Page 159 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

3. Chloride clasts in polymict, volcaniclastic lithic
breccia
Dark green, very irregularly shaped, ragged chloritic
clasts (L) are a minor but conspicuous component of
the volcaniclastic lithic breccia shown in 32.1 and
32.2. They contain evenly distributed, euhedral
feldspar crystals. The chlorite probably replaces an
originally glassy groundmass. Although they may be
relict pumice clasts, no vesicular texture is preserved.
Instead, they could have been glassy but non-
vesicular juvenile clasts (vitriclasts) that have
accommodated to the shapes of adjacent rigid lithic
fragments after alteration to "weak" chlorite.
Mount Read Volcanics, Cambrian; Anthony Road,
western Tasmania.
4. Pink and green banded, crystal-rich sandstone
In detail, the pink-green banding in the crystal-rich
sandstone (St in 32.1) is diffuse and not obviously
coincident with grain size or component changes.
The bands are commonly but not always parallel to
bedding. One interpretation is that albite (pink) and
chlorite (green) alteration has accentuated subtle
shear-induced primary layering in the sandstone
intervals.

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

5. Pink and green banded, crystal-rich sandstone
A. The diffuse banding in this coarse, crystal-rich
sandstone is imparted by pink albite-rich and green
chlorite-rich bands. The sample comes from a 4 m
thick interval that gradation-ally overlies about 10 m
of massive, lithic- and crystal-rich volcaniclastic
sandstone in the same sequence as that illustrated in
32.1. The marked crystal enrichment in the banded
volcaniclastic sandstone may be the result of loss of
original vitric components (pumice, shards) by
sorting according to density during subaqueous
transport.
Mount Read Volcanics, Cambrian; specimen 76812,
Anthony Road, western Tasmania.

B. A photomicrograph from a green band in 32.5A
shows abundant plagioclase and subordinate quartz
crystals and crystal fragments, together with sparse
volcanic lithic fragments. Chlorite in the dark green
bands may be replacing formerly glassy components.
In the pink bands interlocking albite occurs between
the crystal fragments and completely obscures pre-
existing microtextures. Plane polarised light.

Mount Read Volcanics, Cambrian; specimen 76812,
Anthony Road, western Tasmania.

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