Page 191 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

Plate 45 — Altered pumiceous volcaniclastic deposits
1. Altered rhyolitic pumice-rich breccia
These samples are from the base (A) through to the
top (D) of a 190 m thick, rhyolitic pumice-rich,
subaqueous mass-flow deposit affected by feldspar
and sericite alteration. A and B contain tightly
packed, white, feldspar-altered pumice clasts (8 mm),
feldspar crystals with secondary feldspar
overgrowths, dark green relict pumice lenses, and
lithic clasts (L). Relict pumice clasts are transposed
into a strong tectonic cleavage. C is from the
normally-graded upper part of the unit and consists
mainly of smaller (3 mm) pumice clasts. D comprises
sericitic, pumiceous sandstone and dark green relict
pumice clasts interpreted to be suspension-settled
pumice and ash.

Mount Read Volcanics, Cambrian; DDH H1175
(270-467 m), Hercules South prospect, western
Tasmania.
2. Diagenetically compacted altered pumice
Parts of two normally graded, rhyolitic pumice-rich,
subaqueous mass-flow units can be seen in this
outcrop. Relict pumice clasts (pale green; P) have
been extensively replaced by sericite and
subsequently flattened parallel to bedding (S O), now
steeply dipping, by diagenetic load compaction.
Although the breccia units were originally non-
welded, the pumice lenses define a foliation similar
to eutaxitic texture.

Mount Read Volcanics, Cambrian; Hellyer mine
haulage road, western Tasmania.

3. Diagenetically compacted, altered pumice breccia
These two drillcore samples are composed almost
entirely of rhyolitic pumice clasts. However,
alteration is not uniform in intensity, composition or
colour. Least altered pumice is grey and poorly
compacted, whereas strongly phyllosilicate altered
pumice clasts (grey-green) have been thoroughly
compacted and appear as dark grey-green wavy
streaks. Alignment of compacted altered pumice
defines a bedding-parallel diagenetic compaction
foliation.

T 1 Tuffs, Otaki Formation, Miocene; Hokuroku
district, Japan.

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