Page 141 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

3. Granophyric crystallisation in very thick,
welded, intra-caldera ignimbrite
A. Pale, relict pumice fiamme define a weak foliation
in this outcrop of crystal-rich, welded ignimbrite.
Both the pumice lapilli and matrix glass shards are
devitrified (24.3B, 24.3C). The ignimbrite is at least
400 m thick and ponded within a genetically related
caldera complex.

Arringunna Rhyolite, Featherbed Volcanic Complex,
Early Permian; Big Watson Creek, northern
Queensland.

B. Granophyric crystallisation of glass results in a
mosaic of fine-grained quartz and feldspar. This
example of granophyric texture occurs in the
formerly glassy matrix of a densely welded
ignimbrite (24.3A) between feldspar crystal frag-
ments (Fl). Granophyric crystallisation commonly
completely overprints any pre-existing vitriclastic
textures and other criteria must be used to distinguish
a pyroclastic origin. Granophyric texture typically
develops in the interior parts of very thick, welded
ignimbrites that cool slowly. Plane polarised light.
Arringunna Rhyolite, Featherbed Volcanic Complex,
Early Permian; specimen 87301005, Big Watson
Creek, northern Queensland

C. The mosaic of fine quartz and feldspar that
characterises granophyric crystallisation of glass is
well displayed here under crossed nicols. The field of
view is the same as in 24.3B. Granophyric
crystallisation usually overprints both pumice and
matrix shards, and resembles groundmass textures in
some coherent lavas and high-level intrusions.
Crossed nicols.

Arringunna Rhyolite, Featherbed Volcanic Complex,
Early Permian; specimen 87301005, Big Watson
Creek, northern Queensland.

4. Welded rhyodacitic ignimbrite
Pale relict pumice lapilli (P) define a conspicuous
foliation in this massive welded ignimbrite. Both the
pumice clasts and the formerly glassy matrix of the
ignimbrite are totally devitrified and show
granophyric texture in thin-section. Only in
favourable exposures is relict pumice easily
recognised. Otherwise, relict pumice clasts are
represented by patches in which crystal components
are euhedral, and coarser and less abundant than in
the enclosing matrix.
Dundee Rhyodacite, Coombadjha Volcanic
Complex, Late Permian; Ooroowin Road, New South
Wales.

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