Chemistry Reference
In-Depth Information
andalusite/
kyanite/silimanite
Al - Na - 2 Ca - 3 k
staurotide
chloritoid
fixed T, P, Pf
fixed Na/Ca PI
exces of quartz,
muscovite and fiuid
cordierite
gamet
chlorite
Mg
Fe
-2
biotite
Figure 3.28
Thompson's diagram (1957).
Fe and Mg must take into account only the aluminum that is not contained
in muscovite (Al-3 K) (nor in the plagioclase).
This reasoning leads so to reduce the discussion of metamorphic min-
erals in metapelites to a Fe/Mg/Al-Na-2Ca-3K diagram: the diagram of
Thompson (1957) (Figure 3.28). This is a convenient way to represent the
various possible parageneses at a given metamorphic grade and thus to
define precisely metamorphisc facies.
The rigorous reasoning based on the phase rule indicates that the dia-
grams constructed to represent various metamorphisc facies, must satisfy
the following conditions: fixed physical conditions (temperature, pressure,
fluid pressure), fixed composition of the plagioclase (Na/Ca in plagioclase),
excess of quartz, muscovite and fluid. Any change in one of these param-
eters will more or less change the diagram.
Note also that this diagram does not take account of manganese, an
element that may be important in garnet.
A suite of (prograd) metamorphic facies may be so represented by a
series of Thompson's diagrams. Comparison between these diagrams allows
the discussion of the reactions occurring at the isograds.
There are many well documented examples that show the wide variety of
facies and their succession. It is not sensible to theoretically define a standard
succession of facies and isograds. Each case deserves a separate study.
