Geology Reference
In-Depth Information
Andalusite
Hornfels
Diopside
Hornfels
Tremolite
Hornfels
Chlorite
Schist
Kyanite
Hornfels
0
2
1
0.5
1.5
km
Granite intrusion
Shale
Limestone
Metamorphic
aureole
Metamorphic
aureole
Figure 7.24
Schematic example of a metamorphic aureole surrounding a granite
body with a shale protolith on one side and a limestone protolith the other.
granitoid intrusions 10 - 15 km in diameter, where the margins dip at 60 - 70
◦
below the surface. Generally, the aureoles of basic intrusions such as dykes and
sills are much smaller; for example many large sills with thicknesses of up to
several hundred metres may show metamorphic effects up to only a few metres
from the contact. The textures and mineralogical changes developed by contact
metamorphism of common rock-types are summarised in Table 7.2 and high-
lighted in Figure 7.24; these characteristic minerals provide a basis for mapping
and studying aureole zones. For further details of high temperature-low pressure
metamorphism of sedimentary sequences, refer to Fry (this topic series).
7.9 Summary of the Field Characteristics
of Granitic Complexes
Granitic rocks are the final expression of extreme differentiation of the solid
earth. These low density, silica rich materials comprise extensive tracts of earths
middle to upper crust and so far appear unique in the solar system. Problems
surrounding the origin and emplacement of granitic rocks include the rate and
mode of transport from the source region to site of emplacement, and how
space is made for the invading magmas. Recent field and theoretical studies
suggest that plutons are not the giant, amorphous masses once thought but are
instead relatively thin sheets comprised of multiple pulses of magma. In this
way, plutons and batholiths are built up incrementally, perhaps in relatively
short bursts of emplacement activity. If so, then evidence for this should be
preserved in the field in the form of sheeted or pulsed intrusions and evidence
for internal contacts should be pursued judiciously.
Calc-alkaline plutons comprise different proportions of coarse grained rocks:
granite, granodiorite and tonalite, with subordinate diorite and gabbro. Plutons
may show evidence of emplacement by stoping; that is the fracturing of the
roof and incorporation of country rock xenoliths. Together, a large number
