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The Ferrar Large Igneous Province forms a linear outcrop belt for 3250 km across Antarctica, which then diverges into SE Australia and New Zealand. The province comprises numerous sills, a layered mafic intrusion, remnants of extensive lava fields and minor pyroclastic deposits. High-precision zircon geochronology demonstrates a restricted emplacement duration ( < 0.4 myr) at c. 182.7 Ma, and geochemistry demonstrates marked coherence for most of the Ferrar province. Dyke swarms forming magma feeders have not been recognized, but locally have been inferred geophysically. The emplacement order of the various components of the magmatic system at supracrustal levels has been inferred to be from the top-down lavas first, followed by progressively deeper emplacement of sills. This order was primarily controlled by magma density, and the emptying of large differentiated magma bodies from depth. An alternative proposal is that the magma transport paths were through sills, with magmas moving upwards to eventually reach the surface to be erupted as extrusive rocks. These two hypotheses are evaluated in terms of field relationships and geochemistry in the five regional areas where both lavas and sills crop out. Either scenario is possible in one or more instances, but neither hypothesis applies on a province-wide basis. © 2018 The Author(s).
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Apparent synchrony between eruption/emplacement of large igneous province (LIP) magmas and mass extinction has led to the implication of magmatism as a primary trigger of global scale environmental change. Evaluating the efficacy of magmatism as a driver of global change depends on the relative timing of magmatism and environmental change, and the magma effusion/intrusion rate, both of which can be constrained by high-precision geochronology. Early Jurassic (Pliensbachian-Toarcian) global ocean anoxia and acidification, carbon isotope perturbations, and biotic crisis have been linked to "synchronous" eruption and emplacement of the Karoo and Ferrar LIPs. To better constrain the timing and duration of Ferrar magmatism, we apply the single crystal, chemical abrasion U-Pb ID-TIMS method to zircon crystals isolated from twenty Ferrar LIP sills and lavas, and the Dufek intrusion. Dates suggest that both intrusive and extrusive Ferrar magmatism occurred over an interval of 349. ±. 49 kyr, beginning with intrusive magmatism as early as 182.779. ±. 0.033 Ma. Lava eruption was synchronous with, and in some cases postdates intrusion. When coupled with existing geochronology on the Karoo province, our dates confirm broad synchrony between Karoo and Ferrar magmatism, though Karoo magmatism began demonstrably prior to Ferrar magmatism, starting as early as 183.246. ±. 0.045 Ma. The short-lived magmatic history of the Ferrar LIP makes it a plausible trigger for early-Jurassic environmental change. © 2015.
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The Hanson Formation, Antarctica, consists of interbedded sandstones and tuffaceous rocks of Early Jurassic age. The sandstones, pebbly to medium-grained, range between quartzo-feldspathic and volcaniclastic, with some of the former being coarse-grained arkoses that imply proximal sources. Geochronology of detrital zircons provides evidence for source rock ages, whereas sandstone petrology demonstrates a mixed provenance. Tuffaceous strata are reworked fine to very fine-grained tuffs resulting from distal Plinian eruptions. Dated tuffs provide time constraints on the duration of volcanism. The sandstones and tuffs accumulated in a rift environment. Geochemically the tuffs are rhyolitic in composition, and the Sr and Nd isotope data together with the patterns on multi-element diagrams suggest they were derived from a volcanic arc, which is interpreted to have been located along the West Antarctic Gondwana margin. The silicic volcanism extends the distribution and timing of magmatism in the Early Jurassic along that margin. The Early Jurassic extensional regime was delimited by the plate margin region and the East Antarctic craton. The rift valley system along the East Antarctic craton margin, in which the Hanson strata accumulated, was the focus for subsequent emplacement of the intrusive and extrusive rocks of the Lower Jurassic Ferrar Large Igneous Province. The Early Jurassic extensional rifts may have been reactivated during Cretaceous-Cenozoic development of the West Antarctic Rift System. © 2017 Cambridge University Press.
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