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

Abstract

The impetus for this research was the enigma regarding the origin of the extensive silicic volcanic units in the Paraná-Etendeka Igneous Province. Are they rheoignimbrites, lava flows, or a combination of both? The São Joaquim silicic eruptive sequence is comprised of Palmas-Type aphyric dacites and is located in the northern segment of Torres Trough, a dissected mountainous region in southern Brazil. These dacite outcrops form plateaus and remnant hills that are scattered above a basaltic andesitic landscape. The original morphology of the silicic volcanic flows was constrained based on the internal architecture, geometry, and cyclicity of the lithofacies, in conjunction with petrographic and geochemical information. The data suggests the presence of ancient large tabular and lobate silicic lava flows (~100 m thick and ~10-40 km extents). No pyroclastic features were found. Eight interdigitating units were mapped. These large aspect ratios are similar to ones for basaltic flows and those of Snake River-type rhyolites, and they suggest high effusion rates and high temperatures as well as the presence of well-insulated cooled crusts. Lavas would have overflowed from long fissure vents that had created an extensive volcanic silicic surface body composed of conjugated and interdigitated flows above the north segment of the Torres Trough region. KEYWORDS: Extensive silicic lavas; Paraná-Etendeka Large Igneous Province; Serra Geral volcanism; stratigraphy of Torres ARTICLE INTRODUCTIONExtensive silicic units are commonly associated with welded to rheomorfic high temperature (~1,100ºC) ignimbrites (White et al. 2009, Bryan et al. 2002, 2010, Ernest 2014, Wolff & Wright 1981. However, silicic lava flows can also form voluminous units, including those in some Large Igneous Provinces (LIPs) such as the North Atlantic, Madagascar, Snake RiverYellowstone, Chon Aike and Paraná-Etendeka LIPs (Mahoney & Coffin 1997, Ellis et al. 2013, Bonnichsen & Kauffman 1987, Pankhurst et al. 1998. The silicic volcanic units of the Cretaceous Paraná-Etendeka LIP were previously understood to be rheoignimbrites, in which pyroclastic textures would have been masked during emplacement. Therefore, extensive tabular silicic plateaus, high emplacement temperatures, and the presence of circular structures (e.g., Messum Crater in Namibia) led several pioneering authors to infer a pyroclastic origin for these rocks (Whittingham 1989, Garland et al. 1995, Roisenberg 1989, Milner et al. 1992. Since then, much important research on geochemistry and correlation has been conducted within the province, but there is still a lack of detailed cartographic and stratigraphic work, which could help in clarifying the origins of and relationships among the silicic rocks. Therefore, the emphasis of this study is on the physical aspects of the rocks, such as the internal architecture and geometry of bodies and their spatial distribution. The goal of this work is to begin to characterize the geologic framework of the wild and mountainous areas of the nor...

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

“…In view of this, the lava is considered to have reached the surface as extensive flows. Similarly, Besser et al (2018) describe Palmas-type tabular bodies up to 40 km long and 100 m thick in São Joaquim, Santa Catarina. By proposing an effusive origin for these units, those authors argue that high effusion rates and continuous lava flow are determining factors in the generation of such expressive silicic units.…”

“…Discussion on the origin of Palmas-type rocks dates back to the pioneering mapping and geochemical characterization studies of the PIP (e.g., Bellieni et al 1986). However, it was only in the last decade, based on physical volcanology criteria, that more robust models were proposed (Waichel et al 2012, Polo and Janasi 2014, Polo et al 2018a, Besser et al 2018, Luchetti et al 2018a, Benites et al 2020. Luchetti et al (2018b) indicated a pyroclastic origin for Palmas-type rocks that occur in central parts of the PIP (including those described in southwestern Paraná State), characterizing them as high-grade ignimbrites.…”

“…In this case, the abnormally high effusion temperature (930-1,010°C) and low H 2 O content (~1-2 wt.%) would be responsible for the low viscosity values (Nardy 1995, Polo et al 2018a and, consequently, the widespread distribution of the rocks. Effusive models involve extensive tabular flows and lateral coalescence of domes and coulées (Umann et al 2001, Besser et al 2018, Polo and Janasi 2014, Benites et al 2020. Some of these authors (e.g., Umann et al 2001, however, do not disregard the presence of volumetrically subordinate pyroclastic rocks among the sicilic volcanites of the province.…”

Geology and Wine 15. Producing Wine at Altitude: The Terroir of São Joaquim, Brazil

“…The lower flows are formed by low-Ti (~1.75 wt.%) and low Sr/Y (< 6.5) basaltic andesites that form thick rubbly pahoehoe flows. The upper flows are formed by low-Ti (~1.34-1.55 wt%) and high Sr/Y (> 7.5) pahoehoe flows that are typically thinner than the lower sequence (Besser et al 2018). In addition, there are lava flows and some shallow sills of basaltic composition that have high Ti contents (> 3.5 wt.%), referred to as the Urubici magma type, according to Peate et al (1992).…”

“…The felsic volcanic unit (66.58-70.12 wt.% SiO 2 ) is composed of at least eight tabular and lobate lava flows with total thickness of 150 m, occupying an approximate area of 270 km 2 , with an estimated volume of 27 km 3 . They are classified as low-Ti (0.86-1.08 wt.%) dacites with subordinate rhyolites, and are assigned to the Palmas Formation (Besser et al 2018). Table 6 (major elements) and Table 7 (trace elements) give the average composition of the volcanic units in the municipality of São Joaquim, based on the samples collected in this study.…”