Melilite- and Leucite-Bearing Systems - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 326

and (4) Ks ss - Ph ss - Di ss - f. It is noted that the five-phase invariant assemblage

(Ks ss - Ph ss - Kf ss - Di ss - f) is obtained at 770 ± 10 ° C. Since the four-phase uni-

variant assemblage (Ph ss - Kf ss - Di ss - f) common to both the invariant points ' X '

and ' Y ' (Fig. 10.18 ), there has to be a thermal barrier originating from the point

“ O ” (1,140 ± 10 ° C). This univariant assemblage Ph ss - Fo ss - Di ss - f also origi-

nates cat point ' N ' (1,165 ± 10 ° C). Likewise, the univariant assemblage

(Kf ss - Ph ss - Ks ss - f) originates at the point “ P ” (775 ± 10 ° C) (Fig. 10.17 b).

The paragenetic sequence of different simpli ed rock types in the join Fo Lc ak

- -

is similar to the flow sheet diagram as shown in (Fig. 10.18 ). There is some

difference, however, in that the univariant line (Fo ss - Di ss - Ph ss - f) in the join

Fo f). As

merwinite ss reacts with the liquid and is eliminated, the melt composition follows a
divariant surface (Fo ss -

Lc Ak H 2 O, is derived from the assemblage (Fo Mer ss - Di ss -

- - - - ss -

Di ss - Ph ss - f) and finally to the point (Fo ss - Di ss - Ph ss - Kf ss - f)

(Fig. 10.18 ).

10.4 Petrological Signi cance

10.4.1 The Join Forsterite Leucite Studied

under 0.1 GPa [ P (H 2 O) = P (Total)]

Diopside

- -

With reference to Fig. 10.15 , it should be stated that the five-phase point A corre-

sponds to an olivine leucitite termed as ugandite by Holmes (1942). Figure 10.15
further suggests that ugandite can be derived from an olivine italite (along B

A), a

-

leucitite (along D A). With

lowering of temperature, phlogopite ss precipitates along with olivine, leucite ss and
pyroxene ss at 850

A) and a K-rich olivine pyroxene rich magma (along C

- - -

C. This phlogopite-bearing assemblage corresponds to a rock
called missourite or kajanite [equivalent to mica-rich ugandite by Lacroix 1926].
Holmes (1942, 1950) recognised that ugandite is one of the primitive magmas in the
Western branch of the African Rift. Cross, (1897) coined the term wyomingites to a
rock composed of leucite, olivine, augite and phlogopite (also see Yagi and Mat-
sumoto 1966; Carmichael 1967). The assemblage (Ph ss +Fo ss +Lc ss +Di ss + Liq) at
850

± 10 °

C is comparable to woymingite. Barton and Hamilton (1979), studied
melting phase relationships of a natural)madupite composition below 0.4 GPa. At
temperatures below 1,150

± 10 °

C, they obtained the assemblage (Cpx + Ph + Lc + Liq).
This assemblage is similar to that obtained near point

°

C.
In various localities of New South Wales such as Lake Cargello, Begargo Hills,
Flag staff Hills, Bygoloric, and Condobline, there are olivine leucitites containing
phlogopite. Such rocks called missourite are also reported from Gaussberg, Ant-
arctica (Sheraton and Cundari 1980) and from Mount Jugo and Colli-Albani vol-
canic

' A ' and 800 ± 10 °

fields of Italy (Cundari 1973). They however, described these rocks simply as
leucitite ignoring the presence of olivine and phlogopite altogether.