Timing and genesis of the Karoo-Ferrar large igneous province: New high precision U-Pb data for Tasmania confirm short duration of the major magmatic pulse

Abstract

The Karoo-Ferrar igneous province is one of the largest igneous provinces on Earth. It extends from South Africa, along the Trans-Antarctic Mountains to Tasmania and South Australia. Reconstruction of the continents back to the Gondwana configuration in the Early Jurassic reveals a total length of the Karoo-Ferrar province of >5000km. New isotope dilution thermal ionization mass spectrometry (ID-TIMS) single grain U-Pb ages for zircon and baddeleyite from Tasmanian dolerites combined with ID-TIMS literature single grain U-Pb ages from the Ferrar and Karoo suites are consistent with the major pulse of synchronous magmatism throughout the province lasting about 1Ma or less for the major pulse of magmatism at the time of the Toarcian mass extinction event. We argue that the mechanism of synchronization of magmatism over such a short period of time along such a long distance is the major question which has to be answered in search of the correct model for the origin of the Karoo-Ferrar large igneous province. It cannot be reconciled with the lower mantle plume head model with the plume impingement beneath the Karoo. Plume material could not spread beneath the lithosphere at a rate of ~5–10m/yr (5000km per 0.5–1Myr), at least based on the current knowledge of the mantle physical properties. It seems unlikely that the entire Karoo-Ferrar large igneous province formed due to long distance magma migration through dykes from the same mantle plume irrespective on the proposed plume centre location. In such case, magma would have had to cross the boundaries (and thus weakness zones) between three future continents. In the framework of the dyke propagation model we would expect dykes to follow these weakness zones, not cross them. In addition to this, the Karoo and Ferrar contain geochemically different igneous rocks, which were not formed from the same magma source, preventing interpretations based on one single plume. Both the Karoo and Ferrar contain low-Ti tholeiites, which are similar by their trace element patterns to modern arc analogues – the Central Andes and Kamchatka, respectively. Thus, our preferred model for the origin of the Karoo-Ferrar large igneous province is associated with subduction of the Phoenix plate beneath the southern Gondwana. Probably, deep slab dehydration at the depth of the mantle transition zone modulated surface volcanism or the Toarcian tectonic event triggered voluminous but short-term melting of mantle, which was metasomatized by subduction-derived fluids.