Melilite- and Leucite-Bearing Systems - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 329

previous section. With reference to Fig. 10.17 b, it may be observed that the assem-
blage at C) corresponds to olivine-bearing madupite.

Like phase relations in the Fig. 10.16 b, with the lowering of temperature at point

' N ' (Fo 7 Lc 40 Ak 53 ; 1,165 ± 10 °

' N ' ,

the final assemblage near the solids is constituted of forsterite ss , phlogopite ss , diop-

side ss , K-feldspar ss liquid, and vapour (olivine pyroxene-bearing minette). Lloyd

et al. (1985) investigated melting phase relationship of an olivine melilite glass under
variable P-T

-

T conditions, and noted that the above-mentioned assemblage is similar to
that obtained near the solidus close to point

-

(Fig. 10.17 b). If such a liquid coex-
isting with forsterite ss , phlogopite ss , clinopyroxene and K-feldspar ss ascends near the
surface, it would result in an assemblage comprising forsterite ss +

' N '

å kermanite ss + leu-

cite ss + liquid (katungite).
With reference to the study of the join Fo Ak at 2.3 GPa, it may be observed

that Kalsilite-bearing minette, pyroxene-bearing minette and olivine

Lc

- -

pyroxene-
bearing minette are heteromorphic high pressure equivalents of katungite. Likewise
monticellite alnoite

-

kermanite-rich side) is represented at high
pressure by merwinite-bearing websterite.
Appearance of merwinite ss and diopside ss is related to the breakdown of

(Fig. 10.17 a, å

å ker-

manite (Yoder 1973) because of reaction (4) described earlier. (Yoder 1973)

° å

showed that below 0.62 GPa and 750 kermanite breaks down to monticel-

lite + wollastonite. Yoder noted that formation of merwinite and diopside at the
expense of å kermanite and forsterite is valid up to 1.05 GPa and 1,090 ° C, and
above this pressure diopside and merwinite may be the stable products at the
expense of forsterite and

C

kermanite.
Arima and Edgar (1983a) studied melting relationships of a katungite compo-
sition at pressures up to 3.5 GPa and temperatures to 1,300

å

C. They observed that
below 2 GPa in the presence of excess water, olivine appeared as the liquids phase,
but it disappeared at lower temperatures because of reaction with liquid. Their data
indicate that clinopyroxene together with phlogopite has a wide P-T

°

T range of

-

stability. Melilite appeared only below 1.0 GPa and 1,090 C.

At high pressure (2.3 GPa) but low temperatures towards the leucite-rich side,
instead of a breakdown product of

°

å kermanite to form merwinite and diopside,

å kermanite reacts with forsterite, leucite and vapour to produce phlogopite and

diopside. Note the reaction:

5Fo þ 3Lc þ Ak þ 3H 2 O ¼ 3PH þ 2Di

Thus, at 2.3 GPa and low temperature towards the leucite-rich side, above
reactions inhibits the presence of merwinite ss , which disappears and diopside ss
together with phlogopite precipitates as suggested by the above reaction. Merwinite
has not been reported in the K-rich ma c rock, as it is eliminated at low temper-

atures because of the above reaction. Near the surface, equilibrium crystallization of
a liquid lying in the Fo

Ak join has been described by Gupta (1972). He
showed that a katungite magma (point

Lc

- -

' G ' in Fig. 10.17 a), where liquid co-exists