Paleomagnetic field intensity during cooling of the sudbury irruptive 1700 million years ago

Thirty-three oriented cores were taken along a 300-meter section of felsic norite in the Sudbury irruptive, Canada. Polished section, theromagnetic, and electron microprobe investigations show that the norite has a uniform magnetic mineral assemblage of virtually pure magnetite (oxidation states I and II) and ilmenite. This assemblage has a narrow range of blocking temperatures (420–540°C), and was probably formed above the highest blocking temperature. Two specimens from each core were analyzed by the Thellier method and a third by the alternating field demagnetization method of paleointensity determination; 51 acceptable paleointensity and inclination values were obtained. The within-core variation in paleointensity values is significantly less than the between-core variation. The mean local paleofield intensity and inclination for the norite are 0.24±0.06 oe and 64±8°, respectively. The paleofield intensity and inclination variations seem to be caused by variations in the earth's dipole moment except in a small 50-meter part of the section where the variation in intensity can be related to the variations in local inclination.