Geology Reference
In-Depth Information
6
SHALLOW-LEVEL INTRUSIONS
As magma makes its journey from its source (for example, mantle melting) to
the surface it must travel through a pathway we may consider as the 'magma
plumbing system' (Figure 6.1). This pathway may be simple or complex and at
any stage in the field you may be looking at a particular aspect of such a system
depending on the depth of erosion and or the relationships of the igneous rocks
you are considering.
When faced with intrusions in the field you will need to record some key
valuable observations to help interpret their relationships:
•
Contact relationships with country rocks; is this sharp-irregular, is there a
chilled margin, are we intruding through sediments-metamorphic or even
igneous host rocks?
•
Contact relationships with other intrusions; cross-cutting relative age infor-
mation, contacts (again sharp or irregular)?
•
Internal textures; are there crystals and matrix. If there is a clear crystal
population what are its textural features (crystal size, shape, internal zoning,
and so on), what is the size of the crystalline matrix, how do textures vary
from contacts into the intrusions, is there any evidence for vessiculation,
and so on. Are there more than one magma present, for example magma
mingling (see Chapter 9).
Shallow level intrusions also take on many forms which will become apparent
as you map them out (see Figure 6.1). The most important forms are; Sills,
Dykes; Laccoliths and plugs.
In this section we will consider the main types of shallow level intrusions
and the key field relationships you will need to record in order to interpret them
in the context of the magmatic system.
6.1 Sill and Dykes
The commonest form of minor intrusions are termed
sills
and
dykes
(Figures 6.1
and 6.2). A dyke (US, dike) is a tabular intrusion that cuts across horizontal or
gently-dipping planar structures, such as stratification, in the surrounding rock.
