Geology Reference
In-Depth Information
orientation of dark-coloured minerals are predominant. The complex was emplaced
313
±
11 Ma year ago (Zhidkov 1990).
4.12.12 Molbo (59
05
N, 118
49
E)
°
′
°
′
A phlogopite-bearing leucite lamproite (thickness: 1.3
1.5 m) occurs at Molbo
(Aldan Alkaline Province). It cuts lower Cambrian marls, which overlie the Pre-
cambrian basement. The dyke has a zoned structure, with the central portion being
porphyritic and the periphery is aphyric. The core is composed of phenocrysts of
olivine (often altered), leucite and lath-shaped phlogopite. The
-
fine-grained portion
comprises phlogopite, pseudoleucite and microlites of olivine, clinopyroxene,
magnetite and apatite set in a groundmass of phlogopite, chlorite and alkali
amphibole. The lamproite was emplaced 122
92 Ma years ago (Makhotkin et al.
1989). Lamproitic rocks comprising olivine, phlogopite and diopside also occur at
Khani (57
-
°
54
′
N, 120
°
22
′
E), Lyzhnaya (58
°
42
′
N, 125
°
46
′
E) and Ryabinovaya
(58
E). The lamproitic pipe of Ryabinaovaya is associated with
phlogopite-bearing shonkinite and phlogopite minette. The breccia pipe at Kaila
(600
°
40
′
N, 125
°
51
′
700 m) is one of the largest breccia pipes. It intrudes Precambrian crystalline
rocks of the Aldan shield basement complex and dolomites of lower Cambrian age.
The lower part of the pipe is constituted of crystal tuffs, breccias and xenolith-
bearing tuff breccias, which are made up of small fragments of intrusive or auto-
lithic
-
fine-grained lamproite along with large megacrysts of olivine and feldspar
aggregates. The fragmental
lamproites are
fine-grained porphyritic rocks with
phenocrysts of olivine (10
12 %), chrome-diopside (7
15 %) and phlogopite
-
-
(5
12 %) in a
fine-grained groundmass of diopside,
fine laths of phlogopite and
-
K- feldspar. Pseudoleucite is present in the
fine-grained breccia fragments.
4.12.13 Tommot (58
23
N, 125
13
E)
°
′
°
′
Both extrusive and intrusive potassic rocks (167
3 Ma old; Orlova 1990)
were emplaced in Archaean granite and granites gneiss. The extrusive rocks are
found in a depression up to 950 m deep in the western part of the massif. The
volcanic rocks comprise three segments: (1) the lowest one is feldspar-bearing
leucitite, overlapped by (2) peralkaline trachytic lavas and breccias; and
±
5
125
±
-
finally (3)
the peralkaline trachytic ignimbrites, constituting the uppermost part. Leucitite and
leucite tephrite dykes cut across ignimbrites towards the eastern part of the com-
plex. Parallel to the margin of the complex there is a larvikitic body, comprising
strongly oriented tablets of alkali feldspars (50
-
70 vol%) and clinopyroxene
(10
25 vol%) with such accessory phases as amphibole, biotite, sphene etc.
-
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