Phase Relations in the System Leucite-Akermanite-Albite-SiO - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 343

Fig. 11.11 Phase
relationships in the join
leucite-akermanite-
albite 50 anorthite 50 at 1 GPa
(H 2 O-saturated conditions)

8Ca 2 MgSi 2 O 7
akermanite

þ 3NaAlSi 3 O 8

albite

þ 3CaAl 2 Si 2 O 8

anorthite

þ 1KAlSi 2 O 6

leucite ð Þ ð Þ ð Þ

ð Þ

¼ Ca 3 Al 2 Si 3 O 12

grossularite

þ 8CaMgSi 2 O 6

diopside

þ 2CaAl 2 SiO 6

CaTs

þ 6CaSiO 3

wollastonite ð Þ ð Þ ð Þ

ð Þ

þ 3NaAlSiO 4

nepheline

þ KAlSi 3 O 8

K feldspar

ð 11 : 3 Þ

:

ð Þ ð Þ

Nepheline ss, alkali feldspar, wollastonite ss, clinopyroxene, and grossular are
present in the plagioclase-rich side of the join leucite-akermanite-albite 50 anorthite 50
studied under 1 GPa at variable temperatures.
King and Sutherland (1976, quoted in Le Bas (1977)) described coarse- and
medium-grained ijolites from Uyi located in the northeast corner of the Wasaki
peninsula at Angola, located 2 km west of Homa Bay town. These rocks are similar
to those at Kisingiri and Usaki, in that they include nepheline, alkali pyroxene,
garnet, wollastonite, and interstitial alkali feldspar. The Usaki ijolites are also
characterized by the presence of alkali feldspar, wollastonite ss , grossular, and
nepheline ss , as observed in the study of Dwivedi et al. (2007).
It is known that leucite breaks down to form orthorhombic kalsilite and
K- feldspar (Scarf et al. 1966; Fasshauer et al. 1998), which suggests that leucite ss
will be stable at 1 GPa at temperatures above 850

° C. It is also observed that the

field of plagioclase at 1 GPa is contracted, compared to the study on the same join
at 1 atm, where in the anorthite-rich side of the join grossular appears slightly above
the solidus. The disappearance of akermanite ss at 1 GPa under H 2 O-saturated
conditions suggests that melilite ss in leucite-bearing rocks crystallizes, when the
ultrapotassic nepheline-bearing magma enters into the earth

' s crust.