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composition of the melt should move towards E
1
, whereas the liquid in the silica-
undersaturated portion leads towards E
2
. The coexisting assemblages at the
invariant points are diopside + leucite + sanidine + anorthite + liquid (E
1
), and
diopside + sanidine + plagioclase + tridymite + liquid (E
2
). The univariant
assemblages of diopside
anorthite
leucite
liquid (A
E
1
, leucite tephrite), diop-
-
-
-
-
side
sanidine
leucite
liquid (B
E
1
, pyroxene
leucite phonolite), diopside
sani-
-
-
-
-
-
-
dine
anorthite
liquid (C
E
1
, shoshonite), and anorthite
leucite
sanidine
liquid
-
-
-
-
-
-
(D
E
1
, plagioclase
leucite phonolite) lead to E
1
. The univariant assemblages of
-
-
diopside
sanidine
tridymite
l(E
E
2
), anorthite
sanidine
tridymite
liquid (G
E
2
),
-
-
-
-
-
-
-
-
diopside
anorthite
tridymite
liquid
(F
E
2
),
and
diopside
sanidine
anorthite
-
-
-
-
-
-
-
liquid (C
E
2
) move towards E
2
. It should be noted here that at point C, which lies in
the diopside
-
sanidine join, diopside, leucite anorthite and liquid coexist.
However, because of a reaction with the liquid, leucite should disappear and the
resulting assemblage thus should consist of diopside + anorthite + sanidine + liquid.
The univariant assemblages in the silica-undersaturated portion of the system
and the coexisting phases at E
2
correspond to the trachyandesite
anorthite
-
-
quartz
latite series. Potassium-rich rocks consisting of various combinations of leucite, K-
feldspar, clinopyroxene, and plagioclase have been reported from the Somma-
Vesuvius (Savelli 1967), the Vico area of Italy (Cundari and Mattias 1974) and the
Bufumbira region of East Africa (Ferguson and Cundari 1975). Leucite and
nepheline-bearing tephrites have been found at Rothenberg, Epfelsberg and Kruf-
terofen in the East Eifel region by Duda and Schminke (1978).
When Cundari and Mattias (1974) plotted the mineralogical compositions of the
rocks from the Vico area of Italy in a triangular diagram of alkali feldspar (A)
-
shoshonite
-
-
plagioclase (P) and feldspathoid (F), most of the rocks lie in the tephrite
leucite
-
phonolite (TLP)
field (see Fig.
7.3
). This
figure shows that under equilibrium
conditions of crystallization the
leucite phonolite.
Cundari and Mattias (1974) observed a general gradation from tephritic leucitite
(TL) through phonolitic leucitite (PLT) to alkali trachyte (LTR).
Ferguson and Cundari (1975) found that there are two series of leucite bearing-
rocks in the Bufumbira area of East Africa. They underwent the following para-
genetic sequences: Series A: Leucite tephrite, tephritic leucite, leucitite, phonolitic
leucitite and phonolite. Series B: Leucite tephrite, phonolitic leucite tephrite, te-
phritic phonolite, latite, and trachyte.
In their study on the volcanism in the Eolian arc region, Barberi et al. (1974)
found a close relationship between leucite tephrite and shoshonite in Vulcanello, of
which the former rock type was probably produced from the latter by fractionation.
Highly vesicular lava
final product would be a tephritic
-
flows of leucite and K-feldspar-bearing tephrites (E
1
,
Fig.
9.12
), covering extensive areas have been reported by Holmes and Horwood
(1937) from the Sagitwe region of Birunga. Leucite tracyte has been described from
the Karsimbi area of the Birunga region by the same workers.
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