Leucite- and Feldspar-Bearing Systems - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 286

solidus assemblage corresponds to pyroxene-rich nepheline phonolite. The study of
this system therefore, suggests that a pyroxene-rich nepheline phonolite can be
produced from a nepheline leucitite melt, which is a derivative of a melilite

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nephelinite magma. The last magma type itself is a product of an olivine meli-

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lite nephelinite magma. Their study thus supports the conclusions of King (1965),

who suggested that nephelinites and phonolites from the Napak and Elgon volca-
noes (eastern Uganda) were derived from mela-nephelinitic magma by early sep-
aration of forsterite, melilite, and pyroxene. It also supports the conclusion of
Wright (1963), who considered that the differentiation of a mela-nephelinitic
magma should explain the genesis of the Tertiary lava series in the western part of
the rift system in Kenya. These lavas have variable compositions, which correspond
to phonolites, nephelinites, and melanephelinites.

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9.2 Experimental Study of the System
Diopside

Sanidine at 0.1, 1 and 2 GPa
and Variable Temperatures

Nepheline

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Bulk compositions of nephelinite, mela nephelinite, trachyte and phonolite can be

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represented in the join, diopside sanidine, which is a part of the quinary

system, nepheline - Kalsilite - CaO - Mgo - SiO 2 . Gupta et al. (2006) therefore studied
the system diopside

nepheline

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sanidine at 0.1, 1 and 2 GPa [P(H 2 O) = P(Total)]
and variable temperature to determine the signi

nepheline

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cance of the system in the genesis
of nepheline-bearing phonolites (with or without leucite) in association with

melteigite, urtite and ijotite series of rocks.
Nephelinites predominantly contain nepheline and augite. They are usually
associated with mela nephelinite, phonolite (nepheline syenite) and trachyte (sye-

nite) in association with carbonatite (Dawson 1966). They are also observed as a
highly fractionated series derived from lavas of alkali basalt

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phonolite series (Yoder
and Tilley 1962). Association of nephelinites with leucitites, phonolites and trachytes
are also well known from the Birunga volcanic

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field located near the East African rift
Valley (Sahama 1973; Rogers et al. 1998). The bulk compositions of the above
mentioned sodium or potassium-rich silica-undersaturated basic and ultrabasic
igneous rocks lie on the diopside

nepheline sanidine join of the nepheline kalsi-

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lite SiO 2 system. Forsterite appears as a reaction product of nepheline

and diopside in the diopside

CaO Mgo

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nepheline join at 1 atm (Schairer et al. 1962).
Sanidine melts incongruently to produce leucite ss plus a liquid below 0.3 GPa in
the presence of excess water (Tuttle and Bowen 1958). According to them at low
temperature, leucite ss disappears by reaction with liquid, and sanidine ss reappears
near the solidus. Thus, various combination of such phases as leucite ss , nepheline ss ,
forsterite ss and diopside ss are expected to appear in their study of the system diop-
side

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sanidine at low pressure under a condition of [P(H 2 O) = P(Total)]. In
the present study, phase relations in the join diopside - nepheline - sanidine at 0.1 GPa

nepheline

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