The Application of U-Series Systematics in Understanding the Rates and Timescales of Magmatic Processes at a Composite Andesite Volcano, Mount Ruapehu, New Zealand

Abstract

The application of U-Th disequilibria in investigating the timescales of recent magmatic processes is now well established. Processes which fractionate U and Th, and hence disturb secular equilibrium are detectable for ~350 ka. Whilst there appears to be an increased understanding of the behaviour of these isotopes in young arc rocks from ocean-ocean collision zones (e.g. Elliott et al., 1997; Turner et al., 1997), early work on continental arcs has demonstrated that the additional complexity of magma ascent through and residence in continental crust can make interpretation of U-Th data difficult (e.g. Volpe, 1992). This work is a detailed, highprecision study of a single volcano over at least 100 ka, looking at how U-Th data can aid investigation of magma fluxes and crustal residence times over a significant time span. Mount Ruapehu is a composite andesite cone, with a history of volcanism for at least 250-300 ka. It forms the most southerly and largest volcanic edifice in the Taupo Volcanic Zone (TVZ), a zone of extension, high heat flow and volcanism related to the westward subduction of the Pacific Plate beneath the Indian Plate along the Hikurangi Margin. The volcano lies approximately 175 km behind the Hikurangi Trough, and 100 km above the subducting slab. The present day surface geology is dominated by sector collapse structures and glacier-related erosion, which has exposed a number of excellent vertical sequences on which this study concentrates. Compositionally, Ruapehu has been remarkably consistent throughout its known eruptive history. Lavas range from 53-63 wt.% SiO2, but analysed material from this study and others (Graham and Hackett, 1987) is largely dominated by andesitic