The paleomagnetism of the Modipe Gabbro

Samples have been collected from eleven sites in the Modipe gabbro which crops out on the border of Bechuanaland and South Africa. After partial demagnetization in alternating magnetic fields, ten of these sites give almost vertical directions of magnetization, forming a close group which corresponds to a paleomagnetic pole at 33°S, 31°E. The gabbro is intruded by the Gaberones granite, and samples from two sites in the granite show almost horizontal magnetization. On initial measurement gabbro samples collected near the contact yield directions close to those of the granite, which suggests that the gabbro was reheated at the time of emplacement of the granite (2350 m.y. ago). After magnetic cleaning, some of these samples give directions which agree with those at the other gabbro sites. Together with detailed alternating field and thermal demagnetization studies, this agreement provides strong evidence that the magnetically cleaned directions are those of the TRM acquired when the gabbro originally cooled. The gabbro has an age of approximately 2600 m.y. and is to date the oldest rock unit that has been studied paleomagnetically; the results imply the existence of a geomagnetic field in this early stage of the earth's history. Alternating field demagnetization experiments indicate that some of the magnetic grains have very high coercivities. Thermal demagnetization and IRM studies exclude the presence of hematite and show that the magnetic mineral present is magnetite. Microscopic observations and theoretical considerations suggest that the high coercivities observed are due to the shape anisotropy of small elongated single-domain grains of magnetite.