Geology Reference
In-Depth Information
(a) Pre-full crystallisation fabric
(b) Crystal plastic strain fabric
Figure 7.11
(a) Pre-full crystallisation fabrics are defined by passive rotation
and parallel to sub parallel alignment of tabular and platy minerals. (b) Crystal
plastic strain fabrics (formed very late stages of crystallisation or subsequently
after complete solidification) show evidence of straining and change in particle
shape (necking or rounding of feldspars, kinking and warping of mafic minerals).
granitic rocks and other plutons that have been exposed to progressive episodes
of deformation that become more brittle-like with time (Figure 7.12). A good
example of structural development in a granitic pluton is seen in the Miocene
Cordillera Blanca batholith, Peru. Here the granitic magma was emplaced along
a major crustal lineament and the deformation fabrics preserve a history of
magmatic, post magmatic and overprinted tectonic deformation. The relationship
between the internal fabric and structure development inside the pluton and its
bounding faulted margin is shown in Figure 7.13.
7.5 Internal Contacts
It is now known that many large granitic bodies, once thought to be made of
a giant lump of broadly homogeneous material are in fact comprised of sheets
defined by changes in grain-size or composition. Where the temperature con-
trast between two cross-cutting magmas is large, their contact will often appear
sharp or brittle-like in nature. Chilled margins may also develop. Where, how-
ever, interaction comes about in magmas with similar temperatures then their
contact may be more diffuse. Here, contact margins may be gradational and
mineralogical variations between sheets can be subtle. For example, a clear dis-
tinction between layers of tonalite alternating with granodiorite may be difficult
