The Karoo Large Igneous Province

Abstract The Karoo Large Igneous Province (LIP) is mainly situated in South Africa but extends into the surrounding countries and Antarctica. The present area of outcrop in southern Africa is at least 140 000 km2, and the original lava field may have covered two million km2. Added to this is an extensive system of subvolcanic sills and two major dyke swarms. In South Africa, the lava stratigraphy is divided into the Drakensberg and Lebombo groups that have maximum thicknesses of 1.5 and 8 km, respectively. On the broadest scale, the Karoo LIP can be divided into two sub-provinces, and we refer to these geographically and compositionally distinctive magmatic suites as the Drakensberg and Lebombo sub-provinces. This minor modification of previously proposed schemes results in a mutually compatible stratigraphic and geochemical provinciality, that can be extended outside South Africa. Peak eruption and emplacement occurred within 2 Myr to 300 ka at ~183 Ma, with magmatism in the Lebombo sub-province having a longer duration (~6 Myr). Compositionally, the Lebombo sub-province is bimodal in terms of SiO2, being largely confined to the 4 to 5 km-thick sequence in the 600 km long Lebombo Monocline. The mafic magmas are compositionally diverse and can be divided into high- and low-Ti types. All of the lavas and intrusions in the Drakensberg sub-province are low-Ti whereas the Lebombo sub-province has both high- and low-Ti types, with the low-Ti types in the two sub-provinces exhibiting distinct trace element signatures. Trace element and Nd and Sr isotope composition is highly variable for fixed intervals of MgO, and this reflects derivation from a heterogeneous mantle, with the much more diverse high-Ti types having a significant component from the sub-continental lithospheric mantle (SCLM) and the much more uniform low-Ti types potentially including plume-derived components, although the exact nature of the magma sources remains uncertain. The Drakensberg sub-province was emplaced in an intracratonic setting and the Lebombo sub-province is rift-related, which explains the greater thickness, limited areal extent, and the presence of basal nephelinites in the latter. The rift-related rhyolites probably formed by decompression melting of underplated basalt as the crust thinned.