129-11 Plutonic Lithic Fragments as Records of Crystal Mush Processes in the Ohakuri Caldera Magma System, Taupō Volcanic Zone, New Zealand

Abstract:

The Taupō Volcanic Zone (TVZ) in New Zealand is one of the most active silicic volcanic systems, yet the exact structure of the crustal magma plumbing systems and processes that drive eruptive behavior remain debated. In this project, we investigate the magmatic system beneath the Ohakuri Caldera, which, together with the Rotorua Caldera, evacuated >245 km3 of rhyolitic magma in the paired Ohakuri and Mamaku eruptions at 240 ka.

To investigate the Ohakuri magmatic system, we examine plutonic lithics entrained in the eruption. We interpret these as erupted mush fragments, providing direct samples of these otherwise inaccessible mush chambers. Petrographic observations, back-scatter electron and energy dispersive spectroscopy scanning electron microscope analysis of three lithic samples reveal amphibole and glass-bearing granophyric granitoids with preserved cross-cutting textures that reveal at least two distinct crystallization events. The first stage includes crystallization of plagioclase and quartz followed by quartz-feldspar granophyre produced during relatively rapid crystallization of melt within magma chambers under varying cooling conditions. Late-stage textures include interstitial glass and melt pockets of fine-grained amphibole, biotite, pyroxene and opaque minerals within cross-cutting fractures, indicating continued melt movement after initial framework formation. Preliminary amphibole barometry indicates a crystallization event at 180-200 MPa (~7 km depth).

Granophyre and glass coexistence indicates at least two cooling events. Cross cutting interstitial glass suggests a compositionally distinct melt mobilization event during late stage crystallization, occupying fractures through granophyric textures. This is consistent with previous studies discussing vertically extensive chambers beneath the Ohakuri caldera. Interstitial glass compositions differ from those of glass found in pumice from the Ohakuri ignimbrite, indicating that our juvenile melt is of a different type and may have quenched separately from the melt that was erupted. These results suggest a dynamic and interconnected magma system in which crystal-rich chambers undergo crystallization and melt migration prior. 

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