Age of pre-break-up Gondwana magmatism

Extensive outpourings of basalt, and to a lesser extent rhyolite, are closely associated with continental break-up and plume-lithosphere interactions. The Gondwana supercontinent began to fiagment during Early-Middle Jurassic times and was associated with the eruption of over three millionkm’ of dominantly basaltic magma. This intense magmatic episode is recorded in volcanic 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 years, the wider application of single grain 40Ar/39Aarn dor U-Pb geochronology has produced more robust and precise dating of the magmatism. This paper reviews the recent advances in high precision geochronology and provides a full recalibrated 40ArPgAr dataset. Application of these methods across 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 and in the Ferrar provinces at c. 180 Ma, and was dominated by mafk volcanism. Ths peak in volcanism is coincident with a second order mass extinction 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 protoPacific margin, prior to, and during the main phase of break-up. Silicic volcanism was initially coincident with the plume related Karoo-Ferrar provinces, but continued over c. 40 Myr, associated with lithospheric extension and subduction along the proto-Pacific continental margin.

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