Volcanism from fissure zones and the Caldeira central volcano of Faial Island, Azores archipelago: geochemical processes in multiple feeding systems

Abstract

Magmas in Faial Island, Azores (Portugal), were mostly erupted from two fissure zones and the Caldeira central volcano during overlapping periods. The fissure zones follow extensional trends oriented WNW and ESE and erupted nepheline- to hypersthene-normative basalts and hawaiites. The Caldeira central volcano builds the central part of the island, which is cut by the fissure zones. Ne-normative basalts show similar high-field-strength element (HFSE) concentrations but higher large ion lithophile element (LILE) concentrations than hy-normative equivalents. Primitive melts were generated by small (3–5%) degrees of partial melting of garnet-bearing peridotite, variably enriched in incompatible elements. Overall, basalts from Faial show relatively higher LILE abundances and LILE/HFSE ratios than those of the other islands of the Azores and of many other volcanoes in the Atlantic area. This feature indicates the existence of chemical heterogeneities in the mantle sources characterized by variable degrees of metasomatism, both at local and regional scales. Hawaiites evolved from basalts through 30–40% fractional crystallization of mafic phases plus some plagioclase, in deep reservoirs, at about 430–425 MPa (~ 15 km). The Caldeira central volcano rocks range from basalts to trachytes. Basalts, produced under similar conditions as fissure basalts, evolved to trachytes through large degrees of polybaric fractional crystallization (100–760 MPa; i.e. ~ 3.6–26 km), involving olivine, clinopyroxene, feldspar and minor quantities of amphibole, biotite, apatite and oxides. In contrast, mafic magmas from the fissure zones were erupted directly onto the surface from magma reservoirs mainly located at the crust–mantle boundary.

Temporal evolution of a post-caldera, mildly peralkaline magmatic system: Furnas volcano, São Miguel, Azores

“…The observed zoning patterns require more complex reservoir geometries, such as sequential eruption of multiple magma bodies, each acting as an independent evolving system (e.g. Cooper et al 2012;Zanon et al 2013;Pimentel et al 2015). Alternatively, this complex zonation might originate from an irregular chamber geometry which allows lower regions to be tapped first.…”

Magma storage and ascent conditions beneath Pico and Faial islands (Azores archipelago): A study on fluid inclusions

“…This edifice rises where N150° striking tectonic systems interacted with fissure systems and it is possible that this interaction favored the formation of short‐lived and shallow‐level magma reservoirs, leading to the establishment of a centralized feeding conduit or a series of closely spaced feeding dykes [ Miranda et al ., ]. Basalt‐to‐trachyte magmas [ Métrich et al ., ; Zanon et al ., ] were erupted by this volcano starting from ∼120 ka ago [ Hildenbrand et al ., ], and its last eruptive event took place ∼1.2 ka ago [ Madeira , ].…”

“…These latter lavas, known as ankaramites (i.e., dark porphyritic mafic lavas with a high content of pyroxene and olivine crystals and minor amounts of plagioclase), contained also numerous mafic megacrysts. Lavas of this kind are common on many other islands of the archipelago and have already been described for the islands of Corvo [ Larrea et al ., ] and Faial [ Zanon et al ., ].…”

“…), already described in Zanon et al . []. The primary melt was recalculated after equilibrating the sample composition with Fo89, using the geothermometers of Lee et al .…”

Comment on “The insular shelves of the Faial‐Pico Ridge (Azores archipelago): A morphological record of its evolution” by Quartau et al.

“…Besides these two main processes, it is consensual that the rate of magmatic underplating at the Moho versus the deformation in the lithospheric mantle can control the vertical movements in volcanic islands [e.g., Calvert et al ., ; Wolfe et al ., ; Caress et al ., ; Dañobeitia and Canales , ; Cluzel et al ., ; Klügel et al ., ; Leahy et al ., ]. Despite this issue having been already discussed in recent papers about the islands of Pico and Faial [ Zanon et al ., ; Zanon and Frezzotti , ], Terceira [ Zanon and Pimentel , ], and São Miguel Islands [ Zanon , ], it was not consistently taken into account and discussed by Quartau et al . [].…”