Rock magnetic studies of Miocene volcanics in eastern Otago and Banks Peninsula, New Zealand: Comparison between Curie temperature and low temperature susceptibility behaviour

Abstract

Samples were collected from 135 sites in 4 centres of Miocene alkali volcanic activity in the South Island of New Zealand: the Dunedin volcano, the Lyttelton volcano, the Akaroa volcano, and the Diamond Harbour Group. Strong field thermomagnetic behaviour has been used to determine the Curie temperatures and hence the magnetic mineralogy of the samples. The results show that differences in the degree of high temperature deuteric oxidation of the titano-magnetites occur between volcanic centres, flows, and even within flows, and that low temperature oxidation of Ti-rich titanomagnetites is extremely common. The use of low temperature susceptibility as a tool for identifying the magnetic mineralogy is also evaluated. Comparison with the thermomagnetic results shows that samples whose susceptibility continuously increases from liquid nitrogen temperature to room temperature can possess a wide range of Curie temperatures, contrary to previously published interpretations. The use of low temperature susceptibility for preselection of samples for paleointensity is rejected. Use of the technique alone can give only limited information about the magnetic mineralogy.