Origins of large volume rhyolitic volcanism in the Antarctic Peninsula andPatagonia 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 magmati sm 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 eval uate 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 uni form (Sr-87/Sr-86(i) similar to 0.707; epsilon Nd-i similar to -3) andesite -dacite magma was generated as a result of anatexis of 'Grenvillian age' hy drous mafic lower crust, linked to earlier; arc-related underplating. The l ower-crustal partial melts would have mixed with fractionated components of the mafic underplate, followed by subsequent storage and homogenization. E arly Jurassic (V-1) rocks of the southern Antarctic Peninsula are interpret ed as melts of upper-crustal paragneiss, which have mixed with the isotopic ally uniform magma in upper-crustal magma chambers. The V-2 rhyolites are t he result of assimilation-fractional crystallization of the isotopically un iform magma. This occurred in upper-crustal magma chambers involving assimi lants with similar isotopic composition to that of the magma. A continental margin setting was crucial in developing hydrous, readily fusible lower cr ust. Lower-crustal anatexis was in response to mafic underplating associate d with the Discovery-Shona-Bouvet group of plumes, thought to be responsibl e for the Karoo magmatic province. The progression (old to young) of volcan ism, for NE to SW in Patagonia and south to north in the Antarctic Peninsul a is consistent with migration away from the mantle plumes towards the prot o-Pacific margin of Gondwana during rifting and break-up.