Radiometric Age Analyses of Rocks from the Northern Envelope of the Karkonosze Massif, the Sudetes, Poland: A Comparative Geochronological Study

K-Ar determinations have been conducted on the background of the earlier radiometric data in the eastern part of the northern Karkonosze-Izera envelope of the granite massif, composed of different varieties of granites, gneisses and mica schists. The values of the K-Ar age for minerals and rocks from the Karkonosze-Izera block lie in the wide interval ranging from 226.0±6.7 Ma to 386.1±3.0 Ma. The K-Ar age data obtained by volumetric method oscillate between 308±21 and 372±26 Ma. Most biotite K-Ar age values of the Izera granite-gneisses complex give an average age of 316.1±3.3 Ma. They correspond to the cooling age of the Karkonosze pluton while the data below 300 Ma are minimum age values that postdate the last granite intrusion of the Krkonoše-Jizera pluton. The obtained results over 300 Ma are mixed apparent ages between the age of the Karkonosze and the Izera granite ages. The K-Ar dates in the studied region have been compared with fission-track (FT) ages of zircon and titanite in the same area which display another distribution pattern.