Two-stage fractionation history of the alkali basalt-trachyte series of Sete Cidades volcano (São Miguel Island, Azores).

Abstract

A comprehensive data set of mineral and whole-rock major element compositions of the Na-alkaline lavas and pumices erupted by the Sete Cidades volcano (Sao Miguel Island, Azores) is here presented. Mineralogy and geochemistry of the Sete Cidades extrusives indicate a low-pressure fractional crystallization, at relatively high fO2, as the main magmatic process generating the volcanic suite, which is mostly made up of basaltic-hawaiitic and trachytic rocks; very few mugearites are known on Sete Cidades emphasizing the possibility that residual liquids were able to leave the hawaiitic crystal mush only when the density had been lowered by compositional change or volatile exsolution ( i.e. at the trachytic stage). We mainly show how compositional variations within each group of silicates (and non-silicate) and the major-trace element covariances in the mineral structures through the alkali basalt-trachyte series are highly dependent on the sequence of crystallization and the correspondent magmatic environment (temperature, pressure, fO 2 ). Relatively primitive, slightly ne - normative basalts with MgO up to 14 % were tapped through flank eruptions indicating they were not held at any crustal levels, with rapid ascent through the plumbing system. Some of these basalts were held close to the Moho where they evolved by wehrlite fractionation to produce slightly hy - normative hawaiites with less than 7 % MgO. Slightly evolved basalts or hawaiitic magmas leaving the deep crustal reservoir were either erupted or were trapped in high level magma chambers to fractionate through towards metaluminous, q - + an - normative two-feldspar trachytes and thence to slightly peralkaline, one-feldspar, q - + ac - normative trachytes. Evidence of this liquid line of descent is also constrained by the hornblendite-kaersutite gabbro-diorite shallow plutonic roots of the volcano. Geothermobarometric and fO 2 calculations, performed on equilibrated pair of minerals (or mineral/liquid) generally agree with the proposed two-stage fractionation history of the Sete Cidades Na-alkaline magmas occurred within the relatively thick Azorean oceanic crust.