia e sugerir seu manejo por meio da interpretação ambiental. Para a identificação dos geossítios foi utilizada metodologia que orienta a seleção desses locais e para a avaliação quantitativa, foram analisados perante os parâmetros desenvolvidos pela plataforma GEOSSIT. Posteriormente, houve uma nova seleção, na qual cinco geossítios foram explorados no viés educativo. Estes foram escolhidos com base nos resultados da quantificação, questões envolvendo a acessibilidade e seu reconhecimento como atrativo municipal. Em seguida, realizou-se a criação de conteúdos interpretativos com base em temas visando a organização de possíveis assuntos a serem abordados nesses locais. Em escala regional, os geossítios são interessantes ferramentas para contextualizar a história geológica do município e aqueles direcionados para a criação de temas, demonstram o potencial para associar novos elementos à valorização da região e divulgação do conhecimento geocientífico. Palavras-chave. Geoconservação, Inventário, Geossítios, Educação em Geociências, litoral paulista. Abstract. Environmental interpretation applied for the geological heritage of Cananeia, south coast of the state of São Paulo. Geoconservation is a recent branch of Geosciences. It includes methods to analyze which geological sites are represented in the geological context of a region, to evaluate their characteristics, values and uses, as well as to promote necessary measures for their preservation and management possibilities. After selecting these sites, an alternative for the management of geological sites is through environmental interpretation, a tool that contributes to the conservation of nature by establishing relationships with what is observed. Thus, this work aims to present the eleven geosites selected in the inventory of the municipality of Cananeia and suggest their management through environmental interpretation. For the identification of the geosites, a methodology was used to guide the selection and for the quantitative evaluation, the geosites were analyzed using the parameters developed by the GEOSSIT platform. Subsequently, a new selection was made, in which five geosites were explored from the educational perspective, allowing for the creation and organization of interpretive content. These were chosen based on the results of the quantification, issues involving accessibility and its recognition as a municipal attraction. Then, there was the creation of interpretive content to be addressed in these places. On a regional scale, geosites are interesting tools to contextualize the geological history of the municipality. Those that were selected to the creation of themes, demonstrate the potential to associate new elements to the valorization of the region and dissemination of geoscientific knowledge.
The tafone-type cavities in the granitic-gneissic rocky outcrops of Rio de Janeiro had
so far gone unnoticed by researchers despite being frequent forms in the rocky outcrops
of the area in both the coastal and adjacent mountainous areas. Little has been discussed,
until now, about the origin of these forms in Brazil usually attributed to sub-soil and/or
subaerial wathering. This paper presents a brief description of the cavities of the tafone type
found in the northern wall of the Pão de Açúcar dome. They are discussed alternative genetic
hypotheses to the usual for these characteristic geomorphological features of the graniticgneissic
basement of southeastern Brazil.
The paper assesses the contribution of mass movement to the development of landforms in weathered granite and gneiss terrain, using examples from SE Brazil, southern Mexico, SE China, and other locations. The style of slope failures varies depending on local relief and the grade of weathering, which mutually influence each other. Shallow debris slides and boulder falls are common in less weathered rock and steep terrain, whereas rotational slides appear to dominate in highly weathered rock and in more subdued topography. In certain topographic circumstances, slides may turn into debris flows or earthflows. Extreme rainfall is a typical trigger of mass movement in weathered terrain. Mapping of past landslides may considerably assist in hazard assessment, as they usually leave a clear geomorphological record.
This map was composed in 1:250.000 scale from the recognition of the morphostructural (Atlantic Orogenic Belt and the Cenozoic Sedimentary Basins) and morphosculptural domains. The relief compartments were defined in two main categories -aggradation and degradation landforms- using morphological and morphometrical parameters such as: relief amplitude; slope; hillslope geometry; drainage density and pattern.
