Page 194 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

4. Strongly deformed, altered tube pumice breccia in
thin-section

Tube vesicles in pumice clasts in this thin-section of
pumice breccia are clearly defined and have been
replaced mainly by secondary feldspar. After feldspar
alteration and during strong tectonic deformation, the
initially randomly oriented pumice clasts were
realigned subparallel to the regional tectonic
cleavage. Plane polarized light.

Mount Read Volcanics, Cambrian; Hercules South
prospect hangingwall, western Tasmania.

5. False shard texture in altered rhyolitic pumice in
thin-section
This photomicrograph shows an altered pumice clast
in non-welded rhyolitic pumice breccia. Round
vesicles (P) that surround a sericite-altered feldspar
crystal (Fd) have been replaced by secondary
feldspar (white to pale brown). Subsequent sericite
alteration (green-grey) has partially replaced the
margins of the pumice clast, resulting in a false shard
texture (arrow) comprising relict segments of the
original feldspar-altered pumice clast, enclosed by
sericite. Plane polarized light.

Mount Read Volcanics, Cambrian; DDH LB271
(130.5 m), Rosebery mine footwall, western
Tasmania.
6. Augen schist texture in deformed, altered rhyolitic
pumice breccia
The progressive development (A to C) of augen
schist texture is shown by these core samples of
deformed and altered rhyolitic pumice breccia.
Augen schist texture has resulted from strong
cleavage superimposed on competent feldspar-and
quartz-rich alteration assemblages. In A, feldspar-
altered pumice clasts have been silicified and the
larger pumices form prominent white siliceous
patches. Core B shows the same rock and alteration
type with slightly stronger cleavage causing incipient
augen texture. In areas of stronger deformation (core
C), cleavage anastomoses around the competent
silicified pumice clasts resulting in augen schist
texture. Relict fibrous tube pumice textures are
preserved in some siliceous augen and can be
identified with a hand lens but require a careful
search.

Mount Read Volcanics, Cambrian; DDH DP259
(220-330 m), Rosebery-Hercules footwall, western
Tasmania.

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