Geology Reference
In-Depth Information
Figure 7.18
Distinctive granite Tor, Devon, UK. Two joint sets are visible, flat-
lying sheet joints generally parallel to the surface and vertical cross joints.
7.6 Emplacement Timing
Most of the characteristics of calc-alkaline intrusions may be recognised in gran-
itoid suites of all geological ages but, the older the intrusion, the greater is the
probability that more complicated fabrics will have developed due to the addi-
tional effects of tectonic overprinting. In such cases, any tectonism (or associated
metamorphic effects), such as regional jointing or post magmatic foliations,
should be traceable laterally into the country rocks. Granitoids which contain
internal structures that have survived a later stage of post-emplacement deforma-
tion during which new country rock fabrics, developed around the intrusion, are
described as
pre-tectonic
.
Syn-tectonic
intrusions, including pre-tectonic intru-
sions from which all traces of an earlier fabric are lost and intrusions emplaced
during metamorphism. These tend to have internal flow structures parallel to
the fabrics in adjacent country rock schists or gneisses. Post-tectonic intrusions
are generally unfoliated cutting pre-existing fabrics (Figure 7.20).
7.6.1 Case studies
Intrusions with different relationships to regional metamorphism may sometimes
occur at the same level of exposure, for example Figure 7.21 illustrates a typical
deeply-eroded area of southwest Finland where a syn-tectonic calc-alkaline suite
(dated at 1900 Ma), with east-west flow structures parallel to the foliation in local
amphibolitic gneisses, is cut by a later post-tectonic (1600 Ma) alkali granite. The