The Atlantic Orogenic Belt is composed by metamorphic and igneous rocks of Neoproterozoic age affected by an extensional tectonic which occured between the Jurassic and Early Tertiary age and was caused by the South Atlantic opening. This tectonic has generated a sequence of normal faults, producing the coastal ranges and the high escarpments of the Serra do Mar and Mantiqueira.
The uplift of blocks resulting from this tectonic event produced a succession of residual uplands, aligned ridges, structural valleys and depressions that mark the hilly and mountainous morphology that were dissected until the Late Cenozoic in adjustment to locals or to the general base level.
The Cenozoic Sedimentary Basins are composed by sedimentary rocks of Early Cenozoic age and sediments of Quaternary age. The sedimentary rocks were stored in continental basins of Eocene-Oligocene age or next to the ocean on the Northern State, represented by the “Barreiras Formation” This formation, with a characteristic tableland morphology, was deposited until Early Pleistocene time. The sediments of the fluvial-marine and coastal plains were generated by transgressive-regressive cycles during the Quaternary, resulting in a succession of bays, lagoons, fluvial-marine plains (mangroves) and beach-ridges that mark a drowned lowland morphology.
THE BRASILIANO OROGENY AT THE CENTRAL SEGMENT OF RIBEIRA BELT, BRAZIL The structure of the central segment of the Ribeira belt is defined by three large thrust sheets (Lower, Intermediate and Upper) which override the autochthonous domain, towards the southern portion of the São Francisco Craton. Four lithotectonic associations are individualized within the domains: basement rocks (pré-1,8 Ga rocks); orthogneisses of unknown age; Post-1,8 Ga metasedimentary cover and Brasiliano granitoid rocks. Continental tholeiitic deformed dykes and disrupted lenses within the cover indicate exten- sional periods. Investigation of the tectono-metamorphic and magmatic aspects, associated with new U/Pb geocronological data support the subdivision of Brasiliano Orogeny within three main tectonic periods: a) the syn-collisional period (590-563 Ma) is represented by the main deformation (D1+2), coeval with the MI metamorphic stage and γl -γ3 granitoid rocks. Crustal shortening took place initially through ductile thrusts and tight recumbent folds. The late stage of this collisional event is characterized by oblique convergence, with pervasive deformation throughout the belt. MI had its thermal peak during D2, and is characterised by interme- diate to high pressures. The spatial distribution of the metamorphic zones defines an inverted gradient. Defor- med I and S type granitoid rocks occur during this period, b) the post-collisional period (535-520 Ma) is characterized by D3 deformation, coeval with M2 metamorphic stage. During DS phase, the oblique convergen- ce continued, but was accomodated by late to post-metamorphic peak strike slip ductile shear zones. Between these shear zones, shortening was achieved by steep folding of S2 foliation. The Ma stage with lower pressure regime is associated with intense melting of the basement and the cover, resulting in the generation of I-type magmatism and subordinated S-type; and c) the post-tectonic period is represented by the Post-D4 γ5 mag- matism registered between 503 to 492 Ma. This magmatism may be related to melting of crustal rocks associated with mantelic contribution related with high temperature conditions and uplift just after the Brasiliano Orogeny.
The territory of Brazil coincides almost entirely with the South American Platform, the crystalline core of the continent. Its basement is composed of ancient metamorphic and igneous rocks and it has not suffered any tectonic regeneration since the beginning of the Phanerozoic. Sedimentary rocks with almost horizontal bedding cover this crystalline basement. This latter shows ages as old as Early Precambrian, although ages between 500 and 1000 m.y. are conspicuously frequent. The cratonic areas became consolidated more than 1700 m.y. ago, whereas the fold belts formed essentially between 500 and 1700 m.y. ago. The sedimentary cover accumulated from the Early Silurian in three large intracratonic basins, until the platform became completely stabilized. A Late Jurassic to Early Cretaceous reactivation, caused by the break-up of the ancient Gondwana continent, created another basin sequence chiefly along the Atlantic continental margin.
