Rock magnetic studies and a comparison between microwave and Thellier palaeointensities for Canadian Precambrian dykes

A selection of Precambrian dolerite dykes from the Canadian Shield has been studied to ascertain the magnetic field strength at the time of formation. Chilled margins samples from dyke swarms with U–Pb ages between 2450 and 2104 Ma were analysed using the microwave and Thellier palaeointensity techniques. Rock magnetic analyses have been carried out in order to evaluate magnetic mineralogy and grain size. Thermomagnetic analyses show single distinct Curie temperatures at 585 °C indicating pure magnetite. Hysteresis and backfield IRM measurements suggest grains within the pseudo-single domain (PSD) grain size range. Lowrie–Fuller tests suggest that single domain grains carry the remanence. High quality palaeointensity estimates were obtained, showing markedly low palaeointensities throughout the Proterozoic. Corresponding virtual dipole moments (VDMs) were calculated, typically being less than 4 × 1022 Am2, less than half the value of the present field. These data are similar to recently published palaeointensities from the same dyke swarms, determined using the conventional Thellier procedure.