Chapter 5 The magmatism of the Azores islands

Abstract

Abstract Magmas in the Azores (Portugal) were erupted from both fissure zones and central volcanoes during overlapping periods. Fissure zones follow extensional trends oriented accordingly with the regional tectonics, erupting basalts and hawaiites. Central volcanoes are characterized by transtensive tectonics and erupted basaltic to trachytic magmas. Highly porphyritic magmas, consisting of cumulitic olivines and clinopyroxenes, can be also erupted during flank eruptions. Volcanism occurs because of the different direction of the plate movement at the triple junction where islands are located. Magma production and its ascent occur in response to the stresses of the lithosphere over a geochemically heterogeneous mantle, enriched in incompatible trace elements and water. The variability in the radiogenic isotopes results from the interaction of the differently enriched mantle sources with a ubiquitous and slightly depleted Azores mantle. Finally, a contribution of recycled oceanic crust is evidenced by a progressive east–west variation in Pb isotope ratios across the island of São Miguel. Primitive melts were generated by 1–7% partial melting of a garnet-bearing peridotite. Overall, basalts show relatively high large-ion lithophile element (LILE) abundances and LILE/high-field-strength element ratios that point to chemical heterogeneities in the mantle sources.