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Pão de Açúcar, RJ Cartão postal geológico do Brasil

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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.

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... 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). ...
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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.
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SGAN 603 -Bl. I -1º andar -70833-080 -Brasília-DF luizcarlos@aneel
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