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350% 0
aragonite +
magnesian
calcite
sea water
lagoonal
supersaturated
water
35% 0
calcite
meteoric and
subsurface waters
3,5% 0
dolomite
0,35% 0
lacustrine waters
rivers
35ppm
1/10
1/3
3/1
30/1
1/30
1/1
10/1
Mg/Ca
Figure 3.36 Field of precipitation of carbonates: calcite, dolomite, aragonite, magnesian
calcite (after Folk and Land, 1975).
Secondary dolomites are the product of a metasomatic transformation
of limestone. These transformations may occur a long time after the primary
deposit. The source of magnesium responsible for these secondary dolomiti-
zation generally remains unknown.
3.4.2.2 Metamorphic rocks
Metamorphism of basic rocks
Many facies of metamorphism (Figure 1.9) are defined by associations result-
ing from the metamorphism of mafic rocks of basaltic composition (calcic
plagioclase
pyroxene). The anorthite is unstable at low temperature and
high pressure, its role is played in these rocks by various calcium silicates.
In metamorphism of low to medium pressure, the following sequence
is observed:
+
￿
in zeolites facies: heulandite/clinoptinolite (Ca, Na) 2 Al 2 Si 7 O 18 · 6H 2 O ),
then laumontite CaAl 2 Si 4 O 12 · 4H 2 O;
￿
prehnite and pumpellyite in prehnite-pumpellyite facies;
￿
epidote in greenschist then in epidote-amphibolite facies.
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