Rapid magma emplacement in the Karoo Large Igneous Province

Understanding the dynamics of continental Large Igneous Provinces (LIPs) relies on precise dating of basaltic rocks. LIP research has traditionally focused on dating lavas, often neglecting the volumetrically important sill intrusions in underlying sedimentary basins. Here we present U-Pb zircon (and baddeleyite) ages for fourteen new samples of Karoo LIP sills and dykes spaced by as much as 1100 km across the half million square kilometer Karoo Basin. The samples yield remarkably coherent ages ranging from 183.0 ± 0.5 to 182.3 ± 0.6 myr. Probability modeling indicates that basin scale emplacement took place within an interval of about 0.47 myrs (less than 0.90 myrs with 95% confidence), and could even have represented a single magma emplacement event. Combining the new ages with the estimated volume of sills in the Karoo Basin gives an emplacement rate of 0.78 km3/yr, which is higher than previous estimates. Upper crustal magma storage may account for these high rates. The results challenge the view that melt emplacement in a sedimentary basin is a prolonged process, support a scenario of pulsating catastrophic events within a narrow time frame, and strengthens the hypothesis linking LIPs and sill emplacement to global environmental crises.

► This study presents the first basin scale sill U-Pb dating across the Karoo Basin. ► The sills form the sub-volcanic part of the well exposed Karoo LIP. ► 14 dolerite sills gave ages between 183.0 ± 0.5 to 182.3 ± 0.6 myr. ► Probability modeling suggests emplacement within 0.47 myr. ► Combined with sill volume estimates, the average melt flux is as high as 0.78 km3/yr.