Potassium argon studies on the South-east Nelson schists

Abstract

Rock samples from the South-East Nelson Alpine Schist Belt were subjected to petrographic, chemical and potassium-argon analyses. The rock chemistry and petrography indicates that this part of the Alpine Schist Belt is composed mainly of immature greywacke type geosynclinal sediments that have been progressively metamorphosed and subsequently exposed. Potassium-Argon age data was obtained for 26 whole rocks and 5 biotite separates. The potassium and radiometric Ar40 concentration patterns indicate that the Ar40 content of the schists is partially independent of the potassium contents. There appears to be a mobile component of the total Ar40 which has been distributed through the metamorphic sequence in response to some post-metamorphic stress which has resulted in the Ar40 content decreasing with increase in grade. Potassium-argon isochron ages for the biotite and garnet zones give ages of 13.5 and 7.2 million years respectively. These may be regarded as cooling ages. Chlorite grade rocks do not define any isochron. A model for the uplift is proposed to account for the observed structure, mineralogy, and Ar40 concentration patterns of the rocks.