Chemistry Reference
In-Depth Information
At higher pressure, the presence of zoisite or lawsonite in the domains
of higher P/T gradient (Figure 1.9) divides the blueschist facies into two
sub-facies.
Metamorphism of calcic, dolomitic rocks and skarns
Dolomite is a mineral that becomes unstable very rapidly in the presence of
silicates as the metamorphism becomes significant. Prograde metamorphism
of
dolostones
or
dolomitic limestones
siliceous is marked by a fairly con-
stant succession of isograds:
dolomite
+
quartz paragenesis (
+
H
2
O) is transformed into talc
+
cal-
cite
CO
2
from the greenschist facies up; if water pressure is too low,
tremolite is the first mineral to appear, instead of talc;
+
the association tremolite
calcite occurs in carbonate rocks roughly
simultaneously with that of aluminous silicates in metapelites;
+
then comes the diopside isograd
+
calcite;
forsterite (
calcite) (and humite) appear at a high grade metamorphism,
beyond the isograd disappearance of muscovite in meta-pelites;
+
periclase is exceptional in the regional metamorphism.
Impure marbles and calc-silicate-gneisses
are rocks of extremely var-
ied chemical composition (Figure 1.10) by the relative proportions of car-
bonate (calcite and/or dolomite), silica and clay minerals and by variations
in the composition of clay minerals (Si/Al and Fe/Mg). These variations may
occur at very low scale. The presence of halite in the initial sediments may
be responsible for the appearance of scapolite (“couseranite” a local name
dipyre/marialite in the Pyrenean metamorphism). Phenomena of diffusion
between carbonate beds and pelitic beds (reaction-skarn) during metamor-
phism are superimposed to the initial variability of the sediments: such
diffusion phenomena may be (partly) responsible for the banding of the
calc-silicate-gneisses.
The study of the metamorphism of these rocks is not only complicated
by the chemical complexity, but also by the fact that the metamorphism of
rocks depends not only on temperature but also on the partial pressures of
H
2
O and CO
2
.
It is therefore impossible to present a general picture for the prograde
metamorphism of such rocks. Their interest in the study of regional meta-
morphism remains modest.
Skarns
are metasomatic rocks developed at the contact between two
chemically incompatible media, most often between a carbonate rock, lime-
stone or dolostone (but also basic rocks) and siliceous rock (or magma).
This siliceous rock is often an igneous rock of the family of granitoids.
The example of enclaves in the lava of Vesuvius and the famous Scawt Hill
