Page 139 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

B. In thin-section, the matrix of 23.3A displays a
vitriclastic texture accentuated by axiolitic
devitrification of glassy components. In the centre of
the field of view is a relict pumice clast in which the
internal vesicular microstructure has been destroyed.
The brown rims of matrix shards show axiolitic
devitrification. Central parts of shards consist of a
mosaic of fine grained quartz and feldspar. Plane
polarised light.

Sapinero Mesa Tuff, 28.5 Ma; specimen SMTe-1, San
Juan caldera, San Juan Mountains, Colorado, USA.

4. Devitrified and slightly welded shards and
pumice in thin-section
The formerly glassy shards and pumice in this
ignimbrite have devitrified to axiolitically arranged
fibres. Vesicle walls (W) and the internal vesicles
(V) in the pumice clasts are still distinct and show the
effects of slight deformation during incipient welding
compaction. Plane polarised light.

Oxaya Formation, Miocene; specimen OxF, 20 km
west of Parinacota, northern Chile.

5. Vapour-phase alteration of pumice lapilli in
thin-section
Vapour-phase alteration of glassy pumice
fragments in ignimbrite may result in total
destruction of the original vesicular microstructure.
Formerly glassy pumice fragments in this vapour-
phase altered ignimbrite now consist of fine,
radiating feldspar laths (arrow), open cavities (V) and
unaltered sanidine (S) phenocrysts. The glass in the
matrix (M) is also devitrified and no vitriclastic
texture is preserved. Crossed nicols.

Campanian Ignimbrite, ?33 ka; specimen CI, Campi
Flegrei, Italy.

Plate 24 — Welding and granophyric crystallisation textures in ignimbrite

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