Peralkaline acid volcanic rocks of oceanic islands

Occurrences of peralkaline acid volcanic rocks on oceanic islands are reviewed. Peralkaline differentiates are usually associated with mildly alkaline or transitional basalts and often with related sodic intermediate rocks. A compositional gap between basaltic and salic rocks is not invariably present. Although a comenditic end member is more usual in the oceanic suites, pantellerites are particularly well developed on Socorro Island and also on Gran Canaria where they form extensive ignimbrite sheets. There may be a genetic distinction between peralkaline rocks of islands which lie near the crests of oceanic rises and those which are built on broad submarine plateaux.