EARLY PALEOPROTEROZOIC RIFTING OF NORTHEASTERN BALTICA-LAURENTIA

The Archean-Proterozoic boundary was characterized by bimodal anorogenic magmatism that indicates rifting of the Archean parts of the Laurentian and Fennoscandian Shields. The Hearst-Matachewan dykes of North America were intruded at ~2.45 Ga. Several 2.44 Ga layered gabbroic intrusions are found in northern Finland and adjacent Russia. Closely associated with these are some felsic rocks of similar age: (1) granophyres on top of the layered Koillismaa Complex in northern Finland; (2) the 2.44 Ga Kynsijärvi quartz syenite pluton near the Koillismaa Complex; and (3) the 2.45 Ga Nuorunen granite pluton near the Oulanka Complex in Russia. Nd isotopic data on the layered mafic rocks of the Oulanka Complex show epsilon-Nd (at 2.44 Ga) values of -4 to -1 (Glebovitsky et al., 2001) whereas those of the Koillismaa Complex are around -2 to -1. Initial epsilon-Nd value of -4.4 and T-DM model age of 2.93 Ga of the Kynsijärvi quartz syenite are within the limits of the evolution path of the surrounding Archean crust. The corresponding values for Nuorunen granite are -2.0 and 2.76 Ga, which are closer to the values from the mafic rocks. The granophyres show more scatter as their epsilon-Nd (at 2.44 Ga) values range from -2 to -5. The Archean core of the Fennoscandian Shield consists of several separate blocks, the two southernmost being the 2.9-2.6 Ga Karelian granite-greenstone terrane that extends to Finland and the 2.6-2.5 Ga high-grade Belomorian Belt in Russia. The Finnish mafic complexes intrude the Karelian rocks whereas the Oulanka Complex is situated near the suture of the Karelian and Belomorian blocks. The Archean blocks (lithospheric terranes) were adjacent at 2.44 Ga when they both were rifted but their earlier history may well be different.