Age of pre-break-up Gondwana magmatism

Extensive outpourings of basalt, and to a lesser extent rhyolite, are close ly associated with continental break-up and plume-lithosphere interactions. The Gondwana supercontinent began to fragment during Early-Middle Jurassic times and was associated with the eruption of over three million km(3) of dominantly basaltic magma. This intense magmatic episode is recorded in vol canic rocks of the Karoo (Africa), Ferrar (Antarctica) and Chon Aike (South America). K-Ar and Rb-Sr whole rock geochronology has consistently failed to produce reliable ages for these volcanic rocks, but in the last four yea rs, the wider application of single grain Ar-40/Ar-39 and/or U-Pb geochrono logy has produced more robust and precise dating of the magmatism. This pap er reviews the recent advances in high precision geochronology and provides a fall recalibrated Ar-40/Ar-39 dataset. Application of these methods acro ss the majority of the volcanic provinces indicates that approximately 80% of the volcanic rocks were erupted within a short, 3-4 Myr period at c. 182 Ma. This burst of magmatism occurred in the Karoo province at c. 183 Ma an d in the Ferrar provinces at c. 180 Ma, and was dominated by mafic. volcani sm. This peak in volcanism is coincident with a second order mass extinctio n event at the end of the Pliensbachian when c. 5% of marine families were wiped out coinciding with widespread oceanic anoxia in the early Toarcian. A prolonged period of silicic volcanism occurred along the proto-Pacific ma rgin, prior to, and during the main phase of break-up. Silicic. volcanism w as initially coincident with the plume related Karoo-Ferrar provinces, but continued over c. 40 Myr, associated with lithospheric extension and subduc tion along the proto-Pacific continental margin.