Geology Reference
In-Depth Information
(a)
(b)
(c)
subsided central block
uplifted central block
(d)
(e)
Figure 6.5
Schematic diagrams of circular shaped shallow intrusions. (a) A sin-
gle ring-dyke; (b) a set of cone-sheets, idealised to show the features of shape,
outcrop and relationship to horizontally - bedded country rock; (c) block dia-
gram of two minor intrusions emplaced as a result of subsidence of a central
block; (d) cross-section through lopolith; and (e) cross-section through laccolith.
are more commonly in the order of several hundred metres. They may occur
singly or in multiple units, and the overall ring complex may be 5 - 10 km in
overall diameter. Cone-sheets occur as single small intrusions, with outcrops
separated by curved sheets termed 'screens' of country rock. By contrast to
ring-dykes
, the form of cone-sheets must indicate emplacement by
uplift
of the
central conical block and this indicates uplift of the roof of the chamber dur-
ing intrusion. Ring-dykes are much more abundant than cone-sheets, but the
latter have been originally mapped out as common features of the Scottish Ter-
tiary volcanic province. Because the mechanism of emplacement of ring-dykes
involves subsidence of a central block (Figure 6.5a), such ring intrusions are
sometimes associated with relatively flat-lying tabular minor intrusions emplaced
over the block (Figure 6.5c). Ring-dykes and cone-sheets may be medium- or
coarse-grained in texture and are more commonly of mafic rather than felsic
mineral composition. It should be noted that recently many of the circular bod-
ies attributed to be either ring dykes or cone sheets have been shown to be
more like lopolith/laccolith type intrusions (for example, Figure 6.5d,e, see also
Section 6.4), where the tips of the intrusion have been exposed at the surface,
and careful mapping of circular type intrusions is required to fully assess the
type of intrusion present. Where country rocks have been blistered up by the
