Leucite- and Feldspar-Bearing Systems - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 299

Fig. 9.9 The hypothetical
phase diagram in the join
forsterite - leucite - anorthite
under 1 atmospheric pressure

9.4.3 The Join Diopside Leucite Anorthite

- -

Experimental results on this join are summarized in Fig. 9.10 . At A (Di 43 An 35 Lc 22 ),
diopside ss + leucite ss + anorthite ss + coexist with liquid. Optical and X-ray dif-
fraction studies indicate that diopside may incorporate Ca-Tschermark ' s molecule,

whereas leucite probably includes K 2 O 5SiO 2 in solid solution. The com-

position of the liquid thus does not lie in this join, and point A therefore is a
piercing point (1,249

MgO

∙ ∙

± 5 ° C).

9.4.4 Paragenesis

On the basis of the phase diagrams of these four bounding joins, the course of
crystallization of a liquid is the system forsterite - leucite - anorthite, can be described

by a simpli flow sheet diagram as given in Fig. 9.11 , in which a rock

nomenclature corresponding to each assemblage is also shown. This shows that a
leucite-bearing basanite can be produced from an olivine leucitite (m, Fig. 9.11 ),
atephrite (n) or an olivine plagioclase italite (p) or a murambite (o), approaching the
composition of a magma, belonging to the shoshonite absarokite series. Lavas of
this type have been reported by Holmes and Harwood (1937; p. 1450) from the
Kogoma area Birunga.

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