Volcanism of the Siberian Platform — Sobolev 
tion of the trap magma. There are traps both 
older and younger than the alkaline rocks. 
Kimberlite has been formed extensively in 
the northeastern border of the platform, more 
than 100 pipes and dikes being known there at 
present. For some kimberlite bodies the same 
sort of relationship to the traps has been es- 
tablished as for the alkaline basaltoids. Some of 
the kimberlite is not younger than Permian in 
age, since pyrope and diamonds from it occur in 
the Upper Permian deposits. However, there 
undoubtedly are younger kimberlites also, for 
a fragment of a belemnite of Upper Jurassic or 
Lower Cretaceous age was found in one of the 
pipes. 
Kimberlite, as a magmatic rock, belongs to 
the ultrabasic group, its composition ranging 
from a form nearly devoid of alumina and alka- 
lies to one rather high in AI2O3 and especially 
high in K 2 0 in mica-rich varieties. The prin- 
cipal mineral is always olivine, containing up 
to 10% Fe 2 Si0 4 , and is present in at least two 
generations: large crystals, commonly partly re- 
sorbed, and small idiomorphic microphenocrysts. 
Phlogopite occurs in idiomorphic plates and 
ranges widely in quantity. It is unquestionably 
magmatic. Pseudomorphs of pyroxene microlites 
are sometimes seen in the vitreous matrix. The 
latter is always -altered. In the northern regions 
fine-grained monticellite has been found in kim- 
berlite for the first time, chiefly in dikes. Nephe- 
line also is supposed to be present. Pyrope, and 
probably picroilmenite and chrome spinel, com- 
monly belong to the first generation of pheno- 
crysts. Perovskite is a later accessory. 
As in South Africa, the kimberlite has a brec- 
ciated structure and is contaminated by frag- 
ments of various sorts of rocks. There are, on 
the one hand, typical pyroclastic rocks filling 
explosion pipes, and, on the other, magmatic 
breccias with various contents of xenoliths. Since 
the rocks have been altered ( serpentinized and 
carbonatized ) , it is not always possible to prove 
the presence of magmatic cement. 
The fragments of other rocks may be sub- 
divided into: 
1) Fragments of ultrabasites and eclogites 
whose origin is in some way connected with the 
origin of the kimberlite itself; 
2) Fragments of rocks picked up by the 
453 
magma from (a) the crystalline basement for- 
mations, and (b) the sedimentary cover. 
Fragments of the first type include various 
ultrabasites — olivinites, peridotites, and others 
- — often containing pyrope as well as typical 
eclogites. The discovery of diamond-bearing ec- 
logites, resembling the well-known eclogite 
xenolith found by Bonney (1899) in South 
Africa, is of particular interest. Together with 
such eclogites brought up from great depth, 
there are eclogites and eclogite-like rocks (con- 
taining plagioclase) picked up from the crystal- 
line basement and formed by eclogitization of 
hypersthene schists. 
The fragments of rocks picked up by the 
magma vary widely in quantity and composition. 
Xenoliths of gneisses and schists are abundant 
in several of the pipes. This can be taken as 
proof that the "explosion” that formed the pipe 
took place at a level lower than the base of the 
sedimentary cover. Allowing for this, and taking 
into consideration the geophysical data on the 
depth of the crystalline basement in the area and 
also the thickness of the rocks since removed 
by erosion, we can say that the explosion took 
place at a depth somewhere between 2 and 4 
km. The depth is greater than in the case of 
the formation of the trap necks, which was 0.5 
to 1 km. 
As in South Africa, xenoliths of rock forma- 
tions that occur at much higher levels (some- 
times several hundred meters higher ) are found 
among the fragments, proving that there was 
not only an ascending but also a descending 
movement of the material in the pipe. 
The synchroneity of their formation has led 
to the hypothesis of a genetic connection be- 
tween the ultrabasic rocks and kimberlites and 
the trap magma of the Siberian platform. Petro- 
graphic data, however, do not support this hy- 
pothesis. The olivine of the kimberlites and the 
meimechites contains only 10% fayalite, and 
this is proof enough that these rocks could not 
have originated as a result of differentiation of 
the trap magma. The author quite agrees with 
Y. M. Sheinman’s (1955) suggestion of the 
formation, in this case, of magmatic chambers 
at much deeper levels than those of the trap 
magma. 
The development of trap volcanism on such 
