Page 186 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

5. Altered in situ breccia in dacite
This breccia consists of angular, blocky to cuneiform
clasts of altered, fine-grained dacite. The breccia is
strictly monomict and jigsaw-fit texture between
groups of clasts occurs locally. Although the breccia
closely resembles in situ hyaloclastite, it lacks clasts
with quenched margins or "tiny normal joints" and is
confined to a fracture zone, suggesting instead that
hydraulic fracturing was important. Strongly fracture-
controlled silicification (dark) overprints more
pervasive K-feldspar-sericite alteration (pale orange).
Silicification has spread out 1-2 mm from fractures,
thereby forming a dark siliceous pseudomatrix
domain.

Mount Read Volcanics, Cambrian; DDH QR793
(63.96 m), Que River mine, western Tasmania.
6. Altered perlitic rhyolite

Large diameter perlitic fractures (macro-perlite) are
evident in this coherent rhyolite. Chlorite-epidote
alteration (dark green-grey) along and adjacent to the
perlitic fractures overprints earlier, pervasive feldspar
alteration (pale grey). In areas where chlorite-epidote
alteration is narrowly confined to the perlitic
fractures (centre of photograph), the arcuate character
of the fracture network is faintly preserved. However,
in areas where chlorite-epidote alteration has
advanced farther out from the fractures (top right),
the rock has a matrix-supported breccia texture
consisting of pale, feldspar altered pseudoclasts in a
dark, chlorite-epidote altered pseudomatrix.

Mount Read Volcanics, Cambrian; Pieman Road,
western Tasmania.
7. Altered perlitic dacite
The formerly glassy groundmass is cut by arcuate
and concentric perlitic fractures. Fluids responsible
for strong silicification (white) have used the perlitic
fractures as channel ways and silicification has
completely replaced the fractures in most of the rock,
thereby forming a continuous pseudo-matrix domain.
Silicified domains enclose large irregular patches and
small kernels of darker, less silicified, more chloritic
dacite that form pseudoclasts.
Mount Read Volcanics, Cambrian; DDH HL304
(95.6 m), Hellyer mine, western Tasmania.
8. Calcite-flooded basaltic hyaloclastite

Centimeter-sized clasts in in situ basaltic
hyaloclastite are separated by a finely comminuted
matrix of the same composition. The porous breccia
matrix has been partially flooded with calcite
alteration (white). Larger clasts remain relatively
unaltered.

Mount Read Volcanics, Cambrian; DDH HL52
(156.43 m), Hellyer mine, western Tasmania.

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