With the cessation of subduction along the western margin of Antarctica, Mesozoic calc-alkaline activity gradually gave way in the Cenozoic to more alkaline volcanism associated with an extensional regime. Calc-alkaline volcanism persisted well into the Tertiary in the South Shetland Islands and has started to develop in the Quaternary in the South Sandwich Islands, though most of the Pliocene-Recent products of this group are of island-are tholeiite affinity. The Cenozoic volcanic rocks of the Ross Sea and Marie Byrd provinces are generally highly undersaturated basanitoids, alkali basalts and phonolites. In contrast, those of the more northerly parts of the Antarctica Peninsula and its off-lying islands are for the most part mildly alkaline or transitional. However, Paulet Island, the youngest volcano on the northeast side of the Peninsula, is distinctly more alkalic than its Pliocene predecessors. Deception Island, distinctive on account of its strongly sodic differentiates, is probably connected with residual back-arc extension along Bransfield Strait.