Ternary Systems with Feldspathoids - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 263

Fig. 7.8 Three-phase triangles showing the composition of liquid in equilibrium with feldspar ss
and feldpathoids. For example, in case of triangles 4,5 and 6, the composition of liquid 4 is in
equilibrium with the composition of feldspar 6 and triangles show the movement of liquid either
toward the eutectic E 1 or the eutectic E 2 . M denotes the thermal maximum produced by the
intersection of the line nepheline K feldspar with the nepheline ss — sanidine ss cotectic Line (Gupta

- -

et al. 2010)

extended toward the albite K-feldspar join, these should give the compositions of

the primary alkali feldspar. Two such tangents aa and bb show the composition of
feldspar along the line a

—

b. Brotzu et al. (1989) observed that the feldspar com-
positions really lie within this range. With reference to their system (Fig. 7.8 ), the
coetectic should intersect the curve ME 1 at

-

, after which a liquid should move to
the eutectic E 1, as there is no jadeite-bearing syenite at that locality, Brotzu et al.
concluded that the rock types of Morro Redondo, Brazil crystallized at low-pressure
conditions, and the

' u '

final liquid should have composition close to the minimum
point, It may be noted that compositions of the eutectic E 1 and the minimum point
at 0.1 GPa are close to each other. Even if the crystallization took place at higher
pressure, and jadeite did crystallize, it is possible that at lower pressure jadeitic
pyroxene reverted back to nepheline and alkali feldspar.
Points V, W and X (Fig. 7.9 ) represent the bulk composition of nepheline syenite
from the khibiny massif, Kola peninsula, Russia, plotted onto the nepheline —

kalsilite silica plane. A liquid of these bulk compositions would follow i-V, j-W

and k-X curves (the nature of the curve has to be determined from the crystalli-
zation trend of liquids in that petrographic province). In the case of curve i-V,
sanidine should crystallize

—

first, followed either by nepheline or kalsilite depending
on the curvature of the path of crystallization. The final assemblage in the case of

both i-V and j-W should comprise sanidine ss + nepheline ss + kalsilite ss (nephe-
line kalsilite syenite); that is what is found in these rocks. Likewise, from liquid of

composition X,

-

finally nepheline ss will precipi-
tate. The crystallization trend X-k (dotted curve in Fig. 7.9 ) is hypothetical, and

first kalsilite ss, then sanidine ss and