Page 47 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

Plate 4 — Micropoikilitic texture in rhyolite
1. Altered rhyolite with micropoikilitic texture

This massive rhyolitic lava consists of extensively
sericitized feldspar phenocrysts (F) in a granular
groundmass. The granular texture in the groundmass
is imparted by micropoikilitic quartz that encloses
very fine, sericitized feldspar laths (cf. 4.2), and is
similar to that observed in hand specimens of
microspherulitic samples.

Mount Read Volcanics, Cambrian; specimen 74269,
Jukes Proprietary prospect, western Tasmania.

2. Micropoikilitic texture in thin-section
A. Well-developed micropoikilitic texture in this
sample consists of small irregular quartz cores (Q) in
quartz-rich patches with boundaries clearly defined
by concentrations of sericite. The quartz cores are
optically continuous with the remainder of the
quartz-rich patches. The quartz cores may represent
an initial stage in the development of granophyric
texture. Lofgren (1971b) predicted, but did not
observe, a gradation between micropoikilitic and
granophyric texture. The granular texture of the hand
specimen (4.1) is the result of micropoikilitic
devitrification. Crossed nicols.
Mount Read Volcanics, Cambrian; specimen 74269,
Jukes Proprietary prospect, western Tasmania.
B. In plane polarised light, the quartz cores (Q) are
clearly distinguishable within the micropoikilitic
quartz-rich patches.

Mount Read Volcanics, Cambrian; specimen 74269,
Jukes Proprietary prospect, western Tasmania.

40