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Меtallogenic epochs of formation deposits of lithophilic rare elements
coincide in time and space to the periods of tectono-magmatic activity after long
periods of tectonic rest on the platforms. The close geochemical connection of
the terra-rare lithophilic elements and alkaline, especially potassium, testifies to
the united depth of the source. In the opinion of V.N.Larin (1980), the juvenile
sources of these elements are the olivine-pyroxene rocks of the depleted upper
mantle. The extraction of lithophilic elements from pyrolite layer is carried out
by deep intertelluric fluid, the appearance and character of which is determined
by degassing of hydrogen from the external core of the Earth. The accumulation
contents of terra-rare elements in the areas of tectono-magmatic activity is
caused by increasing the thickness of pyrolite layer at upper mantle in
connection with the expansion of the Earth, as well as the corresponding fall of
gravity, reduction of gradient of pressure in mantle and transformation spinel-
garnet peridotite to olivine-pyroxene rocks. It is accompanied by “lattice
downthrown” of isomorphous impurity of lithophile elements and alkaline,
especially potassium.
At the appearance of intertelluric fluidic flow these elements are exposed
by "flotation " and are involved in processes of formation rare metal ores in the
upper parts of terrestrial crust. The sources
of the potassium and water, that were
necessary for generation alkaline-ultramafic melts in the upper mantle and
formation K-richterite-olivine and leucite lamproites, you have to search not in
processes of obduction ocean’s plates and convection, as major researchers
assume (Gurney, 1989; Kirkley et al. 1991), but at the natural evolution of the
pulsating-expanding Earth. Thus, at the consideration of evolution terrestrial
crust and upper mantle of the continents on the basis of the hypothesis of the
primordial hydridic Earth, allows us to come closer to the understanding the
internal reasons of determining foundations of the distribution of lithophile rare
elements and diamond-bearing kimberlites and lamproites in the time and space.
4. EPOCHS OF CRATONIZATION AND KIMBERLITE MAGMATISM
As a rule, the diamond-bearing kimberlite provinces are located within
the largest cratons on the platforms and shields with a powerful cover of
sedimentary-volcanic and metamorphic rocks. This observation was first made
by T.N. Clifford (1966) in Africa and is known as Clifford’s Rule. There are
approximately 5,000 kimberlites, worldwide of which 500 are diamond-bearing,
and almost exclusively occur on Achaean cratons. Diamond-bearing lamproites
are located in Proterozoic mobile belts adjacent to Archean cratons (Janse and
Sheahan, 1995).
All well-known diamond enriched kimberlite and lamproite bodies are
connected with epochs of the greatest cratonization of the Earth, comprising the
interval from 1200 up to 100 Ма (Figure 3). At that time the subcontinents
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Pangea III and II were generated. Except for already known deposits, other
diamond-enriched kimberlites with age 900 and 600 Ма, can possibly be
discovered.
The subcontinent Pangea I, which existed among 2700-1600 Ма, had a
rather small amplitude of elevation, and probably because of it, rare and
insignificant displays of diamonds are connected with kimberlite and lamproite
magmatism. On the contrary, the protoplanetary cycle of the development of the
Earth up to 3100 Ma was differed with its intensive tectonic movements and
formation of numerous rather small cratons (Salop, 1982). Diamond-bearing
garnet peridotites, eclogites and kimberlites could be dated to that cratons, for
example, with age 3300 or 2700 Ma (Table 1, Figure 2, 3).
The most general fundamental law of kimberlites dislocation is their
presence only within the limits of ancient platforms.
The kimberlite volcanism is
infallibly related to the history of geological development of cratons. It is a
product of evolution the upper mantle under the spacious platform areas. The
largest displays of the ultrabasic and alkaline-ultrabasic volcanism on the planet
are the kimberlite provinces, surrounded on their periphery with bordering zone
of pikrites.
The formation and localization of kimberlite provinces (diameter up to 2000
km), their diamond-bearing fields (diameter up to 50 km) and separate pipes of
explosion (diameter up to 1.5 km) were caused by essentially various
of type and
scale of natural processes. So, occurrence of kimberlite provinces was one of
consequences of evolution the upper mantle of the planet. The kimberlite fields
were formed as a result of raising the alkaline-ultrabasic melts along zones with
increased permeability of the earth's crust. The location of explosion pipes and
dikes within the limit of kimberlite fields occurred in the superficies
joints of the
crossing ore-controlling and ore-accommodating dislocations.
According to representations about relationship between the cause-
consequence of kimberlite magmatism with convective currents in the upper
mantle, in particular, with elevation of the Mohorovichic discontinuity and the
subsequent subhorizontal diffluention of warmed up substratum, it was assumed
that at the foot of earth's crust under every kimberlite provinces there arose a
huge "lenses" of intermediate deep crust-mantle substances (Milashev, 1974;
Greenson, 1984).
Mobilization and radial movement of huge masses of heated substratum,
possessing with high viscosity, inevitably resulted in the elevation of extensive
areas of earth’s crust on the platforms and formation cratons. The elevated areas
were exposed to denudation, the size and age of which is reliably established by
geological methods. Predominantly the distribution of kimberlites over the areas
with long, slow and steady elevation is the main regional law of dislocation
kimberlites at the level of provinces.
The kimberlite fields are dated to zones of touchy permeability on the areas,
but with the lower thickness of earth’s crust (Мilashev and Rosengerg, 1974).
Complex researching of rocks megacracks and based on it structural separation
of kimberlites fields of the Siberian and Russian platforms has shown, that
overpowering majority of kimberlite diatrems are located (96 %) within the
limits of blocks with isotropic orientation megacracks, that allows to accept