Magma Evolution of the Sete Cidades Volcano, Sao Miguel, Azores

The Sete Cidades volcano (São Miguel, Azores) is situatedat the eastern end of the ultraslow spreading Terceira riftaxis. The volcano comprises several dominantly basaltic pre-calderaeruptions, a trachytic caldera-forming stage and a post-calderastage consisting of alternating trachytic and basaltic eruptions.The post-caldera flank lavas are more primitive (>5 wt %MgO) than the pre-caldera lavas, implying extended fractionalcrystallization and longer crustal residence times for the pre-caldera,shield-building lavas. Thermobarometric estimates show thatthe ascending alkali basaltic magmas stagnated and crystallizedat the crust–mantle boundary (15 km depth), whereas themore evolved magmas mainly fractionated in the upper crust (3km depth). The caldera-forming eruption was triggered by a basalticinjection into a shallow trachytic magma chamber. Lavas fromall stages follow a single, continuous liquid line of descentfrom alkali basalt to trachyte, although slight differencesin incompatible element (e.g. Ba/Nb, La/Nb) and Sr isotope ratiosimply some heterogeneity of the mantle source. Major and traceelement data suggest similar partial melting processes throughoutthe evolution of the volcano. Slight geochemical differencesbetween post- and pre-caldera stage lavas from the Sete Cidadesvolcanic system indicate a variation in the mantle source compositionwith time. The oxygen fugacity increased from the pre-calderato the post-caldera stage lavas, probably as a result of theassimilation of crustal rocks; this is supported by the presenceof crustal xenoliths in the lavas of the flank vents. The lavasfrom the Sete Cidades volcano generally have low Sr isotoperatios; however, rocks from one post-caldera vent on the westernflank indicate mixing with magmas resembling the lavas fromthe neighbouring Agua de Pau volcano, having higher Sr isotoperatios. The different magma sources at Sete Cidades and theadjacent Agua de Pau volcano imply that, despite their closeproximity, there is only limited interaction between them. KEY WORDS: crystallization depth; fractionation; stratigraphy; Terceira rift; volcanic stages