Well-preserved fallout basaltic tuff in central Paraná-Etendeka Large Igneous Province: pyroclastic evidence of high fire-fountain eruptions

Abstract

The Paraná-Etendeka Large Igneous Province (PELIP) is renowned for its massive and rapidly emplaced flood basalts that dominated the centre of the Gondwana supercontinent during the Early Cretaceous. However, little is currently understood about mafic explosive eruptions, which often occur simultaneously with effusive activity, as observed in young basaltic volcanism. Here, we describe the first well-preserved layer of basaltic tuff interbedded within the high-Ti basaltic lava sequence in the central part of the PELIP, Brazil. The Tapalam Tuff consists of basaltic juvenile glassy components, scoriaceous lapilli and coarse ash, with a cement containing clays, zeolites, carbonates, and iron oxides. The glassy sideromelane fragments range from well-rounded achneliths with smooth, curved surfaces to highly angular, cuspate-shaped or platy shards. Achnelith morphologies include broken droplets (Pele’s tears), thread fragments (Pele’s hair), dumbbells, needles, spheres, ovoids and reticulite. Elongated pyroclasts exhibit a flat orientation, and subtle bedding is defined by granulometric alternation. Our findings suggest the deposits were laid down proximal to the volcanic vent and likely indicate a fallout deposit associated with a fluid-dominated, high (hundreds of meters or more than 1 km) fountain similar to a Hawaiian-type eruption, fed by low-viscosity basaltic magma. Volcanic activity was therefore not exclusively effusive but also involved explosive eruptions, which may have significant implications for understanding PELIP’s volcanic history and its relation to local and global environmental changes.