New age constraints for metasedimentary rocks in southern Finland.

Metasedimentary rocks from nine sampling sites in southern Finland and an intermediate dyke in one of these sites were sampled for analysis of U-Pb zircon age and geochemical composition. The zircons of the metasedimentary rock samples yield 207Pb/206Pb dates ranging from 3281 to 1810 Ma. The nearly concordant dates from apparent detrital zircon cores indicate a prominent source with age around 2.1-2.0 Ga. The maximum depositional ages estimated for six of the metasedimentary rock samples span from 1.96 to 1.89 Ga. The data from zircon rims and overgrowths and metamorphic zircons in the metasedimentary rock samples indicate regional metamorphic events in the study area at least at ca. 1.89 and 1.84 Ga, and possibly also at ca. 1.87 Ga. The minimum age of the deposition (=1.89 Ga) was estimated based on the ages of the regional metamorphism and the intrusive rocks. Some zircon rims and overgrowths yield 21.91 Ga 207Pb/206Pb dates, which are considered to possibly represent metamorphic events in the source areas. [ABSTRACT FROM AUTHOR]