Page 41 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

Plate 1 — Evenly porphyritic and volcaniclastic textures
1. Evenly porphyritic texture in coherent dacite

A. This sample shows porphyritic texture comprising
evenly distributed, moderately abundant, euhedral
plagioclase phenocrysts (P), in a much finer
groundmass. Porphyritic texture is characteristic of
coherent lavas and shallow intrusions.

Mount Read Volcanics, Cambrian; specimen 71425,
Pieman Road, western Tasmania.

B. The groundmass of the porphyritic dacite in 1.1 A
consists of fine-grained quartz, feldspar, chlorite and
carbonate. Plagioclase phenocrysts (P) are prismatic
but their shapes are slightly modified by alteration.
Plane polarised light.

Mount Read Volcanics, Cambrian; specimen 71425,
Pieman Road, western Tasmania.

2. Phenocryst-rich porphyritic andesite
A. Medium to coarse, euhedral, evenly distributed
phenocrysts are abundant in this andesite. Most
conspicuous are black hornblende (H) phenocrysts
whereas pale feldspar phenocrysts are harder to
distinguish from the fine-grained groundmass.

Mount Read Volcanics, Cambrian; specimen 75868,
Anthony Road, western Tasmania.

B. Although altered, the plagioclase (P) and
hornblende (H) phenocrysts are euhedral and
unbroken. Chlorite completely replaces some relict
phenocrysts of a ferromagnesian phase (probably also
hornblende). The groundmass consists of finegrained
feldspar and sericite. Plane polarised light.

Mount Read Volcanics, Cambrian; specimen 75868,
Anthony Road, western Tasmania.
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