Acta Musei Moraviae

Natural radioactivity of metamorphic and igneous rocks and pre-Cenozoic sedimentary rocks on the map sheet 14-41 Šumperk

The aim of the paper is to inform about natural radioactivity of metamorphic and igneous rocks and pre-Cenozoic sedimentary rocks on the map sheet 14-41 Šumperk. The area covering the map sheet belongs to the Lugicum, namely to five groups of the Orlica-Śnieżnik Unit (Młynowiec-Stronie, Śnieżnik-Gierałtów, Nové Město, Staré Město and Zábřeh Groups) including serpentinites and a so-called tonalite sill, and to the Silesicum (Keprník, Branná and Desná Groups, and Šumperk Massif). Unmetamorphosed pre-Cenozoic sedimentary rocks are represented by siliciclastics of the Bohemian Cretaceous Basin (Bílá Hora and Jizera Formations). Contents of potassium, uranium and thorium were measured using a laboratory gamma-ray spectrometer in 1481 rock samples. Data are tabled and discussed. Most rocks outcropping on the map sheet show low values of mass activity of ²²⁶Ra equivalent, lower than that for average Earth crust (about 180 Bq.kg^-1). Increased values of mass activity were found in three types of rocks near Ruda nad Moravou: syenitoid (363 Bq.kg^-1 in average), rodingite (465 Bq.kg^-1) and leucotonalitic pegmatite with high uranium and thorium contents in a plagioclase+quartz+diopside subunit.