Geology Reference
In-Depth Information
Aegirine-augite
(Ca Na)(Mg,Fe)Si
26
80
20
60
40
60
40
20
80
A
865
0
670
0
640
0
B
C
20
40
60
80
(Na,K)AlSiO
4
Nepheline
(Na,K)AlSi
38
Alkali Feldspar
Fig. 9.5 Density distribution diagram for 129 plutonic rocks containing 80 % or more normative
clinopyroxene
-
nepheline
-
alkali feldspar. The dotted area represents the locus of more than 75 %
of all compositions. The shaded area contains 15 % of all the compositions, and remaining 10 % is
represented by unmarked areas. A Di
ss
L (four phase point) determined under 1 atm.
pressure; B same four phase point determined under 1 GPa; C same four phase point determined
under 2 GPa P(H
2
O) = P(Total) (after Edgar 1964)
Ne
ss
San
ss
-
-
-
clinopyroxene, 65
24 wt% nepheline, respectively. These
compositions (points B and C) plot either in the dotted or shaded area (Fig.
9.5
).
Such a liquid should have the mineralogical composition similar to phonolite.
When a phonolitic melt is generated at 2 (60
73 wt% sanidine and 31
-
-
35 km) they are
leucite-free. When they ascend near the surface conditions, leucite
ss
appears as an
incongruent mineral phase of sanidine. The crystallization sequence at 0.1 GPa,
1GPa and 2 GPa can be explained by Figs.
9.6
a and
9.6
b.
Mt. Etinde is a volcano situated on the southwestern
70 km) or 1 GPa (30
-
-
flank of the large Mount
Cameroon. Esch (1901; also see Nkoumbou et al. 1995) described the rock types of
Mt. Etinde, a volcano that is built of leucitites, leucite
nephelinites, and nephelinite.
The assemblages observed in an around Mt. Etinde volcano can be explained from
the lineages established from the present investigation (Fig.
9.6
a). Various crys-
tallization trends of different rock types from Etinde are shown by curves paths
marked E-I, E-II and E-III. It may be observed from Fig.
9.6
a that a phonolite can
be derived from either a trachyte (along the curve K
-
V close to the line MH) or a
-
nephelinite (curve G
H). Some rocks of the trend E-III may hit either nephelinite
(the curve GH) or a leucitite (the curve MH, Fig.
9.6
a) depending upon the bulk
composition.
-
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