Acta Musei Moraviae

Voclanoclastic horizon at Kladoruby near Letovice (Boskovice Basin)

The occurrence of volcaniclastic rock was confirmed for the second time within the Letovice Formation (Asselian to Artinskian?) of the Boskovice Basin. Volcaniclastics are reported from the Kladoruby locality near Letovice, ca. 40 km N of Brno. Volcaniclastic layer was found in paleontological excavations in section Dolní Pepřík (GPS N 49° 33.080′ E 016° 35.843′) of Lubě Horizon. Locality is important for abundant Permian acanthodians, xenacanthid sharks, rare fossils of actinopterygian fish, discosauriscid amphibians, and very abundant plant remains. Three samples representing top, middle and bottom parts of the layer reaching 12 cm were taken. Mineralogical composition shows approximately the same portions of montmorillonite, albite, and kaolinite in all samples (each ca. 20–30 wt.%), with constant admixture of both idiomorphic and angular quartz not exceeding 6 wt.%. Calcite is also very common, assumed completely of secondary origin. Prismatic apatite crystals, well developed zircon crystals, cubic pyrite, limonite, and etched baryte were identified in heavy mineral fraction. Grain size of the rock equals to sandy silts, after the decalcification even to silts. There is a strong evidence for volcaniclastic origin reflected in the composition of clay components (montmorillonite is exceptional in the basin sediments) combined with the well-developed crystals of apatite and zircon. However, there is no credible way how to estimate the share of terrestrial component, and therefore distinguish between tuff and tuffite. TAS classification should not be used for this rock because very high loss on ignition (26%). According to the Zr/Ti vs. Nb/Y diagram the rocks belong to trachyandesites. Zircon saturation thermometry yielded crystallization temperature range 700–750 °C and rhyolite parent magma. It is intriguing that Kladoruby is second locality within the Boskovice Basin, where the deposition of volcanic ash is linked with presence of fosiliferous shales rich in organic matter. However, to link volcanic ash fertilization with organic-matter production leading to the bituminous shale horizons development in the basin needs further investigation. The volcanoclastic rock seems to be suitable for U-Pb dating of zircon and apatite.