Rb-Sr whole-rock isotopic study of late Archaean and early Proterozoic granitoid intrusions, Kainuu, Eastern Finland

A tonalite-granodiorite-granite suite emplaced during the late Archaean D, deformational phase yields a six point isochron age of 2,500 ± 270 Ma and an initial 8 7 S r / 8 6 S r of 0.7044 ±0.0018. A rapakivi-type granite suite emplaced after the late Archaean episode and before the emplacement of early Proterozoic Jatulian dolerites yields a six point isochron age of 2,290 ± 110 Ma and an initial »'Sr/^Sr of 0.7036 ± 0.0085. Both ages are c. 150—200 Ma younger than corresponding U-Pb mineral ages for the same rock: the difference is interpreted as representing continued isotopic homogenization prior to uplift although the mechanism for such large scale, fluid phase mass transfer is not well understood.