Paleoproterozoic (~1.88Ga) felsic volcanism of the Iricoumé Group in the Pitinga Mining District area, Amazonian Craton, Brazil: insights in ancient volcanic processes from field and petrologic data

Anais da Academia Brasileira de Ciências

Ronaldo Pierosan²  Evandro F. Lima¹  Lauro V.s. Nardi¹  Cristina P. De Campos¹  Artur C. Bastos Neto¹  José M.t.m. Ferron¹  Maurício Prado¹ 

[1] ,Universidade Federal do Rio Grande do Sul Instituto de Geociências Programa de Pós-Graduação em GeociênciasPorto Alegre RS ,Brasil

关键词: Amazonian Craton;    caldera complex;    ignimbrite;    Iricoumé volcanism;    surge deposits;    Uatumã magmatism;    Craton Amazônico;    complexo de caldeira;    ignimbrito;    vulcanismo Iricoumé;    depósitos de surge;    magmatismo Uatumã;   

DOI  :  10.1590/S0001-37652011000300012

来源: SciELO

【 摘 要 】

The Iricoumé Group correspond to the most expressive Paleoproterozoic volcanism in the Guyana Shield, Amazonian craton. The volcanics are coeval with Mapuera granitoids, and belong to the Uatumã magmatism. They have U-Pb ages around 1880 Ma, and geochemical signatures of α-type magmas. Iricoumé volcanics consist of porphyritic trachyte to rhyolite, associated to crystal-rich ignimbrites and co-ignimbritic fall tuffs and surges. The amount and morphology of phenocrysts can be useful to distinguish lava (flow and dome) from hypabyssal units. The morphology of ignimbrite crystals allows the distinction between effusive units and ignimbrite, when pyroclasts are obliterated. Co-ignimbritic tuffs are massive, and some show stratifications that suggest deposition by current traction flow. Zircon and apatite saturation temperatures vary from 799°C to 980°C, are in agreement with most temperatures of α-type melts and can be interpreted as minimum liquidus temperature. The viscosities estimation for rhyolitic and trachytic compositions yield values close to experimentally determined melts, and show a typical exponential decay with water addition. The emplacement of Iricoumé volcanics and part of Mapuera granitoids was controlled by ring-faults in an intracratonic environment. A genesis related to the caldera complex setting can be assumed for the Iricoumé-Mapuera volcano-plutonic association in the Pitinga Mining District.

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