Abstract

The Jurassic Mount Poster Formation of eastern Ellsworth Land, southern Antarctic Peninsula, comprises silicic ignimbrites related to intracontinental rifting of Gondwana. The identification of less voluminous basaltic and sedimentary facies marginal to the silicic deposits has led to a reclassification of the volcanic units into the Ellsworth Land Volcanic Group. This is formally subdivided into two formations: the Mount Poster Formation (silicic ignimbrites), and the Sweeney Formation (basaltic and sedimentary facies). The Mount Poster Formation rhyolites are an intracaldera sequence greater than 1 km in thickness. The basaltic and sedimentary facies of the Sweeney Formation are consistent with deposition in a terrestrial setting into, or close to, water. The geochemistry of the Mount Poster Formation is consistent with derivation of the intracaldera rhyolites from a long-lived, upper crustal magma chamber. The basalts of the Sweeney Formation are intermediate between asthenosphere- and lithosphere-derived magmas, with little or no subduction-modified component. The basalt could represent a rare erupted part of the basaltic underplate that acted as the heat source for local generation of the rhyolites. U–Pb ion microprobe zircon geochronology of samples from the Mount Poster Formation yield an average eruption age of 183.4 ± 1.4 Ma. Analysis of detrital zircons from a Sweeney Formation sandstone suggest a maximum age of deposition of 183 ± 4 Ma and the two formations are considered coeval. In addition, these ages are coincident with eruption of the Karoo-Ferrar Igneous Province in southern Africa and East Antarctica. Our interpretation of the Ellsworth Land Volcanic Group is consistent with the model that the Jurassic volcanism of Patagonia and the Antarctic Peninsula took place in response to intracontinental extension driven by arrival of a plume in that area.

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