Potassium-argon ages of metamorphic rocks and granites from Westland, New Zealand

Abstract

Abstract Samples of schist and gneiss from the Southern Alps in Westland have given potassium-argon ages of less than 10 million years, except for one specimen, which gave an age of 76 million years. As the time of metamorphism, on geological evidence, was early Cretaceous or older, these low values for age are interpreted as reflecting argon loss during deep burial prior to late Tertiary uplift along the Alpine Fault. Biotite from a pegmatite within the schists and gneisses gave an age of 25 million years. Argillite from the practically unmetamorphosed rocks east of the Main Divide gave an age of 166 million years. Granite from Rotomanu gave an age of 70 million years; granite from the Paringa River, an age of 286 million years. Since the latter granite is intruded into rocks of the Greenland Series, this series must be pre-Permian.