Excess radiogenic argon in metamorphic amphiboles and biotites from the Sulitjelma region, central Norwegian Caledonides

Abstract

Metamorphic amphiboles and biotites from Lower Palaeozoic rocks of the Sulitjelma region in the central Norwegian Caledonides give grossly anomalous K-Ar dates. 33 out of 44 analyses give dates discordant with respect either to stratigraphy or Rb-Sr chronology or both. The peak of metamorphism occurred about 420-445 my ago, but amphiboles and biotites which grew then give apparent ages of up to 2724 my and 1770 my respectively. It is suggested that excess 40Ar in concentrations of up to 5.4 × 10-5 cm3 STP/g amphibole were incorporated when outgassing of the 1700-1800 my old basement produced a high partial pressure of 40Ar. Muscovites and low-grade phyllites from the area have not given anomalous dates. © 1972.