Geology Reference
In-Depth Information
Classic
'saucer-shaped'
sill
SILL
transgression
fingers
(e.g. Karoo and
North Atlantic)
overlap
curved tips
DYKE
offlap
offset
Figure 6.2
Idealised three-dimensional forms of dykes and sills. See text for
discussion.
It therefore has the same form as a vein (described in Chapter 2). A sill is a tab-
ular intrusion that parallels such horizontal or gently-dipping planar structures.
Dykes are therefore discordant to structures in the country rock whereas sills
are generally concordant between beds of layered rock but may show small side
transgressions and/or larger dyke-like transgressions (Figure 6.2).
These definitions raise complications; in deformed terrains it may be difficult
or impossible to determine whether a given tabular intrusion was emplaced
as a dyke or as a sill (however, extensive sills are commonly emplaced only
within well-stratified sedimentary sequences). Many extinct and eroded vol-
canic areas are characterised by the occurrence of an intricate arrangement of
dykes and sills and, in such cases, these minor intrusions form the plumbing
system to the volcano channelling the magmas that were erupted. The com-
plex plumbing underneath many volcanic systems allows the separate evolution
and mixing/recycling of magma batches, which can be recorded in the eruptive
products, in crystal populations within the intrusions and by the juxtaposition
of different magma intrusions.
Minor intrusions have mineral compositions corresponding to most other
igneous rock types. They range from basalt through andesite and dacite to
rhyolite in composition and may be named after the volcanic equivalents
(cf. Tables 3.5 and 4.1). Thus a fine- or medium-grained minor intrusion of
