Voltar
Artigo Acesso aberto Produção Nacional
BIBTEX RIS

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

2011; Sociedade Brasileira de Geologia; Volume: 41; Issue: 3 Linguagem: Inglês

10.25249/0375-7536.2011413420435

ISSN

2177-4382

Autores

Carlos Augusto Sommer, Evandro Fernandes de Lima, Ronaldo Pierosan, Adriane Machado,

Tópico(s)

Mineralogy and Gemology Studies

Resumo

The Acampamento Velho Formation is constituted by neoproterozoic volcanic deposits of dominantly acid composition, which were emplaced under subaerial conditions on Sul-Rio-Grandense 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 870-978ºC for Taquarembó Plateau and 850-946ºC for Ramada Plateau. The calculated viscosity values vary from 4.77 x 10(7) to 1.39 x 10(8) Pa s for the reoignimbrites, and 1.72 x10(11) to 6.55 x 10(11) 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.

Referência(s)