Abstract
Synclinally folded supracrustal rocks of greenschist-facies metamorphic grade occur in the Grythyttan-Hällefors region. The lower units of the sequence consist of metavolcanic rocks. A quartz-porphyritic K-rhyolite from the upper part of the metavolcanic unit has been dated. Zircons from this rock have a U-Pb age of 1862 Ma, indicating that this volcanic unit in the Grythyttan-Hällefors syncline is younger than the Svecokarelian synkinematic granitoids in general and is as a consequence younger than the rocks of the Leptite formation. Alternatively, the supracrustal sequence in Grythyttan-Hällefors constitutes the uppermost and youngest unit of the Leptite formation.