HR: 09:30h
AN: V51E-07    [Abstracts]
TI: Age and Thermal History of the Bushveld Complex, South Africa
AU: * Renne, P R
EM: prenne@bgc.org
AF: Berkeley Geochronology Center, 2455 Ridge Rd., Berkeley, CA 94709 United States
AU: * Renne, P R
EM: prenne@bgc.org
AF: Dept. Earth and Planetary Science, Univ. California, Berkeley, CA 94720 United States
AU: Feinberg, J M
EM: feinberg@eps.berkeley.edu
AF: Dept. Earth and Planetary Science, Univ. California, Berkeley, CA 94720 United States
AU: Mundil, R
EM: rmundil@bgc.org
AF: Berkeley Geochronology Center, 2455 Ridge Rd., Berkeley, CA 94709 United States
AU: Nomade, S
EM: snomade@bgc.org
AF: Berkeley Geochronology Center, 2455 Ridge Rd., Berkeley, CA 94709 United States
AU: Nomade, S
EM: snomade@bgc.org
AF: Dept. Earth and Planetary Science, Univ. California, Berkeley, CA 94720 United States
AU: Merkle, R
EM: rmerkle@postino.up.ac.za
AF: Dept. Geology, Univ. Pretoria, Pretoria, 0002 South Africa
AB: The Bushveld Complex (BC) is one of the largest, most economically important and well-studied layered mafic intrusions in the world. Despite plentiful radioisotopic studies over the past 30 years, the age and emplacement chronology of the BC are not well-constrained. Biotite $^{40}$Ar/$^{39}$Ar data from the UG-2 chromitite layer yield consistent plateau ages around 2042 Ma (IUGS 1977 constants; 28.02 Ma for FCs here and throughout), implying either a slow cooling rate or systematic error when compared with the available Pb/Pb ages of 2059 to 2061 Ma (Nomade {\it et al.}, 2004, J. Geol. Soc. Lond., 161: 411-420). We are acquiring $^{40}$Ar/$^{39}$Ar and U/Pb data to evaluate the rapid emplacement and cooling suggested by petrological and heat-conduction studies (Cawthorn and Walraven, 1998, J. Petrol. 39: 1669-1687). Biotite and hornblende are present as intercumulus phases in gabbros and also in ubiquitous pegmatoid veins cutting the mafic and ultramafic rocks. Preliminary $^{40}$Ar/$^{39}$Ar results from both the eastern and western limbs of the BC show biotite integrated ages clustering between 2030 and 2050 Ma, slightly older than hornblende plateau ages (2030-2040 Ma). Biotites are locally subject to discordance suggestive of $^{39}$Ar recoil redistribution with an interlayer alteration phase; as in other such cases the integrated ages are more consistent and sensible whereas plateau ages are in some cases impossibly old. Biotite from an Fe-rich ultramafic pegmatoid in the western limb (Karee Mine) yields duplicate ~100% concordant plateaux spectra that average 0.8% older than the average of 4 hornblende plateaux. The cause of this apparent discordance (biotite age > hornblende age) is not understood although it is possible that the biotites have unusually high closure temperatures due to large diffusion radii related to the coarse (~5 mm) grain size. Initial ID-TIMS U/Pb single-zircon analyses indicate an age of 2058 Ma for the late-stage Nebo Granite, as displayed by concordant ages on crystals pre-treated with annealing/chemical abrasion (Mattinson, in review). Slightly younger, discordant ages indicating Pb loss were observed as well. Hornblende from the same sample yields 4 $^{40}$Ar/$^{39}$Ar plateaux averaging ~2035 Ma for single crystals. The new data are consistent with the previously inferred rapid emplacement and cooling chronology and confirm a ca. 1% systematic bias between current calibrations of the $^{40}$Ar/$^{39}$Ar and U/Pb chronometers (Min {\it et al.}, 2000, GCA 64: 73-98). The implied rapidity of injection of the multiple magma pulses is reminiscent of the emplacement timescale of most continental flood basalts (CFB). We concur with previous suggestions that the 3-6 x 10$^{5}$ km$^{3}$ BC represents the intrusive component of a massive CFB province whose eruptive component is largely eroded but locally represented by the Rooiberg Group volcanics. The age of the BC paleomagnetic pole used to anchor Paleoproterozoic Kaapvaal Craton apparent polar wander paths is well established, but it should be considered that this pole may not average geomagnetic secular variation.
DE: 8439 Physics and chemistry of magma bodies
DE: 9305 Africa
DE: 9619 Precambrian
DE: 3640 Igneous petrology
DE: 1035 Geochronology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting