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(a)
(b)
(c)
(d)
(e)
Figure 7.3 Types of igneous contact, as seen in cross-section. The intrusive rock
is blank and the country rock is lined or stippled (f). The contact is straight in
(a), jagged in (b), blocky in (c), blocky and matched in (d) and sinuous in (e). (f)
shows a diffuse contact (dark stipple) between an igneous intrusion (blank) and
the country rock (light stipple). These contact relationships may be observed on
the centimetre to kilometre scale (Figures 7.4 and 7.13).
(f)
examination as they often yield important clues not just about timing but also
magma rheology (see Chapter 9).
7.2.1 Pluton geometry
It was thought that the plutons comprising the Peruvian batholith (and by exten-
sion others elsewhere) extended to great depths. However, in Peru recent gravity
data have shown that each pluton is a relatively thin sheet, extending downwards
to depths of a few kilometres at most (Figure 7.5). Other studies from around
the world have provided additional evidence in support for plutons (and by defi-
nition the batholiths they comprise) as relatively thin, broadly tabular structures
as opposed to the more traditional textbook view of deeply penetrating blobs.
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