Chemistry Reference
In-Depth Information
granulite sub-facies: The pyroxene granulites sub-facies is characterized by
the complete absence of (primary) hydrous minerals.
Pyroxene-hornfels facies (Eskola, 1915) differs from the granulite facies
by the absence of garnets of the almandine - pyrope family.
Sanidinites facies (Eskola, 1920) (or sanidine facies) is characterized
by the presence of sodi-potassic mixed feldspar (sanidine) of very high
temperature (and low pressure). The association of mullite
+
sanidine
+
tridymite (often altered to quartz)
+
cordierite
+
glass in metapelite is
characteristic.
Eclogite facies (Eskola, 1920) is defined by the critical association of a
pyrope-rich garnet and a jadeite-rich pyroxene (omphacite) (
+
quartz). Pla-
gioclases are totally absent in this facies.
The concept of metamorphic facies (the term “mineral facies” would be
more appropriate) may easily be extended to igneous rocks: a granodiorite
crystallizes in the amphibolite facies, charnockite in the granulite facies and
a basalt in the pyroxene hornfels or sanidinites facies ...
The grade of metamorphism may sometimes by assessed in a less precise
and rigorous way, by using the expressions low, medium or high grade met-
amorphism (Winkler, 1965-1979). This concept is actually close to older
concepts of epizone, mesozone and catazone of Grubenmann (1904). In
practice, in the case of metamorphism of low to medium pressure, we refer
here to low grade metamorphism (or epizonal) up to the biotite isograd, of
medium grade metamorphism (or mesozonal) from the isograd biotite in up
to the isograd muscovite out, the limit of high grade metamorphism can be
taken at the disappearance of primary muscovite.
Very generally, this concept of metamorphic facies is interpreted in
terms of temperature - (lithostatic) pressure (considered equivalent to the
depth) (Figure 1.9).
This diagram, very commonly used, however, contains some ambigui-
ties. Some reactions do not involve water, others depend on water pressure.
It is often assumed that the fluid pressure is essentially the water pressure
and that it is equal to the lithostatic pressure. This last point is sometimes
questionable and deserves at least to be discussed. On the other hand, the
assumption that the fluid is essentially hydrated is certainly not proven (and
probably wrong) in the granulite facies, or maybe even from the partial
melting isograd.
1.3.2 Chemical factors
The chemical factors can be classified into parameters more or less linked to
the fluid phase (or at least conveyed by it) and parameters directly related to
the chemical composition of the rock as we it see today.
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