Geology Reference
In-Depth Information
Figure 9.5
Strained enclave in marginal tonalite facies flattened with long axis
in plane of internal foliation, Ardara pluton, Donegal (see also Figure 7.22,
Chapter 7).
It is also worth noting that syn-magmatic fabrics can be spectacularly
developed where host magma and enclave are in close proximity. Shearing
textures appear to be an almost inevitable consequence of magma mingling and
at times the fabric development can be quite intense, resembling deformation
fabrics (for example, mylonites) more commonly associated with tectonic
deformation of completely crystalline rock. However, the highly localised
nature of the sheared fabrics rules out an origin through large scale, pervasive
deformation (Figure 9.7).
9.4 Synplutonic Dykes and Sills
Magmatic enclaves sometimes occur in parallel trains or bands, marking contacts
between two different granite types (sheeting) or as remnants of syn-plutonic
dykes. These are mafic to intermediate magmas that intrude into partially crys-
tallised granite (sensu lato) plutons and are a common feature in the major
Batholith terraines of Peru and Chile, and the Sierra Nevada, USA (Figure 9.8).
Synplutonic dykes are important for several reasons. Firstly they confirm that
crustal extension is an important aspect of batholith construction. In some
regions of the Coastal Batholith Peru, individual plutons have undergone over
100% extension as measured by the intrusion volume of synplutonic dykes.
