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 center 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. In this report, we present a well-preserved layer of basaltic tuff interbedded at the base of the high-Ti basaltic lava sequence in the central part of the province, Brazil. The Tapalam Tuff is well-sorted and consists of basaltic juvenile scoriaceous lapilli, vitroclastic coarse ash, and cement containing clays, zeolites, carbonates, and iron oxides. The glassy fragments (sideromelane-palagonite) range from well-rounded with smooth, curved surfaces to highly angular, cuspate-shaped shards. Achneliths morphologies include broken droplets (Pele's tears), thread fragments (Pele's hair), dumbbells, needles, spheres, ovoids and, an extremely rare shape to preserve, the reticulite. Elongated pyroclasts exhibit a flat orientation, and subtle bedding is defined by granulometric alternation. Our findings suggest proximity to the volcanic vent and likely indicate a fallout deposit associated with a fluid-dominated, high-height (hundreds of meters or more than 1 km) fire fountain Hawaiian-type eruption fed by very low viscous basaltic magma. These results reveal that magmatic activity was not exclusively dominated by purely effusive volcanism but also involved explosive eruptions, which can have significant implications for understanding PELIP's volcanic history and its relation to local and global environmental changes.