Geochemistry of Volcanic Rocks from Faial Island (Azores)

Adriane Machado (Centre for Geophysics of the University of Coimbra-CGUC, Av. Dr. Dias da Silva, Coimbra, 3000-134, Portugal)
José M. M. Azevedo (same address as A. Machado)
Delia P. M. Almeida (Isotope Laboratory of the Federal University of Rio Grande do Sul-UFRGS, Brazil, Av. Bento Gonçalves, 9500, Prédio 43.129, Campus do Vale, Agronomia, Porto Alegre, RS, 90501-970, Brazil)
Farid Chemale (same address as D. Almeida)

volume 5 - nº 1 - 2008

Key-words: volcanism, alkaline rocks, sodic series, Faial, Azores.

Abstract: Whole rock geochemistry of nine rocks from Faial Island (Azores) was used to characterize Faial Island volcanism. Studied rocks are lavas and were classified based on chemical data as basalts, hawaiites and trachyte. These rocks represent five stratigraphic units of island, from the base to the top: Ribeirinha Volcanic Complex, Cedros Volcanic Complex, Almoxarife and Capelo formations. The rocks belong to the sodic alkaline series, with the exception of the trachytic rock, that exhibits potassic affinity. Faial Island volcanism is characterized by low SiO2 contents (45 to 49 %), high TiO2 (2 to 3 %) and P2O5 (0.29 to 0.74 %). MgO shows values varying from medium to high (2 to 15 %). The trachyte has 62 % of SiO2, low MgO (0.42 %), TiO2 (0.53 %) and P2O5 (0.13 %) contents. Al2O3 varies from low to high (11 to 18 %) and reflects the plagioclase abundance variation. Na2O+K2O values increase with fractionation and K2O/Na2O ratio is less than 0.54, with the exception of the trachyte (0.73). Ni content decreases with magmatic evolution, from 356 to 5 ppm. The presence of the cumulate phases in basalts of Almoxarife and Capelo formations explains the MgO and Ni high values. Almost all the samples show Rb, Ba, Ta, Nb, Zr enrichment and Th, Sm impoverishment. The trachyte shows Ba enrichment and K depletion. Zr positive anomaly and U, Sr, P, Ti negative anomalies are observed in the trachyte. The REE patterns are typical of oceanic island lavas, showing LREE enrichment relative to MREE and HREE. The REE patterns suggest a continuous fractionation from a common melt generated by low partial melting rates. The rocks of Faial Island are the result of fractional crystallization and derived from a magma, which has mixed characteristics, mainly reflecting PREMA and EM II reservoirs, with minor contribution of HIMU.

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