Insights into the construction of the Oruanui magma reservoir from a comparison of zircon hosted in plagioclase and whole-rock

Abstract

Silicic magmatic systems play a critical role in the evolution of continental crust, and their eruptions could pose significant hazard to modern civilization. However, the processes of silicic magma storage and assembly prior to eruption are not well understood. We report new ²³⁸U-²³⁰Th ages and trace-element data for both rims and interiors of zircons separated from whole-rock (WR) samples and from bulk plagioclase separates from the 25.4 ka Oruanui eruption, Taupo Volcanic Zone, New Zealand. These data provide petrologic context and a more complete record of zircon crystallization than previous analyses of whole-rock hosted zircon interiors. In particular, whole-rock zircon surfaces are more restricted in composition than other zircon groups and define two clear compositional groups. Preservation of these two distinct compositional groups requires that the zircon must have been incorporated into the erupted Oruanui rhyolite at most ∼2 kyrs prior to eruption. In addition, systematic variations in Eu/Eu*, Hf, and Sc with Ti-in-zircon temperature and ²³⁸U-²³⁰Th age recorded in plagioclase-hosted zircon surfaces indicate that a shift to higher temperatures and less evolved compositions began at ∼40 ka and continued until amalgamation of the final erupted magma body. These new data demonstrate the additional insights available from detailed microanalytical investigations and support other studies indicating that large volumes of silicic magma accumulate in the crust via amalgamation of multiple smaller compositionally distinct portions of the broader magma reservoir.