Radiometric age-determination of basalts, dolerites and related syenite in Skåne, southern Sweden

Abstract

Several intrusive and extrusive rock complexes in Skåne in southern Sweden have been dated by Rb—Sr and K—Ar age-determination methods. The dated complexes comprise two sets of dolerite dykes, massifs of syenite and plugs of basalt. The basalt necks belong to at least two groups aged 167 ± 4 Ma and 108 ± 3 Ma, respectively (K—Ar). North-west-trending dolerite dykes dated by the K—Ar method yielded an average age of 294 ± 4 Ma. A shale adjacent to a dolerite dyke was studied, in order to ascertain the thermal effect of the dyke on the surrounding argon system. Syenite massifs associated with the tectonic boundary between south-eastern and south-western Sweden have a Rb—Sr isochron age of 1210 ± 38 Ma and an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7055 ± 0.0027. K-Ar datings of these rocks and their minerals demonstrate extensive argon diffusion, which causes apparent K—Ar ages of 800–900 Ma. NNE.-trending dolerite dikes have a Rb—Sr isochron age of 1600 ± 230 Ma at an initial⁸⁷Sr/⁸⁶Sr ratio of 0.7033 ± 0.0015. The K-Ar ages have been disturbed by the opening of the system and fall in the interval between 800 and 1500 Ma. The syenite massifs contain xenoliths of NNE.-trending dolerite and consequently the Rb—Sr age of the syenites defines a minimum age for the dolerites.