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 Kandalaksha 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 conventional 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 veinlets in xenoliths, which gave a concordant U-238/Pb-206 age of 378 +/- 8 Ma. These results demonstrate that the dyke belongs to the 360-380 Ma old Kola Alkaline Province. The xenoliths represent a variety of rock types. We have analysed zircon from xenoliths of amphibolite, gneissic granite and granite. Most of these crystals have distinct core-rim structures which are clearly shown by cathodolumenescence (CL) imaging. The majority of SIMS analysed spots in cores are close to concordant at c. 2.65-2.75 Ga, which reflects 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 analysed, 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 also include pre-2.75 Ga rocks. Most rims developed during high grade metamorphism 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 during the Devonian carbonatite magmatism. The ages obtained from xenolith zircons are in excellent agreement with the independently known geological evolution of the Belomorides, (C) 2000 Published by Elsevier Science B.V. All rights reserved.