Melilite- and Leucite-Bearing Systems - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 309

the liquid moves towards the lowest melting point. Study of the join suggests that
the lowest melting point of the tetrahedron, forsterite diopside akermanite leucite,

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lies near the diopside akermanite leucite plane.

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10.1.1.6 Paragenesis of Nepheline-Free Melilite and Leucite-Bearing
Ma c and Ultrama c Rocks

Melilite Leucitite

The join diopside leucite (Fig. 10.1 ) shows that a melilite leucitite,

corresponding to the piercing point B, can be derived from either a melilite italite
(aB), leucitite (bB), or a melilitite (cB). Natural melilites contain a considerable
amount of sodium in solid solution (Schairer and Yoder 1964) but are poor in K 2 O.
The melilitites mentioned here have compositions lying within the tetrahedron
forsterite

akermanite

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leucite. At Capo di Bove in Italy, Washington
(1906) noted the sporadic occurrence of leucitite grading to melilite leucitite
(cecilite). Results of the system diopside

- diopside - akermanite -

- akermanite - leucite indicate that melilite

leucitite may be derived from a leucitite magma.
Figure 10.2 shows that olivine leucitite (piercing point C) can be derived from
either potassium-rich olivine pyroxenite (gC), or olivine italite (1C), or leucitite
(iC). Olivine pyroxenite ordinarily contains very small amounts of K 2 O. The
olivine pyroxenites referred to here have compositions lying in the tetrahedron,
forsterite diopside akermanite leucite. In the Bufumbira region of Uganda,

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Holmes (1937), described a lava flow of olivine leucitite (ugandite). At Katunga in

Uganda, he noted the close association of rocks such as leucitite and olivine leu-
citite, and also reported complete gradation of these rocks from pyroxene-rich
leucitite, which in turn grades into pyroxenite. This

field evidence and the results on

the system forsterite leucite suggest that an olivine leucitite magma can

be a derivative of a potassium-rich olivine pyroxenite liquid. Generation of such a
liquid is possible by the partial melting of phlogopite-bearing peridotite.
Phase equilibria study of the system forsterite

diopside

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leucite (Fig. 10.3 )
suggests that a katungite magma can be derived from a melilite italite (kG) or an
olivine italite (IG). Field evidence of a massive lava

akermanite

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flow of katungite was reported
by Holmes (1937) from Toro-Ankole, Uganda, where it occurs in association with
alnoite. On the basis of this experimental study it appears possible that a katungite
magma may have been derived from an alnoite, which itself may have originated
from a potassium-rich monticellite alnoite.

Olivine Melilitite

Reference to Ferguson and Mervin ak-

ermanite system suggests that olivine melilitite may be produced from a peridotite,
a melilitite, or an alnoite magma.

' s diagram (1919) of forsterite diopside

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