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and sills of ma
c phonolites and minettes. There are also dikes of microsyenite,
micromalignite, pseudoleucite phonolite,
tinguaite and alkali
trachyte. O
'
Brien
et al. also described rare occurrence of olivine kersantite, latite and trachyte.
The last-mentioned rock type is the oldest member of the potassic series having
an age of 53.1
1.8 Ma (Marvin et al. 1980). Fifteen km northeast of Highwood
Mountains, occur an intrusive plug called the Haystock Butte, comprising mon-
ticellite peridotite and minor alnoite.
According to Marvin et al. the Haystock Butte has an age of 48.2
±
1.8 Ma. The
latites are quartz-normative having following phenocrystal assemblage, plagio-
clase + hornblende
±
±
augite
±
biotite in a matrix of sanidine + quartz + biotite +
Fe
c phonolites are constituted of
phenocyrstal salite + pseudoleucite (or analcite) + olivne (Fo
84-65
)
Ti oxide + glass. According to O
'
Brien et al. ma
-
glass
with accessory apatite and titano-magnetite. They found that more felsic leucite
phonolites have less pyroxene, no olivine but more sanidine. The ma
±
biotite
±
c phonolite
has an assemblage similar to olivine leucitite, though pristine leucite phenocrysts
are rare. Salite and olivine phenocrysts contain pristine leucite but whenever
uid
had access to them along fractures they have been analcitized. Conversion of leucite
to analcite was found to be consistent with the hypothesis involving reaction of
leucite with a Na-bearing solution at low temperatures as suggested by Gupta and
Fyfe (1975). The porphyritic texture of ma
flows indicate
continuous crystallization presumably under subvolcanic conditions. According to
them the near solidus crystallization of biotite as well as common occurrence of
leucite implies that this magma crystallizing at shallow depths were enriched in
aqueous
c phonolite dikes and
fluid.
In this locality, there are six major stocks of which four are shonkinites and
malignites. There are more primitive and more evolved ma
c phonolites also.
O
Brein et al. described rocks having textural continuity between shonkinite and
ma
'
c phonolite. The rocks of these intermediate varieties are termed micro-
shonkinite. The assemblage comprising salite + sanidine + nepheline + olivine + bio-
tite + Fe-Ti
plagioclase is predominant in shonkinites. The
microshonkinites often contain spherulitic pseudoleucite phenocrysts. The coarse-
grained shonkinites contain pseudoleucites, characterized by symplectic inter-
growth of sanidine and nepheline. The well-known Shonkin stock contains
missourites and fergusites. The missourite is hypidiomorphic and contains leucite,
olivine (Fo
81
), salite and Ba
Ti oxides + apatite
±
-
Ti-rich biotite (5 cm across). There are dikelets
composed essentially of leucite; hence they are italites. There is also a fergusite dike
within the stock. The fergusite is composed entirely of pseudoleucite and salite with
a small amount of biotite and olivine containing leucite inclusion. There is also a
jumillite (much higher mica/leucite ratio than missourite) dike comprising olivine
(Fo
95
), salite, phlogopite, leucite, sanidine and minor amount of K-richterite.
An olivine kersantite comprises phenocrystal olivine (Fo
90
), diopside and pho-
logopite in a matrix of phlogopite, diopside, apatite and chromite rimmed by
magnetite, analcite and plagioclase. Of the exposed portion of the minettes, 25 %
are dikes and sills. Minettes are usually younger than ma
-
c phonolites. According
to O
'
Brien et al. (1991), the olivine minettes are characterized by porphyritic
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