Leucite- and Feldspar-Bearing Systems - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 298

9.4 The System Forsterite Diopside Leucite Anorthite

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The bulk compositions of plagioclase-bearing potassium-rich silica-undersaturated
rocks such as olivine leucitite (ugandite), leucite tephrites and basanite lie within
the system forsterite

anorthite. These rocks occur in the Somma-
Vesuvius region of Italy (Savelli 1967), the Birunga area of Uganda (Holmes and
Harwood 1937; Ferguson and Cundari 1975), and the East Eifel region of West
Germany (Duda and Schminke 1978). In these regions these rocks are often
associated with phlogopite and/or olivine-bearing leucitites. A study of the system
forsterite

- diopside - leucite -

anorthite may thus help to understand the paragenetic
relationships between leucite-bearing tephrites) and basanites and more ma

diopside leucite

- - -

c

potassic-rich lavas without plagioclase.
The system forsterie

diopside leucite anorthite has four bounding joins:

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(1) forsterite diopside leucite,

- -

(2) forsterite anorthite diopside,

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(3) forsterite leucite anorthite, and

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(4) diopside leucite anorthite.

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Of these four joins, join (1) has already been discussed. The others are discussed
below.

9.4.1 The Join Forsterite Diopside Anorthite

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The join forsterite anorthite (Osborn and Tait 1952) has two piercing

points: one at Di 22.5 An 57.5 Fo 20 and 1,317

- diopside -

° °

C, the other at Di 49 An 43.5 Fo 7.5 and 1,270

°

C. At 1,317 C forsterite ss , spinel ss and anorthite ss coexist with liquid, whereas at

1,270 C, forsterite ss , diopside ss , and anorthite ss are in equilibriumwith liquid and the

assemblage at the end of crystallization consists of diopside ss + forsterite ss +
anorthite + liquid.

°

9.4.2 The Join Forsterite Anorthite Leucite

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This join has not yet been studied. However, results from the bounding joins
forsterite anorthite (Anderson 1915), forsterite leucite (Schairer 1954), and leu-

- -

cite anorthite (Schairer and Bowen 1947) are already known. The phase diagram of

this join can therefore be constructed as shown in Fig. 9.9 . The spinel

-

field in this

case is small, SiO 2

(Anderson 1915), and the association of leucite and Mg-spinel is not reported in
nature, although the latter occurs in xenoliths within potassic lavas of East Eifel
(Duda and Schminke 1978).

judging from the study of the join forsterite anorthite

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