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Abstract
Across Värmland County there stretches in N-S direction a broad belt of black diabases known as hyperites. To the east it borders on a diffuse tectonic zone separating the gneiss complex of southwestern Sweden from the Svecokarelian orogenic belt. To the west, the diabases are partly involved in a late, middle Proterozoic thrust zone, the Mylonite Zone. The hyperite diabases appear as variably boudinated sills and dikes which generally follow the northwest strike of the older gneissic rocks. A pegmatoid variety from the interior of a large dike was collected for radiometric dating. Separated zircons plot on a regression line with an upper intercept with the Concordia at 1550 Ma, which is interpreted to correspond to the crystallization of the hyperite diabase. The lower intercept is at 880 Ma, which coincides with a Rb-Sr total rock-mineral isochron age of 975 Ma within the limits of error. It is concluded that a metamorphic event at 900–1000 Ma has caused Sr isotopic homogenization within the analyzed sample and radiogenic lead loss from the zircons.