Page 190 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

4. Altered, flow-banded and nodular devitrified
rhyolite
Flow foliation in altered rhyolitic lava is defined by
alternating siliceous and phyllosilicate-rich bands.
Siliceous laminae comprise abundant, small (mm)
spherulites. Larger nodular siliceous structures are
interpreted as lithophysae (compare with 44.3). The
nodules are concentrated in bands (top and centre) or are
isolated (arrow). Intervening sericitic bands (dark grey)
may have been formerly glassy. Alteration was strongly
influenced by the pre-existing pattern of devitrification:
spherulitic bands and lithophysae (quartzofeldspathic
devitrified) were replaced by quartz-albite (white) and
glassy bands were replaced by sericite-quartz (dark green).
Mount Read Volcanics, Cambrian; Mount Read,
western Tasmania.
5. Altered spherulitic rhyolitic lava in thin-section
Spherulites have been partially recrystallized to pink
feldspar-quartz mosaics in which only faint relict radial
fibrous structure is preserved. Formerly glassy domains
between the spherulites have been replaced by sericite
(dark green). Such sericitic lenses can be mistaken for
flattened pumice fiamme that occur in a variety of pumice-
rich volcaniclastic deposits. Plane polarized light.

Mount Read Volcanics, Cambrian; Winter Brook
area, western Tasmania.

6. Altered, strongly cleaved, spherulitic rhyolitic lava
Texturally and mineralogically contrasting domains have
been accentuated by weathering. Silicified spherulite
nodules are resistant and prominent, whereas sericite-
altered, originally glassy areas are recessive. The resulting
very uneven outcrop surface contributes to a false clastic
appearance. The trace of a strong cleavage is approximately
parallel to the pen.

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

7. Devitrified, flow-banded dacite
A, B. Both these samples come from a devitrified, altered
dacitic intrusion with well-developed flow banding and
evenly porphyritic texture. Phenocrysts of feldspar are
chloritized and appear as dark spots. In A, flow laminae are
defined by alternating chloritic (grey) and siliceous (cream)
layers. In B, siliceous and chloritic flow banding wraps
around a large patch of coalesced spherulites (S) with
bulbous margins (cf. 44.3). The flow banding is locally
accentuated by hematite alteration (arrow).

Mount Windsor Volcanics, Cambrian; DDH REW803
(A, 60.4 m; B, 62.1 m), Reward prospect, northern
Queensland.

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