Geology Reference
In-Depth Information
inflation of a shallow intrusion a laccolith results (Figure 6.5e), which again are
commonly circular in plain section.
6.2 Working Out Emplacement History
Many of the sheet-like intrusions have been emplaced by the amalgamation of
fingers as the intrusion propagates into the country rock (Figure 6.6). In section,
(a)
(b)
(1) Lateral propagation of proto-sills/dykes
(2)
(1)
proto-sills/dykes
(3)
(2) Overlap of proto-sills/dykes and bridge formation
regional
magma flow
sections
main intrusion
body
(3) Bridge breaking and inflation
The formation of tabular sheet intrusions (sills/dykes) by the
propagation of fingers at the front of the intrusion. This can
result in Bridge structures (see also photo) which can be used
to help interpret the direction of propagation
(c)
Figure 6.6
Emplacement of igneous sheets. (a) Formation of tabular sheets by
propagation of fingers. (b) Stages of linking up of fingers in section. (c) Thin
basalt dyke cutting into gabbro with bridge structure, Isle of Skye, Scotland (Note
chilled margin and conjugate tips).