Facies architecture and volcanological aspects of silicic rocks from the Palmas plateau, Brazil

Abstract

The nature of the extensive silicic units of the Paraná Igneous Province is still heavily debated. The silicic rocks that outcrop in the Palmas plateau (Southwestern Parana State) show aphyric texture and TiO₂ < 0.86%, being classified as Palmas-type rhyolites. Physical volcanology criteria are used here to constrain their origin. Textural and structural variations are observed in these rocks, resulting in the description of nine lithofacies: pitchstone; banded vitreous rhyolite; aphanitic rhyolite; rhyolite with planar disjunctions; massive rhyolite; compositionally banded rhyolite; amygdaloid rhyolite, rhyolite with quartz levels, and altered rhyolite. Folded compositional banding associated with volcanic conduits, well-defined lobes and basal autobreccias attest to an effusive origin for most lithofacies. They occur as domes or extensive tabular bodies, fed by volcanic conduits. For both domes and tabular units, the faciological sequence involves (from bottom to top): altered rhyolite; aphanitic rhyolite or rhyolite with tightly-spaced planar disjunctions; monotonous rhyolite with decimetric-spaced planar disjunctions; and top portions with fast cooling features. Only banded vitreous rhyolite differs due to the presence of shards and gas-escape pipes, aspects that imply in a pyroclastic nature. Therefore, a secondary pyroclastic flow created by lava collapse is suggested as its origin, although further studies are required.

KEYWORDS:
silicic rocks; rhyolites; lava domes; effusive rhyolites; Paraná continental flood basalts