40Ar-39Ar analyses of pyroxenes from dolerite dykes from southeastern Varanger Peninsula, Finnmark

Abstract

Pyroxene separates from two dolerite dykes from eastern Varanger Peninsula (at Komagnes and on the island St.Ekkerøya) have been analysed by the 40Ar- 39Ar method. The Komagnes dykes shows steps converging on a 'plateau' with an age of 344+- 17 Ma. The data from the St.Ekkerøya dyke do not yield any conclusive result; a total fusion age of c.640 Ma is shown to be meaningless, and cannot be considered as a crystallisation age for the dyke. The data are compared with results from (1) an earlier K-Ar study and (2) a palaeomagnetic investigation on these same dykes; the former which yielded dates of c.350 Ma, and the latter a loosely Vendian age. The 40Ar-39Ar results have not provided us with any new reliable informationon the age of these dykes which will resolve the conflict between the radiometric and palaeomagnetic 'ages'. The true crystallisation ages of the dykes remain enigmatic. Other isotopic methods may eventuelly help to provide a clearer indication of the true age of these dykes.