Geoscience Reference
In-Depth Information
Usually the effects of chilling extend inwards no more than a
centimetre or two. The very edge may be glassy, lacking
groundmass crystals of any size, but with groundmass crystals
becoming larger and thus more clearly visible further away
from the contact.
BEWARE! In ancient rocks
the shapes of pillows may
have been deformed, making
them hard to confi rm, and
spheroidal weathering can
lead to a pillow-like
appearance in unpillowed
massive basalt.
To spot a chilled margin from a metre or so away, look for
variation in darkness or colour of the rock (Figure 7.15, p. 150).
A fresh basaltic rock with a chilled margin will be grey in its
interior, becoming darker as the feldspar in the groundmass
becomes smaller and more transparent, and ending up almost
black at the glassy contact. Later alteration and/or weathering
can obscure things, but the story may still be visible as, for
example, a gradual change from green to red at the chilled
margin. Use a hand lens to verify grain-size changes and to try
to confi rm any initial diagnosis based on tone or colour.
7
Chilling can affect only the groundmass. If the magma
contained phenocrysts, some of these may occur in the chilled
margin, surrounded by glass. Likewise, any chilling at the
contact cannot manifest itself in the grain size of an intrusion
that was emplaced as a crystal-rich mush (as hypothesized for
some granites). If you fi nd an intrusive contact that lacks any
signs of chilling, you should ask yourself whether the intrusion
was emplaced as a mush, or whether it was emplaced at depth
and/or into country rock that was too hot to cause rapid
chilling.
When an intrusion occurs into a rock of the same composition
and same average grain size as itself, chilling at the contact
enables the relative ages to be deduced, even if there is no
other evidence (e.g. Worked Example 7.2, p. 151). The rock that
is chilled must be younger than the rock that it chills against.
Close to the contact, there will be a gradual (over a centimetre
or two) change in tone, colour or grain size within the younger
rock, but there will be no such changes as you approach the
contact from the other side.
Figure 7.14
Unusually well-
preserved and easily distinguished
pillow lavas. Oman, Arabia.
(David A. Rothery, The Open
University, UK.)
It is exceptional to fi nd chilled margins inside a pluton, but
there are commonly contacts between batches of magma that
had slightly different composition. These are discernible from a
distance by different tone or colour, and evident at close range
by differences in the abundances or crystal size of the main
minerals. Particularly for felsic magmas, viscosity inhibits
mixing, so that the sharpness of the contact between magma
types may be limited only by the size of the crystals.
Pyroclastic rocks
Pyroclastic rocks are a special subset of volcaniclastic rocks.
The latter consist of fragments of volcanic rock, fragmented by
any
process and transported by
any
agent. However, the
contents of a pyroclastic deposit must by defi nition have been
