CONSTRAINING THE MID-TEMPERATURE THERMAL AND HYDROTHERMAL HISTORY OF THE BUSHVELD IGNEOUS COMPLEX: INSIGHTS FROM APATITE U-PB AND BIOTITE AR-AR GEOCHRONOLOGY

Slow cooling of large layered mafic intrusions, such as the Bushveld Igneous Complex, was originally inferred from coarse grain sizes, large-scale layering and concentrated precious metal deposits. However, recent chronologic data and numerical modelling suggests the contrary [e.g. 1-3]. These studies indicate that the Rustenburg Layered Suite (RLS), the predominately mafic/ultramafic unit associated the Bushveld Igneous Complex, cooled rapidly to temperatures well below the solidus. The numerical models of the RLS thermal history all assume heat loss via conduction only [1,3]. The conduction-only assumption must be questioned, as there is abundant mineralogical evidence for a vast hydrothermal system within the RLS as well as in the surrounding host rocks. We report ⁴⁰Ar/³⁹Ar biotite ages for the Eastern Limb of the RLS encompassing a total of ~6.5 km of stratigraphy. Weighted mean ⁴⁰Ar/³⁹Ar biotite ages lack any systematic variation with stratigraphy and also overlap with published U-Pb zircon ages [1,2], indicating that the entire body cooled rapidly from zircon crystallization temperatures of ~750-850 °C to the ~350 °C closure temperature of biotite. The homogenous cooling of the RLS to the closure temperature of biotite suggests additional advective heat loss due to the hydrothermal system associated with the emplacement of the RLS.

While biotite results confirm rapid cooling of the RLS and indicate additional advective heat loss, the low closure temperature and uncertainty of ⁴⁰Ar/³⁹Ar dating (±2 Ma) limit our ability to resolve the age differences needed to elucidate the cause of this rapid cooling. We report precise apatite U-Pb ages (closure temperature of ~400-550°C) from 3 stratigraphic levels of the RLS, refining current numerical models and further elucidating whether the body cooled quickly due to advection related heat loss and associated hydrothermal circulation. The U-Pb apatite ages also constrain the rate of paleomagnetic reversals during the Paleoproterozoic, as the RLS records 7 well-defined paleomagnetic reversals throughout the ~9 km of stratigraphy [4].

[1] Zeh et al. (2015) Earth Planet Sc Lett 418, 103-114. [2] Mungall et al. (2016) Nat Commun 7, 133385. [3] Cawthorn & Webb (2013) Geology 41(6), 687-690. [4] Letts et al. (2009) Geophys J Int, 179(2), 850-872.