Permian sedimentary sequence containing analcimolite and authigenic aegirine from the Boskovice Basin (Bohemian Massif, Czechia)

The Early Permian freshwater lacustrine sedimentary sequence in the vicinity of Bačov (Czechia) is world‐famous for its rich fossiliferous content, including amphibians, actinopterygians and insects. The detailed sampling of two quarries and nearby outcrops (129 samples) focused on the reasons for this sequence's extraordinary paleontological content. The mineral composition of all samples was determined by powder X‐ray diffraction analysis, accompanied by FESEM and WDS analysis, heavy mineral analysis and bulk geochemical analysis of selected samples. In both quarries, the rock‐forming minerals include quartz, albite, microcline, illite, chlorite and carbonates. The steady content of analcime (average 10 and 6 wt% in both quarries) and aegirine (average 4 and 5 wt% in both quarries) is surprising. The extreme enrichment of the 40‐cm‐thick layer in Quarry No. III in analcime (average 35 wt%, max. 56 wt%) indicates its transition to analcimolite, sedimentary rock with over 50% analcime, known from less than two dozen localities worldwide. The presence of authigenic aegirine, NaFe 3+ clinopyroxene, in the sedimentary sequences makes them even more unique, as it has been reported from only about five localities. The combination of several clues, including the presence of analcimised glass shards, layers rich in microcline (up to 58 wt%), well‐developed prismatic apatite crystals, the presence of smectites and the geochemical signature of alteration‐resistant elements, indicates the presence of low‐temperature tephra systems altered during burial diagenesis at temperatures <240°C. The presence of aegirine could be explained by the reaction of Na + and (SiO 4 ) 4− partially expelled during glass decomposition with Fe 3+ brought to the basin from extrabasinal source areas during arid to semi‐arid weather periods, as evidenced by the accompanying hematite. Ashfall of rather intermediate volcanic rock source fits the synsedimentary volcanism of at least two European volcanic complexes. There is a direct link between ashfalls and fossil accumulations (Konzentrat‐Lagerstätten).