Palaeomagnetism of the montagnais diabase dikes of the west central sector of the Labrador trough

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Eighty-six specimens were collected from eighteen different sites on the numerous north-striking thin diabase dikes of middle-Proterozoic (Helikian) age in the west central portion of the Labrador trough. These diabase dikes, which are the only intrusive rocks found in the area investigated (longitude 66°00′–67°30′W, latitude 54°00′–55°30′N), consist mainly of plagioclase feld-spars, olivine, pyroxenes and magnetite. Magnetite is the magnetic memory carrier, its Curie point (580°C) was determined with an electromagnetic balance. The volume ferromagnetic susceptibility, the intensity and orientation of natural remanent magnetization (N.R.M.) were measured in the presence of DC magnetic fields of 0.5 to 1 oersted. In order to obtain information relative to the stability of the N.R.M. component, stepwise alternating field (AF) demagnetization was conducted at intensities of 50, 100, 150, 200, 250, 300, 400 and 500 oersted (peak-to-peak) on twenty-one specimens, that is roughly 25% of the total number. A palaeomagnetic stability index test allowing one to choose the AF intensity range for which the rate of change in remanence direction of a diabase specimen is minimized was used. The optimum AF intensity, at which the primary thermoremanent component is most thoroughly isolated after most of the isothermal remanent magnetization (IRM) has been removed and before significant anhysteric remanent magnetization (ARM) is introduced, is located in the 100–200 oersted range. The remaining 75% of the specimens was then demagnetizaed at steps of 100, 150 and 200 oersteds. The decrease in intensity of the N.R.M. component in the course of AF demagnetization is quite rapid between 0.5 and 150 oersteds and relatively insignificant between 150 and 500 oersteds or more. A mean paleomagnetic pole position of 137oW, 10oS (α.95=14o,d p =08o,d m =15o,K=17), with at least one reversal, indicates that the Labrador trough was near the equator (Central North Pacific Ocean) at the time of intrusion. the possible implications of the results of this study are discussed in relation to hypotheses on polar wandering and on continental drift.