U-Pb Baddeleyite Geochronology of the Bushveld Igneous Complex

The Bushveld Igneous Complex (BIC) of South Africa is the world's largest layered mafic intrusion and most important primary source of platinum-group elements, and its geology has been thoroughly studied as a result. However, its emplacement age and subsequent thermal history are not well-established and are critical to evaluating the hypothesis that the BIC represents the intrusive component of a continental flood basalt province in addition to basic petrogenetic and metallogenetic issues. The dominantly mafic units of the Bushveld are typically devoid of zircon, making high-closure-temperature ages difficult to establish; however, baddeleyite has been extracted from several gabbros of the BIC's Rustenberg Layered Suite (RLS), previously shown to have cooled rapidly from ca. 500-200°C between ca. 2060 and 2040 Ma based on 40Ar/39Ar analysis of hornblende, biotite and plagioclase using U-Pb normalized constants (Nomade et al., 2004; Renne et al., 2008). U-Pb ID-TIMS analyses of baddeleyite are being conducted in order to clarify the high-temperature thermal history of the RLS, as the colsure temperature of baddeleyite is considered to be at least as high as that of zircon. Our preliminary results from a RLS gabbro suggest that the 207Pb/206Pb age is close to the U-Pb normalized 40Ar/39Ar ages of biotite from the same gabbro, and of hornblende and biotite from other units of the RLS. 206Pb/238Pb ages are slightly younger and indicate an open isotopic system. Therefore, we will attempt to apply the annealing/chemical abrasion method (previously only applied to zircon) of Mattinson (2005) to the baddeleyite crystals in order to remove zones in the crystal that have undergone Pb loss due to small-scale alteration and/or the effects of alpha recoil. Mattinson (2005) Chem. Geol. 220: 47-66; Nomade et al. (2004) J. Geol. Soc. Lond. 161: 411-420; Renne et al. (2008) Gechim. Cosmochim. Acta 72: A787;.