Different Localities of Potassium-Rich Silica-Undersaturated Igneous Rocks and Their Silica-Rich Variants - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 133

The Eocene igneous rocks constitute two groups: (1) a silica-saturated series
ranging from minette to trachytes (their coarse-grained equivalents are shonkinites
and syenite) and (2) a silica-oversaturated trachyte and trachy-dacite series.
The latites contain abundant crustal xenoliths, but the intrusive rocks of the
minette-trachyte group show in-situ differentiation feature. In the Rockyboy stock,
different rock types include biotite pyroxenite, various types of shonkinites
(pseudoleucite, plagioclase and hypersthene), nepheline-aegirine syenite, porphy-
ritic potassic syenite and carbonatite. Macdonald et al. found that with exception of
a carbonatite, the entire range represents a single composite stock, which covers a
complete compositional range of intrusions at Bearpaw Mountains.
The minettes range in composition from 20 to 6 wt% MgO. The more magne-
sium varity is characterized by the presence of forsterite, Cr- spinel, diopside (with
or without phlogopite). Macdonald et al. noted that olivine-free rocks are charac-
terized by following two assemblages: (1) salite + phlogopite + pseudoleucite and
(2) salite + phlogopite + analcites (secondary after leucite). Primary leucite is also
present within olivine as inclusions. The mineral chemistry and in particular, texture
of clinopyroxenes indicate that there was mixing between different varieties of
minette magmas.
Latites are characterized by the presence of phenocrysts of sanidine, plagioclase,
clinopyroxene

± ± ± ±

apatite in a feldspathic
groundmass. The latites contain brownish glass with mafic clots consisting of
following assemblages:

phlogopites amphibole magnetite

(1) Biotite + clinopyroxene,
(2) Biotite + opaque + oxide, and
(3) Biotite + plagioclase + carbonate.

Macdonald et al. considered that the compositional variation is also related to
accumulation of olivine, spinel and clinopyroxene phenocrysts and preservation of
mantle xenoliths in the minette.
With reference to the phase equilibria relation in the system diopside-forsterite-
K-feldspar (Esperanca and Holloway 1987) studied at different pressures (1.0, 1.5,
1.7 and 3.0 GPa), Macdonald et al. concluded that the minette magma was gen-
erated at depths equivalent to that of 3.0 GPa from a source having an assemblage
of olivine, phlogopite and diopside. The primary magma further underwent frac-
tionation of olivine and diopside.
The minettes are characterized by higher concentration of large ion lithophile
elements (LILE) and light rare earth elements (LREE). They contain K 2 Oupto
6.18 wt%, Ba: 5,491 ppm, Sr: 2,291 ppm, and Ce: 99 ppm. The 87 Sr/ 86 Sr ratio lies
between 0.707 and 0.710 and 16. According to

Macdonald et al. latites were formed by fractional crystallization of minette magma
in combination with assimilation of crustal rocks, which resulted in enrichment of
SiO 2 and the Na 2 O/K 2 O, 87 Sr/ 86 Sr ratios. Analyses of Bearpaw minettes are given
in Table 4.9 .

∈ Nd ranges between 10 and −