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

Marcell Leonard Besser,Antonio José Ranalli Nardy,Eleonora Maria Gouvea Vasconcellos

doi:10.1590/2317-4889201820180087

Marcell Leonard Besser, Antonio José Ranalli Nardy + Show 1 more

Open Access

https://doi.org/10.1590/2317-4889201820180087

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Abstract

The impetus for this research was the enigma regarding the origin of the extensive silicic volcanic units in the Parana-Etendeka Igneous Province. Are they rheoignimbrites, lava flows, or a combination of both? The Sao 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.

Highlights

Extensive 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)
A continuity of outcrops is difficult to establish because erosion has shaped the ancient volcanic flows into isolated plateaus or small inselbergs
A tabular silicic volcanic flow eroded into little hills above the underlying silicic flows, giving the false appearance of lava domes (Fig. 3)

Summary

Introduction

Extensive 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). 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. 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, 1995). 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. The goal of this work is to begin to characterize the geologic framework of the wild and mountainous areas of the northern segment of the Torres Trough in the Santa Catarina Highlands by determining the origins of the extensive silicic rocks

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