Leucite- and Feldspar-Bearing Systems - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 295

diopside nepheline join studied at 1 atm by Bowen (1928) and Schairer et al. (1962).

In case of crystallization under 2 GPa a magma should not yield melilite, as this phase
is eliminated (see Fig. 6.4 ) below 1.1 Gpa.
During the eruption of lava at Campi Flagrei (Italy), Melluso et al. (1995) noted
the following paragenetic sequence: trachyte

-

leucite phonolite to trachytic pho-
nolite with amphibole. Reference to Fig. 9.6 a suggests that a trachytic melt (curve
M

→

H) should yield a leucite phonolite at 865 ° C. At 815 ° C after leucite reacts out, at

-

low pressure the final product is a pyroxene-poor trachytic phonolite containing

pargasitic amphibole (K-feldspar + diopside ss + nepheline ss + amphibole + L. This
study also supports the conclusion of King (1965), who suggested that a parental
melanepheline (trend E 1 , Fig. 9.6 a) should yield a nephelinitic liquid (N H), which in

turn should result in the genesis of a phonolitic liquid (point H, Fig. 9.6 a).
Cundari (1973) described the occurrence of leucite-bearing lava

-

flows from New

South Wales and Victoria. The dimension of flows varies from 10 to

28,000 km 2 . The variation of modal mineralogy is as follows: leucite (20

the

33 %),

-

clinopyroxene (30 14 %), alkali feldspar + nepheline (14 %)

and glass, which is very much in consonance with the mineralogy that is expected
to be observed during progressive crystallization from this system at low pressure
(Fig. 9.2 ). Cundari did not describe reaction relationship between olivine and liquid
in these rocks. In the alkali igneous rocks of the Balcones volcanic province of
Texas, Spencer (1969) however, observed reaction relationship between olivine and
liquid in olivine nephelinites, which have paragenetic relationship with nepheli-
nites. These nephelinites grade to phonolites (point H, Fig. 9.2 ).
Close association of trachyte (curve M

- 50 %), olivine (11 -

H, Fig. 9.2 ) and phonolite (point H) has

-

been described from the Phlegrean fields located northeast of Naples (Italy) by

Beccaluva et al. (1990). The lava of Vulsinian district (also of Italy) includes lava

flows grading from trachyte to leucite-bearing phonolite (Holm et al. 1982). A similar
trachyte phonolite association has been observed by Baldridge et al. (1981) from the

same locality. In the quaternary lavas of java ad Bali in the Sunda Arc (Indonesia),
Whiteford et al. (1979) described highly potassic rocks containing various combi-
nations of clinopyroxene, alkali feldspar, potassic pargasite, leucite and nepheline and
olivine (present or absent). Trachytes (nepheline syenites),

-

leucite phonolites

(nepheline leucite syenites), phonolite (nepheline syenite) have been described by

Kogarko et al. (1995) from Yaksha, Baikal. Similar combination of the above-men-
tioned minerals has been described by the same authors from Artem (Primorye
Province, USSR). They also reported the occurrence of an alkali complex from Irisu
(Kazakistan), were rock types grade from leucite

-

pyroxene phonolite to phonolite,
suggesting that leucite formed by incongruent melting of K-feldspar reacted out at low
temperature. The present study at 0.1 GPa suggests that crystallization of rocks from
Indonesia and Tezhar (Caucasus) took place at low-pressure near-surface conditions.
Wright (1963) suggested that mela-nephelinites or nephelinites are parental to
phonolites, the same trend as exempli

-

H (Fig. 9.6 a). Nash
et al. (1969) in their study of the rock from Mount Suswa (Kenya) observed the
crystallization trend from alkali

ed by the trend E 1 - N -

H,
Fig. 9.6 a). In general, they believed that the magmas of Mount Suswa might have

trachytic liquid to phonolite (the trend M

-