Timing of late Palaeoproterozoic metamorphism in the northern Belomorian Belt, White Sea region: conclusions from U-Pb isotopic data and P-T evidence

Brilliant-looking zircon crystals from a garnet-amphibolitic, metamorphosed mafic dyke at Lyagkomina in the Lake Kovdozero area, northern Belomorian Belt, most probably mark the 1875±4 Ma age of peak late Palaeoproterozoic regional metamorphism which followed upon dyke intrusion associated with deformation of the Lapland-Kola orogenic tectonic collage. U-Pb ages of titanites (ca. 1860 Ma) and rutiles (ca. 1750 Ma) help reconstruct the retrograde path and the cooling rates which decreased with time. The new titanite and rutile ages agree well with similar age data previously obtained on a regional scale from the central part of the belt and suggest absence of variation in mineral ages as a function of lithological differences. P-T analysis of the metamorphic parageneses indicates initial sharp decompression after peak metamorphism and jump uplift with unroofing of ca. 9-10 km of the crust in the Belomorian Belt previously thickened by late Palaeoproterozoic orogenic thrusting and stacking.