Origins of large volume rhyolitic volcanism in the Antarctic Peninsula and Patagonia by crustal melting

Voluminous rhyolitic volcanism, along the palaeo-Pacific margin of Gondwana was marked by three principal episodes of magmatism. The first of these (V(1)) is essentially coincident with the main episode of Karoo-Ferrar magmatism at similar to 184 Ma. A younger (V(2)) episode occurred at similar to 168 Ma, and a third episode (V(3)) occurred in the interval 157-153 Ma. We evaluate the origin of V(1) and V(2) rhyolites from the Antarctic Peninsula using major and trace element and isotopic (Sr, Nd, O) data. An isotopically uniform ((87)Sr/(86)Sr(i) similar to 0.707; epsilon Nd(i) similar to -3) andesite-dacite magma was generated as a result of anatexis of 'Grenvillian age' hydrous mafic lower crust, linked to earlier; arc-related underplating. The lower-crustal partial melts would have mixed with fractionated components of the mafic underplate, followed by subsequent storage and homogenization. Early Jurassic (V(1)) rocks of the southern Antarctic Peninsula are interpreted as melts of upper-crustal paragneiss, which have mixed with the isotopically uniform magma in upper-crustal magma chambers. The V(2) rhyolites are the result of assimilation-fractional crystallization of the isotopically uniform magma. This occurred in upper-crustal magma chambers involving assimilants with similar isotopic composition to that of the magma. A continental margin setting was crucial in developing hydrous, readily fusible lower crust. Lower-crustal anatexis was in response to mafic underplating associated with the Discovery-Shona-Bouvet group of plumes, thought to be responsible for the Karoo magmatic province. The progression (old to young) of volcanism, for NE to SW in Patagonia and south to north in the Antarctic Peninsula is consistent with migration away from the mantle plumes towards the proto-Pacific margin of Gondwana during rifting and break-up.