Leucite- and Feldspar-Bearing Systems - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 296

been generated due to partial or total fusion of silica-undersaturated syenitic plu-
tonic parent, as xenoliths of syenite are observed in the lavas of that area.
Strong (1972) carried out a detailed petrological study of the alkaline rocks of
the islands of Moheli, western Indian Ocean. Strong suggested an apparent dif-
ferentiation from alkali basalt through melanephelinite and trachyte to phonolite.
The present study under 0.1 GPa in presence of excess water suggest the following
fractionation scheme: (1) Melanephelinite nephelinite phonolite, and (2)

→ →

alkali basalt trachyte phonolite.

→ →

9.3 The System KAlSi 3 O 8 - CaAl 2 Si 2 O 8 - KAlSiO 4 at 0.5 GPa

Liquidus phase relations in the system KAlSi 3 O 8 - KAlSiO 4 have been

determined by Jones and MacKenzie (1991) for low anorthite contents (<20 wt%
anorthite) at 0.5 GPa in presence of excess water.
The phase diagram for the system is shown in Fig. 9.7 . They observed that a
temperature maximum occurs on the

CaAl 2 Si 2 O 8 -

field boundary of plagioclase and leucite.
They established two ternary eutectics: (1) one occurring at Qz 23 An 14 Ks 63 and
735 C where kalsilite (Ks), anorthite (An) and leucite are in equilibrium; and (2)

the other at a temperature slightly higher than 785

°

° C, where leucite, anorthite and

alkali feldspar coexist in equilibrium with liquid.
Occurrence of leucitite, tephritic leucitite, phonolitic leucite tephrite, tephritic
leucite phonolite, and leucite phonolite has been reported from various localities.
Appleton (1972) not only described the occurrence of these rocks in Roccamon

na,
but tried to describe the genesis of these rocks in the light of the system
leucite quartz.

Figure 9.8 (Jones and MacKenzie 1991) summarizes the phase relation in the
join leucite

- anorthite -

quartz at 1 bar, 0.26 and 0.5 GPa. In Fig. 9.8 , the solid line
describes phase boundary at 1 bar, dashed line demonstrates the phase diagram
derived by Appleton (1972) and the dotted line refers to the phase diagram obtained

- anorthite -

Fig. 9.7 Phase relation in the
join anorthite

An An

CaAl Si i O
228

sanidine

- -

kalsilite at 0.5 GPa in
presence of excess water
(after Jones and MacKenzie
1991). The inset diagram
indicates the portion which
has been studied

30 30

An

20

20

Kalsilite Plagioclase

795

790

An

735

845
10

795 795

M

10

H5 H4 H3 H1

H2

Alkali
feldspar

Leucite

Q

Ks Lc Or

Q

Ks 790 770

SiO 2 10 Lc 790

Or