Subduction cycling and its controls on hyperactive volcanism in the Taupo Volcanic Zone, New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Earth Science at Massey University, Palmerston North, New Zealand. EMBARGOED until 8 August 2024.

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The origin and magmatic evolution of arc magmas are strongly influenced by transcrustal and source processes. Transcrustal processes are often employed to explain the geochemical diversity seen in arc magmas, from mafic to the most felsic endmembers. Source processes are usually used to explain the diversity seen particularly in mafic magmas. Yet, the relative contributions of both processes are highly controversial and difficult to identify. The southernmost volcanic expression of the Tonga-Kermadec-Hikurangi subduction system, the Pleistocene to Holocene Taupo Volcanic Zone (TVZ), is a suitable volcanic area to assess these ideas. Here, the subduction of the unusually thick Hikurangi Plateau has strong effects on tectonic erosion. The TVZ is dominated by rhyolites, which is unusual given the thin (~16 km) basement comprised mostly of the Permian to Early Jurassic Torlesse metasedimentary terrane. In comparison, the southern TVZ, dominated by andesitic volcanism, is located on a thicker (~30 km) crust. The general view of the magmatic evolution of the TVZ corresponds to mafic magmas coming from the mantle, ponding at the base of the crust, where they assimilate crustal material and start to ascend through the crust where more transcrustal processes occur. In this thesis, the impact of assimilation-fractional crystallisation (AFC) on rock composition was assessed by using major and trace element concentrations, Sr-Pb isotope systematics and the Magma Chamber Simulator (MCS), yielding thermodynamically constrained results. It was found that i) variations seen in mafic magmas cannot be reproduced by transcrustal processes alone, ii) some intermediate samples can be explained by AFC and mixing, but others cannot, and iii) large volumes of crustal assimilation (50%) and fractionation (90%) are required to reproduce the signatures of the most felsic endmembers. In Pb isotope space, a broadly linear correlation of the magmas is seen, consistent with the mixing of two endmembers: the mantle and a ‘crustal material’. One possibility would be mixing these two endmembers in the source before the transcrustal ascent of magmas. This idea was examined by analysing samples from the Hikurangi margin provided by the IODP Expedition 375. Through the calculation of the bulk chemical and Sr-Pb-Nd-Hf isotopic compositions of the subducting material, it was found that there is no geochemical correlation between this material and the TVZ. This material is too variable and too radiogenic to generate the broadly linear relation seen in Pb isotopic space, and it is also inconsistent in all other isotopic systems (Sr-Nd-Hf). The material located in the accretionary prism and above the décollement zone is homogenous and strongly correlates in the Sr-Pb-Nd isotopic systems. This material would be subducted if affected by tectonic erosion. Once this material is tectonically eroded, it can contribute to the source from where the magmas are being generated. The isotopic correlations are seen in fluid-mobile and fluid-immobile elements. Thus, the recycled material contributed by releasing fluids and melts or solid material derived from the subducting slab. Whether these interactions occur at the slab-mantle interface and/or during a diapiric ascent remains uncertain. Thus, the isotopic diversity of the TVZ may be controlled by crustal recycling of tectonically eroded material, with subsequent transcrustal processing adding to the diversity generated already in the source. This process would not only limit the amounts of crustal assimilation needed to generate the isotopic signatures of the most felsic endmembers but would also explain the isotopic diversity seen in the most mafic endmembers and the presence of andesites with primitive isotopic signatures. Ultimately, the impacts of crustal recycling in subduction zones can help elucidate the processes of magmatic differentiation, crustal growth, crustal recycling and crustal loss.
Embargoed until 8 August 2024
Magmas, Volcanism, New Zealand, Taupo Volcanic Zone