Acta Musei Moraviae

Natural radioactivity of metamorphic and igneous rocks on the map sheet 14-24 Bělá pod Pradědem

The paper provides detailed information about natural radioactivity of metamorphic and igneous rocks on the map sheet 14-24 Bělá pod Pradědem in the northeastern part of the Bohemian Massif. Crystalline rocks in the northwestern corner of the map sheet belong to the Lugicum, namely to the Velké Vrbno Group (phyllites, mica schists, amphibolites, metamorphosed acid to intermediate volcanites and pyroclastics, and marbles prevail) and to the Staré Město Group (gneisses and quartzites). The rest of the map sheet (roughly 95 percent of its area) is part of the Silesicum whose geological structure is very complex and still debated. All major lithostratigraphic and tectonic units of the Silesicum outcrop on an area of the map sheet: the Desná Group (mainly paragneisses, phyllonites, metagranitoids, blastomylonites and amphibolites), the Keprník nappe (metagranitoids, orthogneisses, blastomylonites and paragneisses), the Vidly brook Group (metasiltstones and meta-arkoses), the Branná nappe (mica schists, phyllites, marbles, quartzites, metadacites and greenschists prevail) and the Vrbno Group (mainly phyllites, mica schists, quartzites, greenschists, metamorphosed acid to intermediate volcanites a pyroclastics). The Silesicum contains Variscan I-type granitoid bodies which are represented on the map sheet by the Mt. Rudná Intrusion.

Potassium, uranium and thorium contents were measured using a laboratory gamma–ray spectrometer in more than two thousand rock samples. Data are tabled and discussed. It is evident from the calculated values of mass activity of ²²⁶Ra equivalent that the natural radioactivity of major rock types is low. Slightly higher values of mass activity were measured in granitoids in the southern part of the Mt. Rudná Intrusion (granites and alkali-feldspar granites with 223 Bq.kg^-1 in average). High uranium and thorium contents were found in small bodies of granitic pegmatite which are spatially and genetically bound to the Mt. Rudná Intrusion.