Different Localities of Potassium-Rich Silica-Undersaturated Igneous Rocks and Their Silica-Rich Variants - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 199

4.12.16 Lomam (57 07 N, 128 05 E)

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Several stocks cutting across Cambrian and lower Jurassic country rocks are
observed at Lomam. The stocks are constituted of mica peridotite, missourite,
shonkinite and fergusite. The last two rock types are constituted of olivine, diop-
side, biotite or phlogopitic mica, leucite, pseudoleucite, K-feldspar and K-richterite
in different proportions. Kalsilite and rarely nepheline are found within leucite.
Apatite, sphene, chromian spinel and magnetite are present as accessory minerals.
The mica peridotites are constituted of olivine, clinopyroxene and phlogopite in
equal amounts. Inclusions of leucite are found within pyroxenes. In missourites, the
content of ma c minerals gradually decreases. Apart from leucite inclusions in

pyroxene up to 30 vol%, this mineral is observed in areas between ma c minerals.

Shonkinites often grade to melanocratic pseudoleucite syenites. The shonkinites are
of variable compositions, consisting of pseudoleucite, feldspar-kalsilite intergrowth
and mica in the following proportion of 6:2:1. They also contain unaltered leucite
crystals with well-de

ned crystalline outline.

4.12.17 Tokko (55 36 N, 130 00 E)

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flows (180 m thick) covering an area of 200 km 2 occur at Tokko on both
sides of the Stanovoy ridge. The lower part of the ma

Ma c lava

c sequence comprises alkali
basalts followed upward by peralkaline and weakly alkaline ma c rocks including

hawaiite, limburgite, nepheline and leucite-bearing ma c rocks (Semenova et al.

1987). These rocks are associated with melanephelinite and phonolitic nephelinite.
The alkaline volcanic rocks contain mantle xenoliths of spinel lherzolite, pyroxe-
nite, wehrlite, websterite and xenocrysts of sanidine, olivine, orthopyroxene, spinel
and clinopyroxene. The peralkaline ma c rocks contain Ti-phlogopite, kaersutite

87 Sr/ 86 Sr ratio ranges between 0.7035 and 0.7045.

and Al-augite. Their

4.12.18 Dezhnevski Complex (66 ° 05 ′ N, 169 ° 47 ′ W)

The Dezhnevski complex (Perchuk 1964) covering an area of 125 km 2 occurs in the
extreme northeast of Russia in the Chukotka province near the Berring Strait. The
igneous body intrudes Carboniferous limestone and is of Palaeocene age. Syenites
with or without nepheline occur in the periphery of a granitic body. Pseudoleucite-
bearing shonkinites are reported within nepheline syenite.