U–Pb geochronological constraints of the late Svecofennian leucogranites of southern Finland

The ∼500 km long zone of late Svecofennian leucogranites in southern Finland is characterized by felsic, in places migmatitic magmatism that crosscuts older Paleoproterozoic crustal segments. SIMS and ID-TIMS U–Pb isotopic data on zircon and monazite from 15 granite and migmatite samples show that the emplacement of individual leucogranite plutons took place between ∼1.85 Ga and ∼1.79 Ga. This is a considerably longer period than what has traditionally been assigned to these granites. The youngest ages are found in the eastern part of the zone, which is the target area of this study. Significant age differences between granite plutons imply that the granites were emplaced in various depths and/or in various stages of the late Svecofennian, in support of a multi-stage accretionary history of the arc complexes of southern and western Finland. The late Svecofennian granites represent recycled crustal material and therefore commonly contain inherited zircons. The problem posed by the heterogeneity of zircon ages is overcome to some extent by the fact that the youngest zircons are invariably as old as the monazites, and thus monazite can be used as an emplacement age indicator. Zircon inheritance patterns vary somewhat between the different plutons within the granite zone. Archean inherited ages range from ∼2.8 Ga to ∼2.5 Ga and Paleoproterozoic 2.1–2.0 Ga inheritance is also commonplace, despite the lack of known exposed source rocks of this age. The contrasting age distribution of inherited zircons suggests that both sedimentary and igneous rocks provided source material for these granites.

► Late-Svecofennian magmatism in southern Finland lasted from ∼1.85 Ga to ∼1.79 Ga.
► U–Pb ages of single zircon crystals reveal variation in inheritance patterns.
► Leucogranite magmatism propagated from west to east.