Palaeomagnetism of the Ferrar dolerite in the northern Prince Albert Mountains (Victoria Land, Antarctica)

Abstract

During the sixth Italian expedition to Victoria Land (Antarctica), Jurassic Ferrar dolerite sills were sampled at 23 sites in the northern Prince Albert Mountains. The rock magnetic properties were similar in all sites. Saturation remanence, remanent coercive force and Curie‐temperature values pointed to titanomagnetite as the main carrier of magnetization. Thermal and AF demagnetizations were both effective in removing weak, secondary magnetization components below 250°C and 20 mT peak field. Further demagnetization resulted in single‐component, stable ChRM. Palaeomagnetic directions were derived after AF demagnetization at 20 mT, and site‐mean VGPs computed for all sites but one (Thern A), where systematic changes of the ChRM inclination have resulted in single‐sample VGPs dispersed astride the equator. The site‐mean VGPs distribution is clearly bimodal: 15 VGPs cluster around a palaeopole (lat. 48°S, long. 226°E) in good agreement with the literature data, whereas seven VGPs are displaced toward a palaeopole at higher latitude (lat. 79°S, long. 181°E). All VGPs can be regarded as Jurassic, because no evidence of substantial secondary components was found, and experimental data showed that the PTVRM (partial thermal viscous remanent magnetization) acquisition rate is too low to account for complete remagnetization of the rocks giving the anomalous VGPs. The Thern A and the southward displaced VGPs plot along a great circle through the East Antarctica Jurassic palaeopole. As a high reversal rate in the Jurassic is well documented by magnetostratigraphy, and the reversal VGP path is often confined to a narrow longitudinal band, we suggest that the great circle is a palaeomeridian and the sills characterized by anomalous VGPs were intruded during transitional phases of the geomagnetic field. Similar anomalous VGPs previously reported from the Ferrar Supergroup rocks of Northern Victoria Land also plot along the same palaeomeridian, which is only 30° away from the palaeomeridian derived from the Jurassic intrusives of the Dufek Massif in the Transantarctic Mountains (Burmester & Sheriff 1980). The Jurassic palaeopole of East Antarctica has been recalculated from 103 VGPs available in the literature. The position is 57°S, 221°E when all VGPs are taken into account, and 51°S, 223°E when the high‐latitude VGPs from Victoria Land are discarded as anomalous. Copyright © 1993, Wiley Blackwell. All rights reserved