HR: 1340h
AN: GP33A-0099    [Abstracts]
TI: Paleomagnetism of the mid-Paleoproterozoic Bushveld Complex Alkaline Granites, South Africa
AU: * Geissman, J W
EM: jgeiss@unm.edu
AF: University of New Mexico, Dept. Earth and Planetary Sci., MSC03 2040, Albuquerque, NM 87131 United States
AU: Ferre, E
EM: eferre@geo.siu.edu
AF: Southern Illinois University, Department of Geology, Mailcode 4324, Carbondale, IL 62901 United States
AB: The ca. 2.06 Ga Bushveld Complex is associated with one of the world's largest series of anorogenic alkaline granite intrusions. The silicic plutons (Lebowa Granite Suite) collectively form a composite laccolith about 350 x 250 km in area, and about 2 km thick. These rocks are typically associated with high bulk susceptibilities(1000 to 6000 mSI). Previously reported anisotropy of magnetic susceptibility (AMS) data define a weak planar fabric that is similar to observed sub-horizontal compositional layering, and a very weak linear fabric. Samples obtained for AMS investigation, which were collected as a detailed well-distributed grid of 176 stations over about 400 km² in the Sekukuni Plateau area, east-central part of the complex, were used for paleomagnetic investigation. Several of the best exposures are at topographically high settings, and many outcrops have been partly to wholly affected by lightning. Those samples that have NRM intensities between about 100 and 10 mA/m and show the progressive removal of one or more magnetization components in a consistent manner in either AF or thermal demagnetization defined over a range of relatively high coercivities and/or laboratory unblocking temperatures (below 580C) typically are of north declination and steep positive inclination (e.g.,D = 332.4°,I = 62.0°,a95= 7.9°,k = 18.8, N = 19samples) or south-seeking and steep to moderate negative inclination(e.g., D=169.2°, I = -60.0°, a95=6.6°, k=21, N=24). These results are very similar to those from the Bushveld Complex itself and suggest that the granites at least locally have preserved primary magnetizations. An additional persistent magnetization (D=272.6°, I=-60.0°, a95=6.3°, k = 20, N =28)is more difficult to explain. Magnetizations of southeast declination and moderate positive inclination (D=131.8°, I=29.5°, a95=8.7°, k=15.1, N=20) are similar to primary magnetizations reported from Dwyka Group strata of latest Pennsylvanian to Early Permian age and may reflect local remagnetization during this common period of remagnetization throughout the world.
DE: 1519 Magnetic mineralogy and petrology
DE: 1525 Paleomagnetism applied to tectonics: regional, global
DE: 1527 Paleomagnetism applied to geologic processes
DE: 1533 Remagnetization
SC: Geomagnetism and Paleomagnetism [GP]
MN: Fall Meeting 2005