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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.

Individual volcanoes of continental monogenetic volcanic fields are generally presumed to erupt single magma batches during brief eruptions. Nevertheless, in two unrelated volcanic fields (the Waipiata volcanic field, New Zealand, and the Miocene-Pliocene volcanic field in western Hungary), we have identified pronounced and systematic compositional differences among products of individual volcanoes. We infer that this indicates a two-stage process of magma supply for these volcanoes. Each volcano records: (1) intrusion of a basanitic parent magma to lower- to mid-crustal levels and its subsequent fractionation to form a tephritic residual melt; (2) subsequent transection of this reservoir by a second batch of basanitic melt, with tephrite rising to the surface at the head of the propagating basanite dyke. Eruption at the surface then yields initial tephrite, typically erupted as pyroclasts, followed by eruption and shallow intrusion of basanite from deeper in the dyke. By analogy with similar tephrite-basanite eruptions along rift zones of intraplate ocean-island volcanoes, we infer that fractionation to tephrite would have required decades to centuries. We conclude that the two studied continental monogenetic volcanic fields demonstrate a consistent history of early magmatic injections that fail to reach the surface, followed by capture and partial eruption of their evolved residues in the course of separate and significantly later injections of basanite that extend to the surface and erupt. This systematic behaviour probably reflects the difficulty of bringing small volumes of dense, primitive magma to the surface from mantle source regions. Ascent through continental crust is aided by the presence in the dyke head of buoyant tephrite captured during transection of the earlier-emplaced melt bodies.

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