U-Pb zircon (SHRIMP) ages for the Lebombo rhyolites, South Africa: refining the duration of Karoo volcanism

U-Pb SHRIMP ages are reported for three rhyolite flows from the Lebombo rift region of the Karoo volcanic province. Two flows are interbedded with the Sabie River Basalt Formation and a third sample is from the overlying rhyolitic Jozini Formation. The interbedded rhyolites yield ages of 182.0 +/- 2.1 and 179.9 +/- 1.8 Ma, whilst the overlying Jozini Formation rhyolite yields an age of 182.1 +/- 2.9 Ma. Combined with existing Ar-40/Ar-39 geochronology, the new SHRIMP data fine-tunes the chronology of the Karoo volcanic province and indicates the 12 km succession of volcanic rocks in the Lebombo rift were erupted in 1-2 million years and lends considerable support to the links between the Pleinsbachian-Toarcian extinction event and the global environmental impact of Karoo volcanism.