Hf isotope evidence for variable slab input and crustal addition in basalts and andesites of the Taupo Volcanic Zone, New Zealand

- Combined Hf-Nd isotopes are used to see through crustal contamination.
- TVZ basalts require sediment addition both at source and in the lithosphere.
- TVZ basalts are not parental magmas to TVZ andesites.
- Ruapehu andesites also require sediment addition at source and in the lithosphere.
- Granulite xenoliths at Ruapehu have compositions consistent with ancient MORB.

Crustal contamination complicates the identification of primary mantle-derived magma compositions in continental arcs. However, when crustal processes and components are well characterised, it is possible to extrapolate through continental arc magma compositional arrays towards the Hf and Nd isotope compositions of uncontaminated primary magmas. This is because of the similar behaviour of Hf and Nd during fractional crystallisation and mantle melting, and the subsequent limited variation in Hf/Nd in mantle-derived magmas and in many crustal lithologies, resulting in linear contamination trends for Hf-Nd isotopes. Here we present new Hf isotope data for a selection of volcanic rocks and crustal lithologies from the Taupo Volcanic Zone (TVZ), New Zealand and propose that the scatter in Hf-Nd isotopes indicates heterogeneity in the parental magmas prior to interactions with crustal lithologies. The observed variations likely represent variability in primary magma compositions as a result of different degrees of sediment addition at the slab-wedge interface. Coupled variations in isotopic composition, LILE/HFSE ratios (e.g. Rb/Zr and Ba/La) and SiO2 also clearly indicate that shallower level crustal interactions have occurred. Andesites from Ruapehu Volcano have more consistent parental magma compositions, and require greater amounts of a source sediment contribution. Notably, the compositions of older Ruapehu lavas can be modelled by interactions between mantle-derived magmas and lower crustal granulites, whereas younger lavas have probably interacted more with mid- to shallow crustal meta-sedimentary greywacke-argillite lithologies of the Permian to Cretaceous composite Torlesse Terrane. Hf-Nd isotopic compositions of meta-igneous granulite xenoliths from Mt. Ruapehu are consistent with previous interpretations that they are derived from oceanic crust that underlies the Torlesse meta-sediments. The results indicate that interactions with sediments at both the slab-wedge interface and in the lithosphere must be considered when evaluating trace element and isotopic variations in continental arcs.

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