Eruptive source and depositional characteristics of the Potaka Tephra, Wanganui Basin, New Zealand

Lewis, Bridgette.

The Potaka Tephra was deposited in a range of conditions across the Wanganui Basin including: nearshore/delta front, restricted-sediment-input, nearshore marine environment, and mid-basin with highly variable sediment supply (paleobathymetry and lcaol current fluctuation controlled). Evidence exists for two stages of volcaniclastic sedimentation at Kai Iwi, the result of a delayed (relative to other localities) major influx of material to the site.
A rapid influx of sediments is suggested by componentry, sedimentary structures, thickness, and a prograding delta environment. The main controls on this process is sediment supply. The Potaka Tephra is a good example of post volcanism sedimentary response in a distal basinal environment.
The chemistry of the Potaka Tephra pumice-glass and ash shards ranges from ~72-78% SiO2. Minor variations in other elements occur non-systemically across all localities. Wholepumice compositions range from dacite to high SiO2 rhyolite. All pumice are peraluminous, and range from medium-high K2O. Phenocrysts make up 6-7.5% of the solid material in Potaka Tephra pumice. Phenocrysts identified are: plagioclase (73%), hornblende (8%), OPX (7%), iron oxides (5%), quartz (3%), biotite (3%), CPX (<0.5%), +/- accessory minerals apatite and zircon. Free crystals in the Potaka Tephra make up approximately 5-85% of the tephra depending on locality. The free crystal assemblage consists of: plagioclase feldspar, hornblende, OPX, CPX, quartz, biotite, muscovite, iron oxides, ± alkali feldspar, apatite, and zircons. The proportion of free crystals present in any one sample is highly varied.
All glass and whole-pumice major and trace element compositions show minor compositional variations consistent with fractionation of the observed mineral assemblage ± monazite, allanite, and xenotime. Overall they look similar, and are suggestive of a single population. The Potaka Tephra trace element composition is nearly identical to that of the Kidnappers Ignimbrite and Fall Deposits. For the first time, a definitive correlation of the Potaka Tephra with the Kidnappers Fall and Ignimbrite Deposits can be made.
Magma evolution in the Potaka tephra was dominated by plagioclase fractionation. No definitive evidence of magma mixing was seen. Trace element ratios suggest that the Potaka Tephra was sourced from two separate batches of magma (melt + crystals). Each batch of magma evolved by fractionation of plagioclase and zircon, but to different degrees. To produce chemical signatures like these, the magma batches would have to have been isolated from each other, either by chemical zoning or physical isolation.

xviii, 304, [110] p. : col. ill., maps ; 30 cm. + 1 CD-ROM.

2007Lewis