Page 48 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

3. Flow-banded rhyolite with micropoikilitic texture
Euhedral feldspar and embayed quartz phenocrysts
are evenly distributed in this devitrified flow-banded
rhyolitic lava. Flow bands are defined by alternating
siliceous, micropoikilitic-textured layers, and slightly
more phyllosilicate-rich layers evident in thin-section
(4.4A, 4.4B). The flow foliation is deflected around
an ellipsoidal lithophysa (L) that consists of a
chlorite core (formerly a cavity) and a rim of fibrous
feldspar crystallites.

Mount Read Volcanics, Cambrian; specimen 41092,
Mount Darwin, western Tasmania.

4. Micropoikilitic texture in thin-section
A. In thin-section micropoikilitic texture comprises
patches of optically-continuous quartz enclosing
variably oriented, sericitized feldspar laths (arrow).
Crossed nicols.

Mount Read Volcanics, Cambrian; specimen 41092,
Mount Darwin, western Tasmania.

B. In plane polarized light, the boundaries between
quartz patches are marked by a concentration of fine
sericite flakes (arrow).

Mount Read Volcanics, Cambrian; specimen 41092,
Mount Darwin, western Tasmania.

5. Micropoikilitic texture in thin-section
A. Micropoikilitic texture is very well-developed in
this rhyolitic lava. Feldspar laths enclosed by
micropoikilitic quartz are extensively replaced by
sericite. Crossed nicols.

Mount Read Volcanics, Cambrian; specimen 41340,
east Darwin area, western Tasmania.

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