Rapid cooling of the Rustenburg Layered Suite of the Bushveld Complex (South Africa): Insights from biotite 40Ar/39Ar geochronology

Despite their importance to understanding magma chamber processes and the formation of economically viable precious metal deposits, the cooling histories of layered mafic intrusions remain enigmatic due to limited geochronologic constraints. We provide a comprehensive 40Ar/39Ar study of biotite throughout the Rustenburg Layered Suite (RLS) of the Bushveld Complex, South Africa. Analyses of individual biotite grains from 10 samples, encompassing ∼5.5 km of RLS stratigraphy, yielded weighted mean plateau ages that all overlap at 2σ (α-95% confidence level) and range from 2056.3 ± 3.2 Ma to 2052.0 ± 7.6 Ma (2σ). A weighted mean of all biotite plateau ages yielded an age of 2054.47 ± 0.84 Ma (2σ, n = 30, mean square of weighted deviates = 0.23, P = 1.00; ±21 Ma fully propagated). The overlap between our 40Ar/39Ar biotite and published U-Pb zircon ages suggests that the RLS cooled rapidly to the closure temperature of biotite, with cooling rates on the order of 1000 °C m.y.–1 throughout the stratigraphy. Thermal modeling requires enhanced heat loss, due to the hydrothermal system associated with the emplacement of the RLS, to produce the inferred rapid cooling rates. Previously reported young 40Ar/39Ar biotite ages from the UG-2 and MG-1 chromitite seams and the Merensky Reef are likely a product of localized late-stage circulation of hydrothermal fluids. The lack of similarly young 40Ar/39Ar biotite ages from the remainder of the stratigraphy suggests that late-stage hydrothermal events were potentially localized to chromitites and the Merensky Reef.