The Pigroot scandal; how a phonolite came to bear mantle xenoliths: an experimental analysis of the Pigroot phonolite, Central Otago, New Zealand
Phonolite is a type of nominally evolved alkaline magma, with some occurrences, however, hosting mantle xenoliths. This project aims to characterise the dynamic conditions of magma evolution of the xenolith-hosting Pigroot phonolite within the Miocene Waipiata volcanic field, New Zealand. Field work established that outcrops range from basaltic column to pillow lavas, and that there is heterogeneity in the distribution of xenoliths between outcrops. The occurrence of pillow lavas and tuff deposits at the base of the sequence gave indications for the presence of a water body(ies) during the eruptive period. Samples from the Pigroot phonolite were analysed by petrographic techniques and x-ray fluorescence which enabled the characterisation of the mineralogical suite and chemistry of the xenolith-bearing host rock. Four rock compositions were found, these are (i) phonotephrite, (ii) trachyandesite, (iii) tephriphonolite and (iv) phonolite. A likely fractionation depth in the upper mantle was determined from the chemical compositions of the host rocks, and the results of the phase equilibria experiments. These produced modal mineralogy and mineral chemistry comparable to the natural samples at temperatures and pressures of upper mantle depth. The crystallization assemblage was dominated by amphibole (kaersutite) with minor clinopyroxene +/- oxides +/- plagioclase. The mineralogy was dominated by k-feldspar species (50-70%), followed by two species of opaque minerals, microlitic aegirine-augite, clinopyroxene, minor amounts of amphibole, nepheline and apatite. Fractionation rather than partial melting as the process generating the phonolite is evident by the SiO2 value of >46% observed in all samples, with the relative abundances of trace elements following normal fractionation trends. The xenoliths are interpreted to have been incorporated within the evolved host melt during a mingling event between the evolved melt with a mafic melt that was carrying the xenoliths within a zoned magma storage area.
Advisor: Brenna, Marco
Degree Name: Master of Science
Degree Discipline: Geology
Publisher: University of Otago
Keywords: New Zealand; Central Otago; Geology; Volcanology; Petrography; Geochemistry; high-pressure high-temperature experiments; Quickpress; Microprobe; SEM; Mantle xenolith; Phonolite; Pigroot
Research Type: Thesis