DACITIC VOLCANISM IN THE COURSE OF THE RIO DAS VELHAS (2800-2690 Ma) OROGENY: A BRAZILIAN ARCHEAN ANALOGUE (TTD) TO THE MODERN ADAKITES

LUIZ CARLOS DA SILVA, CARLOS MAURÍCIO NOCE, LYDIA MARIA LOBATO

Resumo


The evolution of the Rip das Velhas Greenstone Belt followed two main episodes of crustal addition. An initial phase of ocean- floor spreading is associated with tholeiitic to komatiitic magmatism. This early accreted, short-lived oceanic crust underwent rapid subduction, giving rise to widespread arc-related, felsic calc-alkaline volcanism, triggered by the direct melting of the oceanic slab. The felsic rocks comprise lavas and pyroclastic facies, extruded at ca. 2775 Ma and rapidly dispersed through syn- and post-eruptive submarine resedimentation processes. The sequence is coeval with a series of granitoids intrusive into the belt margins, characterizing a tonalite-trondhjemite-dacite (TTD) magmatism. The LILE depleted, Na-rich, chemical signature shown by the volcanic rocks fits well with a genetic model which incorporates aspects of the evolution of adakites in the modern Circum-Pacific arcs, and also the ancient FI dacites from the Barberton Greenstone Belt. Keywords: Rio das Velhas Greenstone Belt, evolution, Archean felsic volcanism, adakites, TTD association

Palavras-chave


Rio das Velhas Greenstone Belt; Evolution; Archean felsic volcanis; Adakites; TTD association.

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