Page 44 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

5. Partly palagonitised basaltic volcaniclastic
sandstone

A. Volcanic glass (or relict glass) is a very common
component of a wide variety of primary,
resedimented and reworked volcaniclastic deposits.
The most abundant grains in this fine to coarse
volcaniclastic sandstone are made of tan
palagonitised basaltic glass. Other components are
plagioclase-phyric basalt grains (black) and sparse
crystal fragments. The sample comes from a 10 m
thick, graded bedded submarine sequence of
resedimented, possibly phreatomagmatic basaltic
debris.
Woolnorth Tuff, Tertiary; specimen CGI, Cape Grim,
northwestern Tasmania.
B. Sideromelane (S) is inclusion-free, fresh basaltic
glass that is clear and isotropic in thin-section.
Alteration of sideromelane produces yellow-brown or
yellow-orange palagonite (P). In the sample shown in
2.5A, palagonite forms rims around the vesicles and
along the margins of blocky sideromelane shards.
Vesicles (V) are lined with radially fibrous zeolite
and infilled by massive zeolite. Plane polarised light.

Woolnorth Tuff, Tertiary; specimen CGI, Cape Grim,
northwestern Tasmania.

6. Glassy crystal-rich ignimbrite
A. The abundant feldspar crystals and crystal
fragments (F) in this rhyolitic ignimbrite are
separated by black glassy matrix. Dark lithic
fragments (L) are also present.

Rhyolitic base of ignimbrite P1, 14.1 Ma; Anden
Verde, Gran Canaria, Canary Islands.

B. In thin-section, densely welded glass shards
(arrow) are evident in the matrix of the sample shown
in 2.6A. Welding compaction of the glass shards has
produced coherent glass with perlitic fractures
(probably due to hydration following welding). The
glass shards are strongly deformed, especially near
crystal fragments (feldspar, F; amphibole, A). Plane
polarised light.

Rhyolitic base of ignimbrite P1, 14.1 Ma; Anden
Verde, Gran Canaria, Canary Islands.

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