U-Pb zircon ages from a Devonian carbonatite dyke, Kola peninsula, Russia:a record of geological evolution from the Archaean to the Palaeozoic

Zircon from a xenolite-rich carbonatite dyke on Telyachi island, near Kanda laksha on the Kola Peninsula, northwest Russia, have been analysed for U-Th -Pb to determine the age of the dyke and study the geological evolution of the Belomorides. Zircon from the ankeritic carbonatite, analysed by convent ional isotope dilution, yielded a concordant U-Pb age of 380 +/- 7 Ma. This age is confirmed by SIMS U-Pb analysis of zircon from thin carbonatite vei nlets in xenoliths, which gave a concordant U-238/Pb-206 age of 378 +/- 8 M a. These results demonstrate that the dyke belongs to the 360-380 Ma old Ko la Alkaline Province. The xenoliths represent a variety of rock types. We h ave analysed zircon from xenoliths of amphibolite, gneissic granite and gra nite. Most of these crystals have distinct core-rim structures which are cl early shown by cathodolumenescence (CL) imaging. The majority of SIMS analy sed spots in cores are close to concordant at c. 2.65-2.75 Ga, which reflec ts the ages of the rocks from which the xenoliths were derived. Some cores lost a large fraction of their Pb during the carbonatite magmatism. One ana lysed, CL-dark rim appears to reflect a high grade metamorphic event which is slightly older than 2.75 Ga. This result requires that the xenoliths als o include pre-2.75 Ga rocks. Most rims developed during high grade metamorp hism 1.7-1.9 Ga ago. These are CL-bright and characterized by very low Th/U ratios, and formed by a rejuvenation process in solid state which resulted in partial or total loss of radiogenic Pb. Most rims have also lost Pb dur ing the Devonian carbonatite magmatism. The ages obtained from xenolith zir cons are in excellent agreement with the independently known geological evo lution of the Belomorides, (C) 2000 Published by Elsevier Science B.V. All rights reserved.