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Figure 9.3 Type example of contemporaneous brittle-ductile magma mingling.
Blocks of angular mafic enclaves appear to be wedged apart by invading, fluid
acid magma (brittle-like) while occupying the same space as cauliform magma
pillows indicative of more ductile, fluid-fluid interactions, Maine, USA.
in the southern and south-western sector of the pluton and range in size from
less than 5 cm to several meters in diameter. They are either ovoid or ellipsoid
in shape, with aspect ratios between 2 : 1 and 50 : 1. From careful field obser-
vation, the magmatic enclaves can be divided into two distinct populations:
(i) megacrystic and (ii) megacryst free varieties. The megacryst free enclaves
are composed largely of amphibole (
20%), albite twinned
interstitial plagioclase ( 45%) and show recrystallisation textures. Enclaves can
be further subdivided into two groups depending on the megacryst assemblage:
(i) plagioclase
30%), biotite (
+
+
±
±
alkali
feldspar. Mantling of the megacrysts by coronas of plagioclase or amphibole
quartz and (ii) plagioclase
hornblende
quartz
±
biotite is common. From this it has been possible to map out on a large scale
using an evenly-spaced 1 m grid, the spatial distribution of enclave type in the
central region of the pluton (Figure 9.4).
9.3.2 Magmatic enclaves as strain markers
Magmatic enclaves in deformed plutons may become elongated parallel to the
plane of foliation and can, with some caution, be used as strain markers in the same
way rigid inclusions are used in tectonically deformed rocks (for strain analysis,
see McClay this topic series). The Ardara pluton, Donegal is a good example of
where enclaves have been used to estimate strain during magma emplacement
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