Mafic rock perspective on the magmatic maturation of a large scale silicic system: a multidisciplinary study of basalt in the Taupo Volcanic Zone, New Zealand

Abstract

Large-scale silicic systems are characterised by eruption of rhyolite composition magma in caldera-forming events. This often produces pyroclastic flows, which can travel >100km from the vent. A key question in volcanology is understanding how and why some magmatic systems evolve into producing large volumes of eruptible silicic magma, and others do not. The flux of magma over time must be sufficiently high to allow accumulation of large bodies of partially molten, eruptible material within the crust, as opposed to forming fully solidified plutons. First, priming of the system must occur, by repeated injection of mantle-derived mafic magma into the crust. Therefore, a great volume of mafic material is stored at depth beneath silicic systems, driving the production of silicic magma. However, the dominant erupted products in silicic systems are overwhelmingly rhyolitic in composition, and are subject to large amounts of crustal processing, obscuring primitive mantle-derived features. Therefore, in comparison to our knowledge on silicic processes, our knowledge on the primitive, mafic portion of the magmatic system is limited.

The Taupo Volcanic Zone (TVZ), New Zealand (NZ) is one of the most productive silicic systems worldwide. It is a rifted continental arc, where both fluids derived from the subducting slab and decompression from rifting drive production of magma. Rifting aids the eruption of volumetrically minor amounts of basalt, which rise from depth along faults parallel to the main rift axis, and caldera rims. Large amounts of mafic magma in the crust are required to drive the production of silicic magma, but are too dense to erupt without the help of normal faulting. This means that although basalt is volumetrically minor at the surface, its composition and crystal cargo can be used to infer much broader processes occurring in the primitive portions of the magmatic plumbing system.

The TVZ is also one of the clearest examples of maturation of a magmatic system, where the shift from andesitic continental arc-type volcanism to rhyolitic volcanism is both spatial and temporal. The arc is propagating southwards with time, and is split into north, central and southern segments. Andesitic volcanism is precursory to rhyolitic volcanism in the TVZ and dominates the south TVZ (S-TVZ), where the onset of volcanism is more recent in comparison with the central TVZ (C-TVZ). Coeval mafic rocks show a shift in whole rock composition coinciding with the shift from andesitic to rhyolitic volcanism.

This thesis uses a multidisciplinary geochemical approach to resolve the shift in mafic rock composition between the different segments of the TVZ. We find that basalt in monogenetic eruptions is much more texturally complex than previously assumed. TVZ basalt inherits antecrysts and xenocrysts from a range of sources, including the lithospheric mantle, cognate cumulate mushes, and hydrothermally altered rhyolites. The type of crystals inherited vary between different segments of the arc, allowing us to attribute shifts in the basalt composition to changes in the maturation of the magmatic system. Finally, we successfully reproduce the observed changes in basalt composition through a series of high pressure/temperature experiments. This PhD project gives insight into changes within the mafic portion of the transcrustal magma system through the waxing and waning of one of the most productive large-scale silicic systems on Earth.