Natural radioactivity of metamorphic and igneous rocks in the map sheet 14–22 JeseníkArticle PDF
Natural radioactivity of metamorphic and igneous rocks in the map sheet 14–22 Jeseník
The paper provides detailed information about natural radioactivity of crystalline rocks in the map sheet 14–22 Jeseník in the northeastern part of the Bohemian Massif. Magmatic and metamorphic rocks on the western edge of the map sheet belong to the Lugicum, namely to the Sněžník-Gierałtów Group (gneisses and granulites dominate), the Staré Město Group (gneisses, quartzites, amphibolites and metatonalite), and the Velké Vrbno Group (gneisses, mylonites, amphibolites and leptynites prevail). The rest of the map sheet belongs to the Silesicum, namely to the Branná Group (phyllites, marbles, quartzites and metadacite prevail), the Keprník Group (mainly gneisses and mica schists) and the Vrbno Group (mainly mica schists, gneisses, quartzites, amphibolites, greenschists, and metamorphosed acid to intermediate volcanites). The Silesicum contains Variscan I-type granitoid bodies: the Žulová Pluton is the largest of them. In the Žulová Pluton and in its mantle (gneisses and migmatites dominate) there are dispersed numerous pegmatite bodies. Calcic skarns composed mainly of grossular, vesuvianite, epidote, diopside and quartz occur in the Žulová Pluton mantle.
Potassium, uranium and thorium contents were measured using a laboratory gamma–ray spectrometer in more than two thousand rock samples. It is evident from the calculated values of mass activity of 226Ra equivalent (am) that the natural radioactivity of major rock types is lower or approximately the same as the natural radioactivity of the average Earth crust (about 180 Bq.kg-1). Slightly higher am values were measured in granitoids of the Žulová Pluton (in average 195 Bq.kg-1), biotite plagioclase gneiss of the Vrbno Group (209 Bq.kg-1) and acid to intermediate metavolcanic rocks of the Vrbno Group (242 Bq.kg-1). The uranium and thorium contents are anomalously high in Ca-skarns in the Žulová Pluton mantle: 104 ppm U and 254 ppm Th in a sample with highest radioactivity (am = 2769 Bq.kg-1), vesuvianite can be considered the main uranium-hosting mineral in the studied rocks.
Jiří Zimák, Department of Geology, Faculty of Science, Palacký University, 17. listopadu 12, 771 46 Olomouc; e-mail: email@example.com