Radiometric Dating of the Tertiary Volcanics in Lower Silesia, Poland. I. Alkali Basaltic Rocks of the Opole Region

K-Ar dating was carried out on the Tertiary alkali basaltic rocks represented by melanephelinite and melabasanite lavas and plugs in the Opole region, Lower Silesia (Poland). The most probable geological age of the volcanic activity in this area is given by the mean K-Ar age of the melabasanites, calculated at 26.5 Ma: Late Oligocene (Chattian, middle part). Some melabasanite plugs yielded K-Ar dates significantly older than those from the lavas which - by good geological evidence - pre-dated them. This might be explained by the presence of excess argon in the plugs.