Radial dikes of Lyttelton Volcano — their structure, form, and petrography

Abstract

Radial dikes of Lyttelton Volcano, Banks Peninsula, New Zealand, are mainly hawaiite, with basalt, mugearite, and trachyte next in abundance. Spatial distribution of compositional types is not regular, one interpretation being that the proportion of trachyte increased with time. Analysis of dike numbers suggests there were 2000 near the volcanic centre, and <500 at the level of the eroded crater rim. Dikes have a blade shape with the long axis horizontal. Flow directions are dominantly along that axis with significant deviations up to 90° either side. Dike traces are marked by bends and offsets of as much as 125 m. Most dikes are <1.5 m thick, but they range up to 20 m. Thickness changes slowly horizontally outwards, but quickly vertically; many dikes thicken upwards to the point where they feed flows and domes, or have been cut off by penecontemporaneous erosion. Dikes played a major role in the construction of the volcano, probably a continuous role. A modern analogue is Etna.

Keywords:

• Lyttelton Volcano
• Miocene
• Canterbury
• radial dikes