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texture with phenocrystal phlogopite, diopside and olivine (Fo
87
-
92
) in a matrix of
sanidine, diopside, phlogopite and Fe-Ti
Ti oxides. The most evolved rocks of the
area include syenite and monzonite. The Highwood Baldy Stock is characterized by
the presence of three different types of alkali syenites (white, brownish grey and
bluish grey) and a monzonite. Several types of trachyte dikes are also present in this
area. Biotite clinopyroxinite xenoliths up to 30 cm across, occurs in the bluish grey
syenite of the Highwood Baldy Stock. Xenoliths composed of phlogopite, diopside
and/or olivine, are common in some minette dikes.
O
-
c
phonolitic rocks of the Highwood Mountains province of Montana. According to
them the potassic ma
'
Brien et al. (1988) made systematic studies on minette, lamproite and ma
c volcanic rocks and intrusives of the Eocene Highwood
Mountains province, display mineralogical, textural and chemical evidence of
several episodes of fractional crystallization and mixing of phonolitic magmas and
minette at shallow depth. They concluded from the composition of the phenocrysts,
bulk rocks and REE abundances of most primitive minette magmas that they were
derived by partial melting of a phlogopite-bearing garnet peridotite mantle.
Although the two groups of rocks are characterized by different phenocrystal
assemblages, the bulk composition of both the groups overlaps signi
cantly.
'
Brien et al. the mineralogical differences observed in both
groups of rocks is related to difference in volatile fugacities during crystallization,
which may indicate low pressure degassing. The Highwood Mountains province is
characterized by an older series of quartz-normative latite
According to O
ows and dykes and a
younger series of K-rich silica-undersaturated ma
flows. There are also
dykes and sills of rare lamproitic intrusions. Shonkinites are intruded as four major
stocks. There are also relatively
c phonolite
fine-grained intrusives of compositions similar to
minettes, which occur as smaller intrusive bodies. The minettes are characterized by
phenocrysts of diopsides (Al
2
O
3
content 1 wt%, mg # 92) and phlogopite with or
without olivine (Fo
89
-
92
) in a groundmass of sanidine, biotite, salite, and Fe-Ti
Ti
-
oxide. The ma
c phonolites are constituted of salite (mg # 76, Al
2
O
3
= 4 wt%),
pseudoleucite and olivine (Fo
77
-
60
) phenocrysts in a groundmass comprising salite,
sanidine and Fe-Ti
Ti oxide. Biotite, nepheline and glass may or may not be present in
the matrix. Leucite-bearing lamproitic rocks also occur rarely.
They are characterized by the presence of phenocrystal phlogopite, diopside,
leucite and, olivine (at present pseudomorphed by talc and serpentine) in a matrix of
phlogopite, diopside, leucite, olivine, and Fe-Ti
-
Ti oxides. Although cumulates and
xenoliths are common in all magmatic rocks, mantle xenoliths are absent.
Although major and trace element abundance does not show any distinction
between ma
-
c phonolites and minettes, the most primitive variety of minettes are
slightly enriched in MgO than the phonolites, which display more evolved com-
position. According to O
'
Brien et al. the lamproitic dyke is chemically similar to
primitive minette. They considered that the syenite trend may be related to frac-
tionation of biotite pyroxenite assemblage from a primitive ma
c phonolite magma.
This is in agreement with the occurrence of large bodies of pyroxenites in the area.
There are two trends converging at MgO = 11.5 wt%. The samples which plot on
the higher CaO branch are representative of shonkinites, rich in salite and biotite.
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