Petrography of alkaline volcanic-subvolcanic rocks from the Brazilian Fernando de Noronha Archipelago, southern Atlantic Ocean

Abstract

A classification scheme for the volcanic-subvolcanic rocks of the Fernando de Noronha Archipelago is presented, based on the petrographic study of a large number of samples. In Fernando de Noronha two main volcanic events were defined by Almeida (1955): an older Remedios Formation (12 to 8 Ma old) composed of domes, plugs and dikes of tephritic-basanitic to trachytic and phonolitic compositions, and a younger Quixaba Formation (3 to 2 Ma), made up mostly by flows of nephelinitic composition (ankaratrites). Most of the rocks of the Remedios Formation appear to belong to two distinct petrographic series, one represented by an undersaturated sodic basanite-tephrite (essexite)-phonolite trend and the other by a potassic alkali basalt-trachyandesite-trachyte series, while the limburgites and lamprophyres are of uncertain ancestry. -from Author