Deciphering magmatic processes in response to growth and destruction at Taranaki Volcano, New Zealand : a dissertation presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Earth Sciences at Massey University (Manawatū campus), Palmerston North, New Zealand
Taranaki Volcano is an atypical back-arc andesitic stratovolcano located on the Taranaki Peninsula of the North Island in New Zealand. Volcanism started c. 200 kyr ago and the edifice went through at least 14 sector collapse events with the same number of corresponding growth cycles, expanding the surrounding volcanic apron over its lifetime, which presently is populated and farmed. Previous studies focussed on the modern edifice (<14 ka), the tephra deposits (<29 ka), and the volcaniclastic stratigraphy over the 200 kyr volcanic history. However, there is a significant knowledge gap in relation to the evolution of the Taranaki volcanic system during successive edifice growth cycles (i.e. inter-collapse states) and the response of the magmatic system to unloading of the edifice. In order to unravel the subaerial and subvolcanic aspects of these growth cycles, the sedimentary textures, lithologies and stratigraphy of the volcaniclastic mass-flow deposits were investigated in the southwestern sector of the Taranaki ring plain, which provides a nearly continuous stratigraphic record of the time period of c. 65-34 ka that comprises three edifice regrowth phases. Volcaniclastic mass-flow deposits were the focus of this study, providing an opportunity to explore sedimentological and geochemical characteristics of eruptive periods of Taranaki Volcano, as there are no proximal sites available close to the modern edifice. Due to the well-exposed volcaniclastic successions along the coastline of Taranaki Volcano, a classification framework was established for the globally applicable categorization of volcaniclastic mass-flow deposits in ring plain settings. Additionally, the development of the stratigraphic model of the time period c. 65-34 ka highlighted the high frequency of widely distributed volcanic mass-flow events, approximately occurring 4-5 times in every 4-10 kyr. As these deposits encompass the characteristics of eruptive periods, vesicular pyroclasts were analysed in order to investigate the time related aspects of the Taranaki magmatic system during edifice growth cycles. Based on the analysis of 220 lapilli-sized pyroclasts, whole-rock compositions were reproduced by a mixing model, indicating that the volcanic rocks originate from melt-mush mixing processes. The mixing ratios varied within the individual growth cycles and further revealed that the melt-mush ratios define the produced whole-rock compositions, where the assimilant endmember is a primitive mush and the melt endmember is a trachyandesitic ascending melt. The temporal variation of the pyroclast geochemistry showed that within the inter-collapse states (i.e. growth cycles), the range of bulk rock compositions display a broadening pattern over time
towards pre-collapse states. This chemostratigraphic pattern was attributed to edifice loading affecting crustal magmatic processes over time. Whole-rock geochemical results demanded a detailed investigation of the crystal mush, from which Taranaki Volcano is fed, producing the basaltic to trachyandesitic magmas. The textural and chemical analyses of the Taranaki crystal cargo revealed reoccurring and specific crystal patterns and proved the antecrystic origin of the majority of the clinopyroxene, plagioclase and amphibole crystals. The observations highlighted that the Taranaki magmatic plumbing system involved repeated magma recharge, melt-mush mixing and crystal convection processes affecting the produced magmas within the time period of c. 65-34 ka. Crystallisation conditions (i.e. P-T-H₂O) were estimated applying thermobarometric modelling on clinopyroxene and amphibole phenocrysts. Results of the clinopyroxene rim equilibration modelling suggested source depths of approximately 26-12 km (±7.5), which outline the mid- to lower-crustal regions and further indicate polybaric rim crystallization processes. In addition, hygrometry approximations indicated that within the individual growth cycles, melts with various properties (2.9-3.7 to 3.9-4.8 wt% H₂O) arrived at different regions of the crystal mush at mid-crustal depths. Altogether, textural, chemical and hygrothermobarometric analyses outlined the spatiotemporal complexity of the Taranaki magmatic plumbing system and the connected magmatic processes of andesitic volcanism. The interconnected sedimentological and geochemical studies of this research provided an understanding of mid-crustal melt-mush mixing processes producing the Taranaki magmas within a complex, interconnected vertical mush domain affected by the temporal influence of edifice loading and unloading during consecutive edifice growth cycles.