Journal of Petrology Volume 42 Number 11 Pages 1971-1993 2001
© Oxford University Press 2001
Petrogenesis of the Post-kinematic Magmatism of the Central Finland Granitoid Complex I; Radiogenic Isotope Constraints and Implications for Crustal Evolution
1DEPARTMENT OF GEOLOGY, PO BOX 64, FIN-00014 UNIVERSITY OF HELSINKI, HELSINKI, FINLAND
2GEOLOGICAL SURVEY OF FINLAND, PO BOX 96, FIN-02151 ESPOO, FINLAND
3DEPARTMENT OF PHYSICS AND EARTH SCIENCES, UNIVERSITY OF NORTH ALABAMA, POBOX 5172, FLORENCE, AL 35632-0001, USA
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 UPb 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:
Nd (at 1·875 Ga)
0, 87Sr/86Sr
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.
KEY WORDS: crustal evolution; granite; Proterozoic; radiogenic isotopes; UPb geochronology
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