Formation Of Crystal-Rich, Mixed, Intermediate Lavas At Pouakai Volcano And The Evolution Of The Taranaki Volcanic Lineament, Western North Island, New Zealand

Over the past-2 Ma, volcanic activity in the western North Island of New Zealand has been focused along the NW to SE-younging Taranaki volcanic lineament (TVL). Geophysical imaging of the convergent boundary between the Pacific and Indo-Australian plates shows that the subducted slab lies at least-200 km below the present TVL and the <200 ka andesitic Mt. Taranaki. Pouakai is the second youngest (-0.25 Ma) volcanic edifice of the TVL and lies immediately NW of Mt. Taranaki. It is deeply eroded with limited exposure, but it has been sampled by a major collapse event that generated the 0.21-0.24 Ma Maitahi debris avalanche deposit. The Pouakai lavas sampled by the Maitahi deposit are medium-K, pyroxene-bearing basalts and amphiboleand pyroxenebearing basaltic andesites and andesites. Plagioclase, clinopyroxene and amphibole are the dominant phenocryst phases. Olivine and orthopyroxene are rare. Compared to younger Mt. Taranaki lavas, the Pouakai volcanic rocks have lower K, P, Ba, Rb, Sr and Zr and higher Sc and V abundances, and they define subtly different trends on TiO2, MgO and K2O versus SiO2 variation diagrams. Most Pouakai eruptives are similar to the older ring plain debris avalanche deposits of Mt. Taranaki, but Pouakai summit lavas are compositionally similar to younger Mt. Taranaki eruptives. For the Pouakai sample suite, the geochemical trends and patterns are broadly consistent with control by fractional crystallisation (FC), or assimilation fractional crystallisation (AFC). However, textural and geochemical evidence indicates that the rocks are not representative of simple magmas related by a common liquid line of descent. Pouakai basaltic andesites and andesites are highly porphyritic and phenocrysts show complex zoning and disequilibrium reaction textures. Mg# [mol. MgO/(MgO + FeO)] of amphibole and clinopyroxene compositions vary beyond the range expected if the minerals had equilibrated with melts having bulk whole rock compositions. Furthermore, geothermobarometry based on amphibole and pyroxene compositions indicates that mafic phenocrysts in all rock types, as well as in individual samples, equilibrated with comparatively felsic melts over a wide range of pressures. Therefore, each lava sample represents a mixed magma, comprising felsic melts derived from a range of crustal and mantle sources hosting a complex, polybaric phenocryst assemblage. The whole rock compositions were derived through varying degrees of bulk fractionation of phenocryst phases with net removal of amphibole and clinopyroxene and consequent concentration of plagioclase phenocrysts. (c) 2020 Elsevier B.V. All rights reserved.