Page 160 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

Plate 33 — Components in subaqueous, volcaniclastic mass-flow deposits
1. Pumice- and lithic-rich breccia
These three samples come from a thick (>25 m),
unstratified, volcaniclastic breccia unit (33.2) in
which the dominant component is relict tube pumice.
The unit occurs within a submarine, below wave base
sequence, and is closely associated with in situ
coherent dacite lava. The pumice clasts are not
necessarily pyroclasts — they may instead be
resedimented from the pumiceous carapace of a
nearby submarine lava dome or flow.
A. Pale green, aphyric relict pumice fragments have
ragged margins and undeformed tube vesicle texture.
They are closely packed and accompanied by pink,
angular, perlitic, dacitic lava clasts (D), and very
sparse feldspar crystal fragments.
B. The relict pumice clasts in this pumice breccia
show tube vesicle texture in random orientations, and
are clearly non-welded.
C. Lower in the unit, angular, pink and brown,
perlitically fractured, dacitic lava clasts (D) are much
more abundant and occur together with relict pumice
clasts.

2. Graphic log of the pumice- and lithic-rich breccia
illustrated in 33.1.

Mount Read Volcanics, Cambrian; DDH SCS2 (A,
127.5 m; B, 13L2 m; C, 134.8 m), Sock Creek South,
western Tasmania.

3. Relict sparsely porphyritic tube pumice
Randomly oriented relict pumice clasts have ragged
margins and show the fibrous texture characteristic
of tube pumice. They occur in a submarine mass-
flow deposit and have been resedimented from a
subaerial or shallow water source.
Mount Read Volcanics, Cambrian; DDH MAC22
(320 m), Hellyer mine, western Tasmania.
4. Relict coarsely porphyritic pumice
Coarsely quartz- and feldspar-phyric relict pumice
clasts are closely packed so that margins of adjacent
clasts are indistinct. The strong bedding-parallel
foliation is the result of diagenetic compaction, not
primary welding, of the pumiceous clasts.

Mount Read Volcanics, Cambrian; DDH HL541
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