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Journal of Petrology Advance Access published online on March 29, 2006
Journal of Petrology, doi:10.1093/petrology/egl014
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© The Author 2006. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org
Received February 18, 2005
Accepted February 27, 2006

Article

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

CHRISTOPH BEIER 1 *, KARSTEN M. HAASE 2, and THOR H. HANSTEEN 3

1 INSTITUT FÜR GEOWISSENSCHAFTEN, CHRISTIAN-ALBRECHTS-UNIVERSITÄT ZU KIEL, LUDEWIG-MEYN-STRASSE 10, 24118 KIEL
2 DEPARTMENT OF EARTH SCIENCES, UNIVERSITY OF AARHUS, C. F. MØLLERS ALLÈ 110, DK-8000 AARHUS C
3 IFM-GEOMAR, LEIBNIZ-INSTITUT FÜR MEERESWISSENSCHAFTEN, WISCHHOFSTRASSE 1-3, 24148 KIEL

* To whom correspondence should be addressed.
CHRISTOPH BEIER, E-mail: chb{at}gpi.uni-kiel.de


  Abstract

The Sete Cidades volcano (São Miguel, Azores) is situated at the eastern end of the ultraslow spreading Terceira rift axis. The volcano comprises several dominantly basaltic pre-caldera eruptions, a trachytic caldera-forming stage and a post-caldera stage 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 fractional crystallization and longer crustal residence times for the pre-caldera, shield-building lavas. Thermobarometric estimates show that the ascending alkali basaltic magmas stagnated and crystallized at the crust-mantle boundary (~15 km depth), whereas the more evolved magmas mainly fractionated in the upper crust (~3 km depth). The caldera-forming eruption was triggered by a basaltic injection into a shallow trachytic magma chamber. Lavas from all stages follow a single, continuous liquid line of descent from alkali basalt to trachyte, although slight differences in incompatible element (e.g. Ba/Nb, La/Nb) and Sr isotope ratios imply some heterogeneity of the mantle source. Major and trace element data suggest similar partial melting processes throughout the evolution of the volcano. Slight geochemical differences between post- and pre-caldera stage lavas from the Sete Cidades volcanic system indicate a variation in the mantle source composition with time. The oxygen fugacity increased from the pre-caldera to the post-caldera stage lavas, probably as a result of the assimilation of crustal rocks; this is supported by the presence of crustal xenoliths in the lavas of the flank vents. The lavas from the Sete Cidades volcano generally have low Sr isotope ratios; however, rocks from one post-caldera vent on the western flank indicate mixing with magmas resembling the lavas from the neighbouring Agua de Pau volcano, having higher Sr isotope ratios. The different magma sources at Sete Cidades and the adjacent Agua de Pau volcano imply that, despite their close proximity, there is only limited interaction between them.

Keywords: crystallization depth; fractionation; stratigraphy; Terceira rift; volcanic stages.
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