Geology Reference
In-Depth Information
4.12.14 Yakokut (58
27
N, 125
29
E)
°
′
°
′
The Yakokut complex (Kanukov et al. 1991) intrudes lower Cambrian dolerites and
Jurassic formation. The complex is characterized by dykes of alkaline and mod-
erately alkaline co-magmatic rocks (Fig.
4.32
). The volcanic rocks occur as relics
within a deeply eroded faulted caldera. The lowermost section (200 m thick) is
composed of feldspar-bearing epileucitite and leucitite. The middle unit
(200
-
250 m thick) is constituted of lavas, braccias and tuffs of leucitites, biotite
leucitites, and melaphonolites. The upper most section is constituted of lavas and
lava breccias of leucite phonolites and trachyphonolites. Lamproites of similar
compositions have been found in the contact zone of the complex. Phologopite-
bearing leucitite is characterized by the presence of phlogopite (20 vol%) and
diopside (10 vol%) in a groundmass of orthoclase and plagioclase. Biotite-bearing
leucite melaphonolites have phenocyrsts of pseudoleucite and diopside. Leucite
phonolites are characterized by the presence of orthoclase, mica and pseudoleucite.
Microshonkinites constitute the earliest intrusive series. They were emplaced
before the eruption of volcanic units. Nepheline and pseudoleucite-bearing syenites
and peralkaline syenites are also observed in the central part of the massif. Within
the central complex, a large number of north-south-trending dykes (1
3 m long and
several small stocks of about 100 m diameter) also occur. They comprise olivine
leucitite, leucitite, leucite and aegirine-bearing phonolites, olivine-leucite-phlogo-
pite phonolites, syenite porphyries and lamproites.
-
4.12.15 Rododendron (58
22
N, 125
36
E)
°
′
°
′
A circular stock (200
300 m diameter) intruding Jurassic sandstones, is found in
Rododendron. The central part of the complex comprises shonkinites and syenites
at the centre of the complex. It is surrounded by shonkinite porphyries and syenite
porphyrites. Volcanic rocks also occur at the margin of the intrusive rocks, which
are constituted of leucite-bearing trachytes, comprising phenocrysts of biotite,
orthoclase, sodic plagiocalse (An
20
-
30
) and apatite in a
-
fine-grained matrix. The
shonkinite porphyries are melanocratic with augite phenocrysts (30
40 %) and a
small amount of biotite (up to 10 vol%) with large amount of pseudoleucite, olivine
and alkaline amphibole. The groundmass is constituted of K-feldspar, biotite,
augite, plagioclase (An
40
-
50
) and small grains of leucite, apatite and magnetite. The
central part of the shonkinite is of two types: one portion containing nepheline, the
other part has pseudoleucite. The age of the stock was determined to be 166
-
±
5Ma
(Eremeyev 1987).
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