Terceira Island (Azores) distinguishes from all the other islands of the archipelago for its noteworthy abundance of lava domes. These silicic lavas are the most expressive volcanic product in the recent (< 20 ka) eruptive history of the island, which occurred mainly in the two active volcanoes: Santa Bárbara and Pico Alto.
The rocks show a relatively narrow compositional range (65 - 70 wt% SiO2), with peralkaline index (P.I.) from 1.1 to 1.8, corresponding to peralkaline trachytes (comenditic and pantelleritic) and rhyolites (comendites). They vary from almost aphyric to porphyritic (2.4 - 20.1 vol% crystal content), with trachytic or glomeroporphyritic textures. Alkaline feldspar (0.4 - 16.8 vol%) is the most abundant phase, followed by plagioclase (0 - 6.3 vol%), clinopyroxene (0 - 1.4 vol%), olivine (0 - 0.3 vol%) and oxides (0 - 0.6 vol%). Amphibole (0 - 0.9 vol%) is only present in Pico Alto samples. The same mineral phases occur also as microphenocrysts.
These lavas show a general decrease in TiO2, Al2O3, MgO, CaO, P2O5, Ba, Sr and Eu and a strong enrichment in incompatible elements such as Cs, Rb, Th, U, Nb, Ta, LREE, Zr and Hf, in respect to SiO2 content. However, diagrams of FeOt, MnO and K2O vs. SiO2 reveal opposite trends in Santa Bárbara and Pico Alto series.
Major and trace elements variation patterns observed are indicative of fractionation of the mineral phases described previously. Though, the different behaviour of some major oxides, in the series of the two volcanoes, was attributed to distinctive fO2 conditions during the final stages of differentiation, resulting from differences in the degassing process. The FeOt and MnO enrichment in the Pico Alto series is typical of magmas that evolved by fractional crystallization at low fO2 conditions (< FMQ) while the Santa Bárbara series seems to reflect slightly higher fO2 conditions. This may be interpreted as the effect of the 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 their volatiles (H2O), which is compatible with the presence of amphibole in the lavas.
The petrological and geochemical data obtained in this study allowed to corroborate the role of fractional crystallization as the dominant process in the evolution of peralkaline silicic magmas of Terceira. Moreover, the differences observed in the final stages of differentiation of the two magmatic series may have significant implications for the eruptive behaviour of these volcanoes in the future.