The timescales of andesite generation at Mount Ruapehu, New Zealand

Abstract

Intermediate composition arc volcanoes present some of the most serious natural hazards because of their tendency towards violent, explosive eruptions. Very little is known about the long term processes which produce such magmas and the timescales over which they operate. U-series systematics have considerable potential for helping to understand these timescales, although very few studies to date have examined individual volcanoes in detail. A detailed series of 230Th-238U and 226Ra-230Th analyses are presented in conjunction with major and trace elements and Sr, Nd and Pb isotopes for lavas sampled from coherent stratigraphic sequences on Mount Ruapehu. This andesitic volcano lies at the southern end of the Taupo Volcanic Zone (TVZ) in the centre of North Island, New Zealand. Volcanic activity at Ruapehu is related to the subduction of the Pacifc Plate beneath the Indian Plate along the Hikurangi Trough. Major element trends are dominated by shallow level crystallisation and assimilation processes. Modelling of crystallisation trends suggests that the magmas were hydrous and may have elevated fO2. Trace element patterns show high LILE/HFSE ratios characteristic of arc lavas. AFC models of isotope data suggest that 50% crystallisation and 10-15% assimilation can explain the observed trends. The assimilant is likely to have been dominated by partial melts of local Torlesse basement. Major element extrapolations suggest that the mantle wedge beneath the TVZ was relatively fertile. Modelling of the trace element budgets in relatively primitive lavas suggests that a three component model is required for magma generation. Partial melts of sediments were added to the mantle wedge at a relatively shallow level, and the modified mantle was then fluxed by fluids derived from subducted oceanic crust at greater depth. 10-90% of many LILE and other elements are derived from fluids and sediments rather than the wedge. U-Th analyses show 1-14% U excess and (230Th/232Th) ratios in the range 0.75-0.85. Data for the coherent stratigraphic sequences fall on linear trends on an equiline diagram. Crystallisation and alteration are ruled out as causes of the observed U/Th variation and although the lavas have been affected by assimilation, links between U/Th and indices of assimilation are difficult to demonstrate. Crustal melts with similar U/Th ratios to the parental basalts and relatively low U and Th abundances are the most likely candidate for the assimilant. It is argued that the U-Th systematics reflect addition of hydrous fluids to the mantle wedge. Whole rock pseudo-isochron ages are derived for the different stratigraphic sequences, and compared with data on eruption ages to estimate combined transit and residence times (CTRTs). Rocks from older sequences dated at -80 ka and -130 ka have very short CïRTs << 40 ka, whereas younger sequences have longer CTRTs on the order of 40-80 ka. It is inferred that the magmas of the younger sequences spent a longer period of time ponded at the base of the crust and underwent limited chemical modification. A detailed model of the behaviour of U-Th-Ra systematics during crustal magmatic processes is also presented. This allows crystallisation, assimilation and replenishment processes to be combined with a time factor to predict magma evolution curves. Whilst detailed systematic studies of individual volcanic centres are required to test the model, the lack of age constraints and complex sub-volcanic history of Ruapehu magmas do not permit the detailed application of this model here