Page 42 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

3. Volcaniclastic texture in crystal-rich sandstone
A. Abundant crystal fragments (feldspar and quartz)
occur together with scattered volcanic lithic
fragments (L) in this crystal-rich sandstone. Unlike
phenocrysts in coherent lavas or intrusions, the
feldspar crystal fragments are angular, unevenly
distributed and range from almost complete,
millimeter-sized euhedra to sub-millimeter chips and
splinters.

Mount Read Volcanics, Cambrian; specimen 91-9B,
Cradle Mountain Link Road, western Tasmania.

B. A thin-section of the crystal-rich sandstone (1.3A)
shows the uneven distribution, angular shape and
broken condition of the crystal components, all
hallmarks of volcaniclastic texture. Abundant
feldspar (F) and less abundant quartz (Q) crystal
fragments are locally closely packed, with only minor
fine, chloritic and quartzofeldspathic matrix. Another
important indicator of volcaniclastic origin is the
presence of relict shards (arrow). Plane polarised
light.

Mount Read Volcanics, Cambrian; specimen 91-9B,
Cradle Mountain Link Road, western Tasmania.

4. Apparent porphyritic texture in crystal-rich
ignimbrite
A. The most conspicuous feature of this hand
specimen is the apparent porphyritic texture,
comprising abundant, evenly distributed, mostly
euhedral feldspar (F) crystals. Although the crystal
abundance is high, the texture in hand specimen
alone could reasonably be interpreted as that of a
coherent lava or intrusion.

Rhyolitic base of ignimbrite P1, 14.1 Ma; Anden
Verde, Gran Canaria, Canary Islands.

B. The sample in 1.4A in fact comes from crystal-
rich welded ignimbrite. In outcrop, the scattered
lithic fragments (dark brown and grey) and subtle
foliation (arrow) of flamme are important clues that a
volcaniclastic origin is correct. (The matrix also
contains abundant welded shards — 2.6B.)

Rhyolitic base of ignimbrite P1, 14.1 Ma; Anden
Verde, Gran Canaria, Canary Islands.

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