Leucite- and Feldspar-Bearing Systems - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 287

(in the presence of excess water) were examined first to establish the paragenetic

relationship between olivine nephelinite, melanephelinite, nepheline leucitite, tra-
chyte and phonolite (with or without leucite ss ). Phonolite by de

nition is a rock
containing alkali feldspar and nepheline ss with accessory clinopyroxene, but in
potassium-rich silica-undersaturated rocks nepheline may not appear and leucite may
take its place instead. Such rocks have also been termed as phonolite by Holmes and
Harwood (1937). In some phonolites both nepheline ss and leucite ss may be present. In
the present study phonolite has been designated as the alkali feldspar-bearing vol-
canic rock containing nepheline with accessory clinopyroxene. If both leucite ss and
nepheline ss are present it has been described in the present study as leucite phonolite
to emphasize the presence of both nepheline and leucite. The plutonic equivalents of
the volcanic rocks relevant to this study are shown within brackets in italics.
The study of this join was further extended to higher pressure (1 and 2 GPa)
under the condition of P(H 2 O) = P(Total), as the magmas related to alkali-rich,
silica-undersaturated feldspathoid-bearing basic and ultrabasic magma are believed
to be generated in the upper (Tilley and Yoder 1968; Clague and Frey 1982).
Conversion of nephelinite to amphibolites has been observed by Tilley and Yoder
(1968), White et al. (1972), Woolley and Jones (1987) and Downes (1987). It was
considered by White et al. (1972) that at suitable pressure [P(H 2 O) = P(Total)]
conditions rocks of these series should be converted to amphibolites by subsolidus
hydration. Direct crystallization of amphibole above solidus was also observed by
Singh et al. (2000) in the diopside nepheline system in the presence of excess

water. The vol.% of alkali amphibole in this join in the subsolidus region obtained
by them was more than 80 %. The diopside

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sanidine join was, therefore,
studied in the presence of excess water to check the possibility of generation of
amphibole-bearing nephelinite, phonolite (nepheline syenite) and trachyte (syenite)
at low temperature and pressure from this system.

nepheline

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9.2.1 Phase Relations in the System Diopside

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Nepheline Sanidine at 0.1 GPa [P(H 2 O) = P(Total)]

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Chattopadhyay et al. (1999) presented only the liquidus phase relations in the system
diopside

sanidine. The subliquidus and subsolidus data for the same
system was not determined earlier. Experimental results for the system at 0.1 GPa in
the presence of excess water and variable temperatures are summarized in Fig. 9.2 ,
which describes liquidus phase relation only. Amphibole always appears as a phase
appearing above or below the solidus. Its presence cannot therefore be deciphered
from the liquidus phase relations (Fig. 9.2 ). The results show that the system inter-
sects the phase volumes of diopside ss , nepheline ss , leucite ss and forsterite ss . As for-
sterite ss and leucite ss appear in this system, it is not a ternary. The electron microprobe
analyses also show that diopside contains Al 2 O 3 as a CaAl 2 SiO 6 molecule. The join

nepheline

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