Geology Reference
In-Depth Information
Figure 7.25
Elongate crystals of andalustite (chiastolite) in contact metamor-
phosed (hornfelsed) slate.
of individual plutons (each 1 - 50 km in cross-section at outcrop, and roughly
tabular), may comprise a batholith. When examining plutons, it is necessary to:
1. Describe and determine the relative proportions of rock types present; use
Table 7.1 if a preliminary tectonic interpretation is required.
2. Examine contact relations to determine age relations.
3. Study the nature and distribution of enclaves (see Chapter 9).
Examination of individual plutons may reveal small-scale layering or lami-
nation (0.01 - 10 m), or zoning on a pluton scale. These may be due to crystal
accumulation under gravity or shearing during flow. Plutons may also contain
igneous rocks derived by multiple injections of different magmas between which
contacts may be recognised, with associated magmatic enclaves.
Important activities in the field are to:
1. Describe the nature and orientation of any planar igneous foliation and iden-
tify timing of formation as either magmatic (pre-full crystallisation) or post
magmatic (crystal plastic strain fabric).
