Page 145 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

3. Flow foliation in high-grade (rheomorphic)
ignimbrite
Flow foliations and flow folds can develop in high-
grade (densely welded) ignimbrite that undergoes
non-particulate flowage (rheomorphism) during and
after deposition. These flowage features closely
resemble those in coherent lavas and evidence for a
pyroclastic origin may be subtle. The rapidly cooled,
less welded bases of such ignimbrites are most likely
to record their pyroclastic character. In this outcrop,
the only suggestion of a pyroclastic origin is the
abundance of angular lithic lapilli (arrow), although
elsewhere vitriclastic textures are preserved in the
matrix. Silicic lavas may also contain lithic
inclusions, but they are typically sparse and evenly
distributed.

Gomez Tuff, Tertiary; Davis Mountains,
southwestern Texas, USA.
4. Flow fold in high-grade ignimbrite
Non-particulate laminar flowage of high-grade
(densely welded) ignimbrite has produced 1-2 cm
thick flow bands that define recumbent isoclinal flow
folds. Silicic lava flows commonly contain similar
flow structures and thick, voluminous lavas can be as
extensive as some ignimbrites. In ancient sequences,
a pyroclastic origin for widespread silicic sheets may
be difficult to establish beyond doubt.

Fisherman Rhyolite, Featherbed Volcanic Complex,
Early Permian; Upper Combella Creek Gorge,
northern Queensland.

5. Fiamme in high-grade peralkaline ignimbrite
The strong foliation in this outcrop is produced by
extremely compacted, lenticular fiamme. The fiamme
were formerly glassy pumice fragments but are now
totally devitrified. At a late stage in the compaction,
the largest fiamma in the centre of the photograph
has responded in a brittle fashion to further
deformation by breaking into segments, while the
still ductile matrix has healed the fractures.

Ignimbrite E, Upper Mogan Formation, Miocene;
Playa de Tauro, Gran Canaria, Canary Islands.

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