Reoignimbritos e ignimbritos de alto grau do vulcanismo Acampamento Velho, RS: origem e temperatura de formação

Reoignimbrites and high-grade ignimbrites of Acampamento Velho volcanism, RS: origin and formation temperature

Resumo:

A Formação Acampamento Velho é constituída por depósitos vulcânicos neoproterozoicos dominantemente ácidos, emplaçados em condições subaéreas sobre o Escudo Sul-Rio-Grandense. Registros deste vulcanismo são encontrados nos platôs da Ramada e Taquarembó localizados no sudoeste do Estado do Rio Grande do Sul. Este magmatismo varia de metaluminoso a peralcalino e apresenta afinidade alcalina sódica. Nos depósitos de fluxo piroclásticos dominam fragmentos juvenis, como púmices, shards e fragmentos vítreos maciços. Estes apresentam um típico elevado grau de soldagem com ignimbritos, com alto grau nas porções de base e intermediárias, e reoignimbritos no topo. Os cálculos de temperaturas pré-eruptivas obtidas na saturação em zircão mostram valores entre 870-978^oC para o Platô do Taquarembó e de 850-946^oC para o Platô da Ramada. Os valores de viscosidade calculados variam de 4,77 x 10⁷ a 1,39 x 10⁸ Pa s para os reoignimbritos, e 1,72 x10¹¹ a 6,55 x 10¹¹ Pa s para os ignimbritos. Os conteúdos de Zr crescem em direção ao topo da sequência piroclástica indicando aumento da peralcalinidade, que determinou a redução da viscosidade dos clastos nas porções superiores dos fluxos. Um modelo agradacional progressivo permite explicar as variações verticais no tamanho e tipos de clastos, e na geoquímica do depósito. Este modelo assume a aglutinação de partículas juvenis quentes durante o deslocamento do fluxo, o que espessa progressivamente o depósito. Portanto, a elevada soldagem e o reomorfismo são sin-deposicionais. Os fluxos piroclásticos de alta temperatura dos platôs da Ramada e Taquarembó foram gerados a partir de baixas colunas de erupção e podem ser interpretados como depósitos ignimbríticos extracaldeira.

Palavras-chave:
vulcanismo; depósitos ignimbríticos; soldagem; rochas ácidas

Abstract:

The Acampamento Velho Formation is constituted by neoproterozoic volcanic deposits of dominantly acid composition, which were emplaced under subaerial conditions on Sul-RioGrandense Shield. The volcanism registration have been found at Ramada and Taquarembó plateaus localized at southwest of Rio Grande do Sul State. This magmatism varies from metaluminous to peralkaline and shows alkaline sodic affinity. At the pyroclastic flow deposits dominate juvenile fragments such as pumices, shards and massive vitreous fragments, which show a typical high welding degree with ignimbrites, being high degree at the base and intermediate portions, and reoignimbrites at the top. The pre-eruptive temperature calculations obtained at the saturation of zircon show values between 870-978^oC for Taquarembó Plateau and 850-946^oC for Ramada Plateau. The calculated viscosity values vary from 4.77 x 10⁷ to 1.39 x 10⁸ Pa s for the reoignimbrites, and 1.72 x10¹¹ to 6.55 x 10¹¹ Pa s for the ignimbrites. Zr contents increase to the pyroclastic sequence top, indicating increasing of peralkalinity, which determined the viscosity reduction of clasts at the upper portions of the flows. The progressive aggradacional model allows explaining the vertical variations at the clast size and type, and at the geochemistry of the deposit. This model assumes the agglutination of hot juvenile particles during the flow movement, which becomes the deposit progressively thicker. Therefore, the high welding degree and the reomorfism are sin-depositional. The high temperature pyroclastic flows of Ramada and Taquarembó plateaus were generated from low eruption columns and can be interpreted as extracaldera ignimbritic deposits.

Keywords:
volcanism; ignimbrite deposits; welding; silicic rocks

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Agradecimentos

Ao CNPq pelas bolsas de produtividade e pelo apoio financeiro (CNPq: 306616/2006-8, 306142/2006-6, 303584/2009-2, 473683/2007, 5470641/2008-8, 470203/2007-2 e 303038/2009-8). Ao IGEO/UFRGS pela logística.

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Datas de Publicação

• Publicação nesta coleção
  Jul-Sep 2011

Histórico

• Recebido
  13 Jul 2010
• Aceito
  13 Out 2011