Igneous intrusions - Introducing Tectonics, Rock Structures and Mountain Belts - page 70

Stocks and diapirs (Figure 8.1E, F)
are much smaller bodies than batho-
liths, and are often given the generic
name of plutons . They typically have
a broadly circular or elliptical outcrop,
10-20 km in diameter. Some high-level
plutons can be shown to have arisen
from a larger body such as a batholith
at depth; these are usually referred to
as stocks . The line of granite stocks
extending across SW England from
Dartmoor to the Scilly Isles is a well-
known example of a series of stocks
fed by a batholith, which is deduced
from gravity measurements to lie at
depth beneath them. Other plutons,
at deeper crustal levels, appear to
have combined to create batholiths.
Many plutons that have been
studied in detail are composed of
several separate intrusions, each
slightly different in composition,
which form a series of concentric rings
or partial rings, becoming younger
inwards. This arrangement suggests
that the pluton has gradually become
larger over time by successive injec-
tions of magma as it moves upwards
through the crust, like the inflation
of a balloon. Such a process is known
as diapirism and the resulting pluton
is termed a diapir (Figure 8.1F). Not
all diapirs need have been entirely
liquid when emplaced; because of the
relatively low density of granite, the
effect of the smaller gravitational load
compared to that on the surrounding
rocks may be sufficient to cause the
granite body to rise to a higher level
in the crust. This process requires
the host rocks to be squeezed aside
in order for the pluton to proceed
upwards, so that diapirs must be
regarded, to some extent, as forceful

intrusions, and the host rocks should
show the results of the resulting
deformation around their margins.
Stock-like plutons that do not
display signs of marginal deforma-
tion are more likely to have resulted
from permitted, rather than forceful,
emplacement. Some stocks, such as
the well-studied Devonian granite
plutons of the Central Highlands of
Scotland, appear to be of this type.
The famous Glencoe ' cauldron sub-
sidence ' is an instructive example
of how this process may work. The
Glencoe structure (Figure 8.4A) is a
circular fault surrounding a down-
faulted area of Devonian lavas and
underlying basement rocks, interpreted
as a volcanic caldera. The central
depressed area is surrounded by a
partial ring of granite that has risen
up along an outwardly dipping ring
fault, as shown in Figure 8.4B. In this
example, the space for the granite has
been created by the depression of the
cylindrical block, which has sunk into
the granite beneath. The granite of
the surrounding ring intrusion would
have been responsible for much of
the vulcanicity within the cauldron.
Several granite stocks in the same
general area, including the Cruachan-
Starav pluton to the immediate south-
west (Figure 8.4A), are composed of
several ring intrusions surrounding a
central circular body. The structures
in the host rocks continue right up to
the margins of these bodies without
any sign of deviation, suggesting that
they are permitted intrusions. They are
thought to have resulted from a similar
process to the Glencoe structure but at
a deeper level, as shown in Figure 8.4C,
where a cylindrical block has sunk

beneath the present exposure level,
allowing successive batches of granitic
magma to fill the space created above
the subsiding block. This process is a
large-scale example of stoping , where
blocks of host rock are broken off the
roof of an intrusion and sink into the
magma (Figure 8.4D). Many granites
contain abundant inclusions of host
rock, especially around their margins,
indicating that this process may have
played an important role in their
emplacement. The main outer granite
of the complex, the Cruachan granite,
has also invaded the Glencoe cauldron,
indicating that the latter has sunk
into the Cruachan magma chamber.
The margin of this granite is much
more irregular than that of the inner
granites and shows extensive veining,
melting and assimilation of host rock.
A good example of a more deep-
seated pluton, interpreted as a diapir ,
is exposed a short distance west of
the Cruachan-Starav complex, on
the north-west side of the Great Glen
Fault (Figure 8.5A). This body, the
Strontian pluton, is a forceful intrusion
composed of three main granodior-
ite components, and has deformed
the Moine metasediments around
its margins. The flow foliation within
the two outer granodiorites defines a
synclinal shape, which can be inter-
preted as the lower half of a diapir, as
shown in Figure 8.5B; this shape has
then been distorted by further inflation
caused by the intrusion of the inner
granodiorite. The intrusion is located
at the intersection of the Loch Quoich
lineament and the Great Glen fault,
which is thought to have provided an
extensional gap through which the
granite magma could flow upwards