HR: 0800h
AN: GP21B-0155 INVITED     [Abstracts]
TI: Plagioclase-Hosted Magnetite Inclusions From the Bushveld Complex
AU: * Feinberg, J M
EM: feinberg@eps.berkeley.edu
AF: Dept. Earth and Planetary Science, University of California, Berkeley, Berkeley, CA 94720 United States
AU: Scott, G R
EM: gscott@bgc.org
AF: Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709 United States
AU: Renne, P R
EM: prenne@bgc.org
AF: Dept. Earth and Planetary Science, University of California, Berkeley, Berkeley, CA 94720 United States
AU: Renne, P R
EM: prenne@bgc.org
AF: Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709 United States
AU: Wenk, H
EM: wenk@seismo.berkeley.edu
AF: Dept. Earth and Planetary Science, University of California, Berkeley, Berkeley, CA 94720 United States
AB: Gabbros from the Main Zone of the 2.064 Ga Bushveld Complex have long been known to possess unusually stable magnetizations due to the presence of high coercivity, exsolved magnetite inclusions in plagioclase and clinopyroxene. The paleomagnetic pole for these rocks has been used to anchor apparent polar wander paths for the Kaapval craton during the Early-Mid Proterozoic. To better understand the rock magnetic properties of silicate-hosted magnetite inclusions, oriented paleomagnetic samples of gabbro were collected from quarries near Belfast and Rustenberg, South Africa, sampling the eastern and western limbs of the Complex, respectively. Plagioclase composition at both sites ranges from An$_{55}$ (rims) to An$_{65}$ (cores) based on optical and electron microprobe data. Four kinds of inclusions are present within the plagioclase: elongate magnetite needles, nanometer-scale magnetite particles (responsible for the "cloudy" appearance of some crystals), translucent brown hematite/ilmenite platelets, and colorless euhedral inclusions of pyroxene and/or feldspar. Magnetite inclusions are most abundant at the cores of the plagioclase crystals. Orientations of the needles and the platelets are crystallographically controlled by the silicate host. Although the elongation direction of the magnetite inclusions can occur in any of five possible orientations, only two or three of these directions dominates each plagioclase crystal. Alternating field demagnetization of bulk samples (NRM = 1.5 x 10$^{1}$ A m$^{-1}$) shows univectorial remanence with average median destructive fields (MDF) of 115 mT (Belfast) and 90 mT (Rustenberg). AF demagnetization of single plagioclase crystals (NRM = 100 A m$^{-1}$) also shows single component remanence with average MDFs $>$150 mT. The NRM coercivity spectra of single plagioclase crystals are indistinguishable from that of the bulk samples. When normalized to their abundance in bulk samples the magnetite-bearing plagioclase fully accounts for the NRM of Bushveld gabbros at both sites. Close examination of the inclusions' interiors using magnetic force microscopy shows no ulv”spinel exsolution as observed in other silicate exsolved titanomagnetites with comparably high coercivities. Consequently, we interpret the high coercivities of the inclusions to be a product of their small size and extreme shape anisotropy. Single crystals of plagioclase demonstrate a strong anisotropy of IRM acquisition (see Scott, et al. this conference). Additionally, electron backscatter diffraction (EBSD) orientation indexing shows a strongly preferred orientation for plagioclase and pyroxene (with (010)$_{plag}$ and (100)$_{pyr}$ parallel to subhorizontal layering) consistent with gravitational settling within a magma chamber. Thus, there are two anisotropies (silicate preferred orientation and magnetite inclusion remanence) to consider when describing the ancient magnetic field present during the emplacement of the Bushveld.
DE: 3625 Descriptive mineralogy
DE: 3640 Igneous petrology
DE: 1518 Magnetic fabrics and anisotropy
DE: 1519 Magnetic mineralogy and petrology
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting