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An
An
50
50
LCT
plag
C
lc
san
TL
PLT
TLP
b
LP
a
C
Q
Lc
Or
wt.%
5 kbar
-2.6 kbar P
=P
1 atm
H
2
O
Total
Fig. 9.8 Phase relation in the leucite
SiO
2
at 1 bar, 0.26 and 0.5 GPa (after Jones and
MacKenzie 1991). Inset diagram represents the equilibrium assemblage at point C. LCT leucite
tephrite, TL tephritic leucitite, PLT phonolitic leucite tephrite, LP leucite phonolite, TLP tephritic
leucite phonolite
anorthite
-
-
by Jones and MacKenzie (1991). It was pointed out by them that under a condition,
when P(H
2
O) < P(total), at 0.5 GPa, the stability
field of leucite expands, and
liquidus temperature is higher than that under a condition, when P(H
2
O) = P
(total) = 0.5 GPa. The fractionation trend of tephritic leucitites, leucite
trachytes
and tephritic leucite phonolite has been described by Appleton (1972) in terms of
anorthite
-
quartz diagram at 0.26 GPa in presence of excess water. Jones
and MacKenzie pointed out that Appleton
leucite
-
-
'
is deduction on phase relationship is
wrong in that he showed that
eld
boundary falling along the entire length. Experimental results have shown that
within the
temperatures on the plagioclase
sanidine
-
field of feldspar in this system the ternary feldspar join acts as a thermal
barrier and temperature decreases from the ternary feldspar plane on both direc-
tions. Their revised phase diagram is shown in Fig.
9.8
along with the
eld
boundary determined at 0.5 GPa. Jones and MacKenzie extended the
eld
boundaries schematically to silica-saturated portion of the system. In the SiO
2
-rich
side expansion of quartz
field relative to that of feldspar with increase of pressure is
expected.
With reference to their study, Jones and MacKenzie considered that a bulk
composition of tephritic leucitite, which plot in the plagioclase
field. It should
precipitate plagioclase
first followed by sanidine and then leucite or vice versa and
remain silica-undersaturated. Only under lower pressure are magmas with com-
positions similar to Roccamon
na lavas are likely to produce silica-saturated dif-
ferentiates. Jones and Mackenzie (1991) considered the same rocks compositions in
terms of quaternary system quartz
kalsilite and showed that
derivative of leucite phonolites may be controlled by fractionation of two feldspars
at pressures high enough to support the stability field of leucite. This phase rela-
tionship however, is not clear from the quartz
-
anorthite
-
nepheline
-
anorthite
leucite system.
-
-
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