RECONSTRUCTION OF TERRANE BOUNDARIES IN THE FINNISH SVECOFENNIAN: OXYGEN ISOTOPES FROM ZIRCON

Oxygen isotope ratios in zircon from 1.88-1.87 Ga post-kinematic magmatism in central Finland and 1.65-1.54 Ga anorogenic (rapakivi) magmatism in southern Finland provide insights into the evolution and growth of the Precambrian crust during the Svecofennian Orogeny. The distribution of these magmatic suites, emplaced shortly after cratonization, helps to define paleosutures and tectonic boundaries between highly metamorphosed accreted terranes. The oxygen isotope compositions from a suite of undeformed 1.88-1.87 Ga granitoid plutons across the Central Finland Granitoid Complex (CFGC) are relatively homogeneous (d¹⁸O_zircon=6.22±0.48‰, n=7), with the exception of three plutons in contact with supracrustal rocks in the adjacent greenstone and metasedimentary belts (d¹⁸O_zircon=7.70±0.09‰). Whole rock d¹⁸O for these rocks would be ~0.8 to 1.3‰ higher than d¹⁸O_zircon. Oxygen isotope ratios of this suite do not correlate with geochemistry or emplacement age. The oxygen isotope composition of the classic 1.65-1.54 Ga anorogenic granites in southern Finland are distinct from the 1.88-1.87 Ga suite and show a shift in d¹⁸O in zircon from north to south, independent of their emplacement age. The three southern-most anorogenic granites have d¹⁸O_zircon=6.14±0.07‰, but the northern plutons have an average d¹⁸O_zircon=7.95±0.68‰ (n=6). This division also correlates with a discontinuity in Nd isotopes (e_Nd=-0.9 to +0.7 for granitoids of various ages in the south vs. e_Nd=1.5 to 3.0 in the north). Boundaries between island arc terranes accreted during the Paleoproterozoic are revealed by the preservation of oxygen isotope ratios in zircons associated with the 1.88-1.87 Ga and 1.65-1.54 Ga magmatism in southern Finland.