ESTABLISHING CONTEMPORANEITY AND DURATION OF MAGMATISM IN THE JURASSIC GONDWANA LARGE IGNEOUS PROVINCE

The sequence and timing of events in the evolution of large igneous provinces (LIPs) are important in understanding the processes of continental break-up. For some LIPs (e.g., North Atlantic) dating has established the timing and duration of flood basalt eruption, pluton and dike emplacement, dipping reflector sequences, and coastal flexuring. In contrast, events associated with initial Gondwana break-up are not well constrained. The Ferrar (Antarctica) and Karoo (southern Africa; Queen Maud Land, Antarctica; Falkland Islands) provinces constitute the Gondwana LIP; the former is notable for inferred long distance transport of magmas and the latter for chemical diversity and large volume. Interbedding of chemically distinct basalts provides constraints on the sequence of Karoo province magmatism, but many areas cannot be correlated via field relationships. The Ferrar province has restricted compositions and, speculatively, may be correlative with a low-Ti geochemical type in the Karoo. Ar/Ar dating of feldspars has shown a short duration (<~1 m.y.) of emplacement at 176.7+/-1.7 Ma for Ferrar lavas and sills and some Queen Maud Land tholeiites. Similar ages, although ranging to slightly older, have been obtained for the Karoo Tabankulu and Insizwa intrusions and a Barkly E. Formation basalt. Previous zircon and baddeleyite U/Pb dating implies that Ferrar intrusive rocks and at least the Karoo New Amalfi Sheet are contemporaneous. Previous Ar/Ar dating of Karoo tholeiites by others, using a different set of monitors, has yielded slightly variable ages that were interpreted to indicate emplacement at 183+/-1 Ma. Allowing for Ar/Ar age calibration issues, the Ferrar and Karoo appear to be synchronous, at least in part; however, the data as a whole are permissive of a more extended (~5 m.y.) overall period of emplacement. Therefore the potential exists for distinguishing the temporal relationships between distinct Gondwana LIP magmatic centers that can be inferred from dike distributions.