Page 84 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

Plate 12 — Hyaloclastite
1. In situ andesitic hyaloclastite

In this example of in situ hyaloclastite, blocky
hornblende-phyric andesite clasts (C) are separated
by small amounts of millimeter to sub-millimeter-
sized granular matrix (Mx) of the same composition,
including splinters of andesite and sporadically
distributed hornblende crystal fragments. Groups of
clasts and splintery fragments in the matrix display
jigsaw-fit texture.

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

2. Altered andesitic hyaloclastite
Part of the outcrop shown in 12.1 is affected by
chloritic alteration. As a result, the boundaries
between clasts (C) and matrix (Mx) have been
modified and there is a distinct lenticular fabric.
However, some clasts still have curviplanar outlines
and jigsaw-fit texture is locally preserved.

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

3. Andesitic hyaloclastite
Parts of the andesitic hyaloclastite outcrop shown in
12.1 are relatively matrix-rich. The larger clasts are
evenly feldspar-and hornblende-phyric and have
curviplanar margins. The matrix includes splinters of
andesite and scattered crystal fragments. Jigsaw-fit
texture is most clearly displayed by clasts
intermediate in size between the largest clasts and the
matrix granules but is otherwise not easy to recognize
because matrix is abundant.

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

4. In situ andesitic hyaloclastite in thin-section
Equant to cuneiform andesite clasts are closely
packed in a recrystallized matrix that contains
scattered crystal fragments. The clasts comprise
chloritized feldspar and pyroxene phenocrysts in a
groundmass of fine-grained feldspar and pyroxene.
Fragment margins are curviplanar and jigsaw-fit
between groups of clasts is common. The margins of
the clasts are more chloritic (formerly more glassy?)
than the interiors. Plane polarised light.

Mount Read Volcanics, Cambrian; specimen 74108,
Burns Peak, western Tasmania.
76