Melilite- and Leucite-Bearing Systems - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 328

is comparable to the end product, crystallizing from a liquid composition ' D ' in the

Fo H 2 O join) (Fig. 10.16 b).

MacDonald et al. (1992) described minettes from Bearpaw Mountains, which are
characterised by the presence of olivine and clinopyroxene phenocrysts in a
groundmass of phlogopite and sanidine. This assemblage is comparable to the

Di Lc

- - -

ve-

phase point (Ph ss +Kf ss +Di ss + Liq + V) (Fig. 10.16 b) and corresponds to

pyroxene-bearing minette. As the composition closed to

' C '

contains only 3 %
forsterite, olivine is consumed in the formation of phlogopite and K-feldspar. See
the reaction below:

' C '

3Fo + 3 Lc ¼ 2Ph þ Kf

(Fig. 10.16 b) is characterised by the assemblage
kalsilite ss K-feldspar ss phlogopite, and liquid. This assemblage is equivalent to
kalsilite-bearing minette. Orlova et al. (1986) described phlogopite-bearing yakutite
(phlogopite + K-feldspar + clinopyroxene + kalsilite) from Malomurunskii ultra-
potassic complex. This assemblage is noted in a strati

The assemblage at point ' D '

ed lopolith. The complex has
a lower layer comprising phlogopite and pyroxene at the bottom followed upward
by a layer with the assemblage consisting of kalsilite, K-feldspar and clinopyrox-
ene. Thus a Kalsilite-bearing minette magma, having an assemblage similar to the

cooled slowly and the phlogopite and pyroxene-rich fraction
settled at the bottom leaving behind a kalsilite

five-phase point ' C '

clinopyroxene K-feldspar-bearing

- -

residue (kalsilite syenite) observed in that locality.
Gupta and Green (1988) studied the system kalsilite forsterite silica at 2.8 GPa

- -

[P(H 2 O) = P(Total)]. They also observed the pressure of a five-phase assemblage of

Ph ss +Ks ss +San ss + L + V similar to Figs 10.16 b and 10.18 . Foley et at. (1986)
added 4 wt% F to see the effect of F on the topology of Fo

SiO 2 phase diagram.
They observed that their phase diagram broadly resembled the water saturated F-
free system of Gupta and Green (1988) in having a large primary phase volumes of
phlogopite, forsterite, kalsilite and sanidine. Foley et al. (1986) found that F-bearing
system differs in that the

Ks

- -

fluorphlogopite has a much greater thermal stability

(maximum 1,490 1,500 ° C) than hydroxyphlogopite (<1,200 ° C; Gupta and Green

-

1988).

10.4.3 The Join Forsterite Akermanite Studied

at 2.3 GPa and Variable Temperatures

Leucite

- -

Phase relations in the leucite-rich section of the join Fo - Lc - Ak (Fig. 10.17 b.) is very

similar to those of the join Fo Di Lc studied at 2.3 GPa (Fig. 10.16 b) in that there are

- -

primary phase elds of forsterite ss , phlogopite ss , K-feldspar ss , kalsilite ss and diopside ss

in the å kermanite-poor region of the join. Comparing Figs. 10.16 b and 10.17 b, it may
be stated that the paragenetic relationship in the leucite-rich but

å kermanite-poor

(<60 wt% å kermanite) section of the join is very similar to that already described in the