Chaotic flow and fragmentation paths in acidic conduits from the Paraná-Etendeka Magmatic Province (PEMP)

Abstract:

This work focuses on recently recognized feeders/conduits of acidid volcanic products in the large igneous province of Paraná-Etendeka, in Brazil (Lima et al. 2012; Waichel et al., 2012). In the region of São Marcos, in RGS, conduits are exposed along a 0.5 up to 1 km wide of fracture/fissure sytem. The combination of detailed field work and determination of fractal dimension from outcrops, suggest at least five main features: a) flow structures with the development of shearzones evolving into funnel-like structures of aphiric/vitrophiric ballooning of melt accumulation, b) filaments of partially devitrified material (melt) in chaotic flow patterns and c) regular regions with the predominance of bubble-rich aphiric /vitrophiric material, which is mostly deformed and fragmented along the flow; d) breccia-like regions with angular and rounded fragmented blocks of partially bubble-rich magma and; e) huge bubbles (> 40 cm!), which complexly refold previous flow lines. Observed patterns suggest two different fragmentation episodes preceding or coeval with melt extrusion: 1) a high temperature event (above glass-transition) and, 2) a lower temperature event (below glass-transition). Additionally, different intrusive moments may be recognized through deformational signs such as: pseudotachylitic veins, progressive development of a strong stretching foliation and stair-stepping-like objects along the flow. Successive melting and remelting products from different effusive and/or fragmentation moments point towards different episodes of crossing glass transition back and forward. Frozen chaotic structures in the exposed conduits depict ancient melt paths.