Chemistry Reference
In-Depth Information
andalusite/sillimanite become increasingly more ferroan when the tempera-
ture increases. Similarly, the biotite of the paragenesis of biotite-cordier-
ite-sillimanite/andalusite become increasingly magnesian with increasing
temperature.
In order to show the variations of the topology of Thompson's diagram
with pressure, let us compare two rocks of the same chemical composi-
tion that were formed at equivalent temperatures. Both are from Montagne
Noire, but one comes from the region of Murat-sur-Vèbre, a domain of low
pressure metamorphism (with the isograds:
silli-
manite, and where garnet or staurolite are practicallyabsents); the other one
comes from the region of Labastide Rouairoux, a domain of medium pres-
sure metamorphism (with the isograds:
+
cordierite
+
andalusite
+
+
garnet
+
staurolite
+
kyanite
+
sil-
limanite) (Figure 3.31).
Al-Na-2Ca-3k
Cd
Gt
Chl
Chl
Fe
Mg
tie lines between actual minerals
(analyses with the micoprobe)
Murat
northern slope of
Montage Noire
(low pressure)
Labastide
sourthen slope
of Montagne Noire
(intermediate pressure)
whole rock analayses
Q-Bt-Ms-Gt-Chl
(garnet zone; Labastide)
Q-Bt-Ms-Cd-Chl
(cordienite zone; Murat)
Biot
Figure 3.31
Shifting of the tie lines with pressure: two regions with different types of
metamorphism, low pressure and intermediate pressure.
