Evolution of high-K arc magma, Egmont volcano, Taranaki, New Zealand: evidence from mineral chemistry

Abstract

Magmas from Egmont volcano contain xenocrysts and glomerocrysts entrained from melt zones at or near the base of the crust. Lava whole- rock geochemistry therefore reflects mixing between these crystals and melt and no longer represents melt compositions. As an alternative to using whole-rock analyses, mineral chemistry is used to determine processes occurring during early stages of magma evolution. Primitive magmas at Egmont volcano were hydrous high-magnesian basalts with original ƒ O 2 2.5-3 log units above FMQ. By the time of eruption this had fallen to 0.8-1.0 log units above FMQ. Early fractionation of olivine (Fo 87) and chromite (Cr# 0.7-0.8 and Fe 3+ # 0.24-0.31), and later olivine + clinopyroxene + titanomagnetite drove the evolution of the magma to a high-alumina basalt composition. At the base of the crust these evolved magmas entered the amphibole stability field and reaction between both anhydrous mafic cumulates and wall rocks crystallised amphibole, buffering the melt composition to basaltic andesite. Tapping of these melts to higher levels took them out of the amphibole stability field, resulting in decompressive melting of amphibole phenocrysts and incongruent melting of amphibole in lower to mid-crustal wall rocks in contact with the melt. K 2O-rich liquids from incongruent melting were a major source of potassium in the Egmont high-K andesites. Some plagioclase fractionation occurred in higher-level magma chambers but melt segregation was also an important process. H 2O-saturated melts fractionated amphibole as the amphibole stability field was again intersected and these melts evolved along a calc-alkaline trend to dacite. In contrast drier melts did not fractionate amphibole and evolved only to andesites. As most of the K 2O present is in the groundmass of the lavas, that is, in the liquid phase, the melts formed by the highest degree of melt extraction are the most potassic and these are the melts which tend to evolve to dacite.