Age relationships in the N–S trending Neoproterozoic (Pan-African) Kaoko belt of northwestern Namibia are still poorly constrained. U–Pb and Pb–Pb zircon age determinations by single grain evaporation, conventional multigrain fraction analyses and ion microprobe (SHRIMP) from a profile along the E–W Hoanib River reveal various episodes of zircon growth, ranging from late Archaean to late Neoproterozoic. From the eastern part of the profile we report crystallization ages of gneiss protoliths from 2645 to 2585 Ma, the oldest ages so far found in Namibia. Sm–Nd isotope data suggest the involvement of still older crustal material in the generation of these gneisses. A thermal event at ca 2285 Ma ago caused new zircon growth in the Archaean rocks, and major igneous activity between 1985 and 1960 Ma is represented by granitoid intrusions. This demonstrates that these gneisses were part of the Congo craton since Palaeoproterozoic times. An emplacement age of ca 1500 Ma was determined for the protolith of a granitic orthogneiss within the central part of the Hoanib River profile. This is the first record of a magmatic event of this age in this region. The western part of the profile consists of granitoid rocks that were generated during the Neoproterozoic Damara orogeny. This terrain has previously been interpreted as a Neoproterozoic metasedimentary domain and subsequently as pre-Damaran basement, but our study proves these rocks to be igneous in origin with emplacement ages ranging from 552 to 656 Ma. Sm–Nd isotope data and zircon xenocrysts demonstrate the involvement of older crustal material in the generation of these granitoids. We interpret a coastal granite, dated at ca 564 Ma, as being emplaced during the last peak of high-grade metamorphism in the Kaoko belt, which is approximately synchronous with the last metamorphic event in the Damara belt of central Namibia. © 1998 Elsevier Science B.V. All rights reserved.
Geochemical and isotopic U-Pb SHRIMP and Sm-Nd studies in three granitic plutons from the Pan-African Neoproterozoic/Cambrian Saldania Mobile Belt, southwestern South Africa supports differences in the sources and timing of emplacement of the main plutons related to the Cape Granite Suite as established by previous researchers. The Phase I, early syn-tectonic Darling Batholith Granitoid reflects an overall peraluminous chemical signature compatible with derivation mainly from melts extracted from paraderived crustal sources (Sa1 association). The Phase II, late to post-tectonic Robertson Pluton, reflects affinities to the Australian I-type granites (1a association). Despite the good structural constraints on the syn-and post-tectonic origin, the U-Pb ages point to a broadly synchronous crystallisation episode at 547 ± 6 Ma and 536 ± 5 Ma, respectively. In addition to U-Pb, Nd isotopic studies were also carried out for both plutons, as well as for the Riviera Granite, another phase II (1a association) pluton. The initial εNd (SSO Ma) based on a depleted mantle model range from -3.5 (Darling), to -3.1 (Robertson) and to -2.6 (Riviera). The Nd mean crustal residence ages are 1559 Ma for Darling, 1626 Ma for Robertson and 1243 Ma for Riviera. Despite the small databank, a dominant Mesoproterozoic ( ~1600 Ma) crust may be seen as the best candidate to explain the model TDM ages obtained. All the data largely overlap with others recently obtained for other plutons within the Cape Granite Suite and cast doubts on the current correlation between Saldania and the southeastern Brazilian, Dom Feliciano Belts.
Geocronologia U-Pb SHRIMP e Sm-Nd na Província Mantiqueira meridional, no Cinturão Saldania (África do Sul) e a evolução do Ciclo Brasiliano / Pan-Africano
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SGAN 603 -Bl. I -1º andar -70833-080 -Brasília-DF luizcarlos@aneel
- Cprm -Serviço Geológico Do Brasil
CPRM -Serviço Geológico do Brasil. SGAN
603 -Bl. I -1º andar -70833-080 -Brasília-DF
luizcarlos@aneel.gov.br
2 aramos@cprmbh.gov.br