A scanning electron microscope study of post-depositional changes in the northeast Nigde ignimbrites, South Central Anatolia, Turkey

The ignimbrites of the northeast Nigde area, which are subdivided into the Lower, Middle and Upper ignimbrites on the basis of their compositional and stratigraphical characteristics, display textural variations from the base to the top. A large proportion of these ignimbrites have been altered and recrystallized by the processes of compaction, welding and devitrification, and as a result of volatile movement. The alteration and recrystallization processes include four stages: hydration, glassy and spherulitic stages ac companied by vapour-phase crystallisation. In the early phases of devitrifi cation, detailed shard shapes are easily recognisable with the scanning ele ctron microscope. but as alteration proceeds clarity of detail disappears b ecause compaction results in collapse of the structure of the rock. Spherul itic and vapour-phase crystallisation usually involves the growth of alkali feldspar and cristobalite. These later stages are more common in the Upper Ignimbrite than the Middle and Lower ignimbrites.