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Instituto de Investigação
em Vulcanologia e Avaliação de Riscos
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Referência Bibliográfica

PIMENTEL A., ZANON V., PACHECO J., (2009) - Peralkaline silicic lavas of Terceira island (Azores - Portugal): constrains on magma evolution and volcanological implications, Geoitalia 2009​ (Poster).​


Terceira Island (Azores), in the North Atlantic Ocean, is a world-class example of peralkaline volcanism. It is located in a geodynamic setting dominated by the slow-spreading Terceira Rift that separates the Eurasian and African plates, east of the Mid-Atlantic Ridge. Terceira differs from all other islands of the Azores archipelago for its noteworthy abundance of domes and coulees. These peralkaline lavas are the most expressive volcanic product in the recent (< 20 ka) eruptive history of the island, occurring in the two younger central volcanoes: Santa Bárbara and Pico Alto.

The Terceira magmatic suite shows a bimodal trend from subalkaline/transitional basalts to peralkaline rhyolites, marked by a compositional gap in the intermediate fields. Dome and coulee rocks show a narrow compositional range (65 - 70 wt% SiO2), with agpaitic index (AI) of 1.1 - 1.8 and are classified as comenditic trachyte, pantelleritic trachyte and comendite.

These peralkaline rocks range from almost aphyric to porphyritic (2.4 - 20.1 vol% crystal content) with trachytic or glomeroporphyritic textures. Mineral assemblage consists of abundant phenocrysts of alkali feldspar (anorthoclase/sanidine), subordinate plagioclase, clinopyroxene, olivine (fayalite) and oxides. Amphibole (arfvedsonite) and aenigmatite are rare and only occur in Pico Alto samples. The same mineral phases occur also as microphenocrysts.

Major and trace elements variations vs. SiO2 are generally scattered but show a general decrease in TiO2, Al2O3, MgO, CaO, P2O5, Ba, Sr and Eu, a strong enrichment in incompatible elements such as Cs, Rb, Th, U, HFSE and LREE, while FeOt, MnO and alkalis reveal opposite trends in Santa Bárbara and Pico Alto series. Post-eruptive F and Cl contents increase with evolutionary degree while residual H2O shows a slight decrease. Overall, there is a higher residual volatile content in Pico Alto than in Santa Bárbara rocks, although S is very low both cases.

The presence of both comenditic and pantelleritic trends in the series of Santa Bárbara and Pico Alto volcanoes suggests a different role played by FeOt, TiO2 and Al2O3 during final stages of evolution, under low fO2 conditions (QFM). The origin of these peralkaline silicic magmas may result from complex and various geochemical processes occurring at different depths involving different starting compositions. Polybaric fractional crystallization starting from alkali/transitional basalts at 5 kbars seems to be responsible, to some extent, for the magmatic evolution towards peralkaline compositions.

The FeOt and MnO enrichment in Pico Alto is typical of magmas that evolved by fractional crystallization at low QFM buffering conditions (QFM-2), while Santa Bárbara magmas seem to reflect slightly higher fO2 conditions. This may be interpreted as the effect of loss of volatiles during the more efficient degassing of Santa Bárbara magma. In turn, the low fO2 conditions in Pico Alto magma suggests it retained more volatiles, which is compatible with the higher post-eruptive halogen content and the presence of amphibole, indicating a higher pH2O for these magmas.
The differences in the two magmatic series may have significant implications on the eruptive behavior of these volcanoes. Peralkaline silicic magmas have lower viscosities than magmas with the same SiO2 content. Furthermore, the effect of halogens in the magma reduces significantly the viscosity. This rheological feature allows peralkaline silicic lavas to flow for larger distances reaching several kms. On the other hand, the higher volatile content of Pico Alto magmas may lead to a greater tendency to erupt explosively, as seen in the past (> 20 ka), producing ignimbrites and fallout deposits.