Page 143 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

B. A thin-section of the ignimbrite in 25.3A shows
that it is densely welded and partly spherulitically
devitrified. Compacted and deformed, still glassy
pumice wisps and shards (S) define a foliation which
is deflected around plagioclase (P), hornblende and
biotite crystals and crystal fragments. The rest of the
matrix consists of glassy patches that contain clusters
of crystallites (G), and spherulites (Sp) that overprint
the welded shards. Remnants of the original welded
vitriclastic texture are recognisable within the
spherulites (arrow), especially close to their margins.
Plane polarised light.

Nelson Mountain Tuff, 27 Ma; specimen NMT4, San
Luis caldera, San Juan Mountains, Colorado, USA.
4. Crystal-rich devitrified welded ignimbrite
A. Crystal fragments (quartz, feldspar, hornblende,
biotite) constitute more than 50 modal % of this
ignimbrite. Additional components are phenocryst-
rich relict pumice and even-grained possibly cognate
igneous inclusions (I). The coarse crystal-rich texture
and compact, massive character result in superficial
resemblance to a coherent porphyritic intrusion.

Fish Canyon Tuff, 28 Ma; La Garita caldera, San
Juan Mountains, Colorado, USA

B. In thin-section, formerly glassy shards (S) are
evident in the matrix of the ignimbrite in 25.4A. The
shards occur between plagioclase (P), quartz and
hornblende (Hb) crystal fragments and show
remnants of axiolitic devitrification texture. Axiolitic
devitrification develops during primary cooling and
crystallisation of hot volcanic glass, and is a good
indicator of primary emplacement of volcaniclastic
deposits. Axiolitic devitrification of shards typically
preserves and accentuates the original shard shape.
Plane polarised light.

Fish Canyon Tuff, 28 Ma; specimen FCT6, La Garita
caldera, San Juan Mountains, Colorado, USA.

5. Recrystallised pumice in welded ignimbrite in
thin-section
Recrystallised, formerly glassy, platy and cuspate
matrix shards are strongly deformed at the margins of
quartz and feldspar crystal fragments as a result of
welding compaction. In the centre of the field of view
is a compacted, recrystallised, relict pumice clast in
which the internal vesicular microstructures are still
discernible. Plane polarised light.

Tyndall Group, Mount Read Volcanics, Cambrian;
specimen CT52, Comstock Valley, western Tasmania.

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