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

Abstract

The evolution of the Rio 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.