Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original paper

Jan Franěk, Michal Filippi, Vladislav Rapprich

Origin and spatial distribution of large amygdales in Permian mafic lavas, Krkonoše Piedmont Basin (Bohemian Massif)

Journal of Geosciences, volume 70 (2025), issue 2, 71 - 86

DOI: http://doi.org/10.3190/jgeosci.0028.25

Spatial distribution and mutual relations of large amygdales were studied in a texturally stratified basaltic lava flow in the Permian Krkonoše Piedmont Basin in northern part of the Czech Republic, Central Europe. Clusters of distinctively shaped large amygdales were observed in lensoidal domains of altered trachybasalt between massive, amygdale-free zones below and scoriaceous lava above. The shapes and orientations of amygdales - from vertically elongated forms in lower parts to horizontally flattened types near the base of the upper scoriaceous zone - indicate a progressive shape evolution influenced by cooling dynamics of the nearly static lava body. Fluids creating the large vesicles likely originated from underlying water-saturated older volcanic rocks or local sediments, while the alteration of surrounding trachybasalt is expected to proceed at least partly as higher-temperature processes in the cooling lava rather than purely by later stage percolation of meteoric groundwater. These results allow for a comprehensive model of formation of large vesicles and their possible immediate filling with agate, carbonates or goethite liberated by water-dominated fluids from cooling lava in their immediate surroundings, potentially explaining also meteoric isotopic signatures observed in amygdales elsewhere, without need for Si-rich nor vapour-rich parent magma. The implications are broadly relevant for studies of large agates, geodes and other amygdales in mafic lava flows in water-rich volcano-sedimentary environments within continental basins.