Scientific publications

Inherited zircons in leucosomes from the Salma and Gridino study areas have been studied. The leucosomes were formed in Palaeoproterozoic shear zones crossw-cutting Archaean tonalite-trondhjemite-granodiorite (grey) gneisses that host amphibolites with eclogite relics . Zircon crystals contain rounded inherited cores whose proportion is usually ≥ 90% of a grain volume. The cores display a fine oscillatory zonation typical for zircon from granitoids and are enveloped by rims that show a coarse zonation and define the highly euhedral prismatic morphology. The cores have been analyzed by the U-Pb SIMS (SHRIMP II) technique. Cores of the zircon from a leucosome from the Salma study area have yielded an upper intercept age of 2740 ± 8 Ma, with most of them having concordant ages. Cores of the zircon from the Gridino leucosome have given a Concordia age of 2831 ± 8 Ma. Both these ages are referred to as the time of crystallization of igneous protoliths of grey gneisses in which the leucosomes occur. In the Gridino study area the grey gneisses seem to have been reworked 2.76–2.73 and 2.71–2.68 Ga ago. All the obtained dates are consistent with ages of the extensive formation of the grey gneisses in the Belomorian region at ca. 2.8 Ga, the emplacement of a new generation of granitoids at 2.74 Ga and a reworking at ca. 2.7 Ga. The oldest ages of individual crystals inherited by igneous protoliths of the Gridino grey gneisses are ~ 3.15 Ga.