Page 50 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks
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3. Relict perlite
A. Relict classical perlite is well-developed within
clasts in this hand specimen of formerly glassy, in
situ dacitic hyaloclastite.
Mount Read Volcanics, Cambrian; specimen
C101159, Sock Creek, western Tasmania.
B. In thin-section, the relict perlitic fractures in 5.3A
are outlined by sericite and confined within the
centers of clasts. Other arcuate fractures oriented
subparallel to clast boundaries may be quench
fractures. The granular-textured apparent matrix
(Mx) consists of finely intergrown quartz, feldspar
and phyllosilicates. In places, the apparent matrix
contains ghosts of former perlitic fractures (arrow),
and may have been originally composed of glass but
has been more strongly altered. Plane polarized light.
Mount Read Volcanics, Cambrian; specimen
C101159, Sock Creek, western Tasmania.
4. Relict perlite in dacite
A. Relict perlitic fractures in this hand specimen of
feldspar-phyric dacite are defined by chlorite.
Although recognizable with a hand lens, they require
a careful search.
Mount Read Volcanics, Cambrian; specimen 401242,
Murchison Highway, western Tasmania.
B. In thin-section, well-developed relict perlitic
fractures are ubiquitous in the formerly glassy
groundmass of the dacite in 5.4A. Fractures are
defined by chlorite, and the glass has been replaced
by quartz, feldspar, sericite and epidote. The
groundmass also contains partially recrystallized
spherulites (arrow). Plane polarized light.
Mount Read Volcanics, Cambrian; specimen 401242,
Murchison Highway, western Tasmania.
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