Petrology of the alkaline volcanism of Gorringe Bank (southwest Portugal)

Abstract

Alkaline volcanic rocks of the Gorringe Bank overlie typical oceanic crust made up of spilitized dolerites, differentiated gabbroic and ultramafic rocks. Two main series are distinguishable in alkaline volcanics: one from nephelinite to phonolite, the other from lamprophyre to trachyte. Each sequence is differentiated by fractional crystallization with separation of clinopyroxene in the first one and kaersutite and mica in the second. An intermediate suite is rich in lamprophyric and trachytic ocelli and in cognate ijolitic inclusions. Hypotheses on the origins of the more mafic lavas and on the influence of fluids like H 2O and CO 2 are discussed. These series have a similar petrology to that of the Sines and Monchique complexes which intrude the continental crust at the same time on the same structural trend. The alkaline rocks of Gorringe are among the most alkaline series of the Atlantic islands, close to those of Fernando de Noronha and Trindade where similar rocks occur.