Geoscience Reference
In-Depth Information
If you can see an exposed contact between the igneous rock
and the adjacent rock (which might also be igneous), then
simple observation may make it apparent that the edge of your
igneous body cuts across the fabric of the neighbouring rock
(Figures 7.1 and 8.22b). If so, then this discordant nature of the
contact points very strongly to an intrusive relationship. If the
shape of the contact is complicated (Figure 7.2) this would be
very clear evidence for discordance, but some discordant
contacts look straight (or nearly planar if they can be
determined in three dimensions). In such a situation you need
to rule out the alternative possibility that the two rock units
have been brought into contact by a fault (Section 8.2).
7
Figure 7.2
A complicated
intrusive contact between a pale
intrusive rock and a darker country
rock (Oman, Arabia). In this
example, the intrusive rock (here a
plagiogranite or trondhjemite) has
made space for itself by the
process known as 'stoping', in
which blocks of the country rock
(dolerite, in this case) are plucked
away and engulfed within the
intrusion (where they may then
become assimilated). (David A.
Rothery, The Open University, UK.)
Weathering, soil and vegetation sometimes conspire to make
intrusive contacts, especially into sedimentary rock,
frustratingly poorly exposed. Crucial details may be hidden, so
that the geometric relationship between the intrusion and
country rock cannot be easily assessed. In such a situation,
comparing your best estimate of the strike and dip of the
contact with the strike and dip in nearby exposures of country
rock is a useful pointer. Unless the two planes are similar, the
contact must be discordant (or a fault). You will need to judge
how close 'similar' needs to be according to your particular
circumstances. Factors to bear in mind include:
•
If the contact is poorly exposed your best estimate of its
attitude could easily be wrong by more than 10° either
way.
•
You can often measure the strike and dip of sedimentary
bedding at any one place to better than ±5°, but is that
representative of the local average strike and dip?
•
Attitude may vary locally because of folding, so how far
apart are the measurements that you wish to compare?
Table 7.1 (p. 142) suggests how to pin down a contact that is
unexposed, but manifested because of differences in resistance
to weathering and erosion, in chemistry, or in permeability
between an igneous body and its country rock.
Having established the geometry of the edge of your discordant
intrusion, you should ask yourself 'where is its other edge?'.
This is easy if you are dealing with a narrow curtain of
intrusive rock (a dyke) because these are mostly less than a
couple of metres wide. Except in very poorly exposed regions,
if you can see one edge of dyke you can usually see its other
edge (Figures 7.1 and 7.3). A pipe-like volcanic neck will be
typically tens of metres across, but if you are at the margin of a
pluton the other side may be many tens of kilometres away.
Coarse grain size, and lack of chilling at the contact, are good
clues that you are dealing with a pluton (Section 7.3).
Figure 7.3
A near-vertical dyke
cutting bedded sedimentary rocks
(Howick Bay, Northumbria, UK).
Both edges (margins) of this
1-m-wide dyke are clear. (Angela
L. Coe, The Open University, UK.)
