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If crystallization proceeds at 1 and 2 GPa, from nephelinitic liquids of compo-
sition a
1
and a
2
(Fig.
9.6
b) the melt composition will reach point b
1
along the line
a
1
-
b
2
(at 2 GPa, Fig.
9.6
b), resulting in the
genesis of a pyroxene-poor nephelinite lava (point b
1
or b
2
). Under plutonic con-
ditions, if crystallization stops at these points an urtite should be generated. If the
crystallization proceeds along the curve b
1
-
b
1
(at 1 GPa) or b
2
along the line a
2
-
L to point L (at 1 GPa) or along b2
Pto
-
P (at 2 GPa), in either case the
final assemblage at L or P should correspond to a
phonolite (Fig.
9.6
b, nepheline syenite). It may be noted that the four-phase point L
or P contains less than 5 % diopside. Thus the
first melt formed either at 1 or 2 GPa
should be pyroxene-poor phonolites. If comparison of the crystallization paths for
composition
(Figs.
9.2
and
9.6
a) is made it is found that olivine nephelinite is a
low-pressure product. A plot of the phonolite bulk compositions around the four-
phase points at 1 and 2 GPa suggests that many phonolitic magmas may be gen-
erated at depths of 30
'
X
'
60 km.
Whereas Lippard (1973) conducted
-
field, petrographic and geochemical studies
on phonolitic rocks of the Kenya rift valley, Jaggerson and Williams (1964) dis-
cussed the genesis of alkaline rocks in the north Tanganyika alkaline district.
Lippard observed that bulk composition of the phonolites from the Kenya rift valley
can be easily expressed in terms of the model diopside
sanidine system,
the crystallization trend (marked R), which is plotted in Fig.
9.6
a. the present study
is, therefore, quite relevant to understanding the paragenetic relationship amongst
these rock types occurring near the Kenyan rift valley. Baker (1987) also studied
the nepheline-bearing rocks from the alkali igneous province of Kenya and northern
Tanzania, both of which are associated with the East African Rift Valley, where the
following spatial distribution was noted: melanephelinite, nephelinites, phonolite
and leucite phonolite (point H) along with trachyte (path M
-
nepheline
-
H, Fig.
9.6
a).
-
b
2
, Fig.
9.6
b)
should yield nephelinites (point b
1
or b
2
), then the liquid should follow the paths b
1
-
Differentiation of melanephelinitic magma (e.g. path a
1
-
b
1
and a
2
-
L
or b
2
-
P and then at point L or P should produce nepheline syenites (Fig.
9.6
b). This
course of crystallization has been observed in Usaki complex, Kenya (Le Bas 1977).
He also described a nephelinite volcano in Kisingiri, Western Kenya. The products of
the volcano comprise a melanephelinite, nepheline-bearing melilitite and nephelinite
agglomeratic tuff. Only 300 km to the north of Kisingiri is a highly eroded Napak
volcano. The lavas make up 3 % of the extrusive succession, and they are melanep-
helinites, olivine and melilite-bearing varites and nephelinites. Figure
9.6
a suggests
that the trend of crystallization (marked K, Fig.
9.6
b) should result in the crystalli-
zation of a melanephelinites
first, followed by an olivine melilite nephelinite and then
melilite nephelinite (below 1GPa). In the system diopside
-
nepheline
-
sanidine, in
compositions close to the diopside
nepheline join, melilite appears in addition to
forsterite, diopside and nepheline. Thus, when the K
2
O content of these liquids is very
negligible (<5 %), at the end of crystallization the
-
final product is nephelinite con-
taining minor amount of pargastitic amphibole. In case of one composition Di
10
Ne
60
forsterite, nepheline, melilite and pyroxene-bearing assemblage appears at 950 and
850
C, resembling the assemblage obtained at Kissigiri. It is noteworthy that a
melilite, olivine and pyroxene-bearing assemblage is predominant
°
in the
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