Final stages of the metamorphic evolution of the Kolvitsa belt and Umba block (southeastern branch of the Lapland granulite belt): U-Pb dating on zircon, titanite, rutile

As a result of isotope U-Pb dating of zircons, titanites and rutiles from rocks of the Por'ya guba area the following sequence of metamorphic events has been revealed for the Kolvitsa belt and the Umba block. The peak of granulite-facies metamorphism connected with the main stage of deformations took place before 1915±7 Ma ago. Decompression and cooling due to starting uplift of rocks began since 1915±7 Ma. Final stages of granulite-facies metamorphism were expressed in widely manifested diaphthoresis at 1909-1898 Ma, that was fixed in formation of high-U zircons. Since 1.92 Ga, metamorphic evolution of the Kolvitsa belt and the Umba block has become joint and synchronous with the Lapland Granulite and the Tanaelv belts. Cooling of rocks to 450 °C occurred 1880-1870 Ma ago, determined by the rutile age. Data on titanite and rutile ages with different closure temperatures for U-Pb system allow to calculate a gradient of cooling of rocks from 650-700° C to 400-450° as ~ 9.1