Page 144 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

6. Altered, formerly glassy clast in welded
ignimbrite in thin-section
The prominent chloritic clast in the centre of the field
of view has ragged, delicately cuspate margins
(arrow) and contains scattered spherulites. The
matrix of the ignimbrite is composed of recrystallised
welded shards. The chloritic clast may have been
compacted glassy pumice, or else, a glassy, dense
(non-vesicular) juvenile magmatic clast (vitriclast).
Plane polarised light.

Tyndall Group, Mount Read Volcanics, Cambrian;
specimen CT30, Comstock Valley, western Tasmania.

Plate 26 — Heat retention in pyroclastic flow deposits and high-grade ignimbrite
1. Columnar-jointed, welded, rhyolitic ignimbrite
Wide, regular, columnar joints in this ignimbrite are
perpendicular to a conspicuous foliation (F) defined
by compacted, elongate, relict pumice lenses (P),
some of which are up to 40 cm long. The columnar
joints developed in response to contraction during
cooling and imply primary emplacement. Together
with the pumice foliation and massive character, the
outcrop shows the typical features of very thick,
welded, subaerial ignimbrite.

Arringunna Rhyolite, Featherbed Volcanic Complex,
Early Permian; Convict Creek, northern Queensland.

2. Columnar-jointed, compositionally zoned
ignimbrite
Compositional zonation within this ignimbrite ranges
from a pale, rhyodacitic lower zone to a brown-grey,
andesitic upper zone. Both zones are texturally non-
welded and uncompacted. Regular columnar joints
and vapour-phase crystallisation have formed in the
andesitic zone and indicate somewhat hotter
emplacement for it than the unjointed, unconso-
lidated rhyodacitic base. Although compositional
zonation is a relatively common feature of small to
moderate volume, Tertiary and younger ignimbrite
sheets, few ancient examples have been described.
Ignimbrite from the 6845 a Mount Mazama eruption;
Sand Creek, Crater Lake, Oregon, USA.

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