ORIGIN OF RHYOLITE FROM CRYSTAL MUSH: PLUTONIC LITHICS FROM THE OHAKURI IGNIMBRITE, TAUPO VOLCANIC ZONE, NEW ZEALAND

In the active Central Taupo Volcanic Zone (TVZ), New Zealand, eruptible silicic magmas are extracted from bodies of crystal-rich, melt-bearing, magma mush. Plutonic lithics from the Ohakuri Ignimbrite allow insights into the underground magma system that is otherwise not exposed at the surface. Optical microscopy, SEM-EDS, XRF, and ICP-MS were used to characterize the textures and compositions of the plutonic lithics. Several samples present cumulate textures, such as high percentages of quartz and plagioclase feldspar phenocrysts surrounded by granophyre. Compositionally, these samples do not fall along the fractional crystallization trend of whole-rock and glass compositions from Ohakuri pumice. Instead, they are enriched in compatible elements (for example CaO, found in plagioclase crystals, plots around 2.2 wt%) giving them a cumulate signature. Additionally, SEM-EDS analysis on glass shards surrounding grains in one distinct sample present a similar composition to glass in pumice from the ignimbrite, where the silica content ranges around 77 wt% and the CaO content is less than 1.0 wt%. The textural and compositional data provide evidence for magma mush, where the glass and granophyre represent the interstitial melt surrounding an interlocking framework of phenocrysts forming the mush. The quartz and alkali feldspar intergrowth composing the granophyre radiate around quartz and plagioclase phenocrysts. This texture is evidence of rapid cooling of the remaining interstitial melt in the mush. Trace and rare earth elements combined with major elements support interpretations of fractional crystallization within the mush body. Additionally, SEM-EDS analyses further characterize the granophyre in ways that contribute to understanding how and why the granophyre formed, its relationship with the eruptible magma, and the extraction process of interstitial melt from mush in magmatic systems.