Depth of burial from remanent magnetization: the Sudbury irruptive at the time of diabase intrusion (1250 Ma)

Abstract

Forty-four oriented drill cores were collected from a 65 m thick northwest-trending olivine diabase dyke (1250 Ma) and the norite (1900 Ma) with which the dyke is in contact. Thermal and alternating field treatments show that the magnetic direction of the dyke and that of the norite up to 45 m from the contact are parallel and conform to directions reported by others for the diabase dyke swarm near Sudbury. Between 45 m and 70 m (end of outcrop) hybrid directions are observed intermediate between that of the dyke and that reported by others for the norite. The NRM unblocking temperature spectrum and the inferred acquisition of VRM at higher temperatures in the norite suggest that a maximum temperature of 450 °C was attained in the hybrid zone at about 50 m from the dyke. Calculation of the thermal effect of the diabase intrusion on the norite in the hybrid zone indicates a temperature increase of 270 °C. Consequently, the ambient temperature of the norite at the time of intrusion was 450 – 270 = 180 °C (± 30). Taking the contemporary geothermal gradient at 20 to 25 °C/km the present erosion level of the norite is estimated to have been at a depth of 7 km (± 25%) about 1250 Ma ago. This suggests that a 7 km thick layer of the pre-Grenvillian part of the Canadian Shield has been removed by erosion during the past 1250 Ma if the Shield can be regarded as a single block that has not been tilted. Similar work elsewhere in the Shield may show the extent of these vertical movements.