THE THERMAL EVOLUTION OF THE BUSHVELD COMPLEX: INSIGHTS FROM 40AR/39AR DATING OF BIOTITE

Slow cooling of large layered mafic intrusions, such as the Paleoproterozoic Bushveld Igneous Complex, was originally inferred from coarse grain sizes, large-scale layering and concentrated precious metal deposits. We provide a comprehensive ⁴⁰Ar/³⁹Ar study of biotite throughout the Rustenburg Layered Suite (RLS) of the Bushveld Complex in South Africa. Analysis of individual biotite grains from 10 samples, encompassing ~5.5 km of RLS stratigraphy, yield weighted mean plateau ages that all overlap within 2-σ range from 2052.0 ±7.6 Ma to 2056.3 ±3.2 Ma (2σ). A weighted mean of all biotite plateau ages yields an age of 2054.47 ±0.84 Ma (2s, n=30, MSWD = 0.23, P = 1.00).Weighted mean plateau ages of individual samples are indistinguishable from published U-Pb zircon ages, indicating the entire body cooled rapidly from the zircon crystallization temperature to the ~350°C closure temperature of biotite. The inferred cooling rates are too fast to fit with conduction-only numerical cooling models. Thus, rapid cooling of the intrusion must be due to advective heat loss from hydrothermal system associated with the initial emplacement of the RLS. Previously published young ⁴⁰Ar/³⁹Ar ages of biotite from the UG-2 and MG-1 chromitites, as well as the Merensky Reef, indicate these PGE-rich regions may be subject to localized late-stage hydrothermal activity.