Page 193 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

haulage road, western Tasmania.
Plate 46 — Altered and deformed pumiceous volcaniclastic deposits
1. Foliated, strongly quartz-sericite altered tube
pumice breccia

This rhyolitic pumice breccia has undergone early
heterogeneous feldspar and sericite alteration and an
overprinting quartz and sericite alteration stage (cf.
45.7). Flattened sericite-altered pumice clasts
(fiamme) are associated with and aligned along a
spaced stylolitic foliation (S 1) which is crenulated by
the steep regional cleavage (S 2). Some sericite-altered
pumice clasts are transposed into the tectonic
cleavage.

Mount Read Volcanics, Cambrian; Hercules mine
footwall, western Tasmania.

2. Relict fiamme in altered rhyolitic pumice breccia

The prominent dark green lenses (L) in this outcrop
are relict fiamme and consist of phyllosilicate-
altered, diagenetically compacted pumice. The relict
fiamme have highly irregular, cuspate terminations
and are feldspar-phyric. Feldspar crystals are equally
abundant in the intervening pale feldspathic domains
but much less obvious. Pumice clasts in the feld-
spathic domains display unflattened round and
tubular vesicles in random orientations (46.3, 46.4),
indicating that the unit is non-welded.

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

3. Altered tube pumice breccia in thin-section
Thin-sections of feldspathic domains in rhyolitic
pumice breccia such as that shown in 46.2 reveal
unflattened tube pumice clasts in various
orientations, indicating that the deposit is non-
welded. Pumice preservation is due to replacement of
vesicle walls and infilling of vesicles by feldspar
soon after deposition. Pumice which was not replaced
by feldspar was phyllosilicate-altered and readily
flattened by diagenetic or early tectonic compaction,
generating the relict fiamme conspicuous in outcrops
shown in 45.7 and 46.2. Plane polarized light.
Mount Read Volcanics, Cambrian; Hercules South
prospect hangingwall, western Tasmania.

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