Geology Reference
In-Depth Information
7
GRANITIC COMPLEXES
7.1 Introduction
Granitic rocks are key components of the continental crust, and have over
geological time helped keep the landmasses of the earth stable and elevated
above mean sea level. Also referred to more generally as
granitoids
, they form
part of a compositional spectrum ranging from basaltic (SiO
2
circa 50 wt%) to
highly acidic (SiO
2
70 wt%) medium- to coarse-grained rocks (cf. Chapter 3).
With increasing SiO
2
content, the plutonic rocks of this spectrum progress
from gabbro-tonalite-granodiorite to granite (senso stricto) and are emplaced
entirely beneath the surface. Their volcanic (extrusive) equivalents form the
complementary association basalt-andesite-dacite-rhyolite, sometimes referreed
to as BARD. Together these rock types define the
calc-alkaline
association, a
distinctive petrological and geochemical typology formed at destructive plate
margins (Chapter 1).
As with all plutonic rocks of the calc-alkaline association, the parent magmas
cool and crystallise at varying depths inside pre-existing crust. For granites this
crust is exclusively continental and this simple fact distinguishes them from
other intrusive members of the calc-alkaine association, notably gabbros, which
occur also within oceanic crust. One major goal in understanding how plutonic
magmas were emplaced is to work out as accurately as possible their intrusion
depth. Coarser grain size is one indicator of depth, but may not always be
reliable as other factors including pressure quenching and water content of the
magma can result in sudden rapid growth of large crystals. The relative amounts
of intrusive rock and country rock available for study depend strongly on the
level of erosion (Figure 7.1).
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7.2 General Features and Occurrence
Most of the rocks in the calc-alkaline association are
granitoids
, that is they are
granites, granodiorites and tonalites all containing
20% quartz, with feldspar,
mica and amphibole in various combinations (Chapter 3, Figures 3.10 and
3.11). They occur together with varying amounts of diorite and gabbro. Over
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