A scanning electron microscope study of post-depositional changes in the Matahina Ignimbrite, North Island, New Zealand

Abstract

The processes of welding and devitrification plus volatile movement and reaction have radically altered the primary textures of the Matahina Ignimbrite. Shard shapes are not recognisable with the Scanning Electron Microscope in the lenticulite or devitrified zones because they collapse under the load of overlying rock. Devitrification and secondary vesiculation also obscure the primary textures. Spherulitic, rather than axiolitic, devitrification textures occur where a few very small shards adhere to shard or vesicle surfaces. Volatile activity has corroded glass and transported, and then deposited, Si, AI, and K in crystals of alkali feldspar and cristobalite.

Keywords:

• Matahina Ignimbrite
• welded tuff
• SEM data
• devitrification
• vapour phase alteration