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Timing of Palaeoproterozoic crustal shearing in the central Fennoscandian Shield according to U-Pb data from associated granitoids, Finland

Keywords: shear zones , granites , deformation , absolute age , U/Pb , zircon , monazite , Paleoproterozoic , Hirvaskoski , Ouluj rvi , Finland

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Granitoids in the Finnish part of the Fennoscandian Shield were created by numerous Archaean and Palaeoproterozoic tectonic events. Late phases of Palaeoproterozoic, Svecokarelian deformation created ductile, crustal-scale shear zones, of which the N-S striking Hirvaskoski Shear Zone and the NE-SW striking Ouluj rvi Shear Zone were strongly affected by the latest deformation stages, D3 and D4. These shear tectonic events deformed ductilely prekinematic granitoids and caused intrusion of synkinematic igneous rocks, but the D3 and D4 fault rocks are also penetrated by postkinematic intrusives.Archaean granitoids behaved mostly as rigid blocks unaffected by Palaeoproterozoic deformation. However, inherited Archaean zircons occur in some obviously Proterozoic granitoids, and others contain heterogeneous zircon populations. Granitoids lacking structures of the earliest deformations, D1 and D2, contain inherited zircon populations partially reset during the Svecokarelian orogeny and have been sheared ductilely during stages D3 and D4. Granitic rocksyielding zircon ages of 1.82-1.80 Ga show no features associated with these shearing events and penetrate the D4 fault rocks setting a minimum age for these deformation events. This age marks also the end of convergent tectonics in the central Fennoscandian Shield. Monazite data from a variety of rocks demonstrates a regional cooling below the 600°C isotherm at 1.8 Ga.


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