Geoscience Reference
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the base of the continental crust, lighter liquids can pass straight upwards to the
surface, but most liquids are probably trapped by their density, the continental
crust being less dense than the magma.
Rising magma carries heat with it. Hot magma collecting at the base of the
crust loses heat to the overlying continent and coals and fractionates, precipitating
minerals such as clinopyroxene together with garnet, olivine or orthopyroxene
(depending on the depth of the melt and on its temperature, composition and
percentage of water). After fractionation, the lighter liquids rise to the surface,
most probably as basalt and basaltic andesite magmas. However, the transfer of
heat into the base of the continental crust also produces melting in the crust.
High-temperature partial melting of deep continental crust at temperatures of
about 1100
◦
C produces tonalite liquids, which are silica-rich melts that can rise
to the surface and erupt as andesites. At somewhat lower temperatures (about
1000
◦
C), in the presence of more water, the product of melting is granodiorite.
Many of the large granitic intrusions of the continents above subduction zones
are granodiorite. In the aftermath of large-scale continental collisions (such as
in the Himalayas), there is often overthrusting of continental crust with partial
melting of the underlying slab. After partial melting, the residual material left
behind in the deep continental crust is granulite, which is depleted of all its low-
temperature-melting fractions. This depletion includes the removal of the heat-
producing elements (U, Th and K), which are carried upwards with the rising
granitoid liquids. Because these heat-producing elements are carried upwards,
the continents are self-stabilizing: heat production is concentrated at the top,
not the bottom, of a continent. This process, which has moved heat production to
shallow levels, has had the effect of reducing the continental temperature gradient,
making it more difficult to melt the crust (see Chapter 7, Problem 13).
It is chemically unlikely that granitic liquids are produced directly from partial
melting of mantle peridotite or subducted oceanic crust; otherwise we should find
granitoids in the oceanic lithosphere. The main geographic location of granitoids
is above subduction zones and in continental collision zones, modern or ancient,
which implies that granitoid generation is strongly linked to the processes of
plate tectonics. In the modern Earth, it is probable that most granitoids are gen-
erated in the presence of water. In the Archaean, when the mantle may have been
hotter, tonalites appear to have been more common, having been generated at
1300
◦
C from dry crust. Today, under cooler and wetter conditions, melts are
granodioritic.
The upper continental crust above a subduction zone is characterized by large
granodioritic (granitoid) intrusions. Above these are andesite volcanoes, which
erupt melt that originated in the mantle above the subduction zone, but which
has fractionated on ascent and perhaps been contaminated by material derived
from the continental crust. However, the broad similarity of volcanism in island
arcs, where in some cases little continental material is present, to volcanism
on continents above subduction zones indicates that magma must be produced
