Page 85 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

5. Perlitic in situ andesitic hyaloclastite
A. Between the well-defined, dark rimmed, blocky
and splintery, feldspar-phyric, perlitic andesite clasts
are closely packed, finer (millimeter to centimeter),
pale clasts of identical material. In detail, the
boundaries of the larger clasts are gradational into
granules in the matrix. The clast-in-matrix texture
develops as a result of variable in situ fragmentation.
More thoroughly fragmented andesite forms the
matrix surrounding intact larger clasts.

Mount Read Volcanics, Cambrian; specimen FPS1,
Hellyer mine, western Tasmania.

B. In thin-section, the groundmass of the larger clasts
and the granules have well-developed classical
perlitic fractures and, although formerly glassy, are
now largely sericite. The clasts and granules locally
display jigsaw-fit texture and are separated by
patches of recrystallized feldspar, sericite and
carbonate that contain sparse crystal fragments. Plane
polarised light.

Mount Read Volcanics, Cambrian; specimen FPS1,
Hellyer mine, western Tasmania.

6. Dacitic hyaloclastite
The mismatch of flow banding orientation in
adjacent, variably altered, feldspar-phyric dacite
clasts indicates that they have been rotated, and
suggests that both autobrecciation and quench
fragmentation have operated.
Mount Read Volcanics, Cambrian; DDH SCS2

(103.1 m), Sock Creek, western Tasmania.
7. Dacitic hyaloclastite
In situ quench fragmentation of this finely feldspar-
phyric dacite is indicated by the well-developed
jigsaw-fit texture, and closely packed, blocky to
cuneiform clast shapes.
Mount Read Volcanics, Cambrian; DDH HP1
(662.6m), High Point, western Tasmania.
8. Basaltic hyaloclastite

A. Pale to dark green, blocky and splintery clasts of
amygdaloidal basalt are separated by fine-grained,
more sericitic and partly recrystallized matrix, and
display jigsaw-fit texture.
Mount Read Volcanics, Cambrian; DDH MAC 19
(523.5 m), Hellyer mine, western Tasmania.
B. Blocky amygdaloidal basalt clasts in this breccia
have the curviplanar outlines and jigsaw-fit texture
that are characteristic of in situ quench fragments.

Mount Read Volcanics, Cambrian; DDH MAC 19
(540.2 m), Hellyer mine, western Tasmania.

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