Chemistry Reference
In-Depth Information
temperature, pressure, fluid pressure, fixed composition of the plagioclase.
The phase rule indicates that any change in any of these parameters induces
a different pattern. A significant change in temperature, pressure, etc. pro-
duces the appearance or disappearance of a new phase: this is what occurs
at an isograd; and the topology of the corresponding diagram is changed. A
moderate change of these parameters does not change the topology of the
diagram but shifts the tie-lines that connect the associated minerals.
Consider for example the paragenesis biotite-chlorite-garnet-(quartz-
muscovite), which is critical for the garnet zone in Saint-Ponais (Mon-
tagne Noire) (Figure 3.30a). Between the garnet isograd and the following
staurolite isograd, as the temperature increases the compositions of these
associated minerals varies and become progressively more magnesian with
the increasing temperature: the diagram gradually changes by shifting the
tie-lines.
Similarly if we consider the association biotite-chlorite-cordierite-
(quartz-muscovite) in the cordierite zone (between the cordierite isograd
and that of andalusite) in the Agly massive (Eastern Pyrenees) (Fonteilles,
1970), in the same way, with increasing temperature, the tie-lines shift: the
associated minerals become progressively richer in iron (Figure 3.30b).
Note that, beyond the andalusite isograd, these trends of shifting of the
tie-lines are inversed: biotites of the paragenesis biotite-garnet-staurolite or
garnet zone
(Southern slope of Montagne Noire)
AI - Na - 2 Ca - 3 K
Cordierite Isograd
(Agly massiv)
AI - Na - 2 Ca - 3 K
Cordierite
chlorite
garnet
chlorite
Fe
Mg
Fe
Mg
XMg biot
44,80
45,40
45,94
47,49
XMg Gt
8,49
9,47
10,94
12,57
XMg chl
49,72
49,88
49,96
53,38
XMg biot
55,76
52,41
50,76
45,62
XMg chl
54,30
51,52
49,60
44,70
XMg Cd
n.d.
n.d.
n.d.
n.d.
at the
andalusite
isograd
at the
cordierite
isograd
at the gamet
isograd
at the staurolite
isograd
biotite
biotite
T
T
Figure 3.30
Shifting of the tie lines with temperature (Demange, 1982 and unpublished
data)(XMg
=
100 Mg/Fe
+
Mg).
