Geoscience Reference
In-Depth Information
Table 7.2
Distinguishing characteristics of veins in igneous rock.
Type of vein
Description
Signifi cance
Pegmatite
Coarse crystals, mineralogy similar to or
more felsic than the host rock, may also
contain large crystals of exotic minerals
such as apatite, tourmaline, beryl
Crystallization of last, volatile-rich, fraction of
the magma. If zoned (mineralogy in the
middle differing from the edges), this
indicates evolving chemistry as the joint
expanded
7
Aplite
Fine-grained, sugary, felsic veins,
common in granites
Crystallization of last fraction of silica-rich
magma, possibly triggered by sudden escape
of volatiles when joints were initiated
Hydrothermal
vein
White quartz or calcite, sometimes
containing metallic sulfi des, epidote or
fl uorite. Sometimes the wall-rock is
altered, but little or no material is
precipitated. Tendency to penetrate
further into country rock than other veins
Chemically charged aqueous fl uids,
circulating between igneous body and
country rock. Could refl ect hydrothermal
circulation as a pluton cools, or could be
younger and unrelated
aqueous or late-stage magmatic fl uid to form veins. Veins in
igneous rocks may be much younger than their host, as they
can be in any rock type, so it is important to try to determine
the signifi cance of any veins that you fi nd. Table 7.2
summarizes distinguishing features.
It is often very easy to determine the strike and dip of
individual joints, and in situations such as those shown in
Figures 7.8 and 7.10 a large number of measurements could be
made. However, you need to consider whether this is necessary
and what the aim of collecting all these data might be. Beware
also that some joints, or sets of joints in certain orientations,
may result from tectonic process or release of confi ning
pressure during exhumation, and therefore be irrelevant to the
igneous history of your exposure.
Figure 7.10
A wave-eroded
exposure cut into the colonnade of
the Fingal's Cave fl ow (about
200 m to the right of Figure 7.9).
Note the clear shapes of the
columns. (David A. Rothery, The
Open University, UK.)
7.2.3 Internal architecture: Other exposure-
scale fabrics
Just because you
can
measure something, doesn't
mean that the measurement
will actually be
useful
. Don't
let yourself be distracted from
scientifi c observation into
collecting a large number of
easy-to-make measurements
unless you can foresee a use
for them.
Other features to seek out and record in ancient lava fl ows
include:
•
General morphology and texture.
Are there any pillows
(indicative of eruption into water or under ice)? Is there a
rubbly top or base, suggesting emplacement on land as an
'a'a (Figure 7.11). Are there glassy 'hyaloclastite' fragments
above or below, or a peperitic base (also indicating water)?
If you fi nd a peperitic top, this cannot be a lava fl ow; it
must be a sill intruded into wet sediment.
