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

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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.