Abstract:
Many andesites erupted at arc volcanoes are the products of magma mixing, and this process could be important in triggering their eruption. Throughout the life of Ngauruhoe volcano (∼2.5 ka), erupted andesites lack macroscopic enclaves or lithological features of mingled magma. However, intricate microscopic features of magma mixing and mingling occur. Plagioclase phenocrysts have a wide range of textures. Their diverse <jats:sup>87</jats:sup>Sr/<jats:sup>86</jats:sup>Sr values (0.7039–0.7060) are mostly discordant with the groundmass (∼80%), and many phenocrysts (>40%) are isotopically zoned. Resorbed calcic cores overgrown by sodic rims are common, and record a gradient of increasing <jats:sup>87</jats:sup>Sr/<jats:sup>86</jats:sup>Sr, the result of progressive mixing with a radiogenic melt. Clinopyroxenes and orthopyroxenes have relict cores that nucleated in silicic melts (∼Mg# 30–40), and their rims record cycles of growth in mafic melts (∼Mg# 50–60). Olivine‐bearing, mafic glassy blebs (mostly <500 μm) that occur in the groundmass are the relicts of the intruding magma that disintegrated during the mixing/mingling process, and liberated crystals. Thus, some andesites that lack banding or enclaves are in fact the product of thorough mixing of contrasting magmas. This may require a regime of frequent intrusion leading to hybridization with the stagnant resident magma. Thus, frequent eruptions during Ngauruhoe's historic episode ending in 1975 CE are likely to have been a manifestation of frequent replenishment. A mixing origin for intermediate rocks at volcanic arcs is likely to be more prevalent than previously documented: further evidence for the rarity of andesite liquids in such settings.
