The Bushveld Complex was emplaced and cooled in less than one million years - results of zirconology, and geotectonic implications

The Rustenburg Layered Suite (RLS) of the Bushveld Complex (BC) represents Earth's oldest large igneous province (>370000km3), and contains the world's largest reserves of platinum-group elements, chromium and vanadium. However, its mode of formation, the exact timing and nature of magma emplacement, solidification and sub-solidus cooling history remain a matter of debate. High precision UPb dates, backed by petrological observations reveal that zircon throughout the RLS crystallised within 1.02±0.63Ma from highly fractionated intercumulus melts at temperatures between 940° and 670 °C. Zircon in quenched Marginal Zone rocks crystallised at 2055.91±0.26Ma, and slightly later at 2054.89±0.37Ma in cumulus rocks in the centre of the RLS. This timing is in agreement with field observations and the results of thermal modelling, which require rapid accumulation of magma at a flux rate of >5km3/yr over less than 100 ka, followed by crystallisation and cooling to below 700 °C within 950 ka. This short period of melt accumulation with an extreme flux rate, leading to a large volume of magma stalled within the upper crust, is suggested to result from active magma pumping, triggered by stress field change within the subcontinental lithospheric mantle (SCLM) at 2.056 Ga. This change was caused by rebounding of the SCLM after elimination of its lowest, eclogite-rich part during a mantle plume up-welling event.