Geology Reference
In-Depth Information
(a)
(b)
foliation
within and
surrounding
intrusion
due to growth
during regional
metamorphism
newly
acquired
foliation
earlier
internal fabric
contact
metamorphic
aureole
(c)
(d)
syn-tectonic
post-tectonic
intrusion discordant
to country rock
foliation
post-tectonic
pre-tectonic
Figure 7.20
The relationship between intrusive rocks and phases of defor-
mation: (a) A pre-tectonic intrusion with an earlier internal fabric, around
which metamorphic fabrics have been developed without affecting the intrusion.
(b) Syn-tectonic intrusion emplaced during regional metamorphism, and con-
trolled in terms of both shape and internal fabric by the tectonic stress regime.
(c) Post-tectonic intrusion emplaced through the discordant to pre-existing fab-
rics. (d) Block diagram showing syn-tectonic and pre-tectonic intrusions (light
shading) developed due to strong vertical and moderate left-to-right compres-
sion in three dimensions. Fine lines indicate mineral lineations in the syn-tectonic
intrusion and host rocks. Solid ornament (right) denotes a post-tectonic intrusion.
calc-alkaline rocks are dominated by
trondhjemite
with lesser amounts of tonalite
and gabbro. This raises a general point about long-term changes in the petrology
of calc-alkaline intrusions; whereas modern batholiths are composed of a granite-
granodiorite-diorite or tonalite-gabbro suite, in older examples from the early
Proterozoic and Archaean (that is,
>
1500 Ma before present) a trondhjemite-
tonalite-gabbro suite is more common. So, in the field, ancient and modern
calc-alkaline intrusions may contrast in both their degree of metamorphism and
in their petrology. However, the contrast is also a function of tectonic setting
and erosion level, because the plutons of young island arcs and much of the
