Understanding the magmatic systems beneath the central Taupō Volcanic Zone, New Zealand: insights from plutonic lithics

<p dir="ltr">The central Taupō Volcanic Zone (cTVZ) (North Island, New Zealand) has seen abundant Quaternary silicic magmatism since ∼1.9 Ma. In regions of such young magmatism, where uplift and exhumation have not occurred, plutonic lithics in volcanic deposits represent the only means by which the plutonic roots of the magmatic systems can be investigated. This thesis presents textural, geochemical, and fluid inclusion results on three suites of cTVZ plutonic lithics ranging from olivine-bearing mafic plutonic lithics through to hydrothermally altered, alkali feldspar-bearing granites, plus a fourth suite of strongly hydrothermally altered rhyolite clasts.</p><p dir="ltr">The first suite is mafic to felsic plutonic lithics, studied alongside the host scoria, from the K-Trig monogenetic scoria cone, NE of Lake Taupō. The plutonic lithics range from olivine-pyroxene-melt-bearing mafic plutonics (44–47 wt.% SiO₂) to alkali feldspar-bearing granites. The mafic plutonics are categorised into three textural groups: groundmass-poor, porphyritic and glass-rich, where, based on major and trace element abundances, the groundmass-poor have experienced crystal accumulation, and the glass-rich have experienced more crystal fractionation. Trace element modelling indicates the K-Trig scoria has undergone large amounts (∼40-60 %) of fractional crystallisation from a primitive basalt as a network of connected mafic magma chambers, similar to other cTVZ high-alumina basalts. The K-Trig mafic suite (mafic plutonics and scoria) has a narrow range in ⁸⁷Sr/⁸⁶Sr isotopic compositions for the gabbros, 0.704853 – 0.70488, indicating source homogeneity. Sr-Pb isotope modelling of cTVZ basalts indicates <5 % hydrous sediment melt input to the mantle wedge and ∼10-15% addition of a Kaweka greywacke contaminant to the cTVZ basalts.</p><p dir="ltr">The second suite is intermediate-felsic plutonic lithics recovered from cTVZ deposits ranging in age from ∼1 Ma to 1314 CE (clasts from the ∼54 ka Rotoiti sequence are considered separately). The cTVZ-hosted plutonic lithics are split into diorites and granitoids (granodiorite-granite), where the diorites typically contain pyroxene, amphibole, and plagioclase ± glass and magnetite, and the granitoids contain quartz and plagioclase ± biotite, alkali feldspar, zircon, and allanite/monazite. The textures within the granitoids are highly variable, ranging from coarse-grain equigranular to granophyric textures. The cTVZ-hosted granitoids follow similar major element liquid lines of descent (LLDs), indicating similar silicic magmatic evolution trends across the cTVZ. The granitoids have highly variable Rb/Sr (∼0.4–4), La/Yb(_N) (∼4-16), and ΣREE(_N) (∼250–600) values. This range in values is suggested to be controlled by fractional crystallisation processes, where the higher values reflect granites formed from cooled, crystallised melt that has been extracted from a crystal mush. Despite this, no definitive evidence of crystal accumulation was found within the cTVZ plutonic suite. Depth estimates for cTVZ diorites are ∼7–12 km, and granitoids are ∼2.5–5.5 km depth. However, none of the diorites or granitoids are definitively co-magmatic with their host eruptives; however, the majority of these can be proven to be xenolithic to their host eruption. This suggests the presence of multiple magmatic bodies, partially molten and/or sub-solidus, that were present within the crust during various cTVZ eruptions.</p><p dir="ltr">The third suite is granitoids (sensu lato) recovered from proximal lithic breccias in the ∼54 ka Rotoiti ignimbrite (Ōkataina volcano). These granitoids (∼68–81 wt.% SiO₂) display a wide range in textures (coarse-grained equigranular to abundant granophyric textures) and trace element values: Rb/Sr (∼0.4–14), La/Yb(_N) (∼4-16), and ΣREE(_N) (∼250–1600) values. At least one granite is identified as having formed from fractional crystallisation, having accumulated plagioclase, biotite, and zircon ± monazite/allanite. The granitoids have been classified into three groups using Sr and CaO, which relate to the degree of fractionation experienced by the granitoid. Multiple, isolated magmatic bodies (partially molten and sub-solidus) present beneath the Ōkataina Volcanic Centre at the time of the Rotoiti eruption explain the abundance and diversity of plutonic lithics within a single deposit.</p><p dir="ltr">The fourth sample suite of lithics is the Rotoiti-hosted hydrothermally altered rhyolite lithics studied in conjunction with hydrothermally altered cTVZ and Rotoiti-hosted granitoid lithics. The mineral assemblages in these rhyolites include sericite, chlorite, adularia, orthoclase, rutile, and rare pyrite. The hydrothermal mineral assemblages in the granitoids include chlorite, albite, orthoclase, and epidote, indicative of propylitic alteration. Hydrothermal mineral assemblages indicate that the hydrothermal fluids were near-neutral pH chloride fluids (>260 °C), suggesting limited contribution from magmatic fluids, typical of present-day cTVZ geothermal waters. Microthermometry of quartz-hosted fluid inclusions in cTVZ plutonic lithics indicates interaction with Cl-rich fluids (∼6-13 wt.% NaCl_equiv), suggesting exsolution, but incomplete degassing, of magmatic volatiles within shallow-level plutonic bodies.</p><p dir="ltr">This thesis considers the three plutonic lithic suites in terms of crustal assimilation, the volcanic-plutonic connection and thermal maturation of cTVZ crust over the last ∼2 Ma. The plutonic lithics (K-Trig, cTVZ and Rotoiti-hosted) all assimilated a Kaweka basement terrane, with most assimilation occurring in the lower crust. There is no change in geochemical characteristics in the cTVZ plutonic suite over time (i.e., within the Old, Young and Modern periods), suggesting thermal maturation of the crust does not apply to the cTVZ.</p>