Characteristic Features of the Volcanism of the Siberian Platform 
V. S. Sobolev 1 
Widespread volcanism is characteristic of the 
Siberian platform, and was especially intense 
during uppermost Paleozoic and lower Mesozoic 
time. The Siberian traps, which occupy an area 
of more than 1,500,000 km 2 , are best developed 
there. These are effusive and hypabyssal rocks 
of basalt-dolerite type, closely resembling trap 
rocks in other parts of the globe, especially the 
Karroo dolerites of South Africa. 
The rise of trap magma began in the upper 
Paleozoic (Permian or even as early as Upper 
Carboniferous) time, and reached its climax in 
the Lower Triassic period. It was accompanied 
by the ejection of much pyroclastic material, 
which formed a thick series of tuffs. Lava sheets 
and hypabyssal intrusions of various kinds and 
sizes were formed. 
The process of volcanism was rather com- 
plicated, and at present M. L. Lurie and V. Z. 
Masaitis distinguish five volcanic phases and 13 
separate intrusive complexes, each having its 
own specific features and pattern of develop- 
ment in various parts of the platform. In spite 
of this, however, the magma had some charac- 
teristic features over the entire area, notably an 
iron content somewhat higher than is usual and 
an especially rapid increase of relative iron con- 
tent during the process of crystallization dif- 
ferentiation. The increase of the iron content of 
the femic minerals in the process of crystalliza- 
tion prevails over the conventional reaction 
series of Bowen. For example, olivine of early 
formation contains about 20% fayalite, that 
characteristic of the usual type of traps contains 
about 40% fayalite, and the iron content of 
olivine in pegmatoid veins is as high as 80%. 
The residue of the differentiation is as a rule 
micropegmatite, either in the mesostasis, or in 
some cases forming veinlets of granite compo- 
1 Siberian Division, Academy of Sciences, Novosi- 
bizsk, U.S.S.R. 
Manuscript received April 19, 1962. 
sition. However, such veinlets are quantitatively 
very small. Much rarer is the formation of alka- 
line rocks, such as teschenite, in the last stages 
of differentiation. 
The above characteristics of crystallization 
differentiation (rapid change of the iron con- 
tent of femic minerals and the subordinate role 
of the discontinuous reaction series ) are typical 
not only of the trap formations, but also of the 
deeper-seated magmatic complexes of the plat- 
form. The gabbro-anorthosite-granite complex 
of the margin of the Russian platform, with 
its characteristic granites of Rapakivi type, be- 
longs to this group. These characteristic features 
distinguish complexes of this type from the 
typical granodiorite complexes of orogenic 
zones, in which crystallization largely corres- 
ponds to the well-known Bowen series. Relative 
increase of the iron content of femic minerals 
is there much slower, as is clearly seen by com- 
paring it with the change in piagioclase com- 
position. 
Along with the trap formation, in part simul- 
taneously and in part a little later, another type 
of volcanism developed widely on the margin of 
the platform, with the formation of ultrabasic 
and alkaline rocks. Differentiated effusive and 
intrusive complexes were formed in some re- 
gions, kimberlites in others. 
A typical example of the differentiated com- 
plexes can be seen in the northern part of the 
Siberian platform in the area of the so-called 
Gulinski intrusion. The effusives range from 
meimechite, the closest extrusive analogue of 
true intrusive ultrabasite, to different kinds of 
alkaline basaltoids containing nepheline and 
piagioclase. Among the intrusives are all kinds 
of rocks from dunite to various alkaline rocks 
rich in nepheline. Carbonatite is also present. 
E. L. Butakova (1956) and Y. M. Sheinman 
(1955) have shown that the volcanic rock for- 
mation here was largely simultaneous with erup- 
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