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I. 2. N u c l e a r s t a g e (4,200-3,б00 Ma). In late Katarchean there
was the formation of protocontinental «greygneissic» crust, which consisted
completely of metamorphic magmatic rocks, represented by andesites,
dacites, tonalites, trondhjemites and plagiogranites, coincident on midlle
chemical composition to diorite with sharp dominance of sodium over
potassium. In a lesser degree, the horizons of metamorphic sedimentary-
volcanic rocks are found.
The greygneissic complexes of rocks with absolute age 3,900-3,200 Ma
are found on all continents of the Earth. The separate dome-shaped
elevations of these rocks vary in diameter from several hundreds up to
thousand kilometers. In the late Katarchean there were «islands» of salic
crusts folded by the greygneissic complexes of rocks rather multiplied
composition, which have received the title «nuclear» (Pavlovskiy &
Gluchovskoy, 1982). It is quite possible, that the large dome-shaped
structure of ancient shields, observed on space snapshots, can be the relics
of salic islands of Katarchean. Among them the dome Ungava (Canadian
shield), Aldano-Timpton dome (Aldan shield), Singbum dome (Indian
shield) and dome Pilbara (West-Australian shield) can be regard (Khain,
Bozhko, 1988).
In late Katarchean there were already aqueous basins, with depositions
of sedimentary rocks and pillow-lava of the serial Isua in Greenland.
However, water pools at that time were, apparently, small and flat,
somewhat sunk in depression between dome-shaped elevations. The water
was free of dissolved sulfates and oxygen, but contained chlorides. The
lower hydrogen ions concentration in comparison with modern (рН = 7.0
instead of 7.5-8.5) precluded depositing carbonates from the water. The
temperature of the water was above modern. The atmosphere was strongly
different from modern and resembled venerian. The rotation rate of the
Earth was considerably higher; the duration of day in Katarchean consisted
of 5 hours. The major closeness of the Moon to the Earth caused power
lunar affluxes. The solar radiating was more intensive (Monin, 1983).
The geodynamic regime of the nuclear stage of development of the Earth
differed by high geothermal gradients (54
o
C/km), waterless granulate
metamorphism and fine-meshed near-surface convection in the upper
mantle and crust. According to many researchers, the striking reaction of
asteroids and meteorites on the surface of the Earth in the period between
4200-3800 M.a. determined the localization upraised of mantle jets and
growth of sialic nuclear above them. In intermediate depressions between
rising domes, there were, probably, downward convective currents and the
accumulation of the sedimentary - volcanogenic depositions in the initial
water basins (Khain, Bozhko, 1988).
From the position of primordial hydridic Earth, as a result of radioactive
warming up exterior layer of the core, the separation of disorderly streams
of protonic hydrogen and the hydrogen blow-down
of bowels of the Earth to
the end of Katarchean and in the beginning of Archean there was the
enrichment by oxygen the rocks of the upper mantle and earth crust with
formation of silicate-oxide lithosphere. The earth’s crust at the depth of 25-
30
30 km appeared composed above by «greygneissic» dioritic complex of
rocks, and below by primitive basaltoic rocks.
Metallogeny of the nuclear stage is less known. It is found, that the
Katarchean «greygneissic” complex is enriched a little by Ni, Cr, Fe, V, but
has low contents of lithophile elements, including U, Th, Rb, Ti, Zr, F, Nb,
Ba, Sr, B, in comparison with normal granites (Khain, Bozhko, 1988). In
the composition of « greygneissic » complex of rocks the layers of
ferriferous quartzites, jaspilites, aluminous quartzites, gneisses and shales
are being found. The most ancient deposit of ferriferous quartzites is found
in a southwest part of Greenland in district Godhab-fiord in the complex
Isua. The absolute age of these quartzites equals 3,760 Ma.,
and the reserves
of iron ores compound here 2100 Mt. The ferriferous quartzites were
generated, apparently, at the expense of redeposition of products of the
chemical weathering basic rocks in adjacent fine aqueous basins. In the
Western Greenland, large scheelites deposits dated to metavolcanic acidic
and basic composition with intercalation of carbonate rocks and metapelites
complex Isua (3,800 Ma) are detected, as well as in younger supercrustal
complex Maline (3,000-2,800 Ma.). Scheelite is dated to аmphibolites with
initial pillow-lava and tightly associates with the tourmaline (Andreev et al.,
1997).
The analysis of available materials allows making a preliminary
conclusion, that the Katarchean metallogenic epoch was specialized on
initial concentration of siderophile (iron, titanium) and,
to a lesser degree, to
lithophile (aluminum, tungsten, phosphorum) elements. At the expense of
the chemical weathering of basalts, gabbro and anorthosites, the
redepositions of products of the airing in superficial aqueous basins and
subsequent processes of regional metamorphism were generated deposits of
the indicated mineral resources.
II. P e r m o b i l e c y c l e (3,600 – 2,000 Ma) of geological
development of the Earth is subdivided into three stages: 1. early Archean
(3,600- 3,000 Ma ); 2. late Archean (3,000 – 2,500 Ma) and 3. early
Proterozoic (2,500 – 2,000 Ma).
The geodynamic conditions and nature of geological processes in
Archean are specified by particular features, when the surface of the planet
was still strongly heated, the hydrosphere is practically absent, and the half-
plastic lithosphere was in stage of forming, that does not allow to make
conclusions on the principles of actualism. At this time complexes of rocks,
unique in composition, were constituted, which are not present any more, or
extremely rare among the later formations. The presence of comatiites,
enderbites, charnockites, аnorthosites, granulites, jaspilites, testify to the
higher thermodynamic level of magmatism, metamorphism and deformation
of rocks. The regarded cycle of development of the earth crust is called by
L.I.Salop (1982) as p e r m o b i l e , which one differed by overall tectonic
mobility of the structures. The latter were represented
by cratons, composed
from granulite-gneisis, granite - gneisis, granulite-greenstone and granite –
greenstone areas, with infrequent and small epicratonic sedimentary
aqueous basins and intracratonic geosynclines.