New high precision U-Pb ages and Hf isotope data from the Karoo large igneous province; implications for pulsed magmatism and early Toarcian environmental perturbations

To better constrain the age and duration of the magmatism associated with the Karoo large igneous province (LIP), we present new U-Pb ID-TIMS dates and εHf values from baddeleyite and zircon grains from Karoo basin mafic sills and from felsic samples from the Lebombo and Mwenezi monoclines, together with an ⁴⁰Ar/³⁹Ar age database of Karoo rocks that has been filtered for true plateau ages with >70% of ³⁹Ar released and in which all ⁴⁰Ar/³⁹Ar ages were recalculated using the current best estimates for the decay constants. Zircon and baddeleyite ages from three Karoo basin sills range from 183.36 ± 0.17/0.27 to 183.06 ± 0.07/0.21 Ma, where the two uncertainties reflect the analytical error and the additional error associated with decay constant uncertainty. Zircon from the Mutandawhe pluton are dated to 176.84 ± 0.06/0.20 Ma, which represents the first high-precision U-Pb age of the late stage Karoo-LIP magmatism in the northern Lebombo-Mwenezi region. Initial hafnium isotopes are close to chondritic for the Karoo basin and central Lebombo samples (εHf from −2 to +3), but more negative for zircon grains from the Mutandawhe pluton (−11.3 ± 1.1, 2SD). In combination with previous studies and in agreement with the updated ⁴⁰Ar/³⁹Ar ages, we show that the sill complex that intruded the Karoo basin was short-lived at ~320 ± 180 ka and that it pre-dated the magmatism of the Ferrar-LIP by around 460 ka, whereas the entire Karoo-LIP was emplaced over a period of ca. 6.5 Ma. Based on high-precision U-Pb geochronology, Karoo-LIP magmatism occurred after 183.36 ± 0.17 Ma and therefore postdated the extinction pulses of the late Pliensbachian and likely the Pliensbachian-Toarcian boundary. However, we support previous conclusions that the start of the Karoo-LIP activity agrees with the onset of the Toarcian oceanic anoxic event and the early Toarcian warming, indicating that these environmental changes were likely a response to the magmatic activity of the Karoo-LIP.