Jeffery, Adam John (2016) Petrogenesis and contrasting eruption styles of peralkaline silicic magmas from Terceira and São Miguel, Azores. Doctoral thesis, Keele University.

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Abstract

The petrogenetic processes through which peralkaline silicic magmas are generated in oceanic island environments, as well as the controls on their eruptive behaviour, are not well understood. This study utilises two such systems, Pico Alto and Furnas, Azores, to elucidate the petrogenesis, storage conditions, and eruption of peralkaline silicic magmas, and employs a variety of complementary approaches, including petrographic analysis, whole rock and mineral chemistry, thermobarometry, and petrogenetic modelling. For both of the investigated systems, shallow-crustal, volatile-undersaturated fractional crystallisation of mantle-derived parental melts is the dominant mechanism of magmatic differentiation. Magma mixing and mingling processes are also shown to play a key role, with abundant evidence for mixing of trachytic magmas with either trachytic or mafic magmas. Furthermore, mingling between trachytic magmas and syenitic crystal mushes is recorded as enclaves in syenitic ejecta which represent the near-complete, in-situ crystallisation of trachytic magmas in a thermal boundary layer at the margins of a magma reservoir. Phase assemblages of the syenitic ejecta range from miaskitic to agpaitic, and include rare zirconosilicates such as eudialyite and dalyite. The observed compositional variability of dalyite is demonstrably linked to the compositional evolution of residual melt, coupled with a nucleation delay introduced by variable pore sizes. At both volcanic centres, peralkaline silicic magmas accumulate in shallow crustal reservoirs and develop vertical compositional zonation. These reservoirs inhibit the ascent and eruption of less evolved compositions, and generate a compositional Daly gap. Contrasting eruptive behaviour observed both within and between the volcanic centres of this study is shown to be linked primarily to the degree of pre-eruptive magma volatile undersaturation and, at Furnas, interaction with external water. The presented models provide new insights into peralkaline magmatic systems, and may be applicable to similar peralkaline silicic systems, both within the Azores archipelago, and at similar oceanic island settings worldwide.

Item Type: Thesis (Doctoral)
Subjects: Q Science > QE Geology
Divisions: Faculty of Natural Sciences > School of Physical and Geographical Sciences
Depositing User: Lisa Bailey
Date Deposited: 11 Nov 2016 09:26
Last Modified: 11 Nov 2016 09:26
URI: http://eprints.keele.ac.uk/id/eprint/2477

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