Geology Reference
In-Depth Information
Figure 9.4
Large scale (1 : 200) map of the central part of the Ross of Mull
Granite, Scotland, showing the distribution on enclave types base on mineralogy,
and structural information on deformation fabrics in the host granite.
(Chapter 7, Figure 7.22). To do this requires measurement of the aspect ratio and
orientation (dip and strike) of the long axis of individual enclaves. The degree
of flattening can then be related to the amount of deformation needed to cause
this (one weakness in the method is that traditional strain analysis assumes an
originally spherical inclusion, most magmatic enclaves in undeformed plutons
may be oblate to start with). When combined with other structural data such as
S-C fabrics, deformed enclaves showing asymmetry can also provide information
about the sense of shear. Careful interpretation of the relationship between enclave
and host rock fabric is required, but with careful detailed mapping it is sometimes
possible to use both to determine the shear sense during emplacement where the
magmatic fabric is unanimously pre-full crystallisation (Chapter 7, Figure 7.11).
It is often the case that deformation, hence enclave elongation, is most intense
at or near contact zones, whether at the margin of a pluton with its country rock
(Figure 9.5) or an internal contact where the pluton is sheeted (Figure 9.6).
