Page 80 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

Plate 10 — Autoclastic breccia and talus
1. Deformed and altered, rhyolitic autoclastic breccia

Clasts in this rhyolitic autoclastic breccia are blocky and
flow banded. Flow banding in adjacent clasts has
different orientations. The clasts are closely packed and
there is a paucity of fine matrix. These features are
typical of autobreccia.

Mount Read Volcanics, Cambrian; Mount Read,
western Tasmania.

2. Subaerial rhyolitic autobreccia

This autobreccia occurs at the margin of a subaerial
rhyolite dome. Matrix comprising more finely
fragmented rhyolite separates blocky and slabby, flow-
banded clasts of the same composition.

Boyd Volcanic Complex, Late Devonian; Eden, New
South Wales.

3. Strongly altered autoclastic breccia
All the clasts in this autoclastic breccia are feldspar- and
quartz-phyric rhyolite. However, the clasts have been
variably silicified, chloritized and K-feldspar altered,
resulting in an apparently polymict clast assemblage.
Phenocrysts in silicified clasts have been recrystallized
and are barely distinguishable from the groundmass. In
adjacent chloritically altered clasts, the relict feldspar
phenocrysts maintain their original shape, size and
distribution. Many clasts are flow banded and most are
blocky but lack the curviplanar outlines that are charac-
teristic of clasts in hyaloclastite. These textural features
are consistent with interpretation as an autobreccia.
Mount Read Volcanics, Cambrian; specimen 42571,
Chester mine area, western Tasmania.
4. Talus associated with a dacite lava dome
Coarse talus deposits are actively accumulating on
the flanks of the dacite lava dome in the amphitheatre
of Mount St Helens. Blocks liberated during growth
of the dome tumble, bounce and roll downslope
under the influence of gravity, commonly breaking
up on the way. The unstructured rubble comprises a
framework of angular dacite lava blocks up to tens of
metres across, and minor granular matrix mainly
derived from attrition of the larger blocks. Note
person for scale (encircled).

Mount St Helens dacite lava dome, AD 1986;
Washington, USA.

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