Geology Reference
In-Depth Information
6.4 High-Level Subvolcanic Intrusions
Although some minor intrusions, and many plutonic intrusions, show no clear
geological relationship with surface volcanic activity, many eroded volcanic
areas reveal small intrusions of fine-grained diorite, granodiorite and granite
composition that were probably subvolcanic in character and which may be
termed high-level intrusions. Such intrusions may be approximately circular or
elliptical in outcrop with a diameter of 1 - 4 km, or might form arcuate outcrops
that may have been emplaced by subsidence of an overlying block as part of
a ring intrusion. A classic example of a sub-volcanic system is exposed on the
Ilse of Elba, Italy (Figure 6.13) where a range of laccoliths and small plutons of
various (mainly granitic) composition are found in an inter-connected system.
High-level subvolcanic intrusions are characterised by association with con-
temporaneous volcanic rocks, sharp chilled contacts and a rarity of features such
as pegmatities and aplites that are generally associated with intrusions at deeper
levels. They may also have small cavities up to several millimetres in size lined
by inward projecting euhedral crystals of the host intrusion, or of hydrothermally
Figure 6.13
Coastline exposure and interpreted sub-volcanic system beneath
Elba, Italy. Nested shallow intrusions like these are typical of sub-volcanic feed-
ers (courtesy of Sergio Rocchi).
