Chemistry Reference
In-Depth Information
FeO/MnO
whole
rock
100
80
60
40
20
0
FeO/MgO
whole rock
0,5
1
1,5
2
2,5
Figure 3.33
Diagram FeO/MgO
vs
FeO/MnO in whole rock - comparison of different
types of metamorphism
(data by Fonteilles and Guitard, 1971, Chopin, 1979,
Demange, 1982)
.
and about 4 wt% at their rim; garnets in the sillimanite
muscovite zone
have CaO contents of 2-1.5%. The anorthite content of plagioclase grows
correlatively.
+
High grade metamorphism of pelitic rocks
At high temperatures, muscovite is destabilized into sillimanite
+
K-feld-
spar
water. This is often called the “second sillimanite isograd”, the
first one being the polymorphic transformation andalusite
+
=
>
sillimanite)
(Figure 1.9).
At higher temperatures, aluminous biotites is gradually destabilized by a
continuous reaction, into garnet
water. Garnet and sillimanite
therefore characterized paragneiss of pelitic origin at high grade metamor-
phism. Kinzigite are paraagneisses containing biotite-sillimanite-garnet-
cordierite-quartz-feldspar-plagioclase (and graphite) that are derived from
peraluminous shales. Prograde metamorphism of metapelitic rocks is no
more emphasized by isograds but by gradual changes in composition of
minerals. A very high grade, in granulite facies, even biotite finally disap-
pears. Spinel (
+
K-feldspar
+
+
quartz) or sapphirine (
+
quartz) can develop at the expense
of cordierite.
