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because they are largely composed of
granites and granite gneisses separated
by areas of 'greenstone', which consist
of low-grade metamorphic volcanic
rocks and associated sediments such
as greywackes ( see Chapter 12).
Figure 9.7 illustrates the main fea-
tures of an idealised granite-greenstone
terrain. The granitic bodies occupy
dome-shaped or rounded outcrops
separated by the synclinal greenstone
sequences, which tend to form cuspate
margins projecting into the granites.
The 'granites' are composed partly
of older gneissose rocks, of predomi-
nantly granitic composition, and partly
of granites intruded into them; they
often exhibit a margin-parallel folia-
tion, similar to that shown by the intru-
sive granites described in Chapter 8.
However, the greenstone sequences in
many cases dip away from the granite
margins and sometimes appear to be
unconformable on them, suggesting a
basement-cover relationship. In the late
1940s Eskola described such bodies in
Finland as ' mantled gneiss domes' .
The interpretation of these granitic
bodies as gravity-driven domes and
diapirs has been controversial. However,
the density difference between granite
and basalt - at about 0.2 Mg/m 3 - is
not too different from that between
salt and most other sedimentary rocks
at around 0.3 Mg/m 3 ; moreover, solid
flow of rock at depth is required by
considerations of isostatic behaviour
( see Chapter 2). A series of famous
experiments by Hans Ramberg in the
1960s using artificial materials in a
centrifuge in order to carry out the
experiments on a workable timescale,
produced structures remarkably similar
to those seen in nature, so there are
good grounds for believing that solid
flow of warm rocks at depth in the crust
can explain the types of granitic struc-
tures that have just been described.
9
78
79
B
Figure 9.7 Diapirism of granite-gneiss basement.
Idealised map and vertical cross-section along
line A-B of an imaginary Archaean granite-
greenstone terrain showing diapirs and domes
of granitic basement 'intruding' into a synclinal
cover consisting of lower and upper greenstone
sequences. The basement consists partly of
granitic gneiss and partly of intrusive granite. The
older foliation in the basement is discordant with
the greenstone cover, which is unconformable
upon it. There is a newer foliation parallel to
the margins of the up-domed basement. Both
basement and cover are cut by a younger,
undeformed granite.
older granite basement
granite dome
greenstone
belt
older
granite-gneiss
younger
granite
upper
sequence
younger
marginal
foliation
older
granite-gneiss
lower sequence
older
foliation in
gneisses
10km
A
B
A
younger
granite
'greenstone'
sequence
older
granite
basement
granite-gneiss
basement
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