Petrogenesis of the post-kinematic magmatism of the Central Finland granitoid complex I; radiogenic isotope constraints and implications for crustal evolution

Abstract

Post-kinematic granitoids of the Central Finland Granitoid Complex (CFGC) were emplaced into the Palaeoproterozoic crust of central Finland shortly after the ~ 1·89 Ga peak of the Svecofennian orogeny. They are found as discordant, generally non-foliated plutons with A- and C-type granite characteristics and a bimodal (maficfelsic) magmatic association. Conventional and ion microprobe U-Pb data show that the post-kinematic magmatism commenced in the northeastern CFGC at ~ 1·885 Ga and gradually moved to the west, where the plutons are dated at ~ 1·870 Ga. Radiogenic isotope (Nd, Sr, Pb) composition of the post-kinematic plutons indicates homogeneous initial ratios: εNd (at 1·875 Ga) ~ O, 87 Sr/86Sr Ο 0·703, and Stacey & Kramers-type Pb with low long-term Th/U (~ 2). These help to constrain the protolith history of the post-kinematic magmatism. Nd and Pb isotopic data for Palaeoproterozoic granitoids, Pb isotopic data for associated sulphides, and Nd and Pb isotopic composition of the classic ~ 1·6 Ga Finnish rapakivi granites reveal several distinct crustal domains in the Finnish Svecofennian orogen, and indicate that the central part of the Svecofennian orogen consists of several terranes (microcontinents) that were amalgamated in Palaeoproterozoic times. The CFGC region is the oldest (~ 2·0 Ga) among these terranes.