Geochronology and geochemical evolution of magma systems in the Taupō-Maroa area between two supereruptions: Whakamaru and Ōruanui

How volcanic systems behave between super eruptions is not well constrained yet is important in determining the factors that lead to supereruptions, such as how magma chambers evolve to supply caldera forming eruptions. This thesis investigates eruption products in the Taupō-Maroa area of the central Taupo Volcanic Zone (TVZ) to document the re-organisation of magma systems between two supereruptions: Whakamaru Group (349 ka) and Ōruanui (25.5 ka). A total of 220 samples of rhyolite lava, pumice and tephra were collected, 14 of those predate the Whakamaru Group. Major element, trace element and Sr isotopes were obtained on whole rock powders and used to distinguish magma types and source characteristics. Petrography and mineral compositions were used to identify the storage conditions of magma bodies. The geochemistry was tied to a geochronology framework using 40Ar/39Ar dates on plagioclase phenocrysts to understand magma system evolution.

The oldest rocks investigated are lavas of the Western Dome Belt (WDB), subdivided into the Northwestern and Western dome complexes (NWDC + WDC) north and south, respectively, of the Waikato River. These lavas consistently yielded 40Ar/39Ar eruption ages of up to 100,000 years prior to the Whakamaru Group supereruption, indicating a long-lived geochemically distinct magmatic system existed before, and after, the supereruption. Domes are progressively younger from the NWDC to the northern shores of Lake Taupō whereas two domes south-west of Lake Taupo occurred immediately post-Whakamaru. Dome distributions are influenced by NW-SE regional and Taupō Fault Belt fault structures and a pre-Whakamaru N-S lineament related to the arc structural margin.

In the pre-350 ka to pre-45 ka time-period sub-surface magma bodies supplying lava domes along the southern and northern shores of the current Lake Taupō and further north, were organised into six independent magma systems based on chronology, mineralogy, mineral chemistry and textures: 1) South-west shore domes 2) South-east shore domes, 3) NWDC + WDC 4) The Maroa dome complex, 5) the Northern shore domes, and 6) the NE dome system initiated at ~45 ka. The Northern shore domes evolved between ~50 to 100,000 years after the Maroa complex. These are discrete systems with one or more evolving into another (e.g., WDC evolving into Northern Shore indicated by shared plagioclase compositions, temperatures and pressures). Two amphibole types and two orthopyroxene populations suggest two different storage areas of magma towards the south-west. A deeper more mafic magma ascended and mixed with a shallow ponded magma beneath the South-west shore domes whereas incorporation of previous crystal mush material is present in the Northern shore domes. Shallow-forming amphibole and the bulk of plagioclase, orthopyroxene, and amphibole chemistry in NWDC and WDC lavas suggest that conditions that formed these phases were similar.

Based on geochemistry, mineralogy, and Sr isotopes, 11 magma types were recognised on the surface. The spatial dispersal of magma types and mostly closed system Rb/Sr fractionation trends suggest eruption at different stages of magmatic differentiation. Barium, Rb and Sr illustrate three decreasing trends whilst Th, Ta, Nb, Zr highlight 3 parental source variations across 11 magma types. These three parental compositional variations have been generated through contrasting crustal domains via assimilation, from a single mantle source. Light rare earth element enrichment indicates different levels of crustal interaction or different crustal protoliths in each parental variation.

87Sr/86Sr isotopic ratios revealed that two large-scale deeper crustal magma systems were operating in the area from ~450 ka to ~45 ka. The northern part of the study area is dominated by the lower isotopic signature, whilst the southern part is dominated by the more radiogenic signature. The Maroa lava domes are derived from a source with similar Sr-isotopic characteristics as the more evolved and older NWDC lava domes. In contrast, the WDC lavas are a mixture of at least three magma types, originating from two magma systems. The south-western lavas form a tight cluster from the higher 87Sr/86Sr magma system whilst the south-eastern domes are a separate magma system influenced by a more primitive basalt parent or different crustal source. The two deep crustal systems appear to have led geochemically and isotopically into two geologically recent magma systems on the northern lake margins (Ōruanui) and north-east of Lake Taupō. Hence, to a first order, the Ōruanui magma type, erupted from ~60-25.5 ka is descended from the higher 87Sr/86Sr system, whilst the modern NE dome magma system is the successor to the lower 87Sr/86Sr system. These findings collectively suggest four conclusions: 1) the typical behaviour of caldera volcanoes is of frequent small-volume eruptions rather than large-scale caldera forming eruptions, 2) the thermal flux changes spatially and temporally, 3) the fingerprint of the original crustal terrane that was assimilated by the parental magmas is preserved in the deposits, thus deposits on the surface can be correlated back to distinct parental magmas as depth and 4) substantial revision is required to models of the Whakamaru caldera structure.