Page 148 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

B. Samples of the ignimbrite in the section shown in
27.2A consist of quartz and feldspar crystal
fragments in a dark brown fine matrix in which
millimetre-sized relict glass shards can be identified
with a hand lens. The ignimbrite is internally massive
and contains scattered centimetre-sized lithic
fragments.

Mount Read Volcanics, Cambrian; specimen 91-10D,
Cradle Mountain Link Road, western Tasmania.
C. In thin-section, recrystallised shards are clearly
evident in the matrix of the ignimbrite (27.2B). Some
shards have recognisable rod and spine shapes, but
those at the edges of crystals of quartz, feldspar (Fl)
and a chloritised ferromagnesian phase (M; probably
amphibole) are strongly deformed (arrow) and
indicate significant welding compaction. Plane
polarised light.

Mount Read Volcanics, Cambrian; specimen 91-10D,
Cradle Mountain Link Road, western Tasmania.
D. Thin-sections also show perlitic fractures (arrow)
in the matrix of the ignimbrite (27.2B, 27.2C) and
indicate that it was thoroughly welded. Perlitic
fractures only develop in coherent glass. In
pyroclastic deposits, perlitic cracks can form after
complete welding compaction of glassy pyroclasts.
The formerly glassy, perlitically fractured matrix has
subsequently recrystallised to a fine-grained mosaic
of quartz and feldspar. Other components are quartz
(Q) and sericitised feldspar (Fl) crystal fragments.
Plane polarised light.

Mount Read Volcanics, Cambrian; specimen 91-10C,
Cradle Mountain Link Road, western Tasmania.
Plate 28 — Subaqueously emplaced pyroclastic flow deposits: northern Wales, UK

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