Generation of block-and-ash flows at the onset of silicic volcanism in the Paraná Magmatic Province (Brazil): evidence from photoanalysis of Caxias do Sul breccias

In this work, we perform textural analysis on volcanic breccias related to the silicic magmatism in the region of São Marcos (southern Brazil), Paraná Magmatic Province. The breccias are matrix-supported, coarse-grained, poorly sorted with angular to rounded fragments in a fine-grained matrix. Juvenile fragments are dense to pumiceous, massive to banded dacites; lithic fragments were not recognised. The analyses were performed on field photographs. Grain size distribution (GSD) analysis showed maximum fragment sizes between − 8 and − 9.5 ϕ and smallest recognised sizes between − 3 and 1.5 ϕ. Due to the spatial resolution limit of the photographs and the weathered condition of the matrix, the fine fragments from the matrix were not analysed. The GSD fractality could be quantified by a single power law, with low/moderate D values of 1.9–2.3. Vesicle size distribution (VSD) and shape parameters of both fragments and vesicles were also measured. The fragments are typically low vesicularity, with median vesicle area fractions around 5% for all samples, but pumiceous fragments with up to 43% area are found. VSDs exhibit single exponential, concave upwards curves, with vesicles becoming more deformed/complex with increasing size, suggesting a single continuous process of nucleation and growth under low degrees of shear. Regarding the fragments’ shape parameters, all samples have high values of solidity which, together with the low-vesicularity of the fragments, suggest brittle fragmentation of poorly vesicular material and the operation of secondary fragmentation processes during transport that round and smooth the clasts. However, field evidence for local conduit systems together with the GSD fractality suggest that these may be proximal deposits, without substantial transport history. Based on a comparison of the deposits’ characteristics with several types of volcanic breccias, we propose a block and ash flow source for these breccias that indicate the presence of nearby lava domes, providing evidence for high-viscosity low-flux dacitic volcanism in the Paraná Magmatic Province.