Experimental Studies on K-Rich Rocks - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 408

3.0

Amph+Opx+Cpx+L

Ol
+
L

5

2.5

1

Opx+Cpx+L

2.0

Cpx+Phl+L

4

1.5

L

Cpx+Opx+Phl+L

Phl+Ol+Cpx+L

1.0

A

2

6

Ol
+
L

3

Ol
+
Cpx
+
L

Phl+Ol+Cpx
+
Arm+L

Ol+
Cpx
+
Arm
+
L

0.5

B

C

0

900 1000 1100 1200 1300

T o

Fig. 13.21 Pressure-temperature plot of the phase relations of an armalcolite-phlogopite lamproite
from Smoky Butte, Montana. Heavy lines indicate well established data boundaries, light lines are
less well-established., dashed lines are inferred. A narrow field of olivine + pyroxene (orthopy-
roxene or clinopyroxene) probably coexist with liquid between the olivine + liquid and
orthopyroxene + clinopyroxene + liquid assemblages (after Edgar et al. 1992). Arm: armalcolite

The lines AB and AC have been drawn arbitrarily and there are no data points to
substantiate the claim of two different assemblages, Ol + Cpx + Arm and
Ol + Cpx + L.
The Armalcolite-phlogopite lamproite has microphenocrysts of olivine in a
groundmass of phlogopite, sanidine, armalcolite, clinopyroxene, chromite, pride-
rite, apatite and abundant glass. The lamproite is SiO 2 -rich and has high F/H 2 O
ratio relative to lamproites that have been investigated in previous experimental
studies. Their data show that with decreasing temperature from the liquidus at
pressure above

1.2 GPa, a melt coexists successively with olivine; orthopyrox-
ene + clinopyroxene; orthopyroxene + clinopyroxene + phlogopite; clinopyrox-
ene + phlogopite; and clinopyroxene + K-richterite. Below 1.2 GPa, the assemblage
succession is olivine; olivine + clinopyroxene; olivine + clinopyroxene + phlogo-
pite; and olivine + clinopyroxene + phlogopite + armalcolite. The main difference
from the natural paragenesis is that the rock does not contain any orthopyroxene,

*

—

a feature, which is rather remarkable as it has and

the rock differs also in that it contains sanidine and priderite. In the experiments,

16 % normative hypersthene —

*