Anders Scherstén

The Sörvik granite forms rounded intrusions in the dominating Revsund granite, north-eastern Jämtland, central Sweden. Laser-ablation single-collector magnetic sector field-inductively coupled plasma-mass spectrometry on zircons was used to date the granite. The zircons are inhomogeneous, some with evident cores. The fractionation curves, ²⁰⁷Pb/²³⁵U versus time (depth), are irregular and often inconsistent with expected curves due to inhomogeneities in the zircon crystals in the Sörvik granite making age determinations difficult. The best-constrained age is a Tera-Wasserburg concordia age of 1776 ± 9 Ma obtained from five points. An upper-intercept discordia age in the ²⁰⁸Pb/²³²Th–²⁰⁷Pb/²³⁵U system obtained from six analyses with <|2.5|% discordance suggests an age of 1.74 ^. 10³ Ma; these analyses give an identical ²⁰⁸Pb/²⁰⁷Pb age (age brackets ∼1.73 and ∼1.80 ^. 10³Ma) supporting the U–Pb result. The suggested minimum age of a core and the inhomogeneities in the isotope composition of the crystals are consistent with a pre-Svecofennian age of the source rocks. These results, together with previously reported results from other rocks in the area, support earlier suggestions that the crust formation is no younger than early Palaeoproterozoic.