Silicic, high- to extremely high-grade ignimbrites and associated deposits from the Paraná Magmatic Province, southern Brazil

Abstract

The Cretaceous trachydacites and dacites of Chapecó type (ATC) and dacites and rhyolites of Palmas type (ATP) make up 2.5% of the~800.000 km 3 of volcanic pile in the Paraná Magmatic Province (PMP), emplaced at the onset of Gondwana breakup. Together they cover extensive areas in southern Brazil, overlapping volcanic sequences of tholeiitic basalts and andesites; occasional mafic units are also found within the silicic sequence. In the central region of the PMP silicic volcanism comprises porphyritic ATC-type, trachydacite high-grade ignimbrites (strongly welded) overlying aphyric ATP-type, rhyolite high-to extremely high-grade ignimbrites (strongly welded to lava-like). In the southwestern region strongly welded to lava-like high-grade ignimbrites overlie ATP lava domes, while in the southeast lava domes are found intercalated within the ignimbrite sequence. Characteristics of these ignimbrites are: widespread sheet-like deposits (tens to hundreds of km across); absence of basal breccias and basal fallout layers; ubiquitous horizontal to sub-horizontal sheet jointing; massive, structureless to horizontally banded-laminated rock bodies locally presenting flow folding; thoroughly homogeneous vitrophyres or with flow banding-lamination; phenocryst abundance presenting upward and lateral decrease; welded glass blobs in an 'eutaxitic'-like texture; negligible phenocryst breakage; vitroclastic texture locally preserved; scarcity of lithic fragments. These features, combined with high eruption temperatures (≥1000°C), low water content (≤2%) and low viscosities (10 4-7 Pa s) suggest that the eruptions were characterized by low fountaining, little heat loss during collapse, and high mass fluxes producing extensive deposits.

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

“…Because of its large dimensions -more than 60 km in extension -, the rock bodies would, according to Bellieni et al (1986), correspond to ignimbrites, despite the absence of typical textures. A similar origin is proposed by Roisenberg (1989), Philipp et al (1994), and Luchetti et al (2018b). Conversely, other authors attribute an effusive origin to the voluminous silicic volcanites of the PIP.…”

Morphology and stratigraphy of Serra Geral silicic lava flows in the northern segment of the Torres Trough, Paraná Igneous Province

“…1). Indeed, locally eutaxitic textures are preserved as described in the Bergsig Formation in Etendeka and in some places of southern Brazil (Miller 2008, Luchetti 2015, Luchetti et al 2017. Volcaniclastic/epiclastic sediments are identified in Rio Grande do Sul as deposits formed of reworked pyroclastic material that was deposited during silicic magmatic activity (Riccomini et al 2016).…”

Supplemental Material: Rapid eruption of silicic magmas from the Paraná magmatic province (Brazil) did not trigger the Valanginian event

“…Santa Maria rhyolite is characterized by a holocrystalline aphyric texture composed by fine-grained sanidine, quartz, microlites of plagioclase (An 42-60 ; Lucchetti et al, 2018), pigeonite, augite and Ti-magnetite. Sub-millimetric spherulites are present as radial aggregates of sanidine crystals (Fig.…”

“…Jacuí dacite is porphyritic, with ca. 5% euhedral to subhedral plagioclase (An 40-60 ; Lucchetti et al, 2018), augite and pigeonite phenocrysts with fewer Ti-magnetite and apatite microlites set in a partially devitrified phaneritic matrix with salt-and-pepper aspect (Fig. S3A).…”