Page 189 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

Plate 44 — Altered devitrified silicic lava
1. False polymict breccia texture in altered coherent
dacite
Overprinting of alteration phases commonly results
in false polymict breccia textures. Heterogeneous
feldspar-quartz (orange and pink) and chlorite-
epidote (dark green) alteration of this coherent dacite
(lava or sill) has been strongly influenced by perlitic
fracturing in the formerly glassy groundmass. Relicts
of early, feldspar-rich alteration (orange) in least
fractured areas form pseudoclasts, enclosed by a
pseudomatrix domain of subsequent feldspar-quartz
alteration (pink) that mainly affected more strongly
fractured areas. Chlorite-epidote alteration has
overprinted feldspar-quartz alteration in some
strongly fractured areas, forming irregular
pseudoclasts of superficially more mafic
composition.

Mount Read Volcanics, Cambrian; Pieman Road,
western Tasmania.
2. Fragmented phenocrysts in thin-section
Although coherent lavas and intrusions are
characterized by euhedral phenocrysts, in situ
fractured and fragmented crystals can occur,
especially in deformed sequences. The sericitized
feldspar phenocryst fragments (A, B, C) in this
example show jigsaw-fit texture and are set in a
formerly glassy ground-mass with relict perlitic
fractures (arrow) defined by chlorite and/or opaque
phases. Crossed nicols.

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

3. Spherulitically devitrified rhyolitic lava

The flow foliation in this rhyolitic lava is defined by
alternating black, glassy bands (obsidian) and grey
spherulitic bands. Large lithophysae (pale pink; L)
occur in concentrations along particular flow bands
and as isolated structures (compare with 44.4).

Obsidian Cliff rhyolite lava flow, 0.18 Ma;
Yellowstone National Park, Wyoming, USA.
174