Insights into the genesis of young (<24 ka) magmas from Mt Taranaki, New Zealand
Rear arc volcanism is typically potassic in composition but its origins are not well understood. In New Zealand, Mt Taranaki stratovolcano is most commonly attributed to subduction-related magmatism but is located 400 km behind the Hikurangi arc-trench system where seismic evidence for a Wadati-Benioff zone is ambiguous. Alternative magma generation scenarios, invoking lithospheric delamination and asthenosphere inflow have also been proposed. In this thesis, the geochemical and isotopic perspective is presented through the analysis of a suite of high-K basaltic to andesitic Taranaki pyroclastic rocks aged between 0.35 to 22 ka. New whole rock major and trace element data have been interpreted to reflect magma evolution driven by polybaric crystal fractionation. The trace elements are characteristic of subduction-related igneous rocks or continental crust, which includes enrichments of Rb, Ba, K and light rare earth elements (LREE) relative to Zr, Y and heavy rare earth elements (HREE), Pb and Sr enrichments relative to Ce, and depletions of Nb relative to K.
New isotopic Sr-Nd-O isotopic data indicate ~0.5% addition of subducted sediment contaminant within the magmatic source. This is confirmed through the Ce/Pb – 207Pb/204Pb multicomponent models which suggests that the subduction component consists of 1-3% sediment melt coupled with 0.5 – 5% AOC-derived fluid. The absence of 87Sr/86Sr, 143Nd/144Nd and δ18O co-variation with indices of differentiation limits the role of upper crustal assimilation from driving the progressive felsification of magmas. Rather, the evolution of bulk rock compositions is primarily driven by fractional crystallisation of a mineral assemblage dominated by plagioclase, clinopyroxene, amphibole and magnetite. To this effect, the mechanism through which Taranaki magmas acquire their radiogenic signature contrasts to the stratovolcanoes within the TVZ, which comparatively show correlations between Sr-Nd-O isotopes that reflect upper crustal assimilation.
The first Uranium-series disequilibria data obtained from Mt Taranaki reveal large and near ubiquitous 230Th excesses that are atypical of most subduction-related volcanoes. Primarily generated via partial melting of garnet-bearing source rocks, slow rates of fractional melting is required for the ingrowth of 230Th in residual phases. The bi-directional horizontal ‘array’ is best explained by the generation of two endmembers – one in 230Th excess, the other in 238U excess – that variably mix towards the equiline via assimilation with the Median Batholith. The coupling of lithospheric delamination with thickening during transpressive movement of the Taranaki Fault may better account for the slow upwelling rates and low but variable influence of slab-derived fluids that are required to explain the bi-directional nature of U-series disequilibria in Taranaki volcanic rocks.
Clinopyroxene phenocrysts from several large-scale eruptions indicate crystallisation pressures that span the crust, indicating that polybaric fractional crystallisation likely takes place during the genesis of Taranaki magmas. Through the use of a novel analytical technique, the same clinopyroxene phenocrysts are used to quantify the magmatic H2O, indicating that the equilibrium magmatic H2O contents range from 0.21 to 3.85 wt%. In contrast, at the arc-front White Island nominally-anhydrous minerals record low magmatic H2O conditions (H2Oliq = ~0.5 wt.%) that reflect the pressure-dependent solubility of H2O in a shallow magmatic system.
