oceanic islands, petrography of alkaline rocks, Fernando de Noronha, dikes and flows
Abstract
A classification scheme for the volcanic-subvolcanic rocks of the Fernando de Noronha Archipelago is presented, following the IUGS recommendations, 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 Remédios Formation (12 to 8 Ma old) composed of domes, plugs and dikes of tephritic-basanitic to trachytic and phonolitic compositions, with essexite porphyries, limburgites and alkaline lamprophyres intruding basal pyroclastic rocks, and a younger Quixaba Formation (3 to 2 Ma), made up mostly by flows of nephelinitic composition (ankaratrites). The lamprophyres were separated in two groups: tephritic or melanocratic lamprophyres and typical lamprophyres, the first ones belonging to a suite characterized by continuous increase in amphibole content. Most of the rocks of the Remédios 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. Some ankaratrite and olivine nephelinite dikes that only cut the Remédios rocks are also attributed to the Quixaba Formation. The basanitic flow with mantle xenoliths of the São José Island, considered by Almeida (1955) to form the youngest São José Formation, is here tentatively interpreted as representing the waning stages of the Quixaba volcanism.
