Geoscience Reference
In-Depth Information
10.3 Metamorphism
Metamorphic transformations affect all rocks drawn down deep into the Earth by subduc-
tion. They are particularly perceptible where continents collide, i.e. when mountain ranges
form. In contrast to hydrothermal reactions, these are largely, although not exclusively,
dehydration reactions under the effect of temperature or excess CO
2
. Metamorphic facies
correspond to given temperature and pressure ranges and these are usually bounded by spe-
cific mineralogical reactions: the greenschist facies (250-450
◦
C), the amphibolite facies
(450-700
◦
C), and the granulite facies (
700
◦
C) correspond to increasing temperatures at
>
usual pressures; at higher pressures (
>
30 km) we speak of blueschist facies, and at higher
temperatures of eclogite facies.
The nomenclature of metamorphic rocks, based on their mineralogy, is fairly straight-
forward. Gneiss contains feldspar and quartz with variable proportions of other minerals
and is frequently similar to granite in chemical composition. Schist contains little or no
feldspar and is typically composed of quartz and mica; it is of similar composition to clay-
stones. Amphibolite contains amphibole, with or without plagioclase feldspar; it is similar
in composition to basalt. Most dehydrating metamorphic reactions could be described by
dehydration reactions of the type described by
(10.2)
and represented by straight lines in a
plot of ln
P
H
2
O
vs. 1
T
K, but it has become customary to represent metamorphic equilib-
ria as curves on a simple pressure-temperature graph (
T
◦
C,
P
H
2
O
). A metamorphic grid
of this sort is commonly used to determine the temperature and pressure conditions pre-
vailing in ancient metamorphic environments. Oxidation-reduction reactions are also used
to determine temperature and oxygen pressure. Other reactions, finally, do not involve any
fluid, as for example the polymorphic transformation of aluminum silicates:
/
⇔
Al
2
SiO
5
⇔
Al
2
SiO
5
Al
2
SiO
5
(10.16)
(andalusite)
(kyanite)
(sillimanite)
Such reactions, not usually directly involving fluids, are represented by straight lines
in pressure-temperature diagrams (
Fig. 10.6
). The position of the Clapeyron curve of
these different reactions in pressure-temperature space must be carefully calibrated by
experiment or obtained by thermodynamics (see
Appendix C
).
At this stage we should look more closely at the relationships between fluid pressure -
the fluid for simplicity we take to be pure water - and the pressure of the surrounding rock.
As water is about three times less dense than rock, the weight of a column of water is
three times less than the weight of a column of rock of the same height. Near the surface,
the pores are interconnected and interstitial fluid pressure is therefore equal to hydrostatic
pressure. The weight of the water column being about one-third the weight of the rock
column, pressure in the rock matrix (lithostatic pressure) is higher than that of interstitial
water. With depth, the rock is compacted and the pores tend to close, progressively isolating
the pore water a few kilometers beneath the surface. Fluid pressure and rock pressure are
then in equilibrium. When the temperature of this unit is raised by a metamorphic event,
as the water has greater thermal expansivity than rock, it is at higher pressure than the rock
