Leucite- and Feldspar-Bearing Systems - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 284

Chapter 9
Leucite- and Feldspar-Bearing Systems

Nephelinites have been considered to be the most perplexing group of igneous
rocks by Bailey (1974). Such rocks predominantly contain nepheline and augite
with other accessory minerals. They are usually associated with rocks of melteig-
ite c lavas, or

carbonatites. Sometimes, they are also observed as a highly fractionated group of
rocks derived from lavas of alkali basalt

ijolite urtite series in close proximity to either leucite-bearing ma

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phonolite series; in such cases, sanidine
becomes an important phase in addition to augite and nepheline.
Addition of sanidine to the diopside

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nepheline join has a very important pet-

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rogenetic signi cance, because bulk compositions of nepheline-bearing phonolites,

mela-nephelinite, trachyte and alkali feldspar-bearing urtite lie on the diopside

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nepheline SiO 2 system. As

sanidine melts incongruently to leucite + liquid, there is a primary phase

sanidine join of the nepheline kalsilite CaO MgO

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eld of
leucite, which is known to be unstable at high pressure (Scarfe et al. 1966; Fass-
hauer et al. 1998). Besides, the incongruent melting phenomenon of sanidine is
eliminated above 0.3 GPa [P(H 2 O) = P(Total); Tuttle and Bowen 1958]. Thus,
experimental studies of the join under 0.1 GPa (in presence of excess water) should
establish the paragenetic relationship between olivine nephelinite, melanephelinite
and nepheline-bearing phonolite (with or without leucite). At low pressure
(<0.3 GPa in presence of excess water), leucite produced as an incongruent melting
phase should be eliminated at low temperatures to generate leucite-free phonolites.
It should be interesting to see whether melilite is present near the nepheline

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diopside join at high water pressure, and amphibole co-precipitates as an additional
phase in the subsolidus region from more ma

c starting compositions in this join.

The diopside sanidine join was, therefore, studied under 0.1, 1.0 and

2.0 GPa and variable temperatures by Gupta et al. (2006).

nepheline

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