Chemistry Reference
In-Depth Information
rich in iron, at the same time the ratio of magnesium/iron increases slightly.
The fact that the manganese content of garnet decreases in prograde meta-
morphism, is due to the fact that the amount of garnet increases and that the
manganese enter into other phases (biotite and probably ilmenite). At higher
metamorphic grade (deep amphibolite facies and granulite facies), garnets
are poor in manganese, and become more and more magnesian, with a very
strong increase of the magnesium/iron ratio.
When garnet appears in an association that cannot be taken in account
by Thompson's diagram, like, for instance, an association garnet-stauro-
lite-andalusite-(biotite-muscovite) in the andalusite zone, it appears that
the garnet does not fall within the previous trend, but is substantially richer
in manganese than “normal” garnets. In this case (only), it may be accepted
that these garnets are stabilized by manganese.
In the case of different types of metamorphism, the trends of the evolu-
tion of garnets with increasing matamorphism, are similar but shifted. With
an increasing ratio P/T, this shift of the main trend is shifter both towards
stronger Mg/Fe ratios, and lower contents of manganese.
Thompson's diagram indicates that the presence of garnet is mainly
controlled by the Fe/Mg ratio of the host rock. Actually, at a given meta-
morphic grade and type, the presence of garnet is determined the Fe/Mg/Mn
ratios. We can define for a given type of metamorphism, the fields of the
rocks where garnet is present or not, as a function of this parameter. The
diagram FeO/MgO
vs
FeO/MgO in whole rock (Figure 3.33) compares the
boundaries of the field of rocks containing garnet and rocks without gar-
net in different metamorphic domains: low pressure metamorphism (Agly,
Albères), intermediate low pressure metamorphism (Canigou massiv, south-
ern Montagne Noire), high pressure metamorphism (Vanoise massiv) and
very high pressure metamorphism (Gran Paradiso massiv). At low pressure
(Agly massiv), garnet is restricted to rocks rich in both iron and manganese.
A higher pressure garnet appears in rocks less rich in iron and/or manganese.
This result is in accordance with the data on the shift of the tie-lines in the
Thompson's diagram as a function of pressure (pictured in Figure 3.31).
Calcium in the garnets of non-calcareous metapelites
In non-calcareous pelitic rocks, calcium is essentially contained in the clastic
plagioclase. Calcic plagioclase are not stable at low temperatures and, in the
condition of a weak metamorphism, the garnet is the main bearer of calcium.
The CaO contents in garnet in prograde metamorphism follow a similar
pattern to that of MnO contents. All the individual garnets show a similar
zonation from a more calcic core to a less calcic rim and there is a general
trend of decreasing CaO content with increasing metamorphuic grade. In
the previous example of Saint-Ponais (Montagne Noire), the core of the
garnets at the
+
garnet isograd, have a content in CaO of about 6.5-7 wt%
