Determining the Age and Cooling History of the World's Largest Layered Intrusion: U-Pb Zircon-Rutile Geochronology of the Merensky Reef, Bushveld Complex, South Africa

The Bushveld Complex, South Africa, is the world's largest layered intrusion and host to the majority of the known resources of platinum group elements, chromium and vanadium. Thermal modeling [1] and Ar-Ar [2] studies suggest rapid emplacement and cooling of the intrusion, however precise crystallization and cooling ages have been difficult to obtain. In a previous contribution [3], we provided a chemical abrasion ID-TIMS U-Pb Concordia age for zircon of 2054.3 ± 2.5 Ma (2sd, decay-constant errors included) from a sample of the platiniferous Merensky Reef (West Mine, Rustenburg Section). This was interpreted as the crystallization age of the sample, corresponding to conditions at or above the solidus of the Bushveld magmas (>950°C). The sample also contains low-U (1-4 ppm) rutile, which occurs as acicular grains with biotite and as inclusions or overgrowths on chromite. New U-Pb data from seven concordant multi-grain fractions of rutile yield a Concordia age of 2054.6 ± 4.9 Ma (2sd, decay-constant errors included) and date the timing of closure to Pb diffusion in rutile (600-700°C). Overlapping U-Pb Concordia ages for co-existing zircon and rutile indicate extremely rapid initial cooling of the intrusion (1000°C/Myr). Based on a comparison with existing geochronology, emplacement of the Bushveld Complex and other intrusions from the widespread Bushveld magmatic event across the northern Kaapvaal Craton is restricted to an interval of several million years at ca. 2054 Ma. Estimated magma emplacement rates are 0.1-1 km3/yr, which is characteristic of the high output rates of flood basalts, and indicates that the Bushveld layered intrusion and other products of the Bushveld magmatic event represent the high-level intrusive roots of a major Paleoproterozoic large igneous province. [1] Cawthorn and Walraven (1998) J. Petrol. 39, 1669-1687; [2] Nomade et al. (2004) J. Geol. Soc. London 61, 411-420; [3] Scoates and Friedman (2006) AGU 87(52), V31D-0611.