Melilite- and Leucite-Bearing Systems - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 319

Gupta et al. 2002; Conceicao o and Green 2004). The high pressure melting

experimental studies have been conducted in the join forsterite

çã

kalsilite quartz at

- -

different P-T T conditions and in the presence of several volatile components (H 2 O,

CO 2 , F) (Modreski and Boettcher 1972; Wendlandt and Eggler 1980a, b; Wend-
landt 1984: Foley et al. 1986; Gupta and Green 1988; Melzer and Foley 1996).
Recently, Conceicao

-

leucite join
to elucidate the genesis of K-rich silica-saturated magmas from lherzolitic source
rocks.
Nag et al. (2007) added diopside or

çã o and Green (2000) added silica to the forsterite

-

å kermanite to the forsterite leucite join to

-

understand the heteromorphic relationship between the K-rich ma c

volcanic rocks with lamprophyric or lamproitic melts originated in the upper
mantle. In either of these two joins, leucite is expected to break down at high
pressures and temperatures by the following reaction (Scarf et al. 1966; Fasshauer
et al. 1998);

c and ultrama

2Lc ¼ K-feldspar (Kf) þ kalsilite (Ks) ð 10 : 1 Þ

Ak under a condition of
2.3 GPa [P(H 2 O) = P(Total)], phlogopite is an additional phase and forsterite reacts
with leucite to produce the following assemblage typical of minette (Yoder 1986):

In the cases of both the joins Fo - Di - Lc and Fo - Lc -

3Fo þ 2Lc þ 2H 2 O ¼ 2Ph þ Kf ð 10 : 2 Þ

Lc at elevated pressures
and temperatures, various combinations of phlogopite ss , diopside ss , kalsilite ss , for-
sterite ss and K-feldspar ss may appear. Such assemblages are typical of lamproitic
and lamprophyric rocks (Gupta and Yagi 1980; Foley et al. 1987; Rock 1991;
Mitchell and Bergman 1991). In case of the join Fo

The above discussion suggests that in the join Fo Di

- -

Ak following reactions are
also expected under high pressures (Yoder 1973, 1986):

Lc

- -

2Ak þ Fo þ 3Lc ¼ 4 clinopyroxene Cpx ð Þþ 3Ks ð 10 : 3 Þ

2Ak ¼ Di þ merwinite Mer ð Þ ð 10 : 4 Þ

Ak þ 5Fo þ 3Lc þ 3H 2 O ¼ 3Ph þ Cpx ð 10 : 5 Þ

Ak, reactions ( 10.2 ), ( 10.3 ), ( 10.4 ) and ( 10.5 ) lead to various
assemblages found in lamprophyres and lamproitic rocks, and melting relationships
in the join Fo

In the join Fo - Lc -

- Lc - Ak have therefore, signi cant bearing on their genesis.