Geology Reference
In-Depth Information
pluton comprising an outer quartz monzodiorite surrounding a central core of
granodiorite. In plan view its geometry is broadly circular but the outer facies
shows evidence for pure flattening (plane strain) that increases in intensity
towards the margin. The interpretation is that the pluton was emplaced as several
magma pulses
in situ
, causing the intrusion to swell outwards on all directions,
pushing and deforming magma against the surrounding country rocks. This type
of emplacement mode is referred to as
ballooning
. S-C fabrics and flattened
magmatic enclaves (Chapter 9) record the kinematic and strain history of pluton
emplacement (Figure 7.22). The country rock (Dalradian metasediments of the
Appin Supergroup) are contact metamorphosed, with an inner zone of silli-
menite, garnet and cordierite, and an outer (lower temperature) zone containing
staurolite, kyanite and andalusite.
7.7 Distinctive Granitoid Textures
7.7.1 Porphyies
One special kind of granitic rock whose location within plutonic terrains often
points to mineralisation are termed porphyries. These rocks are recognised by their
highly porphyritic nature, with large, mainly euhedral phenocrysts embedded in
a fine (quenched) groundmass. Their overall composition ranges from tonalitic
to leucogranitic and analysis of the groundmass can be extremely useful in
determining the primary magma composition, something extremely difficult to
establish in the medium to coarse grained rocks that comprise most granitic
igneous terrains. These rocks are important due to their associated mineralisation.
For example, at El Tienetie, Chile, one of the world's largest copper mines, the
metal ore is hosted in granite porphyry with an 'adakite-like' chemical signature.
Here are brief descriptions of several distinctive, though rather uncommon,
textural varieties that may be encountered in the field, among granitoids of
various types.
Graphic granite
refers to an inter-growth texture which develops in some
quartz-alkali feldspar pegmatites. Aligned, rod-like, or triangular light grey blebs
of quartz are enclosed within large feldspar crystals and take on the appearance
of runic characters. If the quartz-alkali feldspar intergrowths are confined to
smaller crystals, perhaps in the groundmass of a porphyritic granitoid, then the
texture is
granophyric
(and is often only visible in thin section). Rocks with gra-
nophyric texture are usually fine-grained porphyritic granodiorites and granites
('high-level' granites) and they occur most frequently as minor intrusions.
Orbicular granite
contains large ovoid bodies (orbicules), usually up to sev-
eral centimetres in diameter, in which the components are arranged in concentric
layers of leucocratic quartz - feldspar-rich material alternating with mafic layers
consisting primarily of dark hornblende and/or biotite. Each orbicule contains a
core, commonly an igneous xenolith fragment but sometimes composed of the
