Isotopic ages of Lentiira – Kuhmo – Kostomuksha olivine lamproite - Group II kimberlites

The Lentiira-Kuhmo-Kostomuksha triangle, along the Finland – Russian border and within the central part of the Archean Karelian craton, contains numerous examples of phlogopiterich, ultramafic, mantle-xenocryst-bearing and, in some cases, diamond-bearing dike rocks.Laser probe Ar-Ar data on phlogopite from 3 dike rocks on the Finnish side (Lentiira, Kuhmo) all gave ages within error of each other, 1202±3 Ma (2σ), 1199±3 Ma (2σ) and 1204±4 Ma (2σ) while a fourth sample produced mixed ages. Published Rb-Sr dates on mineralogically and chemically similar dikes from the Russian side (Kostomuksha) are 1232±5 Ma. The question remains open whether these represent two distinct age populations or whether differences in isotopic system behavior are the reason for the 30 m.y. age difference.