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

A comprehensive data set of mineral and whole-rock major element compositio ns 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 crystall ization, at relatively high fO(2), as the main magmatic process generating the volcanic suite, which is mostly made up of basaltic-hawaiitic and trach ytic rocks; very few mugearites are known on Sete Cidades emphasizing the p ossibility that residual liquids were able to leave the hawaiitic crystal m ush only when the density had been lowered by compositional change or volat ile 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 s tructures through the alkali basalt-trachyte series are highly dependent on the sequence of crystallization and the correspondent magmatic environment (temperature, pressure, fO2). 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 pl umbing system. Some of these basalts were held close to the Moho where they evolved by wehrlite fractionation to produce slightly hy- normative hawaii tes with less than 7 % MgO. Slightly evolved basalts or hawaiitic magmas le aving the deep crustal reservoir were either erupted or were trapped in hig h lever 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 des cent is also constrained by the hornblendite-kaersutite gabbro-diorite shal low plutonic roots of the volcano. Geothermobarometric and fO(2) calculatio ns, performed on equilibrated pair of minerals (or mineral/liquid) generall y agree with the proposed two-stage fractionation history of the Sete Cidad es Na-alkaline magmas occurred within the relatively thick Azorean oceanic crust.