Page 185 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

Plate 42 — Altered coherent lava and related breccia
1. Incipient alteration of perlitic fractures in thin-
section
Perlitic fracture texture is well developed in this formerly
glassy dacite. The fractures are accentuated by fine-grained
green chlorite resulting from incipient alteration. Plane
polarized light.

Mount Read Volcanics, Cambrian; Mackintosh
Bridge, western Tasmania.
2. Incipient stage alteration of in situ rhyolitic
hyaloclastite
Subpolygonal blocky rhyolite clasts are defined mainly by
broadly curved fractures and show jigsaw-fit texture with
local rotation of faintly flow-banded clasts. Brecciation was
probably caused by quench fracturing of a formerly glassy,
coherent lava or intrusion, producing in situ hyaloclastite.
Intense silicification (white) is confined to the fracture
network and is prominent along the main fractures, but
poorly developed along minor fractures. Weak to moderate
phyllosilicate-quartz alteration occurs between the
fractures.

Mount Read Volcanics, Cambrian; Mount Read,
western Tasmania.

3. Intermediate stage alteration of in situ rhyolitic
hyaloclastite
This breccia is strictly monomict, comprising blocky clasts
of altered, formerly glassy rhyolite that range from
millimetre to decimetre dimensions. The clasts locally
display jigsaw-fit texture (arrow) and many have
curviplanar outlines. The breccia is matrix poor and clast
supported. However an apparent matrix has formed by
strong silicification that has spread out up to 10 mm from
original quench fractures, thereby replacing the margins of
larger clasts and completely replacing some small clasts.
Areas between silicified fractures have weak to moderate
phyllosilicate-quartz alteration.
Mount Read Volcanics, Cambrian; Mount Read,
western Tasmania.

4. Advanced stage alteration of in situ rhyolitic
hyaloclastite
Intense silicification of this breccia has formed an extensive,
fine-grained pseudomatrix (white) by replacing the
margins of all larger clasts and completely replacing many
small clasts. Silicification moved outward 5-20 mm from
all fractures, reducing the apparent size of the clasts, and
changing the original clast-supported breccia texture to an
apparent matrix-supported texture. Areas enclosed by the
silicified domain have moderate to strong chlorite-quartz
alteration (dark grey). Subsequent strong tectonic foliation and
lineation has stretched the chlorite-rich domains into
elongate pseudoclasts, which resemble pumice fiamme in
welded pyroclastic deposits.
Mount Read Volcanics, Cambrian; Mount Read,
western Tasmania.
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