THE TIMING AND DURATION OF THE KAROO-IGNEOUS-EVENT, SOUTHERN GONDWANA

A volcanic event of immense scale occurred within a relatively short p eriod in early Jurassic time over large regions of the contiguous Gond wana supercontinent. Ln southern Africa, associated remnants of thick volcanic successions of lava flows and extensive dike and sill complex es of similar composition have been grouped together as the Karoo Igne ous Province. Correlative volcanic and plutonic rocks occur in Antarct ica and Australia as the Ferrar Province. Thirty-two new Ar-40-Ar-39 i ncremental heating experiments on feldspars and whole rocks from Namib ia, South Africa and East Antarctica produce highly resolved ages with a vast majority at 183 +/- 1 Ma and a total range of 184 to 179 Ma. T hese are indistinguishable from recent, high-resolution Ar-40-Ar-39 an d U-Pb age determinations reported from the Antarctic portion of the p rovince. Initial Karoo volcanism (Lesotho-type compositions) occurred across the entire South African craton. The ubiquitous distribution of a plexus of generally nonoriented feeder dikes and sills intruding Pr ecambrian crystalline rocks and Phanerozoic sediments indicates that t hese magmas penetrated the craton over a broad region. Lithosphere thi nning of the continent followed the main pulse of igneous activity, wi th volcanism focused in the Lebombo-Nuanetsi region, near the eventual split between Africa and Antarctica. Seafloor spreading and dispersio n of east and west Gondwana followed some 10-20 m.y, afterward. The vo lume of the combined Karoo-Ferrar province (similar to 2.5 x 10(6) km( 3)) makes it one of the largest continental flood basalt events. The t iming of this event correlates with a moderate mass extinction (Toarci an-Aalenian), affecting largely marine invertebrates. This extinction event was not as severe as those recorded at the Permian-Triassic or C retaceous-Tertiary boundaries associated with the Siberian and Deccan flood basalts events, respectively. The difference may be due to the h igh southerly latitude and somewhat lower eruption rates of the Karoo event.