Page 92 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

5. Basaltic pillow lava
The adit to Hellyer mine exposes cross-sections
through closely packed pillows that each show
accommodation (arrow) to underlying pillows. The
pillows have distinctly darker, chilled rims and
massive interiors. Pillow lava of the Hellyer Basalt
overlies the Hellyer massive sulfide deposit, and
occurs near the base of and within thick pyritic
mudstone (Que River Shale). It was both erupted and
emplaced in a relatively deep, submarine
environment.

Mount Read Volcanics, Cambrian; Hellyer mine adit,
western Tasmania.

6. Basaltic pillow in drill core
The presence of abundant interpillow mudstone aids
the recognition of pillow boundaries in this section of
drill core. Pillow margins are chilled and pale green.
Intricate interpenetration of mudstone and basalt
occurs along the margin, indicating extrusion of the
basalt into wet, unconsolidated sediment. At the
pillow boundaries chlorite- and chlorite-carbonate-
filled vesicles are small and elongate parallel to the
pillow margins. In the centre, they are larger, less
abundant and more equant in shape. In cases where
there is no interpillow sediment and the pillows are
close-packed, their boundaries are marked by subtle
changes in vesicle shape, size and abundance, and by
textural or colour changes that reflect chilling. The
orange-colored material in the mudstone is pyrite.

Mount Read Volcanics, Cambrian; DDH HL45
(325.5 m), Hellyer mine, western Tasmania.

7. Basaltic pillow lava
This C-shaped pillow lobe segment exhibits a black,
glassy chilled rind which is locally flow wrinkled. In
cross-section, the pillow lobe displays radial
columnar joints (C) that are coated with iron oxides
and carbonate. An asymmetric transverse spreading
crack (arrow) encircles the lobe.

Pillow retrieved from a Holocene(?) basaltic flow on
the floor of the Manus Basin (2400 m below sea
level), Papua New Guinea.
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