Page 45 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

Plate 3 — Spherulites and lithophysae
1. Spherulites in Palaeozoic, flow-banded rhyolite

Pinkish, isolated and coalesced, spherical spherulites
up to about 4 cm across are locally arranged in trains
that help define flow bands (F) in this outcrop of
coherent rhyolite.

Bulgonunna Volcanic Group, Late Carboniferous;
Wyarra Creek, northern Queensland.

2. Spherulites in Quaternary, devitrified rhyolitic lava
The well-defined spherulites in this feldspar-phyric
subaerial rhyolite are sufficiently large for the
radiating crystallites to be easily distinguished.
Cuspate areas of obsidian (Ob) remain between
larger coalesced spherulites. Pale brown patches of
closely-packed microspherulites (arrow) have
irregular bulbous margins.

Ngongotaha lava dome, <140 ka; Hendersons
Quarry, Rotorua caldera, New Zealand.

3. Spherulites and obsidian in rhyolite
A. This rhyolite is substantially devitrified. It consists
almost entirely of large spherulites (S) and
lithophysae (L) within patches of coalescing
microspherulites. Obsidian (Ob) is restricted to
small, cuspate areas. Concentric zones within the
large spherulites reflect variations in the packing of
radial fibres and the abundance of very fine
inclusions. The textural and compositional contrasts
produced by such primary devitrification strongly
influence the textural effects of subsequent
hydrothermal alteration or metamorphism.
Ngongotaha lava dome, <140 ka; Hendersons
Quarry, Rotorua caldem, New Zealand.

B. In thin-section, spherulites such as those
illustrated in 3.3A consist of radial crystal fibres.
This example has grown on a cluster of plagioclase
crystals (C). Fan- to sheaf-shaped microspherulites
occur around the margin of the central spherulite.
Plane polarised light.

Ngongotaha lava dome, <140 ka; Hendersons
Quarry, Rotorua caldera, New Zealand.
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